Particleboard adhesives

With respect to thermosetting, it has been well documented that crude Ca-based SSL can be used as a particleboard adhesive (46). This application has been evaluated on a mill-scale basis in Denmark, Finland, and Switzerland, but has not been adopted on a commercial scale. This is because resin curing required both high press and autoclave temperatures as well as long heating times. While this development was a technical success, it was - according to Nimz (36) - never commercialized due to the frequency of fires experienced during mill-scale trials. 

The use of lignin, essentially as a diluent (extender), has also been investigated (75). In this case, di- or polyisocyanates were reacted with ethylene or propylene carbonates in a solution containing lignin. These mixed ethylene and propylene carbonate-containing organic polyisocyanates were suitable as particleboard adhesives. 

The moisture dependence of the two PF-extended lignin adhesives was subsequently determined. Single-layer boards were prepared with the resinated chips at different moisture contents. The adhesive content (10%) and press conditions (170 °C, 15 min.) were kept constant. The results presented in Figure 4 indicate the deleterious effect of high moisture contents of the chips on the strength of the boards. The application of the lignin particleboard adhesive thus clearly requires strict control of the moisture content of the chips. 

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. 

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. 

Particleboard adhesives 70% success dependent upon application conditions and techniques... 

Pizzi and colleagues (Chapter 7) have cleverly taken advantage of the special circumstances offered in the kraft lignins from bagasse to develop both cold-setting wood laminating and particleboard adhesives. Lignin oxidations... 

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... 

The first attempts to use wattle tannins in particleboard adhesives (114, 170) followed reports by Dalton (50, 51) on the use of Pinus radiata bark extracts as substitutes for phenol-formaldehyde resins in plywood adhesives. Even though the molecular weight of wattle tannin is comparatively low (208), solutions of bark extracts at solids contents required for adhesives (40%-58%) exhibit excessively high viscosities. High-temperature alkaline treatments reduced their viscosity, and... 

The development of wattle tannin-based plywood adhesives occurred concurrently with the particleboard adhesives and, although quite different adhesive properties are required to bond these two types of furnish, many of the basic premises on which their development were based are the same. A series of wattle tannin-based plywood adhesives that provide exterior bond qualities have been described in detail by Pizzi (182, 186). All recent formulations use low-molecular-weight polymers, as first suggested by MacLean and Gardner (131), rather than formaldehyde for crosslinking agents. 

Conifer Bark and Related Uannins as Particleboard Adhesives... 

It would seem that condensed tannins would have good potential for use in adhesives for oriented strand- or flakeboard particularly because of their known tolerance to comparatively high moisture content furnish and because of the potential for more rapid cure rates. At current prices for phenol-formaldehyde resins in North America, interest in these types of adhesives is not particularly strong. However, considering the rapid expansion of this industry, good potential exists if prices for phenol-formaldehyde resins should increase substantially. It should be recognized that amounts of resins applied to oriented strand- or flakeboards in North America are much lower that those used in the above-described exterior particleboard adhesive applications. 

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