Background information
A plastic is made up principally of a binder together with plasticizers, fillers, pigments, and other additives. The binder gives a plastic its main characteristics and usually its name. Thus, polyvinyl chloride is both the name of a binder and the name of a plastic into which it is made. Binders may be natural materials, e.g., cellulose derivatives, casein, or milk protein, but are more commonly synthetic resins. In either case, the binder materials consist of very long chainlike molecules called polymers. Cellulose derivatives are made from cellulose, a naturally occurring polymer; casein is also a naturally occurring polymer. Synthetic resins are polymerized, or built up, from small simple molecules called monomers. Plasticizers are added to a binder to increase flexibility and toughness. Fillers are added to improve particular properties, e.g., hardness or resistance to shock. Pigments are used to impart various colours. Virtually any desired colour or shape and many combinations of the properties of hardness, durability, elasticity, and resistance to heat, cold, and acid can be obtained in a plastic.
There are two basic types of plastic: thermosetting, which cannot be re-softened after being subjected to heat and pressure; and thermoplastic, which can be repeatedly softened and remoulded by heat and pressure. When heat and pressure are applied to a thermoplastic binder, the chainlike polymers slide past each other, giving the material “plasticity.” However, when heat and pressure are initially applied to a thermosetting binder, the molecular chains become cross-linked, thus preventing any slippage if heat and pressure are reapplied.
This is a typical Young modulus for some polymers:
Tables of result
Sainsbury
D= 0.03mm W= 15.0 mm L= 200.0 mm
Tesco
D= 0.02mm W= 15.0 mm L= 200.0 mm
ASDA
D= 0.02mm W= 15.0 mm L= 200.0 mm
Errors
There is an estimated error on each reading:
Ruler: 0.5 mm
Micrometer: 0.005 mm
Weights: 50 g
Evaluation
Finding the Young’s modulus by using this technique could not be accurate enough, as there are many significant errors during the experiments. During the experiments I used 100 g masses, which definitely cause a big amount of errors in my results. It would be more accurate to use smaller masses, however using smaller masses takes more time and need better equipments to measure the extension.
Obviously, by using new improved instruments we can reduce the amount of errors. For instance, we can use digital arms linked to a computer to put pressure on plastic bag and at the same time measuring the extension by the same instrument.
On the other hand, each part of the plastic has a different concentration of polymer, so that lead to another inaccuracy. I would suggest testing the plastic structure before using it to improve our results. Another problem was cutting 9 strips of plastic with the same length and width, which was quite difficult and that is another source of error and as I have explained before we should use better instrument to reduce the amount of errors.
From the graph it is clear that I did not have any malicious results.
The percentage of error in this experiment can be calculated as bellow:
Extension: 0.5 / 18 * 100 = 2.7%
Length: 0.5 / 200 * 100 = 0.25 %
Width: 0.5 / 15 * 100 = 3.33 %
Thickness: 0.005 / 0.03 * 100 = 16.6 %
Analysis
The young’s Modulus can be calculated by finding each graph gradient and multiply it to its initial length and divide that by the area of the strip:
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Sainsbury’s: 6.86N / 17mm x 200mm / 0.45mm = 179.35 N.mm-1
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TESCO: 5.88N / 12mm x 200mm / 0.3mm = 326.67 N.mm-1
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ASDA: 5.88N / 10mm x 200mm / 0.3mm = 391.96 N.mm-1
Final results show that ASDA bag had the biggest Young’s modulus of 391.96N.mm-1 closely followed by Tesco bag with 326.26 N.mm-1 and Sainsbury’s had the lowest amount, 179.35 N.mm-1. As a result, Asda bag is better than Tesco’s and both better than Sainsbury’s plastic bags.
This can be explained easily by the amount of Young’s modulus for each bag. The bigger the Young’s modulus is, the stronger the plastic bag is.
At the molecular level what is happening is the intermolecular forces keep all molecules together and when a force try to separate them, intermolecular forces won’t allow it as long as the force does not exceed the maximum level of the intermolecular forces. Once that happened, plastic bag will breaks.
Companies are trying to improve the strength of the bags by changing the polymer, thickening them, or they put new elements in the polymer and many other things to improve the strength of their product.
Bibliography
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Physics by Ken Dobson, David Grace, David Lovett
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New Understanding Physics for Advanced Level 4thEd by Jim Breithaupt
- http:// www.scienceworld.wolfram.com/
- http://en.wikipedia.org/wiki/Young's_modulus
