Novolac crosslinking

TABLE 7.10 Flame Retardance of Networks Prepared from Phenolic Novolac Crosslinked with Various Epoxies... 

Novolac PEMA Novolac crosslinked with hexamethylene tetramine Semi- and full IPNs prepared two phase behavior 185... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

The term novolac refers to the early use of phenolic to replace expensive shellac-based coatings. Novolacs are now those resins made at formaldehyde-to-phenol molar ratios of less than one-to-one. They are generally, though not always, manufactured under acidic conditions. Sulfuric or oxalic acids are most often chosen as catalyst though aromatic sulfonic acids and phosphoric acid are also quite common. Many other acids are used for special purposes. The finished novolac resin is incapable of further polymerization or crosslinking and therefore... 

An alternative copolymerization is illustrated by the method of Blasius. In this preparation, a phenol-formaldehyde (novolac) type system is formed. Monobenzo-18-crown-6, for example, is treated with a phenol (or alkylated aromatic like xylene) and formaldehyde in the presence of acid. As expected for this type of reaction, a highly crosslinked resin results. The method is illustrated in Eq. (6.23). It should also be noted that the additional aromatic can be left out and a crown-formaldehyde copolymer can be prepared in analogy to (6.22). ... 

Used for crosslinking novolacs or catalyzing resole syntheses, HMTA is prepared by reacting formaldehyde with ammonia (Fig. 7.5). The reaction is reversible at high temperatures, especially above 250°C. HMTA can also be hydrolyzed in the presence of water. 

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... 

Since a small amount of water is always present in novolac resins, it has also been suggested that some decomposition of HMTA proceeds by hydrolysis, leading to the elimination of formaldehyde and amino-methylol compounds (Fig. 7.15).42 Phenols can react with the formaldehyde elimination product to extend the novolac chain or form methylene-bridged crosslinks. Alternatively, phenol can react with amino-methylol intermediates in combination with formaldehyde to produce ortho-or para-hydroxybenzylamines (i.e., Mannich-type reactions). 

Resole resins are generally crosslinked under neutral conditions between 130 and 200° C or in the presence of an acid catalyst such as hydrochloric acid, phosphoric acid, p-toluenesulfonic acid, and phenolsulfonic acid under ambient conditions.3 The mechanisms for crosslinking under acidic conditions are similar to acid-catalyzed novolac formation. Quinone methides are the key reaction intermediates. Further condensation reactions in resole resin syntheses under basic conditions at elevated temperatures also lead to crosslinking. 

Void-free phenolic networks can be prepared by crosslinking novolacs with epoxies instead of HMTA. A variety of difunctional and multifunctional epoxy reagents can be used to generate networks with excellent dielectric properties.2 One example of epoxy reagents used in diis manner is the epoxidized novolac (Fig. 7.34) derived from the reaction of novolac oligomers with an excess of epichlorohydrin. 

A biphenyl diglycidyl ether based epoxy resin was crosslinked with aminecuring agents (d -diaminodiphenylmethane and aniline novolac) and phenolcuring agents (phenol novolac and catechol novolac), and the thermomechanical... 

Benzoxazines are heterocyclic compounds obtained from reaction of phenols, primary amines, and formaldehyde.98,99 As described previously, they are key reaction intermediates in the HMTA-novolac cure reaction.40,43 Crosslinking benzoxazine monomers at high temperatures gives rise to void-free networks with high Tgs, excellent heat resistance, good flame retardance, and low smoke toxicity.100 As in HMTA-cured novolac networks, further structural rearrangement may occur at higher temperatures. 

Novolac hydroxyl groups reacted with cyanogen bromide under basic conditions to produce cyanate ester resins (Fig. 7.41).105,106 Cyanate esters can thermally crosslink to form void-free networks, wherein at least some triazine rings form. The resultant networks possess high s, high char yields at 900°C, and high decomposition temperatures.105... 

Novolac network degradation mechanisms vary from tiiose of resole networks due to differences in crosslinking metiiods. Nitrogen-containing linkages must also be considered when HMTA (or other crosslinking agent) was used to cure novolac networks. For example, tribenzylamines, formed in HMTA-cured novolac networks, decompose to cresols and azomethines (Fig. 7.50). 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

The quantitative nature of the silylamine-phenol reaction has been demonstrated for several different polymer systems (7). In our case, the charged PDMSX content was low to ensure that <1 phenolic group per novolac molecule reacted. This was done primarily to prevent extensive branching or crosslinking, and problems of insolubility and reproducibility associated with network formation... 

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