Technical Bulletin No. 110 GAS BARRIER PROPERTIES OF RESINS

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1 Technical Bulletin No. 110 GAS BARRIER PROPERTIES OF RESINS The most outstanding property of EVAL TM resins is their ability to provide a barrier to gases such as oxygen, nitrogen, carbon dioxide and helium. This property is especially important when considering the packaging of food products. The presence of oxygen causes most food products to degrade, losing flavor, color and quality. On the other hand, packaging such as Controlled Atmosphere Packaging (CAP) or Modified Atmosphere Packaging (MAP) depends on the retention of the gas in the package to retard spoilage. In either case the excellent barrier properties of EVAL TM resins allow the packaging engineer to develop a package that provides a cost/performance advantage over conventional packaging methods. This bulletin describes these barrier properties and how they may be affected by the environment in which they are used. GAS BARRIER PROPERTIES EVAL TM resins are outstanding in their ability to provide a barrier to gases such as oxygen, carbon dioxide or nitrogen under a variety of environmental conditions. The use of EVAL TM resins in a packaging structure enhances flavor and quality retention by preventing oxygen from penetrating the package. In those applications where CAP or MAP techniques are used, EVAL TM resins effectively retain the combination of gases, such as oxygen, nitrogen and carbon dioxide, used to blanket the product. The permeability or the inverse barrier, of any polymer can be determined by considering the equation for Mass Transport of a Gas across a membrane, as shown in Figure 1. P mgas t Mgas / t A p Figure 1 Mass Transport of a Gas = P A p = permeability of barrier = transmission rate = area of barrier = thickness of barrier = partial pressure difference across the barrier EVAL TM Resin and Film Sales Office 2625 Bay Area Boulevard, Suite 600, Houston, TX 77058; EVAL TM Manufacturing and R&D Bay Area Boulevard, Pasadena, TX 77507; Page 1 of 12

2 There are two environmental conditions that will directly affect this equation. The flow of a gas ( Mgas) is directly affected by changes in temperature. The partial pressure differences ( p) is also affected by temperature, and even more so by the difference in relative humidity between the inside and outside of the package. Temperature Effects Table 1 shows the effect of temperature on the oxygen transmission rate of EVAL TM resins and selected packaging materials. Table 1 Oxygen Transmission 0% RH Equiv. cc. 25µ/m 2 24 hr. atm cc. mil/100 in 2 24 hr. atm % C2 5 C 23 C 35 C 50 C 5 C C 50 C EVAL TM L Series F Series H Series K Series E Series G Series Saran VC PVDC Saran MA PVDC Barex 210 AN Oriented Nylon Non-Oriented Nylon MXD6 Nylon Oriented Polyester High Density Polyethylene Low Density Polyethylene Oriented Polypropylene Polystyrene Page 2 of 12

3 As Table 1 indicates, EVAL TM resins are less temperature sensitive than other barrier materials. Even at elevated temperatures, the oxygen transmission rate of EVAL TM resins is superior. Table 2 shows this same relationship for other gases. Table 2 Gas Transmission 0% RH cc. 25µ/m 2 24 hr. atm CO 2 N 2 He EVAL TM F Series 5ºC ºC ºC EVAL TM H Series 5ºC ºC ºC EVAL TM E Series 5ºC ºC ºC cc. mil/100 in 2 24 hr. atm CO 2 N 2 He Saran 5253 PVDC Oriented Nylon 6 Oriented PET 23ºC ºC ºC Moisture Effect The second environmental consideration affecting the mass flow equation is moisture absorption or the partial pressure difference across the barrier layer. Whenever there is a difference in relative humidity between the inside and outside of a package, there will be a difference in partial pressure from one side of the barrier layer to the other. Page 3 of 12

4 Figure 2 is a simple schematic of the phenomena. Moisture flows across the barrier layer until an equilibrium is reached. Once equilibrium is reached, the EVAL TM resin barrier layer is at a constant percent relative humidity as long as conditions do not change. Figures 3 and 4 show the effect of relative humidity on the moisture content of the EVAL TM barrier layer. Page 4 of 12

5 As the barrier layer absorbs moisture, the permeability increases, as is shown in Figure 5. Structural Materials A very effective method to control moisture absorption, and probably the most common method, is the correct selection of a material or combination of materials to form the structural layers of a package. When choosing these materials, the moisture vapor transmission rate (MVTR) is an important criteria to consider. Table 3 indicates the MVTR of commonly used structural materials together with those of selected barrier resins. Page 5 of 12

6 Table 3 Moisture Vapor Transmission Rate, (40 C, 90% RH) g. 25µ/m 2 /24 Hrs. g. mil/100 in 2 /24 Hrs. Structural Materials Biaxially Oriented Polypropylene H D- Polyethylene Polypropylene LD-Polyethylene Biaxially Oriented PET PET Rigid PVC Polystyrene Biaxially Oriented Nylon Polycarbonate Barrier Materials EVAL TM L Series F Series H Series K Series E Series G Series Saran 5253 PVDC Saran XU PVDC Barex 210 Nitrile Selar PA Amorphous Nylon MXD6 Nylon Using the information from Table 3 together with the structural layer thickness, the barrier layer thickness and the environmental conditions under which the package is used, a close approximation of the barrier layer relative humidity can be found. Figure 6 is a model of a multilayer packaging structure of any number of layers, N. Page 6 of 12

7 Figure 7 is a generalized equation for determining the relative humidity of the barrier layer for any multilayer structure. In this equation, the adhesive layers are ignored because their effect is minimal. Figure 7 Relative Humidity of Barrier Layer For a struciton of (N) layers, the relative humidity of the barrier layer in position (i) is determined by: Where: 1/2 (P i-1 + P i )=R.H. of barrier layer P o = R.H. of outside environment P n = R. H. of inside environment i = Position of barrier layer (number of Layers from outside) n = Number of layers A = MVTR Thickness For a five-layer structure, the equation can be simplified as shown in Figure 8. Where: 1/2 (P 2 +P 3 ) = R.H. of barrier layer (%) P 0 = R.H. of outside environment (%) P i = R.H. of inside environment (%) t b = thickness of barrier layer (mils) t i = thickness of inside layer (mils) t 0 = thickness of outside layer (mils) M b = MVTR of barrier layer (g-mil) M i = MVTR of inside layer (g-mil) M 0 = MVTR of outside layer (g-mil) Page 7 of 12

8 To effectively control the relative humidity of the barrier layer, two approaches can be taken. 1. When packaging wet products or products containing considerable moisture and the same structural material is desired throughout the package, a thicker layer of structural material is placed toward the inside of the package and a somewhat thinner layer toward the outside. Considering the law of partial pressures, this offers the highest barrier to the intrusion of moisture from the product into the barrier layer and the moisture that does intrude is allowed to escape to the outside of the package. If dry Products are being packaged, the reverse is used. Consider the following example: (Outside) (Inside) Structure: Polypropylene/Adhesive/F Series/Adhesive/Polypropylene Thickness: 8 mils - 1 mil - 18 mils Inside Package R.H.: 100% Outside Package R.H.: 70% Using the equation in Figure 8, and the data in Table 3, the relative humidity of the barrier layer at equilibrium would be: R.H. (Barrier) = 79.3% 2. A second method of controlling the barrier layer relative humidity is to use a combination of materials as structural layers. Again, using a wet package for an example, the inside structural layer would be the material with the lowest MVTR and the outside structural layer would be the material with the highest MVTR. For example: (Outside) (Inside) Structure: LDPE/Adhesive/F Series/Adhesive/Polypropylene Thickness: 13 mils - 1 mil - 13 mils Inside Package R.H.: 100% Outside Package R.H.: 70% R.H. (Barrier) = 81.5% 3. A third method, and the most effective, is a combination of 1 and 2 above. Here we use two different materials, as in method 2, and a thicker layer of the high MVTR material on the inside as in method 1. Example: (Outside) (Inside) Structure: LDPE/Adhesive/F Series/Adhesive/Polypropylene Thickness: 8 mils - 1 mil - 18 mils Inside Package R.H.: 100% Outside Package R.H.: 70% R.H. (Barrier) = 76.4% Page 8 of 12

9 Generally speaking, in wet packaging, the optimum position for the barrier layer is approximately 20 to 25% of the total thickness from the outside edge. For dry packaging, it is 20 to 25% of the total thickness from the inside edge. As these examples indicate, most food packaging can be designed to maintain a barrier relative humidity of 70 to 80%. Figures 9 & 10 show that within this range, EVAL TM resins offer superior protection to the other high barrier resins. Tables 4 & 5 provide additional examples of package constructions and the effect they have on oxygen transmission rate. These tables reflect what could be expected with packages using ambient, aseptic or hot fill food processing techniques. Page 9 of 12

10 Table 4 Relative Humidity vs. Oxygen Transmission Rate For Various Multilayer Structures Inside Wet (100% R.H.) Structure Outside 65% R.H Outside 75% R.H. Outside 6 mil EVAL TM Resin 1.0 mil Inside 24 mil R.H. of Barrier O 2 TR O 2 TR EVAL TM PVDC R.H. of Barrier EVAL TM PVDC F Series 72.0% % PET F Series 69.2% % PC F Series 65.7% % PS F Series 65.8% % HDPE F Series 75.9% % PC E Series 65.9% % Nylon E Series 66.4% % LDPE E Series 69.8% % E Series 72.2% % TR: cc/100 in 2 /24 68 F PVDC: Saran VC Outside 24 mil Table 5 Relative Humidity vs. Oxygen Transmission Rate For Various Multilayer Structures Inside Dry (10% R.H.) Structure Outside 65% R.H. Outside 75% R.H. EVAL TM O 2 TR O 2 TR Resin Inside 6 R.H. of R.H. of 1.0 mil mil Barrier EVAL TM PVDC Barrier EVAL TM PVDC F Series 24.2% % F Series PET 16.6% % F Series PS 16.9% % F Series HDPE 29.1% % F Series LDPE 21.4% % E Series PC 21.0% % HOPE E Series LDPE 14.5% % TR: cc/100 in 2 /24 68 F PVDC: Saran VC Page 10 of 12

11 The same design principles are used for retort sterilized packages. Although the barrier layer becomes completely saturated with moisture during the retort cycle, it will begin to dry as soon as the retort cycle is completed. Due to the dynamic situation occurring, the equations in Figures 7 & 8 cannot be used to predict the oxygen barrier properties of the package. Instead, a retort computer simulation program is used to predict oxygen barrier and oxygen ingress over time for potential retort sterilized packages. Table 6 shows the effect of layer position and layer thickness for a retort package, based on the simulation results. Table 6 Simulated Retort Sterilization Cycle Inside (mils) (15) (20) (25) (2g) Structure Barrier (mils) F Series (2) F Series (2) F Series (2) F Series (2) Outside (mils) (15) (10) (5) (1) 1 week O 2 TR 0 2 Gain (cc/100 in 2 ) Storage (20 C., 65% RH) 26 weeks O 2 TR 0 2 Gain (cc/100 in 2 ) 52 weeks O 2 TR 0 2 Gain (cc/100 in 2 ) Page 11 of 12

12 HEAT TREATMENT AND ORIENTATION EVAL TM resins are highly crystalline materials. It is this crystallinity that allows EVAL TM resins to offer superior barrier properties. Crystallinity may be affected by both heat-treating and orientation (stretching). The following general improvements are seen when EVAL TM films are subjected to either heat treatment, orientation or a combination of both. Heat treatment alone can improve gas barrier properties, particularly those at high humidity conditions. A combination of heat treatment and orientation will further improve gas barrier properties at high humidity conditions. Improvement by orientation alone without heat treatment is marginal. Table 7 shows the effect of orientation and/or heat treatment Table 7 Orientation and Heat Treatment vs. Oxygen Transmission Rate Processing O 2 TR (cc-25u/m 2 /24hrs/atm) O 2 TR (cc-25u/m 2 /24hrs/atm) Chill Roll Temperature Heat Orientation Treatment F Series E Series F Series E Series 0%RH 100% RH 0%RH 100% RH 0%RH 100% RH 0%RH 100% RH 50 C None None C None None C None 140 C C Uniaxially (3 times) 50 C Uniaxially (3 times) 50 C Biaxially (3x3) 50 C Biaxially (3x3) None C None C The techniques described above can all be used in various packaging systems. By controlling the moisture content of the EVAL TM resin barrier layer through the proper selection of structural materials, heat treatment, orientation or a combination of all three, EVAL TM resin multilayer systems may be used for ambient, hot fill or retortable packaging systems. All information contained herein is presented in good faith and without warranty. Kuraray America, Inc. accepts no liability for damage or loss resulting from the use or misuse of this information (20 C) REV Page 12 of 12