Polymerisation formaldehyde resin

Figure 7.25 Further polymerisation of urea-formaldehyde resins.

The route to crosslinked phenol-formaldehyde resins via resoles corre.sponds to that used by Baekeland in his original commercial technique, They now tend to be used for adhesives, binders, and laminates. The resole prepolymers are made typically in batch processes, using a trace of ammonia (about 2% on phenol) as the alkaline catalyst. Care has to be taken with this process since, despite the molar excess of formaldehyde, there is sufficient of each component present in the prepolymer to permit the formation of a highly crosslinked product. Indeed, such a product will form if the resole is heated excessively, but the problem can be avoided by careful attention to the conditions of reaction and by ensuring that polymerisation is not allowed to proceed for too long. 

This is demonstrated by the example of polystyrene with a mol.wt. of 2 400, which has a Vo of only 2 7. In such a case a decision can often be made by carrying the polymerisation to a further stage. When the particles are globular, practically no change of Vo is then to be expected, but when chain-molecules are present, Vo must increase. The results of such an experiment are illustrated in Table 6, where the F o-increase on polymerisation of globular phenol-formaldehyde resin is compared with that of polystyrene,... 

Examples of this can be found with phenol-formaldehyde, urea-formaldehyde, glyptal, and shellac resins (for formulae see p. 39). In Fig. 22 the change of highly elastic deformability is demonstrated for a cresol-formaldehyde resin and for shellac, showing that in both cases a certain degree of polymerisation is necessary for high-elasticity to be observed (the sol must ... 

Thermochromatography was used to compare the usefulness of different catalysts in the polymerisation processes. Figure 4.2 shows thermochromatograms of the degradation products of urea melamine formaldehyde resins cured with equal amounts of AICI3, NH Cl, and FeClj as catalysts. The thermal destruction of the polymers gives two main products formaldehyde and methanol. 

Although there is a substantial body of information in the pubHc domain concerning the preparation of polyacetals, the details of processes for manufacturiag acetal resins are kept highly confidential by the companies that practice them. Nevertheless, enough information is available that reasonably accurate overviews can be surmised. Manufacture of both homopolymer and copolymer involves critical monomer purification operations, discussion of which is outside the scope of this article (see Formaldehyde). Homopolymer and copolymer are manufactured by substantially different processes for accomplishing substantially different polymerisation chemistries. 

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). 

This is used in manufacture of brake linings and is a polymer based on cashew nutshell liquid admixed with formaldehyde or furfuraldehyde and other ingredients. The polymerised resin mixture is cast into 8 cm thick slabs and then ground finely to produce the friction dust. Several fires have been experienced during bulk storage of the dust, attributed to autoxidation of the still partially unsaturated resin compound. Previously, linseed oil was used in place of the nutshell liquid, but fires were then more frequent. 

The polymers can be categorised as formaldehyde containing and formaldehyde free and as thermoset or thermoplastic resins. Typical formaldehyde containing resins are melamine formaldehyde sulfonamide resins, where the sulfonamide is ortho and para toluenesulfonamide. The sulfonamide, which is a solvent for the dye, undergoes a condensation polymerisation with formaldehyde and melamine, the latter acting as a cross-linking agent. Non-formaldehyde, thermoplastic resins are usually polyamides and polyesters. 

As far as calix[4]pyrrole-based materials are concerned, particular emphasis should be placed on the production of chelating resins. Those obtained by the condensation reaction of calix[4]pyrrole and formaldehyde in the presence of formic acid offer potential for their use as extracting agents for fluoride removal from water. Their advantage relies on the fact that a single-step procedure is required for polymerisation. However, research in this area is in a preliminary stage. Much work needs to be done to establish the full capacity of these materials to take up fluoride from water, the kinetics of the process, and the optimum experimental conditions for fluoride extraction. Although, their ability to extract fluoride reaches a value of 88% from solutions of tetra-n-butylammonium fluoride... 

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