Amino-resins

Amino resins are the resins produced by the reaction between amino group-containing compounds and formaldehyde. The most popular amino resins are urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins. Amino resins are considered to supplement and complement phenolic resins [31, 32]. UF and MF resins are prepared 

Amino resins are condensation thermosetting polymers of formaldehyde with either urea or melamine. Melamine is a condensation product of three urea molecules. It is also prepared from cyanamide at high pressures and temperatures  

The nucleophilic addition reaction of urea to formaldehyde produces mainly monomethylol urea and some dimethylol urea. When the mixture is heated in presence of an acid, condensation occurs, and water is released. This is accompanied by the formation of a cross inked polymer  

A similar reaction occurs between melamine and formaldehyde and produces methylolmelamines  

A variety of methylols are possible due to the availability of six hydrogens in melamine. As with urea formaldehyde resins, polymerization occurs by a condensation reaction and the release of water. 

Amino resins are characterized by being more clear and harder (tensile strength) than phenolics. However, their impact strength (breakabil-ity) and heat resistance are lower. Melamine resins have better heat and moisture resistance and better hardness than their urea analogs. 

The successful use of resins produced by the condensation of formaldehyde with phenol catalyzed the development of other condensation products of formaldehyde with urea and melamine. The urea resins were described by Tollens in 1884 and were patented by John in 1918. 

Contributions to the urea resins technolc were made by Goldschmidt and Neuss in 1921 and by Poliak and Ripper in 1923. Molding resins, based on condensation products of urea and thiourea with formaldehyde (Beetle) were produced in England in 1926. Toledo Scales introduced a urea-formaldehyde molding powder (Plaskon) in 1928. Paper impregnated with urea resin was used as the outer surface layer of Formica decorative laminates in 1931. 

Resins produced by the condensation of formaldehyde with melamine were introduced by Ciba in 1933 and patented by Henkel in 1936. Comparable resins were produced by Palmer Griffith of American Cyanamide Corp. in 1933. This firm produced alpha cellulose-filled melamine molding compounds in 1937 under the trade 

Polymerizations with Formaldehyde Amino Resins (Urea and Melamine) and Phenolics 

These reactions are not very fast at room temperature characteristic reaction times are of the order of minutes. 

The various equilibrium constants have been measured using NMR [202] and a model describing the vapor-liquid equilibrium in that system has been developed. 

Reaction between a water solution of formaldehyde with urea, melamine, and similar molecules (such as acrylamide) leads to hydroxymethylation of the nitrogens 

Since melamine is made from urea and ammonia, it is more expensive. Melamine resins are therefore chosen when one can get an appreciable benefit from their better hydrolytic or thermal resistance. 

The same general comments apply when epoxy resin additions are made to hydroxy acrylic/amino resin combinations. In addition, the presence of epoxy resins in these systems means that the curing temperature must be raised, from 120°C to between 140 and 1S0°C, in order fully to exploit the improvement in performance expected from such modifications. 

Hydroxy functional acrylics will cure with all types of amino resins, including urea formaldehyde, melamine formaldehyde (MF) and benzoguanamine formaldehyde types. Urea formaldehyde resins are faster curing and cheaper than the other two main types, but they are rarely used with acrylics due to inferior film performance (e.g. resistance properties and exterior durability). 

Benzoguanamine based amino resins are not widely used with acrylics due to their inferior flow and compatibility. These characteristics, combined with their cost, make them unattractive crosslinking resins for acrylics. 

Melamine is reacted with formaldehyde to produce up to 6 methylol groups per melamine molecule. 

The properties of the resultant butylated MF resin are governed bv the methylol content, degree of butylation and molecular weight as shown in Table 4-1  

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

Alternatively the methylol groups can react with further amino groups, also evolving a molecule of water in what is another condensation reaction (Reaction 1.11). 

Unlike phenol-formaldehyde polymers, the amino resins are not themselves deeply coloured, but are of a naturally light appearance. They can be easily pigmented to give a variety of shades, which leads to application in uses where good appearance is highly valued, for example in decorative tableware, laminated resins for furniture, and modem white electrical plugs and sockets. 

When crosslinked, amino resins are very resistant to most organic solvents, though they tend to be attacked by both acids and alkalis. Urea-formaldehyde polymers are more susceptible to attack than those prepared from melamine and formaldehyde. 

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Liquid coating resins are prepared by reacting methanol or butanol with the initial hydroxyme-thylureas. Ether exchange reactions between the amino resin and the reactive sites on the polymer produce a cross-linked film. 

Uses. The principal use of adiponitrile is for hydrogenation to hexamethylene diamine leading to nylon-6,6. However, as a result of BASE s new adiponitrile-to-caprolactam process, a significant fraction of ADN produced may find its way into nylon-6 production. Adipoquanamine, which is prepared by the reaction of adiponitrile with dicyandiamide [461-58-5] (cyanoguanidine), may have uses in melamine—urea amino resins (qv) (see "Benzonitrile, Uses"). Its typical Hquid nitrile properties suggest its use as an extractant for aromatic hydrocarbons. 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

Eyrol 51 is a water-soluble Hquid containing about 21% phosphoms. It is made by a multistep process from dimethyl methylphosphonate, phosphoms pentoxide, and ethylene oxide. The end groups are principally primary hydroxyl and the compound can thus be incorporated chemically into aminoplasts, phenoHc resins, and polyurethanes. Eyrol 51, or 58 if diluted with a small amount of isopropanol, is used along with amino resins to produce a flame-retardant resin finish on paper used for automotive air filters, or for backcoating of upholstery fabric to pass the British or California flammabiHty standards. 

Pyrovatex CP coreacts on cellulose with an amino resin in the presence of a latent acid catalyst, to produce finishes durable to laundering (125,126). A higher assay version, Pyrovatex (CP New, has also been introduced. 

Amino and Phenolic Resins. The largest use of formaldehyde is in the manufacture of urea—formaldehyde, phenol—formaldehyde, and melamine—formaldehyde resins, accounting for over one-half (51%) of the total demand (115). These resins find use as adhesives for binding wood products that comprise particle board, fiber board, and plywood. Plywood is the largest market for phenol—formaldehyde resins particle board is the largest for urea—formaldehyde resins. Under certain conditions, urea—formaldehyde resins may release formaldehyde that has been alleged to create health or environmental problems (see Amino RESINS AND PLASTICS). 

The commonly used resins in the manufacture of decorative and industrial laminates ate thermosetting materials. Thermosets ate polymers that form cross-linked networks during processing. These three-dimensional molecules ate of essentially infinite size. Theoretically, the entire cured piece could be one giant molecule. The types of thermosets commonly used in laminates ate phenoHcs, amino resins (melamines), polyesters, and epoxies. 

Formaldehyde. Worldwide, the largest amount of formaldehyde (qv) is consumed in the production of urea—formaldehyde resins, the primary end use of which is found in building products such as plywood and particle board (see Amino resins and plastics). The demand for these resins, and consequently methanol, is greatly influenced by housing demand. In the United States, the greatest market share for formaldehyde is again in the constmction industry. However, a fast-growing market for formaldehyde can be found in the production of acetylenic chemicals, which is driven by the demand for 1,4-butanediol and its subsequent downstream product, spandex fibers (see Fibers, elastomeric). 

Polymers. AH nitro alcohols are sources of formaldehyde for cross-linking in polymers of urea, melamine, phenols, resorcinol, etc (see Amino RESINS AND PLASTICS). Nitrodiols and 2-hydroxymethyl-2-nitro-l,3-propanediol can be used as polyols to form polyester or polyurethane products (see Polyesters Urethane polymers). 2-Methyl-2-nitro-l-propanol is used in tires to promote the adhesion of mbber to tire cord (qv). Nitro alcohols are used as hardening agents in photographic processes, and 2-hydroxymethyl-2-nitro-l,3-propanediol is a cross-linking agent for starch adhesives, polyamides, urea resins, or wool, and in tanning operations (17—25). Wrinkle-resistant fabric with reduced free formaldehyde content is obtained by treatment with... 

Catalysts. The alkanolamines continue to find use as blocked catalysts for textile resins, coatings resins, adhesives, etc. Of particular utifity in curing durable-press textiles is AMP-HCl. Other salts, such as those of the benzoin tosylate or A-toluenesulfonic acid, find utifity in melamine- or urea-based coatings (18) (see Amino resins and plastics). 

Nitrocellulose based lacquers often contain short or medium oil alkyds to improve flexibiUty and adhesion. The most commonly used are short oil non drying alkyds. Amino resins or urethane resins with residual isocyanate functional groups may be added to cross-link the coating film for improved solvent and chemical resistance. The principal appHcations are furniture coatings, top lacquer for printed paper, and automotive refinishing primers. 

Amino resins are thermosetting polymers made by combining an aldehyde with a compound containing an amino (—NH2) group. Urea—formaldehyde (U/F) accounts for over 80% of amino resins melamine—formaldehyde accounts for most of the rest. Other aldehydes and other amino compounds are used to a very minor extent. The first commercially important amino resin appeared about 1930, or some 20 years after the introduction of phenol—formaldehyde resins and plastics (see Phenolic resins). 

Amino resins are manufactured throughout the industrialized world to provide a wide variety of useful products. Adhesives (qv), representing the largest single market, are used to make plywood, chipboard, and sawdust board. Other types are used to make laminated wood beams, parquet flooring, and for furniture assembly (see Wood-BASED composites and laminates). 

Some amino resins are used as additives to modify the properties of other materials. For example, a small amount of amino resin added to textile fabric imparts the familiar wash-and-wear quaUties to shirts and dresses. Automobile tires are strengthened by amino resins which improve the adhesion of mbber to tire cord (qv). A racing sailboat may have a better chance to win because the sails of Dacron polyester have been treated with an amino resin (1). Amino resins can improve the strength of paper even when it is wet. Molding compounds based on amino resins are used for parts of electrical devices, botde and jar caps, molded plastic dinnerware, and buttons. 

Amino resins are also often used for the cure of other resins such as alkyds and reactive acryUc polymers. These polymer systems may contain 5—50% of the amino resin and are commonly used in the flexible backings found on carpets and draperies, as well as in protective surface coatings, particularly the durable baked enamels of appHances, automobiles, etc. 

The term amino resin is usually appHed to the broad class of materials regardless of appHcation, whereas the term aminoplast or sometimes amino plastic is more commonly appHed to thermosetting molding compounds based on amino resins. Amino plastics and resins have been in use since the 1920s. Compared to other segments of the plastics industry, they are mature products, and their growth rate is only about half of that of the plastics industry as a whole. They account for about 3% of the United States plastics and resins production. 

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