Heat pulse monitoring of curing and polymer–gas systems

Abstract

SYNOPSISAn implantable thermal probe is used to study the reaction of molten polymers and curing systems to pulse heat release. At the heating rates T 2 5 -lo5 K/s, a polymer system shows the response to pulse heating that is confined in time and reproducible with respect to temperature. This response is related to the abrupt change in the conditions of the contact between the probe and a substance. The temperature of the response T* is determined by the polymer properties and depends on the pressure and T. The pulse thermal probe method, which includes two measuring procedures, complementing each other, has been used to monitor a number of processes in polymeric systems. The variation of the values of T* and the thermal activity of a polymer is compared with the variation of its molecular weight (A& -10'-lo6), the molecular weight distribution, and the concentration of a low molecular weight component. The method allows one to trace the kinetics of polymerization and curing and the kinetics of dissolution of volatile impurity and polymer devolatilization, and to determine the limit of supersaturation of gas solutions in molten polymers. The pulse repetition frequency is changed from 0.1 to 1 Hz. The heated volume of a substance is mm3. 0 1994 John Wiley & Sons, Inc. INTRODUCTIONThe methods of the study of the substance properties and characteristics of processes based on the recording and analysis of the system response to some perturbation ( mechanical, acoustic, electrical) have found a wide application for polymers.' We used for these purposes the method of pulse heating of a substance on the surface of a wire probe2s3 and revealed the response confined in time and reproducible with respect to temperature of a polymeric system to heat release. The nature of this response is related to an abrupt violation of the conditions of the probe contact with a substance. The temperature of the response depends on the instantaneous composition and physicochemical properties of a substance. Its value is above the traditional temperature range for experiments with polymers. By the character of the response, the observed phenomenon is similar to that of spontaneous nucleation in superheated low * To whom correspondence should be addressed. , but shall discuss the possibilities of the practical use of the method of pulse heating for the evaluation of a number of parameters of polymeric systems and processes, in particular, the value of the molecular mass and polydispersity, kinetics of curing, kinetics of dissolution or removal of volatile impurities, and the boundaries of the stability of a condensed state of an oligomer supersaturated with a gas. The prospects of effective use of the method are based on its speed, microquantity of the analyzed substance, independence of the observed parameter (T* ) on the medium temperature, and compactness and noise im-munity of the equipment employed. The characteristic time of heating is of the order of 10-4-10-5 s, and the heated volume is of the order of 10 p4 mm3. The pulse repetit...

Estimation of the critical constants of long-chain normal alkanes

“…The peculiarities of measuring the temperature of the attainable superheat of polymer melts, and the diagram of the apparatus are given in Ref. 29.…”

“…The dependences of the temperature of the attainable superheat and the pressure p~ on the pulse duration are most probably caused by the thermal decomposition of polyethylene. An increase in the t* value produces the same effect on the values of T* and p~ as the introduction of a low molecular weight component into the initial substance [29,30]. Therefore, a certain concentration of the products of thermal decomposition C can be attributed to every curve T*(p) at a fixed heating time t*, and the values ofpl and T*(pl) may be considered close to the critical constants of the solution.…”

Nucleation of a Vapor Phase and Vapor Front Dynamics Due to Boiling-Up on a Solid Surface

“…Consequently, the boiling site turned out to be localized on the heating surface. Numerous experiments have been carried out with oscillography and high-speed recording of the surface boiling processes [14][15][16][17][18][19][20][21][22][23][24]; extensive data have been obtained on the attainable superheating of liquids during their pulsed heating [4][5][6]. However, there was uncertainty associated with the randomness of the boiling site on the surface and the influence of the thickness of the heated layer (the temperature gradient normal to the heating surface) on the dynamics of the observed processes.…”