Phenol-formaldehyde Resol Resins Research Articles

Organic acid-assisted organosolv fractionation of sawdust and using the fractionated lignin to prepare activated carbon pellets for high-temperature applications

In present, activated carbon (AC) pellets that are commonly prepared by petroleum-based binders have been used to control NOx, SOx, and volatile organic compounds (VOCs) emissions in the iron/steel industry. However, the use of petroleum-based binders leads to great challenges toward sustainability and environment. Alternatively, lignin as a natural bio-binder derived from biomass and organic waste can act as a potential additive for preparing AC pellets. Therefore, in this study, pinewood sawdust was initially fractionated into crude cellulose and lignin by organosolv fractionation method at 90–120 °C for 180 min in an acetic acid/formic acid/water mixed solvent of 3/5/2, 3/6/1, and 5/4/1, vol%/vol%/vol%. Afterward, the fractionated crude lignin was used as bio-additive to substitute 50 wt% of petroleum-derived phenol to synthesize phenol-formaldehyde resole resins (PLPF) and compared with those prepared using 100% petroleum-derived phenol resole resins (PF). PLPF and PF resins were further fabricated to AC pellets by thermal treatment/curing at 150 °C and different loadings of resins (i.e., 10–50 wt%), followed by carbonization at 400 and 700 °C. In the fractionation, it was found that temperature played a role in determining the yield and purity of crude cellulose and lignin, while the effect of solvent composition was insignificant. PLPF resins (loading of 50 wt%)-based AC pellets obtained from thermal treatment at 150 °C followed by carbonization at 400 °C showed a relatively higher compressive strength, which meets the national standard and could ensure their uses as an emission control method in the iron/steel industry. Overall, it can be concluded that lignin-derived from woody biomass by organosolv fractionation in acetic acid/formic acid/water mixture is an alternative bio-binder to prepare AC pellets for emission control at high temperatures.

Catalytic Activity for ORR of Pt Supported on Ordered Mesoporous Carbon with Network Structure

IntroductionIn order to realize carbon neutrality, working toward becoming a hydrogen-based society using renewable energy is essential, and hydrogen fuel cell vehicles will be one of the key devices. For further widespread deployment of fuel cell vehicles, cost reduction and performance enhancement of polymer electrolyte fuel cell (PEFC) systems are urgently needed. Recently, Pt/C-based electrocatalysts using mesoporous carbon as a support material have attracted attention, because well-designed mesopores can act as “accessible pores,” in which direct contact between ionomer and Pt surface is avoided, while the transport of protons, oxygen and generated water is not suppressed. Ordered mesoporous carbon (OMC) with a uniform nanopore structure is one of the ideal candidate materials for a catalyst support with accessible pores. However, with conventional synthesis methods, the typical OMC primary particle size is on the order of micrometers, and mass transport within the nanopores could be a difficult issue. In the present study, we focused on the development of OMC nanoparticles with a network structure. By use of nonionic surfactant micelles with phenol-formaldehyde resole resin as a carbon source, we obtained OMC nanoparticles with a novel hierarchical network structure (ns-OMC). To the best of our knowledge, there are no reports of the synthesis of ordered mesoporous carbon nanoparticles with a rigid network structure. In this study, in addition to the effect of synthesis conditions on the ns-OMC structure, a newly developed technique for the selective deposition of Pt nanoparticles within the ns-OMC nanopores, and the ORR activity of the Pt/ns-OMC catalysts, will be discussed in detail.ExperimentalNs-OMC powder was synthesized with resol-nonionic surfactant micelles as a carbon source and structure directing agent. Resol-F127 (nonionic surfactant) micelles were synthesized by mixing phenol, formaldehyde, water and F127 with sodium hydroxide. After hydrothermal treatment of the mixture, the resulting solid was filtered and washed thoroughly and dried under vacuum. The obtained powder was carbonized at 700 ºC followed by annealing at 1000 ºC. Pt was deposited on the ns-OMC powder by the reverse micelle and colloid techniques. Cyclic voltammetry and linear sweep voltammetry were carried out in N2-saturated and O2-saturated 0.1 M HClO4 solution at 25 ºC using the conventional rotating disk electrode (RDE) technique. Potential step cycling measurements simulating load cycling between 0.6 V and 1.0 V vs. RHE were carried out according to the FCCJ protocol. Coating of the catalyst on a glassy carbon disk substrate was carried out by an electrospray (ES) coating technique developed by our group [1]. N2 adsorption measurement at liquid nitrogen temperature was carried out. The morphology of the Pt/ns-OMC was observed by scanning transmission electron microscope (STEM).Results and discussionAs seen in the STEM images of the ns-OMC powder (Figure 1), the network structure formed by connection of OMC primary nanoparticles to form developed macropores is clearly observed. The OMC nanoparticles were tightly connected via a thick necking structure. The primary particle size of the OMC and neck thickness were controllable via the resol-F127 micelle concentration and the temperature during the hydrothermal treatment. Highly ordered nanopores were observed on the surface of the OMC particles, with a pore size of ca. 5 nm and an interpore distance of ca. 8 nm. The nanopores and the network structure were stable even after high temperature annealing at 1400 ºC. The BET specific surface area for the ns-OMC annealed at 1000 ºC was 814 m2 g-1, and the BJH pore size distribution curve of the ns-OMC showed a mesopore peak at 4.5 nm, which is in good agreement with the STEM observation. When Pt was deposited on the ns-OMC support, most of the Pt particles, with diameters of 4 to 5 nm, were dispersed on the support and, interestingly, located inside or right above the nanopore entrances. Careful STEM measurements revealed that the Pt particles were not deposited deeply within the OMC nanopores, but most were deposited at relatively shallow positions. RDE measurements revealed that the Pt/ns-OMC catalysts showed higher specific activity and mass activity for the ORR and much higher stability for the ECSA values during potential step cycling measurements in comparison with a commercial Pt/CB catalyst. The higher catalytic activity and durability observed for the Pt/ns-OMC catalyst may be due to a near-ideal distance between neighboring Pt particles deposited on the novel support. Aggregation and sintering of the Pt particles were successfully suppressed due to the nanopore structure.[1] S. Cho, K. Tamoto and M. Uchida, Energy Fuels, 34, 14853 (2020).Acknowledgement: This work was supported by the ECCEED’30-FC project from NEDO. Figure 1

Characterization and adhesive performance of phenol-formaldehyde resol resin reinforced with carbon nanotubes

Chemical, physical, thermal properties and bonding quality of phenol-formaldehyde resol resin (PF) synthesized with single-walled carbon nanotubes (SWCNTs) was evaluated at varying ratios from 1 wt% to 5 wt%. The effect of the SWCNTs addition on thermal and chemical properties of the PF resins was characterized by thermal gravimetric analysis (TGA) and Fourier transform infrared (FT-IR) spectroscopy, respectively. FT-IR analysis revealed that the peaks of the modified PF resol resins were similar to those of the reference (laboratory-produced) PF resol resin. These similarities indicated that the synthesis of the resins with phenol, formaldehyde, and carbon nano tubes was successful. The PF resins modified using SWCNTs demonstrated higher thermal stability than the reference PF resin. It was found that the bonding strength of the PF resin containing 3 wt% SWCNTs could reach 12.45 N/mm2 in dry conditions and 7.57 N/mm2 in wet conditions. The bonding test results demonstrated that the SWCNTs were able to improve the bonding performance of the resin under dry and/or wet conditions. This work presents an effective method to improve PF resins with SWCNTs reinforcement for use in the wood and/or polymer composite industries.

Phenol Formaldehyde Resole/Nanosilver Colloid Immobilized Polyester Textile Composite for Antibacterial Applications

AbstractThis work reports the fabrication of an antibacterial phenol formaldehyde (PF)/silver (Ag) nanocomposite coated polyester fabric material for industrial coating applications. Spherical Ag nanoparticles (NPs), with an average particle size of 20 nm were uniformly distributed in the PF resole resin. The polymer nanocomposites with different percentages of Ag NPs (0.2 %, 0.4 %, and 0.6 %) have been coated on polyester fabric by a hand layup procedure. The fabric samples were characterized by scanning electron microscopy (SEM), energy dispersive X‐ray analysis (EDAX) and X‐ray diffraction analysis. The thermal properties of samples have been investigated by differential scanning calorimetry and thermo‐ gravimetric analysis. The effect of Ag NPs on the adhesion features of resole resin coated polyester fabric was evaluated by the peel strength measurement. The optimized composite showed excellent antibacterial properties against Staphylococcus aureus and Escherichia coli bacteria. All the resole PF/Ag nanocomposite coated polyester fabrics are biocompatible. It has been proposed for a variety of applications in the fields of apparel, architecture and construction, insulation, automotive and chemical processing.

Yolk-shell silicon/carbon composites prepared from aluminum-silicon alloy as anode materials for lithium-ion batteries

Silicon is an attractive anode material for lithium-ion batteries due to its ultrahigh theoretical specific capacity. However, its commercial application is largely limited by the poor cycling stability due to its huge volume change during lithiation and delithiation. A low-cost method is developed to prepare yolk-shell silicon@void@carbon composite particles in this study. The synthetic phenol-formaldehyde resole resin was used to bind the graphite to the surfaces of the aluminum-silicon alloy particles, and then was thermal cross-linked and carbonized; aluminum was gently dissolved into ferric chloride etchant, leaving void space between the carbon shell and silicon core. Owing to the presence of the robust carbon shell and internal void space, the yolk-shell composites exhibited significantly better cycling stability than the powder mixture of silicon and graphite. The capacity retention of the silicon component in the powder mixture dropped below 60% after only one cycle, whereas silicon in the composite still has about 70% remaining capacity after 100 cycles.

Rheological Study of Phenol Formaldehyde Resole Resin Synthesized for Laminate Application.

Heat explosions are sometimes observed during the synthesis of phenol formaldehyde (PF) resin. This scenario can be attributed to the high latent heat that was released and not dissipated leading to the occurrence of a runaway reaction. The synthesis temperature and time played important roles in controlling the heat release, hence preventing the resin from hardening during the synthesis process. This study aims to assess the rheological and viscoelasticity behaviors of the PF resin prepared using paraformaldehyde. The prepared PF resin was designed for laminate applications. The rheological behavior of the PF resin was assessed based on the different molar ratios of phenol to paraformaldehyde (P:F) mixed in the formulation. The molar ratios were set at 1.00:1.25, 1.00:1.50 and 1.00:1.75 of P to F, respectively. The rheological study was focused at specific synthesis temperatures, namely 40, 60, 80 and 100 °C. The synthesis time was observed for 240 min; changes in physical structure and viscosity of the PF resins were noted. It was observed that the viscosity values of the PF resins prepared were directly proportional to the synthesis temperature and the formaldehyde content. The PF resin also exhibited shear thickening behavior for all samples synthesized at 60 °C and above. For all PF resin samples synthesized at 60 °C and above, their viscoelasticity results indicated that the storage modulus (G′), loss modulus(G″) and tan δ are proportionally dependent on both the synthesis temperature and the formaldehyde content. Heat explosions were observed during the synthesis of PF resin at the synthesis temperature of 100 °C. This scenario can lead to possible runaway reaction which can also compromise the safety of the operators.

Studies on Synthesis, Characterization of Modified Phenol Formaldehyde Resin and Metal Adsorption of Modified Resin Derived From Lignin Biomass

This study was related to development of economically viable method of extraction of lignin from saw dust in order to produce lignin modified phenolic resin and ecofriendly adhesives (bio-based resin). This study cover to improve the mechanical properties by modification of phenol formaldehyde resole resin using some additives such as boric acid, sulfuric acid and lignin biomass. The synthesis and metal adsorption capacity of resin derived from lignin biomass were explored. Lignin sample was extracted from sawdust of Acacia sp. collected from Batticaloa region by alkali extraction method called delignification process. Qualitative tests were carried out on the extracted alkali lignin and it was used to prepare modified resin. Resin synthesized by using lignin substitution phenol and allowed to condensation reaction in the presence of sodium hydroxide. Boron-modified phenol formaldehyde resin was prepared by using boric acid with formalim method. The above reaction was performed for four hours by refluxing with toluene. Which was produced a high viscous massive resin with 90% yield. The absorbtion peak of B-O bond at 1362cm-1 was observed at IR spectra. Rise in solid mass content leads to produce smooth resin surface without causing cracks and bubbling. Phenol formaldehyde resin was modified into their sulfonated forms to increase their ion exchange capacity, since the ion exchange capacity of virgin resin was found to be zero. Conductivity property shown by sulfonated resin(121mS/cm). The synthesized Lignin based PF resin was used to study the metal adsorption capacity of Cd2+ in aqueous solution. The adsorption capacity of heavy metal Cd2+ ion shown by lignin modified resin (55%) and lignin (86%). In this study sawdust lignin could be best substitution for phenol in synthesis of Phenol-Formaldehyde resin. It’s better due to their sustainability, environmental control, low production cost and their ability to adsorb heavy metals. Phenolic resin was modified with boric acid to improve thermal resistance property and to get smooth resin surface.

TECHNOLOGICAL PROPERTIES OF PLYWOOD BONDED WITH PHENOL-FORMALDEHYDE RESOL RESIN SYNTHESIZED WITH BIO-OIL

ABSTRACT In this study, it was aimed to use of bio-oil as an alternative to petroleum-based phenol in the production of phenol-formaldehyde (PF) resin used for making exterior plywood.Bio-oil obtained from pine wood sawdust using a vacuum pyrolysis reactor at 500 °C. The PF resol resins were produced by substituting up to 20 wt% of phenol with bio-oil by modifying the chemical synthesis process. FT-IRanalysis was performed to characterizethe organic functional groups in the bio-oil modified PF resins. In comparison to the commercial and lab-made PF resins, the bio-oil modified PF resins were found to have larger average molecular weights, higher polydispersity indices, and shorter gel times. Six different types of plywood panels were produced from the experimental PF resins which were commercial PF resin, lab-made PF resin, and PF resins modified with bio oil of 5, 10, 15 or 20 wt% contents, respectively. Plywood specimens produced with the PF resin modified with bio-oil up to 20 wt% had better tensile shear strength (wet condition), modulus of rupture, and modulus of elasticity in bending as compared to the commercial and lab-made PF resins.

Replacing 100% of phenol in phenolic adhesive formulations with lignin

ABSTRACTLignin, produced as a byproduct of pulp and paper and bioethanol industries, is a polyphenolic compound that has excellent potential to be used as phenol replacement in phenolic adhesive formulation. In this study, the phenol portion of phenol formaldehyde (PF) resin has been replaced by an agricultural‐based lignin, which was produced as a byproduct of a cellulosic bioethanol process through dilute‐acid pretreatment and enzymatic hydrolysis from corn stover. The PF resol resin was formulated using isolated lignin under alkaline condition. Chemical, physical, and thermal properties of the isolated lignin, PF resin and adhesive were measured using advanced analytical techniques such as Fourier transformed infrared spectroscopy (FTIR), size exclusion chromatography (SEC), phosphorous nuclear magnetic resonance spectroscopy (31P NMR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The developed 100% lignin‐based adhesive and a commercially formulated phenol resorcinol formaldehyde (PRF, as reference) were used to prepare single‐lap‐joint samples for mechanical testing. The plywood samples were pressed under exactly the same conditions (time, temperature, and pressure) as what recommended for the commercial PRF formulation. According to two‐way ANOVA results, statistically there was no significant difference between the shear strengths of plywood samples made with 100% lignin‐based adhesive and those made with the commercial PRF resin. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45124.

Effect of charcoal on formaldehyde emission, mechanical, thermal and dynamic properties of resol resin

The effect of charcoal (CH), on reducing the emission of free formaldehyde from phenol–formaldehyde resol resins and the properties of the modified resins were investigated. The reduction of free formaldehyde content (FFC) of the resol resin was monitored by Waker’s titration method. The charcoal was found to be very effective in reducing the FFC of the resin due to its high porosity and large surface area. With the incorporation of charcoal the free formaldehyde content of resol resin was almost reduced to zero. The mechanical properties of the PF resin were improved by modification with charcoal. SEM micrographs of tensile fractured surfaces were in agreement with improved mechanical properties. Modified samples exhibited improvements in elastic modulus, loss modulus and tan delta as indicated by dynamic mechanical analysis. The activation energy for the relaxation process in the glass transition region increased with the increase in the amount of charcoal. The thermogravimetric analysis showed that the charcoal had only marginal influence on the thermal stability of the resin.

Characterization of Modified Phenol Formaldehyde Resole Resins Synthesized in Situ with Various Boron Compounds

In this study, three different boron compounds were used together with alkaline catalyst to synthesize phenol formaldehyde (PF) resole resins in situ. The resin curing behavior, molecular structure, bonding performance, and properties of resin-impregnated wood were investigated. Results showed that boron compound-modified PF resins had a lower degree of polymerization than the control PF resin made in the laboratory. The curing kinetics, molecular structure, and functional groups of the modified resins varied depending on the type of boron compounds used. The thermal stability of cured modified PF resins was slightly lower than that of laboratory-made control PF resin. Boron compound-modified PF resins exhibited dry and wet bonding strengths comparable to the those of the laboratory-made control PF resin. Wood impregnated using modified PF resins had comparable dimensional stability, mechanical properties and improved fire resistance than the wood impregnated using lab made control PF resin regardless the type of boron compounds used.

Synthesis and Characterization of Bio-Based Phenol-Formaldehyde Resol Resins from Bark Autoclave Extractives

Abstract With the growing concern for fossil fuel depletion and environmental carbon footprint, there is a strong interest in exploring the renewable biomass materials as substitutes for petroleum-based feedstock. In this study, bark autoclave extractives from the mountain pine beetle (Dendroctonus ponderosae Hopkins)–infested lodgepole pine (Pinus contorta Dougl.) were used for partially replacing petroleum-based phenol in the phenol-formaldehyde (PF) resol resin synthesis. The structural characteristics of the bark autoclave extractives were examined using liquid-state 13C nuclear magnetic resonance (NMR). The curing behavior and curing kinetics, bonding strength, and bond development of the resulting bio-based bark extractive–PF resol resins were investigated using differential scanning calorimetry (DSC), lap shear, and dynamic mechanical analysis (DMA) tests, respectively. Results showed that bark autoclave extractives were a complicated mixture containing tannin, degraded hemicellulose, and degraded lignin components. The bark extractive–PF resins exhibited a higher molecular weight, higher viscosity, shorter gel time, and faster curing rate than the laboratory-made PF resin without bark components. The bark extractive–PF resins had comparable bonding strength to a commercial PF resin even when the phenol replacement rate was as high as 50 percent by weight. Bark autoclave extractives obtained from the beetle-infested lodgepole pine are suitable as a partial replacement of petroleum-based phenol in making PF resol adhesives.

Recent Development in Forest Biomass Derived Phenol Formaldehyde (PF) Resol Resin for Wood Adhesives Application

Recent Development in Forest Biomass Derived Phenol Formaldehyde (PF) Resol Resin for Wood Adhesives Application

Preparation, Acid Curing, and Thermal Stability of Various Formulated Phenolic Resole Resins: Unfilled and Nanocomposites

Several phenol-formaldehyde resole resins were prepared with variety in monomers ratio, catalyst type, and content, having different nanoclay levels; then they were cured using various acids in the wide range of concentration. The acid-curing process was studied, considering gel time (tG) and cure time (tC). In addition, the thermal stability was investigated before and after cure for all samples. With increasing F/P ratio, tG, tC and tG-tC decreased and also CC (critical concentration). By using more catalyst in the synthesizing step, the curing was done more rapidly in the order of NaOH, Ba(OH)2 and NH3. Stronger acids having smaller pKa made a more realizable cure. However, the weak boric acid had no curing effect. Nanocomposites had shorter tG and tC, mainly at lower acid concentration. The structure peak of nanoclay shifted to the lower angles in nanocomposites, especially in the cured state. Crosslinked samples had higher degradation temperature (TD) and lower weight decrease (Δw) related to the primary resoles. For uncured resins, thermal stability increased with decreasing of F/P ratio, and the inverse effect was found for the cured resins. Resins cured with HCL had higher TD and lower Δw. However, at 30% concentration the sample cured by H2SO4 was more stable. With increasing catalyst amount and reactivity, TD increased and Δw decreased. In all acid concentrations, at the presence of nanoclay the better thermal resistance was observed. TD increased and Δw decreased as the nanoclay level increased.

Synthesis of biobased phenolic resins/adhesives with methylolated wood‐derived bio‐oil

AbstractBio‐oil from the hydrothermal liquefaction of Eastern White Pine (Pinus strobus L.) sawdust was methylolated with formaldehyde in the presence of sodium hydroxide. The obtained methylolated bio‐oil (MB) was used to synthesize methylolated bio‐oil–phenol formaldehyde (MBPF) resol resins with a phenol substitution ratio of up to 75 wt %. All of the resins were used as wood adhesives for the production of a three‐layered plywood. The MBPF resol resins were comprehensively characterized for their physical, chemical, thermal, and mechanical properties (e.g., viscosity, nonvolatile content, shelf life, free formaldehyde level, molecular weight and distribution, curing temperature, bonding strength) when used as plywood adhesives. All of the MBPF resins contained similar nonvolatile contents as the reference pure phenol formaldehyde (PF) resin but had higher viscosities and shorter shelf lives, depending on the amount of MB in the MBPF. All of the MBPF resins displayed a lower curing temperature than the reference PF resin, with the main curing peak around 140°C, which was similar to that of the pure PF resin. According to thermogravimetric analysis, the methylolation treatment of bio‐oil improved the thermal stability of the MBPF resins compared to the bio‐oil–PF resol resins, which used untreated bio‐oil. The dry/wet bonding strengths of the plywood specimens glued with the MBPF resol resins with up to 60 wt % phenol substitution exceeded or were comparable to those of the conventional PF resol resin. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

The Study of Divalent Metal Ion Catalysts on Phenol-Formaldehyde Resol Resins

The curing characteristic of phenol-formaldehyde resol resins catalyzed with magnesium hydroxide, calcium hydroxide and barium hydroxide was studied in this study.The effects of catalysts on chemical structure of phenol formaldehyde resin was investigated by fourier transform infrared.The results indicated that divalent metal ions catalysts played an important role influence both the cure rate and cure time.Phenol formaldehyde resin catalysed by diffierent catalysts showed diffierent addition of formaldehyde onto ortho positions of phenolic rings.The order of the divalent metal ions effectiveness studied in alkaline conditions is calcium hydroxide, barium hydroxide and magnesium hydroxide.

Synthesis of Phenol-Formaldehyde Resol Resins Using Modified Alkaline Lignin with Laccase

lignin (L)-based phenolic prepolymers with different phenol (P) substitution degrees were synthesized using the modified(M) L with laccase. It was found that the ML showed lower molecular value, polydispersity and higher reactivity compared to unmodified sample, and up to 30% of phenol can be substituted by MF to give LPF wood adhesive having better bonding strength in comparison to a control unmodified lignin phenol formaldehyde wood adhesive. MF was characterized using FTIR and GPC. FTIR showed that after the treatment of laccase, the methoxyl group increased, which was mainly due to the demethylation, and laccase/mediator system made the ether bonds of L cleave to produce more alcohol hydroxyl, as a result, the reactivity of ML was improved.

Thermal degradation of lignin–phenol–formaldehyde and phenol–formaldehyde resol resins

Resol resins are used in many industrial applications as adhesives and coatings, but few studies have examined their thermal degradation. In this work, the thermal stability and thermal degradation kinetics of phenol–formaldehyde (PF) and lignin–phenol–formaldehyde (LPF) resol resins were studied using thermogravimetric analysis (TG) in air and nitrogen atmospheres in order to understand the steps of degradation and to improve their stabilities in industrial applications. The thermal stability of samples was estimated by measuring the degradation temperature (T d), which was calculated according to the maximum reaction rate criterion. In addition, the ash content was determined at 800 °C in order to compare the thermal stability of the resol resin samples. The results indicate that 30 wt% ammonium lignin sulfonate (lignin derivative) as filler in the formulation of LPF resin improves the thermal stability in comparison with PF commercial resin. The activation energies of degradation of two resol resins show a difference in dependence on mass loss, which allows these resins to be distinguished. In addition, the structural changes of both resins during thermal degradation were studied by Fourier transform infrared spectroscopy (FTIR), with the results indicating that PF resin collapses at 300 °C whereas the LPF resin collapses at 500 °C.

Synthesis of phenol–formaldehyde resol resins using organosolv pine lignins

Lignin was extracted from white pine sawdust by organosolv-extraction using hot-compressed ethanol–water co-solvent. The optimum conditions for extracting lignin from the pine sawdust were found to be at 180 °C with ethanol–water solvent (1:1 wt/wt), where the lignin yield attained ca. 26% with a purity of ca. 83%. The lignin under such conditions was oligomers with a broad molecular weights distribution: M n of 537, M w of 1150 and polydispersity of 2.14. Bio-based phenol–formaldehyde resol resins were synthesized using the resultant lignin as the replacement of petroleum-based phenol at varying ratios from 25 to 75 wt.% by condensation polymerization catalyzed by sodium hydroxide. Upon heating the lignin–phenol–formaldehyde resols could solidify with a main exothermic peak at around 150–175 °C, typical of the conventional phenolic resol resins, and a secondary peak at 135–145 °C, likely due to the exothermic reactions between the free formaldehyde with phenol or lignin to form methylophenols. The replacement of phenol with lignin at a large ratio deferred the curing process, and the introduction of lignin in the resin formula decreased the thermal stability of the resin, leading to a lowered decomposition temperature and a reduced amount of carbon residue at elevated temperatures. For practical applications, it is suggested that the replacement ratio of phenol with lignin be less than 50 wt.%. The thermal stability can however be improved by purifying the lignin feedstock before the resin synthesis.

Online monitoring of synthesis and curing of phenol–formaldehyde resol resins by Raman spectroscopy

The synthesis and curing of phenol–formaldehyde resol resins were monitored online by Raman spectroscopy. The synthesis of the resins (F/P 2.0, alkalinity 4.5wt%) was studied at rising temperature (40–90°C) for 90min and at constant temperatures (80°C, 90°C, 100°C, and 110°C) for 120min. The progress of the curing was investigated isothermally (80°C, 90°C, 100°C, and 110°C) for 120min for three resins with different degrees of condensation. The synthesis and curing of the resins were started in the reactor and the advancement of the methylolation and condensation reactions was followed through the window of the reactor in the wave number region of 2000–400cm−1 with use of a fiber optic probe for the data collection. The Raman spectra of six model compounds (formaldehyde, phenol, 2-hydroxybenzyl alcohol, 4-hydroxybenzyl alcohol, 2-benzylphenol, and 4-benzylphenol) were analyzed to facilitate the interpretation of the spectra of the resins. The consumptions of free phenol and free formaldehyde, as well as the progress of the methylolation and condensation reactions were easily monitored by following the changes in intensity of the characteristic Raman bands. The results for the cured resins obtained by Raman spectroscopy were in good agreement with the structures and residual reactivities studied by CP/MAS 13C NMR spectroscopy and differential scanning calorimetry (DSC), respectively. The results of the study show Raman spectroscopy to be a promising tool for the online monitoring and control of phenol–formaldehyde resol resin synthesis and curing; in addition, Raman spectroscopy offers an effective and fast method for structural study of the solid state resins.