Vol, 47, No, 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
486
index of deivasecl oil contain relatively more treated oil of a given viscosity index. The relationships shown in t,liis paper are based on insufficient data to ensure that all residua Trill fit the same pattern. There are not, many data at the extremes of some properties, such as a t very high. sulfur content, and where unusual coinbinations of constit,uents occur. However, the method of estimation appears sufficiently valid to lie useful for preliminary estimates of t h e value of new residua as Bources of lubricating oil. For accurate estimation of yields, preparation of t’reated oils by the adsorption method has been niost satisfactory.
LITERATURE CITED
(I) Furby, S. TV., Anal. Chem., 22, 876
(1950). (2) Hersh, R. E., Fonske, RI. R., Booser, E. It., and Koch, E. F.. J . Inst. Petroleum, 36, 624 (1960). ( 3 ) Hill, J. R., and Coats, H. B., IND.Esc,. CHEM.,20, 631 (1925). (4) Holde, D., “Examination of H y d r o c a r b o n Oils and of Saponifiable Fats and Waxes,” Wiley, S e w York, 1922. ( 5 ) Van Xes, K., and Van Westen, H. I\., “Aspects of the Constitut.ion of Jlineral Oils,” Elsevier Publishing Co.,New York, 1!X1
RECZIYED for review M a y 18, 1954. A C C E P T E D OCtOllCr 2 7 , 1 Y j l . Presented before the Division of Cilell,istl.y a t t,le i2;ith lIeeting of the AVERICANC H E m c A L Sociwrr, Kansas City. 310.. 1931.
henornena in Silieo E. L. WARRICK
ASD
er
P. C. LAUTERBURl
Mellon Znstitute, PittsbiLrgh 23, P a .
T
HE importance of studying filler phenomena in silicone rub-
ber is tn-ofold. Technologically, there is interest in improviiig the very lo\\- strength of the unfilled polymer gum. Academically, there is hope of contributing t o the knowledge of reinforcement mechanism thwugh the study of a saturated polymer whose low interpolymer forces (3, I O ) permit the detwtion of even very weak filler effects. Many fillers, found empirically, do improve the mechanical properties of silicone rubber. The present, studies show t h a t most of the filler phenomena ohseryetl with organic rubbers also apply t o silicoiie ruhlier, dinleth!-lpol~silosane. The prime function of filler, hoir-e~ci~, is riot clear. Thc simple subst,itution of polymer-filler binding for the lon- interpolymer forces is not, likely the real basis for implovcd mechanical properties. Filler phenomena eommon to both organic and silicone rulihers include the iiiflueiice of particle size, t,he chain structure xithin filler aggregates, the cheinicnl nature of the filler surface: and the ability to form hound polymer. These factors determine the polymer-filler I)inding 11-hich is shon-11 to be of the magnitudc of hydrogen boriding. Polymer-filler links as counted by modulus measurements before :ind after prestress are about equal iii number to the croselinlis established by vulcanization. Mole for mole, primary valence vulcanization links are more effective than polymer-filler links i n supporting a trnsile load. This fact rules out polymer-filler I,indiiig the prime source of rein€orw nient and leaves unsettled the chief function of filler. PARTICLE SIZE: OF FILLERS
The effects of particle size on tensile strength a t break (tensile based on ultimate cross section) while paralleling the findings of Catton and Thompson (6)in neoprene (Figure 1) are more erratic, as illustrated in Figure 2 . Tensile strength a t break is related to the usual values I)ased on original cross sect,ioii 1.):~ the emression (yoe1;;ytiori Tensile a t 1)reali = tensile X T h e materials used by Catton and Thompson were a series of carbon blacks a-ith reasonably similar surface composition. The fillers referred to in Figure 2 range from titania, through aluminum oxide, calcium carbonat.e, and a variety of silicas. It is apparent t h a t other factors, such as the chemical nature of the surface, effects of volume loading, and state of cure influence tensile. The unfilled vulcanized gum has a tensile strength at break of about 125 pounds per square inch so t,he “reinforcing” 1
Now in the Armed Forces.
cffect of virtually any filler is clear. The steepness of t,he curve below 20 mp is greater than that for neoprene and demonstrates the greater dependence of the fundamentally weak dimethylpolysilosane gum on filler particle size for its reinforcement. POLYMER MOLECULAR WEIGHT
SThile the action of a filler in improving the properties of silicone r u b h r is the c,hief interest, the molecular weight of the polymer also affects t e n d c st,rength ai: shown in Figure 3, where 16 volume yo of a silica filler of 21 nip particle diamet,er was used. This curve gives every indication of paralleling the behavior found by Flory (6) far But,yl r u h h r . IIonerei,, the dimethylpolysiloxane elastomer again shows its greater sensitivity to factors that influence tensile. Butyl rubber tends to level OR in tensile st,rength about 300,000 molecular n-eight while silicone ruhber may be leveling off around 1,500,000 molecular reight. The apparent dropping off in tensile at, high molecular weights may he derived from inabilit?. to accomplish adequate milling. STRUCTURE OF FILLISR
One of the f:ict,ors usuallg disc et1 i n the reinforcement of rubber is “structure” of filler. ually t,liis term indicates a chain or network st,ructure that bodies the polymer to an unusual degree. An early view that t,hese structures were achieved by flocculation wit,hin the milled rubber seeins to be replaced by the view that structures present before milling are maintained ( g ) . Structure can be introduced into a fine silica filler by ball milling before incorporating i t into a polymer. hpparently welding of small particles occurs to give large porous aggregates. The data given below were obtained from n silicone rubber cont,aining 16 volume % of such a fine silica (Ilurned type) before and after ball milling. Surface A i ea“,
sq >I./C,.
Silica as prepared Ball milled silica 0 By air permeability ( 1 3 ) . b By electron micrograph.
125 101
mp
Tensile at Break, I.L./Sq. Inch
21 150
4500 836
J’article
Sizeb,
SVliile the surface area, by air pernieabilitj-, is not altered since the particles are porous, the gross aggregates are larger after ball milling. This is reflected in the poor tensile properties and indeed the aggregate size is confirmed by the tensile figure if Figure 2 is used as a basis. Clearly the structure of such aggregates does not seem t o be broken by milling and such silica structure is not useful in reinforcement of silicone rubber.
March 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
stretched and unstretched films, using the monochromatic p i n h o l e technique. T h e n a r r o w collimated beam of x-rays (CuK) penetrated a sample at perpendicular incidence and emerged to fall on flat film a t a distance of 5 em. from the sample. I n this manner pictures were taken
45
487
vided silica of 21 mp obtained from burning silicon tetrachloride which was reacted with (CH&SiCl in the vapor phase to partially remove Si-OH structures on the filler. I n every test of this kind the second stress tends to rejoin the initial stress-strain curve slightly above the point of previous prestress. Bonds seem to be broken progressively without recovering on relaxation. Indeed these curves are reproduced even if one allows 24 hours a t room temperature to intervene between the time of the first and second stress. The number of bonds involved may be studied if modulus at low elongations is measured and the result is used to characterize the system's molecular weight between crosslinks, Mc. The reasonableness of this technique is shown by the modulus figures for the first 150% elongation for relative length 01 = l / l o = 2.5:
-
*g 3
z 2 m
2
Stress Elongation, % 50 75 100 125 150
d
2 15
m
50
Initial
Av.
Modulus, Lb./Sq. Inch Second Third
Final
67 62 57.8 61.5 69.6
39.7 39.5 42.3 45.3 63.2
40.7 33.8 33.7 33.7 38.8
27.5 26.8 26.3 26.3 26 9
63.6
41.7
36.1
26 7
0
E
f
Condition Silicone alone Unfilled, unstretched Silica filled Unstretched 300'% elongation 400% elongation 755% elongation 400% elongation (-180' C . )
4.2A
Diffuse
...
Halo Spacing 7 . 8 to 6 . 7 A.
1 6 . 0 to 15 5 .4.
Y)
P
"b Medium ktrong
Very &ong
Medium weak Strong Strong Medium Very strong Medium Medium Spots showing Spots showing Medium Strong spots showing Strong fiber pattern
5 5
w
0
m
F 4 d
0?3
The spacings corresponding t o polymer identity distances first increase in intensity as alignment begins on elongation and then incipient formation of a fiber pattern occurs a t 400% elongation. Freezing in the stretched position shows a highly crystalline fiber pattern. These changes on stretching are shown in Figure 4 for the 300y0 stretched sample and for the well developed equational spots a t 755% elongation. These spots first appear a t 400% elongation indicative of the onset of fiber orientation. Figure 4 also shows the strong crystalline fiber pattern a t liquid air temperatures and a t 400% elongation. The high strength of natural rubber is often attributed t o crystallinity which sets in a t the higher elongations (6, 12, 15). Here in a silica-filled silicon rubber evidence is presented for the formation of a n oriented structure which may explain in part the moderately high tensile strength. An unfilled silicone rubber cannot reach these elongations a t which orientation seems to begin. STRESS ANALYSIS OF LINKING
Evidence for the existence and the nature of forces between polymer and fillers may be obtained by a type of prestress analysis first shown by Mullins (11) and more recently discussed by Blanchard and Parkinson ( I ) . Samples were stressed t o a given elongation a n d immediately relaxed. On a second stressing the values for elongation were based on the initial gage length, and thus the slight set obtained b y the first stress was ignored. The stress-strain curves for such a repeated stressing are shown in Figure 5 . This sample contained 16 volume yo of finely di-
0
0
z
F W
0
0
2 0 0
0
50
150
100 FWITICLE
SIZE rnp
Figure 2. Effect of particle size of fillers on tensile a t break for siloxane rubber as molded 15- to 25-volume loading
The modulus figures were obtained assuming the form of the theoretical stress-strain relation Force = modulus ( a -
1/012)
t o apply. T h e more complete relationship is
F
= p
rcg(a - 1/01~)(1- 2Mc/M) RT
where p is the polymer density; g is an empirical factor and in the siloxane case seems t o be unity; Mc is the molecular weight between cross links; and M is the molecular weight of the polymer before cross-linking. The number of cross links is Links = 1/2Mc and therefore modulus is directly proportional to the number of links. T h e percentage of links broken by any given prestress
INDUSTRIAL AND ENGINEERING CHEMISTRY
488
may be calculated, and the data shown above are plotted in this n a y in Figure 6. The evtrapolation of the links broken by prestress to 100% of links destroyed goes very near the point of actual rupture of this sample.
7r
0
05
I
15
10 NC,ECLLAF:
?YEIGHT
1
20
25
IO6
Figure 3. Effect of polymer molecular weight at break
011
tensile
Blanchard and Parkinson ( 1 ) use an empirical method of fitting the stress-strain curve to obtain the behavior of linkages on prestressing. A number of linkages may be calculated from the modulus data assuming the theoretical expression to apply over the region where modulus is constant. It is necessary to use the density of the filler-polymer mix and to realize that a theoret,ically derived expression applicable sbrictly to unfilled polymers a t low elongations is being used in a rather approximate way. Gee (8) used modulus data to estimate the number of links in unfilled natural rubber. K i t h such limitations in mind the numbers of links, moles of links per cc. can be calculated as shown in Table I. I s the prestress is increased the number of links effective in modulus drops markedly and seenis t,o be approaching an asyniptotic limit. The area under a stress-strain curve is an energy term and if taken to failure is often considered as energ!. required to rupture all the linkages. The differences in area between two successive stress-strain curves then might be considered to represent the energy required to rupture the links broken in that particular prestress. By this means the data of the last column in Table I were obtained. At l o x prestress weak links break, and at higher prestresses stronger linkages break. -411 the values are less than primary valence link strengths and indeed might be
Figure 4.
Vol. 47, No. 3
those of simple hydrogen bonding. These low energy links are assumed to be those between polymer and filler. They arc broken at' low to moderate prestress and undoubtedly reform in some position more or less in equilibrium for the elongation at the prestress. On relaxing, these bonds are not broken in 24 hours a t room temperature-Le., links are not reformed in old positions----and thus a t elongations lew than the prestress do not count toward the modulus. The iiiodulus of a prestressed sample is then determined solely by links which remain unbroken b y the prestress. The data of Table I are plotted in Figure 7 as the solid*line. ca filler and shoT7 ho\7 The additional points are for anothe t,he strength of link is a function of the particular silica. In fact, the upper three points are for a silica as obtained by burning silicon tet'rachloride in hydrogen. The double-circle points on the line (data of Table I ) are for the same silica with a treatment of (CHr)aSiC1from the vapor phase. Here is an illustxation of hoi7- =Si-OH structures on the filler may be responsible for. the hydrogen bonding to polymer, and, if partially reacted with (CH,),SiCl, the silanols are blocked and hydrogen bonding is decreased. The fourth open point below the line is that for the untreated silica-filled sample after 24 hours aging a t 250" C. Clearly the average strength of link is lowered great1.v. This seems t o result from a great increase in the number of weak links. Xs higher and higher prestresses are reached, an approach to a limiting number of links-the primary vulcanization links-maj~ be expected. The data of Table I show rather well the asymptotic approach to a limit. It should be possible to check this number in tn-o ways. First, the modulus data for an unfilled but vulcanized silicone rubber should yield roughly the same number of links as an asymptotic limit in the prestress experiment. Secondly, swelling measurements which also enable calculation of Jrc values ( 7 ) may confirm the values determined by modulus. Thus welling of a filled but unvulcanized sample may yield n number equivalent to the filler-polymer links broken by a high prestress. The asymptotic limit for the data of Table I seems to be about 2.5 x IO-: moles of links per cc. of sample. Modulus data for
Table I.
Strength of Links Strength
Pres tress, Lb ./ Sq . Inch
Changes on elongation
1\Iolrs
Links/Cc. X 105
Ax-. Link. Cal./Mole of Links
March 1955
489
INDUSTRIAL AND ENGINEERING CHEMISTRY A C T U A L BREAK
/
@
I
,, I
15 -
{ -
6C-
z 5cW
Y
x 0 LL
I
w w
Y VI
+-
s
/
10-
m 40-
5
0
3cW
2 2
2e-
5-
w > 44 >
3-
IO-
0
2-
Figure 5 . Effect of prestress on modulus
I
2
3
4
5
6
STRESS
:,I:
7
S
100 P S I
9
2 1 -
10
100
Figure 6. Effect of prestress on number of links broken
an unfilled but vulcanized sample of a similar degree of cure give Mc = 19,200 which yields 2.52 x 10-5 mole links per cc. of sample. The ultimate properties of this unfilled sample are: tensile, 45 pounds per square inch, and elongation, 175%. Swelling measurements were made on a filler-polymer mix different from that used to obtain the data of Table I. Prestress data on the new mix are summarized: MC Initial Prestressed t o 286 lb./sa. inch Links broken Swelling unvulcanized mix (in toluene) usingp = 0 . 4 3 (4)
8,750
20,600
...
15.900
200
300
400
530
effects of surface composition can be demonstrated, and here materials normally considered inert become reactive a t the high temperature (200" t o 250' C.) of service of silicone rubbers. The reactivity of fillers in the range 200" to 250" C. can be shown by the tensile properties after 24 hours' aging a t 250" C., as a function of the particle size of the filler. The data of Figure 8 are for the fillers cited in Figure 2.
Rloles of Links/Co X 100 5.71 2.43 3.28 3.15
Thus swelling does not serve to break these filler-polymer links h u t seems to count them as effectively as does the change in modulus on prestressing to a moderately high load. The failure of swelling t o break polymer links to an active filler such as silica parallels the findings of Zapp and Guth ( 1 6 ) where bonds of carbon black t o Butyl rubber were not broken by swelling. Bonds between less active mineral fillers and Butyl rubber were broken by swelling. The function of polymer-filler links in what is loosely termed "reinforcement" might be questioned. Even assuming that Bnks broken a t low elongations reform in equilibrium positions a t higher elongations to support the load, there is no reason to assume a change in the energy of polymer-filler bondings. The counting of links by modulus and swelling, as outlined above, would seem to yield roughly equal amounts of vulcanization links and polymer-filler links. The strength of the latter seems to be in the range 2 to 10,000 cal. per mole whereas the vulcanization links, as primary valence bonds, would range from 40 to 90,000 cal. per mole. Even if bindings resulting from crystallization are disregarded, the contribution of polymer-filler links to overall tensile properties is not likely to exceed 15 to 20%. Filler may be performing some other more vital service than that of increasing bindings between polymer chains. NATURE OF THE SILICA SURFACE
As noted already the blocking of Si-OH structures on the silica surface alters the strength of polymer-filler binding. Other
O
I -
0
0
0
100
50
0
I50
pi
Figure 8. Effect of particle size of fillers on tensile a t break for siloxane rubber
-
aged 26 hours a t 250 C. 15- to 20-volume loading
The same general influence of particle size is displayed with most of the small particle materials being the most reinforcing. The exceptions are of the type illustrated by a silica GS 199s prepared by D u Pont with an alkoxy coating. The point for this silica in Figure 2 was one of the very high tensile points. On heating 24 hours at 250' C. the coating is destroyed and the point becomes one of the extremely low points on Figure 8. Further evidence for the effect of surface composition is ap-
INDUSTRIAL AND ENGINEERING CHEMISTRY
490
parent in treating a given silica with a very sinall aniount of gaseous ammonia. Tensile properties drop markedly as shown: Tensile a t Break, Lh./Sq. Inch Silica as prepared "$-treated silica (10-min exposure)
4560
Vol. 47, No. 3
hours a t 25" C., the amount of bound polynier as determined by Soxhlet extraction rose above the level of any efiects of heating for 1 hour. Moreover, after 3 days, the bound polymer rises t o 84y0(Soxhlet). Apparently milling is about equivalent to 1-hour heating a t 150" C., but standing even a t room temperature serves to bind more polymer than any such heating.
1010
One of the ways of studying differences in surface coinposition and their effects on polymer binding is through the measurement of "bound" polymer. The technique used in this work is that of Schweitzer and 1,yon ( 1 4 ) . Solutions of polymer are evaporated a t low temperatures in contact with the fillers and, after 1-hour heating periods a t various temperatures, the mix is flooded with an excess of solvent (toluene in this case). After 3 days an aliquot of the solvent is used to determine the amount of polymer removed from the filler. The amount not removed by solvent in this iiianner is termed "bound" pol! me],
I
I
i
J
Figure 10. Bound polymer effects These outstanding effects of silica in binding siloxane polymers suggest that a reaction is taking place instead of a simple physical adsorption. One might postulate that a reaction such as that shown in Figure 11 is occurring. Certainly siloxane rearrangement is quite common a t higher temperatures, and this reaction niay be catalyzed by traces of alkali adsorbed on or reacted with filler silanol groups. R
Figure 9.
R
4
R
Bound polymer effects
Data for a variety of fillers at 50 parts 1)s weight loading are summarized in Figure 9 The single point for polymer indicates that oxidative gel forniation even a t 250" C. is very slight in 1 hour. Titanox is a t a higher loading (100 parts) than all the other fillers hut it is the least effective in forming bound polymer. A carbon black SRF type is low on the scale and indeed is lolyer than Schweitzer's data for a carbon black in GR-S (double circle). Alumina (Alon) is also lo^ on the scale while silicas in general are quite high. Ball milling a silica serves t o reduce the bound polymer level markedly. The coated silica GS 199s has little bound polymer a t lower temperatures, but heating above 125' C. destroys the organic protective coating and the bound polvmer level rises markedly. Schweitzer likens this heating of polymer and filler to the action achieved in milling. The effects of milling an unprotected silica (as burned), the influence of standing a t room temperatures, and the effects of different loadings may be seen in Figure 10. The curves and solid points are for data obtained by 3chweitzer's technique and repeat some data from Figure 9; the open circles are for individual experiments described below. The effect of increased loading becomes important only a t higher temperatures. A11 these data are for burned-type silica of 21 mp particle size. The milled sample containing 25 parts silica by weight was placed in a Soxhlet immediately and extracted with benzene. When such a sample was allowed to stand 7
\
----ea
H 0
I
L /7T3 I
LJ
Figure 11 The first indications that silanol groups on the filler surface played an important role in reinforcing phenomena came from a study of the infrared absorptions of filler mulls in Nujol. The relative hydroxyl content was obtained by reference to an Si-0 band a t 12.4 ,u: Silica Du P o n t Hyperfine Tamms German Aerosil Bantooel C Linde Vycor glass powder Hydrolyzed SiCla
Relative Hydroxyl 0.026 0.037
Tensile a t Break Lb./Sq. Inch 690 148 1980
0.835
Too weak to test
0.123 0 196 0 238
0,985
2750 3060
270
While these preliminary figures ignored particle size it seemed that the filler could be too rich or too poor in hydroxyl groups. A number of techniques were utilized t o determine the effect of surface hydroxyl content, but the most reasonable seemed t o
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1955
be the use of the Karl Fischer reagent. Gilman (9) used the method t o determine hydroxyl content of RsSiOH structures. The hydroxyl content alone was of little value but the hydroxyl content per square meter of surface proved t o be related t o the tensile strength a t break: as HYdroxyl, G/
Silica Du Pont G S 1998 German Aerosil Santocel C
S. Tensile a t Break Lb./Sq. Inch 24 hours droxyl/ Area As at 106 molded 2 5 O O C .
H ~ -
Area,
100 G.
Size, MM
0.52 0.75 0.8G
12.0 21.4 23.5
T.indn
1.Ro
17.4
n u Pont Hyperfine Hydrolyzed Sic1
0 45 3 28
88.2 44.5
274 133 121 161’
1.90 5.64 7.11 8.39
8890 4560 2390 3060
567 3290 1092 1680
34
13.20. 43.80
1585
785 240
75
270
The hydroxyl content as determined by Karl Fischer reagent on a sample desiccated 3 days at 25” C. is a relative number. Other methods of drying were tried, but, after heating t o 150” C., the absolute level of hydroxyl is too low t o be determined accurately by this method. Accordingly, the desiccation at 25” C. leaves water but presumably only in proportion t o its =SiOH content. Effectively, in drying t o a constant vapor pressure of water, silanol groups are being “tagged” b y adsorbed water.
0 PARTICLE
SIZE
HYDROXYL
49 1
sensitivity t o some variables. Particle size of the filler is of greater importance in silicone rubber than in hydrocarhori elastomers. Structure of the filler seems t o prevent dispersion into ultimate particles and so limits filler benefits. Polymer size influences tensile properties t o a greater extent with silicone elastomers. Polymer-filler bindings of energies similar to that of hydrogen bonding are found, but these seem to contribute only to a minor extent t o ultimate tensile strength. The chemical nature of the filler surface is of considerable importance, and the more inert fillers seem most beneficial. The principal function of a filler must be something other than a means of providing temnorary bindings between polymer molecules. ACKNOWLEDGMENT
The x-ray pictures of silicone rubber films were taken by L. E Alexander of the staff of the Department of Chemical Physics, Mellon Institute, Pittsburgh, Pa. LITERATURE CITED
(1) Blanchard, 8.F., and Parkinson, D., IND.ENG.CHEX, 44,799 (1952). (2) Blanchard, A. F., and Parkinson, D., Rubber Chem. and Technol., 23, 615 (1950). (3) Bondi, A., J . Phys. & Colloid Chem., 55, 1355-63 (1951). (4) Boyer, R. F., Dow Chemical Co., unpublished. (5) Catton, N. L., and Thompson, D. S., IND.ENGCHEM.,40, 1523 (1948). (6) Flory, P. J., Chem. Rev., 35, 51 (1944); IND.ENG.CHEM.,38, 417 (1946). (7) Flory, P. J.,‘and Rehner, J., Jr., J . Chem. Phys., 11, 521 (1943). (8) Gee, G., J . Polymer. Sci., 2,451 (1947). (9) Gilman, H., and Miller, L. S.,J . A m . Chem. Soc., 73, 2367 (1951). (10) Hunter, &I. J., Hyde, J. F., and coworkers, Ibid., 68, 667 (1946). (11) Mullins, L., J. Rubber Research, 16, 275 (1947). (12) Park, Chullchai, and Yoshida, Usabura, Rubber Chem. and Technol., 23, 581 (1950). (13) Rose, H. E., J. A p p l . Chem., 2, 511 (1952). (14) Schweitzer, C . W., and Lyon, F., IND.ENG.CHEM.,44, 125 (1952). (15) Villars, D. S., Rubber Chem. and Technol., 24, 1944 (1951). (16) Zapp, R. L., and Guth,,.:!I IND.ENG.CHEM.,43, 430 (1951). RECEIVEDfor review September 2, 1964. ACCEPTED October 28, I%:+. Contribution from the multiple fellowship sustained a t Mellon Institute, Pittsburgh, P a . , b y the Corning Glass Works and Dow Corning Corp.
P A R T I C L E SIZE
3
HYDROXYL
‘g8
Figure 12. Effect of particle size and hydroxyl concentration o n tensile at break for siloxane rubber as molded The relative hydroxyl content does not correlate with S,the tensile a t break, either as molded ( A M ) or after 24 hours’ aging at 250’ C. However, the grams of hydroxyl per square meter of silica surface do correlate with tensile a t break; the highest tensiles are obtained from the driest silicas. The conclusion is presented clearly in Figure 12 where the tensile at break is plotted against particle size (with open circles) and against hydroxyl content per square meter (filled circles). The curve drawn fits the hydroxyl content quite well, but the particle-size data are still quite erratic. The driest silicas, which show the best tensile properties as molded, are the materials that have organic coatings t o block silanol structures on the surface, such as D u Pont’s GS 199s. This observation would suggest that an ideal filler would be completely inert. SUMMARY
The filler phenomena in silicone rubber parallel effects in hydrocarbon elastomers but exhibit a few deviations and a greater
Radiant Heat Transfer from Flames in a Turbojet Combustor-Correction I n the article on “Radiant Heat Transfer from Flames in a Turbojet Combustion” [Topper, Leonard, IND. ENG.CHEW,46, 2551 (1954)] scales were omitted from some of the figures. The following should be added. Abscissa Scale
Figure
Ordinate Scale 3 Wave length, microns 4 Absorption strength, K L 5 Red-brightnzss temperature, R. 6 Wave length, microns
8
___
. .. . . . .
11 Flame temperature,
12
Emissivity
Tq,
70
Radiation transmitted,
%
Reference ( 2 ) in title Tr%nsmittanoe,
7 Wave length, microns
10
Remarks
Relative luminous intensity, J ~ / e ? pB.t.u. , /(hr.)(sq. ft.)(cnl.1 Emissivity a t wai’e length of 2.0 microns
......
R.
Combustor inlet-air pressure, inches H g abs.
Combustor inlet-air pressure, inches Hg ahs.
From left t o right, A , B, C From left to right, A, B From left to right A, B
