The Materials Analyst, Part 3: Lot-to-lot variation

This series of articles is designed to help molders understandhow a few analytical tools can help diagnose a part failure problem.Michael Sepe is our analyst and author. He is the technical directorat Dickten & Masch Mfg., a molder of thermoset and thermoplasticmaterials in Nashotah, WI. He has provided analytical servicesto material suppliers, molders, and end users for the last 10years. He can be reached at (414) 369-5555, Ext. 572.

Material problems are best handled before a lot of productis made from the raw material. A molder who is aware of the normalprocess conditions for a job, or who takes the time to test productas it comes off the line, is more likely to be in a position tospot problems before they mushroom into a quality-control nightmare.The processor we worked with on this project runs a high-qualityoperation where frequent tests are performed on the end product.When new lots of material failed to produce the expected quality,this company was able to isolate samples from different runs andsubmit them to our laboratory with a detailed account of the problems.

The material in question was an ethylene-vinyl acetate (EVA) copolymer.In order for a blend or a copolymer to perform consistently, theamount of the different ingredients in the compound must be consistent.Vinyl acetate is added to polyethylene in different amounts tomodify certain properties. Increasing the concentration of vinylacetate will increase low-temperature toughness, flexibility,resistance to environmental stress cracking, weathering, and flex-uralfatigue. Adding vinyl acetate will also increase the ability ofthe material to heat weld to itself. On the down side, vinyl acetatereduces modulus, melting point, crystallinity, chemical resistance,yield stress, and barrier properties. With such a wide array ofcritical properties at stake, it is obvious that the concentrationof vinyl acetate must remain as consistent as possible. If it'stoo rubbery, for example, it's very difficult to mold. In fact,most EVAs are extruded. But reducing lot-to-lot variation is importantfor molding too, and testing for the amount of vinyl acetate inthe compound is a good place to start.

One test method that can be used successfully to separate ingredientsin a blend is thermogravimetric analysis (TGA for short). As wesaw in last month's case study, it is a fancy name for a simplemethod that heats a material and measures the change in weightof the material as a function of temperature. At some point, allpolymers become hot enough to decompose. When decomposition starts,the instrument will detect a weight loss.

Figure 1 shows a typical result for a TGA test on polyethylene.One curve plots the actual weight of the sample in percent. Webegin with 100 percent of the sample, and at some point, the materialbecomes hot enough to degrade and leave the weigh pan. Eventually,all of the material is burned away and the graph shows the remainingweight at 0 percent. The second curve is the derivative of theweight curve. It shows the weight loss rate. The fact that therate curve is smooth and symmetrical tells us that this sampleis uniform and probably composed of one polymer.

However, if a material blend is made of two or more ingredientsthat degrade at different temperatures, then they can be separatedby TGA. EVA happens to be one of those materials. If EVA is heatedslowly enough, the vinyl acetate portion of the material willdegrade completely before the more stable polyethylene beginsto decompose.

Figure 2 shows a typical result for a TGA scan of an EVA copolymer.The vinyl acetate degrades at a much lower temperature than thepolyethylene. The weight loss levels off in between the vinylacetate and the polyethylene. It is necessary to achieve thisseparation in order to get good results. If the polyethylene beganto burn away before the vinyl acetate weight loss was complete,then this would not be a good method for quantifying the two ingredients.

Figure 3 shows the results of three TGA tests on three differentlots of EVA copolymer. For clarity, only the weight curves areshown here. All three materials show an initial weight loss followedclosely by degradation of the remaining material. It is easy tosee that the initial weight losses are different for all threematerials. The control batch that the customer wanted to duplicateis labeled #2. The new batches that had come in had either toomuch or too little vinyl acetate and the problems with the variationwere explained.

Once this type of history is developed, a customer can institutean incoming QC test on the material's composition instead of spendingtime and money adjusting the process for each new lot and performinga large amount of product testing after the parts have been produced.