EVOH film

Humidity does not affect the permeabihty, diffusion coefficient, or solubihty coefficient of flavor/aroma compounds in vinyhdene chloride copolymer films. Studies based on /n j -2-hexenal and D-limonene from 0 to 100% rh showed no difference in these transport properties (97,98). The permeabihties and diffusion coefficients of /n j -2-hexenal in two barrier polymers are compared in Table 12. Humidity does not affect the vinyhdene chloride copolymer. In contrast, transport in an EVOH film is strongly plasticized by humidity. 

In the case of both EVAL EP-E105 and SOARNOL D resins, attempts to uniaxially orient to 2.5X were unsuccessful, as the films split and cracked upon drawing. Reducing the stretch rate and increasing the orientation temperature yielded no significant effect. As such, biaxial orientation of these two EVOH films was not attempted. 

The difficulty experienced in orientation of EVOH films is not surprising. Ikari ( .) indeed reports that drawing below the softening point of EVOH resins yields slight unevenness and cracks are likely to appear when the draw ratio is increased. He attributes this to the crystallization accompanying stretch orientation. This is especially true for low ethylene content EVOH grades. 

Films. Three films were included in this study. Low density polyethylene (LDPE) was included as a representative polyolefin. It is not considered to be a barrier polymer. It has permeabilities to selected aroma compounds slightly higher than the permeabilities of polypropylene and high density polyethylene (1). A vinylidene chloride copolymer (co-VDC) film was included as an example of a barrier that is useful in both dry and humid conditions. The film was made from a Dow resin which has been designed for rigid packaging applications. A hydrolyzed ethylene-vinylacetate copolymer (EVOH) film was included as an example of a barrier film that is humidity sensitive. The polymer was a blend of resins with total composition of 38 mole% ethylene. 

In an experiment with the EVOH film and ethylvalerate at 110 C, 0.25 milliliters of water was injected into the three liter flask after the permeation experiment had reached steady state. Within seconds, the permeation rate increased to above the detection limit of the mass spectrometer. This was consistent with prior experience wherein the permeability and the diffusivity in an EVOH film increase by a factor of about 1000 in the presence of moisture (6). No more experiments were done with humidity for this study. 

Fewer esters were used in experiments with the EVOH because the trends observed with the co-VDC film were apparent. Figures 11, 12, and 13 show the permeabilities, diffusion coefficients, and solubility coefficients for the esters at 85"C in the EVOH film. Figures 14 and 15 show the activation energies for diffusion and the heats of solution. 

The activation energies for diffusion in the co-VDC and EVOH films increase as the size of the ester increases. This is expected since cooperative motion of larger zones of the polymer matrix are necessary to create larger holes for passage. The activation energies for diffusion in LDPE were not determined. 

Table 33-02. Physical Properties of EVAL EVOH Films...

Table 33-04. Mechanical and Optical Properties of EVAL and EVOH Films compared with PVDC and BOPP Films...

A high barrier nylon film has been developed by Bayer, also using nanoadditives and Nanocor, USA, has developed an aluminosilicate with platelet-type particles in the nanometer (0.001 pm) size range has been shown to reduce permeability to gases by up to 45 times in polyolefin, PET, or EVOH films. Correctly added, the platelets overlap and present a difficult path to migration of molecules through the film. 

Fig. 5. Layered structure of thermoplastic waxy maize/EVOH film after 3 days of soil burial test (129).

EVOH films provide an excellent oxygen barrier in dry applications, but do not provide a good moisture barrier. In addition, the oxygen barrier diminishes at elevated relative humidity levels. Co-extruded biaxiaUy oriented films containing EVOH have use in processed meat, cheese, and some dry food packaging applications. 

Figure 14.5 Water solubility coefficients (S) (bars) and plasticization factors (lines) of the EVOH amorphous fraction in PP/EVOH films at 25 °C and 0.98 water activity.

Figure 20.9 Diffusion coefficients for water vapor at 25 °C and 0.98 water activity in PP/EVOH films as a function of EVOH content. Reprinted with permission from Ref. [79] 2009, Elsevier.

Figure 6.12 Weight loss of corn starch/EVOH films in a soil burial test as a function of time and of amylose content, (a) 70% amylose, (b) 25% amylose (c) 5% amysose [118].

Measurements were made of the permeation rates of methanol, toluene, a 50/50 toluene/isooctane fuel (fuel C) and fuel C with 10% ethanol or 15% methanol through EVOH films. Large scale methanol and water induced plasticisation was suggested by large solvent uptakes and confirmed by the observation of considerable decreases in Tg. The permeability of fuel C increased significantly when methanol was present, while the high barrier properties of EVOH to hydrocarbons was maintained for fuel C containing ethanol. 9 refs. 

It was found that before retort the OTR of the pure EVOH multilayer film was 0.40 cc/ (m -day) but the OTR of EVOH/MLS multilayer film was 1.10 cc/ (m -day). The WVTR of the EVOH/MLS was 57% lower than the pure EVOH film. Table 4 shows the OTR and WVTR data for the multilayer films. It was unexpected that the pure EVOH samples would have a lower OTR than the EVOH/MLS samples because the nanocomposites are supposed to create a toturous path which the molecules have to follow in order to permeate the material. After the retort the pure EVOH and EVOH/MLS multilayer films were tested with the Ox-Tran five times for consistency and accuracy, but failed the test all five times. A sample failure is when the OTR exceeds the upper limit of the sensor range which is 2000 cc/ (m -day), although there is some flexibility for the actual failure value. [14] EVOH is primarily used as an oxygen barrier since the OTR failed after retort it was not necessary to test the WVTR after retort. The OTR and WVTR of the multilayer films were not found to meet the military specifications. Although the transmission rates did not meet the military food packaging specifications further experimentation with different materials or a PP nanocomposite outer layer could lead to a structure which meets the required specifications. 

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