TY - THES

T1 - Screening Method Development for Die Drool in Extrusion Blow Moulding for PCR PEHD

AU - Jochum, Philip Anton

N1 - embargoed until 03-10-2028

PY - 2023

Y1 - 2023

N2 - This thesis is dealing with the die drool phenomenon associated with high-density polyethylene (PEHD) materials for extrusion blow moulding (EBM). Die drool is the unwanted build-up of material on the edge of the die during extrusion. In the first part of the thesis, selected parameters affecting the die drool formation were investigated while developing a test method for further trials. For this purpose, a test extruder with different dies was used. In the second part of the thesis, the focus was on contaminants in the form of foreign polymers in post-consumer recycled (PCR) PEHD grades and their effects on die drool formation. A design of experiments (DoE) screening was performed to determine the influence of various factors on the die drool formation. The experiments showed that the investigated unimodal PEHD grade was more sensitive towards building-up die drool than the bimodal grade. The reason for this is likely to be found in the molecular structure of the grades. Unimodal grades tend to have more long linear chains, which probably favour the separation of fractions of different molecular weights within the melt. In addition, the slip-stick effect in the tested unimodal grade lead to high accumulations of material at the die. However, the slip-stick effect had no influence on the die drool formation in the tested bimodal grade. The next effect that promotes die drool formation is thermal degradation. This occurs at high temperatures and long residence times in the extruder. This is probably due to low-molecular chains, which are formed by damaging the materials, and which are more easily separated of the melt for entropic reasons. Orientation and relaxation behaviour of the polymers seem to play also a role, since experiments with a short die land length show a higher die drool formation compared to a long die land length. Through the DoE and further experiments, an efficient method was developed, which was used for testing other materials. High temperature, low shear rates during extrusion resulting in long residence times and the shortest die were used in order to gain sufficient die drool quantities. Blends of different grades of PEHD showed that unimodal grades have a higher die drool formation, too. Incorporation of homopolymer PEHD grades lead to a higher die drool formation in both unimodal and bimodal grades. The reason for this is most likely the short linear chains, which are easily separated from the melt. High viscosity unimodal PEHD grades lead to an increased die drool formation only when co-mingled with unimodal grades, whereas in bimodal grades they showed hardly any effect. Model compounds were created to test the effect of the recycling process and foreign polymer contamination. The pure compounded materials have a much higher die drool formation than the virgin materials. This shows that the recycling process itself has a strong influence on the die drool formation. Compounds with polypropylene (PP) had an increasing effect in bimodal grades and a decreasing effect towards die drool formation in unimodal grades. The reason for this is probably due to chain entangling. Low-density polyethylene (PELD) and linear low-density polyethylene (PELLD) have little effect in bimodal grades, while they promote die drool formation in unimodal grades. Polyethylene terephthalate (PET) particles lead to a high die drool formation in both unimodal and bimodal grades.

AB - This thesis is dealing with the die drool phenomenon associated with high-density polyethylene (PEHD) materials for extrusion blow moulding (EBM). Die drool is the unwanted build-up of material on the edge of the die during extrusion. In the first part of the thesis, selected parameters affecting the die drool formation were investigated while developing a test method for further trials. For this purpose, a test extruder with different dies was used. In the second part of the thesis, the focus was on contaminants in the form of foreign polymers in post-consumer recycled (PCR) PEHD grades and their effects on die drool formation. A design of experiments (DoE) screening was performed to determine the influence of various factors on the die drool formation. The experiments showed that the investigated unimodal PEHD grade was more sensitive towards building-up die drool than the bimodal grade. The reason for this is likely to be found in the molecular structure of the grades. Unimodal grades tend to have more long linear chains, which probably favour the separation of fractions of different molecular weights within the melt. In addition, the slip-stick effect in the tested unimodal grade lead to high accumulations of material at the die. However, the slip-stick effect had no influence on the die drool formation in the tested bimodal grade. The next effect that promotes die drool formation is thermal degradation. This occurs at high temperatures and long residence times in the extruder. This is probably due to low-molecular chains, which are formed by damaging the materials, and which are more easily separated of the melt for entropic reasons. Orientation and relaxation behaviour of the polymers seem to play also a role, since experiments with a short die land length show a higher die drool formation compared to a long die land length. Through the DoE and further experiments, an efficient method was developed, which was used for testing other materials. High temperature, low shear rates during extrusion resulting in long residence times and the shortest die were used in order to gain sufficient die drool quantities. Blends of different grades of PEHD showed that unimodal grades have a higher die drool formation, too. Incorporation of homopolymer PEHD grades lead to a higher die drool formation in both unimodal and bimodal grades. The reason for this is most likely the short linear chains, which are easily separated from the melt. High viscosity unimodal PEHD grades lead to an increased die drool formation only when co-mingled with unimodal grades, whereas in bimodal grades they showed hardly any effect. Model compounds were created to test the effect of the recycling process and foreign polymer contamination. The pure compounded materials have a much higher die drool formation than the virgin materials. This shows that the recycling process itself has a strong influence on the die drool formation. Compounds with polypropylene (PP) had an increasing effect in bimodal grades and a decreasing effect towards die drool formation in unimodal grades. The reason for this is probably due to chain entangling. Low-density polyethylene (PELD) and linear low-density polyethylene (PELLD) have little effect in bimodal grades, while they promote die drool formation in unimodal grades. Polyethylene terephthalate (PET) particles lead to a high die drool formation in both unimodal and bimodal grades.

KW - Die Drool

KW - Extrusion Blow Moulding

KW - Polyethylene High Density

KW - post consumer recycled

KW - Düsenbart

KW - Extrusionsblasformen

KW - Polyethylen High Density

KW - post-consumer recycled

U2 - 10.34901/mul.pub.2023.222

DO - 10.34901/mul.pub.2023.222

M3 - Master's Thesis

ER -