INDUSTRIAL A X D ENGIIVEERIXG C H E X I S T R Y
April, 1924
The equation of the curve demands tangency to both the X and Y axes a t infinity. As a matter of fact, the original pellet has a measurable length. This means that the readings taken over the first short time interval will not fall on the curve as demanded by the equation and cannot be used. It has been found by actual experiment that, even in the case of a stiff rubber, the motion has assumed the type demanded by the equzltion in 3 minutes if the determination is made a t a temperature of 100" C. Readings may therefore be safely begun in 3 minutes. 4.0 It is obvious that the value of the plasticity number, K , is a function of the weight applied to the rubber. I n the same manner it is also a function of the size of pellet, since a change in the volume of 2.0 the rubber changes the weight per unit area. This means that both the weight on the rub1 . 0 ber and the ~size of 2 Curves obtaihed pellet must be arbitrarily fixed. Since any 5 with plasticity p ~ s 5 f T i m e i n minutes' plastic material pos0 20 2 30 sesses a certain rigidity, it is possible to reduce the weight to a point where no flow will result, but a weight even approximating this limit does not prove to be practical. With a weight of 1000 grams it was found that readings could not be safely begun in less than 10 minutes, while 3000 grams would not permit readings before 5 minutes. The 2-cc. sample and 5000-gram weight were selected by trial as being best suited for the purpose. The ialue of seems to be a function of the type of apparatus and remains practically constant as the %.eight is changed. Single determinations run on a sample of rubber when wrights of 2000, 3500, 5000, and 5500 grams were used gave vaiues for YL of -0.190, -0.203, -0.197 and -0.203, respectively.
EFFECT OF TEMPERATURE ON PLASTICITY The value of K changes quite rapidly with the temperature. This is especially true of stiff rubber, which may become quite plastic while hot, but upon cooling will again become firm. The relationship between K and temperature has been determined for a number of samples of rubber of different plasticities, and the data are plotted in Fig. 4. Other values may be determined approximately by interpolation. CHANGE I N
K
AFTER h h L I N G
The period of rest after milling has no influence upon the value of K when pure rubber is considered. When first placed on the mill rubber develops a large amount of heat due t o the energy consumed. Because of the rise in temperature the rubber may appear to be quite plastic while hot, but upon being cooled the value of K increases as shown by Fig. 4. After the rubber has once cooled there is no further change in plasticity. Table I gives the plasticity of fohr samples of rubber a t various times up to 60 days after milling. AT looo C. 312 Hours 44 Days 60 Days 4.20 4.22 4.20 2.84 2.83 2.83 1.66 1.70 1.83 1.00 0.95 0.95
TABLE I-PLASTICITY NUMBER K Sample Immediately 24 Hours 1 4 20 4.20 2 2.83 2.83 3 1.67 1.70 4 0.95 0.90
96 Hours 4 18 2.80 1.65 0.95
363
PREPARATION OF L9BORATORY SAMPLE
hlost samples of rubber require preparation before a plasticity determination can be made. Except in the case of slab or similar types of rubber, it is generally impossible to cut uniform pellets with a volume of 2 cc. If the rubber is in the form of thin uniform sheets, it is possible by removing the surface to build up the sheet to a thickness that will permit a suitable pellet to be made. This cannot be done with thin crepes, and it will be found, in general, that a period of milling is necessary. The plasticizing action of a mill depends upon the resistance offered-by the r;bber. Unless resistance is offered by the rubber no work can be done upon it. This means that the power consumption is much larger for a stiff rubber, and that the actual reduction in the value of K for the initial period of milling must be much greater than in the case of a soft rubber. Stiff rubber not only generates the greatest amount of heat, but its plasticity is most affected by changes in temperature. This means that the stiff rubber softens itself temporarily on the mill due to the heat generated by milling. There is, then, an equilibrium which can be reached for each particular case when the rubber will continue to work on the mill without being further plasticized. If the rubber becomes softer due to rise ~ in temperature, ~ the internal resistance will decrease until the energy that is expended on the rubber is not great enough to cause further breaking down. For this reason a stiff rubber may appear to break down as soon as a soft rubber, although this does not mean that the total energy expended has been the same or that the same plasticity has been reached. If the stiff rubber could be cooled during milling to a temperature equal to that of the soft rubber, the breaking down of the stiff rubber would be very much greater. It can be seen that if two samples of rubber having different original values for K were milled under the same temperature conditions, the tendency would be to bring the values of K nearer together. If, however, the rubber is permitted to assume its own equilibrium with the heat generated, the tendency is to preserve the difference in original plasticity and to reduce to an equal power consumption. I n any case where it is necessary to mill the sample of rubber before suitable pellets can be made, the value of K is decreased, although if the heat generated is allowed to remain in the rubber the relation between the K values for two samples of rubber remains practically the same as in the unmilled rubber. This is shown by the figures in Table 11. TABLE 11 K Unmilled Sample Sample ( A ) Smokedsheet 9.50 ( B ) Gristlycrepe 9.90 12.00 (C) L. S. rubber
K Milled Sample 4.72 4.97 6.12
Ratio Unmilled k : B = 0.96 B : C = 0.83 A . C = 0.79
Ratio Milled A : B = 0.95 B : C = 0.81 A : C = 0.77
The following probedure has been used in preparing a sample on a 12 x 6 inch laboratory mill: 8.
6.
5.
Vol. 16, S o . 4
I S D 7;TISTRIALA N D l?hTGIhTEERIh’G CHEMISTRY
364
I
4
FIGURE 4-
1
TABLE111-PLASTICITY OR DIRPERBNT TYPESO F RUBBER K on ExperiK on 84-Inch Mill mental Mill
The sample is milled hot and for a short time in order to preserve the original plasticity as much as possible. Hot water is turned through the rolls with the m’lll running until the temperature of the rolls is just above 60’ C.
hrote.-For measuring 4.. roll temperature a fine copper-constantan thermocou3. ple is soldered to the center of a piece of thin copper sheet which is approximately 13 mm. (0.5 inch) square and curved t o fit the mill 1. roll. The wires are brought through the center of a piece of thin felt about 38 mm. 120 (1.6 inches) square and t h e Five 0 60to 8 cm. (280 or 3 inches) of 100wire are coiled copper against fastened t h e backtoof the t h e felt. felt
3.87 4.48 4.48 4.01 4.30 4.72 4.97 4.53 5.09 4.82 5.90 5.12 6.12
250-Pound Batch Smoked Sheet 4.25 4.70 4.40 4.56 4.35 ‘4.35 Crege Rubber 4.40 4.70 4.95 4.80 5.30 5.10 L. S. Rubber
K
Unmilled 8.40 9.50
9.70
.. .. .. .. .. .. ..
.. ..
..
12.00
..
3.40
Guayule 1.86
z-B
PLASTICITY OF COMPOUNDED STOCKS
The foregoing procedure will, of course, Five different results for the same rubber if conducted on different mills. A temperature of 60” C. and 500 grams of rubber are convenient for a 30-cm. (12-inch) mill. Larger mills would require larger samples. The surface speed and the ratio between the speed of the front and back roll of the mill also influence the power consumption and the plasticizing action. Any mill may be used to obtain comparisons between two samples of rubber. PLASTICITY OF DIFFERENT TYPES OF RUBBER
It is found, in general, that plasticity determinations may be made with compounded stocks as well as with pure rubber. Exceptions are caused by samples that are capable of beginning to cure in a few minutes at the temperature of the determination. I n this case the curve obtained with the press no longer follows the expression Y = KX”, but flattens and will finally become parallel to the X axis when all flow ceases. Iq dealing with this type of stock it is necessary to take the first reading as soon as possible. In this way a value for K is obtained before the curing action has proceeded far enough to affect the determination materially. The initial curing action of accelerators or the tendency of a stock to “burn” while in process may be determined by means of the effect on the plasticity curve. Fig. 5 shows a plasticity curve obtained with an accelerated stock and the calculated curve that would have been obtained had no curing taken place. The calculated curve is obtained by using the K value taken from the 3-minute reading on the experimental curve. If it is assumed that no curing action takes place in 3 minutes, the value 3.50 for K is taken from the curve represented in the true plasticity. If another value is now taken from the 15-minute reading, it is found that the value of K has increased to 3.71, or 6 per cent. This change can be made the basis for a comparison of accelerators providing the amount of accelerator used is not great enough to affect the 3-minute reading and providing the second reading is not. taken too near the flat portion of the curve.
The figures in Table I11 represent variations that have been found in different samples of rubber. There is no relation between the figures in the different columns, since each value of K was obtained from a different lot of rubber. It can be seen that considerable variation in plasticity exists. 4.0 A. Accelerated Sm Although all soft rubber does not give low tensile figures, it is found in general that 3.0 low-grade rubber has a small K value. The average stiffness of the different types of 200 rubber increases in the order: low-grade, smoked sheet, crepe 5 IO 15 20 2 5 30 rubber, and slab rubber or evaporated latex. The values within’ a single type of rubber vary enough for the plasticity numbers to overlap, and it is common, for instance, to find samples of crepe rubber that are much more plastic than some samples of smoked sheet.
The Cellulose Division is planning t o have an exhibit a t the SOCIETY Washington meeting of the AMERICANCHEMICAL during the week of April 21. The gbject of the exhibit is to demonstrate the importance of cellulose chemistry by exhibiting a large number of products t h a t depend for their successful production on a knowledge of this particular branch of chemistry. The exhibits will be divided into three groups: first, pulp products, which will include the various kinds of paper pulps, and products made from them, as well as the so-called “balsam wool” and gelatinized wood developed by the Burgess Company. The second division will comprise products made from cellulose esters and from colloidal dispersions of cellulose, such as cuprammonium solutions and the so-called vulcanized fiber. This will include artificial silk of various kinds, pyroxylin plastics, lacquers, explosives, etc. The third group will consist of decomposition products of cellulose, and will include those made by destructive distillation, such as acetic acid, acetate of lime, acetone, methanol, charcoal, etc. ; the products of alkaline fusion, particularly oxalic acid; and products of acid hydrolysis, such as sugars, cattle food, and alcohol. It is probably true t h a t the chemistry of celluloseis not generally recognized as of major importance; yet cellulose plays an important part in our everyday life. We wear cellulose, we eat cellulose with our food, and we live in houses made of cellulose. Besides these uses which are of fundamental importance to everyone, the wide ramifications of cellulose chemistry will be shown in the various products of the exhibit.
and covered with several additional layers of felt. This is then fastened to a cork or other suitable handle. Temperature is measured with the mill running. Although this method is open to criticisms, the results are but slightly affected by differences in contact pressure with the roll and equilibrium is quickly reached. Whether the temperature recorded is exactly correct or not the temperature conditions can be duplicated.
The water is then turned off, the temperature allowed to fall to 60” C., and a 600-gram sample placed on the mill while the rolls are approximately 2 mm. apart. This leaves only a small bank of rubber between the rolls. The rubber is allowed to remain on the mill for 4 minutes, after which it is taken off and folded to the required thickness. Plasticity measurements may be run as soon as the sample is cool.
Exhibit of Cellulose Division
