The shear strength of Calabrian pine (Pinus brutia Ten.) bonded with polyurethane and polyvinyl acetate adhesives

In this study, the change in shear strength on radial and tangential surfaces of Calabrian Pine (Pinus brutia Ten.) wood having different roughness values as the result of sawing with a circular ripsaw, planing and sanding, and bonded with polyurethane (PU) and polyvinyl acetate (PVAc) adhesives at the pressure levels of 3, 6, and 9 kgf/cm², was studied. Each of the 360 specimens prepared to determine the effect of the variables on bond performance were subjected to shear test in an universal test machine in accordance with the ASTM D 905–98 standards. The values obtained were analyzed statistically and the results were interpreted. The highest shear strength (11.83 N/mm²) for plane of cut was obtained on the tangential surface after sanding and applying PVAc adhesive with a pressing pressure of 9 kgf/cm². The lowest shear strength (6.01 N/mm²) was obtained in the joinings made on the planed surfaces by using PU adhesive and a pressing pressure of 3 kgf/cm². The highest shear strength (9.10 N/mm²) on the radial surface was obtained after sanding and applying PVAc adhesive and pressing with a pressure of 6 kgf/cm². The lowest shear strength (3.76 N/mm²) was obtained in the specimens whose surfaces were sanded and glued with PU adhesive with a pressing pressure of 3 kgf/cm². In general, in the radial surfaces, just like in the tangential surfaces, the specimens bonded with PVAc exhibited a higher shear strength compared with those glued with PU. According to these results, it is definitely necessary to sand the surfaces prior to the bonding process to have a higher shear strength. The bonding process should be made on the tangential surfaces with higher pressures. The PVAc adhesive should be preferred instead of the PU adhesive. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3050–3061, 2006     

Shear strength of calabrian pine (Pinus brutia Ten.) bonded with polyurethane and polyvinyl acetate adhesives

In this study, the change in shear strength on radial and tangential surfaces of Calabrian pine (Pinus brutia Ten.) wood having different roughness values as the result of sawing with a circular ripsaw, planning, and sanding, and bonded with polyurethane (PU) and polyvinyl acetate (PVAc) adhesives at the pressure levels of 3, 6, and 9 kg f/cm² were studied. A total of 360 specimens of each, prepared with the objective of being able to determine the effect of the variables on bond performance, were subjected to the shear test in the universal test machine in accordance with the ASTM D 905–98 standard. The values obtained were analyzed statistically and the results were interpreted. The highest shear strength (11.83 N/mm²) for plane of cut was obtained on the tangential surface after sanding and applying PVAc adhesive with a pressing pressure of 9 kg f/cm². The lowest shear strength (6.01 N/mm²) was obtained in the joinings made on the planed surfaces by using PU adhesive and a pressing pressure of 3 kg f/cm². The highest shear strength (9.10 N/mm²) on the radial surface was obtained after sanding and applying PVAc adhesive and pressing with a pressure of 6 kg f/cm². The lowest shear strength (3.76 N/mm²) was obtained in the specimens whose surfaces were sanded and by using PU adhesive and with a pressing pressure of 3 kg f/cm². In general, in the radial surfaces, just like in the tangential surfaces, the specimens bonded with PVAc produced a higher shear strength compared to those glued with PU. According to these results, it is definitely necessary to sand the surfaces prior to the bonding process to have a higher shear strength. The bonding process should be made on the tangential surfaces with higher pressures. The PVAc adhesive should be preferred instead of the PU adhesive. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4856–4867, 2006     

Low-cost natural binder for particleboards production: study of manufacture conditions and stability

Nowadays, the majority of adhesives used in particleboards (PB) manufacture are formaldehyde-based. In the present research work a low-cost bioadhesive, based on the combination of thick spent sulfite liquor (TSSL) with wheat flour, was tested for the production of three-layer particleboards, at different conditions (particleboards target density, pressing time, pressing temperature, wood type and binder age). It was possible to produce particleboards with densities ranging from 682 kg m⁻³ to 783 kg m⁻³, at pressing temperatures from 180 to 210 °C, and pressing times between 8 and 10 min. All the particleboards produced in these conditions were in accordance with the internal bond strength requirements of standard EN 312 for particleboards type P2 (0.35 N mm⁻²). The best result (0.69 ± 0.01) N mm⁻² was obtained for particleboards pressed for 10 minutes at 200 °C with the recycled wood mix. Regarding resin stability, the particleboards manufactured with the binder, stored for 30 days, presented good internal bond strength ((0.58 ± 0.02) N mm⁻²), above the requirements of standard EN 312 for particleboards type P2.     

A new source of natural adhesive: Acacia mangium bark extracts co-polymerized with phenol-formaldehyde (PF) for bonding Mempisang (Annonaceae spp.) veneers

Acacia mangium is a fast-growing dicotyledonous tree species and has become the dominating plantation in Malaysia. It was grown particularly as a raw material for veneer, pulp, and paper industries. The chemical properties test in this study showed that the A. mangium tree bark contains higher extractive content as compared to the wood portion (sapwood). Tannin extracts from A. mangium tree bark were found to be rich in phenolic compounds and had the potential to replace conventional phenol-formaldehyde (PF) adhesive used in the plywood manufacturing industry. Tannin adhesive (tannin-paraformaldehyde) prepared from A. mangium bark tannin by cross-linking with paraformaldehyde were used for bonding of Mempisang (Annonaceae spp.) plywood board. However, the resulting bonding strength using tannin adhesive was found to be only suitable for interior grade application. Further extension of its application for interior and exterior grade plywood could be achieved with addition of PF (co-polymerization with resol) during the production process. The optimized formulation of tannin adhesive consists of A. mangium solid extracts (90 parts), commercial PF (10 parts), and paraformaldehyde (3%). Results have shown that the plywood shear strength complies with the requirement for European norms EN 314-1 and EN 314-2:1993, which includes the dry test, cold water test, and the boiling test.     

The effect on shear strength of different surfacing techniques in Oriental beech (Fagus orientalis Lipsky) and Scotch pine (Pinus sylvestris L.) bonded joints

The selection of the most appropriate surfacing technique is influential in the success of bonding, painting and varnishing processes. The objective of the study was to determine which surfacing technique was the most appropriate when applied as the final process in the production of structural wood members, which were subjected to shearing. The study also includes the effect on shear strength of the variables, such as type of wood, plane of cut, type of adhesive and pressing pressure, as they are directly related to the main objective of the study. In view of this objective, the changes in shear strength on radial and tangential cut surfaces of Oriental beech (Fagus orientalis Lipsky) and Scotch pine (Pinus sylvestris L.) woods having different roughness values as a result of sawing with a circular ripsaw, planing and sanding, and bonded with polyurethane (PU) and poly(vinyl acetate) (PVAc) adhesives at 3, 6 and 9 MPa pressure, were studied. The 936 specimens prepared with the objective of determining the effects of variables on bond performance were subjected to a shear test in a universal test equipment in accordance with the ASTM D 905-98 standard. The highest shear strength (13.85 N/mm²) was obtained for the Oriental beech specimens cut from their tangential surfaces with a circular ripsaw, which were glued with PVAc adhesive by applying a pressure of 9 MPa. The lowest value (4.22 N/mm²) was obtained in the specimens planed from their tangential surfaces, which were glued with PU adhesive by applying a pressure of 3 MPa. The specimens obtained from Oriental beech wood showed a higher shear strength compared to the specimens obtained from Scotch pine. In general, in both species of wood, the specimens glued with PVAc adhesive, both on the tangential surfaces and on the radial surfaces, produced higher shear strengths compared to the specimens glued with PU adhesive.     

Specific fracture energy at glue joints in European beech wood

The specific fracture energy G at glue joints in hardwood was investigated by using a novel approach of testing and data evaluation. European beech wood was bonded with a specific one-component polyurethane (PUR) adhesive and a specific phenol resorcinol formaldehyde resin (PRF) to produce double cantilever beam (DCB) specimens. These specimens were stored at different relative humidities (50%, 65% and 95%) prior to testing. Additionally, DCB specimens manufactured out of differently aged glulam elements were also evaluated. Special care was given to a constant crack propagation rate during examination, and only the middle part of the specimens was evaluated to avoid influences of the discontinuities at crack initiation and final rupture. The PUR series were tested at three different crack propagation rates to check for a potential influence on G. The PRF specimens failed mostly in the wood, with an average wood failure of 90%. Thus no difference was notable between the PRF and solid wood samples. Their G was around 0.85 N/mm for the regular and dry climates and 1.35 N/mm for the wet climate. All aged series had a 10–15% reduced G, but with lack of significance. The PUR specimens failed in adhesion, independent from climate, ageing or testing speed, and had an average G of 0.24 N/mm. The novel approach, i.e. with constant crack propagation rate and evaluating the data only partially, worked well and can be recommended for further research.     

Microfibrillated-cellulose-modified urea-formaldehyde adhesives with different F/U molar ratios for wood-based composites

The relationship between urea–formaldehyde (UF) adhesives (E0 and E1 classes) and microfibrillated-cellulose (MFC) and its effect on the mechanical properties of laminated veneer lumbers (LVLs) were investigated. Bending strength, modulus of elasticity in bending, and tensile shear strength of the LVLs were determined. The morphology, gel time, viscosity, and acidity of the modified UF adhesives were also investigated. The tensile shear strength of the LVLs significantly increased (2.89 to 3.35 N/mm²) as the MFC suspension was increased to 3.75 g in the 7.50 g E0 class UF adhesive, while it slightly increased (3.10 to 3.16 N/mm²) as the MFC suspension was increased to 2.5 g in the 8.75 g E1 class UF adhesive. The results showed that the bond performance of the E0 class adhesive with the MFC was better than that of the E1 class adhesive with the MFC. The MFC was found to be valuable nanoscale reinforcing filler for the improvement of bond performance of UF adhesive, in particular, E0 class adhesive, in the production of wood-based composites.     

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.     

Bonding performance of melamine-urea-formaldehyde and phenol-resorcinol-formaldehyde adhesives in interior grade glulam

Wood moisture content (MC) affects the glued laminated beam (glulam) bonding strength. Selected adhesives were Phenol-Resorcinol-Formaldehyde (PRF) and Melamine-Urea-Formaldehyde (MUF) adhesives with 1, 1.5 and 2% (w/w) carboxymethyl cellulose (CMC) formic acid solutions. Douglas fir (pseudotsuga menziesii) from North America was used in the test. The bonding behavior of these adhesives with wood at 12 and 18% MC were investigated. The study focused on the effect of 18% MC on shear strength performance of MUF and PRF adhesives and optimizing the formula of CMC formic acid solution. Compressive shear strength of wood with MUF adhesive with 2% (w/w) formic acid solution at 12 and 18% MC stabilized at 10.6 and 10.0 MPa, respectively, which were 17 and 16% higher than that with PRF adhesive at the same condition. At 12–18% MC, MC had a little effect on bonding strength. However, 18% MC wood with PRF adhesive had 52.2% less initial strength increasing rate than that of 12% MC wood. 18% MC wood with MUF adhesives with 1, 1.5 and 2% (w/w) CMC formic acid solutions had 16.0, 15.5 and 27.0% less initial strength increasing rates than that of 12% MC wood, respectively. MUF adhesive using 2% CMC formic solution required the shortest press time at 12 and 18% MC about 1.6 and 2.7 h, respectively. The strength of PRF adhesive was significantly affected by wood MC and enough press time is essential for the proper bonding strength.     

Effect of pressing parameters on the shear strength of beech specimens bonded with low solvent liquefied wood

Liquefied wood (LW) is a naturally based product which has the potential to be used as an adhesive. It can be used as a part of a polymer formulation, as a part of an adhesive mixture with commercial adhesives, or as an independent material for wood bonding. In this study, wood was liquefied at 180?°C using ethylene glycol as the solvent and sulphuric acid as a catalyst. In the first part of research, LW with different pH values was used for the bonding of solid wood at 200?°C for 15?min. In the second part, LW with an optimal pH value was used for bonding at different press temperatures for 15?min. In the third part, the minimum pressing time at the optimal pH value and at the optimal press temperature was determined. Unmodified LW with a negative pH value, a press temperature of 180?°C, and a pressing time of 12?min was determined to be optimal (based on highest shear strength) for the bonding of 5?mm thick wood lamellas with the LW used in this study. At these conditions bonds exhibited shear strength of around 7?N/mm² which was too low to attain standard requirements. Despite this, high wood failure (100%) was observed as a consequence of low pH value and high press temperature which caused damage of the part of beech lamellas where LW was applied.     

Modification of soy proteins and their adhesive properties on woods

Adhesive properties of trypsin-modified soy proteins (TMSP) on woods were investigated. A simple method developed in our laboratory, consisting of measuring the force required to shear the bond between glued wood pieces in the Instron universal testing machine, was used to examine adhesive strength of modified soy proteins on wood. Adhesive strength of TMSP was measured for cold-pressed (ambient temperature for 2 h) and hot-pressed (60, 80, 100, and 120°C for times varying from 0.5 to 2.5 h) woods. Of the woods examined, soft maple gave the highest strength [743 Newtons (N) at a protein glue concentration of 2 mg/cm²]. For soft maple and cold-pressing, TMSP at 2 mg/cm² gave twice the adhesive strength of unmodified protein controls, 743 vs. 340 N. Also, the adhesive strength of TMSP increased from 284 to 743 N as glue concentration was increased from 1 to 2 mg/cm². However, hot-pressing of wood pieces beyond 1 h at 120°C and 30% relative humidity resulted in decreased adhesive strengths of TMSP compared to controls. Further, adhesive strengths of hot-pressed glued wood samples decreased when the relative humidity at which they were kept for curing increased from 30 to 60%. This negative effect of increased humidity on adhesive strengths of glued wood pieces was not observed with cold-pressed TMSP.     

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.     

Effects of Outdoor Exposures and Boron Compounds on Bonding Strength of Desmodur-VTKA Adhesive

This study was carried out to determine the shear strength of an adhesive on weathered wood, impregnated with a boron compound, using a long term dipping method. The shear strength of D-VTKA adhesive on two types of wood species each containing one of two types of the impregnated material, under different weathering conditions, was measured. The results showed that the highest shear strength (11.01 N/mm²) was obtained with the control samples of untreated, oriental beech wood, and the lowest was obtained for Scots pine impregnated with boric acid after two seasons of exposure (six months). Weather conditions affected the shear strength in a negative way. Finally the results showed that if the boron compounds were supported with non-leaching chemicals, they could be recommended as fire-retardant additives and would extend the life of wood bonded with D-VTKA adhesive.     

Cure and performance of castor oil polyurethane adhesive

Aiming at the development of sustainable materials, in this study, a biobased wood polyurethane adhesive (PUA), derived from castor oil (CO), was synthetized and its properties were compared with a conventional wood adhesive. Different NCO/OH ratios have been used to assess its effect on the properties of the ensuing adhesives. FTIR, and DMA were used to monitor the extent of reaction and the glass transition temperature of the adhesive, respectively. In turn, the wood bonding properties of the PUA over time were assessed by lap shear using pine wood specimens. Is was observed that the lap shear strength increases with the increase of the R_NCO/OH up to R_NCO/OH = 2.50. Above this ratio, the adhesive performance decreases slightly, due to the rigidity of the PUA. Comparison with a conventional wood adhesive showed that CO derived adhesives presented similar strength properties but required less time to develop the ultimate bonding strength. The chemical and thermal stability of the most promising CO adhesive was also assessed. Despite of being sensitive to the chemical environment, the castor oil derived adhesives presented higher thermal stability than conventional wood adhesives.Finally, the cure process of CO derived adhesives was studied by differential scanning calorimetry and the Kissinger and Ozawa methods were used to determine the activation energy (Ea). The former afforded a value for Ea = 80.55 and the latter Ea = 87.07 kJ mol⁻¹. Moreover, it was observed that the activation energy is dependent on the degree of cure, increasing slightly up to 0.6 and decreasing significantly afterwards.     

The role of wood extractives in structural hardwood bonding and their influence on different adhesive systems

Bonding of hardwood for structural applications is a complex process. Various factors influence the bond performance and the interface area is considered the most crucial part. The chemical composition of the interface, e.g. wood extractives, is expected to influence the bonding of hardwoods. The subject of this study was to determine the influence of seven model substances that represent common wood extractives on different adhesive systems namely one-component polyurethane, two-component polyurethane, melamine urea formaldehyde and phenol resorcinol formaldehyde. The influence of the model substances on the cross-linking behavior of the adhesives was determined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and on the rheological properties in terms of gel point and storage modulus. In addition, model substances characteristic for selected wood extractives were applied to the surface of European beech wood [Fagus sylvatica L.] before bonding and consequently tested in tensile shear mode according to EN 302-1. The ATR-FTIR spectra showed an influence of some substances on the crosslinking for all adhesive systems. Further, the curing process was mostly accelerated for phenol resorcinol formaldehyde, while melamine urea formaldehyde and polyurethane showed a less negative change in rheological behavior. The mechanical strength of beech wood bonds at room climate indicated only minor influence of model substances, but samples tested in wet conditions demonstrated a significant effect on some adhesive systems. It was concluded, that polyurethane adhesives degrade by acid substances and melamine urea formaldehyde by starch and gallic acid. Phenol resorcinol formaldehyde system was influenced negatively by starch and acids.     

Experimental investigation and prediction of bonding strength of Oriental beech (Fagus orientalis Lipsky) bonded with polyvinyl acetate adhesive

Adhesive bond strength of solid wood plays a key role in the efficient use of wood in a large number of engineering applications. In this study, the effects of amount of adhesive, pressing pressure, and pressing time on bonding strength of beech wood bonded with polyvinyl acetate adhesive were investigated and predicted by developing an artificial neural network (ANN) model. Experimental results have showed that bonding strength of wood samples increased generally by increasing amount of adhesive, pressing pressure, and pressing time. Besides, ANN analysis has yielded highly satisfactory results. The designed neural network model allows predicting the bonding strength of wood samples with mean absolute percentage error of 2.454% and correlation coefficient of 97.8% for testing phase. It is clear from the results that the model has a good learning and generalization ability. This model therefore can be used to predict bonding strength of beech samples bonded with polyvinyl acetate adhesive under given conditions. Consequently, this study provides beneficial insights for practitioners in terms of the safe and efficient use of wood as an engineering material in applications related to the strength of the bond between wood and adhesive.     

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.     

The effects of surface roughness on varnish adhesion strength of wood materials

In this study, the effects of surface roughness of wood material on the adhesion strength of varnish layers have been tested. For this purpose, test samples of beech (Fagus orientalis L.), Scots pine (Pinus sylvestris L.), and cherry (Prunus cerasus) wood species were prepared and sanded with 80, 120, and 180 grit abrasives. The surface roughness values of specimens were determined using a stylus-type profilometer TIME TR 200 according to the ISO 4287 standards. Then, water-based, polyurethane, and acrylic varnish were applied to the surfaces of the samples in accordance with ASTM-D 3023. Layer adhesion strength was determined to the TS EN ISO 4624 standards. As a result, the highest adhesion strength value (2.52 N/mm²) was found in cherry, while the Scots pine had the lowest value (2.32 N/mm²). For the varnish types, polyurethane varnish types gave the highest value (3.15 N/mm²), while the lowest value was obtained from the water-based varnish. Looking at interactions between adhesion strength and surface roughness, the water-based varnish had the strongest correlation with adhesion strength (69%) and acrylic varnish also had a similar result (67%), while polyurethane varnish had the weakest result (33%) in this interaction.     

Effects of boron compounds on the bonding strength of phenol-formaldehyde and melamine-formaldehyde adhesives to impregnated wood materials

Wood materials are increasingly being used in the construction of structural beams, sports equipment, etc. This study was carried out to determine the bonding strength of phenol-formaldehyde (PF) and melamine-formaldehyde (MF) adhesives to impregnated wood materials. For this purpose, brutia pine (Pinus brutia Ten) and elm (Ulmus compestris L.) woods were impregnated with borax (Bx), boric acid (Ba), Bx + Ba (wt:wt 50:50%), di-ammonium phosphate (D), [D + (Bx + Ba)]/(50 + (25 + 25%), w/w) and Tanalith-C 3310 (T-C 3310) using the vacuum method according to ASTM-D 1413-76. The effects of wood species, impregnating material and type of adhesive on the bonding strength were determined. The highest shear strength (11.09 N/mm²) was obtained from elm wood control (i.e., without any impregnating materials) samples with MF; thus, the impregnation process negatively affected the adhesive bonding strength. Impregnating materials, especially those containing oily or similar solutions such as T-C 3310, are not advised for wood elements which are subjected to shear.     

Low Formaldehyde Emitting Biobased Wood Adhesives Manufactured from Mixtures of Tannin and Glyoxylated Lignin

Abstract

Kraft (LN-T-CO₂-2) and wheat straw (CIMV) glyoxalated lignin mixed with mimosa tannin and hexamine as a hardener were used as wood adhesive resins in particleboard fabrication. The adhesive systems proportion used were 40/60 and 50/50 w/w for lignin and tannin, respectively. The gel time test was determined by knowing the polymerization time between the different mixes under the controlled conditions. The results showed a slower polymerization with the kraft lignin/mimosa tannin blending than with the wheat straw lignin/mimosa tannin one. Thermomechanical analyses (TMA) tests were carried out as an indication of the final strength of the adhesive systems revealed by the elasticity modulus (MOE). The MOE results have demonstrated the best mechanical resistance values in 40/60 lignin/mimosa tannin proportion with respectively 3.422 and 3.347 (MPa), for CIMV and LN-T-CO₂-2, and 2.122 (MPa) for 50/50 proportion. Particleboards were prepared and the internal bond (IB) tests were carried out according to the European Standard EN 312. The IB tests confirmed the TMA results. The higher mechanical results of the IB were .43 and 0.53 (MPa), for CIMV and LN-T-CO₂-2 lignin in a 40/60 lignin/mimosa tannin proportion. They were classified as interior panel P2 in according with the standard request EN-312. Free-formaldehyde was determined through the flask method EN 717-3. Particleboards prepared with these natural adhesive resins registered emissions at least 87 and 75% lower than the commercial UF and MUF dhesive resins. The panels were classified as E0.