Dec., 1954
CHEMORHEOLOGY OF SPECIALLY PREPARED SILICONE RUBBERS
pared with 4.92 for Ca(0H)Z; and it was actually possible t o show the existence of both reflections in mixtures containing more CaO than the 3Ca0. 2Ti02 ratio which had hydrated in the atmosphere. It will also be seen from Fig. 3 that in addition to the fact that compositions between 50 and 60 mole per cent. CaO yield patterns which are essentially superpositions of the CaTiOs and 3€,a0. 2Ti02 patterns, there is in some cases a systematic shift in both patterns. It is felt that the inconsistent results obtained were due t o inability to retain the high temperature structure by sufficiently rapid quenching in all cases. A definite shift has been noted in several patterns leading t o the qualitative representation shown in Fig. 2 in which some solid solution is shown in each compound toward the CaO end with the extent of solid solution decreasing a t lower temperatures. No quantitative valu$s can be ascribed to the extent of solid solution but it would appear to be less than 2-3 wt. per cent. in each case. Thus the X-ray diffraction data, though not straightforward, confirm the thermal data regarding the presence of the compound 3Ca0.2Ti02. Optical examination of these end members and mixtures although performed under unfavorable conditions due to their high indices of refraction provided confirmatory evidence for this interpretation. When mounted in solid immersion media of index 2.3, both phases can be detected in the mixtures (3Ca0.2Ti02 as birefringent, subhedral laths less than 2.3; CaTQ as isotropic, or nearly isotropic anhedral to rounded grains of index greater than 2.3). This is in agreement with Fisk's'O data on the refractive indices. Quench runs of the 3Ca02"iO2 composition from all temperatures have the same pattern, and microscopic examination fails to show any other phase. All mixtures higher in CaO than the 3Ca0.
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Fig. 3.-XIRay dflractometer patterns of a series of'mixtures from CaaTi207 to CaTiOa (after melting and quenching) illustrating difficulties in interpretation of tliese data; composition of mixtures in weight %, CaO/TiOz:- a, 51.291 48.71 (CaaTi207); b)' 48.65/51.35; c, 46.00/54.00;. d,.43.60[ 56.40; e, 41.24/58.76 ( CaTiOa). Each sub'divlsion, 0.2 2e cu K ~ .
2Ti02 composition contain 3Ca0.2Ti02 in the same form as the pure compound as determined by both X-ray and microscopic methods. This inteppretation of the data is seen to agree with most of the more recent work such as that of Ershov" and Fisk'O and explain some aspects of the earlier work. Failure to retain high temperature phases on quenching to room temperature and the similarities between the two binary compounds would appear to explain some of the discrepancies Acknowledgment.-This work was performed as part of a program sponsored by the U. S. Army Signal Corps, Contract No. DA 36-039 sc5594, to investigate the synthesis and stability of synthetic crystals.
CHEMORHEOLOGY OF SOME SPECIALLY PREPARED SILICONE RUBBERS BY D. H. JOHNSON, J. R. MCLOUGHLIN AND A. V. TUBOLSKY Contribution from the Frick Chemical Laboratory, Princeton University, Princeton, N . J . Received Mazl IB, 1964
Specially prepared silicone rubbers were formulated from octamethylcyclotetrasiloxane, cross-linking agent and catalyst. These rubbers were extremely labile as evidenced by stress relaxation measurements. The lability could be overcome by tying up the acid catalyst with water vapor or pyridine.
Introduction Ring siloxanes such as octamethylcyclotetraailoxane [(CH&Si0I4 may be catalytically transformed into long chains by shaking with a small quantity of sulfuric acid. l v 2 It was desired to prepare films of lightly crosslinked silicone rubber directly from octamethykyclotetrasiloxane by incorporation of a cross-linking agent and a catalyst. The catalyst wks based on sulfuric acid, but since sulfuric acid is not soluble (1) W. Patnode and D F. Wilcock, J . Am. Chem SOC.,68, 358 (1940). (2) .4 V Tobolsby. F. Leonard and G. P. Roeser, J . Polulner Sci , 3, GO4 (1948)
in the silicones' in the desired concentration range, a means was found to incorporate this catalyst without separation into two phases. The crosBlinking agent was' added to prevent thermoplastic flow. Once these films wire prepared, they were found t o be highlylabile bec&usethe presence of catalyst caused continual .interchange of the Si0 bonds. These 'inteshanges . were -manifest- by a .lo& of weight in the samples. if they were not kept in closed containers. This undoubtedly was due t o the formation of volatile ring siloxanes. The great lability of the rubber films prepared in this way was also manifest by the extremely rapid
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1).H.
JOHNSON,J. R. MCLOUGHLIN ,4ND A. V. TOHOLSKY
stress relaxation that could be observed in these samples under certain conditions. This will be described more fully later in this paper. The rubber films could be stabilized both as regards loss of weight and as regards stress relaxation by addition of suitable stabilizers.
Materials dimethylsiloxanes can be obtained by the hydrolysis of dichlorodimethylsilane and isolated from the resulting mixture of linear and cyclic dimethylsiloxanes by fractional distillation. The total yield of cyclics runs about 50% of the hydrolyzate of which approximately 80% is the tetramer. The linear siloxanes produced by the hydrolysis can be converted to cyclic materials by heating them under vacuum to 350400” at which temperature range the Si-0 bonds undergo thermal rearrangement with the cyclics distilling off. With the use of sodium hydroxide as a catalyst, a 90% yield of low cyclics is obtained of which about 25% is the tetramer. The pyrolysis of these linear siloxanes takes place without observable rupture of carbon-silicon bonds. The cyclotetrasiloxane for this work was obtained by the above procedure and also by fractional distillation of General Electric “Dri-Film” which contains about 35% of the tetramer. This yield was substantially increased by thermal rearrangement of the residue.
0ctamethylcyclotetrasiloxane.-Cyclic
LINEAR DlrF LRENTIAL TRANSFORMER
TRANSFOYMES MOUNT 0,YOT
5
SPRING MOUNTING BLOCW
EXTENSION ROO
LEAF SPRINGS ROD STOP ZERO POSITIONING SET SCREW
ROD SET SCREW
/
HEAVY IRON BASE
RUBBER MOUNT
SUPPORTIN0 BRACKETS
S T E E L WIRE
CLIMP
-OVEN SAMPLE
CLAMP
T E S T TUBE
Fig. 1.-Rubbery
state stress relaxation unit.
Cross-linking Agent.-A polysiloxane cross-linking agent was prepared by the cohydrolysis of dichlorodimethylsilane and methyltrichlorosilane. A 1: 1 mole ratio of the di- and trichlorosilanes was hydrolyzed in a mivture of water, toluene and butanol. The organic layer was separat,ed, washed and rid of solvents hy distillation under reduced pressure. The resulting mixture had a number average molecular weight of 1320 by a cryoscopic measurement. Catalyst.-A mixture of 90 g. of the methyl tetramer and 7.2 ml. of 20% fuming sulfuric acid was vigorously shaken in a separatory funnel. The excess acid was then separated by eithcr long standing followed by decantation or by centrifugation. A clear, viscous liquid results that is very reactive to water, water vapor and basic materials. Imnrediate polymerization results from contact with any of the above substances. The catalyst prepared in the manner described was titratcd against alcoholic KOH using phenolphthalciri YR in-
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dicator and found to contain 7.0-8.0 X 10-3 mole H280, per gram of sample. Catalysts were also prepared using phosphoric, formic p toluenesulfonic and acetic acids. In these cases, however, it was necessary to reflux the mixture to obtain a convenient rate of reaction. Preparation of Samples.-It was decided that the most convenient specimen with which one could study and evaluate the polymer properties would be in the form of films. In order to obtain suitable specimens for this purpose, various methods of casting films were explored. Although the adhesion of silicones to metal surfaces is generally poor, difficulty was encountered in removing coherent films from metal surfaces. It was found very convenient to cast the films on mercury surfaces. Clean mercury was placed in an appropriate diameter crystallizing dish and the polymerization mixture poured on top of the mercury in such an amount as to give a film of the desired thickness. The crystallizing dish was then laced in a desiccator to maintain a controlled humidity and eated to the desired temperature. The relative ease with which films with parallel faces can be prepared by this technique is important for use in measurement of stress relaxation. Using the method of film preparation just described, films of various compositions were repared to determine a formulation suitable for the study orthe polymer properties. The films were prepared in most cases a t 60” since this temperature gives a reasonable balance of rate of cure versus loss of the monomer by va orization. A close control of the humidity during the pogmerization was also important for reproducibility of results. Except where noted, the film formulation used in the succeeding studies was
g
Octamethylcyclotetrasiloxane Cross linking agent (see Materials section) Catalyst (see Materids section)
10.0 parts by wt. 0.3 part by wt. 1.0 part by wt.
The above mixture was polymerized a t a controlled low humidity (“Drierite”) for 24 hours a t 60”. These films show the very interesting property of practically zero shrinkage.2 Stress Relaxation.-Subsequent examination of the physical properties of the silicone rubber films whose preparation has just been described were carried out by the stress relaxation technique.8 The stress relaxometer used is shown in Fig. 1. A complete description of the use of this apparatus is available elsewhere.4 I t was found that samples conditioned and tested at high humidity supported stress indefinitely, whereas samples conditioned and tested in vacuum flowed down around the clamps before the relaxation experiment could be started. In order to make a quantitative study of the effect of relative humidity on the stress relaxation of these rubber films, concentrated solutions of sodium hydroxide were used to control the humidity. The sodium hydroxide solution was poured into the bottom of a large test-tube-like glass container about 12 inches deep and 2 inches in diameter. The glass tube was clamped against a metal plate to seal off the upper end, using a rubber gasket. The rod for extending the sample and a thin wire from the top clamp of the sample to the load-measuring element above, passed through this plate; the wire passing through a capillary sealed into the plate. A rotating shaft equipped with a propeller on its lower end also passed through a bushing in the metal plate and served to keep the air in the container circulating over the sample and the constant humidity solution. The relaxometer shown in Fig. 1 was used to measure and record the stress as a function of time after stretching the sample to a fixed length. Figure 2 shows stress divided by initial stress for samples kept over dry calcium chloride and over solutions of varying NaOH concentrations which produced the relative humidities shown in the figure. The experiments were all performed a t 60”. I t is striking that a t low relative humidities (3) A. V. Tobolsky, I. B. Prettyman and J. H. Dillon, J . Applied Phys., 16, 380 (1944). (4) J . R. McLoiiglilin, P1i.D. Thesis, Princet.on University, 1951. The experimental work contained in this paper in described more fully in this thesis, including other experiments on the atrevs relaxation of silicone rubber sainideu.
THEHYDRATES OF MAGNESIUM PERCHLORATE
Dec., 1954
Fig. 2.-Relaxation
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of stress of special] prepared silicone rubber under various humidities at 21% elongation: (A)
0 satd. CaCI2, 18% R.H.; (B) a, dry Ca&, l%R.H.; (C)o,6.2% R.H., over NaOH; (D) 0 , 3.35% R..H., over NaOH;
(d)e, 1.47% R.H., over NaOH.
the samples showed a very rapid chemical stress relaxation, whereas at high relative humidities the stress delay was slowed up very considerably. Evidently free sulfuric acid (or its esters) in the silicone polymers is capable of catalyzing extremely rapid interchanges manifested by the stress relaxation experiment. Water appears capable of tying the activity of the sulfuric
acid, although this effect is reversible as can be observed by again lowering the relative humidity. When pyridine was allowed to soak into the polymer, it exerted a permanently stabilizing effect,so that low humidities no longer resulted in high rates of stress relaxation. Obviously the pyridine was acting to neutralize the acid catalyst.
THE HYDRATES OF MAGNESIUM PERCHLORATE BY L. E. COPELAND AND R. H. BRAGG Portland Cement Association, Research and Development Division, Chicago 10, Ill. Received M a y 19. 1964
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Aqueous vapor pressures for the equilibri:: Mg(C10&2H20 2H20 = Mg(C104)~4HzO,Mg(C104)2.4HtO 2H10 = mm., Mg(C104).6H20have been measured a t 23 . The equilibrium pressures are 8.15 f 0.54 and 20.9 f 1.1 X mm. respectively. For the equilibrium Mg(C104)2 2H20 = Mg(C104)*.2HzOthe vapor pressure is less than 0.56. X The vapor pressure of the saturated solution is 81 X 10-3 mm. No evidence indicating the existence of a trihydrate was found.
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Introduction The published studies of magnesium perchlorate and its hydrates are due mainly to Smith and his colleagues. 1-3 Smith and Willard prepared the anhydrous salt in 1922 and proposed its use as a drying agent.‘ They also prepared the hexahydrate, a compound which had been reported previo ~ s l y and , ~ reported the preparation of the trihydrate by drying the hexahydrate over phosphorus (1) G. F. Smith and H. H. Willard, J . A m . Chem. Boc., 44, 2255 (1922). (2) G. F . Smith, 0. W. Rees and V. R. Hardy, ibid., S4, 3513 (1932). (3) G. F. Smith, “Dehydration Studies Using Anhydrous Magnesium Perchlorate,” The G. Frederick Smith Cheiniosl Co., Columbus, Ohio, 1934. (4) R. F. Weinland and P. Ensgraber, Z. anorg. allgem. Chem., 84 372 (1914).
pentoxide in vacuo a t 20-25’. I n this latter study their plot of water retained in moles per mole of magnesium perchlorate versus time appeared to approach 3 asymptotically. However, equilibrium had not been reached in 4 months, at which time they discontinued the drying. Apparently, the notion of the existence of a trihydrate arose from the belief of Smith and Willard that the composition of the salt was approaching that of a trihydrate. A further study of the hydrates of magnesium perchlorate was published by Smith, Rees and Hardy in 1932.2 They predicted on theoretical grounds that in addition to the known hydrates, the di- and tetrahydrates should exist also. Two kinds of evidence were presented to support their contention: (1) data on the vacuum dehydration of the hexahydrate, and (2) X-ray diffraction patterns,
