Good mechanical properties in long glass fibre polypropylene materials can be achieved at relatively low fibre lengths in cases of good fibre-matrix adhesion. The primary issue, explains Warden Schijve of DSM, is fibre-effectivity and not critical fibre length.
Long glass fibre PP materials exist in many forms - the best known are the GMT-materials, containing continuous or cut glass fibres. More recently, several materials with long, but not continuous, fibres have come on the market that can be processed by melting granulate in an extruder. It is clear that the average fibre length in the final product is reduced from the initial granule length of 12-25 mm, to 1-10 mm depending on the process used. Despite these shorter fibre lengths, the properties can be raised to levels similar or even exceeding the traditional continuous fibre GMTs. The 'critical fibre length' is often used to describe the minimum required fibre length for a certain material. However, in reality this only indicates the transition of fibre pull-out to fibre failure and is certainly not the optimum fibre length.