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PAINT has been known in various forms since earliest times. The ancient cave painters used naturally occurring materials, such as charcoal and iron oxides. Eventually their successors seem to have found that if they mixed these materials with natural vegetable resins, such as rosin or cellulose, and others derived from insects, such as shellac, it was easier to apply them and they lasted longer.

From humble beginnings and through numerous centuries of development, paint now affects most aspects of our lives – whether it is for decorating the home, preventing our cars from corroding, or protecting machinery from abrasion and chemical attack. And the graffiti on railways and public buildings are a reminder that paint is forever finding new social uses. With few exceptions, paint is a truly international medium with the same types being used from the poles to the tropics.

The range of surfaces, or substrates, to which paint can and is applied is vast and includes metals, timber, plastics, plaster, cement and concrete. And the conditions under which it must protect those surfaces can be just as wide – from corrosive environments to abrasive, hot, cold, wet and brightly lit ones. The sheer diversity of colours, finishes and types of paint available for a single job can be bewildering, as can be their composition and the way in which they are applied.

In the past few years the paint industry has tended to prefer the general term “coating” rather than “paint” for this broad range of products. This takes into account, for example, powder coatings and specialist ones such as sprayed metallic or ceramic coatings. The term “paint” is generally applied to liquid coatings, or more correctly a “formulated” composition (usually based on an organic resin), that dries to form a hard film adhering to a substrate.

Simple protective film

Complex formulations

PAINTS are mainly used to protect and decorate surfaces. The aim is to interpose a film between a substrate and its surrounding environment so as to separate it from that environment. This is achieved by applying different coats of paint to the substrate, and each coat being formulated to impart specific characteristics, such as adhesion and protection against weathering and microbial attack.

The decorative value and protection provided by the finished system depend on both the substrate and the prevailing environment in which it resides -usually called its service exposure. Many paints that are designed to impart a high degree of protection against corrosion often have little aesthetic value as, for example, the highly protective bituminous paints used to protect pipelines, storage tanks and flat roofs. Conversely, many paint systems designed to provide coatings with a high degree of decorative value can often only be used on substrates exposed to relatively mild service environments. Generally, though, a compromise can be achieved between the decorative and protective functions of a coating system.

Not only do the types of substrate to which paint is applied vary widely but also their physical qualities, such as their absorbtiveness (that is to say their ability to absorb), surface finish, state of deterioration and cleanliness. The shape of the substrate and its accessibility must also be taken into account.

There may be wide differences in the severity of the service environments to which the painted surface is exposed. These can range from refrigerated tanks that are held at subzero temperatures, to coatings on the chimney stacks that are used by industry to vent corrosive vapours at several hundred degrees Celsius. In hospitals, operating theatres are painted with coatings containing biocides to maintain maximum sterility.

Despite the complexity of the substrates and the multiplicity of paint materials available to coat them, all paints are basically similar in composition. The earliest ones were suspensions of dyes and pigments in a solvent, but modern paints are a complex mixture of several different ingredients carried in varying proportions. Paints today tend to be composed of three main components: a polymeric resin, known as the binder or vehicle, a finely dispersed solid pigment and a solvent. In addition, other functional ingredients are included to enhance the physical properties, or performance, of the paint system.

Matter of composition

Handle with care

THE BINDER resin (see Box) is the most important component in the paint system because, once it has solidified in the drying or curing process, it should provide a hard surface film with the required attributes of adhesion, flexibility and toughness.

A major function of the pigment is to provide colour and opacity for the paint film, and hence to the surface on which it is applied. Some pigments have other specific functions which, for example, may provide a more durable film coating for metal substrates to protect them against corrosion. Anticorrosion pigments include iron oxide, zinc phosphate and the chromates of zinc, strontium and lead.

The other components are usually incorporated into paints to modify the characteristics imposed by the pigment and resin. Extenders are used to control the cost of a formulation by reducing the amount of expensive pigment (such as titanium dioxide) required, and to improve various physical properties. The extenders are minerals such as whiting (calcium carbonate), barytes (barium sulphate), kaolin, mica and talc (all three are complex silicates). Driers can help to control the drying or curing process of the liquid paint and are usually fatty acid esters (such as naphthenates) of metals such as cobalt, manganese, lead, vanadium and cerium. Some soaps of barium and calcium are also used. The addition of fungicides helps to prevent moulds growing on the final film’s surface.

Volatile solvents, including aliphatic (such as white spirit) and aromatic hydrocarbons (for example xylene and toluene), as well as several oxygenated compounds (ketones, esters, ethers and alcohols) are included to control the application properties of the liquid paint and to modify the way in which the film dries. Halogenated organic solvents are used if other types are not effective. However, water is often now the solvent of choice for general purpose paints.

Health hazards can arise from the presence of certain compounds used in the paint industry, many of which had been used for centuries before the dangers became realised. Perhaps the most notorious of all is red lead, which was once used in rust-resistant paint; however, its use in paints is now prohibited. Lead is now known to be a cumulative poison and that it can act on the central nervous system causing, as the concentration increases, dullness, irritability, headache, loss of memory, convulsions, paralysis and even death.

Other potentially dangerous poisons are the compounds of chromium used in some pigments, as well as mercury and tin compounds used in antifouling paints for treating ships’ hulls. Some resins and the precursor chemicals used to make them are also potentially hazardous, such as the isocyanate component of polyurethanes and amine curing agents used in epoxy paints. If carelessly handled, various resins can cause dizziness and breathing difficulties, so it is advisable to wear protective clothing and a face mask when using them. The bis-chloromethyl ether, which is formed in the reaction of formaldehyde with hydrochloric acid catalyst in urea-formaldehyde resins, is known to be carcinogenic.

Paint solvents, such as hexane, toluene, xylene, esters, ketones and alcohols, can cause various degrees of contact dermatitis and may even act on the central nervous system, leaving victims sensitive to changes in temperature and light, and sometimes suffering memory loss. Some solvents may cause circulatory disturbances and even renal failure, while others, such as henzene, are known to be carcinogenic.

Most resins in a liquid state and solvents present considerable fire hazards and must be handled with great care.

New trends for old

Safer environments

THERE are several different classifications of paint, the most usual being based on the type of resin binder used, by the type of pigment or by the proposed function of the paint. Of these, the generic type of resin is the most widely used. More than a dozen classes of resin are used as paint binders (see Box), and each has its own distinctive attribute and range of end uses.

For general purposes, however, paints tend to be distinguished by their end use in three groups: architectural paints, which are applied on the outside and inside of buildings; paints applied to industrial products in the factory on a production line; and speciality coatings. The last-mentioned include ship paints containing antifouling compounds, coatings for repairing vehicles (refinishing), those used to protect against corrosion, paints for road markings, fire-retardants, electromagnetic shielding, antibacterial purposes and even slippery paint to deter burglars from climbing up drain pipes.

Over the past 25 years, the emphasis of paint manufacture has moved from traditional products to one where paints tend to be developed to meet the specific requirements of users. This applies particularly to the sector concerned with coatings used by industry – that is to say paints used on the production line and ones that can only be applied in special controlled conditions.

This pattern of change is the norm for many modern industries. From an initial dependence on naturally occurring materials, such as paints based on linseed oil and soya oil, the paints industry has evolved through early developments in synthetic resins to the plethora of types that are in current use, such as the polyurethanes that are formed by the reaction of an isocyanate with a polyol – a polyhydric alcohol, which contains several hydroxyl groups.

Two events had a considerable effect on the paint industry in the early 1970s. The first was the recognition of the detrimental effects on the environment of solvent emissions. To combat this, stringent legislation was introduced, initially in the US. This led to the development of novel coatings able to meet these laws, as well as being acceptable to the users. Examples include the so-called high-solid types that use smaller amounts of organic solvents and the systems that replace most of the organic solvent content with water. The ultimate development came with the powder coatings which contain no solvent at all.

Western Europe rapidly followed the US legislation, led by the Scandinavian countries and Germany. This trend, which is now worldwide, was accelerated by the world energy crisis of the early 1970s. The price of crude oil rocketed from below $5 a barrel to a high of about $35 in the early 1980s. Even now, with a world glut of oil, the price is still between $15 and $20 a barrel and seems unlikely to fall in the immediate future.

This escalation in the price of crude oil caused substantial increases in the price of many raw materials used in coatings, as well as the cost of the energy necessary for their production. It gave an impetus for industry to look for alternatives to the conventional low-solid, solvent-based coating.

Search for solutions

Waterborne paints

TRADITIONALLY a paint system used an organic solvent and had a total solid content of around 60 per cent by weight. As a result of the environmental pressures already mentioned, statutory regulations are being introduced by the European Commission to control the amounts of volatile organic compounds (VOCs) emitted into the atmosphere by industry. These regulations comprise the VOC Control Directive that limits emissions from painting shops in a factory (such as a car plant), and the International Pollution Prevention and Reduction Directive. This is more far reaching and provides a series of guidelines to regulate releases of substances.

There has been a change in the preferred organic solvents, with the aromatic and aliphatic hydrocarbons in particular being replaced by more environmentally friendly versions such as the oxygenated solvents typified by the glycol ethers and ketones. However, the move to substitute more acceptable organic solvents has been overtaken by the evolution of water-based coatings, where the solvent is mainly water, albeit containing a small proportion of a compatible organic compounds, such as butyl or propylene glycol ethers.

Water-based coatings (in the paint trade they are called “waterborne”) are now able to match the performance properties, such as abrasion resistance, weathering, chemical resistance, gloss levels and so on, of a solvent-based paint. The advantages of these water-based coatings are generally well recognised, but many users seem reluctant to accept them. They require special precautions when handling, such as the need to isolate any electrical equipment where the paint is being applied for safety reasons, and clean-up procedures tend to be more difficult and time-consuming than with solvent-based systems. More stringent cleaning of the substrate prior to application is also necessary with water-based coatings.

Another class of so-called high technology coatings include the high-solid paints. The solids content is increased substantially to more than 70 per cent by weight for liquid paints using organic solvents.

Water-based and high-solids coatings systems are essentially modifications of the conventional low-solids, solvent-based systems, with the solvent content reduced. This can be achieved by either increasing the relative amount of solid components, or by replacing most of the organic solvent with water. This concept is carried to its extreme in powder coatings, which contain no solvent component and are effectively a solid form of paint.

Powder coatings fill a significant niche in the industrial coatings market and their use is growing, particularly in the motor manufacturing industry. But these coatings and the equipment required to apply them are expensive. Furthermore, difficulties may occur with colour changes, and it may not be possible to use electrostatic applications with complicated shapes if the paint cannot flow uniformly over the whole surface. Contamination is another concern and substrates may need special cleaning procedures.

Since powder coatings are almost totally solid, they can satisfy the very stringent environmental legislation, such as the laws relating to the permissible level of organic solvent content. Also, because they can be applied electrostatically in particulate form, a diversity of polymer types can be used of significantly higher molecular weight than with liquid systems. Thus extremely hard coatings are possible that are highly resistant to chemicals and to abrasion, such as those needed for car finishes.

There are several esoteric but valuable technologies that involve the polymerisation of resins such as acrylics, acrylated polyesters, polyurethanes and cycloaliphatic epoxides to form a surface film by exposing them to radiation – a process known as radiation curing. Many of the techniques are highly advanced, and involve electron beams and ultraviolet light in the curing process. Research into ways of curing paint products continues using X-rays, ion beams and lasers. Advantages of radiation-cured systems include low energy consumption and that the substrate does not become heated.

The number of factors that have to be considered when selecting a coating for a specific use with these advanced technologies has greatly increased. Traditionally, such a choice would be determined by a combination of acceptable costs, the ability to meet the properties required by the customer (such as its durability, life span, resistance to weathering, gloss and so on), as well as ease of application to give a film of uniform thickness. Additional factors can include the ability to meet the statutory regulations governing the use of new materials and the health and safety of operatives. Adequate finance to meet any necessary product or process changes, such as the installation of a waste incinerator or solvent recovery and recycling units, is also an important consideration.

Influential industry

Colourful outlook

THE paint industry is large and one of the major sectors of the chemical industry. It typically accounts for between 10 and 15 per cent of the chemical output (in monetary terms) for most industrialised countries. Unlike other sectors, it is decentralised and most countries have several paint-producing enterprises, ranging from large multinational groups down to small family-owned operations supplying low quality paints to local markets.

Western Europe alone has in excess of 1500 paint factories (excluding ones with fewer than 10 people), employing upwards of 110 000 people. These produce 5.4 million tonnes of paint every year but, despite the presence of so many manufacturers, a handful of groups dominate the world industry. The leading 12 companies together account for around 55 per cent of the total global output. This group consists of four American companies (Dupont, PPG Industries, Sherwin Williams and Valspar), two British (ICI and Courtaulds), two German (Herberts – part of the Hoechst Company – and BASF), two Japanese (Nippon Paint and Kansai Paint), and one each from the Netherlands (Akzo-Nobel) and France (Total Chemie). Other leading players in the world market include Sigma Coatings, which is part of the Belgian Petrofina group and the Norwegian company, Jotun.

Only a few of these major producers are primarily paint companies, and most are part of large and diverse chemical corporations (such as Akzo-Nobel, Dupont, Herberts, BASF and ICI) or petroleum companies (Petrofina and Total Chemie). These have diversified into the paint industry over the years, mainly through acquisitions.

The paint industry offers an exciting future as there is a need for much basic research into surface phenomena. Most of the resins now available have probably been developed to their limits and are unlikely to meet all that is required of them in the 21st century. New resins, new pigments and other components will be required to meet such challenges and to satisfy the call for a safer environment.

Chemical classes of some major paint resin systems

SEVERAL generic classes of resins are widely used in paints, the most important being:

• • Polyester resins (A), include the alkyd resins that are the most widely used group of binders, produced by condensation of esters formed by the reaction of polycarboxylic acids and polyhydric alcohols. Uses: Decorative, industrial, marine, anticorrosion and wood.
• • Polyurethanes (B) are materials based on the reaction of an isocyanate molecule with compounds containing an active hydrogen atom, such as hydroxyl compounds (notably polyesters, polyethers, acrylics, epoxies and alkyds), amines, water and various carboxy-containing compounds. Uses: Automobile, industrial, anticorrosion and wood.
• • Acrylic resins (above) are polymers that contain acrylate and methacrylate esters in their structure, together with certain other vinyl unsaturated compounds, such as vinyl acetate, ethyl acrylate and butyl acrylate. Uses: Decorative, automobile, industrial, anti-corrosion and wood.
• • Vinyl resins comprise straight-chain thermoplastic vinyl compounds prepared from vinyl monomers that are mostly used as copolymers produced from two or three different types of monomers. Uses: Decorative, industrial and wood.
• • Phenolic resins are obtained by reacting various substituted phenols with formaldehyde and the dimer that is formed being subsequently catalysed with acid or alkali to produce a hard resin. The acid-catalysed products (Novalacs) are non-oil-reactive in contrast to the reactive alkali-catalysed resins (Resoles). Uses: Industrial.
• • Cellulose derivatives are obtained by chemically modifying cellulose to produce thermo-plastic materials. The most used are cellulose nitrate (nitrocellulose); other derivatives including cellulose acetate and cellulose acetate butyrate. Uses: Wood and automobile.
• • Amino resins comprise the reaction products of melamine (1,2,5-triaminotriazine) and of urea with formaldehyde. Uses: Industrial and wood.
• • Oleoresinous binders are based on certain vegetable oils, of which linseed oil is the most important, others comprising tung oil, dehydrated caster oil and oiticica oil. Uses: Industrial and wood.
• • Epoxy resins (above) contain the epoxide or oxirane group, the most widely used types being formed by the reaction of bisphenol A and epichlorhydrin. Uses: Automobile, industrial, marine and anticorrosion.
• • Elastomeric resins are based on natural rubber (polyisoprene) and some types of synthetic rubbers. Uses: Anticorrosion.