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Development of a low termperature processing method for the production of natural long fibre filled polypropylene sheet

Final Report Summary - EXTRU CO2 (Development of a low termperature processing method for the production of natural long fibre filled polypropylene sheet for automative applications)

Natural fibres, such as hemp kenaf, flax and jute find an increasing role in providing reinforcement for polymer matrices used in the automotive sector. Currently most of the consumption is geared towards thermoset chemistry, typified by low shear and temperature processes such as resin transfer moulding. Their use in thermoplastic applications is limited to compression or other press moulding operations, principally because the heat and shear required by conventional processes such as compounding and extrusion can damage the natural fibres. Polypropylene, the main automotive polymer resin has a processing temperature of around 200 to 220 degrees Celsius and at these temperatures, particularly in the presence of shear, natural fibres can suffer thermal degradation leading to loss of mechanical performance, discolouration and odour problems.

This issue has stunted the growth of such materials in automotive applications, which is unfortunate since they offer significant benefits over traditional reinforcements such as glass fibre, not least because of their environmental credentials. Thermoplastic materials containing natural fibre reinforcement are recyclable and also offer a move towards sustainable products and processes. Couple this with low weight and the design freedom offered by thermoplastics and the benefits become very obvious.

The stated aims of the project were to develop the necessary extrusion and mixing technologies to allow for low-temperature processing of natural fibre filled polypropylene using scCO2 as a processing aid to produce natural fibre polypropylene compounds suitable for thermoforming.

This overall aim is crystallised in the following objectives.
- to develop a precompetitive industrial process to produce solid, natural fibre filled thermoplastic sheet;
- to produce a precompetitive solid, natural fibre filled thermoplastic sheet;
- to reduce the environmental impact of both the developed process and the sheet materials produced;
- to reduce production costs;
- to increase small and medium-sized (SME) competitiveness.

The two-year project has been divided up into structured work packages (WPs) which were integrated together to achieve the objectives of the project.

The first three months of the project concentrated on reviewing the state of the art with regard to thermoplastic natural fibre composites, defining target material applications and arriving at a specification which could be used to bench mark success for the automotive industry. The targeted applications are lightly loaded internal automotive body panels such as door and trunk linings, head lining and acoustic insulation panels.

Compound formulations have been developed using various grades of polypropylene containing hemp and kenaf natural fibre reinforcement in a range from 10 %w/w-30 %w/w. The role of fibre compatibilisers have been explored and optimum formulations developed to balance the requirements of processability, notably the elastic properties required for thermoforming, and the ultimate property characteristics demanded by the application.

Natural fibre compound manufacture presented its own set of challenges, principally in resolving how best to incorporate the low bulk density fibre into the twin screw compounding process at relatively high loadings. The problem was resolved by developing a fibre pre-pelletisation process which allowed fibre length to be preserved whilst at the same densifying the fibre to achieve a bulk density similar to the resin carrier. This allowed the fibre to be introduced into the process on conventional feeding equipment.

The effect of the scCO2 process was investigated by performing in-line capillary rheometry measurements. A slit die rheometer was specially constructed and attached to a 38 mm single screw extruder which was modified for barrel injection of scCO2. Much effort was devoted to develop and optimise a gas handling and pumping system to control the accuracy and consistency of scCO2 injection. As part of the process several speciality in-line mixers were manufactured to investigate the distribution of the supercritical fluid within the polymer matrix. The work has shown that incorporation of such mixers are beneficial to process stability.

Furthermore it has proved possible to use this reduction in viscosity to progressively lower the processing temperature of the extrusion process post the gas injection system. In this manner a 20 degrees Celsius reduction in melt temperature depresses the melt temperature of polypropylene extrudate below the 200 degrees Celsius mark. The approach is possible because the scCO2 plasticisation effect ensure that the viscosity of the system is still sufficiently mobile for the melt to remain processable.

Demonstration of the scCO2 plasticisation effect enabled sheet extrusion trials to be performed on a pilot scale. Sheet 300 mm wide sheet and up to 3 mm thick was manufactured in combination with scCO2 induced melt temperature reduction.

The sheets produced in this manner were subsequently thermoformed by the consortium partners and accepting for certain limitations such as excessively deep draw angles and high fibre content, the material processed well, producing well defined crisp mouldings.

The project has demonstrated conclusively the plasticisation effect of scCO2 which has in turn has allowed a sheet extrusion process to be developed to manufacture polypropylene natural fibre composites at temperatures below 200 degrees Celsius. In this respect both the prototype pre-competitive process and product have been realised: these provide additional opportunities to expand the remit of natural fibre products beyond the techniques that currently exist - mat technology etc.

The mechanical properties of the composites produced by the route are in-line with the requirements of the automotive industry. Charpy impact properties greater than 25 kJ/m2 have been obtained and tensile modulus values greater than other types of organic fillers have also been achieved.

With respect to competitor material currently available as pre-compounded feedstocks the properties of the products developed are superior and the cost comparison are quite promising.

The environmental credentials of the product have been reviewed by undertaking recycling studies. Again the data is promising with results indicating that up to 50% w/w of recyclate can be re-incorporated without deterioration in mechanical properties. This feature will again help support the process economics.

The principle area for improvement is in maximising mechanical property. So far complete solid profiles have not been attained and it is likely that future effort should be devoted to this are to better understand the complex interaction between the materials and the supercritical fluid.

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