Oct ,, 1958
FOAMING IN LUBEOILS
1269
FOAMING I N LUBE OILS BY L. T. SHEARER AND W. W. AKERS The Rice Institute, Houston, Texas Received March 3, 1968
Low concentrations of substances having a lower surface tension than oil romote foaming markedly in oils if a single phase is present. If two phases are present and the added phase is finely dispersefin the oil the substance is a strong anti-foamant. Photomicrographs and high-speed motion pictures present a visual confirmation of the mechanism of anti-foamants.
Introduction Foaming and the control of foaming in gasliquid systems represents an area in which few quantitative relations have been developed. Since both static and dynamic properties represent major system variables, precise experimental measurements necessary for theoretical analyses are extremely few. This experimental study, largely qualitative in nature, attempts t o elucidate the mechanisms of foaming and foam stability, and hence provide a sound basis for theoretical attack. Four oils, designated as 555, 702, 528 and 529 and having quite different physical properties and foaming characteristics were chosen as subjects for the study. Dow-Corning 100 C.S. and 1000 C.S. silicones were used as anti-foamants.
Foam Studies I n order to evaluate the influence of temperature, physical properties of different oils and concentration of silicone on the foaming properties of the oils, a series of foam tests were carried out in which a measured volume of oil was aerated by nitrogen flowing a t a fixed rate through a porous stone. The temperature was maintained at a desired level by flowing liquid through the jackets of the oil containers. If silicone was to be added, a solution of silicone in n-pentane (0.1%) was added to the oil while stirring in a special high-speed stirrer. The results of these measurements are recorded here. Oil 528.-Samples containing 0, 0.5, 1.0, 2.0, 2.5, 5, 10, and 20 p.p.m. of silicone were aerated over the temperature range of 20 to 120". At 20", none of the samples produced foam; however the bubbles were much smaller than in those with silicones (0.5-3.0 mm., in diameter as compared to 5.0 mm. with silicones). At 60", the sample without silicone began foaming. At 70", the 0.5 p.p.m. sample began t o foam. The number of bubbles floating in the liquid decreased and the size of the bubbles increased with increasing silicone content. At 97" small bubbles began collecting on the surface of the liquid in the other samples. At loo", all samples were foaming, the foam height being approximately 2.5 times the liquid height. There was a slight increase in foam height with increased silicone content. As the temperature was further increased, the foam height slowly dropped in all samples. The samples then were cooled slowly while aeration continued. At 45", the oil containing no silicones had noticeably less foam than the samples containing silicone. There was no observable difference in those samples containing silicone. The addition of 0.05 p.p.m. of silicone t o these samples broke the foam immediately.
Oil 702.-The oil produced a stable foam a t room temperature. Samples containing 1, 5 and 20 p.p.m. of silicone did not. As the temperature was increased, the foam height in the virgin sample decreased until foaming ceased at 43". At 102" all samples began foaming and the height increased with increasing silicone content. The oils were then cooled slowly. Foaming increased in all samples as the temperature dropped. At 40", the oils containing silicones were foaming vigorously whereas that without was foami'ng only slightly. Oil 555.-Samples containing 0, 1, 5 and 20 p.p.m. silicone foamed a t 20", the silicone being an active foam promoter. The heights decreased on heating, the foam disappearing a t approximately 85" in all samples. The heights on cooling were approximately the same as on heating. The addition of approximately 2% silicone inhibited foaming at room temperature in this oil, but the oil became pro-foamant at 45". Oil 529.-Samples containing 0, 1, 5 and 20 p.p.m. of silicone did not foam a t 20". At 103" the virgin sample began foaming. After remaining at 110" for one hour all samples began foaming. On cooling the foam height diminished, the sample without silicone having a noticeably lower foam height. I n order t o gain a better insight into the above phenomena, microscopic examinations of the oil systems were made. If silicones were active as an anti-foamant, it was present as a dispersed phase with particle diameter less than 100 p . As the oils were heated, the silicone phase either disappeared due t o the increased solubility or else, as in the case of prolonged heating or on cooling a single phase, the silicone phase was present as particles much larger than 100 p, Since the number of small dispersed particles had only a minor effect on anti-foaming, this solubility effect explains the sharp break a t which oils became pro-foamant. I n all cases, silicones acted as a strong pro-foamant unless a dispersed phase was present. Surface tension measurements were made on oilsilicone solutions. The surface tension dropped markedly on the addition of silicone until saturation was reached. Further additions had no effect on the surface tension. Foaming tests confirmed the surface tension-solubility relationships. Below saturation silicone acted as a very active profoamant. At saturation, with an excess of silicone as a dispersed phase, the oils were strongly antif oamant . I n order t o get a visual picture of the action of the dispersed silicone phase, high-speed motion pictures were taken through a microscope of the oils during aeration. Pictures were taken a t
L. T. SHEARER AND W. W. AKERB
1270
Vol. 62
TABLE I Sample No. Description Gravity, "API Flash, PM, "F Flash, COG, "F Viscosity, SSU, at 100°F. at 210°F. Viscosity index Color: Lovibond 6 inch cell .Tag Robinson ASTM Pour, "F
528 refined Mid continent distillate 27.0
...
529 refined Mid continent residual stock 27.1
....
594 870 79.4 88
'
...
....
410
1601 116.5 98
50.2
....
139.4 42.5 100
.... ....
10
.... 0
$5
...
702 refined Mid continent distillate 31.9
555
...... 2'/: ....
40
555 refined Coastal distillate 32.5 296
.... 1
-45
...
... 0
Fig. 1.-Inhibited oil: magnification, 1OOx ; camera speed, 1000 p.p.5.
speeds up to 5000 frames per second and with magnifications up t o 200X. Uninhibited, the bubbles appear t o be smooth spheres (Fig. 1). With the addition of silicones, the surface takes on an uneven "jelly-like" appearance. As the bubbles form and rise through the liquid, the silicone particles adhere to the surface and slowly spread (Fig. 2). The bubbles with the silicone particles did not tend to coalesce in the bulk liquid but ruptured almost instantly on reaching the surface. The pictures did not indicate that there was any particular attraction between the bubbles and the silicone particles. I n order to detect any possible electrostatic effects, oil containing both silicone particles and air bubbles was placed between electrodes having a potential difference of up to 30,000 volts. No movement of either the silicones or the bubbles could be detected in either a uniform or a non-uniform field. Thus it was concluded that electrostatic effects, if present at all, were very small and had little inAuence on the system. I n order to determine what properties a dispersed liquid phase must have to be active as an anti-foamant, foam tests were run using the following materials as a dispersed phase in the oil samples : di-n-octyl phthalate, di-2-methoxyethyl phthalate, propylene glycol, styrene oxide, diphenyl oxide, dipropylene glycol, triethylene glycol, polyglycol E-600, triethanolamine, polyglycol P-1000, diphenyl, polyglycol P-750, Dowanol 22-H, Dow-
Fig,2.-Inhibited oil: magnification, 1OOX; camera speed, 1000 p.p.s.
anol 23-H, Dowanol 25-H, and other similar compounds. None of the above were effective as an anti-foamant. It should be noted that none had a surface tension less than that of the oils. The requirement of very limited solubility and a lower surface tension excludes almost all compounds except the silicones for hydrocarbon-oil systems. Conclusion Silicones are active as anti-foamants in lube oil systems if present as a dispersed phase having particles less than 100 p . Otherwise it acts as a pro-foamant. High-speed motion pictures indicate that the silicone particles adhere to the bubble surface, causing rapid rupture of the bubble when it reaches the surface. Acknowledgment.-This work was supported by The Texas Company.
