Continuous thermally initiated bulk

A mathematical model for styrene polymerization, based on free-radical kinetics, accounts for changes in termination coefficient with increasing conversion by an empirical function of viscosity at the polymerization temperature. Solution of the differential equations results in an expression that calculates the weight fraction of polymer of selected chain lengths. Conversions, and number, weight, and Z molecular-weight averages are also predicted as a function of time. The model was tested on peroxide-initiated suspension polymerizations and also on batch and continuous thermally initiated bulk polymerizations. 

Duerksen and Hamielec (26) collated much of the literature data on the energy of activation for the thermal initiation of stryene in bulk or solution which was available in 1968 and concluded that the value of 115 kJ mol l derived from experiments in a Continuous Stirred Tank Reactor was concordant with the existing values which they quoted. Recent work by Bengough et al. (27, 28) gives values of 121 and 118 kJ mol l supporting the median value of 115 kJ mol . ... 

Superabsorbent polyacrylates are prepared by means of free-radical-initiated copolymerization of acrylic acid and its salts with a cross-linker (12,13). Two principal processes are used bulk, aqueous solution pol5unerization and suspension polymerization of aqueous monomer droplets in a hydrocarbon liquid continuous phase (14) (see Bulk and Solution Polymerizations Reactors Heterophase Polymerization). In either process, the monomers are dissolved in water at concentrations of 20-40 wt% and the polymerization is initiated by free radicals in the aqueous phase (15). The initiators, freeradical (qv) used include thermally decomposable initiators, reduction-oxidation systems, and photochemical initiators and combinations. Redox systems include persulfate/bisulfite, persulfate/thiosulfate, persulfate/ascorbate, and hydrogen peroxide/ascorbate. Thermal initiators include persulfates, 2,2 -azobis(2-amidinopropane)-dihydrochloride, and 2,2 -azobis(4-cyanopentanoic acid). Combinations of initiators are useful for polymerizations taking place over a temperature range. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

Description The process involves continuous, bulk-phase polymerization of styrene using a combination of thermal and chemical initiation. A typical unit design consists of separate reaction trains for GPPS and HIPS grades, which have been optimized for each resin... 

High Impact Polystyrene (HIPS) HIPS is a heterogeneous material produced by continuous bulk or bulk-suspension processes, in which a butadiene-based elastomer (polybutadiene (PB), or a block copolymer of styrene-butadiene) is first dissolved in styrene monomer (St) and the resulting mixture is then heated so that the polymerization proceeds either thermally or with the aid of a chemical initiator. At the molecular level, the product is a mixture of free polystyrene (PSt) chains and elastomer chains grafted with PSt side chains. The process yields a continuous (free) PSt matrix containing... 

However, a commercially feasible process for bulk polymerization in a continuous stirred tank reactor has been developed by Montedison Fibre [103,104]. The heat of reaction is controlled by operating at relatively low-conversion levels and supplementing the normal jacket cooling with reflux condensation of unreacted monomer. Operational problems with thermal stability are controlled by using a free radical redox initiator with an extremely high decomposition rate constant. Since the initiator decomposes almost completely in the reactor. 

As the name implies, continuum mechanics is predicated on the hypothesis that one may describe the properties and behaviour of physical systems entirely in terms of continuous functions of position and time, at least for a single pure component within a single bulk phase (gas, liquid, or solid). Continuum mechanics makes no reference to the fact that real materials are composed of atoms or molecules. Strictly speaking, rationalization of transport coefficients (like viscosity, thermal conductivity, and diffusivity) in terms of molecular behaviour lies within the realm of statistical mechanics and molecular simulations. Continuum mechanics begins to lose its validity when the characteristic length and time scales in a physical system become comparable to molecular scales. Continuum mechanics thus cannot describe the initial stages of SEI film formation composed of many simultaneous, discrete, discontinuous molecular events. 

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