Flame retardant natural fibre composites for high performance applications

Natural fibres such as flax, jute and hemp are becoming attractive alternatives to traditional high performance fibres such as glass and carbon fibres for reinforcement in composites. This is because natural fibres are from renewable sources, are biodegradable, and therefore are more environmentally friendly. However, unlike inherently non-flammable glass and carbon fibres, natural fibres thermally degrade and burn readily and their use has been restricted in applications where the fire regulations are stringent. In addition, natural fibres have low compatibility with a polymer matrix, especially hydrophobic polymers such as polypropylene, causing poor fibre/matrix interfacial adhesion in composites. This leads to lower mechanical performance in natural fibre composites. Therefore treatments/modifications of fibre and/or polymer matrix are required to overcome these drawbacks and expand their range of applications. The main aim of this PhD is to develop flame retardant (FR) natural fibre composites for high performance applications by using FR treated fibres and/or polymer matrices with FR treatment. To achieve this, firstly the flammability of different natural fibres and bio/synthetic polymers was studied by using limiting oxygen index (LOI) and cone calorimetry to identify suitable components for producing FR natural fibre composites. Flax was selected as reinforcing component, and three polymers (polypropylene (PP), polylactic acid (PLA) and poly(furfuryl alcohol) (furan resin)) were selected for polymer matrices. Various strategies to develop flame retardant natural fibre composites were adopted. These can be summarised as (1) development of FR composites from FR treated flax/PP and flax/PLA commingled, woven fabrics, (2) surface modification of flax/PP and flax/PLA fabrics for improving fibre-matrix adhesion in FR composites, (3) development of FR bio/synthetic polymer matrix, and (4) Identification of suitable FR strategy involving synergistic combination of different FR components for producing high performance FR natural fibre composites. The fire performance of these natural fibre composites was evaluated by using UL-94 and cone calorimetry, while the mechanical performance was studied by tensile, flexural testing. In order to develop flame retardant flax/PP and flax/PLA composites from commingled flax/PP and flax/PLA woven fabrics, it was established that the best way to render these flame retardant is to apply FR aqueous solutions by using a conventional pad-dry technique, commonly used for textiles. Usually for textile finishes, fabrics are first scoured which helps in improving the pickup of fabrics. As a first step, the effect of fabric pre-treatment (scouring) on fire and mechanical performance of composites was studied. It was concluded that there is no advantage of using an extra process of scouring during FR composite preparation as the improvement in fire and mechanical properties were only marginal. A number of commercially available water soluble flame retardants were used, out of which guanylurea methylphosphonate (GUP), was seen to be the most effective FR to improve fire performance of the composites. However, GUP caused significant reduction in mechanical properties of the composites therefore the effect of flame retardant concentration on fire and mechanical performance of the composites was studied in order to identify an optimised flame retardant formulation that significantly improves fire retardancy (i.e. V-0 rating in UL-94) of each of flax/PP and flax/PLA laminates with minimal effect on their mechanical performances. The optimised GUP content for flax/PP and flax/PLA was identified as those which have respectively 0.9% and 0.6% phosphorus in respective fabric/composite. To improve fibre/matrix interfacial adhesion in these composites, different surface modification treatments of the fabrics were studied including silane! solution application (i.e. triethoxyvinylsilane (VTS) and 3-aminopropyl triethoxysilane (APS) for flax/PP and flax/PLA respectively), exposure to atmospheric argon-plasma and a combination of both. The fibre/matrix interfacial adhesions of these composites were studied by using peeling and flexural tests. The results showed that the combination of plasma and silane treatments was the most effective method to improve the fibre/matrix adhesion for FR treated flax/PP, whereas only plasma treatment was required for FR flax/PLA composites. In order to develop flame retardant polymer matrices, PP and PLA were compounded with different flame retardants and then extruded into fibres. The commercially available organic phosphorus compound (OP) was seen to be the most effective FR to improve flammability of PP and PLA fibres. In furan resin also a number phosphorus based FRs were studied, from which ammonium polyphosphate (APP) and melamine polyphosphate (MPP) were seen to be most effective. These FR components were then used in different combination (i.e. either application of FR on flax, or in polymer matrix, or both) in order to identify the most effective FR strategy to improve fire performance of a composite with minimum effect on mechanical properties. For FR thermoplastic (flax/PP and flax/PLA) composites best results were achieved with the use of FRs in both flax fibre and polymer matrix. On the other hand for FR flax/furan laminates, the addition of FR in the resin provided best results