Chemistry of novolac resins. II. Reaction of model phenols with hexamethylenetetramine

It is proposed that the reactions of hexamethylenetetramine (HMTA) with 2,4-xylenol and with 2,6-xylenol occur by different pathways. The rate of reaction and the final product distribution depend on the initial xylenol : HMTA ratio and are different in the two systems. Measured by HMTA consumption, with 2,4-xylenol the reaction rate increased with increasing xylenol : HMTA ratios, whereas with 2,6-xylenol the rate of reaction decreased with increasing 2,6-xylenol : HMTA ratio. In systems which contain both 2,4- and 2,6-xylenol, a strong preference for reaction of the HMTA with the ortho site of 2,4-xylenol was noted. This preference was apparent even in mixtures in which 2,6-xylenol was present in greater amounts than 2,4-xylenol. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1389–1398, 1997     

Novel flame‐retardant thermosets: Phosphine oxide‐containing diglycidylether as curing agent of phenolic novolac resins

Phosphorus‐containing novolac–epoxy systems were prepared from novolac resins and isobutyl bis(glycidylpropylether) phosphine oxide (IHPOGly) as crosslinking agent. Their curing behavior was studied and the thermal, thermomechanical, and flame‐retardant properties of the cured materials were measured. The T_g and decomposition temperatures of the resulting thermosets are moderate and decrease when the phosphorous content increases. Whereas the phosphorous species decrease the thermal stability, at higher temperatures the degradation rates are lower than the degradation rate of the phosphorous‐free resin. V‐O materials were obtained when the resins were tested for ignition resistance with the UL‐94 test. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3516–3526, 2004     

High pressure DSC of phenolformaldehyde moulding compounds

The influence of pressure on the polycondensation reaction between novolac resin (N) present in commercially available moulding compounds and hexamethylenetetramine (HMTA) was studied up to 80 bars under air and in an inert atmosphere. For a low HMTA content (N/HMTA=98/2 mass ratio) high pressure enables the detection of two successive curing reactions. With increasing HMTA content the peak due to the first curing reaction becomes less pronounced at high pressure, while the enthalpy of the second increases. In an inert atmosphere both curing reactions are well observable even at ambient pressure and for lower HMTA content take place at lower temperatures, as expected. For the sample with N/HMTA=98/2 the curing reaction was followed using TG-MS.     

Curing studies of epoxy resins with phosphorus‐containing amines

The curing system of diglycidyl ether of bisphenol A (DGEBA) with two phosphorus‐containing amine compounds—bis(3‐aminophenyl)methyl phosphine oxide and bis(4‐aminophenyl)‐bis(9,10‐dihydro‐9‐oxa‐10‐oxide‐10‐phosphaphenanthrene‐10‐yl)methane—was studied with differential scanning calorimetry under isothermal and nonisothermal conditions and compared with the DGEBA/diamino diphenyl methane system. The isoconversional method was used to evaluate the dependence of the effective activation energy on the extent of conversion. Modulated differential scanning calorimetry and dynamic mechanical thermal analysis were used to study the phenomena of vitrification and gelation. The thermal and flame‐retardant properties were evaluated, and the limiting oxygen index values of the phosphorylated resins, above 30, confirmed that phosphorus‐containing epoxy resins are effective flame retardants. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1676–1685, 2006     

A new viscoplastic model and experimental characterization for thermosetting resins

Resins as matrix materials for structural composites show nonlinear rate-dependent mechanical behaviors. In the present work, a new viscoplastic constitutive equation based on a potential function is proposed to predict the mechanical response of an epoxy matrix to any three-dimensional loading condition. The proposed potential function is a combination of the second and third invariants of the deviatoric stress tensor as well as the first invariant of the stress tensor, i.e. the hydrostatic stress. Series of tensile and shear constant-rate straining tests were performed on epoxy resin specimens up to the fracture. Under shear loading, the nonlinearity of the stress-strain curve and the rate dependency of the initial modulus and strength are more significant than that under tensile loading. The viscoplastic model parameters are derived from the experimental data, and the fracture patterns of the specimens under tensile and shear loadings are studied. Further, the model predictions are compared with a known rate-dependent model to show the accuracy of the presented model.     

Synthesis of novel boron‐containing epoxy–novolac resins and properties of cured products

Epoxy–novolac resins were synthesized by modifying a commercial novolac resin with epichlorohydrin. These epoxy–novolac resins were characterized and further modified with different contents of bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide or bis(4,4,5,5‐tetramethyl‐1,3,2‐dioxa‐borolanyl)oxide. The boron‐containing epoxy–novolac resins were autocatalytically crosslinked or crosslinked with BF₃MEA and their thermal stability and flame retardancy were determined by thermogravimetric analysis and limiting oxygen index (LOI) values. These LOI values for the bis(benzo‐1,3,2‐dioxa‐borolanyl)oxide derivatives were higher than the boron‐free novolac resins, which shows the benefit of the presence of boron. To test the role of boron in the enhancement of flammability, scanning electronic microscopy and energy‐dispersive X‐ray spectroscopy images were made. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6332–6344, 2006     

The reaction of furfuryl alcohol resins with hexamethylenetetramine: A 13C and 15N high-resolution solid-state NMR study

The reactions between furfuryl alcohol (FA)/poly(furfuryl alcohol) (PFA) and hexamethylenetetramine (HMTA) have been studied by ¹³C and ¹⁵N high-resolution solid-state NMR techniques. Highly crosslinked polymer networks similar to those obtained from the thermal curing of FA resins under acidic condition are formed. Possible reaction pathways are postulated on the basis of changes of chemical structures during the curing; α-substituted furfurylamines are shown to be the initial intermediates. Their further reactions with FA/PFA, together with thermal decomposition, produce methylene linkages between furan rings, resulting in chain extension and crosslinking, which occurs at both the methylene linkages and the 3- or 4-C of furan rings. Various side-products such as amines, imines, amides, imides, and nitriles are also formed during the reactions, and some of these can remain in the resins up to 205°C. The crosslinked network can be heterogeneous with different structures on nanometer scales when higher oligomers of FA resins react with HMTA. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2233–2243, 1997     

Method of model parameter calculation of ion-exchange resins

Russian Journal of Electrochemistry - A method for determination of parameters of a three-wire model and Lichtenecker model on the basis of experimental data of conductivity measurement for...     

Syntheses and characterizations of thermally reworkable epoxy resins. Part I

To meet the need for reworkable epoxy resins, a series of cycloaliphatic diepoxides containing thermally cleavable carbamate linkages were synthesized and characterized. These materials were shown to undergo curing reactions with cyclic anhydride in a similar fashion as a commercial cycloaliphatic epoxide, except that the carbamate group within the diepoxides can act as the internal catalyst. Furthermore, cured samples of the formulations from these diepoxides started to decompose at lower temperatures, i.e., between 200–300°C as compared with 350°C for the cured sample of the commercial cycloaliphatic epoxide, which showed their potential to be used as reworkable underfill encapsulants in the electronic packaging area. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2991–3001, 1999     

Curing behavior and toughening performance of epoxy resins containing hyperbranched polyester

The effects of the hyperbranched polyester with hydroxyl end groups (HBPE‐OH) on the curing behavior and toughening performance of a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) were presented. The addition of HBPE‐OH into DGEBA strongly increased its curing rate and conversion of epoxide group due to the catalytic effect of hydroxyl groups in HBPE‐OH and the low viscosity of the blend at curing temperature. The improvements on impact strength and critical stress intensity factor (or fracture toughness, K_1c) were observed with adding HBPE‐OH. The impact strength was 8.04 kJ m^?1 when HBPE‐OH reached 15 wt% and the K_1c value was approximately two times the value of pure epoxy resin when HBPE‐OH content was 20 wt%. The morphology of the blends was also investigated, which indicated that HBPE‐OH particles, as a second phase in the epoxy matrix, combined with each other as the concentration of HBPE‐OH increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Estimation of Free Radical Polymerization Rate Coefficients Using Computational Chemistry

This study explores the application of computational chemistry to estimate free radical polymerization rate coefficients. The Evans-Polanyi relationship is combined with computed heats of polymerization to estimate copolymerization reactivity ratios for many vinyl monomer pairs, focusing on acrylates, methacrylates and styrene, with accuracy assessed by comparison to experimental values. The effect of different optimization approaches on the values of thermodynamic properties is explored, and it is concluded that a combination of conventional optimization and relaxed potential energy scans was most effective at identifying global minima. The difference between thermodynamic properties calculated using the harmonic oscillator treatment and a hindered rotor model is evaluated for methyl methacrylate polymerization.     

Co-curing studies of ethynyl terminated oligoimide and (methyl) nadicimide resins

The paper describes the co-curing studies of ethynyl and ethenyl end-capped imide resins. The effect of composition and chemical structure of ethenyl end-capped resins (nadicimides) on thermal behavior of ethynyl end-capped resins was evaluated using DSC and thermogravimetric analysis. An increase in char yield was observed on co-curing of few resin formulations. A mechanism has been proposed to account for this observation.Paper presented at Intern. Symposium on Macromolecules at Trivandrum, Jan. 11–13, 1995.     

Estimation of the activity of catalysts and the reactivity of organosulfur compounds

The kinetics of hydrogenolysis of various classes of organosulfur compounds were studied in the presence of MeAlCl ₄ catalysts (Me=Li, Na, K). The reactions were conducted at 100–250°C without introduction of hydrogen into the reaction zone, but in the presence of hydrocarbons capable of acting as hydride—ion donors; these donors were necessary for H ₂ S formation. A series of catalytic activities and optimal reaction conditions for each catalyst were determined from the rate constants of hydrogenolysis at the C-S bond. Reactivities of mercaptans, sulfides, disulfides, and thiophene were estimated.Institute of Chemistry, Bashkir Scientific Center, Ural Branch, Russian Academy of Sciences, Ufa 450054. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 562–564, March, 1992.     

Photophysical processes in aromatic polyimides. Studies with model compounds

Emission mechanism in an aromatic polyimide, PI(BPDA/PDA), derived from biphenyltetracarboxylic dianhydride and p-phenylene diamine were studied with ultraviolet visible absorption and fluorescence spectra of a series of the model compounds. The excitation spectrum of the intermolecular charge-transfer (CT) fluorescence peaking around 550 nm of PI(BPDA/PDA) thin film was completely consistent with the absorption spectrum, indicating that the intermolecular CT fluorescence emission of PI(BPDA/PDA) film is not caused by direct excitation of the CT absorption band, but by light absorption due to structural units in the polymer backbone. The UV-vis. absorption spectra of the model compounds corresponding to the structural units in PI(BPDA/PDA) showed that the longest wavelength absorption band is due to the biphenylbisimide moiety. The band was assigned as π, π* transition with the polarization spectrum of the model compound. The fluorescence spectra of the model compounds changed sensitively depending on the conformation around N-phenyl bond. The lifetime measurement for the model compounds suggested that intramolecular CT process occurs very rapidly. © 1993 John Wiley & Sons, Inc.     

Characteristic and spectroscopic properties of the Schiff-base model compounds

Two series of conjugated aromatic imines (Schiff-base model compounds) with different central groups and various side-group substitutions have been synthesized and characterized by elemental analysis, differential scanning calorimetry (DSC) technique, hydrogen nuclear magnetic resonance (¹H NMR), Fourier transform infrared (FTIR) and ultra-violet and visible light (UV–vis) spectroscopy measurements. The UV–vis absorption of solutions of these compounds in dimethylacetamid (DMA), chloroform and methanol was investigated in the optical range from 240 to 450 nm, where two distinct absorption bands: at 250–280 and 315–360 nm with the different level of absorption have been observed. The influence of compound molecular structure and polarity of solvent on the absorption spectra and the possible optical transitions have been discussed. Structure of diamines in the azomethine models fundamentally affected their spectroscopic properties and conjugation of π-electrons.     

Synthesis and characterization of self‐catalyzed imide‐containing pthalonitrile resins

Some new amino‐ and imide‐containing phthalonitrile compounds with 1:1 molar ratio of amino group to pthalonitrile unit were successfully synthesized. The molecular structures were characterized by spectroscopic techniques. They were thermally polymerized under nitrogen/air, even in the absence of curing additives. The thermal properties of the cured products were characterized by thermogravimetric analysis and differential scanning calorimetry. The 5% weight loss temperatures ranged from 525 to 528 °C and 513 to 520 °C under nitrogen and air, respectively. Char yields (900 °C) were in the range of 62–70%. Rheometric measurements showed that the rate of the cure reaction differs for all the three monomers. The glass transition temperature advances with increasing extent of cure and disappears on postcure at 375 °C. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Reaction of diazoalkanes with unsaturated compounds. 11. Relative reactivity of olefins on catalytic cyclopropanation with diazomethane and palladium catalysts

The relative reactivity of unsaturated compounds on catalytic cyclopropanation with diazomethane in the presence of catalysts based on Pd²⁺ [Pd(OAc)₂, Pd(acac)₂, (PPh₃)₂·PdCl₂] has been studied using competing reactions. An interconnection was shown between the relative reactivity of the olefins and the coordinating activity of the C=C bond depending on its strain and spatial environment.For previous communication, see [1].Institute of Organic Chemistry, Urals Branch, Russian Academy of Sciences, 450054 Ufa; N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 2353–2361, October, 1992.     

Effects of rubber type on the curing and physical properties of silica filled rubber compounds

As environmental regulations are getting stricter, tire industries for automobiles have shown much interest in substituting silica for conventional carbon black partially or entirely. To take full advantage of silica as fillers for rubbers, it is essential to find a reasonable rubber system that shows an excellent performance with silica reinforcement. Therefore, in this study, several different rubber compounds comprising the same amount of silica were prepared with several different rubber systems, respectively. The processability, curing characteristics, and mechanical and viscoelastic properties of the rubber compounds were investigated to analyze the performance of the rubber compounds as tire tread materials. Among the rubber compounds studied, SBR1721 compound comprising natural rubber (NR) and styrene butadiene rubber (SBR) with high styrene content was considered the most appropriate for application to tire tread materials. Copyright © 2008 John Wiley & Sons, Ltd.     

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