i This is to certify that the thesis entitled VARIABLES AFFECTING INTERLAYER ADHESION IN HULTILAYER LAMINATE STRUCTURES presented by TIMOTHYLEE SHARKEY has been accepted towards fulfillment of the requirements for Em degree in PACKAGING v Major professor Date FEBRUARY 27; 1992 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution meme? Et’é’icisigjam stigma. a u r r ‘ w: to ' $.91 Mr:- F ‘3" 8 (Lin ..; in. c: s; :5: 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 MSU Is An Affirmative Actlorfiquel Opportunity Institution owns-9.1 r——__.___ i 7 i VARIABLES AFFECTING INTERLAYER ADHESION IN MULTILAYER LAMINATE STRUCTURES BY Timothy Lee Sharkey A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE School of Packaging 1992 ABSTRACT THE EFFECT OF CORONA TREATMENT AND ADHESIVE SELECTION ON INTERLAYER ADHESION IN MULTILAYER LAMINATE STRUCTURES BY Timothy Lee Sharkey These studies were designed to investigate the effect of corona treatment levels on laminate integrity following surface exposure to selected food and food simulant systems. Low Density Polyethylene / Polyethylene Terephthalate laminates were fabricated using a two component urethane based (CONTROL) adhesive system, and a single component (commercial Adcote— 333) urethane based adhesive system, each at three levels of polyethylene corona treatment. The Control Structure was designed to facilitate delamination and provide samples for the development of analytical procedures to quantify the mode of bond failure. The results obtained with the Control laminate established the general applicability of surface analysis by (ESCA) and (SEM) procedures in determining the mode of bond failure. No evidence of delamination was found for the one component adhesive system, as a result of surface exposure to; lemon juice and hot sauce, and the respective food simulants; 3% (wt/v) citric-acid solution and 3% (v/v) acetic—acid solution. ii DEDICATION This thesis is dedicated to my parents, Thomas and Anne, whose unending support made it possible to complete this work. iii ACKNOWLEDGEMENTS I would like to recognize and thank the following people, without whom, this work would not have been possible: Drs. Bruce Harte and Jack Giacin, for their guidance, support and assistance while serving as Co-Chairmen of the graduate committee. Dr. Ian Gray, for serving on the guidance committee. Dr. William Taylor, for the scholarly advice and use of printing facilities. Dr. Mike Rich, Manager of Research; Composite Materials Research Center, for his counsel, assistance and expertise. Mr. Javad Kalantar, Doctoral Candidate Engineering, for sharing his patience, counsel and expertise. Mr. Mark Schroeder, for his special insight and provision of film for this research project. Mr. David Markgraf, Enercon Industries, for his knowledge and expertise regarding the Corona Treatment Process. The Center for Food and Pharmaceutical Packaging Research, for Project Funding. The Packaging' Education. Foundation, for their fellowship support. The engineers at Cello-Foil for their knowledge and expertise regarding the conversion process used in this research. The E.I. DuPont de Nemours Company, for their donation of film for this research project. The Morton Thiokol Company, for their donation of adhesive products to this research project. iv TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION LITERATURE REVIEW Page vi vii 6 corona Treatment 000......OOOOOOOOOOOOOOO0.00.00.000.06 Product/Package Interactions ........................16 Adhesion/Bond Failure ...............................21 Analytical Tests ........................... ..... ....25 MATERIALS AND METHODS 35 Materials: Laminate Materials ......................35 AdheSives OOOOOOOOOOOOOOOOOOOOOOOOQ0.0.0.36 Laminate Construction ...................37 Food Products and Controls ..............39 Experimental Methods: Laminate Exposure ............41 Analytical: Determination of Locus of Failure ......45 Examination of Locus of Failure ........46 RESULTS AND DISCUSSION 47 Development of Analytical Methodology................47 Control Adhesive System............................47 Control Structure Delamination.....................48 Locus of Failure......................... ....... ...51 Micrograph Interpretation..........................54 Commercial - Adcote 333 Adhesive System..............60 SUMMARY AND CONCLUSIONS 62 FUTURE WORK 64 APPENDICES Appendix A: Composition of Product Additives .......65 Appendix B: Test Cell Construction .................66 BIBLIOGRAPHY 67 LIST OF TABLES Table Page 1 Typical Substrate Surface Tensions. 8 2 Mode of Failure determined with ESCA - 53 Control Adhesive System. 3 SEM Micrograph Figure Legend. 55 4 Mode of Failure determined with ESCA - 61 Adcote 333E Adhesive System. vi LIST OF FIGURES Figure 1 10 11 12 13 14 15 Probable mechanistic pathway for corona discharged films in air. Conventional Corona Treatment Station for Non-Conductive Material. Bare Roll Corona Treatment Station for Conductive and Non-Conductive Material. Cross-section and composition of laminate structure. Schematic of modified ASTM test cell used for laminate exposure. Schematic of cell designation exposure regime. Photograph of relative delamination for acetic acid exposed laminate. Photograph of relative delamination for hot sauce exposed laminate. Output from Electron Spectroscopy for Chemical Analysis for delaminated ply of intermediate corona treatment low density polyethylene laminate sample exposed to hot sauce, with peaks representing oxygen ls, nitrogen ls, and carbon ls atomic bonding orbitals. Virgin LDPE surface at SOOOX. Virgin PET surface at SOOOX. Hot Sauce exposed LDPE laminate at 5000X. Hot Sauce exposed PET laminate at SOOOX. Acetic Acid exposed LDPE laminate at SOOOX. Acetic Acid exposed PET laminate at SOOOX. vii Page 10 12 14 38 42 44 49 49 52 56 56 57 57 58 58 INTRODUCTION Recently, plastic materials have found broad applications in food packaging. Some of the most exciting developments are in multilayer laminations, which allow the incorporation of a wide range of materials, each with its own particular properties, into a single structure. Substantial flexibility has resulted in the common use of laminates for packaging of such foods as meats and cheese, snack foods, condiment packets, beverages and boil-in-bag meals. Along with the significant advantages gained with plastic materials, potential disadvantages have also been identified which must be understood and overcome. For example, interactions between the product and the package play a major role in overall package performance, despite seemingly good package construction. It is therefore critical that material selection and package design take into consideration the product/package system, prior to large scale production. The evolution of plastic packaging materials in the food industry has made convenience a reality. The packaging of convenient food products usually involves the use of polymeric materials. Therefore, it is essential to investigate the potential interactions between these "new" products and the contacting polymeric packaging material. 1 2 One of the major concerns of food packagers is the issue of sorption induced delamination. Specific questions regarding this area include: (1) Are components of food products capable of inducing delamination? If so, what are the common characteristics of these foods?. (ii) HOW’dO these foods cause delamination? (iii) Assuming delamination occurs, is it adhesive or cohesive in nature? and (iv) What can be done to prevent delamination from occurring if these food products are to be packaged in laminate type package systems? The proposed studies will focus on providing answers to these specific questions. The maintenance of overall package integrity is a primary concern when developing a packaging system for a food product. The concept of "overall integrity" goes beyond simply maintaining intact seals. It includes the equally complex consideration of integrity "within" the package system. For example, a package constructed utilizing a multilayer laminate technology relies on the adhesive system to provide adequate bond strength between component plies. This bond strength is based on: (1) Adhesive Bonds: bonds along the interface between the adhesive and adhering film. (2) Cohesive Bonds: bonds within the bulk of the adhesive. The development and maintenance of both of these bond types is critical to the "overall package integrity". Adhesive or 3 cohesive bond failure can lead to a significant deterioration of structure characteristics, resulting in delamination and possible loss of package integrity (Schroeder, 1990). The effect of sorption of flavor volatiles on the adhesive and cohesive bond strength of multilayer laminations was the focus of a study by Schroeder et. al. (1990). Two polyvinylidene chloride coated polyester / polyolef in adhesive laminates were surface-exposed to lemon juice and a hot sauce product, as well as to food simulant systems at 45%L Sorption of flavor volatiles resulted in delamination of both laminate structures, following exposure to the respective food products. Gas chromatography was utilized to identify d- limonene as the primary component sorbed by the laminates upon exposure to lemon juice. Using Electron Spectroscopy for Chemical Analysis (ESCA) and Scanning Electron Microscopy (SEM), adhesive bond failure with the linear low density polyethylene (LLDPE) laminate was found, while cohesive bond failure occurred within the ethylene vinyl acetate copolymer (EVA) laminate. Bond strength was quantified using a peel test. Significant reductions in bond strength were observed upon exposure to the food.products and controls. In addition, significant reductions in bond strength occurred regardless of any observed delamination. Due to the range of variables involved in the production of multilayer laminate materials, and the inability to manipulate these variables in a controlled experiment, the results described by Schroeder et al. (1990) applied only to 4 theatwo laminate systems evaluated, and extrapolationiof these results to similar structures was not recommended. Delamination can be the result of adhesive or cohesive bond failure and can be attributed to a number of factors, to include: 1. Poor adhesion of the adhesive to the outer ply of the lamination. 2. Poor adhesion of the adhesive to the heat seal ply of the lamination. 3. Inappropriate adhesive formulation. 4. Curing conditions. A detailed study of the effects and interactions of these variables would require considerable time and expenditure, and is beyond the scope of the present investigation, iHowever, an in depth study designed to evaluate one or more of these variables can contribute significantly to the general body of knowledge in this area*. An important consideration in developing the protocol for such a study would require that special laminate structures be fabricated to exact specifications. 'This ‘would enable ‘the investigation ‘to eliminate from consideration dependent variables related to laminate web structures or processing conditions which were not part of the experimental design. (*) Such a consideration provided impetus for the present study. 5 The specific objectives of the present study include: 1. Evaluation of laminate intrgrity following surface exposure to selected food and food simulant systems. Develop analytical methodology to establish the mode of interfacial bond failure in multilayer laminate structures induced by contact phase exposure. Develop a correlation between sorption induced interfacial bond failure in the test laminate structures, the mode of failure and the specific variable(s) evaluated in the experimental design. LITERATURE REVIEW CORONA TREATMENT The published literature regarding corona treatment has been reviewed and will be presented in three major sections; (1) Corona treatment and surface tension, general background including surface tension measurement techniques; (ii) Corona treatment systems and electrode developments; and (iii) Corona discharge chemistry. Corona Treatment and Surface Tension Corona treatment has been used for nearly three decades to improve surface wetting characteristics for printing and laminating applications (Podhajny, 1987) . The development of surface treatment of substrates to achieve higher levels of surface tension has permitted higher line speeds with improved adhesion of inks, adhesives, laminations and other coatings (Markgraf, 1986). The necessity for surface treatment is based primarily on the nature of polymeric materials. Polymers in general have chemically inert and non-porous surfaces with low surface tensions that cause them to be non-receptive to bonding with printing inks, coatings, and adhesives Markgraf (1986). For example, polyethylene, a common food contact, heat seal layer in many packages, has the lowest surface tension of commonly used packaging polymers, 6 7 and is one of the materials most often subjected to surface treatments for the improvement of bonding characteristics. Corona treatment is one of the most effective methods for improving the surface tension of thin films in web form. Markgraf (1986) has described the "Wetting Tension Test" (ASTM D-2578), which is the most widely used measurement to determine treatment level. The test consists of applying a mixture of formamide/cellulosic solutions which have gradually increasing surface tensions, in a prescribed series to the surface of a polymeric material. The procedure is complete when the solution spreads or wets the material. At this point, the surface tension of the material is equal to the known surface tension of the "dyne-solution". In this manner, one can accurately determine the surface tension of a particular polymer material to within one dyne. Surface tension is measured in dynes per centimeter squared. A reference table containing the inherent dyne levels of virgin substrates is shown in Table 1. Corona Discharge Chemistry The term "corona" is used to describe the condition of the region between the electrode and.the film roll. .According to Podhajny (1987), the phenomenon of corona begins with stray electrons present in air between the electrode and the film roll. The electrode creates a strong electric field which accelerates the electrons towards the positive electrode. Table 1. Typical Substrate Surface Tensions. POLYMERS SURFACE TENSION @Lneslcm) Hydrocarbons Polypropylene - PP, OPP, BOPP 29-31 Polyethylene - PE 30-31 Polystyrene - PS 38 ABS 35-42 Polyamide <36 Polymethyl methacrylate - PMMA <36 Polyvinyl acetate/ polyehtylene co-polymer (PVA/PE copolymer) 33-34 Epoxy <36 9 The electrons continually strike surrounding air molecules (N2, 02: and H20) located in their path. The impact of these collisions knocks out orbital electrons from the air molecules, leaving a positive ion and another electron. In turn, these constituents strike other air molecules and soon the air is full of electrons, positive ions, excited oxygen and nitrogen molecules, heat and light. The term "corona" refers to the bluish color of the air under these conditions. These excited molecules dissociate to form free radicals, ions and photons. Ozone is produced from the interaction of highly reactive oxygen fragments and oxygen molecules. The ozone then reacts rapidly with polar molecules such as water. The dominant theory concerning corona discharge is that oxygen- containing functional groups are introduced into the polymer surfaces as shown in Figure 1. Several additional considerations exist regarding discharge treatment. The relative humidity of the air surrounding the discharge station directly influences the required discharge time. Discharge time increases with an increase in relative humidity. A variety of active chemical species have also been found to be generated by the Corona surface treatment. Bulk and mechanical properties of the film are also significantly effected by the discharge process. For a detailed account of corona discharge under gases other than air, film additives effect on treatment, surface roughness from corona treatment, and corona over-treatment, the reader is refered to Podhajny (1987) and references cited therein. 10 (U R' R'\ no I H—C-H Eff-2;?- -c—u 9M H—Too: R a “K R );K? I RR'CH—O-O—CHR'R ZR'RCHO (O) smart-H10 O "0 O-O- , Corona . Betta-CH R “Ozone Rflc E CHR /°"°\ RHC\ /CH— 3' Gama: O /oisture O O RC< + RC< OH H Source: Podhajny, 1987 Figure 1: Probable mechanistic pathway for corona discharged films in air. ll Corona Treatment Systems Stobbe, (1987) has reviewed in detail the overall corona treatment process. The most basic corona treatment system consists of a corona discharge station and a high voltage transformer. The transformer is coupled with a power supply or generator which supplies the transformer and converts the plant power to high frequency ‘single phase power, suitable for corona treatment. Although the discharge station itself may be assembled in several configurations, four essential components are necessary in one form or another, they include: an electrode to which the high voltage is connected, an air gap which will be ionized creating corona, a dielectric material which can support the high voltage levels without breaking down, and a ground plane which is usually a metal roll which the film passes over as it proceeds through the station. Mechanical System Overyiey The "conventional" system shown in Figure 2, is the most common configuration used. In this example, the electrode is an aluminum bar, adjustable to various film widths. The dielectric is a non-conducting material such as silicon or rubber, bonded to the ground roll which the film passes over. The air gap is formed by spacing the electrode approximately 1/16th of an inch away from the surface to be treated. When a high voltage potential is applied between the electrode and the grounded roll, the air gap ionizes across the width of the 12 f /— Electrode Dielectric Grounded Roll Source: Stobbe, 1987 Figure 2: Conventional Corona Treatment Station for Conductive ' and Non-Conductive Material. 13 electrode creating corona. Stobbe (1987) described another configuration commonly called the "bare roll" which is depicted in Figure 3. This second system is identical to the "conventional" system above, except that the.dielectric material is bonded to the electrode rather than to the ground roll. This system has a distinct advantage when treating a conductive web such as foil or metallized materials, since the ground roll is bare and at ground potential when the metallized portion of the film is being treated. In general, "bare-roll" systems tend to be of a fixed width type and are inherently less efficient than their "conventional" counterparts. Although an exact understanding of this has not been determined, additional power is required to attain the same level of corona treatment when ‘using’ a "bare-roll" system 'versus a "conventional" system. Electrode and Station Developments Markgraf (1983) has reviewed both. the evolution of electrode configurations and their station assemblies comprehensively, and readers are referred to this reference and references therein, for a detailed review of this topic. Conclusions and Summary of Corona Treatment Through the increase of polar surface energy, corona discharge ‘treatment. has been shown 'to improve substrate wettability and adhesion characteristics. Film surface l4 /— Electrode Dielectric /- Material Grounded Roll Source: Stobbe, 1987 Figure 3: Bare Roll Corona Treatment Station for Conductive and Non-Conductive Material. 15 regions may be crosslinked by corona treatment, resulting in an increase of film cohesive strength. The formation of polar groups and reactive species appears to be the mechanism through which improved surface characteristics are imparted to substrates. Some of these species are: * Peroxides and ozanides: ROZR, ROOH, ROZR * Ions: -NO; , R-CfO * Neutral functionalogroups: ;c=o, ip-on, -qfo, —c§o, -NO OH OR * Ozone * Film Surface Crosslinking The effectiveness of corona treatment on polymeric films depends on the relative humidity during discharge. The chemical constituents formed on the film surface by corona discharge are largely controlled by the composition of the gas between the electrodes, as well as by the film and its additives. Corona discharge treatment improves the wettability and adhesion characteristics of various substrates, such as: foil, metallized paper and polymeric films. Its versatility over a range of substrates and conditions implies that some common mechanistic element(s) do exist (Podhajny 1987, Markgraf, 1986). Additional information regarding corona treatment effectiveness with respect to ink adhesion on polyethylene is located in the literature review of Electron Spectroscopy for Chemical Analysis (Markgraf et. al., 1990). l6 PRODUCT [PACKAGE INTERACTION Some of the most visible advances in plastics technology are in the areas of consumer goods and food products. The phenomenal growth of polymer and plastic materials in these arenas is no coincidence, in fact, a number of key factors have been responsible for the dramatic evolution which has occurred. Based on observation and personal communication, some of these driving forces include: 1. The advancement of science and technology. 2. Interactive packaging and barrier technology. 3. Demand for convenience. 4. Desire for consistency and an increased knowledge of the tools and techniques employed to obtain it. 5. The monetary benefits of shipping and transporting a lighter "product". From commodities to ready-to-eat entrees, numerous food packaging systems consisting of polymeric materials in direct contact with the food product, are on the supermarket shelf. Cereal products, spice packets, soft drinks, milk, flavored yogurts, sour creams, cheeses, meats, juices, chocolates and numerous sauces, are some just to name a few. Of particular interest are those products which. contain ‘volatile, low molecular weight organic compounds. The reason for the 17 interest is two-fold: first, this group encompasses many key flavor and aroma constituents in foods; and secondly, because these components are low molecular weight molecules, they are readily sorbed by the non-polar food contact polymers. (Giacin, 1991). The effect of interactions between a food.product and its packaging material has been the subject.of'a number of studies in recent years, and a considerable understanding regarding the broad spectrum of package/product interactions has resulted. Sorption of food product constituents by packaging material is known as "scalping". The specific effects of this type of interaction are‘well documented in the literature, and discussed below; For example, it has been shown that "sorption" can influence related physical and mechanical properties of polymers. (Hirose, 1988; Imai, 1988; and Konczal, 1989). In addition, sorption has been shown to effect bond strength in multilayer laminate structures. (Schroeder, 1990). Food/package interactions typically involve a cumulative series of events which act to change, alter or compromise the quality of the product or the integrity of the package. In a typical liquid food product system, sorption of flavor or aroma constituents is quantified by determining a partition coefficient (K). (Kwapong and Hotchkiss, 1987). This is a measure of the sorbate (food constituent likely to interact with package) partitioning itself between the polymeric 18 material and the food contact phase at equilibrium. Once the partition coefficient is determined, researchers have a general sense of the potential scalping which may occur. Depending on the nature of the product and the type of polymeric material used, scalping can result in significant changes to product quality and packaging material performance. Sorption Kwapong and Hotchkiss, 1987; Durr, 1981; Marshall, 1985,88; Hernandez et al. 1986; Rogers, 1960; Hirose, 1987; Imai, 1988; Konczal, 1989; and Schroeder, 1989, are among the researchers who have studied sorption. Kwapong and Hotchkiss (1987) described the partitioning of limonene between various plastic films and an aqueous solution by an equilibrium partition coefficient (Kc), as described in equation 1. Kc = [Cpleq / [Cu]... (1) Where: [C29]eq is the equilibrium aroma concentration in the plastic film. is the equilibrium aroma concentration in the aqueous solution. [c.qleq Limonene exhibited a strong affinity for polymeric films such as low density polyethylene and ionomers, which was evidenced by the high Ke values obtained by the authors. Both sorbate composition and. polymer composition jplay’ a role in the sorptive process. (Kwapong and Hotchkiss, 1987). Marshall (1985) and Durr (1981) reported a 40-60% decrease in limonene concentration in packaged orange juice, 19 due to sorption into' the polyethylene lining of carton packages. The rate of loss has been characterized in a two part mechanism: within 3-6 hours, a rapid loss of limonene occurred which was followed by a period of diminishing loss. Surlyn material was also studied by Marshall (1985) who reported that a 45% loss of limonene occurred by the same mechanism as stated for the polyethylene. Earrie; Property Chapges Significant barrier property loss was exhibited when the oxygen permeability of selected films increased between 50% and 350% upon exposure to d-limonene from orange juice. Hirose et al. (1988) Mechanical Properpy Chapges Significant changes in the mechanical properties of sealant films have also been reported by several authors. Hirose et al. (1988) studied the effect of flavor sorption from orange juice on the mechanical properties of polyethylene and two ionomers, sodium and zinc types. Film samples exposed to orange juice for 27 days were found to have experienced the following changes in mechanical properties: (i) up to a 40% decrease in the modulus of elasticity; (ii) seal strength decreases up to 25%; and (iii) decreases in tensile strength, percent elongation and impact resistance. The greatest decrease in modulus of elasticity, tensile strength and seal strength occurred with the sodium type 20 ionomer, which also sorbed the greatest amount of d-limonene. Imai (1988) performed similar studies using low density polyethylene (LDPE), an.iethylene ‘vinyl alcohol copolymer (EVOH) of high ethylene content and co-polyester (Co-PET) sealant films. The latter structures were developmental films and not commercial heat sealant film structures. Sorption of d-limonene affected the modulus of elasticity (EVOH and Co-PET), stress at 100% elongation, impact resistance (EVOH and LDPE), seal strength (LDPE), and yield stress (EVOH). D-limonene sorption by the Co-PET material was reported to be significantly lower than that of the two other films. With the exception of modulus of elasticity, sorption had an insignificant effect on the Co-PET film's other mechanical properties. Konczal (1989) studied.the effect of flavor sorption from apple juice on the mechanical properties of LDPE, EVOH and Co- PET. The results included significant changes in the tensile strength and percent elongation (all three films), modulus of elasticity (LDPE), and finally, yield point and impact resistance (EVOH). The changes exhibited by the three films occurred after a one day exposure period, with little change occurring thereafter. Bond Strength Changes The effect of sorption. of flavor ‘volatiles on the adhesive and cohesive bond strength of multilayer laminations was the focus of a recent study reported by Schroeder et al. 21 (1989). Two polyvinylidene chloride coated polyester/ polyolefin adhesive laminates were surface-exposed to lemon juice and a hot sauce product, as well as to food simulant systems at 45%;. Sorption of flavor volatiles resulted in delamination of both laminate structures following exposure to the respective food products. Gas chromatography was utilized to identify d-limonene as the primary component sorbed.by the laminates upon exposure to lemon juice. Using Electron Spectroscopy for Chemical Analysis (ESCA) and Scanning Electron Microscopy (SEM), adhesive bond failure was identified with the linear low density polyethylene (LLDPE) laminate, while cohesive bond failure occurred within the ethylene vinyl acetate copolymer (EVA) laminate. Bond strength was quantified using a peel test. Significant reductions in bond strength occurred upon exposure to the food products. ADHESION AND BOND FAILURE The topic of "adhesion" has been studied for hundreds of years and many definitions, terms and theories have been introduced to characterize this phenomena. The definition supplied by the American Society for Testing and Materials (ASTM) for adhesion, is the following: "Adhesion, is the state in which two surfaces are held together by interfacial forces consisting of valence forces, interlocking action, or both". 22 In the context of the present study, "adhesion" refers to the bonding of two laminate structures by a commercially available thermoset plastic. The process of adhesion consists of three distinct stages which must be completely performed for proper adhesion to occur. The stages include: wetting, bond formation and solidification. The first stage has been defined by Zisman (1963) as the adhesion on contact between a liquid (the adhesive) and a solid (the adherend); and the extent to which the liquid spreads. Thermodynamically, wetting occurs whenever the free energy change for producing the liquid-solid interface is negative compared to the free energy changes for a loss of the solid-air and liquid-air interfaces. (Schneberger, 1979). Equation 2 describes the thermodynamics involved, such that when summation is negative, wetting will occur: dEW = dEh — (dEh + dB”) (2) 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. As wetting occurs, bonds are formed between the liquid adhesive and the solid adherendc 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 23 attraction. Polar bonding is the result of asymmetry in the electric field around atoms or molecules, resulting in attraction (Mattox, 1978). The final stage in the adhesion process is solidification. Schneberger (1979) has reported two important phenomena which characterize the solidification process. The first is a loss of mass in the adhesive phase due to the volatilization of the solvent or liquid constituent. Shrinkage occurs as a result of this activity which in turn imposes stress at the adhesive/adherend interface. In addition, stresses and cracks within the adhesive may be formed at this stage. The second phenomena pertains to thermally-induced, internal vibratory motion which occurs constantly even after complete solidification and curing. Masuoka (1982) describes this same phenomena and both authors maintain. that. a :minimal amount. of chain flexibility is desirable/necessary within the adhesive, in order to prevent failure of the adhesive. Bond Failure Two distinct types of bond failure can occur within an adhesive joint. (Zisman, 1963; Good, 1978; and Mittal, 1978) define these types as: (i) Adhesive bond failure, is the failure of bonds at and along the interface between the adhesive and the adherend; and (ii) Cohesive bond failure, is the failure of bonds within the bulk of the adhesive. Zisman (1968) reports that under ideal wetting, bonding and 24 solidification conditions, that adhesive bond strength will exceed cohesive bond strength. Good (1978) characterizes bond failure as a two part process consisting of: the initiation of separation, and propagation. Imperfections, flaws and the solidification process itself may lead to stress concentrations within the adhesive/adherend interface. These concentrations of stress can result in the dissipation of energy which causes the initiation of separation. Propagation occurs with the dissapation of energy across the interface. The stress concentrations present within the adhesive joint contribute to this phenomena. The end result of bond failure is a physical separation of the laminate structure, known as delamination. Volatile, low molecular weight constituents of food products partition into and through the food contact layer, Schroeder (1989). Once present in.the interfacial region, these moities act as stress concentrators to "load" or stress the existing adhesive joint imperfections or stresses. Initiation of separation often occurs in these regions of relatively high stress, and is followed by propagation in surrounding regions. This process will continue throughout the laminate until the energy assiciated with the stress buildup is depleted. Oftentimes numerous regions and zones of high stress are created under normal converting processes. The converting process thus inherently incorporates some of the preconditions necessary to facilitate delamination, given exposure to food products containing known stress concentrators. 25 For a more thorough review on adhesion and bond failure, the reader is referenced to Schroeder (1989) and references cited therein. ANALYTICAL TESTS Spanning Electron Microscopy (SEM) The views presented here are in large part those of Flegler, Heckman and Klomparens (1990), who form the nucleus of the Center for Electron Optics (CEO) at Michigan State Universityu This group of researchers is actively involved in the use of microscopy for biological specimen analysis, and write from the standpoint of electron microscopy in general. The scanning electron microscope (SEM) was first introduced into the commercial market in 1965. In the 26 years that have followed, an increasing number of scientists have been able to use this powerful analytical instrument to conduct research in the fields of biology and. material science. The use of SEM for surface morphology research is superior, as it provides these important advantages over other surface analysis techniques: a large depth of field (0.003-1 mm), extremely high resolution (3-4 nm), extremely high range of magnification (lo-100,000X), and high dimensionality image processing which allows results presentation of results that is easily understood through 26 actual photomicrograph recording. The general operation of an SEMC consists of three interrelated functions: image formation/generation, image detection, and image recording. Image FormationLGeneration The SEM utilizes an electron beam which is produced by an electron gun in an evacuated chamber. The beam is directed, condensed and focused respectively by the anode, condenser lens, and the objective lens. The lenses in the electron microscope are circular electromagnets which act to control the beam of electrons striking the sample. Deflection coils physically set within the objective lens act to deflect the beam of electrons in a raster pattern through the use of a varying applied voltage. The electron beam then strikes the sample with resulting production of secondary electrons, backscattered electrons, etc. Image Detection Following generation, these electrons are detected by a detector, converted to a voltage and amplified. This amplified voltage is applied to the grid of a Cathode Ray Tube (CRT) where it modulates or changes the intensity of the spot of light on the surface» .Differences in sample topography are represented by variable levels of electrons generated which result in variable levels of voltage developed. Projections on sample surfaces typically generate large quantities of 27 secondary and backscattered electrons which translate into the generation of a correspondingly large voltage. Finally, a bright spot is generated on the CRT screen. Depressions on the sample surface result in the generation of a small quantity of electrons and voltage. This ultimately results in the generation of a darker spot on the CRT screen. In summary, Flegler et al. (1990) state that the SEM image is thus thousands of spots of varying intensity on the CRT screen, which corresponds to the topography of the sample. Image Recordipg A typical SEM system will employ two types of CRTs, one for image viewing, and one for image photography. The distinguishing factor between the two CRT types is the nature of the phosphor composition. The viewing CRT contains a long persistence phosphor which facilitates a delayed image suitable for viewing. The long persistence phosphor system is characterized by the poor resolution that it exhibits due to the coarse grain nature of its makeup. The second CRT, often referred to as the record CRT, features a short persistence phosphor, with very small grain size. It is impossible to view the record CRT as only scanning lines are present. The very short. persistence jphosphor’ is characterized by 'the absence of a persisting image. However, excellent resolution is obtained for photography and the image is recorded on short persistence film. A camera system which typically uses instant film is mounted in front of the record CRT to 28 photographically record the image. Appligations Scanning Electron Microscopy has been used successfully by materials scientists to investigate the topographical and morphological composition of various polymer and composite structures. Several authors refer to the use of SEM to determine the locus of failure within adhesive joints (Baun, 1978; Packham, 1981; Adamson, 1982; Masuoka, 1982; Brewis, 1985; and Schroeder, 1989). Packham ( 1981), Masuoka (1982), and Schroeder (1989) have used SEM to investigate the mechanism.of interfacial bond failure in multilayer laminates. In all three studies, SEM analysis was performed to determine/confirm the presence of designated residual material on main structures after peeling the laminate system apart. Electron Spectroscopy for Chemical Analysis (ESCA) x-ray Photoeleczion Spectroscopy The views presented here are in large part those of Christie (1989), who explains ESCA from the standpoint of system background, development and theory. Early Development ESCA or XPS has its origins in 1887 with the first observation of the photoelectric effect by Hertz. From 1887 to 1967, 29 Einstein (1905), Siegbahn (1967, 1985) and others have contributed to the development of ESCA. Siegbahn (1967) is noted for his major contributions to what we know as the modern ESCA or XPS system. Christie explains the basic principles and theory most succinctly: "ESCA is one of a large number of instrumental in-situ surface analytical techniques that have been developed over the past.20 years. In each of these techniques, the specimen of interest is excited by some form of controllable: energy’ (the. excitation. source) and its subsequent response to that excitation, in the form of an emission of some species, is observed by some type of spectroscopy or microscopy. In ESCA, the primary excitation is accomplished by irradiating the specimen by a source of (more or less) monochromatic X-rays. The X- rays cause photoionization of atoms in the specimen and the response of the specimen (photoemission) is observed by measuring the energy spectrum of the emitted photoelectrons." Commercial Development The 1970’s 'brought. the improvements and :refinements necessary for the successful introduction of a commercially available ESCA instrument. The development of two final components, an ultra high vacuum system and a spherical sector electrostatic electron energy analyzer, were responsible for this breakthrough. Basic Regpirements Christie relates the basic requirements which all ESCA spectrometers must incorporate: (l) a Ultra High Vacuum (UHV) environment; (2) a controlled source of X-rays; (3) a specimen manipulation system; 30 (4) an electron energy analyzer and detection system; (5) a data recording, processing and output system. A brief discussion of each follows. The Ultra High Vacuum (UHV) environment An UHV environment is essential for surface analysis techniques utilizing X-rays, electron and ion sources, electron analyzers and detectors. For the sensitive surface analysis obtained with ESCA, strict conditions such as these are required in order to minimize any potential specimen and spectrometer interaction. Typical systems employ stainless steel and glass components coupled with a suitable:UHV'pumping capability. x-ray Source Due to the highly technical nature of the information in this section, Christie is quoted directly: "The simplist type of X-ray source for XPS is one which utilises characteristic emission lines from an anode bombarded by high-energy (15 keV) electrons. The necessary characteristics of an X-ray source suitable for XPS are that the X—ray energy should be sufficiently high to excite core-level elements (i.e. 1 keV or more), the X-ray spectrum should be relatively ’clean', with very few satellites or other peaks (i.e. low atomic number anode material), the characteristic X-ray linewidth should be narrow in comparison to the intrinsic core-level linewidths and chemical shifts we wish to study (i.e. <1 keV), and the material must be suitable (conductivity, melting point) for the construction of an anode. Only two elements possess all of the above properties - magnesium and aluminum. IMost commercial XPS instruments are therefore fitted with twin X-ray sources incorporating both aluminum and magnesium anodes. The advantage of having both AlK and MgK radiation available lies in the fact that photoionisation produces, not only photoelectrons, 31 but also, via Auger decay of the electron hole formed, Auger electrons. Obviously, the Auger electron energy is independent of the X-ray energy used to create the hole, whereas the photoelectron energy is related to the X-ray energy via the Einstein relation. Hence, the apparent binding energy of Auger peaks appears to change (by 233 eV) on going from AlK to MgK radiation (or vice versa), whereas photoelectron peaks do not shift in binding energy. This feature of XPS may be used, not only to differentiate between photoelectron and Auger peaks in the spectrum, but also to resolve photoelectron and Auger peaks which may otherwise interfere with each other". The sample Considerable flexibility exists with regard to sample investigation with a modern ESCA system. Many sample types may be studied by ESCA, however all samples are restricted to having a low vapor pressure. ESCA systems commonly employ a circular, carousel-type sample ‘tray' which is capable. of manipulating up to ten different samples by performing storing, rotating and presentation functions. Actual sample size can range up to 1.5 cm2 to 2 cm2. In addition, sample mounting techniques such as clipping, clamping and double sided taping exist to facilitate powder, fibre and bulk sample analysis. The electron energy analyzer The ESCA spectrometer’s most important component is the electron energy analyzer which functions to disperse the photoelectrons emitted from the sample, according to their energies, across a detector array. As mentioned earlier, the electron energy analyzer's configuration consists of a complex 32 spherical sector (actually a concentric hemisphere). This curious looking apparatus has been given the acronym SSA, for spherical sector analyzer. Data recording, processing and output The information in this section is taken directly from Christie as its complete detail is essential for a full understanding of Data recording, processing and output. "Photoelectrons enter an electron detector array following dispersion in the SSA. Incident photoelectrons cause a secondary electron cascade which results in an output pulse of up to 10 electrons with less than a 0.1 microsecond duration. Using conventional current amplification and pulse counting techniques it is possible to operate under count rate conditions of 10 8 per detector. A basic spectrometer with single detector may then be operated in analogue mode by recording the ratemeter output (count rate) versus electron energy. Data processing capability is considerable enhanced, however, by recording the energy spectrum on a dedicated microcomputer. Since the detector output is in the form of discrete pulses, ESCA is ideally suited to conventional digital spectrum.acquisition.and.processing techniques." AW ESCA analyses has been used with success to investigate the elemental surface composition of materials in related research areas. As referenced by Schroeder (1989); Clark et. al. (1975), and Masuoka (1982) have used ESCA to study bond failure. Masuoka (1982) used ESCA to investigate the mechanism of interfacial bond failure in an aluminum/nylon- melt/aluminum joint. Interfaced with ATR Spectroscopy, ESCA analysis was used in an attempt to detect residual nylon on 33 the aluminum surface. When nylon was not detected, adhesive bond failure was determined to be the mode of failure between the aluminum/nylon interface. Clark et.al. (1975) utilized ESCA to detect residual high density polyethylene on aluminum foil following high temperature pressing and peeling. Markgraf and Edwards (1990) utilized ESCA to perform a chemical diagnostic evaluation of flame treated milk carton sealing quality. ESCA was used to investigate material surfaces for the presence of elusive hydrocarbon constituents thought to be responsible for poor sealing quality. Markgraf et. al. (1990) used Angle Dependant ESCA techniques to quantitatively evaluate the effectiveness of several corona treatment processes on printability. The surface elemental composition of carbon and oxygen following several treatment processes was investigated. The following conclusions regarding method effectiveness are quoted directly from Markgraf (1990): "Most Effective - A moderate treatment by the supplier at the time of manufacture followed by moderate treatment on the press just before ink laydown. The final treatment can be varied to optimize ink adhesion with heat sealing characteristics. Second Most Effective - Treatment on the press only. This can lead to difficulties if the polyethylene contains slip additives. In any case, it is much more effective than treating at the time of manufacture as the treated surface will not contact the other side before the printing station thus the full results of the treatment still exists at the point of ink laydown. 34 Least effective - Treatment by supplier only. The problems and difficulties encountered have been covered but a quick summary may be in order: (i) Loss of a substantial portion of the treatment. (ii) Production of pinholes, blocking, scratches, etc., caused by using extremely high power levels at the treater". Although the preceding section describing various corona treatment processes and their relative effectiveness pertains to printability and ink adhesion, the theory regarding over treatment damage may logically be extrapolated to include treatment for bonding adhesives. Schroeder (1989) utilized ESCA to investigate the effects of sorption of volatiles on the adhesive and cohesive bond strength of multilayer laminations. Both adhesive and cohesive bond failure have been identified using ESCA in conjunction with SEM. MATERIALS AND METHODS MATERIALS LAMINATE MATERIALS The film structures utilized in this research were: Polyethylene Terephthalate - PET and Low Density Polyethylene - LDPE. These commercially available films were provided by E.I. DuPont de Nemours & Co., Inc. (Wilmington, Tredegar Films Inc., (Richmond, VA), respectively. Films A. Barrier Layer: * PET (0.48 mil: 0.00048") * One side Corona Treated 44-46 dynes * LBT-Mylar B. Food Contact Layer: * LDPE (1.5 mil: 0.0015") * One side Corona Treated at three Levels (i). 35-36 dynes (Low treatment) (ii). 40 dynes (Intermediate) (iii). 44-46 dynes (High) 35 36 A HESIVES Control Adhesiye System This adhesive is a two component adhesive designed for laminating high slip films. It was specifically chosen for this research due to the converters prior experience, which had shown this adhesive's susceptability to bond failure when exposed to the food products and food simulants selected in this research. The CONTROL Adhesive System was specially formulated by the film converting facility (Cello-Foil Products, Inc.; Battle Creek, MI.) which. prepared the respective laminate structures for study. Based on the converters previous research and expertise an adhesive system was formulated that would be highly susceptable to bond failure when exposed to the food products and simulants used in this research. The purpose of the CONTROL in the research design was to facilitate: (1) likely bond failure] delamination and (2) the development of an analytical methodology to investigate such a phenomonea. The CONTROL adhesive system consists of an an isocyanate terminated polyester urethane adhesive. The co-reactant is a specially formulated polyester resin. The two components are mixed together and diluted with Ethyl Acetate to a 30% solids application ready mixture. 37 Commercial - Adcote 333E Adhesiye Svstem The Morton Adcote 333E System is a utility grade adhesive system commonly used by converters to fabricate multilayer laminate structures. Adcote 333E is a single component, 75% solids, isocyanate terminated, polyurethane adhesive which requires moisture to cure. INA C NST U TI N Commercial - Adcote 333E Adhesive System A. Laminate 1 PET /LDPE (Low Corona Treatment 35-36 dynes) B. Laminate 2 PET [LDPE (Intermediate Corona Tmt. 40 dynes) C. Laminate 3 PET /LDPE (High Corona Treatment 44-46 dynes) Laminates 1-3: 0.48 mil polyethylene terephthalate/ one component. polyurethane adhesive system/ 1.5 mil polyethylene (LDPE) Control Adhesive System D. Laminate 4 PET [LDPE (Low Corona Treatment 35-36 dynes) E. Laminate 5 PET [LDPE (Intermediate Corona Tmt. 40 dynes) F. Laminate 6 PET [LDPE (High Corona Treatment 44-46 dynes) Laminates 4-6: 0.48 mil polyethylene terephthalate/ two component. polyurethane adhesive system] 1.5 mil polyethylene (LDPE) A schematic diagram of the laminate structures is shown in Figure 4. 38 Adhesive Contact Layer Polyester Figure 4: Cross-section and composition of laminate structure. 39 FOOD PRODUCTS and CONTROLS Food Products Lemon Juice Realemon brand natural strength lemon juice from concentrate (Borden Inc., Columbus, OH), was purchased in 32 fluid ounce: glass bottles through ‘University Stores. Immediately prior to filling the cells, a uniform batch of lemon juice was made up by combining product from the individual bottles. The pH of the lemon juice after mixing was 2.65 at 23%:. 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 Louisiana.brand.hot.sauce (Bruce Foods Corp., New Iberia, LA), was purchased in six fluid ounce glass bottles through a local retailer. Immediately prior to filling the cells, a uniform batch of hot sauce was made up by combining product from the individual bottles. The pH of the hot sauce after mixing was 3.1 at 23%:. Product ingredients were listed as: \ Peppers, vinegar, salt. Controls A 3% (w/v) aqueous citric acid solution was used to simulate the citric acid fraction of the lemon juice, minus the low molecular weight, volatile, organic components, such 40 as d-limonene. ,The solution was prepared by mixing 60.0 g of citric acid (assay 99.9%) (Mallinkrodt, Inc., Paris, KY) with 2000.0 mL.of distilled.wateru The mixture (pH 4.0) was placed in a 4.0 liter brown glass bottle and stored at 23°C (1 2°) . A 3% (v/v) aqueous acetic acid solution was used as the hot sauce control, to simulate 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 final volume of 2000.0 ml with.distilled.water, in a 4.0 L‘brown.glass bottle. The pH of the solution was 2.5 when stored at 23°C (i 2°) . Antioxidants The antioxidants, Sustane 20A and Sustane W (UOP Process Division, McCook, IL), and an antimicrobial agent, Sodium Azide (Aldrich Chemical Co., Inc., Milwaukee, WI), were added to both the lemon juice and hot sauce. These additives 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. The food products containing the antioxidant and antimicrobial agents were prepared, prior to laminate exposure. A uniform batch of lemon juice was produced by mixing 3724.0 ml of lemon juice from individual glass bottles in a 4.0 liter brown glass bottle. A Mettler AE 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 41 of the stock lemon juice. The mixture was transferred to the glass bottle, to give.a 0.02% solution (w/v) of each additive. After thorough.mixing, the stock solution was refrigerated at 4°C (1 2°) . A stock solution of 1947.0 mL of hot sauce was mixed with 0.3894 grams (9) of the antimicrobial and antioxidant agents to yieLd a 0.02% solution (w/v) of each additive. The hot sauce was placed in 400.0 mL glass jars with threaded screw caps, and refrigerated at 4°C (1 2°) . EXPERIMENTAL METHODS 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. Eighteen (18) test cells, (Appendix B) were used. A schematic diagram of the test cell is shown in Figure 5. The test cells were:designed such.that.a laminate sample, 15.2 cm by 10.1 cm, could be placed between the gasket and each aluminum plate. Two (2) samples were exposed simultaneously per cell. The material surface area to volume ratio was 3.9 cm /ml, for each exposure. The food contact layer of each laminate was oriented such that it was in contact with the gasket. The food products and controls were placed into the cells with a glass pipette. 'This resulted in 42 SIDE VIE“ Laminates Aluminum —*—" Plate -- - - -- - L O O FRONT VIEW (with plate removed) Plate [ f L Laminate O O a? / \\\\\\\‘Teflon Gasket"”’///”/r Aluminum Plate Source: Schroeder, 1989 Figure 5: Schematic of modified ASTM test cell used for laminate exposure. 43 only surface exposure of laminates to the contact medium, as the gaskets eliminated edge exposure. The cells were held at 45°C (1; 4°) for a period of thirty (30) days. Temperature and cell volume were monitored and maintained throughout the exposure period. Six (6) treatment groups, (two adhesive systems, each at three levels of corona treatment) were exposed to the four (4) media (lemon juice, citric acid, hot sauce, and acetic acid) in triplicate, with each cell containing two (2) laminate samples. Ultimately, (6) total observations per treatment group were made. The exposure regime is diagrammed schematically in Figure 6. Treatment group samples were taken from several layers below the surface of the respective film rolls, which were kept in a controlled environment at 23°C (1 3°) and 50% RH. Upon completion of exposure, each cell was disassembled, and the laminate samples rinsed.with distilled water and.dried with absorbent paper toweling. Following disassembly of the test cells the samples were visually inspected to determine if delamination had occurred. Delamination was characterized by a separation of the laminates which appeared as bubbling, tunneling or channeling. Figures 7 and 8 (Results and Discussion. Section) provide ‘visual. representation. of “the delamination phenomonea. .b b 0 'JJ A: J Hill 3 :5 O 13 0 Jo Adeote 333E Adhesive Syspem Low Corona Treatment Cells: Intermediate Corona Cells: High Corona Treatment Cells: Agcote 519 Adheeive System Low Corona Treatment Cells: J,K,L. 6 films /Contact Media Intermediate Corona Cells: M,N,O. " n High Corona Treatment Cells: P,Q,R " n 6 films /Contact Media I! 0011’ 3:111!!! H'IJO Figure 6: Schematic of cell designation exposure regime. 45 ANALYTICAL Determination of Loops of Eeilure 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), (Christie 1989), to determine the locus of failure. Delaminated samples, measuring approximately 0.3 cm by 0.75 cm, were cut from the exposed laminates using medical tweezers and scissors, and mounted directly to aluminum sample stubs using double sided tape. Samples were then analyzed by the ESCA (Christie 1989) technique to determine the atomic concentration of the elemental composition of sample surfaces. A Perkin-Elmer PHI-54 00 X-ray Photoelectron Spectrometer; ESCA instrument (Perkin-Elmer Corp., Norwalk, CT) was used to examine the opposing, delaminated plies for the presence of nitrogen. The adhesive is the only component within the laminates containing of nitrogen. An elemental analysis of the delaminated surfaces to determine if nitrogen was present was then performed to indicate 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 results 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 46 (Swingle and Riggs, 1975). This plot was then used to determine the elemental composition of the sample surface. Locus of failure was determined by examination of the delaminated surfaces for the presence of nitrogen which is indicative of the adhesive used in this investigation. Examipapiep ef Locus of Faiiure Scanning Electron Microscopy (SEM) Following exposure, and ESCA analysis, selected samples of the lemon juice, citric acid, hot sauce and acetic acid- exposed laminates were examined by scanning electron microscopy (SEM) to confirm the mode of bond failure. Non-exposed samples were also examined, to establish baseline surface characteristics. A JEOL JSM-35C Scanning Electron Microscope (JEOL Analytical Instruments, Inc., Cranford, NJ) was used for surface microscopic analysis of the laminate materials. Lemon juice, citric acid, hot sauce and acetic acid exposed samples measuring 0.3 cm by 0.75 cm, were gold plated to improve resolution. The same sample materials which were analyzed using ESCA were also examined by SEM, and observations were made of the interfacial regions to determine if delamination had occurred. Magnifications ranged from 1000X to 5000X, and areas of interest were photographed. RESULTS AND DISCUSSION The analytical results presented here should in no way be interpreted as standard for LDPE, PET or any other type of laminate structure. DEVELOPMENT OF ANALYTICAL METHODOLOGY Contgel Adhesive Syepem A major objective of this study was to develop analytical methodology to study selected variables which affect bond strength in multilayer laminate structures. As in the development of any scientific methodology, control over key variables is essential. In order to accomplish the objectives set forth in this study, it was necessary to fabricate a laminate structure that would experience bond failure ' under the experimental conditions evaluated. Therefore, a low bond strength adhesive system, known by the converters to be susceptible to bond failure, was formulated and utilized in the fabrication of a laminate structure which served as a control in this study. The control structure was designed to facilitate delamination. and. provide samples for ‘the development. of analytical procedures to quantify the mode of adhesive bond failure. As cited in the literature review, Electron Spectroscopy for Chemical Analysis (ESCA) and Scanning Electron Microscopy (SEM) are valuable analytical techniques 47 48 which can be used for detailed surface analysis of a variety of materials. Based on this knowledge, these techniques were employed in this study as a potential method to monitor and determine the mechanism of bond failure in multilayer laminate structures. Cont t uc e e a 'na '0 Delamination was observed in structures containing the control (low bond strength) adhesive system upon exposure to both food systems (lemon juice; hot sauce), as well as to their respective simulant systems (citric acid; acetic acid). Furthermore, the delamination occurred in all three of the laminate structures (high, intermediate and low corona treatment levels), utilizing this adhesive system. The observed delamination appeared as blisters and elongated channels or tunnels throughout the laminate matrix, with a significantly high proportion of the separation occurring parallel to the machine direction of the film. Figures 7-8 are typical of the differences in relative ply delamination observed between the food and food simulant systems. Clearly, the delamination resulting from exposure to the food systems was more extensive than the delamination resulting from exposure to the food simulant systems. The extent of delamination in samples exposed to the food contact systems (lemon juice and hot sauce) was essentially equivalent. Similarly, the extent of delamination in samples exposed to the food simulant contact systems (citric acid and 49 FIGURE 7: Photograph of relative delamination for acetic acid exposed laminate structure. Figure 8: Photograph of relative delamination for hot sauce exposed laminate structure. 50 acetic acid) was also equivalent. The delamination observed in the fecd product exposed samples is attributed to the presence of low molecular weight, volatile, organic flavor compounds. The delamination observed in the food simulant exposed samples cannot be attributed to these same components. An alternative bond failure mechanism, involving sorption of the aqueous acid solutions themselves by the food contact layer (LDPE) is proposed. Under the accelerated conditions, 45° C for 30 days, diffusion of the aqueous acidic solutions through the food contact layer and into the bulk of the adhesive layer may occur; Sorption.of acidic solutions by the adhesive layer can result in swelling of the adhesive layer in the regions of pre-existing localized stress which commonly occur during the adhesive solidification process. Internal stress and stress concentrations develop within the adhesive, the most common of which is a difference between the thermal expansion coefficients of the adhesive and adherend (Schroeder, 1989). Schneberger (1979) reports that another important phenomena occurs during final drying, where the adhesive itself may appreciably shrink (resulting in a loss of mass) upon release of the solvent phase. The combination of internal stress 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 (Matsuoka, 1982). 51 cc 3 o 'l e c e sc 0 e 'c n 's ESCA The ESCA analyses were conducted at the Composite Materials Research Center at Michigan State University. This technique ‘was used to examine the opposing surfaces of delaminated samples for the presence of nitrogen containing adhesive. Surface analysis of the delamination zones yielded the atomic concentration (on a relative percent basis) of the elements: carbon, hydrogen and nitrogen. In Figure 9 is presented a spectrogram and tabulated atomic concentration levels, (C, H, N), typical of the opposing delaminated surfaces following exposure to both the food and food simulants. The overall results of the ESCA analysis for the Control laminates are summarized in Table 2. The results in Table 2 show that delamination occurred in all samples utilizing the low bond strength control adhesive system. Furthermore, quantification of the surface elemental composition using ESCA indicates that a substantial level of nitrogen from the urethane adhesive system was present on both opposing surfaces for each sample treatment. The results of the Locus of Failure ESCA Analyses indicated that the mechanism of interfacial bond failure was cohesive in nature and this technique provided a means of establishing the mode of bond failure for a urethane based adhesive system. .maeuunuo acacccn auscue ea common use .ma ceoouuwc .en ceouxo ocuucoeeudeu exeem mud: .eosem ea: c» cenomxe cameos enemaaea menu uceSueouu ecouoo eueacoeueuca no >Hm veuecdeeaec ecu eae>aec£ anode-EU ecu anon-0.30090. couuoeau Souu coma—.5 .m ensur— 52 _ ms.m~ _ ss~.s _ mean _ mes _ _ em.~ _ -v.s _ mam _ we: _ _ mv.vp _ mam s . ops? . was _ _ AN. _ eczema _ im\>¢-mzu. _ _ _ co_zmescmocou _ ms_>_zemcmm _ meme _ z=m¢a_m _ 20.28ma.9=a:u o a: an an RV am am a n . + r + L- » -:11- - 4: n . c + a it N .r mH :mwomufic if M r v .r : m r a : t P a UH m“ cmwzxo mH conumo n .: - l I n n I :. a -:t:-::- x v . 3 60'5'3/(3)I 53 Table 2. Mode of Failure determined with ESCA - Control Adhesive System. ‘Contact gMedium . Acetic Acid Polymer Material Corona Corona . Acetic Acid , Hot Sauce 2.10 . Hot Sauce 1.66 i Citric Acid E Citric Acid : Lemon Juice LLemon Juice 54 Scanning Electron Microscopy (SEM) In conjunction with Electron Spectroscopy for Chemical Analysis, Scanning Electron Microscopy analyses (SEM) were also performed, to confirm the mechanism of failure within the laminate structures. The samples used in the ESCA analyses were gold coated and examined by SEM. Representative photomicrographs were obtained for each sample. Additional gold coating was required for analysis of the exposed samples since the initial beam intensity "burned-out" rectangular zones during observation and photography. The micrographs in Figures 10 and 11 represent the "baseline" surface topography of the non-exposed (virgin ) material samples. Delaminated regions of opposing laminate plies (LDPE) and (PET) from Lemon Juice and Hot Sauce exposure as well as from exposure to food simulants were examined using SEM. Representative micrographs taken of the Hot Sauce and Acetic Acid exposed samples, showing delamination (Intermediate Corona Treatment), are presented in Figures 12-15. Table 3 contains the SEM Micrograph Figure legends for the representative micrographs. Micrograph Interpretetien -E as Vir ' ate 'a Sa les The non-exposed material micrographs for LDPE and PET differ markedly in their respective surface topography. As shown in Figure 10, LDPE samples have a slightly wave-like surface. Figure 11 shows the glass-smooth surface topography 55 Table 3: SEM Micrograph Figure Legend 1 ur Film Food Syerem Magpification 10 pg *********** sooox 11 pET *********** 5000X 12 PE Hot Sauce 5000X 13 PET Hot Sauce 5000X 14 PE Acetic Acid 5000X 15 PET Acetic Acid 5000X 56 FIGURE 10: Virgin LDPE surface at 5000X. FIGURE 11: Virgin PET surface at 5000X. St-lm 233333 I 13KU X5:333 57 FIGURE 12: Hot sauce exposed PE laminate at sooox. FIGURE 13: Hot sauce exposed PET laminate at sooox. 58 FIGURE 14: Acetic acid exposed LDPE laminate at 5000X. FIGURE 15: Acetic acid exposed PET laminate at 5000X. 13KU X5,333 59 characteristic of virgin PET samples. In addition, the PET samples appear to have very small particles scattered across the surface. The notably distinct surface characteristics of the non-exposed (virgin) materials provides a useful baseline from which to interpret exposed sample surface topography. Food and Food simulant Exposeg Materiei Semples All of the Food and Food simulant exposed laminate structures displayed surface topography consistent with the results of ESCA analyses. Representative Food System (Hot Sauce) exposed samples are presented in Figures 12-13, and simulant (Acetic .Acid) exposed samples are presented in Figures 14-15, as noted in Table 3. As shown the micrographs of the exposed samples exhibit a distinct difference in surface topography in comparison to the respective non-exposed samples. These differences are manifested in a similar "roughness" occurring on both the LDPE and PET surfaces, at high magnification. This "roughness" is thought to be a result of the presence of adhesive on the opposing laminate surfaces. SEM observation of the Lemon Juice and Citric Acid exposed samples produced similar findings. These confirm the ESCA results which detected nitrogen from the adhesive on the opposing laminate surfaces. The bond failure mechanism is thought to be cohesive in nature. Cohesive bond failure is characterized by failure within the bulk of the adhesive phase. The presence of 6O nitrogen (ESCA results) on both.opposing laminate plies of the exposed samples and the surface topography (SEM results) strongly supports cohesive bond failure as the mechanism responsible for the delamination of the Control laminate structure. The results obtained with the Control lamination also show'the.general applicability of surface analysis by the ESCA and SEM procedures in determining the mode of adhesive bond failure for urethane based adhesive systems, which are commonly employed in the fabrication of multilayer laminate structures used in the food packaging. CIAL - ADCOTE 3 ESIV EM Delamination did not occur upon exposure to either food system (lemon juice; hot sauce) or their respective simulant systems (citric acid; acetic acid), for any of the laminate structures (high, intermediate and low corona treatment levels) utilizing this adhesive system. The results are summarized in Table 4. The absence of cohesive or adhesive bond failure, at all three levels.of corona.discharge treatment (44, 46, 48 dynes), for the laminates containing this adhesive system, indicates that strong bond formation had been established during the converting and curing processes. The ESCA and SEM techniques developed to determine locus of failure could not be employed to analyze bond failure in the laminate structure containing the Adcote 333 adhesive system because delamination did not occur. 61 Table 4. Mode of Failure determined with ESCA - Adcote 333E Adhesive System. Contact Polymer Corona Corona Corona Low Medium Material Acetic Acid Acetic Acid Hot Sauce Hot Sauce Citric Acid Citric Acid ‘Lemon Juice l . (Lemon Juice SUMMARY AND CONCLUSIONS These studies were designed to investigate the effect of corona treatment levels on laminate integrity following surface exposure to selected food and food simulant systems. Low Density Polyethylene (LDPE)/ Polyethylene Terephthalate (PET) adhesive laminates were fabricated using a two component urethane based (CONTROL) adhesive system, and a single component (commercially available - Adcote 333) urethane based adhesive system, each at three levels of polyethylene corona treatment. The Control Structure was designed to facilitate delamination. and jprovide samples for 1the :development of analytical procedures to quantify the mode of bond failure. In this study, Electron Spectroscopy for Chemical Analysis(ESCA) was the primary analytical technique used to determine the locus of bond failure. Scanning Electron Microscopy (SEM) was also used to confirm the results of the ESCA analyses. The results obtained with the Control laminate established the general applicability of surface analysis by the ESCA and SEM procedures in determining the mode of bond failure for urethane based adhesive systems. Following exposure to the food products; lemon juice and hot sauce, and the respective food simulants; 3% (w/v) citric acid solution and 3% (v/v) acetic acid solution, delamination did not occur in the laminate structures using the single 62 63 component - Adcote 333 adhesive system, at any of the (three) corona treatment levels. By comparison, delamination occurred in all Control laminate structures, although. there ‘were significant differences in the extent of delamination observed between the respective food and food simulant systems. Under the conditions of this investigation, corona discharge treatment levels had no significant influence on interlayer adhesion in the Control laminate structures. As shown with the Control structures volatile, low molecular weight/ organic flavor compounds, sorbed by the food contact laminate ply, are capable of effecting the integrity of interlayer adhesion in multilayer laminate structures. Package development scientists must therefore be cognizant of potentially adverse product/package interactions such as those investigated in the present study, and their influence on product quality. 6 4 FUTURE WORK Future work in the area of sorption-induced delamination should also be conducted with laminate materials produced under closely controlled conditions. This step is essential for the elimination of unknown variables such as, laminating conditions, corona discharge treatment conditions, and especially variations in adhesive formulation. In addition, laminate components should. be :Characterized in ‘terms of critical surface tensions, wettability and surface topography - factors capable of affecting adhesive bond strengths. In addition, finished laminate structures should be analyzed (spectrophotometry and chemically), to provide irrefutable confirmation of the identity of the adhesive system employed for conversion purposes. The results obtained in the current study, with the Control lamination, show the general applicability of surface analysis by the ESCA and SEM procedures in determining the mode of bond failure for urethane based adhesive systems. Using this analytical method, the effect of selected other: contact layer compositions, adhesive formulations, and food and food simulant contact phases should be further investigated. Furthermore, the sorptive and diffusive behavior of the adhesive system(s) should.also be examined, in that movement and interaction of sorbed volatiles through the adhesive layer should also play a role in delamination. APPENDICES APPENDIX A 65 APPENDIX A Composition of Product Additives ANTIOXIDANTS (A) Sustane w (UOP, INC.) Ingredients (weight percent) (1) (2) (3) (4) (5) (5) (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 hydroquinone (TBHQ) (20%) citric acid (3%) propylene glycol (15%) edible oil (30%) mono and diglycerides of fatty acids (20%) ANTIMICROBIAL AGENT (C) Sodium Azide (Aldrich Chemical, Inc.) FORMULATION 0.02 percent (w/v) (A) + (B) + (C) was added to the lemon juice and hot sauce products prior to laminate exposure. APPENDIX B 66 APPENDIX B 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. Eighteen test cells were fabricated. The following materials were used in the construction of each cell, as depicted in Figure 4: 2 aluminum plates, 15.2 cm X 10.2 cm X 0.6 cm. 1 U-shaped Teflon gasket, outer dimensions: 15.2 cm X 10.2 cm X 0.6 cm., Cavity dimensions: 13.3 cm X 6.4 cm., centered at the top of the gasket. 9 stainless steel bolts, 3.2 cm X 0.6 cm. 18 steel washers, 1.9 cm. in diameter. 9 steel nuts, 0.6 cm. 1 Teflon gasket plug, 6.4 cm X 2.5 cm 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 Figure . 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. BIBLIOGRAPHY 67 BIBLIOGRAPHY ASTM Designation D 907-70. 1971. 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