Sunday, 26 January 2020

High Performance Polymers: Suitable for Low Temperatures?






At this time of the year winter is all over our places and temperatures can get really frosty. In this post, we discuss the suitability of high performance polymers (polymers with continuous use temperature above 150°C) for lower temperature applications.


When writing down the requirements for your application during the material selection phase, most applications have a temperature range which needs to be fulfilled in order to ensure proper end use. In the automotive industry, for example internal combustion engines (ICE), have a temperature requirements ranging from -30°C to 140°C.


When screening the datasheet values of most plastics, room temperature properties such as mechanical, electrical and chemical properties indicate good resistance against environmental stress cracking. However, decreasing temperatures can lead to a brittle behavior with low values in failure stresses. Therefore, a material’s low temperature behavior also needs to be carefully taken into account during your material selection.


How to test the brittleness temperature of plastics and elastomers?


The brittleness temperature provides us information about the temperature at which brittle fracture is the dominant failure mechanism.


The test method ASTM D746-04 is usually utilized for measuring the brittleness temperature. The obtained temperature is usually found in the onset region of the glassy phase. It is rather difficult to obtain an exact temperature point. Accordingly, a brittleness temperature is estimated. This indicates a 50% probability of a brittle fracture to occur at this temperature.


ISO 974-2000 is another standard test method, which can be also be utilized. It is technically equivalent to ASTM D746-04 though. In case you want to test rubbers for impact brittleness, ASTM D 2137 is the preferred method.


Low temperature properties considerations


Application of rapid loads at room temperature can lead to the same effect of failure than having a higher load at lower temperature. This shows that apart of temperature, time and fatigue play an important role in the brittleness behavior of plastics.


Glassy vs. brittle


In general, polymers with a Tg above room temperature have a glassy state at room temperature. Examples are for instances PS, PMMA, and PET. These are easier to break since they are inherently more brittle. On the other hand, polymers which have a Tg below room temperature have a rubbery state at room temperature. These are, for example, PP and PE (LD, HD, LLDE), which are rather flexible and difficult to break at room temperature.


Fluoropolymers show a different behavior. PTFE has a Tg of 115°C. At room temperature as well as below room temperature one may think PTFE should be brittle. However, this is not the case due to its unique carbon-fluoro bond, which results also in a high melting point of 400°C. As a result, PTFE has good strength at higher as well as lower temperatures, below its Tg.


Service temperatures


The majority of applications have a service temperature range of -20°C up to 60°C and many commodity plastics are available to fulfill those ranges. There are low-temperature applications such as aircraft parts, oil rigs, industrial refrigeration, superconducting magnets, and liquid-helium devices, which are exposed to temperatures down to -270°C. Material selection becomes critical to prevent any part failure at such low service temperatures. At temperatures below -40°C, the choice for plastic materials becomes limited. In this temperature range, resistance to Liquid Oxygen (LOX) also becomes an extremely important application requirement.


LOX compatibility


Among different polymer families, high performance polymers such as fluoropolymers show good to excellent Liquid Oxygen compatibility. PTFE, FEP, and PCTFE show the best in class suitability, followed by PVDF. Altogether, fluoropolymers are good candidates for low temperature applications, since they are excellent insulators and have high chemical resistance.


Most important property of fluoropolymers at low temperatures is their ductility: when reaching the absolute zero temperature point (-269°C), the ductility of these polymers holds at approximately 1%. All in all, fluoropolymers are a good material choice for static seals at low temperatures.


Minimum service temperature of different high performance polymers


As we have already discussed, low temperatures are as challenging as high temperatures. This due to polymers showing brittle behavior, which can lead to fracture and crack formation in the final part. Besides fluoropolymers, other high performance polymers can be used at temperatures down to -50°C. For cryogenic temperature applications where high loads are involved, PAI and PI can also be used.


The graph below shows the minimum service temperature of different high performance polymers.

Summary
For most materials including plastics, low temperatures are a difficult environment. However, polymers such as PAI, PI and fluoropolymers can be a solution. In particular fluoropolymers, with their stable carbon-fluoro bonds, are suitable at extreme low temperatures. This in turn, supports designers to solve low temperature material problems in an efficient manner.


If you want to know more about fluoropolymers and why they are important in the future, you can find an article I wrote already here.



Thanks for reading & till next time!
Greetings, Herwig Juster

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Literature:
[1] http://www.tangram.co.uk/TI-Polymer-Low_temperature_plastics.html

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