Resole phenol-formaldehyde polymers

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. 

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... 

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. 

Phenol-formaldehyde polymers, including novolaks and resoles, have a number of applications in coatings, finishes, adhesives, composites, laminates and related areas. Concerns have been raised regarding the continued use of phenol-formaldehyde resins due to the various toxic effects of formaldehyde. Consequently, there has been active investigation for alternative sources of these types of oligomers and polymers with a consideration for environmental compatibility. 

Resols are intermediates in the formation of highly crosslinked phenol formaldehyde polymers, and they usually have molecular weights in the 300-700 range. A typical resol structure is shown in Fig. 4,... 

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. 

In the manufacture of pure resorcinol resins, the reaction can be violently exothermic unless controlled by the addition of alcohols. Because the alcohols perform other useful functions in the glue mix, they are left in the liquid adhesive. PRF adhesives are generally prepared firstly by reaction of phenol with formaldehyde to form a PF resol polymer, that has been proved to be in the greatest percentage, and often completely, linear [95], In the reaction step that follows the resorcinol chemical is added in excess to the PF-resol to react it with the PF-resin -CH2OH groups to form PRF polymers in which the resorcinol groups can be resorcinol chemical or any type of resorcinol-formaldehyde polymer. 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

Phenol-formaldehyde resins using prepolymers such as novolaks and resols are widely used in industrial fields. These resins show excellent toughness and thermal-resistant properties, but the general concern over the toxicity of formaldehyde has resulted in limitations on their preparation and use. Therefore, an alternative process for the synthesis of phenolic polymers avoiding the use of formaldehyde is strongly desired. 

J. Monni, R Niemela, L. Alvila and T.T. Rakkanen, Online monitoring of synthesis and curing of phenol-formaldehyde resol resins by Raman spectroscopy. Polymer, 49, 3865-3874 (2008). 

Baekeland found that a relatively stable resole prepolymer could be obtained by the controlled condensation of phenol and formaldehyde under alkaline conditions. These linear polymers of phenol-formaldehyde (PF) may be converted to infusible cross-linked polymers called resites by heating or by the addition of mineral acids. As shown in structure 4.80, the initial products obtained when formaldehyde is condensed with phenol are hydroxybenzyl alcohols. The linear resole polymer is called an A-stage resin, and the cross-linked resite is called a C-stage resin. 

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. 

Phenol-formaldehyde type polymers had been the only exterior-durable adhesives for wood bonding, until the recent limited use of isocyanates. Both systems are petrochemical-based. Several researchers substituted carbohydrates for part of phenolic adhesives (1-4) > producing solid, fusible novolak resins. Recently, reaction of carbohydrate acid-degradation products with phenol and formaldehyde has produced liquid resols (5). Gibbons and Wondolowski (6,7) replaced a considerable amount of phenol with carbohydrate and urea to pro-... 

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). 

Including 4-bromophenol in the phenol-formaldehyde resol system impacts the cross-link density of the cured product. In a systematic study of this copolymer, a comparison was made among the polymers obtained using phenol only, a 9 1 mole ratio of phenol to 4-bromophenol and a 1 1 mole ratio of phenol to 4-bromophenol. Comparisons included measurement of interlaminar shear strength and cone calorimetry tests of composites prepared using these phenolic resins and S2-glass fiber plain weave. 

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... 

KI1 Kim, M.G., Nieh, W.L., Sellers, T., Wilson, W.W., and Mays, J.W., Polymer solution properties of a phenol-formaldehyde resol resin by gel permeation ehromatography, intrinsic viscosity, static hght scattering, and vapor pressure osmometric methods,/wJ. Eng. Chem. Res., 31, 973,1992. 

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. 

Derrick and coworkers have investigated polyglycols (PEG, polybutadiene,polystyrene, and poly(methylmethacrylate) by FD-MS. Prokai and Scrivens et al. have looked at various phenol-formaldehyde (novalak and resole) resins. Wile and Cook obtained FD spectra of lactone, lactam, and carbonate polymers." Matsuo et al. investigated polystyrene and poly(propylene glycol) as high mass reference compounds." Faffimer and Schulten studied several hydrocarbon polymers (polybutadiene, polyiso-prene, polyethylene, polystyrene)." Evans et al. used FD-MS to investigate mechanistic aspects of fhe mefal-cafalyzed polymerization of ethylene and other olefins. 

Journal of Applied Polymer Science 69, No. 11, 12th Sept. 1998, p.2175-85 IR SPECTROSCOPY AS A QUANTITATIVE AND PREDICTIVE ANALYSIS METHOD OF PHENOL-FORMALDEHYDE RESOL RESINS Holopainen T Alvila L Rainio J Pakkanen T T Joensuu,University Dynoresin Oy... 

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). 

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. 

Amino Rosins. Amino resins are the reaction product of an aldehyde with a compound containing an amino (—NH2) group (see Amino Resins). Both urea and melamine react with formaldehyde, first by addition to form methy-lol compounds, and then by condensation to form cross-linked polymers through methylene bridge and methylene ether formation. The cross-linking reactions are exothermic and produce water and formaldehyde as volatile products in reactions similar to resole phenolics, illustrated in equation 3. Urea-formaldehyde and melamine-formaldehyde account for the bulk of the amino resins. Their characteristics include water solubility and unlimited colorability with dyes and pigments. Applications include decorative high pressure phenolic laminates, adhesives in the laminated wood and furniture industries, and as an additive to textile fabrics to impart wash and wear properties. 

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