condensed tannins for adhesives

Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 359-369 359

Literature Cited Albright, L. F.; Hanson. C. ACS Symp. Ser. 1976, No. 22, 114, 259. Behr, L. C. J. Am. Chem. Soc. 1954, 76, 3672. Bolleter, W. T. “Symposium Processing of Propellants and Explosives and

Chandler, C. D.; Kohlbeck, J. A. Bolleter, W. T. J. Chromatog. 1972, 64,

Dunstan, I.; Adolph, H. G.; Kamlet, M. J. ”Nltro Compounds, Proceedings of International Symposium“, Warsaw, 1963; pp 431-440; Chem. Abstr. 1965, 63. 17956~.

Flikscheim, 8.; Simon, T. Pr. Chem. SOC. 1907, 23, 163-4; “Beilsteins Handbuch der Organlschen Chemie”; Sprlnger: Berlln, 1933; Voi. 16, p 629.

Gehrlng, D. G. Anal. Chem. 1968, 40(4). 792-795.

Ingedlents”, 1977; 2.4-7.

123- 128.

Gilman, H.; Blatt, A. H. “Organic Syntheses”; Coll. Vol. 1, Wiley: New York,

Industrial Explosives Society (Japan), “The Handbook of Industrial

Low, A.; Balz, E. H. J. Am. Chem. SOC. 1921, 43, 341. Simon, R. L.; Floves, K. A.; Ross, D. S. “Oxidation in TNT Production”, 1977. Waters, W. A. “Mechanism of Oxidation of Organlc Compounds“; Methuen:

Yasuda, S. K. J. Chrometogr. 1964, 73, 78.

1951, p 541.

Explosives”; Tokyo, 1966; p 9.

London, 1964; p 49.

Received for review July 27, 1981 Revised manuscript received March 5 , 1982

Accepted April 21, 1982

REVIEW SECTION

Condensed Tannins for Adhesives

Antonio Pizzi

National Tlmber Research Institute. Council for Scientific and Industrial Research, Pretoria, Republic of South Africa

The current “state of the art” in the manufacture, formulation, and application of tannin-based adhesives is briefly reviewed.

Introduction

Natural tannin extracts have been employed since an- tiquity for the conversion into leather of hides and skins. This traditional “cottage-industry”-type process was dramatically changed during industrialization over the latter half of the last century, resulting in the substitution of most of the traditional ”hydrolyzable” tannins, i.e., alga- robilla, dividivi, tara, myrabolans, oak, and sumach extracts, with “condensed-tannins”, namely wattle (or mimosa), quebracho, and mangrove extracts. The early preference for the “hydrolyzable”-type tannins was probably dictated by their local availability and by their lower “astringency” compared with condensed tannins.

The drift in emphasis to “condensed tannins” was instead due to their greater availability from natural forests, such as for quebracho and mangrove extracts, or from industrial afforestation on an economical basis, such as for wattle (mimosa) extract, to the abundance and richness of these sources, and to the development of tanning methods which overcame the “astringency” factor. The structural differences between “hydrolyzable” and ‘condensed tannins“ are, however, of fundamental im- portance when considering uses of tannins other than leather manufacture. The hydrolyzable tannins are mixtures of simple phenols, such as pyrogallol and ellagic acid, and of esters of a sugar, mainly glucose with gallic and digallic acids. They can and have been used as partial substitutes of phenol in the manufacture of phenol-formaldehyde resins. Their chemical behavior is very similar

0196-4321 l82/1221-0359$01.25/0

to that of simple phenols of low reactivity toward formaldehyde, and their use as partial phenol substitutes in phenol-formaldehyde (PF) resins does not present special difficulties. However, the low level of phenol substitution allowed by the use of hydrolyzable tannins, their low reactivity toward formaldehyde, their now drastically de- creased worldwide production, as well as their lack of polymeric structure in their natural state contributed to severely limit their wider industrial application in processes other than the production of small amounts of specialized types of leather. Condensed tannins, instead, constituting well over 90% of the total world production of commercial tannins ( - 350 OOO tons per year), are both chemically and economically more interesting for the preparation of adhesives and resins.

Condensed tannin extracts which are potentially significant are those manufactured from the bark of the black wattle tree (or mimosa tannin of commerce, Acacia mearnsii, formerly mollissima), from the wood of the quebracho tree (Spanish: Quebra hacha, axe-breaker, Schinopsis lorentzii and balansae), from the bark of the hemlock tree (Tsuga heterophylla), and from the bark of several commonly used pine (Pinus) species. The preparation of wattle and quebracho extracts is a well estab- lished industrial practice and they are freely available in considerable amounts. These two extracts constitute the main part of the bulk of condensed tannins world production. Considerable potential, at least for application in adhesives, is also available in the form of pine and hemlock bark extracts which are not yet commercially exploited to any great extent (1 million ton per year minimum potential).

Condensed tannin extracts, such as wattle and quebracho, are composed of approximately 70% polyphenolic tannins, 20% to 25% nontannins, mainly simple sugars

0 1982 American Chemical Society

360 Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982

and polymeric carbohydrates (hydrocolloid gums), the latter of which constitute 3% to 6% of the extract and heavily contribute to extract viscosity, while the balance is accounted for by a low percentage of moisture (Schar- fetter et al., 1977). For example, in wattle extract the main polyphenolic pattern is represented by flavonoid analogues based on resorcinol A and pyrogallol B rings (I) (Drewes and ROUX, 1963). These constitute about 70% of the tannins. The secondary but parallel pattern is based on resorcinol A and catechol B rings (II)(Drewes and ROUX, 1963). These tannins represent about 25% of the total bark tannin fraction. Superimposed over these two predominant tannin flavonoid mixtures are two minor groups of analogues arising from photosynthetic processes oc- curring in the leaves and immature bark. These are based on phloroglucinol-pyrogallol (111) and phloroglucinol- catechol (IV) flavonoids (5%)(Drewes and ROUX, 1963). These four patterns constitute 65% to 80% of mimosa bark extract of commerce.

OH

L ,OH ,/OH

~

I OH HO 0 OH ffO-,’\., 0 . ~ -

, < , OH OH

I Ii

Similar flavonoid A and B ring relationships, though slightly different and less surely determined, also exist in quebracho tannins, in which the two main substitution patterns are also present (Roux et al., 1975). The main difference is that in quebracho no phloroglucinolic A rings pattern, or more likely a much lower quantity of it, is present. Completely different patterns and relationships do instead exist in the case of pine tannins. All the main pine species studied, with a few exceptions such as Pinus ponderosa and Pinus brutia, present only two patterns. The main pattern is represented by phloroglucinol A rings and catechol B rings structures (V). The other pattern, present in much lower proportion, is represented by phloroglucinol A rings and phenol B rings (VI) Hemingway and McGraw, 1976; Porter, 1974).

V VI

All these flavonoid units are repeated to give the polymeric true condensed tannins which constitute the bulk of the commercially available extracts. In the case of wattle tannins, structures I and I1 are repeated two to eleven times with the different units linked 4,6 with each other with the exception of where a phloroglucinolic A-ring unit is linked. Thus, the more general wattle and quebracho flavonoids natural autocondensation patterns appear to be mainly 4,6 (VII, VIII) between resorcinolic

VI1

HO

A-ring units, following initial 4,8 links (VII) or less fre- quently 4,6 links (VIII) between resorcinolic and lower terminal phloroglucinolic units (Roux et al., 1975).

However, more recently, it has been realized that also ”angular”-type (IX) structures, in which all the possible

reactive positions of phloroglucinolic A ring are already blocked and thus not available for further reaction with formaldehyde during adhesives preparation and curing, are also present in sizeable amounts (Pizzi, 1980). With regard to pine extracts, whose flavonoids are only phloroglucinolic in nature, the accepted continuing pattern of condensation is 4,8-linked (X).

However, “angular”-type patterns of condensation similar to X, but in which all the flavonoid units in the tannin are of phloroglucinolic nature have recently also been postulated for pine tannins (Hemingway, 1980). The proportion of “angular” in relation to ”linear” pine tannins in the extract has been postulated to be 1:2. Such self- condensation patterns result in polymers as large as 10- or 11-linked flavonoids (mol wt = -3000) for wattle tannins (number average molecular mass 1250) and poly- flavonoids of higher average mass range for quebracho

Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 361

relatively high pH. Thus, the B rings do not generally participate in the reaction to any noticeable extent, except a t high pH’s (210), where the reactivity toward formaldehyde of the A rings is so high that the tannin-formaldehyde adhesives prepared have unacceptably short pot-lives (Hillis and Urbach, 1959a,b). In general tannin adhesive practice only the A rings are used to cross-link the adhesive network. However, because of their size and shape, the tannin molecules become immobile at low levels of condensation with formaldehyde, so that the available reactive sites are too far apart for further methylene bridge formation (Pizzi and Scharfetter, 1978). The result is incomplete polymerization which leads to the weakness and brittleness that are characteristic of many tannin- formaldehyde adhesives. Bridging agents such as phenolic, aminoplastic, and urethane resins have been used to help bridge distances too large for interflavonoid methylene bridges. With regard to the pH dependence of the reaction with formaldehyde, the reaction rate of wattle tannins with formaldehyde is slowest in the pH range 4.0-4.5 and for pine tannins in the pH range 3.3-3.9. Also the quantity of formaldehyde which reacts with the tannins in these ranges is minimal.

At neutral pH, rapid reaction of formaldehyde with the 6- and/or 8-positions of the flavonoid units occurs, ac- companied by a considerably slower reaction at positions 2’ and 6’ on the pyrogallolic or catecholic B ring. Form- aldehyde 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 both as liquid formalin solution and in its polymeric form of paraformaldehyde which is capable of fairly rapid de- polymerization under alkaline conditions. The reaction of formaldehyde with the tannins may be controlled by addition of alcohols to the system. Under these circum- stances some of the formaldehyde is stabilized by the formation of hemiacetals, e.g., CH,(OH)(OCH,), if methanol is used. The alcohol is driven off at a fairly constant rate with progressive release of formaldehyde from the hemiacetals once the adhesive is cured at elevated temperature. In this manner the adhesive “pot-life” is extended and less formaldehyde is lost when the reactants reach curing temperature. The reaction kinetics of formaldehyde with both resorcinolic- and phloroglucinolic-type condensed tannins have also been investigated. The same experiments were repeated with resorcinol, phloroglucinol, and catechol as simple model compounds of A- and B- flavonoid rings, respectively.

The reaction of tannins with formaldehyde (Rossouw et al., 1980) follows, under alkaline conditions, a second- order kinetic law in which the reaction rate is rate =

with

k,[HCHO][tannin-OH] + k,[HCHO] [tannin-0-1 (1)

fi tannin-OH - tannin-0-

The reaction is mainly a k2 reaction in the case of pH’s higher than 8.2 where the k, term is negligible and the reaction rate is determined by the reactivity of the tannate formed. It is mainly a kl reaction at pH’s lower than 7.9 where the concentration of the tannate is negligible and the k2 term can be ignored.

The reaction of formaldehyde with already-formed tannin/HCHO condensates also follows a second-order kinetic law

rate = [HCHO]Ck,[(CH,-tannin),] (2) n

X

(average -1800) and for pine (average -4300). Reactivity and Reactions of Polymeric Tannins with Formaldehyde

Condensed tannins exhibit unique reactions, as well as reactions normdy expected of flavan-3-ok units, reactions which are important in the industrial application of tannin extracts to adhesives. The nucleophilic centers on the A rings of any flavonoid unit tend to be considerably more reactive than those found on the B rings. This is due to the vicinal hydroxyl substituents which merely cause general activation in the B ring without any localized effects as those found in the A ring. Formaldehyde reacts with tannins to produce polymerization through methylene bridge linkages to reactive positions of the flavonoid molecules, mainly the A rings. The reactive positions available on the A rings are the 8-position of all the resorcinolic flavonoid units, the 6- and 8-positions of the resorcinolic upper terminal flavonoid unit, and the 6- position of the phloroglucinolic lower terminal unit. The relative accessibility and/or reactivity of flavonoid units has been examined by selected bromination using units of the phloroglucinol and resorcinol tannin series (Roux et al., 1975). The phloroglucinolic A-ring-type (+)-tetra- o-methyl catechin (XI) has been found to have as pref- erential bromination sequence 8 > 6 >> 6’. In its resorcinolic equivalent, (-)-tri-o-methylfustin (XII), the sub-

1 ’\ I? OMe

.? / 3\*oMe Meowme 2 /

XI XI1

stitution sequence is modified to 6 > 8 >> 6’. The pref- erential 8- and 6-substitution in phloroglucinol and resorcinol-type flavonoids is related to the greater accessibility of these positions in each instance. Thus, in the same manner, in the reaction of resorcinol with formaldehyde the position ortho to both hydroxy groups is not favored for formaldehyde attack.

1. Formaldehyde Reactions with Flavonoids A and B Rings and Their Kinetics. Tannins are subject to the same reactions of phenols with formaldehyde either base or acid catalyzed. In condensed tannin, molecules and A rings of the constituent flavonoid units retain only one highly reactive nucleophilic center as the remainder ac- commodates the interflavonoid bonds. Resorcinolic A rings (wattle) show reactivity toward formaldehyde comparable, though slightly lower, to that of resorcinol. Phloro- glucinolic A rings (pine) behave instead as phloroglucinol. Catechol and pyrogallol B rings are by comparison un- reactive and may only be activated by anion formation at


This term is valid, with different k's, for both the condensates in the tannin and tannate form according to the pH range used. Thus, the total reaction rate equation of the tannin/HCHO reaction can then be summarized as

rate = kl+,[ HCHO] [ tannin-OH]" [ tannin-0-1 '-" + [HCHO]Ck,[(CH,-tannir~)~l (3)

Terms dependent on the concentration of the hydroxy- methylated tannins obtained after the initial formaldehyde attack (i.e., k[HCHO] [tannin-CH,OH]) can be excluded, at the flavonoid unit level, as no other positions available for formaldehyde attack are available on the flavonoid A ring. They cannot be excluded, but their contribution is negligible, however, at the flavonoid polymer level because the tannins' resorcinolic and phloroglucinolic methylated A rings are too unstable and reactive to allow further reaction with formaldehyde before condensing with other tannin A rings. For instance, a t 25 "C for the reaction of wattle tannin with formaldehyde (1:l flavonoid unit: HCHO molar ratio) the (kl + k,) = 4.61 X lo-, L mol-' s-' and C,k, = 3.3 X L mol-ls-' at pH 9.0. For the pine tannin/HCHO reaction under the same reaction conditions (k, + k2) = 4.9 X lo-' L mol-' s-' and C,k, = 6.75 X L mol-' s-'.

In acid conditions the reaction also follows second-order kinetic laws. The situation is quite complex as a great variety of species contribute to the total kinetic curve. However, some of the species contributions are negligible because due to the different conditions of acid pH's used, they contribute in varying proportions to the rate of reaction. A simplified rate equation valid at mildly acid pH's can be expressed as rate = kl[HCHO][tannin] + kPIHCHO-released]+

n

[HCHO]Ck,[(CH,-tannin),l (4)

This equation shows clearly that two predominant sets of reactions are present, namely (1) the reaction of formaldehyde with tannin and with low molecular weight tannin/HCHO condensates, which are responsible for the aldehyde consumption; and (2) the release of formaldehyde, which becomes again available for reaction. This reaction appears to be due to the transformation of unstable -CH,-0-CH,- ether bridges initially formed to the more stable methylene-linked (-CH,-) compounds with the release of some formaldehyde in the process.

The values of the rate constants at 25 "C and pH 4.9 for a wattle tannin flavonoid unit reactive points:HCHO molar ratio of 1:l are kl = 6.94 X lod3 L mol-' s-', k , = -1.9 X L mol-' s-', and C,k, = 4.3 X lo-" L mol-' s-', For the pine tannin/HCHO reaction under the same reaction conditions k, = 1.0 X lo-' L mol-' s-', k, = -2.0 X lo-* L mol-' s-', and C,k, = 7 X

To conclude, the reaction rate is also dependent from the concentration of the catalyst, generally NaOH, hence on the pH. For this reason the k values must be given for well-defined pH's. In the case of acid-base-catalyzed reactions, as flavonoid/HCHO reactions are, the rate constant of a reaction can be expressed as

(5)

and as the rate constants were obtained at pH's 4.9 and 9.0 with no addition of OH- and with addition of OH-, respectively, we have kpH4,9 = ko and thus

n

L mol-' s-'.

k = ko + k,+[H+] + ko~-[oH-]

k p ~ g = ko + ko~-[oH-]

As both k p ~ 4 . 9 and kpHg are obtained experimentally, it is

possible to calculate kOH-. Thus, the reaction rate can be expressed, from eq 3, when taking into account the effect of the OH- catalyst as

rate = (kw + ko,[OH-])[HCHO][tannin] + [HCHO] C(kolt + k,oH[oH-])n[(CH,-tannin)n]

2. Metal Catalysis (Fraser et al., 1957a,b; Pizzi, 1979a,b). The ortho-orientating effect caused by bivalent metallic ions in the reaction of phenols with formaldehyde has been used for a long time to produce phenolic resins of high reactivity. Bivalent metallic ions have been found to accelerate the reaction of phenol with formaldehyde in both the resin manufacturing and curing stages, while trivalent metallic ions have been found to retard the same reaction. Such accelerating or retarding effect is due to the formation of metal ion/formaldehyde/phenol complexes as follows.

n

XI11

The rate of metal exchange in solution of the complexes of type XI11 is the determining factor in the accelerating or retarding effect of the metal ion on the reaction of phenols with formaldehyde. Metals having a fast rate of exchange accelerate the reaction while metals having a very slow rate of exchange slow down or even stop simple phenol/formaldehyde reactions. The extent of the accelerating effect given by different metallic ions does not appear to correspond to scales of metal complex stability.

Independently from the formation of the complexes indicated, the metals, as they do not change their valence state during the course of the reaction, behave as hydrogen ions in accelerating the initial attack of formaldehyde on the phenolic nuclei. The acid catalysis due to the metal ions differs only in degrees from that of the hydrogen ions. The effect of the metal is much stronger, hence the accelerating effect, than that of hydrogen ions, since, because of higher charge and greater covalence, its interaction with donor groups is greater. Metal ion catalysts have then, at higher pH values, the same catalytic effect on the preparation and setting of the resins which is shown by higher hydrogen ions concentrations (and hence lower pH values). This allows accelerated setting of phenolic resin adhesives in milder acid conditions without any wood deterioration. The extent of the accelerating effect is directly proportional to the quantity of metallic ions present. Bivalent metals will generally not inhibit the reaction as the complexes they form have high rates of metal ion exchange in solution. Trivalent metals, whose complexes of type XI11 present slow or no metal exchange in solution will generally inhibit the reaction proceeding and slow down the formation of the phenol/formaldehyde resin. The scale of the accelerating effect for simple phenols and for different metals, in order of decreasing accelerating effect and increasing retarding effect is as follows. accelerating Pb" , , Zn'I Cd", Ni" > MnII, Mg", Cu", CoI' >

MnIII Fe"I , retarding

>> Be", Al"', Cr"', Co"'

In the case of condensed tannins the scale of the accel-


erating effect is somewhat different, as follows. accelerating

PbI1, ZnI1 > Mn" > FeIn > Mg'I, Ni", CrIII, AllI1

Thus, the accelerating effect, caused by the presence of metallic ions on phenol/formaldehyde reactions is main- tained in the case of the phenol being a condensed tannin. The retarding effect is lost, however. This latter finding appears to be due to the high steric hindrance characteristic of tannin polymers as a consequence of their close tridimensional structure. The metallic ion can still probably form a complex with formaldehyde and the tannin phenolic nuclei, accelerating the initial reaction by forming a carbocation of strong positive charge with the formaldehyde. The high steric hindrance of the tannin, though, should cause strong stresses on the partly aromatic etherocyclic ring containing the metal, with consequent in- stability of the ring and finally ring opening. Once the metal-containing ring is open, the methylol-phenolic compound formed, not any longer blocked by a slow exchange rate metal, is free to proceed in the reaction and to form methylene bridges. In this way the retarding effect of the metal ion due to the formation of a stable immovable ring is lost, while its accelerating effect, due to its strong charge, is not.

Consequently, all bivalent and trivalent metallic ions function, to a different extent, as accelerators of the flavonoid tannins/HCHO reaction. At the small level of addition of metallic salt used (generally up to 4 % to 5 % of metallic salts such as zinc acetate, by mass on the mass of the adhesive's solids content) a small amount of flavonoid B ring cross-linking is also obtained, with a consequent slight improvement in the strength of the cured adhesive network. Metal catalysis effects on tannin/ formaldehyde adhesives have been successfully used to shorten pressing times of plywood panels glued with tannin-based adhesives, in the shortening of curing times of tannin adhesives used to glue plywood skins onto frames in door manufacturing, and to explain precuring problems of tannin/HCHO water-proofing resins in starch-based corrugated cardboard adhesives caused by the increase of magnesium in the water used.

3. Hydrolysis and Acid and Alkaline Autoconden- sation (Roux et al., 1975). When heated in the presence of strong mineral acids, tannins are subject to two com- peting reactions. One is degradative, leading to antho- cyanidins and catechin formation. The second one is condensative as a result of hydrolysis of heterocyclic rings @-hydroxybenzylether links). The p-hydroxybenzyl- carbonium ions created condense randomly with nucleophilic centers on other tannin units to form "phlobaphenes" or "tanners red". Other modes of condensation, e.g., free radical coupling of B-ring catechol units, cannot be excluded in the presence of atmospheric oxygen.

Predominantly alcoholic conditions, e.g., 80% to 100% EtOH, lead to a preference for hydrolysis and anthocyanidin formation, although self-condensation is not excluded, while under aqueous conditions phlobaphene formation, or formation of insoluble condensates, pre- dominates. In absolute alcohols, e.g., ethanol, n- and 2- propanol, partial etherification of the alcoholic 3-hydroxy groups occurs in the presence of mineral acid, leading to both the anthocyanidin and ita 3-0-alkyl derivative in increased yields. Hydrolysis of the interflavonoid link under these conditions occurs far more readily. Partial autocondensation in strongly alkaline conditions also takes place as observed by the considerable viscosity increases at high pHs. This is thought to be due to reactions of the

Table I. Adhesives Components (A and B ) Used for "Honeymoon" System

component A A l . commercial wattle/

resorcinol/formaldehy de cold-setting laminating adhesive + hardener t fillers (pH 7.5)

resorcinol/formaldehyde cold-setting laminating adhesive + hardener t fillers (pH 8.0)

A2. commercial phenol/

component B B1. commercial wattle

extract fortified with phenol/m- aminophenoll formaldehyde resin, no hardener (pH 9.0)

B2. commercial wattle/resorcinol/ formaldehyde cold-setting laminating resin, no hardener, pH adjusted to 11.4

B3. pine tannin extract, no hardener, pH adjusted to 12.4

B4. commercial wattle tannin extract, no hardener, pH adjusted to 12.6

phenol/resorcinol/ formaldehyde resin, no hardener, pH adjusted to 10.5 to 11.5

B5. commercial

flavonoids C-4 of the lower terminal units with C-6 and/or C-8 of flavonoid units in other tannin polymers according to which of the latter two is readily available. This might be due to the reactivity of hydroxy-group-carrying C-4.

4. Sulfitation (Pizzi, 1979c, 1980; Pimi and Daling, 1980). The sulfitation of condensed tannins is one of the oldest, but nevertheless potentially most useful reactions in the preparation of adhesives based on condensed tannins in that it affects both the chemical and physical properties of condensed tannins. This reaction solubilizes relatively insoluble tannins and reduces the viscosity of tannin extracts dramatically, consequently allowing the use of adhesive solutions of higher resin content.

The course of the reaction with flavan-3,4-diols and with catechin-like (flavan-3-01) tannin units may be predicted.

HO OH

Introduction of the sulfonic group at the 2-position results in heterocyclic ring opening by hydrolysis of a benzyl ether link. The newly formed resorcinol (A ring), though still linked to the modified flavonoid polymer skeleton, should allow a decrease in the amount of resorcinol chemical added to tannin-based laminating and finger-jointing adhesives, by an increase in the level of sulfinitation. Namely, to a 3% sulfiiitation level (3% Na2S03 on tannin extract, mass/mass) corresponds the formation of 1.0% resorcinol on tannin extract (Table I). In theory it should then be possible to reduce the amount of resorcinol on extract (33% resorcinol:66% extract solids) in a standard tannin-based cold-set adhesive for wood laminating.

The results obtained for beech strips glued specimens and for laminated beams cured for 4.5 h at 50 "C indicate


that resorcinol, although still linked to the modified flavonoid polymer skeleton, is truly liberated during sulfitation of flavonoid extracts such as wattle tannins. Wattle extract sulfited at the 7.5 and 10% levels appears to be optimal for glulam adhesives as, at this level of sulfitation, the highest shear strength and especially higher percentage wood failure were obtained. However, levels higher than 5% are not recommended for the less fortified thermosetting adhesives for plywood while unfortified thermosetting adhesives for particle-board do not tolerate more than 2.5% sodium sulfite. The anomalous behavior of the hot-setting adhesives, in which sulfitation also generates resorcinol, is due to their low or nonexistent level of fortification with synthetic resins, causing a lower degree of cross-linking which allows the strong hydrophilic effect of the sulfonic groups to induce rapid deterioration of the cured network in water. Increasing the level of sulfitation generates a progressively increasing amount of resorcinol chemical in situ from the wattle tannin extract.

The levels of resorcinol needed to obtain good results when gluing laminated beams appear to confirm the amounts of resorcinol generated during sulfitation. How- ever, while the amount of resorcinol liberated appears to be consistent with what is expected up to a 20% silfitation (see Table I), levels of sulfitation higher than 10% to 12% introduce enough sulfonate groups to promote sensitivity to moisture with adhesive deterioration. An added advantage of a small to moderate level of tannin sulfitation is the adhesives' higher moisture retention which allows slower adhesive film "dry-out" during gluing and thus allows longer assembly times.

5. Similarities between Tannin/Formaldehyde and Phenol/Formaldehyde Resins during Curing; Dif- ferential Thermal Analysis (Steiner and Chow, 1975). Comparison of differential thermal analysis data for tannin/formaldehyde, phenol/formaldehyde (PF) and phenol/resorcinol/formaldehyde (PRF) resins supports similarities in cure characteristics between condensed tannins/ formaldehyde mixtures and PRF rather than PF resins. PF resins exhibit a characteristic two-endotherm thermogram, the major endotherm peak being due mainly to vaporization of water in the 115 to 130 "C range while a much smaller peak occurs between 150 and 165 "C. In addition to these two endothermic peaks, PRF resins show an exothermic peak at about 180 "C. This exotherm is resin-reactivity dependent, rather than only pH-dependent and relates to the cure properties of the resin, indicating its room-temperature-cure capability. Resorcinolic-type tannin extracts with formaldehyde at pH 5.0 exhibit similar DTA thermograms to PRF resins. With increasing pH or by addition to the extract of resorcinol a t a fixed pH, hence with a resin reactivity increase, the exotherm shifts to a lower temperature while the second endotherm shifts to 170 to 175 "C with, in some cases, a significant increase in magnitude coupled with a corresponding decrease in the size of the first endotherm. The presence of an exotherm at less than 100 "C is desirable in terms of potential room-temperature cure capabilities. At pH's higher than 6.0 there is a significant drop in exotherm temperature from 70 to 49 "C at pH 8.2. Addition of resorcinol to the tannin/formaldehyde mixture as well as grafting of resorcinol onto the tannin before addition of formaldehyde hardener also induces a significant drop in the exotherm temperature. Coupling increased pH's with resorcinol additions increases considerably the ease of room-temperature cure, indicating the capability of suitably modified tannin/formaldehyde mixtures and resins to function as cold- or ambient-temperature-setting adhesives behaving

4000- - 2 - L- I

STRENGTH oftor 2 4 h cdd worm rooklng

romplr.

L- SUCROSE

.?-HYDROCOLLOID GUMS

- - - - - - - X WOOD FAILURE 01 soO&ed

O 10 20 30 4 0 50 C a r b o h y d r o i a s

ADHESIVE S O L I D S : C A R R O H Y D R A T E S R A T I O

Figure 1. Influence of carbohydrates on glued joints quality.

as synthetic PRF resins. Coupled with the exotherm peak-shift phenomenon is an increase in exotherm heat with increasing pH and/or resorcinol addition, which is indicative of accelerating resin reactivity. The shift in size of the first and second endotherm is explained as due to some pre-gelling in the tannin/formaldehyde mixture which forms a loose network binding water to such an extent that its vaporization is delayed.

6. Technology of Industrial Tannin Adhesive Formulations. The purity of vegetable tannin extracts varies considerably. Commercial wattle bark extract and quebracho extract normally contain 70 to 80% active phenolic ingredients while pine bark yields an extract containing only 50 to 60%. The non-tannin fraction, mainly sugars and hydrocolloid gums, generally does not participate in resin formation with formaldehyde. In order to demonstrate the effect of the presence of non-tannins on the quality of adhesives, a commercial phenol/resorcinol/ formaldehyde cold-setting adhesive was adulterated, separately, with various-amounts of glucose and gum ar- abic. The effects on strength, wood failure, and water resistance of the glued joints were that sugars reduced the strength and water resistance proportionally to the amount added. Their effect is only a dilution effect of the resin solids of the adhesive with consequent proportional worsening of the adhesive's properties. The hydrocolloid gums had instead a much more marked effect on both original strength and water resistance of the adhesive (see Figure 1) (Scharfetter et al., 1977).

If it is assumed that the non-tannins in tannin extracts have similar effect on adhesive properties, it is then clear that unfortified wattle tannin/formaldehyde adhesives can at best achieve only *75% of the performance shown by synthetic adhesives, while unfortified pine tannin adhesives only &60%. Fortification of the tannin extract with phenolic, aminoplastic, urethane, and diisocyanate resins is in many cases the most practical and inexpensive approach to reduce the effect of the non-tannins. Addition of a fortifier not only decreases the percentage of non-


It must be noticed that the viscosity of a tannin extract water solution varies also according to the pH. The effect is not dramatic but it is definitely very noticeable; thus, the viscosity increases when passing from pH 4 to pH 10, a t equal extract solids content of the solution.

Tannin adhesives have a tendency to “dry-out” more rapidly than synthetic resin adhesives after application to wood. This is apparently caused by the higher affinity of water for wood than for this particular type of adhesives. Furthermore, the thickening or “drying-out” of the adhesive is related to: (1) the amount of fillers in the adhesive, the problem becoming more acute with heavily extended adhesives; (2) the hydrophobicity of flavonoid etherocyclic rings; opening of these rings has shown lengthening, thus improvement, in “drying-out” times; (3) the use of volative organic solvents, such as methanol, also may cause “drying-out” problems. Elimination of part or most of the organic solvent also reduces “drying-out”.

Industrial Applications 1. Particleboard Adhesives. Unmodified condensed

tannins, when reacted with formaldehyde give adhesives with characteristics which are not really suitable for particleboard manufacture, namely high viscosity, lower strength, and poor water resistance. However, modification of the tannin and of the adhesives application techniques has rendered the manufacture of exterior grade weather- proof particleboard one of the more popular applications of tannin-formaldehyde adhesives. The most common processes used to eliminate the disadvantages mentioned consists of a series of acid and alkaline treatments of the tannin extract which caused hydrolysis of the gums to simple sugars, consequently improving the viscosity, strength, and water resistance of the unfortified tannin/ formaldehyde adhesive (Pizzi, 1978b). Organic anhydrides have been proved to be considerably more effective than simple organic acid in upgrading the performance of treated tannin extracts in their application as adhesives for particleboard (Pizzi, 197813). In addition to acid and alkaline treatments, the most common process used further reduces the viscosity of the tannin extract by the addition of simple aromatic molecules as dynamic hydrogen-bond- breakers. Unfortified tannin extracts which have un- dergone the above process react with formaldehyde to give adhesives which are able to produce excellent exterior grade particleboards (Pizzi, 1978). This type of process is effective only when resorcinolic tannins, such as wattle tannin, are used. Tannin/formaldehyde adhesives fortified with phenol/formaldehyde or urealformaldehyde resins can also be successfully used for the preparation of exterior grade particleboard.

In the case of particleboard manufacture, combination of wattle tannin extracts treated with acid, alkalies, and hydrogen-bond-breakers which have been fortified with a urea/formaldehyde prepolymer (low molecular weight condensate) acting also as the tannin adhesive hardener are just as popular, in certain countries, as the more traditional paraformaldehyde-hardened, unfortified, but treated, tannin adhesive (Pizzi and Merlin, 1981). The favorite application of the UF-fortified tannin adhesives mentioned is for particleboard for use in flooring. Par- ticleboard prepared by using paraformaldehyde-hardened wattle tannin adhesives is instead used for exterior wall cladding, flooring, road signs, and many other applications. Particular gluing and pressing techniques have been developed for tannin adhesives for particleboard to achieve pressing times much faster than those obtained with synthetic PF adhesives (2-3 min for tannin vs. 5-5.5 min for phenolic in boards of equal thickness) (Pizzi, 1979d).

tannins in the mixture to a level where both strength and wood failure are dramatically improved (see Figure l), but it also improves the cured adhesive network cross-linking with consequent strength gains. The various synthetic resins have been found particularly effective if added in sufficient quantity to reduce the non-tannin fraction to below 20 ?& .

Addition of greater amounts of fortifier do not improve strength and wood failure results dramatically and as such the use of higher fortifier percentages does not afford any further technical advantages and it is thus uneconomical. The possibility of refining the extracts, rather than to fortify them, has proved fruitless, because the intimate association between the various constituents makes industrial fractionation difficult. Some interesting progress in industrial fractionation has been done by Australian and New Zealand researchers in the case of pine tannin extracts, the difficulty in handling of which renders a more expensive production process more justifiable. However, excellent and more economical industrial formulations do exist also for pine tannin extract without the need to recur to the added capital expenditure of adding an ultrafiltra- tion facility to the tannin extraction plant.

Since tannin molecules are generally large, the rate of molecular growth in relation to the rate of linkage is high; thus tannin adhesives can tend to have short pot-lives.

From the point of view of both molecular size and reactivity, phloroglucinolic tannins are worse than mainly resorcinolic tannins as shown by their relative gel times at their pH’s of lower reactivity: -950 s for wattle tannin extract (resorcinolic) against -65 s for pine tannin extract (phloroglucinolic). Reduction of the molecular size is difficult and therefore it does not offer a practical solution. The only ways of lengthening adhesive pot-life are to lower the initial viscosity by dilution and to retard the reaction. The latter is done by: (1) adjustment of the adhesive’s pH; (2) addition of alcohols, i.e., methanol, to the adhesive glue mix; (3) addition of additives other than alcohols retarding the tannin-formaldehyde reaction, i.e., sodium sulfite. The first two systems are the ones generally used to best effect in this respect. Adjustment of the pH to values where the adhesive’s pot-life is suitable for industrial application is generally used in the compounding of thermosetting tannin adhesives for particleboard and plywood. Addition of alcohols is more used for cold-setting glulam adhesives in which the pH must be kept within well-defined and narrow limits, to maintain cured adhesive strength, and cannot be easily tampered with.

Compared to synthetic resins, tannin extracts are more viscous at the concentrations normally required in adhesives. High viscosity of aqueous solution of condensed tannins is due to the following causes, in order of impor- tance: (1’) presence of high molecular weight hydrocolloid gums in the tannin extract; (2) hydrogen bonds and elec- trostatic and other secondary tannins-tannins, tannins- gums, and gums-gums interactions; (3) presence of high molecular weight tannins in the extract.

The tannin extract solution’s high viscosity can be reduced in a number of ways, namely: (1) by dilution; (2) by breaking down, totally or partially, the hydrocolloid gums to simple sugars by acid and/or alkaline hydrolysis; (3) by the addition of small amounts of hydrogen-bond breakers; (4) by sulfitation and bisulfitation, which reduce considerably the viscosity of the extract by opening of the flavonoids etherocyclic rings and thus increased tannin molecular mobility. Part of the viscosity reduction is also due to hydrolysis of the gums caused by the sulfite or bisulfite ions.


The succeas of wattle tannin particleboard adhesives relies equally on industrial application technology as on the chemistry and preparation technology of the adhesive itself. For the exact industrial application techniques which are used, that transcend the scope of this review, the reader is referred to more specialized wood science articles (Pizzi, 1979d, 1978b).

In the case of phloroglucinolic tannins, such as pine tannins, completely different adhesives and application techniques are used (Pizzi, 1981,1982; Ayla and Param- eswaran, 1980; Ayla and Weissman, 1982). A melamine/formaldehyde, or a phenol/formaldehyde and a wattle tannin/formaldehyde adhesive to which an excess of formaldehyde has been added (in the form of paraformaldehyde) and unmodified pine tannin extract are separately (simultaneously or subsequently) sprayed onto the wood chips. The proportion of pine tannin extract to fortifying adhesive solid varies in the 70:30 pine extract:synthetic adhesive, to the 10:90 range by mass, with the favorite ratios being in the 50:50 to 40:60 range when maximizing the use of the pine tannin extract. The pine tannin extract can be added to the wood chips all in liquid form as a low resin solids solution, or partly in powder form to avoid viscosity problems. The separate application of the mixture of paraformaldehyde and fortifying synthetic resin adhesive (melamine, phenolic, etc.) and of the pine tannin is necessary in order to overcome any problem of short pot-life and pre-curing of the tannin-formaldehyde network which can easily be experienced when using such reactive tannins.

The best adhesive formulation for phloroglucinolic type tannins such as pine tannin extracts (i.e., Pinus radiata, Pinus patula, and the loblolly pines such as Pinus elliottii and Pinus taeda) is instead an adhesive formulation which is capable of giving excellent results also when using resorcinolic tannins such as a wattle tannin extract. The adhesive glue mix consists only of a mix of an unmodified tannin extract solution to which has been added paraformaldehyde and 4,4’-diphenylmethane diisocyanate (commercial MDI). The proportion of tannin extract solids to MDI is 70:30, respectively, based on mass. If lower viscosities are needed the tannin can undergo the modi- fications already described to decrease ita viscosity. The extract solution, paraformaldehyde and MDI, can be mixed a short time before spraying onto the chips, which is not advisable in industrial application, or sprayed onto the chips by separate application, which means the extract solution and the MDI are sprayed through separate nozzles.

However, the best foreseable system of industrial application would be to have two glue blenders in line to each other, in one of which the pine tannin extract solution is added to the wood chips while in the other the diisocyanate is added to the wood chips as already advised for other adhesives. The order of addition for continuous gluing could be pine extract in the frist “in-line” blender and diisocyanate adhesive in the following “in-line” blender, or vice versa, according to which system is capable of giving the best results.

The paraformaldehyde, in both types of application systems, is better added separately to the chips according to industrial techniques already described for resorcinolic type tannin adhesives (Pizzi, 1979d). In this manner, by fortification of the tannin with MDI the poorer results generally obtained with phloroglucinolic tannins when the cross-linking mechanism used is only based on their reaction with formaldehyde are overcome. This is due to the fact that MDI will cross-link with the alcoholic and phe-

nolic hydroxy groups of the tannins, sugars, and gums present in the extract, forming polyurethanes and increasing the cross-linking obtained by the reaction of the tannins phenolic A rings with the formaldehyde. As a consequence, certain factors limiting the successful use of phloroglucinolic tannins as wood adhesives, such as their usual low content of phenolic material (MDI reacts also with sugars and gums hence these materials will participate to the cross-linking mechanism), their incontrollably short pot-life due to the extremely high reactivity of their phloroglucinolic A rings with formaldehyde (controllable by separate application of paraformaldehyde) will be overcome by the use of a different reaction as the phloroglucinolic nuclei of the flavonoids will not influence the rate of formation of the urethane links.

The results obtainable with this system are quite good and not too different from the results obtainable with some of the wattle tannin adhesive formulations. Furthermore, such adhesive formulation can be used with any type of tannin, phloroglucinolic, or resorcinolic (i.e., wattle), and consequently constitutes a really universal tannin adhesive for particleboard (Pizzi, 1981). The results obtained with tannin adhesives for particleboard are comparable with those obtained using phenol-formaldehyde adhesives and in certain cases even superior to them.

2. Plywood Adhesives. Several tannin-based adhesive formulations for plywood have been used industrially from time to time (Pizzi, 1980). At present only four of all these formulations are in use, the selection having been made on economical, as well as technical advantages. In most of the formulations used 10 to 25% of a fortifying resin are mixed with a resorcinolic tannin extract such as wattle extract. The fortifying resins used in the three fortified plywood adhesives are (i) a low level of condensation phenol/resorcinol/formaldehyde resin, (Pizzi and Schar- fetter, 1978), (ii) a low level of condensation phenol/ formaldehyde resin, and (iii) commercial urea-formaldehyde resins (Pizzi, 1977). The fourth formulation is an unfortified one in which to the tannin has been added 2 to 5% zinc acetate and some acetic acid (Pizzi, 1979a,b). The zinc acetate functions as both an accelerator of the tannin/HCHO reaction and an improver of the extent of cross-linking of the adhesive network by allowing a small amount of extra cross-linking through the flavonoid B rings. Zinc acetate can of course be added also to the other three, fortified, adhesive formulations as an accelerator; 15 to 16% paraformaldehyde and 10 to 25% coconut shell flour (or other high density flour), by mass on tannin extract solids, are used as hardener and filler, respectively. In formulation (ii) Paraformaldehyde can be excluded if a higher amount of methylol-rich PF resin is used.

3. Cold-Setting Adhesives for Glulam (Pizzi and Roux, 1979). A series of different novolak-like materials has been prepared by copolymerization of UF resins or PF resols with resorcinol and/or resorcinolic A rings of poly- flavonoids such as condensed tannins. The copolymers formed have been used as cold-setting exterior-grade wood adhesives complying to the relevant international specification. The formulations used industrially up to now can be schematically described as follows.

(1) Simultaneous Synthesis of Resorcinol/Form- aldehyde and Flavonoid/Formaldehyde Condensates. Simultaneous Synthesis of Resorcinol/UF and Fla- vonoid/UF Copolymers. The final mixture of reaction products is an adhesive that can be set and cured at ambient temperature by the addition of paraformaldehyde.

(2) Grafting of Resorcinol on a Tannin/Form- aldehyde Resole or on a Tannin/UF Copolymer


$

35 - n

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I I- (3

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1 GRADE 8

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T I M E ( h o u r s ) Figure 2. Development of finger joint bending strength as a function of time. Systems A2fA2, A2fB3, A2fB5; wood = S.A. pine.

? I G W 2 D m P U E i l OF PINCP(-JOINT B m I N G STBWGTB AS R FUHCTION OF ‘SI-

Systems R2/R2, W E 3 . R2/B58 W d - S.k. Pine

I 4 h I 24 h I 7 days 1

W A Z 40,3 73 0,476 44.2 85 0,501 66.7 73 0,601 AZD3 38,6 83 0.476 4 5 , l 87 0,512 5 3 , 2 75 0,601

U / S S 57.4 30 0,522 53.6 1M) 0 ,623

Having Free Hydroxybenzyl Groups. The final mixture of products of this system is an adhesive that can be set and cured at ambient temperature by the addition of paraformaldehyde.

(3) Synthesis of Stable Resorcinol/Formaldehyde, PRF, or Resorcinol/Urea-Formaldehyde Resins and Subsequent Addition to Flavonoid Polymers. Resor- cinol/formaldehyde, resorcinol-terminated PF, and resorcinol-terminated UF resins are prepared and subsequently added to flavonoid polymers. The mixture of synthetic resin and tannins is used as an adhesive that can be set and cured at ambient temperature by the addition of paraformaldehyde.

These three systems give the most commonly used cold-setting tannin-based resins with the first one being by far the most popular and easy to manufacture. Other more unusual systems have also been tested and are sometimes used. These adhesives are mainly used for laminated wood beams (glulam) manufacture.

4. Fast-Setting Adhesives for Fingerjointing (Van der Westhuizen et at., 1978; Pizzi et al., 1980; Cameron and Pizzi, 1981). Finger joints are commonly used to product long boards from short length timber. Such joints are acceptable in structural timber and in laminations for glulam. Adhesives generally used in fingerjointing are melamine/urea/formaldehyde and mainly phenol/resorcinol/ formaldehyde resins which require lengthy periods to set a t ambient temperature or 4-6 h curing at 50 OC. There is therefore a considerable delay between fingerjointing and further processing and despatch which is in- convenient and interferes with production flow.

Separate application or “honeymoon” fast-setting adhesives capable of setting faster than conventional adhesives were developed in several countries to glue large components where presses were impractical (Kreibich, 1974). The disadvantage of those adhesives, and the reason for their industrial %on-usen were due to the high cost

consequent of the use of expensive and scarce accelerating chemicals such as m-aminophenol. In South Africa, however, the basic “honeymoonn gluing system was considerably modified by (i) the use of tannin-based rather than synthetic phenolic resins for both or just one of components A and B, (ii) the elimination of the expensive m- aminophenol, and (iii) the complete elimination of the hardener from component B. Thus, the “honeymoonn fast-set adhesives as successfully

and extensively used in South Africa are formed according to the combinatbns shown in Table I. To summarize, component A is a normal slow-reacting PRF or tannin- based cold-set adhesive plus hardener, while component B is a fast reacting phenolic or tannin-based resin containing no hardener. All the possible combinations of the component A and B shown in Table I (ten combinations) give excellent structural-grade finger joints. Formulations in industrial use at present are A2/B4 and A2/B5. Two other formulations also are accepted or have at some time been accepted by the South African Bureau of Standards, namely Al/Bl and Al/B2, although they are not in commercial use at present. The A2/B4 and A2/B5 formulations are excellent adhesives which are able to fully cure respectively at 10 and 5 “C. The A2/B4 is a particularly economical formulation in which half of the expensive PRF resin is substituted by inexpensive, untreated wattle tannin extract a t high pH. It must be noticed that the A2/B3 formulation is just as good as the A2/B4 one and it is not used only for the lack of commercial availability of pine tannin extract. An idea of the performance of the A2 f I34 and A2/B5 formulations in relation to the speed of curing of a traditional PRF adhesive (A2/A2) is shown in Figure 2.

5. Corrugated Cardboard Adhesives (Custers et al., 1979; McKenzie and Yuritta, 1974). The adhesives which have been developed for the manufacture of damp-ply-resistant corrugated cardboard are based on the


addition of spray-dried wattle extract, UF resin, and formaldehyde to a typical Stein-Hall starch formula of 18 to 22% starch content. The wattle tannin/UF copolymer is formed in situ, and any free formaldehyde left in the glue-line is absorbed by the wattle extract. The wattle extract powder should be added at a level of 4% of the total starch content of the mix, that is, carrier plus slurry. Successful results can be achieved in the range of 2 to 12 % of the total starch content, but 4% is the recommended starting level. The final level is determined by the degree of water hardness and the desired bond quality. While certain wattle/formaldehyde systems have given considerable processing problems in the plant and have consequently been abandoned, the wattle extract/UF fortifier system is highly flexible and can be adopted to damp-proof a multitude of basic starch formulations.

Even if strictly quite different from the corrugated cardboard adhesives, small amounts of the same tannin- /UF/HCHO water-proofer have been successfully added, without any starch, to the papier mach6 used for the manufacture of egg trays. The improved stiffness imparted to the paper trays has allowed a decrease in the amount of papier mach6 required to manufacture the tray to specification with a consequent shortening of manufacturing time due to the faster heat transfer caused by having thinner trays.

6. Tannin-Based Phenolic-Type Foams. Tannin- based cold-setting foams can be obtained by the acid- catalyzed condensation of wattle tannins with furfuryl alcohol and formaldehyde (Sperling and Brian, 1976). Low-boiling-point solvents (30 to 45 OC) added to the mixture are used to give a foamed product by means of the heat supplied by the reaction of furfuryl alcohols and tannins. Identical results have been obtained in neutral and alkaline rather than acid environments by using the heat supplied by a small amount of a mixture of resorcinol and m-hydroxyaniline, or of phenolic resins containing such chemicals, to blow a tannin/formaldehyde network into a foam (Pizzi, 19790. Foams of density as low as 0.02 g/cm3 have been prepared in this manner. These foams have good mechanical characteristics and present excellent fire resistance, far better than the polyurethane foams they are aimed at substituting. One negative point is their high water absorption that has caused their use to be limited to the field of floral foams (in which they are extensively used as they have a price advantage over equivalent phenolic foams).

7. Tannin-Based Polyurethane Adhesives and Coatings (Drewes, 1961; Saayman, 1974; Pimi, 1979e,1980). Polyurethane surface coatings with good resistance to weathering have been developed from diisocyanates with partially benzoylated mimosa tannins as the hydroxyl source. Partial benzoylation of tannin units by the Schotten-Baumann reaction serves a variety of purposes: (1) the number of hydroxyl groups per flavonoid unit is reduced, thus limiting reactivity with the isocyanates; (2) partially benzoylated tannins, prepared from the extract, readily precipitate, leaving unreacted carbohydrates in aqueous solution; (3) partial benzoylation confers solubility of the highly polar tannins in those solvent systems required for reaction with diisocyanates.

The bifunctional isocyanates serve to cross-link tannin units to produce films of high gloss and scratch resistance. The tannin presents the lowest cost component in the final product. Resins prepared in this manner can also be successfully used as adhesives. As economics is against condensed tannins being used in this type of adhesive (the amount of tannin used in the adhesive is small as the

tannin must be benzoylated first), another approach has been pursued for adhesive purposes. A tannin-based cold-setting adhesive is added to a commercial polyurethane or to a mixture of polyurethanes, which are compatible with it, to increase the polyurethane strength and heat resistance. The amount of tannin used is much higher than in the previous method though still relatively small (25 % of total adhesive). The tannin/polyurethane mixture is in the emulsion form. This type of adhesive has been successfully used for hot-gluing aluminum to aluminum joints, and the strengths reported have been up to 2300 psi (Pizzi, 1979g). The strong upgrading effect of the tannin/formaldehyde adhesive on the polyurethane is open to discussion. However, it may be assumed that three cross-linking mechanisms are present: (1) the tannin cross-linking with formaldehyde; (2) the polyurethane cross-linking with itself; (3) the polyurethane cross-linking with the tannin by reaction of the free isocyanate group present in the polyurethane adhesive with the alcoholic and phenolic hydroxyl groups present in the tannin. The latter cross-linking mechanism is probably the one giving the considerably higher joint strength than that given by the polyurethane adhesive alone. The presence of water contributed by the tannin-based adhesive will deactivate part of the isocyanate groups, but from the results obtained it can be deduced that such deactivation is indeed small. Other applications, such as polyurethane foams of such a tannin/polyurethane system, can be envisaged.

8. Generation of Resorcinol. All the flavonoid components present in wattle wood (not bark) tannins are based on resorcinol (A rings), and alkali fusion of the wood extract gives resorcinol (also @-resorcilic acid if its decar- boxylation is not complete) and protocatechuic acid. The wood contains about 8% extractives which yield 8% crystalline resorcinol on alkali fusion (Roux et al., 1976). This alternative source of resorcinol to its synthesis based on benzene is not economical, notwithstanding rising commercial resorcinol prices, and the resorcinol produced in this way is approximately 50% more expensive than the one produced starting from benzene.

More interesting and economically more promising is the generation of resorcinol in situ as resorcinol already bound to the polymer. Examples of this approach are sulfitation and/or acid hydrolysis of the flavonoids benzyl ether links already described, in which the A rings pass from the equivalent of m-methoxyphenol to the equivalent of a m-hydroxyphenol (resorcinol) (Pizzi and Daling, 1980; Pizzi, 1980). Sulfited tannins are in effect more reactive toward formaldehyde than unsulfited flavonoids, indicating that the transformation of the A ring to linked resorcinol does truly have potential in adhesives application as re- flected by an increase in reactivity of approximately 20 to 25%. On this basis the resorcinol chemical content of cold-setting adhesives described in the previous paragraphs can be nearly halved by using a heavily sulfited (7.5 to 20%; optimum 10 to 12%) tannin rather than an unsulfited one.

9. Miscellaneous. Considerable amounts of flavonoid-type condensed tannins, such as quebracho and wattle extracts, are used as additives to phenolic resins in order to obtain special effects. The levels of addition are, per- ceptually on the synthetic resin, quite small but the total quantity of tannin used in this manner is considerable. The two most common uses of this type are (i) the use of sulfited quebracho extract mixed as a powder with the paraformaldehyde hardener of phenol-formaldehyde adhesives for plywood (its function is as accelerator of the curing of the PF adhesive), and (ii) the addition of about

Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 369-375 369

4% wattle extract to phenol-resorcinol-formaldehyde cold-setting wood adhesives, a t the end of the resin's process of manufacture as viscosity corrector, free HCHO mop, and resorcinol content active extender.

10. Present Industrial Status and Potential for Expansion. The two most diffused applications of the tannin adhesives described are firstly weather-proof particleboard manufacture and secondly plywood manufacture. It must be kept in mind that tannin adhesives are an economical proposition in (i) countries which produce tannin extract and (ii) countries in which both phenolic resins or their raw materials as well as tannin or tannin resins must be imported. It is then evident that the area of present diffusion and future expansion of tannin adhesives used is limited by this economical considerations to, mainly and broadly, all the countries in the southern hemisphere. Application in the northern hemisphere countries might take place if and only when local sources of tannins will be economically exploited. At present 15000 tons of tannin extracts (on solids basis) for adhesives are used every year, the main quantity (6000 to 7000 tons) going for the manufacture of exterior grade particleboard, 2000 to 3000 tons for the manufacture of marine-grade plywood, 100 to 300 tons for the manufacture of glulam adhesives, 400 to 600 tons for the manufacture of water- proof cardboard containers, and 100 to 500 tons for fast-set fingerjointing. Their use, however, is expanding at a considerable rate with the fingerjointing, plywood, particleboard, and cardboard applications being the ones, in that order, to have the highest growth potential. Literature Cited Ayia, C.; Parameswaran, N. Holz Roh Werkst. 1080, 38, 449. Ayk, C.; Weissman, G. Hok Roh Werkst. 1082, 40 , 13. Cameron, F. A.; Pizzi, A. CSIR Special report HOUT 207. Pretoria, South

Custers, A. J. L.; Rushbrook, R.; Plzzl, A.; Knauff, C. J. Holzforsch. Holzver-

Drewes. S. E. f i s t . Palnt Rubber 1981, 53. Drewes, S. E.; Roux, D. 0. Biochem. J . 1083, 87, 167.

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Hillis, W. E.; Urbach, G. J . Appl. Chem. 1050a, 9 , 474. Hillis, W. E.; Urbach, G. J . Appl. Chem. 195Ob, 9 , 665. Kreibich, R. E. Adhes. Age 1074, 77, 26. McKenzie, A. E.; Yuritta, J. P. Appite 1074, 26. Piui, A. Adhes. Age 1977, 20(12), 27. Puzi, A. J . Appl. Polym. Sei. 1978a, 22, 2717. Pizzi, A. For. Prcd. J . 1078b, 28(12), 42. Piui, A. J . Appl. Polym. Sci. 1070a, 24 , 1247. Pizzi, A. J . Appl. Polym. Scl. 1070b, 2 4 , 1257. Piui, A. ColhM Polym. Scl. 197% 257, 37. Piui, A. Holzforsch. Holzverwert. 1079d, 37(4), 85. Pizzi, A. J . Appl. Polym. Sei. 1070e, 23, 1889. Pizzi, A. J . Appl. Polym. Sci. 107W 23, 1901. Pizzi, A. J . Appl. Polym. Sci. 1070g, 2 4 , 1579. Piui, A. J . Macromol. Sei., Rev. Macromol. Chem. 1080, C78(2), 247. Piui, A. J . Macromol. Sei., Chem. Educ. 1081, A76(7), 1243. Piui, A. Holz Roh Werkstoff 1082, in press. Pizzi, A.; Daling, G. M. E. Holzforsch. Holzvenvert. 1980, 32(3), 64. Pizzi, A.; Merlin, M. Int. J . Adhes. Adhes. 1081, 7 , 261. Pizzi, A.; Rossouw, D. DU T.; Knuffei, W.; Singmin, M. Holzforsch. Holzver-

Piui, A.; Roux, D. G. J . Appl. Polym. Sci. 1070, 22, 145. Plzzl. A.; Scharfetter, H. J . Appl. Polym. Sei. 1978, 22, 1745. Porter, L. J. N. Zeal. J . Scl. 1974, 17, 213. Rossouw, D. DU T.; Pizzi, A.; McGiliivray, G. J . Polym. Sci. Chem. 1080,

78, 3323. Roux. D. 0.; Ferreira, D.; Garbutt, D. C. F. Appl. Polym. Symp. 1978. 28,

1365. Roux. D. G.; Ferreira, D.; Hundt, H. K. L.; Maian, E. Appl. Polym. Symp.

1075, 28. 335. Saayman, H. M. J . Oil Colour Chem. Assoc. 1074, 57, 114. Scharfetter, H.; Pizzi, A.; Rossouw, D. DU T. International Union of Forestry

Research Organizations, Wood Gluing Working Party Symposium, Mer&, Venezuela, Oct 1977.

Sperling, 6.; Bryan, J. 6. H. South African Patent 766507, 1976; German Patent P25-25411.7, 1976.

Steiner, P. R.; Chow, S. Western Forest Products Laboratory, Information report VP-X-153, Vancouver, Canada, Oct 1975.

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wert. 1080, 32(6), 140.

Received for review February 1, 1982 Revised manuscript received April 5, 1982

Accepted May 13, 1982

Wood-Plastic Combinations with High Dimensional Stability

Rudolf Schaudy' and Emii Proksch

Department of Chembtry, Ausfrbn Research Centre Selbersdorf, A-2444 Seibersdorf, Austria

This review covers a period of 15 years, beginning in 1966, when domestic wood species have been investiited following the knowledge and experience available at that time. Spruce, beech, ash, birch, and others were impregnated with methyl methacrylate or styrene-acrylonitrile mixture, cured by y radiation, and the resuiting wood-plastic combinations (WPC) tested. Maple turned out to be suited best. A decisive improvement of dimensional stability was gained by the application of a mixture of acrylonitrile and methyl methacrylate (70:30) together with some additives. The still inherent brittleness of WPC of that type was overcome by the combinations of /so- cyanate and acrylic compounds which yielded WPC of re- markable quality for special applications. Unsaturated poly- esters and acrylic-modified melamines and epoxy resins were investigated for the production of water-resistant chipboards and fibre hardboards using y and electron radiation for curing.

Introduction Wood-plastic combinations or wood-plastic composites

(WPC) of the described type are known since about 1960.

They are to be prepared by impregnation of wood, mainly in compact form, with liquids polymerizable by a radical-type mechanism (monomers or resin solutions) and by in situ curing using heat together with a heat-sensitive free radical catalyst on the one hand or high-energy radiation on the other hand. The resulting product resembles natural wood, its properties are a combination of wood and plastic material, i.e., applied to the wood component: improved hardness, abrasion resistance, compressive and bending strength, dimensional stability, and others. Ma- terial of that type can be produced in almost every di- mension.

In Austria experimental work on radiation-cured WPC has been performed since 1966 (Gutlbauer et al., 1966). At that time initial basic experience had been already made by other working groups in the U.S.A. (Kenaga, Kent, Meyer, and others), U.S.S.R. (Karpov and co-workers), Finland (Miettinen), Denmark, and a few other countries. I t resulted that, despite the considerable improvement gained, only the application of relatively cheap woods and

0196-4321/82/1221-0369$01.25/0 0 1982 American Chemical Society

condensed tannins for adhesives

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