Tannin-Based Bioresin as Adhesives

C H A P T E R

7 Tannin-Based Bioresin as Adhesives Paridah Md. Tahir1,2, Juliana Abd. Halip3 and Seng Hua Lee1 1

Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, Serdang, Malaysia 2Faculty of Forestry, Universiti Putra Malaysia, Serdang, Malaysia 3Faculty of Technology Management and Business, Universiti Tun Hussein Onn Malaysia, Parit Raja, Malaysia

INTRODUCTION Tannin is the generic name for a substance that is easily dissolved in water, whose aqueous solution is highly astringent, and has the property of tanning leather. Chemically, tannin is not a simple substance, but an aggregation of a complex organic compound. Tannin is classified in terms of its chemical property into: (1) hydrozable tannin, that is hydrolyzed by heating with a dilute acid to generate gallic acid, ellagic acid, and the like; and (2) condensed tannin that is polymerized to generate phlobaphene that is insoluble in water. The former, the so-called hydrolyzable tannins, include chestnut, myrobolans (Terminalia and Phyllantus species), and dividivi (Caesalpina coraria) extracts. Condensed tannins, on the other hand, occur in substantial concentrations in the wood and bark of various trees. The most frequently studied species include Acacia mearnsii (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), Rhus (sumac extract), Pinus (radiata pine and southern pine bark extract), and Carya (pecan nut pith extract). Others that are equally

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© 2019 Elsevier Inc. All rights reserved.

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7. TANNIN-BASED BIORESIN AS ADHESIVES

tannin-rich which have not yet been commercially exploited include those from mangrove bark extracts (Rhizophora mucronata, Rhizophora apiculata), Acacia mangium, and Eucalyptus spp. While mangroves are only found in some tropical countries, both A. mangium and Eucalyptus spp. are the main tree species being planted in forest plantation programs throughout South East Asia, particularly in Indonesia, Malaysia, Thailand, Vietnam, and the western part of China. Hence there is an abundance of bark being generated from the processing mills in these regions that are ready to be exploited.

Condensed Tannin Condensed tannins are polymeric flavonoids which are also called proanthocyanidins (Wu¨nsch, del Vedovo, Rosset, & Smiley, 1984). Catechins and some low-molecular weight proanthocyanidins have received considerable attention owing to their various biological activities, in particular their effects on arteriosclerosis (Masquellier, 1988) and their oxygen-free radical scavenger ability (Ricardo-da-Silva, Darmon, Ferna´ndez, & Mitjavila, 1991). Anthony Pizzi has been the key contributor for the development of tannin adhesives. To date, Pizzi has developed several types of tannin adhesives for plywood, particleboard, and laminated products and reviews of these adhesives have been published in many of his books as well as chapters in numerous other books (Brosse & Pizzi, 2017; Pizzi, 1983, 1994, 2000, 2008; Pizzi et al., 2009; Pizzi & Mittal, 2010). Almost all condensed tannin can be found in tree bark. The term bark refers to all tissues of a woody stem or root occurring just outside of the vascular cambium, that is, all tissues that could be stripped away from the woody core. Stems of monocotyledons, especially trunks of arborescent types, often have an outer cover, sometimes appearing like a form of bark, but in these cases the origin of the cover is very different from that of dicotyledons and gymnosperms. Ferns and other seedless vascular plants never form bark of any type. Condensed tannins are made up of C6 C3 C6 units of the flavanoid system, comprising two aromatic rings, joined via a pyran ring (Fig. 7.1). The chemical properties of condensed tannins are largely determined by three factors: (1) the hydroxylation pattern within the flavanol unit; (2) the stereochemistry at the three chiral centers present in the heterocyclic ring; and (3) the location of the interflavonoid bond (Fig. 7.2). The reactivity of, and the orientation of, substitution in the tannin A-ring are controlled primarily by the hydroxylation pattern and steric influences (McGraw, 1989). The chemistry of the B-ring is also determined by the hydroxylation pattern, its properties being characteristic of those of the parent phenol (Laks, 1989). The reactivity of ring C, the pyran ring, is affected by rings A and B and is also influenced by the steric

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FIGURE 7.1 The basic repeating unit in condensed tannins. If R1QR2QOH, R3QH, then the structure is that for ( )-epicatechin. The groups at R1 and R3 for other compounds are indicated below the structure. R2QO-galloyl in the catechin gallates.

effects of the whole condensed tannin molecule (Hemingway, 1989). Together, these characteristics determine the reactivity of the tannins toward formaldehyde and other reactants as well as their potential as precursors for adhesives and other specialty products. With many types of tannin it is often not a question of enhancing, but rather of controlling, reactivity. For example, reactions between formaldehydebased reactants and mimosa or pine tannins have demonstrated that reactivity is a function of species and depends primarily on the difference between the A- and the B-ring substitution patterns (Pizzi, 1983).

Commercial Uses of Tannin Because of their polyphenolic structure, tannins have a range of actual and potential uses; their main applications are listed in Table 7.1. Interest in their use as components of adhesives began about five decades ago and research has been out in various laboratories around the world. The three major areas of work have been: (1) the development of western conifer bark extracts and commercial production facilities on the west coast of North America from 1953 to 1975; (2) the application LIGNOCELLULOSE FOR FUTURE BIOECONOMY

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FIGURE 7.2

Model structure for a condensed tannin. If RQH or OH then the structure represents a procyanidin or prodelphinidin. The 4 6 linkage (dotted line) is an alternative interftavan bond. The terminal unit is at the bottom of such a multiunit structure.

of mimosa tannin in adhesives in South Africa from the early 1970s to the present day; and (3) a resurgence of interest in pine bark as a raw material for tannin-based adhesives, beginning in the middle 1970s. Each of these activities has been characterized by parallel work on the structure of the tannins and development of methods for incorporating them into adhesives. Meanwhile, tannins from other wood species such as spruce, maritime pine, aleppo pine, douglas fir, A. mangium, and Eucalyptus spp. tree barks have been reportedly used for wood adhesives; however, no commercial production has been found so far.

Tannin as Wood Adhesives The use of renewable natural materials as a major component of wood adhesives is not new. Adhesives using protein sources such as blood and casein and vegetable proteins such as soy and wheat were used extensively prior to the large-scale uptake of synthetic adhesives such as phenol-formaldehyde and urea formaldehyde (UF) adhesives following World War II. The definition of renewable wood adhesives is broad and has been interchangeable with other terms such as green and bioderived adhesives. Van Langenberg, Grigsby, and Ryan (2010)

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Potential and Established Uses of Tannin (Pizzi, 2008; Steiner, 1989)

Treatment/modification

Application

Condensation with formaldehyde

• • • • • • • • • • • •

Reaction with diisocyanate

Sulfitation

Mannich-type reaction with formaldehyde and amines Hydrogenolysis/oxidation Complexation with metal/protein/ carbohydrates

Isolation and purification

wood adhesives paper and cardboard binder phenolic molding compounds wood adhesives polyurethane foam coatings drilling mud additive mineral flotation slow-release encapsulation wood adhesives cationic flocculant water treatment

• monomeric phenolics and their derivatives • • • • • • • •

dispersants slow-release biocide bark boards grouting system wood preservative leather tanning antioxidants medical

defined tannin as “materials of natural, non-mineral or non-petroleumbased origin that can be used either in their natural state or after small modification, capable of reproducing the behaviour and performance of synthetic resins.” Wood adhesives from renewable raw materials have been a topic of considerable interest for many years. After the oil crisis in the early 1970s and 2000s increasing oil prices and the high energy requirement associate with the production of synthetic polymers prompted the use of renewable resources such as wood, tree bark, and nut shells, etc., in material applications rather than in energy production and has led to the development of replacement materials for petroleum-derived phenolic compounds from natural resources in the wood adhesive industry. Among the possible alternatives, tannin is an excellent renewable resource which can be used for replacing petroleum-derived phenolic compounds. Currently, the prices of phenol ranges between US$1788 and US$1828 per ton (ICIS, 2017). Most phenol is used in the bisphenol-A (BPA) and phenol-formaldehyde resin industry (Fig. 7.3); Asia Pacific has been the fastest growing market for phenol while European and North American demand has been rather flat (Merchant Research, & Consulting, Ltd, 2018).

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FIGURE 7.3 Expected global phenol demand by application (Merchant Research, & Consulting, Ltd, 2018).

Over 50 years, tannin incorporation has been evaluated with most wood adhesive systems. The reasons for using tannins have been varied which include material replacement, cost reduction, increased crosslinking or adhesive reinforcement, promoting cure, and efforts to improve water resistance. As noted, the commonality of polyphenolic tannins with phenol and resorcinol chemistry and tannin’s reactivity toward formaldehyde have made tannins ideal candidates to be substituted into wood adhesives. However, for a variety of reasons this published research has not led to widespread uptake of tannin-based adhesive technologies in engineered wood products. Tannins have been used in phenolic resin applications, substituting or replacing phenol and resorcinol in typical wood adhesives. Typically, this use has been with A. mearnsii (mimosa or black wattle) tannins, but not exclusively as the use of condensed tannins from pine, spruce, mangrove, A. mangium (mangium), and quebracho species as well as other sources have been described. Paridah and coworkers have extracted tannins from mangrove (Paridah & Musgrave, 1999a, 2006; Paridah, Musgrave, & Zaidon, 2002) and mangium (Hoong, Paridah, et al., 2010; Hoong, Paridah, Luqman, Koh, & Loh, 2009; Hoong, Pizzi, Tahir, & Pasch, 2010) barks for the production of tannin-based adhesives suitable for laminating and plywood adhesives, respectively. Work on Eucalyptus spp. is still on-going, but early results indicate a much inferior performance compared to those of mimosa, mangium, pine, and mangrove. Most of these works used almost exclusively tannin formaldehyde and methylol coupling chemistry although there

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are more recent examples of differing aldehydes substituting for formaldehyde. Other than phenolics, tannin can also be combined with aminoplasts, epoxy and isoscyanates resins, starch, lignin, and proteins with encouraging results. Van Langenberg et al. (2010) reported a comprehensive review of these systems and their applications in wood-based products. The following sections summarize our works using tannin from tropical plant sources, that is, mangrove, A. mangium, and Eucalyptus spp. Comparisons are made with other similar works reported in the literature.

Tannin Extraction Tannin can be extracted from bark and wood of trees at different temperatures and under slightly different conditions than those used on a commercial scale. Industrially, tannins are always extracted by countercurrent extraction using hot water. The comminuted plant material is placed in a series of enclosed or covered vat (autoclaves) and extracted with water at ,100 C using a countercurrent method so that the extracting solution is gradually enriched as it progresses from vat to vat. Lower temperatures (between 70 C and 90 C) are preferred because as the temperature increases the yield of extracts increases, but not the yield of tannin. Instead, other materials particularly carbohydrates, are extracted more. The tannin extracts usually consist of a mixture of mono- and polyflavanoids with a considerable proportion of nonphenolic materials such as simple sugars and polymeric carbohydrates. Brosse and Pizzi (2017) recommended the optimum extraction temperature for any tannin to be 70 C 75 C; however, for some species such as quebracho or pine bark chips, 2% sodium sulfite or bisulfite and 5% sodium bicarbonate are added to increase the yield. In the case of mangrove and mangium tannins, 4% of sodium sulfite and 0.4% sodium bicarbonate are used (Hoong et al., 2009; Paridah & Musgrave, 1999a). As barks of mangrove and mangium trees are relatively thick, it yields between 15% and 25% (w/w) tannins (Hoong et al., 2009; Paridah & Musgrave, 1999a). In another study, Bertaud, Tapin-lingua, Pizzi, Navarrete, and Petit-conil (2010) used acetone/water (70/30, v/v) for extracting tannin from the barks of spruce, aleppo pine, douglas fir, maritime pine, and eucalyptus and found that the yields depend very much on the species, that is, 26.6%, 30.1%, 22.8%, 10.2%, and 6.5%, respectively. There are various patent applications and patented inventions related to exploitation technology for bark and wood extracts that are seeking protection. The most relevant of these are described in Gebert, Pozo, and Fuentes (2009) which were mainly filed in the United States, United Kingdom, New Zealand, Australia, European Union, and Canada.

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Based on their works, Berg and coworkers identified several difficulties. Sulfitation is normally applied in tannin extraction from bark to increase the yield of tannin. The extraction pH level is generally basic (pH 8 11) as alkaline conditions produce the highest yields and the best-quality extracts. Unfortunately the addition of sulfite salts and alkali to the extraction also has negative consequences such as: (1) more tannins are extracted, but with a greater molecular weight so that the viscosity of the concentrated tannin solutions becomes very high and the cured resin is brittle as the three-dimensional reticulation with a crosslinking agent (e.g., aldehyde) is hindered which makes it difficult to use as an adhesive; (2) only one part of the sulfite reacts with the polyphenols extracted, the rest combines physically and chemically with the high molecular weight polyphenols that cannot be extracted from the bark; and (3) the process is less efficient, as well as the remaining barks could not be further used due to its high content of inorganic compounds. To overcome these issues, Gebert et al. (2009) invented a process to obtain low and medium molecular weight polyphenols as well as a standardized solid fuel from tree bark or wood. The process includes bark grinding, solid liquid extraction from bark using a low-molecular weight aliphatic alcohol, concentration of the extract to recover the aliphatic alcohol used in the extraction, decantation, filtration, and/or centrifugation to separate the solid and liquid phases of the extract, drying of the soluble fraction in water of the hydroalcoholic extract, drying of the insoluble fraction in water, and separation of the fine fraction of the bark extracted in a sieve and/or an air fractionation system. The extraction can be carried out continuously or in stages by using either alcohol—that is, methanol or ethanol in a water solution with concentrations varying between 20% and 99% in mass—an acid, or a base in a concentration that can vary between 0.1% and 5% in mass in relation to the total mass of the solution. The process generates hydroalcoholic extracts from both soluble and insoluble fractions which have been used to prepare thermo hardening resins, matrices for extrusion, injection, or any other transformation process characteristic of the plastics industry. In summary, using the method described in Gebert et al. (2009), several advantages can be achieved: (1) the viscosity of concentrated tannin solutions is much lower; (2) the polyphenol concentration is unusually high with respect to impurities present (mainly carbohydrates); (3) the average molecular weight of the polyphenols is low in relation to polyphenols obtained through alternative conventional processes; and (4) the water-soluble fraction of the hydroalcoholic extracts from bark constitutes a significantly higher quality than any of the commercial tannins known to date. Other than extraction temperature, bark chips size and age are also crucial in producing high tannin yields. Paridah and Musgrave (1999b)

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found that mangrove barks stored for # 4 weeks produced a significantly higher tannin yield than those stored for longer. The extracts prepared from these barks contained a significantly larger amount of reactive tannins (Stiasny numbers . 56%). In another study, the extraction of mangium bark using a combination of water and sulfite medium produced 15% 25% tannin (dry weight). In this research, several extraction conditions such as bark size, plantation site, extraction time, and extraction medium were studied. The results showed that by using either hot water or a sulfite medium a reasonable amount of tannin yield could be obtained. Bark sizes of less than 1 mm mesh size gave a relatively high tannin yield irrespective of the extraction medium used. Using a 600:100:2:0.5 (w/w) ratio of water:bark:sodium sulfite:sodium carbonate and reacted at 75 C for 3 hours improved the tannin yield by at least 30%. The extracts were reasonably reactive toward formaldehyde as shown by their high Stiasny number; water extract, 60% 70%, and aqueous sulfite carbonate extracts, 85% 90%. For tannin-based chemicals to gain a share of the adhesive market, a competitively priced, nondisruptive supply must be available having both consistent quality and know reactivity. Because tannins can be obtained from the barks of trees which are available as by-products of wood processing, low material costs are possible. The fewer steps required to isolate and purify tannins and to transform them into usable chemicals, the more attractive these will be in contrast with synthetic phenols. Thus, a simple, onestep extraction process would provide a strong economic incentive for more extensive tannin use (Brosse & Pizzi, 2017).

Characterization of Tannin The study of condensed tannins has been difficult because of the structural complexity of these compounds. Many analytical methods have been used to quantify tannins in plant materials. Commonly used methods include oxidative depolymerization of PA, reactions of the A ring with an aromatic aldehyde, and oxidation reduction reactions. Other methods include acid cleavage reactions, precipitation reactions, enzyme and microbial inhibition, and gravimetric procedures. The next sections review some of the chemical characterizations for various types of tannins, principally those that are available in tropical regions, that is, mangrove, mangium, and eucalyptus.

Molecular Weight Many condensed tannins contain polymeric species with a number average molecular weight corresponding to 5 9 flavanoid units per

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molecule (Steiner, 1989). For instance mimosa tannin has Mn (molecular weight number) 5 1250 3000 and quebracho tannin has Mn 1784129 while an acetylated southern pine bark extract has Mn 5 c.1300.52 A somewhat higher value (Mn 5 1800) was reported for P. radiata (Yazaki & Hillis, 1980). The acetylated product from the water-soluble extract of R. mucronata bark showed low polydispersity (1.512), with Mn 916, Mw 1384, and Mz 1913 (Paridah, Musgrave, & Zaidon, 2002). Table 7.2 summarizes the composition of carbohydrates found in different tannins while Table 7.3 lists the analysis of water-soluble sugars for different tannins.

Analytical and Structural Determination Methods Nuclear magnetic resonance (NMR) and mass spectrometry (MS) techniques provide the tannin chemist with powerful tools to determine the structures and conformations of these complex molecules. The 13C NMR spectra of a concentrated solution, for instance, can indicate the structural characteristics that are of potential importance in the use of these materials as wood adhesives. Pizzi and coworkers have shown that the 13C NMR characteristics of five types of commercial and industrial tannin extracts (i.e., mimosa, quebracho, radiata pine, pecan nut pith, and gambier) which are usable for wood adhesives TABLE 7.2 Composition of Carbohydrates found in Different Tanninsa Sugar content (%) R. mucronata tannin

Types of sugar

Mimosa tannin

P. radiata tannin

Hot water

Acetylated

Sulfited

Rhamnose

Trace

0.3

8.6

5.5

3.8

Fructose

Trace

Trace

0.0

0.0

Trace

Arabinose

0.6

1.3

0.7

0.2

1.6

Xylose

Trace

0.1

0.1

Trace

0.2

Mannose

1.3

0.6

0.2

0.0

0.3

Galactose

0.4

0.5

0.3

Trace

0.7

Glucose

4.0

2.8

0.9

0.2

1.0

Uronic acids

2.3

3.8

0.7

0.8

2.9

Total

8.6

9.4

11.5

6.7

10.5

a

The tannin samples were hydrolyzed in sulfuric acid and the hydrolysate was neutralized, reduced, and acetylated. The total monosaccharides were analyzed as alditol acetates by GLC using allose and inositol as internal standards.

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TABLE 7.3

Analysis of Water-Soluble Sugars for Different Tanninsa Sugar content (%) R. mucronata tannin

Sugar

Mimosa tannin

P. radiata tannin

Hot water

Acetylated

Sulfited

Rhamnose

0.0

Trace

0.3

0.1

0.0

Fucose

0.0

Trace

Trace

0.

0.0

Arabinose

0.4

0.9

0.2

0.0

0.0

Xylose

0.0

Trace

Trace

0.0

0.0

Mannose

1.7

1.5

0.05

0.0

0.0

Galactose

Trace

Trace

0.03

0.0

0.2

Glucose

2.9

4.0

0.05

0.0

0.2

TOTAL

5.0

6.4

0.63

0.1

0.2

a

The tannin samples were dissolved in water, reduced, and acetylated. The soluble monosaccharides were analyzed as Alditol Acetates by GLC.

preparation reveal considerable differences. These include: (1) the relative proportions of fisetinidin-robinetidin units and catechin-epicatechin units; (2) the range of molecular sizes; (3) the extent of the ring-opening of the heterocyclic ring C; (4) the extent of branching of the polymer chain; and (5) the relative proportions of pyrogallol-type and catecholtype B-rings in the flavanoid repeating units. Paridah (1995) applied the fast atom bombardment mass spectrometry (FAB-MS) technique to crude sulfited mangrove tannin and the results were compared with those obtained by 13C NMR spectroscopy and gel permeation chromatography. FAB-MS promises to achieve the same level of importance for structural elucidation. The FAB-MS of proanthocyanidins has so far been done largely on pure compounds, but there is clearly potential for analysis of mixtures via tandem mass spectrometry or MS MS techniques. The characterization of R. mucronata (Mucronata) tannin was carried out using NMR, MS, and the FAB-MS technique. The acetylated product from the water-soluble extract of Mucronata bark showed low polydispersity (1.512), with Mn 916, Mw 1384, and Mz 1913. The 13C NMR spectra of Mucronata was found to be similar to that of Pinus radiata (Radiata) which supports the view that both tannins have phloroglucinolic A-rings with hydroxy groups at C-5 and C-7. They differ in the hydroxylation pattern of the B-ring; Mucronata having hydroxy groups at C-3ʹ, C-4ʹ, and C-5ʹ and Radiata at C-3ʹ and C-4ʹ. Both tannins have interflavanoid links at C-4-C-8, but Mucronata tannin also contains C-4-C-6 linkages. The

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presence of a considerable amount of carbohydrates was indicated by the 13C NMR spectra in both types of tannin. The more abundant free or combined sugars present in the extracts include rhamnose, glucose, arabinose, mannose, galactose, and glucose. The Mucronata extract contains mainly rhamnose, glucose, and arabinose while the Radiata extract contains mainly glucose and arabinose, and the mimosa extract mainly contains glucose and mannose. Uronic acids (i.e., glucuronic acid and galacturonic acid) are found (0.7% 3.8%) in all the tannins. The total carbohydrates content is highest (11.5%) in the hot water extract of Mucronata, while Radiata and the sulfite carbonate extract of Mucronata contain 9.4% and 10.5% respectively. Sulfitation of Mucronata bark reduced the total carbohydrate extracted as well as the rhamnose, but increased the amount of arabinose and uronic acid. The quantity of water-soluble sugars obtained from the sulfite-carbonate treatment of Mucronata in our work is significantly lower than that from Radiata under similar conditions. This suggests that the much higher content of water-soluble sugars in Radiata may influence the amount of tannin extracted. The MALDI-TOF MS and solid-state CP-MAS 13C NMR spectroscopic technique were introduced to characterize mangium tannin. The MALDI-TOF MS illustrated a series of peaks corresponding to oligomers of condensed tannins of up to 11 flavonoid units (3200 Da). Mangium condensed tannins were found to consist predominantly of prorobinetinidin combined with profisetinidin and prodelphinidin. Both the MALDI-TOF mass spectra and the solid-state CP-MAS 13C NMR indicated that the structure of mangium tannins varies according to location; an angular structure, twice-angular structure that are heavily branched, and merely linear structure (Fig. 7.4). The high degree of

FIGURE 7.4 A typical polymer structure of A. mangium tannin: (i) repeating units consist of profisetinidin (A), prorobinetinidin (B), and prodelphinidin (C); (ii) tannin with an angular structure resolving itself into a branched polymer structure; (iii) tannin with a “twice-angular” structure up to 7 units; and (iv) tannin with a “linear” structure.

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polymerization of linear, angular type, twice-angular structures, and longer oligomer (3200 Da) chains have not been observed in other studies of condensed tannins. The spectra also indicated that mangium tannins are more heavily branched and have a higher degree of polymerization ( . 7.0) compared to commercial Mimosa tannin (4.9). The difference in structural arrangement has apparent effects on reactivity toward formaldehyde. It appears that mangium tannin has a lower proportion of polymeric carbohydrates (hydrocolloid gums) and has more branched angular structures compared to mimosa tannin. Intramolecular association, particularly hydrogen-bonding between polyflavanoid units and complex formation with carbohydrates or proteins yield products that behave like much higher molecular weight molecules. Thus, mangium tannin would be expected to be suitable for adhesive applications (Hoong, Pizzi, et al., 2010). Paridah and coworkers did the Fourier-transform infrared spectroscopy measurement of mangium tannin, which is summarized in Table 7.4. The peaks from wavenumber 700 to 1800 cm21 are recorded and assigned to various functionalities in the tannin structure. This assignment follows the methods done by Falca˜o and Arau´jo (2014) to identify tannins from various sources. The result of runs on various TABLE 7.4 Comparative Absorption Wavenumber between A. mangium and Mimosa Tannins (Silverstein, Bassler, & Morrill, 1981) Functional group

A. mangium tannin

Commercial mimosa tannin

CQO phenolic ester/ester lactone

1739s

1740vs

CQC (H) aromatic ring deformation

1615vs

1613m

CQC aromatic ring

1508s

1507w

C C skeletal ring vibration

1452vs

1451m

C OH phenol/lactone deformation

1369s

1369vs

Aromatic C O C valance vibration/ OH (H) deformation

1214s

1213vs

C C asymmetric cyclic ester

1158w

βQC (H) deformation

1032s

1030m

993wb

993wb

OH aromatic tetra substitution

845w

845w

H aromatic

803w

802w

762w

759w

Tetra substitution aromatic

H

Notes: w, weak; m, medium; s, strong; vs, very strong; b, broad.

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tannins by Falca˜o and Arau´jo (2014) has been summarized in Table 7.4. The marker or identification of types of tannin is possible using comparative techniques. The stretching vibration of benzene ring and methylol groups of tannin structures can be seen in the bands at 3300 3800 cm21. This range coincides with pure phenol formaldehyde (PF) with the spectra having broad an absorption peak at 3890 cm21. This augurs well with the structure of phenol, a monomer of PF. The strong and broad absorption observed at a wavenumber of around 3300 cm21 could be assigned to hydrogen bonded (O H) stretching absorption. Both mangium and mimosa tannins show a strong resemblance; the signature absorption of mimosa tannin for OH aromatic tetra substitution of 845 cm21 is obvious for mangium tannin as well. The value 1213 cm21 in mimosa tannin, and 1204 cm21 in mangium tannin can also be used to distinguish them from tannin from cork oak bark, Quebracho, or Catechu (1237, 1245, or 1240 cm21, respectively).

Tannin-Based Adhesive Formulation A “new” adhesive can only be considered industrially successful if it produces products with performance that is comparable or superior to existing products at a lower, effective cost and with fewer or comparable handling problems. In the case for condensed tannin, the mixture of largely monomeric phenols that constitute hydrolyzable tannins would, at first sight, appear to an adhesive chemist like a dream come true. After all, there are always proposals around to depolymerize condensed tannins or lignins to simple monomeric components and rebuild resins from them according to more traditional resin manufacturing routes. So, in hydrolyzable tannins there are monomeric phenols that can be, in theory, reacted with formaldehyde to build phenolic resins exactly as they are made with synthetic phenol. The first problem, however, becomes evident at a more-detailed, second look. The simple phenols available in these tannins are mainly pyrogallol, ellagic acid, gallic, and digallic acids. The distribution of the substituent groups on the benzene nuclei of these phenols ensures that their reactivity toward formaldehyde is lower than that of synthetic phenol. It is then true that hydrolyzable tannin-formaldehyde resins can be prepared in the same traditional manner as the synthetic PF resins of commerce, but their preparation time will be much longer; or at comparable reaction times, the distribution of the condensates formed is shifted toward lower molecular weights negatively affecting the cured strength of the resin; even when methylolated, their reactivity will be lower, hence, the gel time of the resin is bound to be much longer. If one adds that often in these tannins

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gallic and digallic acids are present as esters of a sugar, mostly glucose, then this picture becomes even bleaker. The presence of a large amount of carbohydrates bound to the skeleton of the final resin may well negatively affect the water resistance of the resin as well as interfere with the mechanism of the reaction of formaldehyde with the phenolic nuclei at both the stages of resin preparation and resin curing. Suddenly, from a very promising material, if we approach the problem as a traditional resin chemist would, we have resins that: (1) have lower molecular weight and lower viscosities and hence may suffer from over absorption and glueline starvation; (2) have higher free formaldehyde content and consequently high formaldehyde emission; (3) have a much slower curing rate and much poorer water resistance and exterior durability than synthetic PF resins. If one adds to these problems a cost higher than that of synthetic phenol, we can safely assume that these materials, at least when using formaldehyde resin technology, may well not be as attractive as first thought. These problems can be overcome by using other approaches, such as prehydrolizing the sugars from the phenols or removing the sugar-carrying tannins by membrane filtration, or even using reagents that crosslink the tannins through mechanisms different from formaldehyde. The problem is that all these modifications are expensive and although the final material might be technically acceptable, it also becomes uneconomical for industrial applications.

Mangium Tannin Phenol Formaldehyde for Bonding Tropical Plywood Hoong et al. (2009) prepared tannin-based adhesive by crosslinking mangium tannin with paraformaldehyde for bonding tropical hardwood plywood. The resulting bonding strength of this adhesive was found to be only suitable for interior application. Further improvement for exterior grade adhesives could be achieved by copolymerizing it with phenol-formaldehyde resol during the cocondensation process. The optimized formulation of tannin adhesive consists of mangium solid extracts (90 parts), commercial PF (10 parts), and paraformaldehyde (3%). Based on this formulation, the plywood shear strength complies with the requirement for European norms EN 314-1 and EN 314-2:1993, which includes the dry test, cold water test, and the boiling test. Another formulation using mangium tannin and paraformaldehyde with the addition of low-molecular weight PF resin (10 parts) and PF (10 parts) was attempted. This formulation was found to improve the shear strength of the plywood, from 0.96 to 1.43 MPa after a 72 hours boiling test which is suitable for interior and exterior grade plywood.

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The addition of low-molecular weight PF appears to assist in the extent of crosslinking between tannin, paraformaldehyde, and PF polymer. A shear test was done based on EN 314-1 and EN 314-3:1993. The shear strength of plywood bonded with these adhesives is listed in Table 7.5. The results of the shear test showed that the plywood bonded with TPF-mangium adhesive is relatively superior to that bonded with TPF-mimosa, meeting the minimum requirements for both interior- and exterior-grade plywood. TPF-mangium adhesive seemed to have better crossed linking with the wood veneers as shown by the high percentage of wood failure. The heat properties of cured tannin phenol formaldehydes (TPFs) were analyzed on differential scanning calorimetric. As presented in Table 7.6, all the curves show eutectic peaks typically of a polymer having combinations of varying molecular weight. The mimosa tannin appears to be more thermally resistant with a melting point of 205.79 C (at 20% tannin substitution) while mangium tannin was only 198.02 C (at 30% tannin substitution). Interestingly, both melting temperatures are higher than that of commercial PF resin which is 183.75 C. Nevertheless the ΔH of PF significantly exceeds both TPF resins with 368.8 J g 1 as compared to 115.4 to 240.6 J g 1 in the latter. This suggests that PF is well-crosslinked and, thus, requires much higher energy ( . 100%) to be melted. Contrarily, even though the melting temperatures for both TPF are relatively higher, the lower ΔH recorded implies that the TPF did not have as good a crosslinked structure. A similar trend was observed with Tg values where PF indicates a more wellstructured network and brittleness compared to both TPFs.

TABLE 7.5 Plywood Shear Strength of Mangium and Mimosa Tannin Phenol Formaldehyde

TPF resin TPF-mangium

TPF-mimosa

PF

Tannin substitution (%)

Shear bonding strength (MPa) Dry

CBRa

Wood failure percentage (%)

20

2.58

2.11

100

30

2.12

1.08

80

40

1.71

1.09

30

20

2.14

1.65

40

30

2.5

Delaminated

40

1.32

0.93

40

100

3.41

2.48

50

a

Cyclic boil resistant.

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Delaminated

TABLE 7.6 Melt Temperature, Melting Enthalpies, and Glass Temperatures of TPF Adhesives Type of resin

Tannin substitution (%)

Initial temperature ( C)

Maximum temperature ( C)

ΔH (J g 1)

Temperature, Tg ( C)

Heat capacity, ΔCp (J g 1)

TPF-mangium

20

191.44

194.46

166.7

152.82

0.16

30

194.39

198.02

166.0

151.38

0.31

40

173.31

178.85

240.6

143.72

1.28

20

198.81

205.79

183.9

159.73

0.93

30

189.13

198.84

115.4

156.00

1.31

40

196.08

200.77

150.1

158.36

2.16

181.55

183.75

368.8

149.67

0.26

TPF-mimosa

PF

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Remarkably the heat capacities of all TPFs increase with an increasing addition of tannin which is noticeably higher than PF. This effect of the heat absorption property is also seen in thermogravimetric analysis (TGA). Figs. 7.5 and 7.6 show the TG curves of TPF-mangium and TPFmimosa, respectively, at different tannin substitutions, 20%, 30%, and 40%. The degradation of PF starts at 53 C with the largest mass loss occurring at 472 C to 560 C indicating a total pyrolysis of organic compounds. The presence of tannin clearly expedites the thermal degradation process which starts at about 50 C. The degradation of the TPF-mangium resin shows three main regions of mass losses: the first region is between 40 C and 135 C with a peak at 49 C due mainly to loss of absorbed water and trace formation of volatile compounds such as CO and CO2 released by the resin. The second region between 135 C and 675 C with the peak around 255 C which can be attributed to the degradation of lateral resin chains (Zhang et al., 2011). In fact, between 135 C

FIGURE 7.5 TGA curves at different levels of tannin substitution: (A) TPF-mangium and (B) TPF-mimosa.

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FIGURE 7.6 DTG curves at different levels of tannin substitution: (A) TPF-mangium and (B) TPF-mimosa.

and 255 C, tannin starts to decompose (partial pyrolysis) and release CO2 and CO at 215 C. The third region of mass loss is between 675 C and 765 C with the peak around 720 C and due to the decomposition of rigid segments in the resin with the release of CO, CO2, CH4, and carbon formation (Yoshio, Lee, & Fukua, 2003). It should be noted that the decomposition was not total as about 40% of the initial mass remains. TPF-mimosa produced the most consistent trend in terms of derivative weight loss based on the DTG curve. Evidently, blending of tannin into PF resin gave a more thermal resistant material as proven by the higher Tg values.

Laminating Adhesive From Sulfited Mangrove Tannin Research on using mangrove tannin as a wood adhesive are limited and many of them were carried out before 2000. Suminar and Choong

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(2012) reviewed the utilization of mangrove tannin in Indonesia and concluded that tannin-based adhesives can be used for bonding plywood and particleboard, but more studies need to be done, particularly for new sources of tannins such as catechu (gambier), other mangrove species, pine, acacia, and eucalyptus. Mangrove tannin comes from the prodelphinidin-type flavanoid ring and, hence, is very reactive with aldehyde. Its viscosity is relatively higher than mimosa and pine and, thus, could not be used for bonding wood composites using the normal thermosetting resins. Paridah and Musgrave (1999a, 1999b) attempted to produce a cold-setting adhesive from this tannin, but were hindered by its viscosity which was too high and so prevents the adhesive mixture to be sufficiently absorbed into the wood cells. Even though sulfitation of condensed tannins reduces the viscosity solutions and increases the water solubility of the resulting extracts, it is still not sufficient to slow down the curing rate of the adhesive made from it. As a result, both the joint strength and the moisture resistance of the bonded wood are poor due to precuring the adhesive. Sowunmi, Ebewele, Conner, and River (1996) improved the poor wet strength, brittleness, and poor wood penetration in mangrove tannin adhesive by treating the tannin extract with acetic anhydride and then sodium hydroxide followed by modification with 20% resole-type PF resin. Significant structural changes occurred after the chemical treatment. The heat of the reaction of tannin with formaldehyde was increased while the activation energy was drastically reduced. The premature cure of resin was also reduced. Paridah and Musgrave (2006) examined the effect of alkaline treatment on the gelation rate of the sulfited tannin and bond strength of the bonded wood. Mucronata and Apiculata barks were extracted with 4% aqueous sodium sulfite and 0.4% aqueous sodium carbonate at 100 C for 2 hours. The tannin extracts were slightly acidic and were reasonably reactive toward formaldehyde (SN . 85). The viscosity of the extracts was relatively high ( . 300 cP) and the gel time was very short (,2 minutes). The gel time was successfully increased to an acceptable working condition for a “honeymoon” bonding technique through pretreatment with sodium hydroxide and the shear bond strength was improved by at least 20%. The phenol resorcinol formaldehyde (PRF)-mucronata resin mixture produced better water resistant bonds than those containing PRF-apiculata tannin (Fig. 7.7). They also concluded that the concentration of aqueous sodium hydroxide present in alkaline sulfited tannin solutions greatly influenced the gelation rate of the resulting PRF-ST adhesives. With 5% added aqueous sodium hydroxide the rate of gelation was sufficiently slow to permit the efficient use of such tannin extracts in wood adhesives. The effect of lapsed time on the gelation time of the sulfited tannin from mangrove bark

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FIGURE 7.7 Water resistance of cured mangrove tannin phenol resorcinol adhesive after a week, from left to right: 100%, 30%, 50%, 70%, and 0% tannin.

was more noticeable when at 4% and lower concentrations of aqueous sodium hydroxide were used.

Eucalyptus Tannin as a Copolymer in Urea Formaldehyde and Phenol Formaldehyde Adhesives for Bonding Plywood Eucalyptus is a large genus of mostly large trees of the Family Myrtaceae (Order Myrtales) which is native to Australia, New Zealand, and Tasmania. There are about 600 species based largely on their bark characteristics. The bark is either smooth or rough with varying degrees in each class. Eucalyptus camadulensis is one of the most common eucalypts, where others such Eucalyptus grandis, Eucalyptus pelitta, Eucalyptus deglupta, and Eucalyptus globulus have become more popular as plantation trees at the end of 1990s. The adhesives based on tanninsformaldehyde of Eucalyptus spp. present some limitations such as high viscosity and low resistance of the glueline (Vital, Oliveira Carneiro, Pimenta, & Lucia, 2004). This is due to the fact that tannin extracts contain, in addition to active phenolic substances, other substances such as trace amounts of amino and imino acids and, mainly, sugars and gums of high molecular weight. Another limitation is the size of the tannic molecules which are relatively large and, therefore, have a certain immobility (Pizzi, 1994). We have successfully extracted tannin from E. camadulensis tree barks via sulfitation. The sulfited eucalyptus tannin was fortified with PF and

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UF resins to bond tropical hardwood veneers. The tannin solution (40%) was prepared by applying vigorous stirring which was then left rested for several hours prior to blending with PF resin. The blending was carried out, firstly, by warming the tannin solution at 50 C for 30 minutes. Once the heat was switched off, the PF resin was added to the tannin solution. The mixture, that is, Eucalyptus tannin phenol formaldehyde (ETPF), was stirred for another 30 minutes. The same procedure was used for preparing Eucalyptus tannin urea formaldehyde (ETUF). Both the ETPF and ETUF adhesives were used to make a three-ply plywood. Tables 7.7 and 7.8 tabulate the ingredients of ETPF and ETUF, respectively. The ETPF and ETUF resins were used to fabricate 3-ply plywood with medium-high density veneers and medium density veneers. The shear strength of all the 3-ply plywood bonded with ETPF and ETUF resin has a bonding shear of $ 1.0 MPa, meeting the minimum requirement of standard ISO 12466. Three-ply plywood bonded with ETUF has TABLE 7.7 Eucalyptus Tannin Phenol Formaldehyde Adhesive Mixture for Bonding Tropical Hardwood Veneers Ingredient

Parts per weight

Solid (g)

Water (g)

PF, 42% solid

80

33.6

46.4

Tannin, 40% solid

20

8

12

Flour (30% IWF 1 70% CaCo3)a

28

25.2

2.8

Water

3 5

Total

131 133

3 5 66.8

32.2

a

Assuming 10% moisture content.

TABLE 7.8 Eucalyptus Tannin Urea Formaldehyde Adhesive Mixture for Bonding Tropical Hardwood Veneers Ingredient

Parts per weight

Solid (g)

Water (g)

UF, 42% solid

80

33.6

46.4

20

8

12

28

25.2

2.8

NH4Cl (1.5% of resin solid)

0.6

0.6

Water

3 5

Total

131.6 133.6

Tannin, 40% solid Flour (30% IWF 1 70% CaCo3) b

a

3 5 67.4

a

Assuming 10% moisture content. Resin solid (33.6 1 8) 5 41.6.

b

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32.6

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131

significantly better bonding properties (shear strength) compared to those bonded with ETPF. Boiling-drying-boiling (BDB) assessment was carried out and the results revealed that plywood bonded with ETUF showed total delamination (100%). On the other hand, ETPF-bonded plywood shows superior bonding quality with percent delamination between 18.7% and 94.0%. The BDB test is one of the quality control test procedures which is useful to conduct an evaluation on exterior type bonded-wood products in terms of waterproof adhesiveness evaluation. The test is based on protocols elaborated in ISO 12466 (Part 1) that measure the percentage of wood failure or delamination of wood panels from adjacent ply with a maximum depth of 2.5 mm over the fractured surface after the broken specimens were dried.

CONCLUSIONS Tannin extracted from a number of natural sources showed great potential to be used in the synthesis of wood adhesive that are environmentally friendly. However, in the current state of research, it is impossible to fully replace the traditional petroleum-based adhesives as tannin could only act as fortifier due to viscosity constraint which limits the level of substitution in the synthetic resins. Therefore, it is important to note that all of the technologies that are considered “green” still consist of a large component that is petrochemically derived or the renewable material has been chemically modified with petrochemicals. The move away from petrochemical adhesives to those incorporating natural and renewable materials is currently underway in countries like North America, Europe, Chile, Brazil, Australia, and New Zealand. As the prices of wood adhesives depends very much on petroleum prices, it would be reasonable to expect that within the next few decades these changes will occur within the ASEAN marketplace, especially as consumers become more environmentally aware and the requirements of export markets change. These new technologies have been proven technically and economically feasible in other countries, hence wood-based products stakeholders in ASEAN countries should start looking at a more concerted effort in using our own resources, that is, A. mangium and mangrove tree barks as a source of tannin for biophenolic adhesives. In addition, there has been increasing international attention to the production and use of “green” adhesives due to a number of drivers. The most important of these being government legislative changes to minimize the health effects relating to product emissions of volatile organic chemicals, most notably formaldehyde, and the use of renewable materials as a cost-effective replacement for petrochemical

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components of adhesives. This has been coupled with the increasing environmental awareness of consumers and the drive for sustainability.

Acknowledgments The authors wish to thank the financial support provided by Higher Institution Centres of Excellence (HICoE).

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