Kinetic studies on shear degradation of polystyrene during extrusion

Kinetic studies on shear degradation of a polystyrene ( M̄ n = 115,700 and P̄ n = 1113) during extrusion in a model extruder were made at different temperatures (170, 190, 210, and 230°C) and shear stresses (340,406, and 472 g/cm 2 ) for determination of rate constants for degradation, reaction order, and effects of temperature and stress on the rate constants. Mechanical energy and effective activation energy ( E *) of bond rupture were related to temperature and applied stress. The following results were obtained. The degradation process is found to be satisfied by a second order reaction over the temperatures and stresses studied with respect to the changes in number average chain length ( P t – P ∞ ), thus –d P /d t = k ( P t – P ∞ ) 2 , where k is a reaction rate constant and t and ∞ refer to degradation times. The mechanical energy of bond rupture has a maximum and the rate constant a minimum at about 180°C, indicating that the least effective temperature for mechanical degradation is about 180°C. E * decreases with increasing applied stress (τ) as a linear relationship, i.e., E * = E A – ατ. At τ = 0, E * becomes equal to the activation energy for thermal degradation with the value E A = 48.6 kcal/mole which agrees well with literature data. Temperature and stress effects on the rate constant are well expressed for our study by the Arrhenius equation proposed by Zhurkov, et al. , i.e., k = A exp[–( E a – ατ)/ RT ] where A and α are coefficients.