Lignin–phenol–formaldehyde resin adhesives prepared with biorefinery technical lignins

In this study, four biorefinery technical lignins were used to synthesize lignin–phenol–formaldehyde (LPF) resin adhesives with a proposed formulation that was designed based on accurate analysis of the active sites in lignin with ³¹P nuclear magnetic resonance (NMR). The properties of the LPF resin adhesives and the plywoods prepared with them were tested. The structural features and curing behavior of the LPF resin adhesives were thoroughly investigated by solution‐ and solid‐state ¹³C NMR. Results indicated that the proposed formulation exhibited favorable adaptability for all four of these technical lignins for synthesis of LPF resin adhesives. High‐performance plywood with low emissions of formaldehyde could be successfully prepared with the synthesized LPF resin adhesives. All the LPF resin adhesives exhibited similar structure and curing behavior with the commercial phenol–formaldehyde (CPF) resin adhesive. However, the LPF resin adhesives showed relatively higher curing temperatures as compared with the CPF resin adhesive. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42493.     

Preparation of phenol–urea–formaldehyde copolymer adhesives under heterogeneous catalysis

A new synthetic strategy for PUF copolymers based on three steps was developed. In the first step, two precondensates of phenol with formaldehyde and urea with formaldehyde, respectively, were produced. In the second step, the two precondensates were mixed and condensed using a heterogeneous catalyst in a tube reactor at 90°C. The last step is a vacuum distillation to reach the final copolymer compositions. With regard to the properties, the products can be used as adhesive. The copolymers were analysed by gel permeation chromatography (GPC), ¹³C‐NMR‐spectroscopy, and MALDI‐TOF mass spectrometry. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2946–2952, 2006     

Brominated phenol–formaldehyde resin as an adhesive for plywood

A brominated phenol–formaldehyde resin was investigated as a plywood adhesive to study the effect of bromine on the physical and flammability properties of this resin. The results of these studies showed that brominated phenol–formaldehyde resin of 10% bromine content by weight of the phenol–formaldehyde resin was suitable to be used as a plywood adhesive. The optimal compressing temperature and compressing time were 110°C and 30 min, respectively. The prepared plywood obtained from the optimal condition gave a high shear strength, good flame retardancy, and good resistance to both hot and cold water. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1918–1924, 2003     

13C‐NMR study on cure‐accelerated phenol–formaldehyde resins with carbonates

Both liquid‐ and solid‐state ¹³C‐NMR spectroscopies were employed to investigate the cure‐acceleration effects of three carbonates [propylene carbonate (PC), sodium carbonate (NC), and potassium carbonate (KC)] on liquid and cured phenol–formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure‐acceleration mechanism in the PC‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, while the addition of both NC and KC into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C‐CP/MAS‐NMR spectroscopy was smaller for the cure‐accelerated PF resins than for that of the control PF resin. The result indicated that cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 841–851, 2000     

Novel two‐stage phenol–formaldehyde resol resin synthesis

The 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     

Influence of acrylamide copolymerization of urea–formaldehyde resin adhesives to their chemical structure and performance

To lower the formaldehyde emission of wood‐based composite panels bonded with urea–formaldehyde (UF) resin adhesive, this study investigated the influence of acrylamide copolymerization of UF resin adhesives to their chemical structure and performance such as formaldehyde emission, adhesion strength, and mechanical properties of plywood. The acrylamide‐copolymerized UF resin adhesives dramatically reduced the formaldehyde emission of plywood. The ¹³C‐NMR spectra indicated that the acrylamide has been copolymerized by reacting with either methylene glycol remained or methylol group of UF resin, which subsequently contributed in lowering the formaldehyde emission. In addition, an optimum level for the acrylamide for the copolymerization of UF resin adhesives was determined as 1%, when the formaldehyde emission and adhesion strength of plywood were taken into consideration. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Properties of phenol–formaldehyde resin modified with organic acid esters

Properties of liquid and cured phenol–formaldehyde (PF) resin modified with esters were analyzed in this study. Esters with different carbon chain lengths, both in the acid and alcohol groups, were applied in the experiments. It was found that the modification of phenolic resin with applied esters does not deteriorate its pot life at the temperature of 20°C. It results in an increase of its reactivity at higher temperatures, manifested in the shortening of gel time at 130°C and a decrease of activation energy. Results of FTIR tests of polycondensed modified PF resin showed that products of alkali hydrolysis of esters not only catalyze the curing reaction of resin, but also become embedded in its structure. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Adsorptive removal of uranium from water by sulfonated phenol–formaldehyde resin

Adsorption characteristics of a sulfonated phenol‐formaldehyde resin (SPR) have been studied for U removal from aqueous solution by means of batch method. Adsorption experiments have been carried out as a function of contact time, solution/adsorbent ratio, particle size and pH. Adsorption isotherm has been evaluated by changing adsorbent dosage in the range of 0.04–80 g/L at an initial uranyl nitrate concentration of 0.05 mol/L. The enormous adsorption capacity of 0.29 mol/g estimated from the plateau region of the S shaped isotherm is well comparable the Langmuir capacity of 0.31 mol/g. Equilibrium data are also adequately well described by the Freundlich and the Dubinin‐Radushkevich (D‐R) isotherm equations. The parameters of the isotherms and pH dependency of distribution coefficients (K_D) indicate that polymeric uranyl chains form on bidentate surface complex as a result of solute–solute interactions on the adsorbent surface. Both desorption and elution studies show that uranyl chains are irreversibly bounded on the SPR. Kinetic curves having a fast initial part followed by a slower process well fit both McKay model based on two‐resistance diffusion and Nernst‐Plank model with single diffusion coefficient. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Time–temperature–transformation cure diagrams of phenol–formaldehyde and lignin–phenol–formaldehyde novolac resins

In this study, the time–temperature– transformation (TTT) cure diagrams of the curing processes of several novolac resins were determined. Each diagram corresponded to a mixture of commercial phenol–formaldehyde novolac, lignin–phenol–formaldehyde novolac, and methylolated lignin–phenol–formaldehyde novolac resins with hexamethylenetetramine as a curing agent. Thermomechanical analysis and differential scanning calorimetry techniques were applied to study the resin gelation and the kinetics of the curing process to obtain the isoconversional curves. The temperature at which the material gelled and vitrified [the glass‐transition temperature at the gel point (_gelT_g)], the glass‐transition temperature of the uncured material (without crosslinking; T_g0), and the glass‐transition temperature with full crosslinking were also obtained. On the basis of the measured of conversion degree at gelation, the approximate glass‐transition temperature/conversion relationship, and the thermokinetic results of the curing process of the resins, TTT cure diagrams of the novolac samples were constructed. The TTT diagrams showed that the lignin–novolac and methylolated lignin–novolac resins presented lower T_g0 and _gelT_g values than the commercial resin. The TTT diagram is a suitable tool for understanding novolac resin behavior during the isothermal curing process. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Removal of basic dyes from aqueous solutions with sulfonated phenol–formaldehyde resin

In this study, we aimed to investigate the removal of basic dyes, including Safranin T (ST), Nile Blue A (NBA), and Brilliant Cresyl Blue (BCB), from aqueous solution with a sulfonated phenol–formaldehyde resin. This sulfonated resin was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. The adsorption properties of this resin were investigated under different adsorption conditions with different initial dye concentrations, contact times, and pH values. The adsorption equilibrium data were analyzed with Langmuir and Freundlich models. The adsorption behaviors of ST, NBA, and BCB onto the sulfonated resol‐type phenol–formaldehyde resin were better described by the Freundlich model. The adsorption capacities of the sulfonated resol resin for ST, NBA, and BCB decreased in the following order: NBA > BCB > ST. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

H2O2 as a modifier of phenol–formaldehyde resin used in the production of particleboards

The purpose of the research was to study the influence of H₂O₂ on the properties of fluid phenolic (PF) resin, the curing process, the cured resin structure, and the properties of the particleboards produced with its use. The influence of added H₂O₂ on resin usability at 20°C, on the gel time of the modified PF resin in the temperature range 110–140°C, and on the activation energy of the curing process were studied. Also, the structure of the cured resin was examined by Fourier transform infrared spectroscopy. Finally, the properties of the obtained particleboards were determined. The results indicate that the H₂O₂ modification leads to greater reactivity of the phenolic resin and increases the mechanical properties of particleboards. In contrast, there is no significant influence of H₂O₂ on the water resistance of the particleboards. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3084–3092, 2003     

Matrix‐assisted laser desorption/ionization mass spectrometry of synthetic polymers. VI. Analysis of phenol–urea–formaldehyde cocondensates

Phenolic resoles can be regarded as copolymers of phenol and formaldehyde that are distributed in the chain length and the number of methylol groups per molecule. While other spectroscopic methods like FTIR and NMR only give average structures, MALDI–TOF mass spectrometry is able to resolve the oligomer distribution of phenolic resoles. Using 2,5‐dihydroxybenzoic acid or 2,4,6‐trihydroxyacetophenone as matrices, MALDI–TOF spectra are obtained where each oligomer peak can be assigned to a particular chemical structure. Thus, the degree of polymerization and the number of reactive methylol groups can be determined. For urea‐modified resoles, in addition to phenol–formaldehyde and urea–formaldehyde structures, for the first time, phenol–urea–formaldehyde cocondensate structures can be identified directly. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2540–2548, 2003     

Hydroxymethylation and polycondensation reactions in urea–formaldehyde resin synthesis

Formaldehyde–urea (F/U) reaction products with molar ratios of 1.8, 2.1, and 2.4 were synthesized at pH 8.3, and the last one also at pH 4.5 using 45% formaldehyde aqueous solution. For obtaining the resin, the synthesis of F/U 2.1 was continued by acid‐catalyzed condensation at pH 4.5 and posttreatment with second part of U (F/U 1.05/1) at 70°C and pH 8.3. The products were analyzed using ¹³C‐NMR spectrometry. Higher excess of F increases the dihydroxymethyl content on account of smaller dimethylene ether content. Certain ¹³C chemical shifts in carbonyl and methylene region of spectra were assigned to trishydroxymethylurea, being the main trisubstituted urea compound in hydroxymethylated product. Acid catalyst promotes the formation of methylene groups by polycondensation of hydroxymethyl groups, against the background of similar content of dimethylene ethers in both catalytic conditions. The ratio of linear/branched chains is emphasized in characterizing the resin structure. Higher hydroxymethyl content in acid‐catalyzed polycondensation is an advantage of three‐step synthesis technology. The amount of binding methylene and dimethylene ether groups linked only to secondary amino groups can be increased by transhydroxymethylation with subsequent polycondensation in posttreatment with U in suitable reaction conditions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1673–1680, 2006     

Characterization of phenol–urea–formaldehyde resin by inline FTIR spectroscopy

Phenol–urea–formaldehyde (PUF) resins were synthesized by a two‐step polymerization process. The first step was the synthesis of 2,4,6‐trimethylolphenol (TMeP) from phenol and formaldehyde, under alkaline conditions. In the second step PUF resins were synthesized by the reaction of TMeP with urea, under acidic and alkaline conditions. The influence of temperature on the synthesis of TMeP was investigated. The molar ratio between TMeP and urea was varied to study the composition effect on the second step of the PUF synthesis and final product properties. Synthesis of TMeP and PUF resins were monitored by inline FTIR‐ATR system. Analytical methods, such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, and infrared spectroscopy were used for characterization of TMeP and PUF resins. Obtained PUF resins were cured and tested on flexural strength. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Synthesis–structure–performance relationship of cocondensed phenol–urea–formaldehyde resins by MALDI‐ToF and 13C NMR

Matrix assisted laser desorption ionization time of flight (MALDI‐ToF) mass spectrometry has consistently confirmed on a number of PUF resins that phenol–urea cocondensates exist in phenol–urea–formaldehyde (PUF) resins. A noticeable proportion of methylene‐linked phenol to urea cocondensates were detected in all the PUF resins tried, alongside methylene bridges connecting phenol to phenol and urea to urea. The PUF, PF, and UF oligomers formed were identified. Variations of the PUF preparation procedure did always yield a certain proportion of the mixed phenol to urea cocondensates. Their relative proportion was determined and related the synthesis procedure used. Comparison of the MALDI‐ToF results with a ¹³C NMR investigation showed that in a real PUF resin in which phenol to urea cocondensates were identified the methylene bridge NMR signal at 44 ppm, characteristic of phenol to urea unsubstituted model compound cocondensates, does not appear at all. This confirmed that this peak cannot be taken as an indication of the existence of phenol and urea condensation under actual resin preparation conditions. The peak indicating cocondensation in PUF resins in which the phenolic nuclei and urea are substituted appears instead at 54.7–55.0 ppm. Thermomechanical analysis has again confirmed that the resin gel times greatly accelerates with increasing urea molar content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

13C‐NMR study on cure‐accelerated phenol‐formaldehyde resins with carbonates

Both liquid‐ and solid‐state carbon‐13–nuclear magnetic resonance (¹³C‐NMR) spectroscopies were used to investigate the cure acceleration effects of three carbonates (propylene carbonate, sodium carbonate, and potassium carbonate) on liquid and cured phenol‐formaldehyde (PF) resins. The liquid‐phase ¹³C‐NMR spectra showed that the cure acceleration mechanism in the propylene carbonate‐added PF resin seemed to be involved in increasing reactivity of the phenol rings, whereas the addition of both sodium carbonate and potassium carbonate into PF resin apparently resulted in the presence of ortho–ortho methylene linkages. Proton spin‐lattice rotating frame relaxation time (T_1ρH) measured by solid‐state ¹³C cross polarization/magic‐angle spinning NMR spectroscopy was smaller for the cure‐accelerated PF resins than that of the control PF resin. The result indicated that the cure‐accelerated PF resins are less rigid than the control PF resin. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1284–1293, 2000     

Effects of urea and curing catalysts added to the strand board core‐layer binder phenol–formaldehyde resin

Effects of adding urea to the strand board core‐layer phenol–formaldehyde (PF) resin were investigated in conjunction with cure‐accelerating catalysts. Ten percent urea based on the liquid resin weight was added at the beginning, at three different middle stages of polymerization, and at the end of PF resin synthesis. No significant cocondensation between the urea and PF resin components occurred as identified by ¹³C NMR analyses, which corroborated well with the curing and strand board bonding performance test results. The various urea addition methods resulted in resins that slightly differ in the various tests due to the urea's temporary holding capacity of formaldehyde. The preferred method of urea addition was found to do it in the later part of PF resin synthesis for convenience, consistency, and slightly better overall performance. Some cure‐accelerating catalysts were shown to reduce the thickness swelling of strand boards. This study showed the usefulness of adding some urea to strand board core‐layer binder PF resins of replacing higher cost phenolic components with lower cost urea. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Curing of phenol‐formaldehyde resin mixed with wood preservatives

The effects of preservatives used for glue‐line treatment on the curing of phenol‐formaldehyde resin (PF) were studied by dynamic mechanical analysis and differential scanning calorimetry. Storage modulus, G′, loss modulus, G″, and loss tangent, tan δ, of PF with and without preservatives were recorded as a function of time under isothermal heating. The time required for G′, G″, and tan δ to reach steady values increased with addition of preservative. The G′, G″, and tan δ curves of PF containing benzyl alcohol (used in the preservative as a diluent) were almost identical to those of PF containing preservative. However, the addition of antitermite and anti‐fungal compounds alone had no effect on the curing process. There were no differences in total reaction enthalpy or dependence of activation energy on degree of conversion between pure PF and mixtures. Our results indicate that benzyl alcohol in preservatives plasticizes the curing system for PF. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Controlled synthesis of high‐ortho‐substitution phenol–formaldehyde resins

The relationship between the use of 19 kinds of metal catalysts and the proportion of ortho–ortho links of novolac resins was studied. The proportion of ortho–ortho links of novolac resins was characterized with Fourier transform infrared, ¹H‐NMR, and ¹³C‐NMR. The effects of different catalysts and different reaction conditions, such as the molar ratio of phenol to formaldehyde, the pH value of the reaction, and the reaction time, were examined. Phenol–formaldehyde resins were synthesized with a certain proportion of the ortho position through the adjustment of the reaction conditions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 652–658, 2005