Creep Strength and Toughness of High Performance Plastics

In this blog post, we discuss the creep and toughness behavior of high performance plastics.

Creep – a general definition

In general, creep is the deformation of a material over a certain time under a constant load and constant environmental conditions. Estimation of creep properties is done by using the ISO standard 899-1 or the ASTM D2990. For testing, a defined force is applied onto a tensile bar over a certain period of time (e.g. 1000 hours) and temperature range. Temperature can vary between 23°C to 150°C.

As a plastics design engineer you are interested in such results when you are dealing with application parts which are under high load for a long period of time. Overall, environmental changes impact the creep behavior too. Especially the increase of the temperature decreases creep performance dramatically.

Creep strength of high performance polymers

In Figure 1, different unfilled high performance polymer grades are compared to each other at different temperatures. PAI and PBI show both excellent resistance toward creep. PEEK has good creep resistance at room temperature. However, the creep resistance at temperatures around 150°C decreases down to 8 MPa, since the glass transition temperature of PEEK is reached.

Figure 1: Tensile creep strength of different high performance plastics.

Adding fibers will increase the creep resistance. Carbon fiber filled PEEK will still not outperform unfilled PAI at 150°C in terms of creep resistance. For example, PEEK with 30% carbon fiber shows at 100°C a better creep performance as unfilled PAI. At 150°C, this turns around and PAI has the better creep resistance.


Toughness of high performance polymers

In general, toughness is the combination of polymer strength and ductility. Obtaining the toughness can be done over impact testing. A though polymeric material has a good ability to absorb energy during plastic deformation. There is a direct relation between the brittleness of a polymer and energy absorption.


There are two major tests for estimating impact values: Izod and Charpy test. Both can be done with notched and un-notched specimen. The load is applied at high rate and the absorbed energy is measured (Joule).


Figure 2 shows the maximal and minimal toughness values of high performance polymers. Among the amorphous high performance polymers, PolyPhenylenSulfone (PPSU) shows the highest energy absorption, whereelse PolyPhthalAmides (PPA) show high toughness values among the semi-crystalline high performance polymers.

Figure 2: Toughness values of different high performance polymers.

I hope you found this information useful and can support you in your next design decision with high performance plastics.
Thank you for reading!
Best regards, Herwig

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Literature:
[1] Laurence W. McKeen: The effect of temperature and other factors on plastics and elastomers, 2008
[2] Omnexus.com

Metal Replacement with Polyarylamide (PARA) for Single-Use Surgical Instruments

Today, we discuss an interesting evolving area: the metal replacement of single-use surgical instruments with high performance, semi-aromatic polyamides.

When you listen to the needs of hospitals and small ambulances all over the world, ready-to-use instruments which have optimal mechanical and ergonomic properties are high up on their wish list.

The non-active medical devices supplier Bastos Viegas from Portugal picked up this need in his hands and started a metal replacement program for stainless steel single-use surgical instruments.

The incumbent stainless steel solution fulfills all the mechanical requirements, however weight and cleaning procedures make them in the allover process of handling not attractive.

Requirements of plastic single-use instruments

To replace the stainless steel solution the polymer compound needs to fulfill the following requirements:

- High level of rigidity

- Dimensional stability

- Metal-like strength

- Biocompatibility according to ISO 10993

- Resistance to high-energy gamma radiation without significant change in appearance and physicochemical properties

- Resistance to ethylene oxide (EtO)

- Highly aesthetic surface finish

Material selection

Based on the list of requirements, different materials where analyzed. One material seemed quiet promising for Bastos Viegas from the beginning: the semi-aromatic Polyarylamide (PARA or MXD6).

This high performance polyamide brings all the aforementioned attributes under one roof. It has a metal-like strength (tensile modulus of 22 GPa at 50% glass loading, room temperature), high dimensional stability, half of the water uptake compared to a normal nylon 6.6. and has the best surface appearance among all polyamides (resin rich surface due to fine crystallization). In addition, its excellent flow ability (similar to PPS) and resistance to high-energy sterilization process made it the ideal candidate for such a replacement.

In the Figure 1 the properties of the selected Ixef® grade GS-1022 are compared to a standard nylon 6.6 and other engineering polymers.

Figure 1: Flexural properties of Polyarylamide (PARA) compared to other high performance and engineering plastics.

In Table 1, a comparison of different property values of PARA, Nylon 6.6, Nylon 6 and PBT is done. This comparison makes the advantages of PARA numerically visible: low water absorption in combination with high modulus values.

Table 1: Specific properties of Polyarylamide (PARA) and comparable others engineering plastics.

So far, we have compared PARA to other Nylon materials or other engineering plastics. Since we want to do a metal replacement, we are interest in the comparison of PARA with other metal die cast materials. This is shown in Table 2. As it can be seen, the tensile strength of PARA is at the same level or in some cases higher than that of the casting metals and its E-modulus although inferior is within the same range. As a result, required performance can be easily attained with significant reduced weight as looking at the respective densities of PARA and the metals.

Table 2: Specific properties of Polyarylamide (PARA) and different die casting metals.

Conclusion

The combination of different property data of PARA explains why PARA is such a good candidate for metal replacement. For instances, this is low moisture uptake, superior strength, stiffness, and highly aesthetic surface finish.

In the future, more and more medical metal applications will be replaced by high performance plastics such as PARA, PEEK and Polysulfones.

Below a selection of the different PARA-based Zillion Black® single-use instruments is shown.

Greetings and till next time,

Herwig Juster

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Literature:

[1] https://www.solvay.com/en/press-release/solvays-ixef-para-enables-zillion-black-single-use-instruments

[2] https://www.solvay.com/en/brands/ixef-para