POLYMERS AND VISCOSITY INDEX H . C. EVASS Enjay Company, Inc., 15 West j l s t S t . , .Yew l b r k 4, S. Y.
D. F. YOLEG1 Standard Oil Company of .Yew Jersey, Buyiuay, S. J .
A
SIOSG the iiiiportant temperature; t,oluene,xylene, A graphical interpretation is presented of viscosity index characteristics desired and isoamyl caproate gave improvement by certain polymers, using data obtained blends which, in addition, in niotor oils are lovi conduring a study of the effect of temperature on the viscosity showed increased thickening suniption and ease of startof high-molecular-weight polybutene in pure solvents. at the higher temperatures as ing. Increasing high teniA n explanation is given as to why ability to improve the calculated from the ratio of perature viscosities bo imviscosity index is limited to polymers having definite soluqSp/c a t 25' and 37-7S3 C'. prove, oil consumption and bility characteristics. Data are presented showing that That there is a liniit t o this piston seal, and decreasing the viscosity index of polymer-oil blends can be greatly reversed viscosity effecr is low temperature viscosities enhanced by adding as third components certain synthetic t o iniprbve ease of starting s h o m in Table I1 whew chemicals which reduce the solubility relation between oil can be combined in the same blends of benzene and isoami-I and polymer. These three-component systems produce hutyrate were studied over oil only by improving the oils of abnormally high viscosity index. Polymers of varied their period of transition by viscosity-temperature relacomposition and solubility properties are blended into a obtaining the viscosities ( I \ er tion-in other words, by number of oils varying from -321 to +lo5 viscosity index, smnll temperature itlcre-. raising the viscosity index. Data and curves show that the relative ability of the varimerits. The viscosities of It is generally knon-n t h a t ous polymers to improve viscosity index varies with these blends incrciase oi~ly certain polymers of high during short period,-mers that are soluble in lubricating oil. Therefore, ns the range of decreased relative thic1;ening p o w r at tlic 1owt.r some explanation of the plienomcnon of viscosity indes improvetemperature is reached. ment by certain polymer:: seems necessary in order to underThe blends just described are u i theoretical importance only st,and why other similar polymers are unable to exert this effect. ~1.qsolvent-polymer blends of this type have little practical iigConsiderable data (5)have already been given t'o illustratc the nificance. Holyever, t'he use of polymers in lubricating oils t o viscosity-concentration relation of polybutene in a number 01' improve their viscositJ--tenlperaturc relation is not n e v , and t,he pure solvents. The data indicate that when a s t m d a r d polymer preceding data have a direct bearing on these oil-polymer blends w a ~used, the relative solvating properties of solvents could be i n t h a t t,hey have aided in explaining the underlying principl1.s estimated from the calculated qsp,Ica t a specified concentration. of viscosity index improvement by means of certain polymers. It mas shown t h a t in the case of complete solubility the Arrhcnius On a graph (Figure 1) having as its ases log viscosit,y and conequation might be applicable a t high concentrations: centration, three diagonal parallel lines d , B , and C are dran-n; l0gL = Kc they represent the change in viscosity of a polymer-oil blend as 11 the concentration increases a t three temperatures 1,B , and C, where q' = viscosity of solution provided the polymer has a const'ant thickening pon.cr over the q = viscosity of pure solvent K = experimentally determined constant range of concentration covered. From the precedirig obserc = concentration of polymer in solvent vations on polybutene in pure solvents, it can be assumed that this is not the case, but rather the thickening power decreases The equation did not hold, and deviations from it increased as n-ith increaeing concentration. Some point is reached where it the solvating polver of the solvent decreased. I n t'lie case of such agains reverses itself and t'he thickening power becomes propoor solvents for poIybutene as benzene and toluene, the dcnounced. A flat S-curve results. Such a curve might be drawn viation became quite marked. These published curves indicate as A ' (Figure 1). If n-e can assume from the data that deviation that, at l o v concentrations of polymer in solvent, the pol>-meris n-ill be less a t each concentration at a higher temperature because completely dispersed and eserts its optimum thickening effect. of greater solubility but that an equivalent deviation \vi11 occur Ho\vever: as the concentration of polymer in solvent is increased, a t some higher concentration, it should be possible to drarr curve B' the relative thickening poLver decreases until, at higher concerias shown, remaining closer to B, yet breaking alvay a t the higher trations, further addition of polynier has less effect on viscosity. concentration. The samc reasoning rvas used t o draw curve C'. This range of reduced thickening effect is characterized by abnormal sensitivity to temperature changes. First, viscosities .It loiv concentrations of poLymer in oil curves A' and B' tend of a number of blends n-ere det,ermined a t 25' and 37.78" C. t o coincide ivitli --I and B , respectively. As the concentration of (77" and 100° F.), and the results are given in Table I. Bcnzene iwlynicr in oil increases, the relative thickening power of the yielded polymer blends that increascd in viscosity a t the higher polymer decreases and the curves in each case tend to slope away horizontally from thf respective straight lines, 1 Preeent address, S t a n d a r d Oil Development Company, Elizabeth, N. J . 1676
1677
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
December 1947
' P r
^i
-d I L L ~ ~ C ~ ~ PSLYISE? I G U
Figure 1. Effect of Temperature and Polymer Concentration on Viscosity of High Polymer Solutions in Solvents of Low RIolecular T e i g h t
C O Y C E 2 T R A T I O Y POLY MER Figure 2 . Effect of Temperature and Poljmer Conrentration on 1iscosit? of Lo\+ Polymer Solutions in Lubricating Oils
iii ttir preceding paragralh defines a region \\-ticre a t a givcn At point AV curves d' and B' intersect, and agaiii intersect at I-. -itS and I' there can be no chaiige in viscosity during the temperature the polymer molecules are highly convoluted i n increment tcniperature change A ' t o B'. Betir-een X a n d I' solution. At present it is held t h a t the molecules are convoluted because of the very strong self-association factors for the pol>-mcIr the curves are reversed, and the viscosity increases as the temin a poor solvent. Under the convoluted condition t,hc polyn1r.r perature is raised from A to B. I t is therefore obvious t h a t , as the temperature is raised from '4 t o B t o C betn-een points S exhibits a lon- intrinsic viscosity. Also, the convolut,d polymer may bc easily removed from solution by the addition of a rat1ir.r and I-, the viscosity of the polymer blend first increases in viscosity from A t o B and then decreases in viscosity from B to C. sniall amount of a strong polymer nonsolvent. Esperinic~nta1 proof of this has been given (3). It should be remembered t h a t the data used to d r a n the above series of curves were taken from experiments on pure solvents and K e conclude, t,herefore, t h a t a polymer capable of improving the viscosity indes of a n oil must undergo a, period of 1(.2.s t i ) a polymer nearly ten times the average molecular n-eight of that used in lubricating oil blends. It n-ould be expected, therefore, that tlie stability of such blends n-ould be far more critical than those of an oil of relatively high average molecular weight and a TABLE I. V I ~ C O ~ I T Y - T E ~ ~ P E RRE~,ATIOSS A T C R E OF polymer of lower average molecular weight. Figure 2 shows a POLIBUTESEa I S SOLVEST5 similar series of curves, based on oil and lo~r-er-iiiolecul:ir-~~eig~i~ Viscosity. Centistokes 7 l d C pol>-mer, which are less accentuated than thosc in Figure 1, Solvent 25' C. 37.78' C . 13' C. :37.78' C 1::itio corroborate t h e relation better knoivii t o csist i l l Renzene ' 786 2.47 0.221 0.21iCi 1 34 Toluene 3 4!1 3 27 0 4'9 0 440 I.ll? practice. THEORIES O F FL.4TTESING O F V I S C O S I T Y - C O S C E S T K ~ T I O S CURVE
One explaiiat ion ior tlie flatteiiiiig of tlic tion curve and the unusual temperature-vise of the blends is the view expressed in a n earlier paper (31 : Thc polymer exists in two phases-a niolecularl>- dispersed polymvr in solvent phase and a niechanically dispersed solvent in poi!.iiicjr phase n-ithin the first phase. I t must be pointed out that t h i vien- is considered t o be the very extreme case for t h r p i t viscosity-temperature or viscosity index theory. .I second explanation for the flattening of the viscosirj--roricentration curve and the unusual temperature-viscosity cli:ira[,teri-tics of blends follows. T h e intrinsic viscosity of a poiymer iii a poor solvent is lox-er than in a good solvent. €Ion-ever, tiicrt. is little change in intrinsic viscosity with temperature in a good solvent. n-liereas there is a considerable increase in intrinsic viscosity in a poor solvent. It is necessary, thercforc, that any polymer cnpnhlc of improving the viscosity index of a n oil t o a: grcat c ~ t e i i must t pass through this reduced solubility tra~isitiori a t the temperature of measurement-e.g., x i t h i n t h e rangci 100-210" F. It must shon. reduced viscosity-improving effect at 100" F. ant1 net normally a t 210" F. This is not aln-ays po5siblc, and sonic pol>-merswhich pass through this period of transition at ot,her teniperatures n - o d d do eo unobserved by the measurrnic~iitof "vixosity irides." ' a R d u i . e dsolubility" i i i the polymer-solvent system mentioned
Xylene n-Proprlhenzene IIesitylene Cymene Amylbenzene Isoaniyl caproate
5 67 7.06 6 28 9.7G 13 t i ' 4.86
5.24
G
IjG
.> ail 7 89 10.33 4 00
0.678 1) 690 0 690 0.714 0.784 0,150
0 688 0 GSO
I 01 1 (if1
