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
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
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.
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