Composition of Paints, Varnishes, and Lacquers
Excerpted from Estlander T, Jolanki R. Chapter 61 Paints, Lacquers, and Varnishes
Kanerva’s Occupational Dermatology 2nd ed. 2014
Binder is the main ingredient in paints, lacquers, and varnishes. It is the actual film-forming agent in paints. It keeps the pigments bound and permanently dispersed on the painted surface, and it provides the most important properties of the coating: hardness, flexibility, and speed of drying. Binders include synthetic or natural resins and oils, alkyds, epoxy resin compounds, formaldehyde resins, acrylic resins, polyurethane resins, water-dispersible polymers and other synthetic resins, polystyrene resins, and cyclohexanone resins (Rose and Vance 1997; Fischer and Adams 1999; Estlander et al. 2000).
Natural resins and oils originate from trees, plants, fish, and insects. Dammar, Japanese lacquer, and shellac are suitable for lacquers and varnishes because they dry quickly, although the film formed is brittle. Copal is a fossilized resin which is used in varnishes. Natural oils, such as flaxseed or linseed oil, perilla, tung and pine (tall) oils, soybean and ricin oils are all used in oil-based paints. Since the 1980s, synthetic alkyd resins have widely replaced natural binders (Fischer and Adams 1999; Estlander et al. 2000) .
Alkyds or alkyd resins are condensation products of poly-alcohols; e.g., glycerol, pentaerythritol and sorbitol, and polycarboxylic acids or corresponding anhydrides such as phthalic anhydride, adipic acid, and maleic acid. Common alkyd binders are formed after modification with oils containing unsaturated fatty acids. These include linseed, soybean, sunflower, cottonseed, and pine oil. Linseed oil and similar drying oils can be combined with colophony (rosin) to produce a paint resistant to climatic conditions while also having good color retention.
Polyester resin binders are alkyds containing no modifying oils. Styrene and vinyl toluene are used as cross-linking agents for these alkyds. Epoxidized alkyd resins are alkyds modified with epoxidized oils which are formed by reacting double bonds in unsaturated fatty drying oils with oxygen to form an epoxide ring.
Paints based on alkyd resins need no hardeners. They are hardened by the evaporation of organic solvents or water, followed by the reaction of the binder with the oxygen in the air (Fischer and Adams 1999; Estlander et al. 2000).
Epoxy resin compounds. Paints, varnishes, and lacquers based on epoxy resins are used in various industrial applications because of their strength and durability. A hardener must be added to two-component epoxy paints that cure at ambient temperature before their application. The paint hardeners include polyamines, e.g., diethylenetriamine, triethylenetetramine, trimethylhexamethylenediamine, isophoronediamine, and 1,3-xylylene diamine and often also a catalyst, such as 2-,4-,6-tris(dimethylaminomethyl)phenol (tris-DMP).
One-component epoxy powder paints that are heat-cured contain a hardener which can only be activated by heating. Polyfunctional aromatic amines, solid polyamides, and organic anhydrides can be used as curing agents (Jolanki 1991; Estlander et al. 2000; Ponten 2006). See Chap. 52, “Epoxy Resins” for additional information.
Formaldehyde resins. Urea, melamine, phenol, or substituted phenols can be modified with formaldehyde to produce corresponding resins. An excess of free formaldehyde must be removed in order to prevent interference with the film-forming properties of paint. These resins can also be used to cross-link alkyd resins. The curing takes place by heating. Phenol formaldehyde resins (PFRs) remain stable in temperature variations and are resistant to moisture, acids, and solvents (Fischer and Adams 1999; Estlander et al. 2000). For more information, refer to Chap. 53, “Contact Allergy to Phenol-Formaldehyde Resins” in the textbook.
Acrylic resins. Multifunctional acrylates are used in the formulations of UV-curable coatings. The most commonly used multifunctional acrylates in industrial UV-curable acrylate paints and coatings are trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA), or hexanediol diacrylate (HDDA). A photoinitiator system is also needed.
Multifunctional acrylates can be combined with polyfunctional aziridine (PFA) cross-linking agents. Commercially available PFA cross-linking agents are usually synthesized from ethyleneimine or propyleneimine and TMPTA or PETA. A PFA cross-linker is added to the aqueous acrylic component before its use. The cross-linking reaction is self-curing, but heat or UV radiation may be used to enhance the reaction, resulting in faster drying of the products.
PFA is also used to cross-link water-based polyurethane emulsions, such as lacquers and topcoats. (Kanerva et al. 1995; Fischer and Adams 1999; Estlander et al. 2000; Bjorkner 2006). For more information, refer to Chap. 51, “Acrylic Resins” chapter.
Polyurethane resins. Polyurethane coatings are formed by the reaction of isocyanate groups with hydroxyl groups of polyalcohol components. The polymerization reaction is usually catalyzed by tertiary amines. Mainly di- or triisocyanates, e.g., toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), hexamethylene diisoxyanate (HDI), or their trimers are used in the reaction. Unmodified polyurethane resins can be formulated in one- or two-component systems. They can be modified with natural drying oils, resulting in coatings which dry in the air and are polymerizable, like alkyl resins. Polyurethane resins are durable, heat resistant, and flexible (Fischer and Adams 1999; Estlander et al. 2000; Frick 2006). For more information, refer to Chap. 54, “Polyurethane Resins” in the textbook.
Water-dispersible polymers and other synthetic binders mostly include polyvinyl acetate, polyacrylate, copolymers of vinyl acetate and acrylate, and copolymers of styrene and acrylate. These are used in water-based latex paints. The latexes may contain small amounts of ammonia (0.3% w/w), formaldehyde (0.06% w/w) or other biocides (e.g., mixture of isothiazolinones), surfactants and polymerization inhibitors (e.g., hydroquinone or benzyl peroxide), and even traces of monomers, e.g., mono(meth)acrylates (Hansen et al. 1987; Fischer and Adams 1999; Estlander et al. 2000).
Polystyrene resins are made from polymerized styrene and have good insulating power. Synthetic rubber, known as styrene-butadiene or chlorinated rubber latex, can be used in paints for floor coverings or tank linings (Fischer and Adams 1999).
Cyclohexanone resin can be added to increase the hardness and water resistance of any paint but is most often used in floor paints. Paint may contain 5% cyclohexanone resin. Various manufacturers make cyclohexanone resins (Bruze et al. 1988).
Paint solvents include water and organic solvents. They are used to modify the paint viscosity required for the application methods of brushing, rolling dipping, and spraying. Solvents are chosen for their solvency, evaporation, and suitability for product use (Leira 1997; Estlander et al. 2000).
Solvent-based paints contain about 50% organic solvents, which is the volatile component of paints, Until the 1970s, turpentine was the most often used solvent in many countries in construction paints, but it was later replaced by aliphatic and alicyclic hydrocarbon solvents, such as white spirits. Paints used for other purposes may also contain xylene, toluene, alcohols (e.g., n-butanol and isopropanol), and ketones. The use of D-limonene, the principal component of the oil extracted from citrus fruit rinds, has expanded dramatically. It may also be used in paints and other coatings (van Faassen and Borm 1991; Wieslander et al. 1994; Rose and Vance 1997; Estlander et al. 2000).
Solvent-based paints dominated the construction paint market until the 1970s (Hansen et al. 1987). New environmental regulations and consumer demand have led to the development of low VOC and zero VOC paints and finishes. The first latex-type water-based paint was introduced in 1957 (Rose and Vance 1997; Wieslander et al. 1994).
Water-based paints contain water-dispersible polymer binders. They can also contain alkyl resin and a mixture of polyacrylate and polyurethane binders. Although water is the main solvent in these types of paints, comprising about 30–85% of the raw materials, up to 10% organic solvents may be used to improve the film-forming properties of the paint. Totally solvent-free paints and varnishes (VOC less than 0.1%) for decorate finishes are available. Powder paints are also free from solvents (Wieslander and Norback 1997; Fischer and Adams 1999; Estlander et al. 2000; Kaukiainen 2005).
Pigments are fine ground powders dispersed throughout the paint to give it a color. They also have other properties: they can alter paint flow and provide corrosion resistance. Pigments also have limited solubility to water and solvents, and good color fastness. They must be opaque but may be either inorganic or organic (Rose and Vance 1997).
The most commonly used inorganic pigment is white titanium dioxide, which can be used in combination with zinc oxide. Titanium dioxide is also one of the most broadly applied nanomaterialsand is used in numerous other commercial products (Aitken et al. 2006). Other white pigments include lithopone white (mainly consisting of zinc sulfate and barium sulfate), zinc oxide, and antimony trioxide. The use of white lead (basic lead carbonate) has been banned in most countries because it can cause lead poisoning, although paint with significant lead content may still be used in industry and by the military. Red pigments include iron oxides and Cadmium Red (containing cadmium sulfide and cadmium selenide). Yellow pigments include lead chromate, strontium chromate, zinc chromate, and lead chromate. Chrome Green (containing lead chromate and ferric ferrocyanide) and chromium oxide are examples of green pigments. Blue color is obtained using, e.g., a certain iron oxide, and violet color with, e.g., manganese ammonium pyrophosphate. Carbon black is the most commonly used black pigment (Rose and Vance 1997; Fischer and Adams 1999).
Organic pigments are used for special purposes. They are generally purer but more expensive. Examples include Para Red, Lithol Reds, Pigment Red 170, Toluidine Red, Phthalocyanine Green and Blues, and Dioxazine Violet.
Extenders are fillers used in paint pigment. They do not provide as much cover as primary pigments and have a significant impact on the overall characteristics and performance of the paint, including durability, scrubbability, and the retention of color. Extender pigments are mainly made from clay, silica, talc, and chalk (Fischer and Adams 1999; Estlander et al. 2000).
Additives are used in paints in small percentages to ensure, e.g., their stability, quality, and desired application properties. Additives include biocides, driers, emulsifiers or surfactants, thixotropic agents (thickeners), plasticizers, stabilizers, antioxidant antiskinning agents, photoinitiators, and corrosion inhibitors.
Biocides are used to prevent growth of microbes (bacteria, fungi) mainly in water-based paints. They are used for conserving the binder and the paint during production and storage. Biocides are effective even after the paint has dried, and thus prolong its life. Oil-based paints do not usually contain antimicrobials, but some exterior paints can contain an antimildew agent. A great number of biocides are available for use in paints. Antifouling agents are used in marine paints and are usually toxic to underwater organisms. These include copper, organic tin, tetramethylthiuram disulfide (TMTD), and zinc carbamates (Estlander et al. 2000).
Driers can feature one or more metal salts, e.g., cobalt, manganese, iron, lead, zinc, and tin naphthenates, oleates, octoactes, and resonates.
Emulsifiers or surfactants include sodium pyrophosphates, dioctyl sodium sulfosuccinate, sodium lauryl sulfate, and non-ionic detergents. They help to maintain pigment-particle dispersion in water-based latex emulsions. Antifoaming agents prevent the formation of foam during the manufacturer and application of water-based latex paints.
Thixotropic agents (thickeners) such as polyamides are added to oil-based paints, whereas cellulose derivatives are used for the same purpose in water-based latex paints.
Plasticizers are added to paints to increase the flexibility of the resinous film. They include dibutyl and dioctyl phthalates, adipic and sebacic acids, and their esters, polyester resins, and castor oil. Coalescing agents include pine oil, butyl cellosolve, and tributyl phosphate; these are volatile substances that temporarily plasticize a liquid coating.
Stabilizers have an effect on the heat and light resistance of paint. Examples are benzophenones in nonpigmented coatings and epoxy resin in paints based on vinyl chloride polymers or copolymers. Ammonia and volatile amines are used to stabilize paint at a pH of 8–9.
Antioxidants or antiskinning agents prevent coatings from drying too early. They include oximes, e.g., butyraldoxime, methyl ethyl ketone oxime, and cyclohexanone oxime. Hydroquinone and substituted phenols are used in some specialized industrial paints.
Photoinitiators are needed in UV-curable products to initiate the polymerization process, e.g., benzophenonones.
Corrosion inhibitors in paints protect metallic surfaces from oxidation.