820
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 33, No. 6
for the circulation of sludge to obtain the greatest value from the magnesium oxide employed. In the second stage disodium phosphate is used for reduction of hardness to practically zero. Expected Figure 10 shows the design of a hot-process Silica Content softener for a plant operating a t 450 pounds presRemoved Silica silica as sios, P. P. M. ~ ~ so&ned ~ o silica ~ er Part sure. The raw water is relatively low in hardness SofBlow- Softened Water, Removed Remosil Eemosil with alkalinity exceeding the hardness. Phosphoric River tened down Water P. P. M." P P. M . ' P. P. M: Used 3 3 52 12.0 8.9 5.9 12 0.49 acid is first introduced, release of carbon dioxide 4 3 52 28.4 17.7 14.7 12 1.20 is obtained in the deaerator, and then sodium 50 4 4 . 0 2 3 . 3 1 8 . 3 12 1 . 5 3 4 4 40 42.0 19.8 12.8 12 1.07 hydroxide and magnesium oxide are added for 4 5 48 28.5 16.6 11.6 12 0.97 4 5 48 28.6 16.6 11.6 12 0.97 hardness precipitation and silica removal. Provision is made for recirculation of the partially a Calculated on basis of mixed raw and boiler water. spent magnesium oxide sludge. Because of the advantages possible through recirculation of boiler waterto the softener, provision has also been made for this feature which will further increase the quantity of silica A design of a combination hot-process softener is shown that can be removed per part of magnesium oxide. in Figure 9. This unit was designed for softening and the removal of silica from the make-up water for a 1400-poundLiterature Cited pressure utility installation. Silica is present in the raw water as 33 p. p. m., and the softened water will constitute approxi(1) B e h r m a n , A. S., a n d G u s t a f s o n , H., IXD.ENG.CHEM.,32, 468-72 mately 10 per cent of the boiler feed water. Specifications re(1940). (2) B e t z , L. D., Noll, C. A., a n d M a g u i r e , J. J., Ibid., 32, 1323-9 quire that the silica content of the boiler water shallnot exceed (1940). 10 p. p. m., with total solids not exceeding 1000 p. p. m. Mag(3) K a h l e r , H. L., IND.ENG.CHEM.,Anal. Ed., t o be published. nesium oxide affords the most efficient means of silica removal (4) Maguire, J. J., a n d Tomlinson, W. J., Combustion, 11, 26-32 without increasing the solids content of the make-up water. (1939). Silica and partial hardness removal are accomplished in the I'RESENTBD before the Division of Water, Sewage, and Sanitation Chemistry first stage by magnesium oxide and lime. Provision is made at the 100th Meeting of the American Chemical Society, Detroit, Mich. TABLEVI. SILICA REMOVAL IN CONJUNCTION WITH POT LIME-SODA SOFTENING A N D BOILERWATER RECIRCULATION AT A BOILER PRESSURE OF 225 POUNDS PER SQVARE INCH
;
The System Asphaltic BiturnenRubber Powder J. M. VAN ROOIJEN The Rubber Foundation, Delft, Holland
I
N COMMUKICATIOP\'S 3, 7 , and 15 of the Rubber Foundation the advantage of adding unvulcanised rubber powder to asphaltic bitumen 'was pointed out. I n one of these publications (5) an explanation was given, based on the selective absorption of the light hydrocarbons by the rubber powder, by which a two-phase system is formed, with swollen rubber particles dispersed in a medium of hardened asphalt. An examination was also made into heat resistance, in which it appeared that a mixture containing 5 per cent rubber could be kept heated a t 170" C. for 16 hours without any change in properties. The literature on the subject, however, mentions less stability to heat. Davey ( I ) , for instance, states that by heating a t too high a temperature, the rubber-containing mixtures sometimes become softer than those without rubber, which is attributed to decomposition of the rubber by superheating. At the Research Station a t West-Java (3)it was observed more than once that the penetration increased in the presence of rubber when the mixtures were heated above 100" C. The penetration value of Wonokromo asphalt without rubber powder amounted to 48; that of a mixture of 5 and 7.5 per cent unvulcanised rubber powder (Pulvatex) after being heated a t 125" C. for 2 hours was 62 and 74, respectively,
the surface of the penetration dishes was rough, and the penetration values diverged considerably. These phenomena were not found at first in this laboratory. On the basis of the theory given (6) it was, moreover, not thought probable that softening of the rubber by heat would have an unfavorable influence, since the viscosity of the dispersed phase has little influence on the characteristics of the entire system. When softening accidentally appeared with a special mixture, it appeared desirable to investigate this phenomenon. I n the first place, it was necessary to see in what cases such behavior is to be expected-in other words, what significance this phenomenon has in practice. The second object of the research is the explanation of this phenomenon.
Variables Affecting the Stability of the System Among the factors which may affect the stability of the properties are the percentage of rubber, the type of asphaltic bitumen, the temperature, and the length of the heating period. In the following investigation these factors were varied as follows: 1. Rubber content: 5 , 7.5, and 10 per cent of the mixture. 2. Asphaltic bitumen: normal asphalt 20/30, blown asphalts R85/40 and R75/55.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
A n investigation was made into the heat stability of asphalt-rubber mixtures; the influence of type of asphalt, percentage of rubber, temperature, and time of heating upon the mixtures was examined in regard to penetration and softening point, visual and microscopical appearance. The mixtures containing 5 per cent rubber hardened gradually and looked smooth; under the microscope they showed no peculiarities. The 10 per cent rubber mixtures behaved differently; afLer heating, a "soft stage" in penetration and softening point resulted. This phenomenon was least striking with a normal asphalt and more so with two blown asphalts. In the latter the surface became temporarily granular, while at a high temperature (235" C.) the soft stage occurred sooner and lasted a shorter time than a t a lower temperature (175" C.). Certain microscopic features always appeared to accompany this abnormal behavior. The system originally formed, in
3. Temperature: 235", 200°, 175O, and 150' C. 4. Heating period: up to 24 hours.
Pulvatex, the rubber powder used, was described in detail in a previous communication (6). Asphalt 20/30 is the same as that used in the previous investigation (6). The two blown asphalts were chosen on account of theoretical considerations to be discussed later on; they are distinguished by a high percentage of light hydrocarbons. The most important characteristics are given in Table I.
TABLE I. CHARACTERISTICS OF ASPHALTS %
% Ether
Softening Point,
Petroleum Naphtha Asphaltenew
Asphalt -P ,eneratoitnAsphalDesignation 25' C. 34.9' C. 40' C. O C. tenes 71 63.5 18.5 20/30a 19 46 71 86 20.4 28:s R85/40b 30 57 R'75/65b 55 74 26.8 29.3 a Penetration at 25' C. b The first figure indicates ring and ball softening point in C.; the second figure is penetration a t 25' C.; R is used in Europe t o mean a blown asphalt, 0 Boiling range, 60-80° C.
..
..
TABLE 11. EFFECTOF HEATINGAT 235" C. ON 95 PERCENT ASPHALT20/30 MIXTURE Hours of Heating 0 1 2 3 5
--
25' C. 17 18 15 15 14
Penetration34.9' C. 36 33 30 29 ~.
24
--. C.
40' 51 45 42 37 34
B. & R. Softening Point,
c.
78 80 79 79
which swollen rubber particles are present in a bituminous medium, apparently can undergo a '(reversal of phases", so that the swollen and depolymerized rubber becomes the continuous phase; moreover, it may be accompanied by coagulation of asphaltenes. This reversal of phases is facilitated by the presence of light hydrocarbons and a comparatively high percentage of rubber, since both increase the volume of the swollen rubber; coagulation of the asphaltenes may be expected particularly with asphalts of the blown type, in which the asphaltenes are less well stabilized. Softening, granulation, and syneresis disappear after a longer heating period, probably because chemical changes prevent the rubber from having further coagulating action on the asphaltenes. With mixtures of more than 7.5 per cent rubber difficulties are to be expected, which will increase with length of heating period, increase in temperature, and use of the blown type of bitumen.
Heating took place in an air bath while the mixtures were occasionally stirred. The mixtures were kept a t around 150' C. for about one hour before being heated to a high temperature to enable the rubber t o swell in a normal way; 235" C. was chosen as the highest practical working temperature. The data for the longer heating periods were obtained by remelting and reheating the mixtures that had been heated during a shorter period and then been examined; this interrupted heating yielded the same results as continuous heating.
95 Per Cent Asphaltic Bitumen-5 Per Cent Rubber Mixtures The mixtures prepared from the three asphalts were heated a t 235" C. during gradually increasing periods. The characteristics are listed in Tables 11, 111, and IV. The 95 per cent asphalt R75/55 mixture was also heated a t 175' C. (Table V).
TABLE IV. Hours of Heating
Penetration
25' C.
40° C.
--Penetration2.50
c.
34.90 c.
R. & B. Softening Point, O C. 113 112-120' 120-129" 142 145
7
40' C. 71 65 29-42
84
TABLE V.
Hours of Heating
EFFECTOF HEATINGAT 235" C. ON 95 PERCENT ASPHALT R75/55 MIXTURE
36 56 37 50 6-24 26 23 30 15 8 16 22 The ball does not gradually fall through th? rinq in the normal way, b u t the disk of asphalt drops in its entirety from the ring. 0 1 2 3 4
TABLE 111. EFFECT OF HEATING AT 235" C. ON 95 PERCENT ASPHALT R85/40 MIXTURE R. & ,B. Softening P:int, C.
821
Flow after 20 Hr. a t 75O C., Mm.
Hours of Heating 0 2 4 6 8 10 12 14
EFFECTOF TIMEOF HEATING AT 175' C. CENTASPHALTR75/55 MIXTURE Penetration 25' C. 34.90 c. 36 56 About 28 About 48 38 60 34 56 33 48 42 28 26 39 About 19 24
ON
95 PER
R. & B. Softening Pzint
C.' 113 113 126 128 135 140 143 146
INDUSTRIAL AND ENGINEERING CHEMISTRY
822
OF HEATING ON ASPHALT R75/55 TABLEVI. EFFECT
Hours of Heating
,--Penetration-
2 5 O C.
34.9O C.
R. & ,B. Softening Point, C
4OOC.
At 235O C .
A t 17.5' C . 2
51
85
82
108
90 PER CEKTASPHALT 20/30 TABLEVII. EFFECTOF HEATING MIXTURE AT 235" C. Hours of Heating
0 0.5 1 2 3 4 5 6 7
--
Penetration34.9O C. 33 36 About 100 51 33 34 31 29 26
25O C. 16 24 About 65 30 20 21 17 16 13
R. & B. Softening Point, C. 95 88 87 94 89 82 82 84 88
7
4OOC. 45 54 >170 67 43 45 42 39 36
TABLEVIII. EFFECTOF HEATING90 PER CENT ASPHALT R85/40 MIXTURE AT 235" C. Hours of Heating 0 5 min. 1 2 3 4 5 6 7
--Penetration25O C . 34.9'C.
...
400 29 32 58a 135a 55 3s 24
40°
-. C.
R. & B. Softening Point, O C . 137
...
...
455 27" 40" 113a >220 117 62 37
11Ob
50s 30 544 162a
1056
lOOb
lO5b
...
89 104 127 141
159 85 40
Flow after 20 Hr. at
looo C., &lm.
.. ..
>'io
.. 3
Approximately. these values diverged from 20 to 40 per cent. b The disk of asp'halt fell from the ring in its entirety before the ball had deflected. 0 After 1 hour the mixture slipped off the brass plate as a whole without normal flow having occurred. 0
m
TABLEIX. EFFECT OF HEATING 90 PERCENTASPHALT R75/55 WITH 10 PER CENT RUBBER Hours of Heating
.--Penetration 25' C.
34.90 C.
40' C.
R. & B. Softenjng Point, C.
30-46 28-34 62-115
131 154 1265
At 235' C. 0
1 2 3 4 5
1 2 3 4 6
7
8
10 12 14 16 2 4 6 8 10 12
1-35 2-30 35-125 34->ZOO 100-190 > 150 100-150 76-118 71 43 16 316
38 43 26
23-62 56-143 >165
At 200' C. 20-33 24-31 72-116 >ZOO >210
.. ...
> iho
71 b 35
At 175' C. 62 b 57 79 34 64-154 1856
14 ... 16 >165 ... 18 > 180 ... 20 > 145 ... 23 105-160 ... a The asphalt disk drops from the ring. b Approximately.
...
... ... .
I
.
. .
...
103 37 78b
...
...
.. .. .. ... .
.
I
.
.
... ...
joa,b
55asb 586 60" 680
si5 S6a
1145
Vol. 33, No. 6
For comparison Table VI summarizes the changes that appear with unmixed asphalt R75/55 when heated; as shown, gradual hardening takes place. Tables I1 to V show that the three mixtures are resistant to heating and that only gradual hardening occurs, which is comparable to what happens when asphaltic bitumen is heated without rubber (Table VI). Although the rubber depolymerizes a t the high temperature, this does not result in the fluxing of the asphalt. A slight irregularity occurs in Table IV, where after 2 hours of heating the penetration values diverge considerably as well as the softening point. The appearance of all samples is homogeneous, while the microscopic examination of the mixtures of Tables I11 and IV shows that the rubber particles become finer and finer, which agrees with former experience (6). The microscopic appearance of the mixture of Table IV was more irregular after 1 and 2 hours of heating, but after longer periods a fine structure was visible again. The mixtures with 5 per cent rubber are, in general, resistant to heating and behave in a way similar t o that found when the mixture was heated longer but a t a lower temperature (6).
90 Per Cent Asphaltic Bitumen-10 Per Cent Rubber Mixtures h P H A L T 20/30. Two series of tests yielded similar values; the averages are given in Table VII. After one hour the penetration showed a sharp but only temporary increase. The appearance of the specimen remained homogeneous and smoothsurfaced. The microscopic examination will be discussed in detail later. ASPHALT R85/40. The data of Table VI11 indicate that the phenomenon of temporarily increased penetration appears here to a still higher degree than with asphalt 20/30; and after 4 hours of heating the softening point is lowest. The curves of Figure 1 show these results. Almost i m m e d i a t elg after reaching the high temperature, the mixture becomes granular, while only after about 4 hours of heating, a homogeneous 0 2 4 6 8 mass is formed again. HOURS OF HEATING AT 235OC. This granular condition is not only accompanied by FIGCRE 1. EFFECT OF a considerable divergency HEATING ON THE PENETRATION AND SOFTENING POINT in the penetration values, OF 90 PER CENT ASPHALT but also by syneresis, the R85/40 WITH 10 PER CENT influence of which is eviRUBBER dent in the determination of softening point and flow; an oily layer formed between the asphalt-rubber mixture and the metal plate caused the mixture to leave the contact surface, I n these cases we cannot attach much significance t o softening point values found. ASPHALTR75/55. Table IX shows the effect of heating a t 235', ZOO", and 175' C. on penetration and softening point,
June, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
823
Here the temporary softening appears again, as shown in Figure 2. The penetration and softening point a t the three temperatures are recorded, and the divergence of the observations is indicated. The softest stage (penetration more than 150, softening point around 60" C.) is reached a t 235", 200", and 175" C. after heating for 3, 7, and 18 hours, respectively. The lower the temperature, the later the soft stage appears and the longer it lasts. Granulation of the specimens is again evident, and the surface becomes rough and more or less sticky. At 235" C. this happens after 1 hour; a t 175" C. it begins after 4 hours and then increases until after 20 hours the surface is very granular and sticky. Also in the divergency of the penetration values and the abnormal phenomenon of the softening point this mixture agrees with the two preceding mixtures which contained 10 per cent rubber. It is apparent that these mixtures, in contrast with those containing only 5 per cent rubber, cannot stand longer heating a t a higher temperature.
Reasons for Difference i n Behavior of 5 and 10 Per Cent Rubber Mixtures As explained previously (6),the addition of rubber powder to asphaltic bitumen creates a disperse system, in which the rubber grains that have swollen (for instance, to five times their volume) are dispersed in the bituminous medium. It was pointed out that when this system of rubber in asphalt is heated strongly, no increase in penetration is to be expected, even though the rubber becomes softer. Such a system exists in the 95 per cent asphalt-5 per cent rubber mixtures previously examined, as well as in the mixtures of asphalt 20/30 and R85/40 with 5 per cent rubber. Moreover, such a system is formed in the mixtures with 10 per cent rubber when they are not heated to a high temperature. The microscopic appearance of such a system is shown in Figure 3A. However, as the rubber has greatly swollen, especially if many absorbable hydrocarbons are present, the volumes of the continuous phase and the dispersed phase are about equal in the case of 10 per cent rubber mixtures. The conditions are then favorable for a "reversal of phases", by which quite a different system is formed. The swollen rubber becomes the continuous phase and the asphalt the dispersed phase. This phenomenon may be attended by a second-namely, coagulation of the asphaltenes by the depolymerized rubber hydrocarbon, in the same way as with naphtha hydrocarbons. This phenomenon is, then, connected with the surface tension of these hydrocarbons ( 3 , 4 ) and probably also with their molecular volume (6). Accordingly two types of systems can be distinguished under the microscope: in one the dispersed phase tends toward further dissociation (Figure 3B), in the other, more toward flocculation (Figure 3C). Nellensteyn (4),in a somewhat different connection, observed that in dissociation precipitates small globules are seen, and that if the asphaltenes coagulate an amorphous precipitate results. The reversal of phases will be promoted by the presence of a considerable percentage of light hydrocarbons and a comparatively high percentage of rubber, since both increase the volume of the swollen rubber. Coagulation will take place easily with asphalt of the blown type, as the asphaltenes are less stable and both phenomena are accelerated by a rise in temperature. FIVEPER CENT RUBBERMIXTURES. Microscopic examination showed that the migtuqes of asphalts 20/30 and R85/40 with 5 per cent rubber we always of the rubber-
0
2
4
6
8
IO 12 14 16 18 2 0 22
HOURS OF HEATING
FIGURE2. EFFECTOF HEATINGON THE PENETRATIOS AND SOFTENING POINTOF 90 PER CENT ASPHALT R75/55 A N D 10 PERCENTRUBBER
in-asphalt type and do not undergo any irregular changes in penetration and softening point when heated (Tables I1 and 111). After 1 and 2 hours of heating a t 235" C. the mixture of asphalt R75/55 with 5 per cent rubber (Table IV) showed under the microscope an inclination to immiscibility but without flocculation. After a longer heating period a fine structure resulted which was perceptible only a t a magnification of 500 times. This initial immiscibility may be considered to cause the penetration values to diverge and the softening point disk to drop from the ring. That this takes place with asphalt R75/55 and not with the two others is attributed t o the greater percentage of light hydrocarbons in R75/55 which had been prepared by fluxing 86 parts of a hard blown asphalt with 14 parts of a light lubricating oil extract. The same mixture, heated a t 175" C., showed no reversal of phases under the microscope even after a 12-hour heating period; this is in agreement with the data of Table V which show that no irregularities occurred in penetration and softening point. TENPERCENTRUBBERMIXTURES. With asphalt 20/30 (Table VII) 1 hour of heating a t 235" C. caused an increase in penetration of 50 units. This behavior differs essentially from that of the mixture with 5 per cent rubber, where a proportional increase of 25 units would be expected but does not occur. In agreement, the mixture with 10 per cent rubber shows, after 1 and 2 hours of heating, the microscopic appearance of asphalt in rubber with which softening is evidently connected. During further heating the structure becomes finer and finer, so that after 4 hours, a t a magnification of 500 times, the specimen has become fully homogeneous. The mixture of asphalt R85/40 with 10 per cent rubber (Table VI11 and Figure 1) behaves similarly. Here too, in contrast with the mixture with 5 per cent rubber, the asphalt-in-rubber system is formed as soon as the mixture is
INDU STRIAL AND ENGINEERING CHEMISTRY
824 A . Asphalt RS5/40; 170' C.
heated a t
Vol. 33, No. 6
R. Asphalt R85/40; heated 1 hour a t
gradually become more granular again (like Figure 3C)-for example, a t 235' C. after 4 hours. Only the 10 per cent rubber mixture of asphalt R75/55 shows the soft stage; with the 5 per cent mixture this does not occur, 7.5 PERCENTRUBBER MIXTURE. This mixture of intermediate composition w.as examined (Table X). A divergency of the penetration values and abnormal behavior a t the softening point occur, but there is no softening as with the mixture of 10 per cent rubber. The microscopic appearance bears a greater resemblance t o the mixture with 5 per cent than to that with 10 per cent rubber; the coarse structure that results after 1 hour of heating (like Figure 3B) has disappeared after 4 hours of heating, so that only a t a magnification of 500 times is a fine structure still perceptible. The softening which occurs with the mixtures of 10 per cent rubber is thus not permanent, but when heating is continued, the material hardens again, although the microscopic appearance of asphalt in rubber is maintained. In order to investigate the cause of this rehardening, rubber powder was heated a t 235" C. in the same way as the asphaltrubber mixtures. Soon the rubber began to soften and, after being cooled, a thick viscous mass formed. If-the rubber was C . Asphalt R7W55: heated 1 hour a t D.Asphalt R85/40: heated 4 houis a t 235' C. 235' C. heated for a longer period and then cooled, FIGURE 3. PHOTOMICROGRAPHS ( X 140) OF 90 PERCENTASPHALT WITH 10 no perceptible change in consistency took PERCENTRUBBER place after 1, 2, 3, 5, 7 , and 9 hours. The viscosity may have decreased a little, but a noncontinuous change (e. g., strong depolyheated a t 235" C. (Figure 3B); again abnormal values of merization and then resinification) did not occur, The nonconpenetration and softening point are found. This phase tinuous changes in the asphalt-rubber mixtures on heating are remains essentially the same during further heating, even therefore not a result of the corresponding changes in rubber on though the asphalt particles are a little larger after 4 hours heating. Furthermore, chemical changes do occur in rubber, (Figure 3 0 ) ; after 5 hours the sample reverts to the condias shown by the fact that, when it is heated in the above tion shown in Figure 3B. The various abnormal phemanner i t is not soluble in naphtha and is rather insoluble nomena (penetration, softening point, appearance, and microin benzene. The changes in the asphalt-rubber mixtures scopic condition) are more distinct with this asphalt than with must therefore be more of a colloidal nature, and the rethe preceding type. hardening may then be attributed t o a less strong flocculating I n the mixture of asphalt R75/55 wit11 10 per cent rubber action of the oxidized rubber on the asphaltic bitumen. (Table IX and Figure 2 ) , the initial heating a t 150" C. This cannot be seen under the microscope; the slighter produces the rubber-in-asphalt system, which is maintained flocculation of the mixtures is, however, apparent to the eye, for a few hours (Figure 3 A ) . If the temperature is raised, and after the longer heating period, they become less granular a reversal of phases first occurs (like Figure 3B); later the and show no syneresis. appearance is more that of a flocculate than was the case with the preceding mixtures (like Figure 3 C ) . If the heatAcknowledgment ing is continued, more rounded particles are formed as in It is a pleasure to express appreciation t o A. van Rossem, Figure 3 0 (for example, a t 235' C. after 3 hours); they director of the Research Department of the Rubber Foundation, for his kind assistance in preparing this article. The two blown asphalts were supplied by Rataafsche Petroleum Maatschappij, through the courtesy of J. Ph. Pfeiffer. TABLE X. EFFECTOF HEATISGAT 235' C. ON 92.5 PER CENT ASPHALTR75/55 MIXTURE B. & R. Literature Cited Hours of Penetrittion~Softening 7 -
Heating
a b
250
c.
235'
34.90
c.
A pioxiniately. Tte'asphalt disk drops from the ring
-c.
400
Point.
C.
C.
(1) Davey, W. S., J . SOC.Chem. I n d . , 55, 43-8T (1936). (2) Hoedt, T. G. E., van der Bie, G. J., Ortt, C., and Kerkhoven, R . E., Arch. Rubbercultuur, 22, 23 (1938); van der Bie, G. J., Bergcultures, 13, 784 (1939). (3) Nellensteyn, F. J., Chem. Weekblad, 34,646 (1937). (4) Ibid., 36, 362 (1939). (5) Rooijen, J. M. van, De Ingenieur, 53, 19 (1938). (6) Saal, R. N. J., Chem. Weekblad, 34,648 (1937).
