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The primary aim of this research was to develop and manufacture continuous unidirectional carbon fibre reinforced polyvinylidene fluoride (PVDF) by incorporating atmospheric plasma fluorination (APF) of carbon fibres to confirm that the improvements achieved previously on the single fibre model composite level can be translated to improvements on the composite laminate level. The findings obtained here will hopefully provide novel composites that could be used in applications which require excellent chemical inertness as well as high strength and toughness. The study of composites can be divided into micro-scale where the characteristics of the composite are studied at single-fibre level, macro-scale which covers the properties obtained on composite laminates and finally the application-scale, where the composites are made into a structure and its performance is demonstrated. Within the scope of this research, the macro- mechanical properties of carbon fibre reinforced PVDF laminates manufactured in-house were characterised to understand the effect of APF treatment on the performance of the composites.

Continuous unidirectional carbon fibre reinforced PVDF with a fibre volume fraction of 60 ± 2% were manufactured using a laboratory-scale composite production line via a wet impregnation method. APF treatment of carbon fibres was performed in-line during the manufacturing process prior to the fibres entering the impregnation bath. The manufacturing speed was altered to study the effect of various degrees of fluorination of the carbon fibre surfaces on the composite mechanical properties. It was found that both the flexural and interlaminar shear strength of the composite can be enhanced by as much as 110% and 200%, respectively, for composites made with fluorinated carbon fibres with surface fluorine content of 3.7 at.-% as compared to

165 unmodified carbon fibre reinforced PVDF. Although an attempt was made to measure the critical energy release rate from Mode I, GIC and Mode II, GIIC interlaminar fracture toughness of the

composites manufactured, both tests only indicate a flexural strength of the composite. This is mainly due to the nature of PVDF matrix as being tough at room temperature. Further work has to be done in order to measure the true interlaminar fracture properties of the composite. By carefully design the test specimen (having a thicker arm or using a stiff arm material such as aluminium plate), the problem with tough matrices could be tackled. However, it is worth to note that the improvements from the test results indicates the improvements in the flexural strength of the composite and these improvements can be directly related to the enhancement of the fibre/matrix interface by APF treatment on the fibre surface. In order to understand the axial tension behaviour of this novel composite, UD carbon fibre reinforced PVDF was subjected to tensile tests along the fibre axis. It was observed that the axial tensile strength and Young’s modulus of the composite, being a fibre dominated property was 1,130 ± 53 MPa and 133 ± 6 GPa for the unmodified carbon fibre reinforced PVDF composite. By incorporating fluorine into the carbon fibre surface, the axial tensile strength and modulus improved by 8% and 5% to 1,260 ± 70 MPa and 140 ± 2 GPa, respectively, for composites made with carbon fibres containing fluorine content of 1.7 at.-% on the fibre surface. The results observed at the macro- scale demonstrated that APF treated carbon fibres has a positive impact on the overall mechanical performance of the composites manufactured over those made with as-received fibres. Therefore, the next step was taken to test whether these properties also translate into a composite structure, in this case, a reinforced thermoplastic pipes (RTP) with improved properties. RTPs were made by filament winding of continuous unidirectional carbon fibre reinforced PVDF prepregs onto pure PVDF liners. The winding pattern chosen was helical with a winding angle of ±55°. The composite pipe was cut into 25 mm wide ring sections and subjected to hoop tension and compression tests. The hoop tensile strength of the unmodified carbon fibre reinforced PVDF RTP increased by 8% to 57.2 ± 1.1 MPa from 52.9 ± 0.3 MPa for the PVDF pipe. The hoop tensile strength of reinforced PVDF pipes made with fluorinated carbon fibres containing 3.7 at.-% fluorine content on the fibre surface was improved by 10% to 63 ± 2 MPa. Similarly, the RTP stiffness factor was also enhanced by 11% when APF surface treated carbon fibre containing PVDF was used as compared to the PVDF pipe. All of these improvements show that by tailoring the fibre/matrix interface through APF surface treatment,

166 the mechanical performance of the composite can be improved. This improvement indicates the ability of load to be transferred from matrix to fibre through the improved adhesion at the fibre/matrix interface. This was proven to be true from the micro-scale, macro-scale as well as at the application level.

A separate study was conducted to evaluate the effect of both fibre and matrix modification on the interfacial adhesion between fibre and matrix. The fibre was modified by atmospheric plasma fluorination (APF) while the matrix was modified by addition of maleic anhydride (MAH) grafted PVDF. A synergy between fibres and matrix modifications was observed on the interfacial shear strength (τIFSS). τIFSS was found to improve up to 300% for fibres with 2.8 at.-% fluorine content on its surface and matrix 5 ppm of MAH grafted PVDF. The improvement in τIFSS was found to match the ultimate tensile strength of PVDF. An enhancement was also observed in the interlaminar shear strength of UD carbon fibre reinforced PVDF laminates. These improvements show that the fibre matrix interface can be tailored further to optimise the interfacial adhesion which leads to overall improvement in the composite’s performance. Although the improvements obtained were overwhelming, it was not possible to further characterise the mechanical properties of the laminates. This is mainly due to lack of resources especially of the MAH grafted PVDF. The MAH grafted PVDF that was used in this study was a research grade material that was kindly supplied by Arkema (Serquigny, France) and is not available on the market yet. However, this study showed synergy of both modifications as proof of concept. Further work should be carried out in the future if sufficient material can be obtained from Arkema.