Plastic wrap
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Plastic wrap is a thin plastic film typically used for sealing food items in containers to keep them fresh. Plastic wrap, typically sold on rolls in boxes with a cutting edge, clings to many smooth surfaces and can thus remain tight over the opening of a container with no adhesive or other devices. Common plastic wrap is roughly 0.01 mm (0.0004") thick.
Plastic wrap is known as cling-film in the United Kingdom. Commonly known brands of plastic wrap in the United States include Saran wrap, Glad wrap and Stretch-Tite. In Australia and New Zealand, Glad wrap is the leading brand, known well enough to make its manufacturer concerned about its name becoming a genericized trademark.[1] In Canada, Saran wrap is the genericized trademark. In Hong Kong, a company named Fine Vantage Limited is the major private label LDPE plastic wrap manufacturer.
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[edit] History
As many great discoveries, plastic wrap's history is built on a lab error. Plastic wrap was invented in 1953.
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[edit] Characteristics
An ideal plastic wrap should meet the following user expectations. Some testing labs, such as SGS, can offer plastic wrap testing to certify it is suitable for food contact and has the right tensile strength.
- Easy to pull out of the box and easily separated by the cutter.
- Cling well with glass, ceramics and stainless steel, but will not cling heavily to itself.
- Close to total transparency with no haze, film wrinkles, tension marks, un-even thickness, or gel.
- High tensile strength and puncture resistant so that the film will not easily break apart. This is difficult to achieve, as plastic wrap is only 7 to 15 microns thick.
- Packaged in a food-grade recyclable plastic or paper box printed with soy ink. Historically, about 30% of imported food-related products are fined by the customs officer because of a failure to meet packaging requirements.
[edit] Materials used
Plastic wrap was first made from PVC, which remains the most common material, but non-PVC alternatives are now being sold because of concerns about the risk in transfer of plasticizers from PVC into food. It is also problematic to achieve full polymerization of the material, which could contain remains of vinyl chloride. For food catering applications, PVC is the most common. For household use, LDPE is gaining market share because it is purportedly safer.
More and more countries over the world are concerned about the environmental impact of PVC, as the material is told to be toxic and harder to recycle. Nevertheless, PVC is still used because its stretching properties offer excellent food catering presentation on the shelf, and it clings well to more kinds of surfaces. Even so, some countries are starting to ban the use of PVC in toys for infants and food contact applications.
The original Saran Wrap is made of polyvinylidene chloride (PVdC). In July, 2004 the name Saran Original was changed to Saran Premium and the formulation was changed to low density polyethylene (LDPE). SC Johnson claimed that this change was a result of an initiative to look for more sustainable and environmentally acceptable plastic. The film in Saran Premium Wrap does not contain chlorine. However, LDPE does not possess the same barrier qualities to oxygen, aroma, and flavor molecules that vinylidene chloride copolymers do, making the new formulation a lower quality plastic wrap, as it is not as useful in protecting from spoilage or flavor loss.
The PVC-based films contain plasticizers, most often bis(2-ethylhexyl) adipate (DEHA), but phthalates (most often dibutyl phthalate (DBP) and bis(2-ethylhexyl) phthalate (DEHP), in spite of being prohibited in most countries, also cause concern. The plasticizers were found to migrate to some foods, for example cheeses or fatty fish and meat. In the UK, polymerized plasticizers replaced DEHP in this application, largely eliminating the problem.[2]
A common alternative to PVC is low density polyethylene (LDPE), which is less clingy than PVC, but also does not contain traces of potentially toxic additives. Newer production processes are closing the clingy gap between PVC and LDPE. Linear low density polyethylene (LLDPE) is sometimes added to the material, as it increases the clinginess and the tensile strength of the film.[3] Brands like Glad Cling Wrap or Handi-Wrap are LDPE-based. Saran Premium Wrap, a newer version of Saran Wrap, is based on LDPE as well.
Glad Press'n Seal has its surface covered by microscopic spikes, preventing direct contact of the polymer surfaces, preventing it from sticking to other materials, including itself. To achieve the adhesion, the material has to be subjected to mechanical force, crushing the spikes otherwise people handling the material would feel small pricks.[citation needed]
PVdC has better barrier properties than the more-permeable LDPE, making the foods wrapped in it less subject to freezer burn. However, LDPE is substantially cheaper and easier to make.
Natural polymers of LDPE and PVdC are insufficiently clingy on their own, and they do not adhere to themselves. To achieve the desired clinginess, certain polymers with lower molecular weight have to be added; the most common two are polyisobutene (PIB), and poly[ethylene-vinylacetate] (EVA) copolymer. Their chains readily interact with each other and their lower molecular weight makes them more mobile within the host polymer matrix.[4]
