Brushstrokes - October 2011 - Surface Coatings Association of New ...

the flow and levelling properties are positively influenced. Also
the risk of CO2 bubbles getting trapped in the drying coating
is reduced. In Table 1 the main properties of the waterborne
acrylic polyol used in this study are given.
Table 1. Resin properties.
Hydroxyl value 4.2 % OH
Solids content 47 %
pH 8
Acid value (on solids) 16 mg KOH g mixed
carboxylic and sulfonic
Particle size ± 100 nm
Co-solvent Butyl glycol (approx. 2 %)
Rheology of hand applied WB2K coatings
The paint rheology is one of the most important parameters
for brush or roller application. When comparing the flow-curve
of a conventional solvent-borne 2K paint formulated for hand
application (VOC of ± 500 g/l) with that of a WB2K system, the
initial flow curves immediately after mixing are quite different
(figure 1).
Figure 1. Flow curve of SB 2K and WB 2K paints.
The flow curves also change in a different way as a function of
the time after mixing (graph 2).
Figure 2. Changes in rheology during the pot-life of WB and SB 2K
formulations.
At high shear the viscosity of WB2K paint is much lower,
resulting in an initial brush resistance that is far too low.
Furthermore the viscosity decreases during the pot-life, so
application properties become worse after mixing of the paint.
Under low shear on the other side, the viscosity is much higher,
with relatively poor flow as a consequence. Because of this
rheological behaviour defoaming of the paint after application
also becomes difficult.
In order to adjust the rheological behaviour of the WB2K paint
a high shear a polyurethane thickener is used to increase
the ICI viscosity to a level of 1.6 Poise, similar to that of
the solventborne paint. By adding the thickener the solids
content of the paint could be lowered in order to reduce the
low shear viscosity. This results in adequate brush resistance
in combination with acceptable levelling, flow and defoaming
properties. Most waterborne two pack systems still require the
use of some co-solvent and the influence on the rheological
profile needs to be taken into account. With this specific polyol
it was found that a mixture of butyl acetate, dipropylene glycol
dimethyl ether and butyldiglycol acetate offered the best
results regarding the balance between gloss and levelling. The
flow-curves as function of time after mixing for the adjusted
formulation are given in figure 3.
Figure 3. Time dependant flow-curves of adjusted formulations.
Formulation parameters for clear and
pigmented coatings
A basic formulation for a hand applied WB2K formulation (both
pigmented and clear) is given in Table 2. The white mill base
used in the pigmented formulation was prepared by dispersing
in a pearl mill 20.12 parts of titanium dioxide with 1.03 grams
of a dispersant, 0.16 grams of thickener and 5.8 grams of
demineralized water until a Hegmann fineness of less than 10
microns was obtained.
During this work it was found that proper selection of the high
shear thickener and defoamer is essential to formulate a paint
with a good balance of aesthetical and performance properties.
How to achieve proper defoaming
When formulating without a defoamer, the paint, after brush
or roller application will contain at lot of foam. After drying
of the paint this will result in surface defects such as pinholes
and foam. The selection of a defoamer however is
not straight forward as it should ensure proper defoaming
during application without affecting gloss, flow and levelling.
Preferably the defoamer should be easily introduced into the
base without excessive shear.
SURFACE COATINGS ASSOCIATION OF NEW ZEALAND 13