Permeability PVDC

Poly(vinylidene chloride) (PVDC). The homopolymer is intractable in the conventional plastics processing techniques. For this reason, copolymers of vinylydene chloride with vinyl chloride or terpolymers with vinyl chloride and acrylonitrile are used. Due to its exceptionally low vapour, gas, and aroma permeability, PVDC films are used for packaging and for the barrier layer in composite films. Paper varnished by PVDC is a widespread packaging material. Trade names Diofan (FRG), Ixan (B), Saran (USA). 

The effect of copolymer composition on gas permeability is shown in Table 9. The inherent barrier in VDC copolymers can best be exploited by using films containing Htde or no plasticizers and as much VDC as possible. However, the permeabiUty of even completely amorphous copolymers, for example, 60% VDC—40% AN or 50% VDC—50% VC, is low compared to that of other polymers. The primary reason is that diffusion coefficients of molecules in VDC copolymers are very low. This factor, together with the low solubiUty of many gases in VDC copolymers and the high crystallinity, results in very low permeabiUty. PermeabiUty is affected by the kind and amounts of comonomer as well as crystallinity. A change from PVDC to 50 wt °/ VC or 40 wt % AN increases permeabiUty 10-fold, but has Httle effect on the solubiUty coefficient. 

PVdC-coated PVC increases the sheet s water-vapor permeability resistance by a factor of 5-10. The coating is applied at different loadings per square meter of PVC sheet. The coating is applied on one side and usually faces the product and lidding material. Stability studies will establish whether interactions with the product are taking place. This is usually indicated by discoloration of the blister or the product. 

Water vapour PVDCs have permeabilities evaluated at 0.07 to 0.2 compared to a full range of 0.05 up to 400 for all tested plastics. 

Barrier Properties. Vinylidene chloride polymers are more impermeable to a wider variety of gases and liquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeability. A more subde factor maybe the symmetry of the polymer structure. It has been shown that both polyisobutylene and PVDC have unusually low permeabilities to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values listed in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

Example 9-4. What permeability is obtained in Example 3 when a 10 pm thick PVDC film is used instead of LDPE for sealing the ends of the tube What is the ratio of a) the permeation from the PVDC film from the ends to the permeation from the surface of the HDPE tube and the permeation from the LDPE film from the ends in Example 3 to the permeation from the surface area of the HDPE tube ... 

When the ends of the HDPE tube are sealed with 10 pm PVDC film one gets the permeation, using permeability coefficient value for oxygen through PVDC at 30°C given in Table 9-2, in the following way ... 

A number of copolymers of vinylidene chloride are used in practice. Copolymers with acrylonitrile are used in low flammability fibers (modacrylic fibers). These fibers begin to lose weight when heated at 285-308° C due to dehydrohalogenation [41] but do not ignite easily. A tercopolymer butadiene-styrene-vinylidene chloride is used in fabrics industry and in paper industry. Other copolymers include PVC/PVDC, used for fibers and for films with low permeability to gases and water vapors (barrier films), etc. 

The proportion of crystallinity to amorphous regions has a marked influence on both diffusion and solubility of the diffusing molecules. The coefficient of diffusion decreases as the degree of crystallinity increases. For example, PVdC is very highly crystalline, and PS is very low. In terms of permeability to water, PVdC is very low, whereas PS is very high. 

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. 

The process of manufacture is detailed in Chapter 7. Natural RCF, apart from natural clarity, is similar to paper in its general properties, although it is a continuous film and not fibrous in nature, e.g. it is moisture sensitive and not heat sealable. Moisture permeability can be improved by coatings which can also confer heat sealability. These coatings, which may be applied to one or both sides, are usually based on nitrocellulose or Saran (PVdC). 

Chlorinated and particularly fluorinated derivatives of ethylene usually offer high inertness and good barrier properties. One film, Aclar (trade name) based on polymonochlorotrifluoroethylene (PCTFE), is the most moisture-impermeable commercial film currently known. For a similar thickness it is approximately ten times less permeable than PVdC. Derivatives generally have high melting and softening points. Found both as homopolymers and copolymers. For detail see Chapter 13. 

A magnificent property of PVDC films is their low vapor and gas permeability. Application of even very fine PVDC layers in co-extruded multilayered films imparts them with perfect barrier characteristics. Oriented PVDC films are highly strong, elastic and resist well depression and tearing, although they lose these properties at elevated (60°C) and lowered (—20°C) temperatures. 

Extruded ethylene vinyl alcohol copolymer (EVAL) is used as an oxygen-barrier packaging material. Dry EVAL has extremely low oxygen permeability, but the vinyl alcohol part of the copolymer is hydrophilic, and in the swollen wet state its permeability becomes higher than that of PVDC (Fig. 11.5). 

With the exception of packages for fresh produce, MAP trays or pouches need to be made of materials with low permeability to gases (CO2, Nj, O2). Laminates made of various combinations of polymer as PET, PVdC, PE, and polyamides such as nylons should have low oxygen permeability [175-178]. Respiration in fruits and vegetables leads to a build-up of carbon dioxide that may reduce the rate of respiration and help to prolong the shelf life of the product a reduction in the oxygen... 

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. 

Plotting In P versus l/v gives a straight line, linear regression using the Pq2 and Ifv values of PE. PIB, and PVDF) the permeability in PVDC can be estimated => Pqj in PVDC = 0.036 Barrer. The value given in the reference is somewhat lower (Pq2 = 0.0013 Barrer) but since PE, PVDF and PVDC are semicrystalline this estimate is fairly good. 

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