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.     

Chinese wattle tannin adhesives for exterior grade plywood

Chinese wattle tannin adhesives were prepared in conjunction with the preparation of phenol-formaldehyde (PF) and phenolurea-formaldehyde (PUF) resins as crosslinking agents which can be substitutes for expensive paraformaldehyde. Adhesive formulations and gluing conditions were examined by preparing laboratory plywoods (300×300 mm). Based on the results obtained, plywoods (600×600 mm) ofTilia species and ofPinus massoniana were prepared using wattle tannin adhesives in pilot plant trials. The gluing results revealed that the tensile strength of the plywood can meet the requirements of the Chinese Standard GB9846.4-88 for exterior grade plywood.     

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.     

Decrease of pressing temperature and adhesive content by metallic ion catalysis in tannin-bonded particleboard

The use of bivalent metallic ion catalysts such as zinc acetate in wattle tannin adhesives for exterior grade particleboard results in the attainment of better board properties at lower pressing temperatures without any lengthening of the pressing time of the board. Lower pressing temperatures achieved in this manner also allow a reduction in the percentage resin solids in the board. There are indications that the same effect is present when using synthetic phenol-formaldehyde resins.     

Evaluation of formaldehyde emission of pine and wattle tannin-based adhesives by gas chromatography

Formaldehyde emissions from wood particleboards bonded with pine and wattle tannin-based adhesives, using hardeners paraformaldehyde, hexamethylenetetramine and TN (tris(hydroxyl)nitromethan), were measured by the perforator method (DIN EN 120—European Committee For Standardization 1991). All particleboards made using the wattle tannin systems with the three different hardeners were satisfied grade E₁, while in the case of pine tannin only the use of the hexamine hardener led to grade E₁ being satisfied. This tendency was attributed to the curing mechanism of the hardener, the reactivity of the tannin molecule toward formaldehyde and the fast reactivity toward formaldehyde of pine tannin, due to the inclusion of phloroglucinolic A-rings within its molecular structure. Hexamine was effective at reducing formaldehyde emission in tannin-based adhesives when used as the hardener. Being a more sensitive and advanced method, it was also used. The samples used for gas chromatography were gathered during the perforator method. The formaldehyde emission levels obtained from gas chromatography were similar to those obtained from the perforator method.     

Starch-sugar synergy in wood adhesion science: basic studies and particleboard production

Synergic properties between starch and sugar were discovered. 100% crop-based starch and sugar formulations were studied. Classical preliminary tests such as curing temperature and gel time were measured for different starch-sucrose water suspensions. More significant information was deducted with applied tests such as plywood gluing tests and particleboard production. Wood-to-wood bonding properties were analysed applying NaOH and high temperature (180°C) as catalysts for starch gelification. Comparable strength to commercial urea-formaldehyde and polyvinyl acetate adhesives was achieved in plywood tests. Consequently, the knowledge acquired in wood bonding of flat surfaces was applied to particleboard production. These derived particleboards showed that it is possible to obtain a panel with only carbohydrates as gluing agent. Starch-sugar synergy in alkaline environment increases the internal bond (IB) of the natural panels up to 0.25 N/mm². Possible mechanisms of synergy between these saccharides were proposed. Finally, a blended formulation with Mimosa tannin allows for achieving an internal bond of 0.40 N/mm², which is the required value for interior grade particleboards (DIN EN 312). Starch-sugar-tannin adhesives are 100% environmentally-friendly formulations.     

Structural bonding of ash (Fraxinus excelsior L.): resistance to delamination and performance in shearing tests

The utilization of ash (Fraxinus excelsior L.) allows for a significant enhancement of the load-bearing capacity in structural laminated products. However, such applications fundamentally require high-strength and durable bonds between lamellas and in finger joints. Therefore, the aim of the present survey was to evaluate ash bondings in terms of resistance to delamination (EN 302-2) and shear performance (EN 392). Investigations were performed with five adhesives: phenol-resorcinol-formaldehyde (PRF), melamine-urea-formaldehyde (MUF-1, MUF-2), polyurethane (PUR), emulsion polymer isocyanate (EPI) and varying closed assembly times as key bonding parameter. For all tested adhesives and closed assembly times, the shear test showed high wood failure percentages and bond strength values that compare to solid ash. In contrast, for resistance to delamination, significant differences were found between the adhesives as well as between closed assembly times, with improving resistance to delamination for increasing closed assembly times. The best performance was determined for the PRF-adhesive and long closed assembly times. However, standard requirements for resistance to delamination could not be met by any of the adhesives. The resistance to delamination showed no correlation with the shear performance for any of the adhesives. Microscopic examination of the bonded joints revealed that both the penetration behavior of the adhesives and glueline thicknesses clearly correlated with the closed assembly time.     

Rheology of tannin-added phenol formaldehyde adhesives for plywood

adhesive consumption in double glue line, which is significantly lower than the aprox. 400 g/m² allowed by commercially available phenol-formaldehyde adhesives. The adhesives prepared with resols containing 36% reactive solids, the tannins representing aprox. 20%, has permitted the preparation of eucalyptus plywood boards exceeding the requirements of the European norm for exterior grade plywood.     

Fast vs. slow-reacting non-modified tannin extracts for exterior particleboard adhesives

Non-fortified, non-modified tannin extracts, rather than chemically-modified tannin adhesives intermediates, can be used as effectively to produce excellent exterior grade particleoboard at fast pressing times of industrial singnificance, with considerable advantages in both handling and cost. This can be achieved by simple pH-controlled reactivity adjustments of the tannin extract in the glue mix. This new concept is shown to be applicable to all types of polyflavanoid tannin extracts: Pine, mimosa, pecan, quebracho, and gambier tannin extracts are compared as regards their performance and capabilities. The new concept applied to the faster-reacting tannins, such as pine tannin and pecan tannin extracts, is shown to produce fast hot-pressing rates and excellent high moisture content tolerances which are undreamed of for the slower-reacting tannins, modified or non-modified, which are traditionally associated with thermosetting wood adhesives. Industrially used techniques to control well viscosity and pot-life of the glue-mixes of accelerated tannin adhesives are also presented.     

Rheology of tannin-added phenol formaldehyde adhesives for plywood

2 adhesive consumption in double glue line, which is significantly lower than the aprox. 400 g/m² allowed by commercially available phenol-formaldehyde adhesives. The adhesives prepared with resols containing 36% reactive solids, the tannins representing aprox. 20%, has permitted the preparation of eucalyptus plywood boards exceeding the requirements of the European norm for exterior grade plywood.

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2 je doppelter Klebeline erlaubt. Dies ist bedeutend niedriger als die etwa 400 g/m², die bei kommerziell erh?ltlichen Harzen n?tig sind. Die hier hergestellten Harze enthalten etwa 36% reaktive Harze, wovon ca. 20% Tannine darstellen. Mit diesen Harzen konnten Eukalyptus-Sperrh?lzer hergestellt werden, welche die Anforderungen der europ?ischen Normen für Au?enverwendung übersteigen.

     

Verklebung von Buchenholz für tragende Holzbauteile

The load bearing capacity of glulam could be enhanced by using lamella of European beech (Fagus sylvatica??L.). Essential requirements for the production of those beams are adhesives which allow for a durable und reliable bonding of lamella. The paper shows to what extent commercially available MUF and PU adhesives and adapted bonding parameters ensure gluing of European beech timber for load-bearing timber structures. Both beech wood containing heartwood and beech wood without heartwood were included in the study. With regard to technical approval, the chosen test methods were resistance to delamination according to EN??302-2 (2002) and shear test of glue line according to EN??392 (1995). From the two tested MUF systems it was evident, that delamination decreased by increasing the closed assembly time. The requirements of EN??301 (2006) were fulfilled for adhesive type??I and??II. Without any limitations beech wood containing heartwood could be glued using MUF-1. On the other hand, the results of the PU adhesive were insufficient. Light-optical micrograph examinations show that long assembly times secure the formation of a glue line. It was assumed that the curing process is retarded by beech wood. No relationship was found between the results of the shear strength test and the delamination test. For quality control of glulam the delamination test is recommended. Based on the positive results an application for technical approval of glulam consisting of beech was made. For this reason beech-glulam can be generally utilized in service class??1.     

Pine tannin adhesives for particleboard

Pine tannin-based adhesive formulations suitable for the industrial manufacture of exterior grade particleboard are discussed. The formulations described are a combination of substantial amounts of pine tannin extract with melamine-formaldehyde, phenolformaldehyde and diisocyanate wood adhesives. The pine tannin extract/diisocyanate (MDI) formulation appears to be particularly suitable for industrial application. The results obtained at laboratory level with the latter formulation are excellent. The elimination of problems inherent in the chemical nature of pine tannin extracts was obtained by already used industrial and laboratory “separate application” techniques. The formulations and application techniques presented appear to be suitable for most of the more widely distributed pine species such us Pinus patula, P. radiata. P. taeda. P. elliotii and others.     

Untersuchungen zur Verleimung von Furniersperrholz mit modifizierten Diisocyanat-Klebstoffen

It was the aim of the investigations to find some modified and extended PMDI binders, suitable for bonding vencer plywood of the type AW. The bonding strength had to fullfill the requirements of DIN 68 705 part 3 resp. DIN 53 255. Pretrieatments were a cold water storage and boiling test. Three-ply boards were pressed from spruce, beech and gaboon veneers. Binders were PMDI-glues with and without emulgators and the extender tannin, Organosolv-spruce-lignin, lignosulfonic acid, casein and corn starch. The results showed the following. Veneer plywood of the type AW can be produced applying PMDI together with suitable reactive agents. The veneer wood species, the glue mixtures, the pressing conditions and the fillers have to be adjusted to the PMDI type and to the extender. Plywood made with modified, emulsified diisocyanate showed a better bonding strength and wood failure than plywood bonded with non-emulsified diisocyanate.     

Wood adhesives fromPinus radiata bark

Conditions for extracting bark of radiata pine (Pinus radiata D. Don) were examined with the purpose of obtaining high yield and high quality extractives for wood adhesives. A four-stage squeeze extraction comprising a two-stage extraction with hot water and one-stage extraction with NaOH aqueous (pH 8.3) solution followed by a one-stage washing with hot water doubled the extractives yield obtained by the first two-stage hot water extraction alone and resulted in a total extractives yield of 30% from commercially available samples of the radiata pine bark. Furthermore, high purity of polyflavanoids in the extractives was obtained by the four-stage squeeze extraction and viscosity reduction was achieved by sulphitation of the insoluble fraction of the extractives. Subsequently, unfortified plywood adhesives of sufficiently high quality to satisfy the requirements for Type A bond (fully weather and boil proof) were readily formulated from the high yield extractives.     

Use of tannin adhesive from Stryphnodendron adstringens (Mart.) Coville in the production of OSB panels

Stryphnodendron adstringens (Mart.) Coville is a species from the Cerrado (Brazilian tropical savanna) popularly known as barbatimão. Its main use of economic value lies in the extraction of the tannins found in its bark and leaves. The purpose of this study was to assess the technical viability of using a tannin-based adhesive coming from the bark of Stryphnodendron adstringens (Mart.) Coville for the production of Oriented Strand Board (OSB) panels. Tannin-based adhesives were produced from tannins extracted from the bark of barbatimão, and these adhesives were used in proportions of 0, 25, 50, 75 and 100 % with commercial urea–formaldehyde adhesive. The properties of the pure adhesives and the mixtures were determined. Subsequently, OSB panels were produced with urea–formaldehyde adhesives, tannin–formaldehyde adhesives and their mixtures. The panels were produced with wood from Pinus oocarpa, with 8 % adhesive, a press cycle of 40 kgf cm^?2, and temperature of 160 °C for a period of 8 min. Only the OSB panels produced with 100 % urea–formaldehyde adhesive, 100 % barbatimão tannin adhesive and 25 % barbatimão adhesive combined with 75 % urea–formaldehyde fulfilled all the pre-requisites stipulated in the standard EN 300 (2006) for type 1 panels, thus proving to be technically viable for application in OSB panels.     

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.     

Eigenschaften formaldehydarm verleimter Sperrhölzer

Tests were carried out on both industrial plywood with low content of formaldehyde and laboratory plywood. There were eight series plywood from different industrial manufactures including blockboard and a blockboard with particleboard decks. Twelve lab-series (5×1.5 BU) were produced of different formulae by the technical department of WKI. Bending and bonding properties as well as formaldehyde emissions of plywood were determined. Creep behaviour of plywood will be described in part 2. Plywood showed good bending characteristics dependent on its structure. Bonding strength after tensile and shear tests as well as after prick test complied with IF bonding quality even after alternating climatic storages. The different series, however, differ widely with respect to IW bonding quality. The different potential of formaldehyde emissions of the series was determined according to various methods, correlations between the methods could be proved. The test, however, revealed the difficulties caused by the material to measure formaldehyde emission of plywood.     

Bark extractives from spruce as constituents of plywood bonding agents

The principles of the use of tannins and resins extracted from wood bark have been known since the 1950's. But only in recent years research on tannins has become lively. The literature review at the beginning of this paper concentrates mainly on the latest investigations in this field. The experimental part investigates the amount and quality of spruce bark extracts and their suitability as extenders in plywood adhesives. The use of chemically treated bark (bark dispersion) in plywood gluing is studied in the last part. If spruce bark is extracted with distilled water, the extract yield is 11.2% at the most, but when 10% of NaOH is added the yield increases to 40.0%. The extract content of drum-debarked pulpwood bark was clearly lower than that of unfloated logs, debarked in the sawmill. The results obtained reinforce the earlier belief that spruce bark extract can substitute Quebracho as an extender for plywood adhesives. Bark dispersion can also be used for this purpose. Exterior-grade birch plywood can be manufactured, using glues which contain spruce bark extract and dispersion. The extract and dispersion can also substitute part of the phenolic resin in the glue mixture, up to 20% at best, without any deterioration in the quality of gluing.     

Mimosa tannin resins for impregnated paper overlays

The surface quality of plywood coated with resin impregnated papers based on tannin/furfuryl alcohol resins was studied. Several hardeners such as hexamine, paraformaldehyde, formurea and a mixture of formurea and paraformaldehyde were added in small quantities to a mimosa tannin/furfuryl alcohol resin. Papers impregnated with resin based on tannin + furfuryl alcohol + paraformaldehyde, or + formurea were found to yield excellent results in standard water vapour resistance and cross-cut tests. Matrix assisted laser desorption ionization time of flight (MALDI-TOF) mass spectrometry and cross-polarisation-magic angle spinning NMR (CP-MAS ¹³C NMR) were used to show coreaction of tannin with furfuryl alcohol and even furfuryl alcohol self condensation under alkaline conditions, as well as coreaction of formaldehyde-yielding compounds such as hexamine with both tannin aromatic rings and the aromatic ring of furfuryl alcohol. Some of the more important oligomers formed were identified by MALDI-TOF.     

Investigation on bonding quality of beech wood (Fagus orientalis L.) veneer during high temperature drying and aging

In the present study, the effects of high drying temperature and UV light induced aging on bonding quality of plywood manufactured from untreated and treated veneer layers were investigated. Rotary cut veneers with dimensions of 500 mm×500 mm×2 mm produced from beech (Fagus orientalis Lipsky) log were selected for topochemical, chemical and mechanical analyses. The veneer sheets were oven-dried at 100°C and 180°C after the peeling process. Afterwards, the surfaces were exposed to artificial UV irradiation in an UV chamber for 24 h, 48 h and 72 h representing natural sun irradiation of 2, 4 and 6 months, respectively. Topochemical distribution of lignin and phenolic extractives of the treated and untreated veneers was investigated on a cellular level using UV microspectrophotometry (UMSP). For the chemical characterization of accessory compounds high performance liquid chromatography (HPLC) was used. Furthermore, the shear and bending strengths of plywood manufactured from the treated samples are determined in order to study the bonding quality. The UV microscopic detection shows that after high drying temperature and aging treatment, lignin condensation occurs. With increasing drying temperature and aging duration, more phenolic extractives are situated in parenchyma cells and vessel lumens which can be proved by increased absorbance at 278 nm. The HPLC analysis of the treated tissue showed distinct signals of polymerized compounds such as catechin and 2,6-dimethoxybenzoquinone which are chromophoric compounds in discolored beech wood. The mechanical properties of plywood showed that with increasing drying temperature up to 180°C does not negatively affect shear and bending strengths of samples. After exposure of the veneers to UV irradiation (especially 6 months), decreasing shear and bending strengths of plywood samples can be observed.