Novolac phenolic resins synthesis

Chen-Chi M. Ma, Han-Thing Tseng and Hew-Der Wu, Blocked diisocyanate polyester-toughened Novolac-type phenolic resin Synthesis, characterization, and properties of composites . Journal of Polymer Science, Part B Polymer Physics, 1998, 36,10. 

This chapter emphasizes the recent mechanistic and kinetic findings on phenolic oligomer syntheses and network formation. The synthesis and characterization of both novolac- and resole-type phenolic resins and dieir resulting networks are described. Three types of networks, novolac-hexamethylenetetramine (HMTA),... 

The cross-linked polymers obtained from the addition-cure approach are often a complex arrangement of atoms bonded in heterocyclic and carbocyclic rings. However, the objective in the preparation of these systems is not simplicity. It is to obtain systems with the desirable properties of phenolic resins retained and the undesirable properties improved or removed. Voids are an undesirable result in the synthesis of both resols and novolacs. Hence, addition-cure phenolic resins are designed to avoid this result. Ease and flexibility of processing are also sought in the addition-cure systems. 

A more complicated system exists if a two nucleus phenolic compound is used for the synthesis of phenolics. In 4,4 -(l-methylethylidene)bisphenol, the para positions of the phenolic nuclei are substituted. Therefore, methanal can only react with the ortho positions relative to the hydroxy group. A uncatalyzed reaction of methanal with 4,4 -(l-methylethylidene)bisphenol gives novolac type resins (equation 41). 

Figure 2.1 Reaction mechanism for the synthesis of novolac-type phenolic resin...

I ovolac Synthesis and Properties. Novolac resins used in DNQ-based photoresists are the most complex, the best-studied, the most highly engineered, and the most widely used polymers in microlithography. Novolacs are condensation products of phenoHc monomers (typically cresols or other alkylated phenols) and formaldehyde, formed under acid catalysis. Figure 13 shows the polymerization chemistry and polymer stmcture formed in the step growth polymerization (31) of novolac resins. 

The literature on basic- and acid-catalyzed alkylation of phenol and of its derivatives is wide [1,2], since this class of reactions finds industrial application for the synthesis of several intermediates 2-methylphenol as a monomer for the synthesis of epoxy cresol novolac resin 2,5-dimethylphenol as an intermediate for the synthesis of antiseptics, dyes and antioxidants 2,6-dimethylphenol used for the manufacture of polyphenylenoxide resins, and 2,3,6-trimethylphenol as a starting material for the synthesis of vitamin E. The nature of the products obtained in phenol methylation is affected by the surface characteristics of the catalyst, since catalysts having acid features address the electrophilic substitution in the ortho and para positions with respect to the hydroxy group (steric effects in confined environments may however affect the ortho/para-C-alkylation ratio), while with basic catalysts the ortho positions become the... 

Calculations for a branched advancement synthesis are defined to permit development of well-characterized functionality in the product molecules. In this study, the difunctional epoxy resin monomer used is the diglycidyl ether of bisphenol A, Epon 828, and the multifunctional epoxy phenol novolac resin used is DEN 438. Let ... 

Fig. 13. Polymerization chemistry of phenol—formaldehyde condensation synthesis of novolac resin. The phenol monomer(s) are used in stoichiometric excess to avoid gellation, although branching invariably occurs due to the multiple reactive sites on the aromatic ring.

In the commercial synthesis of Novolac resin there exists the strong possibility that some chain branching will occur (Figure 1). Thus the actual functionality of individual phenols will vary depending on its position in the network. 

While novolac links may form at elevated temperature during resole synthesis (Scheme 1.12), the preferred synthesis of novolac resins takes place under acid conditions (pH from 1 to 4) and with an excess of phenol. In this case the methylol intermediates cannot be isolated since they react rapidly to give methyene-bridged structures with relative molar mass <2000, as shown in Scheme 1.15. 

Thermal synthesis of methylene bridged, substituted phenols in the absence of a catalyst occurs in a melt with paraformaldehyde at temperatures between 150-200°C [33]. A representative reaction is 4,4 -(l-methyl ethylidene)bisphenol reacting with paraformaldehyde at 200°C to form novolacs with the structure drawn in equation 12 [33]. The acidity of the phenolic group is probably sufficient to catalyze the formation of these special novolacs, all of which have a yellow color. These products are prepolymers for the synthesis of epoxy resins [63]. 

Reaction Path for Synthesis of 4-(l, 1 -Dimethylethyl)phenol Novolac Resin (from ref [121]). 

Unlike resoles, which show a definite preference for methylolation and condensation at the para position, ring positions in novolacs are less differentiated. The normal ratio of o,p-, and /j,p-linkages in a novolac will be 1 2 1. This may be affected by the choice of catalyst, and much work has been done to control this aspect of novolac synthesis, with the emphasis on producing highly or/Zio-Iinked resins. In some cases, the judicious choice of protic acid may lead to the desired result. More commonly, a Lewis acid salt is chosen as the catalyst. These are usually divalent metal salts of acetates or similar small carboxylates. Zinc acetate is probably the most common example. Often resins made using these salts cannot be cleanly characterized as resole or novolac. They may have a resole molar ratio and a novolac pH or they may be made near neutral conditions. As mentioned before, commercial phenolic polymers showing 85% ortho linkage are available. Solvent choices may also be important to determination of substitution patterns. 

The epoxy novolac resin was prepared by two step reactions. The first step was involved in the formation of novolac resin through chemical reaction between phenol and formaldehyde in acid environment. In the second step, the epoxidation of previously prepared novolac resin was carried out by reacting epichlorohydrin with a novolac resin in a specific molar ratio. The detail of the synthesis of novolac resin and its epoxidation was described in our previous report [19,20]. The typical properties of the synthesized ENR are given in Table 16.2. 

Pol)merization reaction of phenol-formaldehyde oligomer (novolac resin - SF-0112) with tetraepoxypolyphenylsilsesquioxanes was carried out in melt condition at 125 5°C temperature at various ratios of initial compounds, without catalyst. The samples were tested on contain of epoxy groups during the reaction. It was shown that during synthesis of block-copolymers the reaction rate decreases, which may be explained by presence of free phenols in used phenol-formaldehyde oligomer. In accor-... 

---