Urea-formaldehyde Adhesives Research Articles

Improving the wet adhesive bonding of bamboo urea‐formaldehyde adhesive using styrene acrylate by controlling monomer ratios

AbstractThe utilization of bamboo resources can significantly alleviate the wood shortage and facilitate CO2 neutralization. However, the adhesive bonding performance of bamboo is not ideal, which greatly limits its application. Therefore, the biggest challenge is to explore a simple and effective method to enhance the bonding between bamboo and adhesive. In this study, we synthesized three butyl acrylate‐styrene (BS) emulsions with different monomer ratios and incorporated them into urea‐formaldehyde (UF) resins to prepare butyl acrylate‐styrene modified urea‐formaldehyde (BUF) adhesives. Results show that BS emulsion not only prolongs the workability time of UF adhesives (95 s–108 s), but also reduces the free formaldehyde content (0.38%–0.26%). Specifically, BS emulsion with the highest hard/soft monomer ratio makes BUF adhesive the lowest activation energy, showing the accelerated curing reaction. Specifically, the prepared laminated bamboo‐veneer lumber (LBVL) exhibits the best wet‐bonding strength and meets the Chinese National Standard, suggesting that BS emulsion with a higher hard/soft monomer ratio can enhance bonding performance. Thus, this efficient method can not only enhance various adhesive properties, but also improves the bonding performance on LVBL, which can greatly ease the shortage of wood resources and speed up the development of the bamboo industry.

A Green Resin Wood Adhesive from Synthetic Polyamide Crosslinking with Glyoxal.

Glyoxal is considered to be the most likely substitute for formaldehyde to synthesize resin adhesives for wood bonding due to its reactivity, structural characteristics, being non-toxic, low volatility, and acceptable cost. Regrettably, the performance of the resin synthesized using glyoxal to directly replace all formaldehyde is not totally satisfactory, especially as it has almost no water resistance. This makes such a simple alternative fail to be suitable for industrial production. To prepare an environment-friendly glyoxal-based adhesive with good bonding performance, the work presented here relies first on reacting citric acid and hexamethylene diamine, producing a polyamide, with glyoxal, and then crosslinking it, thus synthesizing a thermosetting resin (namely CHG) adhesive and applying it for plywood bonding. The plywood prepared exhibits excellent dry and wet shear strength, which are better than GB/T9846-2015 standard requirements (≥0.7 MPa), and even after being soaked in hot water at 63 °C for 3 h, its strength is still as high as 1.35 MPa. The CHG resin is then potentially an adhesive for industrial application for replacing UF (urea-formaldehyde) and MUF (melamine-urea-formaldehyde) adhesives for wood composites.

Heat bonding of wood with starch-lignin mixtures creates new recycling opportunities

Engineered wood products are a meaningful approach to overcome some limitations inherent to wood, such as inhomogeneity and dimensional constraints. Wood adhesives play a key role in this concept; however, their bonding chemistry hitherto impedes facile separation of the components at the end of the service life of engineered wood products. Besides lacking recyclability, environmental safety and sustainability are further issues that could be improved. Herein, we present an inexpensive bio-adhesive system that could pave way towards a novel generation of sustainable engineered wood products. Following a four-stage experimental design to assess the impact of various process parameters, it has been shown that aqueous mixtures of equal quantities of corn starch and sodium lignosulfonate (BIOMIX) feature excellent bond characteristics when subjected to hot pressing (150°C, 0.2 MPa, 60 s), comparable with that of commercial urea-formaldehyde (UF) adhesives. Activation of starch beforehand by gelatinization is not required as granule opening, chain disentanglement and plasticization occur during pressing as confirmed by polarized light microscopy and differential scanning calorimetry (DSC). The presence of sodium lignosulfonate is essential due to its water retarding, dispersing and plasticizing abilities. Preliminary recyclability tests of beech veneer assemblies glued with BIOMIX revealed that facile hydrolysis of the glueline (60 min) and subsequent re-bonding without the addition of new adhesive reaches almost the initial shear strength values as investigated using an automated bonding evaluation system (ABES).

Influence of Critical Synthesis Parameters and Precursor Stabilization on the Development of Adhesive Strength in Fructose–HMF–Amine Adhesives*

Abstract The synthesis of an adhesive composed of fructose, hydroxymethylfurfural (HMF), and bishexamethylene triamine was optimized. The intended application of the adhesive is the production of wood composites such as particleboard or medium-density fiberboard. Adhesives synthesized using HMF-rich precursors produced in continuous and batch systems were compared in regard to their tensile shear strength development; no significant differences could be found. The use of sodium dithionite for HMF stabilization during precursor production led to lower resin viscosities, whereas no significant influence on the adhesive strength development was measured. Variation of the critical synthesis parameters' reaction temperature and reaction atmosphere allowed the production of an adhesive with similar or even faster strength development than commercially used urea–formaldehyde adhesives. The ideal reaction temperature was found to be 60°C. Adhesive synthesis under nitrogen atmosphere shifts the curing reaction to lower temperatures, which was demonstrated by differential scanning calorimetry measurements, but did not improve the overall strength development of the adhesive system.

Study of the Effect of Alfa-x to the Urea Formaldehyde Adhesive on Plywood

This study investigated the effect of adding Alfa-x to the of urea-formaldehyde (UF) adhesive. The content of free formaldehyde and the bonding specifications were examined. The three-layered plywood of poplar (Populus x eureamericana) and ekaba (Tetraberlinia bifoliolata) was prepared by adding alfa-x to adhesive mixtures in specified amounts. The content of free formaldehyde, withdrawal shear strength, and bending strength were studied in the experimental plywood. The strength specifications of plywood bonded with this resin were slightly reduced by adding the Alfa-x to the adhesive mixture. There was also a moderate decrease in the content of free formaldehyde.

Preparation and characterization of urea–formaldehyde adhesives modified with glyoxalated tannin

Preparation and characterization of urea–formaldehyde adhesives modified with glyoxalated tannin

Effects of poly(diallyldimethylammonium chloride) addition on the curing kinetics of urea-formaldehyde adhesives for particleboards

Addition of poly(diallyldimethylammonium chloride) (PDDA) on the performances of urea-formaldehyde (UF) adhesives was evaluated in this work. Three types of UF adhesives were prepared, one without PDDA addition, and two types with PDDA addition of 1 and 3 wt.% per dry UF adhesive mass. These UF adhesive systems were used for producing experimental particleboard panels. The addition of PDDA decreased the thickness swelling of the panel samples, while the internal bond of the particleboards increased significantly only at the highest PDDA content (3 wt.%). Differential scanning calorimetry (DSC) was applied to address the influence of PDDA on UF adhesive curing kinetics. DSC scans were performed in non-isothermal regimes using different heating rates (5, 10, and 20 ?C?min?1). The activation energy (Ea) of the curing reaction showed slightly lower values for the UF adhesive systems containing PDDA. However, the peak temperatures and enthalpy of reaction did not change significantly. The Kissinger-Akahira-Sunose and Friedman iso-conversional methods were applied to investigate the effects of PDDA addition on the UF adhesive curing process.

Curing Kinetics of Tannin and Lignin Biobased Adhesives Determined by DSC and ABES

The curing process of two biobased adhesives: pine tanninhexamine (TH) and organosolv lignin non-isocyanate polyurethane (NIPU), suitable for interior nonstructural use, were compared with commercial urea-formaldehyde (UF) adhesive. Changes in chemical structure before and after the curing process were observed with Fouriertransform infrared spectroscopy (FTIR). The process of adhesive curing was monitored with differential scanning calorimetry (DSC) and the automated bonding evaluation system (ABES). Both DSC and ABES measurements confirmed UF as the fastest and NIPU as the slowest curing adhesive observed. Taking into account the ABES results, the optimal pressing parameters for the TH adhesive would be 4 min at 175°C, for the NIPU adhesive 7 min at 200°C and for the UF 1.5 min at 100°C. Strong linear correlation was observed between mechanical and chemical curing for the UF and NIPU adhesives, whereas lower correlation was observed for the TH adhesive. At all observed adhesives, the DSC measurements were underestimating the curing process determined by ABES in the first part and overestimating it at the end. The underestimation was the most evident with the TH adhesive and the less with the UF adhesive. When comparing the uncured and cured FTIR spectra of all three types of adhesives, a drastic decrease in the characteristic band of -OH groups at 3330–3400 cm−1 and an increase in the signal intensity at 2920 cm−1 of aliphatic -CH2-groups were observed. For the UF adhesive, the C=O stretching frequency has shifted from 1632 cm−1 for uncured to three different bands at 1766, 1701, and 1655 cm−1 for cured UF. The sharp band for phenolic alcohols at 1236 cm−1 of C–O stretch and hydroxyl O–H functional group at 1009 cm−1 and at 684 cm−1 of uncured TH adhesive diminished during curing, which indicates that a crosslinking reaction occurs via -OH groups. The peak of the C=O group of urethane bridges at 1697 cm−1 for uncured NIPU shifted to lower wavenumber at 1633 cm−1 for cured NIPU.

Bending Strength Evaluation of Glulam Beams Made from Selected Nigerian Wood Species

Bending strength, was assessed in glued laminated beams made from local wood species bonded with phenol resorcinol formaldehyde, polyurethane and urea-formaldehyde adhesives. The 3- point loading was used as the basis of bending strength assessment along the directions parallel and perpendicular to the glue line. The load orientation parallel to the glue line offered higher bending strength. Furthermore, the characteristic values of bending strength obtained were 35.16 N/mm2, 40.79 N/mm2, 47.34 N/mm2, 60 N/mm2, and 67 N/mm2 for Afara, Akomu, Gmelina, Iroko, and Omocedar wood species. These values are similar to strength values given in EN 1194-1999 for glulam beams. Consequently, the findings of the study are beneficial to architects and structural engineers in exploring the dimensional, strength, and architectural flexibility glulam affords for both beam and column design and holds the potential for creating an industrial hub for enhancing the value chain around engineered wood and allied industries.

Effect of Copper (II) Sulfate on the Properties of Urea Formaldehyde Adhesive.

The purpose of this work is to investigate the effects of copper (II) sulfate on the formaldehyde release and the mechanical properties of urea formaldehyde (UF) adhesive. Copper (II) sulfate has been used as a formaldehyde scavenger in UF resin, and its effects on the physical and chemical properties of UF adhesive have been studied. Moreover, the mechanical properties and formaldehyde release of plywood prepared with modified UF resin have been determined. The UF resin has been characterized by Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). FTIR spectra showed that the addition of copper (II) sulfate to the UF resin does not affect the IR absorptions of its functional groups, implying that the structure of UF is not modified. Further results showed that the free formaldehyde content of the UF resin incorporating 3% copper (II) sulfate was 0.13 wt.%, around 71% lower than that of the untreated control UF adhesive. With a copper (II) sulfate content of 3%, the formaldehyde release from treated plywood was 0.74 mg·L−1, around 50% lower than that from the control UF adhesive, and the bonding strength reached 1.73 MPa, around 43% higher than that of the control.

New formaldehyde-free adhesives for wood manufacturing: In vitro evaluation of potential toxicity of fine dust collected during wood sawing using a new experimental model to simulate occupational inhalation exposure

Formaldehyde mainly emitted from wood adhesives, finishing materials, paint for furniture represents, together with wood dust, a potential carcinogenic risk for wood workers. Aims of this multidisciplinary study are to investigate the possibility of replacing urea-formaldehyde (UF) adhesives in the wood industry with organic and/or inorganic-based glues to obtain a final less toxic product and to evaluate the potential toxicity of wood glued with such new adhesives. For this purpose we selected poplar wood to test an organic new adhesive HBP (Hemp Based Protein), a mixture of hemp flour and cross-linker PAE (polyaminoamide epichlorohydrin), and spruce wood to test an inorganic adhesive geopolymer K-PSS (potassium-polysiloxosialate) plus polyvinyl acetate. For the poplar wood, we also used a commercial panel glued with UF for comparison. We reproduced occupational inhalation exposure during sawing activities of mentioned woods, collected and characterized the wood dusts emitted during sawing and evaluated in vitro their potential cyto-genotoxic and inflammatory effects. We used human lung cells (A549) exposed for 24 h to 20 and 100 μg/mL of collected PM2.5 wood dust. We found that both the new adhesives wood dusts induced a slightly higher apoptotic effect than untreated natural wood dusts particularly in spruce wood. Only geopolymer K-PSS wood dust induced membrane damage at the highest concentration and direct and oxidative DNA damage that could be explained by the different chemical composition and the lower particle sizes in respect to organic HBP adhesive wood dust. We found slight induction of IL-6 release, not influenced by K-PSS treatment, at the highest concentration in spruce wood. For poplar wood, IL-6 and IL-8 induction was found particularly for untreated and UF-treated wood at the highest concentration, where hemp adhesive treatment induced lower inflammation while at lower concentration similar slight cytokine induction was found for all tested wood dusts. This preliminary study shows that natural adhesives used to replace UF adhesives represent an interesting alternative, particularly the organic hemp-based adhesive showing very low toxicity.

THE SOY FLOUR AS AN EXTENDER FOR UF AND MUF ADHESIVES IN BIRCH PLYWOOD PRODUCTION

Formaldehyde emission still remains a major disadvantage of widely applied formaldehyde-containing amino resins such as UF (urea-formaldehyde) resin and MUF (melamine-urea-formaldehyde) resin. The compositions of adhesives for plywood manufacturing have to contain a proper extenders in order to adjust their viscosity. Thus, the aim of the study was to investigate the effect of protein-rich soy flour (SF) as the extender for adhesives. The composition of flours and their ability to absorb the formaldehyde were determined. Properties of liquid resins such as gel time, viscosity, pH and solid content were investigated. The possible chemical interaction between the extenders and resins were assessed with the use of FTIR spectroscopy. Plywood panels manufactured using UF and MUF adhesives with the soy flour introduced as the extender in various concentrations were tested in terms of shear strength and formaldehyde release. Studies have shown that soy flour has a favorable composition and formaldehyde-scavenging ability. The addition of SF affected resins properties such as viscosity and gel time but showed no influence on their pH and solid content. FTIR analysis has not explained the chemical interaction between resin and extender. The application of soy flour in the concentration of 15% for UF resin and 10% for MUF resin allowed to produce plywood characterized by improved bonding quality and decreased formaldehyde emission.

Application of UF adhesives containing unmodified and phosphate-modified cellulose microfibers in the manufacturing of particleboard composites

In recent years, the application of cellulose derivative materials as reinforcement in wood adhesives has attracted considerable attention. This study aimed to explore the characteristics of cellulose microfibers (CMF) and phosphorylated cellulose microfibers (P-CMF) successfully extracted from giant reed plant, and their potential effect as binders in the formulation of particleboard panels-based urea-formaldehyde (UF) adhesive. The influence of fiber content (5%, 7.5%, and 10%) on the mechanical and physical properties of the prepared composites were assessed. Moreover, the fire-retardant behaviour of panels containing the optimal amount of phosphate was evaluated to study phosphate groups' efficiency in improving the resistance character of the panels towards the fire. Results showed that the mechanical properties were improved by the addition of 7.5% of CMF and P-CMF. The physical and fire-retardant properties were found to be significantly enhanced for panels containing 7.5% of P-CMF as a result of the chemical phosphorylation.

Properties of Eco-Friendly Particleboards Bonded with Lignosulfonate-Urea-Formaldehyde Adhesives and pMDI as a Crosslinker.

The purpose of this study was to evaluate the feasibility of using magnesium and sodium lignosulfonates (LS) in the production of particleboards, used pure and in mixtures with urea-formaldehyde (UF) resin. Polymeric 4,4′-diphenylmethane diisocyanate (pMDI) was used as a crosslinker. In order to evaluate the effect of gradual replacement of UF by magnesium lignosulfonate (MgLS) or sodium lignosulfonate (NaLS) on the physical and mechanical properties, boards were manufactured in the laboratory with LS content varying from 0% to 100%. The effect of LS on the pH of lignosulfonate-urea-formaldehyde (LS-UF) adhesive compositions was also investigated. It was found that LS can be effectively used to adjust the pH of uncured and cured LS-UF formulations. Particleboards bonded with LS-UF adhesive formulations, comprising up to 30% LS, exhibited similar properties when compared to boards bonded with UF adhesive. The replacement of UF by both LS types substantially deteriorated the water absorption and thickness swelling of boards. In general, NaLS-UF-bonded boards had a lower formaldehyde content (FC) than MgLS-UF and UF-bonded boards as control. It was observed that in the process of manufacturing boards using LS adhesives, increasing the proportion of pMDI in the adhesive composition can significantly improve the mechanical properties of the boards. Overall, the boards fabricated using pure UF adhesives exhibited much better mechanical properties than boards bonded with LS adhesives. Markedly, the boards based on LS adhesives were characterised by a much lower FC than the UF-bonded boards. In the LS-bonded boards, the FC is lower by 91.1% and 56.9%, respectively, compared to the UF-bonded boards. The boards bonded with LS and pMDI had a close-to-zero FC and reached the super E0 emission class (≤1.5 mg/100 g) that allows for defining the laboratory-manufactured particleboards as eco-friendly composites.

Urea impregnated multiwalled carbon nanotubes; a formaldehyde scavenger for urea formaldehyde adhesives and medium density fiberboards bonded with them

AbstractMultiwalled carbon nanotubes (MWCNTs) were subjected to modification by urea to use as formaldehyde scavenger in urea formaldehyde (UF) adhesive and reducing the free formaldehyde emission of the medium density fiberboards (MDFs). Morphological differences besides elemental analysis was investigated using field emission scanning electron microscopy (FESEM) and energy dispersive X‐ray spectroscopy. The effect of urea impregnated MWCNTs filler on the physical, morphological and thermal properties of the UF resin has investigated. Furthermore, characterization of the mechanical properties, free formaldehyde emission and thickness swelling were carried out for the MDF panels. From the results, the free formaldehyde of the UF resins was significantly decreased. The lowest free formaldehyde was belonged to the sample with 3 wt% of scavenger which was about 71% lower than the value for neat UF resin. Accordingly, the formaldehyde emission of the fiberboards was also showed a descending trend by incorporation of MWCNTs‐U to the composite structure. It was decreased from 9.67 to 3.89 mg/100 g dried board. These results indicated that the prepared nano modifier was successfully performed as a formaldehyde scavenger for the UF resin and could prevent the hazards of the free formaldehyde emission from MDF panels.

Utilization of Birch Bark as an Eco-Friendly Filler in Urea-Formaldehyde Adhesives for Plywood Manufacturing.

The potential of using ground birch (Betula verrucosa Ehrh.) bark as an eco-friendly additive in urea-formaldehyde (UF) adhesives for plywood manufacturing was investigated in this work. Five-ply plywood panels were fabricated in the laboratory from beech (Fagus sylvatica L.) veneers bonded with UF adhesive formulations comprising three addition levels of birch bark (BB) as a filler (10%, 15%, and 20%). Two UF resin formulations filled with 10% and 20% wheat flour (WF) were used as reference samples. The mechanical properties (bending strength, modulus of elasticity and shear strength) of the laboratory-fabricated plywood panels, bonded with the addition of BB in the adhesive mixture, were evaluated and compared with the European standard requirements (EN 310 and EN 314-2). The mechanical strength of the plywood with the addition of BB in the adhesive mixture is acceptable and met the European standard requirements. Markedly, the positive effect of BB in the UF adhesive mixture on the reduction of formaldehyde emission from plywood panels was also confirmed. Initially, the most significant decrease in formaldehyde release (up to 14%) was measured for the plywood sample, produced with 15% BB. After four weeks, the decrease in formaldehyde was estimated up to 51% for the sample manufactured with 20% BB. The performed differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and derivative thermogravimetry (DTG), also confirmed the findings of the study. As this research demonstrated, BB as a waste or by-product of wood processing industry, can be efficiently utilized as an environmentally friendly, inexpensive alternative to WF as a filler in UF adhesive formulations for plywood manufacturing.

Potential of Brewer's Spent Grain as a Potential Replacement of Wood in pMDI, UF or MUF Bonded Particleboard.

Brewer’s spent grain (BSG) is the richest by-product (85%) of the beer-brewing industry, that can be upcycled in a plentiful of applications, from animal feed, bioethanol production or for removal of heavy metals from wastewater. The aim of this research is to investigate the mechanical, physical and structural properties of particleboard manufactured with a mixture of wood particles and BSG gradually added/replacement in 10%, 30% and 50%, glued with polymeric diisocyanate (pMDI), urea-formaldehyde (UF) and melamine urea-formaldehyde (MUF) adhesives. The density, internal bond, modulus of rupture, modulus of elasticity, screw withdrawal resistance, thickness swelling and water absorption were tested. Furthermore, scanning electron microscopy anaylsis was carried out to analyze the structure of the panels after the internal bond test. Overall, it was shown that the adding of BSG decreases the mechanical performance of particleboard, due to reduction of the bonding between wood and BSG particles. This decrease has been associated with the structural differences proven by SEM inspection. Interaction of particles with the adhesive is different for boards containing BSG compared to those made from wood. Nevertheless, decrease in the mechanical properties was not critical for particleboards produced with 10% BSG which could be potentially classified as a P2 type, this means application in non-load-bearing panel for interior use in dry conditions, with high dimensional stability and stiffness.

Effects of different nano fillers on the physical and mechanical properties of medium density fiberboards (MDF)

In this study, the physical and mechanical properties of medium density fiber boards (MDF) produced with urea formaldehyde adhesive reinforced with nano-boron nitride (BN) and nano-titanium dioxide (TiO2) were investigated. 0.5% and 1.5% nano-filled urea formaldehyde adhesives were used in the production of test samples. The density, thickness swelling, water uptake, internal bond strength, bending strength, modulus of elasticity in bending and SEM analysis of the boards prepered were determined. According to the laboratory tests conducted, there was no significant change in the density of the boards after nano filling. Physical properties such as water intake and thickness swelling were determined to decrease with the addition of nano particles. The mechanical properties of the boards increased with the addition of both nano-BN and TiO2. According to the a results of thermogravimetric analysis, it has been determined that the thermal stability of urea formaldehyde adhesive with TiO2 is increased and maximum mass loss temperatures are higher than the addition of nano-BN. The morphological structure of the urea formaldehyde adhesive was visualized with an electron microscope, and the nanoparticle dispersions were displayed and the particles were also analyzed by EDAX analysis. As a result, it has been determined that MDFs prepared with nanoparticle adhesives have improved physical, mechanical and thermal properties. Particularly, all samples with nano particles have been achieved the standards required in their mechanical properties. As a result, it can be sad that the adding nano-particles have positive effect on the physical and mechanical properties of the panels.

The Application of Oak Bark Powder as a Filler for Melamine-Urea-Formaldehyde Adhesive in Plywood Manufacturing

The woodworking industry generates a great amount of bark which has not yet found a wider industrial application. None of the previously conducted research has considered oak bark application (which is one of the most often processed wood species in Poland) as a filler for wood adhesives. Moreover, no studies have determined the properties of bark containing melamine-urea-formaldehyde resin (MUF), which increasingly replaces pure urea-formaldehyde adhesives. Thus, the aim of the study was to determine the possibility of grinded oak bark application as a filler for MUF adhesive in plywood manufacturing. The chemical composition of oak bark was evaluated. Properties of liquid resins, such as viscosity, gel time, pH, and solid content, were determined. Chemical interaction between the filler and resin was assessed with using Fourier-transform infrared (FTIR) spectroscopy. Plywood panels manufactured using MUF adhesive filled with different bark concentrations (10%, 15%, 20%, 25%) were tested in terms of such properties as formaldehyde release and bonding quality. Studies have shown an improvement in liquid resin properties. The course of FTIR spectra did not explain the chemical interaction between the polymer and the filler. The addition of oak bark at a concentration of 15% made it possible to produce plywood panels characterized by reduced formaldehyde release and improved bonding quality.

Larch Bark as a Formaldehyde Scavenger in Thermal Insulation Panels.

The aim of this study is to investigate the formaldehyde content and emissions of bark-based insulation panels bonded with three types of adhesives: urea formaldehyde, melamine urea-formaldehyde, and tannin-based adhesives. These panels were produced at two levels of density—300 and 500 kg/m3—and a thickness of 20 mm, and the influence of the adhesive amount and type on the formaldehyde emissions and content was measured. Other mechanical and physical properties such as modulus of rupture, modulus of elasticity, internal bond, and dimensional stability were also scrutinized. With one exception, all the panels belonged to the super E0 classification for free formaldehyde content (perforator value ≤1.5 mg/100 g oven dry mass of panels). The measurements using the desiccator method for formaldehyde emissions assigned all the testing specimens in the F **** category for low-emission panels according to the Japanese International Standards.