Working with paint
14.2 Paint viscosityThe desired paint viscosity also depends upon the application method
chosen. Obviously, a different viscosity is required in the case of brush
application than with a spray-based application. Various options are available
for adjusting or influencing the viscosity:
adjusting the temperature of the paint
thinning the paint
using thixotropic paints
From the perspective of maintaining the quality and performance
characteristics of a paint product, it is better to ensure that the paint has
the proper application temperature than to thin the paint with solvent.
For example, if the paint is too viscous at a temperature of 8°C, then
the viscosity can be lowered by raising the temperature to about 18-20°C, which
means that less thinning will be required!
If thinning still remains necessary, then we strongly recommend that you
follow the thinning instructions given by the paint supplier which apply
specifically to the paint products concerned. If thinners other than the
recommended ones are used, the polarities of the solvent and binder may differ
too much, therefore requiring more thinner than would otherwise be necessary
and modifying the technical characteristics of the paint.
It goes without saying that in the case of two-component products, the
viscosity of the product after mixing is more important than the separate
viscosities of the primer or curing agent.
Once the colour of the mixed product has become stable and even, you can
be sure that both products have been thoroughly mixed, and only then is it
possible to evaluate the viscosity of the mixed product.
The so-called tixotropic paints or coatings are quite common in the
protective coatings sector for steel. If these paints are stirred inside the
can with a spatula, they initially appear to be very viscous and fall off the
spatula in blobs. However, the harder one stirs or mixes it, the less viscous
the paint becomes. This is because of the energy being transferred to the
paint. If the mixing is stopped and the paint is allowed to rest, it will
regain its original viscosity. This same thing happens if the paint is applied
via spray painting. As soon as the paint is fed into the pump and becomes
‘energized’ it becomes less viscous.
As the paint is forced through the small nozzle opening of the pistol,
it is converted into a fine spray or mist. The result is an optimum
distribution of paint on the object being sprayed.
Once the paint reaches the substrate and is no longer in motion, the
viscosity increases, thereby allowing the paint to ‘stay where it is.’
The great advantage of this method is that it makes it possible to apply
a thicker layer at one go, as the paint quickly becomes viscous enough to
remain stuck on the surface.
14.3 Pot life
Pot life is the length of time that a catalyzed resin system retains a
viscosity low enough to be used in processing. Pot life is measured by
determining the length of time required for the viscosity of a paint product,
after mixing, to double via the chemical reaction. By thinning the paint, it is
possible to influence the pot life. The temperature also plays an important
role in determining pot life. As two-component coatings depend upon chemical
reactions, an increase in temperature will reduce pot life, whereas a decrease in
temperature will often extend it.
For some years now, products have been available with good curing
characteristics even at lower temperatures. As a result, the drying
characteristics are still good even at temperatures as low as 0-5°C, without
this having any influence on pot life.
14.4 Mixing
Proper
mixing is essential for obtaining a product that can be used effectively and
that complies with its performance and other technical specifications.
Mixing
a ready-to-use single-component product is quite different from mixing a
two-component product.
In
the case of a single-component product, one has only to mix the product in the
pot until it is homogeneous and, if necessary, thin it down, after which the
paint is immediately ready for application.
In
the case of a two-component product, the primary component and the curing agent
must first be mixed together to obtain a single homogeneous product which is
evenly distributed throughout the container and has a homogeneous and even
colour.
Paints
sometimes settle out somewhat inside the pot and this can lead to the formation
of clots or thicker pieces. It is then advisable to continue mixing for some
time until any such pieces are not only well distributed throughout the paint
but have also become dissolved should that be necessary.
We
recommend using a clean mixing tool to ensure that no foreign paint or colour
is transferred from one pot to the other.
If
mixing is carried out smoothly but at a high enough rotational speed to
generate an effective vortex, the amount of air trapped inside the liquid will
be minimal and a smoother spray painting pattern will result. Here also, the
temperature is important: it is easier to mix a paint at a temperature of18-20°C
than at 5°C.
14.5 Recognizing different coating
systems
Alkyd
paint systems become harder as they age until they end up becoming brittle. A
fast-drying alkyd paint will become harder more quickly and become more brittle
than one which has normal drying characteristics. This is also why outdoor durability,
flexibility and gloss retention of alkyd paints with normal drying
characteristics are generally better.
For
some colours and types of gloss finish, loss of gloss may occur over time.
Alkyd
coatings and/or varnishes are not very resistant to strong solvent such as
xylene or thinners.
Chlorinated
rubber paint systems tend to become somewhat less brittle than alkyd paints as
they age. However, chlorinated rubber paints tend to lose their gloss and
become yellowish over time.
These
paints dissolve readily in aromatic solvents such as xylene and thinners.
Epoxy paint systems harden well and adhere well to various surfaces.
Over time, especially when used outdoors, some of the gloss will disappear and
yellowing takes place. Whether or not this effect is visible or noticeable
depends also on the specific colour chosen.
These paints have good resistance to solvents, and their resistance to
chemicals depends upon the quality of the paint.
Polyurethane paint systems display great hardness and good outdoor
durability as well as colour fastness and good gloss retention over time.
Resistance to solvents and chemicals is good.
Water-based acrylic paint systems can have flexible as well as hard
surfaces. Gloss retention is good, and yellowing hardly ever occurs.
Resistance to solvents is moderate to good.
Of course, every type of coating is also available in various qualities, in which case some properties will be better and some will be less good.
