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 comparison between lignin modified by ionic liquids and glyoxalated lignin as modifiers of urea-formaldehyde resin

The aim of this research was to compare the influence of modified lignin by ionic liquid (IL) on the physical and mechanical properties of wood-based panels bonded with urea-formaldehyde (UF) resin with the effect of glyoxalated lignin (GL) on UF properties. For this purpose, soda bagasse lignin was respectively modified by 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) IL and glyoxal and then the various content of modified lignins (10, 15, and 20%) were added at pH=7 during the UF resin synthesis instead of the second urea . The changes in the structure and thermal properties of lignin, after and before modification with glyoxal and IL, were analyzed by Fourier transform infrared spectrometry (FTIR) and differential scanning calorimetry (DSC). The physicochemical properties of the prepared resins as well as the water absorption, shear strength, and formaldehyde emission of the plywood panels made with these adhesives were measured according to standard methods. According to the FTIR spectra, the content of C=O bond increased in GL while in the IL-treated lignin the content of C–N bond markedly increased. DSC analysis indicated that lignin modified by IL had lower glass transition temperature (T_g) value compared to those modified with glyoxal and unmodified lignin, respectively. The UF resins containing IL-treated lignin exhibit a faster gel time compared to those prepared with GL. Equally, the plywood panels prepared with an IL had lower formaldehyde emission and higher mechanical strength compared to those made from UF resin containing GL. There were no significant differences in dimensional stability of the panels bonded with UFs modified with GL and those with IL-modified lignin.     

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.     

Influence of nanoclay on urea–glyoxalated lignin–formaldehyde resins for wood adhesive

In this research, the influence of nanoclay on urea–glyoxalated lignin–formaldehyde (GLUF) resin properties has been investigated. To prepare the GLUF resin, glyoxalated soda baggase lignin (15 wt%) was added as an alternative for the second urea during the UF resin synthesis. The prepared GLUF resin was mixed with the 0.5%, 1%, and 1.5% nanoclay by mechanically stirring for 5 min at room temperature. The physicochemical properties of the prepared resins were measured according to standard methods. Then the resins were used in particleboard production and the physical and mechanical properties of the manufactured panels were determined. Finally, from the results obtained, the best prepared resin was selected and its properties were analyzed by differential scanning calorimetry (DSC), Fourier transform infrared spectrometry (FTIR), and X-ray diffractometry (XRD). Generally the results indicated that the addition of sodium-montmorillonite (NaMMT) up to 1.5% appears to improve the performance of GLUF resins in particleboards. The results also showed that nanoclays improved mechanical strength (modulus of elasticity (MOE), Modulus of Rupture (MOR), and internal bond (IB) strength) of the panels bonded with GLUF resins. The panels containing GLUF resin and nanoclay yielded lower formaldehyde emission as well as water absorption content than those made from the neat GLUF resins. XRD characterization indicated that NaMMT only intercalated when mixed with GLUF resin. Based on DSC results, the addition of NaMMT could accelerate the curing of GLUF resins. The enthalpy of the cure reaction (ΔH) of GLUF resin containing NaMMT was increased compared with neat GLUF resin. Also the results of FTIR analysis indicated that addition of NaMMT change the GLUF resins structures.     

Properties of liquefied wood modified melamine-formaldehyde (MF) resin adhesive and its application for bonding particleboards

In this study, we modified melamine-formaldehyde (MF) resin adhesive with liquefied wood (LW) and determined the properties of MF–LW adhesive mixtures. Furthermore, we produced particleboards using prepared MF–LW mixtures and evaluated their mechanical and physical properties. Results showed that with increasing content of LW in the adhesive mixture gel time and peak temperature increased while reaction enthalpy decreased. With increasing substitution of MF resin adhesive with LW the thermal stability of adhesive mixture reduced, namely thermal degradation started at lower temperature and weight loss increased. Properties of particleboards improved with increasing amount of LW in the adhesive mixture up to 20% and then deteriorated. Nevertheless, the properties of particleboard with 30% LW in the adhesive mixture were comparable to the properties of particleboard without LW while they worsen at greater portion of LW. Consequently, MF resin adhesive with 30% LW substitution could be used to produce particleboards with suitable mechanical properties and reduced formaldehyde release content.     

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.     

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.     

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.     

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.     

Ionic liquid modified graphene oxide for enhanced flame retardancy and mechanical properties of epoxy resin

In this work, to improve its dispersion and flame retardancy, graphene oxide (GO) was functionalized by silane coupling agent KH550 and 1-butyl-3-methylimidazole hexafluorophosphate (PF₆-ILs), and characteristics of the PF₆-ILs@GO was obtained by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Then, the synergistic flame retardant of GO or PF₆-ILs@GO and melamine pyrophosphate (MPP) were applied for epoxy resin (EP) materials. Specifically, the limiting oxygen index (LOI) value of EP with 0.1 wt% PF₆-ILs@GO was increased to 29.2% from 27.5% of EP/MPP composites, and the UL-94 test reached the V-0 rating. The CCT results showed that the total heat release (THR) and total smoke release (TSP) of EP/MPP/PF₆-ILs@GO composites were significantly 24.4% and 53.4% lower than that of EP/MPP composites. Besides, the thermal behavior investigated by TGA indicated that the char-forming effect of GO and PF₆-ILs@GO was great, the residual char of EP/MPP/PF₆-ILs@GO composites was as high as 19.5% at 700°C, and its thermal stability was higher than that of EP/MPP composites. On the other hand, the tensile strength of EP/MPP/GO and EP/MPP/PF₆-ILs@GO composites were increased by 15.6% and 28.3% compared with EP/MPP composites. According to SEM analysis, the EP/MPP/GO composites formed a good protective char layer, which can effectively improve flame retardancy of EP. This research represents a new method of flame retardant modified GO to improve the flame retardancy and mechanical properties of polymers.     

Technological performance of formaldehyde-free adhesive alternatives for particleboard industry

Due to their high reactivity, chemical versatility and economic competitiveness, formaldehyde-based poly-condensation adhesives are used in huge amounts - in 2010 in the order of 20 million metric tons - around the globe, primarily in the wood-processing industry. Since the 1970s formaldehyde emissions from products made thereof came under pressure and were reduced continuously. The discussion intensified again initiated by the latest European CLP (classification, labelling and packaging) regulation, which came into force in 2016, classifying formaldehyde as a Carcinogen Category 1B compound. In view of potential and even stronger future restrictions of formaldehyde use, appropriate alternatives to substitute formaldehyde-based adhesive systems such as urea formaldehyde would have to be developed and implemented in the wood processing industry. The present review represents a critical appraisal of formaldehyde-free adhesive systems for particulate wood composites production proposed in literature so far. Adhesive systems analyzed here include both synthetic and renewable-based adhesives. The core of the review is an assessment of the individual adhesive systems based on selected technological (product and process) parameters relevant for wood-particleboard production. Based on this data we evaluated their potential to identify suitable alternative adhesives having a certain probability to meet the requirements of a large-scale processing industry sector. As an overall conclusion, there are still many challenges to overcome to replace formaldehyde. Except for pMDI-based systems, most of the alternative adhesives are considerably less reactive, which would result in dramatically higher production costs. Furthermore, the availability of most components proposed to produce alternative adhesives are currently not available in the necessary quantities. Moreover, toxicological investigations on alternative systems are still missing. As several components replacing formaldehyde are also toxic or hazardous to different extents, it cannot be guaranteed that the individual proposed alternative adhesives are safer during processing and service life compared to conventional systems. Due to the nature of the organic material wood, particleboards will always release a certain amount of formaldehyde, even when produced with formaldehyde-free adhesives.     

Improving the properties of ionic liquid-treated lignin-urea-formaldehyde resins by a small addition of isocyanate for wood adhesive

The aim of this research was to investigate the effect of polymeric 4, 4 diphenyl methane diisocyanate (pMDI) on the physical and mechanical properties of plywood panels bonded with an ionic liquid (IL)-treated lignin-urea-formaldehyde resin. Soda lignin modified by 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) IL was added to a urea formaldehyde (UF) resin during resin synthesis to prepare a lignin-urea-formaldehyde (LUF) resin. pMDI at various contents (2, 4, and 6% on resin solids) was then added to prepare a LUF resin. The thermal and physicochemical properties of the resins prepared as well as the water absorption, shear strength, and formaldehyde emission of the plywood panels bonded with them were measured according to standard methods. DSC analysis indicated that the addition of pMDI decreases the gel onset and curing temperatures of the LUF resin. According to the results obtained, the addition of pMDI significantly increased the viscosity and solid content and accelerated the gelation time of LUF resins. Based on the findings of this research, the addition of pMDI dramatically improves the performance of LUF resins as a new adhesive for wood-based panels. The LUF resins with isocyanate added yielded panels presenting lower formaldehyde emission and lower water absorption content when compared to those bonded with the control LUF resins. Greater dry and wet shear strength can be obtained by a small addition of pMDI to LUF resins.     

Use of NIR for structural characterization of urea–formaldehyde resins

In this paper, the effect of pH and temperature on the structure of urea–formaldehyde resins was studied. GPC, NMR and Raman measurements were performed to elucidate the structural characteristics of the resin systems. Fourier Transform Near Infrared (FT-NIR) spectroscopy via optical fibers was used to monitor the reaction progress in situ. It was found that the reactions of urea and formaldehyde at different temperatures and pH values result in resins with different structures and properties: Resins produced at high temperatures and acidic pH values exhibit higher degrees of condensation, presumably because of the development of more cross-linked structures.     

Studies on modified phenolic resin. Iv: Properties of phenolic resin modified with 4-hydroxyphenyl- maleimide/n-butylacrylate copolymers

Three 4-hydroxyphenylmaleimide/ n-butylacrylate (HPMI/n-BuA) copolymers with different monomer ratios were synthesized. Their average molecular weights, glass transition temperatures (T,_g), and thermal decomposition temperatures were measured. It was found that these copolymers had higher average molecular weights and higher thermal decomposition temperatures than novolac. Modified phenolic resins were prepared by transfer moulding from moulding compounds consisting of novolac, the copolymer, hexamethylenetetramine (hexamine), and glass fibre. Properties of the three kinds of modified phenolic resins were examined by flexural test, impact test, dynamic thermomechanometry, and observation of morphology. It was found that phenolic resin modified with HPMI/ n-BuA (1/3-6) copolymer and modified with HPMI/n-BuA (1/7-0) copolymer showed good toughness and good heat resistance. It was also found that the heat resistance of modified phenolic resins was improved by after-cure, but the mechanical properties were decreased by after-cure: similar behaviour was observed for unmodified phenolic resin.     

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.     

纳米粘土改性酚醛树脂结合剂的制备及表征

通过原位插层聚合方法在苯酚与甲醛缩合反应过程中引入层状钠基硅酸盐粘土,得到1种纳米粘土改性酚醛树脂的结合剂。通过X-射线衍射、透射电子显微镜等手段表征了所制备的纳米粘土改性酚醛树脂结合剂中粘土片层的分散状态。采用这种结合剂所制得的无定形捣打料和铝碳砖的耐压强度以及抗弯刚度均得到较大提高。     

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.     

H2O2 as a modifier of phenol–formaldehyde resin used in the production of particleboards

The purpose of the research was to study the influence of H₂O₂ on the properties of fluid phenolic (PF) resin, the curing process, the cured resin structure, and the properties of the particleboards produced with its use. The influence of added H₂O₂ on resin usability at 20°C, on the gel time of the modified PF resin in the temperature range 110–140°C, and on the activation energy of the curing process were studied. Also, the structure of the cured resin was examined by Fourier transform infrared spectroscopy. Finally, the properties of the obtained particleboards were determined. The results indicate that the H₂O₂ modification leads to greater reactivity of the phenolic resin and increases the mechanical properties of particleboards. In contrast, there is no significant influence of H₂O₂ on the water resistance of the particleboards. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3084–3092, 2003     

Grafting of Methyl Methacrylate and Styrene onto Polychloroprene Latex for Compatibilization of Polychloroprene Latex/Styrene-Acrylate Emulsion Blends

Grafting of methyl methacrylate (MMA) and styrene (St) onto polychloroprene latex (CRL) was carried out successfully using emulsion polymerization. The chemical structure of the grafted copolymer was characterized using Fourier transform infrared spectroscopy. The compatibilizing effects of the grafted polymer CRL-g-(MMA-co-St) on the CRL/styrene-acrylate emulsion (SAE) blend were investigated using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and mechanical testing. The results showed that the mechanical properties of the contained CRL-g-(MMA-co-St) blend improved significantly in comparison to those of the simple CRL/SAE blend. For the contained CRL-g-(MMA-co-St) blend, the maximum tensile strength (6.72 MPa) and the maximum elongation at break (1142.6%) were obtained when the content of SAE was 60% (occupied by the total dry weight). At the same content of SAE, the T-peel and lap shear strengths of the contact adhesive derived from the contained CRL-g-(MMA-co-St) blend were 5.3 N/mm (canvas to canvas) and 1.8 MPa (plywood to plywood), respectively. The grafted copolymer CRL-g-(MMA-co-St) showed a remarkable compatibilizing effect on the CRL/SAE blends by drastically improving their mechanical properties.     

动态DSC法研究BMI改性酚醛树脂固化反应动力学

采用非等温DSC(差示扫描量热)法研究BMI(双马来酰亚胺)改性PF(酚醛树脂)体系的固化动力学,借助升温速率-温度(β-T)外推法和红外光谱(FT-IR)跟踪固化反应过程,确定了BAN(BMI改性PF)体系的固化工艺和固化动力学参数。结果表明:BAN的固化工艺为"120℃/2 h→140℃/2 h→160℃/2 h→180℃/2 h",后处理工艺为220℃/3 h,BAN固化体系的动力学参数是表观活化能Ea=123.4 kJ/mol、频率因子A=1.96×1012s-1和反应级数n=1.05;根据n级动力学反应模型求解出该树脂的反应动力学方程,其计算值与试验值基本吻合,说明该模型能较好描述BAN的固化反应过程。