Formaldehyde resins

Urea is largely used as a fertilizer (ISy ), and as a non-protein feed supplement for sheep and cattle. The most important chemical use, which however accounts for only a small part of urea production, is in the manufacture of urea-formaldehyde resins. U is also used in the manufacture of adhesives, pharmaceuticals, dyes and various other materials. U.S. production 1981 7 0 megatonnes urea resins 1983 6 megatonnes. 

Ion-exchange resins. The constituent phenolic hydroxyl groups in the insoluble phenol-formaldehyde resins react with cations of salts ... 

The formation of various types of phenol - formaldehyde resins (the so-called Bakelite) may be demonstrated by the following experiments. 

The use of hydroxyethyl (also hydroxypropyl) methacrylate as a monomer permits the introduction of reactive hydroxyl groups into the copolymers. This offers the possibility for subsequent cross-linking with an HO-reactive difunctional agent (diisocyanate, diepoxide, or melamine-formaldehyde resin). Hydroxyl groups promote adhesion to polar substrates. 

Phenol-Formaldehyde Resin. Phenol-formaldehyde resin is prepared as follows ... 

The reaction of urea with formaldehyde yields the following products, which are used as monomers in the preparation of urea formaldehyde resin. 

The reaction conditions can be varied so that only one of those monomers is formed. 1-Hydroxy-methylurea and l,3-bis(hydroxymethyl)urea condense in the presence of an acid catalyst to produce urea formaldehyde resins. A wide variety of resins can be obtained by careful selection of the pH, reaction temperature, reactant ratio, amino monomer, and degree of polymerization. If the reaction is carried far enough, an infusible polymer network is produced. 

Some commercially important cross-linked polymers go virtually without names. These are heavily and randomly cross-linked polymers which are insoluble and infusible and therefore widely used in the manufacture of such molded items as automobile and household appliance parts. These materials are called resins and, at best, are named by specifying the monomers which go into their production. Often even this information is sketchy. Examples of this situation are provided by phenol-formaldehyde and urea-formaldehyde resins, for which typical structures are given by structures [IV] and [V], respectively ... 

Table 5.7 Calculated Values of a and f for Phenol-formaldehyde Resins Formed from Different Proportions of Reactants and Based on Experimental Values of n and n .

Urea is also used as feed supplement for mminants, where it assists in the utilization of protein. Urea is one of the raw materials for urea—formaldehyde resins. Urea (with ammonia) pyrolyzes at high temperature and pressure to form melamine plastics (see also Cyanamides). Urea is used in the preparation of lysine, an amino acid widely used in poultry feed (see Amino acids Feeds and feed additives, petfoods). It also is used in some pesticides. 

The consumption of urea for urea—formaldehyde resins has decreased in recent years because of the new findings about the toxicity of formaldehyde slowly released by the resin. 

During the late 1970s, concerns were raised about levels of airborne formaldehyde in buildings resulting primarily from constmction using composite panels bonded with urea—formaldehyde resins and combined with energy-efficient building practices which reduced air losses. 

The synthesis in 1912 of phenol—formaldehyde resins by Baekeland, which forms the basis of many modern day adhesives. 

The first cellular synthetic plastic was an unwanted cellular phenol—formaldehyde resin produced by early workers in this field. The elimination of cell formation in these resins, as given by Baekeland in his 1909 heat and pressure patent (2), is generally considered the birth of the plastics industry. The first commercial cellular polymer was sponge mbber, introduced between 1910 and 1920 (3). 

Urea.—Forma.IdehydeResins. Cellular urea—formaldehyde resins can be prepared in the following manner an aqueous solution containing surfactant and catalyst is made into a low density, fine-celled foam by dispersing air into it mechanically. A second aqueous solution consisting of partially cured urea—formaldehyde resin is then mixed into the foam by mechanical agitation. The catalyst in the initial foam causes the dispersed resin to cure in the cellular state. The resultant hardened foam is dried at elevated temperatures. Densities as low as 8 kg/m can be obtained by this method (117). 

Syntactic Cellular Polymers. Syntactic cellular polymer is produced by dispersing rigid, foamed, microscopic particles in a fluid polymer and then stabilizing the system. The particles are generally spheres or microhalloons of phenoHc resin, urea—formaldehyde resin, glass, or siUca, ranging 30—120 lm dia. Commercial microhalloons have densities of approximately 144 kg/m (9 lbs/fT). The fluid polymers used are the usual coating resins, eg, epoxy resin, polyesters, and urea—formaldehyde resin. 

Data on epoxy lesins can be found in Ref. 132 on uiea—formaldehyde resins, Ref 133. 

Foams prepared from phenol—formaldehyde and urea—formaldehyde resins are the only commercial foams that are significantly affected by water (22). Polyurethane foams exhibit a deterioration of properties when subjected to a combination of light, moisture, and heat aging polyester-based foam shows much less hydrolytic stabUity than polyether-based foam (50,199). 

Mono- and dimethylol derivatives are made by reaction of formaldehyde with unsubstituted amides. Dimethylolurea, an item of commercial importance and an intermediate in urea—formaldehyde resins, is formed in high yield under controlled conditions (62) ... 

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