Engineering biomass into formaldehyde‐free phenolic resin for composite materials

The use of formaldehyde to prepare phenol‐formaldehyde (PF) resins is one of the primary challenges for the world‐wide PF industry with respect to both sustainability and human health. This study reports a novel one‐pot synthesis process for phenol‐5‐hydroxymethylfurfural (PHMF) resin as a formaldehyde‐free phenolic resin using phenol and glucose, and the curing of the phenolic resin with a green curing agent organosolv lignin (OL) or Kraft lignin (KL). Evidenced by ¹³C NMR, the curing mechanism involves alkylation reaction between the hydoxyalkyl groups of lignin and the ortho‐ and para‐carbon of PHMF phenolic hydroxyl group. The curing kinetics was studied using differential scanning calorimetry and the kinetic parameters were obtained. The OL/KL cured PHMF resins were tested in terms of thermal stability, and mechanical properties for their applications in fiberglass reinforced composite materials. The results obtained demonstrated that OL/KL can be promising curing agents for the PHMF resins. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1275–1283, 2015     

Nonisothermal curing of a solid resole phenolic resin

A commercial solid resole phenolic resin was thoroughly characterized with Fourier transform infrared spectroscopy, NMR, and gel permeation chromatography, and its nonisothermal curing reaction was studied systematically with differential scanning calorimetry at a series of heating rates (βs) of 3, 4.5, 5.7, and 10°C/min. The results show that the solid resole had a higher molecular weight than conventional liquid resoles, and its reactive hydroxymethyl (CH₂ OH) and dibenzyl ether (CH₂ O CH₂) functionalities participated in the crosslinking reaction upon heating. The nonisothermal curing reaction of the solid resole exhibited a relatively constant reaction heat, whereas the onset, peak, and end curing temperatures increased gradually with increasing βs. In addition, the reaction kinetics of the solid resole was analyzed with an nth‐order reaction model, the global activation energy was determined with the Kissinger method, and the reaction order was derived from the Crane equation. The obtained rate equation was applied to simulate the reaction time, conversion, and reaction rate, with a good fit achieved between the experimental data and the model predications. In conclusion, this study provided us with new knowledge on solid resoles at a molecular level and was also a great help for the curing procedure design, property optimization, and practical application of this commercial solid resole. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Polymerization of resole resins with several formaldehyde/phenol molar ratios: Amine catalysts against sodium hydroxide catalysts

Triethylamine and sodium hydroxide catalyzed phenol/formaldehyde resole resins were investigated in terms of their behavior during both addition and polymerization reactions. Amine‐catalyzed prepolymers were mainly ortho‐substituted structures, whereas the sodium hydroxide catalyst directed the addition reaction to para reactive sites. During polymerization, triethylamine led to dimethylene ether bridges as the principal linkages between aromatic structures, increasing their final concentration as the starting hydroxymethyl group concentration increased. In contrast, the use of sodium hydroxide reduced dramatically the dimethylene ether bridge concentration, favoring methylene bridge formation. The influence of hydroxyl ions on the stability of quinone methide intermediates could be the reason for those differences. Despite the formation of dimethylene ether bridges, at higher curing temperatures, more oxidized groups started to appear in cured resoles when the formaldehyde/phenol molar ratio was higher. The presence of infrared bands associated with quinones, aldehydes, and/or carbonyl groups, mainly in high‐formaldehyde‐content resins, could indicate a direct oxidation process from dimethylene ether bridges and/or residual hydroxymethyl groups, without the formation of methylene bridges. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2623–2631, 2006     

Graphitization of phenolic resins for carbon-based refractories

The chemical resistance and thermo-mechanical properties of refractories bonded with resole or novolak resins depend on the presence of crystalline carbon phases (preferentially with features close to graphite ones) in their compositions. Although thermosetting resins are commonly classified as non-graphitizing carbon sources, many efforts have been made in recent years in order to find effective routes to induce the in situ graphitization of such components in refractory products during service. This work evaluates the role of processing parameters (mixing, curing and firing temperature) and additives (ferrocene, boric acid and exfoliated graphite) in the graphitization process of two commercial resins (resole and novolak) and a synthesized one (modified-novolak). X-ray diffraction, Raman spectroscopy and thermogravimetric analyses were carried out to identify the microstructural evolution of the compositions. According to the results, carbon graphitization was already detected after firing the samples at 1000 °C for 5 h under reducing atmosphere. Ferrocene addition favored a more effective graphitization of the selected resins, but H₃BO₃ also induced the rearrangement of the carbon derived from the commercial novolak product. The mixing and curing procedures used when preparing the compositions proved to be very important steps as they affected, to a greater extent, the resulting graphitization degree of the fired samples.     

Relationship between curing activation energy and free formaldehyde content in urea-formaldehyde resins

This study investigated the effect on the curing behavior, activation energy (E _a) of the curing reaction, crystalline structure, crosslinking, and free formaldehyde content of the addition of the following scavengers in urea-formaldehyde (UF) resins: medium density fiber board flour, rice husk flour, silica powder, and tannin powder. The scavenger content was 3 and 7?wt% of the UF resin solid content. The curing behavior of UF resins was monitored by differential scanning calorimetry, thermogravimetric analysis, and X-ray crystallography. The curing E _a was correlated to the free formaldehyde content of the scavenger containing UF resins. The thermal stability of the UF resins increased but the curing E _a decreased with increasing scavenger content. After curing, the crystallinity of the UF resins decreased in the presence of scavengers. The unreacted free formaldehyde content was reduced in the tannin powder containing UF resins. The degree of crosslinking affects the formaldehyde emission from wood panels bonded with UF resin. This is especially true for wood panels in service for long periods of time and exposed to high humidity conditions. Once the free formaldehyde which influences considerably the emission has disappeared, the presence of the –CH₂– groups then becomes important. Hence, an increased resin crosslinking indicates a higher concentration of –CH₂– groups present, which may hydrolyze and emit formaldehyde slowly over time.     

Physicomechanical and thermal properties of moldings made from liquefied wood‐based novolak PF resins under various hot‐pressing conditions

The wood powder of Cryptomeria japonica (Japanese cedar) was liquefied in phenol, with H₂SO₄ and HCl as a catalyst. The liquefied wood was used to prepare the liquefied wood‐based novolak phenol formaldehyde (PF) resins by reacting with formalin. Furthermore, novolak PF resins were mixed with wood flour, hexamethylenetetramine, zinc stearate as filler, curing agent, and lubricating agent, respectively, and hot‐pressed under 180 or 200°C for 5 or 10 min to manufacture moldings. The results showed that physicomechanical properties of moldings were influenced by the hot‐pressing condition. The molding made with hot‐pressing temperature of 200°C for 10 min had a higher curing degree, dimensional stability, and internal bonding strength. The thermal analysis indicated that using a hot‐pressing temperature of 180°C was not sufficient for the liquefied wood‐based novolak PF resins to completely cure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Commercial lignosulfonates from different sulfite processes as partial phenol replacement in PF resole resins

Four commercial spruce lignosulfonates representing the most common acidic, neutral, and alkaline sulfite pulping processes and varying significantly in molecular weight characteristics were tested as partial (40 wt %) phenol substitute materials for the manufacture of lignosulfonate‐phenol‐formaldehyde (LPF) resole resins. Similar as recently reported for technical lignins from nonsulfite pulping processes (kraft, soda, organosolv), all lignosulfonates of this study effectuated a faster viscosity gain during resole cooking compared to the lignin‐free reference resin (1000 mPa s after 120 min vs. 250 min to reach 1000 mPa s). Sodium lignosulfonate featuring the lowest weight average molecular weight (M_w 5780 g mol^?1) and dispersity (Ð 6.1) turned out to be superior to the other lignosulfonates with regard to curing rate (B‐time; 3:37 min vs. 6:41–9:08 min) and tensile shear strength development under hot pressing (120 °C; T_S,max = 5.64 N mm^?2 after 8 min) for beech veneer strips glued together with the respective LPF resins. Calcium and magnesium lignosulfonates are less suited with regard to phenol replacement due to the poor performance of the respective LPF adhesives in terms of tensile shear strength (T_S,max = 3.29–3.49 N mm^?2 after 12 min) most likely caused by considerable amounts of side products formed in the course of formose‐type reactions. Phenolation of the two promising lignosulfonates, that is, sodium and ammonium lignosulfonate, did neither considerably increase the rate of PF network formation during resin cooking and curing nor improve tensile strength development during hot pressing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45893.     

Model‐free kinetics: Curing behavior of phenol formaldehyde resins by differential scanning calorimetry

Isoconversional analysis was used to treat nonisothermal DSC data and yield the dependence of activation energy on conversion during the curing process of PF resins. The shape of the dependence revealed that the curing process of PF resins displayed a change in the reaction mechanism from a kinetic to a diffusion regime. In the kinetic regime a comparative DSC experimental analysis between monomer mixtures and PF resins showed that the addition reactions between phenol and formaldehyde had been mostly completed during the synthesis of PF resins and that the main kinetic reactions contained parallel condensations in the curing process. For the diffusion regime a modified equation for the diffusion rate constant, k_D = D₀ exp(?E_D /RT + K₁α + K₂α²), is proposed. This equation is in good agreement with the experimental dependence of E_α on α in the diffusion regime, which shows the effect of both temperature and conversion on diffusion. A prediction of the conversion advancement with the reaction time under isothermal condition for PF resin has been made. This prediction can be useful in practical applications for evaluating isothermal behavior of thermosetting systems from nonisothermal experimental data. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 433–440, 2003     

Cure Characteristics of Phenol-Formaldehyde Resin Catalyzed with Ba(OH)2

Phenol-formaldehyde resins are high performance wood adhesives, and are also used to impregnate base papers which are manufactured high-pressure laminates applied in furniture and interior decoration. In this investigation, PF resins catalyzed with Ba(OH)₂ were studied. Both Ba(OH)₂ content and F/P molar ratio were found to influence the cure rate. The cure rate of the Ba(OH)₂ catalyzed PF resin was faster by 50% than that of ordinary PF resin at 150°C. The DSC results showed that the Ba(OH)₂ catalyzed PF resin was cured at a lower temperature than the ordinary PF resin. And the IR results showed that the Ba(OH)₂ catalyzed PF resin has higher degree of ortho-position coupling.     

Characterization and performance of melamine enhanced urea formaldehyde resin for bonding southern pine particleboard

Urea‐formaldehyde resins modified by melamine were synthesized by four catalysts (H₂SO₄, HCl, H₃PO₄, and NaOH/NH₄OH) with a F/U/M molar ratio of 1.38/1/0.074. Resin structure and thermal behavior were studied by ¹³C‐NMR and DSC techniques. For H₂SO₄, HCl, and H₃PO₄ catalysts, resins were prepared by two stage pH adjustment: the first pH stage was set at 1.25 (H₃PO₄ pH 1.60) and second pH stage was set at 5.0. For the NaOH/NH₄OH catalyst, the resin was set at pH 5.0 from the start. Of the four catalysts, HCl catalyzed resins, with the highest free urea and lowest free formaldehyde, consistently yielded the lowest formaldehyde emission; NaOH/NH₄OH catalyst resulted in the best IB strength tested at dry conditions and also after 24 h cold water soak and the lowest water absorption and thickness swell. The resins catalyzed with H₃PO₄ had the highest free formaldehyde and no free urea yielding the highest formaldehyde emission. Each DSC thermogram was proceeded by a weak exothermic peak and followed by an obvious endothermic peak. The exothermic peak temperatures were 125.0, 131.1, 111.4, and 125.2°C, and endothermic peak temperatures were 135.8, 147.6, 118.9, and 138.4°C, respectively, for H₂SO₄, HCl, H₃PO₄, and NaOH/NH₄OH catalysts. The close proximity of the peak temperatures of the exothermic and endothermic reactions strongly suggests that there is potential interference of heat flow between the exothermic and endothermic reactions which may impact resin curing. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

C NMR study on structure, composition and curing behavior of phenol-urea-formaldehyde resole resins

Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state ¹³C NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network.     

Synthesis of urea-formaldehyde resins: NMR studies on reaction mechanisms

The structural changes which occur during the two step condensation of the liquid urea-formaldehyde resins (UF) have been studied by NMR spectroscopy combined with chemical analyses. In the first step of the condensation (at 95°C and mildly acid reaction), the reaction takes place quickly with the formation of methylene linkages (?N? CH₂? N?) and, to a minor extent, of methylol groups (?N? CH₂OH). In the second step (at room temperature and mildly alkaline reaction) the reaction goes on very slowly, with the formation only of methylol groups. At the end the oxymethylene formaldehyde is only present as ?N? CH₂OCH₂? N? groups.     

Modification of phenol–formaldehyde resol resins by lignin,starch, and urea

Lignin‐based chemicals, starch, and urea were used as modifiers for phenol–formaldehyde resol resins. The effects of the addition stage of the modifiers used in the synthesis of the resins and the type of modification reagent on the structures of the resins and their molar masses and reactivities were investigated. The modifications with corn starch and lignin promoted condensation; this was verified by increased molar masses and high ratios of methylene bridges to the sum of free ortho and para aromatic groups with respect to the corresponding reference resin without a modification reagent. The later the modifier was added to the resin condensation mixture, the more methylene bridges were formed with respect to the amounts of free ortho and para aromatic groups. In addition, when urea or wheat starch was added in the later condensation stage, the final condensation also reached high stages. The modifications with lignosulfonate and starch, as well as the early addition of urea, enhanced p–p′ bridge structures. The lowest condensation stage and, therefore, the highest reactivity were found when wheat starch was added with the starting reagents. The curing heat of the wheat‐starch‐modified resins decreased according to the deferred addition point of starch. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 582–588, 2003     

Improving the curing process and thermal stability of phthalonitrile resin via novel mixed curing agents

High curing temperature (including post‐curing temperature) and long curing time of phthalonitrile resins make them thermally stable but difficult to process. In this paper, novel mixed curing agents (CuCl/4,4′‐diaminodiphenylsulfone (DDS) and ZnCl₂/DDS) were firstly designed for solving these problems. Bisphenol‐based phthalonitrile monomer (BP‐Ph; melting point: 228–235 °C) was synthesized and used as the curing precursor. Differential scanning calorimetry results indicated that BP‐Ph cured with CuCl/DDS and ZnCl₂/DDS exhibited curing temperatures close to the melting point of BP‐Ph with curing ending temperatures of 225.4 and 287.1 °C, respectively. Rheologic investigations demonstrated obvious curing reactions of BP‐Ph occurred with the mixed curing agents at 220 °C. Thermogravimetric analysis showed that BP‐Ph cured by CuCl/DDS or ZnCl₂/DDS maintained 95% mass at 573 or 546 °C, respectively, at a post‐curing temperature of 350 °C for 2 h. Reasonable long‐term thermo‐oxidative stability was also demonstrated. When the post‐curing temperature decreased to 290 °C, char yield at 800 °C of BP‐Ph cured by CuCl/DDS was 77.0%, suggesting the curing procedure can be milder when using mixed curing agents. © 2017 Society of Chemical Industry     

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

The 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     

Novel epoxy resins based on cyclohexanone-aldehyde condensation products

Solid ketone-formaldehyde resins are used in certain coating formulations in order to improve hardness, gloss, and light stability. They are soluble, thermoplastic by nature, and contain limited amounts of hydroxyl groups. We found that their primary hydroxyls can be etherified with epichlorohydrin (ECH) either by a two-step ECH-addition/dehydrohalogenation procedure or by a one-step phase-transfer process. An intermediate of particular usefulness is the crystalline 2,2,6,6-tetramethylol-cyclohexanol (TMCH) made from cyclohexanone and 5 mol formaldehyde, yielding low colored epoxy resins with epoxy values up to 7.5 eq/kg. Depending upon the nature of the curing agent, high T_g solids as well as tough and flexible coatings with good outdoor stability can be made. Upon decreasing the formaldehyde–cyclohexanone ratio, solid condensation polymers melting up to 150°C can be obtained. Phase-transfer glycidylation yielded solid thermoset glycidyl ether resins with M?_n up to 1600, M?_ω up to 13,000, epoxy values up to 3.6 eq/kg, and softening points between 80 and 160°C. Powder coatings formulated with carboxyterminated polyesters are hard, glossy, solvent-resistant but somewhat brittle. In order to overcome this drawback, polycycloacetals have been produced from TMCH and glutardialdehyde, which are terminated by pairs of methylol groups. Powder coatings of the corresponding glycidylethers with carboxyl-terminated polyesters exhibited excellent flexibility and impact strength.     

Preparation of cardanol–formaldehyde resins from cashew nut shell liquid for the reinforcement of natural rubber

Natural rubber was reinforced with a high loading of a cardanol–formaldehyde resin prepared from cashew nut shell liquid. Cardanol–formaldehyde resins, both resoles and novolaks, were synthesized from cardanol, which was extracted from cashew nut shells. This was done by the condensation polymerization of cardanol and formaldehyde in the presence of base and acid catalysts. The cardanol–formaldehyde resole with the highest yield (ca. 75%) was prepared with a formaldehyde/cardanol molar ratio of 2.0 at pH 8.0 and 90°C for 8 h. The cardanol–formaldehyde novolak with the highest yield (ca. 80%) was prepared with a formaldehyde/cardanol molar ratio of 0.8 at pH 2.2 and 100°C for 7 h. Fourier transform infrared and ¹³C‐NMR were employed to characterize the chemical structures of the obtained cardanol–formaldehyde resins. The resins were compatible with natural rubber in various formulations. The cured behaviors of natural rubber blended with the cardanol–formaldehyde resole and novolak resins were investigated. The cured behaviors of cardanol–formaldehyde resole and cardanol–formaldehyde novolak samples were different, reflecting differences in their chemical reactivities. Furthermore, the incorporation of cardanol–formaldehyde resins into natural rubber provided significant improvements in mechanical properties such as the hardness, tensile strength, modulus at 100 and 300% elongation, and abrasion resistance. However, the elongation at break and compression set of the blends decreased as expected. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1997–2002, 2007     

Studies on UV‐stable silicone–epoxy resins

Silicone–epoxy resins were synthesized through hydrosilylation of 1,2‐epoxy‐4‐vinyl‐cyclohexane with 1,3,5,7‐tetramethycyclotetrasiloxane. The silicone–epoxy resins showed high reactivity in the presence of aluminum complex/silanol compound catalysts. Curing of the resins was effected at extremely low concentrations of the aluminum acetylacetonate/Ph₂Si(OH)₂ catalyst to give hard materials with optical clarity. For the silicone–epoxy resins containing Si? H bonds, Al(acac)₃ alone is effective for the curing. The cured silicone–epoxy resins showed excellent UV resistance. An improvement in the lifetime of UV‐LEDs was achieved using the silicone–epoxy compositions as encapsulant. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3954–3959, 2007     

Effect of Synthesis Conditions on the Structure and Curing Characteristics of High-Urea Content PUF Resin

Phenol-urea-formaldehyde (PUF) resins of high urea content were prepared at different hydroxyl/phenol (OH/P) mole ratios and formaldehyde/(phenol + urea) [F/(P + U)] mole ratios. The effect of synthesis parameters including OH/P and F/(P + U) mole ratios on the structure, composition, and curing characteristics of PUF resins were investigated by using both liquid ¹³C nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC). The NMR analysis indicated that an increase in the OH/P mole ratio and/or F/(P + U) mole ratio decreased the amount of unreacted urea and monosubstituted urea, and promoted the formation of polysubstituted urea. The DSC results showed that the higher OH/P mole ratio and/or F/(P + U) mole ratio of PUF resins resulted in a lower curing temperature. The F/(P + U) mole ratio of PUF resins seemed to have a more significant accelerating effect on the curing reaction than the OH/P mole ratio.     

On the chemistry,behavior, and cure acceleration of phenol–formaldehyde resins under very alkaline conditions

The curing acceleration by organic esters of alkaline phenol-formaldehyde (PF) resins can be explained by two different mechanisms. In the course of the work, the unexpected curing behavior of PF resins alone, under very alkaline conditions, was observed, which diverged from past assumptions. PF resins curing, which is supposed to be accelerated by the formation of phenate ions, is instead markedly slowed down by increasingly alkaline pHs and accelerates in the presence of esters. The mechanism of ester acceleration of PF resins curing was clarified and the effect of different esters was quantified and was related to the pK_a and quantity of the acid forming the ester. A second mechanism, which appeared also to explain the peculiar slowing down at increasing pH of PF resin curing, could not be confirmed; some evidence appeared clearly to deny it. The impossibility of isolating an Na⁺ phenol ring complex, theorized by other authors and other evidence, indicates that this second mechanism is unlikely to occur. © 1993 John Wiley & Sons, Inc.