Лако-красочные материалы — производство

Tannins are polyhydroxyphenols of vegetable origin, which are soluble in water, alcohols, and acetone and can coagulate proteins. They are obtained by extraction from wood, bark, leaves, and fruits. Other components of the extraction solutions are sugars, pectins and other polymeric carbohydrates, amino acids, and other substances. The nontannin substances can reduce wood failure and decrease water resistance of glued bonds [176]. The polymeric carbohydrates especially increase the viscosity of the extracts.

The basic structures of condensed or polyflavonoid tannins are [176]

in the A-ring: resorcinol, phloroglucinol

in the B-ring: pyrogallol, catechol, and more rarely phenol.

Depending on the chemical structure of the A-rings two main types can be distin­guished:

resorcinol type: in mimosa/wattle, quebracho, Douglas fir, spruce tannin extracts

phloroglucinol type (pine type): most pine species, e. g., Pinus radiata, Pinus patula, Pinus elliotti, Pinus taeda, Pinus pinaster, Pinus halepensis, Douglas fir, and Pinus echinata. Pinus brutia and Pinus ponderosa are mixed types with predo­minant resorcinol character.

The disadvantage of the phloroglucinol type is the distinct lower yield during extraction as well as the much higher reactivity of the A-ring towards formaldehyde, which if uncon­trolled can cause extremely short pot lives of the glue mix.

The disadvantages of these polyphenols are the high viscosity of the solutions in the range of the concentrations of industrial application, due to the polymeric carbohydrates and high molecular weight tannins [228,229], and in some cases their short pot life. The maximum usable concentration of tannin solutions is approximately 40% by mass, except for mimosa where it can be as high as 50%. By selectively removing the polymeric carbo­hydrates the viscosity can be decreased and with this the possible concentration can be increased. Such purification steps using an ultracentrifuge [229-233] and an acid precipita­tion followed by filtration or centrifugation have been described [185,234]. However, they have not yet been introduced in industrial practice; they are only available at laboratory scale. A further possibility is the optimization of the conditions during the extraction in order to minimize the content of nontannins in the extract.

The viscosity of tannin solutions usually increases at higher pHs [185,235,236], but for some tannin types no clear dependence of the viscosity on the pH is shown. The viscosity of an extract increases with the solids content, especially if carbohydrates are present from the extraction step. There are several ways to decrease the viscosity of tannin extracts:

Dilution (lower solids content): this leads to increased moisture content of the glued particles (which is not necessarily a disadvantage, since tannins need high moisture contents of the glued particles to guarantee proper flow during press­ing) as well as to a decreased content of active adhesive [176].

Degradation of the high molecular carbohydrates, e. g., by NaOH [237,238].

Addition of hydrogen bond breakers, e. g., urea [228,239,240].

Modification of the extract by sulfite or bisulfite [241]: this modification of the extracts will especially decrease the sometimes high viscosity to achieve a better performance, but also a longer pot life and a better crosslinking will be achieved; however, it can give poor results if too high a level of sulfite is used.

Modification by treatment with acetic anhydride or maleic acid anhydride as well as NaOH to decrease the viscosity [228,242-244].

Tannins are used mostly in the southern hemisphere [176]; applications in Europe are only for niche products with special properties. Depending on the resin content applied to wood, tannins can be used for interior or exterior boards. The necessary crosslinking is often done by addition of formaldehyde. This, however, can lead to some formalde­hyde emission, but this is low due to the phenolic nature of the tannin. Sometimes crosslinking is performed by the addition of isocyanate. Hardening by tannin autocon­densation without any aldehyde addition is also possible [245-247].

Tannins from mimosa, quebracho, and pine (Pinus radiata) are actually used on an industrial scale for wood gluing. The extraction itself is only performed industrially in the southern hemisphere. The tannins are produced by water extraction of the wood or of the bark. Suitable solvents are water, alcohols [248], or acetone. Some of the para­meters which influence tannin extraction are:

temperature [240,248-252]

addition of various chemicals, e. g., NaOH or sodium carbonate [185,233,234, 248,249,253-261], sodium sulfite or bisulfite [241,248,250,262], and sulfite/ bisulfite with sodium carbonate with or without urea [240]

duration of the extraction [240,258,260]

concentration of the extraction solution: ratio of the amount of dry bark to the amount of extraction solvent [260]

properties of the raw material: wood species, age, time span between harvesting and extraction, storage conditions, particle sizes [176].

Usually concentrated solutions or spray dried powders are sold [176]. A purification step usually is not done at industrial scale level [176].

As tannins contain many ‘‘phenolic’’ type subunits one may be tempted to think that they will exhibit a similar reactivity potential to that of phenol and, therefore, procedures used in standard PF production can be transferred to those containing tannin. This, however, is not the case; the real situation is that tannin is far more reactive than unsubstituted phenol due to the resorcinol and phloroglucinol rings present in the tannin structure [263,264]. This increase in hydroxyl substitution on the two aromatic rings imparts an increase in reactivity to formaldehyde 10 to 50 times greater as compared to simple phenol. This whilst initially sounding promising creates additional problems with respect to producing an industrially applicable resin, due to limited pot lives of the ready-to-use formulations [227], although these problems have been solved and solved well even at industrial level [263,264].

Besides tannin autopolymerization, crosslinking usually is achieved via methylene or other bridges in a polycondensation reaction with formaldehyde or isocyanates. Tannins react with formaldehyde similarly to phenol, whereby the nucleophilic sites of the A-ring are more reactive than those of the B-ring. Formaldehyde reacts with a tannin in an exothermic reaction forming methylene bridges, especially between the reactive sites of the tannin A-rings. The reactive sites of the B-ring need a pH of at least 10 [265,266] to react. However, at such a high pH the reactivity of the A-ring becomes so high that no useful pot lives of the glue mix are obtained any more. Due to their size and shape the tannin molecules become immobile already at rather low degrees of condensa­tion, so that formation of further methylene bridges is impeded or hindered, causing a low degree of hardening (crosslinking) [266]. The higher the molar mass of the tannin, the earlier this effect occurs.

At neutral pH a rapid reaction of formaldehyde with the sites 6 and 8 on the A-ring takes place. This leads to the advantage that no (high) alkaline pH as for the phenolic resins is necessary to achieve rapid gelling and that a neutral glue line is obtained. A minor disadvantage is the necessary exact adjustment of the pH, because the gelation time varies strongly with the pH [266,267].

From a purely technological point of view the gel time may not be reduced below a certain limit. Decisive factors are the pot life, the viscosity of the tannin solution, and the rate of the steam escaping from the mat and the board during hot pressing. One possible way is the separate addition of the crosslinker, e. g., by dosing paraformaldehyde via a small screw conveyor directly to the particles in the blender. Also a liquid crosslinker, e. g., a urea — formaldehyde concentrate (UFC), can be mixed with the tannin solution in a static mixer just prior to the blender. The higher viscosity of the tannin solution at higher pHs, even without addition of the crosslinker, can be overcome by warming to 30-35° C or by adding water. A higher moisture content of the glued particles is no disadvantage in tannin adhe­sives, on the contrary it helps to guarantee a proper flow of the tannin during hot pressing.

Possible crosslinkers are formaldehyde as aqueous solution [268], paraformaldehyde [263,265,267,269], UFC [270,271], UF resins [272], aqueous formaldehyde solution emul­sified in an oil [273], dimethylolurea [274] or urea and phenol methylols with longer chains to overcome steric hindrance. Tannins can also be hardened by addition of hexamethy­lenetetramine (hexamine) [275], whereby these boards show a very low formaldehyde emission [269,275-281]. The autocatalytic hardening of tannins without any addition of formaldehyde or other aldehyde as crosslinker is possible, if alkaline SiO2 is present as a catalyst at high pH or just as a consequence of the catalysis of the reaction induced by a lignocellulosic surface [282].