Formaldehyde-resorcinol reactions

Further proof of complex reactions between propylene carbonate and phenolic nuclei leading to compounds in which the carbonic acid has attacked the phenolic ring has been presented [23] based on the NMR spectrum of the product of the reaction of resorcinol with propylene carbonate, in the absence of formaldehyde. Resorcinol was chosen as its aromatic ring is a stronger nucleophile than that of phenol and thus if any reaction had occurred this would be less elusive and much more easily observed [23]. The reaction products which appeared to be formed were carboxylic and dicarboxylic species. That they might be present was also derived by NMR [23]. It must be pointed out that such structures need to be only transitory and not permanent to obtain the same effects noted experimentally. Such a subsequent lability could be the reason why it is difficult to observe such linkages in the hardened resin except for faster reacting phenols where they can be observed due to early immobilization of the network which surely occurs. 

It is interesting to note that while -CH2-O-CH2- ether bridged compounds have been isolated for the phenol-formaldehyde [24] reaction, their existence for fast-reacting phenols such as resorcinol and phloroglucinol has been postulated, but they have not been isolated, as these two phenols have always been considered too reactive with formaldehyde. They are detected by a surge in the concentration of formaldehyde observed in kinetic curves due to methylene ether bridge decomposition [19]. 

Paraformaldehyde is used by resin manufacturers seeking low water content or more favorable control of reaction rates. It is often used in making phenol—, urea—, resorcinol—, and melamine—formaldehyde resins. 

Methylene chloride is one of the more stable of the chlorinated hydrocarbon solvents. Its initial thermal degradation temperature is 120°C in dry air (1). This temperature decreases as the moisture content increases. The reaction produces mainly HCl with trace amounts of phosgene. Decomposition under these conditions can be inhibited by the addition of small quantities (0.0001—1.0%) of phenoHc compounds, eg, phenol, hydroquinone, -cresol, resorcinol, thymol, and 1-naphthol (2). Stabilization may also be effected by the addition of small amounts of amines (3) or a mixture of nitromethane and 1,4-dioxane. The latter diminishes attack on aluminum and inhibits kon-catalyzed reactions of methylene chloride (4). The addition of small amounts of epoxides can also inhibit aluminum reactions catalyzed by iron (5). On prolonged contact with water, methylene chloride hydrolyzes very slowly, forming HCl as the primary product. On prolonged heating with water in a sealed vessel at 140—170°C, methylene chloride yields formaldehyde and hydrochloric acid as shown by the following equation (6). 

Resorcinol (1,3-dihydroxybenzene) is more reactive to formaldehyde than phenol itself, the two hydroxyl groups reinforcing each other in activating the o-and p-positions. This allows faster reactions to occur than with phenol, when compared at the same temperature, and under appropriate conditions curing can occur at normal room temperatures. 

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

Fig. 2. Schematic representation of the reaction of resorcinol and formaldehyde to give linear resorcinol-formaldehyde oligomers.

The absence of methylol (-CH2OH) groups in all six lower molecular weight resorcinol-formaldehyde condensates which have been isolated [119] reflects the high reactivity of resorcinol under acid or alkaline conditions. It also shows the instability of its para-hydroxybenzyl alcohol groups and their rapid conversion to jpara-hydroxybenzyl carbonium ions or quinone methides. This explains how identical condensation products are obtained under acid or alkaline reaction conditions [119]. In acid reaction conditions methylene ether-linked condensates are also formed, but they are highly unstable and decompose to form stable methylene links in 0.25 to 1 h at ambient temperature [121,122]. 

The condensation reaction of resorcinol with formaldehyde, on an equal molar basis and under identical conditions, also proceeds at a rate which is approximately 10 to 15 times faster than that of the equivalent phenol-formaldehyde system [16-18,123]. The high reactivity of the resorcinol-formaldehyde system renders it impossible to have these adhesives in resol form. Therefore, only resorcinol-formaldehyde novolaks, i.e. resins not containing methylol groups can be produced. All the resorcinol nuclei are linked together through methylene... 

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. 

Where resorcinol adhesives are not suitable, resins can be prepared from modified resorcinol [128], Characteristic of these types of resins arc those used for tyre cord adhesives, in which a pure resorcinol-formaldehyde resin is used, or alternatively, alkyl resorcinol or oil-soluble resins suitable for rubber compounding are obtained by prereaction of resorcinol with fatty acids in the presence of sulfuric acid at high temperature followed by reaction with formaldehyde. Worldwide more than 90% of resorcinol adhesives are used as cold-setting wood adhesives. The other most notable application is as tyre cord adhesives, which constitutes less than 5% of the total use. 

These are thermoset polymers made from phenol or, less commonly, phenolic-type compounds such as the cresols, xylenols, and resorcinol, together with formaldehyde. They had been known for some time - G.T (later Sir Gilbert) Morgan discovered them in the early 1890s when attempting (unsuccessfully) to make artificial dyestuffs by reaction of phenol with formaldehyde. But this knowledge had not been exploited before 1907, the year in which Leo... 

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. 

Resorcinol differs from other phenols in that it reacts readily with formaldehyde under neutral conditions at ambient temperature. To make stable adhesives, which can be cured at the point of use, they are prepared with less than a stoichiometric amount of formaldehyde. About two thirds of a mole of formaldehyde for each mole of resorcinol will give a stable resinous condensation product. The resin is formed into a liquid of convenient solids content and viscosity. Such solutions have infinite stability when stored in closed containers. Glue mixes formed at the point of use from these solutions, on addition of paraformaldehyde-containing hardeners, will have a useful life of several hours due to two principal factors (1) the paraformaldehyde depolymerizes to supply monomeric formaldehyde at a slow rate, as determined by the pH (2) the availability of the formaldehyde is also controlled by the kind and amount of alcohol in the solvent. Formaldehyde reacts with the alcohol to form a hemiacetal. This reaction is reversible and forms an equilibrium which exerts further control on the availability of the formaldehyde. 

The acid-catalysed reaction of resorcinol with aldehydes leads to cyclic tetra-mers which are grouped in the class of resorc[4]arenes (exo-calixarenes).3 This reaction is quite general with respect to the aldehyde and to the starting resorcinol, which can bear several substituents in the 2-position.15 Niederl and Hogberg showed in a typical example, the synthetic procedure by which an ethanol solution of resorcinol, acetaldehyde and 37% hydrochloric acid maintained at 80 °C for 16 h, afforded 75% of the all cis C-methylresorc[4]arene. A shorter reaction time leads to the kinetic stereoisomer (trans,cis,trans,cis).16 This procedure is general for many aldehydes (Scheme 7.4), but the use of formaldehyde is limited to some special cases.17... 

The reaction of isolated and probably functionalized tannins from natural sources with formaldehyde to yield low temperature-curing thermoset adhesives. These resins may be suitable for use alone or in combination with conventional resorcinol-formaldehyde or phenol-resorcinol-formalde-hyde resins. 

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

In Resin Manufacture. Proper control of the exothermic reaction of formaldehyde with the tannin/resorcinol mixtures is essential. Full water cooling should be applied already at temperatures as low as 25 to 30 °C, otherwise, without it, a 1-ton industrial batch can sometimes pass from ambient to 80 °C in less than 2 minutes I have seen the stainless steel door of a reaction vessel fly 20 meters into the air as a consequence of this mistake, and the operator was lucky The vessel may easily explode or the reaction mixture boil over through the condenser, if the reaction is not properly controlled at its beginnings. 

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