Effect of chemical structure on the crosslinking behavior of bismaleimides: Rheological study

The effect of molecular structure on the cure characteristics of bismaleimides (BMIs) was investigated by rheological measurements. BMI resins of different chemical structures were used, prepared from diglycidyl ether of bisphenol A and N-(3-carboxy phenyl) maleimide or N-(4-carboxy phenyl) maleimide. Temperature dependence of the rheological data was correlated to the Arrhenius equation, from which the activation energy of crosslinking was calculated.Upon heating in dynamic curing, viscosity passed through a minimum then sharply increased due to increasing amount of the crosslinking reaction. The temperature at minimum viscosity increased as the chain length became longer. After passing the minimum point, viscosity increased much faster for meta BMIs (3BE1, 3BE2) than para BMIs, however, the difference in the chain length did not show any appreciable difference in the viscosity increase rate (dη^∗/dT). Nonetheless, the final viscosity was lower for longer chains, because their crosslinking density would be lower.In isothermal curing, the final crosslinking density was expected to increase with temperature, which was shown via the increase in the final viscosity. The gelation time decreased with temperature, and the activation energy of the crosslinking reaction was obtained. Using the reaction kinetics parameters obtained from dynamic scanning calorimeter and Arrhenius type equation, the viscosity change during the isothermal curing was simulated and compared with the measurements.     

Chemorheological study of the curing kinetics of a phenolic resol resin gelled

The changes in the resin viscosity, conductivity, mass, and enthalpy during curing reactions have been studied to obtain kinetic parameters that allow modeling of the resin behavior throughout its industrial application. In this work, isothermal rheological tests of a phenolic resol resin were performed in order to study its complex viscosity during crosslinking reactions. Samples were prepared by a precuring treatment in a heated plate press to reach gel point of the resin. Rheological analyses of resol resin curing were carried out at five different temperatures (80-100 °C), and the kinetic models of Arrhenius and Kiuna were applied. The resol resin curing presented an activation energy of 72.1 kJ/mol according to the Arrhenius model. The Kiuna model was proposed to fit the non-linear evolution of the resin’s complex viscosity at the highest temperatures. This kinetic model was suitable for predicting the changes in the complex viscosity of the resol resin after its gelation, and the process activation energy obtained for the second order polynomial applied in this model was 88.1 kJ/mol. In addition, the profile for the degree of curing of resol resin was determined from measurements of the material’s elastic modulus.     

Allylic monomers as reactive plasticizers of polyphenylene oxide. Part III – Rheological and mechanical properties

The chemorheology of blends of diallyl ortho-phthalate (DAOP) as reactive plasticizer of polyphenylene oxide (PPO) were monitored during their cure with either dicumyl peroxide (DCP) or tert-butyl hydroperoxide (TBHP), and their mechanical properties and morphology were studied. The steady shear and dynamic rheology behaviour was consistent with chemical gelation of DAOP in blends with low concentrations of PPO but the gelation behaviour at higher PPO concentrations was more complex. Dynamic mechanical thermal analysis of the blends of PPO:DAOP cured with either DCP or TBHP indicated a two phase structure. For PPO:DAOP/DCP, the lowest transition (between 150 °C and 200 °C) was attributed to a DAOP-rich phase and its T_g was higher than that for pure DAOP/DCP due to the presence of PPO in the DAOP-rich phase. The smaller damping shoulder near 250 °C was caused by a PPO-rich phase with a T_g that was lower than pristine PPO due to the presence of unpolymerized or polymerized DAOP. In contrast, the glass transition region of the PPO:DAOP/TBHP system was very broad due to an overlap of the transitions for DAOP-rich and PPO-rich phases caused by higher levels of unpolymerized DAOP. SEM observations of the blends revealed a two phase morphology with PPO-rich particles in a poly-DAOP matrix for blends with ?30 wt% PPO, a co-continuous morphology for blends with 40 wt% PPO, and a phase inverted morphology with more than 50 wt% PPO. These SEM observations agree with studies of the swelling, disintegration or dissolution of matrix of the blends in solvent.     

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.     

Chemical shrinkage and thermomechanical characterization of an epoxy resin during cure by a novel in situ measurement method

In this study, a novel technique is presented to enable the characterization of the dimensional changes and evolution of mechanical properties of a resin during cure. This is achieved using an innovative in situ device called thermal flux cell combined with a Dynamical Mechanical thermal Analyser (DMA). With this system, it is now possible to eliminate the sources of error induced while combining two or more instruments. This device consists into a mold containing the resin where the upper and lower surfaces acting as heat flux sensors. Changes in temperature and thermal flux are directly monitored as well as the dynamical displacement and the stiffness during the curing process. In this work, an epoxy DGEBA resin was used to demonstrate the innovative approach. The tested resin was characterized using different vibration frequencies and amplitudes of the DMA. The results were then processed in order to provide accurate data on gel time and cure kinetics behavior. The volume and mechanical changes were also derived from experimental data and linked to the degree of cure. Chemo and thermo-mechanical models were created to predict the changes in chemical shrinkage and stiffness during cure.     

自由基聚合反应挤出过程的化学流变分析

进行了甲基丙烯酸正丁酯在异向旋转双螺杆挤出机内的自由基聚合反应过程数值模拟,描述了单体转化率、重均分子量、流体粘度等物理量的变化特点,分析了其影响因素,获得了材料体系的化学流变规律.模拟结果与实验结果基本吻合.     

Spectroscopic probes for real-time monitoring of polymer modification and degradation reactions

The processing of polymers may be accompanied by oxidation and elimination reactions that affect the quality of the final product. In reactive processing, polymerisation occurs in the extruder or autoclave and the extent of reaction becomes an important process variable. The monitoring of these chemical changes in real-time so that processing may be optimised requires the use of analytical methods such as fibre-optic Near Infrared (NIR) spectroscopy. In this paper the requirements and limitations for this and related spectroscopic probes are described and novel approaches based on infrared emission and transient infrared transmission spectroscopy (TIRTS) are discussed. To cite this article: G. George et al., C. R. Chimie 9 (2006).