Phenol-formaldehyde reaction addition

Although the condensation of phenol with formaldehyde has been known for more than 100 years, it is only recently that the reaction could be studied in detail. Recent developments in analytical instrumentation like GC, GPC, HPLC, IR spectroscopy and NMR spectroscopy have made it possible for the intermediates involved in such reactions to be characterized and determined (1.-6). In addition, high speed computers can now be used to simulate the complicated multi-component, multi-path kinetic schemes involved in phenol-formaldehyde reactions (6-27) and optimization routines can be used in conjunction with computer-based models for phenol-formaldehyde reactions to estimate, from experimental data, reaction rates for the various processes involved. The combined use of precise analytical data and of computer-based techniques to analyze such data has been very fruitful. 

In the phenol/formaldehyde reaction, the rates for the addition and condensation reactions are in the ratio of 1 42. The overall activation energy is 84-100 kJ/mol. The p site in this acid-catalyzed reaction is about 2.4 times more reactive than the o site of the phenol. In general, then, p-methylol phenols are produced, but these are not as attractive commercially as the o,o -methylol-rich novolaks. The curing of novolak should, of course, occur as quickly as possible, which assumes that the reactive p sites are available for reaction (see below). 

Foams of phenol formaldehyde resins can be made from a dispersion of a volatile diluent (isopropyl ether dispersed with the aid of a surfactant) in an aqueous solution of an incomplete phenol-formaldehyde reaction product [46]. Addition of an acid catalyst such as hydrochloric or sulfuric acid causes further condensation of phenol and formaldehyde to give a dimensionally stable, network structure. At the same time the heat of reaction volatilizes the diluent, yielding a foam. The foaming can be done in place. Phenolic foams are used as heat-stable, flame-retardant, thermal insulation. 

Other modifications of the polyamines include limited addition of alkylene oxide to yield the corresponding hydroxyalkyl derivatives (225) and cyanoethylation of DETA or TETA, usuaHy by reaction with acrylonitrile [107-13-1/, to give derivatives providing longer pot Hfe and better wetting of glass (226). Also included are ketimines, made by the reaction of EDA with acetone for example. These derivatives can also be hydrogenated, as in the case of the equimolar adducts of DETA and methyl isobutyl ketone [108-10-1] or methyl isoamyl ketone [110-12-3] (221 or used as is to provide moisture cure performance. Mannich bases prepared from a phenol, formaldehyde and a polyamine are also used, such as the hardener prepared from cresol, DETA, and formaldehyde (228). Other modifications of polyamines for use as epoxy hardeners include reaction with aldehydes (229), epoxidized fatty nitriles (230), aromatic monoisocyanates (231), or propylene sulfide [1072-43-1] (232). 

By far the preponderance of the 3400 kt of current worldwide phenolic resin production is in the form of phenol-formaldehyde (PF) reaction products. Phenol and formaldehyde are currently two of the most available monomers on earth. About 6000 kt of phenol and 10,000 kt of formaldehyde (100% basis) were produced in 1998 [55,56]. The organic raw materials for synthesis of phenol and formaldehyde are cumene (derived from benzene and propylene) and methanol, respectively. These materials are, in turn, obtained from petroleum and natural gas at relatively low cost ([57], pp. 10-26 [58], pp. 1-30). Cost is one of the most important advantages of phenolics in most applications. It is critical to the acceptance of phenolics for wood panel manufacture. With the exception of urea-formaldehyde resins, PF resins are the lowest cost thermosetting resins available. In addition to its synthesis from low cost monomers, phenolic resin costs are often further reduced by extension with fillers such as clays, chalk, rags, wood flours, nutshell flours, grain flours, starches, lignins, tannins, and various other low eost materials. Often these fillers and extenders improve the performance of the phenolic for a particular use while reducing cost. 

Older cook styles called for addition of phenol, formaldehyde, and water followed by alkali. Once the alkali was added, strict temperature control was the only barrier to a runaway reaction. A power or equipment failure at this point was likely to lead to disaster. Every batch made involved a struggle between the skill of the operator and capability of the equipment to control the exotherm versus the exothermic nature of the reactants. Most of the disasters that have occurred were due to utilization of this cooking method. 

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). ... 

BF3.OH2).217 The use of a sulfonated phenol-formaldehyde polymer in conjunction with formic acid is also reported.208 Acids that are ineffective include phosphoric,208 trichloroacetic, dichloroacetic, and acetic acids.134 It is reported that addition of lithium perchlorate to the reaction mixture improves product... 

Processing of phenol-aldehyde oligomers into various articles is based on a polycondensation reaction which leads to solidification of the material at temperatures below 200°C and pressures exceeding 10 MPa. The process is accompanied by volatile product formation. However, phenol-formaldehyde resins of the resol type can be cast without additional pressure and heat. The raw molding reactants contain different organic and mineral fillers and other additives in addition to the basic resin. 

In order to estimate the kinetic parameters for the addition and condensation reactions, the procedure proposed in [11, 14] has been used, where the rate constant kc of each reaction at a fixed temperature of 80°C is computed by referring it to the rate constant k° at 80°C of a reference reaction, experimentally obtained. The ratio kc/k°, assumed to be temperature independent, can be computed by applying suitable correction coefficients, which take into account the different reactivity of the -ortho and -para positions of the phenol ring, the different reactivity due to the presence or absence of methylol groups and a frequency factor. In detail, the values in [11] for the resin RT84, obtained in the presence of an alkaline catalyst and with an initial molar ratio phenol/formaldehyde of 1 1.8, have been adopted. Once the rate constants at 80°C and the activation energies are known, it is possible to compute the preexponential factors ko of each reaction using the Arrhenius law (2.2). 

Phenol-formaldehyde adhesives are produced by a condensation polymerization reaction between phenol and formaldehyde. The phenolics used for exterior particleboard are made at a formalde-hyde/phenol ratio greater than 1.0 i.e., they are classified as resoles and additional formaldehyde is not required to complete the curing reaction to a highly cross-linked network structure. Many characteristics can be incorporated into the adhesives by changes in the F/P ratio, condensation pH, and condensation time. The reactive solids content is normally between kO and 50 percent since the stability and viscosity are adversely affected at higher solids. 

Saligenin (76) is photochemically reactive (254 nm) when irradiated in basic media (MeOH/HeO). The reaction affords phenol/formaldehyde type resins in reasonable yield. The route to the condensation process involves the formation of the enone (77) by photochemically induced expulsion of hydroxide from the phenolic anion generated from (76). This enone then reacts with another anion ultimately to build up oligomers. Evidence for this process comes from the minor products formed during the reaction. These are the ether (78) which is produced by the addition of methanol (the solvent) to the enone (77). Furthermore the diphenylmethane derivatives (79) and (80) are also formed by the condensation of two substrate molecules either with or without the addition of solvent. Products of this type are considered as good evidence for the condensation reaction proposed. ... 

The large group of phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde polymers are also prepared by carbonyl-addition-substitution reactions. Their final crosslinking reaction occurs in the solid state, however, the polymers are amorphous. Crystalline linear polymers have been obtained using parasubstituted phenols. But up to now only relatively low molecular weight polymers have been prepared (5). 

The classical Mannich reaction converts phenols to aminomethylated phenols. The reaction involves the addition of phenols to C=N bonds of imines or iminium salts formed from formaldehyde and primary or secondary amines, respectively . Recent modifications employ the reaction of an aminal in the presence of SO3, which gives a sulfonate ester, followed by o-aminomethylation (equation Sc(OTf)3 catalyzed... 

A third reason for predicting very low emissions of formaldehyde from phenolic panels is that the cured resin is extremely stable and does not break down to release additional formaldehyde, even under extremely harsh environmental conditions ( ). The high resistance of phenolic resins to deterioration under severe service conditions is, of course, a principal reason they are used so widely in making exterior types of wood panel products. Because of their chemical stability the U.S. Environmental Protection Agency has declared that phenol formaldehyde resins represent a consumptive use of formaldehyde, meaning that formaldehyde is irreversibly consumed in its reaction with phenol so that the formaldehyde loses its chemical identity (3). 

Nitrated hydroxyaromatics may enter into the atmosphere from both primary and secondary sources. The formation of nitrophenols and nitrocresols in die combustion processes of motor vehicles has been reported by Tremp et al. (1993). Others primary sources may be combustion of coal, wood, manufacture of phenol-formaldehyde resins, pharmaceuticals disinfectants, dyes and explosives (Harrison et al., 2005). Studies in our and other laboratories have shown that an additional important source of diese compounds in the atmosphere could be the gas-phase OH-radical initiated photooxidation of aromatic hydrocarbons such as benzene, toluene, phenol, cresols and dihydroxybenzenes in the presence of NOx during the daytime as well as the reaction of NO3 radicals widi these aromatics during the night time (Atkinson et al., 1992 Olariu et al., 2002). Once released or... 

Acid-Catalyzed Reactions. When an acid catalyst is used and the pH of the phenol/formaldehyde mixture is lowered to 0.5-1.5, somewhat less complicated products are formed. The initial reaction of addition of formaldehyde to the aromatic ring results in an unstable intermediate that rapidly condenses to three possible dihydroxydiarylmethanes. 

Figure 7.13 Carbonate formation in the reaction with O2 of nitrogen-containing activated carbons prepared from phenol-formaldehyde resin with the addition of aniline. (Adapted from ref. 164.)...

It must be noted that the phenol/aldehyde reaction can be catalyzed by Bronsted acids (protonation of the carbonyl oxygen) as well as by Lewis acids (coordination of the carbonyl oxygen). In the latter case one Lewis centre (e.g. Al ) can accommodate and activate both the phenol and the aldehyde (cq. the benzyl alcohol, in the consecutive reaction). As a consequence, ortho-substitution is favoured [14,15]. The high 2,2 -dihydroxydiphenylmethane selectivity we obtained with homogeneous Al " -catalysis and with 7-alumina is consistent with these data. Additionally, the finding that the H - US - Y catalyzed toluene/formaldehyde-condensation gives a low 2,2 -selectivity, 19% [16], compared to the 32% we obtained with phenol, also indicates the hydroxyl-group plays a role. However, transalkylation, reported to lead to ortho-substitution in condensations of phenol with methanol on both zeolite- and non-zeolite Bronsted acid catalysts [17], can t be ruled out. 

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