The Materials Analyst, Part 57: How stable is your material? (Part 2)

We ended the last article with a brief discussion of materials that can degrade by either prolonged exposure to elevated melt temperature or the presence of excess moisture during processing. Polyesters, polycarbonates, polyurethanes, and polyamides (nylons) are the material families of greatest concern.

Not coincidentally, these are the materials that fail with the greatest frequency due to process-induced degradation. When these failures occur, a variety of methods that measure average molecular weight can verify that degradation has occurred. However, with greater emphasis placed on root cause analysis, it has become increasingly important to determine the exact mechanism that produced the degraded polymer.

While the mode of degradation can sometimes be determined from infrared spectroscopy, experience has shown that degradation must be significantly advanced before the infrared spectrum will show signs of the chemical modifications that can distinguish between thermal and hydrolytic degradation.

In addition, the root cause is not always an either/or proposition. Often excessive heat and moisture levels work together to produce an effect that neither factor alone could.

However, with all of the materials mentioned above, one factor is usually more important than the other. Actual experimentation using the molding process and a simple method for evaluating the molded parts can verify which factor is most critical. This knowledge, in turn, can be translated into a control plan for preventing further difficulties.

Dryers Distract with PBT
In the study we will illustrate here, PET and PBT polyester containing 30 percent loadings of glass fiber were used to make a particular part with a high level of mechanical demand. The parts were qualified for physical properties and the results of the physical tests were correlated with melt flow rate tests that compare the melt viscosity of the raw material with that of the molded part.

In order to evaluate the dual effects of melt temperature and residence time, parts were molded at two different melt temperatures and at a wide variety of resin moisture contents. The moisture content of the raw material was measured using the Karl Fischer method discussed in previous articles. Melt temperatures were verified with a handheld pyrometer.

Historical data had established that when the retained viscosity of the material in the molded parts fell to 30 percent of the melt viscosity of the raw material, the parts began to show signs of mechanical failure. The incidence of these failures increased as the viscosity declined below this point. This dividing line between good and bad parts agrees well with rules for using the melt flow rate or melt viscosity to control part quality in highly reinforced materials.

The results of the study appear in Figure 1. They tell us a great deal about the behavior of the two materials and the factors that cause polymer degradation.

First, let’s look at the behavior of the PBT polyester. This material was molded at 255C (491F) and at 277C (530F). The results show that when the melt temperature of the PBT is kept in the lower range, moisture has little effect on polymer molecular weight. At the low end of the moisture content spectrum, the material run at 255C retains almost 65 percent of its original viscosity. Even when molded very wet, 900 ppm (.09 percent), the material still retains 53 percent of its original viscosity.

In contrast, when run at the higher of the two temperatures, no amount of drying can prevent catastrophe. Even when dried at less than 100 ppm (.01 percent), the material molded at 277C still falls below the dividing line between good and bad production. The conclusion here is that for PBT, thermal effects are a much greater threat to polymer degradation than hydrolysis.

There is great practical significance to this knowledge because it directs our troubleshooting efforts. Much time is spent going over dryers in response to problems with brittle PBT parts because it is assumed that the problem is due to moisture. Worse yet, once these steps are taken, corrective action documents are filed detailing all of the work performed to improve drying. These documents come complete with the assurance that the problem has been addressed and further complications will not arise, when in fact the root cause has not been addressed at all.

Consider All the Variables
The picture for the PET polyester is more complex. The slope of the melt viscosity plots for PET molded at both 277C (530F) and 300C (572F) clearly show that PET is very sensitive to moisture content. At the same time, for any given moisture content the material molded at the lower melt temperature gives better results.

It is interesting to note that for parts molded at 530F, the borderline moisture content is 350 ppm (.035 percent) for this particular process. However, at 572F it is only 150 ppm (.015 percent) even though the general processing guidelines for PET recommend maximum moisture content of 200 ppm (.02 percent). On a relative scale, moisture is obviously a larger problem than temperature for PET polyester, which verifies some of the more scientific studies performed by polymer chemists. However, it is clear that these process variables cannot be considered in a vacuum. They work together either to ensure good product or to cause bad parts.

This same type of study can be performed on any material. Simply vary the molding conditions, making sure that you include some combinations of temperature and moisture content that are known to cause problems. Then observe the effects on physical performance and melt flow rate. Not only will this create a greater understanding of the connection between molecular weight and performance, but it will also clarify issues regarding the root cause of degradation. Root cause analysis only succeeds when it is governed by an adherence to scientific principles. You cannot vote a correct solution into existence—nature cannot be fooled.