Nowadays, we can see many decorative structure around the world which is coated by paint. However, the paint technology that we use widely have been used for many thousands of year, from the cave painting which was dated from the prehistoric age up to the painting of Mona Lisa.
Generally, there are 2 main functions of the paint; as decorative which makes product (car, house, etc) more attractive, so it is easier to sell, and as a protective barrier for example preventing car from corrosion and house from mould formation. The basic formulation of paint consists from 4 main components: pigment, extender, binder, and carrier. Later on we will see the function of those components and how it works in paint formulation (paint technology).
Paint Formulation
Pigments and Extender
Pigment comprise finely divided mineral particles that provide colour, 'hiding power', and opacity. A pigmnet should be insoluble in water, chemically unreactive, and have good UV stability. Moreover, most of the pigments are made from transition metal oxide, and as a fact transition metal can give varies colourful compounds. In table below, there are some examples of common pigments.
From those pigments in the table above, the commonly used white pigment is
TiO2, but other minerals (e.g.
BaSO4, CaC
O3) are less opaque but much cheaper than
TiO2. The manufacturing of
TiO2 pigment are shown below and the process is an energy-intensive process, so the it is an expensive pigments.
To strengthen the colour of the paint and also to reduce the price of paint, an extender is used in paint. Basically, an extender is made of a cheap mineral (e.g.
BaSO4, CaC
O3) and acts as the supplements of the primary pigment. An extender usually presents in powder form and it is insoluble in the paint formulation. Moreover, it has a refractive index less than 1.7 (water = 1.33). Refractive index can be defined as the ratio of speed of light in a vacuum to that in the medium of interest. Besides that, the costs of the extender typically £30 to £500 per metric tonne.
The Binder
In modern paints, the binder comprises soft copolymer latex particles of microscopic dimensions [typically 100 - 500 nm (diameter of a fine hair = 50 000 nm)]. There are several functions of the latex binder in paints such as:
- to bind pigment particles together,
- to ensure adhesion to the substrate,
- to impart UV durability and chemical resistance,
- to form a water-impermeable barrier
The last function is the consequence of the latex binder to form a film layer at the surface of the substrate and the film formation can happen at the condition which is called
minimum film-forming temperature (MFFT). The MFFT is the minimum temperature at which latex binder will form a film and it can be adjusted by copolymerisation of appropriate monomers in varying proportions during the latex synthesis (e.g adjusting the styrene and acrylates monomer can adjust MFFT). If the condition is lower than MFFT, latex particles cannot deform to form a continuous film.
There are many several ways to synthesis latex binders in industry. Firstly, latex binders for solvent-borne paints is made by non-aqueous dispersion polymerisation, and it was invented and developed by ICI (Imperial Chemical Industries) in around 1960. The copolymerisation of styrene and/or acrylic monomers is done by free radical polymerisation in an organic solvent and it requires a polymeric stabiliser for latex formation. One of the result of this technique is shown below.
In the other sides, latex binders for water-borne paints is made by aqueous emulsion polymerisation. Originally, it was developed as part of Allies' effort to manufacture synthetic rubber in around 1940 (World War II). The copolymerisation of styrene and/or acrylic monomers is done by free radical polymerisation in aqueous media and it can adjust MFFT by appropriate comonomer choice and by varying the relative proportions comonomers. This process is very efficient and it gives well-defined latex particles of uniform diameter as show below.
The mechanism of latex film formation is followed 4 main stages (Jones et al., 1995).
 |
| Film formation mechanism |
Firstly, the system consists of 20 - 50% percent of latex solids in aqueous phase and after it is heated to evaporate the water. As the result of the water evaporation, it makes the latex solid are closed together to form more than 55% of the solids. In further heating, the solids becomes closer and closer until the particles create a deformation pressure to form the hexagonal deformation. The deformation pressure can be predicted or calculated as:
Moreover, γ can be calculated by Frenkel's equation as shown below.
So, it can be simplified the deformation pressure is inversely proportional to the radius of the particle, so smaller radius, it produces bigger pressure. Thus, the particle coalescence is easier for smaller latex particles.
In some paint formulations, the absence of pigment is possible to form a
transparent latex film. In the other sides, pigmented latex film
provides the coloured paint as mainly used in nowadays. If the pigment
is evenly distributed within the latex film, the strongest colour effect can be achieved or the best gloss finish.
The Carrier or Solvent
The carrier causes the pigment and binder solids to behave as a fluid for application purposes. The carrier evaporates completely as the paint dries. In modern paint formulation, there are 2 main broads categories:
- In solvent-borne paints, the carrier is usually either an aliphatic hydrocarbon (e.g. mineral spirits) or an aromatic hydrocarbon (e.g. xylene). This type of paint is useful for exterior house paint, high gloss paint, wood coatings, etc.
- In water-borne paints, the carrier is either water or water/glycol mixtures and it produces low odour, so it is preferred by consumers for interior decoration.
Moreover, legislation-led drive towards paints with zero volatile organic compounds (VOCs). Hence, many solvent-borne paints now being reformulated as water borne. However, it is technically much more difficult to achieve high gloss and excellent water resistance using water-borne paints.
How Paint Works
In this section, we will see how paint provides opacity or 'hiding power', and to begin with we start from the pigment.
TiO2 is the brightest, whitest pigment known and it offers excellent opacity (hiding power). The annual worldwide production is around 4 million tonnes and 58% of the production is used in paint. To provide an optimum gloss potential and hiding power, the pigments particle should have the optimum particle size. Based on the figure below, the optimum diameter of the particle
TiO2 diameter is within 0.5 of the wavelength of visible light (400 - 800 nm).
 |
| A plot of light scattering against the particle size |
This optimum size offers maximum light scattering and highest possible opacity. Besides that, uniform particle size also allows better packing within the film, which also increases its gloss potential, but a high proportion of TiO
2 particles with diameter more than 500 nm can reduce gloss potential. Moreover, the roughness of the surface can affect gloss or matt appearance of the paint. If the paint film forms a smooth film, the light will be reflected uniformly, so it produces a gloss appearance. In the other sides, a matt appearance can be form if the surface of paint film is rough, so the light will be reflected randomly (be scattered).
Another factor that provides the opacity of the paint is from the refractive index of the pigments. Generally, the latex binder has refractive index around 1.4 - 1.6, and as the refractive index of the pigment is higher, the opacifying effects increases. As shown in figure below, higher refractive index of TiO
2 ensures strong light scattering, so the underlying substrate is invisible to observer (good hiding power). In the other sides, the extender has lower refractive index (around 1.6), the underlying substrate is visible to obsever.
Critical Pigment Volume Concentration (CPVC)
Pigment volume concentration (PVC) can be defined as the ratio of the volume of the pigment and extender with the total volume of the solid components of the paint without the solvent or carrier.
At the CPVC (or above) abrupt changes in various paint properties are observed. The observation is the paint film becomes lighter in colour, get lower scrub resistance, reduced gloss, poorer mechanical properties and the film porosity increases (due to air voids). At above CPVC, air voids are present in the film and pigment particles protrude from the surface and it reduces gloss effect of the paint. Hence, a matt appeareance is produced with maximum extender and minimal latex.
 |
| Diagram of light reflection when PVC below CPVC (left) and PVC above CPVC (right) |
In the other sides, at below the CPVC, all pigment particles are completely wetted by excess latex binder which leading to a smooth reflective film surface. Hence, a good quality gloss paint finish.\
Moreover, CPVC can be regarded as a phase transition within the paint formulation and household paints have different CPVC values depending on their specific use.
 |
| Phase transition of paint along the increasing of PVC |
Alternative Extender: Hollow Latex Particles
In the 1970's Rohm and Haas developed and patented a new organic extender, hollow latex particles. Hollow latex particles produce micro-voids within the paint film and increase its opacity. The mechanism is by lowering the average refractive index of the film, so it increases the different between the film and the TiO
2 pigment. Hence, it increases light scattering from the high component. The synthesis of hollow latex particle is shown below.
 |
| The synthesis of hollow latex particle |
This optical trick is easiy achieved above the CPVC and it is known as
dry hiding. Besides that, it works well wilt 'silk' (semi gloss) paints, but less effective below the CPVC. Moreover, depending on the precise CPVC, hollow latex particles allows the proportion of TiO
2 to be reduced by up to 10-20% without compromising the 'hiding power' of the paint and this particle is considerably cheaper than TiO
2, so this results in significant cost savings for the paint manufacturer.
Non-Drip Paints and the Other Types of Paint
Latex (and pigment) particles in paint are weakly aggregated or clustered in solution. These aggregates lead to a structured fluid, which helps prevent sedimentation of the relatively dense pigment particles and that weakly aggregated particles break up under shear created by paint brush strokes. Besides that, the potential energy of 2 latex particles and potential energy can be represented as:
 |
| A plot of potential energy interaction curve for 2 latex particles on close approach |
The primary minimum is the most stable distance between two latex particles and the secondary minimum is due to the weak attractive inter-particle forces that produce 'sticky collisions'. Moreover, on removal of the shear force, the dispersed particles re-aggregate to reform the structured fluid and this is the fundamental principle behind a 'non-drip' paint.
Besides non-drip paint, there are another types of paint that commonly used in these days.
- Alkyd resins: High gloss solvent-borne paints for door frames, skirting boards, etc. However, it future is uncertain due to contain VOCs.
- Powder (solventless) coatings
- UV curable coatings
- Maritime paints: It is formulated to minimise biofouling. Tributyl tin is a very effective biocide for barnacles and it is used in maritime paint formulations since 1960's. Unfortunately, tributyl tin slowly leaches out of the coatind and it is toxic to oysters, mussels, and clams. Therefore, it is now being phased out in favour of copper acrylate.
Next Generation Paints
Nanocomposite Particles
 |
| The synthesis of nanocomposite particle |
In 2004, FARBE & LACK prize awarded to BASF, Germany for nanocomposite particle that can be used in paint formulation. The nanocomposite particle is made from 10 - 20 nm aqueous anionic silica sol with styrene, and n-butyl acrylate comonomers in cationic surfactant and initiator to form 100 - 200 nm diameter of nanocomposite particles. The emulsion polymerisation in aqueous media (water-borne paint) with 35% solids content and 20-60% silica content based on total solids. Moreover, the incorporation of silica particles into latex is almost 100%. In picture below is the comparison between nanocomposite particles which was synthesised by BASF and the University of Sheffield.
Moreover, copolymer-silica nanocomposite particles offer many advantages:
- Tougher, harder, more durable coatings
- Reduced water permeability
- Good transparency
- Char formation and no dripping plastic
These new water-borne nanocomposite-based paints are now marketed by BASF (with Akzo Nobel) as next generation exterior architectural coatings. Moreover, the hydrophilic silica makes this paint 'self-cleaning' as the dirt is poorly adherent.
Evoque™ - Pre-Composite Polymer Technology
 |
| The synthesis of pre-composite polymer technology |
It is developed by Dow Chemicals as the polymer nanoparticles adsorb onto
TiO2 pigment and the nanoparticles adsorb via phosphate groups onto
TiO2 and act as a spacer. Moreover, it ensures more uniform
TiO2 dispersion within paint film. Moreover, it allows either greater hiding power and/or less
TiO2 to be used, so it produces cheaper paint.
After, we discuss the chemistry inside the paint formulation, there are future trends that we can predict. Thus, the future trends are:
- Legislation-led drive towards zero-VOC paints.
- More aqueous formulations.
- Reformulation of existing solvent-based paints.
- More emphasis on solventless UV-curable powder coatings.
- Likely future shift away from oil-based feedstock towards renewable feedstock.
- Increasing use of 'nanotechnology' for the production of more sophisticated (nano-structured) pigments and binders.
Generally, there are 2 main functions of the paint; as decorative which makes product (car, house, etc) more attractive, so it is easier to sell, and as a protective barrier for example preventing car from corrosion and house from mould formation. The basic formulation of paint consists from 4 main components: pigment, extender, binder, and carrier. Later on we will see the function of those components and how it works in paint formulation (paint technology).
From those pigments in the table above, the commonly used white pigment is TiO2, but other minerals (e.g. BaSO4, CaCO3) are less opaque but much cheaper than TiO2. The manufacturing of TiO2 pigment are shown below and the process is an energy-intensive process, so the it is an expensive pigments.
To strengthen the colour of the paint and also to reduce the price of paint, an extender is used in paint. Basically, an extender is made of a cheap mineral (e.g. BaSO4, CaCO3) and acts as the supplements of the primary pigment. An extender usually presents in powder form and it is insoluble in the paint formulation. Moreover, it has a refractive index less than 1.7 (water = 1.33). Refractive index can be defined as the ratio of speed of light in a vacuum to that in the medium of interest. Besides that, the costs of the extender typically £30 to £500 per metric tonne.
The mechanism of latex film formation is followed 4 main stages (Jones et al., 1995).
Moreover, γ can be calculated by Frenkel's equation as shown below.
So, it can be simplified the deformation pressure is inversely proportional to the radius of the particle, so smaller radius, it produces bigger pressure. Thus, the particle coalescence is easier for smaller latex particles.
Another factor that provides the opacity of the paint is from the refractive index of the pigments. Generally, the latex binder has refractive index around 1.4 - 1.6, and as the refractive index of the pigment is higher, the opacifying effects increases. As shown in figure below, higher refractive index of TiO2 ensures strong light scattering, so the underlying substrate is invisible to observer (good hiding power). In the other sides, the extender has lower refractive index (around 1.6), the underlying substrate is visible to obsever.
At the CPVC (or above) abrupt changes in various paint properties are observed. The observation is the paint film becomes lighter in colour, get lower scrub resistance, reduced gloss, poorer mechanical properties and the film porosity increases (due to air voids). At above CPVC, air voids are present in the film and pigment particles protrude from the surface and it reduces gloss effect of the paint. Hence, a matt appeareance is produced with maximum extender and minimal latex.
Moreover, copolymer-silica nanocomposite particles offer many advantages:
After, we discuss the chemistry inside the paint formulation, there are future trends that we can predict. Thus, the future trends are:
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