Accuracy:
I will perform the test three times on each plastic bag. This helps to eliminate any anomalous results caused by rips in the strip, or the experiment failing for another reason.
Fair Test:
To ensure that a fair test is carried out, I must:
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Use the same method for each strip of each plastic bag. This ensures that the experiment is reliable because it is repeatable. Deviating from the method for any part of it could vary the results to those which should be found.
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Use the same area of bag, 3cm x 20cm. This will ensure that the results are comparable to each other, and so conclusions can be drawn, and the variables are kept the same.
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Have the same person placing the masses on to the mass stand. This will help make sure that the masses are being added with the same speed and force each time, and so making sure that the chance of the plastic bag strip breaking is the same each time, therefore making the results comparable.
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Use the same apparatus throughout the experiment. This will make sure that the loop of string, for example is always the same, therefore making it a fair test.
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Have the clamp stand at the same height. Although this is not necessary, it could affect the overall results, if only by a small margin. This is because the gravity being exerted on the strip and masses will differ in accordance to height.
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Use the same balance and Vernia Caliper. This is, again not necessary, but will help to keep the test as fair as possible, and therefore as valid as possible.
Results:
Analysis:
From my results, I have found out that the thickness of the plastic bag does not directly correlate with the amount of mass it can hold. For example, one of the thinnest bags, Morrisons (0.01mm), can hold 1500g. This is the same as the Ryman’s bag (0.04mm) and 100g more than the two 0.02mm thick bags. So this proves that the thicker the bag doesn’t always mean that it can hold more mass.
However, the Virgin plastic bag was the thickest, at 0.08mm thick, and it also held the most mass of all the bags, (2700g.)This is almost double the mass of a bag of half the thickness, 0.04mm.
From the results, it is clear that the Morrisons bag must have stronger covalent bonds than the Aldi bag because it can hold more mass, although they are of equal thickness.
The second graph shows ascending thickness, with Morrisons being the thinnest at 0.01mm, and Virgin being the thickest at 0.08mm. This again proves that thicker bags don’t necessarily hold more mass. An example of this is the Argos bag; it is 0.02mm thick, but can hold 300g more mass than the Ryman bag, which is 0.04mm thick.
The reason the Morrisons bag can hold more mass than the Aldi bag is because the polymers are cross-linked, and so the polymer molecules cannot slide over each other as easily, making the bag harder to break. Equally, the Aldi bag (0.01mm thick) can hold more mass than the Disney bag (0.03mm thick.) This is because the Aldi bag will have more cross-links and a longer polymer chain than the Disney bag.
The load per gram column of the results table shows that the lighter the bag is doesn’t always mean that it can hold less mass per gram. For example, The Tesco bag (0.09g) holds more load per gram of bag proportionally than the Aldi bag, which has a mass of 0.2g. One of the reasons this is, is because of cross-linking, which means there are more covalent bonds between the polymers, which make the bag stronger.
Making Plastic Bags:
When fractional distillation occurs to separate crude oil, one of the fractions given off is Naptha (a hydrocarbon,) which is used to make useful chemicals. The hydrocarbon is then broken down in to smaller molecules using a catalyst, this is called Catalytic Cracking, and is an example of a thermal decomposition reaction. You are then left with ethene/ethylene, which is an alkane because all of the Carbon atoms are joined together by single bonds. This also makes it an unsaturated hydrocarbon.
Then addition polymerisation takes place, so that the monomers become polymers, which then add together to form chains.
Cross links then form between the polymer molecules to form strong covalent bonds. This polythene is then turned in to plastic pellets, which are then turned in to plastic bags.
Plasticisers are also added to the polymer. They push the polymer molecules slightly further apart, weakening the forces between them and making the material softer and more flexible. Polythene is not a thermo set polymer, it is a thermoplastic.
Thick plastic bags are made of low density polyethylene (LDPE). These are usually used for higher value items such as clothes. Thin bags are made of high density polyethylene (HDPE). Both types of bags are made from the by-products of fossil fuel production, so they come from non-renewable resources.
The reason why some thinner plastic bags are stronger than some thicker one is due to cross-linking. This is where the polymer chains are chemically joined together in places by covalent bonds. The polymer molecules cannot slide over each other so easily, meaning that the plastic bag cannot be so easily stretched, and so harder to break.
A polymer chain is made up of many polymer molecules that lay side by side. They can slide past each other and uncoil, meaning the material is flexible. Plastic bags with shorter polymer chains are not as strong as those with longer polymer chains, because the forces of attraction are weaker.
Validity:
My results are valid because I performed a fair test and I have drawn substantiated conclusions. I know I have done this because I have checked through my method and fair test list to make sure that I followed each step. A mistake in my method or not carrying out a fair test would mean that the results are not valid, and therefore my conclusions would not be substantiated.
Reliability:
My results are reliable because I repeated the test on each plastic bag to rule out any anomalous results. I also took the average mass taken for each bag.
Arguments For and Against Plastic Bags:
Against: Plastic bags have been popular for many years because they are quite cheap to produce, waterproof and durable. However, because they are so durable, it makes them unsuitable as disposable items. Because they are made of polythene, which comes from the raw material of crude oil, it is a non-renewable resource. They are not biodegradable and are almost indestructible – they take between 500 and 1000 years to break down. However, when they do eventually break down, it becomes "plastic dust" which contaminates animal life, the soil and the water, the oceans and the seas, forever.
Around a trillion plastic bags are used world wide every year (10 billion in the UK alone). They are environmentally unfriendly because they, fill up landfills, block drains, and litter the land and the oceans
Many plastic bags are not recycled; they are dumped in landfills and escape into the environment by wind or water action. In South Africa, plastic bag litter is so common it is called the 'national flower'.
Plastic bag litter has a devastating effect on animals, birds and marine life. They mistake them for food and die after ingesting plastic bags which have obstructed their digestive systems. Birds become trapped in plastic bags hanging off bushes and trees. Whales, seals, turtles and countless fish become tangled in them and then suffocate or starve to death.
Plant life on land is smothered and killed by plastic bag litter. In the oceans, not all plastic floats, lots of it sinks to the bottom where it smothers and kills marine plant life as well.
All of the chemicals used in the process of creating plastic bags are toxic, and cause damage to the environment and people. One of the key ingredients in manufacturing plastic bags is vinyl chloride, which is a proven carcinogenic and may also cause liver, kidney, and brain damage. This chemical has a high toxicity.
For: It takes more than four times as much energy to manufacture a paper bag as it does to manufacture a plastic bag.
In 1999, 14 million trees were cut to produce the 10 billion paper grocery bags used by Americans that year alone.
Paper sacks generate 70% more air and 50 times more water pollutants than plastic bags.
Plastic bags are also convenient, lightweight and easy to carry, and manufacturing them produces little waste.
They can be used for varied purposes, such as to wrap food, store extra things while travelling, and to hold wet or dirty clothes. They are also useful as rubbish bags during journeys. Strength, durability, versatility, and variety are some of the key advantages of plastic bags. They are also used to advertise shops, and so are cost affective.
Jute bags are an alternative to polythene bags, along with bags for life. This is a type of sustainable development, because they cause much less harm to the environment because they are reused, but also fulfil the needs of the current population.
In my opinion, plastic bags serve their purpose well, and are very useful. However, because we now know the damage they cause to the environment, such as plastic dust and high level of toxins, I think it is necessary to find alternatives, such as jute bags and bags for life. I also think that people should reuse their plastic bags as much as possible, instead of getting new ones every time they go to the shops. If we do not cut down on the use and disposal of plastic bags, plant life will die off, and so this will have great effects on human life too.
