SERVICE

PROPERTIES

OF

FUELS

AND

OILS

F O A M I N G P R O P E R T I E S OF H Y D R A U L I C F L U I D S V, N. P r i g o r o d o v

a n d L. V. G o r n e t s

UDC 532.694.1

Existing methods of assessing the foaming properties of liquids, both in the Soviet Union [1] and abroad [2], are relative and do not reflect the processes that actually take place under practical service conditions. Only limited information is available about foaming of nonaqueous solutions, and the influences of temperature and the composition of the gas phase on foaming properties have practically not been studied. In previous works we determined quantitative criteria to assess the foaming properties of liquids and developed an instrument [3] to measure them, The object of the present work was to study special features of the foaming of hydraulic fluids of different kinds operating over a wide range of temperature and other test conditions. The formulae and designation of the liquids that were studied are given in Table 1. The gas phase was dry air. The foaming properties were studied on the instrument described in [3] which is a thermostatted measuring cylinder with a porous filter soldered to its base connected to a device to control the temperature and composition of the gas phase. The studies were made under dynamic conditions with gas delivered continuously to the test liquid in the measuring cylinder. The rate of foam destruction was recorded by a flow-meter wMch measured the gas discharged from the cylinder and the volume of foam was measured from its equilibrium - height in the column. The foaming properties of the liquids were characterized by the stability coefficient O and the maximum foam volume Vsp [4].

.:=,ol

V'k\ -4,0

0

gO

\ 160

2~0

Temperature, ~ Fig. 1. Temperature dependence of foam stability of liquids: 1) PAKhS; 2) PAS; 3) NF; 4) AAF; 5) PAFS.

Figures 1 and 2 give temperature dependences of O and Vsp for various liquids over a wide range with equilibrium distribution of the components between phases with the gas and the liquid at the same temperature. These conditions were achieved by bubbling the gas ttirough the experimental liquid which was thermostatted at the given temperature. As will be seen from Figs. 1 and 2, foaming changes considerably over the temperature range from - 6 0 to 150-250~ In all tests foaming is greatest in the positive temperature range from 150 to 230~ for PAFS and 50 to 100~ for the other liquids where it is twice or three times the mean value. The foam stability of all the liquids

TABLE i. Compositions and Designations of Liquids Composition

Designation

Polyalkylchlorarylsiloxanes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyalkylsiloxanes 50% dioctyl sebacate 50% . . . . . . . . . . . . . . . . . . . . . . . Petroleum fraction 200-300~ containing 8% butyl ester of polyvinyl a l c o h o l . . . Alkylarylphosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polyalkyl-7 triftuorpropyl siloxanes~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PAKhS PAS NF AAF PAFS

Surface tension, dyn/cm 21.7 28.4 27.7 34.1 18.8

All-Union Scientific-Research Institute for Study of Aviation Materials (VIAM). Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 9, pp. 38-42, September, 1970.

9 1971 C o n s u l t a n t s Bureau, a division o f Plenum P u b l i s h i n g Corporation, 227 West 17th Street, New York, N. Y. 10011. A l l rights reserved. This article cannot be reproduced for any purpose w h a t s o e v e r I without permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

688

gSO0 gO00

I500 #a

e looo 500

- 40

0

80

160

Temperature, ~ Fig. 2

200

120

t44

I

I

I

80

80

100

Temperature,

~

Fig. 3

Fig. 2. Temperature dependence of maximum specific volume of foam of liquids: 1) PAKhS; 2) PAS; 3) NF; 4) AAF; 5) PAFS. Fig. 3. Temperature dependence of foam stability of solutions of additive PAFS in liquid PAKhS. The figures on the curves denote the concentration of PAFS in solution (% weight), arrows denote the cloud point. studied, except NF and AAF, increases steadily as the temperature is lowered so that at -80~ the stability coefficient is two to three orders greater than at 100~ It should be noted that foam stability increases particularly sharply at sub-zero temperatures. The general nature of the temperature dependences of foam stability and foaming of the various liquids coincides with the nature of change of these quantities in petroleum oils [5].

;2gO moo

~-

800 BOO CC>

o00 ZOO

] 0

,,d" i-"-~r-IgO ~

4.0

~ BO

~o 80

, 100

Temperature, ~ Fig. 4. Temperature dependences of foam stability of solutions of additive PAFS in liquid NF, The figures on the curves denote the concentration of PAFS in solution (% weight), the arrows denote the cloud point.

The influence on foam formation of temperature and component composition of the phases was also studied. Table 2 gives experimental results for the dependences of the foaming properties of the liquids on the temperature difference between the liquid and gas phases with constant component composition. It will be seen from these results that delivery of cold gas to hot liquid does not influence the foaming properties of the stystems studied whereas delivery of hot gas to a cold liquid appreciably (two or threefold) alters the stability of the foam and the foaming properties of PAKhS, PAS, and NF. The influence of volatile components which may either be present initially (water, manufacturing impurities, contamination, etc.) or that are produced by chemical decomposition in service were also studied.

Table 3 gives foam stability results for liquids in the temperature range 20 to 100~ in the initial state and after blowing for 2 h with dry air at 80 to 90~ It will be seen from Table 3 that for NF and A A F in the initial state there is a clearly expressed m i n i m u m stability in the region 30 to 40~ whereas after blowing with dry air the stability of these liquids varies steadily with temperature. This latter behavior is also typical of liquids PAKhS and PAS for which dry air treatment has no noticeable influence on foam stability. Apparently the presence of volatile components in NF and A A F reduces foam stability. It follows from the above that the foam stability of the liquids is significantly influenced by temperature, by the content of volatile components and by the temperature difference between the two phases. The nature of the influence and also the absolute values of the measured quantities depend on the nature and compos/tion of the liquid. In order to reduce foaming of liquids a study was m a d e of the foam-suppressing properties of PAFS liquids (fluorine substituted silicones) which have low foaming properties and low surface tension when added to liquids NF, P A K h S and PAS which have the most stable foams. As these liquids are used over a wide temperature range it was of interest to study the influence of both additive concentration and temperature on their foaming properties.

689

TABLE 2. Influence of Temperature Difference between Phases on Foaming Properties of [2quids Stability coefficient 0 (cm2/ml) and Stability maximum voL of foam Vsp(ml/ml) at coefficiliquid tli q and gas tg temperatures L i q u i d ent and 1TIaXI1TIUI tliq=95 ~ tli_ =95 ~ :li-= 10~ tliq=Io oc t q--lo ~ tg~60 ~ tg =95 ~ VO1. of gat=o At=0 foam At= "50 ~ At= +80 (

PA Si 0 gsp PAS

0 Vsp

NF

o__ Vsp

AAF

0_2_ Vsp

PAFS

o_ Vsp

180 13

200 12

1340

33,4

2000 15,8

15 6,9

15 6,0

35 6,4

75 5,6

5,0 5,6

7,5 7,6

10,0 3,8

30, 0 9,0

4,7 7,3

5,2 7,6

0,6 3,8

0,7 3,9

18,3 5,0

18,5 5,9

16,0 3,5

15,6 3,2

TABLE 3. Influence of Dry Air Treatment on Foam Stability Liquid

Dry air treatm e n t [

[

Temperature, ~ 20 1 30 1 40 1 501. 601 801 98 !

PAS

Not treated Treated

PAKhS Not treated Treated NF

Not treated Treated

AAF ITreared

2800 2380 140(~1160 870 600 320 2740 2360 1372111801890 620. 305 1210 186 125 220 183 120

92 90

78 40 [ 30 42 28

[ 250 ll0 1~0 11102 23 40 41 ~02 108 76 38 18 2 4 0 160 106

68

40

19

10 10

Figures 3-5 give experimental results that characterize the influence of the concentration of additive PAFS on foaming of the liquids in the temperature range 20 to 100~ It may be concluded from these results that additive PAFS stabilizes foam in liquid PAS and increases the foam stability in the temperature range under consideration. Additive PAFS has a dual action on liquids PAKhS and NF and its action depends very much on the temperatare and concentration. At low temperatures and high concentrations PAFS behaves as a foam suppressor and at high temperatures and low concentrations it is a strong stabilizer and increases foam stability. Moreover, it was noted that with a particular content of the additive in liquids PAKhS and NF there is a narrow (1-5~ temperature range in which the action of the additive reverses. Visual observations of the cloud points of solutions and also additional data on the relative foaming properties and solubilities of PAFS in the liquids under consideration shows that the duality of additive action is associated with its limited solubility. In the molecular dissoNed state PAFS is a powerful foam stabilizer whereas if it is present as an undissolved liquid phase in PAKhS and NF it is an effective foam suppressor.

690

1600[ I000I 1200I 0,6-

These results confirm published information about the impaired effectiveness of foam suppressors at high temperatures [5] and clearly show that their action is related to limited solubility [6]. Comparison of the graphs shows the anomolous shapes of the curves that characterize the dependence of foam stability of solutions of liquid NF on the concentration of the additive PAFS in the molecular solubility range, namely, reduction in stability with increase in additive concentration. This is evidently associated with the circumstance that at low concentrations FS has a foam stabilizing effect at low temperatures where the viscosity of the system is greatest.

lO00I

eoo[

~

0

I

20

I

40

60

80

tO0

Temperature, ~ Fig. 5. Temperature dependences of foam stability of solutions of additive PAFS in liquid PAS, The figure on the curves denote the concentration of PAFS in solution (% weight).

The relationship between the limited solubility and the nature of the action of the additive PAFS in these systems can be explained by study of their surface properties. As was previously shown [7] there is an inflection in the surface tension isotherm at the cloud point t m of the solutions, which indicates that there is a change in the surface properties of the layer and in particular surface stratification may occur, It is evident that the occurrence of such a layer on the surface when t > t m reduces the elasticity of foam films and consequently their stability [8]. On the other hand, because of the low surface tension PAFS is adsorbed on the surface of separation of solutions of the liquids and when t < t m it has a stabilizing effect and increases the stability of the liquid foams.

The investigations indicate the complex and contradictory nature of insoluble foam suppressors. In accordance with this, in selecting stabilizers and foam suppressors we should bear in mind not only the composition and nature of the system, but also the service conditions, because changes in operating conditions, for example, temperature, can alter the effect of the surface-active substance in the system. CONCLUSIONS 1, The foaming properties and nonaqueous working liquids based on petroleum fractions, silicones and phosphororganic oligomers were studied in the temperature range - 6 0 to 150-250~ 2, It was shown that foam stability depends not only on the temperature and nature of the experimental liquid but also on the temperature difference between the gas and liquid phases and the presence of volatile components in the liquid, 3, A study was made of the foam suppressing action of fluorine-substituted silicone additives relative to liquids based on petroleum fractions, alkylarylchlorsiloxanes and mixtures of alkylsiloxanes with dioctylsebacate in the temperature range 20-100~ 4, It was shown that the effect of the additive depends on concentration and temperature. When the additive is in a state of molecular solution it acts as a foam stabilizer and when it is an insoluble liquid phase it is an effective foam suppressor. LITERATURE I. 2. 3.

4. 5. 6. 7.

8.

CITED

Methods of Testing Aircraft Hydraulic Fluids, First Stage (Preliminary Laboratory Tests) [in Russian], Izd. Cos. Standardov (State Committee of Standards) (1968). R, E. Hatton, in; Liquids for Hydraulic System [Russian translation], Edited by V, V, Vainshtoka, Izd. Khimiya (1965). G. L Kichkin, Khim. i Tekhnol. Topliv i Masel, No. 4 (1966). V. N, Pfigorodov, Zavod. Lab,, No. 10 (1969). S, Ross, J. Phys. Coll. Chem., 54, No. 3 (1950). L, T. Shearer and W. W, Alkers, L Phys. Chem., 62 (1958). B, Ya. Teitel'baum, T. A. Oortalova, and S. O. Oanelina, Kolloidn. Zh., 12, No, 4 {1950); B, Ya. Teitel'baum, Kolloidn, Zh,, 12, No. 5 (1950), S, Rossand Haak, J. Phys. Chem,, 62 (1958).

691