Abstract
Sandwich composites being lightweight materials, are increasingly being used in high-performance applications such as in transportation (aviation, marine and railway) due to their high specific stiffness, strength, fatigue, corrosion resistance and thermal insulation. Despite these favourable properties, their poor fire resistance is a major limiting factor for their usage in many engineering applications. Sandwich structures are generally composed of thin composite laminates as skins and thick, low-density materials (e.g. balsa, foam) as cores. Fibres such as glass/carbon used for reinforcement in composite skins are non-flammable and can retain their chemical and physical stability at relatively high temperatures. So, flammability of skins of composites arises mainly from the resin (matrix) part. Hence the preferred way to control the flammability of composites is to choose a resin of low flammability or reduce its flammability by adding additive or reactive flame-retardant chemicals. Blending of a flammable resin with an inherently flame-retardant resin is another simple and effective physical way to reduce its flammability.
The main aim of this research was to investigate the thermal properties and fire resistance of some light weight sandwich composites, that could be potentially used in marine applications.
To achieve this, firstly, resins commonly used for marine and aerospace applications (unsaturated polyester (UP), vinyl ester (VE), epoxy) and some inherently fire-resistant resins (e.g. phenolic resins), which could be used alone or blended with UP and/or VE were selected.
These resins were cast into plaques and characterised for their thermal properties via thermogravimetric analysis and fire performances by means of pyrolysis combustion flow calorimetry and cone calorimetry. The fire safety assessment diagram (total heat release plotted against the flashover propensity values, calculated by dividing peak heat release rate by time-to-ignition) from the cone tests gave an indication of the overall fire safety, the trend being phenolics followed by UP, VE and epoxy, in descending order of fire safety. The fire safety of blends was in between those of the respective phenolic and UP/VE resins. The correlations between several fire parameters collected from different test methods were also assessed.
In the second step, sandwich composites with similar components (glass fibre-reinforced UP composite laminates as skins and balsa wood as a core) but with different compositions were prepared. The design variables consisted of: two different thicknesses of core materials (12.7 mm (0.5 inch) and 25.4 mm (1 inch)), lay-ups (skins on one side or on both sides) and three different sample preparation techniques (resin infused laminates as skins on both sides, hand lay-up laminates as skins on both sides, and hand lay-up sandwich structure in one go). Their fire performances were evaluated by two different standard fire tests - cone calorimetry and propane burner testing at heat fluxes of 50 and 113 kW/m2 , respectively. In both tests, temperatures through the thickness of the samples were measured using thermocouples from which their thermal barrier performances could be studied. The results indicated that there was a minimal effect of the cone heater orientation or sample preparation technique, however, samples containing thicker balsa core or composite laminates on both sides showed better thermal barrier performances (lower heat transfer), the behaviour in the former was due to the physical and thermal thickness of the thick core sample, giving rise to a larger volume of the charred wood. In spite of similar flammability of composite laminates on one side or on both sides, the glass fabric on rear side in the latter reduced the rate of burning, therefore resulted in a lower mass loss rate. Following these observations, the design selected for further study was:
composite laminates on both sides, balsa core thickness of 25.4 mm (1 inch), and sample preparation technique involving hand lay-up sandwich structure in one go.
A number of sandwich composites were then prepared using different resins and resin blends/combinations impregnated as matrices for skins, keeping all other composition variables constant. This also included skins prepared from three layers of UP or UP/VE impregnated glass with a top glass fibre layer impregnated with a phenolic resin. Their fire and thermal barrier properties were studied and results analysed to select parameters that could be used to assess their overall fire-resistant properties. During cone and propane burner tests, the temperature differences in the top and back skin surfaces (measured using thermocouples) were used to calculate apparent thermal conductivities and resistivity values of the sandwich composite’s charred residue, which gave an insight into the thermal barrier properties of the chars from different resin types. The overall fire performances indicated that introducing phenolic resins, either incorporated as blends with UP or VE, or applied as a top layer, helped in reducing the flammability of the composite, the latter approach giving the best results. Using cone parameters the same as those used for cast plaques, fire safety plots were obtained, from which overall flammability of different resin types and their blends could be ranked. However, this approach does not include heat transfer through the thickness of the samples, which affects the time to burn-through in a composite. Therefore, a novel thermal barrier efficiency index, including cone parameters of peak heat release rate, time-to-ignition, and also time taken for the back surface temperature to reach 300 °C in cone or propane burner test, was proposed to combine both the fire and thermal barrier properties, and plotted against total heat release values from cone tests for all sandwich composites. This changed the trend previously seen from cone parameters only.
UP or VE/phenolic blended resins, while performing very well in cone and other flammability tests such as UL-94, show relatively poorer performance in the burn-through test. This was attributed to the higher thermal conductivity of the highly crosslinked char from the phenolic component. Hence, considering both fire and thermal barrier performances, while phenolic blends were still safer than UP or VE resins, the difference was less than that judged on the basis just the burning behaviour.
Among different designs, the best fire performance was exhibited by the sandwich composite prepared with both skins comprising one layer of phenolic resin on top of three UP or VE layered structures. The thin layer of phenolic skin seemed to be very effective in improving the overall flammability of the sandwich structure by either preventing or delaying ignition, whereas it had a minimal effect on the thermal barrier performance (burn-through property) of the composite.