In short:* The EU paint directive has reduced the proportion of volatile organic compounds allowed in paints and varnishes
come in a dazzling array
A greener shade of green
However, TiO2 is not ideal on an environmental front either, as it takes large amounts of energy to manufacture it from ore. 'Recently, paint manufacturers have introduced products that lessen the environmental impact by using clever formulations to reduce titanium dioxide levels,' Collins says. 'As the water dries out of the emulsion, dry hiding takes place - the apparent opacity of the dry film is higher than that of the wet film - as the water is replaced by air, giving microscopic voids in the paint film, which can scatter light, and reduces the refractive index of the matrix.
It's almost like giving each TiO2 particle a boost in terms of its opacifying power. You might even get to the point where you could formulate with little or no TiO2, just using air as your pigment. But there would be many problems to overcome here, including that the paint would look pretty transparent until it dried. TiO2 level is one of the criteria in the EU Ecolabel for paint, which sets stringent requirements for a product's manufacture, use and disposal. While this hasn't had a great uptake in the UK so far, elsewhere in Europe it has been much faster, and Germany was ahead of the game with the Blue Angel scheme that predated the EU Ecolabel.
Earthborn claims to be the only UK company whose paints currently carry the EU Ecolabel flower symbol; these are manufactured for them by specialist German company Ecotec Naturfarben. According to Ecotec's Johannes Putz, the philosophy of natural paints has changed over the past 20 or 30 years. 'It used to be about what was going into the paint - was it renewable or not, he says. 'Now what comes out of the paint is more important.' Germany has been ahead of the game on the environmentally friendly paint front, and according to Putz more than half of the companies globally making eco-paint are German.
We have more comprehensive listings of manufacturers making eco-friendly paints on our 'Safe Wall Paints' pages.
An old idea has provided an excellent way of making eco-paint - using vinegar esters as a binder. 'The idea for these is at least 70 years old - back in the 1930s the German rail board used an acetate-based paint on the trains, but the raw materials were quite expensive,' he says. And this is the problem here - not performance, but cost. 'Now, more customers are willing to pay a higher price for a paint that is environmentally friendly. 'The colour range isn't limited, either. 'With clay paints, there are seven colours from the different types of clay, and then we have a range of 126 colours by using a tint as well,' Putz says. 'These tints are also VOC free, and it takes longer to get the right formulation compared to conventional tints.'
Car paints are also included in the EU paint directive
Va va vroom
Car paints are also included in the EU paint directive, with new lower VOC limits set for products such as metal primers, top coats and special finishes. The ultra-harsh conditions car paint has to withstand mean meeting the new VOC targets for these products has been even more complicated than for wood paint. In the same way as painting wood, the metal of the car is first treated with a primer, to give adhesion and ensure the surface is flat.
However, in cars the top coat is split into two, explains Mark Korsmit, sales manager of the car refinishes division of the Dutch paint company AkzoNobel. First the colour is painted on, followed by a varnish to give gloss and protection from hazards such as ultraviolet radiation and chemicals. 'Because of the high colour specifications and the special effects like metallics or pearlescent paints, splitting the two functions up gives better performance and more degrees of freedom,' he explains.
Performance is even more critical in a Formula 1 car, and AkzoNobel worked with the McLaren Mercedes team to develop a paint system that optimises the car's performance and keeps its chrome finish, while meeting VOC requirements. 'It's not just one colour - as well as the chrome, there is a fluorescent red, grey metallic, black and white,' Korsmit says. As with any car, the first layer is a primer, followed by a sealer to give a high gloss flat surface. On top of this comes six layers of the chrome, and then a high performance clear coat that is formulated with a very low level of VOCs. 'The six layers of chrome are extremely thin,' he says. 'A normal paint layer applied by a brush is close to 100μm thick, and with a spray gun this can be reduced to 25 or 50μm. But this chrome layer is less than 5μm thick. The other colours such as the fluorescent red are then painted on top.'
The paint is cured after every layer, not using an oven but with a special infrared arch. This heats up and cools down in a matterof seconds, giving a very fast throughput. Providing a coating that has sufficient hardness while retaining a low VOC content proved to be a major challenge. 'We made small changes to the binder molecules which made the coating dry slightly faster, and have the right properties,' explains Floor Minne, group leader of research and development at AkzoNobel's colour finishes division. 'Binders are difficult. Normally, if they will be hard in the end, binders have high molecular weights but are difficult to get into solution, but lower molecular weight polymers will be too soft.'
The ideal would be a low molecular weight system that goes hard. And that's difficult.' Finding one involves trial and error - make the likely looking polymers and test them out in a real coating system. 'There are multiple raw materials in the paint and the chemistry is complex, so you really have to try it out on a car,' he says. 'The conditions for applying the paint are also crucial - which temperature, what humidity, as these can influence the end results. It's just like making a cake - it's easy when you read the packet but when you make it, it always tastes different!'
A clear coat that gives an ultra-high gloss finish is the latest AkzoNobel product. -This is important in Formula 1 as the surface needs to be completely flat to reduce air resistance,' he says. 'But that's also what ordinary car drivers like. So we are now launching the coat into the marketplace,' says Korsmit. 'It's not just engineering where developments for Formula 1 have important applications in the real world - we can use all the knowledge we gained from the McLaren project in our regular paint system.'
A different approach to lowering VOC levels, being taken by German company Bayer MaterialScience, is developing polyaspartic raw materials as binders in new top coat technology. This hard-wearing paint is being used in the construction industry - who must also meet these tightened regulations - for applications including coating bridges, concrete and tunnels. 'Polyaspartic coatings are formed by the reaction of the secondary amine aspartics with aliphatic polyisocyanates,' explains Bayer corrosion protection expert Markus Mechtel. 'The reaction speed of both components can be controlled by the variation of steric hindrance of the amino group in the aspartic's chemical structure, leading to a wide range of curing conditions.' Polyaspartics are relatively low viscosity, which allows ultra-high solids coating formulations to be created that can be close to zero-VOC. 'They dry significantly more quickly at room temperature than conventional top coats, which enables faster further treatment and earlier transport of the coated parts,' he says. 'They also enable significantly greater film thicknesses to be achieved, with a dry film thickness of up to 400Î¼m being possible in a single operation.
In practice, this enables a coat to be saved. The increase in efficiency is particularly important if the coating process is part of a process chain; rapid drying and reduction in the number of coats enables a significant increase in productivity to be achieved. Mechtel adds that the coatings have high weatherability, chemical and mechanical resistance. This technology was put to good use in the recent refurbishment of BayArena, the home football stadium of Bundesliga team Bayer Leverkusen in Germany. It has a discus shaped, translucent roof with a diameter of 217m, made of Makrolon polycarbonate sheeting. 'To ensure durability, the steel support structure, weighing around 3000 tonnes, was given an anticorrosion coating based on polyaspartic raw materials, says Mechtel. With large structures such as bridges and power plants, a threecoat paint system is normally needed to provide protection against the ravages of wind and weather. With our polyaspartic coating, two coats were sufficient because of the higher film thickness with each application, Mechtel says. The schedule for the refurbishment was extremely tight - it had to be finished in time for the start of the next football season, and the rapid drying and fewer coats were instrumental in meeting the deadline.
The McLaren 2010 Formula 1 car uses AkzoNobel low-VOC high performance paint
Sarah Houlton is a freelance science writer based in London, UK | Contact: firstname.lastname@example.org
Sarah Houlton, Chemistry World, 2010, 7(3), 46-49,
Reproduced by permission of the Royal Society of Chemistry
Picture credits from top to bottom:
© THINKSTOCK IMAGES
© BAYER, © MRW