The Materials Analyst, Part 12:Performance problems: When the pellets don't match the parts

It is fairly obvious to readers who have been following this series that composition is one of the first things that should be checked when a part fails. Typically when a sample of a failed product comes in for analysis, some details are provided on the supposed composition of the item. Sometimes we are fortunate enough to receive the bad part along with a good one from a previous run. This allows us to compare the composition of the two products. On rare occasions, we will also receive a sample of the pellets from which the part is supposedly molded. This is the best scenario since it allows us to verify that the material being used matches the material that ended up in the molded part.

In this particular case, the customer presented us with all three pieces of the puzzle: the pellets, good parts, and bad parts. There were two different products involved, and both of them were being produced from fairly exotic materials. One part was being made from a 30 percent glass-reinforced, low-viscosity PEI commercially known as Ultem 2310.

The second part was being molded in a material that came from a respected compounder. It was a PPS with some proprietary additives designed to improve wear resistance. Although the supplier's literature typically provided detailed information on the composition of its wear-resistant packages, no specific data was available for this particular grade of material. However, we had pellets, good parts, and bad parts.

Very often, the first tendency is to check for degradation using any one of a variety of viscosity tests. While these are very useful, they are meaningless if the material being tested isn't what you think it is. So a few basic composition checks are important. As in the past, a combination of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was used to characterize both the polymer and the fillers. A good library of standards is indispensable on a project of this type.

We started with the Ultem 2310. Figure 1 shows the results of the DSC runs on the good and bad parts and the pellets. The only event was the typical glass transition near 215C for all three samples.

The TGA, however, was a different story. Figure 2 shows the weight losses for all three samples as a function of temperature. The temperatures for each step in the weight loss process were reasonably consistent, confirming the conclusion from the DSC that there was no contamination. But the glass contents did not match. The pellets tested out at 28 percent-on the low side of 30, but still acceptable. The good part, which was recently molded, came in even closer to nominal with a 29.6 percent.

But the older part that had failed in the field clearly had a problem. The glass content was 20.3 percent. The material was Ultem, but the grade was incorrect. The problem parts had been molded from the Ultem 2200 series, the 20 percent glass-reinforced grade. This reduced the modulus of the material by 30 percent, and when a particular feature was stressed in the application, the resulting deformation exceeded the limits of the material with the lower level of reinforcement, and the feature snapped off.

The second problem was more difficult. First, we did not know the exact makeup of the wear-resistant package used in the PPS. Carbon, Teflon, and silicone fluid are all used in wear-resistant modifiers. Second, none of the PPS parts were performing properly; they all exhibited varying degrees of a performance problem relating to reduced strength and stiffness. We started with the TGA.

Figure 3 shows the test result for the virgin material. The TGA test starts out in a nitrogen atmosphere. The objective is to burn off everything that will decompose in nitrogen-a process called pyrolysis. We then switch the gas stream to air. In nitrogen, anything containing carbon residue is stable. But in air, this residue will combine with oxygen and leave the system. This two-step process is important because every polymer forms a certain amount of carbon during pyrolysis.

Materials like polyethylene and polypropylene form less than 1 percent carbon. Materials with intermediate heat resistance like PET, PBT, and polycarbonate form moderate amounts. High-heat materials like PEI, PPS, and LCP form very large amounts. Each material has a signature amount of carbon that forms during pyrolysis, and it becomes part of the identification fingerprint. Through experience we have learned that for every 1 percent of polymer lost during pyrolysis, PPS forms 1 percent of carbon that will burn off once the air is introduced.

Figure 3 shows both the weight loss and the weight loss rate. In the first step, the PPS pyrolyzes, losing 25 percent of the sample mass. The weight loss process then stops until air is introduced into the instrument. In a traditional filled PPS, we expect to see another 25 percent weight loss due to the carbon and a residue of 50 percent that would be a filler of some kind. In this case, all of the remaining material decomposed. We were able to conclude from this that the remaining 50 percent of the material was part of the wear-resistant package and that it was some combination of carbon fiber and graphite.

Figure 4 shows the weight loss results from the two molded parts compared to the pellet. Clearly we did not have a match. Both parts lost 40 percent of their mass during pyrolysis, and the remaining 60 percent burned off in air. We had a mystery component, and already we knew a lot about it. First, it constituted 15 percent of our compound-the difference between the 25 percent weight loss in the pellet and the 40 percent weight loss in our parts. Second, this component was organic; it burned off in nitrogen along with the PPS. Third, it was a very heat stable material; it decomposed in the same temperature range as the PPS. All of this meant we should be able to find the culprit using DSC.

Figure 5 shows the DSC result for the raw material. The broad exotherm between 90 and 150C is solid state crystallization. Anyone who molds PPS knows it will not completely crystallize during the cooling process unless the mold is kept above a certain temperature. If the material is quenched, then the regions that did not crystallize will do so in the solid state once the material is heated above the glass transition temperature. The broad peak indicates that the strands of material coming out of the extruder were probably water cooled before being pelletized. The endotherm at 281C is the melting point of the PPS. Nothing else can be detected in this scan in spite of the fact that we heated the sample to 340C.

Figure 6 shows a DSC from one of the molded parts. Two items jump out. First, the recrystallization in the part is sharp, well-defined, and energetic. This clearly shows the parts are being produced in a cold mold. This factor alone reduces material properties. The second surprise is an additional melting point at 327C. The position of this event and the shape of the peak is a signature for PTFE (Teflon). PTFE has excellent thermal stability and does decompose at the same temperature as PPS. In addition, PTFE produces no carbon during pyrolysis.

This explains why the 15 percent increase in polymer content was matched by a corresponding 15 percent decrease in the carbon in air content. Finally, when we compared the energy required to melt the PTFE in our molded part with the energy needed to melt pure PTFE, we were able to determine that the PTFE in the molded part was indeed present at a level of 15 percent. Either by accident or purposefully, the parts had been molded from a material that did not match the pellets we had been given. Almost one third of the carbon had been replaced with Teflon. While Teflon has the lowest coefficient of friction of any commonly used material, it is also a relatively soft and weak material that will typically downgrade the strength and stiffness of a high-performance polymer. This helped to explain the product failures.

But the DSC gave us an unexpected bonus. It also led to the discovery of inappropriate molding conditions. The low mold temperatures used to produce the PPS parts led to an inadequate level of crystallinity. This reduced physical properties, chemical resistance, and dimensional stability at elevated temperatures. Ironically, this problem also compromised the very wear resistance for which the compound was being used in the first place.