Can High-Temperature Plastics Really Take the Heat?
Information on data sheets is great for predicting how test bars will react at room temperature to specific conditions. But in the real world? Six of the seven important specifications issues for high-temperature plastics aren't included in typical ASTM or ISO test data.
What Does High-Temperature Mean?
One of the least understood and most erroneously specified areas in the design field is high-temperature plastics. The phrase is widely used but difficult to define. What plastics can genuinely be called “high temperature”? What are high temperatures? The answers can differ dramatically, depending on the application and the plastic.
The minimum definition: They are plastics that have usable mechanical properties at an elevated temperature. The irony is that very few suppliers of high-temperature plastics publish that information on data sheets. Instead, what you find are mechanical properties based on ISO or ASTM tests conducted at room temperature.
“If you’re designing a load bearing part for a high-heat application, you must understand how the operating temperature will affect the properties that are critical to the application,” comments Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. “Although data sheets don’t have specific information like this, you can usually get what you need from your resin supplier.”
The Plastics Performance Iceberg
That’s just the tip of the performance iceberg for the unwary design engineer. These issues can also create obstacles during material selection and the design process:
- Poor understanding of testmethods and what the results really mean
- Failure to factor in the chemical environment and the effects of moisture
- Excessive reliance on single-point data instead of performing mechanical analysis over a range of temperatures and a range of times, when appropriate
- Poor understanding of how changes in temperature affect plastics with different chemistries
- Failure to consider stresses and other problems caused by processing or design (product and mold)
“When you perform an ISO or ASTM test, you end up with results that are directly related to the condition of the test,” comments James Beauregard, president of Plastics Technology Laboratories Inc. of Pittsfield, MA. “But when you take a design out into the field, there are a lot of other things occurring that must be taken into consideration.”
Start your material selection process with a review of polymer fundamentals that can affect product life. Polymers comprise two molecular structures: amorphous (randomly ordered) and crystalline (highly ordered). For all practical purposes, thermoplastics are either amorphous polymers or semi-crystalline polymers, which have both amorphous and crystalline regions.
One of the major differences between these two types is how they respond to changes in temperature.
Amorphous polymers – such as polycarbonates and polysulfones – do not have defined melting points, but rather ranges at which they soften. This is called the glass transition temperature (Tg). Semi-crystalline polymers – such as nylons – have glass transition temperatures (Tg) as well as sharp melting points (Tm).
“This has to be taken into consideration when selecting a material because semi-crystalline polymers lose a significant amount of their mechanical properties when heated above the Tg, especially if they’re not reinforced,” says Solvay Advanced Polymers’ Warkoski. For example, Amodel® polyphthalamide is a semi-crystalline engineering plastic that has a heat distortion temperature of around 120°C (250°F) when unfilled and about 275°C (530°F) when reinforced.
Because Tg is a fundamental property of the base polymer, it usually isn’t included on data sheets. Instead, you find the heat deflection temperature (HDT) of the finished resin. HDT is used to determine short-term heat resistance and distinguishes between materials that are able to support a load at an elevated temperature.
At this temperature, a test bar shows a specified deformation under a load of 264 psi. It is generally acceptable to use 5-10°C (9-18°F) below the HDT as the maximum short-term operating temperature. Specific information on D648-04 Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position is available at www.astm.org. The corresponding test from the International Organization for Standardization (ISO) is ISO 75-1:2004, and is available at www.iso.org.
HDT (also called DTUL, or Deflection Temperature Under Load) is a snapshot of thermal properties at a single temperature point. Maintenance of properties over time at elevated temperatures is crucial for applications with continuous heat. The chemical environment, such as oil or gasoline, can also dramatically affect mechanical properties, particularly at elevated temperatures. These are just a few of the issues that design engineers must consider when specifying plastics for load-bearing applications at high temperatures.