Phenol-formaldehyde basic conditions

A Friedel-Crafts-type reaction of phenols under basic conditions is also possible. Aqueous alkaline phenol-aldehyde condensation is the reaction for generating phenol-formaldehyde resin.34 The condensation of phenol with glyoxylic acid in alkaline solution by using aqueous glyoxylic acid generates 4-hydroxyphenylacetic acid. The use of tetraalkylammonium hydroxide instead of sodium hydroxide increases the para-selectivity of the condensation.35 Base-catalyzed formation of benzo[b]furano[60]- and -[70]fullerenes occurred via the reaction of C60CI6 with phenol in the presence of aqueous KOH and under nitrogen.36... 

Phenolic resins, introduced in 1908, are formed by either base- or acid-catalyzed addition of formaldehyde to phenol to give ortho- and para-substituted products. The nature of these products depends largely on the type of catalyst and the mole ratio of formaldehyde to phenol. In resole formation, excess formaldehyde is reacted with phenol under basic conditions. The initial reaction products are ortho- and para-substituted mono-, di-, and trimethylolphenols ... 

Phenolic Resins. Phenohc resins (qv) are formed by the reaction of phenol [108-95-2] C H O, and formaldehyde [50-00-0] CH2O. If basic conditions and an excess of formaldehyde are used, the result is a resole phenohc resin, which will cure by itself Hberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenohc resin, which is not self-curing. Novolac phenohc resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0] C H22N4. Phenohc resin adhesives are found in film or solution... 

Reactions with Aldehydes and Ketones. An important use for alkylphenols is ia phenol—formaldehyde resias. These resias are classified as resoles or aovolaks (see Phenolic resins). Resoles are produced whea oae or more moles of formaldehyde react with oae mole of pheaol uader basic catalysis. These resias are thermosets. Novolaks are thermoplastic resias formed whea an excess of phenol reacts with formaldehyde under acidic conditions. The acid protonates formaldehyde to generate the alkylating electrophile (17). 

Scheme 4b depicts condensation between a hydroxymethyl group and a phenolic ring where the hydroxybenzyl attacks at a ring position that is already hydroxymethylated. In this case, a methylene linkage is produced between the rings with concurrent loss of one mole each of formaldehyde and water. Both Jones and Grenier-Loustalot et al. demonstrated the occurrence of this reaction pathway beyond doubt under basic conditions. 

Phenolic oligomers are prepared by reacting phenol or substituted phenols with formaldehyde or other aldehydes. Depending on the reaction conditions (e.g., pH) and the ratio of phenol to formaldehyde, two types of phenolic resins are obtained. Novolacs are derived from an excess of phenol under neutral to acidic conditions, while reactions under basic conditions using an excess of formaldehyde result in resoles. 

Alkyl-substituted phenols have different reactivities than phenol toward reaction with formaldehyde. Relative reactivities determined by monitoring the disappearance of formaldehyde in phenol-paraformaldehyde reactions (Table 7.3) show that, under basic conditions, meta-cresol reacts with formaldehyde approximately three times faster titan phenol while ortho- and para-cresols react at approximately one-third the rate of phenol.18 Similar trends were observed for the reactivities of acid-catalyzed phenolic monomers with formaldehyde. 

TABLE 7.3 Relative Reaction Rates of Various Phenols with Formaldehyde under Basic Conditions"... 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

Crosslinking resoles in the presence of sodium carbonate or potassium carbonate lead to preferential formation of ortho-ortho methylene linkages.63 Resole networks crosslinked under basic conditions showed that crosslink density depends on the degree of hydroxymethyl substitution, which is affected by the formaldehyde-to-phenol ratio, the reaction time, and the type and concentration of catalyst (uncatalyzed, with 2% NaOH, with 5% NaOH).64 As expected, NaOH accelerated the rates of both hydroxymethyl substitution and methylene ether formation. Significant rate increases were observed for ortho substitutions as die amount of NaOH increased. The para substitution, which does not occur in the absence of the catalyst, formed only in small amounts in the presence of NaOH. 

Urea and melamine are tetra- and hexa-functional molecules. However, the formation of a network polymer is prevented by adding alcohols such as w-butanol and by condensing with formaldehyde at low temperatures under basic conditions. While phenol resins have better moisture and weather resistance than urea resins, the latter are preferred for light-colored... 

One of the earliest commercial plastics was Bakelite , formed by the reaction of phenol with a little more than one equivalent of formaldehyde under acidic or basic conditions. Baeyer first discovered this reaction in 1872, and practical methods for casting and molding Bakelite were developed around 1909. Phenol-formaldehyde plastics and resins (also called phenolics) are highly cross-linked because each phenol ring has three sites (two ortho and one para) that can be linked by condensation with formaldehyde. Suggest a general structure for a phenol-formaldehyde resin, and propose a mechanism for its formation under acidic conditions. (Hint Condensation of phenol with formaldehyde resembles the condensation of phenol with acetone, used in Problem 26-17, to make bisphenol A.)... 

Phenol-formaldehyde resins find numerous applications in such areas as wood composites, fiber bonding, laminates, foundry resins, abrasives, friction and molding materials, coatings and adhesives, and flame retardants (JL). From a specialty chemicals standpoint, they are also used as developer resins in carbonless papers (2.). Conventional methods of preparation involve condensation of a phenol with formaldehyde under either acidic (novolak) or basic (resole) conditions (2). Their typical molecular weight range is from 800-4000 daltons (D) and includes a wide variety of alkyl or aryl substituted phenols (A)- The... 

In an earlier paper (2), we determined that carbohydrates could replace a significant portion of the phenol-formaldehyde resin used for bonding plywood veneer. Carbohydrates from renewable resources such as wood can replace up to 50% of the phenol and formaldehyde in resins formulated under basic conditions without significant loss of bond quality. Two-ply, Douglas-fir-veneer panels bonded with these carbohydrate-modified resins have shear strengths approximately equivalent to those for panels bonded with unmodified phenol-formaldehyde resin. 

Figure 1 compares the dry- and wet-shear strengths of two-ply, Douglas-fir veneer panels bonded with a commercial phenol-formaldehyde resin (basic), a phenol-formaldehyde resin prepared in the laboratory under basic conditions, and an unmodified neutral resin prepared in the laboratory. The shear strengths obtained with these three resins served as control data for further experiments. The dry-shear strengths of panels bonded with the unmodified neutral resin are lower than those for panels bonded with the resins cured under basic conditions however, the wet-shear strengths of panels bonded with the three resins are all... 

Xylose Modified Phenol-Formaldehyde Resins. Xylose (I) and byproducts streams containing xylose (e.g., wood prehydrolysates from the production of chemical pulps and waste liquors from the wet process for hardboard production) are readily available. Our previous experiments (2) showed that free reducing sugars are not acceptable modifiers for phenol-formaldehyde resins cured under basic conditions. 

Figure 14.7 Representative structures of phenol-formaldehyde resins (a) novolac (formed under acidic conditions), and (b) resole (formed under basic conditions).

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). 

The product depends on the molar ratio of phenol to formaldehyde, pH conditions, and temperature. The mechanism is essentially the same in acidic or basic aqueous solution, but the details of the reaction are different. 

The mechanism and phenol-formaldehyde reaction under acidic conditions is different from that under basic conditions described previously. In the presence of acid the products o- and p-methylol phenols, which are formed initially, react rapidly with free phenol to form dihydroxy diphenyl methanes (Figure 4.25). The latter undergo slow reaction with formaldehyde and phenolic species, forming polynuclear phenols by further methylolation and methylol link formation. Reactions of this type continue until all the formaldehyde has been used up. The final product thus consists of a complex mixture of polynuclear phenols linked by o- and p-methylene groups. 

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). 

Many adhesives cure by step polymerization. Structural phenolic adhesives are based on resoles, which are made by the step polymerization of phenol with excess formaldehyde under basic conditions (see Phenolic adhesives single-stage resoles). They contain... 

Phenol has unique chemical properties due to the presence of a hydroxyl group and an aromatic ring, which are complementary in that they facilitate both electrophilic and nucleophilic reactions. The aromatic ring of phenol is highly reactive towards electrophilic snbstitntion, which assists its acid-catalyzed reaction with formaldehyde. Phenol is a weak acid and easily forms sodium phenoxide (NaPh) in a base-catalyzed medinm. In the presence of sodium phenoxide, the nucleophilic addition of the phenolic aromatic ring to the carbonyl group of formaldehyde occurs. Thus, phenol can react with formaldehyde under acidic or basic conditions, leading to either novolac or resole resins (Weber and Weber, 2010). 

When phenol is carefully treated under basic conditions with formaldehyde (H2CO), substitution occurs at positions ortho and para to the phenolic hydroxyl (-OH). In this way, hydroxybenzyl alcohols (Scheme 8.44) can be produced. Acidification results in the generation of the respective stabilized benzylic carboca-tions (an El reaction, Chapter 7), which can then react with unreacted phenol and/... 

Phenol-formaldehyde condensates were among the first synthetic polymeric materials on the market. It was Baekeland at the beginning of the 20th century who in 1907 defined the differences between basic or acidic reaction conditions and the different molar ratios on the reaction procedure and the resulting molecular structure. He was able to manufacture a thermosetting resin and made applications for a patent [19] (Bakelite). 

A. Reaction of Phenol and Formaldehyde Under Basic Conditions... 

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