Publication

Coatings Technology for Competitive Advantage

Coatings can be a significant source of competitive advantage – or disadvantage – for manufacturers of heavy vehicles. Quality coatings, well applied, can put the finishing touch on eye-appealing design. They can protect vehicles, increasing re-sale values. They can lower production costs and increase production speed. But all this is dependent on the very best standards of application.

Getting the best out of modern coatings demands an understanding of application technology, which is developing as the coatings themselves are developing. The demise of conventional solvent borne coatings has been driven both by environmental and quality pressures. Put simply, modern coatings can do a better job and lower the environmental impact of the coating process at the same time.

The alternative technologies – powder, high solid or water borne coatings – are all used, although high solids coatings tend to be more acceptable in North America and not acceptable in some European countries. For most off-highway OEMs, particularly in Europe, water borne coatings (WBCs) are the favourite for environmental compliance and practicability. However, this technology differs from solvent borne coatings (SBCs) in some important respects.

A typical WBC system will be a two-component (2K) epoxy primer, followed by a 2K polyurethane topcoat, applied over cleaned and ideally pre-treated substrate. Successful application demands understanding some of the technology of that very unusual chemical, water. It influences pre-treatment, pumping, spraying, drying and waste disposal.

Make a clean start

Paint performs best on clean and chemically stabilised substrate and this is particularly critical for WBCs. SBCs can smother and absorb a certain amount of grease on a surface, but any contamination will affect water. Thorough de-greasing is vital and phosphate pre-treatment is ideal. The choice of pre-treatment type is important, since the optimum anti-corrosion and mechanical performance will come from a careful marriage of pre-treatment with primer.

Trojan-horse-style contaminants can also be smuggled into the system unwittingly. Seals, valves etc. must never be lubricated with silicone containing or other surface-active materials since they lead to the cratering of the paint. Water and oil do not mix, and neither do water and silicone!

Applying science

WBCs have different application properties from SBCs, properties that can be exploited to improve application efficiency and coating performance if they are clearly understood. One of the key pieces of science exploited in WBCs is rheology, the way a liquid flows. Figure 1 shows that the viscosity of WBCs is much more variable than for SBCs. The rheological make-up of WBCs can be tailored to provide “thixotropic” and “shear thinning” capabilities.

In storage, WBCs are at a higher viscosity than SBCs. Pumping begins to break this thixotropy, but the high-energy input of spraying shears the WBC to a lower viscosity for good atomisation. Once on the substrate, the WBC is no longer under shear and the thixotropy recovers quickly, because of the rheological properties of the coating, helped by evaporation of some water.

The ability to define the rheological properties is a very helpful characteristic of WBCs: this “pseudoplastic” profile helps prevent settlement in storage, giving a coating that can be readily applied but is resistant to sagging on the component. It can be less helpful at other points in the coating cycle such as pre-conditioning stored paint for application, measuring viscosity itself and spraying.

It is common sense not to risk freezing with WBCs, which should be transported and stored between 5°C and 30°C. What is less obvious is that, the colder the paint, the higher the viscosity and the slower the drying will be. So water borne paint should be always be brought up to the temperature at which it is going to be checked for viscosity and used. Since water has approximately double the specific heat value of solvents, and since the structured nature of the paint damps down convection currents, raising low storage temperatures requires some time to achieve the necessary processing temperature. For example, three days should be allowed to raise the coating from 10°C to 20°C by storing at 20°C in a 1000 litre container without stirring. This has to be taken into account in the planning of stockholding and paint preparation areas.

Viscosity tests prior to spraying have traditionally used cups with a calibrated hole in their base, measuring the time taken by gravity to force the paint through the hole. This simple system will not work for WBCs. To replicate viscosity at spraying conditions, rotation viscometers must be used which apply a similar shear force to the coating sample as it will be subjected to in spraying. Incidentally, the pH value of the coating must also be on specification, otherwise this, too, can affect the viscosity and performance.

Demystifying spraying

Efficient spraying demands a high level of atomisation and high transfer efficiencies – demands that tend to work against each other. The spray systems with the best transfer rates tend to be air-assisted airless or high volume, low pressure (HVLP) systems. However, these tend to give poorer atomisation, with attendant effect on quality of finish. Air-assisted airless systems are the most common application method for heavy vehicles, often in conjunction with electrostatic spray (ES) techniques, which will help both in atomisation and also transfer efficiency, through attraction and wrap.

But here water presents another hurdle – its high conductivity. ES systems apply charges of up to 90K volts to the spray cloud, holding the droplets in the cloud apart while attracting it to the earthed work piece. These voltages can be achieved safely, either by insulating and isolating the whole application system, or (more usually) by using a cascade system which compartmentalises the paint supply, holding part at charge and automatically topping-up using a voltage block or brake to contain the charge. For handspray, the charge is often applied externally – after the nozzle.

Returning to the issue of viscosity, what special treatment do WBCs need to prepare them to form a good spray cloud? As well as the inherent high viscosity of the paint, water has a very high surface tension, which means that yet more work is required to break it down into fine droplets.

Win-win solutions to these problems have been developed from the need for both components in a 2K system to be really intimately mixed for an effective chemical reaction – which of course starts as soon as they are mixed. Small in-line Archimedean screw mixers do a good job and obviate pot-life problems, since paint is mixed and applied within a few seconds. 2K epoxy primers are blended relatively easily and one screw mixer is usually adequate. 2K polyurethane topcoats may require two or three mixers in series, to overcome the naturally hydrophobic nature of some isocyanates. The energy inputs from the mixers and the spray pump will ensure good atomisation.

The use of in-line mixers is particularly valuable with 2K WBCs since, unlike SBCs, their viscosity does not increase markedly as the end of pot-life approaches. The same increase in molecular weight occurs, but without a noticeable increase in viscosity, particularly in primers. However, the performance of the film will be reduced, giving rise to problems that only become visible later, in the field.

Watching paint dry

OEMs rightly demand the shortest possible times from priming through overcoating to handling. Once again, water’s properties are less than completely helpful. Compared with the solvents in SBCs, it is slow to evaporate, especially at low temperatures; it needs more energy input to evaporate; and the rate of evaporation slows with higher atmospheric humidity. Figure 2 shows the effect of relative humidity (RH) and temperature on the drying times of a two pack epoxy primer.

With small components that can be stoved, this is no problem. But larger, air-drying components need an appropriate processing window in terms of RH and temperature. In central and northern Europe this is normally set as 50-80% RH and 20˚C - 30˚C, but slight adjustments to paint formulations can accommodate other ambient conditions. In general terms, especially in high humidity conditions, good airflow is more important than a little more warmth, but care is needed to ensure that air flows are clean and dust-free, and that they do not interfere with spraying.

As can be seen from Figure 2, it is still not possible to overcoat a WB primer or build coat immediately at ambient temperatures, as can be done with some SBCs. Some time must be allowed between coats – or alternatively a flash-off section with some heat and forced ventilation can be introduced, which will allow acceptable production-line overcoating times.

The final stages of film formation prior to full drying and curing are also important. WBCs contain a small amount of a special solvent called a co-solvent that plays a vital part in the formation of the paint film. Another reason for operating within the correct processing window is to ensure that the co-solvents are not evaporated before they have completed their film-forming job.

The end-game

Even when the paint has been successfully applied and the product is drying, the cleaning and waste treatment associated with WBCs need to be considered. When flushing spray guns and cleaning supply systems it is important to realise that the resins in one WBC are often incompatible with binders in other WBCs, and with many organic solvents. The incompatibility can show itself in swelling up, flocculating out or coagulation of the coating materials. Also, WBCs, when atomised, tend to dry quickly on spray guns.

As far as possible it is prudent to clean with water before the coating dries. Dried-on or coagulated material can be removed with certain specific organic solvents or solvent-water cleaning mixtures.

Booth wash water may require defoamer addition to counteract the effect of the surfactants in the WBC. Incidentally, should metallic coatings be involved, the pH of the wash water must not be allowed to become strongly alkaline (e.g. through cleaning), since the aluminium flake may then liberate considerable quantities of highly-explosive hydrogen.

In some circumstances it may be possibly to re-concentrate the overspray using osmotic filtration, but often this is impractical when painting off-highway components. Overspray will require careful coagulation for disposal, bearing in mind that the wash water may contain a proportion of water-soluble coalescing solvents, normally between 0.5 and 1%. Consequently the solvating or swelling of coagulated paint must be controlled. Foam control and coagulation are best achieved with the right package of hardware, defoamers and flocculants to constantly clean the system.

Ecology without tears

If the water route to compliant coatings looks like a minefield, it is worth remembering that the coatings themselves perform extremely well, and can be very efficiently applied. In their own right, they are well worth having compared with the alkyds of old.

It is also worth remembering that, for items that cannot be stoved, WBCs normally offer the best – often the only acceptable – environmental performance. True, the total system needs careful, compatible design and all aspects of application need to be well managed – but this is the cost of success in every competitive area today.

The best safeguard is to work, from the earliest stage possible, with a competent coatings partner who is experienced in off-highway vehicle painting and all the associated issues that stretch way beyond the paint drum. Beckers recognised this over a decade ago when they launched their “Becker GreenLine®” environmental package, providing, in addition to coatings that meet environmental legislation, advice to customers on the implications of the legislation; and full support in order to maximise paint performance and optimise production conditions.

Driving down total cost in partnership is the informed route to competitive advantage in coatings.

Mike Brady is responsible for the development of water borne coatings for Becker Industrial Coatings Ltd. Beckers is Europe’s market leader in supplying coatings to the off-highway vehicle market. He has worked in high performance coatings for Beckers for 18 years.