Effects of oxidation on mechanical and physical properties of ultra-high-modulus and ultra-high-molecular-weight polyethylene fibres

Abstract

The effects of oxidation on gel-spun ultra-high-modulus and ultra-high-molecular-weight polyethylene (UHMWPE) fibres were investigated. The single-fibre tensile properties of the UHMWPE fibres were lowered by all chromic acid/potassium dichromate oxidation reactions studied. The tensile properties were highly reaction-time-dependent for the fibres treated in the acid-assisted reactions with CrO 3 ( I) and with K 2Cr 2O 7 ( IIa) at a higher H 2SO 4 concentration (or a 1:20 K 2Cr 2O 7:H 2SO 4 weight ratio). The tensile properties of fibres treated in the acid-assisted reaction with K 2Cr 2O 7 ( IIb) at a lower H 2SO 4 concentration (or a 1:1.3 K 2Cr 2O 7:H 2SO 4 weight ratio) and the base-catalysed reaction ( III) were slightly lowered, but were independent of reaction time. The fractured fibre ends from reactions IIb and III remained fibrillated as observed on the untreated fibres, whereas brittle fractures were observed on those oxidized by reactions I and IIa. Experimental results indicated that the oxidative agents in reactions I and IIa penetrated more thoroughly into the fibrillar structures of the fibres than in the base-catalysed reaction III and reaction IIb. The changes in the tensile and fracture properties suggested different diffusion and oxidative mechanisms between chromic acid ( I) and potassium dichromate ( IIb) under similar sulphuric acid-assisted conditions. All oxidative reactions reduced superheating, which resulted in the increased heat of fusion but decreased melting temperatures and narrowed melting peak widths.