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technical paper | Foaming polyolefi ns


Do the UMS grades really lower product densities? Do they really give fewer waves and what about the cell structure?


The UMS grades have been commercially available


for two years and are mainly used for the physical foam- ing process with blowing agents such as CO2


and


isobutane. In order to monitor and evaluate the results in


Figure 6: Contour plot of density at t0 as a function of the two relevant variables and material. Left Sabic LDPE 2102TX00 and right Sabic LDPE 2202UMS


practice, samples were extracted from commercial foam extruders and analysed. The results for the 2202UMS grade were compared with a regular PE grade or Sabic foam grade 2102TX00. The following properties of the samples were analysed: 1. Foam density 2. Average cell size (stereo microscope) 3. Cell structure (SEM analysis) 4. Compression stress (ISO 7214) 5. Resilience (ISO 3386-1)


Figure 7: Corrugation expressed as number of waves formed. Left Sabic LDPE 2102TX00 and right Sabic LDPE 2202UMS


Processing with UMS resin In practice, the 2202UMS grade typically replaces a regular grade with a comparable MFR and density. The UMS material is currently running on a commercial basis using single, double and tandem extruders. The recipe is mainly based on PE, talc and glycol mono stearate (GMS). For example, after starting up the foam extruder with 2012TX00 and stabilizing the process settings and validating the product properties, the grade is replaced by 2202UMS. If the machine settings are left un- changed, then the production density typically remains the same and the dimensions of the product become smaller. This is because the cell size decreases. The machine settings need to be changed to make


Figure 8: Realized cell-sizes in 3 mm foamed sheets. Left Sabic LDPE 2102TX00 and right Sabic LDPE 2202UMS


For the response density, a model equation is given


expressing the behaviour and its dependency on different parameters. This also gives the opportunity to predict or calculate values for regions not covered by the experiments.


log(density) = C0 C4


[Iso]2 + C5 + C1 + C1 {Mat} + C2 [Nuc] + C3 {Mat} * [Nuc] + C6 {Mat} + C2 [Nuc] + C3


Waves and cell-size give similar equations: property = C0


[Iso] + {Mat}*[Iso] [Iso] + C4 [Iso] * [Nuc]


What happens in practice? After studying the design of experiments and its results, one can ask the questions: “What’s in it for me in practice” and “Is the prediction of the model correct”.


48 COMPOUNDING WORLD | January 2013


the most of 2202UMS, and there are two main options: increasing the production effi ciency to reduce cost of production, or improving the product quality to create a premium product.


Increasing product effi ciency In practice, it is possible to cool down the cooling zones and the head zones of the foam extruder much more than the regular temperature profi le without getting frozen PE particles in the head or die build-up. If this is combined with adding more blowing agent, it is possible to maintain the machine pressure in the extruder more or less steady. Both adjustments (cooling down and adding blowing agent) will give a lower density for the end product. Another parameter that needs to be adjusted is the dosage of nucleating agent. Because of the fi ner cell structure that is created by 2202UMS for a given recipe, it is necessary to lower the amount of nucleating agent.


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