Лако-красочные материалы — производство

Phenol-formaldehyde core layer resins usually have the highest molar masses and hence show a high reactivity and quick gelation. They contain higher amounts of alkali than face

layer resins in order to keep the resin soluble even at higher degrees of condensation. The higher the degree of condensation during the production process (the higher the viscosity), the shorter the gel time [145]. The upper limit of the degree of condensation of the resin during its production process is given by (i) the viscosity of the resin (the resin must be able to be pumped, and a certain storage stability as well as a proper distribution of the resin on the particles during blending is required) and (ii) the flow behavior of the resin under heat, guaranteeing wetting of the unglued matching second wood surface and a sufficient pene­tration into the wood surface. Too high a moisture content of the glued particles limits the level to which it is possible to dilute the resin and its solids content.

The hardening of a phenolic resin can be seen as the transformation of molecules of different sizes via chains lengthening, branching, and crosslinking to a three-dimensional network with theoretically an endlessly high molar mass. The hardening rate depends on various parameters, such as molar mass, molecular structure of the resin, the portions of various structural elements as well as possible catalysts and additives.

Alkaline PF resins contain free reactive methylol groups in sufficient number and can harden even without any further addition of formaldehyde, a formaldehyde source, or catalysts. The hardening reaction is initiated by heat only. The methylol groups react to form methylene and methylene ether bridges. Under high temperatures methylene ether bridges can rearrange to methylene bridges. The lowest possible temperature for a suffi­ciently fast gel rate is approximately 100°C. In some cases to improve this, potash in the form of a 50mass% solution is added in the core layer resin mix in an amount of about 3 to 5% potash solid based on resin solids content.

Pizzi and Stephanou investigated the dependence of the gel time on the pH of an alkaline PF resin [146]. Surprisingly they found an increase in the gel time in the region of very high pH values (above 10). Standard commercial PF adhesive resins with a content of NaOH of 5 to 10 mass% have exactly such pHs. A decrease of the pH in order to accel­erate the hardening process is not possible, because a spontaneous precipitation would occur with such standard PF resins. A change in pH of the resin, however, might occur when the resin comes into contact with a wood surface. Wood is generally acidic in character, and especially with rather acidic wood species, the pH of the resin could sig­nificantly drop when in contact with the wood surface [147].

Lu and Pizzi [148] showed that lignocellulosic substrates had a distinct influence on the hardening behavior of PF resins, whereby the activation energy of the hardening process was much lower than for the resin alone [149] and the hardening rate much faster [149]. The reason is a catalytic activation of the PF condensation by carbohydrates such as crystalline and amorphous cellulose and hemicellulose. Covalent bondings between the PF resin and the wood, especially lignin, play only a minor role, however.

The gelling process can be monitored via DSC, ABES, or dynamic mechanical analysis (DMA). The chemical hardening can be followed also by solid state NMR, looking (i) at the increase of the amount of methylene bridges based on the amount of aromatic rings [123,150,151], (ii) at the portion of 2, 4, 6-three-substituted phenols [151], or (iii) at the ratio between methylol groups and methylene bridges [152,153]. This degree of hardening, however, is not equal to the degree of hardening as monitored by DSC. Plotting one of these degrees of chemical hardening versus the degree of mechanical hardening, as measured, e. g., via ABES or DMA, reveals the hardening pattern of a resin [151,154,155].

An acid- rather than alkali-induced gelling reaction of PF adhesive resins can cause severe deterioration of the wood substrate at the interface and, therefore, its use has lost its significance in the application of PF resins to bond wood. Pizzi et al. [156] describe, however, an effective procedure for the self-neutralization of acid hardened PF glue lines. The system is based on a mixture of a complex formed by morpholine and a weak acid in the presence of para-toluene sulfonic acid. The complex decomposes with heat and re­forms on cooling to a complex in which the weak acid has been exchanged with the weak base, yeilding an almost neutral glue line. The system prevents, to a considerable extent, the acid deterioration of the wood substrate. Several other attempts based instead on incorporating the acid chemically into the resin or fixing the hardeners physically in the glue line have failed [157].

The acceleration of the hardening reaction is possible by using as high a degree of condensation as possible. Another approach is the addition of accelerating esters [146,158], among which, for example, is propylene carbonate [158,159]. The mechanism of this acceleration is not yet completely clear; it might be due to the hydrogen carbonate ion after hydrolysis of propylene carbonate [160] although this has been shown to be unlikely [146,159] or due to the formation of hydroxybenzy alcohols and temporary aro­matic carbonyl groups in the reaction of the propylene carbonate with the aromatic ring of the phenol as in the Kolbe-Schmidt reaction of CO2 with phenol to give salicylic acid [146]. The higher the addition of esters such as propylene carbonate, the shorter the gel time of the PF resin [146]. Other accelerators for PF resins are potash (potassium carbo­nate), sodium carbonate [95,161], guanidines, or sodium and potassium hydrogen carbonate. Also chemicals inherent to wood might have an accelerating influence on the hardening reactivity of PF resins [161].

Since phenolic resins for wood bonding harden only thermally, postcuring during hot stacking is very important. In contrast to UF-bonded boards, PF-bonded boards should be stacked as hot as possible to guarantee a maximum postcuring effect. The strength of the panel improves during hot stacking due to continuous slow curing of the PF resin. On the other hand, very high temperatures during stacking might cause partial deterioration of the wood, seen as discoloration.