Plastic wrap

Background

Plastic wrap is a form of food packaging consisting of a thin film of flexible, transparent polymer that clings to itself and to food containers to form a tight seal. The plastic keeps the food fresh by protecting it from air and by preventing dry foods from absorbing moisture and wet foods from losing moisture. It also seals in odors to prevent them from spreading to other foods stored nearby.

Plastics are artificial polymers; that is, they consist of gigantic molecules formed by combining thousands of small molecules of the same kind into a long chain. These small molecules are known as monomers, and the process of combining them is known as polymerization. Natural polymers include such familiar substances as silk, rubber, and cotton.

The first plastic was made by the British chemist Alexander Parkes in 1862, who produced a substance he called Parkesine from cotton, nitric acid, sulfuric acid, castor oil, and camphor. Two years later in the United States John Wesley Hyatt improved this product and named it celluloid. Celluloid was a tremendous success and was used to make many different products, but it was highly flammable.

The first completely artificial polymer (unlike celluloid, which was a derivative of the natural polymer cellulose) was Bakelite, which was produced from phenol and formaldehyde by the Belgian chemist Leo Baekeland in 1908. Many other polymers were developed during the 20th century, including such important products as artificial rubber and artificial fibers such as nylon.

The first plastic used for wrapping was cellophane, another derivative of cellulose invented by the Swiss chemist Jacques Brandenberger in 1911. It had the advantage of being transparent, and was used for packaging as early as 1924. Cellophane was the most common form of plastic film made until 1963, when it was overtaken by polyethylene.

Polyethylene was discovered by accident by research workers at the British company Imperial Chemicals Industries in 1933, when they mixed benzene and ethylene at high temperature and pressure. Polyethylene was first used chiefly for electrical insulating material. It was first made into a film in 1945 by the Visking Corporation in the United States, and has grown in popularity ever since.

Polyvinyl chloride (PVC) was produced before World War II and was originally used as an inferior substitute for rubber, but films of this substance were not made in any quantity until the 1950s. PVC is used today in many different products such as pipes, flooring, electric cables, shoes, and clothing, as well as plastic wrap.

Polyvinylidene chloride (PVDC) film was developed by the Dow Chemical Company during World War II for military use. It offered a high degree of protection from moisture and resistance to oils, greases, and corrosive chemicals, so it was used to package sensitive equipment such as optical devices and aircraft engine components. In 1952 it was offered to the public under the familiar trade name Saran Wrap.

Raw Materials

Most household plastic wrap is made from polyethylene, PVC, or PVDC. These polymers are all derived from simple hydrocarbons such as methane or ethylene, which are produced from natural gas or petroleum. Polyethylene is made directly from ethylene. PVC is made from vinyl chloride, derived from ethylene, or from acetylene, derived from methane. PVDC is made from vinyl chloride and vinylidene chloride, a derivative of 1,1,2-trichloroethane, which in turn is derived from ethylene or acetylene.

Some plastic films, including cellophane, are derived from cellulose, which is obtained from wood pulp or from linters, tiny fibers that cling to cotton seeds after the longer cotton fibers have been removed by a cotton gin. At least one form of plastic film (Pliofilm, a trademark of Goodyear) is derived from rubber.

The Manufacturing
Process

Processing the raw materials

• The chemicals needed to synthesize polymers are usually obtained from petroleum. Crude oil is heated in a furnace to about 752°F (400°C). Vapors from the oil move into a fractionating column, a tall tower containing a series of chambers. The chambers are warmer at the bottom of the tower and cooler at the top. The various substances that make up petroleum rise through the chambers as gases until they reach the temperature at which they become liquids. Since each substance has a different boiling point, they liquefy in different chambers and can be separated and collected.
• Most polymers begin with very simple hydrocarbons that have low boiling points. These substances do not liquefy in the fractionating column, but instead remain in the form of gases which can be removed from the top of the tower. They may also be obtained from natural gas, which is mostly methane. Another source for these chemicals is naphtha, a mixture of liquid hydrocarbons, obtained from a fractionating column, which are heavier than gasoline but lighter than heavy oil. Naphtha is heated under pressure to break down the liquid hydrocarbons into smaller molecules, a process known as cracking. A catalyst is added to enable cracking to take place at a lower temperature and pressure than it would without it. The catalyst may be a natural or artificial clay (a mixture of alumina and silica or a zeolite (any of various minerals containing aluminum, silicon, oxygen, and other elements in combination with water.) Catalytic cracking usually takes place at a temperature of about 932°F (500°C) under a pressure of about 100 kilo-pascals. The cracked naphtha is then distilled in a manner similar to that in a fractionating column to separate its components.

Polymerization

• Polyethylene is polymerized from ethylene, which is obtained from cracking. Ethylene is heated in a pressure chamber to about 338°F (170°C) at a pressure of about 200,000 kilopascals in the presence of a small amount of oxygen. The oxygen breaks the ethylene down into free radicals, which combine with each other to form chains of polyethylene. About one percent of a nonreacting gas such as propane is added to prevent the chains from becoming too long.
• PVC is polymerized from vinyl chloride, which can be obtained either by mixing acetylene with hydrochloric acid or ethylene with chlorine. Ethylene is more commonly used because it is efficiently obtained from the cracking of naphtha. If acetylene is used it must first be synthesized by heating methane to about 2732°F (1500°C) or through various other chemical reactions. Vinyl chloride is mixed with water and agitated to form a suspension, much as oil and vinegar are mixed to form salad dressing. Various suspending agents such as starch and gelatin are added to keep the mixture from separating. The temperature of the suspension is raised to about 104°F (40°C) or 122°F (50°C) and an initiator, usually an organic peroxide, is added to start the reaction. The vinyl chloride molecules react with each other to form chains of PVC. The mixture is cooled and particles of PVC are separated from the water in a centrifuge and dried in an oven.
• PVDC is polymerized from a mixture of about 15% vinyl chloride and about 85% vinylidene chloride. To produce vinylidene chloride, first 1,1,2-trichloroethane is made by mixing acetylene, hydrochloric acid, and chlorine, or by mixing ethylene and chlorine. The 1,1,2-trichloroethane then reacts with calcium hydroxide or sodium hydroxide to produce vinylidene chloride. Polymerization of PVDC proceeds in much the same way as PVC.
• Polyethylene is naturally flexible, but PVC and PVDC must have plasticizers added or they will be hard and rigid. Various organic and inorganic esters can be used as plasticizers. Generally the liquid plasticizer is slowly sprayed into dry polymer powder and heated to about 302°F (150°C) to form a homogeneous mixture.

Making plastic wrap

• Plastic wrap is made by extrusion. In this process granules of plastic are heated until they melt at about 212°F (100°C) for polyethylene and about 392°F (200°C) for PVC and PVDC. The liquid is then forced through a die to form a tube of warm, stretchable plastic. At regular intervals compressed air is blown into the side of the moving tube to form large bubbles. This stretches the plastic to the desired thinness. The thin plastic cools rapidly, and the bubble is collapsed between metal rollers to form a film. The film is wound around a large metal roller to form a roll that may hold several kilometers of plastic wrap. The plastic film on these rolls is then unrolled, cut to the proper length (usually about 49 feet [15 m]) and width (about 1 foot [0.33 m]), and rerolled onto small cardboard tubes. (This rolling, unrolling, and rerolling tends to give the plastic wrap a slight negative charge of static electricity, that helps it cling.) The cardboard tubes of plastic wrap are placed in cardboard boxes that have a serrated edge at the opening so that the consumer can tear off the desired length. Some also have a sticky spot on the box to catch the edge of the plastic wrap so it doesn't stick to the tube. The boxes of plastic wrap are then stacked in cartons and shipped to retailers.

Quality Control

A variety of standard tests exist to ensure that plastic wrap is effective. The most important are tests for permeability, impact resistance, and tear strength.

Water vapor permeability is measured by filling a dish with calcium chloride, a highly water-absorbent substance. It is covered with a sample of plastic wrap and weighed. The dish is then placed in a chamber with a controlled temperature and humidity. After a measured amount of time the dish is weighed again. The increase in weight shows how much water vapor has passed through the plastic. This test can also be done by filling the dish with water instead of calcium chloride and measuring the decrease in weight to see how much water vapor has escaped. These tests are performed at 73°F (23°C) with a relative humidity of 50%, at 90°F (32°C) with a relative humidity of 50%, and at 100°F (38°C) with a relative humidity of 90%.

Gas permeability is measured by placing a sample of plastic wrap between two chambers. The upper chamber contains a pressure of 100 kilopascals, and the lower chamber contains a vacuum connected to a tube containing liquid mercury. As the air in the upper chamber passes through the plastic wrap it increases the pressure in the lower chamber and forces the level of mercury to drop. The change in the level reveals how much air has penetrated the plastic.

Impact resistance is measured by dropping weights of increasing size on test samples until half of them break, at which point the weight is recorded. It can also be measured by filling bags made from the plastic wrap that is being tested with sand and dropping them on a hard surface from increasing heights until they burst. The height at which this occurs is then recorded. Impact resistance is also measured by shooting a small steel ball propelled by pressurized air through a sheet of plastic wrap and measuring how much the plastic slows it down.

Tear strength consists of tear initiation strength (the force required to start a tear) and tear propagation strength (the force needed to continue a tear). To measure tear initiation strength a sample shaped like a shallow V is pulled between two jaws until it begins to tear. This unusual shape is selected to provide a 90 degree angle that provides a controlled starting point for the tear. Tear propagation strength is measured by pulling apart a sample containing a precut slit.

In general, PVDC is stronger and less permeable than polyethylene, which is less permeable than PVC.

Environmental Concerns

Since plastic wrap is difficult, if not impossible, to recycle and is rarely reused, it does contribute to waste. One consumer group, considering such factors as the energy and raw materials needed for manufacture, the wastes released during manufacturing and disposal, the ability to be recycled, and the typical amounts used, has rated plastic wrap as "Good." By comparison, reusable plastic containers were rated as "Excellent," plastic bags as "Very Good," aluminum foil and freezer bags as "Good," and freezer papers as "Poor." Another concern is the possibility that exposure to certain plasticizers in plastic wrap could be harmful. These chemicals are absorbed from plastic wrap into hot and fatty foods. Although they have never been shown to cause harm in humans, plasticizers have been proved to cause cancer when fed in large amounts to lab animals. PVC wrap can consist of as much as one-third plasticizers, PVDC wrap consists of about 10% plasticizers, and polyethylene wrap usually contains no plasticizers.

Where To Learn More

Book

Gait, A. J. and E. G. Hancock. Plastics and Synthetic Rubbers. Pergamon Press, 1970.

Periodical

"Keeping Food Fresh." Consumer Reports, March 1994, pp. 143-47.

[Article by: Rose Secrest]

The ability of this versatile food wrap to cling to both food and containers makes it superior for forming an airtight seal. There are many varieties of plastic wrap, some of which are thicker, cling better and have better moisture-vapor retention than others. Most plastic wraps are made of polyethylene, whose components are not absorbed by foods to any degree. The wrap that is considered to have the best cling and moisture retention is made of polyvinylidene chloride, another leading brand is made of polyvinyl chloride (PVC). For added flexibility, both require the addition of plasticizers that, if in direct extended contact with food, can be absorbed. However, the USDA has approved their use with food and, though little is known of the effects of human ingestion of plasticizers over a prolonged period of time, there is no current evidence that they are harmful. There is some concern, however, that wraps containing plasticizers can transfer their components to food during lengthy heating in a microwave oven.

The noun has one meaning:

Meaning #1: wrapping consisting of a very thin transparent sheet of plastic

This article is about a plastic layer used for short-term preservation of domestic foods. For a plastic wrap often used in bulk packaging and manufacturing, see Shrink wrap.

A roll of LDPE plastic wrap in a box. The lid of the box has a serrated edge for cutting the film.

Plastic wrap, cling film, or cling wrap, is a thin plastic film typically used for sealing food items in containers to keep them fresh over a longer period of time. 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 without adhesive or other devices. Common plastic wrap is roughly 0.01 mm thick.

Contents 1 Names 2 History 3 Materials used 4 References 5 See also 6 External links

Names

Plastic wrap is known as cling-film in the United Kingdom, and in the United States as cling wrap, or plastic wrap. The most commonly used term for plastic wrap in Australia is Glad wrap, whereas in the U.S. saran wrap and Stretch-Tite are the most commonly used terms. The term Glad wrap is not entirely unused in the United States. 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 and much of the American midwest, is the genericized trademark.

History

Plastic wrap's history is built on a lab error and serendipity. Plastic wrap was invented in 1953 by a scientist who was trying to make a hard plastic cover for his car; his experiment was completely unsuccessful but he then found the usefulness of plastic wrap which he happened to create instead. The original cling wrap material was Saran, the commercial name for polyvinylidene chloride (PVdCl). The material was given approval for direct dry food contact and for paperboard coating for contact with fatty and aqueous foods.

In Australia, the original cling wrap material was polyethylene with a "stickifier" in the form of edible gum. The product was introduced under the brand name "Glad" in 1966.

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. Indeed, it is problematic to achieve full polymerization of the material, which could contain remains of the vinyl chloride monomer. 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 effect of PVC, as the material is said 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 flavour loss.^{[citation needed]}

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 shaped dimples, which hold the adhesive away from the surface. While being handled, the wrap is unsticky, but when pressure is applied the dimples are flattened and the adhesive pushed against the contacting surface, sticking them together.^[4] The adhesive used is a special edible type similar to chewing gum.^[5]

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.^[6]

References

1. ^ GLAD WRAP[R] trade mark advic
2. ^ Endocrine Disrupting Chemicals in Plastic Wraps Letter From Consumers Union to FDA 5jun98
3. ^ Cling Wrap Explained
4. ^ http://patimg1.uspto.gov/.piw?Docid=06489022&idkey=NONE&Input=View+first+page Patent for Glad Press 'n' Seal
5. ^ GLAD - Plastic Wrap FAQs
6. ^ Re: Hi, I really need to know the way SARAN WRAP/CLING WRAP works, thank you

See also

• Aluminium foil
• Mummification (BDSM)
• Wax paper
• Polywrapping

External links

Wikimedia Commons has media related to: Cling film

• "Cling film - a revolution in the food industry", European Council for Plasticisers and Intermediates

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