MAY, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
process of cooling. Rapid cooling, as by passing the material countercurrent to a stream of air in a rotary drier, will also be advisable in order to prevent loss of ammonia by decomposition of the diammonium phosphate present or of the urea when use is made of urea-ammonia liquor. The substitution of double superphosphate for ordinary superphosphate in a fertilizer mixture of average rornposition mill increase its plant food content about 50 per cent. Thus a 8-10-4 mixture containing 1100 pounds (500 kg.) of 18 per r m t superphosphate per ton (0.907 metric ton) d l become a 4..5-15-G mixture by replacing the superphosphate with an equivalent quantity (440 pounds, or 200 kg.) of a 45 per cent double superphosphate. If a mixture of this kind is assumed to contain G per cent of moisture, then the heat developed on adding 35.2 pounds (16 kg.) of ammonia (S per cent of the double superphosphate) will raise the temperature of the mixture from 20" to about 100" C. Thiq temperature i, likewise
567
sufficient to cause decomposition of urea or dianimonium phosphate unless provision is made to dissipate the heat within a reasonable time after ammoniation. 9rapid dissipation of the heat if accoinpaiiied by agitation of the inass, as in a rotary drier, offers the further advantage that it will bring about a partial granulation of the entire mass, thereby reducing the tendency of the mixture to segregate and greatly improving it,udrillability. Literature Cited i l l Bassett, 2. anory. Cliem., 5 9 , l (1908). ( 2 ) Keenen, IND. ENG.CHEJI.,22, 1378 (1930). (3) Ross, Jacob, and Beeson, J . Assoc. Oficial d g r . Cherfc., 15, 2 2 i (1932). (4)Ross and hlehring, IXD. EKG.GHEJI.,Sews Ed., 1 2 , 4 3 0 (1934). ( 5 ) Ross, Mere, and Beeson, J . Assoc. Oficinl.4gr. C'he?n., 18 (May 15, 1935).
RECEIVED February 5 , 1935.
Paraffin Wax Tensile Strength and Density at Various Temperatures W.F. SETER AND KURA\IITSU IUOUYE, Cni+ersity of British Columbia, Vancoutrr, Canada Data on the clen5itj 01 d d A R A FF I S The tensile or breaking strength of comparaffin wax are rathei meager. wax is cornrllercial p a r a ~ l n , ~ was a ~ investigated at monly reThe density appears to depend temperatures betiteen -10' and 30" C. It 170tll upon the meltingpointalld garded a$ a solid. It possesses \ a r k s considerabl? with the temperature$ the m y in nhich the wax has h t h a cryitalline c t r u c t u r e reaching a inaxinium \ alue of 32 kg. per been prepared. Thus according and the property of resisting to Carpenter (3),who measured deformation when wbjected to sq. cnl. at :&o11t 3" C. The density \+-as the densities of paraffin a t varihhearing stresaec. There are, also nleasured o,er the silme temperature our temperatures, some paraffin howerer, two classes of solids. the interval. There appeared to be no direct may contain as much a' 20 per ordinary crystalline type and linear relationshiPr between the two Wancent b y volume of entraimed air. plastic solids which are really Ilorri. and Adkinq (i). deterviscous liquids. According to tities %neasured. niined the density over a range of Bingham ( I ) , "if a body is contemperature froin 15.5O to 54.4' C., but in view of the specific tinuously deformed by a very small shearing stress, it is a nature of this property their results mere not directly appliliquid; whereas if the deformation stops increasing after a time, cable to the present nriters' tensile measurements; it was the substance is a solid." Thus, as Clerk Maxwell has therefore deemed expedient to ineasiire the densities of the pointed out, sealing wax, although much harder than paraffin particular samples used throughout the investigation. wax, must be classed as a liquidwhile thelatter. because of the fact that increase of deformation stops after a time vhen it iq subjected to a shearing stress, can be classed as a true solid. Tensile Strength It will be shown later that paraffin wax can fulfill both condiFor the determination of tensile strength of paraffin wax tions, depending upon the temperature. The critical temperathe apparatus shonn in Figure 1 was constructed ture for ordinary commercial wax, with a melting point of 33 ', was found to be about 30" C.; above this temperature it beIt consisted of a quadripod, a stirrer, a balance, a holdei for the haved like a liquid and below like a solid. Thus when the specimen, a Denar flask, and a thermometer. The quadripod, temperature lies above this critical point, paraffin wax cannot made of t n o brass plates and foul glass rods, nas rigidlj attached he said to possess a breaking or tensile strength. to a framenork. The balance was placed on heavy, wellbraced table of auch a height that a cord through the tip of the Carpenter ( 3 ) seems t o have been the first to invent a balance arm could pass vertically to the center of the bottom method of testing the breaking strength of paraffin wax. As plate of the quadripod. One end of the cord was fastened to the his method was intended for only a comparative study of a screw cap ( a ) which was designed t o fit one end of the holder of qeries of waxes under the same general conditions, it could the specimen whose shape and dimensions are given in Figure 1. After the specimen had been put into place and the balance adnot be used for obtaining the tensile strength in absolute justed, the former was immersed in the water by raising the Deunits. Hence a special method based on those procedures nar flask. The stirrer was then ut in motion and the temperawhich are used for measuring the tensile strength of other ture regulated by the addition opeither hot or cold water a$ the materials, had to be devised. experiment demanded. The temperature was measured by an
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ordinary thermometer reading to 0.1" C. and previously calibrated against a standard resistance thermometer. The paraffin used for this work was the commercial material called "Parawax" whose melting point is approximately 54.4" C. At an early stage of the work, attempts were made to prepare test
pecirnen t o k Tusted
VOL. 27, NO. 5
following manner: After a preliminary test had been made to determine the order of strength of the sample for a particular temperature, a new test piece was put in place and a 2kg. weight placed on the scale pan. Then, depending upon the strength of the piece, one or two 1-kg. weights were added, followed by a 500-gram or such 100-gram weights as were necessary. The final adjustment was made by running small lead shot into a beaker on the scale pan until rupture took place. The whole procedure was timed to occupy about a minute. It was soon discovered that specimens having a mottled appearance or other visible flaws gave very low values, so that thereafter such pieces were discarded when such imperfections were discovered after the test piece had been prepared. It was also found that the tensile strength was much lower if the tests were carried out immediately after machine operations than if the pieces were allowed to cure for several days. (The terms "tensile strength" and "breaking strength" appear to be synonymous for this type of material.) Yet in spite of all the precautions taken to treat every specimen in exactly the same manner, both in preparation and in actual breaking operations, no concordant results could be obtained even when the pieces were from the same block of paraffin. Small variations in the rate of applying the load seemed to make no difference. I n all these respects the results are similar to those obtained with metals except that the elongations after the elastic limit had been reached were negligible in the case of paraffin. So small was this elongation that i t was not possible to read it with the
Fig 1-APPARATUS FOR
DETERMI~ING
RNSl LE STRENGTH o r PARAFFIN WAX
pieces by melting the wax and then pouring it into a mold in order to give it a form similar to that illustrated in Figure 1. This proved unsatisfactory because the paraffin contracted when change of state took place. To prevent the wax from sticking to the molds, the brass of which it was composed was first amalgamated but this did not help. I t therefore became necessary to make the test pieces from a block of paraffin that had already set. A slab from each pound package was cut into four bars and then rounded with a pocket knife. These were placed in a lathe fitted with a special knife and trimmed to such a size that the diameter was between 0.5 and 0.6 em., because it was difficult to reach any exact size. For a cooling medium below 0' C., a mixture of salt and ice was used. The temperatures at which the breaking strength was to be obtained were all approached very slowly so as to bring about the minimum amount of strain in the test piece. Thus the temperature was always raised or lowered by decrements of 0.5" C., and after the desired temperature had been rearhed it was held constant from 1to 2 hours, depending upon the temperature, Thus at 0" C., the maximum strength appeared to have been reached after about 3 hours, whereas at 10" C., 1 hour appeared to suffice (Table I). TESTOF PARAFFIN WAXAT 0" AND 10' TABLE I. CURING Temp.
c. 0
10
Load
Diameter
Kg. 5.817 6.917 6.267 6.267 6.817 5.717 5.717
0.549 0.544 0,542 0.549 0.545 0.544 0.544
Cm.
Tensile Strength K g . / s q . em. 24,50 25,50 27.20 27.00 24.80 24.60 24.60
1 1 Balance -I
Stlrrer
-==L
Oewar Flosk
latinurn Resistance Thermometer Ther morneter Sinker plus paraffin suspended
tube
- Fig.2
APPARATUS FOR
DETERM IN I NG
DEN SlTY OF PAR A FF IN WAX
c.
Time Hours 1 2 3 4 1 2 3
As in the case of metals, fatigue was also of importance. Test pieces subjected to loads somewhat less than the breaking load never gave high values for the breaking strength. The point of rupture was some complicated function of the time the load acted, the amount of strain, and the temperature. The application of the load was carried out in the
help of a cathetometer graduated to 0.1 mm. This applied to temperatures below 25" C., for above 30" the elongation was so great, even with very small loads, t h a t there was no real breaking point, the material behaving as a plastic body. The loads a t the breaking point at various temperatures, t h e loads in kilograms per square centimeter, and the time t h e test piece had been immersed before applying the test are given in Table 11. Determination of Density The density of the wax was measured by finding the loss in weight which took place when a cylindrical block of paraffin
INDUSTRIAL AND ENG INEERING CHEMISTRY
MAY, 1935
TABLE11. BREAKINGOR TENSILESTRESGTHAT VARIOUS TEMPERATCRES Temp.
c. -9.5 -9 -8 c
--I
-6
-5 -4 -3
-2 0
2 5
7 8 10
12 13 15 16
17 18 20
22 23 25
26 27 30
6.417 6.517 6.017 6.917 7.677 6.717 6.717 7.321 6.717 6.767 7.017 6.517 6.617 6.717 6.017 5.317 6 017 5.317 6.517
Cm. 0.536 0.538 0.526 0.533 0.536 0.532 0.532 0.554 0.538 0.526 0.536 0.530 0.530 0.528 0 555 0.557 0 538 0.539 0.526
Tensile Strength Kg./sq. cm. 28.50 28.70 28.80 31.10 34.10 30.20 30.20 30.40 29.50 31.20 31.10 29.60 30.20 30.60 24.80 21.80 26.50 23.40 30.00
Time Hour8 3.0 1.5 3.5 1.5 3.0 5.0 4.0 1.5 1.5 4.0 3.0 5.0 4.0 4.0 1.5 1.0 1.5 4.0 4.0
7.227 7.117 7.421 7.421 7.017 7.017 fi.717 7 . i67 5.817 5.917 6.267 6.267 5.717 6.867 7.721 7.621 6.717 4.917 6.317 7.167 6.817 6.017 6.117 5.817 5.717 6,267 5.717 7.421 6.017
0.555 0.538 0,576 0.576 0.538 0.555 0.526 0.545 0.549 0.544 0.542 0.544 0.537 0.574 0.581 0,585 0.540 0.530 0,520 0,545 0.545 0.526 0,544 0.545 0.544 0 551 0.544 0.570 0.560
29.80 31.30 28.50 28.50 30.90 28.90 30.90 30.80 24.50 25.50 27.20 27.00 25.30 26.50 29.10 28.30 29.40 22.30 29.80 30.70 29.30 27.70 26.40 25.00 24.60 26.30 24.60 29.10 24.50
1.5 2.5 1.5 1.5 2.5 3.0 4.0 1.0 1.0 2.0 3.0 4.0 4.0 1.5 1.5 1.5 2.5 4.0 4.0 1.5 2.0 3.0 3.5 1.0 2.0 2.5 3.0 1.5 4.0
6 317 5.312 6.11, 6.117 6,605 6.505 6.817 5.417 5,457 6.017 4.751 4.751 5.517 4.817 4.501 4.646 5.017 3.101 3.258 4.517 3.917 2.301 3.300 0.701 2.151
0 553 0.526 0 521 0.544 0,383 0.583 0.585 0.530 0.553 0,549 0.581 0.584 0,528 0.517 0,548 0.520 0.544 0.500 0.512 0.528 0.517 0.449 0.508 0.415 0.534
26.20 24.50 28.70 26.30 24,80 24.40 25.40 24.50 22.60 25.40 18.00 17.70 25.20 22.90 19.10 21.90 21.60 15.80 15.80 20.60 18.60 14.50 16.30 5.10 9.60
1.5 4.0 4.0 1.5 1.5 1.5 4.0 4.0 1.5 1.0 1.5 1.5 4.0 4.0 1.5 4.0 1.5 1.5 1.5 4.0 4.0 1.0 4.0 1.5 4.0
Load
Kg .
Dittmeter
569
in a loop of very fine platinum wire which was, in turn, suspended from the bottom of a balance pan of an ordinary analytical balance. The platinum wire had previously been platinized a t the point Ti-here it cut the surface of the water in order to reduce the forces of surface tension.
FIGURE 3. PHOTOGRAPH OF DENSITY APPARATUS For finding the loss in weight, distilled water was used above 0" C. and a solution of calcium chloride for temperatures below. The temperatures were measured by a standard resistance thermometer immersed in the liquid near the cylinder of paraffin. In every case below 15" C., a 2-hour period w:is necessary to bring the temperatures to the desired point before weighings were taken. By this time the weights had been found to be constant. Above 15' C. one hour was considered sufficient. Finally the paraffin cylinder mas removed by melting the wax, and the brass sinker vias cleaned by immersing it in petroleum ether. The sinker and connections were then weighed in water by themselves. Knowing the weight of these in air ryith and without the paraffin made it simple to calculate the densities. Density values for calcium chloride solutions were obtained from the International Critical Tables. All weighings were corrected to uucuo. The densities are recorded in Table 111. The figures have been given to the fifth decimal place (merely for what they are worth) in the matter of showing whether there exists a temperature lag or a change in crystalline form in going from one temperature to another. While the method was sufficiently sensitive to warrant the accuracy expressed in the figures, paraffin is not wetted by water and too great reliance must not be placed on the values given. ~~~
TABLE111. DENSITIESCORRECTED
attached to a brass sinker was weighed in water at various temperatures: The wax cylinder was 1.3 cm. in diameter and 8.2 cm. in length. A thin brass wire 0.078 cm. in diameter was fastened to the brass sinker, whose cross-sectional area was about t8hatof the paraffin block and whose length was 1.4 cm., and then was passed through the long axis of the paraffin cylinder (Figures 2 and 3 . A little space about 0.5 cm. vias reserved to prevent the possib e adherence of air bubbles between the sinker and paraffin. One end of the brass wire was bent into the form of a hook and hung
?
Tempera- Density Density ture, C. Ascending Descending 30 0.90155 ..... 28 0.90542 0.90531 25 0.90863 0.90863 20 0.91241 0.91218 15 0.91580 0,91565 a With CaClz Bolution.
FOR
Temperature, C. 10 5 0 -2' 5"
-
~~~~
BUOYANCY O F AIR Density Ascending 0.91928 0.92235
..... ..... .. ...
Denaity Descending 0.91922 0.92233 0,92447 0.92685 0.92818
Discussion of Results In Figure 4 tensile strength is plotted against temperature. A curve is drawn through the points lying to the extreme right. The justification is that these points represent maximum tensile strengths; in other words, they represent cases where conditions mere nearest the ideal, and where internal strains and flam were a t a minimum. The curve shows that
INDUSTRIAL AND ENGINEERING CHEMISTRY
570
rCrWP€RATUR €,
VOL. 27, NO. 5
CEWT/GRAD€
F I G U R E 4. TENSILE STRENGTH O F P A R A F F I N W A X AT VARIOUS TEMPERATURES
FIGURE
5 . DENSITY US. TEM-
6. RELATION BETWEEN TENSILE STRENGTH AYD DENSITY OF PARAFFIN FIGURE
PERATURE OF P A R A F F I N W A X
the tensile strength increases rapidly below 30" C. until about 5" is reached; then the increase begins to fall off and a maximum is apparently reached a t about 0". (The authors do not wish to emphasize the slope of the curve a t 0", even though the measurements so far made with ordinary samples of commercial wax do show a decline in strength according to the measurements carried out under the conditions of the experiment a t the indicated temperatures.) At about -6" C. the strength suddenly declines and a t -9" is very low. Below this temperature the tensile strength became so low that it could not be measured accurately. One test carried out a t -7" C. gave an extraordinarily high value, and its position on the graph could conceivably be an extension of the curve from the point where the latter is cut by the perpendicular drawn upwards from the temperature axis a t about 7". Numerous attempts were made to secure values which would enable such an extension t o be made but without success. I n one particular case the cooling from 15" to -6' C. was spread over 10 hours in the hope of preventing the development of internal strains or other flaws due to unequal cooling, but it was no more successful than were those cases where cooling was more rapid. The falling off in tensile strength appeared to be due to the development of tiny cracks. These were visible to the naked eye below -6" C. where the tensile strength was rapidly approaching a very low value; hence it is presumed that they were being formed even a t 6" or 7" above 0" C. and were responsible for the decline in the strength of the paraffin. These observations appear to lend weight to the internal flaw theory which has been put forward to explain the divergence between actual and theoretical tensile strength of crystals. It has already been mentioned that above 30" C. the paraffin behaved like a plastic solid. The explanation for this change in property must lie in the fact that parafin is a mixture of hydrocarbons having different melting points; those with the lowest melting points become liquid a t about 30" and thus allow internal slipping to take place. Figure 5 shows that below 25" C. the density of paraffin wax varies in a linear manner with the temperature. Apparently the volume increase, due to the development of cracks, is not sufficient to influence the density as far as the accuracy of the measurements show. When the densities are plotted against values of the tensile strengths taken from the smooth curve in Figure 4, the curve
\VAX
in Figure 6 is obtained. It is seen that the relationship between the two quantities is not a linear one. This fact is not a t all surprising when it is taken into consideration that paraffin wax is a conglomerate not only of various hydrocarbons but also of different crystal forms. Thus Carpenter (3) claims that the paraffin wax obtained from Burma crude has a transition point some 10" to 15" C. below the melting temperature, and that a t this point the crystalline needles change into plates. H e further states that the solvent used for crystallization appears to affect the quantity and nature of the crystals formed. Opinions are rather divergent regarding the nature of the crystalline formation in wax. Rhodes, Mason, and Sutton (Y) maintained that there are two types of crystals-needles and plates-and that the needles are formed from the plates. Buchler and Graves (2) stated that the purified paraffin crystallizes in plates and the wax containing impurities produces needles. Graves (j),in another communication, claimed he had obtained crystallized plates entirely from purified paraffin wax. Again Ferris, Cowles, and Henderson (4) found three broad types of crystallization-viz., plate, needle, and mal-crystalline-in purified paraffin obtained from a Midcontinent petroleum. I n view of all these statements, it is probable that both the tensile strength and the density should be affected by the nature of the crystal matrix formed. The unstable nature of this mixture would in all probability account for the slight differences in density found (Table 111) between that obtained when a particular temperature mas approached from above and from below. The influence of the entrapped air on the density and strength of the paraffin wax is being further investigated.
Literature Cited (1) Binghani, E. C., "Fluidity and Plasticity," p. 216, New York, McGraw-Hill Book Co., 1922. (2) Buchler, C. C., and Graves, G. D., IND.EXG.CHEM.,19, 718 (1927). (3) Carpenter, J. A,, J . I n s t . Petroleum Tech., 12,288 (1926). 14) Ferris, S. W., C o d e s , H . C., and Henderson, L. M., IND.EXG. CHEM., 23, 681 (1931). (5) Graves, G. D., Ibid., 23, 762 (1931). (6) Morris, F. J., and Adkins, L. R., I b i d . , 19, 301 (1927). (7) Rhodes, F. H., Mason, C. W., and Sutton, W.R., Ihid., 19, 936 (1927). RECEIVED December 10, 1934.
