Pollutec Recycling

ShortTerm Mechanical Testing

Experiments for measuring the effects of reprocessing have now been discussed. MFI as a method of monitoring degradation has already been mentioned, next the methods of mechanical evaluation will be considered namely tensile and impact properties.

3.5.1 Tensile Testing

Tensile testing can be thought of as a stretching test. Tensile properties are a method used widely to analyse the short-term stress-strain response of a material. Computer controlled tensile test machines are commonly employed, allowing one simple test to give a variety of information regarding strength, elongation and toughness. For this, a dumbbell-shaped test specimen is required such as the samples shown in Figure 3.5. Standard procedures are

Figure 3.5 Standard tensile test pieces

Strain -

Figure 3.6 Examples of stress-strain behaviour used such as the International Organization for Standardisation method ISO 527 [3] or the American Society for Testing and Materials method, ASTM D638 [4] to ensure comparison can be made between materials. The shape and gradient of the curve produced describes the way the sample stretches or breaks, its strength and rigidity, and whether a material is brittle or ductile (pliant). Examples of such curves are given in Figure 3.6.

The way a stress-strain curve is used to determine mechanical property data is shown in Figure 3.7. This shows a stress-strain curve and indicates how values are obtained for:

• Tensile strength at break (the maximum stress).
• Elongation at break (the distance the material stretches (strained) before breaking).
• The rigidity of the material is calculated from the gradient of the curve. This value is sometimes shown as the tensile modulus.

Strain -

Figure 3.6 Examples of stress-strain behaviour

. Strength of the plastic /

Elongation, %

Figure 3.7 How mechanical properties are calculated from stress-strain data

Elongation, %

Figure 3.7 How mechanical properties are calculated from stress-strain data

Degradation of the plastic will cause the mechanical strength (stress in Figure 3.6) to drop, as well as causing premature failure of the material.

3.5.2 Impact Testing

Impact tests measure the ability of a material to withstand a high velocity impact, for example, as might be experienced by a plastic kettle dropped from a kitchen worktop. Again, standard methods (e.g., ASTM D256 [5]) and specimens are employed, the results give a measure of the toughness of materials. Generally, the methods used fall into two categories. In one, a pendulum strikes a sample and the energy required to break the test piece in one pendulum swing is noted. This is most often seen on data sheets as Charpy or Izod impact tests. Both Charpy and Izod use a standard striking energy (see Table 3.2).

Table 3.2 Standard striking impact energies
Impact test type	Striking energy (J)
Charpy	300
Izod	167
Drop test	Depends on height and weight

Samples can also be notched, the size and shape of the notch is also standardised. This ensures that the samples fracture.

The second method involves dropping free falling weights onto samples. Free falling drop tests allow higher velocities and impact energies to be achieved. Useful data can also be gathered by doing impact tests at different temperatures as often this reflects more accurately the kind of environmental conditions the plastic will be subject to. For example, how a car bumper performs under impact at 25 °C may be different to how it performs at -10 °C. It may become more brittle at low temperatures. Whatever impact test method is employed, polymer degradation should, like tensile testing, show up as a reduction in the property measured.

3.5.3 Tensile and Impact Testing of Recycled Expanded Polystyrene

To show the kind of effects that are observed due to degradation, the following example shows the results of a study using Method 1A on recycled expanded polystyrene. This method is typically used for foam cups and packaging.

Figure 3.8 clearly illustrates mechanical degradation as a result of reprocessing. The properties of recyclate materials are intrinsically linked to their processing history. Equipped with this knowledge, the stages of recycling are the focus of the next two chapters.

Figure 3.8 Mechanical performance of EPS after repeated cycling by injection moulding [2]

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