Phenol-formaldehyde polymers novolac

Synonyms P-F-R-2 resol phenol formaldehyde resin phenol formaldehyde phenol polymer with formaldehyde formaldehyde-pnenol polymer paraformaldehyde-formaldehyde-phenol polymer paraformaldehyde-phenol polymer phenol-formaldehyde polymer Novolac Resole resol based on phenol, resorcinol, and formaldehyde... 

Phenol - formaldehyde polymers are the oldest synthetic polymers. These are obtained by the condensation reaction of phenol with formaldehyde in the presence of either an acid or a base catalyst. The reaction starts with the initial formation of o-and/or p-hydroxymethylphenol derivatives, which further react with phenol to form compounds having rings joined to each other through -CH2 groups. The initial product could be a linear product - Novolac used in paints. 

Let us apply Eq. (3.86) to phenol - formaldehyde polymers synthesized in an acid medium with a phenol excess (novolacs). Phenol is a trifunctional reactant (A3), the functional groups being the aromatic hydrogens located in positions 2, 4, and 6 of the phenolic ring. Formaldehyde acts as a bifunctional monomer (B2), forming methylene bridges between the reactive positions of phenol. Novolacs are synthesized with a phenol excess, such that gelation does not occur at full formaldehyde conversion. From Eqs (3.83) and (3.86), we obtain... 

Fig. 5-1. Phenol-formaldehyde polymers, (a) Character of resole -lype re.sins normally produced with excess formaldehyde under alkaline conditions, (b) Novolac"-lype resins nomially made with excess phenol under acidic conditions.

Two families of phenolic-based adhesives are to be found in industry those formulated with phenolic resoles and those with novolacs. Although the starting chemistries for both resins are very similar, both are phenol/formaldehyde polymers, the different manufacturing routes leading to resins with significantly dissimilar properties. This article is concerned with resoles novolacs are considered in Phenolic adhesives two-stage novolacs. 

There are two types of phenol-formaldehyde condensation polymers resoles and novolacs (117). Phenol-formaldehyde polymers prepared from the base-catalyzed condensation of phenol and excess formaldehyde are called resoles. In most phenolic resins commonly used with epoxies, the phenolic group is converted into an ether to give improved alkali resistance. At elevated temperatin-es (>150°C), resole resins react with the hydroxyl groups of the epoxy resins to provide highly cross-linked polymers. 

Most processors of fiber-reinforced composites choose a phenol formaldehyde (phenoHc) resin because these resins are inherently fire retardant, are highly heat resistant, and are very low in cost. When exposed to flames they give off very Htde smoke and that smoke is of low immediate toxicity. PhenoHc resins (qv) are often not chosen, however, because the resole types have limited shelf stabiHty, both resole and novolac types release volatiles during their condensation cure, formaldehyde [50-00-0] emissions are possible during both handling and cure, and the polymers formed are brittle compared with other thermosetting resins. 

Phenol-formaldehyde prepolymers, referred to as novolacs, are obtained by using a ratio of formaldehyde to phenol of 0.75-0.85 1, sometimes lower. Since the reaction system is starved for formaldehyde, only low molecular weight polymers can be formed and there is a much narrower range of products compared to the resoles. The reaction is accomplished by heating for 2 1 h at or near reflux temperature in the presence of an acid catalyst. Oxalic and sulfuric acids are used in amounts of 1-2 and <1 part, respectively, per 100 parts phenol. The polymerization involves electrophilic aromatic substitution, first by hydroxymethyl carboca-tion and subsequently by benzyl carbocation—each formed by protonation of OH followed by loss of water. There is much less benzyl ether bridging between benzene rings compared to the resole prepolymers. 

The second process (Fig. 1) uses an acid catalyst and excess phenol to give a linear polymer (novolac) that has no free methylol groups for cross-linking. In a separate second part of this two-stage process, a cross-linking agent is added and further reaction occurs. In many instances, hexamethylenetetramine is used, which decomposes to formaldehyde and ammonia. 

The inherent thermal stability of the phenol formaldehyde chemistry is preserved but with the crosslinking characteristics of the epoxy groups. However, epoxy novolacs also form very rigid and brittle polymers when fully cured because of their high crosslink density. For this reason, they are often used as modifiers in epoxy adhesive systems rather than as the base polymer. 

Some of the factors identified in determining the final properties of these resins are the phenol-formaldehyde ratio, pH, temperature and the type of catalyst (acid or alkaline) used in the preparation of the resin. The phenol-formaldehyde ratio (P/F) (or formaldehyde to phenol ratio, F/P) is a most important factor as it leads to two different classes of synthetic polymers, namely Novolacs and resoles. The first class of resins, Novolacs, is produced by the reaction of phenol with formaldehyde with a P/F > 1 usually under acidic conditions (Scheme 2a). Resoles are produced by the reaction of phenol and formaldehyde with a P/F <1 usually under basic conditions (Scheme 2b). 

Novolacs are linear polymers. Metacresol, a very reactive derivative of phenol, is typically used to prepare novolac resins. The presence of a methyl group at the meta (3 or 5) position of the henzene ring of phenol enhances the reactivity of the compound toward polymerization with formaldehyde. Novolac resins made with metacresol are also more moisture resistant than those with phenol. After preparation, novolac s ability to resist further polymerization is attributed to the fact that the chains terminate with phenol groupings, having been prepared with an excess of phenol. ... 

Phenol formaldehyde resins (pol3miethylene phenylene) are polymers with irregular cross-linked structure. During friction of resol- and novolac-t3rpe phenol formaldehyde pol3miers two processes take place simultaneously, namely structuring (aftercure) via recombination of weak ester groups... 

Since the cross-linked polymer of phenol-formaldehyde reaction is insoluble and infusible, it is necessary for commercial applications to produce first a tractable and fusible low-molecular-weight prepolymer which may, when desired, be transformed into the cross-linked polymer [14,44,45]. The initial phenol-formaldehyde products (prepolymers) may be of two types resols and novolacs. 

In a positive-tone resist, the areas that are exposed to the radiation develop away into the solvent faster than unexposed areas, resulting in a positive-tone image of the mask. The majority of commercial, positive-tone, nonchemically amplified resists used today are variations of the well-known, two-component, diazonaphthoquinone-novolac resist [5]. Novolac is a name generally given to acid-catalyzed phenol formaldehyde condensation polymers of the type shown in Fig. 2. 

Bifunctional monomers, such as A-A, B-B and A-B, yield linear polymers. Branched and crosslinked polymers are obtained from polyfunctional monomers. For example, polymerization of formaldehyde with phenol may lead to complex architectures. Formaldehyde is commercialized as an aqueous solution in which it is present as methylene glycol, which may react with the trifunctional phenol (reactive at its two ortho and one para positions). The type of polymer architecture depends on the reaction conditions. Polymerization imder basic conditions (pH = 9-11) and with an excess of formaldehyde yields a highly branched polymer (resols. Figure 1.8). In this case, the polymerization is stopped when the polymer is still liquid or soluble. The formation of the final network (curing) is achieved during application (e.g., in foundry as binders to make cores or molds for castings of steel, iron and non-ferrous metals). Under acidic conditions (pH = 2-3) and with an excess of phenol, linear polymers with httle branching are produced (novolacs). 

SiC nanofibers by melt-spinning of polymer blends have been prepared from PCS as a SiC ceramic precursor and a novolac-type phenol-formaldehyde resin [121]. These nanofibers were amorphous, about 100 nm in diameter, more than 100 (un long, and were rich in oxygen. 

The phenol/formaldehyde molar ratio, coupled with the type of catalyst used, determines whether the polymer is phenol terminated or methylol terminated phenol-terminated resins are referred to as novolacs, or two-step resins. Because phenol is nominally trifunctional and formaldehyde is difunctional, novolacs are produced from a reaction mixture having a formaldehyde/phenol molar ratio between 0.5 and 0.8 in the presence of an acid catalyst. Such resins are not heat reactive until a second ingredient is added that supplies additional formaldehyde needed for a cure. Novolacs are employed as solid products. 

The acidity of the reaction medium appears to be the most important factor governing the reactions between phenol and formaldehyde. The rate of the phenol-formaldehyde reaction at pH 1 to pH 4 is proportional to the hydrogen ion concentration, but above pH 5 it is proportional to the hydroxyl ion concentration, indicating a change in reaction mechanism. Four types of polymers can be obtained novolacs, high ortho-ortho novolacs, resoles, and high ortho-ortho resoles. 

Other well known condensation polymers include phenol-formaldehyde resins, the prototype of which is Bakelite (Figure 13.15 C). Such structures were known as early as the 1870s, and in the early 20th century these tough, durable thermosets were among the first synthetic polymers of commerical importance. More modern versions of this type of polymer are known as Novolac. This chemistry is also that which make.s calixarenes (Chapter 4), which are cyclic tetramers rather than linear polymers. 

The early history of polymers is really the conversion of natural polymers into useful materials. Examples include the vulcanization of rubber (Goodyear, 1839), celluloid (which is plasticized cellulose nitrate—Hyatt, 1868), and cellulose-derived fibres, e.g. cuprammonia rayon (Despeisses, 1890) and viscose rayon (Cross, Bevan and Beadle, 1892). The first truly synthetic polymer, that is, one made from laboratory chemicals, was Bakelite (Bakeland, 1907). This was made from phenol and formaldehyde. Bakeland probably did not know the chemical structure of the Bakelite, but he did realize that organic chemicals containing multiple functionality yielded insoluble materials. The various phenol-formaldehyde resins (PF), e.g. Bakelite and novolacs, were thus obtained in an empirical manner. 

Phenol-formaldehyde resins have been widely used for brown and black electrical fittings, and they represent a thermosetting condensation polymer. Depending upon the ratio of phenol to formaldehyde and the pH of the reaction mixture a resole or a novolac is formed by the substitution of formaldehyde molecules at different positions around the benzene ring. In the case of resoles, phenol groups are... 

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