Tannin extract adhesives

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]. 

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. 

Selection of the End-Joint Profile. Because the purpose of this work was to determine whether a portion of the phenol-resorcinol-formaldehyde adhesive used in structural end-joints could be replaced by tannin extracts from southern pine bark, it was necessary to use a structural end-joint profile design. The profile chosen was the design used by many U.S. plants for the manufacture of structural end-joints. A reproduction in natural size with a drawing showing the exact dimensions is shown in Figure 1. 

Adhesives from Diisocyanates and Tannins. The tannin extracts were mixed as powders or solutions with nonemulsifiable or emulsifiable diisocyanate. The potlife of the formulations of extract powders with nonemulsifiable diisocyanate was sufficient the viscosities of the mixtures remained nearly constant for more than 5 hours. With extract solutions, the viscosity of the glue increased... 

Four types of condensed tannins were studied in the adhesive dips 1) extracts from pecan nut pith obtained by digestion with aqueous sodium sulfite-sodium carbonate solutions, 2) purified tannins from southern pine bark, 3) extracts from southern pine bark obtained by digestion with aqueous sodium sulfite-sodium carbonate solutions, and 4) tannins extracted with acetone-water solutions from peanut skins. 

Figure 2. Pullout forces of polyester cords for various tannin-containing adhesive dips as a function of percent resorcinol. 1, pecan pith sulfite extract 2, purified pine bark tannin 3, pine bark sulfite extract 4, peanut skin tannin.

Condensed tannins have considerable promise as substitutes for resorcinol used in resin formulations for bonding of nylon or, particularly, polyester cord to rubber. Although much more work needs to be done, preliminary results suggest that refinement of extract properties and adhesive formulations could lead to a large, high-value market for condensed tannin extracts. 

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 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... 

Figure 1. Consistency in the viscosity of tannin extract solutions and adhesives made with them.

The major effort in understanding the chemistry of these complex derivatives in attempts to use them most effectively in adhesives is exemplified by the work of Pizzi (JO, JJ, 28). A detailed study of the composition of tannin extracts and their reaction characteristics... 

A wide range of adhesives are used. For the three panel types that are the focus of this discussion synthetic adhesives are used, although tannins extracted from bark... 

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. 

Natural phenolic compounds are used as both replacements for substantial portions of synthetic phenol in plywood adhesive resins and as glue mix additives to improve performance 4 to 6% is added, based on phenolic resin solids. They bring about improvements in assembly time tolerance and flow with no significant change in adhesion. Glue mix additions of wattle tannin or other condensed flavonoid tannin extracts with or... 

Formaldehyde is generally the aldehyde used in the preparation, setting, and curing of tannin adhesives. It is normally added to the tannin extract solution at the required pH, preferably in its polymeric form of paraformaldehyde, which is capable of fairly rapid depolymerization under alkaline conditions, and as urea-formalin concentrates. Hexamethylenetetramine (hexamine) may also be added to resins due to its potential formaldehyde releasing action under heat. Hexamine is, however, unstable in acid media [24] but becomes more stable with increased pH values. Hence under alkaline conditions the liberation of formaldehyde might not be as rapid and as efficient as wanted. Also, it has been fairly widely reported, with a few notable exceptions [25], that bonds formed with hexamine as hardener are not as boil resistant [26] as those formed by paraformaldehyde. The reaction of formaldehyde with tannins may be controlled by the... 

In recent years the importance of the marked colloidal nature of tannin extract solutions has come to the fore [27,36-45]. It is the presence of both polymeric carbohydrates in the extract as well as of the higher molecular fraction of the polyphenolic tannins which determines the colloidal state of tannin extract solutions in water [26,36]. The realization of the existence of the tannin in this particular state affects many of the reactions that lead to the formation and curing of tannin adhesives, to the point that reactions not thought possible in solution become instead not only possible but the favored ones [26,36], while reactions mooted to be of determinant importance when foimd on models not in the colloidal state have in reality been shown to be inconsequential to tannin adhesives and their tannin applications [43,44]. 

The viscosity of bark extracts is strongly dependent on concentration. The viscosity increases very rapidly above a concentration of 50%. Compared to synthetic resins, tannin extracts are more viscous at the concentrations normally required in adhesives. The high viscosity of aqueous solutions of condensed tannins is due to the following causes, in order of importance ... 

The high viscosity of tannin extract solutions has also been correlated with the proportion of very-high-molecular-weight tannins present in the extract. This effect is not well defined. In most adhesive applications such as in plywood adhesives, the viscosity is not critical and can be manipulated by dilution. 

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... 

Another application of condensed tannin extracts that has proved technically successful is as tire cord adhesives. Both thermosetting tannin formulations [68] and tannin-resorcinol-formaldehyde formulations have been experimented with successfully. 

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