Surface Treatment of Hydrated Silica
Pigments for Reinforcement of Stocks R. S. STEARNS
AND
B. L. JOHNSON
The Firestone Tire & Rubber Co., Akron, Ohio present, and + is a constant representative of the reinforcing ability of the filler, + may be considered as a measure of the effect of the filler on the configurational entropy of the polymer chain during deformation as a result of the immobilization of of the polymer at the polymer-pigment interface. It has been shown (IO)t h a t
MA‘
TY finely divided solids have been added to elastomers t o effect reinforcement of the cured stocks. It is generally agreed t h a t the more finely divided the solid and the greater the degree of physical adhesion (wetting) between the surface of the filler and the polymer chains, the greater will be the general over-all reinforcing effect (8). However, recent work on the mechanism of reinforcement has indicated that an important, if not limiting, property of the filler, in a t least modulus reinforcement, is the immobilization of polymer segments on the solid surface (IO). The immobilization is effected by the formation of strong chemisorptive bonds, if not covalent bonds, between the polymer and the filler. The application of this concept to carbon blacks has been discussed in detail by the authors, and similar conclusions have been reached by Blanchard and Parkinson ( 4 ) and by Barton, Smallwood, and Ganzhorn (3). Up t o the present time carbon blacks are unique as reinforcing agents in Hevea and GR-S type polymers. The numerous inorganic fillers ( I , b, 9 ) which have been studied are generally inferior t o the carbon blacks in one or more respects. The rubber chemist and technologist is chiefly interested in such properties as modulus, tensile strength, abrasion resistance, and resilience of the compounded stock. Tensile strength and resilience as commonly measured are primarily a function of the extent of surface, the particle size distribution, and the degree of dispersion of the added pigment or filler. The chemical nature of the solid surface appears t o be of secondary importance. Abrasion resistance is a difficult quantity to measure, especially in the laboratory. Therefore, in studying the mechanism of reinforcement and the reinforcing effect of different types of fillers, measurements have been confined t o a n evaluation of the so-called “equilibrium rnodulu~,”from which an apparent work of retraction may be calculated. It has been found t h a t the work of retraction is highly sensitive to changes in the chemical nature of the solid surface and the results may be interpreted in terms of the kinetic theory of elasticity. I n principle, the work of retraction is measured in the following manner (IO). The test specimen is first conditioned at the highest temperature and the largest deformation to which the sample is t o be subjected. The test specimen is then retracted in small increments, allowing about 45 minutes for stress recovery after each 10% reduction in elongation. The area under the resulting “equilibrium” stress-strain curve is then measured with a planimeter t o give the work of retraction. The limits of integration were kept between 0 and 125y0elongation throughout this work and the maximum initial elongation was limited t o 150% of the unstrained length. The temperature was kept constant a t 30’ C. Under the experimental conditions outlined above, the work of retraction as a function of the filler loading may be described by the equation
+ = b(g)1/2
(2)
where b is a constant, and g is the surface concentration of active sites capable of, and available for, the immobilization of polymer segments. More generally, g = Zrini, where rt is the degree of d
reactivity of a site and ni is the number of such sites. In applying Equations 1 and 2 to a series of data it is essential t h a t all the curing ingredients and conditions be standardized so t h a t W i will be constant. For GR-S stocks containing carbon blacks, it has been shown t h a t the value of @ is positive and t h a t the furnace blacks have a higher value of 4 than channel blacks of equivalent particle size. The nonreinforcing pigments, such as Graphon and hyequal t o zero. drated silicas, were found to have a value of According t o Equation 2, this means t h a t for these fillers there are no sites on the surface for immobilization of polymer when stress is applied at the polymer filler interface as the result of the gross deformation of the sample. The inorganic fillers differ from carbon blacks in two respects. First, the essentially spherical carbon black particle is composed of a n aggregate of parallel layer groups having a disordered graphitelike laminar structure. Secondly, the carbon blacks contain considerable hydrogen and oxygen, the carbon-hydrogen and hydrogen-oxygen ratios being a distinguishing feature between different types of carbon blacks. It has been suggested that the carbon black particle may be considered as made u p of polynuclear benzenoid hydrocarbons of high molecular weight, The surface of such a particle is capable of reacting with oxygen during milling and sulfur during cure to bond the polymer to the black surface through carbon-sulfur-carbon bonds, carbonoxygen-carbon bonds, or carbon-carbon bonds resulting from free radical type reactions (S, 6). If i t is assumed t h a t immobilization of polymer segments a t the pigment or filler interface is a necessary condition for successful reinforcement, then for stocks containing inorganic fillers the value of +, or the inherent reinforcing ability of the pigment, should be enhanced if the surface of the particle could be treated so as to provide sites whereby the polymer molecule could be immobilized. This immobilization, as a result of the effective increase in the number of cross-linked units in the polymer pigment matrix, would appear as an increase in the modulus of the cured stock. The experimental work t h a t is described below is an attempt t o apply the concepts already outlined t o hydrated silica-type pigments. This work was completed in October 1950, before attention was called t o the work of Stelling (11 ). The conclusions reached b y Stelling and covered in his patent appear t o be identical with the conclusions reached by the present authors.
+
where W Ris the work of retraction of the losded stock between the indicated limits, W i is the work of retraction of the gum stock between the same limits, v2 is the volume fraction of filler 961
962
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 -Si-0-gi-0o/i I
0
qSi-OH H
p
H*O
-si-o-S,i-O-S /O\
p
0
--+
#HeO?
I
l
?
l
?
?
Oh
OIH
-Si-O-Si-0-Si-OH
-Si - 0 -Si
/O\ ?I - 0 - S i -0 -S,i -R
Table I. Compounding Recipe
i-OH
9
9
9
0
0
0 0
-s/-o~,i-o-sli,
-di-o-Si-o-~i-~
-Si-0 -Si-0-Si-OH
I
Vol. 48, No. 5
-& 0-4l -0 -s'il / I 9 9 9 -Si-0-Si-0-Si-OH oh dH
-S,I
- 0 -$I,o,-0-SI
OF'-R
- 0 -S
I
ditives were used, they were adsorbed onto the pigment from a slurry, so that in the cured stocks the pigment-glycol or -amine ratio was a constant as the volume loading of filler increased. The compounding recipe employed is given in Table I. Bccause GR-S I000 does not undergo crystallization on deformation and GR-S has a very flat cure curve, it was chosen as the polymer in which the various fillers n-ere dispersed. In studies of the type undertaken, it is important that the basic compounding recipe not be altered, so that W7;may be considered constant. 0
r
1
CI-Si-CI R
OH
GI
RD-TOR' R
.
"=
0.25
0
Figure 1. Reaction of silica with silanes to produce a reinforcing-type pigment
-I
u 4 n
N
-,00.20
The hydrated silica pigments, as well as the aluminas and clays, contain on their surface a large number of reactive hydroxyl groups in the form of bound water. The number of these rea.ctive hydroxyl groups may be varied by heat treatment of the pigment ( 6 ) . The chlorosilanes provide a number of compounds which will react with hydroxyl groups. Therefore, by varying both the bound water content of a pigment such as the finely divided hydrated silicas and the organic radical in the trichlorosilanes, the chemical properties of the surface may be varied within wide limits. The general course of the reaction has been drawn in the schematic diagrams of Figure 1. The remaining chlorine atoms may be removed by reaction with water, alcohols, glycols, or amines. EXPERIMENTAL
The finely divided hydrated silica used was prepared by the controlled addition of salt and acid to water glass. The area as determined by low temperature nitrogen adsorption was 150 square meters per gram. The particle size from electron micrographs was 21 mp. The &io of the surface area calculated from the particle diameter to the nitrogen area is considered to give a measure of the surface roughness or that part of the total surface which is unavailable for adsorption of polymer. For the silica used this amounted to 40 t o 50% of the total surface. The usable area was therefore about 75 to 90 square meters per gram, which compares favorably with the available areas of HAF and E P C carbon blacks. The silanes were commercial products, with the exception of two laboratory preparations, the aHy1- and the cyclohexenylethyltrichlorosilanes ( 7 ) . The silanes are most easily applied when added t,o a slurry of the silica in such unreactive solvents as carbon tetrachloride and petroleum ether. However, they may also be applied in the vapor phase or incorporated in the polymer pigment dispersions on a rubber mill or in a Banbury. The unreacted chlorine at'oms wcre removed with methanol. Generally it was found advisable to wash the pigment in dilute aakali in order to destroy completely all adsorbed hydrochloric acid. Pigment was then dried a t 50" C. until it reached a constant weight. It is often desirable to add triethanolamine or dipropylene glycol to the stocks, or preferably to the pigment, in order to obtain good cures (6). I n this work when such ad-
-
3!
0 15 0
0.05
0.10
0.15
0.20
v2 / ( I - V 2 P
Figure 2.
Vork of relraclion as a fuiicLion of the volume loading of silica
The details of the equilibrium modulus measurements have been described ( I O ) . The increase in the work of retraction as filler is added is assumed, on the basis of the kinetic theory, to mean an increase in the number of cross-linked units in the threedimensional polymer-pigment matrix. The constant @ represents the increase in the number of cross-linked units due only to the presence of the filler a t a fixed level of cure. The constmarit +/Wl is to a first approximation independent of the degree of cross linking in the rubber matrix itself-Le., the degree of cure of the unloaded gum stock. The constant is obtained from the as :L slope of the line in plots of the work of retraction [TVR];~~ function of the volume loading of filler, Vp, as illust,rated i n Figure 2.
+
DISCUSSIOR' OF RESULTS
The value of 0 has been considered as indicative of the inherent modulus-reinforcing ability of a finely divided solid incorporated in a cured rubber matrix. On this basis the increased modulus reinforcement obtained by the introduction of silanes onto a silica surface is shown very clearly in Table 11. At low concentration of silanes the presence of triet'hanolamine and dipropylene glycol is necessary in order to promote the cure. Apparently the amines and glycols serve to reduce the adsorption of accelerator by the silica surface ( 6 ) . However, when the concentration of silane is about 10% or greater the silica surface has been sufficiently shielded by hydrocarbon chains t o reduce the accelerator adsorption t'o a point where good cures are obtained. The change in (6 as the number of vinyl groups introduced on the silica is increased is shown in Figure 3 . The dependence
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1956
Table IT. Heat Treatment,
C.
315 120 120 500 500 120 120 None 120 120 120 250
Effect of Silanes o n Modulus Reinforcement of Finely Divided Silica i n GR-S Stocks Silane,
Trichlorosilane None Vinyl Vinyl Vinyl Vinyl Vinyl Vinyl Vinyl Ethyl Cyclohexenyl Cyclohexyl Allyl
Yo
on Silica
Mole Silrqne/G. Silica X 104
..1
1 1 2 5 5 10 10
10 10 15
0:e 2 0.62 0.62 1.24 3.10 3.10 6.20 6 11 4.28 4.60 8.55
Additive, % TriDiethanol- propylene amine glycol
.. .. 1
1
..
.. 5
[+I&*‘
Cal./ Cc. 0.00 0.11 0.18
0.30 0.27 0.00 0.23 0.33 0.10 0.20 0.00
3
3
.. 5 .. ..
.. ..
..
963
Table IV.
Effect of Reactivity of Silane on Reinforcing Ability of Treated Silicas Heat Dipropylene bIfZ6, Treatment, TrichloroMole Silane/ Glycol, Cal./ c. silane Gram Silica % cc. r+/w;12 500
X-CYC~O.~ (.Ahexenylethyl
1 . 2 5 X 10-4
5
0.5
12.0
500 500 500 250 500
Cyclohexyl Ethyl Vinyl Phenyl None Carbon black Carbon black
1 . 2 5 X 10-4 1 . 2 5 x 10-4 1 . 2 5 x 10-4 1 . 2 5 x 10-4 1 . 2 5 X 10-4
5 5 5 5 5
0.39 0.33 0.24 0.24 0.10 0.35 0.60
5.3 4.4 3.1 2.2 0.5 5.5 16.0
EPC HAF
0.36
Table 111. Effect of Heat Treatment on Modulus Reinforcement of Silica in GR-S Stocks Heat Tre$tment.
Dipropylene Glycol,
Triethanolamine,
%
Cal./Cc.
315 315 315 None 500
.. .. 4 ... .
6’ ..
0.00 0.00 0.00
C.
%
6 5
MIP,
0.08 0.10
0
I
2
3
between 4 and the number of vinyl groups per gram of silane (MOLS S I L A N E / g S l L I C A ) ” 2 X IO2 is identical with the dependence found with respect t o # and Figure 3. Effect of amount of vinylsilane on the number of reactive sites on carbon blacks. T h a t silanemodulus reinforcement of a finely divided silica treated silicas follow Equation 2 lends support t o both the “chemical theory” of reinforcement and the validity of Equation 2. The results obtained are not dependent on either the heat The data presented here support the view t h a t reinforcement treatment of the silica previous t o introduction of the silane or does involve immobilization of polymer segments on the surface the presence of either triethanolamine or dipropylene glycol. of the finely divided solids used as reinforcing agents. This This is illustrated by the data in Table 111. immobilization is apparently the result of the cross linking of The effect of the “activity” of silica surface on reinforcement the polymer chains t o the solid surface predominantly through was further investigated by variation of the organic radical in vulcanization-type reactions. Inactive surfaces such as silicas, the silanes attached t o the silica. The activity of five such clays, and aluminas may by various treatments be converted silanes, added to silica and compounded with GR-S 1000, has into active surfaces. This has been accomplished in the case of been compared by the “equilibrium modulus” technique. silioas studied in this paper b y reaction of the silica surface with The molar quantity of silane added is equivalent to 2y0 silanes having reactive hydrogen atoms. The degree of modulus of the vinyltrichlorosilane. The relative activity of the silanes reinforcement can be correlated with the activity of the organic is expresse: by the figures in the last column of Table IV. The silane radical attached t o the silica. The modulus-reinforcing ratio +/WR takes into account the small variation in W i , ability of silicas treated in this manner compares very favorably which had a value of 0.15 rt: 0.03 cal. per cc. for the stocks used. with that of the HAF and EPC carbon blacks, as shown by the As the data follow Equation 2, a t least t o a first approximatioon, last two entries in Table IV. and the molar quantity of silane is constant, then i t is ( @ / W R ) ~ which is a relative measure of the immobilization of polymer ACKNOWLEDGMENT segments on the silica surface, assuming that the silanes will The authors wish t o thank F. W. Stavely and members of the exhibit the same behavior (Equation 2) with respect t o 9 and Firestone research staff for their interest and assistance; esthe number of potential sites for cross-linking reactions as do the pecially L. J. Kitchen for the preparation of the silanes, and carbon blacks. The Firestone Tire & Rubber Co. for permission t o publish The effect of the silanes on the modulus-reinforcing properties this work. of silica is evidently due t o the fact t h a t the silica surface has been provided with sites, whereby the polymer chains may be imLITERATURE CITED mobilized at or near the silica surface predominantly through sulfur bonds. The polymer-pigment dispersion is thereby cross(1) Allen, E. M., Gage, F. W., Rubber Age ( N . Y . ) 65, 297 (1949). (2) Barrett, C. E., Jones, H. C., IKD.ENG.CHEM.41, 1518 (1949). linked into a continuous three-dimensional matrix. The order (3) Barton, B. C., Smallwood, H. M., Ganzhorn, G. E., J . Polumer of reactivity observed in Table IV may be explained if it is Sci. 13,487 (1954). assumed t h a t the carbon atom adjacent to a silicon atom behaves (4) Blanchard, A. F., Parkinson, D., IND. ENG.CHEW.44, 799 (1952). in a manner similar to a carbon atom adjacent to a double bond, (5) Garten, V. A., Nature 173, 997 ( 1 9 5 4 ) ; P r e p r i n t , Third Rubber and t h a t the reaction with sulfur proceeds both through an Technology Conference, London, 1954; Abstract, Rubber A g e a-methylene hydrogen atom, double bonds, and by ordinary 75, 534 (1954). chain transfer. On this basis the p(As-cyclohexenyl) ethyl com(6) Hausch, W. R., India Rubber Wor2d 130, 59 (1954). pound has six a-methylene hydrogen atoms, the cyclohexyl (7) Kitchen, L. J., unpublished work. (8) Rehner, John, Jr., J . Polymer Sci. 7, 519 (1951). compound has one very active hydrogen on a tertiary carbon (9) Schmidt, E., IND.ENG.CHEW43, 679 (1951). adjacent t o the silica, the ethyl has two a-methylene hydrogens, (10) Steams, R. S., Johnson, B. L., I b i d . . 43, 146 (1951). and the vinyl is reactive through the double bond and the phenyl (11) Stelling, O., Swed. Patent 123,381 (Nov. 23, 1948). as a weak chain transfer agent. On this basis the values in the RECEIVED for review September 27, 1954. ACCEPTED December 23, 1955. last column of Table IV are a t least of reasonable relative magniDivision of Rubber Chemistry, 26th Meeting, ACS, New York, N. Y. tude. September 1954.
