Acid catalysed formaldehyde resin

Novolac resins are generally prepared by the acid-catalysed reaction of phenol and aqueous formaldehyde under reflux. Although strong acids such as sulphuric and hydrochloric acid can be used, the weaker oxalic and phosphoric acids give a less exothermic and more... 

Bio-based composite materials were obtained by acid catalysed curing of ESO in the presence of terpene modified phenol-formaldehyde resin or rosin modified phenol-formaldehyde resin. These composites exhibited improved mechanical properties when compared with those of the ESO... 

Acrysol WS-68 is a small particle size emulsion of an acrylic copolymer containing acid groups which solubilise the polymer when the amine is added. Cymel 303 is monomeric melamine-formaldehyde resin which crosslinks the acid groups in the acrylic copolymer. The p-toluene sulphonic acid catalyses the crosslinking reaction after the DMAE evaporates during the stoving operation. 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Fig. 21. Hydrolysis of acetals at 20°C on a Dowex 50W X10 resin catalyst [513]. Rate coefficients of the resin-catalysed reaction (feres) versus rate coefficients of the reaction catalysed by dissolved inorganic acid (fehom)- 1 Formaldehyde dimethylacetal 2, formaldehyde diethylacetal 3, formaldehyde di-2-propylacetal 4, acetaldehyde ethyleneacetal 5, acetone ethyleneacetal 6, acetaldehyde dimethylacetal 7, acetaldehyde diethylacetal. The slope for acetals 1—3 is 1, for the acetals 3—7 0.5.

The two examples illustrative of the section, however, involve alternative procedures for introducing the required substituent into the a-position of the acet-amidomalonate reagent. In the first (Expt 5.183) the hydroxymethyl group is introduced by a simple base-catalysed condensation with formaldehyde subsequent hydrolysis and decarboxylation yields serine (R = CH2OH). In this case, acidic conditions are preferred for the final hydrolytic stage, and the use of a weakly basic ion exchange resin to obtain the halide-free amino acid from a solution of its hydrochloride is described. 

Acrylamide based acrylics are capable of self crosslinking, when subjected to temperatures in excess of ISiy C. Crosslinking is a complex mixture of condensation reactions and side reactions involving the liberation of water, formaldehyde, primary alcohol and ethers. The reaction is catalysed by acids. Bisphenol A epoxy resins are often incorporated into acrylamide acrylic coating formulations to improve performance. This epoxy resin also takes part in the curing reaction. Figure 4-2 lists the major reactions which take place. 

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