Novolak resin cross-linking

Between 10 and 15 parts of hexa are used in typical moulding compositions. The mechanism by which it cross-links novolak resins is not fully understood but it appears capable of supplying the requisite methylene bridges required for cross-linking. It also functions as a promoter for the hardening reaction. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

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. 

Other Reactants. Other reactants are used in smaller amounts to provide phenoHc resins that have specific properties, especially coatings appHcations. Aniline had been incorporated into both resoles and novolaks but this practice has been generally discontinued because of the toxicity of aromatic amines. Other materials include rosin (abietic acid), dicyclopentadiene, unsaturated oils such as tung oil and linseed oil, and polyvalent cations for cross-linking. 

Novolaks. Novolak resins are typically cured with 5—15% hexa as the cross-linking agent. The reaction mechanism and reactive intermediates have been studied by classical chemical techniques (3,4) and the results showed that as much as 75% of nitrogen is chemically bound. More recent studies of resin cure (42—45) have made use of tga, dta, gc, k, and nmr (15). They confirm that the cure begins with the formation of benzoxazine (12), progresses through a benzyl amine intermediate, and finally forms (hydroxy)diphenyknethanes (DPM). 

The multiepoxy functionality of the epoxy novolaks (2.2 to >5 epoxy groups per molecule) (3) produce more tightly cross-linked cured systems having improved elevated temperature performance and chemical resistance than the difunctional bisphenol A-based resins. 

The novolak resins themselves contain no reactive methylol groups and do not form cross-linked structures on heating. If, however, they are mixed with compounds capable of forming methylene bridges, e.g. hexamethylenetetramine or paraformaldehyde, they cross-link on heating to form infusible, thermoset structures. 

The PE fibres are produced by melt spinning a novolak resin of molecular weight ca 1000 and then cross-linking the molecules by exposure to gaseous formaldehyde at 100-150°C for 6-8 h or with a formaldehyde solution. The fibres were introduced under the tradename Kynol by American Kynol Inc., a subsidiary of Carborundum AG. 

Novolak resins with epichlorohydrin are thermally cross-linked, and the resultant polymer layers show excellent mechanical and dielectric properties... 

Chlorinated Novolak Resins. Mixtures of a cresol formaldehyde Novolak resin and a photoactive compound cross-link at electron doses far smaller than the dose required for the Novolak resin alone (11). The reason for this accelerated cross-linking is the reactions between the ketene (an intermediate formed from the photoactive compound upon irradiation) and the Novolak resin. This reaction may be reduced by using a Novolak resin modified for this purpose, or by using certain additives. The rationale for developing a halogen-substituted Novolak resin is the control of the reaction between the intermediate ketene and the Novolak. 

Imide blends are usually cross-linked and inherently stable strucmres. The additivity rule seems to be appropriate for blends with Novolak resins [Kundu et al, 1986] and the addition of a phosphine oxide to bismaleimide lowered the curing temperature without sacrifice of thermal properties [Varma and Mittal, 1989]. 

Crosslinking in phenol-formaldehyde resins is carried out on essentially linear prepolymers which have been formed by having one of the components in sufficient excess to minimise cross-linking during the initial step. These prepolymers may be one of two kinds the so-called resoles or the so-called novolaks. 

Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. A novolak resin has no reactive methylol groups in its molecules and therefore without hardening agents is incapable of condensing with other novolak molecules on heating. To complete resinification, further formaldehyde is added to cross-link the novolak resin. Phenolic rings are considerably less active as necleophilic centers at an acid pH, due to hydroxyl and ring protonation. 

Novolaks are mixtures of isomeric polynuclear phenols of various chain lengths with an average of five to six phenolic nuclei per molecule. They contain no reactive methylol groups and consequently cross-link and harden to form infusible and insoluble resins only when mixed with compounds that can release formaldehyde and form methylene bridges (such as paraformaldehyde or hexamethylenetetramine). 

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