School of Engineering and Materials Science
Research Student Awards
PhD Thesis: Recyclable All-Polypropylene Composites: Concept, Properties and Manufacturing
Author: CABRERA, Norbert
Supervisor(s): Ton Peijs
The ambitious objective of the PURE project to which belongs the work presented in this manuscript was to offer an alternative to the most successful ever cost-performance composite material: glass fibre reinforced polypropylene (GFRPP). Their success relies on the unique performance/price ratio of polypropylene among engineering plastics as well as that of glass fibres among fibrous reinforcements. An important aspect of glass fibre reinforced polypropylene is that their properties can be tailored by adjusting the volume fraction of glass and the length of the fibre. Increasing the fibre content (although for GFRPP only in the range 20-50 vol.%) and fibre length improves the mechanical properties but lowers the processability of the composite. If the versatility of these materials is a clear advantage, the initial driving force for their development was the possibility they offer in terms of recycling when compared to thermoset matrix composites. However, recycling of glass fibre reinforced polypropylene is an issue as recycling companies are reluctant to take reclaimed polymers with high filler content. Here lies the opportunity for an all-polypropylene composite to become an attractive alternative. In spite of the unquestionable recycling advantage, it is essential to assess both the processability and the performance of all-polypropylene composites and that was the objective of this work. In the light of the current technology, all-polypropylene composites can only be manufactured from woven or non-woven continuous tapes unlike GFRPPs. Indeed the rather low compaction temperatures and low matrix content involved prevents flow moulding processes to be used. Apart from that, all-PP composites can be transformed by other traditional composite manufacturing processes such as non-isothermal stamping, filament winding and lamination of sandwich constructions. The co-extruded tapes are woven into a fabric and belt-pressed in a continuous process to form semi-finished sheet product. The plates can then be used to make sandwich panels where two thin sheets of all-PP composites are separated by a thick and light polypropylene foam or honeycomb. Alternatively, the sheets can be stamped into shell products in a non-isothermal process. The fabric can also be directly thermoformed without the need of the expensive continuous belt-pressing process. More complex geometries can be formed as the tape can be further drawn when compared to brittle continuous glass fibre reinforcements. There is however a clear disadvantage of these thermoforming processes as they require a trimming operation where easily up to 20 to 30% of material can be wasted. The low investment and simple filament winding process allows manufacturing of pipes and pressure vessels without any material waste. Furthermore the thermal shrinkage of the tapes allows a self-compaction pressure to build up during the process. The filament winding process for tapes or fabrics is particularly interesting as manufacturing of GFRPPs pipes remains a difficult process. Finally large parts or prototypes can be manufactured by a vacuum bagging process although the compaction pressure (1 bar) is rather limited. The tensile properties of all-PP composites based on bi-directional woven fabrics lies in between that of GMT and that of continuous woven glass fibre reinforced PP. And its low density (up to 50% that of GFRPPs) makes the material particularly attractive and competitive. It was shown how all-PP structures can be efficient when the performance to weight ratio is optimised. However the low compressive strength and the rapid decrease of stiffness and strength with temperature are severe weak points of that material and unfortunately intrinsic to the fibrillar structure of polypropylene. However the strongest point of all-PP composites is their impact performance as they clearly outperform glass fibre reinforced polypropylene.
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