Potential of tannin extracts as resorcinol replacements in cold cure thermoset adhesives

Blends of phenol resorcinol formaldehyde (PRF) resin with tannin obtained from bark offers the potential to minimise the quantity of resorcinol required in conventional two pot, cold set PRF wood adhesives. In the presence of ammonia, both quebracho and radiata pine bark extracts, resorcinolic and phloroglucinolic type tannins respectively, showed accelerated adhesive cure behaviour with formaldehyde, in a similar manner to PRF adhesive Greenweld technology for modifying glued timber processing rates. Segregation of pine bark components established that only tannin-containing fractions exhibited accelerated adhesive cure behaviour. Results from adhesive testing indicated adhesive blends of quebracho extract and PRF resin failed to attain satisfactory bond strength at full cure. Of the pine bark components, only a pure tannin fraction produced acceptable bond strength when blended with PRF (3:1). The study indicated pine bark extract can be substituted into PRF adhesives and adhesive cure rate accelerated using ammonia. However, extracts containing crude tannin or carbohydrate materials in adhesive blends led to inferior adhesives.     

Environmentally friendly wood adhesives based on chestnut (<Emphasis Type="Italic">Castanea sativa</Emphasis>) shell tannins

The aim of this work was the formulation of adhesives for particleboards based on tannins extracted from industrial lignocellulosic wastes, namely chestnut shell, chestnut bur and eucalyptus bark. The interest was centred on the possibility of completely removing formaldehyde from adhesive formulations. For this, hardener alternatives to formaldehyde were used: tris(hydroxymethyl)nitromethane (TRIS), glyoxal (GLY) and hexametilentetramine (HEX). The influence of the type and concentration of the hardener and pH on adhesive gel time and pot-life were studied. A comparative structural characterization of the adhesives was performed by 13C-NMR spectroscopy. Thermomechanical analyses (TMA) tests were carried out as an indication of the final strength of the adhesive systems. Particleboards type 2 of interior use (EN 312) were manufactured with adhesives based on chestnut shell tannins alone or mixed with chestnut bur/or eucalyptus bark tannins. Free-formaldehyde was determined, and boards were prepared with TRIS, GLY and HEX tannin adhesives were classified as E0.     

Low formaldehyde emission,fast pressing,pine and pecan tannin adhesives for exterior particleboard

Exterior grade particleboard satisfying the requirements of the V100 test, of formaldehyde emission well within the emission standard E1 class, and of pressing times as fast as 8.5 sec/mm can be produced at higher moisture contents using fast-reacting non-modified and non-fortified procyanidin/prodelphinidin tannin extracts such as pine bark tannin extract and pecan nut pith tannin extract. Such results, under the conditions reported are not achieved with the slower reacting tannins traditionally associated with the practice of tannin adhesives. The results reported establish that the faster is the reaction of the tannin with formaldehyde, the lower is the amount of formaldehyde needed to achieve V100 standard, the lower is the formaldehyde emission, the faster is the pressing time achievable and the higher is the tolerance of the adhesive to high moisture content of the resinated wood particles.     

Utilization of bark flours as additive in plywood manufacturing

Increasing demand for wood based panel products and shortage of wood as raw material have triggered many efforts to utilize residues generated annually by the forest industries including a large portion of bark in panel production. In this study, the effects of using bark flours as additives obtained from different wood species (walnut, chestnut, fir and spruce), having much polyphenol content, on some physical and mechanical properties and formaldehyde emission of plywood panels were examined. Wheat flour, which has been used widely as additive in plywood manufacturing, served as control. Plane tree (Platanus orientalis) logs were obtained for veneer manufacturing. Urea formaldehyde (UF) resin with 55 % solids content was used as adhesive. The bonding shear strength, bending strength, modulus of elasticity (MOE), density, equilibrium moisture content and formaldehyde emission of plywood panels were determined according to related standards. It was found that the use of flours obtained from the barks of chestnut and fir trees in the glue mixture decreased the formaldehyde emission of panels. The bonding strength values of the test panels made using the glue mixture including the flour of walnut and spruce barks as additive were lower than those of the panels with adhesive containing the flour of fir and chestnut barks. The panels manufactured with adhesives including the flour of fir bark gave the highest bending strength and modulus of elasticity values.     

Effect of tannin on increasing UF adhesive performance at high temperature investigated by TMA and TGA analysis

Tannins have been largely studied as wood additives and wood adhesives. In a perspective of a desirably increased utilization of natural raw material in the wood timber industries, this study provides further and specific information on the use of condensed tannins to improve the heat resistance of urea–formaldehyde (UF) resins. The results obtained by thermomechanical and thermogravimetric analyses show that high percentage (33 and 50 %) of quebracho tannin in UF-tannin blend dramatically improves the overall UF thermal resistance, but 50 % of tannin decreases the maximum value of modulus of elasticity (MOE) of the bonded joint. The best balance between mechanical performance and decreased degradation at high temperature is obtained with 33 % of quebracho tannin. The MOE value of UF + 33 % of quebracho tannin at 260 °C is seven times greater than MOE of UF control. Thermograms show that quebracho tannin has a beneficial effect on increasing the resistance of glue-mix to degradation induced by temperature. When the content of tannin in the glue-mix is increased up to 50 %, the resin degrades completely at about 670 °C, i.e. more than 50 °C higher than neat UF resin. This improvement is related mainly to the resorcinol-type ring structure of tannin and the alkaline pH of UF + quebracho tannin glue mix that confer a higher resistance against joint degradation to UF even at elevated temperatures.     

Hydrothermal treatment of strand particles of pine for the improvement of OSB panels

The aim of this study was to evaluate the effect of hydrothermal treatment in strand particles of pine used for oriented strand boards (OSB) production. Strand particles of pine were hydrothermally treated at 130, 150 and 170 °C for 7 and 21 min, for the determination of chemical composition, pH, equilibrium moisture content, particles mass loss, and contact angle of these particles with phenol–formaldehyde resin. Afterwards, OSB panels were produced using 8% phenol–formaldehyde resin, with a nominal density of 0.7 g/cm³, and pressing cycle at 170 °C and 3.14 MPa for 8 min. Then, the panels were kept in climate chamber until mass stabilization for the determination of their physical and mechanical properties, and for comparison with the European standards. The hydrothermal treatment in the particles decreased carbohydrate, especially mannan, xylan and arabinans, resulting in reduced equilibrium moisture content, pH, and contact angle, and increasing buffer capacity and mass loss. In OSB panels, treated particles caused the drop in the equilibrium moisture content and reduction of the thickness swelling of the panel, without reducing the mechanical strength, making the hydrothermal treatment very attractive. The hydrothermal treatment at 170 °C for 7 min allowed the resulting OSB panel being classified into the categories 1 and 2 of the European standard, expanding its range of use.     

Sustainable particleboards with low formaldehyde emissions based on yeast protein extract adhesives <Emphasis Type="Italic">Rhodotorula rubra</Emphasis>

Nowadays, there is great interest in developing adhesives that do not generate formaldehyde emissions. The objective of this work was to prepare adhesives based on yeast protein extract from Rhodotorula rubra (Rr) to make particleboards (PBs). The adhesives were characterized, and the physical and mechanical properties of the PBs were evaluated. The results were compared with those obtained using a commercial urea–formaldehyde adhesive (UF). Adhesives viscosity varied from 420 to 600 cP, with pH ranging from 4.35 to 4.73, and solid content of around 50%. The UF had a viscosity of 305 cP, pH of 7.53 and solid content of around 65%. Dynamic mechanical analysis showed that the wood-adhesive using protein extract Rr with reinforcing agent gave a good performance and stiffness, comparable to that using UF. The internal bond strength values of PBs varied from 0.36 to 0.46 N/mm², while swelling thickness reached values of 24 and 38%, respectively. Density values of the PBs fluctuated between 682 and 693 Kg/m³. The formaldehyde emission of the PBs manufactured with Rr extract based adhesive had values between 0.2 and 0.3 mg/100 g, while formaldehyde emission for UF particleboard was 6.8 mg/100 g. Properties of the PB made of Rr based adhesives presented similar properties to particleboard manufactured with UF, but with lower level of formaldehyde emissions. The results from this research validate the manufacture of particleboards with Rhodotorula rubra yeast protein extract based adhesives. These PBs may have the same applications as the traditional PBs, with the advantage of being able to fulfill the high standards of formaldehyde emission demanded by the sustainable market.     

Density related properties of bark insulation boards bonded with tannin hexamine resin

Building owners are increasingly interested in a healthy and sustainable living environment, which is a trend favoring ecological building materials with outstanding structural physical parameters. Insulation boards from particles of larch bark (Larix decidua Mill.) bonded with a formaldehyde-free tannin resin were pressed and evaluated for their mechanical and physical properties. It could be shown that light (target density 250 kg/m³) boards can be pressed, and their thermal conductivity is low (0.065–0.09 W/(m*K)). With regard to mechanical characteristics, the influence of panel density was studied, and it was found that a certain compaction (ρ ≥ 400 kg/m³) is necessary to meet the requirements of the relevant standard. Interestingly, the resin amount did not influence the mechanical board properties as strongly as expected, and panel density is the most important variable in this respect. The study proved that tree bark cannot only be used for substantially upgraded insulation panels but can also be bonded with a formaldehyde free tannin resin.     

Increased pine tannins extraction and wood adhesives development by phlobaphenes minimization

The phlobaphenes formation and precipation during sulphite/water extraction of pine tannins from pine bark was minimized by blocking tannin self-condensation by the addition of small amounts of a strong nucleophile such as phloroglucinol, m-phenylenediamine and urea, the latter due to its low cost for industrial application. These inhibitors reacted preferentially with the reaction intermediates formed in the tannins self-condensation mechanism, stopping the reaction reaching the molecular rearranged precipates known as phlobaphenes. The principle of inhibition appears to be effective at both the laboratory and industrial level. The results of laboratory and industrial extractions and the results of the adhesives from the modified pine tannin extract/urea adducts obtained are presented. The pine tannin/urea extracts proved to yield good thermosetting wood adhesives for panel products. The minimization of phlobaphenes precipitation increased the yield from 19% to 25% industrial scale.     

Characterization of a formaldehyde-free cornstarch-tannin wood adhesive for interior plywood

This study investigated the physical properties (rheological and thermogravimetric analysis) of cornstarch-tannin adhesives and the mechanical properties (dry tensile strength and 3-point bending strength) of plywood made using cornstarch-tannin adhesives. This adhesive was evaluated for its utility in interior plywood manufacture. The optimum cure temperature and cure time of cornstarch-tannin adhesives were 170°C and 4?min, respectively. Plywood bonded with formaldehyde-free cornstarch-tannin adhesive exhibited excellent mechanical properties comparable to commercially available phenol-formaldehyde plywood adhesives. It was found that cornstarch-tannin panels which do not contain formaldehyde and with an emission equal to that of heated but unbound wood can be obtained by the use of hexamethylenetetramine (hexamine) as hardener. The work has indicated that an environmentally friendly wood adhesive can be prepared from a natural renewable resource (cornstarch and wattle tannin) for bonding interior-type plywood.     

Chinese wattle tannin adhesives suitable for producing exterior grade plywood in China

Wattle tannin adhesives were successfully formulated from tannin extracts, which were commercially produced from black wattle (Acacia mearnsii) bark in China. Using the Chinese wattle tannin adhesives a total of 153 plywood panels were prepared in order to study effects of pH, paraformaldehyde, glue spread, hot pressing temperature and time, and closed assembly time on bonding strength and their bonding quality was assessed according to the Chinese Standard for exterior grade plywood. Statistical analysis results revealed that Chinese wattle tannin can be used for producing exterior grade plywood under Chinese plywood factory conditions. One of the specific characteristics of these Chinese wattle tannin adhesives was that their use resulted in plywood of high quality bonding over the wide range of closed assembly time from 30 minutes to 16 hours. These Chinese wattle tannin adhesives have a great potential for commercialization in China.     

Colourless formaldehyde-free urea resin adhesives for wood panels

Amino resin precursors prepared by the addition of a new, colourless, non-volatile and non-toxic aldehyde, dimethoxyethanal (DME), to urea gave resins for boards that while able to harden were underperforming due to the lower reactivity of DME in relation to formaldehyde. Urea reacts with one and even two molecules of DME to form UDME and U(DME)₂ (called DU) but the subsequent cross-linking reaction to form bridges between two ureas, although existing as observed by CP MAS ¹³C NMR, was too slow at temperatures lower than 140 °C to be of significance for wood panel adhesives. However, addition of 20% isocyanate (pMDI) contributed to cross-linking of DU by its reaction with pMDI to also form urethane bridges, their existence being confirmed by CP-MAS ¹³C NMR. The adhesive resins so formed had excellent performance, were colourless, and produced boards that satisfied well the requirements of the relevant norms for interior panels (EN 120 and EN 312). The results were good enough to decrease the proportion of pMDI to 14% at pressing times starting to be of significance for industrial panel products. Formaldehyde emission, by perforator method was down exclusively to the formaldehyde produced by heating the wood chips. The panel emission was sufficiently low to even satisfy the relevant F^**** JIS A 5908 Japanese standard (JIS A 5908, 1994 ). These adhesives are colourless, as UF resins.     

Macro- and microtechnological studies on beechwood panels bonded with Pinus brutia bark tannin

The suitability of hot water extracted tannins from the bark of Pinus brutia from Turkey as a binder either alone or in combination with phenol formaldehyde was examined. Using formaldehyde-hydrochloric acid about 77% of the active phenolic components could be precipitated. Because of the high molecular weights of the polyphenols only small amounts of formaldehyde were necessary for a complete curing. On the basis of the tannin along with a hardener, such as paraformaldehyde or hexamethylenetetramine, boiling-water-resistant gluing of beechwood panels was obtained (DIN 53254, DIN 68602). Electron microscopical and microanalytical studies of such tannin-bonded panels showed the localization of the adhesive, penetration of the adhesive into the wood cells as well as the interaction between the tannin adhesive and the wood cell walls.     

Biological properties of an OSB eco-product manufactured from a mixture of durable and non durable species and natural resins

OSB panels were manufactured with a mixture of pine and cypress heartwood and lignin and tannin based resins in order to propose an eco-friendly wood composite. The resistance of OSB panels against Reticulitermes santonensis was tested according to EN 118 and EN 117 as well as in a field test. OSB made from cypress showed more resistance against the tested termite, the resistance decreased as the percentage of pine increased. The degree of attack differed according to the choice of standard procedure. The field test results revealed that in outdoor conditions and when the termites had a feeding choice, all mixtures showed the same behaviour. The decay resistance was assessed according to EN 12038, and the results revealed that the resistance increases when the percentage of cypress increases.     

Contact thermal post-treatment of oriented strandboard to improve dimensional stability: A preliminary study

The oriented strandboard (OSB) has less dimensional stability than plywood, but they are competitive panels and have been used for similar ends. The wood-water relation variables, such as thickness swelling and water absorption, express this OSB dimensional instability and can be explained by two main factors: wood hygroscopicity and imposed hot-pressing stresses. The objective of this present paper was to propose a thermal post-treatment as a method to improve OSB dimensional stability by decreasing wood hygroscopicity and releasing hot-pressing stress. OSB panels from Pinus taedawood were produced in laboratory, and their characteristics were: single layer, 0.8 g/cm³; 8% phenolic resin and without wax. The OSB panels were treated in a laboratory press at 250 °C for about 4, 7 and 10 minutes. The wood-water relation variables, thickness swelling (TS), water absorption (WA), equilibrium moisture content (EMC) and springback or permanent thickness swelling (PTS) were determined and compared with untreated panels. The results showed that the proposed thermal treatment was effective to reduce TS, EMC and PTS, but didn’t affect WA which was affected by panel density reduction. The longer the treatment the higher the dimensional stability, and panel weight loss could be used as predictive variable for the efficiency of the treatment.     

Tannin–resorcinol–aldehyde cold-set wood adhesives with only formaldehyde as hardener

Based on standard tannin adhesive formulations, tannin-resorcinol-glyoxal and tannin-resorcinol-glutaraldehyde cold-setting adhesives were prepared for wood glulam in this paper. Only less para-formaldehyde was added as hardener of adhesives. Both of them showed a rival tensile strength with tannin-resorcinol–formaldehyde adhesive. However they exhibited lower wood failure. The proportion of formaldehyde of the two are drastically decreased.     

Effect of radiata pine juvenile wood on the physical and mechanical properties of oriented strandboard

This work analyzes the impact of radiata pine (Pinus radiata D. Don) juvenile wood on the physical and mechanical properties of oriented strandboards (OSB). Radiata pine logs were obtained from 10 trees of a 26-year old managed stand located in the 8th Region of Chile. The experimental design considered the proportion of juvenile wood and strand orientation as independent variables. OSB panels of 0.4 m×0.4 m×12 mm were produced and tested. The results show that the juvenile wood proportion has a significant impact on the physical and mechanical properties of OSB. Strands orientation had a significant impact on all the properties studied with the exception of the modulus of elasticity in bending. However, this impact was small in all cases and would not change panel grade with the exception of linear expansion. In this case, panels made from tangential strands showed a higher linear expansion. According to these results, radiata pine juvenile wood can be used for the manufacturing of OSB up to a proportion of 70% of the oven-dry wood weight without significant losses of the physical and mechanical properties if the juvenile wood strands are located in the surface layers.     

Characterization of <Emphasis Type="Italic">Ficus sycomorus</Emphasis> tannin using ATR-FT MIR,MALDI-TOF MS and <Superscript>13</Superscript>C NMR methods

The Ficus sycomorus bark tannin was extracted and characterized using Attenuated Total Reflectance Fourier Transform Middle Infrared (ATR-FT MIR) spectra in the 1800 and 600 cm^?1 range, Matrix Assisted Laser Desorption Ionisation Time of Flight Mass Spectrometry (MALDI-TOF MS) and Carbon-13 Nuclear Magnetic Resonance (¹³C NMR) spectroscopy methods. Ficus sycomorus species have a very high percentage (46%) yield of tannin. These three characterization methods have shown that the Ficus sycomorus bark tannin is condensed and particularly a procyanidin type. It is composed of quercetin, apigenin, fisetinidin, catechin, gallocatechin, chalcone, radicinin, catechin gallate units with the presence of carbohydrates, quinone and gallic acid residues. The analysis results revealed that quercetin are the major constituents of Ficus sycomorus bark tannin. This procyanidin type tannin can give good wood adhesives.     

Improving characteristics of melamine–urea–formaldehyde resin by addition of blocked polyurethane prepolymer

In this paper, the effect of blocked polyurethane prepolymer (BPUP) with four R values (the ratio of –NCO to –OH) and different blending proportions on the characteristics of melamine–urea–formaldehyde (MUF) resin were first investigated. The properties of the modified adhesive systems were examined by dynamic wettability and shear strength. The results indicated that the bonding performance and the wettability of high R value blending modification were superior to that of low R value case. In addition, the optimal bonding performance came from the MUF resin modified by 15% BPUP (R = 5) addition. Besides, the K value on wood substrates with different moisture contents (MC) was calculated by wetting model to interpret the wetting kinetics. The polymer blending adhesive has a potential capacity for making wood-based panels from high-moisture raw materials.     

Non-toxic, zero emission tannin-glyoxal adhesives for wood panels

In this study different tannin-glyoxal glue mixes for particleboard were studied. Gel times at 100 °C of 45% water solutions of commercial pine tannin extract (Pinus radiata ex Diteco Ltda, Chile) with respectively 8% paraformaldehyde and 9% glyoxal were done at different pHs. Duplicate one layer laboratory particleboards were prepared by adding 12% total resin solids of adhesives composed of commercial pine tannin extract (Pinus radiata ex Diteco Ltda, Chile) with paraformaldehyde, or glyoxal or polymeric 4,4’ diphenylmethanne diisocianate (pMDI), respectively. It was found that tannin-glyoxal panels which do not contain formaldehyde and with an emission equal to that of unbounded wood can be obtained by the use of glyoxal as hardener.