Paint
7
7.1 The composition of paints
The basic function of a paint, that of protecting a surface from the action of light, water, and air, is
achieved by the application of a thin resistant impervious, flexible film to the surface. The film usually
contains components to hide and decorate the surface.
Paints have a number of basic components, liquid and solid. In general, the liquid components
are known as the vehicle, and the solid as the pigment, as shown in Figure 7.1.
PAINT
VEHICLE
Resin/
Binder
PIGMENT
Solvent
Primary
Filler/
Extender
ADDITIVES
FIGURE 7.1 The basic components of paint
The main functions of the vehicle components are to:
•
provide a means for the paint to get onto the brush/roller/spray head
•
provide adhesion of the paint film to the substrate
•
hold the pigment together
•
protect the painted surface
The main functions of the pigment components are to:
•
provide hiding power, stain resistance and colour
•
control the gloss of the film
•
decrease moisture penetration
•
improve weather resistance
•
help control corrosion and mildew of the surface
The main functions of the additives are to:
•
improve useability
•
improve drying characteristics
•
reduce breakdown by light, chemical or biological attack
The proportions of the various components are:
•
solvent 30-80%
•
binder 20-60%
•
pigment 2-40%
•
additives 0-5%
Colourant
7. Paint
Solvent
The fluidity of paint allows easy application and penetration even into the most intricate crevices. It is
achieved by dissolving the binder into a solvent (oil-based paints, where the solvent is a hydrocarbon
mixture), or by colloidal suspension of both pigment and binder in a diluent (where the “solvent” is
water).
Resin/Binder
These are resin polymer materials which bind together the pigment particles and hold them onto the
surface. They may also be referred to as "film formers".
Water-based paints, known as acrylics, are an emulsion which contains tiny spheres of acrylic
polymer held in suspension in water. When a thin film is applied to a surface, the water evaporates
and the spheres are forced together by capillary pressure. With coalescents present to soften the
surface, the spheres fuse together to form a continuous film of polymer, which is then insoluble in
water after all the volatiles have evaporated from the film (see Figure 7.2).
FIGURE 7.2 Film formation in water-based acrylic paints
By comparison, oil-based paints, where the resins – known as alkyds and based on unsaturated
vegetables oils such as linseed oil – are complex monomers, which polymerise on exposure to the air
and on evaporation of the solvent. This gives a harder surface than the plastic film in water-based
paint, but the reaction with oxygen continues during the lifetime of the paint, causing it to
progressively embrittle. This leads eventually to failure by cracking and peeling, particularly over
substrates which have any tendency to move.
Industrial Products (Testing)
7.2
7. Paint
Primary pigment
This is usually the intensely white powder, titanium dioxide, which provides the main hiding power of
the pigment. It is one of the reasons why paint is relatively expensive, since the source of TiO2 – rutile,
a titanium mineral in black sand – is not that common, and the separation and purification is time- and
energy-consuming.
Filler/extender
Extender particles do not provide colour. These improve adhesion and film strength, make sanding
easier and control gloss and viscosity. They include talc and calcium carbonate.
Colourant
This obviously gives any non-white colour to the paint, and is sometimes inorganic, but more often
these days, a mixture of organic dyes.
Additives
These are small quantities of substances added to carry out special functions in the paint. They may
enhance the performance, or provide a new characteristic. Additive functions include:
•
thickener
•
wetting agent
•
plasticiser
•
antimould agent
•
antifungal agent
•
catalyst
•
flame retardant
7.2 Physical testing
Those physical properties routinely tested in the paint industry include:
• density
• viscosity
• fineness of grind
• coverage
• colour
• durability
Density
The measurement of density is relatively simple, involving the determination of the mass of the paint
in a vessel of known volume. The vessel may be a sophisticated one, such as a pycnometer, or a simple
one, such as a bucket. It depends on who is doing the test. Hydrometers can also be used, and are
the quickest (and easiest to clean afterwards!).
But why measure the density? It doesn’t seem like a property that makes much difference to
how the paint performs. It is simply a production test to relate the mass added to the can to the
volume that it is supposed to be. If the filling mechanism works on mass, but the can says four litres,
then the density needs to be known. It is also a simple check that all the major ingredients are present.
Non-volatile content
Another simple test for checking that the formulation has the right amount of ingredients, this simply
involves evaporating the solvent from a known amount of paint.
Industrial Products (Testing)
7.3
7. Paint
Viscosity
A wide variety of instruments are available to measure this particular physical property. In recent times
paint technologists have realised that a single viscosity measurement is not sufficient for analysis.
Instead a viscosity profile involving a series of measurements is required to give a complete picture of
the rheological properties of the material.
Specialised instruments have been developed to measure viscosity-related characteristics of
paint such as:
• levelling or flow out
• resistance to sagging
• brushability
Capillary viscometers were mentioned in a previous chapter. In the paint industry, these are less
commonly employed, partly because of the cleaning difficulty, but mainly because they do not really
duplicate the usage conditions of a paint. The more common device is known as a Brookfield
viscometer, and employs a stirring blade which is rotated in the paint. The force required to turn the
spindle is related to the viscosity.
Other specialised devices are also employed, which attempt to duplicate the operating
requirements of brushes, rollers and spray guns.
Coverage (hiding power)
This may be qualitatively defined as the property of a paint that enables it to obliterate beyond
recognition any background over which it may be spread. Quantitatively the volume of paint required
to cover a certain area with a machine-controlled brush is measured.
Pigment particle size
Size, size distribution and the shape of pigment particles greatly influences the optical and other
physical properties of paints. Relatively large particles may protrude through the surface and detract
from the gloss. Extreme fineness is however, not always desirable as their is an optimum size below
which hiding power decreases.
There are numerous methods for determining particle size, including microscopy and light
scattering photometers. Sieving is generally not suitable because the particle sizes are too low.
Colour
This may sound simple, but it is not just a matter of a visual check or running a visible absorption
spectrum. Our brains have too much control over what our eyes are seeing: for example, the light
from either a tungsten globe or a fluorescent tube is not white, but our brain filters out the orange or
green tints.
Another possible method could be a visible absorption spectrum, but measured as a reflectance
spectrum, because paint is not transparent. A special reflectance cell is used. A more specialised
method is the tristimulus value, which is related to the proportions of the three primary colours needs
to match a particular colour. This would be a simpler QC test for a batch of paint than a full absorption
spectrum.
Durability
These tests are designed to measure the effect of sunlight, moisture and temperature on surface
coatings applied to a variety of substrates.
Industrial Products (Testing)
7.4
7. Paint
7.3 Chemical testing
A typical chemical analysis of paint would involve firstly separation of the individual components. Table
7.1 shows the methods for identifying the components once separated.
CLASS EXERCISE 7.1
How would you obtain separate samples of the solvent, binder and pigment from an oil-based paint?
TABLE 7.1 Identification and quantification of paint components
Component
Analytical techniques
Solvent
GLC
Binder
IR
Pigment
- inorganic
- organic
XRF, ICP
IR, UV/VIS
What You Need To Be Able To Do
•
•
•
•
list and describe the functions of the components of paint
outline the method of testing, and the significance of the property, for a range of physical
properties of paints
describe how the chemical composition of paints can be determined
outline how samples of paints can be obtained
Industrial Products (Testing)
7.5