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Better water-based paints

   
 
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Quality problems with water-based paints continue to hamper their use as environment-friendly alternatives to coatings based on organic solvents. In general, water-based coatings are more permeable than their conventional counterparts and give a poorer surface finish.
This project set out to learn more about the factors affecting permeability and surface roughness in aqueous coatings. The partners, a group of five specialist coatings research laboratories, learned a lot about choosing polymers, pigments and additives to give the best product. In the 18 months since the project's results were published, European paint manufacturers have started to put these lessons into practice to produce better-looking and more durable coatings.

Water-based paints and other aqueous coatings are slowly gaining ground as environment-friendly alternatives to coatings based on organic solvents, as increasingly stringent local enviornment regulations are being brought forward to reduce emissions from Volatile Organic Compounds that create pollution. Unfortunately, their take-up has been hampered by the fact that they are often more permeable than solvent-based paints towards water vapour and airborne pollutants, with the result that they are not as durable as conventional paints. In addition, water-based paints often give a relatively poor finish because the surface of the coating is not smooth enough.
The partners set out to learn all they could about how to form smooth, pore-free films from waterborne coatings. The Belgian Coatings Research Institute (CoRI) led the project, and the other partners were all research institutions with an interest in coatings: the Irish state technology organisation FORBAIRT, the UK's Paint Research Association (PRA), TNO Centre for Coatings Research in the Netherlands and the Institute for Surface Chemistry in Sweden (YKI).
The results confirmed that getting good performance from water-based coatings poses serious challenges for coatings manufacturers and polymer chemists. The project yielded mathematical models that successfully describe the film formation process and the distribution of pigment within the coating. These showed, for instance, that the polymers in traditional water-based coatings are often too hard to give a good film.
Industrial take-up of the results was rapid because the project partners were able to make the information available to their own industrial sponsors. In this way, a total of 25 companies played a part in sponsoring the project. Partly as a result of this effective dissemination, manufacturers of the latest generation of water-based coatings are using innovative chemistry to overcome some of the problems associated with previous coatings.

Flocculation and films

The film-forming part of an aqueous coating is a latex, a dispersion of very small spherical particles of high-molecular-weight polymer in water. Surfactants and protective colloids added to the latex stop the particles from coalescing during storage. When the coating is applied to a surface, how- ever, the water starts to evaporate and the polymer particles are forced closer and closer together. Once the particles are packed tightly together they start to coalesce into a continuous film.
During this film-forming process the polymer particles are pulled together by surface tension and capillary forces, while their own rigidity tries to keep them apart. The trade-off between these two factors determines whether the final film is smooth and pore-free or rough and full of microscopic holes.
CoRI developed a successful mathematical model of the film formation process. When supplied with measurements of polymer properties and details of the temperature, humidity and co-solvents present, the model predicts the quality of the resulting film.
The model confirmed that for good film formation it is important for the polymer particles to be fairly soft, though the cured film needs a harder polymer in order to give effective protection. At the moment many water-based coatings also contain some organic solvents, and these help soften the polymer so that it forms a smooth film. Afterwards the solvent evaporates and the polymer hardens to give a tough film.
Increasingly stringent rules governing the use of volatile solvents, however, are likely to mean that future coatings will not be able to use these co-solvents. Instead, the latest generation of water-based coatings uses innovative chemistry to make the polymer particles soft during film formation and then harder as the film cures.

Pigments and rheology

Paints rely on pigments for much of their colour and durability. To work best the pigment particles should be evenly sized and well distributed through the film, but instead they often clump together (flocculate). This is a problem because it effectively wastes pigment as well as giving a low-quality finish. Paint manufacturers include additives known as dispersants to modify the surface properties of the pigment particles and keep them evenly suspended in the paint.
Water-based coatings give particularly poor pigment distributions because the presence of the latex particles restricts the motion of the pigment particles. PRA worked on this, first producing a mathematical model of these physical effects and testing it in the laboratory. They then added in the effects of chemically treating the surface of the pigment and polymer particles, adding dispersants and changing the ionic strength of the mixture. The result is a good prediction of the quality of the dispersion.
YKI also studied how well dispersants are absorbed onto the surface of pigment particles - important work because thickeners and other additives can compete with the dispersants so that the effect of each is reduced. The YKI researchers found they could avoid this 'competitive absorption' by matching the chemical characteristics of the different additives so that they work together instead of in competition. This will help paint manufacturers when they are designing new recipes.
The flow behaviour or rheology of the coating is also important. Paint has to be thin enough to be applied easily and to be self-levelling, yet it must adhere well to the surface and be thick enough not to drip. CoRI, TNO and FORBAIRT worked on this, studying many different combinations of latex, additive and solvents. Unfortunately the researchers found it difficult to come up with a universal model to predict rheology.

Going public

The project did not solve all the problems associated with water-based coatings, say the partners, but it did a good job of identifying them and directing research into new areas. It also highlighted the fact that there are some very fundamental problems with water-based coatings concerning durability, permeability and quality. Partly as a result of the models developed during the project, paint manufacturers are developing new products using more sophisticated chemistry to try to overcome problems with film formation.
The results of the project have been published and there has been no direct commercial benefit to the partners. This certainly does not mean, however, that the results have not been commercially useful. The project partners were research organisations with active support from their industrial members - who in effect acted as 25 industrial partners. This meant that spreading the word on the project findings was very quick, and coatings manufacturers have seen a corresponding benefit.

 

Project Title:  
Environmentally friendly high performance waterborne coatings

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)


Contract Reference: BE-4191

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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