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Foaming polyolefi ns | technical paper


John Krist and Emanuel van der Ven of Sabic describe the optimisation of polyethylene foam formulations by design of


experiments and validation with production results


Optimising foamed


polyolefi n formulations using DOE and practical processes


The foaming of thermoplastic polymers is a process with a huge number of parameter settings and variables. For materials development and research programmes, many tests are needed to get an under- standing of material properties and foaming behaviour. Executing all these tests is hard to do on production lines with outputs larger than 100 kg/hour. Therefore, there is the question whether tests executed using a small laboratory line at 10-20 kg/h can be representa- tive for production on commercial lines. A Design Of Experiments (DOE) can be used to minimize the number of experiments and to obtain statistical information about accuracies. DOE is a mathematical approach to obtain as much possible quantitative information as possible out of a test matrix with the least number of possible tests. The results of these experiments at a laboratory scale are discussed in relation to practical experiences and production using commercial lines. The laboratory scale tests were used to study


physical foaming processes with variation of param- eters such as cell stabilizers, nucleating agents and blowing agents. Different foam resins were also compared during these tests, including Ultra Melt


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Strength (UMS) grades. The processing and performance of the relatively


new UMS resins was validated in practice and related to the properties of the end product. These properties were compared with the properties of products produced with regular foam grades. The work addressed the following main questions: (1) What are the most important parameters for the production of foam with UMS grades? (2) Does DOE supply useful information to build a model and what are the most infl uential parameters for the foaming process?


(3) How can these parameters be applied in practice? (4) What are the advantages for the market of these UMS grades?


Setting up the design A classical approach of experiments would be based upon “one variable at a time” (OVAT). For polymer foaming, a vast range of parameters can be applied. Therefore, parameters such as output, die pressure and temperature settings are kept constant. Only basic things are changed, for example: nucleating agent at 0.25, 0.5, 1.0 and 2.0%; blowing agent at 6, 10


January 2013 | COMPOUNDING WORLD 45


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