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July 4, 2000

11 Min Read
The Materials Analyst, Part 34:  Identifying that last special ingredient

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This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure problem. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years. 

Last month we talked about the primary techniques used to determine the type of polymer or polymers present in a commercial compound. But there is a lot more to a material than this. Even in the simplest material there are traces of stabilizers to prevent the polymer from breaking down during processing. There will also usually be lubricants designed to promote proper feeding of the material into the screw and facilitate release from the mold. There are colorants that are either incorporated into the material or supplied as color concentrates. There may be special additives such as flame retardants. And of course there are fillers and reinforcements that can range from calcium carbonate and talc on the low end to carbon fibers on the high end. 

Frequently we get a request from a client that starts with, "I need a complete breakdown of everything in my compound." Less than 1 percent of the time this ends up being a real requirement. Usually at the root of this request is the desire to determine the nature of a particular processing or performance problem. It’s not that analysts don’t like to do the complete workup; it’s fun to probe the deep, dark secrets of a material. But it is also very expensive. And the rule of thumb is that 95 to 99 percent of the cost of analysis is spent on the last 1 to 5 percent of the ingredients, kind of a magnified version of the 80/20 rule. So let’s start with a brief description of a worst-case scenario and then work backwards to some more practical solutions. 

A Complete Examination
The exact techniques required to do a full analysis may vary depending upon the material. But in general this is how it works. First we dissolve the polymer in a solvent. We then perform some variation on a technique that we have discussed before, gel permeation chromatography (GPC). 

All chromatography is fundamentally the science of separating materials that may seem irretrievably mixed together. Some of you may remember doing an experiment in high school or junior high school chemistry in which different colors of ink were applied to pieces of paper that were hung vertically. The inks were then dabbed with a solvent, usually a simple alcohol. Over time some of the inks began to separate into multiple colors. Black was particularly good for this type of experiment. This is chromatography at its simplest. It employed a difference in solubility to separate the various colors that were mixed together to produce what appeared to be a monochromatic material. 

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Figure 1. Idealized test result from a GPC test. 


In GPC we are using differences in molecular size to separate materials from one another. When we inject the dissolved material into a column, the various sizes of molecules make their way through the column at different rates; the large ones have the lowest affinity for the column medium and go through the fastest while the smaller molecules take longer. Figure 1 shows an idealized result of such an experiment. We end up with three distinct regions. First we get the polymer, which is made up of a range of molecular sizes or molecular weights. 

The rule of thumb is that 95 to 99 percent of the cost of analysis is spent on the last 1 to 5 percent of the ingredients.

There is a great deal of information in this section alone as we have discussed in the past. Then we get some lower-molecular-weight material called oligomers. These are usually made up of the same material as the polymer, but during polymerization they never grew up to be full-fledged polymer chains. They typically serve as flow enhancers. In a degraded material they may be present in an unusually high concentration, one of the trouble signs in diagnosing causes of brittle behavior. 

Finally, there will be residual monomer and the various additives. While this diagram shows these as a single peak, a more discerning analysis may reveal as many as a dozen separate compounds in this section. As these materials come through the column they can be collected and analyzed using one of several techniques. This will result in an impressive laundry list of ingredients that have been measured down to hundreds of even tens of parts per million. Knowledge of the chemistry of these various materials can then be used to determine the function of each substance in the compound. 

As cool as all this may sound, the reality is that it can take a long time even if performed on a rush basis, can cost thousands of dollars, and in the end still may not pinpoint the problem that led to the desire for the analysis. So when a request for a complete analysis arrives, the first thing to do is ask some questions of the client. This is analogous to going to a physician for an ailment. It is certainly technologically possible to measure every detail of every bodily function, but a responsible doctor starts by simply asking, "Where does it hurt?" Often the answers to that question dictate the problem-solving approach. So let’s look at a few examples of alternatives. 

Stabilizers, Colorants, and Fillers
One of the most common requests is one for the amount of stabilizer present. This is usually some form of antioxidant without which few processes could run. The chemical names for some of these materials can take up an entire line on this page, but the bottom line is how effective the stabilizer is. To determine this, the wire and cable industry developed a test almost half a century ago called the oxidation induction time (OIT) test. It works particularly well on materials like polyethylene and polypropylene, where these questions often arise. A small sample of material is placed in a DSC cell and heated to a particular temperature in a highly oxygenated atmosphere. This aggressive environment rapidly consumes the antioxidant in the compound. 

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Figure 2. Comparison of poypropylenes with different stabilizer levels. 


Once the antioxidant is gone, the polymer degrades rapidly. Figure 2 shows a comparison of a properly stabilized material and a sample that has lost all of its antioxidant package. It takes 50 minutes for the good material to degrade while the bad material begins to react almost immediately. The number of parts per million of antioxidant present in these two samples, while perhaps of great academic interest, is seldom the point when it comes to rapidly identifying a real-world problem. This type of analysis also works for UV stabilizers and radiation-resistant stabilizers since these systems all operate by essentially the same mechanism. 

The elimation of heavy metal systems has made the analysis of colorant systems far more challenging.

Colorants often present problems in both processing and performance. The elimination of heavy metal systems based on elements such as cadmium and lead has made the analysis of colorant systems far more challenging. In the good old days most of these systems were stable at temperatures well above the degradation point of the polymer and could easily be collected by simply ashing the compound and collecting the residue for an analysis of the elements. Now many of the dyes and organic pigments degrade along with the polymer, making such a separation impossible. But there are some situations in which simple solutions still work. 

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Figure 3. Comparison of white pigment content by TGA. 


Figure 3 shows a TGA analysis of three white polypropylene parts. Two of these parts were brittle while the third performed satisfactorily. Almost all white pigments are still based on titanium dioxide, which does not degrade in normal TGA test conditions. In an unfilled material, therefore, any residue can be attributed to the colorant. The brittleness in this case was caused by an overmixing condition; adding too much concentrate interfered with the ductility of the polymer.

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Figure 4. TGA pigment analysis of white concentrate. 


This test technique works even better on a concentrate in which the colorant is present at a much higher level. Figure 4 shows a result for a white concentrate designed for high-impact polystyrene. In this figure we show both the weight loss and the weight loss rate. Here we see some details that might be missed in the molded part. As expected, the polymer burns off first, leaving behind an ash of 55 percent. Most of this is titanium dioxide, as expected. But there is a small additional weight loss above 700C that starts slowly and picks up speed as it nears completion. Experience has taught us that this is the partial decomposition of calcium carbonate being added, probably in an effort to reduce cost. 

Even if we did not know about this distinctive fingerprint, we could confirm this identification by performing a simple test on the residue known as energy dispersive X-ray spectroscopy (EDS or EDX, depending upon whom you talk to). This technique breaks down the sample into its constituent elements by capitalizing on the fact that each element interacts with X rays in a unique way. In a sample of pure TiO2 we would see only titanium and oxygen. In the ash from the sample in Figure 4 we will also see a trace of calcium. 

This same technique can also help us with filler analysis. Determining the amount of a filler is fairly straightforward using TGA, as we have shown in previous articles. Identifying this filler can be a little more difficult. The best way to start is simply to put the residue under a microscope and look at it. Glass fibers and beads have a distinctive appearance. If you have a microscope with a digitized grid, you can even take measurements of the length and diameter of the fibers. The ratio of these two quantities, known as the aspect ratio, is a key component of how well a reinforcement does its job. Mica also has a distinct plate-like appearance that is easily recognized. 

Table 1. Composition of common fillers by weight (based on ideal chemical formulas)

Element

Magnesium

Aluminum

Calcium

Silicon

Oxygen


But it gets interesting when we get into the various minerals that are used in the industry. We have already shown how calcium carbonate behaves. But most other minerals are very stable at the temperatures used in a TGA and are therefore difficult to distinguish. Here the EDS technique is very useful. The important mineral fillers that are used in addition to calcium carbonate are kaolin clay, talc, and wollastonite. Each of these contains a unique balance of light elements that includes magnesium, aluminum, silicon, and calcium. Table 1 shows the breakdown for these three fillers by weight. Identifying a particular filler is generally a matter of matching the profile of the unknown residue to these tables. Most older EDS instruments do not even detect the oxygen, and the newer ones that do still do not see it in direct proportion, so the actual numbers will emphasize the other elements in the compound—but you get the idea. 

In the real world these proportions are not so predictable, and the exact balance of these elements can vary. But it is fairly easy to see from this table that there is a unique major component in each of these three minerals that is not present in the other two. This makes the test a useful one in distinguishing between fillers that might otherwise just look like nondescript white powders. 

Glenn Beall has made the point that good design is a matter of knowing the difference between what is possible and what is practical. The same can be said for material analysis. Being able to use the high-end techniques in those rare cases where they are needed is a wonderful thing. But usually a little questioning leads to a simpler, faster, and less expensive solution. In these days when both time and money seem to be in short supply, a parsimonious approach is usually the best one.

Contact information
Dickten & Masch Mfg. Co.
Nashotah, WI
Mike Sepe
Phone: (262) 369-5555, ext. 572
Fax: (262) 367-2331
Web: www.dmanalytical.com
E-mail: [email protected]

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