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 Polymer Processing Society 14th Annual Meeting, Yokohama, Japan, 8th-12th June 1998, paper 9-03.
S F Bush with F G Torres
Introduction
Discrete long glass fibre (LGF) reinforcement of thermoplastics has been applied successfully to conventional polymer processing techniques, such as extrusion, injection moulding and blow moulding. The results of the studies covering those processing techniques and their advantages with respect to other types of fibre reinforcement have been summarised elsewhere1. One of the most important potential applications of discrete long glass fibre (LGF) reinforcement of thermoplastics is sheet extrusion. The fibres can be induced to form mats of more or less defined mean orientations in the melt state, so that extruded sheets of controlled anistropy can be obtained. The sheets can then be processed by thermoforming.
Thermoplastics, especially, PP have many desirable solid state properties for a thermoformed product. However it presents a very narrow processing window for thermoforming, because the stretching process of the sheet has a tendency to be unstable. High melt strength PP (HMPP)2 and reinforced or filled PP have been used in order to improve the stretchability and broaden the processing window. In this context, discrete long glass fibre (LGF) reinforcement has proved to be one of the best methods of achieving these goals, increasing the melt viscosity (shear and elongational)3, and in addition increasing strength and stiffness in the solid state, in particular at high temperatures.
In the present paper, Dynamic Mechanical Analysis (DMA) is used as a method for measuring the properties of the extruded sheets in a wide range of temperatures.
References
[1] BUSH S F, “Long glass Fibre Reinforcement of Thermoplastics: Results from Injection and Blow Moulding, Sheet and Pipe Extrusion”, Poly Proc Soc (PPS Euro Mtg) Gothenburg, Sweden, 19-21 Aug (1997)
[2] MORAD J J, “High Melt Strength Polypropylene for Large Part Thermoforming”, ANTEC 95, Vol I, Boston, USA, May (1995)
[3] TORRES F G, “Modelling of Polymer and Polymer Composite Flows: Molecular Chain Orientation and Reinforcing Fibre Orientation”, MPhil Thesis, UMIST, Manchester, UK (1997)
[4] ERDOGAN E S, Private Communication, UMIST 1997, also in PhD Thesis, UMIST 2000.
