Mechanical Properties of Thermoplastics & Thermosets

Written by beth robinson
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Mechanical Properties of Thermoplastics & Thermosets
Knotted net is similar to a thermoset's internal structure. (Hemera Technologies/ Images)

Thermoplastic and thermoset plastics have different internal structures and so tend to have different mechanical properties. Imagine the structure of a thermoplastic as a pile of tangled strings, somehow without knots. Imagine the structure of a thermoset as more like a three dimensional net with knots or cross links at the intersections. In general, the molecules of a thermoset cannot move as freely as those of a thermoplastic, but will transfer force more evenly. A thermoset is therefore stronger and more brittle than a thermoplastic with a similar chemical make-up.

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Tensile and Compressive Strength

The strength of the material is the amount of force that can be applied over a certain area before it fails. Tensile strength is a pulling action and compressive strength is a pushing action. The tensile strength of a thermoplastic is typically below 50 MPa. The tensile strength of a thermoset will range between 50 and 100 MPa. This difference is primarily because the force applied to a thermoset is more efficiently distributed throughout the structure due to the additional interconnecting chemical bonds of the cross links.

Modulus - Stiffness and Flexibility

Modulus is essentially a measure of stiffness. The stiffer the material, the higher the modulus. A stiff material can be deformed with greater force and still return to its original shape when the force is removed. A flexible material can bend further and return, but the amount of force applied is typically lower. Thermosets typically show a modulus of 5-20 MPa while thermoplastics typically show a modulus of under 5 MPa.

Per cent Elongation at Failure

When a plastic sample is pulled apart it stretches some amount before it fails. This value is measured in terms of a percentage of the initial length of the relevant portion of the sample. Thermosets tend to have very low per cent elongations at failure because the cross linked molecules are not free to move. Thermoplastics can also fail without stretching very far, depending on the chemical structure, but many can reach 200 to 500 per cent of their initial length before they fail. The polymer chains align themselves and move during the stressing process, allowing the material to remain in one unbroken piece.

Effect of Time and Temperature

Mechanical properties of plastics are highly dependent on testing conditions. This is why there are standardised testing methods, such as ASTM D638. Among other details, ASTM D638 describes the rate at which the pulling of a tensile test should take place. This is necessary because the faster you pull a plastic apart the more brittle it appears.Most laboratory testing takes place near 23.9 degrees Celsius as the resulting values will change at elevated or reduced temperatures.

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