‘IIHNIHHIHHWIWWWII“(lWIWWIHHlHHl 133 174 This is to certify that the thesis entitled Evaluation of the Effect of Relative Humidity on the Per- meation of Toiuene Vapor throgh B a r r 1 e rprr-eseiiwefii by Kenny Jae-Kae Liu has been accepted towards fulfillment of the requirements for Master degree in Packaging 0 We, mum Date 8115/ '86 0-7639 MS U is an Affirmative Action/Equal Opportunity Institution mini 3112: "iii Will L 3 44 Ni It, lViESI.) RETURNING MATERIALS: Place in book drop to LIBRARIES remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. ' ti Tee :1. FEB . 51:49? -. 0 EVALUATION OF THE EFFECT OF RELATIVE HUMIDITY ON THE PERMEATION OF TOLUENE VAPOR THROUGH BARRIER FILMS By Kenny Jae-Rae Liu A THESIS Submitted To MICHIGAN STATE UNIVERSITY In Partial Fulfill of the Requirements for the Degree of MASTER OF SCIENCE School of Packaging 1986 file 251‘ '1‘ To My Parents and Lovely Wife ii ABSTRACT EVALUATION OF THE EFFECT OF RELATIVE HUMIDITY ON THE PERMEATION OF TOLUENE VAPOR THROUGH BARRIER FILMS By Kenny Jae-Rae Liu The effect of relative humidity on the diffusion of toluene vapor through a multi-layer coextrusion film structure containing moisture sensitive hydrophilic barrier layers (i.e. nylon and EVAL) was evaluated. Two experimental test methods were developed. In method I, the effect of relative humidity on the diffusion of toluene vapor was evaluated, when the test film was preconditioned to a fixed water activity prior to test. In method II, the effect of water vapor as a co-permeant was evaluated. Studies carried out by method I showed the concentration dependency of the diffusion process and the importance of relative humidity on the diffusion of organic vapor through barrier structures. Water vapor was found to exhibit strong interactive effects with the moisture sensitive polymer layers of the laminate which resulted in an increase in the diffusion of toluene vapor through the barrier structure. iii When the effect of water vapor as a co-permeant was evaluated (method II), the direction of permeation(i.e. surface exposed to high relative humidity and vapor concentration) markedly influenced the resultant barrier properties. iv ACKNOWLEDGEMENTS I would like to express my appreciation to the follow- ing people for their help in this study. Dr. Jack R. Giacin, for his guidance and professional advice throughout my research. I would also like to thank members of the graduate committee, Drs. Eric Grulke and Sue Selke, for their advice and review of the manuscript. Mr. Stephen Tan, Larry Bauer and Ruben Hernandez for the sharing of knowledge and skills. I also acknowledge film samples and financial support for this study provided by Cryovac Division, W. R. Grace & Co. LIST OF CONTENTS ABSTRACT ------------------------ LIST OF TABLES ------------------------ LIST OF FIGURES ------------------------ INTRODUCTION ........................ LITERATURE REVIEW ........................ 5-1. Permeation theory ------------------------ 5-2. The Effect of Penetrant Concentration on the Permeability of Organic Vapors -------- 5-3. The Effect of Water Vapor on the Permeability of Permeant Gas and Organic Vapor through Barrier Films ---------- MATERIALS AND METHODS ------------------------ 6-1. Analytical ------------------------ A 6-2. Procedures ------------------------ 6-3. Operation ------------------------ RESULTS AND DISCUSSION ------------------------ 7-1. The Effect of Relative Humidity on the Permeation of Toluene Vapor through Cryovac Test Film FDX 1570 ................... 7-2. Equilibrium Sorption Isotherm of Cryovac Test Film FDX 1570 .................... 7-3. Moisture Content vs. Exposure Time of Preconditioning Film .................... vi 9. 10. 11. 7-4. The Permeability Constant vs. Exposure Time of Preconditioning Film ----------------- 7-5. The Effect of Water Vapor as a Co- Permeant on the Diffusion of Toluene Vapor through Cryovac Test Film FDX 1570 ------ 7-6. WVTR of Cryovac Test Film FDX 1570 ----------- 7-7. Calculated Results ------------------------ CONCLUSION ........................ RECOMMENDATIONS ....................... APPENDICES ........................ BIBLIOGRAPHY ........................ vii 49 54 58 61 65 67 69 72 LIST OF TABLES Table page 1 The Effect of Water Vapor on Gas Transmission at 30°C, (Meyer et a1., 1957) --------------------- 14 2 The Effect of Water Vapor on Gas Transmission through Nylon 6 Film at 25°C (Meyer et a1., 1957) -- 15 3 The Effect of Water Vapor on the Diffusion of Gases in Polymers (Ito, 1961) -------------------- l8 4 The Effect of RH on the Diffusion of Toluene Vapor Through Cryovac FDX 1570 Film at 81 ppm (wt/v) -------------------- 36 5 The Effect of RH on the Diffusion of Toluene Vapor Through Cryovac FDX 1570 Film at 62 ppm (wt/v) --------------------- 41 6 Equilibrium Sorption Isotherm of FDX 1570 Film -—- 45 7 Moisture Content of Cryovac FDX 1570 Film ---------- 47 8 Permeability Constant (E) vs. Exposure Time of Preconditioning ----------------------------------- 50 9 The Effect of Water Vapor as a Co-Permeant on the Diffusion of Toluene Vapor through Cryovac er 1570 Film ---------------------------------- 56 10 The WVTR of Cryovac FDX 1570 Film ----------------- 59 viii Figure 10 11 12 LIST OF FIGURES page Transmission Rate Profile ---------------------- 7 The Effect of Toluene Vapor Concentration on the log F for Polypropylene and Saran (Baner, 1985) --------------------------------- 12 The Relative Permeability Constant of Cellulose Films at 25 C to Oxygen as a Function of Relative Humidity. (Pilar, 1960) --- l6 Permeability as a Function of Relative Humidity for Polythene/Cellophane/Nitrocellulose (Notley, 1963) -------------------------------- Relative Permeability of Cellophane to Hydrogen as a Function of Relative Humidity. (Kaniya and Takahashi, 1977) ----------------------------- Diffusion Coefficient of Ethyl Alcohol in Keratin at 35°C as a Function of Volume Swelling. (Watt, 1964) --------------------- Permeation Cell System ----------------------- Schematic of Permeation Test Apparatus -------- Transmission Rate Profile of Cryovac FDX 1570 Film at 81 ppm (wt/v) Toluene Vapor Concentration (Varying %RH) ----------------- Plot of Permeability Constant of Cryovac FDX 1570 Film vs. RH% at 81 ppm ---------------- Transmission Rate Profile Of Cryovac FDX 1570 Film ---------------------------------------- The Equilibrium Sorption Isotherm of Cryovac FDX 1570 Film --------------------------------- ix 19 20 22 27 29 37 38 42 44 13 14 15 16 17 18 Plot of Moisture Content Gained by Cryovac FDX 1570 Film vs. Exposure Time for Preconditioning ---------------------------- 48 Plot of F of Cryovac FDX 1570 Film vs. Preconditioning Time ------------------------ 51 Transmission Rate Profile of Cryovac FDX 1570 Film at 97 ppm (wt/v) Toluene Vapor Concentration 57 WVTR Profiles of Cryovac FDX 1570 Film -------- 60 Plot of F of Cryovac FDX 1570 Film vs. RH% ---- 63 Plot of 5 vs. Equilibrium Moisture Content Gained by Cryovac FDX 1570 Film ---------------é 64 INTRODUCTION Due to considerations of convenience and economy, polymeric packaging materials are increasingly put to use for all kinds of food products through storage and distribution and estimates for future usage show continued growth. Knowledge of the transmission rates of permeants such as carbon dioxide, oxygen and water vapor through polymeric packaging films have been utilized in selecting packaging systems which would prevent or inhibit deterioration of food products. In contrast to the extensive studies conducted on the permeability of non-interactive gases (i.e. oxygen) and water vapor through plastic packaging films, data describing the permeability of organic vapors are limited. The shift from absolute barrier packaging systems, such as metal cans and glass bottles, to semi-permeable plastic packages has created a need to develop a better understanding of the transport properties of polymeric packaging systems to organic vapors and aroma constituents. For food products packaged in polymeric materials, the permeability of the packaging system with respect to organic vapors becomes increasingly more important, as the loss of specific p2 = 0, so that Q/t can be calculated from p2/t, obtained from a plot of the pressure increase at the low pressure side versus time. The permeability constant then is calculated as, F = Q-L/t-A-pl (6) Where Q is the quantity of gas at STP which has permeated in the time interval t during steady-state flow, A is the effective area of the film, and L is the average film thickness. Typical units for the permeability constant are cubic centimeters of gas at standard temperature and pressure passing per second through a 1 sq. m area of a film 1 mil thick, when the pressure differential across the film is 1 atm. When a penetrant diffuses through a film, there is an interval from the time the penetrant first enters the film until the steady state of flow is established. The intercept on the time axis of the extrapolated linear steady—state portion of the curve is called the lag time (9) (see Figure 1). Using appropriate solutions of the diffusion TOTAL QUANTITY PERMEATED Unsteady State . Steady State TIME Figure 1 TRANSMISSION RATE PROFILE equation, Barrer (1939) has shown that the time lag is directly related to the diffusion coefficient D. Using a pressure increase method or other appropriate techniques, data pertaining to the rate of attainment of steady-state flow can be readily obtained. The steady state measurements serve to determine the permeability constant F, and the corresponding time lag from the same experiment serves as a means for calculation of D. Since F is defined as the product of D x S, the solubility coefficient S can also be calculated as the quotient, F/D. This general method for the estimation of D and S from permeation time lag data is very convenient and is capable of giving accurate estimates of D and S for non-interactive penetrants (Crank 1975). In a typical experimental case (Rogers et al. 1956b), the film is initially free of penetrant, and diffusion flux occurs through the film into a reservoir of essentially zero concentration of penetrant, so that, e=L/ 6-D (7) 5-2 The Effect of Penetrant Concentration on the Permeability of Organic Vapors The diffusion of organic vapors in polymers differs from that of simple gases (i.e. helium, hydrogen, oxygen). For organic vapors, the diffusion coefficient (D) at a given temperature is not constant as in simple gases but is an increasing function of penetrant concentration (Pace et a1., 1979). Several investigations have shown that the diffusion of simple gases (i.e. helium, argon, oxygen and hydrogen) in rubber (Barrer, 1939) and plastics (Reitinger, 1944) obeys Henry's law, with a diffusion coefficient independent of concentration. The ideality of the diffusion process with simple gases is due to their relative insolubility, so that at ordinarily accessible concentrations the quantity of gas dissolved in the polymer is insufficient to alter its properties to any observable extent and encounters between gas molecules within the polymer are rare (Meares, 1954). This contrasts with the behavior of organic vapors which have strongly concentration-dependent diffusion coeffi- cients. Several investigations of the diffusion of organic vapors in polymers have established that in most systems the diffusion process is strongly dependent on the concentration of the penetrants in the polymer (Barrer, 1957; Prager et a1, 1951; Meares, 1958). Meares (1965) indicated that organic vapors are usually freely absorbed by polymers, and the absorbed molecules diffuse by a random exchange of vapor causing the polymer to swell and to change the configuration of the polymer molecules. These configurational changes are not instantaneous : they are controlled by the retardation time of the chains. If these are long, stresses may be set up which relax slowly. Thus, the absorption of a vapor is accompanied by time-dependent processes in the polymer which are slower than the micro-Brownian motion which promotes diffusion. There are a number of literature references describing the phenomenon of the concentration dependent permeation of organic vapor through barrier films. Zobel (1982) described such finding in studies carried out on the diffusion of benzyl acetate through coextruded oriented polypropylene and Saran coated oriented polypropylene, at various penetrant concentrations. Baner (1985) in studies carried out with toluene vapor with a Saran and with a Saran coated-oriented polypropylene found similar results, with the permeability constant (F) increasing with an increase in penetrant concentration. Figure 2 shows a plot of the 10 {79 permeability constant (F) as a function of toluene vapor concentration for polypropylene, Saran, and Saran-coated polypropylene and illustrates such a concentration dependency. 11 1000.0? Log P (g- structure/sq.m-day- 100ppm) Oriented Polypropylene 100.0 I- ' Saran Coated Polypropylene 10.0'- Saran 1.0!- b d I .001 .0001 IE+LLL411LLLLI 10 30 50 70 90 110130 Toluene Concentration (ppm) (wt/v) Figure 2 THE EFFECT OF TOLUENE VAPOR CONCENTRATION ON THE Log P FOR POLYPROPYLENE, SARAN AND SARAN-COATED POLYPROPYLENE (Baner, 1985) 5-3 The Effect of Water Vapor on the Permeability of Permeant Gas and Organic Vapor through Barrier Films The presence of water vapor often accelerates the diffusion of gases and vapors in polymers with an affinity for water (Barrie, 1968). The effect of water vapor on the gas transmission for a large number of polymer-gas systems has been studied by Simril and Hershberger (1944). The results obtained with nylon, at varying relative humidities, have shown that the presence of water vapor can markedly affect the rate of transmission of various gases such as nitrogen, oxygen and carbon dioxide. The effect of relative humidity on the permeability of nitrogen, oxygen and carbon dioxide through polyethylene (PE), polyethylene terephthalate (PET) and the hydrophilic film , nylon 6 was studied by Meyer et al.(1957). These investigators found little effect of relative humidity on the gas permeability of PE and PET films(Table 1). 13 Table 1 The effect of Water Vapor on Gas Transmission at 30°C. (Meyer et a1, 1957) Film Gas RH% P (ccvmm / sq. cm-sec-cmHg) PE Nitrogen O 2.1 36 2.1 56 2.2 63 2.6 Oxygen 0 6.6 32 6.2 66 6.6 Carbon 0 26 Dioxide 35 26 61 28 -3 PET Nitrogen 0 5.9 x 10 -3 35 6.0 X 10 -3 61 6.1 X 10 The results obtained for nylon 6 with carbon dioxide as the penetrant are given in Table 2. It is interesting to note that for the hydrophilic film the transmission rate of gases increases with an increase in relative humidity. 14 Table 2 The Effect of Water Vapor on Gas Transmission Through Nylon 6 Film At 25°C ( Meyer et al. 1957 ) Film Gas %RH P(cc-mm / sq. cm-sec-cmHg) Nylon 6 Carbon 0 0.12 Dioxide 42 0.17 97 0.28 100 0.28 Pilar (1960) also found that an increase of relative humidity from 0 to 100 % RH resulted in a ZOO-fold increase in the permeation constant of cellophane to oxygen. Figure 3 showed the profound effect of relative humidity on the permeability constant of oxygen through cellophane. As shown, at a relative humidity range of 70- 80%, the permeability constant increases in an exponential manner. This corresponds to the results of Simril and Smith (1942) who showed that around this value of relative humidity, the mechanism of water sorption appears to be changing and is accompanied by extensive structural changes within the polymer. Similar studies were carried out by Kunz and Cornwell (1962) who found that an increase in the relative humidity caused an increase in permeability of oxygen through cellophane films. In general, polymers which swell greatly in water 15 16 200 160b 120- 80 I RELATIVE PERMEABILITY CONSTANT (Qr = Q/Qo) 40 I l o 26 a?) so so 160 PERCENT RELATIVE HUMIDITY Figure 3 THE RELATIVE PERMEABILITY CONSTANT or CELLULOSE FILMS AT 25°C To OXYGEN AS A FUNCTION OF RELATIVE HUMIDITY. (Pilar, 1960) Q = P'at a given relative vapor pressure Qo= P of the dry film will always show increasing diffusivity with increasing relative humidity. For nylon and polyvinyl alcohol (PVOH) this is illustrated by the results of Ito (1961), summarized in Table 3. It is apparent that when water is highly sorbed, as in nylon 6 and polyvinyl alcohol, the film is plasticized by the water, which leads to greatly increased permeability rates. Crank and Park (1968) also indicated that the addition of water vapor, like the addition of a plasticizer to a polymer, decreases the cohesive forces between the polymer chains resulting in an increase in polymer chain segmental mobility. Jabarin and Lofgren (1986) found that water absorbed by high barrier acrylonitrile-based polymers influences both mechanical and thermal properties of the polymers. They proposed that water decreases the effectiveness of the intermolecular forces in the polymer, and therefore reduces the rigidity of the three dimensional structure. The plasticizer (water) then reduces the polymer intermolecular forces by neutralizing the polymer polar groups with its own polar group or by increasing the distance between polymer molecules. This suggests that the glass transition temperature (Tg) is then greatly reduced. The reduction in Tg is directly proportional to the amount of absorbed water. It is clear that this should result in an increased rate of 17 diffusion and a lower activation energy for the diffusion process. Table 3 The Effect of Relative Humidity on the Diffusion of Gases in Polymers (Ito, 1961) _ 11 (a) Polymer Gas T (°C) %RH P x 10 Nylon 6 Carbon 30 0 1.0 Dioxide 30 44 1.7 30 95 2.9 Polyvinyl Carbon 23 0 0.01 Alcohol Dioxide 23 84 52 23 94 119 (a) P = CC°(STP)/Sq. cm -sec-cmHg Notley (1963) has also measured the effect of water vapor on the oxygen permeability of cellophane laminates. Similarly, large increases in the permeability of oxygen through the cellophane laminates were obtained when water was present (see Figure 4). Notley proposed that water vapor causes swelling of the cellophane structure and consequently increases the permeability of gases through cellophane films. Kamiya and Takahashi (1977) used a laminate composed of cellophane and a hydrophobic polymer film (i.e. polyethylene) for studying the permeability of moistened cellophane to gases. They found that the effect of water vapor on the films permeability, as shown in Figure 5, 18 100 10 OJ Log P (cc-structure/sq.cmosecocmHg) L o 25 so 73 150 L l PERCENT RELATIVE HUMIDITY Figure 4 PERMEABILITY AS A FUNCTION OF RELATIVE HUMIDITY FOR POLYTHENE/CELLOPHANE/NITRO- CELLULOSE.(Notley, 1963) 20 3.0 - 2.0 - «I In :a‘. O! o A 1.0 - O0 63 100 PERCENT RELATIVE HUMIDITY Figure 5 RELATIVE PERMEABILITY OF CELLOPHANE TO HYDROGEN AS A FUNCTION OF RELATIVE HUMIDITY.(Kamiya & Takahashi, 1977) ‘P 3 PERMEABILITY COEFFICIENT AT VARYING RH P*= PERMEABILITY COEFFICIENT AT 0% RH was very similar to the effect obtained by Pilar (1960). Also, Petrak et a1. (1980) measured oxygen permeability through a number of hydrophilic films (i.e. polyacrylic acid, polyvinyl alcohol, carboxylcellulose, etc.) at varying relative humidities. They reported a similar effect, namely a large increase in the permeability coefficient is observed because of the plasticizing effect of water on hydrophilic polymers. Long and Thompson (1953, 1955) found that organic vapor diffused more rapidly into polymers containing sorbed water vapor than into the dry polymer, provided the polymer absorbed water to an appreciable extent. They point out that the most plausible explanations for the accelerating effect of water vapor are : (a) the water vapor diffuses rapidly into the polymer and (b) water present in the polymer acts as a plasticizer and leads to a higher diffusion coefficient for the organic vapor than would be found for the dry polymer. Watt (1964) has determined the rates of diffusion of ethyl alcohol in keratin at various level of swelling with the use of water vapor as a swelling agent. He showed that swelling of the polymeric material by water vapor resulted in an increase in the rate of diffusion of the ethyl alcohol. A plot of calculated diffusion coefficients as a function of the percentage volume swelling is shown in Figure 6. 21 ~13 DIFFUSION COEFFICIENT x 10 (sq.cm/sec) 22 120- 100 I 60 40 20 l 4.— 0 S 10 15 20 PERCENTAGE VOLUME SWELLING Figure 6 DIFFUSION COEFFICIENT OF ETHYL ALCOHOL IN KERATIN AT 35°C AS A FUNCTION OF VOLUME SWELLING. (Watt, 1964) Different results were obtained by Pye et al. (1976) who found that water vapor reduced the permeability of gases through amorphous aromatic polyimides. The experimental data indicated that there was a significant reduction in the permeability of methane and hydrogen gases through wet films, relative to the dry structure. A possible explanation for the observed permeability reductions is that a small amount of water binds to active sites (Carbonyls) within the polymer bulk phase which effectively reduces the microvoid content of the films and the available diffusion paths for the nonreactive gases. Such interaction may also reduce the solubility of the penetrant at elevated relative humidity levels. It has long been known qualitatively that the presence of water vapor can have a marked accelerating effect on the rate of diffusion of organic vapors in certain polymers (Long et a1., 1953; Praeger et al., 1951). However, there is a paucity of literature data available which describes in detail the effect of water vapor upon the permeability of organic vapors through polymeric materials and elucidates the mechanism of the effect of water vapor on the barrier properties of multi-layer laminate structures containing a hydrophilic barrier layer. 23 MATERIALS AND METHODS Film Sample : Barrier film to be tested was supplied by the Cryovac Division of W. R. Grace & Company (P. O. Box 464, Duncan, SC 29334), coextrusion film sample FDX 1570 (100 gauge film, PE/nylon/EVAL/nylon/PE). The diagram of the cross section view of the coextrusion film is presented in Appendix A. Toluene : Toluene with purity greater than 99.8%, boiling point of 110-110°C, from Burdick and Jackson Laboratory Inc. was used as the permeant. Nitrogen : High purity dry nitrogen 99.98% was provided by Union Carbide Corporation, Linde Division, Daudery, Connecticut. 6-1 Analytical Analysis for penetrant concentration was based on a gas chromatographic procedure. In all cases, a standard curve of response vs. penetrant concentration was constructed from standard solutions of known concentration. The penetrant concentration was determined by reference to 24 the calibration curve. Analyses were carried out on a Hewlett Packard Model 5890 Gas Chramatography, equipped with flame ionization detection. The gas chromatographic conditions are presented below : Injection Temperature: 200°C Column Temperature: 125°C FID Temperature: 350°C Area Reject: 0 Carrier Gas Flow Rate: 30 cc/min (Nitrogen) Column: 6'x 1/8" O.D. stainless steel, packed with 5% SP2100 on 100/120 mesh Supelcoport (Supelco, Inc. Bellfonte, PA) To elute toluene at 0.56 min. 6-2 Procedure Two test methods were developed for considering the effect of water vapor on the diffusion of organic vapor through barrier films. In Method I, the effect of relative humidity on the diffusion of organic vapor through barrier membranes is considered and the film to be tested is preconditioned to the required relative humidity employing a preconditioning system prior to test. In Method II, the effect of water vapor as a co—permeant on the diffusion of organic vapor is evaluated. In both methods, analysis is based on an accumulation or quasi-isostatic procedure and utilizes gas chromatographic analysis for quantifying the amount of organic vapor that permeated through the film sample. 25 The permeability cells are of our own design and are constructed of either stainless steel or aluminum; the cells are composed of two cell chambers and a hollow center ring. A schematic diagram of the permeability cell is shown in Figure 7. The right and left cell chambers each have a volume of 50 cc, while the volume of the center cavity is approximately 50 cc. In Operation, the test films are placed in the cell so that the film effectively isolates the center cell chamber (i.e. the high concentration cell chamber) from the low concentration cell chambers. Hermetic isolation is achieved by the compression of overlapping Viton "0" rings on the film specimen. Viton is a fluorocarbon elastomer which is resistant to attack and swelling by most organic vapors. As shown in Figure 7, both cell chambers and the center ring are equipped with inlet and outlet valves and a sampling port. The films to be tested are mounted in the permeability cell and the cell assembled. A constant low partial pressure of permeant vapor, adjusted to the desired relative humidity, is then flowed continuously through the cell chamber. This allows the permeability of two film specimens to be determined concurrently under identical conditions. Figure 8 presents a schematic diagram of the permeation test apparatus designed for this study. As shown, to perform multiple runs concurrently, a series of four cells can be attached to a dispensing manifold. This allows 26 .862 2.31m 30.“. .30 use 333 O». I n. 27 :oo 59: n. 0: :00 3:30 n. 00 :00 :04 n. o.— atoa aczefium n. o, 03.3 0:302 fl> 33:33: I a cone) accosted .u >a Ii .flflnnxmx‘w .. . ./ / , @~ .: zmsmsm sumo onsamzmmm s unseen delivery of a constant concentration of permeant vapor to each cell. To deliver a constant low partial pressure of humidified permeant vapor to the permeability cells, a vapor generator system was designed. As illustrated in Figure 8, the vapor generator system consisted Of three gas washing bottles with fritted dispersion tubes, connected as shown. One gas washing bottle (Bl) contained the pure organic liquid penetrant. A second gas washing bottle (B2) was filled with a mixture of distilled water and the organic liquid penetrant. Gas washing bottle BB contained pure distilled water. Flow meters were used to provide a continuous indication that a constant rate of nitrogen flow was maintained through the respective gas washing bottles. Gas flows were regulated with NU PRO needle valves, type B- 286. When the vapor generator system is assembled as shown, this design allows for evaluation of the diffusion of organic penetrants over a broad range of organic vapor concentrations and relative humidity values. To obtain a lower than saturation concentration of penetrant vapor, the humidified organic vapor stream is mixed with another stream of pure carrier gas (nitrogen). The vapor generator system was mounted in a constant temperature water bath maintained at 1°C above ambient temperature so as to avoid condensation after the permeant vapor stream passed through the glass reservoirs. For studies carried out to evaluate the effect of 28 SCHEMATIC OF PERMEATION TEST APPARATUS FOR EVALUATING THE EFFECT OF RELATIVE HUMIDITY ON THE PERMEATION OF ORGANIC VAPOR T IIII III [Perm Precmditioned eation (2le ' Permeation CellsJ Iio II: (a) (b) Br Organic vapor bubbler N-Needle valve 82-Organic/water vapor bubbler R - Rotarneter B, -wqter bubbler Fig-Regulator Cr-Four way valve T- Nitrogen tank H - Hygrorneter 13,- Three way valve Ito-Hood W-Water bath Figure 8 29 relative humidity on the diffusion of organic vapor through a barrier film, the film samples to be tested were preconditioned to the required relative humidity employing a preconditioning system (see Figure 8). To precondition the test films, the film samples are mounted in the permeability cells and are equilibrated to the required relative humidity by continuously flowing a humidified nitrogen stream through the test cell for a period of time (i.e. approximately 1-2 weeks) considered appropriate to have equilibrated the film to the surrounding relative humidity level. The permeation cells containing the preconditioned film samples are then removed from the preconditioning system and affixed directly to the permeability test apparatus. The organic vapor stream which is flowed continuously through the high concentration cell chamber is adjusted to the same relative humidity to which the film sample was preconditioned and the rate of diffusion of the organic penetrant determined by the lag time diffusion method. In this study, the high and low concentration cell chambers and the test film are maintained at a constant relative humidity throughout the period of test. To evaluate the effect of water vapor as a co- permeant in the diffusion of organic vapor through barrier membranes (i.e. Method II), the film sample to be tested is stored over a desiccant prior to initiating the permeability 30 studies. The film sample is then mounted in the permeability cell and the cell affixed to the organic vapor permeability test apparatus. The organic vapor stream, adjusted to the desired relative humidity, is then flowed continuously through the high concentration cell chamber of the permeation cells. Again, the permeation rate through the film, under the test conditions of a fixed relative humidity and organic vapor concentration, is determined by the lag time diffusion method. In Method II, both water vapor and the organic vapor permeate simultaneously and, therefore, the effect of water vapor as a co-permeant is evaluated. As shown in Figure 8, the relative humidity of the organic vapor stream which is flowed through the high concentration cell chambers is monitored upstream from the three way valve (Tv) to the cell by using a special hygrometer sensor, designed for use in an organic vapor atmosphere (Hydrodynamic, Inc., Silver Springs, MD). Unless otherwise stated, permeability runs were carried out at 23°C. All runs were carried out in duplicate. A low, but constant, penetrant concentration (i.e. less than 100 ppm, wt/v) was employed. Studies were carried out at four relative humidity conditions, namely 0% RH, 23% RH, 50% RH, and 85% RH. Unless otherwise stated, the upper surface (outer surface) is always exposed to the high concentration of toluene vapor. 31 Two methods were employed in these studies to determine the water content of the test films, namely gravimetric and titration procedures (Pande, 1974). Good agreement was obtained between these two methods. The equilibrium sorption isotherm for the test film was determined according to ASTM E 104-51 standard method of test. Humidity conditioning was achieved by storing samples at ambient temperature in relative humidity chambers ranging from 12 to 93 percent RH. These humidity conditions were maintained using various standard salt solutions. 32 6-3 Operation The quantity of permeant vapor that has permeated through the polymer membrane into the lower concentration chamber with time is quantified by gas chromatography with flame ionization detection. At predetermined time intervals, an aliquot (0.5 ml) of headspace gas is removed from the right and left cell chambers with a gas-tight syringe and injected directly into the gas chromatograph. A constant total pressure of 1 atm is maintained throughout the run in both the high and low permeant concentration cell chambers. A constant pressure in the high concentration cell chamber is maintained by continually flowing the vapor stream through the high concentration cell chamber and discharging it at atmospheric pressure. TO maintain a constant total pressure in the low concentration cell chamber, the volume of headspace gas removed for analysis is replaced with an equal volume of humidified nitrogen gas removed from the preconditioning test system (see Figure 8b) for the studies. Residual levels of penetrant in the lower concentration cell chamber are checked before each run to ensure no interference. Samples are removed from the lower concentration cell chamber at predetermined time intervals and the permeation-time plot (i.e. transmission rate 33 profile) monitored until a steady state permeation rate is obtained. The resultant transmission rate profile is related to the permeation rate of the film specimens. Measurements are continued until sufficient data is collected to ensure steady-state kinetics. Care is taken so that the permeant concentration in the lower concentration cell chamber does not exceed 3% of the permeant vapor concentration in the high concentration cell chamber (i.e. center chamber). This is done to assure a constant driving force of the penetrant throughout the course of the run. 34 RESULTS AND DISCUSSION 7-1 The Effect of Relative Humidity on the Permeation of Toluene Vapor through Cryovac Test Film FDX 1570 (Method I) Table 4 and Figure 9 summarize the data which were obtained for the studies on the diffusion of toluene vapor (81 ppm, wt/v) through Cryovac FDX 1570 samples as a function of relative humidity. For this film sample, good agreement was obtained between replicate runs and the results reported are the average of replicate studies. The results indicated that the test film was an excellent toluene vapor barrier at 0% RH. However, as shown, the permeability constant (P) for this film structure, was dependent upon relative humidity, with P increasing with an increase in relative humidity at a constant permeant level. To better illustrate the effect of RH on the transmission rate for the test films, the results are presented graphically in Figure 10, where the permeability constant is plotted as a function of relative humidity. Figure 10 serves to illustrate the profound effect of relative humidity upon the permeability of toluene vapor through the FDX 1570 film. 35 TABLE 4 THE EFFECT OF RELATIVE HUMIDITY ON THE DIFFUSION OFOTOLUENE VAPOR THROUGH CRYOVAC FDX 1570 FILM AT 81 PPM, 72 F (a) _la)(C) P P O Permeability Permeability Lag RH Rate Constant Time (%) (9/hr/SQ- m) (g/SQ- m-daY-looppm) (hr) (b) 0 No Detectable Level ------ --- -3 -2 23 2.7 X 10 7.2 x 10 6.9 -3 -2 3.1 X 10 8.4 X 10 5.9 -3 -3 -2 -2 x = 2.9 X 10 i 0.2 X 10 7.8 X 10 i 0.6 X 10 6.4 -3 -1 49 6.4 X 10 1.90X 10 1.1 -3 -1 5.8 X 10 1.70X 10 1.3 -3 -3 -1 -1 X - 6.1 X 10 i 0.3 X 10 1.8 X 10 i 0.1 X 10 1.2 -2 -1 85 1.08X 10 3.20X 10 0.8 -2 -l 1.06X 10 3.12X 10 0.8 -2 -3 -1 -2 x = 1.07x 10 i 0.1 x 10 3.16x 10 i 0.4 x 10 0.8 -2 -1 89 1.98X 10 5.85X 10 0.6 -2 -1 2.54x 10 7.49X 10 0.6 -2 -2 -1 -l x = 2.26X 10 i 0.28X 10 6.67x 10 i 0.82X 10 0.6 (a) Average of Replicate Runs (b) No Detectable Level of Permeation After 2 Weeks (c) Permeability Coefficient Normalized to 100 ppm (wt/v) Toluene Vapor Concentration 36 6 TOTAL QUANTITY PERMEATED (g x 10 ) 37 : 89% RH 150- 851 'RH . 49% RH . e .. A 100— _ ' a 50.. 23% RH O '. L L ; HI 1 1 0 _ Z 4 6 8 TIME (Hours) Figure 9 TRANSMISSION RATE PROFILE OF CRYOVAC FDX 1570 FILM AT 81 PPM, 72°F. (Method I) PERMEABILITY CONSTANT (gostructure/sq.m-day~100 ppm) 38 0.7 I- 0.5 I 0.5 I 0.3 0.2- PERCENT RELATIVE HUMIDITY Figure 10 PERMEABILITY CONSTANT VERSUS RELATIVE HUMIDITY OF CRYOVAC FDX 1570 FILM AT 81 PPM, 72°F. For the Cryovac FDX 1570 film, the relationship between the permeability constant for toluene vapor (81ppm) and the relative humidity of test is given by equation 8 which was derived by the cubic spline method. Equation 8 is a high order polynomial expression which gave a correlation coefficient of 99.9%. 4 3 2 Y = 5.18X - 7.66x + 2.96x + o.27x - 0.06 (8) Further experiments were carried out at a lower (62 ppm, wt/v) concentration of toluene vapor with varying RH environments for the FDX 1570 film. A summary of the experimental results is given in Table 5. In over 14 days of continuous testing, there was no measurable permeation at relative humidities of 0%, 23% and 49%. The permeability coefficients at these relative humidities were below the detectable threshold level for this method, which is determined by the sensitivity of the gas chromatography flame ionization detector. However, an upper limit value was estimated and is presented in Table 5. The procedure of estimation is given in Appendix B. As can be seen, the permeability constant is much lower than that for studies conducted at 81 ppm (wt/v) concentration of toluene vapor at comparable relative humidities. The vapor concentration dependency of the diffusion process is illustrated graphically in Figure 11, 39 where the total quantity of toluene permeated (Q) is plotted as a function of time for studies carried out at 81 ppm (wt/v) and 62 ppm (wt/v) toluene vapor concentration respectively with both runs being carried out at 85-86% relative humidity. As shown in Table 5, the effect of relative humidity on the diffusion of toluene vapor is similar to that obtained at 81 ppm (Table 4), namely, that the transmission rate increases with an increase in the relative humidity at which the test was performed. Further, at a constant RH of 85-86%, the diffusion process also appears to be dependent upon the concentration of penetrant vapor (from 62ppm to 81ppm, wt/v). The Observed concentration-dependent permea- bility constant suggests penetrant/polymer interaction, resulting in configurational change and alteration of polymer chain configurational mobility and thus penetrant diffusivity. 40 TABLE 5 THE EFFECT OF RELATIVE HUMIDITY ON THE DIFFUSION OF oTOLUENE VAPOR THROUGH CRYOVAC FDX 1570 FILM AT 62 PPM, 72 F (a) ._(a)(b) P P e Permeability Permeability Lag RH Rate Constant Time (%) (g/hr/sq- m) (g/sq- m day-looppm) (hr) -7 -6 (c) 0 P < 2.1 X 10 P < 8.4 x 10 --- No Detectable Level After 14 Days (0) -7 -6 23 P < 2.1 x 10 P < 8.4 x 10 --- NO Detectable Level After 14 Days -7 -6 (c) 49 P < 2.1 x 10 P < 8.4 x 10 --- No Detectable Level After 14 Days -4 -2 86 5.1 x 10 1.94x 10 10 -4 -2 9.1 x 10 3.46x 10 7 -4 -4 -2 -2 x = 7.1 x 10 i 2.0 x 10 2.7 x 10 i 0.76x 10 8.5 (a) Average of Replicate Runs (b) Permeability Coefficient Normalized to 100 ppm (wt/v) Toluene Vapor Concentration (c) Represents An Estimated Upper Limit Value 41 TOTAL QUANTITY PERMEATED (g X 10 ) 140- 81 ppm 85% RH 120' 100' 80‘ 60- 20' TIME (Hours) Figure 11 TRANSMISSION RATE PROFILE OF CRYOVAC FDX 1570 FILM AT 72 F, VARYING VAPOR CONCENTRATION. 7-2 Equilibrium Sorption Isotherm of Cryovac FDX 1570 Test Films The equilibrium sorption isotherm of the FDX 1570 test film is presented in Figure 12. As shown, good agreement was obtained between the two test methods, namely the gravimetric and titration methods for quantitating moisture content (Pande, 1974). Table 6 gives a summary of the isotherm results. Since the test film (FDX 1570) contains the hydrophilic polymers nylon and EVAL in the coextrusion structure, it is necessary to obtain the isotherm in order to have a better understanding of the relationship between relative humidity and water vapor absorbed by the test film. A high order polynomial equation describing the relationship between the equilibrium moisture content and the relative humidity (Figure 12), is given below : 4 3 2 Y = 30.85X - 23.00X + 6.77X + 0.33X (9) Equation (9) gave a correlation coefficient of 99.9% (cubic spline method). 43 oo— \ 2033:... .mo Nb Ed EHK onma xom mom EmZBOmH ZOHEmOm ZDHKmHa—HDON seHonsz m>He