Phenol-formaldehyde reaction products

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

The initial phenol-formaldehyde reaction products may be of two types, novolaks and resols. 

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. 

The reaction conditions, formaldehyde-to-phenol ratios, and concentration and type of catalyst govern the mechanisms and kinetics of resole syntheses. Higher formaldehyde-to-phenol ratios accelerate the reaction rates. This is to be expected since phenol-formaldehyde reactions follow second-order kinetics. Increased hydroxymethyl substitution on phenols due to higher formaldehyde compositions also leads to more condensation products.55... 

Resorcinol/phenol-formaldehyde condensation products prepared by Durairaj et al. (3) using zinc acetate as the reaction catalyst contained 2% / -//-phenolic, 16% -p -phenolic, 64% o-o -phenolic-4-4 -resorcinolic, 16% 2-4 -resorcinolic, and 2% 2-2 -resorcinolic methylene bridges. 

Because of the aforementioned circumstances, the loss of control of the phenol-formaldehyde reaction has been the cause of a number of severe incidents in chemical batch reactors during the last decades [12], These incidents have caused many injuries and, in the worst case, even fatalities among the plant operators. Other severe consequences have been the evacuation of residents in the surrounding area due to chemical contamination and a protracted stop in the plant production. 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

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. 

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. 

Table 1.1 (see page 8) shows that polymerization reactions featured in by far the most incidents, followed by nitration, sulphonation and hydrolysis reactions. Of the polymerization reactions, 20% involved phenol-formaldehyde condensations. In view of the number of incidents with phenol-formaldehyde resin production the British Plastics Federation (BPF) came forward with an exemplary approach to the problem in its publication Guidelines for the Safe... 

The primary causes of accidents in the chemical industry are technical failures, human failures and the chemical reaction itself (due to lack of knowledge of the thermochemistry and the reaction kinetics) [156]. As discussed previously, polymerization reactions are subject to thermal runaway, so that it is not surprising to learn that polymerization reactions (64 from 134 cases) are more prone than other processes to serious accidents [157]. Among the polymerization processes, the phenol-formaldehyde resin production seems to be the worst case, although incidents have been reported for vinyl chloride, vinyl acetate and polyester resins polymerization processes. 

Resoles are usually mixtures of a number of methylol phenols, with small amounts of higher condensation products involving methylene and benzylic ether linkages. The kinetics of base-catalyzed phenol-formaldehyde reactions have been extensively researched. At high dilution and at a pH not exceeding 10, the rate equation can be expressed by Eq. (13), where [PhO ] denotes the concentration of phenoxide anion and [F] the concentration of unreacted formaldehyde, determined titrimetrically. " ... 

Condensation products of dialkylphenols with formaldehyde Reaction products of phenol with styrene 1,1 -Methylene-bis -(4-hydroxy -3,5 -tert-butylphenol) 2,2 -Methylene -bis-(4-methyl-6-tert-butylphenol)... 

Urea 1 (/=4) and melamin 2, 2,4,6-triamino-l,3,5-triazin (/=6) under basic or acidic conditions react with formaldehyde (/=2) rather similar to the phenol-formaldehyde reaction. The reaction products are called aminoplastics. 

Freeman and Lewis [23] published one of the first complete kinetic investigations of phenol with formaldehyde. The hydroxymethylation at 30°C was followed by analysis using quantitative paper chromatography with detection of five products (2-hydroxymethylphenol, 4-hydroxymethyl-phenol, 2,4-dihydroxymethylphenol, 2,6-dihydroxymethylphenol, and 2,4,6-trihydroxymethylphenol). More recently, Zavitsas and Beaulieu [24] used GLC to investigate the kinetics of the phenol-formaldehyde reaction using only catalytic amounts of base and at pH ranges where the second-order rate expression was shown to be valid. 

Urea-formaldehyde reaction products were described as early as 1908, but the first useful commercial product, a molding compound invented in England by Edmond C. Rossiter, did not arrive until almost 20 years later. It was a fairly complex formulation using purified cellulose fiber as reinforcement. The amino resin contained equimolar amounts of urea and thiourea. The new product could be supplied in light translucent colors. The molded products had a hard, stain resistant surface, and there was no objectionable phenolic odor. In short, the product was unique for its time. 

The reactivity of the ortho and para hydrogen atoms in the phenolic nucleus is the predominant factor in phenol-formaldehyde reactions.. Uthough there is some indication that the meta hydrogen atoms may l)ecome involved w hen other pcsitions are blocked (page 180), this has not been conclusively demonstrated. Xuclear derivatives are the principal reaction products and formals, such as are obtained from alcohols and fonnaldehyde, are seldom isolated. 

Phenol-formaldehyde resins are produced by a system of chemical reac tions in which the linkage of phenol molecules by meth dene groups plays a predominant part. How ever, because of the rapidity nith Yhich con-ieeutive phenol-formaldehyde reactions usually take place, the isolation d simple methylene derivatives containing onb two or three phenolic nuclei is often extremely difficult. This is particularlj true of phenols in which the active oi tho and para positions are unsubstituted and in which pnups which tend to lower reactivity, such as nitro and carboxyl radicals, are absent. Since in these cases a large number of simple isomeric products can be formed, the separation of any one isomer in a pure state is difficult. The isolation of the simple methylene derivatives is noteworthy, since it clarifies our knowledge of the mechanism by which the more complicated products are produced. 

Epoxy novolac resins are produced by glycidation of the low-molecular-weight reaction products of phenol (or cresol) with formaldehyde. Highly cross-linked systems are formed that have superior performance at elevated temperatures. 

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. 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

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