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.

Paper to the 19th Annual Meeting of the Polymer Processing Society, Melbourne, Australia, 7th-10th July 2003

S F Bush with D R Blackburn and K J Jamieson

Abstract

This paper describes a new process for the production of certain types of smart composite materials, which under prescribed temperature fields spontaneously adopt prescribed shapes. These shapes are quite stable at room temperature plus about 50 ^oC. At or near the forming temperatures the shapes may revert to their original forms. Articles of this type can thus be seen as the polymer composite equivalent of bimetallic strips or shape memory metal alloys.

The purpose of this process which is under commercial development under the acronym SMARTFORM^© is to be able to make shapes from straight rods and flat sheet feedstock which are either impossible to mould or very expensive to do so by conventional processes. The key to the new process is the placing of mixtures of heat shrinkable synthetic and natural fibres in precise positions in the cross-section of pultruded profiles – notably rods and sheet. When subsequently cut to size, the rods or sheet elements are passed on belts through a series of heating zones for prescribed times which cause them to curl or twist into the shapes required. The process is economical since both the pultrusion stage and the thermal forming stage are continuous, not requiring manual intervention.

Paper to the 5th International Conference on Manufacturing, Processing Composite Materials, Plymouth University, 12th-14th July 1999.

Published in the journal: Composites Part A: Applied Science and Manufacturing (incorporating Composites and Composites Manufacturing) ISSN 1359-835X, Volume 31, Issue 12, December 2000.

S F Bush with F G Torres

Abstract

The present paper summarises the main aspects and the developments in sheet extrusion and thermoforming of discrete long glass fibre (LGF) composites using the SAFIRE (Self Assembling Fibre Reinforcement) technology. During extrusion the long glass fibres are organised into coherent fibre mats which persist into the solid state, and are able to withstand the deformation process that takes place during thermoforming. A process analysis has been performed for extrusion and thermoforming indicating the main individual operations. Both processes have been studied with regard to their performance with the materials used in the studies, namely polypropylene homo and copolymer, with and without LGF reinforcement. Significant improvements in mechanical properties relative to the unreinforced materials have been found for the extruded sheets and the thermoformed products. Major improvements in processability relative to unreinforced PP have been found for the LGF materials. These are discussed in terms of the coherent fibre mat concept.

Paper to the Polymer Processing Society 15th Annual Meeting, ‘s-Hertogenbosch, Holland.

Published in International Polymer Processing XV (2000) 2.
S F Bush with F G Torres

Abstract

This paper lays out the main procedures for performing morphological characterisations of Long Glass Fibre (LGF) composites with particular reference to the sheet extrusion and thermoforming processes as they may be configured for production. The techniques used, including optical microscopy, scanning electron microscopy and image analysis, are described both with regard to their laboratory application to these materials and to their potential for monitoring the performance of the industrial manufacturing process. Results obtained from the different techniques at the three various stages of the manufacturing route are presented and discussed in terms of the structure property relationships obtained and the reinforcing fibre mat system typical of these types of material.

Paper

S F Bush with F G Torres

Abstract

Thermoforming is a major process with a wide range of applications in several fields. One of the most interesting possibilities is the thermoforming of PP. It is well known that Long Glass Fibre (LGF) composites present better mechanical properties than unreinforced PP. In addition to that, long fibres increase the thermal stability and the melt strength of the unreinforced polymer. In this paper, the thermoformability of LGF reinforced PP is studied using dynamic mechanical analysis (DMA), hot tensile tests, sheet sag tests, and microscopical techniques for the characterisation of 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 LGF reinforced materials present a much lower degree of sag and a higher resistance to localised heating than the unreinforced polymers. Finally, scanning electron microscope (SEM) pictures are presented to verify the mat deformation processes occurring during thermoforming.

Paper to the 7th International Conference on Fibre Reinforced Composites, FRC’98, University of Newcastle-upon-Tyne, UK, 15th-17th April 1998

Published by Woodhead Publ, Ed A G Gibson, pp 237-244, ISBN 1855 73 3757.

S F Bush with F G Torres and E S Erdogan

Abstract

The mechanical properties of discrete long glass fibre (LGF) reinforced polymer sheets have been studied pre and post thermoforming. Extruded sheets have been produced using different grades of PP and PE at different fibre concentrations. Novel fibre management devices have been employed in order to control fibre mat formation during sheet extrusion. Tensile and dynamic mechanical properties of the sheets are reported. In addition, thermoformability studies have been carried out by varying the processing parameters. The coherence of the reinforcing fibre mat has been observed before and after uniaxial and biaxial stretching in the thermoforming process. The operating temperature window for these materials has been defined.