Novel processing aids for extrusion of polyethylene

Abstract

Commercially available thermoplastic elastomers (TPEs) based on block copolymers of diisocyanates and polyols (i.e., TPUEs) were used to delay sharkskin and stick-slip instabilities in the extrusion of linear low density polyethylene (LLDPE). When TPUE is added in a small mass fraction to LLDPE, it deposits at the die surface during extrusion and may postpone the onset of sharkskin instability to a 12-20 times higher rate of extrusion. Substantial delay of sharkskin was also achieved under conditions without slip of molten PE inside the die coated by TPUEs. Efficiency to delay the instabilities depends on elasticity of TPUE at processing temperature. The TPUEs could be a cost-effective substitution of fluorinated polymers such as processing aid, lubricant and release agent in the processing of polyethylene by extrusion, blow molding, and injection molding.

Thermoplastic silicone elastomer lubricant in extrusion of polypropylene wood flour composites

“…Moreover, decomposition of polymer blends containing fluorinated polymers may release hazardous gases containing fluorine, which can contribute to the destruction of the ozone layer. Kulikov 17 has recently discussed the applicability and effectiveness of different thermoplastic elastomers to delay the appearance of sharkskin in extrusion of linear low-density polyethylene. Commercially available thermoplastic silicone elastomers (TPSE) could be promising materials to be used as lubricant/PPA in WPC.…”

Novel processing aid based on modified Silly Putty®

“…The propagation of cracks along a rigid wall of the die upstream is unstable, whereas it is stable in the opposite direction. We know from the literature [4] and from our own experiments [13] that sharkskin can appear at relatively low rate of extrusion in the conditions of partial slip between the melt and the die wall having low surface energy. In the same die, we see no sharkskin when the melt slips at high rates of extrusion during super-flow.…”

“…We used the die with a narrower gap for high-temperature extrusion to avoid bubble instabilities of the blown film. The film produced from the blend of LLDPE with the processing additive accumulates much lower static electrical charge in comparison with the film produced with the additives of thermoplastic elastomers based on siloxanes [13].…”

“…The sharkskin onset is delayed when the extrusion die is coated by a material, which is immiscible with LLDPE and being more elastic or viscous in comparison with the molten LLDPE, [see 9 -13]. The sharkskin can be delayed both with and without slip of the molten LLDPE along the coated surface [12,13], but it is preferable to use that processing additive, which causes the polymer to slip, that is an outer lubricant. In addition to obvious reduction of the pressure inside the extruder, the lubrication helps to lower gel formation in processing at elevated temperatures.…”

“…The use of functionalized siloxanes having better adhesion to metal in comparison with methyl end-blocked siloxanes is more efficient. The use of hydroxyl-functional silicones (silanols) as processing additives is known from [16], and of organo-modified silicones based on block-copolymers is known from [13,17,18]. The use of a processing additive and a processing adjuvant to enhance the processing of thermoplastic polymers is proposed in [19 -21].…”