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 discussed the effect that some pigments can have on the thermal stability of a base resin and how this loss in thermal stability can result in a narrower processing window and the risk of losing key propertiesâparticularly impact resistance. This month we will turn our attention to another route to poor impact performance that does not affect molecular weight at all and therefore cannot be diagnosed using the tests that we used in last monthâs study of the problem.
This monthâs case involves a sudden loss in ductility in parts molded from a polypropylene copolymer. The failures appeared initially in a living hinge detail, but an examination showed that the problem was not limited to the hinge area.
This client had done a significant amount of work in material selection during the original launch of this product five years earlier and had discovered an excellent correlation between living hinge performance and the results of instrumented falling dart impact tests. Many different polypropylene base resins had been reviewed during this development period, resulting in the selection of a particular copolymer with a melt flow rate of 12 g/10 min based on its excellent impact resistance. The development work had been performed using the color for the final product, which was white. Now a newly introduced blue product was failing badly.
When an application using color concentrates displays performance problems, it is always best to start with an examination of the concentrate. Most color concentrates are somewhere between 25 and 50 percent color and the rest is a carrier resin, a polymer that binds the color system and makes it easy to incorporate into the base resin through mechanical mixing of some kind. Recommended letdown ratios of color concentrate vary widely depending upon a lot of factors, but 25 parts of base resin to one part of color concentrate is not uncommon. In such a mixture, 4 lb of color concentrate will be added to 100 lb of natural virgin material. If the color concentrate is 50 percent color and 50 percent carrier resin, then 2 lb of that color concentrate is carrier resin.
|Table 1. Effects of color on impact performance|
This raises a question. What is that carrier resin? Many color houses use so-called âuniversalâ carriers, secure in the belief that you can add 2 to 3 percent of almost anything to a pure polymer without noticing a change in properties. But as many in the industry will attest, the act of adding the color itself represents contamination, so the least we can do is be sure that the composition of the carrier resin does not depart dramatically from that of the virgin material. Many living hinge applications have been done in by the apparently innocent substitution of polyethylene for polypropylene as the carrier resin. So when a performance problem arises with a new color system, it is prudent to test the colorant to determine if the carrier resin is compatible with the base resin.
In this case the carrier resin was a polypropylene and our first line of investigation was a dead end. Melt flow rate tests performed on the molded parts also showed no problems. The base resin used to produce the first blue parts had a melt flow rate of 11.8 g/10 min and the molded parts tested out at 12.56 g/10 min, a change of only 6.5 percent.
Next, the client wanted to determine if the correlation between impact test results and living hinge performance held up. The assumption had been made that if the white parts were working well the blue ones would perform equally well, so no impact tests had been run before releasing the parts. Material was mixed at the prescribed ratio and molded.
Table 1 shows the results of the original qualification tests on the natural and the white material from five years earlier, along with the results for the failing blue product. The problem with the hinges was clearly reflected in the results from the standard impact tests. Not only was the energy required to fail the samples reduced by almost 80 percent, but the failure mode had changed from ductile to brittle. This correlated well to the lack of stress whitening around the cracks in the failed parts.
Property changes can occur in fully compounded systems just as readily as they do in natural-plus-concentrate mixtures.
Now, with all of the revolutionary activity in polymer catalysts, particularly in the polyolefins, there was no guarantee that the base polypropylene was the same as it had been five years earlier. Remember that the impact test results for the natural and the white in Table 1 were not current results. So to close the loop on this line of investigation, the current lot of natural material was molded with a current lot of white concentrate and tested. The results produced an average value of 24.62 ft-lb and a ductile failure. The agreement with the original tests could not have been better; the base resin was not at fault.
With no contamination from the color concentrate, no degradation, and no change in the properties of the base resin, attention turned to the crystallinity of the polypropylene. Polypropylene copolymers have been used for years to extend the low-temperature performance of the polypropylene family. While there are a number of types of copolymers, they all achieve their improved impact by reducing the structural regularity of the polypropylene molecule. This structural regularity is the source of the materialâs crystallinity.
Polypropylene homopolymers have a relatively high degree of crystallinity and provide the best strength, stiffness, and elevated temperature resistance within the polypropylene family. But they also tend to be brittle, even at room temperature. By inserting another chemical component into the polymer, this crystallinity is reduced. The desirable properties of the homopolymers are also reduced, but the payoff is improved impact resistance. With the addition of sufficient comonomer, the brittle polypropylene matrix can be turned into a soft and very tough material.
One of the simplest methods for measuring the relative degree of crystallinity in any semicrystalline material like polypropylene is differential scanning calorimetry (DSC). As the crystallinity of the material changes, the energy required to melt the crystals also changes. This property is called the latent heat of fusion (Hf) and is measured as the area under the curve associated with the phase change from solid to liquid. We compared the melting behavior of a sample from a part molded in natural material with one molded in blue.
|Figure 1. Comparison of melting between natural and blue polypropylene.|
The results are shown in Figure 1. The area under the curve for the blue part was 9 percent greater than for the part molded in natural, even though the melting points (Tm) for the two differed by less than half a degree. The white part, not shown in the graph, had an area under the curve that was only 2 percent greater than that of the natural product.
Frequently, when DSC is used in this type of investigation, the analysis ends at this point. The analyst points confidently to the difference in crystallinity and declares the problem solved. But this comparison is made on molded parts. The results reflect a difference that may be due to the properties of the material, but they may also reflect differences in the molding process.
The degree of crystallinity in a molded part is determined in part by the process, in particular by the rate at which the molded part is cooled. In order to obtain a full picture of what is happening, the material must also be compared as it cools from the melt and is then reheated. If the differences seen on first heat disappear on second heat, then the problem can be attributed to the molding process. But if the differences are still there on second heat or if they have been magnified, then they can be assigned to the material itself. Often the cooling process that takes place between the first and second heat sheds some light on the source of the problem.
|Figure 2. Comparison of recrystallization between natural and blue polypropylene.|
This proved to be true in this case. As the samples were cooled from the melt under controlled conditions, a striking difference was observed. The part molded in natural material reached its peak recrystallization temperature (Tc) at 110C while the blue part achieved this same peak at 127C as shown in Figure 2. The white part reached its peak at 113C. In addition, the energy released by the three materials during recrystallization (Hc) shows that the differences in crystallinity observed on first heat were magnified in the blue material.
|Color||1st heat Tm (C)||1st heat Hf (J/g)||Tc (C)||Hc (J/g)||2nd heat Tm (C)||2nd heat Hf (J/g)|
|Table 2. DSC results for natural, white, and blue PP copolymer|
Table 2 shows the phase change peak temperatures and the heat of fusion values for all three colors. While the white part shows small differences from the natural, the real story is in the blue product. On recrystallization the difference that was 9 percent on initial heating increases to 21 percent. The difference between the blue and the natural remains high on second heat at 18 percent.
So what is happening here and how does it explain the impact performance problems? The increase in the recrystallization temperature observed in the blue product represents a pheno-menon known as nucleation. Nucleation is employed intentionally in materials like polypropylene and nylon to speed up crystallization and decrease cycle time. If the material begins to crystallize at a higher temperature then it will become rigid enough to eject from the mold that much sooner.
Clearly, the blue material crystallizes at a much higher temperature than the natural. In fact, the crystallization process in the blue product is essentially over before the natural product begins to solidify. In this case the blue pigment system is acting as a powerful nucleating agent on the polypropylene.
If you have worked with nucleated materials, then you know that like most things in life they represent a trade-off. Cycle times are indeed shorter and the strength and stiffness of a nucleated material is typically greater than that of a standard compound. The one thing that does not get better when nucleation is employed is (you guessed it) impact strength.
Material suppliers go to great lengths to tailor various levels of copolymerization in polypropylenes that improve toughness without unduly compromising the strength and stiffness side of the equation. But colorants can shift the balance. The increased level of crystallinity in the blue material represents a stronger and stiffer but also a more brittle material. The white material shows some of the same effects, but they are small and do not change the overall property balance significantly. The magnified differences in cooling and second heat confirm that the differences seen on first heat were material driven and not a function of the process.
Hopefully, these last two articles have served to illustrate a few of the unintended consequences of adding color to a polymer. It is a process that is often taken for granted. Property changes can occur in fully compounded systems just as readily as they do in natural-plus-concentrate mixtures. As marketing becomes more creative and special effects colorants become more popular, an awareness of the effects of color on final properties will become that much more important.
Dickten & Masch Mfg. Co.
Phone: (262) 369-5555, ext. 572
Fax: (262) 367-2331
E-mail: [email protected]