Curing kinetics of boron‐containing phenol–formaldehyde resin formed from paraformaldehyde

A boron‐containing phenol–formaldehyde resin (BPFR) was synthesized from boric acid, phenol, and paraformaldehyde. The curing reaction of BPFR was studied by Fourier‐transform infrared spectrometry and differential scanning calorimetry. According to the heat evolution behavior during the curing process, several influencing factors on isothermal curing reaction were evaluated. The results show that the isothermal kinetic reaction of BPFR follows autocatalytic kinetics mechanism, and kinetic parameters m, n, k₁, and k₂, were derived, respectively. In the latter reaction stage, the curing reaction becomes controlled mainly by diffusion. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 638–644, 2002     

Thermodynamic properties of phenol–formaldehyde resin

Characterization measurements of a commercial phenol-formaldehyde resin prepared from constituents including phenol and formaldehyde in the molar ratio 1:1.33 are reported. The measurements consist of (1) the linear thermal expansion coefficient between approximately 85 and 270°K; (2) the specific heat capacity between approximately 6 and 100°K; (3) the Young's modulus at room temperature. A critical examination of the data reveals that: (1) the vibrational behavior is predominantly that of a three-dimensional assembly; (2) as far as data available from other sources permit an assessment to be made, the principle of additivity appears to be applicable to the specific heat capacity between approximately 50 and 100°K; and (3) the data lie near the limit of an empirical relationship observed between the Young's modulus and linear thermal expansion coefficient of other polymers.     

Development of novel flame‐retardant thermosets based on boron‐modified phenol–formaldehyde resins

Boron‐containing novolac resins were prepared through the modification of a commercial novolac resin with different contents of bis(benzo‐1,3,2‐dioxaborolanyl) oxide. Their thermal and flame‐retardant properties were measured. Then, they were crosslinked with hexamethylenetetramine, and their thermal, thermodynamomechanical, and flame‐retardant properties were evaluated. Their modification degree was related to the segmental motion of the materials. The crosslinking of the boron‐modified novolac resins with hexamethylenetetramine was slower and not as extensive as that of commercial novolac resins because the nitrogen from intermediate species coordinated with boron. The thermal degradation of the boron‐containing novolac resins generated boric acid at high temperatures and gave an intumescent char that slowed the degradation and prevented it from being complete. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3503–3512, 2006     

A study on the kinetics of condensation reaction of cardanol and formaldehyde,part I

Novolac resins having cardanol‐to‐formaldehyde mole ratios of 1:0.4, 1:0.5, and 1:0.6 were prepared by using aromatic sulphonic acid as the catalyst at four different temperatures ranging between 90°C and 120°C, with an interval of 10°C. Free formaldehyde and free phenol contents were determined at regular time intervals to check the completion of the reaction. The synthesized novolacs were characterized by Fourier‐transform infrared spectroscopic analysis, nuclear magnetic resonance, and gel permeation chromatography. The reaction between cardanol and formaldehyde was found to follow second‐order kinetics. The overall rate constant (k) increased with the increase of temperature. On the basis of the value of k, various other activation parameters such as activation energy (E_a), change in enthalpy (ΔH), entropy (ΔS), and free energy (ΔG) of the reaction were also evaluated. It was found that the condensation reaction of cardanol and formaldehyde with aromatic sulphonic acid was nonspontaneous and irreversible. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 559–572, 2009     

Studies on formaldehyde condensation resins. XIII. Gelation of acetoguanamine–formaldehyde resin

The gelation reaction of acetoguanamine with formaldehyde was investigated in the light of the gelation theory for tetrafunctional amino resins described in the previous paper. The gel time and extents of reaction of formaldehyde, amino groups, and imino groups varied with the molar ratio in the feed, but values of K (the ratio of the rate constant for condensation to that for addition) and k (the ratio of the rate constant for addition of the imino group to that of the amino group) were nearly constant. When the catalyst concentration was increased, the gel time, extents of reaction of each functional group, and the values of K and k varied; in particular K increased markedly. From the results of varying the molar ratio and concentration of acidic catalyst, it was found that the number of methylol groups per molecule of acetoguanamine at the gel point was influenced by the reaction conditions but the number of methylene linkages per molecular of acetoguanamine was nearly constant at about 0.6, regardless of reaction conditions. The number-average molecular weights up to the gel point varied with the reaction conditions, but at the gel point they were all nearly constant at about 385.     

Analysis of the base-catalyzed phenol–epichlorohydrin condensation

The amount of abnormal addition of phenoxide to epichlorohydrin and phenyl glycidyl ether was determined as a function of reaction conditions. The isomer distribution resulting from attack at either position of these unsymmetrical epoxides was quantitatively measured by vapor phase chromatography of the resulting product mixtures. While both epoxides reacted predominantly in the expected normal fashion, small quantities of abnormal addition products were found whose concentrations were temperature-dependent. The analytical method also permitted a study of the effect of temperature on side reactions involving hydrolytic solvents.     

Chain-configurational properties of novolac phenol–formaldehyde resins

Rotational isomeric-state chain-configurational calculations have been applied to the novolac phenol–formaldehyde structure. Steric interference allows the chain to be considered with a twofold potential energy barrier model. Computations that fit the observed dipole-moment data over a range of molecular weights indicate that the conformational angle is near ±80°, with the g^±g⁼ states on the average being 155 cal/mole below the g^±g^± states. The limiting dipole-moment ratio is computed to be 1.47, compared to the experimental value of 1.48. A negative temperature coefficient agrees in sign with the experimental value, and the characteristic ratio of the end-to-end molecular dimensions is calculated to be 3.27, as compared to the experimental value of 4.76.     

Griffith fracture of a phenol–formaldehyde polymer

Cast samples of a phenol–formaldehyde polymer with a crack of length defined by a metallic foil inclusion were fractured in tension. The stress at fracture was inversely proportional to the square root of the crack length, in agreement with the Griffith equation for brittle fracture. The behavior did not conform to the Griffith equation with respect to the experimental value of surface free energy, which was several orders of magnitude higher than a theoretically calculated value. However, as the temperature of tensile testing was raised, the experimental value did approach the calculated value. Consistently the appearance of the fracture surface was observed to change from one showing evidence of plastic deformation at room temperature to a featureless appearance, characteristic of brittle fracture, at higher temperatures.     

A novel photosensitive ternary complex consisting of phenol‐formaldehyde resin,sodium dodecyl sulfate,and diazo resin

A novel photosensitive ternary complex formed from phenol‐formaldehyde resin (PR), sodium dodecyl sulfate (SDS), and diazo resin (DR) was developed. In the presence of SDS, PR remains soluble in water when its solution is neutralized. The PR‐SDS‐DR ternary complex forms when DR is added to the PR‐SDS solution. The ternary complex dissolves in polar solvents such as dimethylformamide and is sensitive toward UV light or heating. It was confirmed that the ionic bond of  SO +N₂ between SDS and DR converts to a covalent bond after the decomposition of the ⁺N₂ group under UV irradiation or heating. As a result, the solubility of PR‐SDS‐DR changes dramatically; that is, after treatment with UV light or heating, PR‐SDS‐DR becomes insoluble in polar solvents. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2566–2571, 2000     

An electrochemical route to phenol–formaldehyde precursors

With the aid of VPC and NMR, the electroinitiated polymerization of phenol or p-tert-butyl phenol with formaldehyde in the presence of basic electrolytes has been investigated over a range of current densities. Results from an electroinitiation study were contrasted with base-catalyzed thermal polymerizations and, except for yield, found to be essentially invariant. GPC of the electroinitiated and thermally polymerized resins indicates similar results with low molecular weight species of relatively narrow molecular weight distribution being the principal products. An electroinitiation mechanism, in agreement with the mechanism for base-catalyzed thermal polymerization is proposed to describe these experimental results.     

Alkaline hydrolysis of cured phenol–formaldehyde resins at high temperatures

Hydrolysis of cured phenolic resin, either straight or oil- or epoxy-modified, in dilute caustic at 280–320°C, gives novolac resins in 70–80% recovery. Infrared analysis indicates these novolacs have the same composition as conventional novolacs made by the acid-catalyzed phenol–formaldehyde condensation. Like the conventional novolacs, these recovered resins are soluble in oxygen-containing solvents and form crosslinked infusible thermoset materials when heated with 10% hexamethylenetetramine. Molecular weights of the recovered novolacs, determined by high-pressure liquid chromatography, are dependent on both time and temperature of the hydrolysis. The weight-average molecular weight varied from 270 to 770. The Lowry index values indicate that the molecular weight distribution is narrower for the recovered novolacs than that normally obtained for conventional novolacs. The recovered novolacs have higher melting temperatures than the corresponding commercial novolacs. This is believed to be due to the lower free phenol content of the recovered novolacs.     

Dipole moment characterization of phenol–, o-cresol–, and p-cresol–formaldehyde resins

Dipole moments and their temperature dependence have been measured in p-dioxane for fractionated novolac phenol–, o-cresol–, and p-cresol–formaldehyde polymers. The phenol–formaldehyde fractions covered a molecular weight range of 200 to 6100, and the limiting dipole moment ratio 〈μ²〉/xm² is 1.48. The p-cresol–formaldehyde dipole-moment ratio at a DP of 4 is 2.47, whereas the phenol–formaldehyde dipole-moment ratio is 1.40. That for o-cresol–formaldehyde is intermediate in value. The dipole-moment temperature coefficients are positive for p-cresol chains and negative for the phenol–formaldehyde chains. These results indicate that the hydroxyl groups along the p-cresol–formaldehyde polymer are highly ordered, with the aromatic rings closer to the sterically hindered planar position than in the phenol–formaldehyde polymers.     

Curing kinetics study of melamine–urea–formaldehyde resin

The influence of the preconditioning at different temperatures on the cure kinetics of melamine?Curea?Cformaldehyde resins coated on stone wool was investigated under acidic conditions using differential scanning calorimetry and thermogravimetry. The higher pre-treatment temperature was applied, to which resin-coated stone wool was exposed, the lower was the mass loss during the experiment. Kinetic model parameters were determined in two different manners, with the parameters being independent of preconditioning temperature and dependent on the latter. The apparent orders of reaction were approximately two (all of them being within the range 0.96?C2.33), which would imply that cross-linking predominantly proceeds via the bimolecular reaction of either melamine or urea and formaldehyde. Nonetheless, the apparent orders of reaction decreased as a function of preconditioning temperature. The apparent activation energies varied less with preconditioning temperature, assuming values between 64.2 and 78.5?kJ?mol^?1. The applicability of nth-order reaction kinetic models was consequently validated for two other dynamic thermal regimes.     

DFT study on the hydrogen bonds of phenol–cyclohexanone and phenol–H2O2 in the Baeyer–Villiger oxidation

Hydrogen bonds of phenol–cyclohexanone and phenol–H₂O₂ in the studied Baeyer–Villiger (B–V) oxidation have been investigated by HF, B3LYP, and MP2 methods with various basis sets. The accurate single‐point energies were performed using CCSD(T)/6‐31+G(d,p) and CCSD(T)/aug‐cc‐pVDZ on the optimized geometries of MP2/6‐31+G(d,p). It has been confirmed that B3LYP/6‐31+G(d,p) could be used to study such hydrogen bonds. Energetic analysis of complexes was carried out using the Xantheas method with BSSE corrected by CP method. Orbital energy order (ε) illuminated that phenol with good hydrogen donor‐acceptor property can interact with cyclohexanone or H₂O₂ to form hydrogen bound complexes, and the binding energies (BE) range from ?4.38 to ?14.06 kcal mol^?1. NBO analysis indicated that the redistribution of atomic charges in the complexes facilitated nucleophilic attack of H₂O₂ on cyclohexanone. The calculated results match remarkably well with the experimental phenomena. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Synthesis and curing kinetics of cardanol‐based curing agents for epoxy resin by in situ depolymerization of paraformaldehyde

Three novel cardanol‐based phenalkamines with good stability have been successfully prepared by Mannich reaction using phenolic compounds with paraformaldehyde and hexamethylenediamine (or its mixture with other amines). The structure of the prepared phenalkamines has been analyzed using liquid chromatography‐mass spectrometry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy. The curing kinetics of the prepared epoxy resin/phenalkamine systems has been investigated using differential scanning calorimetry (DSC), and determined by Kissinger, Flynn–Wall–Ozawa, and Crane methods. Furthermore, the thermal properties of the cured materials have been evaluated using DSC and thermogravimetric analysis, and the mechanical properties of the cured materials have been analyzed systematically. The results demonstrate that the phenalkamine 1 (PAA1) had a lower reactivity and better toughness than phenalkamine 2 (PAA2) and phenalkamine 3 (PAA3). In addition, PAA1 is a solid curing agent, while PAA2 and PAA3 are liquid curing agents, which were more convenient for practical usage. Results indicate that the properties of the prepared phenalkamines strongly depend on the structures. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 472–480     

A 13C NMR study of the methylol derivatives of 2,4′‐ and 4,4′‐dihydroxydiphenylmethanes found in resol phenol–formaldehyde resins

A total of 13 of the 16 possible methylol derivatives of 2,4′‐ and 4,4′‐dihydroxydiphenylmethane have been synthesized, isolated, and identified. These compounds are found as intermediates in the cure process of resol phenol–formaldehyde (PF) resins. Analysis of the ¹³C NMR spectra (in acetone‐d₆) of these compounds provided a way to evaluate the seven methylolphenol ring types (methylol derivatives of 2‐hydroxyphenyl and 4‐hydroxyphenyl rings) found in typical resol PF resins using the ipso carbon region from 150 to 160 ppm. A simple diagnostic test was developed using the chemical shift values of the methylol methylene carbon atoms to identify the presence of intermediates containing either a 2‐hydroxyphenyl or a 4‐hydroxyphenyl ring. Using these data it is now possible to analyze the major components in extracted prepreg PF resins. Copyright © 2002 John Wiley & Sons, Ltd.     

Fractography of a phenol—formaldehyde polymer

Samples of a phenol-formaldehyde polymer with a deliberately introduced flaw were fractured in tension. The appearance of the fracture surface near the flaw suggested the disruption of particles pre-existing in the polymer. At a greater distance from the flaw, a featureless surface was observed which was succeeded by one showing interference colors. At still greater distances, linear features were observed to be lying in the direction of crack propagation which, in most areas, were regular and evenly spaced. There were indications that these features were formed by the curling of a surface film. The above observations are interpreted as providing evidence that tensile fracture is accompanied by plastic defórmation at the fracture surface.     

Polymer structure of cured alkaline phenol–formaldehyde resol resins with respect to resin synthesis mole ratio and oxidative side reactions

A wood adhesive-type phenol–formaldehyde (PF) resol resin synthesized with a typical formaldehyde to phenol mol ratio of 2.10 was thoroughly cured and studied by the solid-state crosspolarization/magic angle spinning ¹³C nuclear magnetic resonance (CP/MAS NMR) spectroscopy. The methylene group/phenol mol ratio values found were between 1.35 and 1.46, close to the value of a completely cured PF polymer structure. The amount of formaldehyde emitted during resin curing was very small. Other formaldehyde-derived groups determined from CP/MAS NMR spectra and relatively high levels of oxidation products of formaldehyde determined from water extracts of cured resin raised the total formaldehyde-derived groups/phenol mol ratio value to close to that of the synthesis mol ratio. Technological implications of these findings are discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3275–3285, 1997