Optimization of process conditions, characterization and mechanical properties of silane crosslinked high-density polyethylene

In this work, the effect of process conditions such as temperature range and rotation was evaluated through mechanical properties of yield stress and elastic modulus. The objective of the assessment was to optimize extrusion process conditions of silane crosslinked high-density polyethylene, aiming at maximizing mechanical properties. Initially, the process was conducted in a Haake 9000 mixing chamber, at different screw speeds (50, 60 and 70 rpm). The different screw speeds employed did not cause significant differences of torque values. Screw speeds of 50 and 60 rpm that caused lower torque values in the stability region were combined in a single-screw extruder to several temperature ranges in order to promote HDPE grafting with 3% and 4% silane content. The crosslinking step was performed immersing the material in water, in a closed system at about 120 deg. C, under pressure. The process condition that resulted in higher yield strength and Young's modulus for HDPE and crosslinked HDPE (PEX) was considered optimum. FTIR analysis allowed confirming the formation of siloxane crosslinks in PEX with 3% and 4% silane. X-ray diffraction, analysis showed that those crosslinks did not cause changes in PEX crystallinity; though an increase of PEX thermal stability when compared to HDPE was observed by thermogravimetric analysis. Crosslinking also improved the mechanical properties of HDPE. The results showed that after crosslinking PEXs presented higher stress values and smaller short-term creep than HDPE. This was attributed to the presence of siloxane crosslinks restricting chain movement and flow and ensuring the higher resistance of crosslinked materials.