When a paint is applied to a substrate it will
have striations in its surface caused by, for example, the bristles of a
brush or the force of the air gun during spraying. After application, the
striations need to flow out in order to form smooth surface – this process
is known as levelling.
However, if the paint is being applied to a vertical
surface, the paint cannot flow too much as this will result in sagging. This
may manifest itself in ‘tear-drops’ being left behind in the dried paint
film. This indicates that the paint should possess well balanced viscoelastic
properties. On the one hand, after the paint has been sheared during application,
it should have predominantly fluid-like behaviour to allow levelling to occur.
On the other hand, at some point, the paint should
re-structure to avoid sagging occurring, i.e. it becomes solid-like.
All of the experiments reported in this article
were performed on a Bohlin rheometer.
Figure 1 shows the curves for two paints – a non-drip
gloss (ndg) and a gloss finish (gf) – which have been analysed using stress
viscometry. In this experiment a stress is applied to the material and the
resultant shear rate is measured. The viscosity is calculated using the
= shear stress/shear rate
The stress is increased stepwise during the experiment
and the shear rate is measured at each stress. The curves show a number of
interesting points about the properties of the two paints:
a) in the low shear region the viscosity of the
ndg is much higher than that of the gf paint
b) the ndg paint shear thins at a much higher stress
than the gf paint, and much more dramatically,
c) the ndg paint shear thins to a lower high shear
viscosity than the gf paint.
1. Stress Viscometry On ndg And gf Paints
The ease of structure breakdown of the ndg paint
means that it is more likely to flow in to the bristles of a paint brush
when it is dipped into the paint. Also, the lower high shear viscosity of
the ndg paint means that it will be easier to apply than the gf paint. In
order to avoid sedimentation it is necessary to build into the paint a high
low shear viscosity. This enables the paint to withstand the force of gravity
causing particles to sediment. As can be seen from the curves in Figure 1,
the ndg paint is more likely to be stable than the gf paint.
Figures 2 (a) and 2 (b) shows the viscoelastic
response of the materials after rotational shear. In the experiment, the
material is sheared at a reasonable high shear stress to simulate the process
of being brushed on to the substrate (pre-shear). The shear is ceased and
the material is subjected to an oscillatory low stress (at a fixed frequency)
to simulate the structure re-build which occurs in the paint. It is important
that the paint flows sufficiently to allow levelling to occur, but not so
much that sagging occurs (see earlier). In terms of the rheological properties
of the material, this means that immediately after shear the viscous modulus
(G’) should dominate the elastic modulus (G’), but at some finite timescale,
G’ should dominate G’.
2(a). Non-drip Gloss, Presh Oscn At 20deg
2(b). Gloss Paint, Presh Oscn At 20deg
The curves indicate that the ndg paint (Figure
2 (a)) should exhibit good levelling and sag properties, while the gf paint
(Figure 2 (b)) would also show good levelling but excessive sag behaviour.
However, the rheological experiment does not allow for either absorption
of the carrier solvent into the substrate (e.g. a plaster wall) or evaporation
of the carrier solvent after the paint has been brushed out to a thin film,
which would change the rheological properties quite markedly and have a large
influence on the structure re-build properties of the paints.
These two simple experiments yield a lot of useful
information about the performance behaviour of paint systems. They are quantitative,
reproducible and non-subjective, in contrast to some of the more traditional
tests used in the coatings industry. The tests could form the basis of both
quality control and development protocols.