Automated Bonding Evaluation System Research Articles

Effect of the percentage of MUF adhesive coverage on shear strength when bonding different wood species

Due to climate changes, it is necessary to consider the use of other wood species to replace currently used woods. This work deals with the determination of the shear strength of bonded veneers (eight European wood species: spruce, larch, pine, beech, oak, poplar, birch, and alder) with Silekol® 311 melamine-urea-formaldehyde adhesive (MUF) with a variable coverage on the surface of the samples: 10, 15, 20, 25, 30, 50, 75, and 100%. The Automated Bonding Evaluation System (ABES) was used to evaluate and compare adhesive bond strengths. The larch, beech, and oak samples exhibited greater single-lap shear strength than the control samples from spruce. There was no statistically significant difference in shear strength regarding the adhesive coverage from 100% to 20% on the surface of the samples, for almost all wood species. The results of the project provide basic information about the bonding strengths with different coverage in the adhesive layer, comparing non-commonly used wood species in wood-based composites such as oriented strand board and particleboard.

Relationship between thermal curing behavior and bond strength development in formaldehyde-based resins with different molecular weights

Urea–formaldehyde (UF) and phenol–formaldehyde (PF) resins are the most widely used adhesives in the production of wood composites, and are cured through hot-pressing to provide adhesion strength with the composites. This study investigates the relationship between thermal curing behavior monitored by differential scanning calorimetry (DSC) and adhesive bond strength development measured by an automated bonding evaluation system (ABES) for UF and PF resins with different molecular weights (M w). DSC results demonstrated that UF resins cured faster than PF resins, as indicated by a lower apparent activation energy (E a). In addition, UF and PF resins with lower M w cured faster in the early stages of curing and slower in the latter stages of curing as compared to higher-M w resins. Furthermore, according to the ABES results, when increasing the molecular weight, both UF and PF resins exhibited greater adhesive bond strength, irrespective of pressing parameters (temperature and time). The isothermal DSC and ABES results demonstrated that the adhesive bond strength increased with an increase in the degree of cure of the resins, indicating a correlation between them. It also showed that UF resins attained optimal adhesive bond strength at low temperature (110–130 °C) in roughly 180 s while PF resins attained it at high temperature (120–130 °C) in about 300 s. However, the degree of cure under isothermal scans was inconsistent with temperatures. These results indicate that the degree of cure is closely related to the adhesion strength development of UF and PF resins which are curing at low and high temperatures to obtain the optimum adhesion strength.

Exploring Fully Biobased Adhesives: Sustainable Kraft Lignin and 5-HMF Adhesive for Particleboards.

Most adhesives used in the wood-based panel (WBP) industry are petroleum-based and are associated with environmental impact and price fluctuations. Furthermore, most have potential adverse health impacts, such as formaldehyde emissions. This has led to interest from the WBP industry in developing adhesives with bio-based and/or non-hazardous components. This research focuses on the replacement of phenol-formaldehyde resins by Kraft lignin for phenol substitution and 5-hydroxymethylfurfural (5-HMF) for formaldehyde substitution. Resin development and optimization was carried out regarding varying parameters such as molar ratio, temperature or pH. The adhesive properties were analyzed using a rheometer, gel timer and a differential scanning calorimeter (DSC). The bonding performances were evaluated using an Automated Bonding Evaluation System (ABES). Particleboards were produced using a hot press, and their internal bond strength (IB) was evaluated according to SN EN 319. Hardening of the adhesive could be achieved at low temperatures by increasing or decreasing the pH. The most promising results were obtained at pH 13.7. The adhesive performances were improved by adding filler and extender (up to 28.6% based on dry resin) and several boards were produced reaching P1 requirements. A particleboard achieved a mean IB of 0.29 N/mm2, almost reaching almost P2 requirements. However, adhesive reactivity and strength must be improved for industrial use.

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).

The bond strength development of coconut wood in relation to its density variations

ABSTRACT This study examines the influence of density variations in coconut palm wood (Cocos nucifera L.) on bond strength development using three types of adhesives: urea formaldehyde (UF), melamine urea formaldehyde (MUF), and polyvinyl-acetate (PVAc). A special testing device called Automated Bonding Evaluation System (ABES) was used to investigate the bond strength development between coconut veneers as a function of adhesive type, wood density (moisture content of 10%), pressing time, and bond-temperature development. Coconut wood density was shown to greatly affect the bond line. The bond strength of medium-density material (600–700 kg/m3) was lower than that of high-density material (900–1,000 kg/m3). Interestingly, the medium-density coconut veneers achieved higher bond strength with shorter pressing times, while the high-density coconut veneers reached better results with longer pressing times. Moreover, the bond strength improved by increasing the hot-pressing time resulting in higher bond-line temperatures. A comparison was made between coconut wood and beech wood for similar bonding parameters.

Improved Wood-Bond Strengths Using Soy and Canola Flours with pMDI and PAE.

The surprising lack of literature on using the very common wood adhesive polymeric methylenediphenyl diisocyanate (pMDI) with protein adhesives may be because of perceived poor improvement of protein wet strength. Reacting pMDI with the flour (soy or canola) before adding water unexpectedly improves wood bonding compared to adding the pMDI to an aqueous protein slurry. Mixing the liquid pMDI with the oilseed flour produces a free-flowing powder with up to 50% of pMDI to flour by weight. The mixture slowly reacts since the isocyanate band in the infrared spectra remains for several days but diminishes with time. Adding pMDI increases the dry and wet strength of wood bonds using Automated Bonding Evaluation System (ABES) testing and levels off at about 50%. Similarly, adding the polyamidoamine-epichlorohydrin (PAE) cross-linker to the oilseed flour increases dry and wet bond strength, but the effect levels off at about 20% of PAE. However, the combination of these two cross-linkers added to the flours results in greater dry and wet shear strength than either one alone. In addition to tests using ABES (ASTM D 7998), the increase in strengths is also observed—but with a diminished effect—in bonding plywood using the interior plywood strength test ASTM D 906.

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.

Lignosulfonate-Based Polyurethane Adhesives.

The feasibility of using lignosulfonate (LS) from acid sulphite pulping of eucalyptus wood as an unmodified polyol in the formulation of polyurethane (PU) adhesives was evaluated. Purified LS was dissolved in water to simulate its concentration in sulphite spent liquor and then reacted with 4,4′-diphenylmethane diisocyanate (pMDI) in the presence or absence of poly(ethylene glycol) with Mw 200 (PEG200) as soft crosslinking segment. The ensuing LS-based PU adhesives were characterized by infrared spectroscopy and thermal analysis techniques. The adhesion strength of new adhesives was assessed using Automated Bonding Evaluation System (ABES) employing wood strips as a testing material. The results showed that the addition of PEG200 contributed positively both to the homogenization of the reaction mixture and better crosslinking of the polymeric network, as well as to the interface interactions and adhesive strength. The latter was comparable to the adhesive strength recorded for a commercial white glue with shear stress values of almost 3 MPa. The optimized LS-based PU adhesive formulation was examined for the curing kinetics following the Kissinger and the Ozawa methods by non-isothermal differential scanning calorimetry, which revealed the curing activation energy of about 70 kJ·mol−1.

Curing characterization of tannin-hexamine adhesive by automated bonding evaluation system, dielectric analysis, and dynamic mechanical analysis

The characterization of the curing process allows the determination of the optimal pressing parameters, which is essential for the economical production of wood-based composites. In this study, an automated bonding evaluation system (ABES), dielectric analysis (DEA), and dynamic mechanical analysis (DMA) were used to determine the curing parameters of biobased pine tannin-hexamine adhesive at five temperatures ranging from 75 to 175 °C. This study aimed to compare the three above methods and to find correlations between them. All methods showed the same trend of the curing process, which became faster with increasing temperature. Due to various heating rates among the different methods, the curves representing the degree of cure were shifted to the left for the period in which nearly isothermal conditions were reached. It was determined that these methods could be mutually comparable. The ABES was regarded as the reference method; the DEA was regarded as a method that overestimates the curing process and that describes the beginning of the curing process more precisely; and the DMA method was regarded as a method that underestimates the curing process and that describes the end of the curing process more precisely. Linear trend lines were found between the observed methods.

Organosolv Lignin for Non-Isocyanate Based Polyurethanes (NIPU) as Wood Adhesive

A non-isocyanate-based polyurethane (NIPU) wood adhesive was produced from organosolv lignin, which is a bio-sourced raw material, available in large quantities and produced as a by-product of the paper industry. The formulation of this new lignin-based NIPU adhesive, which is presented, was chemically characterised by Matrix-Assisted Laser Desorption Ionization Time of Flight (MALDI ToF) mass spectrometry and by Fourier Transform Infra-Red (FTIR) spectrometry analyses. The oligomers formed were determined and showed that the three species involved in the NIPU adhesive preparation were formed by the co-reaction of the three reagents used: lignin, dimethyl carbonate, and hexamethylene diamine. Linear and branched structures were both identified. Mechanical properties of the adhesive were determined using the Automated Bonding Evaluation System (ABES) and internal bond (IB) strength test of the laboratory particleboard bonded with it. The adhesive has shown satisfactory mechanical properties after hot pressing at 230°C. Such a temperature is used industrially in the most modern particleboard factories, but since it is hardly feasible for more conventional wood bonding equipment, the reactivity of the NIPU adhesive was successfully increased with the addition of a small percentage of a silane coupling agent. With the addition of the silane, the proposed NIPU adhesive could also be used at a hot-pressing temperature lower than 200°C.

Development of polyvinyl acetate adhesive bond strength when gluing veneer

This article presents the development of the shear strength of two polyvinyl acetate (PVAc) adhesives: Mekol D3 and Mekol SPECIAL when bonding veneer depending on pressing time. Besides testing the strength of the adhesive bond after hot pressing, we also investigated the effect of cooling on the final strength of PVAc adhesive bonds. The shear strength development of the PVAc adhesive bond during hot pressing was determined using an Automated Bonding Evaluation System (ABES). The pressing temperature was constant and was 80 °C, while the pressing time was extended from 30 to 300 seconds in 30-second intervals. The effect of cooling on the final strength of the PVAc adhesive bond was determined after cooling times of 5 and 30 seconds. Based on the investigations, we found that the shear strength of PVAc adhesive bonds increased with the pressing time and approached a constant value after a certain time. We found significant differences in the strength of adhesive bonds glued with Mekol D3 and Mekol SPECIAL and a statistically significant effect of cooling on the strength of the adhesive bonds.

The effect of diammonium phosphate and sodium silicate on the adhesion and fire properties of birch veneer

Abstract In built environments the combustibility of wood is a great concern, which limits the use of wood as a building material due to legislation. The reaction-to-fire properties of wood can be altered with the use of fire-retardant chemicals, and most of the commonly used fire retardants already have a long history of use. However, only limited information is available on the impact of different fire retardants on the adhesion properties of wood. Additionally, comparative studies between chemicals from different groups of fire retardants is scarce. The objective of this study was to investigate and compare the effects of two commonly used fire retardants, sodium silicate (SS) and diammonium phosphate (DAP), on veneer properties, the focus being especially on thermal behavior and adhesion. Thermal properties and combustibility were studied using thermogravimetric analysis (TGA), flame test and calorimetry. Glue bond strength was analyzed with an automated bonding evaluation system (ABES) and the leaching of chemicals was determined according to EN84. Additionally, the surface characteristics of modified veneers were imaged with scanning electron microscopy (SEM). Results revealed notable differences in the thermal properties of SS and DAP, with DAP having better fire-retardant performance in all thermal testing. SS also affected thermal properties and combustibility of modified veneers, but the effect was only moderate compared to DAP. Neither SS or DAP had any significant resistance against leaching but ABES testing showed a notable increase in the glue bond strength of DAP modified veneers.

A new methodology for rapidly assessing interfacial bonding within fibre-reinforced thermoplastic composites

A new testing methodology is presented for determining the adhesion strength and failure at composite interfaces. An automated bonding evaluation system (ABES) has been adapted to process and test thermoplastic sandwich composites formed with various natural solid phase materials such as wood. Composites can be rapidly formed and then tested in situ over a range of temperatures to provide information on interfacial adhesion and shear strength between the thermoplastic and the substrate. This methodology is demonstrated for HDPE, PP, PLA and TPU composites comprising wood as solid phases. The interfacial properties depend on processing and testing temperatures, individual thermoplastic phase properties, and wood species. It is expected that this new test method will have applicability in thermoplastic composite design, manufacture and service life predictions for reinforced or filled thermoplastics.

Surface activation of wood by corona treatment and NaOH soaking for improved bond performance in plywood

In plywood manufacturing, the surface characteristics of veneers play a critical role in achieving appropriate bonding performance. An inactivated wood surface caused by oxidation or migration of wood extractives has been shown to lead to an insufficient bonding quality. In this study, inactivated birch and spruce veneer surfaces were activated with corona and chemical NaOH treatments. The effects of the treatments were determined by contact angle measurements and bond quality tests conducted with Automated Bonding Evaluation System (ABES). In addition, the mechanical properties of the plywood produced from the treated veneers were evaluated. The results showed that the corona treatment remarkably increased the wettability of the veneer surface and bond quality of both the spruce and birch veneers evaluated by ABES. The corona treatment also improved the mechanical properties of the birch plywood, but the spruce plywood properties were not affected as much. Soaking veneers in NaOH improved the wettability, but the bond strength was lower than that of the references.

Surface modification of birch veneer by peroxide bleaching

This study examined effects of surface modification with hydrogen peroxide (H2O2) on adhesive bond performance in birch veneer bonded with phenol–formaldehyde resin. The veneer was treated with 5% of H2O2 at 80 °C in the presence of alkali with the objective of improving adhesive bond performance and reducing the resin demand. The effects of the surface modification were determined by surface color measured with a spectrophotometer, bond performance tests with ABES (automated bonding evaluation system) and surface hydrophobicity with sessile contact angle measurements. Results demonstrated that veneer surface became significantly whiter, which also increased in lightness and decreased in redness and yellowness. ABES test revealed that a remarkable increase in bond performance in the treated veneer and the maximum bond strength with the treatment at 60 min, which was nearly twice as high as reference sample (5.42 → 9.94 N/mm2), was obtained. A notable decrease in contact angle measurements was also observed in the treated veneers (0° in 0.5 s). The surface modification of birch veneer with H2O2 + alkali demonstrated greater potential for the improvement of physical characteristics in veneer surface. Other aspects of H2O2 consumption during the treatment are also discussed.

Lignin Phenol Formaldehyde Resoles: The Impact of Lignin Type on Adhesive Properties

Lignin-phenol-formaldehyde (LPF) resoles were prepared using different types of lignin at various levels of phenol replacement by lignin (0 to 40 wt.%). Adhesive properties including thermal behavior as determined by differential scanning calorimetry (DSC), time-dependent development of bond strength during hot pressing as determined by automated bonding evaluation system (ABES), tensile shear strength of solid beech wood lap-joints, and free formaldehyde content of the adhesives were investigated. Preparation of phenol-formaldehyde (PF) resole was accomplished using molar ratios of formaldehyde/phenol and NaOH/phenol of 2.5 and 0.3, respectively. Four different types of technical lignins were studied: Sarkanda grass soda lignin, wheat straw soda lignin, pine kraft lignin, and beech organosolv lignin. The synthesis of the resoles was optimized for 20 and 40 wt.% phenol replacement by lignin. Increasing substitution of phenol resulted in faster gain of LPF viscosity for all studied lignins. The best curing performances of the LPF resoles were observed for pine kraft lignin at both 20 and 40% phenol replacement. The amount of formaldehyde not consumed during cooking increased with increasing level of phenol replacement. However, no differences in free formaldehyde content were observed between the different lignin samples at comparable levels of phenol replacement.

Lignins from enzymatic hydrolysis and alkaline extraction of steam refined poplar wood: Utilization in lignin-phenol-formaldehyde resins

In this study, a steam refining process, optimized earlier with respect to carbohydrate yield, was used to extract lignins from non-debarked poplar wood from short growth plantations by enzymatic hydrolysis (EHL) and alkaline extraction (AEL) for the utilization in lignin-phenol-formaldehyde resins (LPF). Three different pretreatment conditions were chosen at 190°C and 16min, 210°C and 15min and 200°C, 15min, 2.5% SO2. Characterization of the lignins revealed that the carbohydrate content decreased about 47% (210°C, 15min) to 58% (200°C, 15min, 2.5% SO2) with intensification of the pretreatment conditions. Furthermore, the average molecular weight and polydispersity increased. The chemical and physico-chemical properties of lignin-phenol-formaldehyde resins synthesized with these lignins were evaluated and gluebond performance was tested by the Automated Bonding Evaluation System (ABES). LPF resins with AEL and EHL (190°C, 16min) were used to produce particleboards. LPF resins with Organosolv lignin (OL) and Kraft lignin as well as neat PF resins served as references. The mechanical properties of these panels were evaluated. The results showed high dry (EHL: 0.64Nmm−2, AEL: 0.73Nmm−2) and wet internal bond strength (EHL: 0.21Nmm−2, AEL: 0.25Nmm−2) of LPF bonded particleboards. LPF resin with AEL fulfills the requirements for the relevant standard specifications P7 according to DIN EN 312:2010-12.

Geopolymers as potential new binder class for the wood based composite industry

Abstract Geopolymer binders are an emerging class of mineral polymer that can be manufactured from natural raw materials and industrial byproducts containing high amounts of silica (Si) and alumina (Al) in mineral compositions. Various ratios of materials used for manufacturing geopolymer binder have been tested to evaluate the bonding performance of geopolymers with wood by means of tests performed on an automated bonding evaluation system (ABES). Tests with a binder based on sodium silicate water glass (Na 50T) are partly promising, which resulted only 10% lower shear strength than that based on urea formaldehyde. The binder characteristics were significantly influenced by changing the ratio of SiO2:M2O (M=Na or K) and the ratio solid content to chemical base in the water glass. Expectedly, increasing press temperatures and pressing times showed a positive correlation with the curing performance of geopolymer binder. It was also demonstrated that the binder properties can be changed in wide ranges to obtain binders which fulfill the minimum requirements set by industrial users.

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.

Impact of thermal treatment on bonding performance of UF/PVAc formulations

Polyvinyl acetate (PVAc) adhesives present several advantages: ready to use, short setting time, flexible joint, easy to clean and long storage life. On the other hand, formaldehyde-based adhesives are less expensive and give a rigid bond.In this work, the combination of a PVAc glue and UF resin for surfacing MDF with wood veneer was assessed. The reactivity of different mixing ratios was evaluated using ABES (automated bonding evaluation system) machine as well as the wettability of beech veneers after different heat pre-treatments. The obtained results have shown that a mixture of PVAc/UF could be used for surfacing MDF with beech veneer without loss of mechanical properties, when compared with pure PVAc.