Solution‐state NMR analysis of hydroxymethylated resorcinol cured in the presence of crude milled‐wood lignin from Acer saccharum

Resorcinol‐formaldehyde adhesives can reinforce stress fractures that appear from wood surface preparation. Researchers have found that applying the resorcinol‐formaldehyde prepolymer, hydroxymethylated resorcinol, to the surface of wood improves the bond strength of epoxy and polyurethane adhesives to wood. Hydroxymethylated resorcinol is thought to plasticize lignin components and stabilize stress fractures through reactions with lignin subunits and hemicelluloses in wood. In this study, a dilute solution of hydroxymethylated resorcinol (HMR) is cured in the presence of a crude milled‐wood lignin (cMWL) from Acer saccharum and subsequently dissolved in dimethylsulfoxide‐d₆ to delineate reactivity with lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan using solution‐state NMR spectroscopy. ¹H–¹³C single‐bond correlation NMR experiments revealed that the HMR only formed 4,4′‐diarylmethane structures with itself in the presence of the cMWL; the 2‐methylols that formed remained free and did not crosslink with resorcinol. Cured HMR resin formed both 4,4′‐ and 2,4‐diarylmethane structures, confirming that the presence of lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan hinders crosslinking at the C‐2 position. No evidence of reactivity between HMR and lignin subunits was found. New peaks consistent with ester linkages were observed in ¹³C‐NMR spectra of the cMWL sample treated with HMR that may be attributable to HMR moieties condensing with glucuronic acid substituents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45398.     

Effect of F/P and OH/P molar ratios and condensation viscosity on the structure of phenol‐formaldehyde resol resins for overlays—A statistical study

A two‐level full factorial experimental design with three variables, formaldehyde‐to‐phenol (F/P) molar ratio, hydroxyl‐to‐phenol (OH/P) molar ratio, and condensation viscosity was implemented to determine the effect of the variables on the structure of phenol‐formaldehyde resol resins for paper overlay impregnation. Ten resins were prepared with F/P molar ratios between 1.9 and 2.3, OH/P molar ratios between 0.09 and 0.13, and condensation viscosities between 60 and 180 mPa s. The effect of these three independent variables on the chemical structure was analyzed by ¹³C‐NMR spectroscopy, on the molecular weight distribution by gel permeation chromatography, and on the reactivity by differential scanning calorimetry. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2942–2948, 2004     

Nanoencapsulation mechanism of fragrant oil: effect of formaldehyde − melamine molar ratio

Encapsulation of essential oils by in situ polymerization is commonly used to contain the oil and thus ensure its controlled release. Melamine resin formaldehyde is one of the most widely used shell materials due to its thermal and chemical stability. One of the factors that influences the properties of the capsules is the molar relationship between monomers. The effect of formaldehyde ? melamine (F/M) molar ratios 3, 4 and 6 on the nanoencapsulation, morphology and properties of nanocapsules was investigated. The morphology and particle size were investigated by the scanning electron microscopy and atomic force microscopy techniques. The composition of the formaldehyde ? melamine resins was determined by Fourier transform infrared spectroscopy, and the thermal stability of the nanocapsules was analysed by differential scanning calorimetry and thermogravimetric analysis. Increasing the formaldehyde content reduced the nanocapsules' chemical stability. The capsule sizes obtained were nanometric at all melamine ? formaldehyde ratios studied, with a non‐significant variation in particle size and shape. © 2017 Society of Chemical Industry     

Thermoplastic and moisture-dependent behavior of lignin phenol formaldehyde resins

Bonding kinetics of thermosetting adhesives is influenced by a variety of factors such as temperature, humidity, and resin properties. A comparison of lignin-based phenol formaldehyde (LPF) and phenol formaldehyde (PF) adhesive in terms of reactivity and mechanical properties referring to testing conditions (temperature, moisture of specimen) were investigated. For this purpose, two resins were manufactured aiming for similar technological resin properties. The reactivity was evaluated by B-time measurements at different temperatures and the development of bonding strength at three different conditions, testing immediately after hot pressing, after applying a cooling phase after hot pressing, or sample conditioning at standard climate. In addition, the moisture stability of the two fully cured resins was examined. The calculated reactivity index demonstrated that LPF requires more energy for curing than PF. Further results indicate that lignin as substituent for phenol in PF resin has a negative impact on its moisture resistance. Additionally, the known thermoplastic behavior of lignin could also be detected in the behavior of the cured resin. This behavior is relevant for the adhesive in use and necessitates a cooling phase before testing the bonding strength development of lignin-based adhesive systems. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48011.     

The effect of formaldehyde/phenol (F/P) molar ratios on function and curing kinetics of high‐solid resol phenolic resins

A series of high‐solid resol phenolic resins (HSRPRs) were synthesized with different molar ratios (1.6, 1.8, 2.0, 2.2, and 2.4) of formaldehyde to phenol using calcium oxide and sodium hydroxide as catalyst. The effects of F/P molar ratios on physical properties, free formaldehyde and phenol, activity, structure, and thermally resistant properties of HSRPRs were fully investigated by chemical assays, liquid and solid ¹³C‐NMR, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The curing kinetics of different F/P molar ratios were explored with differential scanning calorimeter at four different heating rates (5, 10, 15, 20°C/min) from 35 to 200°C. Overall, HSRPRs with F/P = 2.0 had excellent comprehensive properties. The study was significant in solving the wastewater problem during the process of industry‐scale preparation of HSRPRs. We believed that the experimental findings would provide a new avenue for further study and application of HSRPRs. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Preparation and epoxy curing of novel dicyclopentadiene‐derived Mannich amines

Phenol/dicyclopentadiene (DCPD) adducts were prepared from the BF₃‐catalyzed reaction of p‐nonylphenol and dicyclopentadiene at molar ratios of 2 : 1 and 3 : 2. The phenol‐terminating adducts were consequently reacted with diethylenetriamine and formaldehyde using Mannich reaction conditions. These products containing phenol, amine and tricyclodecane functionalities in the same molecule can be used as epoxy curing agents. The diethylenetriamine was add to the phenol via Mannich reaction at approximately 50% theoretical equivalent. The multiple N H groups in amines and the O H groups in phenols provide crosslinking sites for epoxy resins. The cured epoxy resins show improvement in tensile strength and elongation in comparison with those cured by the poly(oxypropylene) diamine (400 molecular weight) or diethylenetriamine. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2129–2139, 1999     

Modification of an o‐cresol formaldehyde epoxy resin by the thermotropic liquid crystalline polymer

Two novel liquid crystalline polymers, polydiethyleneglycol bis(4‐hydroxybenzoate) terephthaloyl and block copolymer (PDBH), were synthesized by condensation reaction. The blends of the two liquid crystalline polymers and o‐cresol formaldehyde epoxy resin were prepared by linear phenol‐formaldehyde resin as curing reagent. Both mechanical and thermal properties of the blends containing liquid crystalline polymer were improved to a certain extent. By adding 5 wt % PDBH, the impact strength, bending strength, and the glass transition temperature were enhanced by 128%, 23.84%, and 28°C, respectively, compared with the unmodified version. The results of curing kinetics showed that the curing reaction active energy of the modified system by PDBH decreased from 79.70 to 70.26 kJ/mol. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1626–1631, 2005     

Epoxy adhesive with high underwater adhesion and stability based on low viscosity modified Mannich bases

A series of Mannich base curing agents were synthesized by using different structured phenols and amines. The phenols were phenol, o‐cresol, and nonylphenol, and the amines were diethylenetriamine, m‐xylylenediamine, and JEFFAMINE D230 polyetheramine. The chemical structures of Mannich bases were confirmed by ¹H nuclear magnetic resonance and Fourier transform infrared spectroscopy. The curing agents had low viscosities in the range of 19.9 to 1549 mPa s. The curing behavior of diglycidyl ether of bisphenol A with these curing agents was studied by differential scanning calorimetry. The mechanical properties and thermal properties of the cured epoxies were also investigated and compared. Lap‐shear adhesion in air and underwater was studied by using stainless steel substrates, and the adhesion after ageing in hot water (80 °C) was also investigated. Adhesive cured with phenol modified m‐xylylenediamine showed the highest underwater adhesion around 5.9 MPa, which was comparable to other ones tested in air. In addition, the high adhesive strength was kept after ageing in hot water for 7 days. The structural influence of the curing agents on mechanical property and adhesive performances were discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45688.     

Properties of compression‐molded plates made from wood powders impregnated with liquefied wood‐based novolak‐type phenol‐formaldehyde resins

Novolak‐type phenol‐formaldehyde (PF) resins with solution form were prepared by reacting phenol‐liquefied Cryptomeria japonica (Japanese cedar) wood with formalin in the presence of methanol. Wood powders of Albizzia falcate (Malacca albizzia) impregnated with these resins were air dried followed by an oven‐dried at 60°C. DSC analysis showed the PF resin existing in wood powders could be melted, and could be cured if hexamine was mixed and heated at high temperature. Compression‐molded plates made with PF resin impregnated woods had a high degree of curing reaction. However, compression‐molded plates hot‐pressed at 180°C for 8 min or 200°C for 5 min had better internal bonding strength and dimensional stability than others. Premixing hexamine with PF resin and impregnating into wood powders simultaneously could enhance the reactivity of PF resin, but it was not useful for improving the properties of compression‐molded plates. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and properties of novel phosphorus‐containing hardener for epoxy resins

A novel phosphorus‐containing curing agent, 2‐(6‐oxid‐6H‐dibenz〈c,e〉〈1,2〉oxa‐phosphorin‐6‐yl) phenol formaldehyde novolac [OD‐PN], was prepared from phenol formaldehyde novolac resin (PN) and a reactive 2‐(6‐oxid‐6H‐dibenz〈c,e〉〈1,2〉oxa‐phosphorin‐6‐yl)chloride (ODC) while ODC was synthesized through reaction between o‐phenylphenol and phosphoryl trichloride. The compound (OD‐PN) was used as a reactive flame‐retardant in o‐cresol formaldehyde novolac epoxy resin (CNE) for electronic application. Owing to the rigid structure of ODC and pendant P group, the resulted phosphorus containing epoxy resin exhibited better flame retardancy, higher glass transition temperature, and thermal stability than the noncyclic P‐containing curing agent or the bromine containing flame‐retardant epoxy resin. UL 94‐VO rating could be achieved with a phosphorus content of as low as 1.21% (comparable to bromine content of 6%) in the cured resin, and no fumes and toxic gas emission were observed. The relationship between the structure and flammability for both phosphorus containing curing agents OD‐PN and TP‐PN (triphenyl phosphate‐phenol formaldehyde novolac reaction product) are also examined. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1636–1644, 2000     

Studies on curing and thermal behavior of diglycidyl ether of bisphenol‐A and benzoxazine mixtures

This article describes the synthesis and characterization of benzoxazine monomers prepared by reacting the mixture of amine (aniline [A], 3‐chloroaniline [C], o‐toluidine [T]) and phenol (bisphenol‐A [B], phenol [P]) with formaldehyde [F]. The benzoxazine monomers prepared by reacting B and F with A, C, and T have been designated as BAF, BCF, and BTF, respectively. Structural characterization of benzoxazines was done using FTIR, ¹H NMR, and elemental analysis. The curing behavior of benzoxazine monomers was investigated by differential scanning calorimetry in the presence of diglycidyl ether of bisphenol‐A (DGEBA). In all the samples, the molar ratio of benzoxazine monomer:DGEBA, was varied as 1 :0, 3 : 1, 1 : 1, and 1 :3. The peak exotherm temperature (T_p) was lowest in all the samples having molar ratio of 3 : 1 benzoxazine:DGEBA and highest in samples having molar ratio of 1 : 3. The heat of polymerization (ΔH) was found to be maximum during curing of mixture of DGEBA and benzoxazine monomer prepared from phenol, aniline and formaldehyde (PAF). Thermal stability of benzoxazines:DGEBA mixture cured isothermally was evaluated by recording thermogravimetric traces in nitrogen atmosphere. The char yield was highest for a mixture having benzoxazine : DGEBA in the ratio of 3 : 1. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

A machinable carbon aerogel composite with a low thermal conductivity and enhanced mechanical properties

ABSTRACT

Carbon aerogels are prepared via the sol–gel polymerisation of resorcinol with formaldehyde, followed by supercritical drying and carbonisation. The fabricated carbon aerogels have low densities in the range 0.028–0.196?g?cm^?3, ultra-low thermal conductivities in the range 0.0259–0.0707?W?(m?K)^?1 and high specific surface areas (>520?m²?g^?1). The carbon aerogel composites are reinforced with short carbon fibres by adding the carbon fibres to the resorcinol–formaldehyde solution to reduce their brittle nature and improve their machinability. The compressive strength of the composites containing 2?wt-% carbon fibres is 1.75?MPa, which is 56% higher than that of pure carbon aerogel. Both fracture toughness and compressive strength of these composites are improved. These composites also have good machinability, with the ability to maintain their shape after being machined with traditional steel tools. Furthermore, the composites with nanoporous structure exhibit ultra-low thermal conductivity up to 1400°C.     

Microwave‐assisted hydrothermal liquefaction of lignin for the preparation of phenolic formaldehyde adhesive

This study aimed to produce phenolic formaldehyde (PF) adhesives using the liquefaction product of bagasse lignin (LPBL) as a partial substitute for petroleum‐based phenol. Lignin was extracted from bagasse using 93% acetic acid solution and was rapidly degraded in hot‐compressed water by microwave heating using oxalic acid as catalyst. The liquefaction yield reached 78.69% under the optimal reaction condition. Gas chromatography–mass spectrometry (GC/MS) analysis showed that the main chemical compounds of the liquefaction product included mono‐substituted and bis‐substituted phenols, such as 2,6‐dimethoxyphenol, 4‐hydroxybenzaldehyde, and so on. The LPBL was employed to replace a portion of phenol at varying ratios from 0 to 20 wt % in the preparation of PF adhesives. The molecular weight, viscosity, and adhesive strength of LPBL‐PF adhesives were found to be lower than those of pure PF adhesives. With the phenol replaced by LPBL up to 20%, the viscosity and adhesive strength of the resin were 4.046 Pa s and 1.017 MPa, respectively. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44510.     

Evaluation of block shear properties of selected extreme‐pH structural adhesives by short‐term exposure test

Nine structural adhesives with varying pH were selected to examine the effect of adhesive pH on wood–adhesive bond quality. The adhesives evaluated included four highly alkaline phenol–formaldehyde, one intermediate pH phenol–resorcinol–formaldehyde, two acidic melamine–urea–formaldehyde, and two acidic melamine–formaldehyde resins. Block shear specimens were prepared using Douglas‐fir and black spruce wood. The adhesive performance was evaluated by measuring the shear properties (strength and wood failure) of the specimens tested at the dry and vacuum–pressure–redry (VPD) conditions. Adhesive pH, test condition, and wood species showed significant effects on shear properties. The different adhesives performed differently at the dry and VPD conditions. The high‐pH adhesives (phenol–formaldehyde and phenol–resorcinol–formaldehyde) showed similar high wood failures at both test conditions and performed better than the low‐pH adhesives (melamine–formaldehyde and melamine–urea–formaldehyde), especially after the VPD conditioning. The low‐pH adhesives showed high wood failure at the dry condition, but wood failure decreased significantly after VPD conditioning for both species, indicating that the low‐pH adhesives were less durable than the high‐pH adhesives. High‐pH adhesives did not have a negative impact on the strength of the bonded specimens. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Influence of chemical composition on the rheological behavior of condensation reaction resins

In this paper, the chemorheological and dynamic mechanical behavior of melamine‐formaldehyde (MF) resins of four different formaldehyde/melamine (F/M) molar ratios (1.25, 1.5 1.75 and 2.00) are investigated. MF resins polymerize via a polycondensation reaction involving formation of up to 10 wt% of H₂O on cure. This typically results in rapid and extensive foaming of the resin when it is cured under atmospheric pressure. Experimental adaptation for the foaming behavior of MF resins is used to gather rheological information concerning the curing kinetics and the mechanical response of neat MF resins of different molar ratios. Likewise, the procedures developed allow curing of the resins under atmospheric pressure, hence allowing volatile evacuation as occurs during venting procedures (commonly used during compression molding of MF molding compounds) or as a result of absorption by hydrophilic fillers or substrates. The results show that increased moisture content in the B‐stage leads to faster reaction rates and greater foaming. Gelation and vitrification times are identified for each molar ratio, and are found to increase with decreasing molar ratio. The dynamic mechanical behavior of carefully molded neat MF samples of different molar ratios is studied using DMTA. T_g is found to be 200°C for the resin with the lowest formaldehyde content (F/M = 1.25), and around 230°C for the other resins. The storage shear modulus above T_g is studied, and the results show that the crosslink density increases with increasing molar ratio.     

Renewably sourced phenolic resins from lignin bio‐oil

Traditional lignin pyrolysis generates a bio‐oil with a complex mixture of alkyl‐functionalized guaiacol and syringol monomers that have limited utility to completely replace phenol in resins. In this work, formate assisted fast pyrolysis (FAsP) of lignin yielded a bio‐oil consisting of alkylated phenol compounds, due to deoxyhydrogenation, that was used to synthesize phenol/formaldehyde resins. A solvent extraction method was developed to concentrate the phenolics in the extract to yield a phenol rich monomer mixture. Phenolic resins were synthesized using phenol (phenol resin), FAsP bio‐oil (oil resin), and an extract mimic (mimic resin) that was prepared to resemble the extract after further purification. All three phenolic sources could synthesize novolac resins with reactive sites remaining for subsequent resin curing. Differential scanning calorimetry and thermogravimetric analysis of the three resins revealed similar thermal and decomposition behavior of phenol and the mimic resins, while the oil resin was less stable. Resins were cured with hexamethylenetetramine and the mimic resin demonstrated improved curing energies compared to the oil resin. The adhesive strength of the mimic resin was found to be superior to that of the oil resins. These results confirmed that extracting a mixture of substituted aromatics from FAsP bio‐oil could synthesize resins with properties similar to those from phenol and improved over the parent bio‐oil. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44827.     

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     

The role of wood extractives in structural hardwood bonding and their influence on different adhesive systems

Bonding of hardwood for structural applications is a complex process. Various factors influence the bond performance and the interface area is considered the most crucial part. The chemical composition of the interface, e.g. wood extractives, is expected to influence the bonding of hardwoods. The subject of this study was to determine the influence of seven model substances that represent common wood extractives on different adhesive systems namely one-component polyurethane, two-component polyurethane, melamine urea formaldehyde and phenol resorcinol formaldehyde. The influence of the model substances on the cross-linking behavior of the adhesives was determined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and on the rheological properties in terms of gel point and storage modulus. In addition, model substances characteristic for selected wood extractives were applied to the surface of European beech wood [Fagus sylvatica L.] before bonding and consequently tested in tensile shear mode according to EN 302-1. The ATR-FTIR spectra showed an influence of some substances on the crosslinking for all adhesive systems. Further, the curing process was mostly accelerated for phenol resorcinol formaldehyde, while melamine urea formaldehyde and polyurethane showed a less negative change in rheological behavior. The mechanical strength of beech wood bonds at room climate indicated only minor influence of model substances, but samples tested in wet conditions demonstrated a significant effect on some adhesive systems. It was concluded, that polyurethane adhesives degrade by acid substances and melamine urea formaldehyde by starch and gallic acid. Phenol resorcinol formaldehyde system was influenced negatively by starch and acids.     

Effects of crosslinking degree and carbon nanotubes as filler on composites based on glycidyl azide polymer and propargyl‐terminated polyether for potential solid propellant application

Triazole crosslinked polymers were prepared by reacting glycidyl azide polymer (GAP) with the propargyl ‐ terminated poly(tetramethylene oxide) (PTMP) at different molar ratios of azide versus alkyne. Based on the optimum mechanical properties of the GAP/PTMP ‐ 2.5, a series of GAP/PTMP nanocomposites reinforced by either multi ‐ walled carbon nanotubes (MWCNTs) or carboxy ‐ functionalized multiwalled carbon nanotubes (MWCNTs ‐ COOH) were prepared with different mass ratios. The glass transition temperatures (T_{g, PTMP}) assigned to PTMP of the GAP/PTMP composites almost kept at a constant range when the molar ratio of azide versus alkyne was from 1.0 to 2.5. When the loading MWCNTs was 1.0 wt %, the tensile strength and elongation at break achieved a maximum of 1.77 MPa and 36.3%, respectively. The nanocomposites with nearly similar T_{g, PTMP} indicated no phase separation in the crosslinked polymers. The results revealed that the improved properties of GAP ‐ based materials could be achieved by changing the molar ratio of azide versus alkyne and the nanofillers content. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45359.     

Synthesis,characterization, and properties of low viscosity tetra‐functional epoxy resin N,N,N′,N′‐ tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane

Tetra‐functional epoxy resin N,N,N′,N′‐tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane (TGDEDDM) was synthesized and characterized. The viscosity of TGDEDDM at 25°C was 7.2 Pa·s, much lower than that of N,N,N′,N′‐tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM). DSC analysis revealed that the reactivity of TGDEDDM with curing agent 4,4′‐diamino diphenylsulfone (DDS) was significantly lower than that of TGDDM. Owing to its lower viscosity and reactivity, TGDEDDM/DDS exhibited a much wider processing temperature window compared to TGDDM/DDS. Trifluoroborane ethylamine complex (BF₃‐MEA) was used to promote the curing of TGDEDDM/DDS to achieve a full cure, and the thermal and mechanical properties of the cured TGDEDDM were investigated and compared with those of the cured TGDDM. It transpired that, due to the introduction of ethyl groups, the heat resistance and flexural strength were reduced, while the modulus was enhanced. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40009.