ANALYTICAL EDITION
106
Vol. 1: No. 2
A Temperature-Recording Micropress for Studying the Course of Vulcanization‘ James C. Walton BOSTONWOVEN
H O S E AND
RUBBERCOMPANY, CAMBRIDGE, MASS.
A new temperature-recording, electrically heated and in which, second, the samSURVEY of the extenmicropress for use in connection with the microscopical ple could be kept a t any desive literature on the observation of vulcanization is described. This instrusired temperature. n a t u r e of vulcanizament permits the duplication of press cures on a small The micropress described t i o n reveals the fact that, scale and renders it possible to determine the temperabelow utilizes a thermocouple almost without exception, ture at which various changes take place during cure. to measure the temperature work on this problem has been The vulcanization of several non-sulfur mixtures is of the sample and thus fulfils confined to purely analytical studied, certain differences in behavior being noticed both of the requirements outm e t h o d s of a t t a c k . This between this type of cure and that of mixtures containlined above. classical chemical Drocedure ing sulfur. shows only the find result of Description of Micropress vulcanization-that is, t h e The micropress, shown in modified cross section in Figure sum of all changes that have taken place-and thus ignores changes in the degree of dispersion, an inversion of phases, and 1, consists of three parts: the top portion, A , around which other changes which may be of fundamental importance but the chromel heating coil is wound, the middle member, B, in which the thermocouple is incorporated, and the bottom which cannot be detected by the older chemical methods. Microscopic investigation, on the other hand, offers a new part, C, which holds the sample. The top part is shaped like an annular ring and is grooved method of attack, with many possibilities, inasmuch as it permits observation of the progressive changes that take on the outside to permit the winding of the chromel resistplace throughout the vulcanizing process and leads to a ance which constitutes the heating coil. I n order to show better knowledge of colloidal changes, which may in them- more clearly the construction of the apparatus, it has been selves be sufficient to explain vulcanization. Weber2 was drawn without the asbestos lagging, which in practice covers the first to recognize the insufficiency of analytical methods, the top and side of the member A in order to confine the heat and he introduced the microscope in the study of the bloom- and to protect the objective of the microscope. The middle member, B, is cup-shaped to permit the focusing of sulfur. This technic was later utilized by Breuil,8 Loewen,4 and Regnaud. All these investigators first heated ing of the microscope on the sample. A thermocouple is their samples in an oven and then subjected them to micro- incorporated in this member by inserting an iron and a conscopical examination; hence no simultaneous curing and stantan wire in this member so that their ends are just flush with the metal surface which fits over the sample. microscopical observation was possible. The bottom section, C, is recessed to receive the sample, It was only during the past year or so that further work was done along this line by Hauser and by Dannenberg. which is placed between a circular slide and cover glass. The micropress developed by Hauser6 employs steam as the A small hole is bored through both bottom and middle memheating medium. I n this equipment the temperature of the bers in order to allow the light from the condenser to pass sample is found from the temperature-pressure relation of through the sample and up into the microscope. It is evident that the middle member, which is made of saturated steam, and accordingly this apparatus can only be used for work above 100’ C. Since the press was designed mild steel and has inserted in it an iron and a constantan for curing a t one definite temperature, it is not adaptable to wire, acts as the hot temperature measurement where the temperature is rapidly junction of the therchanged. Dannenberg’ devised an electrically heated mjcro- m o c o u p l e . B o t h press which permits the examination of the sample through- wires are i n s u l a t e d out the curing operation. With this equipment the tem- and attached to the perature of the sample is found by inserting a thermometer terminals of a milliin a small hole in the apparatus midway between the heating v o l t m e t e r , 0 to 10 coil and the sample. Because of the location of the ther- m i l l i v o l t s r a n g e , mometer and because no account is taken of such factors as which act as the cold rate of heating and radiation and convection losses, this junction of the couple. T h e thermocouple method of measurement does not give the true temperature as constructed records of the sample. The ideal equipment for microscopic vulcanization would t h e temperature of be one which, first, would permit the accurate recording of only that part of the the temperature of the sample over a range of 30” to 170” C., hot junction exposed Figure 1-Cross Section of Micropress to the air, and since 1 Presented under the title “Microscopical Observations on the Vulthis is far from the temperature of the sample it was necescanization of Sulfur-Free Rubber Compounds” before the Division of Rubsary to calibrate it. This was done by placing organic crysber Chemistry at the 76th Meeting of the American Chemical Society, tals of known melting point between a slide and cover glass Swampscott, Mass., September 10 to 14, 1928. 2 Weber, “Chemistry of India Rubber,” p. 110,London, 1919. in the same position as the rubber sample. Current was 8 Breuil, Caoutchouc, 2,82, 118, 158,197 (1905). applied to the heating coil and the melting of the crystals 4 Loewen, Gummi-Ztg., 27, 1301 (1913). was observed microscopically, the millivoltmeter reading 6 Regnaud, Chimie et industrie, 18, 93T (1927). being taken at the time the crystals melted. A number o f 0 Hauser, “Colloid Symposium Monograph,” Vol. VI, p. 207 (1928). crystals of wide melting range were used in the calibration, 7 Dannenberg, Kautschuk, 8, 104,128 (1927).
A
INDUSTRIAL A N D ENGINEERING CHEMISTRY
April 15, 1929
Figure 2-Mixture
Figure 4-Mixture
I1 before Cure.
750 X
I1 after Cooling Following Cure.
750 X
Curing Agent
Most of the work done in the study of vulcanization has been carried out with sulfur as the curing agent. Ostromuislenskii,* st even^,^ and Fisher and GrayLohave shown, however, that it is possible to vulcanize rubber with nonOstromuislenskii, J . SOC.Chem. Ind., 85, 59, 369 (1916). Stevens, I b i d . , 36, 107 (1917). 1 0 Fisher and Gray, IND.END.CAEM., 20, 294 (1928). 9
I1 Heated.
Figure 3-Mixture
and a curve was determined for the temperature range usually encountered in vulcanization-via., 30" to 170" C. I n using the apparatus, the sample is placed between a circular slide and cover glass and is placed in the recess in the bottom member. The middle portion is next put in position over the sample and the bottom screwed into the top member. These parts are screwed together until the sample is compressed or squeezed out sufficiently thin to permit light from the condenser to pass through the sample.
8
107
Figure 5-Mixture
750 X
750 X
I V before Cure.
sulfur compounds such as m-dinitrobenzene and benzoyl peroxide, the physical properties of the cured rubber being similar to those of rubber vulcanized with sulfur. Experimental
The following experimental work is by no means complete, but was performed in an endeavor t o demonstrate the possibilities of micro-vulcanization in the equipment described. Incidentally, it shows the effect of non-sulfur compounds as curing agents. Five mixtures were cured, mixture I being a control to judge the state of vulcanization of those mixtures cured with non-sulfur agents. I
Smoked sheets Sulfur Litharge m-Dinitrobenzene Diphenylguanidine
100 3 10 .
..
111 100
I1 100
.
IV 100
v
.. ..10 .... .... 10
..
3
3
1
..3
100 3
1
,
ANALYTICAL EDITION
108
Figure 6-Mixture
,
IV Heated.
750 X
These mixtures were cured 60,90, and 120 minutes a t 145" C. in a press. During cure mixture I showed no increase in dispersion of the litharge, the only change observable being a coating of the litharge particles by a layer of lead sulfide, with a consequent darkening of the slide. This conforms with Pohle'sll assertion that during cure some compounding ingredients cause a chemical reaction which is confined to the surface of the reacting substances. I n the case of this compound there is no reappearance after cure of colloidal sulfur and no recrystallization of this element. A further study of this type of mixture would probably throw some light on the fact, known in the rubber footwear and topping industries, that litharge stocks are much less susceptible to bloom than mixtures cured with organic accelerators. Mixtures I1 and I11 were found to cure, but no cure was effected in the case of mixtures IV and V, the criterion of cure being that the vulcanized mixture had nerve, elasticity, and tensile strength of the same order of magnitude as the rubber-sulfur-litharge control mixture. The photomicrograph (Figure 2) shows mixture I1 before cure, the m-dinitrobenzene being present as light-colored needles, while the dark particles are litharge. When the mixture is heated (Figure 3) the m-dinitrobenzene melts at 40" C. and a flow similar to that observed with sulfur cures takes place. No change in the degree of dispersion is discernible. When the mixture cools down after cure (Figure 4) minute colloidal droplets appear, but no recrystallization of the mdinitrobenzene takes place. Figure 5 shows mixture IV, rubber and m-dinitrobenzene, before cure. When this mixture is heated a t 145" C. for 2 hours, the m-dinitrobenzene melts and an optically empty picture is obtained (Figure 6). No vulcanization takes place, and on cooling an immediate reappearance of m-dinitrobenzene crystals is observed (Figure 7). Mixture V acts in the same manner, no cure taking place and the m-dinitrobenzene recrystallizing when the mixture cools. It would amear that an activator such as a metallic oxidee. g., litharge-is necessary to secure a cure with m-dinitrobenzene, and that diphenylguanidine, a substance which accelerates vulcanization with sulfur, has no appreciable effect with the compound above.
Figure 7-Mixture
VOl. 1, No. 2
IV on Cooling Following Cure.
750 X
The following two mixtures were cured in order to observe the vulcanieation of rubber with benzoyl peroxide: VI Smoked sheets Benzoyl peroxide Diphenylguanidine
100
6 '18 ....
VI1 100 6 */a
1
Each mixture was vulcanized 15 minutes a t 130" C., and 15 minutes a t 145" C. in a press. A cure was effected according to the criterion set forth above. Benzoyl peroxide more closely resembled sulfur in that it was capable of vulcanizing rubber without the necessity of an activating agent such as litharge. On heating the benzoyl peroxide melted a t 54" C., and upon cooling down after cure it recrystallized in a manner very similar to sulfur recrystallization from a rubber-sulfur mixture. Diphenylguanidine had no effect on cure and no difference was discernible between mixtures VI and VII. Conclusions
A microscopic study of the course of vulcanization offers many possibilities, especially since, by its aid, the progressive changes which take place during vulcanization can be followed. Using the micropress described above, it is possible to observe these changes and to record the temperature at which they take place. A rubber-sulfur-litharge mixture after cure exhibits no recrystallization of sulfur even when cured for a very short time. A further microscopical study of the vulcanization of this type of rubber mixture may yield an explanation of the bloom-preventing properties of litharge. m-Dinitrobenzene and benzoyl peroxide both vulcanize rubber in the absence of sulfur. The former requires an activator such as litharge and does not recrystallize from the mixture when a cure is effected, whereas the latter does not require an activator and recrystallizes after cure in a manner very similar to sulfur.
a *
11
Pohle, 2.wiss. Mikrosko#., 44, 183 (1927).
Acknowledgment
The writerwishes to take this occasion to express his thanks to E. A. Hauser for the interest he has taken in this work and for the helpful suggestions he has given.
