This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years.

Heat is not the only factor waiting to attack plastic properties over the long term. Have you considered chemical exposure?

When it comes to assessing the heat resistance of plastic materials, the only attempt that most data sheets make to capture this capability is heat deflection temperature (HDT). We have reviewed the problems with this test in previous articles (October 2000 IMM, www.immnet.com/articles?article=1372). However, one of the flaws contained in the HDT test that we have not addressed is its failure to capture the effect of time on the properties of a polymer. Time is such an important consideration in judging the fitness-for-use of a compound that the term “continuous use temperature” has no meaning because the phrase never defines the time scale of “continuous.”

There are two major considerations in evaluating the effects of time on the properties of a polymer. The first is the chemical effect of prolonged elevated temperature exposure on a product, the focus of this month’s article. The second is the effect of a constant load or strain, a topic we will cover in Part 2.

Oxidation and Stabilizers

All polymers are organic materials. In this respect they are not that different from you and me, carbon-based systems arranged into long chains that derive their properties from their unique molecular arrangements. One of the most pervasive problems for these types of materials is oxidation, a process that breaks down the molecular structure, often producing free radicals that further accelerate degradation.

Protection from such processes comes from stabilization packages, known as antioxidants or heat stabilizers. These are additives that represent only a fraction of 1% of most formulations, but govern to a great extent the effective lifetime of the product.

One of the best examples of how these stabilizers determine the life span of a product comes from the world of polypropylene. The load-bearing characteristics of a polypropylene depend upon a number of factors including the composition of the polymer (homopolymer or copolymer) and the type and amount of any fillers or reinforcements that may be incorporated into the compound (glass fiber, mica, talc, calcium carbonate, and so forth). But the ability of the material to withstand exposure to sustained elevated temperatures has little or nothing to do with these factors. Instead, this property is governed by the amount and effectiveness of the stabilizer.

Long-term sustainability in a high-heat environment can be captured by a number of tests, most of which are designed to accelerate oxidation by employing extreme conditions. This approach can provide an answer about the fitness-for-use of a material in weeks or months rather than years.

A Quick Result

An example of this is the oven aging test employed by the automotive industry. Under this protocol, molded samples are exposed in an air-circulating oven to a constant temperature of 150°C for up to 336 hours (two weeks). Every 24 hours, the samples are checked for signs of deterioration. These signs are seldom subtle. Polypropylene, if not under a load, can sustain exposure to this temperature because it is below the melting point of most commercial compounds. But once the antioxidant package in the material has been exhausted by the elevated temperatures, the material quickly deteriorates. Samples that may appear to be pliable and intact on day eight may crumble on day nine.

The automotive industry does not really expect any PP material to stand up to this temperature profile in a real application. The engineers who designed this test are capitalizing on a well-defined relationship between temperature and time to failure. While the precise mathematics governing this relationship are specific to the compound under consideration, in general the relationship between temperature and the natural log of time to failure is a straight line. The rule of thumb states that for every increase of 10 deg C, the anticipated time to failure is halved. Since the relationship is exponential, an increase of 30 deg C will reduce the lifetime by a factor of eight, a rise of 40 deg C will cut it by a factor of 16, and so on. So when a test is conducted at 150°C for two weeks, an engineer is attempting to obtain a quick assurance that constant exposure to 90°C, for example, will allow for a minimum lifetime of 64 weeks—a little more than 10,000 hours.

A more rigorous approach to this same assessment forms the basis for the UL relative thermal index (RTI). The RTI has a very specific meaning: It is the temperature at which certain key room-temperature properties are diminished to 50% of their original values after continuous exposure for 60,000 hours. It does not consider load-bearing properties, nor does it evaluate the behavior of a material at elevated temperatures. It simply evaluates resistance to processes such as oxidation that slowly sap a material of its initial performance.

Thankfully, the test does not run for 60,000 hours—which is 6.85 years, in case you were wondering. Instead, four temperatures much higher than the anticipated RTI are used to provoke the desired failure mode over time frames of two months and one year. With four data points relating temperature and time to failure, a straight line can be reliably drawn and extrapolated to the 60,000-hour mark. Again, the result is not governed by short-term considerations such as heat deflection temperatures or softening points. What distinguishes a polypropylene with an RTI of 60°C from one with an RTI of 115°C is the quality and quantity of the antioxidant package.

A Quicker Result

Fortunately, there is an even faster way to obtain a relative assessment of this property. It is known as the oxidation induction time (OIT) test. It is a special use of differential scanning calorimetry (DSC) that involves placing a sample of material in a highly oxygenated environment at an elevated temperature. Some methods call for further intensification of the test environment by pressurizing the oxygen.

When held at a constant temperature, the plot of heat flow vs. time is a horizontal line until the antioxidant is exhausted by the aggressive test environment. When the polymer starts to oxidize, there is a rapid and vigorous release of energy that is detected as an exotherm. This process is shown in Figure 1. The onset time of the oxidation event is the OIT. All by itself it is just a number. However, there is a relationship between the time to failure in the OIT test and the long-term resistance of the material to embrittlement by oxidation.

While the accelerating aspects of the OIT test reduce the time to failure to a very low number, the value derived from the test is a good relative indicator of time to failure. If a material exhibits an OIT value that is twice that of another material, it will also last approximately twice as long in the field under equivalent application conditions.

Why is this important to a processor? Very simply, the molding process consumes some of the antioxidant that is incorporated into the raw material. This is an expected consequence of injection molding. A well-developed molding process will reduce the OIT of the raw material by approximately 30% to 50%. If the correct material has been selected for the application, then the resulting molded part will still contain the level of antioxidant required for good long-term performance. However, a poor process will exhaust an inordinate amount of the stabilizer, leaving the molded product inadequately protected for the intended time and environmental conditions. This deficiency may not be detectable by more traditional methods such as a measurement of melt-flow rate or physical properties because the catastrophic damage has not been done yet. But the stage has been set.

Figure 2 shows the result of an OIT test for a raw material with very good oxidative stability. The value for the raw material is just short of 300 minutes. To put this in context, the highest value that is typically obtained under these test conditions is approximately 600 minutes, and materials with that type of resistance are slated for demanding environments.

Figure 3 shows an OIT test on a part molded from this material. The reduction in OIT from pellets to part is approximately 30%, and this part survives in a hot, wet environment for a sustained period of time.

Figure 4 shows the test result for a part molded at an elevated melt temperature in a molding machine with a much larger barrel capacity. This results in a longer residence time and a higher circumferential velocity during screw rotation. The part has lost more than 90% of its oxidative stability and actually fails a qualification test that is performed before the part is released to the field.

So in selecting a material, it is critical to consider the chemical effects of the long-term environment on material integrity. The additives that constitute a small fraction of the formulation are a key part of this performance. Material selection is just the beginning; the process must maintain the level of stabilization required for adequate service life.

Next time, we will discuss the mechanical aspects of the time-temperature relationship and explain why it is that designers familiar with the behavior of metals can look at an FEA that is a disaster waiting to happen and never see it coming.