Design and Scale up of a Mixing Process for a Novel Automotive Coating Formulation

With every 10% of weight reduction in a typical vehicle, improvements of 6% in fuel economy can be achieved. For electric cars this corresponds to 14% additional driving distance.

Plastics have typically been chosen as a light-weight material and their usage has increased from around 50 kg per vehicle in the 1970’s to more than 150 kg/vehicle today.

As plastic surfaces are more prone to damage compared to metal, this move has led to the need for high performance coatings to provide enhanced durability and physical robustness.

In addition, the range and complexity of functions that require driver interactions with the vehicle have increased in the recent years with the introduction of touch screens, such as integrated satellite navigation and control systems on the dashboard. It is highly desirable for such surfaces to be sufficiently resistant to damage and effects of common consumer products.

The development of high performance coatings

These changes have brought up the need to develop high performance coatings for interior surfaces to enhance properties such as hardness, scratch resistance and abrasion resistance, as well as anti-fingerprinting whilst maintaining the desirable optical properties, such as clarity and colour.

Typically, multi-layer coatings are employed to ensure both protection and desired aesthetic appearance to the finish. The appearance of the top clear coat is prone to degradation under harsh conditions. There have been promising developments, which suggest that abrasion-resistance coatings could be obtained by incorporating metal oxide particles into formulations.

Our research is studying the process of blending a colloidal dispersion of metal oxide particles into coating formulation to develop a design that can be used for large scale manufacture.

The study has been performed with the overall aim of developing a new formulation automotive coating of superior performance in quantities beyond formulation scale and the associated process.

The three main strands of the project were: 

• Product development which was performed at small scale (1-2 litres) for formulation development, material screening and characterisation;
• Process development to evaluate blending performance, process optimisation, scale-up; and finally;
• Performance testing comprising the assessment of scratch resistance, hardness and optical quality. 

Of these, the second is covered in this case study, i.e. the step in which a new additive in the form of a colloidal dispersion is blended with the resin.

The comparative blending performance of two types of impeller, one employed in industrial practice and an alternative, was investigated. Due to the hazardous nature of the actual formulations, we used a simulant liquid that replicates the rheological properties. A wider range of viscosities allowed covering flow regimes of interest at different scales.

The study was performed at scales of operation corresponding to that used for formulation, another comparable to manufacturing scale and an intermediate one.

Formulation scale (V=1.5L, T=0.13m), Pilot scale (V=20L, T=0.3m), Large scale (V=160L, T=0.58m)

Different techniques employed allowed both a visual appreciation of how the added liquid propagates in the bulk and also for correct positioning of the probes. Mixing times could thus be determined through qualitative and quantitative measurements.

Example experiment from 20 litres scale showing the propagation of the added liquid.

Blending performance of the impellers from different scales could then be evaluated and also compared to predictions from mixing time correlations. Mixing time correlations that have been shown to hold for a wide range of impellers indicate that mixing time values are comparable for different impellers so long as they are of a given impeller-to-tank diameter ratio and comparisons are made on the basis of specific power input.

An excellent agreement was noted for one of the impellers across the whole range of scales. With the other, for which there is limited previous work on its blending performance, a dependency on the scale of operation was noted. Whilst of a similar performance at formulation and intermediate scales, much longer mixing times were required with this impeller at large scale, i.e. significant implications in terms of scale up/ large scale design.

The outcome of the research on formulation development and performance testing cannot be elaborated due to its confidential nature. The part included in this case study has provided useful guidance for the design and scale up of this particular process and can also be applied to many others. Further details can be obtained from our publications listed below.