Melting behaviour of gelspun/drawn polyolefins

The melting behaviour of gelspun/drawn UHMW‐PE and UHMW‐PP fibres was investigated. Unconstrained UHMW‐PE and UHMW‐PP fibres melt at 142°C and 170°C, respectively. Upon constraining, by holding the fibres at a fixed length or by embedding the fibres in a matrix, an increase in the melting temperature of both fibres is observed. In the case of UHMW‐PE fibres a solid‐solid phase transition in polyethylene at 155°C from the orthorombic to the hexagonal crystal structure occurs. Above 155°C, the fibres can not sustain any load. This solid‐solid phase transition at 155°C sets an upper limit to both the maximum curing and continuous use temperature of PE‐fibre reinforced composites. In gelspun/drawn UHMW‐PP, such a detrimental solid‐solid phase transition is absent, and therefore the increase in melting temperature can be utilized effectively. For example, heating of UHMW‐PP fibres for 30 minutes at 200°C does hardly affect the room temperature Young's modulus and tensile strength if the fibre is constrained during heating.