Chemorheological analysis of a gelled resol resin curing under non-isothermal conditions by shear strain

The chemorheological behavior of curing of a resol resin was analyzed under non-isothermal conditions beyond the gelation point. Two heating ramps (0.5 and 1 °C/min) from 0 to 100 °C were performed. The rheological measurements of the resin were performed using oscillatory shear strain. The obtained profiles for the resin’s complex viscosity were applied, after treatment by two calculation methods, to the four- and six-parameter Arrhenius models. These models allow one to establish the viscous flow region of the resin and the kinetic parameters of the material’s curing process. The six-parameter Arrhenius model was selected as the best method for modeling of the resin’s rheological behavior during its curing process. The viscous-flow activation energies determined for the gelled resol resin curing were 67.1 and 58.3 kJ/mol for the 0.5 and 1 °C/min heating rates, respectively. The activation energies of the resin curing process were 41.7 and 67.0 kJ/mol for each temperature ramp.     

Determination of kinetic parameters and analytical pyrolysis of tobacco waste and sorghum bagasse

The thermal decomposition of tobacco waste and sorghum bagasse was investigated by non-isothermal thermogravimetric analyses, applying slow heating rates and well-defined conditions. The purpose of evaluating the decomposition was to estimate the kinetic parameters of the analyzed materials. Activation energies and Arrhenius exponential factors were inferred by different estimation methods: the classical methods of Ozawa and Starink and the independent parallel reactions model. The analytical pyrolysis was performed in a micro-pyrolyzer coupled to a gas chromatographer/mass spectrometer. Values of activation energy obtained with single step reaction models by the Ozawa method were: 103.94 kJ/mol for tobacco waste and 120.01 kJ/mol for sorghum bagasse, and by the Starink method - 135.95 kJ/mol for tobacco waste and 148.91 kJ/mol for sorghum bagasse. The independent parallel reaction model presented energy activation values of 39.7-272.0 kJ/mol for tobacco waste and 35.7-220.0 kJ/mol for sorghum bagasse. In analytical slow and fast pyrolysis of tobacco residue and sorghum bagasse, holocellulose and lignin-derived compounds were identified, as well as hydrocarbons and aromatic hydrocarbons. The kinetic behavior of the materials are presented and discussed. Our findings may be helpful in evaluating other types of lignocellulosic biomass.     

酚醛树脂/蒙脱土纳米复合材料的制备及固化反应动力学研究

利用自制的有机蒙脱土 ,采用浇模固化成型法制备酚醛树脂 /六次甲基四胺 /蒙脱土纳米复合材料 ,并用XRD观察有机蒙脱土分别在热塑性和热固性酚醛树脂中复合行为 .研究发现 ,由于两种树脂的固化反应机理不同 ,热固性酚醛树脂与蒙脱土复合 ,可得插层型纳米复合材料 ;而采用热塑性酚醛树脂进行固化 ,则得到部分剥离的纳米复合材料 .通过DSC进一步研究热塑性酚醛树脂 /蒙脱土复合体系的固化反应动力学 .运用Kissinger ,Flynn Wall Ozawa ,Crane方法求出活化能和反应级数等动力学参数 .结果发现 ,加入蒙脱土使固化反应活化能下降 ,反应级数减小 ,从而有利于固化工艺的实现 ,便于纳米复合材料实际应用 .     

Chemorheology and Curing Kinetics of a New RTM Benzoxazine Resin

The chemorheology and curing kinetics of a new high performance resin transfer molding benzoxazine resin was investigated. A chemorheological model based on a modified Arrhenius equation that describes the resin viscosity as a function of temperature and time was proposed. The model, which agreed well with the experimental data, can provide theoretical support for the mold-filling stage in the resin transfer molding process. The average activation energies of the polymerization reaction were obtained by means of gelation times at different temperatures based on the Arrhenius equation and from dynamic differential scanning calorimetry (DSC) results based on the Kissinger and Ozawa methods; the values were 96.0,84.0 and 87.8 KJ/mol, respectively. A plot of activation energy vs. conversion in the curing process was obtained using the Flynn-Wall-Ozawa model. The reaction orders were estimated from isothermal DSC based on a modified Kamal kinetics model which can describe both the autocatalytic and diffusion-controlled curing mechanism.     

Investigation of curing kinetics of various cycloaliphatic epoxy resins using dynamic thermal analysis

Curing reactions of three cycloaliphatic epoxy resins with methyltetrahydrophthalic anhydride (MTHPA) was investigated by differential scanning calorimetry at different heating rates. Activation energy was calculated based on Kissinger method and varied in the range of 67-72 kJ/mol depending on sample. The curing kinetic behavior was well described by Sestak-Berggren (SB) model and the order of the curing reaction is observed to be from 0.02 to 2.11 according to sample.     

Kinetics of the degradation of polystyrene in supercritical acetone

The kinetic analysis of the degradation of polystyrene (PS) in supercritical acetone has been studied using the nonisothermal weight loss technique with heating rates of 3, 5 and 7 °C/min. The weight loss data according to degradation temperature have been analyzed using the integral method based on Arrhenius form to obtain the kinetic parameters such as apparent activation energy and overall reaction order. The kinetic parameters obtained from this work were also compared with those of the thermal degradation of PS in nitrogen atmosphere. From this work, it was found that the activation energies of PS degradation in supercritical acetone were 73.3-200.7 kJ/mol and lower than those of the thermal degradation in nitrogen atmosphere.     

Effect of three boron flame retardants on thermal curing behavior of urea formaldehyde resin

The purpose of the study was to investigate the effects of three kinds of flame retardant (FR), boric acid, zinc borate, and borax on the thermal curing behavior of urea–formaldehyde (UF) resin. Both pH value and gel time were measured to study the curing characters of the UF resin with different loading levels of FR. In addition, differential thermal analysis was also used to obtain kinetic analyses parameter. The results showed that boric acid decreased pH value of UF resin, and reduced gel time of the UF resin. There are no significant changes of the UF resin curing characters with different loading levels of FR. The activation energies for curing reaction of UF resins in the presence of boric acid, zinc borate, and borax, were 84.37, 84.41, and 118.4 kJ/mol, respectively, higher than that of the control one (75.38 kJ/mol). All FRs showed adverse effect on the curing behavior of the UF resin.     

Effect of P/Si polymeric silsesquioxane and the monomer compound on thermal properties of epoxy nanocomposite

The unique polymeric silsesquioxane/4,4′-diglycidyether bisphenol A (DGEBA) epoxy nanocomposites have been prepared by sol-gel method. The structure of nanocomposites was characterized by attenuated total reflectance (ATR) and solid state ²⁹Si NMR. The characteristic intensity of trisubstituted (T) structure was higher than that of tetrasubstituted (Q) structure from solid state ²⁹Si NMR spectra of 3-isocyanatopropyltriethoxysilane (IPTS) modified epoxy. The activation energies of curing reaction of epoxy system and IPTS modified epoxy system are 28-66 kJ/mol and 57-75 kJ/mol, respectively, by Ozawa’s and Kissinger’s methods. The triethyoxysilane side chain of IPTS modified epoxy might interfere the curing reaction of epoxy/amine and increase the activation energy of curing. The thermal degradation of nanocomposites was investigated by Thermogravimetric analysis (TGA). The char yield of nanocomposites was proportional to the 2-(diphenylphosphino)ethyltriethoxysilane (DPPETES) moiety content at high temperature. A higher char content could inhibit thermal decomposition dramatically and enhance the thermal stability. Moreover, the nanocomposites possess high optical transparency.     

In-situ investigations of the curing of a polyester resin

Quasi-isothermal curing of a polyester resin was studied at different catalyst concentrations and temperatures in-situ by ¹H-NMR relaxometry and NIR spectroscopy simultaneously. Sample and probe temperatures were also recorded. An autocatalytic kinetic model, optionally including a diffusion term, was successfully applied to describe and predict the curing kinetics of the polyester resin as a function of temperature and catalyst concentration, although the diffusion effect is relatively weak in the investigated system under the experimental conditions. The corresponding kinetic coefficients and the reaction activation energy were obtained by fitting the models to the data, assuming an Arrhenius relation.     

Lifetime predictions for semi-crystalline cable insulation materials: I. Mechanical properties and oxygen consumption measurements on EPR materials

Long-term accelerated aging studies (up to 7 years of aging) were conducted on four typical EPR materials used as cable insulation in nuclear power plant safety applications with the goal of establishing lifetime estimates at typical aging conditions of ∼50 °C. The four materials showed slow to moderate changes in mechanical properties (tensile elongation) until just before failure where abrupt changes occurred (so-called “induction-time” behavior). Time-temperature superposition was applied to derive shift factors and probe for Arrhenius behavior. Three of the materials showed reasonable time-temperature superposition with the empirically derived shift factors yielding an approximate Arrhenius dependence on temperature. Since the elongation results for the fourth material could not be successfully superposed, consistency with Arrhenius assumptions was impossible. For this material the early part of the mechanical degradation appeared to have an Arrhenius activation energy E_a of ∼100 kJ/mol (24 kcal/mol) whereas the post-induction degradation data had an E_a of ∼128 kJ/mol. Oxygen consumption measurements were used to confirm the 100 kJ/mol E_a found from early-time elongation results and to show that the chemistry responsible before the induction time is likely to remain unchanged down to 50 °C. Reasonable extrapolations of the induction-time results indicated 50 °C lifetimes exceeding 300 years for all four materials.     

Influence of temperature on the thermo-oxidative degradation of polyamide 6 films

The thermo-oxidative degradation of polyamide 6 (PA6) was studied at relative high temperatures (between 120 and 170 °C) using oxygen uptake and hydroperoxide determination methods, chemiluminescence, FT-IR and UV-VIS spectroscopy as well as solution viscosity and tensile property measurements.The relation between the results of the different analytical techniques and influence of temperature on these relations was determined. Arrhenius plots of the degradation determined with the different methods are linear; however the activation energies determined from these plots depend on the analytical method used. For oxygen uptake measurements and changes in UV absorbance (at 280 nm) and solution viscosity an activation energy of about 120 kJ/mol was calculated, for the increase in carbonyl index of about 80 kJ/mol and for the decrease in elongation at break of about 150 kJ/mol.The changes in oxygen uptake UV absorbance and solution viscosity are probably due to the same chemical process. The lower activation energy from changes in the carbonyl index is attributed to the formation of gaseous products, which play a larger role at higher temperatures. The higher activation energy from the elongation at break measurements was ascribed to the contribution of physical changes that play the largest role at the highest temperatures.     

Cure kinetics of epoxy-amine resins used in the restoration of works of art from glass or ceramic

The cure kinetics of two epoxy/amine resins, Araldite 2020 and AY103-HY956 widely used as adhesives in the restoration of works of art from glass or ceramic was investigated using FTIR spectroscopy. These resins are two-part adhesives, consisting of a resin - A, based on a diglycidyl ether of bisphenol A, and a hardener - B which is either a cycloaliphatic amine (isophorone diamine) for Araldite 2020, or a mixture of three aliphatic amines in HY956. The study was based on the collection of IR spectra, in the middle range (4000-600 cm⁻¹), of mixtures of resin and hardener at different proportions and isothermal temperatures (22-70 °C) as a function of curing time. A kinetic model was employed to simulate the experimental data using two kinetic rate constants. Diffusion control was incorporated to describe the cure behaviour at high degrees of conversion. From fitting to experimental data the kinetic and diffusional parameters were estimated, together with the activation energies of the kinetic and autocatalytic rate constants. It was found that higher degrees of curing are obtained at higher temperatures and increased amounts of hardener. Differences in the performance of the two adhesives are explained based on the type of the amines used as hardener.     

Curing kinetics of arylamine-based polyfunctional benzoxazine resins by dynamic differential scanning calorimetry

In this study, the curing kinetics of polyfunctional benzoxazine resins based on arylamine, i.e. aniline and 3,5-xylidine, designated as BA-a and BA-35x, respectively, were investigated. Non-isothermal differential scanning calorimetry (DSC) at different heating rates is used to determine the kinetic parameters and the kinetic models of the curing processes of the arylamine-based polyfunctional benzoxazine resins were proposed. Kissinger, Ozawa, Friedman, and Flynn-Wall-Ozawa methods were utilized to determine the kinetic parameters of the curing reaction. BA-a resin shows only one dominant autocatalytic curing process with the average activation energy of 81-85 kJ mol⁻¹, whereas BA-35x exhibits two dominant curing processes signified by the clear split of the curing exotherms. The average activation energies of low-temperature curing (reaction (1)) and high-temperature curing (reaction (2)) were found to be 81-87 and 111-113 kJ mol⁻¹, respectively. The reaction (1) is found to be autocatalytic in nature, while the reaction (2) exhibits nth-order curing kinetics. In addition, the predicted curves from our kinetic models fit well with the non-isothermal DSC thermogram.     

Synthesis of phenol-formaldehyde resol resins using organosolv pine lignins

Lignin was extracted from white pine sawdust by organosolv-extraction using hot-compressed ethanol-water co-solvent. The optimum conditions for extracting lignin from the pine sawdust were found to be at 180 °C with ethanol-water solvent (1:1 wt/wt), where the lignin yield attained ca. 26% with a purity of ca. 83%. The lignin under such conditions was oligomers with a broad molecular weights distribution: M_n of 537, M_w of 1150 and polydispersity of 2.14. Bio-based phenol-formaldehyde resol resins were synthesized using the resultant lignin as the replacement of petroleum-based phenol at varying ratios from 25 to 75 wt.% by condensation polymerization catalyzed by sodium hydroxide. Upon heating the lignin-phenol-formaldehyde resols could solidify with a main exothermic peak at around 150-175 °C, typical of the conventional phenolic resol resins, and a secondary peak at 135-145 °C, likely due to the exothermic reactions between the free formaldehyde with phenol or lignin to form methylophenols. The replacement of phenol with lignin at a large ratio deferred the curing process, and the introduction of lignin in the resin formula decreased the thermal stability of the resin, leading to a lowered decomposition temperature and a reduced amount of carbon residue at elevated temperatures. For practical applications, it is suggested that the replacement ratio of phenol with lignin be less than 50 wt.%. The thermal stability can however be improved by purifying the lignin feedstock before the resin synthesis.     

Addition effect of erbium(III) trifluoromethanesulfonate in the homopolymerization kinetics of a DGEBA resin

Solid bisphenol-A epoxy resin of medium molecular weight was cured using a Lewis acid initiator (erbium(III) trifluoromethanesulfonate) in three different proportions (0.5, 1 and 2 phr). A kinetic study was performed in a differential scanning calorimeter. The complete kinetic triplet was determined (activation energy, pre-exponential factor, and integral function of the degree of conversion) for each system. A kinetic analysis was performed with an integral isoconversional procedure (model-free), and the kinetic model was determined both with the Coats-Redfern method (the obtained isoconversional E value being accepted as the effective activation energy) and through the compensation effect. All the systems followed the same isothermal curing model simulated from non-isothermal ones. The “nucleation and growth” Avrami kinetic model A_3/2 has been proposed as the polymerization kinetic model. The addition of initiator accelerated the reaction having higher influence when low temperatures were applied.     

A kinetic study of the decross-linking of cross-linked polyethylene in supercritical methanol

The recycling of cross-linked polyethylene (XLPE) by a decross-linking reaction in supercritical methanol was studied using a batch reactor. XLPEs with initial gel contents of 45, 55 and 65% were employed and subjected to reaction temperatures between 320 and 360 °C. Complete decross-linking of XLPE was achieved in 10 min in supercritical methanol at 360 °C and 15 MPa. For the first time, chemical kinetics for the decross-linking reaction is proposed based on the gel concentration, and applicable to the reactor design. With respect to the gel concentration, the first-order reaction model agreed well with the experimental results. The evaluated kinetic constant was 0.0867 ± 0.0082 cm³/mg min at 350 °C, and the activation energy was 578 ± 25 kJ/mol.     

Effect of silica nanoparticles on solid state polymerization of poly(ethylene terephthalate)

In the present study, the effect of silica nanoparticles, on the solid state polycondensation (SSP) kinetics of poly(ethylene terephthalate) (PET) is thoroughly investigated. At silica concentrations less than 1 wt% and reaction temperatures between 200 and 230 °C higher intrinsic viscosity (IV) values were measured, compared to neat PET at all reaction times. However, with 1 wt% of nanosilica (n-SiO₂), the IV increase of the nanocomposites was similar to that of neat PET and a further increase to 5 wt% n-SiO₂ resulted in significantly lower IV values. A simple kinetic model was also employed to predict the time evolution of IV, as well as the carboxyl and hydroxyl content during SSP. The kinetic parameters of the transesterification and esterification reactions were estimated at different temperatures with or without the addition of n-SiO₂. The activation energies of both reactions were determined together with the concentration of inactive end-groups. From the experimental measurements and the theoretical simulation results it was proved that n-SiO₂ in small amounts (less than 1 wt%) enhances both the esterification and transesterification reactions at all studied temperatures acting as a co-catalyst. However, as the amount of nanosilica increases a number of inactive hydroxyl groups were estimated corresponding to participation of these groups in side reactions with the nanosilica particles. These side reactions lead initially to branched PET chains and eventually (5 wt% n-SiO₂ concentration) to crosslinked structures.     

Effect of activated carbon black nanoparticles on solid state polymerization of poly(ethylene terephthalate)

Solid state polycondensation (SSP) is a conventional method used to increase the molecular weight of poly(ethylene terephthalate) (PET) in order to become more suitable for applications as carbonated soft drink bottles, etc. In the present study, the effect of activated carbon black (ACB) nanoparticles, on the SSP kinetics is examined. TEM micrographs revealed that ACB was finely dispersed into PET matrix as individual nanoparticles without creating agglomerates. Intrinsic viscosity (IV) measurements revealed that at temperatures 210 and 220 °C the activated carbon black does not influence the IV increase. However, at 230 and 240 °C an accelerating effect was found and higher intrinsic viscosity values were measured, compared to neat PET. Furthermore, a simple kinetic model was employed to predict the time evolution of IV, as well as the carboxyl and hydroxyl content during SSP. The kinetic parameters of the transesterification and esterification reactions were estimated at different temperatures with or without the addition of ACB. From the experimental measurements and the theoretical simulation results it was proved that ACB enhances the esterification reaction at all studied temperatures acting as a co-catalyst. However, the transesterification reaction remains unaffected by the presence of ACB at elevated temperatures (230 or 240 °C), while it is reduced at lower values (210 and 220 °C). Finally, the activation energies of both transesterification and esterification were determined together with the concentration of inactive end-groups.     

Application of isoconversional and multivariate non-linear regression methods for evaluation of the degradation mechanism and kinetic parameters of an epoxy resin

The thermo-oxidative degradation of an epoxy resin obtained by curing of an industrially produced DGEBA mixture with 4,4′-methylene-dianiline (MDA) and used as electric insulator has been investigated by TG + DTG + DSC simultaneous analyses performed in static air atmosphere, at five heating rates. TG, DTG and DSC curves showed that, in the temperature range 25-900 °C, a glass transition followed by three thermo-oxidative processes occur. The processing of the non-isothermal data corresponding to the first process of thermo-oxidation was performed by using Netzsch Thermokinetics - A Software Module for Kinetic Analysis. The dependence of the activation energy, evaluated by isoconversional methods, on the conversion degree and the relative high standard deviations of this quantity show that the investigated process is a complex one. The mechanism and the corresponding kinetic parameters were determined by multivariate non-linear regression program and checked for quasi-isothermal data. It was pointed out that the first process of thermo-oxidation of the investigated resin consists in four steps, each step having a specific kinetic triplet. The obtained results were used for prediction of the thermal lifetime of the material corresponding to some temperatures of use and the end point criterion 5% and 10% mass loss.