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.

Published in the Journal of the Technical University of Gabrovo, Vol. 29 ‘2004 (49-54) March 2004.

Written by E. Selcuk Erdogan from Trakya University, Mech. Eng. Dept., Edirne, Turkey, and S. F Bush from UMIST, Manchester, UK.

Abstract

The mechanical properties of long glass fibre reinforced thermoplastics have had a wide area and importance in industrial works in recent years. In this work, the tensile and impact properties of extruded homopolymer polyproplylene and polypropylene-polyethylene co-polymer were studied, related to fibre length and fibre concentration. After burn off tests fibre length and fibre concentration of each sample have been found. From the tensile and impact tests it has been found that tensile strength and impact strength of extruded homopolymer polypropylene increase with increasing fibre length and fibre concentration. It has also been found that even small filling of glass fibre by polyethylene blends is increased with increasing fibre length and fibre concentration. As a result of impact tests it has been found that impact strength of extruded co-polymer polypropylene-polyethylene blends is decreased for the low fibre concentrations, then reached almost constant value with increasing fibre concentration and fibre length. It has also been found that even small filling of glass fibre by volume gives higher tensile strength properties.

Prosyma Research Ltd report to Plastics (Manchester) Ltd.

S F Bush

Introduction

Plastics (Manchester) Ltd wish to supply 3 mm thick thermoformed polypropylene cladding to cover 40 mm of rock wool insulating stainless steel tanks. The tanks are suspended by 70 mm x 70 mm x 5 mm carbon steel box sections welded to the tank and attached to a frame.

Over most of the area to be covered by the PP cladding its temperature will be close to atmospheric by virtue of the rock wool insulation. However, where the suspending box sections emerge from the rock wool, it may be expected that the temperature in the steel will be significantly above this. Because the small gap between the PP cladding and the steel box section is to be filled with a silicone sealant, the PP will be in local thermal contact with the steel.

The objective of the following calculations is to establish the temperature at the steel/sealant/PP contact point. As will be seen, this temperature is strongly affected by wind flow around the tank and over the suspending box sections. Without measurements of these flows only very approximate estimates can be made.

Paper to the Polymer Processing Society European Meeting, Stuttgart, Germany, 26th-28th September 1995.

S F Bush with E S Erdogan

Introduction

The work reported here results from a long-term programme of research under the generic title of Self Assembling Fibre Reinforcement (SAFIRE) processes. This work is aimed at providing means by which deformable lace-like structures are obtained within resins and melts in ways which do not interfere with established processing methods such as extrusion, injection moulding and blow moulding. Earlier results, principally on the resultant solid properties, have been reported at PPS conferences ⁽¹, ²⁾ and elsewhere³. The processes involved are now patented world-wide ^{(4, 5)} and products dependent on the processes have entered commercial production.

The SAFIRE processes provide: (a) masterbatch granules consisting of the chosen polymer matrix in which fibres of any defined length are disposed to minimize breakage during processing, and (b) fibre management devices which when placed in the melt flow, manoeuvre the filaments into deformable but coherent lace-like structures which persist into the solid state^{(4, 5)}. The devices are introduced upstream of dies or moulds in ways which do not interfere with the functioning of the process or add materially to its mechanical complexity. Criteria for obtaining fibre mat or lace structures, and factors affecting the polymer-fibre interface are described elsewhere^{(2, 3)}. Hitherto work has concentrated on extruded pipe^{(1, 3)} and injection mouldings^{(2, 3)} using polypropylenes and high, medium and low density polyethylenes, and polybutylene. The present paper describes the application of the SAFIRE concepts to sheet extrusion. As it stands this application is not optimized so the results should be seen as preliminary ones only.

References

[1] Bush S F, Extrusion of Melts Containing Semi-Coherent Fibre Structures, 5th Poly Proc Soc Ann Mtg, Kyoto, Japan (1989) paper 11-02.

[2] Bush S F, Ademosu O K, Blackburn D R and Yilmaz F B, Factors Affecting the Strength of Long-Fibre Reinforced Injection Moldings, Poly Proc Soc, European Mtg, Prague (21-24 Sept 1992) paper 6-06.

[3] Bush S F, Yilmaz F and Zhang P F, Impact Strengths of Injection Moulding Polypropylene Long Glass Fibre Composites, VI Inst. Mats. Conf. Fibre Resin Comp., Newcastle (29-31 March 1994) paper 1-05.

[4] Bush S F, Fibre Reinforced Polymer Compositions and Process and Apparatus for Production Thereof. US Patent 5,264,261 (23 Nov 1993)

[5] Bush S F, Filament Separation in Liquids, US Patent 5,035,848 (30th July 1991)

Paper to the Institute of Materials 4th International Conference on Automated Composites, University of Nottingham, 6th-7th September 1995, pp 134-241.

S F Bush with E S Erdogan

Abstract

Broadly, reinforcement may be introduced into polymeric materials either by mixing discrete fibres into a polymer before injecting the fibre containing melt into a shaping mould, or by constructing a predetermined textile form from staple or continuous fibres and then bringing it into contact with the polymer or resin. The first approach readily lends itself to all the high speed automatic processes which have been devised for unreinforced thermoplastics, but carries the penalty of inefficient reinforcement. The fibres are short (typically 0.3 to 0.6 mm), distributed and oriented more by the adventitious action of flow in the mould than in the directions needed to resist imposed stresses and strains.

The second approach on the other hand can give large increases in tensile strength and stiffness in directions which can be closely matched to the imposed strains, but this reinforcement efficiency is obtained at the expense of extended manufacturing times and therefore cost.

This paper describes results obtained from processes designed to marry the high-speed advantage of the first approach to the reinforcement efficiency advantage of the second approach. By means of specially designed fibre management devices, well-wetted glass filaments, typically 6-15 mm long, are caused to assemble themselves into semi-coherent mat-like reinforcement structures which are not seriously disrupted when carried by the polymer melt into a mould or die. Such structures are readily observable in the solid artefact by burning off the polymer to leave a three dimensional fibre structure which faithfully reflects the artefact’s shape.

Earlier papers ^{(1, 2, 3, 4)} describe the features of the fibre-mat reinforcing structures and the mechanical properties obtainable from these Self Assembling Fibre Reinforcement (SAFIRE ©) processes.  The present paper describes the heat distortion (HDT) temperatures (BS2872 method 121A) obtained from glass fibre reinforced polypropylene injection moulded plaques, with and without SAFIRE fibre management devices, at three different glass fibre concentrations.

The results show that there is a skin-core effect, though smaller than with short-fibre compositions. As a result of this effect the measured HDTs are a few percent greater in the transverse direction than in the flow direction. This low degree of anisotropy is similar to that found previously for tensile³ and impact⁴ strengths and is a product of the fibre-mat structures obtained^{(1, 2)}.

The general result is that for polypropylene the HDT rises by about 6 ^oC for every 1% fibre volume. This is of considerable commercial significance for polypropylene since mass-produced, largely isotropic, high speed long fibre reinforced mouldings, thermoformings, and extrusions can now be obtained with heat distortion temperatures of around 120 ^oC (with 25% glass w/w reinforcement) instead of 55 ^oC for the unreinforced polymer, greatly extending the range of potential applications.

References

[1] BUSH S F, Control of Fibre Structures in Melt Extrusion, 36th Conf. Can. Soc. Chem. Eng., Sarnia, Canada, 6-10 Oct. 1986, paper 32d.

[2] BLACKBURN D R and ADEMOSU O K, Factors affecting Fibre-matrix Contacting in Fibre-filled Granules, Poly. Proc. Soc. IX Ann. Mtg., Manchester, England, 5-8 Apr. 1993, paper 06-14.

[3] BUSH S F, Factors Affecting the Tensile Strength of Long-fibre Reinforced Injection Mouldings, Poly. Proc. Soc. Eur. Mtg., Prague, 21-24 Sep. 1992, paper 6-06.

[4] BUSH S F, YILMAZ F B and ZHANG P F, Impact Strengths of Injection Moulded Polypropylene Long Glass Fibre Composites, VI Fibre Reinforced Composites Conference, Newcastle, England, 29-31 Mar. 1994, paper 5.

Paper (6-2-8) to the 10th International Annual Meeting of the Polymer Processing Society, Akron, USA, 5th-8th April 1994.

S F Bush with D R Blackburn, O K Ademosu, F B Yilmaz and P F Zhang

Introduction

Earlier papers^{(1, 2, 3, 4)} have described the factors affecting fiber-matrix cntacting, the organization of long fibers into coherent lace-like or mat-like structures, and the dependence of tensile strength on these two classes of variable. Equations for the mean number of touches¹ in the fiber structure and tensile strength² have been proposed which allow for the main variables present in moldings and extrusions, including differing matrix properties. Fiber management technology developed under the generic acronym SAFIRE⁴ (Self Assembling Fiber Reinforcement) has recently been applied commercially to practical moldings such as hard hats and pallets, in both of which examples impact strength is a key property.

Accordingly, a wide series of experiments has been carried out using the Izod method to determine the variation of impact strength as a function of the fiber length, fiber concentration, the fiber-matrix interface, fiber reinforcement structure and matrix properties. Both commercially available long-fiber granules and laboratory-compounded types have been used with different mold configurations and molding conditions for a variety of polypropylene and polyethylene matrices.

References

[1] S F Bush, Control of Fiber Structures in Melt Extrusion, 36 Ann Mtg Can Soc Chem Eng, Sarnia, Canada (1986) paper 32d.

[2] S F Bush, O K Ademosu, D R Blackburn, F B Yilmaz, Factors Affecting the Strength of Long-Fibre Reinforced Injection Moldings, Poly Proc Soc Eur Mtg, Prague (1992) paper 6-06.

[3] D R Blackburn and O K Ademosu, Factors Affecting Fiber-Matrix Contacting in Fiber-filled Granules, Poly Proc Soc, 9th Ann Mtg, Manchester 1993, Paper 6-14.

[4] S F Bush, Self Assembling Fibre Reinforcement (SAFIRE) processes, “Textile-reinforced Composites for Engineering”, Bolton Institute, Bolton, England, 12-14 January, 1994.

Paper (5) to the Institute of Materials 6th International Conference on Fibre Reinforced Composites, Newcastle, England, 29-31 March 1994.

Published in Plastics, Rubber and Composites Processing and Applications, Vol 24, No 3 (1995).
S F Bush with F B Yilmaz and P F Zhang

Abstract

A wide series of experiments has been undertaken to measure impact strength as a function of fibre length and concentration, the fibre/matrix interface, and induced fibre-mat structure and matrix properties. Both commercially-available long-fibre polypropylene granules and in-house polypropylene and polyethylene glass-fibre compounds have been used where the interface conditions are known and can be varied. For the fibre-mat structures achieved, notched impact strengths rise with fibre lengths and with fibre concentration, giving in all cases an improvement on the virgin polypropylenes – for some conditions a five-fold improvement at 25% w/w concentration.

Paper to the Polymer Processing Society 9th Annual Meeting, UMIST, 5th-8th April 1993, 02-35.

F B Yilmaz with S F Bush

Introduction

There are a number of different techniques for the production of long glass-fibre reinforced polypropylene (LGFRPP) granules for injection moulding. Dispersion and wetting of glass-fibre in the granule will be different according to the type of polypropylene grade and the compounding methods. Most of the materials used in this study were produced by the melt coating method and contain two bundles of filaments. The granules which are used in this study are given in Table 1.

* They have coupling agent.

Injection moulding is a two-step operation which involves first the plasticization of solid materials in a screw extrusion unit, followed by the high pressure pumping of molten material into a mould cavity. Most fibre breakage occurs in the screw extrusion unit with injection moulding of fully dispersed LGFRPP granules[1]. There is in fact an advantage in using melt coated granules. When glass-fibre bundles are travelling in the screw unit, they protect each other against severe fibre breakage. On the other hand to disperse the fibre bundles a separator is necessary. In tis study a proprietary dispersion device was used between the mould and the screw unit.

References

[1] R S Bailey and H Kraft, Int. Polymer Process. 22, p94-101 (1987)

[2] S F Bush, O K Ademosu, D R Blackburn, F B Yilmaz, “Factors Affecting the Strength of Long-fibre Reinforced Injection Moldings”, Polymer Processing Society 1992 European Meeting, Prague, 21-24 September, paper 06-06.

Report 6 from a series of 5 Prosyma Research Ltd reports to Ametex AG, 15th February 1989 to 14th January 1990.

Report 6 dated 31st May 1989, Report 7 dated 11th July 1989, Report 8 dated 24th July 1989.
S F Bush

Summary: Part 1

1. During the period under review, SAFIRE project A has followed up the breakthrough in finishing technology achieved in the previous six-month period.
2. On the process itself follow-up has concentrated on establishing the design and operating parameters which determine acceptable and indeed exceptional finish with the new technology, at the same time making many metres of pipe for burst and creep testing.
3. Both polypropylene (PP) and HDPE SAFIRE pipe can be made with fine finishes on a routine basis. While not of direct benefit to the project, the finishing technology gives quite remarkable mirror finishes to virgin pipe.
4. In the period under review, SAFIRE project C has concentrated on producing SAFIRE granules in a wide range of combinations of fibre and polymer, and evaluating these for use in pipes.
5. The signs are that the evaluations made for this purpose will also give a good indication of the commercial potential for SAFIRE granules in combination with Fibre Separating Devices (FSD) sold in their own right. The significance of this potential has become clear from recent information on present commercial products.