Allmlst.
192.5
ISDUSTRIAL A N D EXGI,VEERI.YG CHEMISTRY
833
Effect of Humidity in Rubber Testing' By R. B. Stringfield THBGOODYUR TIRE& RUBBERCo., AKRON,OHIO
T IS well known that the moisture content of uncured
(4.5 X 6.5 inches), and after any desired treatment were cured in four-cavity molds to test sheets of that size and 2 mm. thickness. The temperature of the curing press was such that the mold was exactly 141.5' C. (286.7' F. or 40 pounds steam), the press having been previously calibrated by means of a test mold carrying a standardized thermometer reading to 0.1 ' C., and the steam temperature controlled during cure to within 0.14" C. (0.25' F.), using Tycos Dubl-Duty regulators and thermometers graduated in 0.5' F. Test strips were died out to 1.0-cm. width, using a die with standard Kavy ~ h o u l d e r sand , ~ stress-strain curves were secured on a testing machine mounted in a vault where the temperature was held constantly between 23.9' and 25.6" C. (75' and 78' F.), enabling temperature effects to be It was first noticed some time ago, in the course of some neglected. For early experiments various humidity conditions were very careful physical testing work, that atmospheric humidity had a decided effect on the tests, The following resume secured by means of large desiccators in which the test sheets will give an idea of the precautions taken to eliminate other could be suspended, using fused calcium chloride and water, variables and of the method used in studying humidity effects. respectively, for 0 and 100 per cent humidities, and securing Most of the stocks used were put up for the special purpose intermediate percentages by means of sulfuric acid of proper of studying variations in physical testing, the formulas being c~ncentration.~These desiccators were maintained a t 23.9' C. (75' F.) in the testing vault for most of the work, and the as follows: intermediate humidities were checked by means of a hair Hexa Std. TPG Std. D P G Std. hygrometer which had been calibrated by a sling psychromRubber 100 0 100 0 100 0 6 0 Zinc oxide 6 0 6 0 eter. For later work in which lower temperature was de4 0 2.75 Sulfur 3 0 Hexamethylenetetramine sired, two small ice boxes were made practically air-tight 1.0 ... ... (Hexa) and fitted with cooling coils, by which the temperature was Triphenylguanidine (TPG) ... 3.0 ... Diphenylguanidine (DPG) ... ... 0.40 maintained a t 12.8" C. (55' F.), again securing intermediate TorAL 110.0 113.0 109.15 1 humidities by means of sulfuric acid. 1:OO/141.5° C. (40pounds steam) Sheet cures
I
rubber varies with the atmospheric humidity and with the temperature. Much work has also been done on the moisture content of sheets and crepe with reference to mold and rust, and on the effect of various times and temperatures of soaking and washing on the rate of cure of the dried sheets.2 There seems to be no reference in the literature, however, to the effect of humidity on the physical testing of rubber compounds, and as this effect a t times is very large, and in view of the present interest in accurate physical testing, this paper is presented even though some of the data are necessarily incomplete. Testing Precautions
The physical characteristics of the first two are shown in Figure 1, the ratios used being such as to give decided differences in modulus and slope for small changes in cure. To insure absolute uniformity of mixing and gage, a 45.4kg. (100-pound) batch of each stock was milled on a 152-cm. (60-inch) mill, using special care to get thorough mixing, and was then calendered into Holland a t 2.5-mm. gage. From these batches sheets were cut to exactly 114 X 165 mm.,
* Presented before the Division of Rubber Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 t o IQ, 1925. 2 Whitby, "Plantation Rubber and the Testing of Rubber," 1910, pp. 163, l i 7 , 183, and 289. Longmans, Green & Company.
To present all the available data would make this paper unduly long. The few examples given, however, are typical of many others, and the conclusions drawn have been thoroughly checked. Figure 2 shows the maximum variations obtained with the Hexa and T P G standard stocks. The curves shown are on sheets of identical composition, handled under identical conditionsexcept for humidity, and cured 1:00/141.5' C. Sheets were aged a t the relative humidities shown for 48 hours before 3
4
Bur. Standards, Circ. 38, 5 0 (1921). Wilson, THIS JOURNAL, 18, 328 1 9 2 1 ) .
IiVD USTRIAL AND ENGIXEERIXG CHEMISTRY
834
cure a t room temperature and for 48 hours after cure a t 24.5' C. (76' F.). As the humidity in the average testing laboratory is between.40 and 60 per cent at least 90 per cent of the time, a parallel series of sheets was run a t these humidities a t the same time. The difference between load per square centimeter a t 500 per cent and a t 700 per cent elongation is
Vol. 17, So. 8
The effect of humidity after cure is easily reversible, as shown by the following slope figures taken from sheets of the Hexa standard cured a t the same time and tested a t 24-hour intervals, varying the humidity as shown. Sample 1 2 3
DAY-Humidity Slope 0% 52 Open air 52 100% 45
DAY---. Humidity SloDe 100% 42 0% 57 Open air 52
r-2ND
-3RD DAYHumidity Slope Open air 50 100% 41 0% 60
The effects before cure, however, are not so readily changed, although if given time to come to equilibrium they would probably be proportionate. Moisture contents are only a few tenths of a per cent and the small amount of work done to compare the moisture content with the humidity has not given satisfactory results. The effect of humidity is apparently entirely a physical one, as combined sulfur determinations on sheets of the same stock of widely differing physical properties show no differences. ,--HI UIDITYBefore After cure cure STOCK
Hexa TPG
the simplest basis for comparing the curves of pure gum or very lightly loaded stocks. This difference will be called "slope," and is shown in the following table: -RELATIVE HUMIDITYBefore cure After cure Per cent Per cent
0
0
---SLOPE, -Hexa-(a)
KG. PER SQ. CM.(h) 60 45 78
56 100 44 0 0 74 100 58 57 100 100 56 56 40 40 60 51 51 40 40 53 54 60 60 4s 50 60 ( a ) 24 hours after cure: ( h ) 4 8 hours after cure.
-TPG-(a) 62
52 50 47 60 57 51 54
(h)
64 48 51 43 56
1
1
0
C C C
100 0 100
C
0 0
0 0
Combined sulfur
Slope
s;b K g h
60 78 64 51
cm.
%
1 28 1 26 1 44 1.45
As a very appreciable time is required for a sheet of stock to come to equilibrium a t any humidity, it is evident that variations in the amount of surface exposed, circulation of the air, etc , will affect the result. This is shown in Figure 4 where Sheets 1 and 2 were covered with Holland, as is often done, and Sheet 3 was fully exposed on one side.
57
53
RELATIVE HUMIDITY ---Before cure, kg per sq em.--0% 43%" 100%H ." H 49 60 78 55 43 73 34 50 62
A similar set of sheets on a lightly loaded litharge stock gave the following slopes, the humidity effects being in the same direction as with Hexa: After cure 0% H 58% H 1 0 0 7 ~H
1
70
52
It will be seen that higher humidity before cure increases the slope of the Hexa stock but decreases that of the TPG stock, whereas higher humidity after cure decreases the slope of both. The variations between 40 and 60 per cent humidity, however, are comparatively small, especially after cure. The effect of varying humidity on the DPG standard is shown in Figure 3. These data are taken from a series of nine sheets, of which three were aged a t each of the humidities shown a t room temperature for 48 hours before cure, cured 1:00/141.5' C., and then one sheet from each set was aged a t the second humidities shown a t 23.9' C. (75' F.) for 48 hours after cure. It will be noticed that the humidity effect on DPG is in the same direction before cure as the effect on TPG. Slopes are tabulated below. After cure 0% H 47% H 100% H
1
Cure 00/141.5° 00/141.5" 00/141.5° 00/141.5°
-
RELATIVE HUMIDITY -----Before cure, kg. per s q . cm 0% H 49% H 100% H 113 111 92 103 103 84 98 94 81
Stocks with mercaptobenzothiazole acceleration and with no acceleration were found to show effects in the same direction as Hexa stocks, the effects with pure gum, however, being small. Loaded stocks show the same effects but in smaller degree. 8
THRM
nupiirm
LAID ON
amnr
AT CUR[
m ~ m
mor romrn MD 4s noom CUR[. LAB .usnRr 8 0 7 6 0 % ~LAB 90'F M % X EWLf? M HIGH HUMIC-
N CNGITICN PCRCEN?
The effect of the same humidity a t different temperatures is to change the slope of the stock a t the higher temperature in the direction caused by increasing humidity, presumably caused by the greater absolute humidity. One example of this effect before cure is shown in Figure 5 . The effect before cure, however, is very slow and the effect after cure is not large. For practical testing purposes both effects can usually be neglected. Work t o Be Done Much quantitative work remains to be done on the whole subject of humidity effects. Many accelerators have not been touched, the relation between the percentage of rubber present and the effect, the influence of pigments and softeners, if any, the mechanism of the effect, the ratios between time, humidity, temperature, and moisture content, and many other factors should be studied. It is to be hoped that other laboratories will contribute to this work.
August, 1925
I-VD CSTRIAL A S D ENGIiVEERIA+GCHEMIXTR Y
As the relative humidity of the testing laboratory is usually between 40 and 60 per cent, it is found sufficient for ordinary work to secure uniform exposure for all test sheets by laying them singly on trays or shelves where one side is fully exposed to the air for 24 hours both before and after cure, and to keep a record by means of a recording thermometer and hygrometer
835
Elimination of Surface Devitrification on Laboratory Quartzware' By Frank C. Vilbrandt UNIVZRSITY OF NORTH CAROLINA, CHAPELHILL, N. C.
I
of the daily variations in temperature and humidity. As previously noted, the testing machine is mounted in a vault where the temperature is maintained constantly between 23.9" and 25.6' C. (75' and 78" F.) to eliminate the effects of temperature, which in the case especially of pure gum stocks are very large. Summary Differences in the relative humidity of the air to which rubber test sheets are exposed greatly influence the physical properties of the cured sheets. The effect on the slope of pure gum stocks (slope taken as the difference between the load per square centimeter a t 500 per cent and a t 700 per cent elongation) may be as great as 70 per cent and is often as large as 10 to 20 per cent. Loaded stocks exhibit smaller effects, the variation probably being proportional to the rubber content for stocks of the same type of acceleration. Humidity affects a stock both before and after cure. With stocks accelerated with diphenylguanidine or triphenylguanidine, the higher the humidity to which they are exposed before cure, the lower the modulus and slope of the cured stock. With all other stocks so far tested, which includes stocks accelerated with hexamethylenetetramine, mercaptobenzothiazole, and litharge, the higher the humidity before cure, the higher the modulus and slope of the cured stock. With all stocks, the higher the humidity to which they are exposed after cure, the lower the modulus and slope. The effect is reversible after cure and with difficulty before cure: that is, a sheet exposed to dry air and then to wet air will exhibit the characteristics of high humidity, and vice versa. Exposure to the same humidity a t a higher temperature increases the effect slightly in the direction of increasing humidity. The effect is a physical and not a chemical one, combined sulfurs on sheets of the same stock of widely differing physical properties being identical. Ordinary laboratory humidities usually fall between 40 and 60 per cent. Within this range the differences a t equilibrium are comparatively small, 5 to 10 per cent, and greater variations come from lack of uniform conditioning of the sheets. The actual moisture variations in the sheets are small, and occur over a period of hours and days rather than minutes. Uniform exposure of sheets on trays for 24 hours before cure and 24 hours after cure eliminates the major portion of the variation.
T IS common knowledge among users of fused quartz
laboratory apparatus that a visible deterioration occurs on the surface of the ware after continued usage, a phenomenon akin to devitrification in old glass. The devitrification appears on the quartzware a t first as a slight frost, which increases in intensity on further use, and finally results in the development of deep cracks and in breakage of the apparatus. Treatment of the surface with the common acids, such as hydrochloric, sulfuric, nitric, or combinations of these, or fire-polishing, fails to remove this visible deterioration. According to MichieZ the devitrification of quartzware can proceed so far that it can be crumbled between the fingers. He claims that the devitrification is due to faulty selection of raw materials and that the presence of zirconium and thorium oxides increases this tendency. Thomas,3 however, contends that the presence of these oxides greatly enhances the valuable properties of fused quartz. Callender' attributes the devitrification to the formation of crystalline quartz in the fused mass, the frosting, or surface phenomenon, appearing when the tiny crystals split off portions of the fused quartz, on account of their greater coefficient of expansion. Crookes5 attributes this same devitrification to an anisotropic expansion of the fused quartz in the presence of radium salts, since he found such effects on apparatus in which he was treating solutions containing radium salts. The present study was undertaken to investigate the conditions necessary to mitigate or correct this deterioration and to find some remedy. Studies were made on the production of this devitrification under conditions wherein ready slagging of the silica with chemicals was not so apparent as in combustion tube work. Common pieces of fused quartz apparatus were subjected to continued and intermittent evaporations and ignitions. The progress of the devitrification in each case was followed by microphotographs. Water Evaporation Tests
Four cleaned and microscopically clear-surfaced quartzware dishes of 250 cc. capacity were subjected to twentyfive consecutive evaporation tests by repeatedly evaporating 200 cc. of redistilled water, drying the dishes a t 106' C. between each evaporation. The distilled water used in all this work was a redistilled product analyzing 0.28 part per million ignited solids and but 0.09 part chlorides. Microphotographs of the inner surfaces showed considerable frosting and checking (Figure 1). The treatment was then continued to the fiftieth evaporation, a t which time the frosting was easily visible to the eye. Microphotographs revealed a more.decided checking and cracking, almost to bulbosity. Two of the dishes undergoing this evaporation treatment mere immersed momentarily in hydrofluoric acid containing a small amount of sulfuric acid. At first a very rapid evolution of gas occurred on the areas covered with the frost, but the action soon ceased. The dishes were rinsed and Received January 27, 192;. Chem. Zfg., 37, 589 (1913). 8 Ibid , 36, 25 (1912). 4 Chem. T r a d e J , SO, 509 (1901). Chem Sea's. 83, 151 (1901). 6 Chem News, 101, 205 (1912). 1 2
