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No bags, thanks!
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while heating diazomethane.
The environmental issues associated with plastic shopping bags have featured frequently in the news. Plastic shopping bags have a surprisingly significant impact for something which is seemingly innocuous.
As well as being an eyesore (next time you are outside, take a look around), plastic bags kill large numbers of animals each year. In fact, marine creatures can mistake plastic bags for jellyfish. This makes the plastic bag pollution in marine environments particularly dangerous, as birds, whales, seals and turtles ingest the bags and then die from intestinal blockages. Unfortunately, plastic bags are believed to be the most common man-made item seen by sailors at sea.
The biggest problem with the bags is that they do not readily break down in the environment, with estimates for the time it takes them to decompose ranging from 20 to 1,000 years. One of the disquieting facts stemming from this is that plastic bags can become serial killers.
Once an animal that ingests a plastic bag dies, it decays at a much faster rate than the bag. When the animal has decomposed, the bag is released back into the environment more or less intact, ready to be eaten by another misguided creature. The incredibly slow rate of decay of plastic bags also means that each bag we use compounds the problem, because the bags simply accumulate.
Plastic bags clog drains and waterways, threatening not only the natural environments, but also urban ones. In fact, plastic bags in drains were one of the reasons for the severe floods in Bangladesh in 1988 and 1998. As a result, in 2002 a ban was imposed on plastic bags over there.
Plastic bags are made from ethylene, a gas that is produced as a by-product of oil, gas and coal production. Ethylene is made into polymers (chains of ethylene molecules) called polyethylene. The ethene molecule (known almost universally by its non-IUPAC name ethylene), C2H4 is CH2 = CH2.
Polyethylene is created through the polymerization of ethene. It can be produced through radical polymerization, anionic polymerization and cationic polymerization. This is because ethene does not have any substituent groups which influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.
Polyethene is a plastic. It is a white, waxy polymer composed of only carbon and hydrogen. It burns well, does not conduct electricity and is insoluble in water and most organic solvents. Polyethene is a thermoplastic. Thermoplastics melt when heated and can thus be remodelled.
Polyethylene was first synthesized by the German chemist Hans von Pechmann, who prepared it by accident in 1898 while heating diazomethane. When his colleagues, Eugen Bamberger and Friedrich Tschirner, analysed the white, waxy substance he had created, they recognized that it contained long -CH2- chains and termed it polymethylene. The first industrially practical polyethylene synthesis was discovered (again by accident) by Eric Fawcett and Reginald Gibson at ICI Chemicals in 1933.
Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde, once again they produced a white waxy material. Subsequent landmarks in polyethylene synthesis have centred on the development of several types of catalysts (like ziegler and mettalocences) that promote ethylene polymerization at more mild temperatures and pressures.
Polyethylene (PE) is classified into several different categories based mostly on its mechanical properties. The mechanical properties of PE depend significantly on variables, such as the extent and type of branching, the crystal structure and the molecular weight:
1. UHMWPE (ultra high molecular weight PE);
2. HDPE (high-density PE);
3. LDPE (low-density PE);
4. LLDPE (linear low-density PE)
UHMWPE is a polyethylene with a molecular weight numbering in millions. The high molecular weight causes a very good packing of the chains into the crystal structure. This results in the production of a very tough material. UHMWPE is made through metallocene catalysis polymerization.
HDPE has little branching and thus, stronger intermolecular forces and tensile strength. The lack of branching is ensured by an appropriate choice of catalyst (for example, Ziegler-Natta catalysts) and reaction conditions.
LDPE has many more branches than HDPE, which means that the chains do pack into the crystal structure as well. It does not, therefore, have very strong intermolecular forces, as the instantaneous-dipole and induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. LLDPE is a substantially linear polymer, with significant numbers of short branches, commonly made by co-polymerization of ethylene with longer-chain olefins.
UHMWPE is used in high modulus fibers and in bulletproof vests. The most common household use of HDPE is in containers for milk, liquid laundry detergent, etc., while LDPE is used in plastic bags. LLDPE, on the other hand, is used primarily in flexible tubing.
Recently, a great deal of research has focused on the long-chain branched polyethylene. This is essentially HDPE, but has a small amount (perhaps 1 in 100 or 1000 branches per backbone carbon) of very long branches. These materials combine the strength of HDPE with the processability of LDPE.
Polyethene or polythene is made into pellets which are used by plastic manufacturers to produce a range of items, including plastic bags. Supermarket bags are made from high-density polyethylene (HDPE), while the thicker bags are made from low-density polyethylene (LDPE). Unlike HDPE, LDPE can not be recycled.
Recycling your plastic shopping bags is one of the most obvious courses of action. There should be central collection points for recycling. This is because HDPE bags can not be put out for collection with other household recyclables, and there is no separate curbside collection for them as the volume does not support the cost. Instead, bags must be taken to central recycling collection points, such as supermarkets, where there are special bins to collect the bags. Even at these central collection points, there is a risk that the bags may be unsuitable for recycling due to a range of contaminants, such as LDPE bags, ink, food, even supermarket dockets if they are left in the bags.
Prior to recycling, of course, the aim should be to reuse your bags. This list of possible uses for plastic shopping bags is almost as long as the lifespan of the bag itself. If you don’t want to take your bags back to the supermarket to use them again the next time you buy your groceries, there are many ways you can use them around the house. One thing they should not be used for is lining garbage bins. It doesn’t matter if you put them straight in your bin as waste or put your other garbage in them, the plastic bags will still end up in landfill, and potentially at large in the environment.
Given the costs and inconvenience associated with recycling, and the fact that reuse only delays the plastic from entering the environment, the most sensible option is to cut down on the number of plastic bags that you use, or stop using them altogether. It is estimated that it takes the average Pakistani family six shopping trips to accumulate 60 plastic shopping bags. If everyone accepted one less plastic bag every time they went shopping, the number of bags used would be reduced substantially.
There are a range of alternatives to plastic bags. Some retailers save the cardboard cartons in which stock is packaged, so customers can use them to pack their groceries. Others may offer paper bags. Some major supermarket chains have string or calico bags available for sale at a very small price. These bags can be kept in the car and used again and again. The advantage of calico bags is that they are stronger than plastic bags, and also much easier to carry. It takes a little effort to get used to bringing your own bags, but it is an easy habit to fall into and is such a relief not to have to pack the groceries away, and then find room to pack away the plastic bags as well.
There are, of course, situations where you can’t help but use a plastic shopping bag, such as when buying meat or other “messy” items. Thankfully, technology is catching up with the need for a replacement for polythene bags. It was recently reported that supermarkets will be able to introduce biodegradable bags made from tapioca starch. These bags will look and feel like polythene bags, but will decompose in three months.
So the next time you go shopping, hold your head up proudly as you reuse or refuse a plastic bag. You may not be in a rubber dinghy chasing a whaling boat or pursuing ivory poachers, but by refusing to use plastic bags, you can certainly make a contribution to conserving the environment.
Muhammad Jawwad Saif is working as a research scholar at the Institute of Chemistry, Punjab University, Lahore, and Hira Khalid is his research fellow
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