Cross-linking formaldehyde reactions

As noted previously (Section 12.4.2), the conversion of novolaks into network polymers can be accomplished only after the addition of a cross-linking agent. Although novolaks can be cross-linked by reaction with additional formaldehyde or with paraform, hexamethylenetetramine is almost invariably used for this purpose. The mechanism of the curing process is not fully understood. 

Linear polymers having functional groups can be cross-linked using suitable polyfunctional agents. Thus cellulosic fibers (cotton, rayon) are cross-linked by reaction of the hydroxyl groups of cellulose with formaldehyde, diepoxides, diisocyanates, and various methylol compounds such as urea-formaldehyde prepolymers, NiiV -dimethylol-N,AT -dimethylene urea, and trimethylomelamine. Cross-linking imparts crease and wrinkle resistance and results in crease-resistant fabrics. 

Epoxy novolac resins are produced by glycidation of the low-molecular-weight reaction products of phenol (or cresol) with formaldehyde. Highly cross-linked systems are formed that have superior performance at elevated temperatures. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

Methylol Formation. Polyacrylamide reacts with formaldehyde to form an /V-methylo1 derivative. The reaction is conducted at pH 7—8.8 to avoid cross-linking, which will occur at lower pH. The copolymer can also be prepared by copolymerizing acrylamide with commercially available A/-methylolacrylamide [924-42-5] C4H2NO2. These derivatives are useful in several mining appHcations (49,50). They are also useful as chemical grouts. 

In the case of phenoHcs, it is possible to make linear thermoplastic polymers called novolaks, but this is done by reaction of less than one mole of formaldehyde with one mole of phenol the resulting resin has a large excess of free phenol. Usually in appHcation hexamethylene tetramine (HEXA) is added to the novolak. When heated, the HEXA breaks down into ammonia and formaldehyde and enters the reaction to form a light degree of cross-links in the final product. 

Many friction material formulations contain 5—15 wt % of friction particles, the granulated cross-linked products of the reaction of CNSL, a phenol substituted at the meta position with a unsaturated side chain, and formaldehyde. Friction particles range in size from 50 to 500 p.m. They reduce frictional wear and increase pedal softness (86). 

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

Amino Resins. Amino resins (qv) include both urea- and melamine—formaldehyde condensation products. They are thermosets prepared similarly by the reaction of the amino groups in urea [57-13-6] or melamine [108-78-1] with formaldehyde to form the corresponding methylol derivatives, which are soluble in water or ethanol. To form plywood, particle board, and other wood products for adhesive or bonding purposes, a Hquid resin is mixed with some acid catalyst and sprayed on the boards or granules, then cured and cross-linked under heat and pressure. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

Nail hardeners have been based on various proteia cross-linking agents. Only formaldehyde is widely used commercially. Contact with skin and inhalation must be avoided to preclude sensiti2ation and other adverse reactions. The popularity of products of this type is decreasiag because the polymers used ia nail elongators can be used to coat nails to iacrease the mechanical strength. 

The first type includes vulcanising agents, such as sulphur, selenium and sulphur monochloride, for diene rubbers formaldehyde for phenolics diisocyanates for reaction with hydrogen atoms in polyesters and polyethers and polyamines in fluoroelastomers and epoxide resins. Perhaps the most well-known cross-linking initiators are peroxides, which initiate a double-bond... 

The liberation of small amounts of formaldehyde has been detected in the initial stage but it has been observed that this is used up during later reaction. This does not necessarily indicate that formaldehyde is essential to cross-linking, and it would appear that its absorption is due to some minor side reaction. 

Chcmically, Bakelite is a phenolic resin, produced by reaction of phenol and formaldehyde. On heating, water is eliminated, many cross-links form, and the polymer sets into a rocklike mass. The cross-linking in Bakelite and other thermosetting resins is three-dimensional and is so extensive that we can t really speak of polymer "chains." A piece of Bakelite is essentially one large molecule. 

Baekeland had to make important discoveries before he could bridge the gap between the initial concept and final products. In particular, he found that the base-catalysed condensation of phenol and formaldehyde can be carried out in two parts. If the process is carefully controlled, an intermediate product can be isolated, either as a liquid or a solid, depending on the extent of reaction. At this stage, the material consists of essentially linear molecules and is both fusible and soluble in appropriate solvents. When heated under pressure to 150 °C, this intermediate is converted to the hard, infusible solid known as bakelite . This second stage is the one at which the three-dimensional cross-linked network develops. 

Fraenkel-Conrat H, Olcott HS. Reaction of formaldehyde with proteins. VI. Cross-linking of amino groups with phenol, imidazole, or indole groups. J. Biol. Chem. 1948 174 827-843. 

Fraenkel-Conrat H, Olcott HS. The reaction of formaldehyde with proteins. V. Cross-linking between amino and primary amide or guanidyl groups. J. Am. Chem. Soc. 1948 70 2673-2684. 

Some fixatives work by combining with tissue molecules, hence the term addition reactions. This may continue as cross-linking, whereby the original adducted (added-onto) molecule attaches to another portion of the same molecule or to an adjacent molecule. A small branched polymer is thus created. Formaldehyde is the prime example of an additive and cross-linking fixative. 

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