Effect of orientation on oxygen permeability

Table 11. Effect of Orientation on Oxygen Permeability for Certain Polymers...

Polystyrene (PS), 7 610t 23 326, 348, 358. See also Polystyrenes Styrene Styrene plastics biodegradation of, 23 376 brominated, 11 470-474 chain transfer to, 23 383 colloidal suspensions, 7 275 crystalline syndiotactic, 23 388 decomposition of, 14 109 effect of orientation on oxygen permeability, 3 393t... 

The effect of orientation on oxygen permeability of the medium and high barrier resins is seen to be dependent upon the morphological nature of the barrier resin prior to orientation. A plot of the oxygen transmission rates as a function of the overall draw ratio (figure 3) illustrates this clearly. While the semicrystalline polymers, VDC copolymer, and aromatic nylon MXD-6, show little change in the permeability with moderate amounts of orientation in the solid state, orientation of the amorphous polymers SELAR PA 3426 and XHTA-50A causes reduction in the permeability by 5-30% in both resins, depending upon the overall level of orientation. 

It is, therefore, important to evaluate the effects of orientation on the permeabilities of competitive medium to high oxygen barrier polymers. This study was initiated to fill that void. 

The effect of orientation on amorphous barrier resins SELAR PA 3426 and XHTA-50A are shown in Table III. The measured oxygen permeability of 1.4 cc-mil/100 in 24 hr. atm for unoriented extrusion cast SELAR PA 3426 film is reduced by 19% upon uniaxial orientation, while the permeability of biaxially oriented films are 28% lower than that of extrusion cast film. Similarly, the decrease in oxygen permeability with 2X and 2.5X uniaxial orientation in XHTA-50A are 5% and 9% respectively, from the measured value of 3.1 cc-mil/100 in 24 hr. atm for unoriented film. The corresponding reduction in permeability with 2X x 2X biaxial orientation is 12%. 

TABLE 12.1 Effect of Orientation on the Oxygen Permeability of Some Commonly Used Polymers at 23°C... 

The permeation of simple gases in polar polymers is also dependent on relative humidity because of the water s strong interaction with the polymer. Particularly large is the effect of humidity on the oxygen permeability of barrier layers made from EVOH. At 20 °C and 100 % relative humidity the permeability is 300 times higher than at 0 % relative humidity at the same temperature. By orientation and thermal treatments the barrier properties at high relative humidity can be significantly improved (up to a factor of 10). 

Incorporation of silicate nanolayers in semi-crystalline polymers like polypropylene ean have a two-fold effect on the barrier properties, (1) well oriented large aspeet ratio platelets will increase the tortuosity of the diffusion path and (2) the nanolayers will affect the crystalline order (size and interlamellar spacing) and possibly affect the barrier properties. The extent of orientation is greater in blown film than in extmsion cast film and this leads to similar trends in barrier properties of polypropylene nanocomposites with 7wt.% 1.3 IPS (silated) clay as reported by Qian et al. With cast films, the nanoeomposite had a lower permeability to oxygen by a faetor of 1.5 compared to neat polypropylene. With blown films, the nanocomposite permeability to oxygen was lower by a factor of 2.5 compared to neat polypropylene. However, Ellis and D Angelo were able to prepare only intercalated polypropylene nanocomposites with the same 1.31 PS and obtained no improvement in permeability to a solvent over that for neat polypropylene. This underlines the greater sensitivity of barrier performanee to the level of dispersion and orientation. 

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