Poly(vinylidene chloride) has a very regular structure:
It is therefore a highly crystalline material and has a crystalline melting point
of about 200°e. At this temperature the polymer has a high rate of decomposition. Copolymerization reduces the molecular regularity and thus lowers
the softening point, which for 85% vinylidene chloride-15% vinyl chloride copolymer is about 140°C. Since crystallization is thermodynamically
favoured even in the presence of liquids of similar solubility parameter and
since there is little interaction between the polymer and any liquid, there are
no effective solvents at room temperature for the homopolymer. The copolymers, however, are soluble in ethers and ketones and the solutions may be
used for coating applications. The copolymers are resistant to most other
organic solvents and to acids; there is some attack by alkalis.
Even in the copolymers there is extensive crystallization and so the
materials have high specific gravity (1.7 for 85% vinylidene chloride-15%
vinyl chloride copolymer) and low permeability to moisture vapour and
gases. The copolymers also have high tensile strengths, namely about 70 MPa
(10000 Ib/in2 ) for unoriented material and up to 290 MPa (40000 Ib/in2 ) for
oriented filament. It may be noted that the copolymers are not readily
plasticized as all but small amounts of plasticizer exude from the finished
product.
The main application of the vinylidene chloride - vinyl chloride copolymers is for packaging film, which is made by extrusion and biaxial stretching.
The film has very good clarity, toughness and moisture and gas impermeability. The copolymers are also used for filaments, which are made by
melt extrusion and drawing. The filaments are used for such applications as
car upholstery, garden chair fabrics and filter cloths where toughness, durability and chemical resistance are required.
The main use of the vinylidene chloride - acrylonitrile copolymers is as
coatings for materials such as cellophane, paper and polyethylene. The
coatings confer moisture and gas impermeability and they are heat-sealable.
