I
T. E. BOCKSTAHLER, A. P. KOTLOBY,
E. M. BEAVERS, F. H. ZIMMERLI, and 0.C. KOHLER
Rohm & Haas Co., Philadelphia, Pa.
Onset Test for Stability of Uncatalyzed Polymerizable Materials The polymerization curve obtained is characteristic of the system under study and can be used by manufacturer and fabricator in control of new and stored resins
FOR
safely handling monomers and polyester low-pressure laminating resins under varied conditions of large scale manufacture, shipment, and storage, a reliable accelerated stability test is needed. The test should be simple, inexpensive, and recordable automatically. Previous approaches, usually involving a catalyst to speed up reaction rates (77 7), have obvious limitations for predicting stability without catalysts. The, liberation of heat which accompanies polymerization of vinyl monomers provides a means of detecting the onset of polymerization. The relative minuteness of the exotherm which occurs after a period a t ambient temperatures in the absence of catalyst has been a deterrent to the development of tests of the type described here. However, simple thermal initiation of polymerization a t elevated temperatures eliminates this objection and permits good correlation between induction intervals determined at higher temperatures and those predicted a t lower temperatures. I n the onset test, samples are not degassed nor are additives needed ; thus, except for temperature changes during the test, the material is in the same condition as during its storage and handling.
Apparatus The onset test depends upon inexpensive thermocouples to detect the beginning of polymerization. The simple equipment consists of standard test tubes, Teflon thermowells and stoppers, an iron-constantan differential thermocouple system, a sensitive but commercially available recorder, and a constant temperature bath. The differential thermocouple system is similar to that employed in differential thermal analysis-one detector is in the test sample and the second in a nonpolymerizable sample kept always a t bath temperature. Differences in temperature sensed by the two detectors are determined and recorded.
The equipment now in use in these laboratories is shown in Figure 1. The six-point recorder is so wired through a junction box that samples can be tested a t any two temperatures simultaneously. For example, two samples can be tested a t 80’ C., while four others can be run a t 100’ C. Samples and Sample Tubes. Sample tubes of borosilicate glass with rim, 150 X 45 mm. in outside diamkter, were glass is inert toward systems, and the m charge of resin or monomer in the approximate dimensions of a cube. The size of the tube and the amount of the sample are important. A 50-gram sample in the prescribed test tube is a practical compromise betweeen maximum exotherm (in the center of the sample) and economy of sample. The test is based particularly on the fact that poor heat transfer in the sample allows the heat of polymerization to build up. in the center of the mass, so that a thermocouple can detect it. For many commercial polyester resins, an exotherm of about 4’ C. is measured a t the center of a 50-gram sample when polymerization starts a t 100’ C. A smaller temperature rise is noted when a 32-gram used in the same tu for a 5-gram samp found also as the thermocouple is moved
Figure 1 .
closer to the wall of the sample tube. Use of the onset test requires careful adherence to a standardized technique. The reference sample should approximate the test sample in volume and heat transfer chardcteristics. I t is permanent and must be heat-stable and inert. Silicon fluid is satisfactory, whereas saturated aliphatic esters decompose during extended usage. The reference sample should always remain in the bath a t temperature or be allowed ample time to reach temperature equilibrium before tests are begun. Thermowells and Stoppers. Thermowells are necessary to shield the sample from the metal of the thermocouple junction and to suppbrt and locate the wires and sensing tip. When the materials to be tested are thermosetting in nature, it may be difficult to recover a thermowell from the center of a “casting” which results when samples are allowed to remain i.n the bath after the gel point has been reached. Teflon proved to be inert and satisfactory for use in semipermanent thermowells in this application. Neither the gelling nor the gelled resin adheres well to it. Normal life expectancy for continued use is from 3 to 6 months. The thermowell is conical in shape and is drilled lengthwise for the thermocouple. The walls of the well a t the sensing tip should be uniform and no thicker than ‘/16 inch. Stoppers may be turned from clean
Equip-
ment for the onset test can be
of economical space and rugged in construction
.
VOL. SO, NO. 10
OCTOBER 1958
158 J
minutes at 90' and 100' C., respectively. A plot of these values leads to a pre-
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corks (not rubber) or, better still, from Teflon. These may not only be :ecessed to rest in the top of the test tube but threaded to hold the thermowell also. Sufficient projection into the test tube should be allowed to protect the resin completely from light after the tube is inserted up to the rim in the bath cover hole. Should light reach the test sample, results will be erratic. The tip of the thermowell must always be centered in the resin sample. Thermocouple System. Only the iron-constantan couple has been tested. "umber 30 duplex wire with glass insulation may be used. However, the commercial pencil-type iron-constantan thermocouple, l / g inch in outside diameter, with iron-constantan jacks and plugs proved rugged and dependable. The differential system used in the onset test involves two junctions, the iron leads of both being connected to each other and the remaining two constantan leads being connected to the reactor terminals. One junction is inserted into the test sample and the other into the reference sample. Because of the directional property of the thermocouple output, interchange of the junctions or of the lead wires will reverse the movement of the recorder pen. Recorder. Both strip and circular chart instruments have been tested and found useful. However, the strip chart recorder is preferable for materials which show mild exoTherms. For the present work a hlinneapolis-Honeywell Electronik strip recorder, Type 153, is used. I t has a single pen with a speed of 12l/2 seconds and a span of -0.5 to +0.5 mv. direct current designed for iron-constantan thermocouple use. A 2X amplifier and a basic chart speed of 2 inches per hour are specified [Model No. Y153X11V-X-27 (V) for 115-volt alternating current operation]. The span of this instrument is centerzeroed. Thus, during an experiment, when the temperatures of the test and reference samples are identical, a straight line is produced down the center of the paper (zero output). During the test sample heat-up period, a curve is produced on the negative side of the center zero. I n actual practice, the zero point may be shifted to a point about one
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third of full scale. This proportioning increases the amount of the span available for recording the exotherm. Constant-Temperature Bath. The bath must be controlled at temperatures u p to 110' C. within 10.25' C. maximum range. I t is better to shield it completely against the entrance of light, than to attempt to shield each sample separately. Use of a close-fitting cover, in which circular holes slightly larger than the sample tubes have been cut, is adequate. The thermowell supporting stopper provides shielding of the upper part of the test sample itself. Test holes not in use are covered with individual metal plates.
Applications of Test In Figure 2 are reproduced the onset test exotherm curves of four experimental low-pressure laminating resins having distinctly different compositions. The values differ markedly from each other, which permits a manufacturer of such resins to set up unique specifications for each one. Replication of the characteristic values for any sample is within 3% when all test conditions are properly controlled. Thus, close quality control is made possible. A fundamental knowledge of factors governing the stability of these products may be gained also when variations are introduced deliberately and effects noted. One of the main applications of onset times is the prediction of stability at lower temperatures, at which measurements are impractical or time-consuming. For example, the onset times of a given material can be measured at several elevated temperatures. The logarithms of the onset times are then plotted against the reciprocals of the absolute temperatures of the measurements. In agreement with Arrhenius theory, the data show a straight-line relationship when so treated. By extrapolation of this line, one can obtain predicted values for stqbility of the material at storage and processing temperatures. A reasonable correlation has been demonstrated between values predicted in this way and those actually observed. For example, a typical low-pressure laminating resin gave onset values of 540 and 300
INDUSTRIAL AND ENGINEERING CHEMISTRY
dicted stability of 400 hours at 50" C. Samples of the same resin stored in sealed ampoules at 50" & 2' C. showed first evidence of polymerization after 390 hours. Gelation occurred during the next 16 hours. Onset times more closely approximate the start of polymerization than do gel times in uncatalyzed systems. This results in better agreement between observed values of onset times and those predicted from the Arrhenius straightline relationship than is true of observed and predicted gel times. The theoretical basis for the relationship requires that the gel time be equated to the time to destroy inhibiting impurities plus the time for polymerization to proceed to the gel point. This last term is usually assumed to be negligible. on the basis that gelation often occurs very shortly after polymerization starts. The assumption is reasonable at high temperatures but becomes less so at low temperatures, at which uncatalyzed polymerization rates are slow. Use of onset times reduces the error introduced by this assumption.
Acknowledgment Temperature and gel-time relationships were developed originally in these laboratories by E. M . Beavers; more recently, Serge Gratch confirmed their theoretical basis. The skillful assistance of Matthew Tomkowicz and Michael Reno is gratefully acknowledged. Photographs of the test equipment were taken by S. C. Chmielewski.
literature Cited (1) Bengough, W. I., Melville, H. W., Proc. Roy. Sac. (London) A230, 429 (1955). (2) Burnett, R. E., Division of Paint, Varnish, and Plastics Chemistry, 122nd Meeting, ACS, Atlantic City, N. J., September 1952. ( 3 ) Burrell Corp., Fifth Ave., Pittsburgh 19, Pa., "Castor Gelometer." (4) Cass, W. E., Burnett, R. E., IND. EXG.CHEM. 46.1619 11954). (5) Fujii, S.,B@. Chem. Soc. (Jafian) 27, 216. 238 119541. (6)20, FLjii, 409 S (1956). :, Tanaka, S., J . Polymer Sci.
Gordon, M., Grieveson, B. M., Ibid., 17, 107 (1955); 1 8 , 4 9 7 (1955). (8) Majury, T. G., Melville, H. W., Proc. Roy. Soc. (London) A205, 496 (1951). (9) hliyama, H., J . Chem. Soc. (Japan) 77. 196 (1956). (10) 'Nichols, F. S., Bliss, C. H., Modern Plastics 29, No. 9, 124 (1952). (11) Society of the Plastics Industries, Proceedings of Sixth Annual Technical Session, Reinforced Plastics Division, New York, N. Y . , 1951. (7)
RECEIVED for review September 25, 1957 ACCEPTED June 26, 1958 Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on New Developments in Alkyds and Other Polyesters, 131st Meeting, ACS, Miami, Fla., April 1957.
