7. Paint
Industrial Products (Testing)
• basic function of a paint
• protecting a surface from the action of light, water, and air
• achieved by the application of a thin resistant impervious, flexible film to
the surface
• usually contains components to hide and decorate the surface
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liquid components are known as the vehicle
solid as the pigment
these components are essential for the paint
additives to improve various performance aspects may be solid or liquid
Component functions
Vehicle
• 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
Pigment
• 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
Additives
• improve useability
• improve drying characteristics
• reduce breakdown by light, chemical or biological attack
Solvent
• fluidity of paint allows easy application and penetration even into the most
intricate crevices
• the binder is dissolved 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
Binder/resin
• polymer materials which bind together the pigment particles and hold them
onto the surface
• also be referred to as "film formers“
Water-based paints/acrylics
• an emulsion which contains tiny spheres of acrylic polymer held in
suspension
• water evaporates
• spheres are forced together
• fuse together to form a continuous film of polymer
Oil-based paints
• contain complex monomers
• polymerise on exposure to the air and on evaporation of the solvent
Pigment
Primary
• usually the intensely white powder, TiO2
• provides the main hiding power of the pigment
Filler/extender
• do not provide colour
• improve various performance characteristics
Colourant
• gives any non-white colour to the paint
• sometimes inorganic (more common in older paints)
• more often these days, a mixture of organic dyes.
Additives
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not essential
enhance the performance
provide a new characteristic
include:
• thickener
• wetting agent
• plasticiser
• antimould agent
• antifungal agent
• catalyst
• flame retardant
Physical testing
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more important than chemical testing
density
viscosity
fineness of grind
coverage
colour
durability
Density
• mass in a known volume
• hydrometer
• a production test to relate the mass added to the can to the volume that it
is supposed to be
• the filling mechanism works on mass (usual case)
• the can says four litres
• the density needs to be known
• also a simple check that all the major ingredients are present
Non-volatile content
• simple test for checking that the formulation has the right amount of
ingredients
• evaporate the solvent from a known amount of paint
Viscosity
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measure of flowability of paint
can’t be too thick or thin
a single viscosity measurement is not sufficient for analysis
viscosity profile involving a series of measurements is required
specialist devices to measure viscosity-related performance:
• levelling or flow out
• resistance to sagging
• brushability
• most common standard device is Brookfield viscometer
• employs a stirring blade which is rotated in the paint
• force required to turn the spindle is related to the viscosity
Coverage (hiding power)
• defined as the property of a paint that enables it to obliterate beyond
recognition any background over which it may be spread
• the volume of paint required to cover a certain area with a machine-cont
Pigment particle size
• “large” particles may protrude through the surface and detract from the
gloss
• an optimum size below which hiding power decreases.
• measured by microscopy and light scattering photometers
• Sieving is generally not suitable because the particle sizes are too low
Colour
• reflectance visible absorption spectrum - paint is not transparent
• tristimulus value - related to the proportions of the three primary colours
needs to match a particular colour
Durability
• designed to measure the effect of sunlight, moisture and temperature on
surface coatings applied to a variety of substrates.
Chemical testing
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can’t put through a GC or HPLC directly
need to separate components
solvent – GC
binder – IR
pigment – ICP, XRF, IR, UV/VIS
Exercise 7.1
Separate oil-based paint into three fractions
• solvent – distillation
• dilute paint in organic solvent
• centrifuge to separate solid pigment
• dry remaining solution to produce polymerised binder