Tannins particleboard

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. 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

Interest in pine bark as a source of adhesive components began to accelerate following the oil crisis of 1973. Sodium hydroxide extracts of southern pine bark were successfully used in replacing up to 40% of the phenolic resin for bonding of particleboards, oriented strandboards, and composites with a flakeboard core and veneer facing (50f51). Similar results were obtained with extracts from patula pine (52). Encouraged by results of this type, the New Zealand Forest Products Ltd. Corporation expanded their radiata pine bark tannin pilot plant to full-scale operation in 1981 to produce an extract trademarked Tannaphen. This material was crosslinked with paraformaldehyde and used as an adhesive... 

Attempts to combine the natural glues with diisocyanates have led to success. Several tannin adhesive formulations fortified with diisocyanates for particleboards and beechwood have been described in the literature 1-4) ... 

Formulations for particleboard and plywood adhesives based on combinations of diisocyanates and compounds from renewable resources like tannins, starch, and proteins have been developed and tested at the Fraunhofer-Institute. All in all, the results of gluing tests indicated the potential for using diisocyanates combined with natural polymers for adhesive purposes where each natural product used alone will fail. More attempts will be necessary to find precise mixtures to produce successful adhesives for panel products. The development of adequate particleboard and plywood glue formulations based on diisocyanates and natural polymers has to be accompanied by more sophisticated technologies than those used in conventional production processes. The results presented here may give some hints on how these technologies can evolve. 

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. 

Wattle tannin resins are also used to manufacture other resins, such as foams comparable to phenolics, as waterproofing additives, and binders for corrugated cardboard or charcoal briquettes. This discussion, however, deals only with particleboard, plywood, glulam, and finger-jointing exterior-grade wood adhesives. Formulations of the adhesives will be mentioned ad hoc, if at all necessary, as they have already been extensively discussed in articles and reviews in the relevant literature.(7)... 

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

Longterm Performance of Exterior-Grade Tannin-Bonded Particleboard... 

Figure 4. Photographic comparison of 12 mm thick exterior grade commercial particleboards manufactured in the same industrial plant and bonded with MUF, wattle tannin-formadehyde, and PF resins The panels were exposed at the CSIR testing site (4,500 feet above sea level) at Pretoria, South Africa for 15 years. Note the extensive degeneration of MUF board (commerically imported MUF resins from West Germany). Also note the more marked edge and surface degeneration of the PF in comparison to the TF-bonded boards.

Tannin-Induced Formaldehyde Release Depression in Urea-Formaldehyde Particleboard... 

Addition of tannin extract to UF resins in particleboard appear to decrease HCHO-emission over periods of time proportional to the amount of tannin added. The addition of tannin extract appears only to be a stop-gap short to medium term measure because, once the capability of the tannin to absorb and react with HCHO fumes slowly released by the board has been exhausted, the board revert to emissions similar to those of the UF controls. 

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. 

The first point indicate clearly that the addition of tannin is only a stop-gap measure to decrease HCHO-emission from UF-bonded particleboard as the period of lowered HCHO emission is directly proportional to the amount of tannin extract (or better of phenolic matter in the tannin extract = 80 Z) added. Once the tannin has,... 

Table 1 Unfortified Tannin-Formaldehyde Adhesives Obtained by Acid-Alkali Treatment for Exterior-Grade Particleboard Example of Industrial Board Results...

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... 

Table 2 Properties of Particleboard Manufactured Using Pine Tannin Adhesives... 

The main parameter for the application of tannins as adhesives for wood-based panels is the content of reactive polyphenols and the reactivity of these components towards formaldehyde. Tannins can be used as adhesives alone (with a formaldehyde component as crosslinker) or in combination with aminoplastic or phenolic resins. These resins can react chemically with the tannin component in a polycondensation reaction, form only two interpenetrating networks, or both. The simplest adhesive mix formulation consists of the tannin solution and powdered paraformaldehyde as crosslinker [283]. The addition of paraformaldehyde can cause in the short term a relatively high level of formaldehyde emission. Glue mixes using paraformaldehyde for the production of particleboards with low formaldehyde emission are described and used industrially [284]. In the literature a large number of papers describe the combinations of tannins with synthetic resins (Table 14). 

Compared to previous gluten based adhesives the present resins are applicable in liquid form, thus without any need for modifications of the application systems in particleboard factories. Relative to other protein adhesives such as soy-based adhesives based on reaction with formaldehyde some of the resins presented here have several advantages (i) they cannot and do not produce any aldehyde emission as neither formaldehyde nor any other volatile aldehyde was used in some of the formulations (ii) the percentage of natural materials was increased up to 70% for one type of formulation and up to 95% for others. Furthermore, in relation to resin formulations based on different cross-linking reactions other than those with formaldehyde the resins presented here have other advantages they are competitive with alternate natural resin systems such as those based exclusively on tannins and/or lignins. 

The results for wood particleboard prepared with the mixed formulations of tannin/hexamine + glyoxalated lignin are shown in Table 4. The internal bond (IB)... 

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