Monday, 8 June 2020

Design Properties for Engineers: Tensile Properties of High Performance Polymers

In this blog post we compare the tensile properties (tensile strength, elongation, and modulus) of unreinforced and reinforced high performance polymers, including high temperature comparisons (150°C and 200°C).

Tensile properties are suitable to characterize the mechanical behavior of plastics. They describe the relationship between applied force (expressed in stress) and the deformation resulting out of this force (expressed in elongation).

The most used test is the tensile test according ISO 527. In this test, tensile bars are stretched under constant velocity. As a result, tensile stress (MPa or N/mm2) and tensile elongation (%) are obtained. The ratio of stress to strain is called Young modulus.

Tensile properties of high performance polymers can vary depending which polymer type was tested. The table below shows the tensile properties of unreinforced high performance polymers.
Comparison of tensile modulus, tensile strength and tensile elongation of unreinforced high performance polymers

Thermoset polyimides such as PBI and PI show brittle and high strength behavior. On the other end of the polymer spectrum we have PTFE which has a low strength, however shows a high elongation capability. PEAKs and Polysulfones reflect the typical tough and hard behavior of thermoplastics. PAI has a high strength and can already be used as an unreinforced polymer for several applications. It is able to handle high forces and still has good elasticity properties. Overall, PAI has balanced mechanical properties.

PTFE has a tensile modulus between 750 and 800 MPa and a tensile strength of less than 50 MPa with an elongation of over 250%. All the other high performance polymers show higher tensile strength values, combined with tensile moduli of over 2 GPa. However, the elongation is much lower compared to PTFE. PBI is the counterpart to PTFE with a high tensile modulus of nearly 6 GPa and a elongation at break of only 3%.

PPS, PEEK, and PAI show balanced tensile properties and all three have high tensile moduli of 4 GPa. So far, we considered only unreinforced high performance polymers. Adding glass- or carbon fibers will change the tensile properties. Tensile strength is increasing and elongation is decreasing. In the table below we can see improvements in tensile strength of all polymer classes; however reinforced PTFE will not reach the same tensile strength level as the other high performance polymers, especially when they are reinforced too.
Comparison of tensile modulus, tensile strength and tensile elongation of reinforced high performance polymers

Influence of temperature 


The base strength of high performance polymers can be increased with reinforcement fillers. However with increasing temperature, mechanical values are decreasing. This is shown in the next chart below.

Amorphous and thermoset polymers behave differently compared to semi-crystalline polymers when exposed to temperature. Up to the melting point, amorphous and thermoset materials show a nearly linear strength curve. Semi-crystalline polymers have a step-curve behavior. Before semi-crystalline polymers reach their continuous use temperature (CUT), mechanical properties have already decreased. Mechanical performance can change within few temperature differences. This behavior is clearly shown in the chart below: PAI, PBI and PI show at 200°C still good mechanical properties, and even as good as engineering polymers at room temperature. The tensile strength of PPS and PEEK has reduced significantly. PEEK has a tensile modulus of 3 GPa at 150°C. Increasing the temperature up to 200°C results in a decrease of under 1 GPa. PAI, PBI and PI have still high moduli values at 200°C and this reinforces the need of selecting properly polymers when temperatures are high.
Tensile modulus of high performance polymers at 150°C and 200°C

I hope you found the tensile property data of  unreinforced and reinforced high performance polymers, including high temperature comparisons (150°C and 200°C) useful and they can be helpful for you next part design or specification project.

Further design property data of high performance polymers are listed on my start here page.

Thank you for reading!
Till next time!
Herwig Juster

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New to my Find Out About Plastics Blog – check out the start here section

Literature:
https://www.polytron-gmbh.de/materialeigenschaften.aspx

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