The Materials Analyst, Part 15: Distinguishing among fillers

Frequently, we are asked to identify the material used to make a particular product. In some cases, a customer may observe a particular property in a product, such as clarity, toughness, or abrasion resistance, that would be particularly useful in a new product he or she is developing. A few simple tests to determine the composition of the material can save days of guesswork, sampling, and testing.

In other cases, an existing product suddenly starts to fail or to perform in an uncharacteristic manner, and the client wants to know if the formulation of the material has been changed. This month, we deal with examples of each type of detective work.

Polypropylene at Work
Both of our examples revolve around polypropylene. While polypropylene is often thought of as a commodity material, the creative use of various fillers and reinforcements has helped to reincarnate this material as a low-end engineering resin. Polymer selections can vary widely from stiff homopolymers to very tough block copolymers. Fillers such as talc, calcium carbonate, and mica can be used in different loadings to adjust the balance of strength, stiffness, and impact resistance while still maintaining a reasonable cost.

At the upper end of the performance scale, short or long glass fibers can be used to provide properties that frequently allow polypropylene to compete with other high-end resins such as nylons, polyesters, and PPO. Ironically, this rich assortment of options can cause problems. When trying to decide on a formulation to achieve a particular cost/performance balance, there are almost too many choices. And if a formulation changes, it can be difficult to detect the alteration.

Balancing Stiffness and Buoyancy
Our first case came from a client who needed to make a product for sailboarding. It had been designed with relatively thick walls for good rigidity, but in order for it to be useful, it had to float. Even with the thick walls, the client was having difficulty finding a material stiff enough to bear the load of the application without adding so much filler that the part sank. Because the part was thick, a blowing agent was being used to eliminate sink marks and reduce weight. But this only reduced the density by 5 or 6 percent, not enough when the material contained the 30 to 40 percent filler needed to achieve the desired stiffness.

After consuming many hours in an 850-ton press sampling a variety of compounds, our client found an off-the-shelf product with the desired properties. Rather than continuing to eat up valuable press time, it was sent to us for analysis.

The part had obviously been produced with a blowing agent. It was quite thick and had the telltale splay marks. A DSC test quickly confirmed the polymer was polypropylene. Most other materials have densities greater than one even before they are filled. Polypropylene, with a density of .901, is one of a few that could float after being filled. The hard part was identifying the filler. With so many options and the added possibility of combining fillers in a compound, the chances of guessing the content were slim.

Thermogravimetric analysis (TGA) was used to determine the filler content. Once the material was heated to the point where the polymer had been removed, 13 percent of the compound remained. We then placed this residue under a microscope. The thin, shiny slivers told the story; the filler was short glass fiber. Previously, our client had been using lower cost fillers such as talc. While this improved stiffness, it could not achieve the needed modulus even at 40 percent loadings. By then, the density was over 1.2, and the part sank like a stone. With 13 percent glass fiber, the product was stiff enough to do the job, and the density before foaming was 1.04. The blowing agent could accomplish the remaining density reduction. The mystery had been solved in less than 48 hours, and a lot of unnecessary sampling time and field testing had been eliminated.

Loss of Hardness and Scratch Resistance
The second case involved an existing product that suddenly began to perform poorly. The product was being made from a 20 percent talc-filled polypropylene selected for a balance of good surface hardness, high modulus, and moderate impact strength. Quite abruptly, the molded parts had become too flexible, and the surface was easily scratched. The customer suspected a reduction in filler content and sent a sample for a determination of the talc content.

Once again, TGA is the method of choice to determine filler content. Figure 1 shows a TGA scan for a known polypropylene containing 20 percent talc. Weight loss and weight loss rate are shown. As the material is heated in nitrogen, the polymer begins to decompose near 450C. By the time the temperature reaches 500C, the polymer is gone. The atmosphere is switched over to air to burn off any carbon that might have formed. In polypropylene, this step usually yields nothing unless the material has been pigmented with carbon black. Any material remaining is filler.

The sample we had, shown in Figure 2, was different. The polymer weight loss portion was normal enough. When the test reached 500C, the polymer was gone, leaving behind a residue of a little over 20 percent. So far, so good.

Then, between 700 and 750C, a new event started. A new weight loss occurred. It accounted for an additional 9.35 percent of the material and left us with a filler content of only 12.18 percent. Figure 3 compares the weight loss process for 20 percent talc-filled material and the customer sample. Not only did we not have enough talc, we had contamination as well!

The problem is that no polymer that flows at polypropylene molding conditions can last in air at such a high temperature. So what was the mysterious weight loss? Well, experience is a great teacher, and routine tests had taught us this pattern was a signature for calcium carbonate.

Unlike talc, which is completely stable at temperatures well over 1000C, calcium carbonate breaks down partially into calcium oxide (lime) and carbon dioxide. The lime stays behind as residue, but the CO2 leaves the sample chamber. If we do the math and can remember just enough high school chemistry to be dangerous, we can figure out that 56 percent of the calcium carbonate stays as lime and 44 percent leaves as carbon dioxide.

Our residue had been 21.53 percent before the second weight loss began. The final residue of 12.18 percent constituted 56.60 percent of the initial residue. The identification was done.

The filler content was not the problem in this situation; there was 20 percent filler in the material, but it was the wrong filler. If you are a student of filled polypropylenes, you can completely understand the problem. Calcium carbonate is a very useful filler in polypropylene. However, its primary attribute is its ability to maintain or even enhance impact properties. In doing so, it reduces strength and stiffness and creates a softer surface.

In Conclusion
With customers constantly trying to improve the balance between cost and performance, fillers play an increasingly key role in this endeavor. Controlling the filler content and using the correct filler or reinforcement are absolutely critical to optimizing properties and controlling cost. The right analytical technique can usually save significant time and money in the areas of quality control and material selection.