Vinylidene chloride copolymers oxygen permeability

Fig. 4. Oxygen permeability in a vinylidene chloride copolymer film at selected levels of plasticizer (Citroflex A-4). Plasticizer level in parts pet hundred resin...

The effect of plasticizers and temperature on the permeability of small molecules in a typical vinylidene chloride copolymer has been studied thoroughly. The oxygen permeability doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeability are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a litde more complicated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeability and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeability doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

For oxygen, the permeabilities increase about 10% per degree in polymers that are above their T such as vinylidene chloride copolymers and polyolefins. The permeabilities increase about 5% per degree in polymers that are below their T such as acrylonitrile copolymers, EVOH, and PET. 

Fig. 15. Oxygen permeability versus 1/specific free volume at 25 °C (30). 1. Polybutadiene 2. polyethylene (density 0.922) 3. polycarbonate 4. polystyrene 5. styrene-co acrylonitrile 6. polyethylene terephthalate) 7. acrylonitrile barrier polymer 8. poly(methyl methacrylate) 9. poly(vinyl chloride) 10. acrylonitrile barrier polymer 11. vinylidene chloride copolymer 12. polymethacrylonitrile and 13. polyacrylonitrile. See Table 1 for unit conversions.

Vinylidene chloride polymers are more impermeable to a wider variety of gases and liquids than other polymers. For example, commercial copolymers are available with oxygen permeabilities of 0.03 nmol/m s-GPa. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase m either tends to reduce permeability. Permeability is affected by the kind and amounts of comonomer as well as crystallinity. A more polar comonomer, e.g., an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. All VDC copolymers, are very impel meable to... 

This relationship is shown in Figure 13 where Polymer 1 has a permeability 1000 times higher than that of Polymer 2. Published data have small negative deviations from this theoretical relationship. Part of the deviation can be explained by densification of the blend relative to the starting components. Random copolymers, which are forced (by covalent bonds) to imitate combinations of two materials, have permeabilities that are similar to miscible blends. However, the deviations from equation 18 tend to be positive. A series of styrene—methacrylonitrile copolymers were studied (11) and slight positive deviations were found. Figure 14 shows the oxygen permeabilities of a series of vinylidene chloride— -butyl acrylate copolymers [9011-09-0]. 

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