Polyvinylidene chloride permeability

Certain copolymers of this type have been found to have excellent gas barrier properties, with the dry polymer having an oxygen permeability only about 1/lOth that of polyvinylidene chloride. Unsurprisingly, the copolymer has a high moisture absorption and a high moisture vapour transmission rate. Where the material is swollen by water, gas permeability is also higher. 

Barrier Plastics. When plastics replace metals and glass in packaging, their permeability is often a limiting property. Barrier performance generally increases with density and crystallinity. The most promising barrier plastics include ethylene/vinyl alcohol, polyvinylidene chloride, polyacrylonitrile, and polyethylene naphthoate. These are used most efficiently by laminating them to commodity plastics such as polyethylene and polyethylene terephthalate. 

Polyvinylidene chloride is primarily extruded as a film that has low permeability to water vapor and air and is therefore used as the familiar clinging plastic food wrap, Saran Wrap. 

The permeation for gases N2, 02, C02 is usually of the order of 1 4 20. Plastics with low permeabilities include polyvinylidene chloride, polyvinyl chloride, ethylene vinyl alcohol (if dry), nylon and PET. 

The use of other plastics tends to be related to specialised needs and whether their advantages justify the additional cost, e.g. Aclar (trade name) may cost twenty times more than PET but is the nearest approach to an inert plastic and is approximately ten times less permeable than Saran (PVdC—polyvinylidene chloride) which is widely used as a film coating. However foil, even when thin (0.006 mm and above) remains the best barrier material, with newer techniques of film metallisation, especially where two contact layers are used, coming a close second best. 

Figure 9-33. Selectivity of different polymer membranes to He-N2 separation as a function of nitrogen permeability (n, incm /(cm x atm x s)) (1) polyvinylidenechloride (2,4)polyethylene terephthalafe (3) polyvinylfluoride (5) polyvinylchloride (6) polyamide (7) plasfified polyvinylidene chloride (8) cellulose nitrate (9) polypropylene (lO)fluoroplast (26) (ll)co-polymer of isoprene (74%) and acryl-nitryl (26%) (12, 18, 20) different co-polymers of butadiene and acryl-rritryl (13) polyacrylate (14) polycarbonate (15) polyisobutylene (16) bulyl latex (17) co-polymer of vinyl chloride and vinyl acetate (19, 37) butyl acetate of cellulose (21) polyethylene vinyl acetate (22) polybutadiene (23) special polymer SKI-3 (24) natural latex (25) nitryl silicon latex (26) dimethyl silicon latex (27) special polymer SKS-30 (28) special polymer SKMS-50 (29) special polymer SKMS-30 (30, 34, 35) high-density, medium-densily, and low-density polyethylene (31) polyethylene with 5% soot (32) co-polymer of ethylene (90%) and propylene (10%) (33) co-polymer of ethylene (96.5%) and vinyl acetate (3.5%) (36) triacetate of cellulose (38) acetate cellulose (39) polystyrene.

Comparison of the dependence of dry oxygen permeability on temperature with some conventional barrier polymers (after Watanabe)(15) are presented In Figure 9 In normalized units of cc/mV24 hours for a one mil (25.4 micrometers) film thickness. The temperature coefficient of the grafted slloxane polymer appears to be comparable to that of BAREX 210 and ethylene vinyl alcohol. The EVAL shown (EVAL-E) Is 44% by weight ethylene. The silane Is considerably better than that of the polyvinylIdene chloride (Saran 468) sample shown In the figure. The data are plotted so that the temperature Increases to the right on the abscissa. 

The optical properties, clarity and gloss, are also improved and biaxially oriented polystyrene and polypropylene films are used for envelope windows and overwraps. Biaxially oriented polyvinylidene chloride film is also used for food packaging in view of its good mechanical and optical properties, but for economic reasons it is only used if its low permeability to gases, especially oxygen, is required. The electrical properties are improved by biaxial orientation and the applications of polyester, polycarbonate and polypropylene films in capacitors are expected to show a rapid growth. In these electrical applications crystalline films are always annealed in order to improve the dimensional heat stability. 

Polyvinylidene chloride appears as a crystalline polymer excelling in chemical resistance and low permeability, but it is difficult to process and thermally unstable. Both homopolymers vinyl acetate and vinyl alcohol ap-... 

Hard PVC is used for blister packages. It is preheated in a blistering machine and then round or capsule-like cavities are being formed in a moulding station. PVC is relatively permeable to water vapour and therefore not suitable to contain hygroscopic tablets or capsules. However, water permeability is greatly reduced when the PVC-film is combined with a thin layer of PVDC (polyvinylidene chloride). 

A blister pack or strip is a container with a foil that is shaped so that it can contain separate dosages [26]. An aluminium lidding foil closes the form foil. Blisters are mostly used to package tablets or capsules. For blister packs a form foil of PVC or laminate of PVC/PVDC (polyvinylidene chloride) is used. The form foil is warmed in a blister machine and a mold station makes round or capsule-Uke pockets with compressed air. PVC is permeable to water vapour and therefore not suitable for tablets or capsules that are moisture sensitive. To package such products the PVC form foil should include an outside layer of PVDC. PVDC permeates less water vapour. Dependent on product sensitivity to moisture, different PVDC-thicknesses can be chosen. When complete resistance to water vapour is necessary a form foil that consists of a formable aluminium laminate (consisting of polyamide, soft aluminium and PVC) has to be used. 

The Raschig Super Grid support plate is prei ted in Fip. 12 and 13. The spm er plates on toe upper side of toe plate ensure fliat toe random packing do not block any orifices. Owing to toe spacer cams, they are mainly inclined, just as they are in toe packed bed itself. Consequent toe support plate displays almost toe same permeability toe packet M it supports. The device is manufactured of various thermoplastic materials, such as polypropylene, polyvinyl chloride, polyvinylidene fluoride, etc. 

---