Phenol-formaldehyde Resol Research Articles

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.

Extraction of titanium with phenol-formaldehyde resol oligomer Yarresin B from peroxide solutions

Extraction of titanium by nitrogen-containing phenol-formaldehyde oligomer Yarresin B from peroxide sulfate solutions was studied. Molecular dynamic (MD), molecular mechanic (MM), ZINDO/1 semiempirical (single-point calculations; program package HyperChem, Release 7.52, Hypercube Inc.) calculations, and the Cambridge Structure Database (ConQuest 1.8) were used to solve the structure of the extracted complex [Ti(O)2OR(OH)2]2SO4.

Processing conditions analysis of Eucalyptus globulus plywood bonded with resol-tannin adhesives

Phenol-formaldehyde resol containing mimosa tannin extract was employed to produce plywood panels with two plies from Eucalyptus globulus veneers. The effect of processing conditions and tannin content on the gelation time of the adhesive in the glue line was evaluated by dynamic-mechanical analysis (DMA). These results were related with shear strength and wood failure of glue line in the final panels. Hazardous petrochemical phenol could be partially substituted in resols in industrial applications by addition of mimosa tannin extracts.

Structure of polycondensates from hydroxymethylphenols

AbstractThe structure of oligomers obtained from mono‐hydroxymethylphenols in melt condensation at 120°C was determined using 13C NMR spectra in CD3OD solution. Alongside of methylene region of spectrum, valuable information was obtained from signals of aromatic carbons. Noncatalytic conditions promote the formation of dihydroxydibenzyl ethers in equilibrium with ortho‐ and para‐benzoquinones of oxymethylene derivatives. The final methylene linked oligomers are formed, mainly, by splitting the ether intermediates with free aromatic positions. In alkaline conditions, highly nucleophilic phenoxide ions of ortho‐hydroxymethyl compounds are responsible for substitution in free aromatic positions. The most favored reaction in the mixture of both hydroxymethylphenols is the formation of p,p′‐methylene. In condensation of para‐hydroxymethylphenol, formation of p,p′‐methylene groups occurs with simultaneous release of formaldehyde. High content of alkali stabilized ortho‐hydroxymethyl groups of fully substituted methylene linked oligomers determines the curing behavior of resol phenol–formaldehyde resins. The role of hemiformals in reactions was insignificant. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Effect of alkylresorcinols on curing behaviour of phenol-formaldehyde resol resin

Abstract Thermal behaviour of cure-accelerated phenol-formaldehyde (PF) resins was studied using the addition of commercial mixture of water soluble oil shale alkylresorcinols (AR) to PF resin, 5-MR being as model compound. The acceleration effect of AR is based on the promotion of condensation of resin methylol groups and subsequent reaction of released formaldehyde with AR. Commercial PF resins SFŽ-3013VL and SFŽ-3014 from the Estonian factory VKG Resins have been used. The chemical structure of resins was characterised by 13C NMR spectroscopy. TG-DTA analysis was carried out using labsysTM instrument Setaram. By TG-DTA measurements, the shift of exothermic and endothermic peaks and the changes of mass loss rate in the ranges of 1.5–10 g AR/100 g PF resin were studied. The effect of AR on the curing behaviour of PF resins was also followed by gel time. Testing of the plywood when using PF resin with 5 mass% of AR shows that the press time could be reduced by about 15%.

Effects of silane and the lowering of pH on the properties of phenol–formaldehyde resol resins and resin coatings

AbstractPhenol–formaldehyde resol resins were modified by the addition of silane (3‐aminopropyltriethoxysilane) and the lowering of pH (formic acid). The effects of the modifications on the properties of the resins during storage were studied through comparison with the parent resins and by viscosity measurements, NMR spectroscopy, ultraviolet–visible spectroscopy, and differential scanning calorimetry. Resin coatings on paper were prepared to determine the influence of discoloration of the resin solution on the color of the cured resin. A decrease in the pH of the NaOH‐catalyzed resin solutions lightened the color of the solutions and corresponding coatings, whereas silane additions made the coatings slightly more yellow. The lowering of pH increased the viscosities and decreased the reactivities of the resin solutions compared with the unmodified reference resins during storage. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1933–1941, 2007

Cure rate of Phenol–Formaldehyde (PF) resol resins catalyzed with MgO

One of the main drawbacks that has prevented a wider use of phenol–formaldehyde (PF) resins in the manufacture of impregnated paper and wood composite panels is their relatively slow cure rate. In this study, the curing characteristic of PF resol resins catalyzed with MgO was studied with various formaldehyde (F)/MgO/phenol (P) ratios at various pH values. The results indicated that the pH value, nature of pH regulator and synthesis parameters, including the F/P ratio and MgO content, all influence the rate of cure. The pH value played an important role in affecting both the cure rate and cure time. The cure rate was fast when pH was below 7.5. The cure time decreased as the pH value decreased at all F/MgO/P ratios. The MgO/P ratio had a definite influence on the cure rate, the cure time decreased with the increase of MgO/P molar ratio, and the F/P ratio had no significant influence on the cure rate. Differential scanning calorimetry (DSC) results showed that MgO catalyzed PF resin can cure at a low temperature.

Master curve and time–temperature–transformation cure diagram of lignin–phenolic and phenolic resol resins

AbstractThe aim of this work is to generate both a master curve of resol resins based on the time–temperature superposition principle and their TTT cure diagrams. The samples used for this purpose were lignin–phenolic and phenol–formaldehyde resol resins. A TMA technique was employed to study the gelation of resol resins. In addition, a DSC technique was employed to determine the kinetic parameters through the Ozawa method, which allowed us to obtain isoconversional curves from the data fit to the Arrhenius expression. Establishing the relationship between the glass‐transition temperature and curing degree allowed the determination of the vitrification lines of the resol resins. Thus, using the experimental data obtained by TMA and DSC, we generated a TTT cure diagram for each of resins studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3362–3369, 2007

Novel two‐stage phenol–formaldehyde resol resin synthesis

AbstractThe synthesis of phenol–formaldehyde resol resins was carried out in two stages to facilitate the start of a conventional batch process. In the first stage, the starting material solution was preprocessed in a continuous‐flow stirred‐tank reactor with a 5‐min residence time. In the second stage, synthesis was continued in a batch reactor. Samples were analyzed by titrimetric methods, gas chromatography, nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. Most of the starting materials were consumed in the preprocessing reactor, which allowed better control of the reactivity of the prepolymer solution in the second stage. The methylolation and condensation reactions proceeded steadily during the production process in the batch reactor. The results of the study indicated that dividing a conventional one‐stage batch process into two stages could facilitate the control of the initial stages of resol production. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103:371–379, 2007

A study on the effects of reaction conditions on the characteristics of resol resins for foams by statistical analysis

A two-level full factorial experimental design with three variables, formaldehyde-to-phenol (F/P) molar ratios, hydroxy-to-phenol (OH/P) wt%, and reaction temperature was implemented to determine the effect of the variables on the properties of phenol-formaldehyde (PF) resol resins for insulation foam. Ten resins were prepared with F/P molar ratios between 1.5 and 2.5, OH/P wt% between 0.5 and 1.5 and reaction temperature between 80 and 90 ‡C. The effect of three independent variables on the resin properties was analyzed by using three-way ANOVA of SPSS. All three production variables had a significant effect on resin properties. The F/P molar ratio and OH/P wt% were found to have the most frequent effect on resin properties. F/P molar ratio and OH/P wt% had an increasing effect, while reaction temperature showed a decreasing effect on free formaldehyde. The present study confirms that experimental design is a very valuable and capable tool for evaluating multiple variables in resin production.

Gelation and isoconversional kinetic analysis of lignin–phenol–formaldehyde resol resins cure

The curing process of a commercial phenolic (PF) and a lignin–phenolic (LPF) resol resins was studied by thermal mechanical analysis (TMA) and differential scanning calorimetry (DSC) techniques. The main conclusion that we wish to point out is that the gelation can be associated with an isoconversional principle. Thus, the curing kinetics analysis was performed applying non-isothermal DSC data to the isoconversional methods: Ozawa, Kissinger–Akahira–Sunose, and Friedman. These isoconversional methods allowed the determination of the dependence of activation energy and pre-exponential factor on the degree of curing for both resins. Friedman's method presents different results in relation to other methods, but both resins exhibit a similar tendency for kinetic parameters.

Transformation of dynamic DSC results into isothermal data for the curing kinetics study of the resol resins

Kinetics of thermosetting polymers curing is difficult to study by isothermal methods based on the differential scanning calorimetry (DSC) technique. The difficulty is due to the low sensitivity of the equipment for total reaction heat measurements during high temperature process. The aim of this paper is to display the equivalence between a dynamic model, the Ozawa method, and an isothermal isoconversional fit, which allows predicting the isothermal behavior of the resol resins cure through dynamic runs by DSC. In this work, lignin–phenol–formaldehyde and commercial phenol–formaldehyde resol resins were employed. In addition, the isothermal kinetic parameters for both resins were performed by means of transformation of the data obtained from the dynamic Ozawa method.

Phenol-formaldehyde resols modified by boric acid

Phenol-formaldehyde resol resins with improved thermal stability and flame retardancy were prepared by introducing boric acid into the matrix of PF precondensates at the condensation stage. The modified PF resols had small amounts of free monomers and high solid mass content. The thermogravimetry of cured, modified PF resols showed enhanced thermal stability in comparison with unmodified ones indicating the role of boric acid in improving thermo-oxidative resistance. Besides, the determined limited oxygen index values indicated the significantly decreased flammability of the modified resins.

Fracture cleavage analysis of PVAc latex adhesives: Influence of phenolic additives

Abstract Mode I fracture cleavage testing was conducted on wood bonded with poly(vinyl acetate-co-NMA) latex adhesive containing two types/degrees of cross-linking: (1) cross-linking through AlCl3 catalysis of N-methylolacrylamide (NMA) comonomer; and (2) additional cross-linking using a phenol-formaldehyde resol additive, in addition to AlCl3 catalysis. The formulation lacking the phenolic additive performed well under dry conditions; but it completely failed during testing as a result of accelerated weathering. In contrast, fortification with the phenolic additive provided durability against accelerated weathering. In an effort to understand the effects of accelerated weathering, parallel-plate dynamic mechanical analysis was applied to freestanding neat adhesive films and to wood-bonded films (composites). Accelerated weathering dramatically altered the viscoelastic response of films and composites that lacked the phenolic additive; weathering caused a second, broad and reversible relaxation near 100°C, which might be because of softening of the poly(vinyl alcohol) interfacial agent used during emulsion polymerization. In contrast, samples formulated with the phenolic additive only displayed the typical base-polymer glass transition. Correlation of the fracture testing and the rheological analysis suggests that the phenolic additive promotes adhesive durability by cross-linking the interparticle boundaries, where poly(vinyl alcohol) is believed to reside.

Rheology of latex films bonded to wood: influence of cross-linking

Abstract Parallel-plate rheological analysis was conducted on two types of cross-linking poly(vinyl acetate) latex films: dry freestanding films, and dry films bonded directly to wood (composites). For each sample type, three levels of cross-linking were used: (1) little or no cross-linking of unaltered latex; (2) substantial cross-linking through AlCl3 catalysis of N-methylolacrylamide co-monomer; and (3) greater cross-linking from a phenol-formaldehyde resol additive, in addition to AlCl3 catalysis. Simple thermal scans revealed a strong wood/adhesive interaction; wood increased the base polymer T g by ∼5°C in all adhesives. Relative to the simple thermal scans, time-temperature master curves provided more insight and information about the wood/adhesive interaction. Storage modulus and tan δ master curves both indicated that wood retarded adhesive cross-linking. Using time-temperature superposition, a segmental coupling analysis demonstrated that wood actually narrowed the breadth of the glass transition, or reduced segmental coupling. Cross-linking influenced segmental coupling, but in a fashion that was dependent on the presence or absence of wood. Wood-induced reductions in cross-linking and in segmental coupling were attributed to the diffusion of water-soluble reactive compounds away from the adhesive layer and into the bulk wood. Time/temperature equivalence provides a sensitive means to detect interactions between wood and viscoelastic adhesives.

Comparison of model-free kinetic methods for modeling the cure kinetics of commercial phenol–formaldehyde resins

For many industrial processes it is important to model the cure kinetics of phenol–formaldehyde resoles. Yet the applicability of common model-free kinetic algorithms for the cure of phenolic resins is not known. In this study the ability of the Friedman, Vyazovkin and Kissinger–Akahira–Sunose (KAS) model-free-kinetics algorithms to model and predict the cure kinetics of commercial resoles is compared. The Friedman and Vyazovkin methods generate consistent activation energy dependences on conversion compared to the KAS method. In addition, the activation energy dependency on conversion is of higher amplitude with these two methods than with the KAS method. Hence, the Friedman and Vyazovkin methods are more adequate for revealing the cure steps of commercial PF resoles. Conversely, the KAS algorithm is easily amenable to dynamic cure predictions compared to the Friedman and Vyazovkin methods. Isothermal cure is equally well predicted with the three. As a result, the KAS algorithm is the method of choice for modeling and predicting the cure kinetics of commercial phenolic resoles under various temperature programs.

Determination of curing kinetic parameters of lignin–phenol–formaldehyde resol resins by several dynamic differential scanning calorimetry methods

The curing kinetics of lignin–phenol–formaldehyde (LPF) and phenol–formaldehyde (PF) resol resins was studied by non-isothermal differential scanning calorimetry (DSC) at different heating rates. The data were fitted by means of the Borchardt–Daniels, Ozawa and Kissinger methods, which allow determining the kinetic parameters of both resins. The kinetic study has been realized to evaluate the effect of methylolated ammonium lignin sulfonate in the curing process of the lignin–phenolic resins. The results showed a slightly higher activation energy for LPF resin. It is attributed to the presence of modified lignosulfonate in this case. The obtained data, similar in both resins, indicate that the phenol can be replaced partially by a modified lignosulfonate in the phenolic resins.

Eucalyptus bark lignin substituted phenol formaldehyde adhesives: A study on optimization of reaction parameters and characterization

AbstractThe major adhesive resin worldwide used in the manufacture of plywood is phenol formaldehyde resole (PF) resin. The raw material for this kind of adhesive is derived from petroleum oil. Because of rising prices of crude oil and the scarcity of petroleum products, their replacement by natural resource–based raw material has become a necessity. In the present work, the possibility of replacing phenol in PF resin with lignin was explored. The parameters for preparation of bark lignin substituted PF (LPF) adhesive, such as lignin concentration, formaldehyde to phenol molar ratio, catalyst concentration, reaction time, and reaction temperature, were optimized. It was found that up to 50 wt % of phenol can be substituted by lignin to give an LPF adhesive with better bonding strength compared to that of control PF resin. Prepared resins were characterized using IR, DSC, and TGA. IR spectra of LPF adhesive showed structural similarity with that of PF adhesives. Thermal stability of LPF adhesive was found to be lower compared to that of control PF (CPF) adhesive. DSC studies revealed a lower curing temperature for LPF resin than that for CPF resin. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3514–3523, 2004

Fully quantitative carbon-13 NMR characterization of resol phenol–formaldehyde prepolymer resins

Different resol phenol–formaldehyde prepolymer resins have been synthesized with different Formaldehyde/Phenol (F/P) ratios or different catalysts and characterized by 13C NMR spectroscopy in solution. A fast quantitative measuring protocol is proposed based on the use of chromium(III)acetylacetonate as a relaxation agent. APT (attached proton test) and DEPT (distortionless enhancement by polarisation transfer) spectra were acquired to enable proper resonance assignments, especially in the regions with severe signal overlap. Equations are presented in which the methylene bridges (MB), the methylol groups (MG) and the dimethylene ether bridges (DMEB) of resol resins are quantitatively taken into account. Important structural factors determined quantitatively for resol prepolymer resins are the F/P ratio after reaction, the degree of polymerization ( n), the number average molecular weight ( M n) and the content of free ortho and para positions.

Curing process of powdered phenol–formaldehyde resol resins and the role of water in the curing systems

AbstractThe curing behavior of two kinds of commercial powdered resol phenolic resins was studied by differential scanning calorimetry. Liquid‐state 13C‐NMR spectroscopy was used to aid in understanding the curing behavior by detecting the structure of powdered resins. The reaction mechanism was interpreted with the dependency of activation energy on the degree of conversion. The results indicate that there are differences in the curing mechanism between core and face phenolic resins. The curing process of core resin was faster than that of face resin at the same reaction temperature. The water added in the curing system played an important role of plasticizer or diluent according to different curing stages and water content. In the initial curing stage, water mainly diluted the system and retarded the curing reactions. However, at the higher degrees of conversion, water played the role of plasticizer to decrease the effect of diffusion on the curing reactions to make the curing reactions more complete. The excess water added in the curing system played the role of diluent at almost all stages during the curing process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1371–1378, 2003