Industrial tannin adhesives

Tannin adhesives are the class of modified natural adhesives which are used and have been used industrially for more than 25 years, mainly in South Africa and Australia, but also in Zimbabwe, Chile, Argentina, Brazil and New Zealand. It is mainly the still limited raw material supply that limits their use to approximately 30,000 tons resin solids per year [16,17]. 

Wattle (mimosa extract) tannin adhesives have been industrially produced and used in South Africa for many years. From the first, mostly unsuccessful, attempts in 1968 has evolved the consistent manufacture, industrial application,... 

The experience of tannin extract manufacturers, and of companies dealing with tannin adhesives is even more extensive, but results are similar to those illustrated in Figure 1. Thus, the first feedback from any industrial application is that the formulations must smooth and reduce the greater variability of the natural raw material. It is amazing, but in all the articles presented by all the research groups, this point is only hinted at, or not mentioned at all Many... 

Where does the secret of technical industrial success with tannin adhesives lie It varies for certain adhesives, it is in the formulation of the adhesives only for others, it lies in the application conditions and techniques only for most, success requires a good balance of both. While excellent correspondence between laboratory and plant results can often be obtained in adhesives that depend for success on resin formulation, this is usually not the case when success is more dependent on application techniques and conditions (Table I). 

Table II. Comparison of Approximate Industrial Conditions of Application for Particleboard Bonded with Wattle Tannin Adhesives...

In conclusion, it is worthwhile to note that in 1970-71, when I started to be involved with tannin adhesives, there were no more than three laboratories, academic or industrial, conducting research on tannin adhesives. By 1986, I have had direct or indirect contact with 42 different laboratories in many countries that have carried out at some time, or are now carrying out, research on these bonding materials. The increase in industrial usage has also been fast but for various reasons has lagged behind the technical and scientific interest. However, after practically zero consumption in 1970, indications are that about 12,000 tons of exterior-grade tannin resin solids per year are now produced and consumed in the timber industry based on reliable information from several... 

Wood panel adhesives predominantly, or even exclusively, composed of natural materials already exist. Some of these have now been in commercial/industrial use in a few countries for a rather long time. Among these it is sufficient to briefly mention tannin adhesives [1-3], commercial since the early 1970s, variations on tannin/lignin adhesives that are coming to the market at present [4, 5], some types of lignin adhesives [6-9] and different adhesives based on soy protein flour and hydrolysates [10-13] also on the way to early industrialisation. In all these adhesives, synthetic polymers are present in very small amounts or are totally absent. 

More recently, a modification of the system described by Kreibich has been used extensively in industry with good success. Part A of the adhesive is again a standard phenol-resorcinol-formaldehyde (PRF) cold-setting adhesive, with powder hardener added at its standard pH. Part B can be either the same PRF adhesive with no hardener and the pH adjusted to 12, or a 50 to 55% tannin extract solution at a pH of 12-13, provided that the tannin is of the condensed or flavonoid type, such as mimosa, quebracho, or pine bark extract, with no hardener [118,135-137], The results obtained with these two systems are good and the resin not only has all the advantages desired but also the use of vegetable tannins and the halving of the resorcinol content makes the system considerably cheaper [118,135-137]. 

Tannins are polyphenols that occur only in vascular plants such as leaves, needles, barks heartwood, seeds and flowers. Tannins exist primarily in condensed and hydrolysable forms. Natural tannin extracts have been employed since the turn of the last century in leather industry. Recent development of their industrial uses as adhesives, flocculants, depressants, viscosity modifier agents and more recently as corrosion inhibitors reflects their importance as industrial raw materials. Electrochemical studies have shown that tannins extracted from the barks of mangrove trees are excellent corrosion inhibitors of steel at very low pH. The mechanism of inhibition at this pH was due to the chemisorption of tannin molecules while at higher pH, inhibition was achieved via formation of ferric-tannates [6]. 

The rapid growth of the pulp and paper industry following World War II coupled with a renewed scientific interest in utilization of bark and wood residues led to investigative programs on bark and wood tannins. The leather industry was continuing to decline in importance, so other alternatives were needed. One of these was replacement of phenol in whole or in part in phenol-formaldehyde adhesive formulations. This work progressed to the point where commercial quantities of polyphenolic extractives were made and sold for adhesive application. Excessive capacity and low petrochemically derived phenol prices in the 1960 s led to the demise of this effort in the United States (2,5). 

Research vs. Industrial Practice with Tannin-Based Adhesives... 

Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

The amounts of lignins, tannins, and carbohydrates available as residues from processing of forest trees dwarf the commodity adhesive market. At the same time, the forest products industry is especially reliant on adhesives, since over 70% of all wood products are bonded, and their production consumes about 45% of all phenolic and 85% of all urea-formaldehyde resins produced in the United States. 

Tamarind seed is a by-product of the tamarind pulp industry. Presence of tannins and other dyeing matters in the seed testa make the whole seed unsuitable for consumption, but they become edible after soaking and boiling in water. Tamarind seed is also used as raw material in the manufacture of polysaccharide (jellose), adhesive and tannin. Seeds and kernels are high in protein content, while the seed coat is rich in fiber and tannins (anti-nutritional factors). Seeds are gaining importance as an alternative source of proteins, and are besides a good source of fatty acids and rich in some essential minerals, such as Ca, P, Mg and K. 

The tannins of some woods, acacias, chestnuts and oaks - and also their barks -were once of importance for the tanning of leather. Yet they have adverse influence on gluing (inhibiting polymerization of adhesives) and in pulping. Today, in the state of Rio Grande do Sul, Brazil, 100,000 ha of Acacia mearnsii provide short fibre for chip export while the 27-28% tannin in its bark supports another industry. 

The emission of formaldehyde fumes from particleboard manufactured using urea-formaldehyde resins, and its decrease, have now been topics of interest in the timber and wood adhesives industry for a long time. Many solutions, some very effective, to this problem have already been advanced by many authors. In this brief article we do not pretend to present yet another successful or less successful method to control HCHO emission but to show the decrease in the amount of formaldehyde emitted by UF-bonded particleboard, over a period of time, to which tannin extract has been added in small amounts. Tannin extract is an inexpensive commodity in Southern Africa as well as in many other countries in the southern emisphere such as Brazil, Argentina and New Zealand. The method presented, if not completely effective may be an inexpensive system of control of HCHO emission over a limited period of time. 

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