Process and product development for a novel automotive coating

Improving fuel efficiency has been a major drive for the automotive industry in recent decades, and this has led to the increasing demand for light weight plastic components to reduce the weight of the vehicle and fuel consumption. The surfaces of plastic parts, on the other hand, are more prone to damage and degradation compared to heavier material such as metal. This has led to the need to develop high performance automotive coatings, and a growing demand for coated polycarbonate film products of MacDermid Autotype Ltd. (sponsor) has been observed.

The research presented in this thesis had the overall objectives of developing a high performance automotive coating formulation that will provide the coated film’s hardness and the easy-to-clean features which are superior to the current products, and to develop a process route that will allow the novel formulation to be produced in larger quantity beyond the laboratory scale. The research program therefore had three main strands: Product Development, Process Development, and Performance testing of the final product.

The new development product is a hard-coated polycarbonate film for Film Insert Moulding (FIM) automotive application. After a detailed material screening to develop a dual-cure formulation to achieve high scratch resistance (2H pencil hardness), 3D formability, and UV-resistant nanocomposite hard-coats, a new formulation has been developed. This has provided a balanced dual-cure coating of 3D-formability and surface smooth-out properties during the thermal forming process, and a high-hardness surface for the final 3D part respectively.

The new formulation is a solvent based polymeric lacquer that has been prepared using a sawtooth impeller in both laboratory formulation and production scale at MacDermid Autotype. To support the process development, the comparative blending performance of this impeller has been studied in comparison to using a pitched blade turbine (PBT) in model liquids that mimic the rheology of the added colloidal dispersion of nanoparticles into the bulk premix lacquer. The study was performed at three scales of operation from 2 to 160 litres corresponding to those used for formulation and manufacture in industry with an additional intermediate size tank.

It could be shown that the blending performance is comparable in the turbulent regime at a given volumetric power input in T= 0.30 m; however, in the large scale of T= 0.58 m diameter tank, the time to reach homogeneity is much longer with the sawtooth impeller as was demonstrated with both the conductivity or colourisation-decolourisation techniques. Mixing time values obtained through conductivity measurements were in agreement with those estimated using mixing time correlations for both impellers at the intermediate scale of T=0.30 m and only for the PBT at the large scale. This deviation in the performance of the sawtooth impeller at large scale, has highlighted the difficulty in making reliable estimates of blend time with this impeller when designing for larger volume manufacture. The poorer performance noted at larger scale would result in product properties being compromised. This underprediction could be resolved by using a higher value for the constant in the blend time correlation: 11.1, instead of 5.2.

The development coating of the new formulation has been manufactured in production scale to meet customer needs and market opportunities. Performance testing was carried out against the international standards. Key coating characteristics as per project’s objectives, including high scratch resistance has been achieved on samples processed both internally and at customer facilities via FIM. The additional automotive weathering specifications of temperature-humidity resistance (Volkswagen AG), chemical resistance (Volkswagen AG), and Xenon-arc sunlight UV resistance (Fiat Chrysler Automobiles Group) have also been studied. All tested samples have passed the humidity resistance requirement, but the UV resistance showed a dependency on the coating thickness of the FIM part: the thinner the coating, the better sunlight UV resistance was achieved. Although the UV resistance performance may limit the applicability of this development in a vehicle, it is rated as a high performance formable coating that is superior to the standard FIM product by MacDermid Autotype.

The present work has shown that the development product of a new nanocomposite, formable coating of superior scratch resistance has been successfully scaled up and will be ready to be taken to the market for automotive interior coating applications via the FIM process.