Jan., 1920
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
37
Fig. XI undoubtedly shows the real explanation of the variability of the pigments from the volume increase standpoint. They show that, with the exception of zinc oxide, the greater the mean diameter of the pigment particles, the greater is the volume increase under strain. A confirmation of this statement was t h e result of a test on four samples of barytes compounds containing equal volumes of barytes of different degrees of fineness. The different grades were prepared by allutriation in water. The compound containing the finest grade of barytes showed the least volume increase under strain. This was a critical test which eliminated every other variable but the size of the pigment. .___
% Pigmen f FIG.X
crease of hysteresis, due to the friction when the rubber body is distorted, among those particles which are in dry contact with each other. Conversely, any means of reducing the agglomeration of pigment should also be the means of reducing the hysteresis, such as t h e method of milling, and the use of solvents t o reduce the viscosity of the rubber while mixing in the pigment. Fig. X shows the percentage volume increase at failure on a base of percentage pigment, plotted up t o the point of maximum volume increase, which takes place, as mentioned before, when local contraction of area occurs. These maximum points, as shown for the three pigments, barytes, whiting, and zinc oxide, lie approximately on a circle having the origin as center. The lampblack curve cannot be extended sufficiently far to reach the maximum volume increase due t o the inability of the rubber t o absorb large quantities of lampblack. The curves for china clay, red oxide, and carbon black are not shown, but as far as they have been obtained, they fall between whiting and zinc oxide, the china clay lying highest and t h e carbon black lowest of the three.
THE EXPANSION OF RTJBBER COMPOUNDS DURING VULCANIZATION1 By C. W. Sanderson FISKRUBBERCOMPANY, CHICOPEEFALLS,MASS. Received October 29, 1919
The extent to which rubber compounds will expand when subjected to the heat of vulcanization is of importance in the molding of rubber goods. As far as published results go, however, there are no available data on the value of the coefficient of expansion of rubbers of different compositions. The work was undertaken by us primarily from a practical point of view, with the object of determining whether or not use could be made of an expansion test in distinguishing between stocks which move freely and mold well and those which do not mold well and are characterized as being “dead.” Other possibilities suggested themselves and although the results are by no means complete or beyond question, they are set forth as we have found them. APPARATUS
A special apparatus was designed for the investigation (Fig. I). It consists of a hollow steel cylinder which may be filled with rubber and which is surrounded by a steam-jacket. The top surface of the rubber is the only surface free to move when the rubber is heated, and it acts against a piston and spring. The motion is transmitted through a magnifying lever t o a recording pencil. The piston, spring and recording mechanism were taken from a Crosby steam indicator gauge and made over t o fit the apparatus. Above the piston is a place for a spring t o keep pressure enough on the rubber so that it will not “blow.” Ten-, 50- and Too-pound springs were used. The Io-pound spring was hardly strong enough to prevent blowing. The use of the jo- and Ioo-pound springs will be mentioned later. I n making the determinations of the expansion the rubber was heated a t a constant rate, usually 20’ per 5 min. The recording cylinder was advanced at each interval and from the values on this graph smooth curves could be drawn. The dimensions of the steel cylinder were: Diameter, 0.7978 in. Height, 2 .o in.
FIQ.XI
1 Paper presented before the Rubber Division at the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 to 6, 1919.
38
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Vol.
12,
No.
I d
”CIRCUUFERENCE
1.3099 ”DIAM. OF WE
f
zd TEETH
j..
REVOLUTION
xc
8h
U
3
FIQ. 1
These dimensions were chosen so t h a t a t room temperature (Soo F.) t h e volume of the contained rubber was one cubic inch. The motion of the recording pencil was reduced t o the motion of the piston by dividing by six. The cubical coefficient of expansion was derived from the regular forms.
a = Vt-vo a n d V t = V, vo t and worked out as follows:
a=
{ [0.3989
where
(I
(I
+ at)
+ 0.0000061t)]~X3.1416X(2 +:)]--I IXt
t = Temperature change x = Expansion recorded on the chart
I
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Jan., 1920
39
straight line. The beginning of its curve showed a lower slope because of the fact t h a t the rubber was not thoroughly heated through or perhaps had not been put in so as entirely t o fill its cavity. Therefore, all measurements taken t o determine the value of the coefficient were taken over the range of 2 0 0 ' t o 300'. Some of the lower grade compounds, especially those with high sulfur contents, showed a lessening of the slope and even a falling off in the curve after 300 O had been reached. INFLUENCE O F T H E RUBBER
Two samples were made up from the same formula, FIG. ~ - C V R V ESHOWING EFFECT OF RUBBER ON RATEOF EXPANSION 1698 FINEPARA vs. 1698 BROWN CREPE using in one fine hard P a r a and in the other a soft grade of brown crepe (Fig. 2). Each was subjected This reduces t o as nearly as possible t o the same amount of milling. (0.3989 0.0000024 t ) z (6.2832 0 . 5 2 3 6 ~) I The Para sample gave a coefficient of 2.337 X IO-^, a = 1 while the brown crepe gave 4.4.9 X 1 0 ~ ~ . If we neglect the expansion of the steel shell and approximate, t h e formula may be reduced t o a much simpler form:
+
+
a=-
X I 2
t
It was found t h a t t h e differences in the results obtained with t h e exact and with t h e approximate formula were less t h a n the experimental errors. The approximate formula was therefore used in our work. VALUE OF THE COEFFICIENT
Rubber Specific Gravity b y Vol. Coeff.1 Per cent Raw Cured No. USE 4.680 X 10-4 1 . . . . Tube 90 0.89 0.96 2 . . . . . Tube 4.023 X 10-4 94 0.95 0.95 3.. .. Pneumatic Tread 3.365 X 10-4 70 1.18 1.23 3.322 X IO-' 4 . . . . Semi-Hard 48 1.41 1.47 5 . . .. , Pneumatic Tread 3.000 X 66 1.39 1.40 6 . . . . Sidewall 2.807 X 10-4 66 1.50 1.53 2.828 X 10-4 7 . . , , Solid Tire 71 1.83 1.87 2.73 X 10-4 8 . . . , Pneumatic Tread 59 1.44 1.53 Hard Rubber 1.121 X 10-4 9.. .. 58 1.31 1.33 1 Steel, 1.83 X 10-5; Ebonite (Smithsonian table), 4.6 X 10-5.
. . .. .
I n general t h e results show t h a t the higher the rubber content, t h e higher t h e coefficient of expansion. It is, however, difficult t o show any definite relation because, as will be shown later, other factors enter in.
FIG.~ - C V R V ESHOWING EXPANSION AT CONSTANT TEMPERATURE I N F L U E N C E O F MILLING
Other parts of t h e same batch with the Pararubber were subjected t o further milling and a sample gave a coefficient of 4.680 X IO-^, while another sample which had been milled t o excess on hot rolls gave a value of 4.786 X IO-^ (Fig. 3). Then it is seen t h a t milling increases the expansion and brings the value for Para up t o t h a t for the brown crepe. V O L U M E C H A N G E AT CONSTANT T E M P E R A T U R E
One of the first questions which arose in connection FIG.3-cURVE SHOWING EFFECT OF MILLING O N RATEOF EXPANSION with the expansion of the rubber was whether or not there was a break in the curve or, in other words, a NATURE O F T H E EXPANSION volume change a t the point of vulcanization. We The graphs obtained showed t h a t after t h e first know t h a t a physical change takes place and t h a t the 1 5 min. the expansion curve was practically a specific gravity increases and therefore i t was only
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
40
natural t o look for a drop in the expansion. The curves obtained with a rising temperature failed t o show any drop so samples were tried using a constant temperature or, in other words, subjecting the sample t o the same condition as an ordinary cure (Fig. 4). The graphs failed t o show any drop with the exception of the cases already mentioned. Even in those cases t h e drop was very gradual and did not start a t t h e point of vulcanization. Theref ore, the method could not be used as a method of regulating or determining the cure of a sample of rubber.
FIO.5-CURVE
SHOWlNQ
EFFECT OF DIFFERENT PRESSURES ON T W O
R U B B E R S , ONE OF WHICHSHOWS A LARGE AND THE OTHERA SMALL DIFFERENCE BETWEEN R A W AND CURED SPZCIFIC
GRAVITIES
Since the curves failed t o show any volume contraction during the heating process, the increase in specific gravity which always occurs must come as t h e result of the pressure and the subsequent contraction on cooling. Since rubber is nearly incompressible, the decrease must come as the result of the squeezing of air out of the raw rubber. If this be true, then a compound showing a big difference between raw and cured specific gravity will expand more against a light spring than against a strong one. This was tried out and found t o be the case (Fig. 5 ) . -EXPANSION
STOCK
AGAINST
50 lb. 3.81 X 10-4 2.17 x 10-4
............
1520 355 . . . . . . . . . . . .
SPRING-
100 lb. 3.81 X 10-4 1.36 x 10-4
SPBCIFIC GRAVITIES
STOCK 1520.... 355..
..
RAW 0.95 1.457
CUREDIN EXP. APPARATUS
CUREDIN
- -
HYDRAULIC PRESS 50-lb. sDrina 0.957 0.956 1.568 1.568 ,
~-
100-lb. sorina 0.956 1.577
EXPANSION V S . CONTRACTION
The increase in specific gravity shows t h a t the contraction is greater than the expansion. Therefore t h e question arises whether or not the values obtained apply t o the cured rubber. I n order t o determine this, measurements were made on samples cured in a disc mold.
....................
Aluminum mold. No. 6 Sidewall Diameter of mold, h o t . . . . . . . . . . . . . . . 3 8 1 / 8 2 in. Diameter of rubber, c o l d . . . . . . . . . . . . . . 320/s2 in.
a =
Lt - Lo -
Lo
f
0.0625
39063
a (cubical)
= 2.20
(218)
X IO-^
12,
No.
I
This compares with 2.8 X IO-^ obtained the other way. This is a check considering the fact t h a t t h e rubber was of a different sample and was from mill stock, while t h e expansiometer sample was from calendered stock. The conclusion may be drawn then t h a t the coefficient determined between 200' and 300' may be taken as sufficiently accurate for the coefficient for vulcanized rubber from the same sample. CONCLUSIONS
Although i t is not yet possible from the results obtained with this apparatus t o state definitely whether or not a stock will mold well in factory practice, i t does give us a means of distinguishing between batches of the same stock which may have different properties, due either t o rubber, milling, or other conditions. The results may be summarized as follows: I-The values of the coefficient of cubical expansion for different grades of rubber have been determined. 11-The higher the rubber content the greater the expansion. 111-The harder the crude rubber the less the expansion. IV-The more the rubber is milled the greater t h e expansion. V-There is no break in the expansion a t the point of vulcanization. VI-The increase in t h e specific gravity is caused by t h e pressure and not by physical change or internal contraction of volume.
THE MOISTURE CONTENT OF CEREALS By 0. A. Nelson and G. A. Hulett LABORATORY O F PHYSICAL
CHEMISTRY. PRINCETON UNIVERSITY, TON, N E W JERSEY
PRINCE-
Received July 29, 1919
The determination of t h e moisture content in colloidal organic substances presents peculiar difficulties, as has been pointed out in previous work on the moisture content of coa1s.l The layer of water adsorbed on the surface of colloids has quite different properties from t h a t of ordinary water. Its vapor pressure is much lower, and if this layer is of molecular dimensions it cannot be removed by the best dehydrating agents in a vacuum desiccator.2 I n order t o get an idea of the situation, consider a cubic centimeter of t h e organic materials t h a t go to make up coals, plant materials, cereals, etc., divided into little cubes 10-6 cm. on each edge. The area of the faces of these little cubes would be 6 0 0 sq. m. If these surfaces were covered with a layer of water approximately I x 10-8 cm. thick, there would be 0.06 cc. of water, or over 5 per cent of the weight of the substance, and this water would be in a condensed condition so that it would show practically no vapor pressure. If the layer were one-tenth the thickness of the assumed cubes, i t would make up 50 per cent of the weight, and still the vapor pressure would be less than t h a t of normal water, so the sub1
a = o.oooo733g
Vol.
E. Mack and G . A. Hulett, Am. J . Scz.,
174. 2 LOC.
Cil., p. 92
4S (1917), 89, a n d 4 6 (19181,
