Novel wood adhesives from condensed tannins and polyethylenimine

Novel wood adhesives from condensed tannins and polyethylenimine

ARTICLE IN PRESS International Journal of Adhesion & Adhesives 24 (2004) 327–333 Novel wood adhesives from condensed tannins and polyethylenimine K...

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ARTICLE IN PRESS

International Journal of Adhesion & Adhesives 24 (2004) 327–333

Novel wood adhesives from condensed tannins and polyethylenimine K. Li*, X. Geng, J. Simonsen, J. Karchesy Department of Wood Science and Engineering, Oregon State University, 119 Ricahrdson Hall, Corvallis, OR 97331, USA Accepted 17 November 2003

Abstract The wood composites industry is a major manufacturing sector in the United States. Increasing concerns about emissive formaldehyde to human health and our heavy dependence on petroleum and natural gas promote the need for a formaldehyde-free wood adhesive from renewable resources. Marine mussel adhesive protein is an excellent example of a formaldehyde-free adhesive from renewable resources. To cope with turbulent tides and waves, mussels stick to rocks or other substances in seawater by secreting an adhesive protein, commonly called marine adhesive protein. The two key functional moieties in the marine adhesive protein are 3,4-dihydroxyphenylalanine (a catechol moiety) and lysine (an amino moiety). Since marine adhesive protein is not readily available, condensed tannins, which contain large amounts of catechol groups, were considered for use in this adhesive system. This study revealed that a mixture of procyanidin-type condensed tannin and polyethylenimine (PEI) was an excellent surrogate for the marine adhesive protein. A tannin–PEI mixture performed successfully as a formaldehyde-free wood adhesive. Two-ply wood composites bonded with tannin–PEI adhesives had very high shear strengths and were very water resistant. The following variables of the tannin–PEI adhesives were investigated in detail for their effects on the shear strength and water-resistance of wood composites bonded with the tannin–PEI adhesives: the mixing time of tannin and PEI, the total solids content of the tannin–PEI adhesives, the weight ratio between tannin and PEI, the cure time and temperature of the tannin–PEI adhesives, the storage time of the tannin–PEI adhesives, and the molecular weight of PEI. r 2003 Elsevier Ltd. All rights reserved. Keywords: A. Condensed tannins; B. Wood adhesives; C. Marine adhesives; D. Formaldehyde; E. Polyethylenimine

1. Introduction The wood composites industry is one of the largest manufacturing sectors in the United States. Wood adhesives are essential components in wood composites. In 1999, about 7.2 billion pounds of formaldehydebased wood adhesives, i.e., phenol–formaldehyde and urea–formaldehyde resins were consumed in the US and Canada [1]. Formaldehyde-based wood adhesives are derived from petrochemicals, which are non-renewable and therefore ultimately limited in supply. Moreover, formaldehyde is a suspected carcinogen and formaldehyde may be emitted in the production and use of wood composites bonded with formaldehyde-based wood adhesives [2–4]. Therefore, the wood composites industry would benefit greatly from the development of a *Corresponding author. Tel.: +1-541-737-8421; fax: +1-541-7373385. E-mail address: [email protected] (K. Li). 0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.11.004

formaldehyde-free wood adhesive from renewable natural resources. Mussel adhesive protein is an excellent example of a formaldehyde-free adhesive from renewable resources. To cope with turbulent tides and waves, mussels stick to rocks and other substances in seawater through a byssus [5]. The byssus consists of a bundle of threads that attach on one end to hard surfaces through an attachment plaque and merge on the other end with a stem that is deeply rooted in the base of the mussel foot [6]. The attachment plaque is composed of proteins, commonly called marine adhesive proteins (MAP). The attachment plaque is remarkable in that it is able to form a strong and opportunistic attachment to wet surfaces. Investigations on the attachment plaques from various mussels reveal that one of the key components of the attachment plaques is a protein consisting of a decapeptide (Ala-Lys-Pro-Ser-(Tyr/DOPA)-Hyp-HypThr-DOPA-Lys) [5,7]. The decapeptide may repeat up

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to 80 times. Because there is no genetic codon that encodes DOPA (3,4-dihydroxyphenylalanine), DOPA is believed to derive from hydroxylation of tyrosine in a post- or co-translational process. The presence of the unique amino acid DOPA in MAPs has prompted extensive studies on the role of DOPA in the strong and water-resistant adhesion of MAPs. It has been well established that DOPA plays an essential role in the adhesion of MAPs [8–10]. The phenolic hydroxyl groups in DOPA can chelate with metal ions, form strong hydrogen bonds with various adherend substrates and undergo various oxidation reactions that may lead to the formation of crosslinked products and orthoquinones [11]. The ortho-quinones can further react with the amino groups in the MAP such as the amino group in lysine residues. All these reactions eventually result in the formation of water-insoluble, three-dimensional MAP networks [11]. Our recent studies have revealed that grafting the amino group of dopamine to carboxylic acid moieties of soy protein converts the soy protein to a strong and water-resistant wood adhesive[12]. Poly (N-acryloyl dopamine) has also been shown to bond wood very strongly [13]. All these results imply that the two adjacent phenolic hydroxyl groups, i.e., the catechol moiety, rather than the backbone of the polymer, play an important role in the observed strong and water-resistant adhesion. Unfortunately, MAPs are not readily available and synthetic polymers or chemically modified proteins with the catechol moiety are relatively expensive for applications such as wood adhesives. Therefore, the discovery of an abundant natural product containing the catechol moiety would facilitate the development of a formaldehyde-free wood adhesive using a MAP as a model. Condensed tannins (also called polymeric proanthocyanidins) are natural polyphenolic substances found in abundance in many woody plants [14]. Certain condensed tannins such as ‘‘quebracho’’ and ‘‘wattle’’ are produced commercially from woods and barks and they have been used as a raw material for the production of

tannin–formaldehyde wood adhesives since the 1970s [15,16]. Efforts have been made to reduce formaldehyde emissions from the production and use of wood composites bonded with tannin-based wood adhesives by the replacement of formaldehyde with formaldehyde derivatives such as hexamethylenetetramine and tris(hydroxylmethyl)nitromethane [17,18]. Structurally, condensed tannins are flavanoid polymers [14]. Structures vary based on biosynthetic source and they can be further subclassified in a number of structural groups based on the hydroxylation pattern of the repeating flavanoid monomer units. The most common in nature are the procyanidins. A representative chemical structure of procyanidins is shown in Fig. 1. Such procyanidin-type condensed tannins are abundant in the barks of the most commercially important timber species as well as some agricultural residues in North America. Because of their abundant catechol groups (Fig. 1), this study investigated whether such condensed tannins in the presence of polyamine could be used to mimic the MAP adhesive system and lead to a new formaldehyde-free wood adhesive system that would be strong and water-resistant.

2. Experimental 2.1. Materials A 50% (w/v) aqueous polyethylenimine (PEI) solution (average Mw ¼ 750; 000) was purchased from Sigma-Aldrich (Milwaukee, WI) and used as received. PEIs with molecular weights (Mw ) of 70,000 and 10,000 were purchased from Polysciences Inc (Warrington, PA). The condensed tannin used in this study was a procyanidin polymer isolated from Douglas-fir inner bark as previously described and purified over Sephadex LH-20 [19,20]. The extending flavanoid units of this

OH HO

B

O

OH

A OH

OH

OH HO

B

O

OH

A

n OH

OH OH HO

O

B OH

A

H2N

CH2CH2N m CH2CH2NH n CH2CH2NH2 CH2CH2NH2 Polyethylenimine

OH OH Condensed tannin

Fig. 1. Representative structures of a condensed tannin and polyethylenimine.

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polymer are epicatechin while the terminal units are mixed catechin and epicatechin units. Maple veneer was contributed from States Industries (Eugene, Oregon). The veneer was conditioned at 20 C and 65% relative humidity for a minimum of 1 week. 2.2. Preparation of tannin–PEI adhesives and preparation of two-ply wood composites bonded with the tannin–PEI adhesives Tannin was dissolved in water and the pH value was adjusted to 7.0. For determination of the proper mixing time for tannin and PEI, an aqueous tannin solution (2.2 g of oven-dry tannin) was mixed with a PEI (Mw 10,000) in a weight ratio of 1.76:1 tannin to PEI using a magnetic stirrer. The resulting adhesives were mixed for different times, then used to bond wood veneer specimens. The effect of total solids content of the tannin– PEI adhesives on shear strength of the wood composites was investigated for a weight ratio of 2:1 tannin to PEI (Mw 10,000). The effects of the weight ratio of tannin to PEI (Mw 10,000) on shear strength and water resistance were also evaluated for adhesives with 12% total solids content. Maple veneer with a thickness of 0.6 mm was cut to rectangular specimens 7.6  17.8 cm2. The various tannin–PEI adhesives were applied to the maple veneer. The spread rate was 4.0 mg/cm2 (dry weight basis). The application area was 1  17.8 cm2 for each veneer. Two pieces of the adhesive-coated veneer were lapped together with the grain parallel and the longitudinal direction along the long axis of the veneer. The assembly was then pressed at a load of 277 psi. The press temperature and press time were varied for determination of their effects on the strength and water-resistance of wood composites bonded with tannin–PEI adhesives. After hot-pressing, the two-ply composite samples were cut to 6 specimens. Each specimen had a bonded area of 1  2.54 cm2. The shear strength was determined by fracture in tension on an Instron TTBML universal testing machine at a crosshead speed of 1 mm/min. The load at fracture was measured, converted to psi, and reported as shear strength. 2.3. Evaluations of shear strength and water resistance of the two-ply wood composites A water-soaking-and-drying (WSAD) test was performed by soaking the specimens in water at room temperature for 24 h, drying at room temperature in a fume hood for 24 h, and then measuring the shear strength of the specimens. Boiling-water tests (BWT) were performed in accordance with voluntary standard PSl-95 (published by the US Department of Commerce through the Engineered Wood Association (APA), Tacoma, WA), i.e., the

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specimens were boiled in water for 4 h, dried for 20 h at 6373 C, boiled in water for 4 h, and cooled in tap water. The shear strength of some specimens was evaluated when they were wet. The shear strength determined in this fashion was defined as BWT/wet strength. Some specimens were further air-dried in a fume hood for 24 h and evaluated for shear strength. This test was defined as BWT/dry strength. 2.4. Characterization of the tannin–PEI adhesive systems with differential scanning calorimetry (DSC) Calorimetric measurements were obtained on a DSC2920 (TA Instruments, Inc, New Castle, DE) with argon as a purge gas. Argon flow was adjusted to a rate of 40 ml/min. The calorimeter was calibrated against indium (m.p. 156.6 C, DH ¼ 28:45 J/g) at 10 C/min. The upper temperature limit was set at 230 C. PEI (Mw ¼ 10; 000) was first freeze-dried under vacuum and then mixed well with dry tannin powder. An aliquot of the tannin–PEI mixture (1:1 weight ratio) of ca. 2–5 mg was weighed in standard aluminum DSC pans with lid closures. An empty aluminum DSC pan with lid was used as a reference. For the first runs, the samples were first cooled to 3–5 C with ice and the thermograms were then recorded at a heating rate of 10 C/min between 5 C and 230 C. For the second runs, the samples at the end of the first runs were cooled to 3–5 C with ice at an approximate rate of 50 C/min and the thermograms were recorded again at a heating rate of 10 C/min between 5 C to 230 C. The Universal Analysis V3.3B software supplied by TA Instruments, Inc. (TA Instruments, Inc, New Castle, DE) was used to plot and analyze the thermal data. The DSC spectra have been normalized to represent 1 g of the samples.

3. Results and discussion 3.1. Effects of mixing time on shear strength Both tannin and PEI are soluble in water. An aqueous solution of tannin was mixed with an aqueous solution of PEI to form tannin–PEI adhesives. The mixing time did not have a significant impact on the shear strength, which implied that tannin–PEI adhesives were easy to prepare (Fig. 2). 3.2. Effects of the total solids content of tannin–PEI adhesives on shear strength The viscosity of the tannin–PEI adhesives increased with an increase in their total solids content (data not shown). When the total solids content was higher than

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3.3. Effects of tannin/PEI weight ratios on shear strengths and water-resistances of wood composites bonded with tannin–PEI adhesives

Fig. 2. Effects of mixing time on dry shear strength (Tannin/PEI, 1.76/ 1; total solids content, 18%; PEI, Mw ¼ 10; 000; hot-press conditions: 277 psi, 140 C, 5 min). Data are the mean of six replicates and the error bars represent one standard deviation.

Fig. 3. Effect of total solids content of tannin–PEI adhesives on dry shear strength (Tannin/PEI, 1.76/1; mixing time=15 min; PEI, Mw ¼ 10; 000; hot press conditions: 277 psi, 140 C, 5 min). Data are the mean of six replicates and the error bars represent one standard deviation.

24%, the tannin–PEI adhesives became so viscous that they were difficult to apply. In the range of 12%–24%, the total solids content had little effect on the shear strength of wood composites bonded with the adhesives (Fig. 3). A total solids content of 12% was a convenient viscosity for preparation and application of the adhesives and was used in all subsequent experiments. As a matter of fact, low solid content adhesives are not desirable for the production of wood composites because too much energy is consumed in evaporating water during hot pressing. Further research to investigate increasing the total solids content of the tannin– PEI adhesives is under way in our lab through the addition of extenders such as wheat flour and walnut shell powder.

Neither tannin nor PEI (Mw ¼ 10; 000) by themselves were effective at bonding maple veneer. However, the combination of tannin and PEI was observed to bond maple veneer strongly. The weight ratio of tannin to PEI had a great impact on the shear strength and waterresistance of wood composites bonded with tannin–PEI adhesives (Fig. 4). At a 4:1 weight ratio of tannin to PEI, wood composites bonded with the tannin–PEI adhesives had moderate dry shear strength (ca. 400 psi) and had fairly high water resistance to a WSAD treatment or a BWT/dry treatment. The wood composites did not delaminate and retained reasonably high shear strength even after they experienced a BWT/wet treatment. At the 3:1 tannin:PEI weight ratio, wood composites bonded with the tannin–PEI adhesives had comparable shear strengths and comparable water-resistances with those at the 4:1 tannin/PEI weight ratio. When the weight ratio of tannin to PEI decreased from 3:1 to 2:1, dramatic increases in shear strengths and water resistances of the wood composites were observed. However, when the weight ratio of tannin to PEI further decreased from 2:1 to 1:1, the shear strengths and water resistances of the wood composites bonded with tannin–PEI adhesives decreased. The shear strengths for dry, WSAD-treated, or BWT/dry-treated composites decreased to a lesser extent than those for BWT/wettreated composites. When the weight ratio of tannin to PEI decreased below 1:1, the shear strengths and water resistances of wood composites continued to decrease. The optimum tannin:PEI weight ratio observed in these experiments was about 2:1.

Fig. 4. Effects of the weight ratio of tannin to PEI on shear strength (total solids content, 12%; PEI, Mw ¼ 10; 000; mixing time=15 min; hot-press conditions: 277 psi, 140 C, 5 min). Dry (L); WSAD (N); BWT/dry (’); BWT/wet (\). Data are the mean of six replicates and the error bars represent one standard deviation.

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3.4. Effects of hot-press temperature on shear strengths and water resistances of wood composites bonded with tannin–PEI adhesives A DSC run on a tannin/PEI mixture showed a large heat absorption peak at about 100 C, which indicated that the tannin–PEI mixture experienced extensive reactions at about 100 C (Fig. 5). This heat absorption peak disappeared when the tannin–PEI samples were run the second time, suggesting complete cure of the tannin–PEI blend. These DSC results implied that tannin–PEI adhesives would not be fully cured if wood composites bonded with a tannin–PEI adhesive were pressed at a temperature o100 C. Indeed, the shear strength of wood composites bonded with tannin–PEI adhesives at 80 C was significantly lower than those at temperatures equal to or above 100 C (Fig. 6), consistent with the DSC results. It appeared that the

dry shear strengths were comparable as long as the press temperature was equal to or above 100 C. After the wood composites bonded with tannin–PEI adhesives underwent a WSAD treatment, the shear strength gradually increased when the hot-press temperature increased from 100 C to 180 C. Shear strengths of wood composites after a BWT/dry treatment were comparable to each other at the hot-press temperatures of 100 C, 120 C, 140 C, and 180 C, with the shear strengths at 160 C being somehow slightly lower (Fig. 6). When the hot-press temperature changed from 100 C to 180 C, the shear strength of wood composites after a BWT/wet treatment fluctuated slightly, but remained at a relatively high level. 3.5. Effects of hot-press time on shear strengths of wood composites bonded with tannin–PEI adhesives The shear strengths of wood composites bonded with tannin–PEI adhesives and hot-pressed for 1 min was significantly lower than those at longer hot press times. At hot-press times equal to or longer than two minutes, there was no significant difference in the resulting shear strengths (Fig. 7).

500

Heat flow (mW/g)

331

0

-500

3.6. Effects of storage time of tannin–PEI adhesives on shear strengths of wood composites bonded with tannin– PEI adhesives

-1000

-1500 0

100

200

300

Temperature Fig. 5. DSC characterization of tannin:PEI (1:1 weight ratio) mixtures. PEI, Mw ¼ 10; 000; (- - - -) first run and (—) second run tests.

Fig. 6. Effects of hot-press temperature on shear strength (total solids content=12%; PEI, Mw ¼ 10; 000; weight ratio of tannin: PEI=2:1; mixing time=15 min; hot-press conditions: 277 psi and 5 min). Dry (L); WSAD (N); BWT/dry (’); BWT/wet (\). Data are the mean of six replicates and the error bars represent one standard deviation.

Tannin was mixed with PEI for about 15 min. and then stored for various times before being applying to veneer in an effort to determine how the storage time affected the shear strength. With increasing storage time, the shear strength decreased (Fig. 8). When the tannin–PEI adhesives were stored at room temperature for one day, the shear strengths, although being

Fig. 7. Effects of the hot-press time on shear strength (total solids content=12%; weight ratio of tannin:PEI=2:1; PEI, Mw ¼ 10; 000; mixing time=15 min; hot-press conditions: 277 psi and 140 C). Data are the mean of six replicates and the error bars represent one standard deviation.

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Shear strength (psi)

1200 1000 800

R 2 = 0.98

600 400 200 0 0

2

4

6

Time (days) Fig. 8. Effects of storage time of the tannin–PEI adhesives on dry shear strength. Total solids content=12%; weight ratio of tannin:PEI=2:1; PEI, Mw ¼ 10; 000; mixing time=15 min; hot-press conditions: 277 psi, 140 C, and 5 min. Data are the mean of six replicates and the error bars represent one standard deviation.

significantly lower than those at the storage time of 1.5 h, were still relatively high (ca. 675 psi). The reduction in shear strength of the adhesive appeared to follow an exponential decay rate. The data were fit well by the following equation: s ¼ 922 e0:300t where s is the shear strength, psi and t the time, days. The adjusted R2 value was 0.98, indicating a good fit. The decline of strength with storage is still not fully understood. It is likely that the reactions between condensed tannin and PEI continued during storage and thus increased the molecular weight of tannin–PEI adhesives, making the spreading of the adhesive on and the penetration of the adhesive in wood less efficient. 3.7. Effects of the molecular weight (Mw) of PEIs on shear strengths of wood composites bonded with tannin– PEI adhesives PEIs with three different molecular weights (Mw ¼ 10; 000; 70,000, and 750,000) were investigated. In this experiment, the solids content (12%) and the tannin–PEI wt ratio (1:1) were held constant for all three molecular weights of PEIs. The dry shear strengths were comparable for all four PEIs (Fig. 9). The shear strengths of the wood composites after a WSAD treatment were comparable for all PEIs. Overall, the PEI molecular weight appeared to have only a small and variable impact on the shear strengths and water resistances of wood composites bonded with tannin– PEI adhesives. In other words, PEIs with Mw ranging from 10,000 to 750,000 all formed excellent wood adhesives with tannin.

Fig. 9. Effects of the molecular weight (Mw ) of PEI on shear strength. Total solids content=12%; weight ratio of tannin: PEI=2:1; mixing time=15 min; hot-press conditions: 277 psi, 140 C, and 5 min. Dry (L); WSAD (N); BWT/dry (’); BWT/wet (\). Data are the mean of six replicates and the error bars represent one standard deviation.

3.8. Some possible reactions between condensed tannin and PEI Exact reactions between condensed tannin and PEI are not fully understood. Some possible reactions between them are suggested in Fig. 10. It is well known that a catechol moiety 1, i.e., the B-ring structure of condensed tannin, is highly susceptible to oxidation resulting in the formation of an ortho-quinone 2, especially at an elevated temperature such as the hotpress temperature in the preparation of wood composites. The ortho-quinone 2 can readily react with amino groups in PEI to form Schiff bases 3 and 4. The orthoquinone 2 can also undergo a Michael addition reaction to form 5 that can further react with amino groups of PEI to form Schiff Bases 6 and 7. These reactions will give rise to a water-insoluble three-dimensional tannin– PEI network. The catechol moiety can form strong hydrogen bonds with amino groups of PEI (see a representative structure 8 in Fig. 10) and form strong hydrogen bonds with hydroxyl groups of wood components (see a representative structure 9 in Fig. 10). Amino groups of PEI can also form strong hydrogen bonds with hydroxyl groups of wood components. Hydrogen bonds between the tannin–PEI network and wood components presumably play an essential role in the strong adhesion of these compounds to wood.

4. Conclusions The condensed tannins from Douglas fir inner bark were used with PEI to form tannin–PEI wood adhesives. Tannin–PEI adhesives were easy to prepare. A simple mixing of a tannin solution and a PEI solution for about

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PEI

N

Tannins

NH

Tannins Tannins

PEI

N

O

Tannins

O

PEI

Tannins

Oxidation OH O

H-bonding H

B

NH

B H-bonding

1

PEI

O

O

H OH

Tannins

8

N O

Tannins

2

B

HN

PEI

6

Tannins

O

NH

PEI

3

H

N

5

O

Tannins

PEI

7

O Michael addition

Schiff base formation

N

O

PEI

4

NH

PEI

PEI

N

333

H

HO

OH wood

represents a hydrogen bond

9

Fig. 10. Some possible reactions between condensed tannin and PEI.

10 min provided a high strength wood adhesive. In the absence of extenders, tannin–PEI adhesives at a total solids content of 12–24% was easily applied to wood veneer. Wood composites bonded with tannin–PEI adhesives were very strong and very water resistant. The tannin–PEI weight ratio that resulted in the highest shear strength and the highest water resistance appeared to be about 2:1. Extensive reactions between tannin and PEI occurred at about 100 C, which implied that a hotpress temperature of 100 C or higher fully cured tannin– PEI-bonded wood composites. About 2 min of hot-press time appeared to be sufficient to fully cure the two-ply wood composites. Tannin–PEI adhesives could be stored for hours before being applied to wood samples without large losses in adhesive bond strength. However, long-term storage of tannin–PEI adhesives greatly decreased the shear strength of wood composites bonded with the adhesives. Due to their ease of mixing, tannin–PEI adhesives appeared to be well suited for in situ applications. A broad range of PEI molecular weights (Mw ) gave similar shear strengths in tannin– PEI-bonded composites.

Acknowledgements This research was supported, in part, by a grant to K. Li from the national research initiative competitive grants program of USDA (award number: 2001-3550410993). We would like to thank States Industries (Eugene, Oregon) for providing us with maple veneer.

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