Gross adhesive penetration in furfurylated, <Emphasis Type="Italic">N</Emphasis>-methylol melamine-modified and heat-treated wood examined by fluorescence microscopy

This study investigated the radial penetration of three conventional cold-set wood adhesives [emulsion polymer isocyanate (EPI), poly (vinyl acetate) (PVAc), one-component polyurethane (PU)] into various degrees of furfurylated and N-methylol melamine-modified (NMM) Scots pine, and heat-treated Scots pine and beech based on measurements of effective (EP) and maximum penetration (MP) from microscopic observations. EP of EPI adhesive decreased after modification with higher concentration of furfuryl alcohol while an improved penetration was recorded for PVAc into furfurylated wood. A deeper penetration was observed for all adhesives into wood treated with lower concentration of furfuryl alcohol. The EP of EPI and PU adhesives reduced after NMM treatment but it increased in the case of PVAc. In spite of reduction of EP of PU after NMM treatment, it represented a deeper penetration among all adhesives possibly due to its lower molecular weight. For Scots pine, increasing the treatment temperature improved EP of all adhesives while for beech, the EP of PU and PVAc increased largely in the case of samples treated at 195 °C. Visual analysis of fluorescence microscopy pictures provided more detailed information on modality of penetration. The results are useful for understanding the interaction among common adhesives and modified materials, and can be used in future research to explain the bonding behavior of modified wood.     

植物油料蛋白木材胶黏剂改性的研究进展

植物油料蛋白胶黏剂具有来源广泛、无毒、可再生及制备简单等优点,但胶接强度低、耐水性能差、易霉变等问题限制了其发展和应用。综述了近年来几种主要植物油料蛋白胶黏剂如大豆蛋白胶黏剂、花生蛋白胶黏剂、棉籽蛋白胶黏剂等在木材行业的研究和应用现状,重点阐述了提高植物油料蛋白胶黏剂耐水性能的改性方法,包括物理改性、变性剂改性、接枝改性、交联改性、仿生改性、酶改性、纳米材料改性、复合改性等,指出了植物油料蛋白胶黏剂存在的问题,并对未来发展方向和应用前景进行了展望。     

Bonding performance of heat treated wood with structural adhesives

Bonding of untreated, intermediate (hydro-thermolysed) and heat treated wood with melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF) and polyurethane (PUR) adhesives was studied. An industrial heat treatment process (Plato ^®) was used, which included two separate heat treatment stages and a drying stage in-between. Laminated beams having four lamellas were prepared from untreated and treated timber for mechanical testing of the bond lines. The results of the tests showed that heat treatment affected the shear strength and the delamination of the laminated wood depending on the adhesive system used for bonding. The PUR and MUF adhesives performed in a rather similar way, and better than the PRF adhesive. The shear strength of laminated wood bonded with the waterborne MUF and PRF adhesives decreased for the specimens made of hydro-thermolysed timber and decreased further for the specimens made of fully heat treated timber. The difference in adhesive bond shear strength between untreated, intermediate and fully treated wood was less obvious in the case of the PUR adhesive. Delamination of the PRF bond line decreased drastically for all the specimens made of heat treated timber.     

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.     

生物质胶粘剂在人造板中的应用

分析了生物质胶粘剂在人造板中的应用现状,综述了近年来大豆蛋白基胶粘剂、淀粉基胶粘剂和木质素胶粘剂的研究进展.针对目前生物质胶粘剂在胶合强度、耐水性和改性工艺以及在人造板应用等方面存在的问题,笔者认为:应从生物质胶粘剂的宏观粘结特性和微观结构变化出发,深入研究其胶合机理并采用更有效的改性方法和生产工艺,使之广泛应用于人造板工业.     

Thermal stability of glued wood joints measured by shear tests

The thermal stability of glued wood joints is an important criterion to determine the suitability of adhesives in the field of engineered wood. During their product life, glued wood joints can be exposed to high temperatures in various ways (direct exposure to the sun, fire, etc.). Thereby the cohesiveness of the adhesive must not degrade. This raises the question of how the strength of bonding changes under thermal load. The current investigation covers the influence of temperature (T=20 to 220°C) on the shear strength of glued wood joints. Different adhesive systems were investigated. With increasing temperature, the shear strength of solid wood and also of glued wood joints decreased. There were big differences in thermal stability and failure behaviour between the adhesive systems as well as within the polyurethane group. The thermal stability of one-component polyurethane systems can be greatly varied by modifying their chemical structure. Well adapted one-component polyurethane adhesives reach a strength similar to that of phenol resorcinol resin.     

Experimental analysis on small-scale finger-jointed specimens at elevated temperatures

Finger joints have to meet strength performance requirements for the use in structural engineered wood products such as glued-laminated timber beams (glulam). Among these, the thermal stability of the adhesive used in the finger joint is an important criterion to determine the suitability of the connection. The influence of adhesives on the load-carrying capacity of glulam beams and finger-jointed members can be assessed by large-scale fire tests; however, there are obvious benefits in using small-scale specimens tested at elevated temperatures as an alternative. In Europe, there is currently no small-scale test available to test the fire performance of adhesives in structural timber members. The work presented in this paper addresses this issue and was supported by members of a European standardisation committee to develop such a small-scale test. This paper presents, as a first step, the results of a series of tensile tests on small-scale finger-jointed specimens tested at elevated temperatures. The tests were performed with different types of adhesives which are currently also tested in large-scale fire tests. In the small-scale tests here, structural adhesives showed nearly no weaknesses with increasing temperature up to 140 °C. Further, MUF and PRF adhesives showed mostly wood failure even up to temperatures of 220 °C. In the next step of this investigation, the results obtained from the small-scale tests at elevated temperature will be compared to large-scale fire test results in order to find a possible link between both testing methods.     

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.     

大豆蛋白改性和黏合性研究

为解决石油生产的黏合刑的环境毒性问题，以大豆分离蛋白(SPI)为原料，经加热(H)、胰蛋白酶(T)、碱(A)、尿素(U)和盐酸胍(GH)改性，分别制得改性大豆蛋白H-SPI、T-SPI、A-SPI、U-SPI和GH-SPI。将其用作不同硬度木质的黏合剂，经试验测定表明它们的黏合强度和阻水性都优于未改性SPI。还对几种改性方法进行了比较，结果表明尿素改性和碱改性是比较好的两种方法。     

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.     

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.     

化学改性大豆蛋白木材胶粘剂研究进展

该文综述大豆蛋白胶粘剂粘接及耐水机理、化学改性方法及改性大豆蛋白胶粘剂在秸秆板中应用情况;最后指出化学改性大豆蛋白木材胶粘剂存在问题及今后发展方向。     

Novel adhesive system based on 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and hyperbranched polyglycerols

Two hyperbranched polyglycerols bearing 1,1,1-tris(hydroxymethyl)propane or Bisphenol A core and terminal hydroxyl functionality were examined as components of novel wood adhesive systems. Two 1,3-dimethylol-4,5-dihydroxyethyleneurea resins (DMDHEU) were used as crosslinkers. Shear strength tests revealed that the adhesives containing up to 75 wt% of renewable glycerol-derived polyglycerols retained performance comparable to that of neat DMDHEU. The results give way to extending the area of application of hyperbranched polyglycerols in the field of wood adhesives.     

Resistance of thermally modified ash (<Emphasis Type="Italic">Fraxinus excelsior</Emphasis> L.) wood under steam pressure against rot fungi,soil-inhabiting micro-organisms and termites

Thermal modification processes have been developed to increase the biological durability and dimensional stability of wood. The aim of this paper was to study the influence of ThermoWood^® treatment intensity on improvement of wood decay resistance against soil-inhabiting micro-organisms, brown/white rots and termite exposures. All of the tests were carried out in the laboratory with two different complementary research materials. The main research material consisted of ash (Fraxinus excelsior L.) wood thermally modified at temperatures of 170, 200, 215 and 228 °C. The reference materials were untreated ash and beech wood for decay resistance tests, untreated ash wood for soil bed tests and untreated ash, beech and pine wood for termite resistance tests. An agar block test was used to determine the resistance to two brown-rot and two white-rot fungi according to CEN/TS 15083-1 directives. Durability against soil-inhabiting micro-organisms was determined following the CEN/TS 15083-2 directives, by measuring the weight loss, modulus of elasticity (MOE) and modulus of rupture (MOR) after incubation periods of 24, 32 and 90 weeks. Finally, Reticulitermes santonensis species was used for determining the termite attack resistance by non-choice screening tests, with a size sample adjustment according to EN 117 standard directives on control samples and on samples which have previously been exposed to soil bed test. Thermal modification increased the biological durability of all samples. However, high thermal modification temperature above 215 °C, represented by a wood mass loss (ML%) due to thermal degradation of 20%, was needed to reach resistance against decay comparable with the durability classes of ‘‘durable’’ or ‘‘very durable’’ in the soil bed test. The brown-rot and white-rot tests gave slightly better durability classes than the soil bed test. Whatever the heat treatment conditions are, thermally modified ash wood was not efficient against termite attack neither before nor after soft rot degradation.     

碱木素／尿醛树脂胶黏剂的合成研究

利用制浆黑液中的碱木素为主要原料,经过羟甲基化改性,甲醛缩合制备纤维板胶黏剂,探索改性木素胶黏剂粘结效果。用正交试验方案设计对碱木素改性、胶黏剂制备工艺进行优化。研究表明：将碱木素进行羟甲基化改性,其活性明显提高。最佳的优化条件：甲醛用量7mmol·g-llignin,pH值10,反应温度70℃,反应时间120min;改性碱木素最佳的加入量比例为20％。用碱木素制备改性木素胶黏剂可利用造纸黑液木素资源,而且具有广泛的应用前景。     

Physical,mechanical and biological properties of thermo-mechanically densified and thermally modified timber using the Vacu<Superscript>3</Superscript>-process

Densification and thermal modification change wood properties in different ways depending on the treatment conditions and the wood species. In the presented investigations, densification and thermal modification were applied consecutively. The primary objective of this treatment combination was the compensation of reduced mechanical properties due to the thermal modification by densification. The combined processes were applied to five European wood species: poplar (Populus nigra L.), beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), English oak (Quercus robur L.) and European ash (Fraxinus excelsior L.). Depending on the mean density of the species, a thermo-mechanical densification of 43 or 50% was imposed to improve mechanical strength parallel to the grain. Subsequently, the densified material was thermally modified in the so-called Vacu³-process at 230 °C and 20 or 80% vacuum and at 240 °C and 20% vacuum. The thermal modification resulted in changing wood colour, mechanical strength, hardness, dimensional stability and durability. All the wood modification processes were carried out at industrial scale after pre-tests at laboratory scale. The modified material was characterized regarding flexural properties, static and dynamic hardness, structural integrity, abrasion resistance, moisture dynamics, dimensional stability, and durability against white, brown and soft rot fungi. In summary, the test results showed that the consecutive application of thermo-mechanical densification and thermal modification leads to significantly improved durability whilst mechanical properties at least for beech, ash and poplar remained and the material is dimensionally stable.     

Bonding of densified beech wood using adhesives based on thermally modified soy proteins

The bondability/glueability of aged and sanded thermo-hydro-mechanically (THM) densified beech wood (Fagus sylvatica L.) was tested and compared with undensified sanded beech wood as a control. THM and control specimens were bonded with five different soy protein isolate (SPI) based adhesives. Commercial SPI powder was thermally modified in the vacuum chamber at 50 or 100?°C and pH adjusted (to pH 10.0) dispersions in water prepared at 24, 50 or 90?°C. Wettability was determined with measuring the sessile drop contact angles of water. Effective penetrations (EPs) and tensile shear strengths of THM and control specimens were determined. THM and control wood had similar wettability. Although THM wood had lower moisture content than control wood, it absorbed the water more slowly than control wood. THM specimens showed lower EPs than control specimens when comparing individual adhesives due to increased density of THM wood. Adhesives prepared with SPI thermally modified at 50?°C showed statistically significantly lower tensile shear strength of bonded THM specimens than that of bonded control specimens. THM densification had no significant effect on the bonding strengths of adhesives prepared with non-modified SPI and SPI thermally modified at 100?°C.     

我国木材工业用胶黏剂研究与应用现状及发展趋势

介绍了我国木材工业用胶黏剂在生产原料、工艺与技术、工业化设备等方面的研究与技术革新,重点综述了环保型脲醛树脂、酚醛树脂、异氰酸酯类胶黏剂、大豆蛋白胶黏剂、淀粉胶黏剂以及高性能、多功能胶黏剂的研究进展与应用现状,并对我国木材工业用胶黏剂的发展趋势进行了展望。     

Wood Adhesive from Crosslinked Poly(Vinyl Alcohol) and Partially Gelatinized Starch: Preparation and Properties

An environmentally friendly wood adhesive was developed by crosslinking cornstarch and poly(vinyl alcohol) (PVOH) with hexamethoxy-methylmelamine (Cymel 323). Citric acid was used as a catalyst and latex (UCar 443) was added to improve moisture resistance. The adhesive was evaluated for its utility in plywood manufacture. It exhibited excellent mechanical properties comparable to many of the commercially available urea-formaldehyde plywood adhesives used for interior applications. The viscosity of the adhesive at 27 % (w/v) was 7000 mPas, allowing easy application to wood surfaces by brush. The minimum concentration of crosslinking agent needed to achieve good mechanical properties in plywood was 15 % (w/w proportion of total solids). Optimum curing temperature and curing time were 175 °C and 15 min, respectively. Addition of latex to the adhesive formulation improved both moisture resistance and physical properties of plywood test samples. Samples prepared with an optimal adhesive formulation, when completely immersed in water for 2 h or exposed at 93 % or 50 % relative humidity (RH) for 30 days, exhibited > 90 % failure in the veneer as opposed to < 10 % failure in the adhesive joints.     

Delaminierungseigenschaften und Scherfestigkeiten von verklebten rotkernigen Buchenholzlamellen

The paper deals with investigations on dry shear strength and delamination behavior of gluelines between beech wood laminations showing no or distinct red heartwood. The investigations covered 36 parameter combinations comprising besides the red heartwood two widths of the laminations, two different adhesive families and two delamination procedures related to verification of adhesive type I or II. The tests revealed no influence of red heartwood on shear strength which for all parameter combinations exceeded almost throughout 10 N/mm². In contrary thereto the delamination tests revealed a distinct dependency of the glueline integrity on red heartwood and width of the adherents. No parameter combination conformed to the requirements of adhesive type I. In case of laminations without red heartwood the requirements of adhesive type II were met. Gluelines between two laminations with red heartwood showed throughout very pronounced delamination above 50%. The integrity of gluelines versus delamination represents one of the essential normative requirements for adhesives used in glued load bearing wooden elements. This proof was not possible for gluelines of beech wood laminations with red heartwood when applying today’s European delamination test procedures and performance requirements developed and established for gluelines of softwood laminations.