Rates of thermal degradation of polystyrene and polyethylene in a vacuum

Abstract

When heated in a vaccum at 325° to 375°C., polystyrene yields mainly the monomer and some dimer, trimer, tetramer, etc., indicating that the breaks in the molecular chains occur largely at the ends. Polyethylene, under similar conditions, but at a some‐what higher temperature, yields fragments of an average molecular weight of about 700, indicating that the molecular chains break, mostly at random positions. A study was made of rates of thermal degradation of these two polymers by measuring rates of loss of weight of samples by means of a very sensitive tungsten spring balance enclosed in a vaccum. The samples were limited to 5–6 mg. in order to avoid spattering during degradation. Rate curves plotted against per cent loss of weight by volatilization indicate that in the case of polystyrene the process is intermediate between a zero and first order reaction, while in the case of polyethylene, the process approximates a first order reaction. Activation energies were calculated on the basis of rates of degradation at various temperatures. The values thus obtained for polystyrene and polyethylene are 58 and 68 kilocalories, respectively.

Thermal degradation of polymers at low rates

“…The procedme used in the present work is in gen eral the same as that used previously with the spring balance, and has been d escribed in detail [1,2]. In the present method it also required about 15 min from the time the fmnace was raised in position, for the thermocouple R to reach the required tempera tme ( fig .…”

“…The m ethod [1 , 2]2 co nsisted in heating a 4 to 6 mg sample of polym er in a platinum crucible suspended from a tungste n spring balan ce [1,3] and observing at intervals th e loss of weight sustained by th e sample, as indicated by the position of a crossline on th e extensio n of the sprin g . For convenience, the rate experiments were usually planned to b e co mpleted within about 8 to 9 hrs.…”

Factors affecting the thermal stability of polytetrafluoroethylene

“…Polyethylene decomposes appreciably, 0.16 percent p CI' minute at 385 0 C; poly tetrafluoro ethylene decomposes at the same rate at 489 0 0, only 104 deg higher [9,25]. For comparison, bond-dissociation energies in related simple molecules are: for C2H 6 and other lower paraffin , C-C = 80 kcal, C-H = 101 kcal ; for C2F 6 , C-C = 124 kcal, C-F = 107 kcal.…”

Pyrolysis of cellulose in a vacuum

“…The pyrolysis apparatus is the same as that used in the thermal degradation of polytetrafluoroethylene [6], and is shown diagrammatically in figur e 1. In figure 1, A is a quartz tube, elosed at one end and fitted into a larger quartz tube, B , also closed at one end. Tube B is attached to the vacuum system by means of a ground joint, C. A vacuum of about 10-3 mm of mercury could be obtained by means of a rotary oil pump , not shown in the figure , a liquid nitrogen-cooled trap, P , and a m ercury-diffusion pump E. A cylindrical electric heater, not shown in the figure , could be moved in a fixed horizontal position so that tube A fitted approximately in the center of the heater muffle.…”

“…Similarly, experiments 5a to 5f, inclusive, were carried out on the same specimen of h ydrocellulose, and experiments 6a to 6f on the same sp ecimen of viscose rayon . These three series of exp eriments were carried ou t at 280° C. A sample of cotton impregnated with Na' 2 C03 (7%) 6 was pyrolyzed in two consecutive step s, at 251 ° and 291 ° C. A sample of cotton impregnated with N aCl (8%) was pyrolyzed in one step at 280 0 C. Fraction s V25 , on mass-spectrometer analysis, were found to con sist mainly of water mixed wit.h a small amount of acetaldehyde, as is shown later . Fractions V -80 and V -190, also on mass-spectrometer analysis, were found to consist of CO2 and CO, respectively.…”