Лако-красочные материалы — производство

Phenols condense initially with formaldehyde in the presence of either acid or alkali to form a methylolphenol or phenolic alcohol, and then dimethylolphenol. The initial attack may be at the 2-, 4-, or 6-position. The second stage of the reaction involves methylol groups with other available phenol or methylolphenol, leading first to the formation of linear polymers [1] and then to the formation of hard-cured, highly branched structures.

Novolak resins are obtained with acid catalysis, with a deficiency of formaldehyde. A novolak resin has no reactive methylol groups in its molecules and therefore without hardening agents is incapable of condensing with other novolak molecules on heating. To complete resinification, further formaldehyde is added to cross-link the novolak resin. Phenolic rings are considerably less active as necleophilic centers at an acid pH, due to hydroxyl and ring protonation.

However, the aldehyde is activated by protonation, which compensates for this reduction in potential reactivity. The protonated aldehyde is a more effective electrophile.

+ H*- ———- ► °H

The substitution reaction proceeds slowly and condensation follows as a result of further protonation and the creation of a benzylcarbonium ion that acts as a nucleophile.

Resols are obtained as a result of alkaline catalysis and an excess of formaldehyde. A resol molecule contains reactive methylol groups. Heating causes the reactive resol molecules to condense to form large molecules, without the addition of a hardener. The function of phenols as nucleophiles is strengthened by ionization of the phenol, without affecting the activity of the aldehyde.

Megson [2] states that reaction II (in which resols are formed by the reaction of quinone methides with dimethylolphenols or other quinone methides) is favored during alkaline catalysis. A carbonium ion mechanism is, however, more likely to occur. Megson [2] also states that phenolic nuclei can be linked not only by simple methylene bridges but also by methylene ether bridges. The latter generally revert to methylene bridges if heated during curing with the elimination of formaldehyde.

The differences between acid-catalyzed and base-catalyzed process are (1) in the rate of aldehyde attack on the phenol, (2) in the subsequent condensation of the phenolic alcohols, and (3) to some extent in the nature of the condensation reaction. With acid catalysis, phenolic alcohol formation is relatively slow. Therefore, this is the step that determines the rate of the total reaction. The condensation of phenolic alcohols and phenols forming compounds of the dihydroxydiphenylmethane type is, instead, rapid. The latter are therefore predominant intermediates in novolak resins.

Novolaks are mixtures of isomeric polynuclear phenols of various chain lengths with an average of five to six phenolic nuclei per molecule. They contain no reactive methylol groups and consequently cross-link and harden to form infusible and insoluble resins only when mixed with compounds that can release formaldehyde and form methylene bridges (such as paraformaldehyde or hexamethylenetetramine).

In the condensation of phenols and formaldehyde using basic catalysts, the initial substitution reaction (i. e., the formaldehyde attack on the phenol) is faster than the subsequent condensation reaction. Consequently, phenolic alcohols are initi­ally the predominant intermediate compounds. These phenolic alcohols, which contain reactive methylol groups, condense either with other methylol groups to form ether links, or more commonly, with reactive positions in the phenolic ring (ortho or para to the hydroxyl group) to form methylene bridges. In both cases water is eliminated.

Mildly condensed liquid resols, which are the more important of the two types of phenolic resins in the formulation of wood adhesives, have an average of fewer than two phenolic nuclei in the molecule. The solid resols average three to four phenolic nuclei but with a wider distribution of molecular size. Small amounts of simple phenol, phe­nolic alcohols, formaldehyde, and water are also present in resols. Heating or acidifica­tion of these resins causes cross-linking through uncondensed phenolic alcohol groups, and possibly also through reaction of formaldehyde liberated by the breakdown of the ether links.

As with novolaks, the methylolphenols formed condense with more phenols to form methylene-bridged polyphenols. The latter, however, quickly react in an alkaline system with more formaldehyde to produce methylol derivatives of the polyphenols. In addition to this method of growth in molecular size, methylol groups may interact with one another, liberating water and forming dimethylene ether links (-CH2-O-CH2-). This is particularly evident if the ratio of formaldehyde to phenol is high. The average molecular weight of the resins obtained by acid condensation of phenol and formaldehyde decreases hyperbolically from over 1000 to 200, with increases in the molar ratio of phenol to formaldehyde from 1.25:1 to 10:1.

Thermomechanical analysis (TMA) on wood joints bonded with phenol-form­aldehyde (PF) adhesives has shown that, frequently, the joint increase in modulus does not proceed in a single step but in two steps, yielding an increase in the modulus first derivative curve presenting two major peaks rather than the single peak obtained for mathematically smoothed modulus increase curves [3]. This behavior has been found to be due to the initial growth of the polycondensation polymer leading first to linear polymers of critical length for the formation of entanglement networks. The reaching of this critical length is greatly facilitated by the marked increase in concen­tration of the PF polymer due to the loss of water on absorbent substrates such as wood, coupled to the linear increase in the average length of the polymer due to the initial phase of the polycondensation reaction. The combination of these two effects lowers markedly the level of the critical length needed for entanglement. Two modulus steps and two first derivative major peaks then occur, with the first peak due to the formation of linear PF oligomer entanglement networks, and the second one due to the formation of the final covalent cross-linked network. The faster the reaction of phenolic monomers with formaldehyde, or the higher the reactivity of a PF resin, the earlier and at lower temperature the entanglement network occurs, and the higher is its modulus value in relation to the joint modulus obtained with the final, covalently cross-linked resin (Fig. 1).