technical paper | Foaming polyolefi ns
Figure 1: Example of OVAT experimental set up with 36 experiments
Figure 2: DOE of two materials and three factors. Sabic LDPE 2202UMS and 2102TX00 grades (left and right respectively) are analysed using a total of 32 experiments
and 13 %; and cell stabilizer at 1, 2 and 3 %. With this example, 36 experiments are needed for each material with OVAT (Figure 1). Using DOE instead of OVAT provides several
signifi cant advantages: OVAT provides no statistical information (accuracy/ deviations);
OVAT does not work in the presence of interactions; DOE provides more information from the same amount of experiments (deeper insight). When the DOE set-up is used, the total amount of
tests for two materials is 32 experiments, or 16 per material (Figure 2).
Laboratory experiments and results A laboratory single-screw foam extrusion line with an annular die and a maximum capacity of 20 kg/h was used to produce foamed LDPE sheets with 3 mm thickness (Figure 3). Two foaming grades were compared: Sabic LDPE
2102TX00 and 2202UMS. The sheets were produced with 10 kg/h output and with optimised (lowest) temperature settings for each combination of factors and or materials.
Figure 3: A lab extrusion line with an annular die was used to produce the
foamed LDPE sheets
As already mentioned, a set of 32 experiments was
set up for the two foaming materials (Table 1). By changing the full the range of parameters for blowing agent, nucleating agent and cell stabilizer, the resulting densities and cell sizes were observed, as well as a wide range of corrugation levels. Several models were calculated using the data
generated by the experiments to analyse density, corrugation and cell-size. Some statistical details of the obtained models are shown in Table 2 with the overall fi t statistics R2 and Q2 being the most important. In general, the properties R2, Q2 and R2Adj should be as high as possible and close to each other. The key fi gure R2 increases when more terms are added, R2Adj has an optimum and is used to signal a suffi cient model whereas Q2 is cross-validation based and signals the robustness of the model. Figures 4 and 5 show the relative contribution of each
factor and interaction for density of foams. Besides this, the fi t performance is illustrated by a graphical repre- sentation (the observed vs predicted plot). In the observed vs predicted plot. a straight line is
shown together with all measured points clustered around that line. A good model will have all the points
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COMPOUNDING WORLD | January 2013
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