About Paint Technical Information:
Solvent paints, alkyds, oil-based paints, water-based paints, acrylics, PVAs, latex paints, emulsion paints ................... what does it all mean?
Paint can be classified as a part of a broader category of substances called coatings. Coatings may be protective and/or decorative or may serve other specific functions such as road marking, insulation, print enhancing etc. Coatings other than paint are not discussed here but varnishes, polishes, and waxes are examples of other commonly used protective and/or decorative coatings. Paper coatings and printing inks are also closely related to paints.
Types of Paint:
- Paint classification according to the type of liquid and binder (includes an explanation on the difference between Alkyd/Solvent paints and Water Based paints
- Paint classification according to application
- Paint classification according to appearance
- The components of water based acrylic paint and their impact on quality
According to the type of liquid and binder:
Binders are an essential component of paint as they hold it together and bind it onto the surface. Without binders paint would dry into powder which would not adhere well. Certain types of binders need to be used in certain types of liquid medium.
- Oil-based binders (used in alkyd paints/solvent paint/oil based paints)
- Acrylic, Vinyl Acrylic and PVA binders (which are used in water based paints)
- Other binders used in water-based paints e.g. PUD paints (Polyurethane dispersions) and alkyd emulsion paints.
Oil-based binders are not discussed further than in this paragraph. The classical binders used in these paints were vegetable oils such as Linseed and Soya oil. Today these paints are often referred to as Alkyd paints as manufacturing has moved away from only using vegetable oil binders to using modified oils known as alkyds. Some draw backs of this type of technology are high costs, slow drying, paints that tend to yellow in sunlight and paints that are typically hard but brittle and thus chip easily. They have high VOCs (volatile organic compounds) and high AOXs (absorbable organic halides) which have negative environmental and health impacts. Some components used in these paints in the past have been found to be carcinogenic (cancer causing) and Lead was a common component of these paints.
Acrylic, Vinyl Acrylic and PVA binders have led to most water-based paints being called Acrylic Paint or PVAs. These binders are large polymers (long chain complex molecules) dispersed in water. The micro particles of acrylic binders are in fact 'plastic' particles and are dispersions, not solutions. The particles are, however, very tiny and usually their size is measured in nanometres. Developments in plastics technology led to the development of water-based paints using these binders. When the paint dries and coalesces the individual 'plastic' polymers bind to the substrate, to other paint components and to each other to form a continuous plastic film.
Alternative names for these water based paints include Latex Paints and Emulsion Paints. The use of the term latex developed due to the similarity of acrylic substances to the original latex obtained from rubber trees. The term Emulsion Paints is sometimes used as the acrylic binders are emulsions and not solutions (not dissolved but present as small particles).
Other binders used in water-based paints include Polyurethane binders which give rise to paint called PUD (polyurethane dispersion). Polyurethane binders are sometimes used together with acrylic binders. These are less common and are not covered in this web site. They have gained acceptance in glass coatings, plastic coatings, wood floor coatings, automotive coatings, soft-touch coatings and some other applications. There is little doubt that their application will increase. Simply having urethane rheology modifiers, which are commonly used, is very different to paint with polyurethane binders. The use of the term, "contains urethane" may be misleading (possibly deliberately intended to mislead).
Alkyd emulsions are new oil based binders that can be used in water based paints. The technology is aimed at using alkyd binders without the negative solvent impacts associated with normal alkyd paints.
More about water-based acrylic paints
The most common binders used in water-based acrylic paints are VINYL ACRYLIC, STYRENE ACRYLIC and PURE ACRYLIC. PVA (polyvinyl acetate) binders are no longer widely used but water based paints are still frequently referred to as PVAs.
Water-based acrylic paints took the world by storm having steadily gained momentum since the 60s. In a relatively short period they have captured 80% of the world's total paint market. The driving force behind this dramatic industry change has been lower cost, excellent durability and other quality improvements, improved ease of use and environmental and health considerations.
There should be no misconception that water-based paints are not as strong or durable as oil-based paints. Chemistry developments in plastics have been the technical driving force behind water-based paints and the developments in polymer chemistry, used in plastics, have been dramatic over the past years. The high flexible nature of water-based acrylic paints frequently makes them even more durable than oil-based paints which are more prone to being chipped and less able to withstand the expansion and contraction of surfaces. Water based paints are also more permeable so some dampness behind the paint layer can escape giving less tendency to peel than with solvent based paint. In general hardness and flexibility are trade offs; when one increases the other decreases. Acrylic paints tend to have a better balance of these two properties by not being so hard or brittle that they chip easily and having higher flexibility. There are applications where solvent paints have tended to perform better; corrosions resistance, wood coatings, bonding liquids and high gloss paints are examples. Water-based technology is rapidly closing any remaining gaps in these fields of application.
According to the application:
- Architectural Paint refers to paints commonly used to paint the interior and exterior of houses and buildings; also called decorative paints.
- Industrial Paint includes a wide variety of specialised paints. Automotive Paints, Anti-corrosion Paints and Road Marking Paints are examples of industrial paints. It is interesting to note that even these, including metallic automotive paints, have rapidly moved to water-based technology.
- Primers and Bonding Liquids
- Interior Paints
- Exterior Paints
- Roof Paints
Many of the required paint properties are similar for both interior and exterior paints. Exterior paints have additional specific requirements of UV resistance (of paint and paint colour) and provide more of a protective function. They may have higher resistance to water and a higher resistance to mildew, algae, fungi and moulds. Interior Paints have a higher requirement for stain resistance and for some applications higher sheen paints are preferred for interior use.
Primers used as first coats on cement plaster need to fill a number of roles. Alkaline resistance, strong bonding, penetration, sealing, filling and hiding are normal requirements of these primers. The need to seal against liquid water differs depending on the environment e.g. more important in exterior applications. The ideal paint layer will seal out liquid water while still being able to 'breathe' water vapour.
Primers for other substrates have different functions. For many metals corrosion resistance is the key function while for wood, penetration and resin resistance are desirable. On many hard to paint surfaces adhesion is the primary function of a primer.
Bonding liquids are usually clear and have more free binder to donate to the substrate. Some Primers can function as bonding liquids but not all. There has been a move towards water-based masonry primers and bonding liquids due mainly to cost and odour benefits although solvent based products still dominate this section of the market in SA. Solvent based Plaster Primers are referred to ARPs (Alkali resistant primers).
According to the appearance:
A common way of classifying paints is according to the degree of gloss which they generate. Gloss can also be referred to as sheen or shine and is basically a measure of the amount of light the paint reflects with a reflected angle that equals the angle of incidence.
So while total light reflectance determines the colour, reflectance where the angle of reflected light is equal to angle that it strikes the surface determines the gloss. A basic requirement for high gloss is thus high smoothness.
- Flat and Matt
- Low Sheen (also called Eggshell)
- Medium Sheen (the term Silk is commonly used)
- Sheen (the terms Silk and Velvet are commonly used)
- Semi-Gloss (also called High Sheen.)
- Gloss (high gloss paints are often referred to as enamel paints)
- Pastel Base (used to make light colours)
- Deep Base (used to make medium deep colours)
- Clear Base (used to make very deep colours)
White paints can generally be used as a base to produce pastel colours by adding coloured dyes but very different bases are required for making deep shades. This does not imply that all white paints are compatible with all colourants or that they will give a repeatable shade each time the same amount of the same colourant is added. For this it is a safer bet to use a Pastel Base.
Components of Water-Based Acrylic Paint:
- Pigments and extenders
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.)
- Titanium Dioxide (TiO2)
- Calcium Carbonate (CaCO3)
- Clay (Hydrous aluminium phyllosilicate)
- Talc (Magnesium Silicate)
- 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.
- 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.
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.
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 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.
- Vinyl Acrylics
- Styrene Acrylics
- Pure Acrylics
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).
- The binders are actually very small plastic type particles dispersed in water. These particles are far smaller than pigments and extenders. As paint dries they undergo coalescence to form a continuous plastic film that binds the pigment/extender powder together and binds the paint to its substrate.
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).
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.
A large number of minor additives are used to achieve a variety of paint properties.
The most common are introduced below:
- 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.
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.
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.