Лако-красочные материалы — производство

The purity of vegetable tannin extracts varies considerably. Commercial wattle bark extracts normally contain 70 to 80% active phenolic ingredients. The nontannin fraction, consisting mainly of simple sugars and high-molecular-weight hydrocolloid gums, does not participate in the resin formation with formaldehyde. Sugars reduce the strength and water resistance in direct proportion to the amount added. Their effect is a mere dilution of the adhesive resin solids, with consequent proportional worsening of adhesive proper­ties. The hydrocolloid gums, instead, have a much more marked effect on both original strength and water resistance of the adhesive [2,22,42]. If it is assumed that the nontannins in tannin extracts have a similar influence on adhesive properties, it can expected that unfortified tannin-formaldehyde networks can achieve only 70 to 80% of the performance shown by synthetic adhesives. In many glued wood products, the demands on the glue line are so high that unmodified tannin adhesives are unsuitable. The possibility of refining extracts has proved fruitless largely because the intimate association between the various constituents makes industrial fractionation difficult. Fortification is in many cases the most practical approach to reducing the effect of impurities. Fortification generally con­sists of copolymerization of the tannin with phenolic or aminoplastic resins [21,22,42,46]. It can be carried out during manufacture of the adhesive resin, during glue mix assembly, just before use, or during adhesive use. If added in sufficient quantity, various synthetic resins have been found effective in reducing the nontannin fraction to below 20% and in overcoming other structural problems [21,22]. The main resins used are phenol-formalde­hyde and urea-formaldehyde resols with a medium to high methylol group content. These resins can fulfill the functions of hardeners, fortifiers, or both. Generally, they are used as fortifiers in between 10 and 20% of total adhesive solids, and paraformaldehyde is used as a hardener. Such an approach is the favorite one for marine-grade plywood adhesives. These fortifiers are particularly suitable for the resorcinolic types of condensed tannins, such as mimosa. They can be copolymerized with the tannins during resin manufacture, during use, or both [2,21,22,42,44]. Copolymerization and curing are based on the con­densation of the tannin with the methylol groups carried by the synthetic resin. Since tannin molecules are generally large, the rate of molecular growth in relation to the rate of linkage is high, so that tannin adhesives generally tend to have fast gelling and curing times and shorter pot lives than those of synthetic phenolic adhesives. From the point of view of reactivity, phloroglucinol tannins such as pine tannins are much faster than mainly resorcinol tannins such as mimosa. The usual ways of slowing them down and, for instance, of lengthening adhesive pot life are:

1. To add alcohols to the adhesive mix to form hemiacetals with formaldehyde and therefore act as retardants of the tannin-formaldehyde reaction.

2. To adjust the adhesive’s pH to have the required pot life and rate of curing.

3. To use hexamine as a hardener, which under the current conditions gives a very long pot life at ambient temperature but still fast curing time at higher temperatures.

The viscosity of bark extracts is strongly dependent on concentration. The viscosity increases very rapidly above a concentration of 50%. Compared to synthetic resins, tannin extracts are more viscous at the concentrations normally required in adhesives. The high viscosity of aqueous solutions of condensed tannins is due to the following causes, in order of importance:

1. Presence of high-molecular-weight hydrocolloid gums in the tannin extract [42,45]. The viscosity is directly proportional to the amount of gums present in the extract [42,45].

2. Tannin-tannin, tannin-gum, and gum-gum hydrogen bonds. Aqueous tannin extract solutions are not true solutions but, rather, colloidal suspensions in which water access to all parts of the molecules present is very slow. As a consequence, it is difficult to eliminate intermolecular hydrogen bonds by dilu­tion only [42,45].

3. Presence of high-molecular-weight tannins in the extract [28,42,45].

The high viscosity of tannin extract solutions has also been correlated with the proportion of very-high-molecular-weight tannins present in the extract. This effect is not well defined. In most adhesive applications such as in plywood adhesives, the viscosity is not critical and can be manipulated by dilution.

In the case of particleboard adhesives decrease of viscosity is, instead, an important prerequisite. When reacted with formaldehyde, unmodified condensed tannins give adhe­sives having characteristics that do not suit particleboard manufacture: namely, high viscosity, low strength, and poor water resistance. The most commonly used process to eliminate these disadvantages in the preparation of tannin-based particleboard adhesives consists of a series of subsequent acid and alkaline treatments of the tannin extract, causing hydrolysis of the gums to simple sugars and some tannin structural changes, thus improving the viscosity, strength, and water resistance of the unfortified tannin- formaldehyde adhesive [2,44]. Furthermore, such treatments may cause partial rearrangement of the flavonoid molecules that causes liberation of some resorcinol in situ in the tannin, rendering it more reactive, allowing better cross-linking with formalde­hyde, and ultimately yielding an adhesive which without addition of any fortifier resins gives truly excellent performance for exterior-grade particleboard [1,2,44]. Such re­arranged “phlobatannin” structures have formula as follows:

This modification cannot be carried out too extensively, but only to a limited extent to avoid precipitation of the tannin from solution by the formation of ‘‘phlobaphenes.’’ Typical results obtained are shown in Table 1.

Particular gluing and pressing techniques have been developed for tannin particle­board adhesives [47,48] to achieve pressing times much faster than those traditionally obtained with synthetic phenol-formaldehyde adhesives, although recent advances in syn­thetic phenol-formaldehyde resins have markedly limited such an advantage [49,50]. Pressing times of 7 s/mm of panel thickness have been achieved and pressing times of 9s/mm at 190 to 200°C press temperature are in daily operation: these are pressing times comparable to what is obtainable with urea-formaldehyde or melamine-formalde­hyde resins at the same pressing temperatures. The success of these simple types of particleboard adhesives relies heavily on industrial application technology rather than just on the preparation technology of the adhesive itself [42,47,51]. A considerable advan­tage is the much higher moisture content of the resinated chips tolerable with these adhesives than with any of the synthetic phenolic and amino resin adhesives. In the case of wood particleboard and of oriented strandboard (OSB) panels the technology so developed allows hot pressing at moisture contents of around 24% against values of 12% for traditional synthetic adhesives, and presents other advantages as well [42,51,52].

The best adhesive formulation for phloroglucinolic tannins such as pine tannin extracts is, instead, a comparatively new adhesive formulation that is also capable of giving excellent results when using resorcinolic tannins such as a wattle tannin extract [53-56]. The adhesive glue mix consists only of a mix of an unmodified tannin extract in 50% solution to which has been added paraformaldehyde and polymeric nonemulsifiable 4,40-diphenylmethane diisocyanate (commercial pMDI) [53-56]. The proportion of tannin extract solids to pMDI can be as high as 70:30 based on mass, but can be much lower in pMDI content. This adhesive is based on the following peculiar mechanism, by which the MDI, in water, is hardly deactivated to polyureas [54,56]:

+ HCHQ

+ Catechm

+MDl(OCN-R-NCO)

Cafcechin—formoldehyde polymers

Catechm—MDl urethane

‘CH 5-O—C-NH—R—N H-C—0—CH.

Catechin-WDl polyurethane

The properties of the particleboard manufactured with this system using pine tannin adhesives are listed in Table 2. The results obtainable with this system are then quite good and not too different from the results obtainable with some of the other tannin adhesives already described. In the case of phloroglucinolic tannin extracts being used, no pH adjustment of the solution is needed. One point that was given close consideration is the deactivating effect of water on the isocyanate group of pMDI. It has been found

that the amount of deactivation by water of this group when in a concentrated solution (50% or over) of a phenol is much lower than previously thought [53-56]. This is the reason that aqueous tannin extract solutions and pMDI can be reacted without substantial pMDI deactivation by the water present.

The quest to decrease or completely eliminate formaldehyde emission from wood panels bonded with adhesives, although not really necessary in tannin adhesives due to their very low emission (as most phenolic adhesives), has nonetheless promoted some research to further improve formaldehyde emission. This has centered on two lines of investigation: (i) autocondensation of tannins (see Section IV. D below), and (ii) the use of hardeners that do not emit at all simply because no aldehyde has been added to the tannin. Methylolated nitroparaffins and in particular the simpler and least expensive exponent of their class, namely trishydroxymethyl nitromethane [57,58], function well as hardeners of a variety of tannin-based adhesives while affording considerable side advantages to the adhesive and to the bonded wood joint. In panel products such as particleboard, medium density fiberboard (MDF), and plywood the joint performance that is obtained is of the exterior/marine grade type, while a very advantageous considerable lengthening in glue mix pot life is obtained. Furthermore, the use of this hardener is coupled with such a marked reduction in formaldehyde emission from the bonded wood panel as to reduce emission exclusively to the formaldehyde emitted by heating just the wood (and slightly less, thus functioning as a mild depressant of emission from the wood itself). Furthermore, trishydroxymethyl nitromethane can be mixed in any proportion with traditional formal­dehyde-based hardeners for tannin adhesives, its proportional substitution of such hard­eners inducing a proportionally marked decrease in the formaldehyde emission of the wood panel without affecting the exterior/marine grade performance of the panel. Medium density fiberboard industrial plant trials confirmed all the properties reported above and the trial conditions and results are reported [57,58]. A cheaper but as equally effective alternative to hydroxymethylated nitroparaffins is the use of hexamine as a tannin hardener. This sometimes causes problems of early agglomeration in some tannins [59] and a better solution proposed which overcame such problems was rather to use as a hardener a mix of formaldehyde coupled with an ammonium salt.