Vinylidene Chloride Polymers and Copolymers

Vinylidene chloride polymerises spontaneously into polyfvinylidene chloride), a polymer sufficiently thermally unstable to be unable to withstand melt processing 

By copolymerising the vinylidene chloride with about 10-15% of vinyl chloride, processable polymers may be obtained which are used in the manufacture of filaments and films. These copolymers have been marketed by the Dow Company since 1940 under the trade name Saran. Vinylidene chloride-acrylonitrile copolymers for use as coatings of low moisture permeability are also marketed (Saran, Viclan). Vinylidene chloride-vinyl chloride copolymers in which the vinylidene chloride is the minor component (2-20%) were mentioned in Chapter 12. 

Vinylidene chloride is a clear mobile liquid which is highly inflammable and with the following physical properties  

Boiling point Specific gravity Refractive index Specific heat Heat of polymerisation 

Although miscible with many organic solvents it has a very low solubility in water (0.04%). 

A considerable number of other chlorinated polymers find commercial use as a means to impart fire retardancy into a product. Poly(vinylidene chloride) (PVDC) and its copolymers are described here, particularly because of their gas and vapour barrier properties. 

Copolymerization by suspension or emulsion methods, with vinyl chloride or acrylonitrile for example, reduces the regularity in the chain, increasing flexibility and permitting processing below the copolymer s degradation temperature. Copolymers are readily crystallizable, and care is needed in the setting of process temperatures for extruded (e.g. pipe) and moulded goods. 

The polymer and its copolymers are mainly exploited for their barrier properties, both as film and coating materials. The major use for homopolymer and vinyl chloride copolymer is a surface coating for PETP bottles, to enhance their oxygen barrier properties, when the bottle is to be filled with a carbonated alcoholic beverage alcohol may be oxidized to acetaldehyde, which taints the flavour of the drink. The outer surface of the bottle is dip-coated in polymer as a latex, and dried at about 60°C (i.e. below the Tg of PETP), to form a bonded, coherent film on its surface. The biaxially oriented film has excellent mechanical and optical properties. The copolymer is also made into fibres which have excellent mechanical properties, durability and chemical resistance (they are used in deck-chair fabrics for example). 

The acrylonitrile copolymers find limited use in the production of cast film for coated cellophane, polyethylene and paper. 

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Vinylidene chloride copolymers were among the first synthetic polymers to be commercialized. Their most valuable property is low permeabiUty to a wide range of gases and vapors. From the beginning in 1939, the word Saran has been used for polymers with high vinylidene chloride content, and it is still a trademark of The Dow Chemical Company in some countries. Sometimes Saran and poly (vinylidene chloride) are used interchangeably in the Hterature. This can lead to confusion because, although Saran includes the homopolymer, only copolymers have commercial importance. The homopolymer, ie, poly (vinylidene chloride), is not commonly used because it is difficult to fabricate. 

Both methyl acrylate and butyl acrylate have been used to prepare vinylidene chloride copolymers with sufficient stability to permit thermal processing. The presence of alkyl acrylate units in the polymer mainchain limits the size of vinylidene chloride sequences and thus the propagation of degradative dehydrochlorination. More importantly it lowers the melt... 

The most widely used vinyfidene chloride polymer b a copolymer of vinylidene chloride (90%) and vinyl chloride (10%) (Saran). This copolymer... 

Heat stabilizers protect polymers from the chemical degrading effects of heat or uv irradiation. These additives include a wide variety of chemical substances, ranging from purely organic chemicals to metallic soaps to complex organometallic compounds. By far the most common polymer requiring the use of heat stabilizers is poly(vinyl chloride) (PVC). However, copolymers of PVC, chlorinated poly(vinyl chloride) (CPVC), poly(vinylidene chloride) (PVDC), and chlorinated polyethylene (CPE), also benefit from this technology. Without the use of heat stabilizers, PVC could not be the widely used polymer that it is, with worldwide production of nearly 16 million metric tons in 1991 alone (see VlNYL polymers). 

Poly( vinylidene chloride) does not dissolve in most common solvents at ambient temperatures. Copolymers, particularly those of low crystallinity, are much more soluble. However, one of the outstanding characteristics of vinylidene chloride polymers is resistance to a wide range of solvents and chemical reagents. The insolubility of PVDC results less from its polarity than from its high melting temperature. It dissolves readily in a wide variety of solvents above 130°C. 

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... 

Vinylidene Chloride Copolymer Latex. Vinylidene chloride polymers are often made in emulsion, but usually are isolated, dried, and used as conventional resins. Stable latices have been prepared and can be used diiecdy for coatings (171—176). The principal applications for these materials are as barrier coatings on paper products and, more recently, on plastic films. The heat-seal characteristics of VDC copolymer coatings are equally valuable in many applications. They are also used as binders for paints and nonwoven fabrics (177). The use of special VDC copolymer latices for barrier laminating adhesives is growing, and the use of vinylidene chloride copolymers in flame-resistant carpet backing is well known (178—181). VDC latices can also be used to coat polyethylene terephthalate) (PET) bottles to retain carbon dioxide (182). 

Estimate if separation occurs between coating and substrate in the case where flame-treated polypropylene film, coated with vinylidene chloride/methyl acrylate copolymer is immersed in a solution of sodium n-dodecyl sulphate (concentration 0.5%). The data of polymer, coating and liquid are the following (determined in separate... 

Polyco [Borden], TM for a series of thermoplastic polymers in the form of water emulsions or solvent solutions, applied to vinyl acetate polymers and copolymers, butadiene-styrene copolymer lat-ics, polystyrenes, vinyl and vinylidene chloride copolymers, acrylic copolymers, and water-soluble polyacrylates. 

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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