Mechanical properties

Notes: The properties, in general, can be affected by many factors that are not easy to account for in tables such as this: e.g., % crystallinity, orientation and morphology in semi-crystalline polymers; degree of crosslinking/grades in TS resins; hardness of elastomers. Polyurethanes, depending on the type of polyols (polyester or polyether polyols) and the processing conditions, enable the production of a wide spectrum of products, which include flexible and rigid foams, plastics and elastomers, coatings and adhesives. TS polyester may be based on orthophthalic or isophthalic isomers: isophthalic produce higher modulus and strength. ABS has grades of medium, high and very high impact. Furthermore, the test conditions, such as the rate of straining (the cross-head speed), obviously, also affect the values measured.

The properties are affected by the weather/service conditions such as UV radiation, heat, moisture and temperature and the mechanical loading conditions, e.g., speed. These conditions can be simulated by employing accelerated-weathering devices and subjecting the specimens to one or a combination of the weathering elements for a period of time as indicated by standards (e.g., ISO 4892:2006, ASTM D256 -99 (2008), ASTM D4587-11) prior to testing and/or by placing a suitable environmental chamber on the testing machine and conducting multiple tests at different settings.

6.2.1 Effect of testing speed and temperature

The polymeric materials are viscoelastic and, therefore, their mechanical properties are time and temperature dependent. Accordingly the parameters of temperature and the speed of testing should be recorded for tests (it is a good practice to record the relative humidity as well, because of sensitivity of some polymers to moisture). Most tests are conducted under standard laboratory conditions (temperature = 23 ± 2 °C; relative humidity = 50 ± 5%) on dry-as-moulded samples.

The speed of testing as recommended in the standards range from 1 to 500 mm/min and is selected depending on the property measured, for example, for the elastic modulus measurement it is normally 1 mm/min. Figures 6.6 and 6.7 show how the extension rate (speed of testing) and temperature affect the stress-strain behaviour of high-impact polystyrene (HIPS). The elongation to break decreases and the strength (both tensile and yield) increases as the temperature drops and as the deformation rate increases. The temperature effect is significantly greater than the influence of the deformation rate.

Introduction to Polymer Science and Technology

Mechanical properties

The same trends apply to other polymeric materials, therefore, one can generalise as shown in Figure 6.8. Although not very obvious in these graphs for HIPS, however, the elastic modulus trend is similar to that of strength: increases as temperature falls and/or as the speed of testing increases. Modulus variations become particularly pronounced at T of materials, a drop of as high as 1000 N/mm² can result as material changes from being glass-like to being rubber-like. This is particularly critical in amorphous thermoplastics in that the material loses its structural rigidity and hence its load bearing ability significantly. Whereas, the semi-crystalline thermoplastics and thermosetting plastics/elastomers maintain significant structural stability because of molecular crystallinity and chemical cross-linking, respectively.

500 mm/min

10 0

30 40

strain (s), %

Figure 6.6Stress-strain curves for HIPS at extension rates from 0.1 mm/min to 500 mm/min (tests conducted at 23 °C/50 % RH) (source: Ehrenstein (2001, pi 82)

Date: 2015-12-11; view: 1089