Influence of organic fillers on rheological behavior in phenol-formaldehyde adhesives

Phenol-formaldehyde resoles (PF) formulated with wheat flour and organic fillers have long been established for the manufacture of veneer-based wood-composites, and yet much remains unknown about these complex fluids. The rheology of PF/filler formulations was studied as a function of filler type and particle size. Corn cob (Zea mays) residue fillers behaved differently from those made from alder bark (Alnus rubra) and walnut shell (Juglans regia). It was shown that viscoelastic network structures formed within the liquid formulations as a function of shear history, filler type, and filler particle size. The precise nature and origin of these effects is unknown but could involve disintegration of filler particle aggregates on a non-colloidal scale, and/or colloidal effects within the liquid PF medium. In the latter case colloidal structures could form among associated PF chains and also from proteins, polysaccharides, and lignins that leach from wheat flour and filler particles. Relative to alder bark and walnut shell, the unique behavior of corn cob residue was discussed with respect to chemical composition. Many implications for impact on industrial practice are feasible and should be the subject of future research.     

Rheology Study of Sugar Cane Bagasse Lignin-Added Phenol–Formaldehyde Adhesives

The rheological properties of sugar cane bagasse lignin–phenol formaldehyde (PF) (30% lignin – PF) resins were studied using oscillation tests. The bagasse lignin was introduced in the classic adhesive formulation in order to supply a part of PF. Rheological qualities of optimal lignin–PF (30% lignin – PF) resins and commercial PF resin were assessed by using a rotary rheometer (ARES). Dynamic rheological measurements, performed at low strain in the linear range, are useful to characterize the network properties of resins.

The results obtained showed that the time sweep indicates excellent structural stability of optimal lignin–PF (30% lignin–PF) resins and commercial PF resin. The elastic modulus is greater than the viscous one showing a remarkable elastic character of the resins, and the performed frequency sweeps show a typical spectrum of a “weak gels” structure. The time dependence at 125°C shows that the optimum cure time is 7.5 min.     

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.     

A New Methodology to Evaluate the Cure of Resin-Impregnated Paper for HPL

This paper presents a study on the curing conditions of several resin-impregnated papers and its impact on the performance of HPL (high-pressure decorative laminate). A new methodology for evaluating the bond strength development between the different layers of a HPL(overlay, decorative, and kraft papers) was developed using ABES (Automated Bonding Evaluation System) equipment. The proposed method can be applied to the study of the curing step of the different impregnated paper and the development of bonds between them (overlay paper on decorative paper, decorative paper on kraft paper, and kraft paper on kraft paper) trying to simulate the hot-pressing of an industrial HPL. This will permit to establish a more adapted temperature gradient in hot-press in order to achieve the same curing rate for all layers and provide a good final overall product quality.     

聚合物/石墨烯复合材料流变学的研究进展

流动与变形是材料在生产及应用过程的普遍现象,而聚合物复合材料往往具有较为复杂的流动与变形行为。基于石墨烯的聚合物复合材料是近年来材料领域的研究热点之一,以流动与变形为研究对象的流变学对于深入探究这类复合体系的结构与性能具有重要意义。综述了针对聚合物/石墨烯复合材料的流变学的研究进展,并展望了聚合物/石墨烯复合材料流变学研究的发展方向。     

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.     

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.     

A novel fiber–veneer-laminated composite based on tannin resin

Natural nonwoven fiber was impregnated with a tannin resin and laminated with wood veneer for preparation of laminated composites. The tannin resin used showed a good compatibility with the natural fiber, and was easy to assemble with the wood veneers. The tannin resin penetration into the wood veneer was observed by light microscopy. The laminated composite shows very good mechanical properties and water resistance. Shear force–displacement testing demonstrates that the laminated composite had a ductile behavior under wet testing conditions. The laminated composite was prepared using 100% natural biorenewable raw materials and had good properties compared to conventional plywood bonded with synthetic resin.     

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.     

Improvement of storage stability of UF resins by adding caprolactam

During storage, the structure of urea-formaldehyde (UF) resins suffers modifications due to reactions between monomers, oligomers, polymer and free formaldehyde, leading to increase in viscosity and decrease in pH. Eventually, viscosity reaches a value that renders the resin unusable, and it must be disposed off. This aging process is accelerated if storage temperature increases.The aim of this work is to obtain UF resins with long storage stability, even when exposed to relatively high temperatures, such as 40 °C. The main strategy adopted was the addition of a chain growth blocker, caprolactam. This monofunctional compound reacts with end groups, blocking them and therefore reducing the polymer's reactivity. In addition, a weak base was added to adjust the pH value, instead of the traditional strong base, sodium hydroxide, therefore hindering the Cannizzaro reaction.The storage stability of UF resins with formaldehyde to urea molar ratio of 1.6–2.0 was monitored by pH and viscosity measurements. Caprolactam was added in different amounts and at different reaction stages. It was found that 10% addition at the beginning of condensation led to the best results, giving a much higher storage stability at 40 °C (2 months when compared to 4 days for a commercial UF resin with low F/U molar ratio). As expected, the resin reactivity decreased with caprolactam addition, demanding for longer pressing times for wood-based panel manufacture. These verified the internal bond strength specification for EN 312 - P2 standard class. Formaldehyde content in the panels was above the E1 class limit when fresh or one month old modified resins were used, implying addition of formaldehyde scavengers. The resin stored for 2 months allowed producing panels within E1 limit. These preliminary results demonstrate the concept that addition of an end-group blocker during UF synthesis is an effective strategy for improving storage stability, encouraging future work on alternative compounds and synthesis conditions optimization.     

Influence of Resin Molecular Weight on Curing and Thermal Degradation of Plywood Made From Phenolic Prepreg Palm Veneers

The present work evaluates curing and the thermal behavior of different molecular weight phenol formaldehyde (PF) resins used to prepare PF prepreg oil palm stem veneers. The physical properties (solid contents, gelation time, pH, and viscosity) of PF resins were determined. The molecular weight of resins was characterized by gel permeation chromatography, whilst thermal properties were determined by differential scanning calorimetry and thermogravimetric analyses. The average molecular weight of PF resins were 526 g/mole (low), 1889 g/mole (medium), and 5178 g/mole (control - commercial). Among the resins, medium (MMwPF) gives better thermal stability with a retained weight of 48.9% at 300°C. High (Commercial PF) had a low decomposition temperature (109.3°C) which occurred within 11 min. Both low (LMwPF) and MMwPF started to melt at ≥120°C. Based on strength and shear values, phenolic prepreg palm veneers can be prepared using either low or medium molecular weight PF but with varying results. In all cases, the mechanical properties of palm plywood made from PF prepreg veneers were superior to those made from PF-bonded plywood using the commercial process.     

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.     

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.     

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.     

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.     

Wood adhesives derived from hyperbranched polyglycerol cross-linked with hexamethoxymethyl melamines

Wood adhesive system based on hyperbranched polyglycerol cross-linked with hexamethoxymethyl melamines was described. The obtained results showed that it was possible to develop polyglycerol-based interior-grade adhesive exhibiting dry shear strengths of the joints exceeding that of the solid wood. Gelling times of the studied formulations were comparable to those of the industrial thermosetting systems (60–80 s at 100 °C). It was also found that wet shear strengths depended on functionality and F/Me molar ratio in the cross-linking resin.     

Eco-economical polyurethane wood adhesives from cellulosic waste: Synthesis, characterization and adhesion study

This study reports the preparation of polyurethane adhesives using polyols obtained from cellulosic waste and detailed study on its adhesive strength in wood joints. Keeping in view the environmental hazards related to the huge paper-waste generation across the world, low-viscosity polyols have been prepared using magazine paper waste and vegetable oils with different physicochemical properties and were used to prepare two-component polyurethane adhesives for wood bonding. Polyurethane was analyzed by FTIR spectroscopy and TGA was used for the analysis of thermal properties. The adhesive strength was measured and compared with commercially available adhesives under different environmental conditions. The synthesized adhesive with NCO/OH ratio of 1.2 and curing time of 5 days was found to be superior to the commercial adhesives Fevicol™ and Araldite™ when compared simultaneously for the single-lap shear strength in different environmental 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.     

Insights into the development of crystallinity in liquid urea-formaldehyde resins

Differently from most thermoset materials, urea-formaldehyde (UF) resins display the appearance of crystalline domains. In the effort of understanding the mechanism of formation of such crystals, wide angle X-ray diffraction (WAXD), infrared spectroscopy and transmission electron microscopy (TEM) were applied. Liquid UF resins with two different F/U mole ratios (i.e. 1.6 and 1.0) were investigated as a function of hardener level and curing times at room temperature. The WAXD results showed that the liquid UF resin with a low F/U mole ratio had a greater crystallinity than the one with a high F/U mole ratio. An advance in crystal formation in the low F/U mole UF resins was visible, especially in the first phases of curing. However, there were no significant differences in the degree of crystallinity as a function of hardener level. IR spectroscopy highlighted the important role of methylolated species in the formation of crystals. TEM results also confirmed the presence of crystals in all the considered liquid UF resins. The concentration of the hardener and the curing time were critical in shaping morphology and particle dispersion. As a function of the curing conditions, the globular structures present in the samples can aggregate into different morphologies, which can be fibrillar and also lamellar. The obtained results stress the importance of controlling the subtle interplay between crosslinking and formulation for the obtainment and control of the size, quantity and morphology of crystals in UF resins, and therefore for an effective tuning of their properties.     

Ionic liquid modified lignin-phenol-glyoxal resin: a green alternative resin for production of particleboards

The aim of this research was to evaluate the properties of particleboard panels bonded with ionic liquid treated lignin- phenol- glyoxal (LPG) resin. For this purpose, soda bagasse lignin was modified by 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) ionic liquid and then various contents of virgin and modified lignin (20, 30 and 40 wt% based on weight of phenol), phenol and glyoxal were used for synthesis of LPG resins. After resin synthesis, thermal and physicochemical properties of the synthesized resins such as curing behavior, gelation time, viscosity, solid content and density were measured. Finally, the resins so prepared were used for laboratory particleboard manufacturing. The panels physical (water absorption, thickness swelling) as well as mechanical (MOE, MOR and internal bond strength) properties were measured according to standard methods. The resins tests indicated that modification of lignin with ionic liquid not only can accelerate the gelation time and increase viscosity, density and solid content of LPG resins but also decrease the temperature required for curing the LPG resins. Based on the results of this work, the mechanical strength and dimensional stability of the particleboards bonded with a LPG resin can be improved by using modified lignin. The particleboards prepared with the LPG resin, using either modified or virgin lignin, presented higher water absorption as well as weaker mechanical strength than those prepared with the control PF resin. However, there does not appear to be any statistically significant difference between the some properties of the panels bonded with the control PF resin and those bonded with the LPG resin containing modified lignin.