The very perfection of these surfaces, however, causes the unforgiving human eye to notice even the slightest mechanical ”injury” to the car’s outer skin, which are inflicted by the frequent application of car wash brushes, protruding twigs and branches, or the hands of high-spirited children. Naturally, the driver of the car would also like the paint to offer more protection in this respect as well, or even to show ”self-healing powers.” Accordingly, improving the scratch resistance is the uppermost goal of development.
Present-day clearcoats still do not go far enough toward meeting these high expectations. To optimize their scratch resistance means suffering reductions in all other quality features — but this is tolerated neither by the automobile companies nor their customers. The challenge is therefore clear: to develop clearcoat systems whose every property meets the quality level already attained, for example with respect to acid or chemical resistance, but which also have a much-improved scratch resistance. This is a task akin to that of squaring the circle — at the very least, it demands ever greater levels of research and development.
The difficulty of the task is illustrated by the following example. When a just-washed vehicle is gleaming in the sunshine, the problem is obvious. All around the reflected image of the sun are numerous fine, circular scratches in every color of the rainbow, a phenomenon that cannot be seen when the sky is cloudy. What the rainbow colors suggest is proven by microscopic analysis: the scratches are no more than 3 µm deep — one 30th of the diameter of a human hair — and are so close to the wavelength range of sunlight that when they are viewed at a certain angle, interference gives rise to these color effects, which cannot be hidden from the eye.
It should also be noted that a scratch is not simply a scratch! In some cases, the unwanted damage can be a result simply of deformation of the film surface. Scratches of this kind can be ”healed” by heating the paint —even the warmth of the sun may be sufficient. Accordingly, one way in theory of improving the scratch resistance is to reduce the so-called glass-transition temperature (Tg) by changing the paint formulation to lower the temperature required to ”heal” the scratches. Unfortunately, this route has proven to be a blind alley, because the new paints suffer a direct deterioration in chemical resistance.
In other cases, more severe stress may actually remove coating material from the surface, leaving permanent scratches. This type of damage is typically the result of particles of dirt in car wash water, especially if the water is used several times rather than filtered before each new wash.
Formulating a strategy to avoid scratches requires in-depth knowledge of the chemical structure of paint and its physical properties. In order to bring further improvements to a high-tech product such as an automotive clearcoat, it is necessary to look more closely at the way it is produced. The liquid coating material contains building blocks that carry what are known as functional groups. When the coating is cured, these functional groups react chemically with one another so as to construct a three-dimensional network. Ultimately, it is the selection of the building blocks, functional groups and curing conditions that determines the properties of the coating.
In the course of investigating model clearcoats with different scratch resistance, it was found that the mesh size of the network is a crucial parameter. No great surprise: the closer the ”knit” of the network, the better its scratch resistance. In particular, the number of permanent scratches is lower.
To reconcile a high level of scratch resistance with good chemical resistance, therefore, new building blocks are needed that offer a large number of crosslinking sites, so that a closely knit network can form. Chemically, this need can be met in a number of different ways, of which the following three currently offer the best prospects of success.
The first example will be familiar to anyone who owns one of Schott’s Ceran cooktops, which consist of a special rolled-glass ceramic material. This extremely hard surface can be cleaned using steel wool and abrasive cleaning products without showing any scratches. BASF Coatings’ research is aimed at transferring this ceramic concept to coatings. The key factor is to overcome the brittleness of the glasslike system in order to achieve the elasticity required for automotive applications. A solution to this problem might be what is known as inorganic-organic networks, where the inorganic component provides hardness while the organic component lends elasticity. In laboratory experiments, this approach has already been implemented and a distinct improvement in scratch resistance obtained. At the present time, bodywork components coated additionally with this material are being subjected to extreme scratching stresses (see photos on p. 84-85). Before this coating takes to the streets, however, it must pass the tests of numerous other disciplines — including the standard long-term weathering tests conducted over at least three years in the sunny and humid climate of Florida.
Other variants include the coatings that are already used by the furniture industry to coat desktops and kitchen worktops and that, accordingly, meet a high profile of requirements. In contrast to the present car finishes, which require temperatures of up to 140°C, curing in this case takes place by means of UV radiation — which holds the additional attraction of energy savings. However, light always casts a shadow: because of the complex three-dimensional structure of a car body, the UV light is unable to reach into every corner. Only where light strikes the liquid coating material is the latter cured. It therefore makes sense to combine conventional thermal curing with UV.
Here, too, it is only when the extensive range of tests has been successfully passed — once again, the long-term weathering, in particular — that the system can expect to enter the realm of automotive coatings. What is needed here is the development of even ”more intelligent” light stabilizers, which are required to exhibit a particular behavioral ambiguity: Before chemical crosslinking, they must not disrupt the curing effect of UV light, but afterwards they must protect the organic structure against the energetic rays of the sun.
A particularly elegant but also very complicated development is the use of nanoparticles in paint formulations. These tiny particles, with diameters of just a few billionths of a meter, can have the same chemical basis as glass, so giving the coating the corresponding properties such as hardness and scratch resistance. Before they are used on a car production line, however, there are a number of issues which need to be clarified. Firstly, it must be ensured that an exactly identical particle structure can be reproduced industrially; secondly, there are still considerable improvements to be made in the embedding of the particles, and their currently inadequate stability in the coating material, before production on an industrial scale becomes a possibility. Furthermore, there is still a lack of knowledge about whether these submicroscopic particles might trigger unknown and undesirable phenomena or effects.
Among the approaches sketched out in this article to durably enhancing the scratch resistance of automotive clearcoats, experience suggests that only one will prove to be the real breakthrough. Which one it is remains to be seen.
For more information on scratchproof clearcoats, call +49/2501-140, fax +49/2501-143373, visit www.basf-coatings.de.