Carbon Fibres and Carbonaceous Fillers in Engineering Thermosets

Doktorsavhandling, 2004

In view of the excellent properties of carbon fibres (CF) and the growing demand for multi-functional materials e.g., electrically conductive composites, engineering thermosets with carbon fibres are highly attractive. Similarly, electrically conductive fillers are of great importance. Engineering thermosets constitute an important group of polymers commonly used as matrices for polymeric matrix composites (PMCs). Requirements for these materials, particularly mechanical properties, fire resistance and high-temperature performance, are steadily increasing. Two thermosets offering particularly advantageous properties are melamine-formaldehyde (MF) and phenol-formaldehyde (PF). Composites based on MF and PF are studied here. Novel composites of MF reinforced with carbon fibres and MF with carbonaceous fillers (carbon black and mineral-carbon shungite) were prepared and investigated in terms of their microstructure, mechanical and electrical properties. A non-vented manufacturing technique resembling film stacking was developed based on MF moulding compounds with cellulose. CF veils were impregnated with a suspension of alumina trihydrate (ATH) in aqueous MF, dried and processed similarly to sheet moulding compounds (SMCs). Also up-scaled production by injection moulding was used to prepare CF/PF composites. Carbon fibres improve stiffness and strength of MF composites compared to glass reinforcement. Interfacial CF/MF adhesion is low judging from the interfacial shear and transverse tensile strengths. Functional grading in the form of flexibilized MF coating applied to dispersed fibres improves both strength and strain at break by providing more favourable conditions during processing and crack propagation. Using softer ZnS pigment replacing hard Ti02 also improves mechanical properties. For injection moulded PF composites, particularly rubber modified grades, post-curing (proprietary) improves stiffness retention and fatigue performance. Short carbon fibre composites (proprietary) show improved stiffness, strength and fatigue performance, but reduced strain at break, as compared to glass reinforcement. During fatigue, these composites show a linearly decreasing cyclic modulus up to failure. This may enable the prediction of fatigue lifetimes from shorter-time tests, given that the final fatigue modulus can be established. However defects such as undispersed fibre bundles introduce significant scatter in fatigue lifetimes, making such predictions difficult. Fibre breakage occurred in both carbon- and glass fibre composites during quasi-static tensile loading and fatigue. Composites of MF/cellulose and carbon black exhibit electrical conductivity and improved stiffness at low average loadings of the filler, this attributed to the concentration of CB along the cellulose fibres. Composites of MF/cellulose and shungite filler exhibit electrical conductivities in the dissipative range. Compared to CB, shungite is easier to handle and water-ethanol instead of pure ethanol may be used during sonication. Mechanical properties are however rather poor, indicating weak shungite/MF interfaces.