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.     

Evaluation of urea-formaldehyde adhesives performance by recently developed mechanical tests

In the open literature, two main strategies can be found for synthesizing urea-formaldehyde (UF) resins. One is the alkaline-acid process, which takes place in three steps, usually an alkaline methylolation followed by an acid condensation and then the addition of a final amount of urea. The other process consists of four steps, the main difference being an initial condensation in strongly acid environment.In this work, we evaluate the curing behaviour of four resins produced using the aforementioned processes by the Integrated Pressing and Testing System (IPATES) and the Automated Bonding Evaluation System (ABES).The characterisation of the bond strength development during hot pressing by ABES and IPATES shows that the four resins will have different performances in the bonding process of wood-based composites. For each resin, the effect of pressing parameters such as temperature, adhesive and hardener ratios on shear strength (ABES) and internal bond (IPATES) during hot pressing is put into evidence.     

The preparation and application of a new formaldehyde-free adhesive for plywood

In this study, we developed a new formaldehyde-free adhesive prepared by in-situ chlorinating graft copolymerization for plywood. The main ingredients of this adhesive include maleic anhydride (MAH) and high density polyethylene (HDPE) that is MAH grafted onto HDPE (PE-cg-MAH). The reaction between this adhesive and veneer, the optimum formulation to bond veneer and the optimum hot-press conditions to prepare the plywood were investigated. A boiling water test was employed to evaluate the strength and water resistance of plywood bonded with this adhesive. The results showed that the properties of the resulting plywood using PE-cg-MAH as an adhesive can meet the standard of Type I plywood and the optimum hot-press conditions were 160-165 °C and 5 min. When the chlorine contents of PE-cg-MAH was about 3% (wt%), the plywood panels had a higher shear strength after boiling water test above the hot-press conditions.     

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.     

Adhesive distribution related to mechanical performance of high density wood fibre board

In a full-scale mill experiment two groups of high density wood fibre boards were produced. While one group was bonded with a standard melamine reinforced urea-formaldehyde adhesive, a second group was bonded with a modified adhesive yielding systematically improved board properties at identical amounts of adhesive applied. By means of a novel fluorescence-microscopic method using the dye Acriflavine to colour the cured adhesive after board production, adhesive distribution within the industrial produced boards was evaluated and quantified. Very clear differences in the size distribution of the two adhesives were found, leading to the conclusion that a relationship exists between adhesive distribution and mechanical board performance.     

Influence of glyoxal on curing of urea-formaldehyde resins

In the present paper, the effect of glyoxal on the gel formation within the adhesive systems based on urea-formaldehyde (UF) resins is shown. A reduction of formaldehyde content in wood-based panels by decreasing the formaldehyde/urea molar ratio in the UF resins leads to increasing of the UF resin gel time, and impairing the qualitative characteristics of the UF-based wood materials. Glyoxal is shown to speed up the crosslinking of the macromolecules as well as significant reduction of gel time of adhesive composition. The first reason is the result of reaction between glyoxal and ammonium ion leading to protons releasing. Another reason is that glyoxal and its interaction products react with macromolecules of the UF resin forming a three-dimension cross-linked structure. The gel time and the pot life of the UF resin are measured by the oscillatory viscometer. Formation of the UF cross-linked resin structure with glyoxal and a curing catalyst (ammonium sulfate) is studied using dispersion Raman scattering spectroscopy. Particleboards (PB) are produced using different amount of glyoxal and formaldehyde/urea molar ratio in the UF resin. The properties are evaluated according to the European Standards and include density, internal bond, thickness swelling moisture content and formaldehyde content.     

Improving performance of polyvinyl acetate (PVA) as a binder for wood by combination with melamine based adhesives

In this study blending PVA with MUF and MF was evaluated as an approach to enhance the performance of PVA towards water and elevated temperatures. MF and MUF were added to PVA at different proportions: 15%, 30%, 50%, 70% and 100%. Blends of PVA with MF and MUF were used as adhesives to bond wood joints. The shear strength of wood joints was measured at dry and wet states, and elevated temperatures. Thermogravimetric analysis was used to study thermal stability of PVA and its blends with MF and MUF. The structural changes caused by the inclusions were characterized by Fourier transforms infrared spectroscopy (FT-IR). The results showed that shear strength of wood joints were improved by the addition of MF and MUF to PVA in all conditions. Adding small amounts of MUF or MF (as low as 15%) enhanced the performance of wood joints towards water and elevated temperatures. The extent of improvement was sometimes so high that the strength of glue line surpassed strength of wood in wet conditions leading to wood failure rather than glue failure. MF had more effectiveness in improving shear strength of wood joints than MUF in all conditions. Thermal stability of PVA was increased by MF but the effect of MUF on thermal stability of PVA was dependent on MUF proportions and temperatures. FT-IR analyses showed that there are some chemical bonds between PVA and MF. Considering costs, effectiveness and formaldehyde emission, adding 15% MF to PVA seems the optimal proportion of MF in the PVA blends.     

The chemical,kinetic and mechanical characterization of tannin-based adhesives with different crosslinking systems

Network formation, cure characteristics and bonding performance of tannin-based resins were investigated in order to establish structure–property relationships between the stage B and stage C. Tannin–aldehyde and base-catalyzed autocondensed tannin resins were synthesized and characterized for molecular weight distribution, cure kinetics and cure chemistry by means of GPC, DMA and ¹³C CP/MAS NMR spectroscopy and solvent stability tests. The resins performance as wood adhesives was further established from lap-shear tests and microscopic observation of the bondline. Resins prepared with highly reactive aldehydes, such as formaldehyde or glyoxal, exhibited a significant extent of hetero-condensation reactions, fast cure kinetics, a high storage modulus and good solvent stability of the stage C-resin. In contrast, resins prepared with bulky aldehydes of low reactivity, such as citral, were dominated by autocondensation reactions, and exhibited slower cure kinetics, a lower storage modulus and solvent-stability of the stage C-resin, alike those neat autocondensed tannin resins. However, all resin systems fulfilled the standard requirements for wood adhesive bonding for interior applications. Additionally, storage modulus increase during cure was found to be a good predictor of the stiffness of the wood-bonded assembly, useful to discriminate between the autocondensation and heterocondensation cure chemistries.     

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.     

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.     

Chemical composition of melamine-urea-formaldehyde (MUF) resins assessed by near-infrared (NIR) spectroscopy

Melamine-urea-formaldehyde (MUF) resins are commonly used in the production of wood-based panels. The composition of the resin influences many properties of the final product. In industrial production, some properties, such as viscosity, pH, solid content, or molar ratio, are assessed after resin production in order to evaluate if they are within the desired parameters. These properties are useful for quality control of amino resins. However, almost no information is obtained if a certain type of reagent or filler is wrongly added to the formulation, even though the resin's final adhesive performance will be affected. Evaluation of the molar ratio of the reagents might be the only of the few industrially used tests capable of making this assessment. Near-infrared spectroscopy (NIR) is a fast and reliable technique for quality control of amino resins and can give a wide range of information regarding chemical composition of these products. This work intends to test the capability of NIR to assess several properties related to MUF resins’ chemical composition. The approach considered two types of problems: 1) whether there was a flaw on resin manufacture process and 2) which raw-material (amount or kind) was incorrectly added to the reactor. Using NIR spectra of a wide range of MUF resins, several models were established to predict the molar ratio of formaldehyde and urea (F/U), molar ratio of formaldehyde and melamine (F/M), molar ratio of formaldehyde and amino groups (F/(NH₂)₂), total urea (% U) and total melamine (% M). These models were constructed using the multivariate technique of Partial Least Squares (PLS) and could successfully determine the properties of a set of industrial resins. The coefficients of variation (CV) obtained were equal or lower than 5%, except for the property of F/M, which was 17%. A more thorough analysis of the established models reveals that spectral components of melamine are harder to extract by PLS than components of formaldehyde or urea.     

Characterization of the ageing process of one-component polyurethane moisture curing wood adhesive

The aging resistance of five different one-component polyurethane (1C-PUR) adhesives with different mechanical properties has been investigated. The glue layer has been modeled as a superposition of three different layers representing the pure adhesive layer, the interaction layer between the wood and the adhesive and the plain wood layer. The modification of the mechanical properties of each layer was studied with a specific specimen for each layer. Also, a comparative analysis between natural and artificial weathering was conducted. The artificial weathering consists of a cyclic hydro-thermic treatment (treatment A5 according to DIN EN 302-1). The influence of a long time (5 months) mid-range temperature (70 °C) treatment was tested. Additionally, Fourier transform infrared spectroscopy (FTIR) analyses were conducted to determine the hypothetical chemical modification of the bond line. Generally, the strength properties of the samples decreased with the duration of the artificial weathering. However, significant differences were observed between the adhesive of the same system, especially during the temperature treatment. Furthermore, no relevant chemical degradation of the bond line was measured after 5 years of natural weathering (tested from 10 mm thick glued samples). The chosen method found that in the majority of cases, the wood is the weak layer in the bond line. Therefore, for moisture and temperature solicitation, there are no reasons to doubt the durability of the 1C-PUR glue layer for long term use. Nevertheless, further investigations are needed to better characterize the life expectancy of the wood and adhesive interface layer and to establish the influence of parameters, such as UV-radiation, wood extractives and moisture-induced stress, on the life expectancy of the bond line.     

The impact of defects on the capacity of timber joints with glued-in rods

Glued-in rods are an increasingly used technical solution for numerous structural applications in timber engineering, and demonstrate the potential of adhesively bonded connections. During the insertion process the adhesive fills a very narrow gap over significant anchorage contact area, raising concerns that manufacturing defects may impact the structural performance of the bonded joint, namely the possible lack of adhesion resulting from inadequate preparation of the joint on site. Previous studies on the effect of bonding defects on the capacity of bonded joints identified a nuanced relationship that depends on the ductility of the adhesive.This paper presents experimental evidence that sheds light on the relationship between defects and capacity of glued timber joints. Joints composed of softwood glulam members and mild steel glued-in threaded rods were manufactured with two types of defects likely to be encountered on-site: i) rods placed at an angle inside drill hole instead of aligned with the joint axis, and ii) rod placed against the side of the drill hole instead of fully centered. To establish performance benchmarks a first phase studied the influence of the anchorage length and the rod diameter using three different adhesives. The effect of these defects on joint capacity was investigated with three different adhesives in combination with three different rod anchorage lengths. The investigations demonstrated that joints with sufficient rod anchorage (herein 10 times the rod diameter) do not exhibit a statistically significant loss of capacity, if compared to defect free joints. These results can contribute towards better understanding of the influence that the studied parameters have on the performance on timber joints with glued-in rods, as well as to translate this information to promote the development of further applications.     

Creep performance of glued-in rod joints in controlled and variable climate conditions

Presented in this study are the results of creep tests on joints with single glued-in rods performed in controlled and variable climate conditions. The joints were prepared using steel threaded rods of 8 mm in diameter, wood of two species: Black spruce (Picea mariana Mill) and Norway spruce (Picea Abies L.), and two adhesives: polyurethane (PUR) and epoxy (EXP). The first test campaign was performed under two constant climate conditions: 20 °C / 65% relative humidity (RH) and at 50 °C / 72% RH. The applied load corresponded to a stress level of 50% of the static breaking load. The moisture content of wood remained constant during the experiments. Results revealed that the joints were subject to creep with major differences between the adhesives. Low data variability was observed for specimens with the EPX and strong correlations were found between the initial stiffness and the stiffness at 10 days and 25 days. The EPX joints developed lower creep than the PUR joints at 20 °C. At the temperature of 50 °C, the creep of the EPX joints increased but there were no failures observed within 60 days, whereas all PUR joints reached failure at the same stress level within days. The second test campaign was launched using the EPX to study the creep of the joints in variable climate conditions at the stress ratios between 60% and 78% of the static strength. The variation in ambient conditions appeared to affect the creep: humidification phase preceded by drying generated significantly higher creep. The humid and dry cycles may govern the load duration and cause the rupture. In addition, this study revealed that the wood drying from 18% to 10% created some damaging cracks at the wood-adhesive interface. The propagation of stable cracks induced an increase of the slippage in the joints and reduction of the residual strength.     

Effect of surface conditions related to machining and air exposure on wettability of different Mediterranean wood species

Wettability of 6 different wood species commonly used in the woodworking industry in the Mediterranean region was evaluated in this study. The species were Norway spruce (Picea abies Karst.), umbrella pine (Pinus pinea L.), oak (Quercus sp.p.), chestnut (Castanea sativa Mill.) beech (Fagus sylvatica L.) and poplar (Populus sp.p.), and their surfaces were machined according to 3 different processes: planing, sanding and disc-sawing. Measurement of dynamic contact angle and extractives (evaluated by means of GC–MS analysis) were carried out on freshly cut and 24 h air exposed surfaces, in order to also evaluate the effect of ageing on wettability. The parameterisation of the contact angle vs. time curves allowed for the systematic statistical elaboration of data, in order to find the relationships existing between the four parameters characterising the dynamic curves and the considered factors (species, machining, ageing). The evaluations evidenced a different influence of these factors on the chosen parameters and hence some of them could be used to reliably assess both wood wettability and the effects of the factors here considered. In general softwoods showed higher contact angles than hardwoods due to the different anatomy and to the presence of resins and terpenes in addition to fatty acids and phenolic compounds, also present in hardwoods. After 24 h air exposure a shifting upwards of dynamic contact angle curves was observed but, despite the variation in surface composition, this shifting was imputable to other inactivation factors. Also machining appreciably influenced wettability, and the sanded surfaces were the most wettable as compared to both the planed and the disc-sawn ones. On the other hand, these observed differences diminished after ageing due to the levelling effect of inactivation that overcame surface inhomogeneities.     

Use of organosolv lignin in phenol-formaldehyde resins for particleboard production: II. Particleboard production and properties

The objective of this work was to demonstrate the utility of lignin-based resins designed for application as an adhesive in the production of particleboard. Bond qualities of lignin-phenol-formaldehyde resins, phenolated-lignin-formaldehyde resins and commercial phenol-formaldehyde (PF-com) resin were assessed by using an automatic bonding evaluation system, prior to production of particleboards. In order to evaluate the quality of lignin-based resins, particleboards were produced and physical and mechanical properties were investigated. These physical properties included internal bond, modules of rupture and modulus of elasticity. Thickness swell and water absorption properties of particleboards bonded with lignin-based resins were also determined. The lignin-based resins have been reported previously in Part I of this study. The results showed that particleboards bonded with phenolated-lignin formaldehyde resins (up to 30% lignin content) exhibited similar physical and mechanical properties when compared to particleboards bonded with PF-com. The work has indicated that phenolated-lignin formaldehyde resins (up to 30% substitution level) can be used successfully as a wood adhesive for constructing particleboard. The performance of these panels is comparable to those of boards made using PF-com resin.     

Preparation,characterization and properties of an organic siloxane-modified cassava starch-based wood adhesive

A new method of preparing a cassava starch-based wood adhesive with high performance using hydrogen peroxide, acrylamide, butyl acrylate (BA), and an organic siloxane as an oxidant, a hard co-monomer, a soft co-monomer, and crosslinking agent, respectively, is proposed. The effects of various parameters on the shear strength, the water resistance, and the viscosity of the adhesive were investigated. The results showed that the shear adhesive strength in dry state and wet state of cassava starch-based adhesive could reach 6.11 MPa and 3.05 MPa, respectively. The organic siloxane, when added, could reduce the content of the hydroxyl on the starch molecule, and promote the crosslinking of the starch molecules, improving the bonding strength and water resistance.     

Bio-adhesives from soy protein concentrate and montmorillonite: Rheological and thermal behaviour

The incorporation of different amounts of montmorillonite (MMT) to soy protein concentrate (SPC) was used to improve the performance of the bio-nano-adhesive obtained. X-Ray diffraction, rheology, thermogravimetric analysis and scanning electronic microscopy were carried out to characterize the adhesives, and dry and wet strength was used to determine the adhesion strength. In the rheological measurement, the incorporation of up to 3 wt% of MMT did not modify the consistency index values of the SPC, while an increase in the flow consistency index for higher concentrations can be observed due to a strong interaction between MMT and the protein. Besides, the flow point values increase four times with respect to the value obtained for SPC alone. The decomposition temperature of SPC increases with the addition of MMT, which provides a tortuous pathway that obstructs the diffusion of volatile products out of the bio-nano-adhesive. Further addition beyond 5 wt% led to the formation of agglomerates, as verified by SEM. Moreover, the roughness of the fractured surface of the matrix can explain the decrease of the net adhesion of the nano-particles to the SPC suspensions.     

Bond integrity of cross laminated timber from Acacia mangium wood as affected by adhesive types,pressing pressures and loading direction

Cross laminated timber (CLT) was fabricated from Acacia mangium wood by using phenol resorcinol formaldehyde (PRF) and one component polyurethane (PUR) as binders. The purpose of the study was to evaluate the bond integrity of A. mangium CLT produced using different working parameters. The assemblies were pressed at 30 °C for 30 min using three pressing pressures (0.9 N/mm², 1.2 N/mm², and 1.5 N/mm²). Delamination and block shear tests were conducted on the CLT according to European Standards, EN 391 and EN 392, respectively. The results revealed that PRF-bonded CLT experienced lower percent delamination compared to that bonded with one component PUR. It appears that a higher clamping pressure i.e. 1.5 N/mm², is needed to sufficiently bond A. mangium lumbers as indicated by a marked increase in bond shear strength with an increase of pressing pressure. PRF was found to be a more superior adhesive than PUR irrespective of cramping pressure and loading direction. A. mangium wood is relatively dense thus requires quite high pressure, 1.5 N/mm², irrespective of adhesive used. PRF appears to bond A. mangium wood better compared to PUR with shear bond strength of 21% and wood failure percentage of 220% higher.     

Improvement of the water resistance of soybean protein-based wood adhesive by a thermo-chemical treatment approach

In order to extend the applications of wood composites and products bonded by soybean protein adhesive from interior to exterior fields of application, this study proposes a novel approach for improving the water resistance of soybean protein-based wood adhesives using thermo-chemical treatment of soybean protein. The soybean protein formed stable three-dimensional networks due to repolymerization or self-crosslinking during thermo-chemical treatment, confirmed by both increases in the water-insoluble content of the treated soybean protein and the improved hydrothermal-aged wet bond strength of the resulting soybean protein adhesive. Thermo-chemical treatment in the presence of 1 wt% sodium sulfite (which cleaves disulfide bonds) and 1 wt% sodium dodecyl sulfate (which destroys the hydrophobic interactions of proteins) released active groups buried within the globular structure of soybean protein via unfolding. This release both promoted the repolymerization of the soybean protein molecules and exposed more active sites for effective crosslinking by the post-added crosslinker EMPA. Plywood bonded by the optimal soybean protein adhesive possessed a good hydrothermal-aged bond strength of 1.22 MPa, exceeding the value required for structural use according to the JIS K6806-2003 commercial standard.