Determination of Functionality in Methyl Silicone Systems GEOKGE F. KOEIIEL., Genercil Electric Research Lciborcctnry, Srhrnrrtnciy. Oil tiscosity and gel test methods are described for determining small amounts of mono- and trifunctional methj-lsiloxane units in primarily difunctional methylsiloxane systems. Such information is important in the preparation of silicone oils and silicone rubber. The methods are hased on the fact that methylpoljsiloxane sj stems equilibrated with sulfuric acid form gels when the functionalit) is greater than 2, oils when the functionality is less than 2. The tiscosity of the oils prepared is related
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to the mono-, di-, and trifunctional u n i t concentra-
tion. The addition of a known amount of monofunctional material to an oil of unknown di- and trifunctional unit composition permits upon equilibration and viscosity determination definition of the system by reference to a standard viscosity curve. In the gel test method the amount of monofiinctional units required for just preventing gelation i b directly related to the trifunctional unit concentration of the original oil.
5 THE preparation of polymeric systems it, is important that
in varying but k u o \ ~ namounts, the visco'ity o f the oil after Suifuric acid treatment can be plotted as a function of the original
the functionality be carefull>-controlled. This is especially true in tlie preparation of methyl silicont~oil and rubbrr where small amounts of cross-linking and chain ending groups markedly affect the properties'of the final product. I t is therefore crscntial that Incthylchlorosilanes used as starting inaterials in t,he preparation of silicone oil arid rubbcr be of high purity. This riecessitat(+ cawful fractionation, particularly i n the separation of 1nethyltr.ichIorosilaIle and dimethyl(lichlor.o~il~ne which hoil at 6 6 " a n d 70"C., respectively. .ilthough the hydrolyzable chlot,iiie arid tleiisity dettarminations used t J y this laboratory ( 1 , Z ) for general control measuremcnts are satisfactory for most binary nieth~lchlorosilaiiemixtures, they are neither positive nor sufficiently sensitive for determining small amounts of methyltrichlorosilaiie (0 to 0.5%) in dimethyldichlorosilane. The oil viscosity and gel test methods de in this paper permit the most accurate and precise measurement ~ t the ' small amounts of mono- and trifunctional material that suffice to modify t hc propertips of difunctional silicone systems.
trifunctional unit concentration. The t>rifunctionalunit content of a n oil of unknown di- and trifurictional unit composition can then be readily asoertainrd by equilibration with the same quantity of innnofunctional material and rcfcrence to the standard rurve.
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THEORY
\\-hfw the niet hylchlorosilanes arc' hydrolyzed and the result,ing niethylpol?-silosancs, or silicone oils, are shaken with sulfuric acid, catalytic rrarrangenient takes place and equilibrium is est,ablished, providing the functionalit,y of the system is sufficiently low t o allow adequate mobility of the molecules. The general terhnique of equilibrating methylpolysiloxanes with sulfuric acid has been dtwribed by Pat node and Kilcock ( 3 ) . The equilibria involved hare brei1 studird arid discussed by Kilcock ( 7 ) and Scott is). Table I .-:iion-e t h c b polynic~i~ir miits obtained upon hydrolysis of t h e i n c t h ~ l ~ ~ l i l o r o s i lwith a~ic~ whicali this papcr is concerned.
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Polyiiieric Hydrolysis Unit (CHa)aSiOi/?
(CHa)zSiO
CHaSIOli/ L
0.io
HOLE% CH3Si01'/2
Figure 1. Viscosity us. Mole Per Cent of Trifunctional Units
In the gel test method graduated amounts of monofunctional units are added t o a n oil composed of di- and trifunctional units to cover the transition range of oil to gel. The amount of monofunctional units required to just prevent gelation is a function of the trifunctional unit concentlation of the original oil. Both methods can also be used to determine small amounts of monofunctional units in an oil composed of mono- and difunctional units bv the addition of a knoun amount of trifunctional units.
Table I. Polymeric Units Obtained Jlrrh~ lchlorosilane iC Ha) iSiC1 (CH+SiCl: CHaSiCli
o!
Functionality 1
2
3
The oil viscosit) and gel teat method5 aie both based on the fact t h a t methylpolysiloxane systems with a functionality of less than 2 form oils upon treatment Rith sulfuric acid, whereas thosp with a functionality greater than 2 form gels with sulfuric acid. I n the oil viscosity method advantage is taken of the proportionality that exists between the viscosity of an equilibrated silicone oil and its mono-, di-, and trifunctional unit concentration. If a constant number of monofunctional units are added to a sene3 of silicone oils composed solplv of di- and trifunctional units
OIL VlSCOSlTY METHOI)
Standard Curve. The compounds which were used as the source of pure mono-, di-, and trifunctional units are listed in Table 11. The systems composed of known di- and trifunctional unit coilcentration were prepared by dissolving varying amounts of crystalline compounds containing trifunctional units-the inethylpolycyclopolysiloxanes (CH,) ,&3L,O, and (CHJ 1oSi8011of Table 11-in octamethylcyclotetrasiloxane. To these stock solu-
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ANALYTICAL CHEMISTRY
706 Table 11. Source of Pure RIono-, Di-, and Trifunctional Units Siloxane Hexainethyldisiloxane ( 4 ) Octamethylcyclotetrasiloxane (5) A tiiethylpolycyclopolysiloxane (6)a A methylpolycyclopolysiloxane (6) b b
Polymeric Unit Forniula Composition ( C II3)aSiOSi (CHa)a 2(CII3)3SiO1/? [iClHt)?SiO]l
4(CHs)?Si0
( ckI3)10%01
4 (C HdnSiO ; ?C HiSiOi i i z
(CHd loSisOil
2 (CHa)tSiO; GCI13SiOii/ 2
Coinpound B of Scott's Table I. Coinpound E of Scott's Table I.
Table 111. Viscosit) of Oils Composed of Di- and Trifunctional Cnits upon Equilibration with 1.10 Mole 7~of (CHI) SiOl/? Sainple
Mole % CIIaSiO
11,s
Viscosity". Centistokes a t 1 0 0 O
F
ane represents 0.1 inole of (CH,),SiOI;,]. The same precautions in adding the hexamethyldisiloxane described under the oil viscosity method should be observed here. Pipet 0.4 ml. of concentrated sulfuric acid into each bottle. Shake vigorously by hand to disperse the acid, place on a reciprocating mechanical shaker, and equilibrate for at least 3 houm at room tvmperature. Originally the period of shaking was a! least I3 hours but subsequent tests showed that with ordinary hydrolyzed oils 3 hours sufficed. When the sourcc~of trifunctional units is compounds such as the methS-lpol!.cyclopolysilosancs of Table I1 more time is required. Add 10 nil. of 65Vc sulfuric acid and shake vigorously. Place on shaker for about 16 hours. Chvck solubility of solid polymrrs by shaking 1-gram portions \vith 20 nil. of toluene in the presence of an equal volume of water. The water prevents possible depolymerization. Record formation of gel, soluble solid or semisolid polymer, 01' oil. The amount of monofunctional units added t o the sample tvhich is just soluble [the addition of a little less (CH2).8iOl;2gives a highly insoluble g d ] wpresents the mole per cent of (CH:i)sSiOl:2a t incipient gelation. Read the mole per cent of CHsSiOilfrof the original sample from Figure 2.
Ob Ob Ob
Ob
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0.12; 0.30 0.35:
9 0.50 10 0.59d Ostwald-Cannon-Fenske viscometer. b [(CHa)zSiOl4. -. C-Source of trifunctional units, the methylpolycyclopolysiloxane(CHa)laQ
YiaViI. d
Source of trifunctional units, the methylpolycyclopolysiloxane(CHa)lo-
SisOi.
tions containing 0.0 to 0.59 mole % of CH3SiOlI/, were addeq 1.10 mole % of (CH3)3Si01/1in the form of hexamethyldisiloxane. T h e mixtures were equilibrated with concentrated sulfuric acid. T h e series in which the methylpolycyclopolysiloxane (CH,),&5a07 was employed as the source of trifunctional units was shaken for 40 hours. With the methylpolycyclopolysiloxane ( CH3),oSisOll as the source of trifunctional units, equilibration was continued for 70 hours. Upon dilution with water the oils were shaken further for 2 hours, centrifuged, and filtered. The viscosities obtained for these oils containing known amounts of trifunctional units are listed in Table 111. K h e n the mole per cent of trifunctional units is plotted against fluidity a straight-line relationship exists, as is shorn by Figure I. The equation for the straight line is: Mole % CH,SiOll/? = 1 26-309/viscosity (centistokes a t 100" F.)
Procedure. Weigh 30.0 grams of hydrolyzed oil into a 2-ounce (60-ml.) screw-cap bottle. Add 363 mg. of hexamethyldisiloxane [1.10 mole 70 of (CH3)3Si0~/2]to the oil on the balance. Addition of the hexamethyldisiloxane should be rapid, as the oil slowly loses weight when permitted to stand uncapped. It is also desirable to cap the bottle and shake the hexamethyldisiloxane into the oil before making the final weighing, as the disiloxane has a n appreciable vapor pressure at room temperature. Add 1.2 ml. of concentrated sulfuric acid and shake the mixture vigorously by hand to disperse the acid. Place on a reciprocating mechanical shaker and shake at room temperature. The time required for equilibration depends on the type of mixing obtained and should be determined for the equipment to be used. With a common mechanical shaker an equilibration peiiod of 24 hours has been found adequate for hydrolyzed oils. \Vhrn the source of trifunctional units is concentrated in compounds such as the methylpolycyclopolysiloxanes of Table 11, longer periods of equilibration have been found necessary. Gpon equilibration add 3 ml. of water and shake for 2 hours. Centrifuge and filter the oil layer. I n some cases opalescence persists after centrifuging. Shaking with a little anhydrous sodium carbonate before filtering will give clear oils. Determine the viscosity of the oil in centistokes a t 100' F. GEL TEST METHOD
Procedure. Weigh 10.0-gram samples of hydrolyzed oil into 1-ounce screw-cap bottles. Add graduated amounts of hexamethyldisiloxane t o the weighed samples [ 11 mg. of hexamethyldisilox-
MOLE% CH3S1011/2
Figure 2.
Relationship of 3Iono- and Trifunctional Units at Incipient Gelation
Discussion. To obtain an insoluble gel using the sulfuric acid' polymerization t,echnique, a slight, excess of tri- over monofunctional units is required. This is illustrakd in Table IV and in Figure 2 constructed from the data in the table. The amount of CH3SiOlll2 in a silicone system composed of di- and t r i f u n c t i o d units necessary to produce a gel lies between 0.05 and 0.08 mole % of CH,Si011,2. As the trifunctional unit content of the oil increases the tri-/'monofunctional unit ratio at incipient gelation increases. DISCCSSIO\ O F RESULTS
These method> can be uqeti to give high accuracy rn ith well decapable of cxtenqion h w o n d the fined silicone svStcms and
Table IV. Relationship of \lono- and Trifunctional Cnits a t Incipient Gelation Saniple 1
CHiSiOi, 0.04n
2 3 4
0.12b 0.13c
5 6
7
31ole
C;
n. o w
2
>tole "0 (CH,)aSiOi, z ai Incipient Gelation 0 (no aei) 0 (gel) 0.07 0.08 0.18 0.28 0.27
0.37 8 0.48 9 0.48 10 a Source of trifunctional units, (CH3)aSijOe (Compound A of Scott's Table I) ( 5 ) . b Source of trifunctional units, the tnethylpolycyclopo~ysiloxane(CH3)laSieOi of Table 11. C Determined by oil viscosity method on mixture of [(CHa)&iO]a and_'a high CHsSi0i1/z containing oil of di- and trifunctional unit composition.
V O L U M E 20, NO. 8, A U G U S T 1 9 4 8 range discussed in this paper. The precision is such that with care thc mole per cent trifunctional unit values can be determined to 10.01mole yoof CHISiO1 \\’hen both the oil viscosity and gel test methods arc applied ,tern of di- and trifunctional units obtained by nisture of meth?ltrichlorosilane and diniethyltlichlorosilane, the results may vary somen-hat. This indirates the prc’scnce of other than di- and trifunctional units in thc hydroty, for example, will produce a ority rcxsult than in the gel tcst rc~sull. In gcliieral, it is dc~sirableto use the oil viscosity IncLihod n-hcn ihv silicone is to be usrti for making siliconca oil.
