Beryl, who was an A2 student, just got an internship in a plastic manufacturing factory during the spring holiday.
In the team she stayed, the designers were asked to design a new type of plastic shopping bags which was used in supermarkets.
The workers said that the bags should have the ability to carry a heavy load, as well as being light as possible, and inexpensive.
One day, one of the designers found a wonderful plastic material and he thought it would be perfect to meet the requirements.
However, the problem was that he had no idea of how to test it and prove his thoughts.
Since Beryl had learnt some relevant knowledge, she decided to help him…
Some ideas come up…
Ability to carry a heavy loads?
In order to test this, Young’s Modulus should be used:
So a laboratory experiment to investigate how the extension of the sample of plastic varies with the tensile load is needed.
Equipments and materials required:
A top-pan balance;
A pair of scissors;
Two plastic binder clips;
A. Cut out the plastic sample with a pair of scissors.
B. They should be cut with parallel sides into several strips in the same length
C. On each end of the strip, use a plastic binder clip to hold it.(Before this,mark 1cm intervals on the strip)
D. To hang the strip, it must be vertical and attach a known weight and measure how much it stretches.
You should measure the extension of the strip after being loaded.
N.B: the width and thickness of the strip should be measured as well which could be done with a ruler and a micrometer,respectively.
Stress/ Pa Strain Cross-sectional area/m 2 Weight/N Thickness of cross section/m Extension of the strip/m Mass of the loads/kg
Weight=mass of the load x g
Cross-sectional area= thickness x width
Stress=load weight/cross-sectional area
Having obtained the figures, what to do next is to draw a stress-strain graph for the sample.
The graph is expected to be a curve with constant gradient initially but the rate gradually decreases and finally becomes almost zero.
The YM could be obtained from the gradient of the linear part of the graph.
During the experiment, you should be careful about the loads hung to the binder clip. Either it falls on the floor or on your feet is not a good news. It would be better and safer if you prepare a big cushion under the load.
Ideally the properties of the perfect plastic shopping bags should be:
a. the plastic material is able to resist tearing;
b. it can be stretched with large loads;
c. it needs to be biodegradeable to avoid damaging the environment.
Some facts of plastic shopping bags
Plastic shopping bags , or carrier bags or plastic grocery bags, are a common type of shopping bag in several countries. Most often these bags are intended for a single use to carry items from a store to a home: reuse for storage or trash is common. Heavier duty plastic shopping bags are suitable for multiple uses as shopping or storage bags.
Plastic shopping bags are usually made of polyethylene. This can be low-density, resin identification code 4, or most often high-density, resin identification code 2.
Although not in use today, plastic shopping bags could be made from Polylactic acid (PLA) a biodegradable polymer derived from lactic acid.This is one form of vegetable-based bioplastic. This material biodegrades quickly under composting conditions and does not leave toxic residue. However, bioplastic can have its own environmental impacts, depending on the way it is produced. Recyclability of this experimental material is unproven: resin identification code 7 is applicable.
Bags made of biodegradable polythene film, which decompose when exposed to sun, air, and moisture, and are also suited for composting have been proposed as an alternative to conventional plastic shopping bags. However, they do not readily decompose in a sealed landfill and represent a possible contaminant to plastic recycling operations.
Compared to paper bags
The United States Environmental Protection Agency says paper bags are worse for the environment than plastic. Plastic bags require much less energy to manufacture, ship and recycle and because of this plastic generates less greenhouse gas. Plastic bags cause less air and water pollution. They compress to a small size in landfills.
Plastic bags are durable, strong, low cost, and water and chemicals resistant.
They can be welded.
The light weight of plastic bags results in fewer atmosphere emissions compared to paper bags if both are land-filled.
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, transport and recycle; however these studies also note that recycling rates for plastic are significantly lower than for paper.
Plastic bags can be incinerated in appropriate facilities for waste-to-energy.
Plastic carrier bags can be reused as trash bags or bin bags.
Plastic bags are complimentary in many locations (but are charged or "taxed" in others).
Plastic bags are made from ethylene, a by-product of natural gas or from naphtha, a by-product of petroleum. Reserves of gas and oil are limited and using them to make bags means they may compete with other end uses.
Plastic bags are flimsy and often do not stand up as well as paper or cloth for certain tasks.
When disposed of improperly, they are unsightly and represent a hazard to wildlife. See environmental impacts section below.
Plastic bags, conventional or "biodegradable", do not readily biodegrade in a sanitary landfill.
Plastic bags (particularly thin dry cleaning bags) can cause unsupervised infants to suffocate.
Beginning on June 1 2008, China, prohibited the use of ultra-thin plastic bags and required retailers to charge for plastic bags. Stores must clearly mark the price of plastic shopping bags and are banned from tacking that price onto products. The production, sale and use of ultra-thin plastic bags - those less than 0.025 millimeters, or 0.00098 inches, thick - are also banned. The State Council calls for "a return to cloth bags and shopping baskets.In this environment, there is a Chinese environmental protection bag manufacturers, they have their own brand: the source of one mind, their company, based in Beijing, the capital of China.