Polyvinylidene chloride

Polyvinylidene chloride is a polymer derived from vinylidene chloride.

History
Ralph Wiley, a Dow Chemical lab worker, accidentally discovered polyvinylidene chloride in 1933. While cleaning laboratory glassware, he came across a vial he could not scrub clean. Dow researchers made this material into a greasy, dark green film, first called "Eonite" and then "Saran".

The military sprayed Saran on fighter planes to guard against salty sea spray and carmakers used it for upholstery. Dow later devised a formulation of polyvinylidene chloride free of unpleasant odour and green colour.

The most well known use of polyvinylidene chloride came in 1953, when Saran Wrap, a plastic food wrap was introduced. In 2004, however, the formula was changed to low density polyethylene due to environmental concerns about the chloride.

Production methods
Saran fiber is manufactured by melt spinning vinylidene chloride copolymer. Saran is pigment dyed before fiberspinning if color is desired.

Fiber properties
Saran fiber is a heavy fiber. It is a remarkable barrier against water, oxygen and aromas, has superior chemical resistance to alkalis and acids, is insoluble in oil and organic solvents, has very low moisture regain and is impervious to mold, bacteria, and insects. Saran fiber has a high elastic recovery and resists wrinkling and creasing. Because it is pigment dyed before fiberspinning, it has excellent colorfastness and high light permeability. Saran is also flame-retardant and self extinguishing, it may soften or char in flame, and decomposes in moderate heat.

Fiber Types
Saran fiber comes in monofilament, multifilament-twist, and as a staple fiber. It is also available in thermochromic (color changing) and luminescent (glow in the dark) fibers.

Packaging
Polyvinylidene chloride is applied as a water-based coating to other plastic films such as biaxially-oriented polypropylene (BOPP) and polyester (PET). This coating increases the barrier properties of the film, reducing the permeability of the film to oxygen and flavours and thus extending the shelf life of the food inside the package.

Household
Cleaning cloths, filters, screens, tape, shower curtains, garden furniture.

Industry
Screens, artificial turf, waste-water treatment materials, underground materials.

Miscellaneous
Doll hair, stuffed animals, fabrics, fishnet, shoe insoles.

Disadvantages
While extremely useful as a food packaging agent, the major disadvantage of Saran is that it will undergo thermally induced dehydrochlorination at temperatures very near to processing temperatures. This degradation easily propagates, leaving polyene sequences long enough to absorb visible light, and change the color of the materal from colorless to an undesirable transparent brown (unacceptable for one of Saran's chief applications: food packaging). Therefore, there is a significant amount of product loss in the manufacturing process, which increases production and consumer costs.

Trademarks (Producers)
Saran TC and Saran LS (Asahi-Kasei), Saran Wrap and Saranex (Dow Chemical)