Alternative glazing for automotive vehicles : executive summary

The first approach utilises a thin film of acrylic that is moulded onto the outside of a polycarbonate substrate. It was found that the gate of the injection mould cavity must be of uniform cross section otherwise local shear heating can occur and melt the acrylic film. The injection gate must also be located entirely on one side of the mould cavity otherwise the film is punctured by the molten polycarbonate and free to float within the cavity. Any mixing of the two materials will lead to opaque components due the difference in the refractive indices. The film was found to improve the UV resistance of any component, acting as a protective buffer for the polycarbonate. A new variety of hardcoat was applied to film-backed samples to impart abrasion resistance and samples were found to outperform commercially available alternatives under recognised laboratory conditions. The film-backed samples also exhibited excellent impact resistance when impacted upon the film-face. However, similar components failed at extremely low energy levels when impacted from the non-film face because flaws in the acrylic film caused cracks to be initiated when the film was placed into tension. The level of adhesion between the film and the polycarbonate has been found to be critical and if the failure mechanism could be guaranteed, then intruder resistant glazing that could be broken from the inside in an emergency becomes a possibility. Such a product would address the identified consumer concern of being trapped in a vehicle. The second approach utilises simultaneous dual injection moulding (2K), which has previously only been used to manufacture coloured components. A successful feasibility study was undertaken to demonstrate the concept of producing transparent components via such a process. This showed that much greater control is required for transparent applications otherwise the skin and core materials mix and opaque components are produced. The generally accepted academic principles associated with the process have been shown to be too simplistic and cannot be relied upon to guarantee good results. The ratio of viscosities of the skin and core materials appear to be more dominant than previously thought and the relative injection speeds of the two materials has a direct influence upon interfacial mixing and haze generation. It was also found that haze could be avoided if the refractive indices of the skin and core material were matched to within ±0.002, but this is impractical. A third area of research examined the feasibility of introducing structured glass fibres weaves into transparent components to improve rigidity. The study resulted in the construction of a transparent glass fibre pre-preg that could be moulded onto the outer surface of polycarbonate components. Flexural tests revealed that a single layer of glass fibre increased the flexural modulus of test samples by a factor of 3, whilst transparency and clarity were retained. Two patents have been filed as a direct result of this work.