Thermal polymerization of p-methyl styrene at high conversions and temperatures

An experimental investigation of the kinetics of the thermally initiated free radical polymerization of p-methyl styrene in bulk at temperatures of 120°, 140° and 160° in the conversion range, 0–96% is reported. Conversions were measured gravimetrically and by gas chromatography and molecular weight distributions and weight-average molecular weights by size exclusion chromatography (SEC) and low angle laser light scattering photometry (LALLSP). A kinetic model which accounts for diffusion-controlled termination and predicts conversion/time and molecular weight and long chain branching development for nonisothermal polymerizations has been developed. This model should find use in the design, simulation and optimization of poly(p-methyl styrene) reactors.     

Thermal polymerization of styrene — the formation of oligomers and intermediates, 1. Discontinuous polymerization up to high conversions

During the thermal polymerization of styrene small quantities of dimeric and trimeric styrenes are formed which affect essential properties of the polymer. The kinetics of the oligomer formation are investigated under industrially important reaction conditions and the concentrations of the intermediates formed during the initiation reaction are determined by UV-spectroscopy. The results show that no gel effect occurs in the reactions investigated. Using the kinetic fomulae and reaction rate constants obtained it is possible to describe the formation of oligomers and intermediates at temperatures of 137°C and 180°C up to 97% monomer conversion.     

Thermal polymerization of styrene

An experimental investigation of the kinetics of bulk thermal polymerization of styrene in the temperature range of 200°–230°C is reported. Conversions and molecular weight averages were measured by gel permeation chromatography. At elevated temperatures, oxygen in the polymerization mixture appears to have negligible effect on the rate of polymerization and the molecular weights of the polymer. Experimental evidence suggests that the molecular weight development of the polymer is strongly influenced by transfer reactions.     

Radiation-induced polymerization of styrene at high dose rates

A kinetic equation was derived for the radiation-induced polymerization of styrene under the assumption that both radical and cationic polymerizations take place concurrently throughout the whole range of the dose rate of radiation and the water content of the styrene. The equation enables one to calculate rates of the total, radical, and cationic polymerization at a given dose rate and water content and agrees satisfactorily with experimental results, which cover dose rates from 4.2 × 10 to 2.1 × 10⁵ rad/sec and water contents from 3.2 × 10^?3 to 3.5 × 10^?2 mol/l. Experimental estimation of the contribution of radical and cationic mechanisms was done by GPC curves of polymers obtained under various conditions. When the contribution of ionic mechanism is expressed in weight percent, it changes from 0% to 100% in the range of the experiment; on the other hand, if it is expressed in mole percent, it is independent of the dose rate and remains constant throughout the whole range of the experiment.     

Free radical polymerization of methyl methacrylate at high temperatures

Free-radical polymerization of methyl methacrylate in a tubular reactor has been conducted at above-Tg temperatures. A salient feature of these experiments is the very efficient control of reactor temperature by vapor-liquid equilibrium of the polymerizing mixture via monomer evaporation. The system pressure thus provides a powerful control variable, restricting the temperature in the entire reactor by changing the monomer evaporation rate. In the range of our experimental conditions, the temperature and pressure in the reactor follow the Antoine equation closely. High temperature runs also reduce the length requirement of the reactor. However, molecular weight averages of the products are not impressive, unless slow-burning initiators are used. Modeling of above-Tg reactions has been attempted at two-levels of sophistication. A plug-flow model gives predictions in good agreement with our experimental temperatures and conversion data. The predicted molecular weights are also consistent with the experimentally observed values. However, the more elaborate rheokinctic model suggests that the superficial agreement between model and experiment is due to initiator burn-out, which limits the final conversion to within 40 percent. The liquid layer next to the reactor wall can never be so viscous as to form a stagnant deposit, due to this conversion limitation. The velocity profiles are thus not very much distorted, and a plug-flow model is adequate. With a slow-burning initiator and a sufficiently long reactor, skewing of velocity profile and reactor channeling will eventually emerge. Hence, the rheokinetic model must be evoked to model the system under such conditions.     

Thermal properties of gypsum plasterboard at high temperatures

Light timber frame wall and floor assemblies typically use gypsum‐based boards as a lining to provide fire resistance. In order to model the thermal behaviour of such assemblies, the thermo‐physical properties of gypsum plasterboard must be determined. The relevant literature and the chemistry of the two consecutive endothermic dehydration reactions that gypsum undergoes when heated are reviewed. The values determined for the thermo‐physical properties are modified to create smooth enthalpy and thermal conductivity curves suitable for input into a finite element heat transfer model. These values are calibrated within a reasonable range and then validated using furnace and fire test data. The type of plasterboard used in these tests is an engineered product similar to the North American type C board. The temperature at which the second dehydration reaction occurs is altered to be consistent with later research with little apparent affect on the comparison with test results. Values for specific heat, mass loss rates and thermal conductivity for gypsum plasterboard that are suitable for use in finite element heat transfer modelling of light timber frame wall and floor assemblies are recommended. Copyright © 2002 John Wiley & Sons, Ltd.     

Thermal stability of elastomeric networks at high temperatures

Stress relaxation experiments were carried out in a dry nitrogen atmosphere on polydimethylsiloxane and an ethylene-propylene terpolymer. Relaxation times much longer than expected were observed, and plots of In f(t)/f(0) versus time were linear in all cases.     

An experimental investigation into the gasification reactivity of chars prepared at high temperatures

The gasification activities of three kinds of Binxian chars with carbon dioxide were studied at 1000–1300 °C and under atmospheric pressure in self-made thermal balance. The specific surface area of coal or chars was determined with BET methods during gasification. The results showed that the reaction rate of two rapid pyrolysis chars increases at the beginning and decreases subsequently with increasing carbon conversion at relatively high temperatures. The heating rate of coal has a significant effect on the gasification process. The activation energy of slow pyrolysis char varies between 160 kJ/mol and 180 kJ/mol during gasification. The activation energy of the two rapid pyrolysis chars displays a linear trend when the carbon conversion is less than 40% and decreases slowly afterwards.     

An experimental study for property control in a continuous styrene polymerization reactor using a polynomial ARMA model

By using a multivariable nonlinear model predictive controller (NLMPC), the control experiments for the monomer conversion and the weight-average molecular weight are conducted in a continuous styrene polymerization reactor. Instead of a complex first-principles model, a polynomial auto-regressive moving average model (ARMA) is used to describe the nonlinear behavior of the polymerization reactor. The pseudorandom multilevel input signals mounted on the jacket inlet temperature and the feed flow rate are applied to the polymerization reaction system to identify a polynomial ARMA model. In the experiments of identification and control, the monomer conversion and the weight-average molecular weight are measured by on-line densitometer and viscometer with appropriate correlations. The on-line measurements are found to be in good agreement with the off-line analysis by the gravimetry and the gel permeation chromatography. Since a polynomial ARMA model is expected to give a higher order objective function of input variables, we employ the extended Kalman filter based NLMPC scheme to reduce the computational requirement in the control experiments. The NLMPC based on the polynomial ARMA model is found to perform satisfactorily for the control of the polymer properties during a grade-transition period as well as under the steady-state operation.     

Thermal polymerization of styrene in the presence of TEMPO

To investigate aspects of the contribution of (thermal) self-initiation in nitroxide-mediated radical polymerization (NMRP) of styrene, selective styrene polymerizations with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) in the absence of initiator were carried out at 120 and 130 °C. The results of these experiments (including conversion data, molecular weight averages, polydispersity and molecular weight distribution information) were compared with regular thermal polymerization of styrene and NMRP of styrene in the presence of a bimolecular initiator (benzoyl peroxide; BPO). It was observed that although the thermal polymerization of styrene can be controlled to some extent in the presence of TEMPO to provide polystyrene with low polydispersity, the polymerization was never as controlled as that obtained by a BPO-initiated NMRP.     

Thermal conductivity of Strontium-Barium-Niobat under high pressure and at low temperatures

A new method to meassure thermal conductivity at lot; temperature and under high pressure is developed to investigate ferroelectric materials. Results at Strontium-Barium-Niobat (SBS) shah; the influence of the pressure to the glasslike behavior. Also a strong anisotropie of thermal conductivity is detected at this substance.     

原子转移自由基法合成乙丙橡胶接枝物中的热聚合研究

采用原子转移自由基聚合法(ATRP)合成了三元乙丙橡胶与苯乙烯的接枝共聚物(EPDM-g-St),动力学研究表明聚合过程为“活性”聚合。在接枝聚合过程中发现了明显的苯乙烯热聚合现象。对接枝聚合中得到的均聚苯乙烯进行表征的结果表明,苯乙烯在ATRP接枝体系中的热聚合过程在一定程度上受到ATRP机理的控制;升高温度和延长反应时间使得热聚合更为显著。     

An e.s.r. study of some coals under rapid passage condition at high temperatures and pressure

The e.s.r. spectra of Miike and Taiheiyo coals were measured under rapid passage conditions in the temperature range 150–450 °C. A parameter, R, was defined by a ratio of out-of-phase and in-phase spectral amplitudes and the ratios for Miike and Taiheiyo coals were estimated in this temperature range. It was found that the temperature dependence for R was different for the two coals examined, and the reason for the difference was discussed. Further, the ratios for those coals mixed with pyrene were estimated in the temperature range 150–400 °C at atmospheric and 1.1 MPa nitrogen pressure. Based on the temperature dependence of the ratio the coal liquefaction ability of pyrene for those coals was discussed.     

Theoretical study of hydrocarbon dehydrogenation at high temperatures

A theoretical study of hydrocarbon dehydrogenation in state-of-the-art packed tubular membrane reactors and in impermeable tubes is performed. Conditions of the efficient operation of apparatuses are found under which a high degree of hydrocarbon conversion and yields of products are attained. The possibility of experimentally determining a number of constants for the such processes is shown. Results of theoretical calculations are compared to experimental data for isobutane dehydrogenation.     

ESR study on radical polymerization of styrene

Summary Propagation rate constant (k_p) for styrene was evaluated at different chain lengths of the polymer radical based on the steady-state concentration of the polymer radical determined by means of ESR spectroscopy at 70°C. Over a range of degree of polymerization of the polymer radical from 40 to 410, the value of k_p, 480±10 L/mol·s, remained constant. A considerable increase in viscosity of the polymerization mixture did not affect this value.     

A rubber-modified thermoplastic where the morphology produced by phase-separation induced by polymerization disappears at high conversions

An unexpected experimental finding is reported where the primary morphology developed during polymerization-induced phase separation in a rubber-modified thermoplastic practically disappears at high conversions. This process was evidenced by light scattering (LS) and scanning electron microscopy (SEM) for a particular composition of solutions of polyisobutylene oligomers (PIB) in isobornylmethacrylate (IBoMA), during the free-radical polymerization of the monomer. The primary phase separation produced a dispersion of domains rich in PIB containing significant amounts of the monomer (IBoMA). Polymerization of the monomer in these domains occurred at high overall conversions producing the filling of dispersed domains with a PIBoMA-PIB blend that could not be distinguished from the matrix. Under these conditions the final material had the appearance of a homogeneous blend.     

Quantification of spontaneous initiation in radical polymerization of styrene in aqueous miniemulsion at high temperature

The spontaneous (thermal) initiation rate (R_i,th; no added initiator) in radical polymerization of styrene in aqueous miniemulsion at 110 and 125 °C with sodium dodecylbenzenesulfonate or poly(vinyl alcohol) as surfactants (colloidal stabilizers) has been estimated using a novel approach based on the total number of chains. In qualitative agreement with previous work at lower temperatures, R_i,th was found to be 3.1-15.1 times greater than that in bulk. According to the activation energy and conversion dependence of R_i,th, the radical generation mechanism differs from that in bulk. The experimental evidence is consistent with the enhanced R_i,th in miniemulsion being related to the oil-water interface, with radical generation in the aqueous phase playing a negligible role. The implications with regards to nitroxide-mediated radical polymerization in aqueous dispersed systems are discussed.     

Thermal expansion of Portland cement paste,mortar and concrete at high temperatures

Results of measurements of thermal expansion of Portland cement paste, mortar, concrete and a dolomitic rock aggregate are presented. Test temperatures ranged from 27 to 871°C. Cement paste contracted when subjected to temperatures from 204 to 871°C. Thermal expansion of mortar and concrete was dominated by thermal expansion characteristics of the mineral aggregate. However, expansion was moderated by contraction of the cement paste matrix at elevated temperatures. Average coefficients of expansion, over specific temperature ranges, are given for the materials tested. Equipment developed to control automatically the rate of heating and record strain vs. temperature relationships is described.     

An exhaustive study of chain-length-dependent and diffusion-controlled free radical and atom-transfer radical polymerization of styrene

A finely tuned-up and high-performance Monte Carlo simulation, which takes diffusion-controlled and chain-length-dependent bimolecular termination reactions into account, is developed to thoroughly simulate and compare free radical (FRP) and atom transfer radical polymerization (ATRP) of styrene. It is found out that the termination rate constant falls and eventually plateaus upon the increase of the chain length of radicals. In addition, average termination rate constant greatly decreases during ATRP; nonetheless, it remains almost unchanged and smaller in FRP. Moreover, there is an accumulation of CuBr₂ in the reactor as the ATRP proceeds, while the concentration of CuBr decreases and finally plateaus. Polymer chains are entirely initiated at the beginning of the ATRP, whereas initiation of chains continues throughout the free radical polymerization up to the end of the reaction. Also, the dead polymers are much lower in concentration (only 20%) in ATRP as compared to FRP (about 50%). In addition, a shift (toward higher molecular weight) in the location of the peaks of molecular weight distributions can easily be seen for the ATRP system, whilst the chain length distribution of free-radically generated polymers remains the same throughout the free radical polymerization. The molecular weight distributions narrow as the atom transfer radical polymerization progresses. Finally, the simulation results correspond closely to the experimental data.