Plastic bag
From Wikipedia, the free encyclopedia
A Plastic bag is a bag made of thin, flexible, plastic sheet. Plastic bags are used for containing waste for disposal, and for storing and transporting non-waste.
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[edit] Composition
There are four main types of plastic bags in widespread use:
Polyethylene is an economical plastic so it is commonly used for disposable bags. Plastic bags can be made having various colours, including translucent or even transparent in some cases.
Bags made of HDPE (High Density Polyethylene) are typically translucent, but not fully transparent. Bags made out of LDPE (Low Density Polyethylene) can be quite clear, but are still usually not as totally transparent as other plastics.
HDPE and LDPE bags are manufactured from a waste product of the petroleum industry.
[edit] Shapes and sizes
Many plastic bags are shaped like two identically-sized rectangular plastic sheets fused together on three of the sides, with one side open. This type of shape allows for simple, economic manufacturing and compact storage of the bags before use. Sizes vary from a few centimeters on a side, for containing small items for sale such as buttons and small electronic components, to several meters on a side for large items.
Bags are also made with carrying handles, and in other shapes. Some bags have provision for resealable hermetic or non-hermetic closing; others are sealed, often by heating the open edge, once filled, and can only be opened by destroying the packaging.
The thickness of the material used for rubbish bags, for example, varies from about 0.02mm for a very light-duty bag to ??? (this paragraph on the thickness of the material used for plastic bags, is a stub which needs expanding)
[edit] Environmental issues
Plastic bags have advantages and disadvantages when compared to alternatives such as paper bags and cardboard boxes. The durability, strength, lesser energy requirements in manufacture and light weight are advantages of plastic bags. However, non-biodegradable bags fill landfill sites and make for long-lasting litter, which in particular is dangerous to wildlife away from centers of human population.
Many studies comparing plastic versus paper for shopping bags show that plastic bags have less net environmental effect than paper bags, requiring less energy to produce and recycle; however these studies also note that recycling rates for plastic are significantly lower than for paper[citation needed]. Paper is also made from a renewable resource (trees), whereas plastic is non-renewable (petroleum-based). Additives have been developed that allow plastic to degrade and biodegrade within a few months in landfill (as opposed to an estimated 500-1000 years for non-degradable plastic). Plastics made with these additives are called oxobiodegradable, and have been adopted by many 'ethical' retailers, e.g., the Co-op in the UK. However, some argue that oxo-biodegradable plastics contribute more to global warming as they release their carbon as carbon dioxide and methane far more quickly than plastics in landfill. In April 2002, the Marine Conservation Society reported that a dead Minke whale that had washed up on the coast of Normandy was found to have had 800 kg of plastic bags and packaging (including two U.K supermarket bags) inside its stomach.
At present the most widely implemented solution to these problems is to reduce the use of plastic bags — for example, a plastic bag tax was introduced in Ireland in 2002, after which plastic bag usage decreased by almost 90%. [1] The government levy on plastic bags was €0.15 as of 2006. Many retailers in Ireland switched to supplying (untaxed) paper bags, or simply stopped supplying bags. Most supermarkets continued to supply plastic bags, subject to the tax.
[edit] Biodegradability and recycling
Most plastic bags are discarded, often after a single use. Recycling of plastics after final use is possible, but plastic bags, in particular, are rarely recycled. According to the UK-based Ban the Bag campaigning group, 0.5% of plastic bags are recycled [2].
Standard plastic bags may take between 500 and 1000 years to decompose [3]. However, such figures are only ever estimates because plastics have not existed for long enough for the precise decomposition time to be measured [4].
According to the UK government department DEFRA, there are several problems with recycling plastics, and in particular plastic bags: [5]
- the high volume to weight ratio of plastic means that the collection and transport of this waste is difficult and expensive
- there are often high levels of contamination in plastic making the recyclate less useable, especially where food products are involved
- there is a very wide range of plastics in use and segregation is difficult
- the market for using recycled plastic is underdeveloped
Therefore, most of the focus in tackling the environmental problems associated with plastic bags is on reduction rather than recycling.
[edit] Oxo-biodegradable plastic bags
Oxo-biodegradable and other degradable plastic bags have certain useful applications when used as rubbsih bags. Organic waste can be put into oxo-biodegradable plastic sacks and put straight into the composting plant, unopened, thus reducing smells, disease transmission by insects, and handling hazards. The resulting compost may be used by farmers and growers. Since oxo-biodegradable plastic (unlike the starch-based alternative) releases its carbon slowly, it produces high quality compost. Oxo-biodegradable plastic does not degrade quickly in low temperature "windrow" composting, but it is suitable for "in-vessel" composting at the higher temperatures required by new animal by-products regulations. Oxo-biodegradable plastics become peroxidised and embrittled, and behave like natural waste. It is bio-assimilated by the same bacteria and fungi, which transform the degraded plastic products to cell biomass, like lignocellulosic materials. Oxo-biodegradable plastic is designed to fragment by a process which includes both photo-oxidation and thermo-oxidation, so it can degrade in the dark.
[edit] Bag types
[edit] Shopping bags
Open bags with carrying handles are used in large numbers worldwide. A common size for general shopping is about 35 x 40cm, but sizes range from a bag to hold a couple of greeting cards, to bags which can hold large appliances. Bags are often supplied without charge by retail outlets, and usually printed with the store's identity, to serve as mobile advertising. Shopping bags may be reused for their original purpose, although there is little incentive to do so as new ones are usually supplied without charge. Used shopping bags are used for a multitude of other purposes, from reuse as rubbish bags, to manufacture of craft items, but ultimately the overwhelming majority are discarded. Taxes on bags are being introduced and discussed in several countries; once there is a cost, albeit small, associated with bags, the number used drops drastically.
[edit] Point-of-sale packaging
Many products are packaged for sale in plastic bags which are usually sealed in some way. This packaging is usually discarded without any reuse or recycling.
[edit] Rubbish bags
Plastic bags are a convenient and sanitary way of handling rubbish, and are widely used. Plastic rubbish bags are fairly lightweight and are particularly useful for messy or wet rubbish, as is commonly the case with food waste, and are also useful for wrapping up rubbish to minimize odor. Plastic bags are often used for lining litter or waste containers or bins. This serves to keep the container sanitary by avoiding container contact with the rubbish. After the bag in the container is filled with litter, the bag can be pulled out by its edges, closed, and tied with minimal contact with the waste matter.
Plastic bags for rubbish or litter are sold in a number of sizes at many other stores in packets or rolls of a few tens of bags. Wire twist ties are sometimes supplied for closing the bag once full. In the mid-1990s rubbish bags with draw strings for closure were introduced. Some bags have handles which may be tied, or holes through which the neck of the bag can be pulled. Most commonly, the rather soft, flexible plastic used to make rubbish bags is LDPE (Low Density Polyethylene) or, for strength, LLDPE (Linear Low Density Polyethylene). HDPE (High Density Polyethylene) is sometimes used.
Rubbish bags are supplied in a range of sizes, from wastebasket size to lawn and leaf bags for garden waste. Some localities require the use of biodegradable plastic bags or large paper bags for disposal of garden waste.
