Types of Coatings
Epoxy coatings are generally two-part curing systems that are mixed with a hardener before application. They are widely used as coatings in (moderate) corrosive environments and are often used for industiral applications applied either in solid or liquid form. The most common coating resin is bisphenol A epoxy. Liquid grades are usually solvent-free systems cured with aliphatic amines, amine adducts, amine terminated polyamides or blends thereof. Bisphenol A epoxies have good chemical resistance to a broad range of chemicals and excellent heat resistance, whereas hardness, abrasion resistance, toughness and adhesion properties depend largely on the cure conditions, hardener-resin ratio and type of hardener. A major drawback of liquid epoxy resins is their relative high viscosity. For this reason, reactive diluents are often added which not only reduce the viscosity but also improve room temperature cure. However, diluents can have a detrimental effect on the performance properties such as hardness and chemical resistance. Diluent-free systems are formulated with bisphenol F resins which have lower viscosities. However, these resins are typically more expensive, and result in less flexible and tough coatings. Other important epoxy resin systems include epoxy esters, novolacs, and epoxy phenolics. The later two resins improve heat, solvent and chemical resistance but result in decreased flexibility and often require heat cure to fully crosslink the resin.
Phenolics (also called Bakelite phenolics) are some of the oldest coating resins. They are often used for very corrosive and high temperature environments and are usually applied in thin coats which have to be baked. When fully cured, they provide very hard, smooth and durable coatings. For example, some baked grades can withstand battery acid, transmission fluids among many other corrosive chemicals. However, unmodifed phenolics have poor resistance to alkalis and are difficult to repair when damaged. For this reason, they are sometimes blended with other resins such as epoxy, alkyds, and polyesters. Phenolic resins adhere well to most metal substrates including steel, tin and aluminum. Due to their low cost and outstanding corrosion and heat resistance, they are widely used as industrial coatings that have to withstand high temperatures and/or corrosive environments. Important applications include tank, drum, pipe and can linings. They are also used for electrical device protection and insulation of wires, motors and wound coils and as linings for food and beverage cans (epoxy-phenolics).
Acrylics are among the most popular and widely used coating resins. Most acrylic paints are water- or solvent based and are available as emulsions (latices), lacquers (lower volume solids), enamels (higher volume solids) and powders (100% solids). Acrylic paints possess excellent color and gloss retention as well as excellent durability and good adhesion to a large number of substrates. In fact, acrylic paints are the most widely used architectural paints. Solvent-based acrylic paints (lacquers and enamles) have generally better weathering and chemical resistance than water-basd acrylics. However, these paints are mainly used outdoors and have been largely replaced by water-based acrylic paints because of a trend towards tighter restrictions on the volatile organic content (VOC) which are implemented across the world. Water-based acrylic latex paints compete with oil-based (alkyd) paints which have better adhesion and stain blocking but paint thinner is required for clean-up and, therfore, are not recommended for indoor applications. Solvent-borne (pigmented) acrylic enamels produce a superior high gloss when compared to water-based acrylic paints and are extensively used for industrial floor, appliance, coil, and metal furniture coatings and as automobile top coats.
Alkyds are oil modified polyester paints. They are popular inexpensive architectural paints which are often chosen when a high gloss, versatility, and high durability is required. They have better adhesion to heavily chalked surfaces and greater coverage, and coat hiding when compared to acrylic latex paints. However, their color retention is not as good as acrylic paints, and they are usually more difficult to apply due to greater brush drag and slower drying. They also do not adhere well to galvanzied metals unless suitable primer-sealers are applied first.
Alkyds are typically classified as long, medium, and short oil alkyds or oil-free alkyds. These terms represent the oil length in the resin. Common oils including sunflower, linseed, and dehydrated castor oil among many others. Long alkyds can be easily applied by brush and are the main type used in architectural paints whereas short oil alkyds are typically used as industrial (oven) paints. They have greater drying power and produce a harder surface. However, heat is usually required for effective drying. Besides house paints, alkyds are often used for making marine top-coat paints. Very long emulsified oil-drying alkyds are also added to synthetic latex house paints to improve adhesion to chalky painted surfaces, whereas medium to short oil alkyd resins are sometimes blended with silicone resins with high phenyl content for air-dried or baked coatings to improve heat or weather resistance. Some commercially important chemical modifications of alkyd resins include vinylated alkyds, silicone alkyds, urethane alkyds, phenolic alkyds, and polyamide alkyds.
Polyurethanes (PUR) are very versatile polymers with properties that can be tailored over a wide range for a large number of
coating applications. They are known for their excellent chemical
resistance, very good clarity, high gloss, and very good mechanical
properties including hardness, flexibility and abrasion resistance. The main building blocks are diisocyanates and polyols. High
performance urethane coatings are often formulated with aliphatic
isocyantes which impart good light stability and weathering
resistance to the coating whereas aromatic isocyantes have a
tendency to yellow on prolonged exposure to sunlight. The
elastomeric portion of the polyurethane is usually a polyester or
polyether. Polyester-based urethane coatings have better oxidative
stability and higher heat
and abrasion resistance than polyether-based urethane coatings, but
have lower hydrolytic stability and low-temperature flexibility.
PUR coatings are available as either one-part and two-part curing resins. The one-part urethanes harden by either air-drying/solvent evaporation, moisture-cure or heat-cure. Moisture curing urethanes are fequently used in interior and exterior wood coatings whereas heat curing one-part urethanes are often chosen for more demanding coating applications because they show very good overall performance. However, a heat cure process is more costly and inefficient. The two most important two-part urethane systems are 2k catalyzed and 2k ambient-cured urethanes. The later is more common. Both systems are often used for auto refinishing, maintenance and protective coatings.
Acrylic aliphatic urethanes are frequently used in high performance coatings. They are known for their excellent chemical and corrosion resistance, high clarity, and outstanding gloss and color retention. Because of their durability and outstanding weatherability including acid-edge resistance, they are frequently used as OEM automotive clearcoats. The most common acrylic-urethane coating is the two-part (2k) system, where a polyisocyanate resin is mixed with an acrylic polyol. These resins harden via crosslinking and solvent evaporation. Their cure speed can be tailored by varying the catalyst and activator level, and the solvent concentration. Another popular resin system are UV curable (1k) acrylic-urethanes. These resins cure much faster than 1k and 2k acrylic urethane catalyzed coatings and contain little to no VOCs and, thus, are more environment-friendly and economical. However, UV curable resins are often not as tough and their weatherability is less good whereas their scratch resistance is excellent and superior to that of 2k urethanes.
Chlorinated rubber paints are one-part, easy to apply and fast drying solvent-borne thermoplastic resin systems that adhere well to many surfaces. When dried, they form hard, inert and very durable films that withstand dilute acids, alkalis, oxidizing agents and are resistant to mold, bacteria and fungus. The dried chlorinated rubbers are also nontoxic and nonflammable. Despite all these advantages, chorinated rubber paints have been replaced in many applications by other paints because they release large amounts of solvent during drying and, thus, do not meet today's requirements of low VOC. But there are still some applications where chorinated rubber paints find wide-spread use due to their excellent corrosion resistance and antifouling properties. Noteworthy applications include swimming pool coatings, roadway markings, printing inks, and marine coatings.
Slicones are an important class of inorganic resins that are known for their high temperature and oxidative stability, excellent low temperature flexibility, and high resistance to weathering and many chemicals. Pure silicone paints are fully transparent, have outstanding UV resistance and retain their physical properties over a wide temperature range. They can be formulated to withstand temperatures upto 1200ºF (650ºC). However, these coatings are rather expensive and do not meet the requirements of all applications. To improve certain properties and to lower cost, they are often blended with organic resins such as alkyds, polyesters, acrylics and epoxies. However, this comes at the expense of their heat and chemical resistance.
Silicone coatings find many uses in a large number of industries including building & construction, marine, industrial machineries, medical devices, and automotive & transportation. Important applications include release coats, glazes for baking pans, trays and small kitchen appliances, water-repellent masonary/roof coatings and anti-fouling protection.