27.5: Polymers

Polymers are very widely used in many areas today. They have a range of properties that can often be controlled by additives, blending or copolymerisation. Many structures and chemical compositions are seen in polymers, but they can be separated into three main groups:

Thermoplastics: These are the most widely used polymers due to the ease of processing (especially for injection molding). Thermoplastics can, once they have been set (solidified) for the first time, be re-melted and remoulded (unlike thermosets). Some examples of thermoplastics are: polyethylene, polystyrene and PET.

Thermosets: These differ from thermoplastics in that they do not re-melt after they have been set (or cured). This is due to the long polymer chains forming cross links on curing. One example is melamine formaldehyde, which is used in domestic electrical plugs.

Elastomers: These polymers have a glass transition temperature below room temperature (see The Glass Transition in Polymers TLP). Rubbers are examples of commonly used elastomers (for more information see the Stiffness of Rubber TLP)

Techniques for identifying polymers

Polymer tests

The polymer tests are a simple way to identify polymers, or at least to narrow down the possibilities. Some of the steps rely on recognising smells, which can be difficult, and it is important to remember that some tests (for example transparency) can be unhelpful due to additives like dyes.

The polymer identification chart goes through a series of simple tests, which should be carried out on a small sample of the polymer. Below is an interactive version of the identification chart. It is important to connect the results of the test to the function and cost of the item.

Infra Red (IR)

In an IR spectrometer IR radiation excites covalent bonds, causing them to vibrate at their resonant frequency. This frequency depends on the exact nature of the bonds (e.g. single/double and the atomic masses of the elements involved). The output is a graph of intensities at different wavelengths (and therefore energies) of infrared radiation. This plot shows the transmitted intensities, so at resonant frequencies, where the energy is absorbed, there is a peak. This allows the bonds to be identified and therefore the sample identified.

Here is a collection of IR spectra for some common polymers:

IR spectrometry is often a very quick method of polymer identification (depending on the equipment available). Preparing a sample for IR spectroscopy may be very simple. A small sample of the polymer with any coatings removed should be placed in the IR machine, and analysed. If it is likely that the plastic contains plasticizers and colours, placing the polymer in ether for an hour and then fully drying it may remove them prior to carrying out IR spectroscopy on the sample. (Test this with a small piece of your sample polymer first though, as some polymers are soluble in ether).

Differential Scanning Calorimetry (DSC)

DSC measures specific heat capacity and how it varies with temperature. As a polymer is heated through its glass transition point, it experiences a sudden change in heat capacity, as chain rotation allows it to take up more energy. This means that DSC allows us to identify the glass transition temperature of a polymer. It can also aid the interpretation of the type of a copolymer (e.g. block copolymer, random copolymer, graft copolymer).

See Macrogalleria’s page on DSC for an explanation of this technique.

Examples of processes

Polymers are usually processed by moulding methods:

Process	Description	Features
Injection moulding	Polymer granules are melted and forced into a mould. This is extremely widely used to mass produce small, precise polymer components.	It gives a good finish and the injection points where excess material has been cut off are often visible. In transparent polymers a residual stress field may be visible under crossed polars (see the Introduction to Photoelasticity TLP).
Blow moulding	Cylinders of polymer are inserted into a die and hot air is forced in, pushing the polymer out to the walls of the die.	Gives hollow components, such as bottles or containers. It is only used for thermoplastics.

For further examples see plastics processes on Plastipedia.

Additives and Blends:

Polymers very often have some form of additive, even if it is simply to add colour. These may or may not impair the ability to identify the polymer. When identifying any material it is important to think about the cost and properties, but blending or additives can change the properties of a polymer.

One very common example of a polymer commonly found both with and without additives is polyvinylchloride (PVC). This polymer is used in its rigid, un-plasticized form in plastic guttering and water and gas piping, but is also often found with added plasticizers in a variety of applications from clothing to coating electrical wires.