Classify and identify different polymers to determine their physical properties and uses.

Science 2010
EEI Science Report

Aim
Classify and identify different polymers to determine their physical properties and uses.

Background
Polymers are long chain molecules that are made up of repeating smaller chemical units called monomers. The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part). Polymers are commonly made up of the chemicals carbon and oxygen however some polymers also include oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous, and sulfur. Polymers are made from monomers containing a double bond. A chemical reaction bonding monomers together to make a polymer is called polymerisation. A polymer can be formed in two ways. One formation is addition (a single product). This can be represented as A+A+A+A = A-A-A-A. Another formation is condensation. This occurs when a polymer and small molecule form, like water. There are both naturally occurring and synthetic polymers. Natural polymers include such things as tar and shellac. Synthetic polymers are produced commercially on a very large scale and have a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers. All materials used in this experiment are synthetic.

Polymers are common in our everyday life – in DNA, in proteins and starch, in the wheels on skateboards and the tyres on bikes and cars. In fact, polymers can be found every day, everywhere in the world.

Polymers can be classified into two different groups. These two groups are thermoplastic polymers or thermosetting polymers. Thermoplastic polymers, when warmed, can reshape easily. The molecules of these polymers are tangled together. When they are heated, they slide past each other easily. An example of a thermoplastic polymer is an ice-cream container.

Polymers that burn and char when heated are called thermosetting polymers. The molecules of these polymers are cross-linked so they are held together more rigidly. This means that the polymer chains cannot slide past each other when they are heated. Materials such as the soles of tennis shoes and foam cups are examples of thermosetting polymers.

Polymers are useful because chemists can change their structure by changing the length of the molecular chain, changing the chemical composition of the monomer units, or by changing the way the monomer units are arranged. The length of the polymer chains and the presence or absence of cross-links between chains determines the physical and chemical properties of polymers.

Polystyrene is used to make many types of containers such as videocassette cases and compact disks cases, tableware (forks, knives and spoons), and cafeteria trays. A foamed form of polystyrene is used to make heat resistant coffee cups, grocery store meat trays, and building insulation. To minimize fumes in the laboratory, this procedure uses a styrene casting resin. Unexpanded polystyrene is commonly used in thermal insulation systems (wall, roof and sub-floor), ceiling panels and other decorative surfaces, pipe insulation, protective packaging, flotation and buoyancy applications. Polystyrene is commonly used because of its ability to be a lightweight and relatively strong thermal insulator. In the four experiments two types of polystyrene are tested – unexpanded polystyrene and expanded polystyrene.

Nylon was the result of research directed by Wallace Hume Carothers at du Pont. The research team was interested in duplicating the characteristics of silk. Nylon gained rapid acceptance for its use in stockings and in making parachutes. The success of nylon products led to the invention of rayon, dacron, orion, and polyester. Nylon contains nitrogen and oxygen. It is a lightweight flexible material. When bound with other pieces of nylon the strength of it increases.

Polyethylene is a plastic with a range of uses including food packaging and gas pipes. There are two types of polyethylene – high density polyethylene or HDPE and low density polyethylene or LDPE. High-density polyethylene is a hard rigid plastic. A low-density grade is tough and flexible. The plastics can be injection molded or extruded. In our experiment we are using HDPE, which is the high-density form of polyethylene.

UPVC, Unplasticised Polyvinyl Chloride, is often used in the building industry as it is a low maintenance material. It is cheap and has ...

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UPVC, Unplasticised Polyvinyl Chloride, is often used in the building industry as it is a low maintenance material. It is cheap and has a good acid and alkali resistance. UPVC is flame-retardant and stiff and strong. It has good vapour barrier properties and UV resistance. One disadvantage of UPVC is that it has a difficult melting process as it is a thermosetting polymer. UPVC is commonly used for pipe and pipefittings and building products e.g. gutters, cladding and window frames. It has almost entirely replaced the use of cast iron for plumbing and drainage.

Perspex is often used as a light or shatter-resistant alternative to glass. It is an economical alternative to polycarbonate (PC) when extreme strength is not necessary. It is often preferred because of its moderate properties, easy handling and processing, and low cost, but behaves in a brittle manner when loaded, especially under an impact force, and is prone to scratching compared to glass. The molecular formula of perspex is (528)n. Its melting point is 160 °C and its density is 1.19 g/cm3.

Hypothesis
A similitude between the properties and the polymer’s everyday uses can be established through various experiments testing the polymer’s quality of either thermosetting or thermoplastics, density, acid resistance and rigidity.

Apparatus

Experiment 1:

Polymers

UPVC
Expanded polystyrene
Unexpanded polystyrene
Nylon
HDPE
Perspex

Matches
Bunsen burner
Heatproof mat
Tongs
Stop watch
Lab coat
Safety glasses

Experiment 2:

Polymers

UPVC
Expanded polystyrene
Unexpanded polystyrene
Nylon
HDPE
Perspex

250mL beaker
200mL water
Lab coat

Experiment 3:

Polymers

UPVC
Expanded polystyrene
Unexpanded polystyrene
Nylon
HDPE
Perspex

Heatproof mat
Hydrochloric acid 0.1M
Lab coat
Safety glasses

Experiment 4:

Polymers

UPVC
Expanded polystyrene
Unexpanded polystyrene
Nylon
HDPE
Perspex

Ruler
Sticky Tape
50g weights
Rod for weights
Screwdriver
Desk
Lab coat

Method

Experiment 1 – Thermoplastic and Thermosetting Test:

1. Set up the bunsen burner and heat proof mat.

2. Turn on the bunsen burner. Make sure the collar is closed so the safety flame is burning.

3. Adjust the collar so the working flame is now burning. Use the tongs to lift one polymer at a time into the flame.

4. Burn each polymer separately until the experiment is completed. Some polymers have a strong, sickening smell, so it is advised that the windows be open.

5. Record the results (ie. whether it burns, melts or chars).

Experiment 2 – Testing Density:

1. Fill one beaker with 200mL of water.

2. Put each polymer in the water separately.

3. Record the data (ie. whether it floats or sinks).

4. From the results collected, work out the densities.

Experiment 3 – Acid Resistance Test:

1. Place each polymer on the heatproof mat.

2. Put 3 drops of hydrochloric acid 0.1M on each polymer. If the acid touches your skin, wash it off immediately.

3. Let the polymers sit for 2-3 minutes to see the result.

4. Record the results. Once the testing is complete is it important to wash the heatproof mat to ensure that all traces of acid have been erased.

Experiment 4 – Rigidity Test:

1. Tape the ruler to the desk. It is important to ensure that the ruler is completely straight and vertical.

2. Make a hole at the end of each polymer using a screwdriver. The hole must be positioned in the same place on all the polymers. The hole only has to be big enough to fit a medium size hook through it.

3. On the first measurement, connect the 50 g rod to the polymer.

4. Hold each polymer on the desk and measure the distance each one bent. It is important to try and make each polymer the same length hanging from the bench.

5. For each subsequent measurement add a 50g weight to the rod. Do not take the rod off the polymer as if the pressure is released the polymer will elasticise and the experiment will not be accurate. Continue adding weights until the polymer snaps. Stop when you reach the 2kg weight.

6. Record the results.

Results

Experiment 1 –Thermoplastic and Thermosetting Test

Experiment 2 – Testing Density

Experiment 3 – Acid Resistance

Experiment 4 – Rigidity Test

Discussion

These four experiments have revealed the polymers properties which determine the particular use of the polymer.

In Experiment 1, testing polymers’ reactions when placed into a burning flame, some polymers reacted to a greater extent than originally expected, however most of them reacted in a way that was not unusual. For example, expanded polystyrene was expected to burn as it is not a very strong, versatile material, but not expected to disintegrate into a black dust. This reaction, of the polymer disintegrating into black particles, also occurred in the unexpanded polystyrene. Similarly, the UPVC burnt instantly with a pungent, strong odour. It expanded and crumbled into a black ash-like texture. These results occurred as these materials are thermosetting polymers, therefore, they are cross-linked and are held together more rigidly. This means the polymer chains cannot slide past each other when they are heated so they just burn and char. As well as this, it is obvious that these polymers would not be used for high temperature purposes, as they burn easily and almost instantly. It was obvious that the nylon and perspex were both thermosetting polymers, however, both burnt in completely different ways. The perspex, which is reasonably thick, was only charred, however the nylon, a rather scrawny material, disintegrated within 15 seconds. Once again, these thermosetting polymers burnt and charred because of their molecular make-up. HDPE was a thermoplastic material, hardening after about 1 minute. These materials could be used for piping due to their easy application to be joined and sealed together. Unlike thermosetting polymers, thermoplastic polymers have molecules that are tangled together and are not cross-linked. When they are heated, they slide past each other easily, allowing the polymer to melt and re-set. This experiment proved that not all the polymers tested belonged to the same group, therefore, showing that the materials could not be used for the same uses as some are thermoplastic and others thermosetting polymers.

In Experiment 2, testing the densities of our polymers, the results were fairly predictable. Nylon, which had a density of 0.087g/cm3, HDPE, which had a density of 1.106g/cm3, expanded polystyrene which had a density of 0.073g/cm3, unexpanded polystyrene which had a density of 0.362g/cm3 and UPVC which had a density of 0.602g/cm3, all floated. It was obvious that the polymers would float because the materials had a density less than 1g/cm3, which is less than water. An error occurred in the measurement of the HDPE, saying that it had a density of 1.106g/cm3, when its density is actually 0.941g/cm3. This error could be due to inaccurate measurement and calculation of its density. The polymers that floated could be used in life rafts, surfboards and eskies, as they are less dense than water. However some unexpected results occurred when the unexpanded polystyrene, which has a density of 0.362g/cm3 , floated and sank. This unusual outcome was a result of the way it was placed into the water. When the unexpanded polystyrene was put on the water horizontally, it floated. However, when it was inserted vertically, it sank. The perspex with a density of 1.696g/cm3, not surprisingly, sank. This was because the perspex had a greater density level than 1g/cm3. After the results for this experiment were collected, the densities were calculated. Some materials, including the nylon, were difficult to measure with all the voids in the weave making it stretch and give varying dimensions. This inaccuracy affected the measurement of density for this material. It was evident through this experiment that most polymers floated and had a density less than 1g/cm3. The ones that did not float could not be used for the same uses.

Experiment 3, which measured the acid resistance of the polymers, showed that all the materials were resistant to the Hydrochloric Acid 0.1M. This acid had a pH level of 1. This particular hydrochloric acid was more acidic than lemon juice. It is clear from the results of this experiment that all these polymers can be used in conjunction with transporting hydrochloric acid as they are resistant to it. In order to further extend our results, experiments using more powerful acids could be performed. This would have further extended knowledge on the subject as it would have been possible to establish what acids were not resistant to the particular polymers and therefore determine the polymer’s appropriate uses. One problem that was encountered during this experiment was that some of the acid slid off the materials, resulting in an unfair test. In order to fix this, the materials were washed, and the experiment re-started. This experiment determined that all these polymers could be used with Hydrochloric Acid 0.1M.

Experiment 4 measured the flexibility and rigidity of the polymers. In this experiment it was clear that accuracy was a problem. It was very hard to make sure that the polymers were an equal length and width. As well as that, it was hard to hold the polymer with the weights on the desk. This meant that sometimes, the polymers were not an equal length when hanging from the desk. Another problem encountered was that sometimes, the rod had to be taken off the plastic in order to add another weight. This meant that the plastic elasticised and its length decreased. In order for the test to be fair, the affected polymers had to be re-measured. The focus was on fixing these problems and making the experiment as accurate as possible. The nylon had an obvious result, stretching rapidly, until it eventually broke at 500g. This result shows that nylon is not very strong and would not be an appropriate material for heavy-duty uses such as drainpipes. The HDPE increased steadily until it reached 300g where it stayed stretched at 14.2cm it reached 500g. When the 2kg weight was added, the HDPE still did not break. It was evident that the HDPE had elasticised, as at 500g it was bent 14.2cm and when the 500g was replaced by the 2kg weight, it was only bent 9.5cm. The expanded polystyrene was bent at 14.3cm until it broke under pressure at 200g. It was clear that the expanded polystyrene is not a suitable material to be used for purposes that need strong and sturdy equipment. The unexpanded polystyrene only split when the 2kg weight was attached to the material. The material exhibited signs of stress at the point where the mass hung. There was a possible change in the material’s structure as it became white. The UPVC pipe didn’t break or bend. The pipe was heavy duty due to the roundness that distributed the mass. The same observations were seen with the Perspex – 60+ kilograms would not have bent it. Both the UPVC pipe and Perspex were stronger as they have more cross-links. The UPVC pipe and Perspex could be used for purposes such as drainpipes and a strong, sturdy alternative to glass, respectively. Experiment 4 was successful in determining that all the polymers have different rigidity levels, and consequently, cannot be used for the same purposes.

From the results and discussion above, it is clear that the polymers differing properties determined what they could be used for and although there is some similitude (for example in experiment 3) between the properties and the uses of polymers, each of the polymers tested were better suited to a particular use. The various tests showed the polymer’s weaknesses, strengths and differences. Most of the polymers have different uses due to their range of diverse properties. For example, the HDPE was a thermoplastic material therefore useful for moulding pipes together to make them stronger. This is not possible with the thermosetting polymers as they are much more difficult to melt. Also, a wide range of differences were established within experiment 4 and the polymer’s rigidity. It was evident that many of the polymers had very different rigidity levels, consequently, differing their everyday uses. All polymers had the same resistance level to the Hydrochloric Acid 0.1M, meaning they could all be used for similar purposes involving Hydrochloric Acid 0.1M. Experiment 2 also showed different properties within the polymers – some floated and others sank. Through these results, it was proven that polymers are used for different purposes according to their varying range of properties.

Conclusion

It was established that the differing properties of various polymers determine the purposes for which they can be used. The relevant properties include differences between thermoplastic polymers, which disintegrate with heat compared to thermosetting polymers which melted and re-set with heat; differing densities of polymers but which were generally all less than 1g/cm3 meaning they float in water; all polymers were acid resistant; and most polymers demonstrated elasticity and flexibility.

Prudence Burnett pg