Phenolic resins, HMTA curing

Fig. 50. CP MAS 13C NMR spectra of phenolic resin cured with 13C-labeled HMTA (left) and 15N NMR spectra of the same resin cured with 15N-labeled HMTA (right) for 1 h at the temperature indicated. Asterisks denote natural-abundance peaks (adapted from Ref. 218))...

Phenolic resins are prepared by a step-growth polymerisation of formaldehyde and phenol or phenol derivative using an acid or a base catalyst. The product type and the quality largely depend on the ratio of the reactants used and the nature of the catalyst. Phenolic resins are available in two varieties 1) novolac, which is a thermoplastic type and can be used as it is or can be cured with hexamethylene tetramine (HMTA) to get a crosslinked structure. This can also be viewed as a reactive intermediate, and can be transformed into other groups so different types of structures can be generated and 2) resole, which is a multifunctional reactive compound and can be cured thermally without a catalyst or an acid catalyst. 

For this reason, HMTA cured novolac networks are generally brittle in nature and have low fracture energies and impact strengths, probably due to the voids combined with the high crossliiik densities. Curing novolac resins with epoxy resins can produce networks without volatile by products. These can be reinforced with glass or carbon fibers to yield tough, void-fiee composites, which also retain much of the flame-retardant properties of phenolics [39],... 

Phenolic resins such as resol-type resins generate water and formaldehyde during curing as by-products of condensation reactions. Hexamethylenetetramine (HMTA) can be used as a source of formaldehyde for curing novolac-type resins, producing formaldehyde and ammonia as by-products. During curing. 

The most common crosslinking agent for novolac resins is HMTA which provides a source of formaldehyde. Novolac resins prepared from a phenol-formaldehyde (F/P) ratio of 1/0.8 can be cured with 8-15 wt % HMTA, although it has been reported that 9-10 wt % results in networks with the best overall performance.3... 

Hexamethylene tetramine is an important chemical. Its most important application is as a source of formaldehyde for crosslinking phenolic molding powders, shell molding resins, and two-step curing resins for chip board. Quatemization of HMTA with an alkyl chloride (R-Cl) gives a family of bactericides (developed by Dow) for use in latex paints and as a dermatitis preventative in water-soluble cutting oil. Hexamine is used in pharmaceutical formulations to combat urinary tract infections and also as an intermediate in the production of chloramphenicol. 

The substitution of HMTA, normally used to cure novolac-type resins, with epoxies has reduced void content. To retain the fire-retardant properties of phenolics, a network was formed with a higher phenol than epoxy content. 

Due to the acidic nature of phenol, carbonium ions are generated from these a-amino alcohols, which then interact with phenol to form secondary and tertiary benzylamine in a Mannich-type reaction containing chain molecules. This mechanism has been supported by the fact that the reaction rate increases considerably as the pH of the system decreases. Moreover, the presence of water and free phenol has been found to enhance the reaction rate to a great extent. The HMTA hydrolysis reaction and the formation of aminomethylene ions are the slowest reactions and are therefore considered to be the rate-determining step. Formation of oligomers results in the evolution of a gas which consists of about 95% NH3. The cured resin may contain up to 6% chemically bound nitrogen. 

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