One’s less expensive and simpler, but the other generates more comprehensive results. Which is right for you?
The incorporation of fillers and reinforcements into both thermoplastic and thermoset polymers is an important route to modifying the properties of a given base polymer. The amount and type of filler has a significant influence on the final properties of the compound. Consequently, the filler content is an important aspect of quality control monitoring as well as a consideration in failure analysis programs.
This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe, our analyst and author, is an independent materials and processing consultant based in Sedona, AZ. Mike has provided analytical services to material suppliers, molders, and end users for 20-plus years. You can reach him at firstname.lastname@example.org.
Figure 1 shows the dependence of modulus on temperature for five different grades of nylon 6. Note that the curves for all five grades have essentially the same shape. However, the actual value for modulus at any given temperature is dependent on both the amount and the type of filler used. Generally, the stiffness of the material increases with increasing amounts of filler. If we were to perform tensile tests on these five materials we would also observe that the strength of the material increases while the elongation at failure declines as toughness declines.
A close examination of the curves shows that the improvement in properties is not dependent merely on the amount of filler, but also on the type. A 40% glass fiber and mineral system, with actual proportions of 15% glass fiber and 25% mineral, does not provide as much of an increase in rigidity as a 33% filled material that uses only glass fiber. Fibrous materials, particularly when properly coupled to the polymer, provide true reinforcing qualities that offer a higher strength-to-weight ratio than a particulate like talc or calcium carbonate (CaCO3). However, mineral fillers tend to be less expensive and can reduce problems with warpage due to differential shrinkage in the flow and transverse directions.
Because filler content is an important aspect of composition, material suppliers will set minimum and maximum values around a nominal target. For example, a nominal 35% glass-filled material will typically have a tolerance of ±2-3% to ensure consistent performance of the material. The material supplier will perform periodic checks on the compound to monitor this parameter and processors also may wish to confirm the composition of the material that they are purchasing. In a case where a part molded in a filled material fails to perform as expected, the filler content of the molded part is likely to be part of the verification of composition that is always an important step in failure analysis.
Two types of analysis
There are two key methods that can be used to perform these measurements and they each have their advantages and disadvantages. One is a traditional ash test. This involves placing 2-3g of sample in a crucible and heating it in a furnace to a temperature