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The main
paint components can be classified as follows:
-
Pigments and extenders
-
Binders
-
Liquids
-
Additives
In each
category there are an enormous number of products
available. Given the huge number of options it is
almost impossible that any two paint formulation
chemists have ever come up with identical
formulations using identical raw materials. Only
some of the most important aspects of paint
components are covered here.
Pigments and extenders
These are fine powders that have usually been
crushed from naturally occurring mineral deposits.
It is the pigments and extenders that make up most
of what is left of paint once it has dried. (Of
course if they were all that were left after drying
they would simply blow or wash away.)
The most
common pigments and extenders used in paint include
the following:
-
Titanium Dioxide (TiO2)
-
Calcium Carbonate (CaCO3)
-
Clay (Hydrous aluminium phyllosilicate)
-
Talc (Magnesium Silicate)
-
Silica
-
Mica
-
Synthetic pigments (manufactured rather than
naturally occurring white pigments)
-
Colouring pigments (organic and inorganic,
natural and synthetic)
Pigments
can be regarded as those fine powders which have the
primary role of imparting whiteness, opacity (opaque
or non-translucent nature often referred to as
hiding power), colour or other special
characteristics.
Extenders
are lower cost pigments that extend or make up the
balance of the powder component of the paint.
Extenders also have significant quality impacts.
Amongst other things they space the expensive
pigment particles to maximise their benefits.
Some
pigments and extenders are discussed below.
Titanium Dioxide (TiO2)
This is a very expensive pigment essential in white
and lightly coloured paints. The amount of TiO2 in
these paints is a key parameter in determining the
paint quality. Primarily it gives these paints their
opacity or hiding power so that less paint is
required to hide the surface below.
The key
quality benefits of TiO2:
-
Imparts highly opaque nature (hiding power) to
white and light paints.
-
Imparts protection to the binders from the suns
UV radiation and thus improves the colour
stability and life expectancy of these paints.
Colouring
pigments in deep shades of paint should perform the
same functions.
If one
considers that TiO2 costs about 8-10 times more than
most extenders then it is easy to establish a link
between paint quality and cost.
TiO2 is
very effective at creating opacity due to its fine
particle size and high refractive index.
Bastion Paint paints contain high amounts of TiO2
in
the White and Pastel Bases.
Calcium Carbonate (CaCO3)
This is an extender that is widely used. The best
origin of calcium carbonate is marble deposits that
are mined and crushed into a fine powder. Chalk and
limestone deposits are other sources of calcium
carbonate.
Generally
the finer the powder particles the better the
quality. Particle size is measured in micrometers.
Finer particles contribute more to smoothness and
hiding power than coarse particles although there
are a number of reasons why some coarser particles
are also required in paint.
Bastion Paint uses only calcium carbonate from a
marble source and only grades of calcium carbonate
with an average particle size of 0.8, 2 and 5
micrometers.
Smooth
matt, Silk and higher sheen paints require fine
extenders while cheaper rough and textured paints
can use coarser extenders.
Incidentally any advert claiming that Marble is
included for strength or stretch is very far fetched
and aims only to lure people through their
ignorance.
Clay
(hydrous aluminium, phyllosilicate)
Clays are extenders that are less white than calcium
carbonate. Clays are sometimes used as a cheap way
of getting smoothness and developing a degree of
sheen. Clays are a broad and often confusing family
of chemicals. There are 4 main classes of clay of
which Kaolin is probably the best know in the paint
industry.
Clays are
often modified to develop certain properties.
Driving off structural water and volatile materials
at high temperature are common processes. A useful
paint extender is calcined anhydrous aluminium
silicate. The calcining process induces "air voids"
in the crystals thus improving the light scattering
characteristics.
Pyrophillite is an aluminium silicate hydroxide.
Clays are
inorganic but some have organic groups attached
which completely change their nature.
Bastion Paint paints only utilise special clays as
extenders. Anhydrous Aluminium Silicates and
Calcined Anhydrous Aluminium Silicates are included
in our paints to improve hiding power and to impart
special hue characteristics to our tint bases. All
our paints contain a small amount of an organic
containing clay that imparts a thixotropic nature to
the paint. This reduces things like liquid phase
separation (syneresis) of paint during storage and
assists to give good brush loading without the
negative impacts of having very thick (high
viscosity) paint.
Talc
(Magnesium Silicate)
Talc is a very useful extender. There are usually
some other minerals present in talc e.g. aluminium
silicate and pyrophillite. Talc crystals tend to
have a platy nature but different sources of talc
differ in the degree of platy nature depending on
their purity. Talc reduces the tendency of paint to
crack when drying, reduces brush and roller drag,
enhances paint flow properties and aids in water
resistance thus creating some additional weather
resistance of the paint.
Bastion Paint products use 2 different talc types to
fully optimise the performance.
Mica
and Silica
These extenders or pigments are used in some paints
to impart some specific characteristics. Mica has
been well advertised as a valuable component of a
well known local brand. Mica is expensive and quite
difficult to use and too much can have negative
effects on the paint. As such Mica has been
minimised or removed from many formulations. Mica
does, however, have a useful platy structure to aid
weather resistance and assist in ways similar to
platy talc.
Silica is
widely used as an extender in many paints. One
specific application is road-marking paints. Its
coarse nature creates a low-slip surface while its
hardness makes it suitable to endure traffic flow.
There are numerous silica qualities that give it a
wide range of applicability.
Synthetic Pigments
These are synthesised particles rather than crushed
and ground natural substances. A common form is the
hollow sphere while hollow spheres with manipulated
surfaces to increase light scattering are also used.
Being hollow means that light passing through the
small particle has extra surfaces to reflect off and
hence less light passes through so paint opacity is
improved.
They are
seldom present in high quantities but the driving
force behind their use is that they offer good
opacity (hiding power) while being cheaper than
TiO2. A draw-back is that they usually have to be
purchased and added together with a lot of water. As
their primary role is to impart an opaque nature
they are regarded as pigments rather than extenders.
They also have the added benefit of being completely
colour stable and have almost perfect
light-fastness. New trends are moving towards nano-particles
which are 1000 times small than conventional
pigments thus providing a much higher surface area.
Synthetic
pigments are usually present for the purpose of
maintaining quality while reducing costs but in the
case of nano-particles entire new paint applications
are arising.
Colouring Pigments
Almost any colour or shade can be achieved by adding
various coloured pigments. The modern decorative
paint industry relies heavily on colour
formulations, shade matching and on the assigning of
rather exotic names to colours to conjure up
something in the imagination.
Exterior
pigments need a high degree of light-fastness
to ensure that the colour does not change over time
from exposure to sunlight. Exterior paints also
require coloured pigments that have a high
weather-fastness. All reputable colour pigments
have a rating for the above properties but
unfortunately when the consumer purchases coloured
paint this is not often seen as an important
feature.
You can
find out more about Bastion Paint colourants under
the web site section "Colouring". Our 16 colourant "Colour Works" system has exceptional exterior
options available to get non-fading colours.
Binders
Binders were introduced earlier under "Types of
paint" and should you want to understand the
difference between binders used in oil/solvent based
paints and those used in water-based paints you
should scroll up to the top of this section.
In this section we deal only with binders used in
water based acrylic paints.
There are a large number of binder types available
for water based paints. The most common are listed below:
-
Vinyl Acrylics
-
Styrene Acrylics
-
Pure Acrylics
-
Polyurethanes
Poly Vinyl
Acetates (PVAs) are less common nowadays. The term
PVA has, however, stuck to water-based paints. VA
Viova is used and can be thought of as an upgraded
PVA (from the combination of Vinyl Acetate (PVA) and
Visitric Acid).
Without
binder the paint pigments and extenders would simply
blow or wash off the surface once dry.
Pure
binder without pigments forms a thin transparent
glossy film. This film will, however, not be stable
for very long as UV radiation will penetrate right
through it and damage it. Pigments offer protection
to the binder film from UV radiation.
The
selection and amount of binder used is a key
parameter in determining paint quality and cost.
Styrene
acrylic binders are most suitable for interior
paints. Over time they tend to be broken down by UV
radiation and thus 100% Acrylic binders are
preferred for more durable exterior paints.
100%
Acrylic binders are more expensive but offer
excellent stability against UV radiation, and the
lowest tendency to collect dirt.
Within
each of these binder categories there are numerous
types with different binding powers and different
film forming temperatures (FFTs).
All
binders require time to form a proper film so the
rapid drying of water-based paints can be a
drawback. Some solvent is usually added to
water-based paints to assist film formation. This is
discussed as the next raw material below under the
heading 'Liquids'.
Generally
more durable paints have more binder (as a ratio to
pigments and extenders) but exceeding a certain
amount results in the durability again decreasing as
too little pigment is available to protect the
binder from UV radiation.
As binder
amount increases the paint gloss degree increases.
Paints high in binder have lower pigment and
extender amounts which decreases hiding power and
coverage. In good paint this is compensated for by
improving the pigment quality (e.g. high TiO2 levels
with few or no cheap extenders) but this becomes
expensive. Sheen and gloss paints thus often have a
lower hiding power than matt paints.
The amount
of binder can be expressed as the Pigment Volume
Concentration (PVC). Only the dry volume of
pigment and binders are used in this calculation.
Higher binder levels relative to pigments and
extenders decrease the PVC value while paints with
low binder levels have high PVC. Few paint companies
advertise or even declare the PVC content of their
products partly because the public is ignorant and
prefer to see bright colours and partly because they
would be quite embarrassed about how little binder
some of their products contain.
Matt and
flat paints may have much lower binder contents
resulting in lower price but unfortunately also in
lower durability. These may have PVC ratios as high
as 85% (indicating a very low amount of binder for
the amount of pigment/extender used). Such paints
are best described as "white wash".
Super
Acrylic Matt paints in SA usually have a PVC of 65 –
75%. Sheen paints usually have a PVC of around 35%
(more binder to give the sheen).
Liquids
The following discussion refers only to Water-Based
Acrylic Paints.
The liquid
component of a water-based acrylic paint is not
surprisingly mainly water. The chemistry must be
designed to allow drying at a specific rate and
while this drying takes place the binder forms bonds
basically creating a 'plastic' layer in which the
solid particles are firmly held. This bond forming
is known as coalescence.
Some
specific alcohols (or other liquids) are added to
most water based acrylic paints. These are actually
solvents and serve to reduce the film forming
temperature (FFT) of the paint allowing the binders
to coalesce at lower temperatures. They also
evaporate from the paint more slowly than water
which increases the time that the binder has to
coalesce fully. Excess addition of these solvents
can lead to paint remaining slightly tacky for too
long and thus susceptible to picking up dirt and to
abrasion and water damage. The modern
environmentally driven trend is to be less reliant
on these coalescing solvents by using modified
binders or plasticizing agents.
Alcohols
(usually different ones to the coalescing solvents
discussed above) slow initial drying to ensure that
the painter can keep a wet edge when painting i.e.
paint can still be spread after some time. This is
referred to as the paints open time.
Additives
A large number of minor additives are used to
achieve a variety of paint properties.
The most
common are introduced below:
Thickeners and rheology modifiers
Without thickener the blend of pigments, extenders,
binders and liquids would have a water-like
viscosity. By thickening the paint a number of
aspects are improved:
-
More paint adheres to the brush or roller
-
Splashing, dripping and running are greatly
reduced
-
Air entrainment is reduced
-
Painted film thickness is increased
Rheology
modifiers don't simply thicken; they change the
paint flow characteristics over a range of
conditions. These conditions are measured as the
shear force applied to the paint. For example the
shear force is basically zero when paint is stored
but is quite high when a roller is used and is
higher when paint is sprayed.
Hundreds
of thickener and rheology modifier chemicals are
available. The most common classes of thickeners are
Cellulose based and Associative thickeners (more
correctly called RMs, short for rheology modifiers).
Associative thickeners have three classes with the
two most important being HASE and HEUR. The details
are not discussed here but on request this
information can be provided.
Bastion Paint products utilise a combination of
modified cellulose and associative thickeners. The
response of the thickeners and rheology modifiers to
the addition of colourants and their sensitivity to
water are also important considerations in our
formulations.
The
behaviour of paint under different shear forces is
known as rheology. Fluids can be classified
according to their rheology and there are plenty of
terms to describe different fluid behaviour under
conditions of stress (e.g. pumping, stirring,
brushing, rolling, spraying etc). Fluids can be
Newtonian or Non-Newtonian. Newtonian fluids do not
change their viscosity or flow characteristics when
shear/stress is applied. Some common behaviours of
Non-Newtonian fluids are Dilatent, Pseudo-Plastic,
Rheopectic and Thixotropic. Again the details are
not discussed here but on request this information
can be provided.
In-can
preservatives
Without preservatives paint would be attacked by
bacteria in a short time. The pH would change, the
strength would decrease, the paint may tend to
settle and an odour would be generated. Bacteria
predominantly attack the binders and certain
thickeners thereby reducing the paint strength and
viscosity but very few paint components are totally
immune to biological degradation.
Long-term film biological inhibitors
These are added to keep paint free from mildew,
algae, fungi and moulds for a long period after
painting is complete. Typical application of these
additives is in premium exterior paints and bathroom
paints.
Air removal additives (frequently called
defoamers)
These are added to reduce the entrained air. If air
is present it causes small little bubbles in the
paint film which become a small hole that allows
water penetration decreasing the paint durability.
Dispersants
These are
added to keep the pigment and extender particles
apart in storage and thus present as individual
small particles. If the particles start to bump into
each other during storage they form larger particles
that tend to settle. Dispersants can be ionic or
non-ionic in nature. The normal ionic dispersing
mechanism is to make all the particle surfaces
negatively charged so that they repel one another
and don't touch so larger particles are not formed.
During
paint manufacture wetting agents make it easier for
the small pigment particles to contact water while
energy is used to shear them to the required size.
Dispersants then keep the small particles apart to
give a stable paint. All three of these processes
are required.
If
colourants are to be added then dispersants that are
compatible with the colourant particles are
required. If this is not achieved then the colourant
particles do not remain apart but flocculate and
colour intensity is lost. Since there are a large
number of colourant chemistries this is a complex
area of paint formulation.
Compatibility between the type of dispersant and the
type of thickener used is also an important
consideration. |
