（Jinwei Chemical car paint manufacturing base.）
The resin is the film-forming component that identifies the paint. A variety of resins and polymers (materials that will undergo reaction to form a resin) are used in paints. The formulation of a paint material and the particular chemicals used are influenced by the particular resin or resin combination used. The blend of resin (sometimes referred to as the binder) and solvent is sometimes referred to as the paint vehicle.
Paint binders may be referred to as convertible and nonconvertible types. Convertible paints are materials that are used in an unpolymerized or partially polymerized state and undergo reaction (polymerization) to form a solid film after application to the substrate. Nonconvertible paints are based on polymerized binders dispersed or dissolved in a medium that evaporates after the coating has been applied to leave a coherent film on the substrate surface.
Convertible binders include oils, oleoresinous varnishes, alkyds, amino resins,epoxy resins, phenolic resins, polyurethane resins, and thermosetting acrylics.
Nonconvertible resins such as cellulose, nitrocellulose, chlorinated rubber, andvinyl resins will not be covered in this text, because they are usually used in lowsolid,high-solvent content coatings that are not compliant for the majority of modern industrial applications.
OILS AND OLEORESINOUS VARNISHES
Oils were commonly used in paint formulation but have declined as improved polymers were developed that could be used for a broader variety of coatings. With the exception of limited use of refined linseed oil and linseed stand oil in certain types of primers for steel and timber, oils are rarely used as the binder in an industrial paint. However, oils are often used as modifying components in
the preparation of oleoresinous varnishes and especially in the oil-modified alkyd resins, one of the most widely used binders in modern paint technology.
Vegetable oils are generally classified as either drying or nondrying oils on the basis of whether they will react with air temperatures to form a rubbery, poor solvent-soluble film.
For example, linseed and tung oil are referred to as drying oils, because they will readily dry when exposed to oxygen. Another group of oils that includes soybean oil is called semidrying oils, because they do not dry as quickly.
Some oils, such as castor oil or olive oil, are classified as nondrying oils. Nondrying oils like castor may be converted to drying oil by heating to 280◦C for several hours to break down some of the constituent acids of the oil.
The drying process is a complex oxidation reaction involving the centers ofunsaturation in the carbon chains of the fatty acid triglycerides of the drying oil. The reaction is referred to as oxidative polymerization or auto-oxidation. Oxidation of a drying oil and film formation will occur naturally, but the rate of reaction is slow and so it is usually accelerated by the addition of driers such as cobalt or lead naphthenates.
Linseed oil as extracted from the flax plant is dull and cloudy and needs to be refined to remove the natural impurities. The method of refining depends on the purpose for which the oil is to be used. The oil can be treated with alkalis, acids, or bleaching processes to lighten the color of the film.
Alkali refining is more commonly used than acid refining. In this process, the oil is treated with sodium hydroxide solution in a calculated quantity to neutralize the acid value of the oil. The acid value is the number of free acid groups in a polymer. The number is actually the milligrams of potassium hydroxide required to neutralize one gram of the polymer. The resultant soap is then removed by centrifuging. The refined oil is then washed with water to remove all traces of the alkaline material. Although certain treatments are needed to clean up linseed oil, it is still used in an
The refined linseed oils are usually low in viscosity and need to be heat treated to produce thicker linseed stand oil. The oil that is processed by heat-treating (270–300◦C) will thicken from polymerization. This makes a more flexible and durable coating, but one that takes longer to dry.
Alkyd resins are modified with a number of oils including soya, linseed, dehydrated castor, and coconut. They in turn can be combined with such resins as acrylics, vinyl toluene, silicones, and amino-resins. The latitude of compatibility of the oil-modified resins makes them popular for use in industrial coatings. They are fairly inexpensive and they have a variety of properties.
Alkyds can be prepared directly from oil (triglyceride), a polyol, or an acid. The percentage of oil contained in an alkyd classifies the end use of the alkyd and affects such properties as speed of drying, flexibility, durability, and so forth. Two manufacturing methods are used in the preparation of alkyd resins: the alcoholysis process, by which oil-modified alkyds are produced, and the fatty acid process, which is used to prepare fatty acid-modified alkyd resins.
The fatty acid resins tend to have a paler color than the oil-modified resins because of the greater purity of the fatty acids. In the fatty acid process, all three components,the fatty acid, the polyhydric alcohol, and the polycarboxylic acid, are heated together at temperatures in the range of 240◦C until esterification of polymerization is complete. In the alcoholysis process, the polyhydric alcohol and the modifying oil are first reacted together and then the polycarboxylic acid is added. Both processes yield oil-modified alkyds, classified on the basis of the amount and type of oil used as shown in Table 2.1.
Drying and semidrying alkyds cure by auto-oxidation, so they are good resins to use in formulation of air-dry coatings. Nondrying oil alkyds, particularly the short-oil bresins, require exposure to heat for curing. They can also be used as plasticizers for other synthetic resin systems. The composition and simplified structures of a short, medium, and long oil alkyd are shown in Table 2.1.
Polyester resins are typically used in heat-cured coatings that need to be high in paint solids and low in solvent content. They have extremely good color retention that provides good over-bake protection and very good UV resistance, and they can be applied using a wide range of spray equipment. Polyesters are very similar in chemical structure to the earlier-discussed Type I oil-free alkyd resins. Polyesters are often formulated in two-component products that are used for plastics. The catalyst system in the 2K products allows them to be cured at low temperature, and the finished film has excellent appearance and UV resistance.
In the coatings and plastic fields, the term “acrylic” resin applies to the polymers and copolymers of the esters of methacrylic and acrylic acids. Copolymers of these esters with nonacrylic monomers such as styrene, butadiene, or vinyl acetate are also referred to as acrylic resins.
Acrylic resins are very versatile and popular for industrial liquid coatings. They provide toughness, good weathering ability, and resistance to abrasion and chemical attack. They are also considered to be better than alkyd resins for gloss retention.Acrylic thermosetting resins that are cross-linked with epoxy or amino resins are used in the appliance industry because of their excellent physical and chemical properties.
Solid and solution polymers are used in lacquer finishes for metal, wood, leather,ceramic, and plastic surfaces. Acrylic emulsions, which are manufactured directly in the latex form, are used in both indoor and outdoor paints. Along with the stability of the acrylic resins and their toughness and chemical resistance, they also provide an appealing, high-quality coating. The hardness and slip properties can be varied over a wide range to suit the demands of the application. Adhesion and solvent resistance
can be built into acrylic coatings by adding other functional monomers into the resin system by copolymerization.
Acrylic polymers are used for decorative and protective functions. For decorative applications, they provide high gloss, good pigment binding characteristics,and clarity. For protective applications, they provide good adhesion, hardness, and durability.
Acrylic lacquers are used for both ferrous and nonferrous substrates. Lacquers are solutions of resins in organic solvents that harden as the result of the evaporation of the solvent. This can be forced or accelerated with heat. The acrylic can be either the major or minor portion of the lacquer.
There are thermoplastic lacquers that allow thermal reflow and better dispersability of metallic colors than thermoset enamels. Applied over a primer and baked, the acrylic lacquers resist many years of outdoor exposure.
Harder acrylics made of almost straight polymethyl methacrylate resin systems are used to make hard, durable finishes for toys. Automotive touch-up lacquers in aerosol cans are another large market. Plastic substrates are a growing market for acrylic coatings, especially since there are now more grades of acrylic polymers that are soluble in solvents that do not attack the plastic. These solvents include ethanol, isopropanol, naphtha, and straight-chain hydrocarbons. The resin systems are usually
low to medium molecular weight homopolymers and copolymers of ethyl and n-butyl methacrylate.
Thermosetting acrylics are generally harder, tougher, and more resistant to heat and solvents than the thermoplastics. They are less resistant to UVlight and of course,they must be heat cured to obtain cross-linking. Thermosetting acrylics are easierto apply. The relatively low molecular weight copolymers of methacrylate with other acrylic or nonacrylic monomers constitute the uncured resin. These functional monomers provide sites for subsequent cross-linking, usually by reaction with nonacrylic additives. These low molecular weight copolymers make it possible to apply high solids, low volatile organic compound (VOC) coatings, which will level much better before cross-linking and result in a smoother coating.
Most thermosetting acrylic resins available for commercial coating applications contain relatively high amounts of styrene, vinyl-toluene, epoxies, or amine resins to enhance their in-use properties, to lower their cost, or to effect cross-linking. The term“acrylic thermoset” is even applied to coatings that have only the acrylic monomer present to establish the curing or cross-linking sites for the coating. Because of the nonacrylic nature of these coatings, they may not exhibit the characteristic properties normally sought in an acrylic coating.
Amino resins are generally used in baked coatings as cross-linking agents. They are used in proportions up to 50% of the total vehicle binder. They can be used with alkyds, polyesters, epoxies, thermosetting acrylics, phenolics, and other heat reactive resins. Melamine and urea–formaldehyde are the most common examples of this resin.
Epoxy resins are known for their excellent corrosion and chemical resistance. Because of their tendency to fade and chalk when exposed to sunlight, they are used for interior topcoat applications or as primer for exterior applications.
Excellent corrosion resistance can be obtained with film thickness as low as 0.5 mil. The epoxy resin is usually cross-linked with melamine or urea resin at curing temperatures of 175–218◦C. Epoxy coatings are characterized by excellent adhesion, a high degree of impact and abrasion resistance, and resistance to chemicals and solvents. This combination of properties makes the epoxy formulas a good fit for chemical laboratory furniture and similar applications. They also have good insulating
properties, making them a good fit for the electrical industry, and they can provide excellent wear on tools.
Epoxy resins are rarely used as-is in paint formulations because of their low molecular weight. The low molecular weight does not allow adequate film builds, so they are usually cured into higher molecular weight complex polymers called epoxy glycides. The terminal or reactive groups are epoxy and secondary hydroxyl in nature. These complex molecules are then cross-linked with commonly used curing agents.
The most commonly used epoxy paint system is the two-component amine-cured coating where the resin and hardener are mixed just prior to application. The hardener creates a reaction that will create a hardened film after the components are mixed. The pot life (time that it takes for the coating to harden) can range from a few minutes to several days depending on the formulation. Heat is often used in industrial applications to accelerate the cure process.
Epoxy formulas are used as maintenance coatings, can coatings, pipeline coatings, tank linings, and so forth. They have also been formulated using special extenders and thixotroping agents to allow one coat film builds of 6–40 mil. Epoxy formulas are also often used as a primer like auto paint sealer.
Epoxy resins can also be used in epoxy-acrylic thermosetting systems to provide a combination of film hardness, mar resistance, gloss and color retention, and chemical resistance. These formulas make good coatings for major household appliances and automobiles.
Urethane is the accepted description for a group of polymers that are sometimes called polyurethanes. Urethane resins are very popular with formulators, providing a combination of chemical resistance, toughness and abrasion resistance, and exterior durability. Effective application on plastics has led to substantial growth in urethane technology.
Urethanes are the reaction products of isocyanates with materials that have hydroxyl groups. They contain a significant number of urethane groups regardless of what the rest of the molecule may be. The basic chemistry of isocyanates and urethanes has been known for over a hundred years. In 1848,Wurtz prepared methyl and ethyl isocyanates by reacting potassium cyanate and alkyd iodides. He also found that ethyl isocyanate reacted with ethyl alcohol to form ethyl carbamate, which was
later named urethane.
The modern industrial development of urethane polymers stems largely from the pioneering work of Professor Otto Bayer in Germany in 1937. The basic raw materials, the isocyanates, are produced in a number of different forms, but the most widely used is toluene diisocyanate (TDI). The TDI has a noticeable vapor pressure and has an irritant effect on the mucous membranes and so requires special handling.
Table 2.2 compares some of the properties of the various resins discussed. Many different vehicle systems are used for various applications and environmental conditions with a wide range of properties as shown above.
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