Thermoformability of Discontinuous Long Glass Fibre (LGF) Reinforced Polymer Composites
April 13th, 2024
L197 25th October 2005 by S F Bush and F G Torres, University of Manchester Institute of Science & Technology (UMIST)
Abstract
Thermoforming is a major process taking about 7% of all thermoplastics, principally polystyrene and acrylonitrile-butadiene styrene alloys. Typical markets are blister packs (PS) trays; cases, automotive and aircraft interiors and building.
In recent years polypropylene (PP) has enjoyed considerable growth in thermoforming but applications have been constrained by the limited processing window for most grades. The SAFIRE long-glass fibre (LGP) programme for polymer processing, as previously reported, has demonstrated that subject to conditions on the aspect ratio (l/d), the fibres can be self-assembled within the polymer flow into coherent mat-like structures which confer exceptional increases in melt strength even at fibre concentrations as low as 1%v/v. This increased melt strength is particularly marked near the melting point of a polymer, in particular for PP where it translates into a much wider thermoforming temperature range. This in turn increases the size of sheets which can be used and also reduces the sensitivity to hot spots in the heater arrays.
In this present paper, previously reported work on the process and morphology of LGT thermoforms is carried further, by linking dynamic mechanical analysis (DMA), hot tensile tests, sheet sag tests, and viscosity directly to thermoformability and the fibre mat deformation process. DMA is used to characterise the anisotropy and the softening behaviour of the LGF extruded sheets. Hot tensile testing is used for assessing stretchability. Sheet sag studies under Infra-red (IR) conditions showed that particular LGF reinforced PPs used give a much lower degree of sag and a higher resistance to localised heating than the unreinforced polymers. Scanning electron microscope (SEM) and optical microscope pictures are presented to verify the mat deformation processes occurring during thermoforming.
Finally, the wall thickness distributions found for three materials (one unreinforced and two reinforced at the 3%v/v and 6%v/v levels) are given for different thermoformed shapes. These distributions correlate well with the results of the different tests on the sheets before thermoforming, thus providing a comprehensive understanding of the main factors determining the thermoformability of LGF-PP.