THE RATE OF ISOTHERMAL HEAT EVOLUTION BETWEEN 150-DEGREES-C AND 230-DEGREES-C OF LIGNOCELLULOSIC SHEET MATERIALS IN AN AIR STREAM

Experimental data on the rate of isothermal heat release from lignocellulosic and delignified boards are analysed.The following parameters are evaluated as a function of temperature between 150° and 240°C: 1) the initial maximum or plateau rate of heat release 2) its subsequent change with time including corresponding rate constants 3) the overall heat release extrapolated to infinite time. The initial rate of heat release around 200°C is 5 to 10 times higher for lignocellulosic boards than for the delignified boards. The activation energy of this initial rate varies from 15 kcal/mole for bleached kraft board to 30 kcal/mole for groundwood boards, with commercial boards of thermomechanical pulps in between. For lignocellulosic boards the initial maximum rate of heat release declines — approximately according to first order kinetics. The decline is only moderately faster at higher temperature.The activation energy of the rate constant varied around 5 kcal/mole for hard and semi-hardboards, which indicates a reaction rate limited by diffusion, probably into the fiber or fibrous walls. Accordingly the total heat release extrapolated to infinite time, i.e. the total oxidation reaction increases very significantly with increasing temperature. For delignified boards, a constant or at higher temperatures an increasing rate of heat release with time is interpreted as a radical initiated oxidation, maybe catalysed by some of the intermediate oxidation products — but retarded by radical scavengers formed in the degradation of lignin. The corresponding chemical reactions are discussed and also exemplified by the rate of wet strength development and of chain cleavage. Oxidative formation of carboxylic and carbonylic groups is considered to be the main source of heat release. © 1990, Walter de Gruyter. All rights reserved.