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

the shrinkage factor) and is thus irrespective of rheological
properties, thickness, applied surface profile or surface tension.
The shrinkage factor can thus be modelled as:
where ρ solids is density of solids and SC-0 is initial solids
content. Using this formula we can ascertain that by either
increasing the solids content or by decreasing the density of
the solid, the degree of shrinkage can be reduced (See graph
below) In solvent borne coatings, both the solids content is
higher, and due to the higher resin component, the density of
the solids is lower, hence much better telegraphing resistance.
The Development of an Experimental Open
Time Binder
As we have seen, the application and appearance of self
cross linking acrylics are inferior to their solvent borne alkyd
predecessors. To try and minimize the shortfall in open-time,
paint chemists commonly add humectants such as glycols or
surfactants; with conventional logic being that these will hold
back water and therefore extend open time.
However propylene glycol does not form an azeotrope 1 with
water, and it is the belief of the authors that propylene glycol
only improves the wet edge of the paint and not the ability for
a paint to flow out after brush/roller corrections. Significantly
greater gains in open time have been found by slightly reducing
the volume solids content rather than by increasing the
surfactant and solvent concentrations, with only a minimal
reduction in telegraphing resistance.
1 An Azeotropic mixture is defined as a liquid mixture of two or more
substances, that when mixed behave as a single species as the
vapour produced from partial evaporation of the liquid has the same
composition as the liquid (Turner, 1950.) Whilst propylene glycol is
infinitely dissolvable in water, the resulting liquid exhibits two distinct
boiling points.
Even when the volume solids of a coating with a Newtonian
rheology profile is around 35%, the open time using the Nuplex
Cross Open Time (COT) method will be at best between three
and four minutes. This is because the curing mechanism of the
resin is the overriding factor in determining the open time in
waterborne acrylic enamel systems.
The self cross-linking acrylic polymers commonly used in the
manufacture of trim enamels are classified as dispersions.
In the wet state these resins exist as semi solid particles
suspended in water, and film for through deformation or
coalescence of these polymer particles. The dry film retains
some of the structure of the polymer particles which reduces
the gloss, particularly at 20°. In general the acrylic polymer
particles are high molecular weight and cannot easily move
past one another, so the viscosity increases rapidly on drying,
hence the short open times. These polymers get the majority of
their performance properties from their high molecular weight
and not from cross-linking, which is why they appear to cure
at a much faster rate than alkyd systems. It is also why acrylic
systems need to have higher glass transition temperatures than
alkyd systems to get the same blocking resistance.
There are two other waterborne technologies that are currently
available for use in the waterborne trim enamel market; alkyd
emulsions and solutions.
Alkyd emulsions are supplied as alkyd droplets in water (an oil
in water or o/w emulsion.) As water evaporates, the viscosity
increases until the resin inverts from an oil in water emulsion to
a water in oil (w/o) emulsion, and the viscosity decreases. The
dry time of alkyd emulsions is similar to solvent borne alkyds,
as they both require oxidative cross-linking for chemical and
mechanical properties.
Unlike acrylic dispersions, the resin droplets in alkyd emulsions
are liquid, which gives these binders a higher degree of fluidity
at the same solids content, and as alkyd emulsions do not
coalesce, they do not require a coalescing solvent and they
contain renewable (but yellowing) oils. This yellowing and the
slow dry times are two reasons why this technology has fallen
from favour in the Australasian trim market.
Currently the only other technologies that can be utilised are
water soluble polymers. Traditionally the trim market was
dominated by (alkyd solvent borne) solutions, which have
issues with odour, dry time VOCs and yellowing, but have good
open time and telegraphing resistance (due to being able to
flow above 70% volume solids.) Water soluble polymers also
have excellent gloss and open time but perform very poorly
when exposed to water, often re-dissolving within two minutes.
They mostly have poor mechanical properties.
The Experimental Binder EM8683
These deficiencies in current resin technology have led Nuplex
to develop EM8683, an experimental polymer designed to
have much better open time than current technologies on the
market. As we have already seen, the open time of the current
waterborne self cross-linking acrylics is limited by the binder
system. Nuplex have addressed this by developing a fast curing
acrylic copolymer emulsion. Unlike traditional self cross-linking
acrylics, this system is oil modified and oxidatively cross-links,
hence the MFFT is significantly reduced without compromising
mechanical properties.
To ensure that the performance of this experimental polymer is
SURFACE COATINGS ASSOCIATION OF NEW ZEALAND 9