Tannin Adhesives Research Articles

Tannin Adhesive from Stryphnodendron adstringens (Mart.) Coville in Plywood Panels

The aim of this study was to evaluate the technical viability of using tannin adhesives derived from Stryphnodendron adstringens (Mart.) Coville barks in the production of plywood. 0, 25, 50, 75, and 100% tannin-based adhesives (TF) derived from barbatimao barks were combined with commercial phenol-formaldehyde (PF) adhesive. The properties of the adhesives were determined, and plywood was produced. The panels were produced with five crossed layers, an adhesive grammage of 360 g/m² (double line), and an assembly time of 10 min. A pressing cycle at a temperature of 150 °C, specific pressure of 12 kgf/cm², and duration of 10 min was used. With the exception of the parallel modulus of elasticity, panels produced with 25, 50, 75, and 100% barbatimao-derived tannin adhesive met NBR 31:000.05-001/2 standards (ABNT 2001). The tannin barbatimao adhesive proved feasible for use in plywood panels destined for both humid and dry environments.

Oligomers distribution at the gel point of tannin–formaldehyde thermosetting adhesives for wood panels

Tannin–formaldehyde (TF) thermosetting wood adhesives for panel products for the building industry were examined to determine which oligomers were formed during the reaction between tannin and formaldehyde. The reaction products were examined by Matrix Assisted Laser Desorption/Ionization Time of Flight mass spectrometry. The analysis revealed that at the gel point, the adhesive was composed of unreacted flavonoid monomers and some oligomers, and due to reaction with formaldehyde, of methylolated tannin monomers and oligomers as well as flavonoids linked by methylene bridges. The slow gelling conditions used to prepare the TF hydrogel caused the unusual occurrence that not only the flavonoid units A-rings but also pyrogallol-type B-rings of the flavonoid units started to react with formaldehyde at the low pHs between 6 and 7 used for tannin adhesives and contributed to the network formed. This reaction was observed for the first time in this study.

Phenolic resin adhesives based on chestnut (Castanea sativa) hydrolysable tannins

Chestnut hydrolysable tannins are phenolic materials that have been considered too unreactive to compete in the phenolic resin adhesives market for exterior boards for the building industry. However, an article in 1973 describing 3 years industrial application of chestnut hydrolysable tannins during the first oil crisis indicated that this was not the case. We have extended this old work by using superior phenolic resins formulations and producing phenol–formaldehyde–chestnut tannin adhesives where a substitution of up to 80% of the phenol is possible with remarkably good results. The reactions involved were clarified by 13C NMR and MALDI-TOF mass spectrometry.

Phenolic resin wood panel adhesives based on chestnut (Castanea sativa) hydrolysable tannins

Chestnut hydrolysable tannins are phenolic materials that have been considered too unreactive to compete in the phenolic resin adhesives market for exterior boards for the building industry. However, an article in 1973 describing 3 years of industrial application of chestnut hydrolysable tannins during the first oil crisis indicated that this was not the case. This previous work has been extended by using superior phenolic resins formulations and producing phenol–formaldehyde–chestnut tannin adhesives where a substitution of up to 80% of the phenol is possible with remarkably good results. The reactions involved were clarified by 13C NMR and MALDI–TOF mass spectrometry.

Bioadhesives for Wood and Fibres

Recent developments and trends in the field of eco-friendly bio-based adhesives are reviewed. The more recent developments in tannin adhesives without the use of aldehyde-yielding compounds under the conditions of processing, or even without the use of hardeners are described. Lignin adhesives are discussed next as well as protein adhesives such as soy protein adhesives. Some trends in carbohydrate adhesives, as modifiers of existing adhesives, by forming degradation compounds or even in the form of liquified wood are addressed. Unsaturated oil adhesives based on epoxidized unsaturated vegetable oils are also described as well as an example of a different approach using a modified cashew nut shell oil to yield an adhesive by self-condensation of the material.

Parâmetros cinéticos de adesivos produzidos a partir de taninos de Anadenanthera peregrina e Eucalyptus grandis

Devido à crise do petróleo, na década de 70, pesquisas tiveram o objetivo de substituir parcial ou totalmente os adesivos convencionais, à base de petróleo. Uma forma de substituição foi a utilização de fontes naturais, como os taninos, provenientes de algumas espécies arbóreas. Porém, os taninos apresentam algumas limitações de uso, que podem ser superadas através da sulfitação e da hidrolise ácida. Portanto, o objetivo deste trabalho foi avaliar o efeito da hidrólise ácida e sulfitação de taninos extraídos da casca de Eucalyptus grandis e Anadenanthera peregrina, nos parâmetros cinéticos dos adesivos tânicos, através da técnica de calorimetria diferencial exploratória (DSC). Os taninos foram hidrolisados com ácido acético ou ácido clorídrico a 10N, empregando-se três valores de pH e três tempos de reação. A sulfitação foi realizada em solução de sulfito de sódio, à 5%, em três tempos de reação. A energia de ativação, entalpia e temperatura de cura dos adesivos foram reduzidas pela hidrólise e sulfitação dos taninos, porém apresentaram banda larga de polimerização. O tempo de reação dos taninos não teve efeito sobre os parâmetros cinéticos.

Industrial production of pine tannin-bonded particleboard and MDF

This article reports on the industrial particleboard and MDF production and trial results obtained over a period of nine years using pine tannin adhesives in Chile. The industrial panels obtained clearly showed that pine tannin extract, a renewable raw material, can be industrially used as an adhesive to manufacture particleboard and medium density fiberboard (MDF) of excellent quality and of very low formaldehyde emission, and this at a reasonable cost. The technology revealed itself suitable both for very old multi-daylight particleboard lines and for more modern single daylight MDF lines. Panels were manufactured initially with addition of small percentages of pMDI to the pine tannin adhesive. Later, adhesive formulations in which pine tannin alone without pMDI was used still showed good results. Industrial results where either paraformaldehyde or hexamethylenetetramine was used as hardener are reported. In the former case the panels satisfied with ease the E1 formaldehyde emission class of European norms. In the case of either pMDI-added adhesives, or hexamine-hardened adhesives without pMDI the emission was lower than the requirements of the F**** Japanese standard.

Thermomechanical analysis of the tannins of Acacia Nilotica spp. Nilotica as a rapid tool for the evaluation of wood–based adhesives

In this article, autocondensation reactions of Acacia nilotica spp. nilotica (Ann) tannin extracts solutions have been studied at several pH values by thermomechanical analyzer (TMA). TMA has been chosen for this study as it has been shown to give results more rapidly and more precisely than panels. It also tends to exaggerate the differences in results which render it an excellent tool for comparison. Therefore, autocondensation reactions were studied and compared with its polycondensation reactions upon addition of different hardeners such as paraformaldehyde, urea, and PMDI at same pH values. The aim was to evaluate the tannin’s reactivity and therefore its suitability for the production of commercially and technically viable tannin adhesives for wood products. The results of autocondensation showed that the maximum Young’s modulus values (3300 and 3600 MPa) were obtained at pH 4. However, these values have been achieved at high temperature (160 and 208 °C). Results of copolymerization reaction of the tannins with different hardeners revealed that some of these co-reactants were found to depress tannins autocondensations, while others appear to enhance the formation of the final networking. Polycondensation of the Ann tannins with 8% paraformaldehyde and 10% PMDI gave the maximum Young’s modulus values at lower temperature (91–101 °C) and acidic pH5. However, the addition of 20 and 30% PMDI achieved the best Young’s modulus values 2300 and 3300 MPa, respectively, at pH 4. Furthermore, the obtained values were comparable to those obtained by the addition of urea and the self condensation reactions. This is very important for particleboard production from economical and technical point of view. It has been noticed that the addition of hardeners lowered the temperature of hardening, and the obtained values were more consistent with the pH acidity. The study concluded that the tannins of Ann were very reactive, and therefore, it could be a potential precursor for particleboard adhesives. The results also indicated that the tannins can be used alone, and it would possibly produce zero emission environmentally friendly particleboard.

Effect of layers relative moisture content on the IB strength of pine tannin bonded particleboard

Relative percentage of moisture content of surface and core layers in pine tannin-bonded particleboard influences markedly the internal bond (IB) strength of the board. This is due to the differential compression exercised on the layers related to their moisture content. The moisture content differential between surface and core layers in particleboard during pressing is a determining parameter for the internal bond (IB) strength of particleboard. In the case of tannin adhesives where these differences can be more pronounced than for synthetic adhesive the effect is more easily observable.

Development and optimization of a new formaldehyde-free cornstarch and tannin wood adhesive

The development and optimization of a new, environment-friendly adhesive made from abundant and renewable cornstarch and tannin is described in this study. At present, the production of wood composites mainly relies on petrochemical- and formaldehyde-based adhesives such as phenol-formaldehyde (PF) resins and urea-formaldehyde (UF) resins. Formaldehyde-free cornstarch plus tannin adhesives were evaluated for mechanical and physical properties (shear strength, rheological characterisation and thermogravimetric analysis).

Propriedades de chapas de aglomerado fabricadas com adesivo tânico de angico-vermelho (Anadenanthera peregrina) e uréia-formadeído

O principal objetivo deste trabalho foi produzir chapas de aglomerado substituindo-se parcial ou totalmente os adesivos à base de uréia-formaldeído por adesivos de taninos extraídos da casca de angico-vermelho (Anadenanthera peregrina) e hidrolisados. As chapas de aglomerado foram fabricadas com partículas de Pinus elliottii e dimensões aproximadas de 40 x 40 x 1 cm, contendo 8% de sólidos de adesivo em relação à massa seca de partículas, sendo 100% de adesivo de uréia-formaldeído puro ou contendo 12,5, 25 e 37,5% de adesivo tânico. Foram produzidas, também, chapas com adesivos tânicos puros, constituindo um total de cinco tratamentos, com três repetições. As chapas foram fabricadas com densidade média de 0,7 g/cm³. As propriedades físicas e mecânicas das chapas foram determinadas em conformidade com a norma ASTM D-1037 (1993). Os resultados dos testes mecânicos foram comparados com os valores mínimos estabelecidos na norma ANSI/A 208.1-1993 (Wood Particleboard), enquanto os resultados dos testes de absorção de água e inchamento em espessura, comparados com os valores máximos estabelecidos na norma DIN 68m761 (1)-1961. A resistência mecânica das chapas de aglomerado produzidas com adesivos contendo taninos de angico-vermelho, com exceção do módulo de elasticidade, ultrapassou os valores mínimos requeridos na norma ANSI/A1-280/93. A absorção de água e o inchamento em espessura, após 2 e 24 h de imersão, de todas as chapas excederam o valor máximo estabelecido. Conclui-se que os adesivos de taninos de angico-vermelho podem substituir parcial ou totalmente o adesivo comercial de uréia-formaldeído, e as suas misturas são indicadas para uso interior, em que a resistência à umidade não é requerida.

Environment-friendly adhesives for surface bonding of wood-based flooring using natural tannin to reduce formaldehyde and TVOC emission

The objective of this research was to develop environment-friendly adhesives for face fancy veneer bonding of engineered flooring using the natural tannin form bark in the wood. The natural wattle tannin adhesive were used to replace UF resin in the formaldehyde-based resin system in order to reduce formaldehyde and volatile organic compound (VOC) emissions from the adhesives used between plywoods and fancy veneers. PVAc was added to the natural tannin adhesive to increase viscosity of tannin adhesive for surface bonding. For tannin/PVAc hybrid adhesives, 5%, 10%, 20% and 30% of PVAc to the natural tannin adhesives were added. tannin/PVAc hybrid adhesives showed better bonding than the commercial natural tannin adhesive with a higher level of wood penetration. The initial adhesion strength was sufficient to be maintained within the optimum initial tack range. The standard formaldehyde emission test (desiccator method), field and laboratory emission cell (FLEC) and VOC analyzer were used to determine the formaldehyde and VOC emissions from engineered flooring bonded with commercial the natural tannin adhesive and tannin/PVAc hybrid adhesives. By desiccator method and FLEC, the formaldehyde emission level of each adhesive showed the similar tendency. All adhesives satisfied the E1 grade (below 1.5mg/L) and E0 grade (below 0.5mg/L) with UV coating. VOC emission results by FLEC and VOC analyzer were different with the formaldehyde emission results. TVOC emission was slightly increased as adding PVAc.

Recent developments in eco-efficient bio-based adhesives for wood bonding: opportunities and issues

Recent developments and trends in the field of eco-efficient bio-based adhesives are reviewed. Eco-efficient means efficient with regard to both ecological and economical aspects. The recent developments in tannin adhesives without the use of any aldehyde-yielding compounds under the conditions of use, or even without the use of any hardeners, are described. Lignin adhesives are discussed next. Developments in protein adhesives, particularly the recent development in soy protein adhesives, are then addressed. Several new trends in carbohydrate adhesives, namely as modifiers of existing adhesives, by forming degradation compounds such as furanic compounds which can then be used as adhesives building blocks, and directly as wood adhesives, even in the form of liquified wood, are addressed. Unsaturated oil adhesives based on epoxidized unsaturated vegetable oils are also described, as well as an example of cashew nut shell oil modified by a new and inexpensive method of ozonolysis to yield an adhesive by self-condensation of the material. The review addresses last the new process of solid wood welding by mechanical friction without the use of any adhesive, in which the wood interfacial region itself is used as the binder.

Análise cinética da cura de adesivos de taninos das cascas de três espécies de Eucalyptus por calorimetria diferencial exploratória (DSC)

O objetivo deste trabalho foi analisar a cinética de cura de adesivos à base de taninos de Eucalyptus grandis, Eucalyptus saligna e Eucalyptus urophylla por calorimetria diferencial exploratória (DSC), comparando-a com a cinética de cura de adesivos comerciais: fenol-formaldeído e de taninos de acácia-negra (Acacia mollissima D.Wild). Verificou-se que o adesivo de taninos de Eucalyptus urophylla apresentou os parâmetros cinéticos (energia de ativação, entalpia, temperatura de pico e ordem de reação) mais próximos aos do adesivo comercial de taninos de acácia-negra, que foram totalmente diferentes do adesivo à base de fenol-formaldeído. Com base nestes parâmetros constatou-se que, em relação aos outros dois, o adesivo de taninos de Eucalyptus urophylla é o mais adequado para colagem, uma vez que em condições industriais ele necessitará de uso mínimo de energia e de tempo de prensagem durante o processo de colagem.

A no-aldehyde emission hardener for tannin-based wood adhesives for exterior panels

Methylolated nitroparaffins, and in particular the simpler and least expensive exponent of their class, namely tris(hydroxymethyl)nitromethane (TN), function well as hardeners of a variety of tannin-based adhesives while affording considerable side advantages to the adhesive and to the bonded wood joint. In panel products, such as particleboard, medium density fiberboard and plywood, the joint performance which is obtained is of the exterior/ marine grade type, while a very advantageous and very considerable lengthening in glue-mix pot-life is obtained. Furthermore, the use of this hardener is coupled with such a marked reduction in formaldehyde emission from the bonded wood panel to reduce emission exclusively to the formaldehyde emitted by just heating the wood (and slightly less, thus functioning as a mild depressant of emission from the wood itself). Furthermore, TN can be mixed in any proportion with traditional formaldehyde-based hardeners for tannin adhesives, its proportional substitution of such hardeners inducing a proportionally marked decrease in the formaldehyde emission of the wood panel without affecting the exterior/marine grade performance of the panel. Medium density fibreboard (MDF) industrial plant trials confirmed all the properties reported above, and the trial conditions and results are reported. In the case of cold-setting adhesives for finger-jointing and glulam, tris(hydroxymethyl)nitromethane (TN) can be used to good effect only in radiofrequency curing of such joints, hence also by heat activation, its performance instead not coming up to the requirements of relevant standards in the case of ambient temperature curing due to higher temperature activation characteristics of this hardener.

Tannery row - The story of some natural and synthetic wood adhesives

Tannin adhesives are a relatively old but limited speciality which has become again topical with the very marked environment-friendly outlook of wood science at the beginning of the new century. Although the lecture will start by intermingling the technical experience of the author in this field mainly with some humorous stories on the early and not so early beginnings of this speciality, this will only be a brief introduction to more recent scientific/technical developments on mainly synthetic thermosetting adhesives, although natural adhesives will not be forgotten. The lecture will also relate in a very brief form, and without being limiting, some of the author's developments in synthetic (and some natural) resins wood adhesives, and in other fields of wood science and wood technology which might have or have had an impact on the advancement of this field.

Considerations on the interface between lignocellulosic materials and tannin-based and other thermosetting adhesives

The use of the equation f = km/α E, correlating number of degrees of freedom m of polymer segments between cross-linking nodes in polycondensation networks to the energy of interaction polymer segment/polymer segment, both within the same polymer and at different polymers interfaces, hence directly 'in situ' in a lignocellulosic joint, through measures of deflection in bending by dynamic thermomechanical analysis, yields a number of consequences of interest in the field of polycondensation hardened networks and of their process of hardening. From this equation, regression equations correlating only two parameters are obtained. The use of these equations allows the easier determination of the parameters which are more difficult or lengthy to obtain by experimental means. The process of networking, hence of the reaction of polycondensation between the gel point and complete hardening of the network, can be followed by the determination of the average number of degrees of freedom m of the polymer segments between cross-linking nodes obtained through these equations. This is all the more useful as it allows the following of the progress of networking of thermosetting adhesive and in particular of the more difficult to follow networking of oligomeric tannin adhesives 'in situ' in the wood joint.

Fortified mangrove tannin-based plywood adhesive

Mangrove bark tannin adhesives are based on a renewable resource. They are potential substitutes or supplements for phenol–formaldehyde (PF) wood-bonding adhesives which are derived from petroleum, a finite natural resource. However, mangrove tannin adhesives exhibit poor adhesive properties including poor wet strength, brittleness, and poor wood penetration. These problems were addressed by treating 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. Heat of reaction of tannin with formaldehyde was increased while the activation energy was drastically reduced. Premature cure was also reduced. The fortified formulations had good plywood adhesive properties. © 1996 John Wiley & Sons, Inc.

Factory trials of Chinese wattle tannin adhesives for antislip plywood

As the final step in the series of studies on the development of Chinese wattle tannin adhesives, factory trials were carried out in order to evaluate the feasibility of this technology for producing exterior grade plywood in China. More than 1 tonne of wattle tannin adhesives were formulated using PF and PUF resins developed as cross-linking agents. A total of 460 sheets of plywood (1830 × 915 × 9 mm) were produced during four and a quarter working shifts totalling 34 working hours in the plywood factory. The results of gluebond assessments strongly indicate that the technology developed would be extremely feasible for application in the Chinese plywood manufacturing industry.

Production of tannin adhesives from bark of Nigerian trees

Bark of Khaya ivorensis and Avicennia alba was found to contain 27% and 24%, respectively, of extractable tannins. The performance of adhesives from the tannins showed that the latter could replace phenol in synthetic adhesives. Further improvement was attained by incorporation of formaldehyde. The adhesives are best applied at 25 °C for 5 mins.