The Materials Analyst, Part 56: How stable is your material? (Part 1)

One of the most common causes of plastic part failure is polymer degradation during melt processing. In most materials, this degradation results in a reduction in the average molecular weight of the polymer. This reduction is readily measured by a variety of techniques, the simplest being the melt flow rate test.

When presented with a test result that shows degradation, many processors ask how they can determine a material’s stability. Often the question is couched in terms of recyclability: How many times can a material be reground and returned to the molding process before it has been pushed too far? The answer is, it depends on the polymer.

All polymers are subject to thermal degradation. The temperature required to produce a melt of manageable viscosity involves a significant input of energy. But remember, polymers are organic materials. They have a limited tolerance for prolonged exposure to elevated temperatures. Time and temperature work together; a melt temperature that may be appropriate for a given material for 5 minutes may cause problems if extended to 10 minutes.

Overall, thermal stability in the melt is assessed by measuring the change in molecular weight after processing. Some material suppliers process the same batch of material five times in order to study the cumulative effect of melt processing on property retention. However, these studies are fragmented and often not well documented, so it is hard to gain an overview of where all the various polymers rank.

Temperature Limits
As is usually the case, the overarching rules that govern such a question come down to chemistry. In general, amorphous materials are more thermally stable than semicrystalline ones. This is due to the simple fact that there is a wider temperature window between the minimum processing temperature and the degradation temperature for materials that do not have a crystalline melting point.

Amorphous materials lose their structural integrity when they pass through the glass transition. On average, approximately 200 deg C separates the glass transition temperature (Tg) from the onset of degradation. But it usually only takes a temperature rise of 100 deg C above the glass transition temperature to achieve a melt viscosity that permits injection molding of an amorphous material. This leaves a window of approximately 100 deg C that a processor can operate within. PC, acrylic, PS, and many other amorphous thermoplastics exhibit these generous processing windows.

Most semicrystalline materials also possess this window of 200 deg C between Tg and the degradation temperature. However, semicrystalline materials do not soften at Tg. Instead, because of the crystalline structure, it often requires a temperature increase of 150 deg C just to go from Tg to the melting point. This leaves, at maximum, only 50 deg C with which to operate. Acetal, PET and PBT polyesters, PPS, and most nylons provide thermal processing windows of 50 deg C or less.

Chemistry
A second set of rules addressing the particular chemistry of certain polymers must be added to this scheme of semicrystalline and amorphous structure. The most thermally stable materials are the saturated hydrocarbon-based ones. As the name would suggest, hydrocarbon-based polymers are those made entirely from carbon and hydrogen. This means that all the chemical bonds within the polymer are either carbon-carbon (C-C) or carbon-hydrogen (C-H). Saturation means that all of the C-C bonds are single bonds. These are among the strongest and most stable bonds in organic chemistry and therefore very difficult to break.

PE and PP are members of this family, and even though they are semicrystalline materials they tend to be at the top of the food chain when it comes to thermal stability. This is not a violation of the rule that favors amorphous polymers over semicrystalline ones. It merely means that when chemistry and structure are both considerations, chemistry wins.

PS is also a hydrocarbon polymer, but impact grades add butadiene rubber. This introduces unsaturation, reducing the material’s thermal stability.

Acrylics and ABS resins add oxygen and nitrogen to the system, respectively. These attachments provide new capabilities in terms of physical performance, but they increase the thermal sensitivity of the materials. Fortunately, in these materials, these sensitive bonds remain along the side of the polymer chain.

Things get really interesting when we place these same oxygen and nitrogen atoms in the polymer backbone. Polyesters and nylons incorporate such chemistries and performance benefits are immense. But the melt’s thermal stability becomes an increasing concern, particularly since these materials require higher temperatures to melt and process.

At the low end of the spectrum we have materials like PVC. Though amorphous in structure, PVC contains chlorine. The carbon-chlorine (C-Cl) bond is not as strong as most of the other bonds discussed, and the chlorine atom is relatively easy to remove from the polymer chain. Processors experienced with PVC are all too familiar with where the chlorine goes.

Stability Ladder
We can arrange the major polymer families in a semiquantitative way into a hierarchy from most stable to least stable, as shown in Table 1. This list is based on the degree of change in the molecular weight or other important structural characteristics that the polymer undergoes each time it is processed at midrange temperature conditions using reasonable residence times. It is not related to how high the process temperature can go before problems occur, which is why PP is judged to be more thermally stable than nylon or PPS.

It also assumes no special modifiers are present, such as flame retardants or impact modifiers, which of course can change everything. This list is only meant as a general guide.

The problem of degradation is complicated for some polymers by moisture. Only a few polymers actually undergo significant degradation in the presence of moisture, but they are workhorse materials so the consideration is important (Table 2).

When degradation problems occur with one of these materials, the inevitable question arises: Was it temperature or moisture? It is an important question because it determines the troubleshooting strategy in the plant. In Part 2, we will illustrate a method that anyone with a molding machine, a melt flow rate tester, and a little patience can use to determine the interaction between melt temperature and moisture content in this select group of materials.