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 hte technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. He has provided analytical services to material suppliers, molders, and end users for the last 10 years. He can be reached at (414) 369-5555, Ext. 572.
An important measure of the user friendliness of a thermoplastic material is the processing window. The lower end of the processing window is defined by the minimum temperature at which the material can be melted and homogenized by the combined influence of the barrel heater bands and the mechanical action of the screw. The upper end is defined by the temperature at which the material begins to degrade. Degradation typically results in a reduction in molecular weight and this in turn leads to a brittle molded product.
If the temperature difference between these two events is larger, then the material has a wide processing window and it is relatively easy to process. Runner scrap and parts that are out of specification can be reclaimed and re-used freely in these cases without undue concern over property losses. Materials like polyethylene fall in this category. However, if the gap between melting and degradation is small, then the material can be very unforgiving. Melt temperatures and residence times must be monitored carefully, and recycle must be used sparingly or not at all. The best known example of this type of material is PVC, but other resins like PBT polyester and some of the new high-temperature nylons can also fall into this family.
|Figure 1. DSC scans show the melting points for certain semicrystalline thermosplastics.|
Detecting the Onset of Degradation
In previous articles, we have illustrated the use of a technique known as DSC to identify the melting point of a material. It is also possible to pinpoint the temperature at which a material begins to degrade. The melting event appears as a sudden downward departure from the baseline. The melting process is endothermic; that means it takes heat from the surrounding environment to break up the crystalline structure in a material like polyethylene or acetal. Figure 1 shows melting points for four common semi-crystalline materials: HDPE, acetal homopolymer, PBT polyester, and nylon 6/6.
Degradation is often the result of a chemical reaction between the polymer and oxygen. Materials contain antioxidants to prevent this reaction, but every material has its limits. The oxidation process is exothermic; it gives off heat. The DSC curve will rise rapidly from the baseline when this reaction occurs.
Figure 2 illustrates this event for a polypropylene that has been tested in air. The material melts at 163C (325F) and begins to degrade at around 240C (465F) if the sample is tested in air. The temperature region between the two events defines the processing window.
|Figure 2. DSC showing oxidation and melting point of polypropylene.|
No Processing Window
A customer of ours was struggling with a nylon-based elastomer. He was purchasing the material in its unstabilized state and then adding his own antioxidants to the material in a compounding step before molding the compounded material into a medical product. But the material seemed to have no processing window. Almost as soon as it softened, the color of the material darkened and it became difficult to control. A sample of the compounded material was sent in and we evaluated it by DSC.
The test result is shown in Figure 3 and the problem is clear. The melting process is not complete until the material reaches 180C (356F). Almost as soon as the melting process is complete, oxidation begins. By the time the material reaches 195C (383F), oxidative degradation is well under way.
|Figure 3. Comparison of acceptable and unacceptable parts.|
The resulting window is only 15 deg C (27 deg F). If this seems terribly narrow, consider that in the DSC there is no shear and no significant residence time. So as bad as the results appear in the DSC, in the reality of the molding machine they will be even worse.
Once the customer understood the nature of the problem, he reformulated the material with increasing amounts of antioxidant. These additives can be expensive, and a proper cost/performance balance is important. The original material contained only .05 percent stabilizer. Figure 4 shows the results of increasing the stabilizer concentration to .1, .25, and .5 percent.
|Figure 4. DSC shows improvement in resistance to oxidation with increased antioxidant concentrations.|
From an initial degradation temperature of 195C the oxidation onset rose to 210C, then to 240C, and finally to 250C. The small improvement of 10 deg C gained by doubling the antioxidant from .25 percent to .5 percent was not deemed cost-effective. With the formulation set at .25 percent, the window between the end of melting and the onset of degradation had increased from 15 deg C (27 deg F) to 60 deg C (108 deg F).
With this improved window, the barrel temperatures could be set at a point where a homogenous melt could be achieved and the viscosity needed to fill the parts could be attained without leading to the cosmetic and structural problems associated with degradation. The use of DSC testing enabled the customer to optimize the cost/performance balance of the compound and solve the molding problem as well.