JOURNAL OF T H E AMERICAN CHEMICAL SOCIETY YOI.L\IE
00
NLMBER 3
MARCH 26, 1947
[ C J h T R I B C T I O S FROM THE RESE.4RCH LABORATORY O F THE
GEXERAL ELECTRIC COMPANY]
Liquid Methylp~lyailoxane~ Systems BY DONALD F.WILCOCK An eyuilibriium rearrangement of methylpolysiloxanes has been shown t o occur upon contact of these materials with concentrated sulfuric acid.2 The viscosities of the polymeric mixtures, or silicone oils, resulting from the equilibrium rearrangement of octamethylcyclotetrasiloxane with small amounts of hexamethyldisiloxane were found to be dependent in a direct and reproducible manner upon the average functionality of the system. This paper is concerned with the introduction of trifunctional units derived from the hydrolysis of methyltrichlorosilane into these methylpolysiloxane systems, and its effect upon their properties. The molecular size distribution in these polymers anti its relation to their physical properties are considered from both the theoretical and experinicnl.al viewpoints. The statistical distribution of molecular sizes is derived for the equilibrated methylpolysiloxLines following the theories of Flory and Stocktnayer, which are based upon the assumptions that a11 rcacting groups are equivalent and t h a t 1 1 0 cyclic inolccules arc formed. This distribution agrccs with that found in experimental siloxane pol>-mersonly within ;t limited range of viscosity and cotripsition. The molecular size distribution i n the more viscous polysiloxanes and in those contaiiiing a large proportion of trifunctional rinits is coiripli~~atcd by the presence of cyclic and polycyclic molecules which cannot be treated adequately by a simple theory of statistical size ttistributlon . Functionality If some of the bifunctional units, ( C H 3 ) 8 i 0 , in an equilibrated methylsiloxane polymer are For a disciission of urg;~nosiIic~mnomenclature. see R . 0. C h c m . P X . 21, 303 i l n i l ) . ( 2 ) \ V i n t o n 1’:itnorle ;tnd I h m a l d I:. \Vilc,>ck. ’THIS J v u n N n r . . 68, 1 !I I i i ) . L 1)
~ L ~ I P .T. T .
replaced by equimolar amounts of trifunctional units, CH3Si03/,, and monofunctional units, (CH3),SiOl/,, the average functionality remains unchanged. However, the physical properties of the resultant polymeric mixture are altered. The viscosity and the melting range are lowered, the viscosity-temperature coefficient3 is increased, and the proportion of low molecular weight material is greater when this replacement has been made. The density remains substantially unchanged. These changes in physical properties are the results of a high degree of branching as well as of considerable cyclization in the polymeric mixtures containing trifunctional units. In order t o facilitate discussion, the abbreviations M, D, and T will be used to indicate the polymer units : Functionality
Composition
l l n i t weight
M D
1
(CH3)sSiOi;,
2
T
3
(CH,)rSiO CH,Si03/,
81.16 71.13 67. 09
Unit
Thus, hexamethyldisiloxane is M?, octamethylcyclotetrasiloxane is DJ, and the linear polymers arising from their equilibration are M?D,-?, where x is the number of units. The mole fractions of M, D and T units in a polymer will be denoted by m , d and t. In a system comprising M, D and T units the average functionality, F, is related t o the mole gractions by the equation F
=
3t
+ 2d + nt = 2 - ( m - t ) = 2 -
01
(1)
The constant a! = ( m - t ) expresses the mole fraction of M units available for growth termination (assuming complete absence of cyclic material). In the polymer system comprising M , D and T units, the nature of the polymeric mixture
-IT7
(3) 11. I‘.\\’ilcc,ck, I l e c i i . 7i2g,, 1,. 7X!l (1!144);
p .2l!2 (I!li.?).
DONALD F. U'ILCOCK
47s
is dependent not only upon the average functionality but also upon the relative proportions of D and T units, These proportions may be held constant within a series of compositions where F is varied, and such a series will be characterized by a constant 7,defined as f/(d
7
+ t)
Preparation of Fluid Polymers Several series of polymers were prepared, each series being characterized by a constant value of T . The oils in each series were prepared by equilibrating va:rious amounts of Mz with a stock oil containing D and T units in the desired proportions. The stock oil for each series was obtained by the hydrolysis of a weighed mixture of methylchlorosilanes. Over the range 0 < 7 < 0.11 these stock oils contained only D and T units. Over the range 0.11 < T 5 1 they contained equimolar amounts of M and T units in order to avoid gelation upon hydrolysis. The stock oil for the T = 0.103 series was prepared from 3481 g. (27.0 .moles) of dimethyldichlorosilane containing 0.3 mole per ,cent. of methyltrichlorosilane, mixed with 448 g. :3.00 moles) of methyltrichlorosilane. This mixture was hydrolyzed in batches by running 605 cc. slowly beneath the surface of 2650 cc. of water over a period of half an hour. The water was stirred rapidly and maintained between 10 and 15" by an ice-bath. After washing once wii.h 600 cc. of water, the hydrolyzate was centrifuged until clear, and filtered. It was then neutral, as tested by shaking with neutral ether, distilled water and methyl orange. The stock oil for the T = U series was the compound D,,prepared in the following manner. Dimethyldichlorosilane was hydrolyzed as described above, and the hydrolyzate was then distilled through a 15 inch column, one half inch in inside diameter, packed with one sixteenth inch stainless steel helices. The fraction boiling between 173 and 177' \+asretained and carefully redistilled through the same column a t high reflux ratio. The D4 fraction boiling a t 175.i-175.9' (uncor.) a t 758 mm. was separated.
The compositions for equilibration were prepared by weighing the desired proportions of stock oil and M:! on an analytical balance. Equilibration was carried out by adding 4y0 by volume of concentrated sulfuric acid and shaking on a reciprocating shaker for. twenty-four to fortyeight hours. Preliminary experiments indicated that constant viscosity was attained in sixteen hours. Equilibration was stopped by the addition of 20:;; by volume of water followed by shakTABLE I PROPERTIES OF EQUILIBRATED POLYSILOXANES Kinematic viscosity a = (In - 1)
'1NO
V2LO
VTC
(Centistokes)
Densi$y a t 20
Per cent volatile (7 dFys a t 150')
T = o
0.055 .0393 .032 .0135 ,0079
10.8 14.7 49.7 20.6 18-L 73.2 459 185 25.9 36.0
0.58
.59 .59 .60 .60
.OOU!)
I620
.U00
(Gelled upon hydrolysis)
646
.A0
0.960
.,
. ..
13.4
.965
..
. .. ... ...
11.4 11.0 10.6
T
.134 .036 .021 .01& .008 ,005 .004 ,002
8.8
35.4 61.0 72.0 149 223 432 387
= 0.025
T.=
,100
. 0-10 ,015 .008U
.0026 .0014
.0000
.57
3.8 14.4 24.7 28.7 55.8 85.2 164 136
.59 .595 .60 .62 .62 .62 .BO
. !I42 .%E
x.0 13,5
...
11.8 12.7 11.1 10.5 11.2 12. -5
... ... .OT3 . I .
...
O.05l)
12.2 ;7.36 .3fi 27.5 11.0 .GO 61.0 24.4 .iiO 108 $3.:3 .(il 335 129 . (i.5 253 96.3 .ti2 (Cklled upon hydrolysis)
24 :1 17.7 15 2 14.3 14. 1
T = 0.109 1.21 0.702 ..12 -.. 4.6 ..)I ,102 10.6 26.9 .OM 11.0 .59 43.0 17.2 .BO .020 51.3 ,015 20.3 .(io 69.3 27.0 .A1 ,010 .oi149 88.9 35.5 .60 44.7 .6t ,002 1 115 .0008 127 49.5 .61 .0000 (Gelled upon hydrolysis) (Gelled before hydrolysis) .010
.54
T
14.6 36.3 37.3 59.7
6.35 13.6 13.4 21.9
,024 1 . 0083 .0068 .006 1
25.0 36.8 41.6 41.1
T = 0.516 .63 9.35 13.4 .64 .6S 14.7 14.7 . (i4
T
11.8 tti 0
33.7
.0:30
.0127 .011 .0098 .0094 .OOF"
. uu1
2.44 6.06 9 00 9 02 12.1 11 8 14 7 17.5 15.4 14.8 15
a
(Gelled)
=
3.61 5.34
5. 39 7.70 7
.360 ,150 .074 ,040 .036
28.5
23.7 22.9 21.6 20.7 19.5
= 0.397 .3i .63 .64
,0625 ,0178 IO083 .OK37
5 ) . 1s
Oi 9
4U. -4 33.3 33.8 31.0
.63
,970 ,976 ,982
,978
O.ii97 .fil .fX . (i.5
,M
= 1.00u
1.20 2.49 3.31 3.73 4.04 4.0
4.97 5.34 5.19 4. n:1 4 . 73
.5l
69.9
.59
.63 .59
.67 .66 .66 .70 .GO ,fi7 ,ti!)
0.958 ,962 .9rio
...
63.0
65.0 54.5
. 972
...
,989 K 4
50.6
9 T4 953
1s.1
LIQUID~ I E T ~ I Y I . P ~ L Y S I LSYSTEMS ~X,\NE
Ifarch, 1 9 I;
ing for :in additional hour. The acid was centrifuged from the equilibrated polymer, which was then dried and neutralized with anhydrous potassium carbonate and filtered. A typical oil with the constants 7 = 0, (Y = 0.0135, was prepared by weighing b y difference on a n analytical balance 0.530 g . (0.00327 mole) of Mz into 35.453 g. cO.ltYii mole) of D, in a two-ounce bottle. The bottle x a s capped durit;.g the final weighing t o prevent evaporation of M1. The two components were mixed by shaking the bottle and then transferred to a four ounce bottle for equilibration. After 1.5 cc. of concentrated sulfuric acid had been added, the bottle was capped quickly and placed i n it shakcr for equilibration.
The compositions and properties of the equilibrated polymers are given in Table I . Several compositions which did not yield liquid polymers :ire included. Some gelled during equilibration, while others, with lower average functionalities, did not gel until the siloxane-sulfate esters were hydrolyzed a t the end of the equilibration period. The kinematic viscosities were determined in modified Ostvmld pipet viscometers a t 100 and 210'F. The viscosity-temperature coefficients3 (VTC) ;WP calculated from the relation \TC = ( h o -
tlad/rl,oo
(4) Dutn i n thi3 re-tion a d d e d April, 1946. in revisian of original {)',per. f.i) ili)
I). 1'. Wilcock. T i m J O I ' R N A L . 68, W l (194Ik). K. 0 Sailer ! h i d , 68, 954 (194fi).
shorter linear compounds, M2D, 2, are sufiicietit to warrant use of this type of analysis. The results obtained on three low viscosity polymers, with 7 equal t o zero, 0.10 and 1.0, are given in Table 11. The calculated values will be discussed later. In analysis IIC it was not possible to resolve the compounds boiling higher than
M,T. TABLE I1 ANALYSIS OF Low VISCOSITY POLYMERS Weight per cent. Found Calcd.
Compd.
A.
01
= 0.670.
M? M?D MZD, MD:,
7
=
:o. (i
1 61
lii 4 10. I
MjD, M:jDz
2.ti
B.
01
= 0.54,
7
?,-I I
6.0 :3.4
= 0.10
24.4 18.1 13.1
M 2
MZD M?Dzand MIT
c.
0
31.4 24.R 19.2 3 .t i
(2)
The per cent. Tolatiles were determined in a ventilated oven a t l30", by exposing 2 g. of the liquid polymer in a IO-cc. beaker, enclosed in a 23-cc. beaker t o minimize error due t o creepage. The major portion of the volatile materials was lost during the first few hours after which.the rate of loss became very small. Results are reported for ;I 7-day period. Determination of Molecular Size Distribut i ~ n s . ~ - T h e molecular size distribution in a i!uinber ,of equilibrated siloxane polymers was tletermiried by careful fractional distillation, for i t was of great interest as a check on the theoretical distributioii. The very low viscosity polymers were analyzed directly by fractional distillation of.a :%OO g. charge through a packed column similar t o that described for the purification of D,. In order to analyze the more viscous polymers, the volatile material was removed from a large quaritity of polymer by stripping. This volatile material was then fractionally distilled t o determine the amounts of the individual compounds. At the normal distillation pressures the boiling points of the cyclic arid linear compounds alternates so that the compounds are recovered in the order M,, MZD, Dd, MZD?, Ds,MzD3, etc. The intermediate fractions between flats contain only two compounds, which have widely different average functionalities, so t h a t determination of the average functionality by the specific refraction method" permits accurate assignment of the intermediate fractions t o the proper compounds. IVhere nc, cyclic compounds are present, the differences in average functionality between the
-1 79
01
= 0.504,
7
=
23.0 1;. 3
12.7 1.00
M% 49.5 3 7 .8 MJT b/AV II;, be the weight fraction of molecules containing x units, i of .which are T units +Ifx.be the weigh1 average molecular weight
.If
11 7'
The number of ways of obtaining a. given distribution is given by
(8)
where wxi is the number of ways of forming an x,
i-mer from .u units, i of which are T units and (i 2 ) are M units. The distribution of greatest probability, obtained by maximizing (8) is
+
=
A x ,Zl, ("2 7r--2;-2 Y' (i 2)1 (x - 2i -2)! i!
+
= 2s--2%--23*(x - 2)!
t=-
?VZ
(10)
B(x
- 2)! (2q)'-2&-2 (3P)z
T =
B6 3(1
(W
li'rt
1=0
where 6, is Kronecker's delta. The weight average molecular wei ql it is m
AI\, =
2=2
r=2
[a(i
+ 2) + b ( x - 2r
!a(?
+ 2 ) + b ( x - 2L - 2) t LL]n,L
- 21
z
Combining (9) and (10) -I->, = (11) ( r + 2)! ( x - 2; - 2q! i! wherc B = [?-A, arid @ = [y. U, and 8 are obtained by substituting (11) in ( 5 ) , ((i)and ('7) and carrying out the summations. The results are
.L--2-6< -
=
(9)
where -4, $, 7 and y are Lagrangian multipliers. Remembering t h a t the extent of reaction is unity, wzzcan be shown t o be equal t o 73,
Tlic weight fraction of niolcculcs c ~ i i t ~ ~ i ~ iYi i i q units is
+
L?]2nzI.
-
I
(271
The values of the six individuJ sums involvctl i n (27) are
CCn,, = 1 2r CCinzI= F ,
'
- 27) ( 1 - 8 7
where 6 is given by the lowest positive real root of the eauatioii
CC(x - 2i .\lgehrnic solution of equations (1 '7) t o ( I 5 ) givcs
Substituting equations (2s) in ( 2 7 )
117)
Substitutioii in (11) gives, in terms of mole fractions
In terms of
CY
ant1 T
The weight fraction is given by
+ 2) + b(x - 2i - 2) + ciln,,
[a(i If;t= -__~ m
(24)
where a, b and c are the molecular weights of the M, D and T units, equal, respectively, to 81.16, i4.19 and 67.09. Performance of the summation :md substitution of CY gives
+ +
(bx tz - c ) i 2 - a ) ( x - 2)/ pi+2p-*i--ri TI:. - --____-___-_______ [b a ( a - c ) ] (i 2)1 ( x - 2i - 2)! i! TL
+
(25)
Values of ,:CI calculated by means of equation (%), may be compared with experimentally determined weight fractions in Tables 11 and 111. The polymers analyzed in Table I1 were of low viscosity and a major portion of each oil is represented in these analyses of the lower boiling constituents, e. g., the compounds MZ t o M2DS represent 93yoof polymer A. The comparison between the theoretical and experimental weight fractions is good for polymers ii and 13. However, in the case of polymer C, an MT polymer, the theoretical and experimental distributions are somewhat different. The compound M,T, could not be isolated, and it is suspected that the balance of this polymer, with an average functionality close to two, contained many cyclic and polycyclic compounds. The presence of cyclic compounds would account for the excess, over the calcuhted amount, of M units present as Mz in polymer C. The trend toward this excess is already noticeable i n polymer B, a t T = 0.10, where the percentage of M Bfound exceeds t h a t calculated by 1.4%. The expected tendency of experimental error is toward finding a smaller amount of MI than calculated, as in polymer A, because of the low boiling point and low heat of vaporization of this compound
LIQVID~Il?TIIYLPOLYSILOX.4NESYSTEMS
1\Inrcli, 1047
-18.:
Scott13 has isolated a compound analyzing as MTg from an equilibrated MT system, indicating that highly condensed compounds are present. Consequently it seems probable that in the low vi,scosity polymers the proportion of cyclic compounds increases from zero at 7 = 0 t o a large value at T = l.OJ and the basic assumption that only coinpounds of the type formula Mi+zD,-zT; are forrned is strictly satisfactory only for MD systems. The a.nalysis of a higher viscosity polymer with a low value of 7,Table 111, reveals wide deviation from the theoretical distribution in that large proportions of the simple cyclic compounds, D,, are present. ’The proportions of the compounds of the series MS13,-2 are larger than calculated, but :ire in correct :proportion, with the exception of the lower boiling compounds MZ and MZD. How the weight fraction of the cyclic compound Dd rapidly increases with the viscosity is shown in Table IV. Thus an increase in viscosity as well as an increase in T has a serious effect upon the form of the molecu’iar size distribution.
Molecular Weight-Viscosity Relations I n view of the observed deviations from the theoretical distribution, it is interesting to see how near the log viscosity vs. dri curve is to a straight line relation and how nearly the viscosity oi a inixture falls on this same curve, for these considerations lent weight t o the deduction that viscosity is 3 function of .lI\vin other polymer systems. The values o f N,,.for a number of M D polymers have been calculated by equation (29) in Table VI. Log viscosity is plotted vs. d3Twin Fig. 2 . The straight line is a good approximation above a weight average molecular weight of 5000. However, as the proportion of M units is increased, and ciecrciases below ,5000, the log viscosity L I S , -v%i. curve must deviate from the straight line to :ipproach the limiting values of viscosity and MTvrepresented by the point M?. The dotted curi‘e was plotted from equations (4) and (291, using --O.OO% for the value of k . This curve, (Ierivc(1 froni the enipiricnl relation ( 4 ) ,
0.1 l
j
moo
17 25, 9 ::o .7 36 il 49.7 ,79,ii ,
1x4
459 lli20
LX50
FOR
Fig. 2.---Log viscosity ~8.y. J2; 0 , coiiipoti~i(~\ MzD,-,; 0 , equilibrated polymers, 7 = 0; f,i i i i ~ t u r e ~ of r = i)’polymers; lo:, equiii~xatec~ poiyniers, 7 = i1,10:{.
closely approximates the straight line above *1lw = 3000 and at the same time deviates from it below Mw = 5000 to pass through the point for M,. Xixtures of equilibrated polymers, while somewhat lower in viscosity than equilibrated polymers having the same weight average molecular weight, are quite well approximated by the straight line relation between log viscosity and d:iz. Points for three such mixtures are plotted in Fig. 2 (crosses). The J/, values were calculated as outlined i n Table VI1 from the relatiori (:HI)
fi
111
I).
07ii
. I) 0.3) good agreement with the statistical theory a t least up t o T = 0.1 is obtained with no measurable trace of cyclic compounds being found. On the basis of a viscosity-ll/,. relationship, one should expect such polymers, with a value of Mw less than about 1000, t o have viscosities very nearly those predicted from the viscosity-Alf5vcurve for the pure compounds, whereas in fact the viscosities are more nearly those predicted from the viscosity vs.
Yol.
c;n
a curve for these same compounds (see Fig. 1). Consequently, it seems doubtful that an exact relationship between viscosity and -21,exists for siloxane polymers. -~ Gelation Within the limit of experimental error, gelation occurs during the equilibration process whenever a < 0, i. e., when nz < t. When a = 0 ( m = t ) , a gel-like material forms during the hydrolysis of the siloxane-sulfate esters a t the end of the equilibration process. These gel-like materials, however, are soluble in solvents and are not true gels. When a < 0, a gel forms of which a t least a portion is insoluble in solvents. 1Vhen a is only slightly less than zero, this gel forms upon hydrolysis of the sulfate esters, but when a is less than about -0.005, it forms during the equilibration process. According to the statistical relatioris developed for these siloxane polymers according to the theories of Flory and Stockmayer, gelation should be noted when the calculated weight average molecular weight is infinite. From equation (29) i t is evident that as a is decreased from the value of 1, this condition first occurs when p - Y = 0, that is, when m = 3t. When p - Y < 0, equation (29) no longer is applicable. In the composition region t < wz I 3t, gelation is predicted by the theory, but is not found experimentally. The explanation is t o be found in the presence of cyclic and polycyclic molecules in the polymers. These cyclic compounds have a high average functionality, two or larger, and hence can leave the balance of the polymer above the theoretical gelation composition, rn > . 3t. Flory14 found a similar difference between the theoretical and experimental incipient gelation compositions, and ascribed the difference to the presence of intramolecular condensation.
Other Properties Like the viscos.ity, other physical properties of the equilibrated siloxane polymers are also related t o the composition. These are the density, the per cent. volatile, the viscosity t.emperature coefficient, and the melting range. The density is a function of the average functionality, as shown in Fig. 5, but appears to be independent of the molecular size distribution and of the proportion of T units. The densities of the pure linear compounds and of the equilibrated polymers fall on the same curve, The per cent. volatile is a function not only of the average functionality but also of the proportion of T units. The variation with both 01 and r is shown in Fig. 4. For any constant value of r , the per cent. volatile varies from nearly 100 a t 01 = 0134 or higher to a constant limiting value as a is decreased. This limiting value is higher the greater the value of T . Since the volatile fraction represents the lower molecular weight compounds (14) P. J. Flory, THISJ O U R N A L , 63, 3088, 3091 (1941)
_\r:Lrch, l!) I;
LIQUID~IETIIYLPOLYSILOXANE SYSTEMS
4 8 .i
Fig 4
in the equilibrated polymers, it should approach zero as a approaches zero if the statistical distribution represented by equation (20) is correct. Thus the limiting values of per cent. volatile are indicative of the deviations from this theoretical distribution caused by the presence of cyclic molecules. The limiting value of the per cent. volatile increases rapidly with increase in r. The viscosity temperature coefficient (VTC) likewise reflects the increase in cyclic content as the proportion of T units is increased. The cyclic compounds hare been observed t o have higher viscosity temperature coefficients,".' as expressed by their energy of activation for viscoils flow, than the corresponding linear compounds. I n the equilibrated polymers, the VTC is higher the larger the value of T , varying from a maximum of O.[iO for polymers with 7 = 0 t o a maximum of O . ( i i for polyiiiers with T = 1. This increase is attributed to an increasing proportion of cyclic molecules in the polymers as 7 is increased. For comparison, the VTC of petroleum oils ranges from 0.8 to 0.9. The melting range of the methylpolysiloxane
polymers is greatly reduced by the introduction of small proportions of T units into the polymers. This is contrary t o what might be expected from the increase in cyclic content with 7,for while the linear compounds, MzD,-*, are reported2 t o have melting points of -70' or lower, the corresponding cyclic compounds have relatively high melting points,? l5 above -3s'. The introduction of T units reduces the melting range from -25 t o -58' for polymers for which r = 0 to something below the temperature of Dry Ice for polymers with T = 0.05. In the TDM polymers the tendency to crystallize appears to be suppressed by the greater molecular complexity caused by the branch units. Removal of the volatile fractions neither prevents the solidification of D M poIymers nor permits the solidification of TDM polymers at temperatures above Dry Ice temperatures. Acknowledgment.-Thanks are due LCiss B. SJllivan of the Schenectady Works Laboratory for the viscosity determinations, and Mr. E. Chace of the same laboratory for the preparation (15) J F Hyde, el al , THISJ O U R N A L , 68, 687 (1946)
-4 sc,
L. B.FALPENRURG, H. 31.TEETER AND J. C. COWAN
of several of the DM oils. The author is also appreciative of the helpfulness and advice of Dr. Jh‘. I. Patnode, Dr. R. 0. Sauer and Dr. D . W. ?cott during the conduct of this investigation. Summary The physical properties of a polymeric system arising from the equilibrium rearrangement of methylpolysiloxanes are a function not only of the average functionality of the system but also of the proportion of trifunctional units it contains. As the average functionality is increased toward the value of two, beyond which gelation occurs, the viscosity, density and viscosity temperature coefficients increase, and the proportion of volatile niaterial decreases. As the proportion of branch units is increased, a t constant average functionality, the viscosity and melting range decrease, the viscosity temperature coefficient and per cent. volatile increase, and the density and incipient gelation composition remain substantially unchanged. The statistical theory of molecular size distribution based on the assumptions that only linear or branched molecules are present and that all reacting groups are equally reactive, has been adapted to the TDM polymers from the theories of
Val. 69
Stockmayer. Distillation analysis of these polymers reveals good agreement with the theory at low viscosities) whereas a t higher viscosities considerable proportions of cyclic compounds are found. The viscosity vs. weight average molecular weight curve, calculated with the aid of statistical theory, serves to estimate the viscosity of mixtures of D M polymers, but does not accommodate the compounds M2D,-2. For this reason, the existence of a true relation between viscosity and weight average molecular weight in polysiloxanes is questioned. Gelation occurs a t average functionalities greater than two, rather than when iiz I3 as predicted by the statistical theory, due to the perturbing effects of the presence of cyclic and polycyclic molecules. The increased proportion of cyclic and polycyclic structures t h a t accompanies the increase in proportion of T units, together with the resultant increase in mdecular complexity and the deviation from the theoretical size distribution, accounts for the changes in the physical properties t h a t are observed as T units are introduced into D M polymers. GENERAL ELECTRIC Co., THOMSON LABORATORY 920 \VESTERN AvE., VEST I,uux, MASS. RECEIVED OCTOBER9. 1945
]COUTRIBUTION FROM THE NORTHERX RE(;IOSAL
RESEARCH LABORATORY1]
C-18 Alcohol Esters of Orthosilicic Acid BY L. B. FALKENBURG,~ H. 11. TEETER AND J. C. COWAN In connection with studies of new types of synthetic drying oils derived from the component fat acids of linseed and soybean oils, the orthosilicic acid esters of the alcohols corresponding to these fat acids were prepared. Since these esters would have a functionality3 of four, it was anticipated that they would possess superior drying properties. Stearyl, oleyl and linoleyl orthosilicates were also prepared. The drying times of thin films of refined linseed and soybean oils with and without driers have been compared with those of the orthosilicates of the corresponding alcohols. The drying times of the oils since they give soft tacky films were determined by the “finger” method which defines “set-to-touch time” as a period of drying after which oil will not adhere to the finger when the film is touched gently; those of the orthosilicates were determined with a Sanderson Drying The driers were added as naphthenates. The results are given in the table. (1) One of t h e laboratories of t h e Bureau of .4gricultural a n d Industrial Chemistry, Agricultural Research Administration. U. S. D e p a r t m e n t of Agriculture. Article n o t copyrighted. ( 2 ) Present address: T h e h l a y t a g C o m p a n y . Newton, Iowa. ( 3 ) Bradley, 1n.l. E n g . Chem.. 29, 440 (1937). (-1) S a n d e n o n , Proc. A m . SOL. Testing . M n t e r i d , 26, P a r t 11, 407 (1925): ibid.. 26, P a r t T I , d56 (1926).
TABLE I DRYING TIMES IS HOURS 7 -
Oils-
*.E% 0.1% hln CO
--Orthosilicates 0.570 Pb 0.05% None Mn
8 16-24
13 96-120
0.570
Drier
None
Linseed Soybean
144
72
6 12-14
2.75 4.5
--.0.1% Co
2 3
The hard dry orthosilicate films developed aftertack rapidly a t a rate roughly proportional to the original rate of drying, finally becoming soft and greasy or pasty. This effect may be caused by hydrolysis with atmospheric moisture possibly through catalysis by lower fatty acids resulting from oxidation. If the films were both dried and. kept in- a desiccator over phosphorus pentoxide, they retained their hardness indefinitely. Prolonged desiccation (six months) was found to inhibit materially the tendency to develop aftcrtack. When baked three and one-half t o five hours a t 3 20°, films of the orthosilicate of soybean alcohols (containing 0.35% lead and 0.0570 manganese) showed no change after thirty days. Films were almost water-white and had a hardness of 10 as measured with a Sward hardness rocker. They
