Лако-красочные материалы — производство

The same chemical mechanisms and driving forces presented for PF resins apply to resor­cinol resins. Resorcinol reacts readily with formaldehyde to produce resins 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 [4]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [5]. However, reaction with the remaining reactive but sterically hindered site (2-position) between the hydroxyl functions also occurs [4]. In relation to the weights of RF 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, cognizance must be taken of the fact that the first mentioned pair represents two condensation sites relative to one. The difference in reactivity of the two types of sites (i. e., 4- or 6-position relative to the 2-position) is then 5:1 [4]. Linear com­ponents always appear to form in preference to branched components in A-stage resins [4]; that is, terminal attack leads to the preferential formation of linear rather than branched condensates. This fact can be attributed to:

1. The presence of two reactive nucleophilic centers on the terminal units, as opposed to single centers of doubly bound units already in the chain.

2. The greater steric hindrance of the available nucleophilic center (nearly always at the 2-position) of the doubly bound units as opposed to the lower steric hin­drance of at least one of the nucleophilic centers of the terminal units (a 4- or 6- position always available). The former is less reactive as a result of the increased steric hindrance. The latter are more reactive.

3. The lower mobility of doubly bound units which further limits their availability for reaction (Formula 1)

The absence of methylol (-CH2OH) groups in all six lower-molecular-weight RF condensates which have been isolated [4] 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 para-hydroxybenzyl carbonium ions or quinone methides. This explains how identical condensation products are obtained under acid or alkaline reaction conditions [4]. 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 [6,7].

From a kinetic point of view, the initial reaction of condensation to form dimers is much faster than the subsequent condensation of these dimers and higher polymers. 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 PF system [8]. The high reactivity of the RF system renders it impossible to have these adhesives in resol form. Therefore, only RF novolaks, thus resins not containing methylol groups can be produced. Thus all the resorcinol nuclei are linked together through methylene bridges with no methylol groups being present and generally without any presence of methylene-ether bridges either.

The reaction rate of resorcinol with formaldehyde is dependent on the molar ratio of the two constituents, the concentration of the solution, pH, temperature, presence of various catalysts, and amount of certain types of alcohols present [9-12]. The effect of pH and temperature on the reactivity and gel time of the RF system presents the same trend as for all phenolic-formaldehyde reactions, with a minimum of reactivity at around pH 4 and the rate of reaction becoming rapidly faster at progressively more alkaline and more acid pH values [12,13]. Methanol and ethanol slow down the rate of reaction. Other alcohols behave similarly, the extent of their effect being dependent on their structure. Methanol lengthens gel time more than other alcohols, higher alcohols being less effective. The retarding effect on the reaction is due to temporary formation of hemiacetals between the methanol (or other alcohols) and the formaldehyde. This reduces the reaction rate because of the lower concentration of available formaldehyde [10,11]. Other solvents also affect the rate of reaction by forming complexes or by hydrogen bonding with the resorcinol [10,13].

In the manufacture of pure resorcinol resins the reaction would 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 glue. 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 [4]. This can be represented as follows (Formula 2)

In the reaction that follows the resorcinol chemical is added in excess, in a suitable manner, to polymer I to react with the — CH2OH groups to form PRF poly­mers in which the terminal resorcinol groups can be a resorcinol or any type of RF polymer (Formula 3).

In reality, as resorcinol is expensive, the resin manufacturers tend to limit the amount of resorcinol grafted onto the PF resol. This is then generally lower than what is necessary to form structures as shown in Formula 3. Structures as these do occur but the majority of the polymers present in the PRF resin are of the type (Formula 4)

where the residual third reactive site of the resorcinol is still free and is the site through which cross-linking takes place by reaction with the added formaldehyde hardener (see Chapter 8, Fig.1).

Where straight resorcinol adhesives are not suitable, resins can be prepared from modified resorcinol [13]. Characteristic of these types of resins are those used for tire cord adhesives, in which a pure RF 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 tire cord adhesives, which constitutes less than 10% of the total use.

Figure 1 Effect of pH and temperature on the reactivity of the resorcinol-formaldehyde system.

Various adhesive formulations can be used for the manufacture of laminated wooden beams and fingerjoints for structural purposes. Only those adhesive formulations that at some time or other have been used in industrial applications will be described. All these formulations are based totally or partially on resorcinol, and the hardening process is carried out at ambient temperature, to 50°C [14].