99 2
C11.41 1 A I 3 E K 1 ' .ZSI) J. I I ( )i!'AllH2,L.,.2.Naturally, the refiner hecame convinced of the superiority of his compounded oils over straiglit hydrocarbon nils, and this idea has been maintained so persistently, it still prevails very geiicrall!. with consumers of lubricating oils. But within the last ten !.ears, other varieties of petroleum have been found t o yield lubricating oils with superior viscosity and wearing qualities which makes it no longer necessary to rely on cnmpoiinclvd oils ci!.hcr for use on bearings or in cylinders. This is of especial importance with reference t o cylinder oils, for it is well understood t h a t thc conditions of high temperatures and highly heated steam in cylinders lead t o saponification of the animal or vegetable oil used in compounding, with consequent corrosion of the cylinder. As is well known, castor oil is one of the very best lubricating oils, espccially for durability, but its general use is precluded by i t s high cost. It is now possible to prepare straight hydrocarbon oils fully equal in viscosity and wearing qualities t o castor or any other high viscosity vegetable oil. T-iscosity is generally accepted 3 s a standard of value in classifying luhricating oils, but it is not certain t h a t it is rcliable a s indicating t h r durability a n d wearing qiiiilitic-s of oils differing widcly in composition. l'hcrc is lit tlr dniiht 1 lint :i coiifirniation r)f \.iscosity I)!, cIicniic:iI t h t a
ON VISCOSITY AND LUBRICATION.
993
and frictional durability tests may be depended on t o giveaccurate information for commercial use. The viscosity of lubricating oils has received much attention and several methods and forms of apparatus have been suggested for its determination, but for the most part of arbitrary construction and comparison, and differing so essentially t h a t determinations made with different instruments are not easily and readily comparable. As thus determined, viscosity is but an arbitrary standard based on a n assumption t h a t the outflow of a liquid through an orifice, influenced a s it is by several physical conditions, is a correct measure of surface viscosity between bearing surfaces. It is merely a relative comparison with an oil arbitrarily selected a s a standard, or with water or by means of a metal apparatus arbitrarily constructed. But with the use of water, evidently, the conditions of temperature must be stated in t h e results, for the viscosity of water is quite different a t different temperatures, and slight variations in temperature have likewise a n important influence on the viscosity of oils. It is, therefore, necessary t o know the temperature coefficient of water, and it would be interesting t o follow out a series of observations with a n homologous series of hydrocarbons, although the possibility of such a n investigation is almost precluded by the immense labor necessary in separating in a n acceptably pure form the individual hydrocarbons. We have on hand, members of the different series C,H,,,,, C,H,,, C,H,,-,, C,LH,,-, and we have made a series of observations on some of the individual hydrocarbons. Interesting results have also been obtained on the viscosity of mixtures, showing the influence of the hydrocarbons of t h e different series. I n attempting t o arrive a t a series of determinations which should avoid t h e errors in methods in which differences in specific gravity, and accurate observations of temperature are neglected, it was evidently inexpedient t o use a n y of the commercial methods, especially since, a s explained above, the data afforded by those methods are merely empirical, and with no definite relations t o a common standard. The well-known method of Ostwald was selected, therefore, a s best suited for these determinations, and the apparatus employed needs no detailed description. I n this method a definite volume of liquid flows through a capillary tube under a definite head. In the calculation, the pressure under which t h e liquid flows through the capillary, is corrected for its density in the Ost ald forrrula :' Ostwald-I uther, Physico-Chemische Messungen., p. 20. 7 = The viscosity of the liquid examined, S = Density of the liquid examined; t = Time of outflow of the liquid examined; to = The viscosity of the standard liquid; S, = Density of the standard liquid; to = Time of outflow of standard liquid; t to St/Sok.
-
994
C€IARI,ISS I?. RI.1UIIRY
. U I ) J. I l c ) W A R I ) ~ 1 . ~ ' 1 7 1 ~ ~ ~ ~ s ,
The values thus obtained, express t h e ratio of t l w viscosit!. of the liquid under examination to ;L standard liciuid used f o r rcicrence. Ii'ater is the standard liquid most conimoiil!. clioscn. a i i t l tlic \.alucs ui are refcrretl t o as "Specific \.iscosities," 1( e . , t h e r a t i o of viscosit!. t o that o i m t c r a t tliat p r t i c u l a r teiiipcraturc. The acl\xiit:igc oC usiiig jvatc'r consists i n t11c. ease with which it ma!. Iw ol)t:iiiie(l willcicntl!- p i i r ~ and ~ iii tlic iact t11;it tlic value iii ;il)solute units for t h e \,iscoiit!. of water is t h e h ~ s kt n o ~ 1 oi 1 any liquid, :md spccific viscosities ilia!. I)c coii~crtetlcasilJ, into al).olutc. units. 'l'lic specific viscositicvs ol)t;tilic.tl :it tliffvrvnt tcmperaturc s arcs not comparabk, since t1ic.y csprcss o n l y the ratio a t the particular temperature chosen arid talx no account o f t lies chaiigc- in Y o l u t n e of the apparatus. especiallJ. in t h e size of the capillai-J,. 'lo compare the results obtainctl a t different ternperattires. it is iicccssar!. to coii\-ert tile valuvs iiito ahsolute units, using t h e knon.n values ior tlic, viscosit!. oi ivatcr a t t l i c s tcnipcraturc, used. From iiiterpolation aiid estrapoIation of tlic result oiitained by 'l'liorpc a n d Rodger' we obtained the \xlucs 7 = C.).OIOO.; a t zoo C. a n d 17= 0.00462>jat Go0 C., where 7 is t h e coefficieiit of viscosity in aljsolutc uiiits. i. e . , dynes per square centimetcr. I%ig tlie vxluc's ol)tainetl a t 20' and 60' b!- thc,sc numbers, conipara1)le rcsults arc obtained. Constant ternpcraturc was maintainctl by placing the \.iscosiinetcr ill a glass thermostat through \vliicli oliscrvations could 1~ made, aiid which was supplicd with Ivater from a larger thvriiiostat maiiitainccl a t a coiistnnt temperature I)!. nic:ins of a n electric tlicrmo-rcgulator. l'he watcr was pumped from tlic larger t o tlic. smaller tlieriiiostat li!. nieans oi :L small liit-pump operated 1)). ;i liot air ciiginc. l'he temperature in t h e glass thermostat was held a t 20' ( & o .0 2 ) ; since t h e viscositl- of t h e oils, like t h a t of most liquids, changes alioitt 2 per cent. per degree, tliis smnll fluctuation is negligible. The measurements at 6 o o , were matlc iii a glass thermostat of ahout 10 litcrs capacit!. iii which tlie tcriipcr:iturr. was Inaintairitcl by superheated steam injected a t t h e liottoiii through a small orifice. 'I'lii. steam could he easily regulated a n d t h v tcxniperature readily niaintaincd at G o o (50.02). In observations on the paraffin hydrocarbons, it was found t h a t yiscosity increases with sonic' regularity in the homologous series with decreasing percentages oi liytlrogc~n. Thcsc 1iydrocarl;ons were obtained by long-continued fractional separations under sJ-stematic conditions, a n d their identity was shown I)>-analysis a n d critical examination j hut it is doubtful whether the homologues can be completel?- separated c\-cn h y very prolonged distillation unless much larger amounts of material arc used than is possiblc on a lahoratory scale. I'roc. Roy. SOC.,1894; Z. physih. Ciiom., 14, 361.
ON VISCOSITY A N D LUBRICATION.
TABLEI
(20').
B. P.
Sp. gr.
98-100' 125' 172-173' 174-1 75 O 163' 209-210' 2 I 2-2 14' I jS-I 59' (jo m m . ) 1j 5-1 j S ' 174-175'
0.724 0.735 747 0,753 0.745 0.762 0.769 0.793 0.796 0.7c)Y
19'.--200'
0.813
I'
"
995
'
Specific viscosity, 0 . j1
o 60 0.96 0.95 0.89 1.25 1.49 2,79 2 . j j
3.35 5 97
I n Table T , it will be observed t h a t viscosity increases somewhat irregularly with every increment of CH,, and t h a t the change is greater with t h e increase in molecular weight. Since the proportion of hydrogen t o carbon apparently influences materially the value of viscosity, it seemed desirable to compare the viscosity of hydrocarbons of differcnt series. In Table 2 , are given the values for hydrocarbons with the same boiling points, b u t members of different series. TABLE2 (60') Series.
CnHzn--z CnHzn-z CnHzn tz Cn Hzn
B. P.
294-296' (jo mm.) 294-296' '' 2 74-2 76' 2j4-276' ''
Sp. gr.
781 o 841 0 0
---
0
835
I
I5
Specific viscosity.
88 23 8.51 15 63
IO 21
The greater viscosity of the hydrocarbons poorer in hydrogen, is clearly shown. I n comparing t h e viscosities of t h e two hydrocarbons boiling a t 294'-296' it will be observed t h a t the difference is greater than the difference between the viscosities of the two hydrocarbons boiling a t ~ 7 4 O - 2 7 6 ~ . This demonstrates the influence of a decreasing percentage of hydrogen since in the first set, the change is from 2 n t 2 t o 272-2, whereas in t h e second set, the change is only from 2 n 1 2 t o 2n. Both viscosity and specific gravity increase with the decreasing hydrogen. Another possible influence must not be overlooked, however, namely, the internal structure of the different hydrocarbons. It is reasonable t o assume t h a t t h e straight or open-chain structure of the paraffin hydrocarbon CnHzn+2 behaves differently under the stress of internal forces on which viscosity depends, from the ring or cylic structure, which must be accepted for tlic other series, until more is definitely known concerning their constitution. Certainly this is plainly shown in lubrication, where tlic paraffin hydrocarbons are of comparatively little value. If, then, the lower series furnish lubricators with greater viscosity, t h e addition of a member of a higher series should give a mixture lower in Of approximately this composition.
C)
90
1:.
M.ll
