Evaluation of block shear properties of selected extreme‐pH structural adhesives by short‐term exposure test

Nine structural adhesives with varying pH were selected to examine the effect of adhesive pH on wood–adhesive bond quality. The adhesives evaluated included four highly alkaline phenol–formaldehyde, one intermediate pH phenol–resorcinol–formaldehyde, two acidic melamine–urea–formaldehyde, and two acidic melamine–formaldehyde resins. Block shear specimens were prepared using Douglas‐fir and black spruce wood. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum–pressure–redry (VPD) conditions. Adhesive pH, test condition, and wood species showed significant effects on shear properties. The different adhesives performed differently at the dry and VPD conditions. The high‐pH adhesives (phenol–formaldehyde and phenol–resorcinol–formaldehyde) showed similar high wood failures at both test conditions and performed better than the low‐pH adhesives (melamine–formaldehyde and melamine–urea–formaldehyde), especially after the VPD conditioning. The low‐pH adhesives showed high wood failure at the dry condition, but wood failure decreased significantly after VPD conditioning for both species, indicating that the low‐pH adhesives were less durable than the high‐pH adhesives. High‐pH adhesives did not have a negative impact on the strength of the bonded specimens. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Elastic properties of adhesive polymers. III. Adhesive polymer films under dry and wet conditions characterized by means of nanoindentation

The mechanical properties of cured wood‐adhesive films were tested in dry and wet states by means of nanoindentation. A fluid cell was used to monitor possible property changes as a function of changing moisture content. Under wet conditions, the elastic modulus and hardness of all tested adhesives (i.e., urea formaldehyde, melamine urea formaldehyde, phenol resorcinol formaldehyde, and one‐component polyurethane) were reduced to about half of their values in the dry state. After renewed drying, all adhesives largely recovered their initial mechanical properties. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde and polyurethane adhesives

The effect of the heat treatment of spruce wood on the curing of melamine–urea–formaldehyde (MUF) and polyurethane (PUR) adhesives was monitored by measuring their rheological properties by means of a rheometer. Instead of the standard aluminium discs, wooden discs, made from heat-treated wood with different degrees of thermal modification and conditioned in different climates, were used. The wooden discs provided more realistic curing of the adhesives compared to the real-life bonding of wood, because of solvent absorption. The results of the rheological measurements suggested that the modified wood inhibited the curing of MUF and PUR adhesives. The curing of the MUF adhesive was slower because of the reduced absorption of water from the adhesive. The curing of the one-component PUR adhesive was affected by the lower moisture content (MC) of the modified wood.     

Influence of wood extractives on two-component polyurethane adhesive for structural hardwood bonding

ABSTRACT

When bonding wood for structural applications, the wood–adhesive bond is influenced by a variety of factors. Besides the physical and mechanical properties of wood species, their chemical composition, e.g. wood extractives, can play a role in bonding wooden surfaces. A two-component polyurethane system (2C PUR) was chosen to better adapt to the current adhesion problem. The influence of extractives on crosslinking was determined by Attenuated Total Reflection-Fourier Transform Infrared Spectrometer (ATR-FTIR) and on the rheological behavior in terms of gel point and storage modulus. Therefore, 2C PUR was mixed with 10% of eight common wood extractives separately. Furthermore, the mechanical properties of beech wood (Fagus sylvatica L.) bonded with extractive enriched adhesive were tested by means of tensile shear strength tests and evaluation of wood failure. These results of ATR-FTIR clearly show that the majority of crosslinking was terminated after 12 hr. Acetic acid and linoleic acid expedited the isocyanate conversion during the first 2.5 hr. The curing in terms of gel point and storage modulus of 2C PUR was accelerated by starch, gallic acid, linoleic acid, and acetic acid. Heptanal, pentanal, 3-carene, and limonene decelerated the curing. All extractives lowered the storage modulus determined after 12 hr. The bonding of beech wood with extractive–adhesive blends showed a slight decrease of the mechanical properties, with the exception of a marginal increase in the case of linoleic acid and pentanal.

In summary, it can be said that 2C PUR is sensitive to the influence of wood extractives and can therefore be partly held responsible for adhesion problems occurring when extractives in surface-wide and higher contents are available.     

Influence of humidity and frequency on the energy dissipation in wood adhesives

In this paper, the frequency dependent energy dissipation of typical wood adhesive under cyclic stress was studied on film adhesive samples. Three moisture-curing one component polyurethane (1C-PUR) adhesives with relative ductile behavior, one melamine formaldehyde (MF) and one phenol formaldehyde resorcinol (PRF) adhesives both with a more brittle behavior were prepared to study the viscoelastic properties at different relative air humidities (RH). Dynamic Mechanical Analysis (DMA) in tensile mode was used to determine loss modulus, storage modulus and loss factor Tan Delta on free standing adhesive films. It has been shown that 1C-PUR adhesives dissipate proportional more of the stored energy than MF and PRF adhesives. Humidity increased the dissipative processes in all PUR adhesives, especially in the polyamide fiber filled adhesive. PRF adhesive is less influenced by humidity. While for all other tested adhesives the dissipative processes generally increased with higher humidity, humidity decreased the damping of the investigated MF adhesive. The influence of the frequency on the energy dissipation is low for all tested adhesives in the investigated frequency range. Further fatigue tests with glued wood samples are needed to confirm the results observed on the free standing adhesive films.     

两步碱催化法制备尿素和三聚氰胺改性PF胶粘剂

采用两步碱催化法探讨了尿素(U)、三聚氰胺(M)和无机黏土等对酚醛树脂(PF)胶粘剂性能的影响.结果表明:适宜的n(甲醛):n(苯酚)比例是3.00:1或2.00:1;尿素改性可降低PF胶粘剂的生产成本,但其粘接强度减弱;三聚氰胺改性可提高PF胶粘剂的粘接强度,但其成本较高;无机黏土的加入可改善PMUF(苯酚-三聚氰胺...     

木材用聚氨酯胶黏剂的研究进展

聚氨酯胶黏剂由于具有无甲醛,施胶量少,胶接强度高等众多优点,使其在木材工业中的使用量逐步增加.但由于价格因素,聚氨酯胶黏剂在木材用胶黏剂中所占的比例仍然较小.研究聚氨酯胶黏剂与木材的胶接机理是从原理入手,掌握聚氨酯胶黏剂的胶接特点,用最少的原料来达到最好的胶接效果,以希望降低使用价格,扩大应用范围.简述聚氨酯胶黏剂的优点,开发和改性研究的基本思路,重点综述了国内外聚氨酯胶黏剂与木材胶接机理的研究进展.     

木材工业用胶粘剂的现状及发展趋势

主要介绍了脲醛树脂、酚醛树脂、三聚氰胺－ 甲醛树脂、木质素、单宁、异氰酸酯等木材工业用胶粘剂的研制状况及发展趋势。     

Impact of curing condition on pH and alkalinity/acidity of structural wood adhesives

Nine formulations were selected for evaluating the effect of different curing methods on pH and alkalinity or acidity of various structural wood adhesives. These included four phenol–formaldehyde (PF) resins with high pH, one phenol–resorcinol–formaldehyde (PRF) resin with intermediate pH, two melamine–urea–formaldehyde (MUF) resins, and two melamine–formaldehyde (MF) resins with low pH. The four curing methods used in the study were: (1) curing at 102–105°C for 1 h (based on CSA O112.6‐1977), (2) four‐hour curing at 66°C followed by 1‐hour curing at 150°C (based on ASTM D1583‐01), (3) curing at room temperature overnight (based on ASTM D 1583‐01), and (4) cured adhesive squeezed out from glue lines of bonded shear block samples. The effect of the different methods on pH and alkalinity/acidity of the cured adhesive depended strongly on the individual adhesives. For the PF, the alkalinity was different for the different formulations in the liquid form, while in the cured form, the difference in the alkalinity depended on the curing method used. The MF and the MUF were the adhesives most affected by the method used. In particular, the MUF showed much higher cured film pH values when cured by method 2 compared to the other three methods, while both the cured MF and MUF exhibited quite variable acidity values when cured with the different methods. The PRF showed reasonably uniform cured film pH but varying acidity values when cured with the different methods. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

胶粘剂国内外发展概况

本文综述了胶粘剂国内外发展概况,其中包括市场需要概况,主要工业化品种主要发展趋势和发展建议。涉及的胶粘剂类型有：三醛胶、水基胶、反应型胶、溶剂型胶、热熔胶和橡胶型胶等等。     

Wood processing variables and adhesive joint performance

In order to obtain a bonded wood product from the tree, wood must be subjected to a series of processes. These include, among others, vital processes like drying of the green wood and machining of the surfaces of the dried wood in preparation for bonding. However, when wood is machined with dull blades, thermal degradation of the wood frequently occurs. The effects of kiln drying and thermal degradation of wood on the performance of the adhesive joints subsequently formed from such wood were investigated. Tapered double cantilever beam (TDCB) test specimens obtained from hard maple and phenol–resorcinol–formaldehyde (PRF) adhesives were used in the investigations. The enhancement of fracture energy due to the reduction in the moisture content of wood, the improved performance of adhesive joints due to rejointing of wood surfaces before bonding, and the reduction of adhesive joint strength resulting from thermal degradation of wood are presented and discussed.     

国内有机硅改性胶黏剂的研究进展

综述了近年来国内有机硅改性胶黏剂的研究进展,对有机硅改性环氧树脂、丙烯酸树脂、酚醛树脂、脲醛树脂和聚氨酯胶黏剂的制备和性能进行了较为详尽的叙述,展望了有机硅改性胶黏剂的发展前景。     

The effect of steam treatment on bonding strength of impregnated wood materials

In wood materials, the species of wood, its humidity, and the type of the adhesive have an important role to make the wood material durable for a long period both in inner space and outer space. In this study, it is aimed to determine the resistance characteristics of adhesive using different impregnation chemicals and different types of adhesives before and after steam treatment. In this study, beech and poplar as wood materials; mixture of Protim-WR 235, Tanalith-C, and Celcure-AC 500 as impregnation materials; and poly vinyl acetate (PVAc), urea formaldehyde (UF), and desmodur vinyl trie ketonol acetate (D-VTKA), resistant to water, as adhesives materials were used. All samples were kept in a steaming equipment for 2, 6, 12, 24, 48, and 96?h, afterwards the maximum force of the samples were measured for each waiting period, and then their bonding strength was determined. According to the results of the study, beech control samples had higher strength than poplar control samples. Control samples bonded with D-VTKA is the least affected one compared to all original control samples in the steam test. D-VTKA can be offered as the proper adhesive for humid places. The results also showed that Tanalith-C is the least affecting impregnation material on the bonding strength.     

Morphologic accessibility of wood adhesives

Accessibility of wood adhesives was determined using a potassium bromide-embedding technique during resin cure. Cured resin was then extracted with water, followed by x-ray analysis of residual potassium bromide retention in the adhesive matrix. Cured urea–formaldehyde (UF), phenol–formaldehyde resol (PF), and phenol–resorcinol formaldehyde (PRF) accommodated large quantities of potassium bromide. However, only PRF adhesive showed crystallization of trapped potassium bromide, suggesting the presence of large pore sizes in the polymer lattices. Both UF and PRF adhesives, after room-temperature soaking, were equally resistant to water access. At higher temperature, water accessibility of UF increased greatly which may be attributed to thermal softening of the adhesive, while the accessibility of water to the PRF adhesive matrix was mainly a surface phenomenon and was not very temperature dependent. PF was highly accessible to water, even at room temperature, as a result of swelling from the presence of sodium hydroxide (catalyst). Accessibility of PRF adhesive was greatly increased by base (NaOH) solutions but not by acid (H₂SO₄) solutions. The UF resin showed slight increase of accessibility with increase in acidity and basicity of the water solution, but the accessibility increase of this adhesive in base solution was substantially less than that of PRF. The acid hydrolysis influence on accessibility increase of UF adhesive was far less severe than that of temperature.     

The Improvement of Bond Strength Properties and Surface Characteristics of Resinous Woods

Apitong (Dipterocarpus spp.) and Caribbean pine (Pinus caribaea Morelet) contain high amounts of extractives that contribute to poor bonding. To reduce, if not to eliminate, the effects of these extraneous substances, surfaces of small wood blocks were Soxhlet-extracted for 8 hours by different solvents. Wettability of the wood surfaces was then measured by droplet and dynamic methods using water and dilute NaOH as liquids. Tensile shear strengths of extracted wood bonded with aqueous vinyl polymer isocyanate (API), resorcinol formaldehyde (RF) and polyvinyl acetate (PVAc) resin adhesives were also measured. Results revealed that although Caribbean pine had much higher resin content than Apitong, the former had better wettability than the latter. Solvent extraction of the adherend with either hexane or ethanol-benzene (1:2) for 8 hours was not enough to improve its wettability but enough to improve its gluability. However, successive extraction with hexane, methanol and ethanol benzene rendered the wood satisfactorily wettable. Generally, a direct relationship between wettability and bond strength could not be observed. In a separate experiment to improve bonding strengths, test specimens were either overheated or autoclaved for 4 minutes at 125°C during the pressing period. Autoclave treatment was found to be useful in increasing the bond strengths of API, RF, PVAc and urea formaldehyde (UF)-bonded Apitong and Caribbean pine.     

The Improvement of Bond Strength Properties and Surface Characteristics of Resinous Woods

Apitong (Dipterocarpus spp.) and Caribbean pine (Pinus caribaea Morelet) contain high amounts of extractives that contribute to poor bonding. To reduce, if not to eliminate, the effects of these extraneous substances, surfaces of small wood blocks were Soxhlet-extracted for 8 hours by different solvents. Wettability of the wood surfaces was then measured by droplet and dynamic methods using water and dilute NaOH as liquids. Tensile shear strengths of extracted wood bonded with aqueous vinyl polymer isocyanate (API), resorcinol formaldehyde (RF) and polyvinyl acetate (PVAc) resin adhesives were also measured. Results revealed that although Caribbean pine had much higher resin content than Apitong, the former had better wettability than the latter. Solvent extraction of the adherend with either hexane or ethanol-benzene (1:2) for 8 hours was not enough to improve its wettability but enough to improve its gluability. However, successive extraction with hexane, methanol and ethanol benzene rendered the wood satisfactorily wettable. Generally, a direct relationship between wettability and bond strength could not be observed. In a separate experiment to improve bonding strengths, test specimens were either overheated or autoclaved for 4 minutes at 125°C during the pressing period. Autoclave treatment was found to be useful in increasing the bond strengths of API, RF, PVAc and urea formaldehyde (UF)-bonded Apitong and Caribbean pine.     

Adhesive bonding of 3D-printed ABS parts and wood

Additive manufacturing, also known as 3D printing technology, has experienced massive growth in the last decade. Instead of printing the entire product, 3D printing can be used to produce only the most complex parts, which are then combined with simple, non-printed parts from other materials to make the final product. In addition to mechanical connections, adhesive bonding is most commonly used to combine printed parts with other elements. In this study, the influence of 3D-printing parameters on the bond shear strength of 3D-printed Acrylonitrile-butadiene-styrene copolymer parts bonded to beech wood was investigated. Three printing settings with different layer thicknesses (0.39, 0.19, 0.09 mm) and a posttreatment method that utilized acetone vapour were used. The three different adhesives applied were commercial one-component polyurethane adhesive, hot melt adhesive for edge bonding, and a two-component polyurethane adhesive. The results show that the type of adhesive had the biggest influence on the strength of the bond. The highest bond strength was achieved using a two-component polyurethane adhesive. The type of failure (failure in wood, plastic, adhesive, or cohesive failure) depended greatly on the type of adhesive and thickness of the printed layer.     

Effect of steam on bonding strength and dimensional stability of laminated veneer lumbers manufactured using different adhesives

Laminated veneer lumbers (LVLs) manufactured from wood with different adhesives are being increasingly used in the construction of furniture frames and buildings. Yet there is little information available concerning the dimensional stability and shear strength of LVLs after being exposed to steam. In this study, LVLs were manufactured from poplar and beech veneers with phenol/formaldehyde (PF), poly(vinyl acetate) (PVAc), Desmodur-VTKA (D-VTKA) and urea/formaldehyde (UF) adhesives. Dimensional stability of LVLs was measured after being exposed to steam for 2, 6, 12, 24, 48 and 96 h, according to the Turkish Standard (TS) 3639, and also shear strength was measured (according to BS EN 205). The highest initial density of 0.93 g/cm³ was for beech LVL with VTKA adhesive. After exposure to steam for 96 h, the highest weight increase of 65.7% was for poplar LVL with PVAc, the highest radial swelling of 5.7% was for beech LVL with UF, the highest tangential swelling of 7.9% was for beech LVL with PF and the highest longitudinal swelling of 0.5% was for beech LVL with VTKA. The highest shear strength value of 15.8 N/mm² among all samples without exposure to steam was obtained for beech LVL with PVAc adhesive and the lowest shear strength was obtained as 4.48 N/mm² for poplar LVL with UF adhesive.     

木质素在胶粘剂中的应用研究进展

介绍了木质素的结构、分类与性能以及改性方法,综述了木质素在酚醛树脂胶粘剂、脲醛树脂胶粘剂、三聚氰胺甲醛树脂胶粘剂、环氧树脂胶粘剂以及聚氨酯胶粘剂中的应用研究进展,并对今后的研究方向作了展望。     

Development and characterization of a wood adhesive using bagasse lignin

Bagasse is spent fiber left after extraction of sugar. It is mainly used as a fuel to concentrate sugarcane juice. In the present work, the possibility of preparing wood adhesives from bagasse has been explored. The parameters for the preparation of a lignin phenol formaldehyde (LPF) adhesive, (lignin concentration, formaldehyde to phenol molar ratio, catalyst concentration, reaction time and reaction temperature) have been optimized. It was found that up to 50% of phenol can be substituted by bagasse lignin to give LPF wood adhesive having better bonding strength in comparison to a control phenol formaldehyde (CPF) wood adhesive. Prepared resins were characterized using IR, DSC and TGA. IR spectra of LPF resin showed structural similarity with CPF resin. Thermal stability of LPF resin was found to be lower as compared to CPF resin. DSC studies reveal a lower curing temperature for LPF adhesive in comparison to CPF adhesive. A shelf-life study reveals that LPF exhibits consistent behavior as compared to CPF in respect to adhesive strength.