When Plastic Part Investigations Change Direction

By Michael Sepe, Materials Analyst

Checking for melt-flow rate might be the easiest test, but not always the most accurate.

One of the most common complaints associated with part failure is brittle behavior. Even when a part is made from a material with little inherent ductility such as general purpose polystyrene or SAN, the customer still has an expectation of a certain stress-strain response. When this is not realized, some type of evaluation is warranted.

The most common cause of brittle behavior is polymer degradation, the reduction in the average molecular weight of the polymer brought about by some combination of elevated melt temperature, extended residence time, and in some cases inadequate moisture removal prior to processing.

Because reductions in average molecular weight are rather easily documented by melt-flow-rate (MFR) testing, this test is often the first and sometimes the last step in determining a root cause for failure. However, sometimes an evaluation that starts in one direction takes a sudden turn when the results of the first test do not provide a sensible answer. This is one of those cases.

WHEN MFR WON'T TELL THE WHOLE STORY
We start with a simple problem: two sets of parts produced from a 13% glass-fiber-reinforced nylon 6/6. One set of parts functions as expected, the other set exhibits obvious brittle performance at typical operating conditions.

A lot of analysts eschew the MFR test on a material like this for a couple of reasons. First, the melt viscosity of nylon polymers is influenced significantly by moisture content even within the range of moisture content values associated with dry material. A nylon material dried to a moisture content of .20% may flow 30-40% faster than that same material dried to .02%. This is a difference that is almost as large as the change we are trying to document as an indicator of degradation.

The second complicating factor is the presence of the glass fibers. The glass fiber increases the melt viscosity of the compound, which reduces the MFR. When a glass-fiber-reinforced material is injection molded, two things contribute to the change in viscosity associated with processing. One is a reduction in the average molecular weight of the polymer and the other is the reduction in the length of the glass fibers. Both are inevitable consequences of high-shear melt processing, but the MFR result represents a compound response.

Both of these limitations can be overcome with a little work. First, the relationship between MFR and moisture content for any given nylon compound is nearly linear. Therefore, with three or four measurements of MFR at different moisture content values, it is possible to establish a calibration curve. This allows for actual values obtained on unknown samples to be normalized as long as the moisture content of the sample being tested is known. Of course, this requires that moisture content be measured by a device that actually measures moisture and only moisture.

The ability to judge the combined effect of glass fiber breakage and reductions in average molecular weight comes with experience. Repeated trials have shown that a compound containing 13% glass fiber can sustain an increase in MFR from pellets to parts of approximately 100% before performance problems set in.

Both of these limitations in the MFR test can be overcome by resorting to a solution viscosity technique such as relative viscosity or intrinsic viscosity. However, this test involves using very aggressive solvents such as formic acid or chlorinated aromatic compounds. Not everyone is comfortable handling such chemicals. In contrast, the MFR test is relatively simple; therefore, there is a real incentive to make it work.

IT WENT DOWN?
While no raw material was provided with the good and brittle parts in this case, historical data show that the typical MFR for 13% glass-fiber-reinforced nylon 6/6 raw material is 17-23 g/10 min when the material is dried to a moisture content of .10%. The MFR result for the good parts was 30.9 g/10 min, an increase of approximately 55% and well within the acceptable range for good processing. However, the brittle parts gave an unexpected result, an MFR of 5.75 g/10 min.

While downward movement in MFR during processing is not unheard of, it typically involves unusual changes in polymer structure. Some of these, like solid-state polymerization, can have a positive effect on the properties of the polymer, but often involve an increase in melt viscosity that the molder has difficulty managing.

Other causes of a decrease in MFR are not so welcome. One of these is known as oxidative crosslinking, a process that starts out as straightforward polymer degradation but involves subsequent crosslinking of the reduced chain lengths to produce a system that is very high in viscosity but at the same time very brittle.

There is always the possibility that the part is made from a different grade of raw material. Higher glass loadings will decrease MFR; in fact, the MFR value that was obtained in the test on the failed part was quite consistent with a 43% glass-fiber-reinforced nylon 6/6. In addition, this higher glass loading would result in a stronger and stiffer but less ductile part. But the appearance of the fractures in these parts suggested that the material was not homogeneous.

AN INTRUDER IN THE NYLON
Often, departures from expected composition require multiple techniques to pin down the root cause. This is one of those cases. The first test performed on the brittle part was differential scanning calorimetry (DSC), a technique that characterizes a material according to its softening, melting, and recrystallization behavior. Figure 1 (download Word document with figures) shows a first heat scan for one of the good parts. It shows a single melting point at 265°C with a heat of fusion of 57 J/g, typical results for a nylon 6/6 with this filler loading. Figure 2 shows the same result for the brittle part.

There are some clear differences, the most obvious of which is the presence of a second melting point with almost the same strength as that of the primary constituent. This extra melting point appears at 218°C, a temperature that may be associated with a number of semicrystalline polymers including nylon 6, nylon 6/12, and PBT polyester. The strength of the melting event associated with the nylon 6/6 has been reduced by more than 50% compared to the good part; in addition, the melting point has been reduced by 10 deg C.

Mixing polymers can have unpredictable effects on signature events like melting and recrystallization temperatures; therefore, the shift in melting point is not a definitive clue regarding the deviation in composition. However, the consequences of this altered composition depend greatly on what the mystery ingredient is. Nylons can often be melt-blended with other nylons without producing a significant change in toughness. Nylon 6 and 6/6 are frequently mixed to produce grades sold as nylon 6/6 materials that are easier to process. Typically, the only consequence is a slight reduction in melting point.

However, if the second material is PBT polyester, the consequences are far more serious. First, PBT polyester begins to undergo thermal degradation when it is processed at temperatures greater than 260°C. We already know that the melting point of the nylon 6/6 is 265°C and typical melt temperature for processing 13% glass-filled nylon 6/6 is 277-293°C. At these conditions, PBT degrades rapidly. To make matters worse, one family of by-products from PBT degradation is a group of organic acids that can attack the nylon, causing further damage.

Figure 3 shows the infrared spectrum for the failed part along with library spectra for a nylon and a polyester. Infrared spectroscopy (FTIR) cannot distinguish between nylon 6 and nylon 6/6 or between PET polyester and PBT polyester, but the DSC has already accomplished that with melting point determinations. The FTIR has confirmed, however, that our failed part is a mixture of nylon and polyester.

HIGHER LOADINGS, LOWER MELT-FLOW RATE
The last piece of the puzzle involves the change in MFR. Typically when PBT polyester and nylon 6/6 are mixed and processed at normal nylon 6/6 melt temperatures, the degradation in both polymers is severe and the MFR can be expected to increase significantly as a result. However, in this case, the MFR went down dramatically.

The reason for this is revealed by the third test. Thermogravimetric analysis (TGA) evaluates materials by controlled decomposition. Figure 4 shows the result of a TGA scan on the brittle part. There are several features in this result that would not be present in a clean nylon and that confirm the presence of the PBT polyester.

However, the result of greatest interest to our inquiry comes at the end of the test. The total noncombustible residue for this sample is more than 23%, instead of the expected 13%. This tells us that not only was the contaminant a PBT polyester, but it was also a highly filled PBT. Judging from the relative strength of the DSC melting points, we can estimate a filler content of 30-35% in the contaminating material.

The higher glass loading explains the decline in MFR. While it is very likely that the polymer portion of the compound did degrade significantly, the additional glass fiber increased the melt viscosity sufficiently to counter the effects of polymer degradation. The TGA test also provides critical information to determine the source of the contamination, which is a key component of the root cause reporting that is typically part of today's corrective action cycle.

Finally, knowing the source of the problem allows for a simple technique to quarantine and disposition any remaining inventory. The specific gravity of 13% glass-filled nylon 6/6 is approximately 1.28, while the specific gravity of a 30% filled PBT is 1.57. A simple check of part weight will distinguish clean from contaminated product. More importantly, an ongoing check of part weight during production can flag these types of contamination issues before they develop into full-blown failure analysis inquiries.

April , 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.