Why You Must Consider Continuous Heat and Atmosphere
You’re designing a plastic part that must maintain specific load bearing properties at 100°C. Is that 100°C for a very short time, or continuously? What is the environment the part will be used in? What about exposure to moisture, automotive fluids or other chemical environments?
There is data available to help you solve those puzzles, but you probably won’t find too much of relevance on a typical data sheet. And the answers vary by plastic type because of variations in polymer chemistry. And they also differ by type of plastic compound, that is the base material plus its additives, ranging from reinforcement and heat stabilizers to pigments and flame retardants.
Design factors, such as ribs, as well as molding conditions can also affect a part’s fitness for a given application. And by the way, your Chief Technology Officer just announced a corporate drive to “commonality” that may limit your options.
Benefits Outweigh the Complexity
This may seem complicated compared to designing with a metal alloy, but plastics are worth the effort. Their dizzying array of combinations and moldability create an amazing palette of design opportunities that can reduce costs, improve aesthetic appeal and dramatically improve performance.
Part One in this series focuses on the importance of basic chemical structure and short-term temperature tests in assessing the fitness of a plastic for a high-temperature application. We’ll continue in that vein in this installment with a look at the role of chemical environment and extended high-heat conditions on plastic part design.
How To Use RTI
One of the most important tests to determine a given plastic’s fitness for long-term ambient heat is the UL Standard 746B, Polymeric Materials, Long-Term Property Evaluation. As a data point, it’s shortened to Relative Thermal Index (RTI). Underwriters Laboratories Inc. (UL) developed RTI to test the deterioration of insulating materials in electrical devices over time.
RTI gives an indication of the aging temperature that a material can endure for 100,000 hours and still retain at least half of the initial property being measured. Different materials’ properties decay at dissimilar rates. As a result, one caveat in using the data “is that you could track the wrong properties,” cautions James Galipeau, laboratory manager for Plastics Technology Laboratories in Pittsfield, MA. “You don’t want to use test results on impact as an indication of flexural modulus or bending results.” RTIs indicate a given plastic’s electrical properties and specific mechanical properties with and without impact. UL gives RTI ratings for each thickness of the plastic tested.
Sets of test specimens are placed in ovens at four different preset temperatures. At intervals, specimens are removed and tested for specific mechanical or electrical properties. The results are plotted on a time versus property graph until the property being tested declines to 50 percent or less of its initial value. The 50 percent value is referred to as the “half-life” for that property. The half-lives are then plotted against the reciprocal of the absolute aging temperature, resulting in a straight line that can be extrapolated to indicate the half-life of a property at other temperatures. That line is called an “Arrhenius” type of plot. Results are compared to a material with known aging performance, hence the term Relative Thermal Index or RTI.
Here’s how the test applies to design of a plastic part, say a toaster oven. Start with a requirement for how long the oven would actually operate. “Most manufacturers would want a warranty of at least 1,000 hours,” comments Greg Warkoski, process technology manager for Solvay Advanced Polymers, Alpharetta, GA. “So you go back to the RTI for the appropriate mechanical property and find the point that gives you 1,000 hours of use and still maintains 50 percent of the original property value. And then you design in a safety factor of two, three, or four times – which you should do.”
You must also consider the possibility of heat aging in the presence of water or some type of chemical environment. Hydrolytic stability, or resistance to attack by water, is particularly important because water is ubiquitous and can be very aggressive to many types of plastics. One test is to immerse test samples in boiling water and then test mechanical properties after 10 days.
Plastics obviously face potential for a wide variety of chemical attack when used in moving equipment or devices. At times in the past, there was inadequate testing for known exposures, such as when certain plastics were first introduced for exterior automotive applications in Europe. It’s a dicey proposition due to the difficulty in predicting the exact effect of chemical exposure on a polymeric component because the reagent, the exposure time, the temperature of the reagent, the temperature of the polymeric component and the stress on the component all affect the extent of attack and any change in performance. The only reliable method to predict performance is to test a prototype part under actual conditions. Plastics producers perform screening tests to provide general guidance and compare materials.
“An interesting rule of thumb is that the rate of deterioration from chemical attack will double with every increase of 10°C,” notes Warkoski. “What you sometimes find is that engineers will design a test with an artificially high temperature in a chemical environment, reach a failure and then assume this is a linear relationship. It is not.” These examples show the importance of careful consideration when specifying a high-temperature plastic. Post-use testing, as done with the Dodge Neon thermostat housing (see below) shows that quality engineering really pays off.
Arrhenius Plot – Named for Swedish Nobel laureate Svante Arrhenius, this is a plot of the log of some quantity against inverse temperature. See text for detailed description of test protocol as applied to plastics.
Hydrolytic stability – Resistance to hydrolysis, or attack by water.
Thermal oxidative – This refers to the combined effects of heat and oxygen on polymer property retention.
Neon Thermostat Housing: Treasure in the Trash
How well do performance tests, when properly conducted, predict actual behavior of engineering plastics in ground-breaking high-temperature applications in cars? Well the best way to find out is to perform an automotive autopsy. LDM Technologies, a Michigan-based Tier One specialist in highly engineered plastic automotive applications, recovered a pioneering part it had designed and produced – a 2.0-liter thermostat housing molded from Amodel® polypthalamide (PPA) – from a crashed Dodge Neon found in an automotive scrap metal recovery site.
Amodel PPA had replaced die-cast aluminum for the application because of its lighter weight, corrosion resistance, and overall savings. The housing had been in service for seven years and 85,000 miles on the recovered Neon. Engineers analyzed the housing on a coordinate measuring machine using a 122-point dimensional layout. The housing made from Amodel® PPA showed no measurable change: no surface degradation, no dimensional changes and no change in the performance of the polymer.