Sustainable Polymers Analysis

The properties of polymer materials are highly influenced by temperature. As this stress-strain data shows, increasing temperature affects the material’s properties in a variety of ways. The acetel material shows that increasing temperature results in a lower modulus (indicated by the slope of the initial line), lower strength (lower peak stress) and higher elongation (strain at break). Additionally, the material begins to have distinctly different behavior at higher temperatures and strains. Testing at different temperatures is very important to understand how the material will behave across the range of expected storage and operating temperatures.

Material properties are typically tested in ideal conditions. This serves to provide a consistent and reproduceable measurement, but it can also tend to overstate a material’s ability to perform in a given application. A good example is that standard tensile test “dogbone” samples are molded with a single gate providing optimal material flow and entanglement of the molecular chains. Real parts rarely can replicate the material performance in dogbone form. This data shows the effect on the strength and durability of a material when the sample is molded where the material must flow together in the mold creating a knit line.

This tensile testing data shows a marginal loss of strength, but there is a significant loss of elongation. The fatigue testing data is represented as an S-N curve which plots the cyclic stress level (y-axis) to the cycles to failure (x-axis). At a given stress level, the dual gate sample had significantly lower cycles to failure. Other manufacturing artifacts such as voids or filler orientation could have similar effects. It is important to understand a material’s performance after it is processed and formed into the real part.

Polymer materials come in a wide variety of formulations with many options for different blends, different types and amounts of fillers, and various additives for processing and other performance characteristics. Their properties on the datasheet might look very similar, but that does not mean that they will have the same performance.

This data shows three different ABS resin blends and the base SAN resin. The three ABS resins have similar responses in their tensile tests, but significantly different fatigue performance. It is also clear that the best material for fatigue could not be predicted from the tensile testing data.

The viscoelastic nature of polymer materials means that they continue to deform under a steady load. This behavior is called creep. Increasing temperatures result in accelerated creep behavior due to the higher mobility of the molecular chains. Creep resistance is often overlooked as an important property of polymer materials. Creep can cause component failure in multiple ways. One way is that the material deforms under constant load and is no longer able to perform its function. The second creep failure mode is creep rupture, where the material has a brittle fracture due to creep.

A more complete picture of a material’s creep resistance can be seen by performing successive creep measurements at increasingly elevated temperature. The ElectroForce Apex 1 Instrument is particularly valuable for these types of experiments with its high force capacity and long motor stroke, helping ensure that measurements can be completed on a wide range of materials and use cases.