Phenol-formaldehyde Adhesives Research Articles

Evaluation of synthetic bio-phenol resins derived from biomass gasification in bio-adhesive formulation

Bio-phenol formaldehyde adhesives formulated from sustainable biomass sources is an excellent alternative to phenolic petroleum-based adhesives with lower pollution level. This study aimed to formulate bio-adhesive from two types of biomass namely palm kernel shell (PKS) and Leucaena sp. Wood. The intention was to find the best process condition which results in maximum content of bio-phenol in the product. The bio-based phenolic resins (bio-oils) were produced from gasification process and their physical and chemical properties were determined. Both produced bio-oils were involved in formulation of bio-based phenol formaldehyde adhesives (resinification) at different operating conditions (temperature, time and catalyst loading). The chemical functional groups and individual compounds of the phenol resins and bio-adhesive samples were identified by GC-MS. The results indicated that temperature has persistent increasing effect on phenol percent of the bio-oil from Leucaena sp. wood however for the bio-oil from PKS the increase of phenol was until the temperature of 85 °C. Reaction time and catalyst loading were observed to have similar effects on resinification of both bio-oil samples. The bio-adhesive produced under best operating condition has the highest amount of bio-phenol and therefore is considered an environmental friendly adhesive with lower cost and pollution than the petroleum-based types.

Modification of urea- and phenol-formaldehyde adhesives by natural fillers for the production of plywood

For the manufacture of plywood and chipboard are used urea-formaldehyde and phenol-formaldehyde hot curing adhesives. When it is necessary to reduce the content of toxic substances, to increase the strength and reliability of gluing, other properties of adhesive bonding heat urea and phenol-formaldehyde resins are modified. Urea-formaldehyde and phenol-formaldehyde resins were used for research. Urea-formaldehyde resin was modified with technical lignosulfonates with particle sizes of 0.01-0.2 mm, shungites with particle sizes of 0.2-0.8 mm, black shales with particle sizes of 0.005-0.2 mm and aluminosilicates with particle sizes of 0.2 - 0.8 mm. Phenol-formaldehyde resin was modified with pectol, black shales with particle sizes of 0.005-0.2, technical lignosulfonates with particle sizes of 0.01-0.2, and cold sludge from aluminum production with particle sizes of 0.01-0.2. A comparative analysis revealed that all the modifiers under study improve the physicochemical parameters of urea-formaldehyde and phenol-formaldehyde resins, increase the performance properties of glued materials from wood. Plywood was tested for shear strength over the adhesive layer and for the content of free formaldehyde in accordance with Russian standards. Modification of urea and phenol-formaldehyde resins with the proposed substances increases the strength of plywood, at the same time reducing the free formaldehyde content in products.

Environmentally-Friendly High-Density Polyethylene-Bonded Plywood Panels.

Thermoplastic films exhibit good potential to be used as adhesives for the production of veneer-based composites. This work presents the first effort to develop and evaluate composites based on alder veneers and high-density polyethylene (HDPE) film. The effects of hot-pressing temperature (140, 160, and 180 °C), hot-pressing pressure (0.8, 1.2, and 1.6 MPa), hot-pressing time (1, 2, 3, and 5 min), and type of adhesives on the physical and mechanical properties of alder plywood panels were investigated. The effects of these variables on the core-layer temperature during the hot pressing of multiplywood panels using various adhesives were also studied. Three types of adhesives were used: urea–formaldehyde (UF), phenol–formaldehyde (PF), and HDPE film. UF and PF adhesives were used for the comparison. The findings of this work indicate that formaldehyde-free HDPE film adhesive gave values of mechanical properties of alder plywood panels that are comparable to those obtained with traditional UF and PF adhesives, even though the adhesive dosage and pressing pressure were lower than when UF and PF adhesives were used. The obtained bonding strength values of HDPE-bonded alder plywood panels ranged from 0.74 to 2.38 MPa and met the European Standard EN 314-2 for Class 1 plywood. The optimum conditions for the bonding of HDPE plywood were 160 °C, 0.8 MPa, and 3 min.

Tannin-based adhesive cross-linked by furfuryl alcohol-glyoxal and epoxy resins

To prepare a natural tannin-based adhesive with good water resistance, an environment friendly furfuryl alcohol-glyoxal resin (FG) synthesized in the laboratory was developed as a cross-linker for tannin-based adhesives. 13C Nuclear Magnetic Resonance (NMR) and Matrix-Assisted Laser Desorption-Ionization Time-of-Flight (MALDI-TOF) mass spectroscopy results indicated that furfuryl alcohol and glyoxal reacted under acidic conditions and that the -CH-(OH)- groups could be shown to be the ones involved in the cross-linking of the tannin-furfuryl-glyoxal adhesive (TFG). The results for the wet shear strength of TFG-bonded plywood showed that the cured TFG was improved and better than that bonded with a tannin-furfuryl alcohol (TF) adhesive. Moreover, the TFG adhesive cross-linked with 12% epoxy resin (EPR) presented a good water resistance. It had a modulus of elasticity (MOE) higher than that of tannin-furfuryl alcohol-formaldehyde (TFF), TF and phenol-formaldehyde (PF) adhesives.

Lignin-based adhesive crosslinked by furfuryl alcohol–glyoxal and epoxy resins

Abstract In this work, a novel environmentally friendly lignin-based adhesive crosslinked with a furfuryl alcohol–glyoxal resin and epoxy resin (EPR) was used to bond particleboard. The results of 13C nuclear magnetic resonance and matrix-assisted laser desorption–ionization time-of-flight mass spectroscopy indicated that furfuryl alcohol and glyoxal reacted under acidic conditions and that–CH–(OH)–groups may be involved in the crosslinking of the lignin–furfuryl–glyoxal resin, which also easily crosslinked with EPR. The IB strength of the particleboard bonded with LFG + 9 % EPR adhesive was better than that of the particleboard bonded with lignin–furfuryl alcohol (LF) adhesive. Moreover, the modulus of elasticity of the LFG + 9 % EPR adhesive was higher than that of the LF and phenol-formaldehyde adhesives.

The effect of DMDHEU modification on physical and biological properties of parallel strand lumbers

In this study, the effect of dimethylol dihydroxyethyleneurea (DMDHEU) modification with four different concentration levels on several properties of parallel strand lumbers (PSLs) produced from poplar veneer strands with phenol and melamine formaldehyde adhesives was studied: density, moisture content, water absorption, thickness swelling, dimensional stability, wettability and decay resistance against C. puteana and T. versicolor attacks. Accelareted aging test was conducted in order to evaluate any loss in effectiveness in decay resistance and dimensional stability due to possibility of cleavage of adhesive bonds in lumbers in outdoors. The PSLs produced with modified strands presented higher density and dimensional stability related with their weight gain but lower moisture content, thickness swelling and water absorption than the controls. DMDHEU modification of PSLs was found efficacious in suppressing brown and white rot fungi attacks, but the protective effect was low when lumbers were aged. PSLs manufactured from rotary peeled veneer strands modified with 5% weight gain of DMDHEU and phenol formaldehyde adhesive showed the most promising results for the investigated properties.

Preparation of highly phenol substituted bio‐oil–phenol–formaldehyde adhesives with enhanced bonding performance using furfural as crosslinking agent

Highly phenol substituted bio‐oil–phenol–formaldehyde (BPF) adhesives were prepared via the phenolization‐copolymerization method, in which furfural was used as a novel crosslinking agent to improve the bonding performance. The effects of bio‐oil percentage, furfural content, curing temperature, and pressure on the performance of BPF adhesives have been studied in detail. A BPF adhesive with 75% bio‐oil percentage and 15% furfural loading (named 75BPF_15) was synthesized, which was cured at 180 °C and 4 MPa. Compared to the conventional phenol–formaldehyde adhesive, the wet tensile strength of 75BPF_15 adhesive reached 2.84 MPa, which was significantly higher than the 1.54 MPa of PF. A possible mechanism of BPF adhesives crosslinked with furfural is proposed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46995.

The current and emerging sources of technical lignins and their applications.

Technical lignins are bulk feedstocks. They are generated as byproducts from pulping or cellulosic ethanol production. Since lignin undergoes significant structural changes in the chemical and physical treatments, all technical lignins are unique in terms of chemical structure, molecular weight, polydispersity, and impurity profile. Kraft lignin is potentially the largest source of technical lignin as new isolation technologies have been implemented on industrial scale in recent years. Lignosulfonate has been an integral product in sulfite pulping biorefinery. It has a well-established market in construction industry. Organosolv-like lignin production is increasing as cellulosic ethanol has been promoted as the substitute of fossil fuel. It may have unique applications because it has low molecule weight and is free from sulfur. Technical lignin application is expected to expand as the characteristics are improved with fractionation or chemical modification. The application of technical lignin has been focusing on developing products equivalent to those made by petroleum chemicals. The recent development in technical lignin supply should increase its market share as additives in polyurethanes and as the substitute of phenol-formaldehyde adhesives. Quality improvement of technical lignin may also encourage the study of lignin as an alternative feedstock for carbon fiber. In addition, technical lignin depolymerization has been extensively explored to provide renewable aromatic chemicals. Starting from controlled pyrolysis and thermal liquefaction as the baseline technologies, many different chemical depolymerization have been invented with a wide range of underlying chemical principles.

Термопластичні полімери у виробництві фанерної продукції: переваги, можливості та перспективи застосування


 Одними з основних клеїв, які застосовують для виробництва фанерної продукції, є карбамідо- та фенолоформальдегідні клеї. Однак, маючи багато позитивних характеристик, такі клеї є токсичними. Низкою попередніх досліджень доведено, що виділення формальдегіду може призвести до пошкодження органів дихання, очей та нервової системи і навіть до раку та лейкемії. Всесвітня організація охорони здоров'я також недавно дійшла висновку, що формальдегід є канцерогеном для людини. Тому зменшення токсичності деревинних матеріалів має величезне значення для довкілля, а особливо – для здоров’я людини. Впродовж останніх років стали жорсткішими вимоги до виробів, з якими контактує людина, а також до тих матеріалів, які використовують для їх виготовлення. У цій оглядовій статті проаналізовано переваги, можливості та перспективи використання термопластичних полімерів для виготовлення фанерної продукції. Однією з основних переваг таких полімерів є їх не токсичність.
 Розглянуто поверхневе оброблення фанери полімерними плівками на основі поліетилену та поліпропілену. Крім того, з’ясовано вплив прискорення старіння поверхні фанери, опорядженої поліетиленовими та поліпропіленовими плівками, на її властивості. Для поліетилену та поліпропілену властива низька адгезія до деревини. Проаналізовано можливості підвищення адгезії між термопластичними полімерами та шпоном/фанерою шляхом хімічного і термічного модифікування як полімерів, так і шпону.
 Окремо розглянуто можливість використання відходів термопластичних полімерів у виробництві фанери, переробка яких зменшить витрати первинного полімеру, що вирішить проблему захисту довкілля. Загалом невисока вартість та сприятливі екологічні показники сукупно відкривають широкі можливості використання термопластичних полімерів у виробництві фанери.

Properties of red maple laminated veneer lumber impregnated with waterborne micronized and two soluble copper formulations

ABSTRACTCertain important quality parameters of red maple (Acer rubrum) laminated veneer lumber (LVL) impregnated with three waterborne formulations: copper azole (CA-B), micronized copper azole (MicroCA or MCA) and alkaline copper quaternary (ACQ-D) bonded with phenol formaldehyde or cross-linked polyvinyl acetate (XPVAc) adhesives were evaluated. Pre-dipping of veneers before LVL production and two post-manufacturing procedures, viz., vacuum-pressure and post-dipping of LVL, were applied. Maximum copper retention in pre-dip-treated, vacuum-pressure and post-dip-treated LVL was 1.4, 9.7 and 1.7 kg/m3, respectively. Copper retention in MCA-treated LVL was relatively lower than soluble formulations. Various physical, mechanical and bonding properties of treated LVL such as density, water absorption, swelling, flexural properties, hardness, tensile shear strength, delamination and wood failure (%) were studied and compared with untreated LVL. Little to negligible deleterious effect was observed on properties of LVL due to these chemical treatments. Analysis of variance results showed that most of properties of red maple LVL were not significantly different compared with those of untreated LVL. Therefore, vacuum-pressure impregnation process can be used to treat the red maple LVL with novel micronized copper formulations for increasing the service life of such products against biodegradation without affecting techno-mechanical quality parameters.

Integrating Multiscale Studies of Adhesive Penetration into Wood*

Abstract The development of new and improved wood adhesives is hindered by an incomplete mechanistic understanding of what makes a wood–adhesive bond able to withstand changes in moisture levels. Although common methods are established to test and report the bulk-level properties of bond lines, such as bond-line shear strength and wood failure analysis, the development of experimental tools to study wood–adhesive bond lines and the effects of moisture at the nanometer to millimeter length scales remains an active area of research. Here we introduce and briefly review four recently developed tools that, when combined, we have found capable to facilitate the study of bond lines and the effects of moisture across these length scales. The tools are X-ray computed tomography, X-ray fluorescence microscopy, nanoindentation, and small-angle neutron scattering. Their combined utility has been demonstrated by studying model bond-line systems made using loblolly pine and phenol-formaldehyde adhesives. The results led to a new insight that adhesive infiltration into cellulose microfibrils at nanometer length scales likely plays an important role in the moisture durability of wood–adhesive bond lines. A new model to facilitate the discussion of potential interactions between adhesive and wood nanostructure was also developed.

Using Surface Modified E-Glass Fiber Cloths to Enhance Poplar Laminated Veneer Lumber Composites: Effects of Modification Conditions, Gluing Processes, Hot-Pressing Parameters, and Assembly Patterns on Physical-mechanical and Interfacial Properties

To develop construction and building materials from low-quality fast-growing wood, poplar laminated veneer lumber composites reinforced by E-glass fiber cloths (GFC) were prepared with phenol-formaldehyde adhesives. The GFCs were modified with γ-aminopropyltriethoxysilane solution (concentration: 0.8%, 1.3%, and 1.8%). Between each veneer, 260, 330, or 400 g/m2 adhesives were spread on one or both sides of veneers. The composites were hot-pressed under different temperatures (130, 145, and 160 °C) and gauge pressures (0.7, 1.2, and 1.7 MPa). The GFC dosages and positions in composites were varied. To evaluate the effects of variables on composites, analytical methods included the contact angle, scanning electron microscopy, and physical-mechanical measurements (flexural strength, flexural modulus, horizontal shear strength, internal bond strength, thickness swelling, and dip peel). The results showed that γ-aminopropyltriethoxysilane made GFC more hydrophilic and compatible with adhesives, enhancing their interfacial adhesion. Considering most of the physical-mechanical and interfacial properties, the optimum parameters for preparing composites were determined to be 1.3% (γ-aminopropyltriethoxysilane), 330 g/m2 (adhesive, double-sided gluing), 160 °C (temperature), and 1.2 MPa (pressure). Varying GFC dosages and positions in composites had positive or negative effects on different physical-mechanical properties. Models were developed to correlate flexural data with GFC dosages and positions.

Synthesis of Humin-Phenol-Formaldehyde Adhesive.

Humins are low-value-added byproducts from the biomass acid hydrolysis process. In the present work, humins were first employed as a phenol replacement for synthesis of modified phenol-formaldehyde adhesives through a two-step process. In this process, humins were first utilized to obtain alkaline soluble products, mainly consisting of phenolics, through a hydrothermal process. The obtained alkaline soluble products then reacted with phenol and formaldehyde to produce humin-phenol-formaldehyde adhesive (HPFA). The physicochemical properties of HPFA, including viscosity, bonding strength, pH, free formaldehyde level, free phenol level and solid content, met the requirements of the GB/T 14732-2006 Chinese National Standard.

Demethylation of Wheat Straw Alkali Lignin for Application in Phenol Formaldehyde Adhesives.

Lignin is a natural biopolymer with a complex three-dimensional network. It is the second most abundant natural polymer on earth. Commercially, lignin is largely obtained from the waste liquors of pulping and bioethanol productions. In this study, wheat straw alkali lignin (WSAL) was demethylated by using an in-situ generated Lewis acid under an optimized demethylation process. The demethylation process was monitored by a semi-quantitative Fourier Transform Infrared Spectroscopy (FTIR) method. The demethylated wheat straw alkali lignin (D-WSAL) was further characterized by Proton Nuclear Magnetic Resonance (1H NMR), Gel Permeation Chromatography (GPC), and titration methods. After the demethylation process, it was found that the relative value of the methoxy group decreased significantly from 0.82 to 0.17 and the phenolic hydroxyl group increased from 5.2% to 16.0%. Meanwhile, the hydroxyl content increased from 6.6% to 10.3%. GPC results suggested that the weighted averaged molecular weight of D-WSAL was lower than that of WSAL with a smaller polydispersity index. The D-WSAL was then used to replace 60 wt % of phenol to prepare lignin-based phenol formaldehyde adhesives (D-LPF). It was found that both the free formaldehyde content and the free phenol content in D-LPF were less than those of the lignin-based phenol formaldehyde adhesives without lignin demethylation (LPF). Gel time of D-LPF was shortened. Furthermore, the wet and dry bonding strengths of lap shear wood samples bonded using D-LPF were higher than those of the samples bonded using LPF. Therefore, D-WSAL has shown good potential for application in phenol formaldehyde adhesives.

Effect of Log Soaking and the Temperature of Peeling on the Properties of Rotary-Cut Birch (Betula pendula Roth) Veneer Bonded with Phenol-Formaldehyde Adhesive

Heating logs prior to peeling positively affects the surface properties of veneer as well as the wood-adhesive bond strength. However, the mechanism behind this increase in strength is not fully understood. The aim of the present study was to separate the influence of soaking temperature and peeling temperature on the physical surface properties and bonding quality. Rotary-cut birch (Betula pendula Roth) logs were soaked at 70 °C, and half of them were subsequently cooled to 20 °C prior to peeling. Surface roughness measurements, scanning electron microscopy (SEM), surface integrity testing, color measurements, and wood-adhesive bond testing were conducted with an automated bonding evaluation system. The results showed that logs soaked at 70 °C and peeled at 20 °C had roughness, color, integrity, bond strength, and wetting properties more similar to logs soaked and peeled at 70 °C than those soaked and peeled at 20 °C. In every test conducted, the effect of soaking temperature was greater than the effect of peeling temperature. High-temperature soaking not only caused softening of the material during the peeling process, but it also caused irreversible changes in the wood material, which affected the veneer surface characteristics and bond strength development.

Influence of organic fillers on surface tension of phenol-formaldehyde adhesives

Organic fillers derived from biomass waste streams are commonly formulated with phenol-formaldehyde resoles (PF) in the production of veneer-based wood composites such as plywood and laminated veneer lumber. The surface tensions of PF/filler formulations were studied as a function of filler types: flours of walnut shell (Juglans regia), red alder (Alnus rubra) bark, and corn cob (Zea mays) furfural production residue. Surface tensions were measured using the drop weight method (with the Harkins–Brown and the Lee–Chan–Pogaku corrections), and also the drop shape method. In these non-Newtonian liquids, viscosity effects on surface tension were determined over a 10-fold range in shear rate. Viscosity effects were minor or negligible, but measurements at the lowest shear rates were considered most reliable. All fillers reduced PF surface tension by 17–25% with effects greatest in alder bark and walnut shell. For all fillers, room temperature aging resulted in further reductions in surface tension. Surface tension reductions roughly correlated to the chemical compositions of the fillers, and probably resulted from the release of surface active compounds extracted from the fillers in the alkaline PF medium.

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.

Preparation and performance of lignin–phenol–formaldehyde adhesives

Steam explosion lignin phenol formaldehyde (SEL–PF) adhesives were prepared by ternary gradual copolymerization. The parameters for the phenolate of steam explosion lignin (SEL) and preparation of SEL–PF adhesives were optimized. Under the optimum phenolate conditions, the phenolic hydroxyl content of lignin increased by 130%, whilst the methoxyl content was reduced by 68%. The SEL–PF adhesives were used to prepare plywoods by hot-pressing. The pH value, viscosity, solid content, free phenol content and free formaldehyde content of SEL–PF adhesives were investigated. The bonding strengths of the plywoods glued with SEL–PF adhesives were determined. The maximum SEL replacement percentage of phenol reached 70wt%, and the properties of adhesives and plywoods met the Chinese National Standard (GB/T 14732-2006) for first grade plywood.

Adhesive characteristics and bonding performance of phenol formaldehyde modified with phenol-rich fraction of crude bio-oil

Commercial phenol–formaldehyde (PF) adhesive was gradually substituted by increasing amount of the phenol-rich fraction (PRF) of crude bio-oil up to 40 wt%. The effect of substitution level of the PRF on the chemical, curing, morphological scanning electronic microscope (SEM), and bonding characteristics of the PF adhesive was determined. The tensile-shear strength of single lap-joint wood specimens bonded with the modified PF adhesives was investigated under indoor and outdoor exposure conditions. The chemical composition of the PRF was investigated using some chromatographic and spectroscopic techniques. Further structural analysis of PRF-modified PF adhesives was determined using Fourier transform infrared spectroscopy (FTIR). The PRF resol had a similar molecular structure to commercial pure phenol resol adhesive. The PRF could be partially substitute for the petroleum-based phenol in commercial PF adhesives with inexpensive phenols derived from lignocellulosic wastes.

Thermal degradation performance of bark based phenol formaldehyde adhesives

Neat phenol formaldehyde adhesive (neat PF), bark bio-crude synthetic bio-based phenol formaldehyde adhesive (bio-crude synthetic BPF) in which 50% phenol was substituted by bio-crude during synthesis, bark bio-crude formulated bio-based phenol formaldehyde adhesive (bio-crude formulated BPF) that was prepared through direct blending of aqueous bio-crude with neat PF at the weight ratio of 50:50, and bark extractives synthetic bio-based phenol formaldehyde adhesive (extractive synthetic BPF) in which 50% phenol was substituted by alkali extractives during synthesis were characterized by thermogravimetry analyzer coupled to Fourier transform infrared spectroscopy (TGA/FTIR), to investigate the stability, thermal degradation kinetics, structure change and the evolved gaseous products. It was found neat PF and BPFs all went through typical three stages degradation. Introduction of bio-crude into BPF reduced the thermal stability for the resulted BPF to a greater extent than extractive introduction. With respect to residual weight after degradation at 800°C, adhesives thermal stability followed the order: neat PF>extractives synthetic BPF≫bio-crude formulated BPF>bio-crude synthetic BPF, which is consistent with the activation energy for the adhesives degradation in the main degradation stage. Auto-oxidation occurred during all the adhesive thermal degradation. But bio-crude based BPFs are more auto-oxidation resistant than neat PF or extractive synthetic BPF. Gaseous products evolved from neat PF degradation mainly consisted of water, phenol, cresol, CO2, CH4, CO and carboxylic acids. During BPFs degradation, CH4, phenol, cresol were yielded at lower temperatures than neat PF degradation. Besides, xylenol not yielded during neat PF degradation was evolved from BPF degradation.