Kinetics of styrene removal from polystyrene by thermal treatment in air

Abstract

SynopsisThe influence of certain factors on the kinetics of styrene mass transfer from polystyrene in air at elevated temperature is investigated. It is established that this process can be simulated by a model including molecular diffusion of styrene to the surface of the solid material and evaporation from that surface. The effective coefficients of molecular diffusion at different temperatures and the evaporation coefficients under different hydrodynamic conditions are determined. The data obtained can be used for estimation of the monomer removal by thermal treatment of the polymer. INTRODUCTIONFrom theoretical and practical points of view, the kinetics of monomer mass transfer from polystyrene in air at elevated temperature is of great interest. Such transfer is expected to take place both in the polymer granulation and in the granulate processing into products. Depending on the rate of the process, on one hand, the monomer contents in the product will change, which is of great importance when it is used for food packaging, and on the other hand, the surroundings will be polluted with styrene to different extents.Studies on this matter carried out so far'-3 are insufficient to predict the rate of the process, since the monomer in them is removed by a liquid with undefined hydrodynamics.Under these circumstances we set ourselves the task of studying the kinetics of styrene transfer from polystyrene in air at elevated temperature. THEORYMonomer transfer in the gas phase around the polystyrene is expected to comprise two stages: diffusion of styrene to the surface of the polymer, and evaporation of the monomer from the surface into the gas phase. Should this mechanism be assumed, the process of transfer from an infinite plane sheet of polystyrene will be described by the differential equation If the styrene initial distribution in the sheet is uniform, the total amount of the monomer leaving the sheet up to time t of contact with the gas phase will be expressed by the equation which results from the mathematical solution of Eq. ( 1 ) at the boundary condition (2).4In the above expression /In are the positive roots of the equationThe evaporation coefficient a depends mainly on the hydrodynamic conditions in the surrounding fluid.5 At suf?iciently large gas flow rates a > > D and L -, 00. Then the mass transfer rate will be determined entirely by the diffusion in the sheet and Eq. (3) In-=In, --tUsing the above equations and appropriate experiments makes possible the determination of the molecular diffusion coefficient and the coefficient of evaporation, needed for estimation of the process. EXPERIMENTALIn order to study the process, plates of impact-resistant polystyrene Bustrene with length 40 111171, width 20 pm, and thickness 1.7 mm were utilized. They were put into a glass tube with regulated external heating. The flow rate of the air passing through it was measured with a rotameter. The desired temperature of the air before and behind the sheet was obtained by varying the voltage applied to the heater coil...