Coatings
History
Paints (as surface coatings are more commonly termed) were first prepared 100,000 years ago by man’s ancestors, primarily as an art form. These were primitive compounds consisting of charcoal, and natural inorganic pigments, such as red iron oxide and yellow ochre, combined with a natural organic substance, often animal fats and blood, to provide binding properties for the ingredients and adhesive properties to the surface being decorated, for example cave walls, and as body adornment.
After the neolithic period, during the establishment of civilisation, other binders were used, such as milk, beeswax, and the yoke of eggs, and the scope of paint usage was extended to decorative purposes such as walls and statues during the reign of the Pharaohs around 5,000 years ago.
Paints were further developed over time with the first ‘Oil based’ paint, prepared using natural vegetable oils, making its first appearance in Afghanistan in circa 450 AD. The first modern paints became widely available only during the industrial revolution, some 1,400 years later.
Today there are a plethora of different paints available, using both organic and inorganic pigments, with a multitude of different binders being used in solvent based, solventless, and water based paints. Today’s paints are a far cry from the originals based on naturally occurring components. The crude oil industry was established in the 1800s and, apart from the well-known oil and gas products that have been produced from this source, a sizeable ‘down-stream’ industry arose, the petrochemical industry, producing a variety of chemicals as by-products of the oil and gas industry. The availability of these chemicals has allowed chemists to develop binders tailor-made for specific applications by careful selection of the petrochemicals used in their formulation. Many binders are not only composed of petrochemicals but also naturally occurring substances, such as vegetable oils, thus combining the properties of both to develop ‘hybrid’ binders with unique properties.
Unfortunately, it is impossible to list all the types of binders that have been developed over the last 400 years, and I can only cover a subset of them. Even within each binder subset there are multiple compositional possibilities so what inevitably follows is an overall generalisation of the major binder groups.
Polymers
Today the majority of binders are composed of what are chemically referred to as 'Polymers'. The etymology of 'polymer' is from the Greek words 'Polu', meaning many, and 'Meros' meaning 'A share'. Combining the two words we end up with 'polymeros' (having many parts), now simply referred to as 'Polymers'. In short, Polymers consist of many molecules joined together to form very large 'chains', termed 'macromolecules'. There are many types of polymers available but they tend to be identified as:
- Thermoplastics: Those that can be melted and used in processes like injection moulding. Examples of thermoplastics, are 'plastic' bags, small electrical appliance housings, tool boxes, storage tanks etc.
- Thermosets: Those polymers that react further at room, or elevated, temperatures, to produce products with an increased polymer size. Examples of thermoset polymers include some coatings and adhesives.
Resins
Resins are a subset of polymers that may either be thermoplastic or thermoset and be either solid or liquid in their raw state. The original resins were gums and saps originating from natural sources and used in the first coatings. They have been (mainly) replaced by those polymers prepared from both natural and synthetic chemicals, or synthetic chemicals alone, to produce polymers that exhibit superior properties to the solely natural products. Because they were developed to replace natural resins, the original name of 'resin' has been adopted for these products as well. The term 'resin' is generally used by the manufacturers of these products. When used in coatings they are more likely to be termed as 'binders' as this is one of their most important attributes, as a base to 'bind' the other components of the coating together.
The following is a synopsis of the major solvent based resins as used by the coatings industry
Acrylic Resins
Acrylic resins are produced from synthetic chemicals known as acrylates and methacrylates. These monomers are polymerised by addition reactions to form polymers with carbon to carbon bonds, one of the strongest known organic bonds. As such these polymers are highly resistant to chemicals and UV degradation. There are two main types of acrylic resins, those that are classified as 'Thermoplastic, where the drying of the coating is solely due to solvent evaporation, and 'Thermoset', where the initial drying is also from solvent evaporation, but with a subsequent secondary reaction with a catalyst. As the coatings made using acrylic resins tend to be very fast drying they are usually applied by spray or by other mechanical means.
Thermoplastic acrylics are utilised in a number of coatings. Examples of their use include some 'ready to use' canned spray paints, plastic coatings, architectural wall coatings (particularly external ones) and even some nail polishes.
The thermoset variety of acrylic resins, as mentioned are designed for secondary reactions with other reactants. There are thus two separate technologies which depend on the application conditions:
- Single pack coatings contain a 'built in' reactant that requires heat to ensure the reaction takes place. After solvent evaporation the items are placed in an over at elevated temperatures to ensure reaction. These coatings are used industrially for example for appliance and automotive finishes.
- Twin pack coatings are those that supplied in 2 packs, the acrylic coating and a reactant, that are mixed prior to use. The reactant is formulated such that it reacts with the acrylic at room temperature, hence the necessity to keep the two components apart until immediately before use. The reactant is normally a polyisocyanate and the bonds formed during reaction are called 'urethanes' hence these products are commonly referred to as 'polyurethanes' Application areas include OEM, and refinish, vehicle enamels, exterior wall coatings in hot climates, aircraft finishes, plastic coatings etc
Alkyd Resins
Alkyd resins are produced from both natural (mainly natural oils and fatty acids) and synthetic chemicals by condensation reactions. There are 3 main types of alkyds. Long Oil Alkyds, where the quantity of oil in the polymer is above 70%, Medium oil Alkyds, where the oil quantity is between 40 and 60%, and short oil Alkyds, where the quantity of oil is below 40%.
These resins vary from soft to hard solids in their raw state and are supplied in a suitable solvent to improve handling and usage. The solvent used to dissolve the alkyds depends on their oil length and application. For further information on solvents used in the manufacture of alkyd resins, please refer to the Solvents Page
- Long Oil Alkyds are used mainly in household/architectural coatings such as gloss and satin/eggshell enamels, varnishes and as undercoats and primers for wood, applied by brush or roller. The coating solvent is most likely to be white spirits (aka mineral spirits or mineral turpentine)
- Medium Oil Alkyds are used mainly in household/architectural coatings, primarily for satin/eggshell finishes and as undercoats and primers for wood, applied by both brush and roller. These resins are also used for industrial 'machinery' enamels and primers, applied by brush or spray. For household use, the normal solvent is white spirits, whereas for industrial use, the solvent might be be white spirits or one with greater solvency power, and faster drying
- Short Oil Alkyds are used industrially for a variety of applications, mainly by spray application. Short oil resins are used in the manufacture of industrial wood finishes and metal or plastic coatings. The finishes produced by short oil alkyd coatings are much harder and, as they tend to dry quickly, they can only be applied by spray or alternative fast mechanical coating system.
Modified Alkyd Resins
The basic alkyds described above can be further modified to improve their properties or make them more suitable for other applications. The modification of the standard alkyds cane be achieved either the addition of a chain stopping monomer, by reaction with any free hydroxy groups from the original manufacturing process or by 'attaching' other monomers to the backbone of the alkyd polymer:
Chain Stopped Alkyds
The chain stopped alkyds are more a simple adjustment to the resin chemistry of the standard medium and short oil alkyds. They are produced in the same way and use the same solvents as their parent resins. Where they differ is in the use of a monoacid, or monohydric alcohol (those monomers containing one acid, or alcohol group respectively) to stop the growing polymer chain - hence the name 'chain stopped'. The purpose of stopping the growth of the polymer chain is to limit the molecular mass of the resins produced and the viscosity of the resulting resin once it is solubilised in an appropriate solvent. Depending on the monomer used as the chain stopper, additional benefits may also be achieved if the chain stopping monomer imparts additional hardness, and a shorter touch dry time, to the applied coating. The nomenclature of the chain stopped alkyds follows that of the parent alkyd. For example one has 'medium oil chain stopped' alkyds and 'short oil chain stopped' alkyds. The applications for these resins is mostly in industrial coating systems where the combination of additional hardness and faster drying is of benefit.
Urethanated Alkyds
The Urethane resins are produced from long and medium oil alkyds by reacting a 'free' hydroxyl on the alkyd backbone with an isocyanate to form a urethane link. This reaction is exothermic, is known as a urethanation reaction, and does not produce any by-products during the reaction. Based on the name of the reaction, these polymers are termed polyurethanes. Polyurethane alkyds are widely used in industry for the manufacture of various coatings, especially for wood, plaster primers and floor paints. The polyurethanes show excellent chemical and hydrolytic resistance, are especially strong and durable, and exhibit some degree of fire retardancy.
Vinylated Alkyds
Vinylated alkyds are produced by chemical modification of the alkyd polymer carbon chain with one or more vinyl monomers. The vinyl monomers are a class of chemicals that have a reactive double bond in their molecules. These may be chemicals such as styrene monomer, vinyl toluene or any of the acrylic monomers.
Alkyds that have been modified with styrene are know as styrenated alkyds, those with acrylic monomers and those with vinyl toluene, simply VT alkyds.
- The styrenated alkyds are used for very fast drying spray paints for temporary steel coatings, bicycle frames etc where the coating is sprayed on.
- The acrylated alkyds are utilised in more specific applications such as metal decorating (e.g. can) coatings and car refinish primers and enamels. Application is by either roller or spray.
- The vinylated alkyds are used where fast drying brushing primers and enamels are required in combination with mineral spirits solubility.
Polyamide resins
Polyamide resins are produced from both natural (fatty acids) and synthetic chemicals by condensation reactions, in a similar way to the alkyd resins. There are 2 types of polyamide resins, those which are solids at room temperatures, and thermoplastic in nature, and those which are semi-solid or liquid at room temperatures used in the formulation of thermoset coatings.
The thermoplastic variety of polyamide resins are used in 2 main areas:
- Where they are applied in a molten state, via heat guns or associated mechanical means, at temperatures of circa 100°C or so. The applications are generally in 'hot melt' adhesives for bonding dissimilar items together, for instance rubber to metal.
- Where they are dissolved in certain solvents to produce inks for coatings, especially for plastic packaging.
The thermoset polyamides (also known as 'reactive polyamides') are used in conjunction with other polymers with which they react to form extremely strong coatings with good durability and chemical resistance. The usual associated polymer with which the polyamides are reacted are the epoxy group of polymers. Hence products based on polyamides as a reactant are more often referred to as 'Epoxies'. These products react at room temperature, but are sometimes subjected to curing at elevated temperatures to produce a more rapid reaction. As with the 2 pack acrylics/urethanes above, the two components are supplied separately and must be used a short period after they are combined. These '2 pack epoxies' are used in two distinct areas:
- As adhesives, either in liquid or 'putty form' to join a multiplicity of items together, such as plastics, rubber, metals etc, and they have even been used to join metal and/or fibreglass panels together in automotive applications, and repair cracks in bridges and dams.
- As coatings, particularly metal primers, where a very high adhesive strength is required, or as 'solvent free' floor coatings for use in, for example, industrial floor finishes which require a tough, resilient coating.
Saturated Polyester Resins
Saturated polyester resins are more commonly referred to as 'polyester resins' and are produced from (mostly) synthetic acids, diacids and polyhydric alcohols by condensation reactions. The term 'Saturated' is used to distinguish them from the Unsaturated variety as they do not have any unsaturated monomers in the polymer backbone.
The polyester resins are the simplest of the condensation resins, and can be considered the 'base group' for all other condensation reactions, as found in the alkyds, modified alkyds, polyamides, unsaturated polyester and polyamide resins. These resins are not used as a single binder but are subsequently reacted with a second monomer, or polymer, to achieve the desired coating properties.
Together with a suitable reactant, the saturated polyesters are used to produce 'polyurethane' foams and floor coatings as well as industrial primers and finishes for prefabricated metal sheets used in roofing, cladding etc. Coatings produced from saturated polyesters tend to have good to excellent weathering properties and toughness.
Unsaturated Polyester Resins
Unsaturated polyester resins are produced from (mostly) synthetic chemicals by condensation reactions. The use of Unsaturated polyesters is not something one would normally associated with the coatings industry, their use mainly confined to the manufacture of glass reinforced plastics (GRP), in combination with glass fibre and certain other fibrous materials, to fabricate articles such as boats, tanks and even ship hulls. However, there is a substantial use in car repair products in the form of 'body fillers, otherwise known as body putties. In certain other countries of the world, unsaturated polyesters are used in industrial wood primer/surfacers and UV, or electron beam, cured wood finishes. Invariably the solvent used in the preparation of these products is styrene monomer, which also polymerises into the finished product providing a (more or less) 100% solids coating when fully cured.