Saran (plastic)

Saran is the trade name for a number of polymers made from vinylidene chloride (especially polyvinylidene chloride or PVDC), along with other monomers. Since its accidental discovery in 1933, Saran has been used for a number of commercial and industrial products. When formed into a thin plastic film, the principal advantage of Saran, when compared to other plastics, is its very low permeability to water vapor, flavor and aroma molecules, and oxygen. This oxygen barrier retards food spoilage, while the film barrier to flavor and aroma molecules helps food retain its flavor and aroma.

History

Polyvinylidene chloride (PVdC) was discovered at Dow Chemical Company (Michigan, USA) in 1933 when a lab worker, Ralph Wiley, was having trouble washing beakers used in his process of developing a dry-cleaning product. It was initially developed into a spray that was used on US fighter planes and, later, automobile upholstery, to protect them from the elements. Dow Chemical later named the product Saran, and eliminated its green hue and offensive odor.

In 1942, fused layers of original-specification Saran or PVDC were used to make woven mesh ventilating insoles for newly developed jungle or tropical combat boots made of rubber and canvas.^[1][2][3] These insoles were tested by experimental Army units in jungle exercises in Panama, Venezuela, and other countries, where they were found to increase the flow of dry outside air to the insole and base of the foot, reducing blisters and tropical ulcers.^[2][3] The Saran ventilating mesh insole was later adopted by the United States Army for standard issue in its M-1945 and M-1966 Jungle Boots.^[1][2] In 1943, John Reilly (Ralph Wiley's boss)^[4] and Ralph Wiley of The Dow Chemical Co. completed the final work needed for introduction of Saran (polyvinylidene chloride), which had been invented in 1939.^[5] Saran monofilaments were also extruded for the first time.^[6] The word "Saran" was coined by a combination of John Reilly's wife's and daughter's names, Sarah and Ann Reilly.^[7]

In 1949, Dow introduced Saran Wrap, a thin, clingy plastic wrap that was sold in rolls and used primarily for wrapping food. It quickly became popular for preserving food items stored in the refrigerator. Saran Wrap was later acquired by S. C. Johnson & Son. However, today's Saran Wrap is no longer composed of PVDC, supposedly due to environmental concerns with halogenated materials, and is now made from ordinary polyethylene.^[8] However, polyethylene has a higher oxygen permeability, which in turn affects food spoilage prevention. For example, at 23 °C and 95% relative humidity polyvinylidene chloride has an oxygen permeability of 0.6 cm³ μm m⁻² d⁻¹ kPa⁻¹ while low-density polyethylene under the same conditions has an oxygen permeability of 2000 cm³ μm m⁻² d⁻¹ kPa⁻¹.^[9]

After the end of the Vietnam War, the U.S. military phased out Saran insoles in favor of Poron®, a microcellular urethane, for its jungle and combat boots.^[2] However, the British Army continues to use Saran insoles in its combat boots, primarily because of its insulating properties.^[10]

Trademark status

In some jurisdictions, the name Saran is a registered trademark of the Dow Chemical Company, while in others, it has lost trademark status and become a generic term for these polymers. In Japan, Dow's trademark rights in Saran Wrap were assigned to Asahi Kasei.

References

External links

• Saran Wrap - The History of PVDC (from About.com)