Adhesive tannin-based

In this chapter, the development of a thermosetting adhesive from soda bagasse lignin is described. The research has concentrated on the development of interior-grade adhesives for particleboard. The local market for exterior boards is smaller than that for the interior panels, and adhesives for exterior boards are already covered by an excellent range of tannin-based adhesives. 

Interest in use of condensed tannins as components of adhesive formulations began about three decades ago. While research studies have been carried out in widely scattered laboratories around the world, three major areas of activity can be distinguished. These are 1) development of bark extracts and commercial production facilities on the west coast of North America, 1953 to 1975 2) application of tannins in adhesive formulations in South Africa based on indigenously produced mimosa (wattle) tannin, early 1970 s to the present and 3) a resurgence of interest in pine bark as raw material for tannin-based adhesives, beginning in the middle 1970 s. Each of these activities has been characterized by parallel efforts on structural identification of the tannins and development of unique methods for incorporating the isolated tannins into adhesives. 

Tannin-Based Adhesives for Finger-Jointing Wood... 

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. 

Figure 5. World production and use of wattle tannin-based adhesives.

Progress in the use of condensed tannins in adhesive formulations might be expected to be more rapid than is the case for lignins because of the impetus provided by the commercialization of wattle tannin-based adhesives and because of the extraordinarily high reactivity of tannins in reactions with formaldehyde. This reactivity offers an opportunity to substitute tannin for resorcinol (currently priced at about 1.80/lb) instead of phenol (about 0.40/lb). Now that wattle tannins have been successfully introduced, their application can be expected to continue to expand. The situation remains difficult, on the other hand, for use of conifer bark tannins in adhesives. Herb Hergert is certainly... 

The loose term renewable resources adhesives has been used to identify polymerie eom-pounds of natural, vegetable origin that have been modified and/or adapted to the same use as some classes of purely synthetic adhesives [1]. At present two classes of these adhesives exist one already extensively commercialized in the southern hemisphere and the other on the slow way to commercialization. These two types of resins are tannin-based adhesives [2] and lignin adhesives [3 ]. Both types are aimed primarily at substituting synthetic phenolic resins. In some aspects, such as performance, they closely mimic, or are even superior to, synthetic phenolic adhesives, while in others they behave in a vastly different manner from their synthetic counterparts. In this chapter we focus primarily on tannin-based adhesives because they have already been in extensive industrial use in the southern hemisphere, in certain fields of application, for the past 20 years. These adhesives are of some interest not only for their excellent performance in some applications but also for their mostly environmentally friendly composition. Lignin adhesives are treated briefly here and in detail in Chap. 28. 

Condensed tannins are often extracted and/or allowed to react at alkaline pH in the course of manufacture of speciality polymers such as tannin-based adhesives. These preparations invariably exhibit increased acidity and lower reactivity towards aldehydes than those obtained by neutral-... 

Tannin-based adhesives have been studied by many researchers for the past 30 years [3, 7-14]. Nowadays, tannins are commercially produced e.g., mimosa tannins in Brazil, South Africa, India, Zimbabwe and Tanzania. On the other hand, quabracho tannin is produced in Argentina, mangrove tannin in Indonesia, whilst both Italy and Slovenia produce chestnut tannin [15]. 

All the adhesive formulations in the study used tannin solution and resins of 40% solids content. Eight different adhesive formulations were prepared where 7 were tannin-based adhesives. For control (Formulation A), only LmwPF and wheat flour (100 and 15 parts by weight, respectively) were mixed. For formulations B to H, small amounts (10, 30, 50 parts) of LmwPF were first added to the tannin solution (40% solids) followed by wheat flour (15 parts) and paraformaldehyde (3 and 5%). The adhesive formulations used in the study are given in Table 1. 

The main natural resins used as wood panel binders are vegetal tannin adhesives, lignin adhesives and more recently also soy protein adhesives [1]. Of these, tannin-based adhesives have been used commercially the longest, since 1971. They offer the advantage over the other two types of not needing any reinforcement with an oil-derived synthetic resin [1]. Lignin [2-5] and soy binders [1, 6-8], however, still require between 20% and 40% of the total resin to be either phenol-formaldehyde or most often PMDI (polymeric isocyanate) to satisfy the requirements of relevant board standards. 

Work in South Africa and in South America has resulted in development of adhesives based largely or entirely on tannins. Tannins, which like lignins are renewable natural products, have the advantage of affording adhesives with low or no formaldehyde emissions, but tannin-based adhesives tend to be brittle. A useful tannin-based particleboard adhesive was reported using tannin extract... 

Most of the recent efforts to develop uses for the condensed tannins have centered on their application in wood adhesives. Reviews by Pizzi (182, 186) and others (15, 78, 87, 93, 208) provide references to several hundred papers and patents on this subject. Despite world-wide research efforts on other sources of tannins, particularly since the 1972-1973 petroleum shortage, the mimosa or wattle tannins extracted from the bark of black wattle Acacia mearnsii) remain the major source of condensed tannins exploited commercially for adhesive manufacture. Of the approximately 100000 tons of wattle tannin produced annually, only about 10000 tons are used in wood adhesives, predominantly in South Africa but also in Australia and New Zealand (186). The extensive use of wattle tannins by the wood products industry of South Africa is impressive indeed, as these tannins have partly replaced phenol and resorcinol usage in adhesives for bonding of particleboard, plywood, and laminated timbers (182, 186, 213). Three factors have contributed to the success in use of wattle tannin-based adhesives, namely the comparatively high costs of phenol and resorcinol in the Southern Hemisphere, their resorcinolic functionality and low molecular weight and, perhaps most importantly, the commitment by the research and industrial communities of these countries to reduce the reliance of the forest products industry on petroleum-based adhesives. 

Table 10.3.1 Properties of exterior particleboards bonded with wattle tannin-based adhesives (136)...

Pizzi A 1979 Phenolic and tannin-based adhesive resins by reaction of coordinated metal ligands. II. Tannin adhesive preparation, characteristics and application. J Appl Polym Sci 24 1257-1268... 

Pizzi A 1980 Tannin-based adhesives. J Macromol Sci Rev Macromol Chem C 18(2) 247-315... 

Pizzi A, Scharfetter H O 1978 The chemistry and development of tannin-based adhesives for exterior plywood. J Appl Polym Sci 22 1745-1761... 

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