Architects memo no.
28: February 1983
alkyd v. acrylic
The pressure on the market of two distinctly different classes
of paint each vying for the same surfaces must of necessity cause some
confusion in the mind of the specifier. The purpose of this memo is
to lay before you the inherent differences in the two species so that
the choice can be made more logically.
In order to do this, we are going to have to stick our toes into the
deep water of polymer chemistry, but before we take that plunge it is
absolutely vital to clarify our thoughts in the area of history and
development. Alkyd resins have been on the scene for about fifty years.
Theirs is a mature technology with no significant developments having
occurred in decorative alkyds for the last twenty years. Acrylics on
the other hand are still developing and at a very rapid rate. The result
of this tends to make past experiences meaningless as an assumption
based on the use of any acrylic 10 years ago would have little relevance
Any polymer having useful film properties must have a high molecular
weight; within one polymer type higher molecular weight will give tougher,
more abrasion resistant, and more durable films. Alkyds and acrylics
achieve their high molecular weights in very different ways. Acrylic
polymers are grown to massive molecular weights in a reaction kettle
by growth of long chain-like molecules. When they are discharged from
the kettle, all the reaction is spent and the polymers can correctly
be called non-reactive. Such massive molecular weight polymers can only
be handled in the form of an emulsion; a solution would be unusable
sticky and viscous. Alkyd resins on the other hand are cooked only to
medium molecular weights but contain, in their ingredients, reactive
sites (polyunsaturated oils) that will further react to bring the molecular
weight up to the necessarily high level. This further reaction is induced
in the paint film by the addition of catalysts (driers) and the presence
of oxygen in the air. The reaction proceeds by the linkage of adjacent
molecules into a three dimensional lattice.
The difference described is important for two reasons:-
- The three dimensional nature of the alkyd is much more rigid, when fully
cured, than the chain like acrylic. This results in the alkyd becoming
more brittle than the acrylic, and
- Not all of the reactive sites are used up during the curing of an
alkyd - these later become sites for future breakdown
These two reasons provide the theory behind the observed fact that
acrylics are more durable than alkyds. Life, of course, is never quite
as simple as a yes/no alternative and there are often other factors
to consider apart from just durability. The list of factors in this
case is quite long but we will confine ourselves to three major areas:-
Film build (or volume solids)
Because the alkyd starts off at a lower molecular weight you can get
more of it in the can. A good gloss alkyd will have a volume solids
of about 55% whilst the equivalent gloss acrylic will be about 40%.
This means that at equal spreading rate, the alkyd gives a film 37.5%
thicker than the acrylic. Conversely at equal film build the alkyd
will have a 37.5% greater spreading rate.
Alkyds and acrylics form films by entirely different mechanisms. Alkyds
initially lose solvents by evaporation during the 'tack-off' period
then cure by the catalytic/oxidation reactive mentioned earlier. The
rate of this reaction is temperature dependent, slowing down in the
cold. The reaction rate will pick up however, when the temperature
rises. Damage that can occur to the film during cold temperature
drying is usually confined to some gloss loss.
Acrylics however, form a film by the physical fusion of plastic
particles after loss of water by absorption or evaporation. Temperatures
below a certain critical level will cause the particles to become
too hard to deform and coalesce into a film. This is particularly
true when high humidities delay the evaporation process possibly
into the cooler part of the evening. Lack of film coalescence in
acrylics is an irreversible phenomenon.
Acrylics are a particular material carried in a water medium. If
these are applied to a porous surface water will be absorbed preferentially
leaving the vehicle sitting on the surface. If the surface is sound,
this is a very desirable state of affairs as it leaves all of the
protective or decorative film exactly where it should be.
Alkyds on the other hand, due to their lower molecular weight, are
more penetrative of porous surfaces, which can lead to loss of film from
the surface but can also help in binding and conditioning less than
perfect surfaces. This does not prejudice the need for proper surface
preparation but does give the edge to alkyds over doubtful surfaces.
To sum up, where an external surface requires coating, if the surface
is sound and the weather conditions favourable, an acrylic will generally
give the better performance; if the surface is doubtful an alkyd undercoat
followed by an acrylic is a very good system.
NB: In May 1996 Resene launched two waterborne enamels, Resene
Enamacryl and Resene
Lustacryl, the first of their type in the world.
Contact Resene for further information.
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