Plastic bag is a generic name for a flexible, thin-walled container made from plastic and used for carrying products or wastes.
Types
There are four main types of plastic bags in widespread use:
- High Density Poly Ethylene (HDPE)
- Low Density Poly Ethylene (LDPE)
- degradable
- compostable
Manufacture
Most this web site is a piece of crap plastic bags are shaped like two identically-sized rectangular plastic sheets fused together on three of the sides, with one side left for the opening. This type of shape allows for simple, economic manufacturing and compact storage of the bags before 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.
Environmental Issues and concerns
Plastic bags have both positive and negative environmental impacts. Compared to other disposable alternatives (such as paper bags and cardboard boxes), the durability, strength, low energy intensity and light weight make plastic bags preferred. However, their longevity in landfills and their propensity to be easily littered (owing to their light weight), cause them to be targeted as an environmental problem.
Degradable bags
The most useful and economic of the new technologies produces plastic which has the same strength and other qualities as conventional plastic, but degrades by a process of oxidation, initiated by putting a small amount of additive into the conventional plastic. There is little or no additional cost and there is no need to re-equip factories nor re-train the workforce. Oxo-biodegradable plastic can be safely used for direct contact with food. Unlike PVC, the polymers from which oxo-biodegradable plastics are made do not contain organo-chlorine, nor do they emit methane nor nitrous oxide when they degrade.
The plastic is consumed by bacteria and fungi after the additive has reduced the molecular structure, and it is therefore biodegradable. This process continues until the material has biodegraded to nothing more than CO2, water, and humus, and the time taken for oxo-biodegradable plastic to degrade can be ‘programmed’ at manufacture to be as little as a few months or as much as a few years. Degradation starts immediately after exposure to air, but products can be vacuum-packed for delivery to suspend degradation until needed for use.
Hydro-biodegradable or “starch-based” plastics made from crops are not a preferred alternative. They are much too expensive, they are not strong enough, and cannot be recycled in a normal plastic waste stream. They will not readily degrade unless in a highly microbial environment such as a compost heap, so are not much use in the fight against litter. Nor are they really “renewable” because hydrocarbons are burned by autoclaves used to ferment and polymerise the material and by the agricultural machinery and road vehicles employed; also by the manufacture and transport of fertilisers and pesticides. Further, hydro-biodegradable plastic can emit both methane and carbon dioxide (which are both greenhouse gases) when it degrades. Methane is 23 times more potent for global warming than CO2.
Policymakers need always to consider what happens to waste plastic products which escape collection, as it is impossible for industry and government to ensure that they are all collected - and even if collected to ensure that they are all recycled. They should all therefore be made from oxo-biodegradable plastic, except for very long-life items. If collected they can be recycled, composted, or incinerated with energy recovery, but if not collected they will degrade and disappear, leaving no harmful residues.
Biodegradable Bags
Biodegradable bags made from PLA or other Biobased products like Bio-Batch made plastic are a great alternative to conventional plastic bags. PLA based plastic is compostable/biodegradable with a short shelf life, but is served in many applications today. Bio-Batch made plastics are 99% petroleum based and 1% additive based, says their manufacturer Bio-Tec Environmental, with an indefinite shelf life till placed in a landfill and biodegrades anerobically and aerobically. Both are healthy alternatives to traditional plastic bags for the environment.
Rubbish bags
Although there was a time when plastic bags were not available and people made do, they are a more convenient and more sanitary way of handling rubbish and are widely used today. Plastic rubbish bags are fairly lightweight and are particularly useful for messy or wet rubbish as is commonly the case with food leavings and 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 conveniently pulled out by its edges, closed, and tied.
Plastic bags for rubbish or litter are normally sold in a number of sizes at grocery and many other stores in packets of almost a dozen to several dozen. The size and thickness of the bags are often specified on the packet, as well as the number of bags. The simple rectagularly-shaped bags come with twist ties in the packet to be used for closure once the bag is filled. Perhaps ten years ago or so, rubbish bags with draw strings have been introduced. The draw strings can be conveniently pulled to close up the bag after it is filled with litter. 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) has been used in some cases which gives a more crinkly (less soft) texture or feel.
The largest size plastic bags commonly available to the general consumer are lawn and leaf bags for yard waste. Unless the plastic bags are biodegradable, some localities require special large paper bags for disposal of yardwaste. The next largest size commonly available to the general consumer is a rubbish can size made to be suitable lining rubbish containers. This size is very commonly used for disposal of rubbish. Other large sizes are often used in business and institutions. Tall kitchen rubbish size is another somewhat smaller size routinely available to consumers for lining kitchen rubbish containers. Then there are even smaller sizes available for lining wastebaskets.
Increasingly, people are switching to oxo-biodegradable rubbish sacks and bin-liners. Organic waste can be put into oxo-biodegradable plastic sacks in homes, restaurants, hospitals, etc. and put straight into the composting plant, so smells, disease transmission by flies, and handling hazards to humans are effectively minimised. The bags do not need to be opened and disposed of separately. Oxo-biodegradable plastic is particularly useful for “back-of-store” use in supermarkets, as waste bread and other products wrapped in oxo-biodegradable plastic packaging can be put into oxo-biodegradable sacks and put straight into a composting plant.
Oxo-biodegradable/compostable bags can be safely assimilated into the green waste stream, and do not need separate collection. The resulting compost is a valuable resource for farmers and growers, and 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 ideal for “in-vessel” composting at the higher temperatures required by new animal by-products regulations.
It is not of course acceptable to apply conventional plastics to the soil even if they are fragmented, since physical shredding does not transform plastic into a biodegradable product. However, the properties of peroxidised and embrittled oxo-biodegradable plastic are quite different from those of the original plastic. The transformed plastic behaves in the same way as nature’s wastes. It is bio-assimilated by the same bacteria and fungi, and they 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 will work even in the absence of light.
Other uses
Economical disposable plastic shopping bags for bagging purchases at grocery stores or supermarkets are usually made of HDPE.
Smaller plastic bags are commercially available for containing and storing food items and can be used for holding other items as well. A company that started making them years ago called them baggies and since then this name has become commonly used generically for such small bags. These kinds of bags are commonly made of LDPE. Another brand name that has been used in connection with such food bags is Ziploc which refers to the zipper-like sealing mechanism on one side (edge) of the bag. Such simple sealing mechanisms made in one piece with the plastic of the bag are common on food plastic bags. There is generally no need to make rubbish bags see-through, but other types of storage bags, such as for food, are commonly clear so the user can easily identify what is inside. Simple, economical, soft, thin-walled, transparent plastic bags without sealing mechanisms are usually available without charge at grocery stores for bagging produce such as fruits and vegetables. Twist ties or tying the opening end of the bag in a knot can close up such bags.
Since they are economical and convenient, plastic bags of various sizes are also widely used for commercial packaging applications. Small, usually clear, plastics bags are used for containing small items or goods, especially small parts which would otherwise be easily lost. Often such bags are sealed, stapled, or taped shut until they are torn or cut open by the consumer. Plastic linings similar to plastic bags are commonly used inside cardboard boxes or other harder containers, especially for holding powder or granular material.
All these products can and should now be made from oxo-biodegradable plastic. As noted above, they can, if collected, be recycled, composted, or incinerated with energy recovery, but if not collected they will degrade and disappear, leaving no harmful residues.
See also
External links