muouoxzon. .4 I r . 3133.33! oily-3... it! .I .\n f. 31.4- lilv.‘ (Lu-null. 1. 7...} t... v0.5.7. '!.0 V 130M 7/0 MICHIGAN STATEU "HES... I“ II I III II “ IIIIIIII IIIIIIIIIIIIIIIIII 293 00579 1524 This is to certify that the thesis entitled The Effect of Sorption of Flavor Volatiles on the Adhesive and Cohesive Bond Strength of Multilayer Laminations presented by Mark Alan Schroeder has been accepted towards fulfillment of the requirements for M. S. degree in Packaging @wé? MW a rofessor Bruce I? jofigrte , Ph . D . Date May 11, 1989 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution LIBRARY Mi("39ml State University PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE , it; {,3 2 1:51.! ‘I I‘T—T‘: MSU Is An Affirmative ActiorVEqual Opportunlty Institution THE EFFECT OF SORPTION OF FLAVOR VOLATILES ON THE ADHESIVE AND COHESIVE BOND STRENGTH OF MULTILAYER LAMINATIONS BY Mark Alan Schroeder A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE School of PaCkaging 1989 ‘o‘Io n )A 501 ABSTRACT THE EFFECT OF SORPTION OF FLAVOR VOLATILES ON THE ADHESIVE AND COHESIVE BOND STRENGTH OF MULTILAYER LAMINATIONS BY Mark Alan Schroeder The effect of sorption of flavor volatiles on the adhesive and cohesive bond strength of multilayer laminations was investigated. Polyvinylidene chloride (PVDC)-coated polyester/ethylene vinyl acetate copolymer (EVA), and PVDC-coated polyester/linear low density polyethylene (LLDPE) adhesive laminates were exposed to lemon juice and hot sauce products and controls, in a surface exposure experiment, under accelerated conditions. Sorption of flavor volatiles resulted in delamination of both laminates, following exposure to both food products. Using Dynamic Headspace Concentration, interfaced with Gas Chromatography Analysis, d-limonene was identified as the primary component sorbed by the laminates upon exposure to lemon juice. Using Electron Spectroscopy for Chemical Analysis and Scanning Electron Microscopy, adhesive bond failure was found within the LLDPE laminate, and cohesive bond failure in the EVA laminate. Bond strength was quantified using a Peel Test, and significant reductions occurred upon exposure to the food products. DEDICATION This thesis is dedicated to my wife Chris, for her support, love and enduring patience throughout this work. Also, to my parents, for their assistance and support through all my academic endeavors. iii I ACKNOWLEDGEMENTS would like to thank the following people, without whom, this work would not have been possible: Drs. Bruce Harte and Jack Giacin, for their guidance, support and expertise while serving as Co-Advisors. Dr. Ruben Hernandez, for his technical expertise while serving on the guidance committee. Dr. Dr. The for Dr. and Ian Gray, for serving on the guidance committee. Heidi Hoojat, for her assistance and patience. Center for Food and Pharmaceutical Packaging Research, project funding. John Gill, for his statistical know-how. the "Varsity Club", for their friendship and inane ability to maintain my sanity. iv TABLE OF CONTENTS Page LIST OF TABLES vi LIST OF FIGURES vii INTRODUCTION 1 LITERATURE REVIEW 4 Multilayer Laminations ...............................4 Product/Package Interactions .........................9 AdheSion OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOllg Adhesive/Cohesive Bond Failure .... .................. 27 Analytical Tests .............. ..... . .............. ..40 MATERIALS AND METHODS 50 Materials: Laminate Materials .......................50 Food Products and Controls ... ............ 51 Experimental Methods: Selection of Laminate Materials and Food Products ...55 Laminate Exposure .............56 Analytical: Quantification of Bond Strength ........58 Determination of Locus of Failure ......62 Examination Of Locus of Failure ........64 Gas Chromatographic Analyses .. ......... 65 RESULTS AND DISCUSSION 71 Multilayer Laminate Delamination ...... . ............. 71 Bond Strength ......OOOOOOOOIOOOOO ....... O ........... 73 Locus of Failure ................ ........... . ........ 89 sorption ......OOOOOOOOOOOOOOOO0.00.00.00.00.0.0.00.0099 CONCLUSIONS 108 FUTURE WORK 112 APPENDICES Appendix A: Composition of Product Additives ......113 Appendix B: Initial Laminate Exposure Study .......115 Appendix C: Test Cell Construction .......... ...... 120 Appendix D: Improved Bonferroni t-Test ............ 121 BIBLIOGRAPHY 123 Table 10 11 12 13 14 15 LIST OF TABLES Page 1985 U.S. shipment of converted flexible packaging materials. 11 U.S. shipment of converted multilayer flexible packaging materials for 1977, 1983, and 1985. 12 Classic Food & Drug Administration food simulants. 41 Test conditions for bond strength determination. 61 Gas chromatograph conditions used in analysis of flavor sorption by laminate materials. 67 Dynamic headspace concentrator conditions used in analysis of flavor sorption by laminate materials. 69 Linear low density polyethylene laminate bond strength values for samples exposed to food products and simulants. 75 Statistical comparison of treatments applied to linear low density polyethylene laminate samples. 77 Percent change in bond strength in the linear low density polyethylene laminate, for samples exposed to food products and controls. 79 Ethylene vinyl acetate copolymer laminate bond strength values for samples exposed to food products and simulants. 84 Statistical comparison Of treatments applied to ethylene vinyl acetate copolymer laminate samples. 86 Percent change in bond strength in the ethylene vinyl acetate copolymer laminate, for samples exposed to food products and controls. 87 Composition of sixteen multilayer laminations exposed to three food products and Food & Drug Administration food simulants in initial exposure study. 116 Food products used in initial study and results of laminate exposure. 118 Food & Drug Administration food simulants used in initial study and results Of laminate exposure. 119 vi LIST OF FIGURES Figure 10 11 12 Flow diagram Of typical dry adhesive laminating process. Breakdown Of 1985 U.S. packaging industry shipments by material type. Cross-section of multilayer lamination, with components and regional classification. Cross-section and composition of laminate structures selected for research. Schematic of modified ASTM test cell used for laminate exposure. Material flow diagram for testing of laminate samples. Schematic of Tekmar dynamic headspace concentrator gas flow system. Example of output from Instron tensile tester for peel test Of non-exposed, heated laminate samples. Relative change in bond_strength among LLDPE comparisons for exposure to heat, citric acid and acetic acid control solutions, lemon juice and hot sauce. Relative change in bond strength among EVA comparisons for exposure to lemon juice and hot sauce. Output from Electron Spectroscopy for Chemical Analysis for delaminated ply of linear low density polyethylene laminate sample exposed tO hot sauce, with peaks representing oxygen ls, nitrogen ls and carbon ls atomic bonding orbitals. Cross-section of linear low density polyethylene laminate roll stock sample. vii Page 10 29 52 57 59 70 74 80 88 90 94 13 14 15 16 17 18 19 20 21 22 Cross-section Of ethylene vinyl acetate copolymer laminate roll stock sample. Cross-section Of delaminated hot sauce-exposed linear low density polyethylene laminate sample. 800x magnification Of delamination in Figure 14. Cross-section Of delaminated lemon juice exposed ethylene vinyl acetate copolymer laminate sample. isoox magnification of delamination in Figure 16. Gas chromatograph Of lemon juice exposed linear low density polyethylene laminate sample, via dynamic headspace concentration, with d-limonene peak at 10.97 minutes. Gas chromatograph of linear low density polyethylene laminate roll stock sample, via dynamic headspace concentration. Gas chromatograph of hot sauce exposed linear low density polyethylene laminate sample, via dynamic headspace concentration. Gas chromatograph Of non-exposed ethylene vinyl acetate copolymer laminate sample, via thermal distillation. Gas chromatograph of lemon juice exposed ethylene vinyl acetate copolymer laminate sample, via thermal distillation, with d-limonene peak at 11.35 minutes. viii 94 95 95 97 97 100 101 103 105 106 INTRODUCTION Plastics have had a major influence on the packaging industry, with particular impact on food packaging. Plastic films can provide precise control of gas and moisture permeation, grease resistance, printability and a broad range Of mechanical properties. Technological improvements have resulted in new forms and means Of plastic packaging. Laminates are combinations Of two or more films or other non-plastic materials into a single unit (Manypenny et al., 1988). This is accomplished by means of a suitable adhesive which allows the selection and incorporation of the above properties into a single structure. This flexibility has resulted in the commonplace use Of laminate structures for the packaging of such foods as meats and cheeses, snack foods and candies, beverages and boil-in-bag meals. At the same time it has become necessary to understand and overcome the disadvantages involved with plastic packaging materials. Product-package interactions are a potential problem capable of causing changes in both product quality and package performance (Hirose et al., 1988). Sorption of product components from the food phase 2 into the package wall is one mode of interaction common to plastic food packaging systems (Kwapong and Hotchkiss, 1987). This mass transfer process results not only in a reduction in concentration of components in the product, but may effect the morphology of the polymeric packaging material. Sorption Of flavor volatiles may result in changes in the material properties of the packaging medium (Imai, 1988). One of the primary concerns with any food product is the maintenance of package integrity. Loss of package integrity could result in loss Of product quality and a potential loss of market share, as well as the development of a potential health hazard. This is of added concern with laminates. In addition to intact seals, a package constructed from a laminate relies on the adhesive to provide adequate bond strength between the component plies. This bond strength is derived from the following: (1) Adhesive bonds: bonds along the interface between the adhesive and the adhering films. (2) Cohesive bonds: bonds within the body of the adhesive itself. Failure Of either of these bond types can lead to a significant reduction in material strength, which may result in delamination and the possible loss of package integrity. 3 This study was designed to determine the effect of sorption of flavor volatiles from a food product, on the adhesive and cohesive bond strength Of multilayer laminations. Contact phase exposure of two food packaging laminates to two commercial food products was carried out to evaluate possible effects of sorption-induced delamination. Investigation into the nature of the bond failure, as well as the process of delamination, was also determined. The specific objectives of this study were as follows: (1) To determine if sorption of flavor volatiles from a food product is capable of causing delamination in a multilayer laminate structure. (2) To determine if bond failure caused by sorption of flavor volatiles is adhesive or cohesive in nature. (3) To quantify the reduction in bond strength due to sorption of flavor volatiles. (4) To determine the effect Of contact layer composition on the reduction in bond strength due to sorption of flavor volatiles in multilayer laminations. Understanding the effect of product-package interaction, as it relates to the delamination Of multilayer material structures, should lead to more informed materials selection and package development processes. LITERATURE REVIEW Mnltilayer Laminations Multilayer laminations are combinations of two or more films or other non-plastic materials into a single structure (Manypenny et al., 1988). Ashley (1983) stated that requirements for the protection of food products are often such that an adequate balance of barrier properties is not achieved by the use of a single film. Hence the need to laminate two or more materials together, each of which provides part of the barrier requirement, or some particular aspect of package performance, such as heat sealability or toughness. Multilayer laminates owe their properties and behavior to the properties Of the respective components (Scop and Argon, 1967). Although each of the two or more layers contributes its particular property to the total performance of the multilayer laminate, there appears to be no interaction among the individual layers to enhance the laminate's mechanical properties (Killoran, 1974). Laminations allow the inclusion Of non-plastic materials, such as paper and foil, as well as the placement of printing between layers (Mushel, 1984). Galloway (1988) 5 referred to the benefits of laminates over their traditional glass and metal counterparts as one reason for their increased usage. These include: lighter weight, lower costs, reduced shipping charges and ease of use: including opening, dispensing and disposal. Ashley (1983) classified food packaging laminates as follows: General purpose. For packaging of products where barrier requirements are not critical, shelf-life may be short to medium, and the package is not subject to severe product ingredients or hostile environments. High performance. For packaging of products with critical barrier requirements and/or resistance to difficult products or extreme environmental conditions is required. A typical general purpose laminate, such as those used for the packaging of snack foods, meats, and cheeses, consists of an outer barrier layer laminated to an inner heat seal layer. High performance laminates, such as boil-in-bags, typically consist of ultra-high barrier materials and/or high performance adhesives. Fries (1984) listed the common materials used in multilayer laminations. These include polyester, nylon and polypropylene (all coated or uncoated), polyethylene, metallized films, polyvinylidene chloride-coated cellophane, paper, glassine and aluminum foil. The final 6 choice of materials is generally governed by consideration Of physical properties, processability, barrier properties and material costs, as related to product protection requirements (Ashley, 1983). Prior to the laminating process, films are typically pre-treated by corona discharge, a surface alteration process resulting in improved adhesion through an increase in polar surface energy (Podhajny, 1987). Lamination is typically achieved by one of two common processes: dry bond adhesive lamination or extrusion lamination (Galloway, 1988). Dry bond adhesive lamination is accomplished by application of an adhesive to one of the webs, which is then passed through an oven or dryer to remove the solvents carrying the adhesive. This web is then combined with a second under pressure. A typical laminating technique is schematically presented in Fig l. Extrusion laminating is a similar process, whereby an extruded plastic is used in place of the adhesive to combine the individual webs. Adhesives must conform to food and drug administration regulations, have both good adhesion to the surfaces they bond and good internal cohesion, and must be able to retain these characteristics during aging (Bentley, 1986). Fries (1984) classified laminating adhesives as either emulsion adhesives or solution adhesives. Emulsion adhesives are water based and include acrylics (1 or 2 component) and polyvinylidene chlorides. Solution adhesives are solvent 1. lst web unwind. 2. Adhesive application roller. 3. Hot air oven. 4. 2nd web unwind. 5. Hot nip unit - rubber/steel roller. 6. Cooling roller. 7. Laminate rewind. Sorce: Ashley, 1983 Figure 1. Flow diagram of typical laminating process. 8 based and include polyurethanes (1 or 2 component) and polyesters. Polyurethanes are the most widely used adhesives for laminations used in food packaging (Bentley, 1986). Typical 1-component polyurethane adhesives are the reaction product of an isocyanate and a polyOl - (Schollenberger, 1977). Two isocyanate monomers commonly used for packaging applications are toluene di-isocyanate (TDI), and 4-4' Di-isocyanate diphenyl methane (MDI). Ashley (1983) stated that while TDI was most commonly used for high performance adhesives, and MDI for general purpose grades, health concerns with TDI have lead to increased use Of MDI based polyurethanes. Most currently used polyurethane adhesives are applied from solvent based solutions at approximately 30% solids content. Ashley (1983) also reported that the urethane/isocyanate system has the following advantages: i) Good adhesion to a variety of plastic films. ii) Good thermal performance. iii) Room temperature cross-linking. iv) Ease of application. v) Good solvent release. vi) Good chemical resistance. vii) Good color and clarity stability. viii) Good aging characteristics. In 1985, material shipments of the United States (U.S.) packaging industry were $55.84 billion (Rauch, 9 1985). Of that figure, $29.7 billion (53.2%) went to the food and beverage segment of the industry. Industry shipments, by material type, are shown in Fig 2. Flexible packaging accounted for $7.48 billion, or 13.4% of total packaging sales. Of that figure, total U.S. shipment of multilayer flexible packaging materials was $1.95 billion. U.S. shipment of converted flexible packaging materials (single and multilayer) is presented in Table 1. Shipment of multilayer flexible materials increased at an average rate Of 6.0% per year from 1980 through 1985, with a projected 1990 value Of $2.67 billion. U.S. shipment of converted multilayer flexible packaging materials, by laminate type, for the years 1977, 1983, and 1985 is detailed in Table 2. Shipment of film/film multilayer laminates far surpassed that of any other type of laminate produced. Product/Package Interactions Polymeric packaging materials typically exhibit greater product interaction than do their glass and metal counterparts. The effect of package compatibility on product quality has become an area of intense study (Durr et al., 1981: Mannheim et al., 1987: and Imai, 1988). Parliment (1987) noted that volatile, low molecular weight, organic compounds, such as aroma constituents, though Total: $ 55.84 Billion % P99930993 «a. Source : Ranch , 1985 Figure 2. Breakdown of 1985 U.S. Packaging Industry shipments by material type. 11 Table 1. 1985 U.S. shipment of converted flexible packaging materials. Million Dollars Materia1_Txne Valuer Pg;ggn§agg___ Multilayer $ 1950 32.4% Single Layer Plastics 2965 ' 49.2 Paper 785 13.0 Foil 325 5.4 4075 67.6 Total $ 6025 100.0% Source: Rauch, 1985. 12 Table 2. U.S. Shipment of converted multilayer flexible packaging materials for 1977, 1983 and 1985. Million Dollars Mn1t11ayer_Laminate_Iynell. 1977 1983 1985 Paper/paper $ 112 $ 265 $ 230 Paper/foil Extrusion laminated 172 187 195 Adhesive laminated 153 260 290 Paper/film 72 234 250 Paper/film/foil 16 50 75 Film/film 344 647 750 Film/foil 59 __112 160 Total $ 919 $ 1755 $ 1950 Source: Rauch, 1985. 13 present in minute quantities, play a major role in overall product quality. Flavor is not dictated by a single component, but is the result of a delicate balance of compounds within the product (Gianturco et al., 1974). Loss of this balance results in a change in the quality of the product flavor (Ikegami et al., 1987). Several authors (Durr et al., 1981: Kwapong and Hotchkiss, 1987: Shimoda et al., 1988) have studied the extent to which various polymer films are capable of absorbing flavor components. Marshall et a1. (1985) reported a decrease Of over 60% in the concentration Of d-limonene (a major flavor component in citrus products) as the end result of exposure of an orange juice product to low density polyethylene (LDPE): and a 45% decrease upon exposure to an ionomer. Mannheim et a1. (1987) found similar losses over a period of just six hours, upon exposure of d-limonene solutions to LDPE. Kwapong and Hotchkiss (1987) described the partitioning of limonene between various plastic films and an aqueous solution by an equilibrium partition coefficient (Ke), as described in equation 1. Ke = [Cpleq / [Caqleq (1) Where: [Cpleq is the equilibrium aroma concentration in the plastic film. [Caneq is the equilibrium aroma concentration in the aqueous solution. 14 The resulting Ke values were indicative of limonene's strong affinity for plastic films, such as low density polyethylene, and ionomers. Ikegami et al. (1987) found that partitioning is dependant upon the functionality of both the plastic film and the organic sorbant. Characterization of product-package interactions, with respect to organic solubility in the polymer structure, is essential to control "flavor scalping" by the package material (Hernandez et al., 1986). Rogers et al. (1960) noted that sorption, an absorptive process resulting in the movement of compounds from the food phase to the polymer phase, is strongly dependant upon the solubility Of those compounds in the polymer phase. The authors further noted that a solubility coefficient (S) can be calculated from sorption data, as shown in equation 2. s a Meg/WxB (2) Where: Meq is the mass of vapor sorbed at equilibrium for a given temperature. W is the weight of the polymer sample. B is the vapor concentration. Mohney et al. (1988) examined the solubility of limonene in a high density polyethylene (HDPE)/sealant laminate, and a wax/polyvinyl alcohol (PVOH)/glassine/PVOH/ wax structure, as a function of limonene vapor concentration. The solubility coefficient of limonene for the HDPE laminate was found tO be significantly greater 15 than that of the glassine structure, temperature and vapor concentration being equal. Rogers et a1. (1960) found that the solubility coefficient is not a constant for a polymer-sorbant system, but is a concentration and temperature dependant process. As organic vapors are sorbed by polyolefin structures, swelling Of the polymer matrix results in alteration of the. polymer chain configurations, affecting solubility as well as diffusivity and permeability (Bagley and Long, 1958: Fujita, 1961: Crank and Park, 1968: and Berens, 1977). Sorption of limonene has been found to increase the absorption of additional compounds into the polymer phase (Marshall et al., 1985). In addition, DeLassus and Hilker (1987) reported that organic molecules may act as plasticizers in certain polymeric films. Much work has been reported concerning flavor sorption by the sealant (food contacting) layer of multilayer laminations. Kwapong and Hotchkiss (1987) evaluated the sorptive properties of low density polyethylene and two ionomers in contact with five individual food flavoring compounds. Each film exhibited a differing equilibrium partition coefficient for each compound, indicating that both contact layer composition and sorbant composition play a role in the sorptive process. Marshall et al. (1985) examined flavor absorption by the contact layer of multilayer aseptic packaging materials, and found similar results. The loss of d-limonene into the sealant layer, 16 between the juice and polyvinylidene chloride barrier, was directly related to the thickness of the contact layer and not the oxygen permeability of the film. DeLassus and Hilker (1987) considered the effect of barrier layer location (in a multilayer laminate) on the sorption and permeation of organic molecules. Although permeation rates were not altered by the order of the layers in the laminate, the authors found that total sorption by the package wall can be altered dramatically. Since the total pressure drop of the permeant across the laminate will be the sum of the pressure drops across each layer, and since the permeant concentrations in the wall layers will remain constant at steady state, the authors found that equation 3 can be used to define the total pressure drop (PX) across the laminate. PX a Mx/TA (L/P1 + L/Pz + L/P3 + ....) (3) Where: Mx/T is the transport rate of the organic compound across the film. A is the film area. L is the film thickness. P is the permeability, with subscripts identifying each layer. Since P commonly varies among the layers by a factor much greater than that Of L, most of the pressure drop occurs across the barrier layer. The authors suggested that since the concentration of a sorbed permeant in a layer is proportional to the permeant pressure in the layer, the 17 barrier layer should be in close proximity to the food product in order to minimize total sorption by the package wall. Composite aseptic juice packages are multilayer laminates, typically of polymer/paper/foil/polymeric sealant (outside to inside). Hirose et a1. (1988) determined that sorption of d-limonene from orange juice by aseptic sealant films had a significant effect upon the films' barrier and mechanical properties. Oxygen permeability increased 50% to 350%, depending upon the film studied. Modulus of elasticity showed a decrease of up to 40%, while seal strength decreased a maximum of 25%, after 27 days of product exposure. Tensile strength, percent elongation, and impact resistance were also negatively effected. Film samples studied were low density polyethylene and two ionomers, sodium and zinc types. The sodium type ionomer, which sorbed the greatest amount of d-limonene, showed the greatest decrease in modulus of elasticity, tensile strength, and seal strength. Imai (1988) performed similar research using low density polyethylene (LDPE), ethylene vinyl alcohol copolymer (EVOH) of high ethylene content, and co-polyester (Co-PET) sealant films. Sorption of d-limonene affected modulus of elasticity (EVOH and Co-PET), yield stress (EVOH), stress at 100% elongation (LDPE and EVOH), seal strength (LDPE), and impact resistance (EVOH and LDPE). Sorption of d-limonene by Co-PET was significantly less 18 than that of the other two films, and had an insignificant effect upon that film's mechanical properties, with the exception of modulus of elasticity. Konczal (1989) examined the effect of flavor sorption from apple juice on the mechanical properties of LDPE, EVOH, and Co-PET. Immersion Of the film samples in the juice resulted in a significant change in the yield point (EVOH), tensile strength (all three films), percent elongation (all three films), modulus of elasticity (LDPE), and impact resistance (EVOH). All three films exhibited the changes after a one day exposure period, with little change occurring during extended exposure. Snow (1974) categorized chemical attack on packaging materials by product components as a major cause of incompatibility between packaging materials (single and multilayer) and contained food products. Marshall et al. (1985) noted that due to their lipophilic nature, citrus flavorings have the potential to interact with the polymer coatings and sealants present in aseptic laminate materials. The authors found significant localized swelling in low density polyethylene (LDPE) exposed to orange juice containing d-limonene. It was theorized that this sorption-induced swelling might be responsible for the observed delamination of an aseptic laminate material at the LDPE/aluminum foil interface. Excessive localized stresses, within the LDPE-foil region, may develop as a direct result of swelling, leading to layer separation. 19 Adhesion Adhesion, as defined by the American Society for Testing and Materials (ASTM, 1971), is the state in which two surfaces are held together by interfacial forces consisting Of valence forces, interlocking action, or both. Good (1976) believed that adhesion between two phases has both macroscopic and microscopic aspects which are complimentary to each other. Macroscopic aspects involve properties such as elastic modulus, and plastic deformation, as well as the transfer or dissipation of work. Microscopic aspects include the binary atomic and molecular contacts, as well as the short and long range interatomic attractions and repulsions present in such a system. Good (1976) also defined an "adhesive joint" as a system consisting Of two solids of macroscopic dimensions, which may or may not be Of the same material, and a layer of adhesive, such that two effectively parallel interfaces exist. McBain and Hopkins (1925) reported that any fluid which wets a particular surface and is then converted to a solid mass by cooling, evaporation, oxidation, etc., must be regarded as an adhesive for that surface. The precise mechanisms involved in adhesion have been described by many authors (Huntsberger, 1967: Allen, 1969: Raevskii, 1973: Mittal, 1975). Mittal (1978) stated that there is no single theory or mechanism capable of explaining all 20 adhesion behaviors. Good adhesion, regardless of the mechanism involved, relies on the following three qualities attributable to the adhesive: (a) wetting, (b) solidification, and (c) sufficient deformability to reduce the build-up of elastic stresses in the formation Of a joint (McBain and Lee, 1927). Zisman (1963) defined "wetting" as the adhesion on contact between a liquid (the adhesive) and a solid (the adherend): and the extent to which the liquid spreads. Sufficient wetting is required between an adhesive and adherend in order to Obtain proper adhesion. Young (1805) related the contact angle (0) between a drop of liquid and the solid upon which it rests, to the wettability of the solid by that liquid, according to equation 4: Tas ‘ Tls ‘ Tla COS 9 (4) Where: Tas is the surface tension at the interface of the air and solid phases. T13 is the surface tension at the interface of the liquid and solid phases. Tla is the surface tension at the interface of the liquid and air phases. In this relationship, the contact angle between a liquid and the solid upon which it rests is an inverse measure of the wettability of that solid by the liquid. Shafrin (1963) defined the lowest surface tension a liquid can have, yet still exhibit a contact angle greater than zero degrees on a solid, as the critical surface 21 tension for spreading. Critical surface tension (Tc) is directly dependant upon the chemical composition of the surface in question. As an example, the author cited polyethylene, with a Tc of 31 dynes/centimeter. Substitution Of a chlorine atom for a hydrogen atom results in an increase in the Tc to 39 dynes/centimeter, while substitution Of a fluorine atom decreases the value to 28 dynes/centimeter. Levine et al. (1964) found a close relationship between the strength of an adhesive joint and the critical surface tension of the polymers involved. Surface tension may be explained as the net attraction of surface molecules into the bulk phase, in the direction normal to the surface, as the direct result of attractive forces from adjacent molecules. This attraction reduces the number of molecules in the surface region and extends the intermolecular distance (Fowkes, 1964). Such extension requires work, and is responsible for the presence of a surface free energy. In order for spreading to occur, the surface free energy of the liquid must be less than that of the solid (Zisman 1963). Thermodynamically, wetting occurs whenever the free energy change for producing the liquid-solid interface is negative compared to the free energy changes for loss of _the solid-air and liquid-air interfaces (Schneberger, 1979). Equation 5 describes the thermodynamics involved, such that when summation is negative, wetting will occur: dEw = dEls - (dEla '1' dEas) (5) 22 Where: dB is the change in free energy. w is wetting. ls is liquid-solid interface. la is liquid-air interface. as is air-solid interface. The author further suggested that wetting is due to the mixing of the electron clouds between the liquid and solid, resulting from a chemical affinity between the two phases. As wetting occurs, bonds are formed between the liquid adhesive and the solid adherend. Mittal (1978) defined basic adhesion as the summation of all intermolecular or interatomic interactions. Such interactions may be due to chemical, electrostatic, or polar attraction. Chemical bonding occurs when the interaction involves the transfer or sharing of electrons. Electrostatic bonding is due to a charge separation which results in an electrostatic attraction. Polar bonding is the result of asymmetry in the electric field around atoms or molecules, resulting in attraction (Mattox, 1978). Mark (1979) described the forces responsible for intermolecular attraction as follows: London Dispersion Forces. London dispersion forces act between all atoms and are the intrinsic result Of a resonance between virtual dipoles. Since these forces are proportional to the number Of atomic nuclei in the contributing group, and since 23 there is no vast difference in the number Of electrons in the common groups, there is very little difference between groups in regards to this type of interaction. London dispersion forces are a short range interaction, with the energy decreasing according to the sixth power of the distance at short distances and the seventh power at larger distances. The average range of dispersion forces is between 3.5 and 4.5 atomic units (AU). Interactions of Permanent Dipoles. Permanent dipole interactions lead to forces dependant upon the strength of the dipoles involved. These forces are important for all molecules which contain polar groups, and also decrease with the sixth power Of the distance between the dipole centers. Hydrogen Bonds. Hydrogen bonds are strong forces resulting from the interaction between the negative pole of a strong dipole and the positively charged end of a second dipole consisting of a hydrogen atom. The range of hydrogen bonding is between 2.6 and 3.0 AU. Induction Forces. Induction forces are the result of a strong dipole of one molecule approaching a 24 polarizable area of another molecule. A secondary dipole is induced and attracted to the first dipole, as discussed above. The range of these forces is between 3.5 and 4.5 AU. None of the preceding forces fit every situation. Frequently, several bond types appear to play a role in bonding (Schneberger, 1979). Fowkes (1964) introduced the theory of additivity of intermolecular forces, whereby the total surface free energy is equal to the sum of components attributable to the various intermolecular forces involved, as described in equation 6: Etotal 3 Ed + Ep + Eh (5) Where: E is free energy. d is dispersion forces. p is polar forces. h is hydrogen bonding. The author considered hydrogen bonding interactions to be highly specific and of particular importance. Zisman (1963) and Fowkes (1964) made specific references to the importance of London dispersion forces in adhesive bonding. Skeist and Miron (1977) considered the secondary, or Van der Waals' bonds (the most significant of which are the London dispersion forces), to be the most important of the bonds in adhesion. The attractive forces involved in adhesion are limited in the distances over which they may occur. Although the 25 London dispersion forces are effective over greater distances than other intermolecular forces, all require intimate contact between the two phases (Fowkes, 1964). Zisman (1963) discussed the importance of localization of the attractive fields of force around solid surfaces, especially those comprised of covalently bonded atoms. London dispersion forces are known to become unimportant in simple molecules at a distance of only a few atom diameters. It is assumed that with solids and liquids comprised of such molecules, atoms below the surface layers contribute little to adhesion.l Strong evidence was presented showing the attractive forces from metallic and non-metallic solids to be insignificant at a distance as small as 24 angstroms. Following wetting and bond formation, solidification of the liquid adhesive is essential to adhesion. Since attractive forces are effective in both the adhesive and adherend for little more than the depth of one molecule, they will be unaffected by changes in state (provided allowance is made for any changes in surface chemistry and molecular orientation at the interface. Zisman, 1963). As solidification proceeds, internal stresses and stress concentrations develop within the adhesive, the most common cause of which is a difference between the thermal expansion coefficients of the adhesive and adherend. Schneberger (1979) noted that with solidification comes shrinkage of the adhesive phase due to loss of mass through 26 solvent release. The practical result of shrinkage is the appearance of additional stresses at the adhesive/adherend interface, as well as the possible formation of cracks and voids within the adhesive itself. Although solid after curing, all adhesives undergo constant internal motion through thermally-induced vibration (Schneberger, 1979). A minimal amount of chain flexibility is desirable within the adhesive, in that it imparts resiliency and toughness to the adhesive film. Zisman (1963) found that soft adhesives are better able to adjust to shrink induced stress than more rigid adhesives, due to their greater flexibility. Such flexibility is compounded into the adhesive based on its glass transition temperature (T9). T9 is the transition point, above which, materials exhibit soft and flexible behavior, in terms of chain mobility. Below Tg, materials exhibit rigid glass-like behavior (Nielson, 1974). If an adhesive is used at a temperature too high above its Tg, creep may be experienced. If used too far below its T9, the adhesive may be too brittle to adapt to induced stresses and deformations. Flexibility also determines the extent to which an adhesive will be able to deform under load. Masuoka (1982) described the rOle of adhesive deformation in the prevention of adhesive failure. Deformation was found to occur as part of the interfacial failure process, with rupture of the adhesive taking place when its ability to 27 deform was diminished. The author concluded that the mechanical and rheological response of the adhesive is paramount to the maintenance of adhesion. Packham (1981) confirmed the importance of deformation in his analysis of adhesive bond failure. Kinetic energy, which otherwise might lead to bond failure, is dissipated through plastic deformation. Increased adhesion is seen as a direct consequence of increased energy dissipation through deformation. Longworth (1983) added that deformability is closely related to the thickness of the adhesive layer involved. Adhesive/Cohesive Bond Failure Bond strength is not necessarily limited by adhesion, in that the cohesive strength of an adhesive is at least as important as its adhesive strength (Schneberger, 1979). Cohesive strength is the result of several factors, including molecular weight (Mark, 1979). A general relationship exists between mechanical properties (such as tensile strength and resistance to rupture) and molecular weight or degree of polymerization. Below a critical degree of polymerization, significant mechanical properties do not develop. Once exceeded, however, an increase in mechanical performance is Obtained for a given increase in molecular weight. 28 A second factor influencing the mechanical behavior of a polymer is its atomic organization (Mark, 1979). Polymeric materials consist of long, flexible chains randomly entangled or laterally ordered. The response of such a system to an applied force depends not only on the strength of the molecular chains and intra-chain covalent bonding, but also on the degree of lateral interaction between chains. Axial orientation of the polymer chains improves lateral interaction, while a very regular structure allows the formation of a high degree of lateral order, or crystallinity. Chemical cross-linking of the polymer chains will further increase the lateral attachment through strong, localized bonds. The range of covalent bonding forces is approximately 1.5 angstroms, with 5.0-6.0 x E -12 ergs per bond required to cause failure. Weaker intermolecular forces, such as hydrogen bonds and Van der Waals forces (which also exist between polymer chains), require 0.6-3.0 x E -13 ergs per bond for failure (Mark, 1979). Skeist and Miron (1977) noted that when two materials are bonded together with an adhesive, the resulting composite has at least five elements: adherend number 1, interface number 1, adhesive, interface number 2, and adherend number 2. In Fig 3, a cross-sectional view of a composite, is graphically presented, as well as the five "regions" associated with an adhesive joint. Good (1978) classified regions 1 and 5 as within the respective bulk 29 Adherend 1 Interface 1 < Region Adhesive ’,,Region 4 Interface 2 €k\\\~<+Region Adherend 2 “~Region (————Region Source: Good, 1978. Figure 3. Cross-section of multilayer lamination, with components and regional classification. 30 phases, far from the interface. A continuous gradient of properties and behavior into regions 2 and 4 occurs, such that in those regions, localized mechanical behavior is influenced appreciably by the presence of the interface, as well as the second phase. This influence increases as the interface is approached. The interface (region 3) may be molecularly sharp or diffuse, and the mechanical properties may exhibit sudden changes over distances as little as two atoms or molecules. Localized properties associated with the ultimate strength of the composite may change more gradually, even in the presence of a sharp interface. It should be noted that although regions 1 through 5 refer to the properties and behavior within the adhesive and adherend 2, as a function of distance from interface 2, the same classifications of property and behavior would be expected within the adhesive and adherend 1, as a function Of distance from interface 1. Mattox (1978) classified interfaces as being mechanical, monolayer to monolayer, compound, diffusion, or combinations of these types, defined as follows: Mechanical Interface. The mechanical interface is characterized by mechanical locking Of one material with the rough surface of a second material. The strength of this interface is dependant upon the mechanical properties of the materials involved. 31 Monolayer to Monolayer Interface. This interface is characterized by an abrupt change in composition from one phase to the other over a distance of 2 to 5 angstroms. This interface may be formed when there is no diffusion and little chemical reaction between the two phases. Compound Interface. The compound interface is characterized by a constant composition layer, many lattices thick, created by the chemical interaction between phase materials. This interface may have abrupt physical and chemical property discontinuities associated with abrupt phase boundaries. Diffusion Interface. The diffusion interface exhibits a gradual change in composition and lattice parameters across the interfacial region. Gradual changes in the mechanical properties of the interfacial region may also be associated with this interface. The type Of interface formed is dependant upon substrate surface morphology, the presence or absence of contamination, chemical interactions, the energy available during interface formation, and the nucleation behavior of the depositing atoms. 32 Mittal (1975) distinguished between the actual interface and the interfacial region (referred to as the "interphase"). The interphase possesses a defined thickness, in which the mechanical properties differ from those of the contiguous phases. Interphases may be present on the adhering phases (as layers Of oxides or oils) or may be formed as the result of an interaction between adhering phases. Several authors (Zisman, 1963: Skeist and Miron, 1977: Good, 1978: and Mittal, 1978) refer to two distinct types of bond failure within an adhesive joint: Adhesive Bond Failure and Cohesive Bond Failure. These authors have defined adhesive bond failure as the failure of bonds at and along the interface between the adhesive and the adherend. Cohesive bond failure is defined as the failure of bonds within the body Of the adhesive. In theory, the adhesive bond strength of a composite will exceed cohesive bond strength, resulting in cohesive bond failure upon excessive loading (Zisman, 1963). Such a situation will only occur when conditions of complete wetting, and freedom from the formation of gas pockets and voids exists. Longworth (1983) examined the failure of a nylon/ionomer composite which indicated that under Optimum conditions, cohesive failure is expected to occur. Packham (1981) found that cohesive failure may be the result of scission of intermolecular Van der Walls' bonds, as opposed to intramolecular covalent bonds. 33 Mark (1979) observed, theoretically, that the strength of all adhesive bonds is limited by imperfections. Packham (1981) referred to interfacial voids, such as bubbles due to incomplete wetting, or air entrapment by the adhesive while in the liquid phase, as common sources of interfacial imperfections. Surface indentations and roughness are also common causes Of imperfections, in that they may result in air or solvent pockets along the interface (Schneberger, 1979). Solid surfaces slowly undergo chemical and physical changes, even though they are isolated from the environment. These changes may include oxidation, rearrangement of water or gas molecules, and rearrangement Of bonds with adhesive components. Interfacial imperfections lead to the development and concentration of stresses within the interfacial region. Schneberger (1979) found that stresses may be mechanical, chemical or thermal in nature. Such stresses may be considered to be influences capable of straining a bond beyond its ability to resist, resulting in failure. Pores and cracks in the interfacial region are of particular importance in that they not only act as stress concentrators, but absorb and hold corrosive liquids, allowing access to the region (Mattox, 1978). Solidification also leads to stress development, as the result of differences in the thermal expansion coefficients of the adhesive and adherend (Zisman, 1963). Schneberger (1979) found that differences in expansion between the two 34 phases can lead to high levels of stress at the bond interface, making particular bonds risky or impossible. The author further added that stress distribution within an adhesive under load is not uniform. The combination of internal stresses and stress concentrators may lead to bond failure upon loading. Bond failure will initiate at a flaw, which allows a stress concentration or a bond weakening, and will propagate in a plane of weakness under a critical stress (Masuoka, 1982). Interfacial failure occurs when an adhesive layer has a high resistance to cohesive rupture. As a result Of such resistance, deformation of the adhesive in the vicinity Of the interface occurs, leading to rupture of the adhesive in the interfacial region, and bond separation between the adhesive and the adherend. Good (1978) described bond failure as having two stages: Initiation of Separation, and Propagation. Initiation and propagation will not necessarily originate from the same location within the adhesive joint. Initiation may occur as an initial flaw at the site of an imperfection: whereas the energy associated with growth of the flaw may originate from a separate location. In regards to the energetics Of growth, Gent (1971) noted that bond failure occurs when the energy stored elastically in the adhesive in the vicinity of the flaw, and released by growth of the initial flaw, is sufficient to meet the energy requirements for growth. 35 Good (1978) stated that there exist two accepted theories on adhesive and cohesive bond failure. The first theory is based on fracture mechanics, as expressed by the Griffith-Irwin Criterion, equation 7. (sc)2 - K x EG/l (7) Where: Sc is the critical stress to initiate failure. K is a constant E is the elastic modulus. G is the dissipated work per unit area of crack extension. 1 is the length of the longest pre-existing crack. If a crack starts to propagate under stress, through the conversion of stored elastic energy into dissipated work, transport of that energy from the area around the crack to the tip of the crack must occur. If the failure occurs in the region Of the interface, energy is drawn from both phases, and E and G are average values, weighted according to the relative volumes of the two phases involved. According to this model: (1) If no weak boundary layer material exists, and strong intermolecular bonds exist across the interface, then failure will occur in such a way that a thin layer of one phase will be found on the other. (2) If no strong bonds exist across the interface, then failure at the interface is most probable. (3) If interfacial bonding is strong, then failure will occur 36 within the weaker of the two phases, distant from the interface. The second theory refers to a weak boundary layer, as proposed by Bikerman (1959, 1973, 1978). Bikerman suggested that on theoretical grounds, true adhesive bond failure can never take place, and that if a system exhibits such failure, said failure actually occurred in a "weak boundary layer" in close proximity to the interface. The author defined a weak boundary layer as any layer of material at the interface having a low cohesive strength. Bikerman differentiated four types Of weak boundary layers: i) .air or vapor ii) coating material iii) substrate iv) any combination of the above Such a layer may consist of low molecular weight polymeric material which has migrated to the surface of a high molecular weight polymer (for example, a minor component of one phase, such as an antioxidant, which has absorbed at the interface), or contamination. Zisman (1963) stated that theoretically, a film Of oxide or organic contamination as thin as one molecule is capable Of causing a significant decrease in adhesion. Desorption, the displacement of adhesive from the interface by a chemical from the environment, or from within the substrate, is often the cause of "apparent" adhesive 37 failure (Schneberger, 1979). Most surfaces develop a tightly bound layer of water when bonds are formed. Such bonds are able to resist desorption only when the interfering compounds are unable to penetrate the film, or if the adhesive-adherend bond is thermodynamically or kinetically favored over interaction between the substrate and desorber. Brewis (1985) described a situation whereby improper corona discharge treatment had an adverse effect on heat sealability. Excessive treatment resulted in the formation of a layer of degradation products of low molecular weight, which interfered with the sealing process. Carley and Kitze (1978) found that additives such as slip agents and antioxidants were capable of adversely affecting the adhesion process. Such compounds, when allowed to migrate to the surface prior to adhesive bonding, can interfere with the corona discharge treatment. Lai et al. (1985) examined the effect of temperature and relative humidity on the adhesive strength of pressure sensitive adhesives. The authors determined that the adhesive bond strength of pressure sensitive adhesives was significantly reduced by an increase in temperature. The effect of relative humidity was found to be related to the amount of water sorbed by the adhesive, as well as the plasticizing action of the water molecules (which was significant only at relative humidities in excess of 75%). Brewis (1985) also related temperature and relative 38 humidity to improper adhesion. The author referred to both conditions during the establishment of adhesion, and conditions during use as significant considerations. Fries (1981) described processing induced changes (such as change in web length during lamination) which result in a loss of adhesion in multilayer laminations. Laminates usually involve webs with different degrees of resistance to the stresses and resulting strains encountered during the lamination process. The result is a greater stretching of one web relative to the other, which upon relaxation after rewind, results in the familiar phenomenon of "tunneling," or the formation of elongated blisters. Tunneling is prevented when the adhesive involved has sufficient cohesive strength immediately following lamination. Solvent based urethane adhesives are mobile, semi-solids at the time Of laminating, and lack the strength to resist web slippage (Fries, 1981). Killoran's (1974) analysis of multilayered materials supports Fries work. Tunneling delamination was especially prevalent in large sized pouches. Borch (1982) analyzed the affect Of sizing additives on the adhesion of paper and polymer films. It was determined that sizing negatively affected adhesion through a decrease in the surface energy of the paper. The application of sizing results in an increased resistance to wetting of the paper surface. 39 Bisset (1979) investigated the effect of printing inks on bond strength. Interlayer printing results in a layer of ink between the adhesive and the outer ply, such that the adhesion and cohesion of the ink is crucial. Ink formulation requirements for toughness, slip, and non-blocking properties oppose the requirements for wettability - a property essential to good adhesion. Ink formulations tend to inhibit the cross-linking of adhesives, and are capable of migrating into the adhesive after curing. The result is reduced bond strength. Killoran (1974) studied the effect of irradiation on multilayered laminations used in the packaging of thermoprocessed foods. When exposed to ionizing radiation, polyolefins undergo scission of main chains and creation of free radicals, cross-links, double bonds, and end-links. Pouches were formed from multilayer laminations and exposed to gamma radiation while empty. They were then filled with beef slices, vacuum sealed and retorted at 118°C for 40 minutes. Non-irradiated pouches showed a significant decrease in bond strength as a result of retorting, while no change was found for the irradiated pouches. It was determined that while the irradiated pouches showed no delamination, the non-irradiated pouch material delaminated _between the two outermost layers. Analysis of the samples showed that adhesive bond failure occurred between the layers of the non-irradiated materials, and cohesive bond failure occurred between the layers of the irradiated 40 materials. A more detailed analysis showed that the strong adhesion which developed between the layers of the irradiated pouches was due to the formation of intermolecular cross-linking extending across the interface. Analytical Tests Product-Package Compatibility Product-package compatibility is essential to proper packaging. Incompatibility may lead to detrimental changes in both the product and package. Snow (1974) performed compatibility tests on various multilayer laminations by forming heat sealed pouches from laminate materials and filling them with specific products. The pouches were stored at a temperature of 100°F, and removed at various time intervals. Sample pouches were then Opened and visually examined for delamination. Schwarz (1987) described the use of food simulants in compatibility testing by the Food & Drug Administration (FDA). Classic food simulants, and the food types which they represent, are presented in Table 3. Recent changes were suggested by the FDA to improve the correlation between the simulant and its corresponding food product. 41 Table 3. Classic Food and Drug Administration food simulants. (a) simulant Food Type Distilled Water Aqueous foods, pH above 5.0 3% aqueous acetic acid Foods with pH 5.0 and below 8% or 50% aqueous ethanol Foods containing alcohol Heptane Fatty foods (a) Heptane and 3% Aqueous Acetic Acid no longer recommended. Source: Schwarz, 1987. 42 Gas Chromatography Both quantitative and qualitative studies Of flavor compounds may be achieved through the use of a gas chromatograph equipped with a flame ionization detector (FID) (Giacin et al., 1980). The electrical conductivity of charged particles in a gas is directly proportional to the concentration Of the particles within the gas. Quantification of flavor compounds is based on this principle, relating electrical detector response to the concentration of the flavor compounds injected into the gas chromatograph. A standard curve, relating known concentrations to detector response, allows quantification of an unknown quantity, based on the detector response for the sample injection. Qualitative analysis is based upon retention time, the time period from sample injection to detector response. Separation of sample compounds, based on the interaction between the sample injected into the gas chromatograph column, and the column material, results in a unique retention time for each compound. FID detection is sensitive to virtually all organic compounds, yet insensitive to both air and water, making it a suitable technique for the analysis Of both liquid and gaseous samples. Toebe (1987) described a headspace concentration technique which used gas chromatography for flavor analysis. The technique utilized an automated dynamic headspace concentrator to purge the sample of organic 43 volatiles, which were then collected on an adsorbent polymer trap. Subsequent heating of the trap resulted in release of the compounds, which were then cryofocused prior to introduction to the gas chromatograph. The purge cycle allows the retention Of the complete volatile contents of the sample vapor phase. Such a technique is especially useful for analysis of samples containing components of low concentration. Peel Test Bond strength is defined as the unit load applied in tension, compression, flexure, peel, impact, cleavage or shear, required to break an adhesive assembly with failure occurring in or near the plane of the bond (ASTM, 1974). Several methods for measuring the bond strength of an adhesive joint exist, specific to the type of joint in use. Peel tests have been referred to by several authors for the determination of bond strength in multilayer laminations (Bikerman, 1978: Mittal, 1978: Packham, 1981: Gent, 1982: Masuoka, 1982: and Longworth, 1983). Typically, the test is performed by peeling one ply from an adhering ply, through the use of a loading device, while measuring the load required for separation. Gent (1982) observed that the elastic energy of deformation does not change as peeling proceeds, and that if the sample is not stretched appreciably, all of the energy supplied by the loading device is devoted to 44 detachment. Peel strength, as determined by bond strength, is measured in terms of force per sample width required to maintain continuous detachment at a specified peel rate (Mittal, 1978). Peel strength may also be expressed in work or energy units. Packham (1981) expressed peel strength as the sum of the energy dissipation processes which occur during debonding. These include the thermodynamic work involved in adhesion or cohesion (dependant upon the mode of failure), viscoelastic losses in polymers that are strained, losses due to plastic deformation of the polymer, and minor loss terms, as expressed in equation 8: P - Wa (or We) + Lve + Lpd + .... (8) Where: P is the peel strength. Wa is the work of adhesion. We is the work of cohesion. Lve is viscoelastic losses. Lpd is losses due to polymer deformation. Packham (1981) noted that peel strengths are dominated by the energy loss terms, as opposed to the thermodynamic work terms, and that the strength of interfacial bonds has a strong influence on the energy dissipated in polymer deformation. Masuoka (1982) reported that peel strength increases with the thickness of the adhesive layer. Longworth (1983) confirmed this, and found that peel strength is additionally dependant upon peel angle, in an inverse 45 manner. Rate of peel has been found to affect peel strength values in some materials, and it must be noted that strength values will include the force required to bend the component plies (ASTM, 1984). Since peel strength is dependant upon the previously mentioned factors, direct comparisons between laminates of differing composition, or even between similarly composed laminates of differing thicknesses, should not be made (ASTM, 1984). Scanning Electron.Microscopy Several authors refer to the use of Scanning Electron Microscopy (SEM) to determine the locus of failure within adhesive joints (Baun, 1978: Packham, 1981: Adamson, 1982: Masuoka, 1982: and Brewis, 1985). SEM is widely used in the study Of surface morphology (Adamson, 1982). The process can be described as the scanning of a surface by a focused electron beam, resulting in the production Of secondary electrons. The intensity of the secondary electrons is monitored by a detector, the output of which is coupled to a cathode ray tube. The resulting picture is a replicate of the scanned surface. The intensity Of any point on the picture is related to the intensity of secondary electron production at the corresponding point on the surface. The surface character can be resolved to a few thousand angstroms. Masuoka (1982) utilized SEM to study the mechanism of interfacial bond failure in an aluminum/nylon laminate. By 46 studying the surfaces of the laminate for the presence or absence of adhesive, the technique revealed that adhesive bond failure occurred within the composite upon peeling. Packham (1981) referred to the use of SEM in the examination of residual polyethylene on a copper surface after peeling. Infra-red Spectroscopy In addition to Scanning Electron Microscopy, Infra-red Spectroscopy (IRS) has also been utilized in the analysis of multilayer laminations. Attenuated Total Reflectance (ATR), a form of IRS also known as Internal Reflection Spectroscopy, is particularly useful in analyzing thin layers of polymeric materials. The ATR technique consists of pressing the sample of interest against a crystal having a high index of refraction. An infra-red beam is then passed through the crystal, with multiple reflections into the sample surface taking place. The intensity Of infra-red radiation leaving the crystal is then recorded, resulting in a spectrum of the sample surface in contact with the crystal (Mirabella, 1986). During analysis the sample is exposed to a broad range of wavelengths, and the radiation is either transmitted or absorbed. The wavelengths at which absorption by the sample takes place are recorded. The resulting spectrum is a plot of energy transmitted versus wavelength (or frequency), with absorption of specific wavelengths related to the molecular 47 composition and functional groups present in the sample (Giacin et al., 1980). Killoran (1974) utilized ATR spectroscopy to analyze the effect of irradiation on multilayer laminations. Chemical changes (induced by gamma radiation) in a variety Of multilayer laminations were easily detected . Electron Spectroscopy for Chemical Analysis More recently, Electron Spectroscopy for Chemical Analysis (ESCA), also referred to as x-ray Photoelectric Spectroscopy (XPS), has been used for detailed analysis of surface composition. Several authors (Baun, 1978: Bikerman, 1978: Good, 1978: and Mittal, 1978) have reported on the superiority of the ESCA technique in the analysis of various surfaces with respect to adhesion. The basic technique involves exposure of the sample to a flux of monoenergetic X-rays of known energy. Absorption of the x-ray photons by the atoms of the sample material results in emission Of electrons from the sample material, which originate from the core atomic orbitals (Swingle and Riggs, 1975). These electrons have a kinetic energy (E) as described by the Einstein equation (Brewis, 1985): E=hv-Eb-0 (9) Where: hv is the energy of the X-ray photoelectrons. Eb is the binding energy of the emitted electron, 48 characteristic of the atom from which it was emitted. 0 is the angle Of incidence. Binding energies for a particular core level can vary by a few electron volts. Since the atomic structure of each element is unique, measurement of the binding energies of one or two atomic orbitals for each element in the sample is usually sufficient to establish the elemental composition of the sample. Swingle and Riggs (1975) noted that the sampling depth of ESCA ranges from 30 to 100 angstroms. Masuoka (1982) utilized ESCA to analyze the mechanism of interfacial bond failure in an aluminum/nylon melt/ aluminum joint. The technique, used in conjunction with ATR Spectroscopy, failed to detect residual nylon on the aluminum surface following peeling. As a result, failure of the laminate was classified as adhesive bond failure at the aluminum/nylon interface. Clark et al. (1975) studied the bonding of high density polyethylene to aluminum foil by pressing the polyethylene onto the foil at 200°C in a nitrogen atmosphere. Upon peeling of the aluminum foil from the polyethylene, ESCA analysis was used to detect a thin layer of polymer on the aluminum surface. Briggs et al. (1977) performed a similar study using ESCA, and found that pressing conditions affected the thickness of the residual polymer layer. 49 Dwight (1977) studied the adhesion of fluorocarbon polymers to copper, and detected polymeric material on the metal surface after peeling. When the peel strength was low, the residual polymer layers were extremely thin, and ESCA was necessary for detection. MATERIALS AND METHODS MATERIALS Laminate Materials The laminate materials utilized in this research were commercially produced and supplied by a flexible packaging converter. The only known difference between the two laminates was the composition of the sealant layer. The laminates were constructed as follows (outside to inside): Laminate A: 0.6 mil polyvinylidene chloride (PVDC) coated polyester/ink/one component polyurethane adhesive based on MDI/2.0 mil ethylene vinyl acetate copolymer (EVA) film, containing 6% vinyl acetate. Laminate B: 0.6 mil PVDC coated polyester/ink/one component polyurethane adhesive based on MDI/2.0 mil linear low density polyethylene (LLDPE) sealant. The thickness of the polyester films was 1.75 mil. Both the EVA and LLDPE films were corona discharge treated following production, and retreated prior to lamination. 50 51 Printing of laminates A and B was performed in-line with, and immediately prior to, the laminating process. Five differently pigmented inks were used, with both the inks and printing pattern identical in both laminates. Morton Adcote 333 was utilized as the adhesive in both structures, at a level of 25%-35% solids in methyl ethyl ketone. Application was via the direct gravure method, at a weight of 1.25 pounds per 3000 square feet of substrate. A cross-sectional view of laminates A and B is presented in Fig 4. Food Products and Controls Food Products Lemon Juice The lemon juice utilized in the study was Realemon brand natural strength lemon juice from concentrate (Borden Inc., Colombus, OH), purchased from a local retail source. Packaged in 32 fluid ounce glass bottles, product ingredients were listed as: Water, lemon juice concentrate, lemon Oil, 1/40th of 1% sodium benzoate (preservative), 1/40th of 1% sodium bisulfite (preservative). Hot Sauce The hot sauce used was Louisiana brand hot sauce (Bruce Foods Corp., New Iberia, LA), purchased from a local 52 Ink Adhesive PVDC Contact Layer Composition: A. EVA Copolymer B. LLDPE Contact Layer I Polyester Figure 4. Cross-section and composition of laminate structures selected for research. 53 retail source. Packaged in six ounce glass bottles, product ingredients were listed as: Peppers, vinegar, salt. Controls A 3% (w/v) aqueous citric acid solution was used as the lemon juice control, to simulate the citric acid fraction of the lemon juice, minus the low molecular weight, volatile, organic components, such as d-limonene. The solution was prepared by mixing 60.0 g of citric acid (assay, 99.9%) (Mallinckrodt, Inc., Paris, KY) with 2000.0 ml of distilled water. The mixture was placed in a 3.8 liter brown glass bottle and held at 23°C (+/- 2°). A 3% (v/v) aqueous acetic acid solution was used as the hot sauce control, which simulated the acetic acid fraction of the hot sauce, minus the organic components. The solution was prepared by bringing 60.0 ml of glacial acetic acid (J.T. Baker, Inc., Phillipsburg, NJ) to a volume of 2000.0 ml with distilled water, in a 3.8 liter brown glass bottle. The control was then stored at 23°C <+/-2°). Antioxidants Two antioxidants, Sustane 20A and Sustane W (UOP Process Division, McCook, IL), and an antimicrobial agent, Sodium Azide (Aldrich Chemical Co., Inc., Milwaukee, WI), 54 were added to both the lemon juice and hot sauce. These additives functioned to prevent product oxidation and microbial growth during the storage and exposure periods (Hirose et al., 1988). The composition of these additives is detailed in Appendix A. A mixture Of food products with the antioxidant and antimicrobial agents was prepared, prior to laminate exposure. A uniform batch of lemon juice was produced by mixing 3724.0 ml of lemon juice from several primary packages, into a 3.8 liter brown glass bottle. A Mettler AB 160 analytical balance (Highstown, NJ) was used to measure 0.7448 grams (9) each of sodium azide, Sustane 20A, and Sustane W, into 50.0 ml of the stock lemon juice. The mixture was returned to the glass bottle, which gave a 0.02% solution (w/v) Of each additive. After thorough mixing, the stock solution was transferred back to the primary packages in order to minimize loss of product volatiles to the contained headspace, resealed, and refrigerated at 10°c (+/- 2°). A stock solution of 1947.0 ml of hot sauce was produced using 0.3894 g of the antimicrobial and antioxidant agents to give a 0.02% solution (w/v) of each additive. The hot sauce was then placed in 400.0 ml glass jars with threaded screw caps, and refrigerated at 10°C (+/- 2°). 55 EXPERIMENTAL METHODS Selection of Laminate Materials and Food Products In an initial study, sixteen (16) food packaging laminates, of differing composition, were exposed to three (3) food products and three (3) food simulants. The laminate materials, food products and simulants studied, and results of this initial study are detailed in Appendix B. This study was carried out to assist in the selection Of laminates and food products for further research. Heatsealed pouches, measuring 5.1 by 12.7 cm, were filled with the products and simulants, and held at 23°C (+/-2°) for a period Of one month. Following exposure, the pouches were opened, emptied, and qualitatively examined for signs of delamination. Based on the results of the primary exposure study, two (2) laminates and two (2) food products were selected. Selection Of the laminate materials was based primarily upon similarity of structure, and response to the various food products and simulants. A response was the presence or absence of delamination, and the extent to which delamination was present, upon visual examination Of the laminates following exposure. Selection of food products/simulants was based on the ability of the product/simulant to initiate delamination in the sixteen laminates. 56 Laminate Exposure A modification of the test cell described in ASTM F 34-76 (ASTM, 1976) was used to expose the laminate materials to each of the food products and control solutions. Twenty four (24) test cells were constructed, as described in Appendix C. Fig 5 is a schematic Of the test cells used throughout this study. The test cells were designed such that a laminate sample, 15.2 by 10.1 cm, could be placed between the gasket and each aluminum plate. Thus, two (2) samples were exposed simultaneously per cell. The material surface area to volume ratio was 3.9 cmZ/ml, per cell. The food contacting layer of each laminate was arranged such that it was in contact with the gasket. The food products and control solutions were loaded into the cells with a glass pipette. This resulted in surface exposure of each laminate to the treatment liquid. Edge exposure was eliminated through use of the gasket. Each laminate was exposed to four (4) treatments using the test cells: Hot Sauce, Acetic Acid Control Solution, Lemon Juice, and Citric Acid Control Solution. The cells were held at 45°C (+/- 2°) for a period of thirty (30) days. Temperature and cell volume were monitored and maintained throughout the exposure period. Twelve (12) cells were used to test each laminate: four (4) treatment groups, with three (3) test cells per treatment. Each test cell contained two laminate samples. 57 SIDE VIEW FRONT VIEW (with plate removed) Plate Laminates - l [ I # Aluminum — Plate 1 J Laminate :1: O O 22‘: O O O / r 1" \\\\\\\‘Teflon Gasket”/////////r Aluminum Plate Figure 5. Schematic Of modified ASTM test cell used for laminate exposure. 58 Within treatment groups, each laminate sample was taken from a unique location on the package surface. The total laminate surface area within each treatment group represented 90% of the package surface. The laminate samples used between treatment groups were identical, in terms Of the package location from which the material was selected. Additional non-exposed laminate samples were held at 45°C (+/- 2°) for a period of thirty (30) days. All samples utilized throughout the research were printed over 100 % of the laminate surface. Upon completion of exposure, each cell was disassembled, and the contained laminate samples rinsed with distilled water and dried with absorbant paper toweling. Instrumental methods of analysis were carried out immediately following disassembly of the test cells. ANALYTICAL In Fig 6 is a schematic presentation of the flow of sample material from exposure through the various analytical procedures. Quantification of Bond Strength A Universal Testing Instrument (Instron Corporation, Canton MA), Model 4201, equipped with a 1 kN static load cell and an X-Y recorder, was used as described in ASTM designation F 904-84 (ASTM, 1984) for quantification of 59 Exposed Non-Exposed Roll Laminates Laminates Stock (45°C) (45°C) (22°C) Peel Test "——_"(Bond Strength)'——-—J I IR Analysis EL ESCA GC Analysisfr SEM Scanning Electron Microscopy ESCA Electron Spectroscopy for Chemical Analysis GC Gas Chromatography IR Infra-red Spectroscopy Figure 6. Material flow diagram for testing Of laminate samples. 60 laminate bond strength. Testing conditions are listed in Table 4. Following cell disassembly and sample rinsing and drying, five (5) laminate samples per treatment/laminate combination were prepared (with the longer sample dimension in the machine direction) using a JDC Precision Sample Cutter, model 25 (Thwing-Albert Instrument Company, Philadelphia, PA). In addition to samples from the eight (8) treatment/laminate combinations, non-exposed samples (stored at 45°C (+/- 2°) for a period of 30 days), and samples taken directly from roll stock (22°C +/- 2°), were also tested. Ply separation was mechanically initiated by creating a slight cut in the laminate, at the corner of the sample, with a pair of scissors. The cut was then manually propagated, which resulted in a stretching of the ply materials, with a greater degree of stretching for the sealant ply, than for the barrier ply. The sealant ply was then peeled from the barrier ply, using a pair of laboratory forceps. Initial separation of the laminate samples was 3.8 cm. All samples were backed on both sides with cellophane tape (3M, St. Paul, MN) according to recommendations described in the appendix to ASTM F 904-84 (ASTM, 1984). The function of the cellophane tape was to prevent tearing of the sample during testing. When the test sample was loaded, the unseparated portion of the laminate was allowed to hang freely, and the 61 Table 4. Test conditions for bond strength determination. Crosshead Speed 12.7 cm/min Full Scale Load 14.5 kg Full Scale Extension 19.1 cm Initial Grip Separation 1.3 cm Temperature 22.5°C Relative Humidity 50.0 % 62 force required to separate the bonded plies recorded on the X-Y recorder. The average force of separation, recorded as the bond strength, was determined according to ASTM F 904- 84 (ASTM, 1984). The standard calls for disregarding the values for the first inch of separation, and averaging the positive and negative peak values over the next two inches of separation. Determination of Locus of Failure Electron Spectroscopy for Chemical Analysis (ESCA) Delaminated areas of the exposed laminate materials were removed and prepared for examination by Electron Spectroscopy for Chemical Analysis (ESCA, also referred to as X-ray Photoelectron Spectroscopy, XPS) to determine the locus of failure. Delaminated samples, measuring 1.0 by 2.0 cm, were cut from the exposed laminates, and placed in petri dishes containing an anhydrous desiccant (W.A. Hammond Drierite Co., Xenia, OH). The petri dishes were then held at 45°C (+/- 2°) for a period of 24 hours,_ to remove any residual compounds which might out-gas during the vacuum analysis. A Perkin-Elmer PHI-5400 X-ray Photoelectron Spectrometer (Perkin-Elmer Corp., Norwalk, CT) was used to examine the Opposing, delaminated plies for the presence of nitrogen. Since the adhesive and the ink were the only components within the laminates containing significant levels Of nitrogen, an initial analysis was performed using 63 ink and adhesive samples, in order to differentiate between nitrogen from the ink and nitrogen from the adhesive. An elemental analysis of the delaminated surfaces for the presence of nitrogen was then performed to indicate the presence or absence of adhesive on the delaminated plies. The delaminated surface of individual plies was exposed to a flux of monoenergetic x-rays Of known energy, which resulted in emission Of electrons from the core atomic orbitals Of the surface materials. The emitted electrons were measured as counts per energy level, which was plotted as a function of binding energy (Swingle and Riggs, 1975). The plot which resulted, was then used to determine the elemental composition of the sample surface. Three (3) samples per food product/laminate combination were examined. Locus of failure was determined through examination for adhesive on both peeled surfaces. A more detailed analysis was subsequently used, on each of the samples mentioned previously, to determine whether the identified elements were present in an appreciable thickness, or as a surface deposition. Samples were examined in a manner similar to the initial analysis, except that the angle between the sample surface and X-ray beam was changed. Analyses were carried out at 10, 45, and 90 degrees incident to the X-ray beam (initial examination occurred at an angle of 45 degrees). Output was registered as the ratio of the detected signal for nitrogen to that of carbon and oxygen. The ratios at each angle were then 64 compared to determine the relative thicknesses of the surface elements. A constant ratio through the three angles was indicative of a relatively thick nitrogen coating, with a maximum measurable thickness of 100 - Angstroms. A significant reduction in the ratio, as the angle was varied (from 10 to 90 degrees), was indicative of a relatively thin coating on the sample surface, while an increase in the ratio was indicative of an increasing concentration as a function of sample depth. Examination of Locus of Failure Scanning Electron Microscopy Following exposure, randomly selected samples of the lemon juice and hot sauce-exposed laminates were examined by scanning electron microscopy (SEM) to determine the mode of bond failure. Non-exposed roll stock samples were also examined, to verify initial adhesion. A JEOL JSM-35C Scanning Electron Microscope (JEOL Analytical Instruments, Inc., Cranford, NJ) was utilized for cross-sectional microscopic analysis Of the laminate materials. Hot sauce and lemon juice exposed samples, as well as roll stock samples, 1.0 by 3.0 cm, were cryogenically prepared by cooling to -90°C, by exposure to liquid nitrogen, followed by a controlled warming to -65°C for a period of three (3) minutes to remove surface frost. Samples were subsequently recooled to -90°C and gold plated to improve resolution. The sample materials (containing areas of delamination) 65 were then examined, and the interfacial regions observed for signs of delamination. Magnifications ranged from 10X to 1500K, and areas of interest were photographed. Infra-red Spectroscopy Following quantification of bond strength, peeled samples were examined by Infra-red spectroscopy (IR) to confirm locus Of bond failure. A Perkin-Elmer Model 1330 Infrared Spectrophotometer (Perkin-Elmer Corp., Norwalk, CT) was used in the attenuated total reflectance (ATR) mode to examine the sample surface. Two (2) samples, 4.0 by 1.5 cm, were placed in a sample holder, such that the peeled surfaces were in contact with a crystal (KRS-S) having a high index of refraction. The output which resulted was a plot of energy transmitted versus wavelength (or frequency). Surface composition of the analyzed samples was then determined based on absorption of specific wavelengths, as indicative of the functional groups present in the sample. Gas Chromatographic Analyses Thermal Distillation Desorption studies were performed using thermal distillation and gas chromatography. Following exposure, the laminate materials were washed with distilled water and dried with absorbant toweling, and samples, 10.0 by 10.0 cm, were placed in 250 ml septa-seal glass vials. The 66 vials were then sealed with teflon-coated septa and tear away aluminum crimp caps, and held at 120°C for a period of 24 hours. An air-tight syringe (Hamilton Co., Reno, NV) was subsequently used to remove 500 microliters of headspace from the vial, and to inject the sample into a gas chromatograph (GC) (Hewlett Packard, Model 5890A, Avondale, PA) equipped with a flame ionization detector (FID). A splitless injection port was used with a fused silica capillary column (Supelcowax 10, Supelco, Inc., Bellefonte, PA). GC conditions are listed in Table 5. Qualitative analysis of product components sorbed by the laminate materials was then made, based on the characteristic retention time exhibited for each desorbed component detected by the GC detector. Dynamic Headspace Analysis Exposed laminate samples were also examined by a headspace concentration technique, using a Tekmar Dynamic Headspace Concentrator, Model 4000: a Heated Sample Module, Model 4100: and a Capillary Interface, Model 1000 (Tekmar Co., Cincinnati, OH). Following thirty (30) days of exposure to the food products, 1.0 milligram samples were placed in a glass sample tube, and the sample purged with helium while being heated at 130°C. This resulted in volatilization of the sorbed compounds. The purge gas was continually passed through the sample vessel and into a porous polymer trap (Tenax-TA, Tekmar Co., Cincinnati, OH). 67 Table 5. Gas chromatograph conditions used in analysis of flavor sorption by laminate materials. Hewlett Packard Model 5890A Gas Chromatograph: Flame ionization detector: Splitless injection port. Column: 60 meter: 0.25 mm Inner Diameter: Fused silica capillary column: Supelcowax 10 (Supelco, Inc., Bellefonte, PA): Polar bonded stationary phase. Carrier Gas: Helium: Flow rate of 30 ml/minute. Temperature: Injector temperature - 200°C: Detector temperature - 250°C: Initial oven temperature - 75°C: Initial time - 8.0 min: Temperature program rate - 4.0°C/min: Final temperature - 200°C: Final time - 4.0 min. 68 Following completion of the purge cycle, a desorption cycle was accomplished through heating Of the adsorbent trap to 185°C, which resulted in release of the adsorbed sample compounds. The desorbed compounds were then transferred from the trap through a six-port valve and directed to the capillary column of the gas chromatograph via the cryogenic focusing unit. Gas Chromatograph output 'was monitored as retention time versus detector response, and utilized in the qualitative analysis of product sorption by the multilayer laminate materials. Gas chromatographic conditions were identical to those listed in Table 5. In Table 6 are shown the analytical conditions used for dynamic headspace concentration. In Fig 7 is shown a schematic presentation Of the Tekmar gas flow system. Gas Chromatography/Mass Spectrometry Samples of the hot sauce were submitted to the Mass Spectrometry Laboratory at Michigan State University for identification Of the primary component/s present in the product. A Shimadzu GC-9A gas chromatograph with splitless injection port was used with the identical column utilized in the gas Chromatographic analyses previously described. Conditions were identical to those listed in Table 5. A Model LKB 2091 Magnetic Sector Electron Impact Ionozation/70 eV Mass Spectrometer was used, in conjunction with the GC, to analyze hot sauce headspace samples. 69 Table 6. Dynamic headspace concentrator conditions used in analysis of flavor sorption by laminate materials. Tekmar Model 4000 Dynamic Headspace Concentrator: 6 port valve temperature - 130°C: Transfer line temperature - 130°C. Purge: Helium gas: Flow - 40 ml/min: Time - 7.0 min. Purge ready temperature - 30°C: Desorb: Time - 4.0 min: Preheat temperature - 100°C: Desorb temperature - 185°C: Bake: Time - 12 min: Temperature - 225°C. Tekmar Model 4100 Heated Sample Module Line temperature - 130°C: Mount temperature - 130°C: Sample temperature - 130°C: Prepurge time - 0.0 min: Preheat time - 2.0 min. Tekmar Model 1000 Capillary Interface Cooldown medium - liquid nitrogen: Cooldown temperature - negative 150°C: Heatup time - 10 seconds. 70 §<—-’ Source: Toebe, 1987 Figure 7. Schematic of Tekmar Dynamic Headspace Concentrator gas flow system. RESULTS AND DISCUSSION Due to the range of variables involved in the production of multilayer laminate materials (such as the level of corona discharge treatment, wetting of the polymer components by the adhesive, and component surface roughness), and the author's inability to manipulate those variables in a controlled experiment, the results obtained throughout this research apply only to the two laminate systems studied. The analytical results presented here should in no way be interpreted as standard for LLDPE, EVA or any other type of laminate structures. Multilayer Laminate Delamination Delamination Of the two multilayer laminate structures occurred as a result of exposure to both the lemon juice and hot sauce food systems. Due to the mode of exposure (surface of the food contacting ply only), it was assumed that the resulting delamination was sorption-induced. NO delamination was evident for laminate samples exposed to the citric acid, or acetic acid control solutions. This suggested that volatile, low molecular weight flavor 71 72 compounds present in the food products (such as d-limonene in the lemon jUice and ester-based compounds from the peppers in the hot sauce) were responsible for the delamination. Qualitative, visual examination of the samples, following exposure, indicated that the flavor compounds present in the hot sauce were more aggressive than those found in the lemon juice. Delamination (ply separation within the structures, appearing as elongated blisters or tunnels) was present to a greater extent in the hot sauce- exposed samples than in the lemon juice-exposed samples. The number of tunnels per surface area Of sample, as well as the length and width of the tunnels, was greater in the former system, as opposed to the latter. In both cases, tunnels were predominantly oriented in the cross-direction (perpendicular to web length) of the laminate material. Five differently pigmented inks were used in the printing Of the laminate materials. By comparing the extent of delamination in each of the differently pigmented regions, it was determined that ink color did not appear to influence delamination. NO difference in the amount of delamination was visually apparent between areas of differing color. 73 Bond Strength The average force responsible for laminate ply separation was measured using the peel test. In Fig 8 is a plot of force applied to propagate separation of the bonded plies versus distance of separation, typical of the output A from the Instron Tensile Tester. Laminate bond strength was determined for each of the treatment-laminate material combinations. The bond strength of oven control samples (non-exposed laminate samples maintained at 45°C (+/- 2°)) and roll stock samples (laminate samples from roll stock maintained at 22°C (+/- 2°)), was also determined for each material. The presence or absence of delamination was found to be a poor indicator of bond strength, in that samples showing no delamination often had a significant reduction in bond strength. Linear Low Density Polyethylene Laminate The mean bond strength values for each of the treatment combinations, oven control samples and roll stock materials, is summarized in Table 7. Means ranged from a maximum of 120.8 newtons force/meter sample width for the acetic acid-exposed samples, to a minimum of 14.0 newtons force/meter width for the hot sauce-exposed samples. TO determine the significance of the treatment effects, an Improved Bonferroni t-test was used to compare individual mean values for statistically significant differences 74 A C ..D H V m u - -- c-oo-— ——-o-— _.—..P.--- U .- -. ..._._—._ .-.. .. .... m .. I..- ”H“_.r,__n. O 6- . -...-- .-.- --....— ”...—4....“ II: I . IEPPLIQECEZ _.__1I__ ...... .. .:‘:.':. l.0_ 'IiEa'fZL o.- --<}--— EXTENSION (in.) Figure 8. Example Of output from Instron Tensile Tester for peel test Of non-exposed, heated laminate samples. 75 Table 7. Linear low density polyethylene laminate bond strength values for samples exposed to food products and simulants. Treatment (a) Mean Force Std. (Newtons /meter width) Deviation Roll stock (23°C) 110.3 5.2 Oven control (45°C) 92.8 7.0 Acetic acid (45°C) 120.8 10.5 Citric acid (45°C) 115.6 19.3 Lemon Juice (45°C) 21.0 5.2 Hot sauce (45°C) 14.0 3.5 (a) 5 replicates per treatment. 76 (Gill, 1988). In Appendix D, the statistical analysis is detailed. Based on the experimental design, five comparisons of means are statistically valid. They are: 1. Roll stock samples vs. oven control samples, to examine the effect of heating on bond strength. 2. Oven control samples vs. acetic acid-exposed samples, to determine the effect Of exposure to acetic acid. 3. Oven control samples vs. citric acid-exposed samples, to determine the effect of exposure to citric acid. 4. Acetic acid-exposed samples vs. hot sauce exposed samples, to determine the effect of exposure to flavor volatiles found in the hot sauce. 5. Citric acid-exposed samples vs. lemon juice exposed samples, to determine the effect of exposure to flavor volatiles found in the lemon juice. In Table 8 are presented the statistical comparisons at a confidence level of 99%. Each of the comparisons was found to be statistically significant. Heating, exposure to the control solutions, and exposure to the food products resulted in a significant effect on bond strength within the linear low density polyethylene laminate structure. 77 Table 8. Statistical comparison Of treatments applied to linear low density polyethylene laminate samples. Treatment Code Roll stock (23°C) 1 Oven control (45°C) 2 Acetic acid (45°C) 3 Citric acid (45°C) 4 Lemon juice (45°C) 5 Hot sauce (45°C) 6 Comparison Difference* Significance 1 vs. 2 2.76 ** 2 vs. 3 4.44 ** 2 vs. 4 3.72 . ** 3 vs. 6 17.06 ** 4 vs. 5 15.46 ** * Difference between the tabled Bonferronni t-statistic and the calculated t-statistic. Minimum significant difference (24,5).01 = 0.12. ** Statistically significant at an alpha level of 0.01. 78 Loss of bond strength was particularly noteworthy in the comparisons of acetic acid-exposed samples to hot sauce-exposed samples, and citric acid-exposed samples to lemon juice-exposed samples. In Table 9 the percent change in bond strength for each treatment comparison is shown. A graphical presentation of the difference in bond strength among comparisons is shown in Fig 9. The greatest reduction in bond strength, 87.8%, occurred as a result of exposure to hot sauce, in comparison to acetic acid exposure. Since the primary difference between the hot sauce and the acetic acid solution was the presence of low molecular weight flavor volatiles, it is highly probable that sorption of those components was responsible for the Observed reduction in bond strength. A loss of bond strength of 82.6% occurred as a direct result of exposure to lemon juice, in comparison to citric acid exposure. The primary difference between the lemon juice and citric acid solution was the presence of flavor volatiles in the lemon juice. D-limonene, in particular, has been shown to exhibit a high degree of interaction with polymer films (DeLassus and Hilker, 1987), and is likely to have been an important factor in the loss of bond strength upon exposure to this food system. Exposure to heat was also observed to negatively affect laminate bond strength. A reduction of 13.6% occurred when the oven control materials were exposed to a 79 Table 9. Percent change in bond strength in linear low density polyethylene laminate, for samples exposed to food products and controls. Treatment Code Roll stock (23°C) 1 Oven control (45°C) 2 Acetic acid (45°C) 3 Citric acid (45°C) 4 Lemon Juice (45°C) 5 Hot sauce (45°C) 6 Comparison Percent Difference 1 vs. 2 13.6 % reduction 2 vs. 3 21.1 % increase 2 vs. 4 18.1 % increase 3 vs. 6 87.8 % reduction 4 vs. 5 82.6 % reduction 80 Roll 3% 0... :10 Control .1: I U E N _— g a E E 2 E E . E E ”I E =55: III E - E Roll stock vs. Oven control bond strength Citric acid = Oven E Control = Relative 2 Change .-5...§_§_m.§_§-§_§_§_§_§_§j IIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIII I Oven control vs. Citric acid bond strength Figure 9. Relative change in bond strength among LLDPE comparisons for exposure to heat, citric acid and acetic acid control solutions, lemon juice and hot sauce. 81 Acetic acid 31% Oven Control i3 Relative 2 Change "’.§LEEJE=$ EFEE‘EIiL IIIIIIIIIIIIIIIIIIIIIIIIIII III Oven control vs. Acetic acid bond strength Figure 9 (Cont'd) 82 Citric Acid 0 ”I 00 £10 0 :=== N 1:] = o E 3. 5r —— 3 m :2 33- E Lemon ”I E Juice III -—-—"=: Citric acid vs. Lemon juice bond strength Acetic Acid 1335.51? IIIIIIIIIIIII Relative 2 Change Hot WI“ Acetic acid vs. Hot sauce bond strength Figure 9 (Cont'd) 83 temperature of 45°C (+/- 2°) for a period Of 30 days. The most likely explanation for this bond strength reduction was aging of the laminate/adhesive system - a changing of the physical properties of an adhesive through the application Of heat, over time (Manypenny et al., 1988). A statistically significant increase in bond strength occurred for both the acetic acid-exposed and citric acid- exposed samples, in comparison to the oven control samples. While the large standard deviation observed with the citric acid samples may be responsible for the apparent increase (18.1%) in that system, no explanation could be found for the increase (21.1%) Observed for the acetic acid-exposed samples. Ethylene Vinyl Acetate Copolymer Laminate In Table 10 are presented the mean bond strength values for each Of the treatments applied to the ethylene vinyl acetate copolymer laminate structure. Bond strength values could not be determined for the oven control and roll stock samples, because ply separation could not be propagated. Separation, once initiated, resulted in tearing of the sample material under an applied load by the Instron Tensile Tester. This indicates that the adhesive bond strength between the individual plies exceeded the cohesive bond strength of the polymer laminate components. Treatment means ranged from a maximum of 92.8 newtons force/meter width for the acetic acid-exposed samples, to a 84 Table 10. Ethylene vinyl acetate copolymer laminate bond strength values for samples exposed to food products and simulants. Treatment (a) Mean Force(b) Std. (Newtons/meter width) Deviation Acetic acid (45°C) 92.8 29.8 Citric acid (45°C) 63.0 8.8 Lemon juice (45°C) 42.0 10.5 Hot sauce (45°C) 15.8 3.5 (a) Values for roll stock and oven control samples could not be determined due to inability to peel samples without tearing. (b) 5 replicates per treatment. 85 minimum of 15.8 newtons force/meter width for the hot sauce exposed samples. Two valid comparisons exist: 1. Acetic acid-exposed samples vs. hot sauce exposed samples, to determine the effect Of exposure to flavor volatiles found in the hot sauce. 2. Citric acid-exposed samples vs. lemon juice exposed samples, to determine the effect of exposure to flavor volatiles found in the lemon juice. The statistical significance Of the two comparisons is examined in Table 11. Both comparisons resulted in a statistically significant difference at a confidence level of 99%. Exposure to the volatile flavor compounds present in each of the food products had a highly significant effect on bond strength in the ethylene vinyl acetate copolymer laminate system. In Table 12, the percent change in bond strength for each treatment comparison is listed. In Fig 10, a graphical presentation of the difference in bond strength among comparisons is shown. Exposure to hot sauce resulted in a loss of bond strength Of 82.1%, compared to acetic acid exposure. A loss of bond strength of 33.5% occurred as a direct result Of exposure to lemon juice, as compared to samples exposed to citric acid. It is highly likely that aggressive flavor compounds present in the food products were responsible for loss of bond strength. 86 . Table 11. Statistical comparison of treatments applied to ethylene vinyl acetate copolymer laminate samples. Treatment Code Acetic acid (45°C) 1 Citric acid (45°C) -2 Lemon juice (45°C) 3 Hot sauce (45°C) 4 iComparison Difference* Significance 2 V8. 3 2.06 ** 1 vs. 4 7.39 ** * Difference between the tabled Bonferroni t-statistic and the calculated t-statistic. Minimum significant difference.(16,2).01 - 0.19. ** Statistically significant at an alpha level of 0.01. 87 Table 12. Percent change in bond strength in ethylene vinyl acetate copolymer laminate, for samples exposed to food products and controls. Treatment Code Acetic acid (45°C) Citric acid (45°C) Lemon Juice (45°C) Hot sauce (45°C) humid Comparison Percent Difference 2 vs. 3 33.5 % reduction 1 vs. 4 82.1 % reduction 88 Citric Acid Relative 2 Change IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII II Citric acid vs. Lemon juice bond strength Acetic Acid II |IlllllI||||l|IIIIIIIIIIIIIIIIIIIIII|||l|ll Acetic acid vs. Hot sauce bond strength Figure 10. Relative change in bond strength among EVA comparisons for exposure to lemon juice and hot sauce. 89 Locus of Failure Electron Spectroscopy for Chemical Analysis (ESCA) ESCA analyses were conducted at the Composite Center at Michigan State University. Using ESCA, the opposing surfaces of delaminated samples were examined to determine if adhesive was present on their respective surfaces. If adhesive was present, the relative thickness Of the layer was then determined. Both lemon juice and hot sauce exposed samples were examined. In Fig 11 is an output typical of the ESCA analysis. The vertical axis is counts per energy level, while the horizontal axis is binding energy, specific to the identity of the atoms present on the surface of the laminate ply. Peaks representing the Oxygen ls, Nitrogen ls and Carbon ls atomic bonding orbitals are labeled, and are the result of electrons emitted from those specific atomic orbitals. The presence of the identified peaks verifies the presence of those elements on the delaminated surface. As previously discussed, adhesive bond failure refers to the failure of bonds along the interface between the adhesive and the adhering film. Cohesive bond failure refers to the failure of bonds within the body of the adhesive. Mode of failure results for the LLDPE and EVA laminate structures should not be viewed as standard for such laminate systems: these results apply only to the two laminate materials studied here. 9O .ooneaeu eamuunuo 9:99:09 auscus m— sonneu ass a. sensuous .o— cecal: seduce-eueeu execs £993 .oeseu no; Ou oocccxo cusses sacs—ecu occuasue>~oa 999.com sen second we age oeuecussueo new ewuafiec< Headsego uOu aaeuacuuueam couuueum lean usauso .uu ensuum >9 $853 9.3:: 9.9 9.8— 9.99~ 9.999 9.99.. 9.999 9.999 9.92 9.999 9.999 9.999. 41:11.11}: . 1 1 1 1 1 1 1 1 1 1 .1 1 1 1 . 9 1.5 9 . a $_ . :z n :c v . _ m _ a A . . 9 a .8 1 1 1 1 1 1 1 1 1 1 1 1 1 1 .I 1 1 1 1 . 2 YO” 91 Linear Low Density Polyethylene Laminate The LLDPE laminate was constructed as follows: PVDC coated polyester/ink/adhesive/LLDPE sealant. Initial analysis Of delaminated LLDPE laminate samples indicated that bond failure was adhesive in nature: failure occurred in the region of the adhesive-ink interface. A Locus of failure was the same for samples exposed to both food systems. An adhesive thickness of at least 100 angstroms was present on the LLDPE contact layer, with small amounts of ink also present on the surface of the adhesive. A relatively thick layer of ink was present on the polyester ply, with slight traces of adhesive present on the ink surface. Based on the analyses, failure appears to have Occurred within a "boundary region" surrounding the adhesive-ink interface (Bikerman, 1978). It is the speculative opinion of this author that "desorption," the displacement of adhesive from the interface by a chemical from the environment, or within the structure, may have played a role in the apparent adhesive bond failure of the LLDPE laminate structure (Schneberger, 1979). The chemical, or chemicals, responsible for such action may be sorbed flavor volatiles from the food products (which are aggressive in nature), or components from the inks used in the printing of the polyester plies (Bisset, 1979). 92 Ethylene Vinyl Acetate Copolymer Laminate The EVA laminate was constructed as follows: PVDC coated polyester/ink/adhesive/EVA copolymer film. Analysis of EVA delaminated samples indicated that bond failure was cohesive in nature: failure occurred within the bulk of the adhesive layer, in samples exposed to both food systems. A minimum thickness of at least 100 angstroms of adhesive was detected on both the EVA and polyester layers. NO ink was detectable on either of the component plies. It is the speculative opinion of this author that swelling within the adhesive layer (induced by the presence of sorbed flavor volatiles), in combination with the plasticizing action of volatiles present within the adhesive, may have played a role in the cohesive bond failure observed in the EVA laminate structure (Bagley and Long, 1958: Delassus and Hilker, 1987). Such swelling would result in the development of stresses, both within the adhesive and around the adhesive/contact layer interface. In theory, the adhesive bond strength of a composite exceeds its cohesive bond strength (Zisman, 1963). The expected result, then, would be cohesive failure within the body of the adhesive. Scanning Electron Microscopy (SEM) In addition to Electron Spectroscopy for Chemical Analysis, Scanning Electron Microscopy (SEM) was utilized to determine the locus of failure within the laminate 93 structures. Samples were prepared and cross-sectional images examined and photographed. In Fig 12, a typical micrograph of a non-exposed sample of the linear low density polyethylene laminate material is shown, at a magnification of 60X. In Fig 13, a micrograph of a non-exposed sample Of the ethylene vinyl acetate copolymer laminate material is shown, at a magnification Of 1ooox. Examination Of both micrographs indicated complete contact and adhesion between the component laminate plies. NO signs of delamination were detected. Linear Low Density Polyethylene Laminate Material Subsamples of the LLDPE laminate material, containing regions of delamination, were analyzed by SEM. Samples exposed to both food systems were included.. In Fig 14, an 80X magnification of a delaminated hot sauce-exposed sample is shown, while in Fig 15, a view of the same sample is presented, at a magnification of 800x. The left side of both micrographs is the PVDC-coated/reverse printed polyester ply. The LLDPE food contacting layer is present on the right side. Analysis of both micrographs indicated that failure was adhesive in nature. The analyses, however, did not allow for determination of the exact locus of failure. Similar results were Obtained for the lemon juice-exposed samples. No evidence was found, in any Of the LLDPE samples, which would suggest that failure was cohesive in nature. This is in agreement with, and 94 Figure 12. Cross-section of linear low density polyethylene laminate roll stock sample. Figure 13. Cross-section Of ethylene vinyl acetate copolymer laminate roll stock sample. 95 Figure 14. Cross-section of delaminated hot sauce exposed linear low density polyethylene laminate sample. Figure 15. 800K magnification Of delamination in Figure 14. 96 confirms, the results of the ESCA analysis Of the LLDPE laminate structure. Ethylene Vinyl Acetate Copolymer Laminate Material Subsamples of the EVA laminate material, containing areas of delamination, were removed from the exposed samples and examined by SEM. In Fig 16, a cross-sectional view is presented, at a magnification of 10X, of a lemon juice-exposed EVA sample. The edge of a delaminated tunnel area, running perpendicular to the sample, is visible in the upper half of the micrograph. In Fig 17, a magnified view, at 1500X, of the top of the tunnel area in Fig 16 is shown. On the left side Of the micrograph is the PVDC coated/reverse printed polyester, and a defined thickness of adhesive (left to right). 0n the right side of the micrograph is a second defined thickness of adhesive in contact with the EVA film (left to right). The dark area through the center of the sample is the primary locus of delamination in the EVA laminate material. Evident in the micrograph, on both sides of the laminate ply separation, are areas of secondary separation, appearing as dark elongated regions. The larger of the two, located to the left of the locus of delamination, is an area of separation between the PVDC-coated/printed polyester and the adhesive deposited on its surface. The smaller area of secondary separation, located to the right of the locus of delamination, is an area of separation 97 Figure 16. Cross-section of delaminated lemon juice exposed ethylene vinyl acetate copolymer laminate sample. Figure 17. 1soox magnification of delamination in Figure 16. IBIU WISBQ BUGS 16.8H [£089 98 between the EVA film and the adhesive deposited on its surface. These secondary areas of separation positively identify the lemon juice-induced delamination as being the result of cohesive bond failure within the body of the adhesive. This is in agreement with, and confirms, the results of the ESCA analysis of the EVA laminate material. Similar results were Obtained for the EVA laminate samples exposed to hot sauce. Again, minor areas of secondary separation around the primary locus of failure indicated that delamination was due to cohesive bond failure within the body of the adhesive. Adhesive bond failure, between the adhesive and either of the two component plies, was not apparent. Infra-red Spectroscopy (IR) Exposed laminate samples were examined by IR Spectroscopy for locus of bond failure, following performance of the peel test for bond strength. Attenuated Total Reflectance mode (ATR) was used, in conjunction with a crystal of high refractive index, for the determination of adhesive or cohesive mode of failure. The results of the analyses of both laminate materials, exposed to both food products, were inconclusive, in that analyses Of samples with a known layer of adhesive present on their surfaces failed to verify the presence of that adhesive. 99 SORPTION Dynamic Headspace Concentration A dynamic headspace concentrator, interfaced with a gas chromatograph, was used to qualitatively analyze the product components sorbed by the laminate materials. Desorption, and subsequent chromatographic analysis of the exposed laminate samples, provided a visual profile of the compounds sorbed by the laminate structures. Lemon Juiced-Exposed Samples Both EVA and LLDPE laminate samples, exposed to lemon juice, were qualitatively analyzed for sorbed flavor volatiles. In Fig 18 is a typical output from the gas chromatograph for a lemon juice-exposed and desorbed sample. A range of peaks, each attributable to a unique organic volatile compound, appears on the chromatogram. Further analysis of non-exposed laminate materials allowed for differentiation between volatile organic compounds derived from the laminate material, and volatile flavor compounds derived from the lemon juice. In Fig 19 is an output from analysis of the non-exposed laminate material. The predominant peak in Fig 18 (retention time of 10.97 minutes) is attributable to d-limonene, the primary flavor volatile present in the oil fraction of the lemon juice (Marshall et al., 1985). Gas chromatographic analyses, using d-limonene alone and in combination with 100 355 -'1L“ R33 13:3 ELI ZEN IIAI 1L0 3J3 Figure 18. Gas chromtograph of lemon juice exposed linear low density polyethylene laminate sample, via Dynamic Headspace Concentration, with d-limonene peak at 10.97 minutes. 101 Figure 19. Gas chromatograph of linear low density polyethylene laminate roll stock sample, via Dynamic Headspace Concentration. 102 both roll stock and exposed laminate samples, verified that the primary component sorbed by the laminate structures was, in fact, d-limonene. Both materials appeared to behave similarly, in terms of sorption Of volatile flavor compounds. NO detectable difference was observed between materials. Hot Sauce-Exposed Samples Hot sauce-exposed EVA and LLDPE samples were also qualitatively analyzed for sorbed flavor compounds, using the dynamic headspace concentrator system. In Fig 20 is a typical output from the gas chromatograph for a hot-sauce exposed sample. Comparison with Fig 19 allowed for the differentiation between compounds derived from the laminate structure, and volatile flavor compounds sorbed from the hot sauce. Hot sauce headspace samples were subsequently analyzed by Mass Spectrometry, in an attempt to identify the major components sorbed from the product. Positive compound identification could not be made, due to insufficient resolution of component peaks on the gas Chromatograph output. Gas chromatographic analyses of the hot sauce, alone and in combination with both roll stock and exposed samples, did verify that the flavor volatiles desorbed from the exposed laminates originated in the hot sauce product. Again, both laminate structures appeared to behave similarly, with respect to sorption of volatile flavor 103 jl l H l EBA. Figure 20. Gas chromatograph of hot sauce exposed linear low density polyethylene laminate sample, via Dynamic Headspace Concentration. 104 compounds. No detectable difference between laminate structures was observed. Thermal Distillation Laminate samples, exposed to both products, were also analyzed using thermal distillation, in conjunction with gas chromatography. Samples from each of the four Ilaminate/food product combinations were qualitatively examined for sorbed flavor volatiles, through the heating of exposed samples in a contained volume. Volatiles released from the samples during heating were subsequently withdrawn from the volume and directly injected onto the gas chromatograph column. In Fig 21, a typical output from the gas chromatograph for a non-exposed EVA sample is presented. In Fig 22, output from a lemon juice-exposed EVA sample is presented. Comparison with Fig 21 allowed for the differentiation between compounds derived from the laminate structure, and volatile flavor compounds sorbed from the lemon juice. The primary product volatile (retention time of 11.35 minutes) is attributable to d-limonene. Subsequent analyses using d-limonene alone, and in combination with the laminate material, verified the identity of the compound as being d- limonene. No detectable difference was observed between the LLDPE and EVA laminate materials. Similar analyses using hot sauce-exposed samples failed to provide a gas chromatographic profile of the 105 11111 Ill 1 l €61.82 ELM 33 “AU“ 9!! Figure 21. Gas chromatograph of non-exposed ethylene vinyl acetate copolymer laminate sample, via Thermal Distillation. 106 Figure 22. Gas chromatograph Of lemon juice exposed ethylene vinyl acetate copolymer laminate sample, via Thermal Distillation, with d-limonene peak at 11.35 minutes. 107 flavor volatiles sorbed from the hot sauce. Both LLDPE and EVA laminate materials performed similarly under thermal distillation analysis. No detectable difference was Observed between materials. This technique verified the results determined through dynamic headspace concentration for the lemon juice exposed laminate samples. Due to the lack of sensitivity, in comparison to the dynamic headspace concentration technique, the latter would be a more analytical method to monitor flavor sorption by multilayer laminate materials. CONCLUSIONS These studies were designed to evaluate the effect Of flavor sorption on the adhesive and cohesive bond strength of multilayer laminate materials. PVDC coated polyester/EVA, and PVDC coated polyester/LLDPE laminates were exposed to lemon juice and hot sauce food products and controls. Sorption induced delamination was demonstrated in both laminate structures, as a direct result of exposure to each food product. Volatile flavor compounds, present in the food products, were responsible for ply separation within the laminate structures. D-limonene was identified as the primary component sorbed by the laminates, when placed in contact with lemon juice. Specific compounds sorbed from the hot sauce could not be identified. Those compounds were, however, shown to originate from the hot sauce product. The nature of the bond failure was determined to be cohesive (failure within the bulk of the adhesive) for the EVA laminate structure. Adhesive bond failure (failure in the region of the ink-adhesive interface) was determined to be the cause Of delamination in the LLDPE laminate. Due to 108 109 the range of variables involved in the production Of laminates, and the inability to manipulate those variables in a controlled experiment, generalizations about the nature of sorption induced delamination should not be made, based solely on the results of this study. The mode of failure demonstrated in this research applies only to the two laminate structures studied. A speculative Opinion was proposed by the author, as to the process and mode of failure Observed within the laminate structures. As flavor volatiles were sorbed from the food products by the contact layer of the laminate structures, those volatiles diffused through the contact layer and into the adhesive, passing across the adhesive/contact layer interface. Volatile-induced swelling of the laminate components resulted in a stress development within the adhesive, as well as in the region of the interface (Bagley and Long, 1958). As the developed stress surpassed the critical stress level needed for 'failure, failure occurred within the bulk of the adhesive, in combination with the weakening of the adhesive layer due to the plasticizing action of the flavor volatiles (Delassus and Hilker, 1987). Failure may have occurred in the bulk, as Opposed to at the interface, in compliance with the theory that the adhesive bond strength of a composite exceeds its cohesive bond strength (Zisman, 1963). Such a theory would explain the cohesive bond failure observed in the EVA laminate structure. It does 110 not, however, explain the adhesive bond failure Observed in the region Of the ink/adhesive interface of the LLDPE laminate structure. It was further speculated that "desorption," the displacement of the adhesive from the ink/adhesive interface by sorbed flavor volatiles, might be responsible for the observed adhesive bond failure (Schneberger, 1979). Both theories would be compatible, if migration of ink components, or degradation products (due to improper corona discharge treatment), into the adhesive in the LLDPE structure, had partially weakened the adhesive bonds at the interface within that system (Bisset, 1979). The result could be desorptive failure at the interface, prior to the development Of a critical stress for failure within the bulk of the adhesive. Significant reductions in laminate bond strength were demonstrated, as a result of exposure to lemon juice and hot sauce. Loss of bond strength was not always visually apparent in the form Of delamination. Due to both the similarity of the contact layer compositions and the range of variables involved in the production of the laminate materials, the effect of contact layer composition on loss of bond strength could not be determined. In terms of sorptive behavior, both laminate materials studied performed similarly. No detectable differences were observed. 111 In this research, a procedure has been developed for the examination of sorption-induced delamination in multilayer laminate materials. Electron Spectroscopy for Chemical Analysis (ESCA) was particularly useful in the determination Of locus and mode Of failure within the laminate structures. Scanning Electron Microscopy served as an independent method for confirmation Of the ESCA results. Dynamic Headspace Concentration, in conjunction with Capillary Gas Chromatography, proved to be an extremely efficient method for the qualitative analysis of flavor volatiles sorbed by the laminate structures. Laminate Peel Tests allowed for the quantification of bond strength, as well as a means of monitoring loss of bond strength due to the various treatments studied. Sorption of volatile flavor compounds by the laminate contact layer is capable of effecting the integrity and bond strength of polymeric multilayer laminate materials. As a result, selection of laminate components, in regards to their sorptive behavior and the nature of the food product to be packaged, should be closely monitored during the package development process. FUTURE WORK Future work in the area of sorption-induced delamination should be conducted with laminate materials produced, in the laboratory, by the researchers involved. This would eliminate unknown variables, such as laminating conditions, corona discharge treatment conditions, and ink and adhesive variations, which can not be controlled during analysis Of laminates Obtained from commercial sources. At the same time, laminate components should be Characterized in terms Of critical surface tensions, wettability and surface topography - factors capable of affecting adhesive bond strengths. The effect of contact layer composition on sorption-induced delamination should be further investigated, in that the sorptive behavior of the laminate contact layer plays an important role in the delamination process. The current study, if replicated, should include laminates with contact layers which differ to a greater extent, than those examined here. The sorptive and diffusive behavior of the adhesive should also be examined, in that movement and interaction of sorbed volatiles through the adhesive layer should also play a role in delamination. The theory behind this study should also be applied to coextruded and coated structures. in that sorption-induced swelling within those structures might lead to structural and/or performance irregularities. 112 APPENDICES APPENDIX A APPENDIX A Composition of Product Additives ANTIOXIDANTS (A) Sustane W (UOP, Inc) Ingredients (weight percent) (1) (2) (3) (4) (5) (6) (7) mono-tertiary-butyl-r-hydroxy anisole (BHA) (10%) 2, 6-di-tert-butyl-para-cresol (BHT) (10%) n-propyl-3,4,5-trihydroxy benzoate (PG) (6%) citric acid (6%) propylene glycol (8%) edible oil (28%) mono and diglycerides of fatty acids (32%) (B) Sustane 20 A (UOP, Inc.) Ingredients (weight percent) (1) (2) (3) (4) (5) tertiary-butyl hydroguinone (TBHQ) (20%) citric acid (3%) propylene glycol (15%) edible oil (30%) mono and diglycerides of fatty acids (20%) 0.02 percent (w/v) (A) + (B) was added to the lemon juice and hot sauce products prior to laminate exposure. 113 114 ANTIMICROBIAL AGENT (C) Sodium Azide (Aldrich Chemical, Inc.) 0.02 percent (w/v) (C) was added to the lemon juice and hot sauce products prior to laminate exposure. APPENDIX B Table 13. Table 14. Table 15. APPENDIX B Initial Laminate Exposure Study Composition of sixteen multilayer laminations exposed to three food products and Food & Drug Administration food simulants in initial exposure study (Schwarz, 1987). Food products used in initial study and results of laminate exposure. Food & Drug Administration food simulants used in initial study and results of laminate exposure. 115 116 Table 13. Composition of multilayer laminations used in initial exposure study. CODE COMPOSITION 0.6 mil Polyvinylidene chloride (PVDC) coated polyester/ink/l-component urethane adhesive/2.0 mil white pigmented ionomer film. 0.6 mil PVDC coated polyester/ink/l-component urethane adhesive/2.0 mil coextrusion of polyethylene and ionomer. 0.6 mil PVDC coated polyester/ink/l-component urethane adhesive/2.0 mil ethylene vinyl acetate (EVA) copolymer film containing 6% vinyl acetate. 0.6 mil PVDC coated polyester/ink/ethylene acrylic acid copolymer/0.48 mil metallized polyester/ethylene acrylic acid copolymer/2.0 mil coextrusion of polyethylene and ionomer. 0.6 mil PVDC coated polyester/ink/1-component urethane adhesive/2.0 mil low density polyethylene film. 0.75 mil biaxially oriented polypropylene/ink/water based acrylic adhesive/1.5 mil low density polyethylene. Ink/paper/polyethylene/aluminum foil/ionomer extrusion. Lacquer/ink/paper/polyethylene/aluminum foil/ polyethylene extrusion coating. 0.75 mil biaxially oriented polypropylene/ink/white pigmented polyethylene/foil/ionomer. 0.48 mil polyester/ink/white pigmented polyethylene/ foil/2-component urethane adhesive/EVA copolymer extrusion coating. 0.48 mil polyester/z-component urethane adhesive/ 2.0 mil linear medium density polyethylene. 1.0 mil Nylon/2-component urethane adhesive/4.0 mil linear medium density polyethylene. 1.0 mil Nylon/2-component urethane adhesive/2.0 mil medium density polyethylene. 117 Table 13 (Cont'd) N 0.7 mil PVDC coated biaxially oriented polypropylene/ink/l-component urethane adhesive/ 1.5 mil low density polyethylene. O 0.6 mil PVDC coated polyester/ink/1-component urethane adhesive/2.0 mil polyethylene ionomer coextrusion. P 0.6 mil PVDC coated polyester/ink/1-component urethane adhesive/2.0 mil linear low density polyethylene film. Laminates C and P were selected for further research. 118 Table 14. Food products used in initial study and results Of laminate exposure. LEMON JUICE ORANGE JUICE HOT SAUCE STRUCTURE A YES YES not tested B YES YES NO C YES NO YES D NO NO NO E NO NO YES F YES YES YES G not tested* not tested not tested H not tested not tested not tested I N0 NO YES J YES YES YES K NO NO NO L NO NO NO M N0 NO NO N YES YES YES 0 NO YES YES P NO YES YES YES indicates delamination. 1 month exposure at 45°C via test cells. See Table 14 for laminate structure composition. Lemon Juice and Hot Sauce were selected for further study. *Due to limited web width. 119 Table 15. Food & Drug Administration food simulants used in initial study and results of laminate exposure. 3% AQUEOUS 50% AQUEOUS ACETIC ACID ETHYL ALCOHOL HEPTANE* SOLUTION* SOLUTION STRUCTURE A NO NO NO B YES NO YES C YES NO NO D NO NO NO E YES NO NO P YES YES YES G YES YES YES H YES YES YES I NO YES NO J NO NO NO K NO NO NO L NO NO NO M NO NO NO N YES YES NO 0 YES NO NO P YES NO NO YES indicates delamination. 3 month exposure at 23°C via pouches, 2" X 5". See Table 14 for laminate structure composition. * NO longer recommended by FDA as food simulants. APPENDIX C APPENDIX C Test Cell Construction The test cells used throughout the study were a modified construction of the American Society for Testing and Materials (ASTM) test cell described in ASTM F 34-76. Twenty four test cells were constructed. The following materials were used in the construction of each cell, as depicted in Fig 5: 2 aluminum plates, 15.2 x 10.2 x 0.6 cm. 1 U-shaped Teflon gasket, outer dimensions 15.2 x 10.2 x 0.6 cm., cavity dimensions 13.3 x 6.4 cm., centered at the top of the gasket. 9 Steel bolts, 3.2 x 0.6 cm. 18 steel washers, 1.9 cm. in diameter. 9 steel nuts, 0.6 cm. 1 Teflon gasket plug, 6.4 x 2.5 x 0.6 cm. The U-shaped gasket was placed between the two aluminum plates, clamped in place, and holes drilled through the cell in accordance with the placement on Fig 5. Each test cell was then capable of holding two laminate samples: one sample between each plate and the gasket. Using the plates as templates, matching holes were placed through the laminate samples. Once assembled, the cell was clamped together with the washers, nuts and bolts, filled, and the gasket plug put in place. 120 APPENDIX D APPENDIX D Improved Bonferroni t-Test Treatment Code Roll Stock, 23°C 45 C for 1 Month Acetic Acid Control at 45°C, 1 Month Citric Acid Control at 45°C, 1 Month Lemon Juice at 45°C, 1 Month Hot Sauce at 45°C, 1 Month min-“UMP ANALKSIS OF VARIANCE FOR EVA DATA Treatment 3 4 5 6 Replicate 1 .462 .442 .273 .075 2 .668 .305 .240 .100 3 .401 .390 .286 .120 4 .375 .343 .267 .100 5 .754 .340 .145 .080 Yi. 2.660 1.820 1.211 0.475 '§i. 0.5320 0.3640 0.2422 0.0950 2.. -- 6.166 37.. = 0.3083 Total (Uncorrected) Sums of Squares = 2.556012 Correction Term = 1.900978 Source df SS MS Total 19 0.655034 Treatment 3 0.515051 .171684 Error 16 0.139983 .008749 121 122 ANALKSIS OF VARIANCE FOR LLDPE DATA Treatment 1 2 3 4 5 6 Replicate 1 .645 .561 .677 .856 .104 .067 2 .652 .519 .721 .630 .081 .106 3 .652 .544 .587 .574 .153 .084 4 .594 .566 .755 .664 .113 .102 5 .608 .532 .708 .599 .127 .063 Yi. 3.151 2.722 3.448 3.323 0.578 0.422 SH” 0.6302 0.5444 0.6896 0.6646 0.1156 0.0844 2.. 1 13.644 37'. . = 0.4548 Total (Uncorrected) Sums of Squares = 8.231806 Correction Term = 6.205291 Source df SS MS Total 29 2.026515 Treatment 5 1.950966 .390193 Error 24 0.075549 .003148 IMPROVED BONFERRONI T-TEST t = XI-xz W MINIMUM SIGNIFICANT DIFFERENCE (MSD) LLDPE Laminate Material MSD (24,5).01 a T (table)(24,5).01 xv2fi§E75 = 0.1230 EVA Laminate Material MSD (16,2).01 = T (table)(16,2).01 xVZMSE/S = 0.1923 T (table) values are derived from the Two-sided Table for Control of FWI Based on Sidak's Multiplicative Inequality. 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