Curable organosiloxane compositions that cure by a platinum group metal catalyzed hydrosilation reaction to yield machinable elastomers exhibiting a high resistance to erosion contain an alkenyl-functional organosiloxane copolymer as the reinforcing agent together with quartz and organic or inorganic microspheres as the non-reinforcing filler. The concentration and density of the microspheres are preferably within specified limits.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to machinable organosiloxane elastomers. More 
particularly, this invention relates to organosiloxane compositions 
yielding erosion-resistant elastomers suitable for use as abradable seals 
between the moving and stationary elements of motors and turbine type 
compressors. 
2. Background Information 
The use of filled organosiloxane resins to form abradable seals in axial 
flow compressors of the type used in turbines for aircraft jet engines is 
taught in British Patent No. 791,568, which issued on Mar. 5, 1958. The 
purpose of the seal is to minimize the clearance between tips of the rotor 
blades and the housing of the compressor. The clearance is formed by 
applying a layer of curable resin on the housing that is slightly thicker 
than the distance between the tips of the rotating blades and the 
compressor housing. During initial operation of the compressor the 
rotating blades undergo a thermally induced expansion and cut a groove in 
the resin layer to a depth that corresponds to the clearance between the 
edges of the blades and the housing. To ensure continued operation of the 
compressor at maximum efficiency, once the initial groove is formed there 
should be no erosion of material from the resin layer other than that 
resulting from contact between this layer and the rotating blades. During 
operation of turbine-type aircraft engines various materials, including 
abrasives such as sand, grit and other types of debris, water, and 
relatively large objects such as birds are drawn into the intake of the 
compressor stages. The physical properties of the cured material used to 
form the abradable seal should be such as to resist erosion resulting from 
the impact of these foreign materials. 
To achieve the physical properties desired for abradable seals various 
types of reinforcing and non-reinforcing fillers have been incorporated 
into the material used to form the seal. Reinforcing fillers are typically 
finely divided forms of precipitated and fumed types of silica. These 
fillers can be used in combination with non-reinforcing fillers such as 
quartz, calcium carbonate, talc and microspheres formed from glass or 
organic resins. 
A shortcoming of prior art organosiloxane resin and elastomer compositions, 
including those described in the prior art as suitable for forming 
abradable seals for turbine type compressors on aircraft, is the tendency 
of some of these compositions, particularly those containing finely 
divided silica as the reinforcing filler, to erode relatively rapidly 
during operation. 
The objective of this invention is to improve the machinability and 
increase the erosion resistance of cured organosiloxane compositions used 
as abradable seals without adversely affecting other desirable properties 
of the cured material. 
SUMMARY OF THE INVENTION 
The objectives of the present invention are achieved by using a specified 
class of organosiloxane copolymers containing alkenyl radicals as the 
reinforcing agent and using the combination of quartz and organic or 
inorganic microspheres as non-reinforcing fillers. To achieve 
machinability without a substantial decrease in erosion resistance of 
cured materials, the concentration of the microspheres should be 
maintained within specified limits. 
DETAILED DESCRIPTION OF THE INVENTION 
This invention provides an improved organosiloxane composition for 
preparing machinable elastomers, said composition comprising 
A. a curable polyorganosiloxane containing at least two alkenyl radicals 
per molecule; 
B. an organohydrogensiloxane containing at least three silicon-bonded 
hydrogen atoms per molecule in an amount sufficient to crosslink said 
polyorganosiloxane; 
C. a curing catalyst selected from the group consisting of metals from the 
platinum group of the periodic table and compounds of said metals in a 
quantity sufficient to promote curing of said composition; and 
D. sufficient quantities of reinforcing and non-reinforcing agents to 
impart machinability and erosion resistance to said composition. 
The improvement comprises the presence as the said reinforcing agent of a 
resinous organosiloxane copolymer comprising R.sup.1.sub.3 SiO.sub.1/2, 
R.sup.1.sub.2 R.sup.2 and SiO.sub.4/2 units; and the presence as the 
non-reinforcing fillers of from 5 to 60 weight percent of finely divided 
quartz, based on the weight of said composition, and at least 20 percent, 
based on the total volume of said composition, of thermally resistant 
microspheres, wherein each R.sup.1 is individually selected from 
monovalent hydrocarbon radicals and R.sup.2 is an alkenyl radical, with 
the proviso that at least 2 weight percent of the hydrocarbon radicals in 
said copolymer are alkenyl radicals. 
Characterizing features of the present compositions are the use of a 
specified type of resinous organosiloxane copolymer as the reinforcing 
agent; and the presence of quartz and a specified concentration range of 
thermally resistant microspheres as non-reinforcing fillers. The material 
from which the microspheres are formed should have sufficient thermal 
stability to resist decomposition and/or softening at temperatures to 
which the cured elastomer is exposed during use, which are typically about 
200.degree. C. 
When used as abradable seals for turbines, the present combination of 
organosiloxane copolymers and fillers imparts the desirable combination of 
a smooth surface in the groove that is cut by the rotating blades of the 
turbine and resistance to erosion of the elastomer. 
The Curable Polyorcanosiloxane (Ingredient A) 
An alkenyl-containing polyorganosiloxane, referred to hereinafter as 
ingredient A, is the major curable ingredient of the present compositions. 
To achieve curing, ingredient A contains at least two silicon-bonded 
alkenyl radicals in each molecule. 
Suitable alkenyl radicals contain from 1 to about 10 carbon atoms and are 
exemplified by, but not limited to, vinyl, allyl and 5-hexenyl. The 
silicon-bonded organic groups other than alkenyl radicals present in 
ingredient A are typically monovalent hydrocarbon and halogenated 
monovalent hydrocarbon radicals exemplified by, but not limited to, alkyl 
radicals such as methyl, ethyl and propyl; aryl radicals such as phenyl; 
and halogenated alkyl radicals such as 3,3,3-trifluoropropyl. 
The molecular structure of ingredient A is not critical to the present 
invention and is determined by the physical properties desired in the 
cured composition. To achieve a useful level of tensile properties in the 
elastomers, the molecular weight of this ingredient should be sufficient 
to achieve a viscosity at 25.degree. C. greater than about 0.1 Pa.s. The 
upper limit for the molecular weight of ingredient A is not specifically 
restricted, and is typically limited only by the processability of the 
curable organosiloxane composition. The present polyorganosiloxanes 
typically exhibit a viscosity of from 100 to 100,000 centipoise (0.1 to 
100 Pa.s). 
Preferred embodiments of ingredient A are polydiorganosiloxanes represented 
by the general formula I 
##STR1## 
wherein each R.sup.3 is individually selected from monovalent hydrocarbon 
radicals, R.sup.4 represents an alkenyl radical, and x represents a degree 
of polymerization equivalent to a viscosity of at least 100 centipoise 
(0.1 Pa.s), preferably from 1 to 100 Pa.s. A viscosity of at least 100 
centipoise is considered necessary to obtain cured elastomers exhibiting 
the desired combination of physical properties. 
As used in the present specification, monovalent hydrocarbon radicals 
include, but are not limited to, alkyl radicals containing from 1 to about 
20 carbon atoms such as methyl, ethyl, n-hexyl and n-dodecyl; alkenyl such 
as vinyl and allyl; cycloalkyl such as cyclohexyl; aryl radicals such as 
phenyl and naphthyl; aralkyl such as benzyl; and alkaryl such as tolyl and 
xylyl. 
The hydrocarbon radicals represented by R.sup.3 are unsubstituted or can 
contain substituents such as halogen atoms that will not adversely affect 
the storage stability and curing of the present compositions or the 
properties of cured articles prepared from these compositions. 
The two R.sup.3 substituents on each of the silicon atoms in formula I can 
be identical or different, and can contain from 1 to about 20 carbon 
atoms. A range of from 1 to 10 carbon atoms is preferred based on the 
availability of the corresponding monomers. Most preferably, at least one 
of the hydrocarbon radicals on each silicon atom is methyl, and any 
remainder are alkenyl radicals such as vinyl and 5-hexenyl, phenyl and/or 
3,3,3-trifluoropropyl, this preference being based on the availability of 
the reactants typically used to prepare the polydiorganosiloxane and the 
properties of cured elastomers prepared from these polydiorganosiloxanes. 
For the same reasons, the alkenyl radicals represented by R.sup.4 are 
preferably vinyl or 5-hexenyl. 
Representative embodiments of ingredient A containing ethylenically 
unsaturated hydrocarbon radicals only at the terminal positions include, 
but are not limited to, dimethylvinyl-siloxy-terminated 
polydimethylsiloxanes, dimethylvinylsiloxy- terminated 
polymethyl-3,3,3-trifluoropropylsiloxanes, 
dimethylvinylsiloxy-terminated-dimethylsiloxane/3,3,3-trifluoropropylmethy 
lsiloxane copolymers and dimethylvinylsiloxy-terminated 
dimethylsiloxane/methylphenylsiloxane copolymers. 
Methods for preparing ingredient A of the present compositions by 
hydrolysis and condensation of the corresponding halosilanes or by 
condensation of the cyclic polydiorganosiloxanes are sufficiently 
disclosed in the patent and other literature that a detailed description 
in this specification is not necessary. 
For applications requiring high levels of physical properties such as 
tensile and/or tear strength it may be desirable to include in the curable 
organosiloxane composition a second polydiorganosiloxane containing 
ethylenically unsaturated hydrocarbon radicals bonded to both terminal and 
non-terminal silicon atoms. 
The Organohydrocensiloxane Curing Agent (Ingredient B) 
The preferred curable organosiloxane compositions of this invention contain 
at least one organohydrogensiloxane that functions as a crosslinking agent 
for ingredient A. In the presence of the hydrosilation catalyst, referred 
to as ingredient C, the silicon-bonded hydrogen atoms in ingredient B 
undergo an addition reaction, known in the art as hydrosilation, with the 
silicon-bonded alkenyl groups in ingredient A, resulting in crosslinking 
and curing of the composition. 
Ingredient B must contain at least 2 silicon-bonded hydrogen atoms in each 
molecule. If ingredient A contains only two alkenyl radicals per molecule, 
ingredient B must contain an average of more than two silicon-bonded 
hydrogen atoms to achieve a crosslinked structure in the final cured 
product. 
The silicon-bonded organic groups present in ingredient B are selected from 
the same group of monovalent hydrocarbon and halogenated hydrocarbon 
radicals as the organic groups of ingredient A. The organic groups in 
ingredient B are preferably substantially free of ethylenic or acetylenic 
unsaturation. The molecular structure of ingredient B can be straight 
chain, branch-containing straight chain, cyclic, network or a combination 
of these. 
While the molecular weight of ingredient B is not specifically restricted, 
viscosities in the range of 3 to 10,000 centipoise (0.003 to 10 Pa.s) at 
25 degrees Centigrade are preferred. 
The concentration of ingredient B is sufficient to provide a molar ratio of 
silicon-bonded hydrogen atoms to alkenyl radicals in the curable 
composition of from 0.5 to 20. A range of from 0.5 to 2 is preferred. 
When the curable composition contains less than 0.5 moles of silicon-bonded 
hydrogen atoms per mole of alkenyl radicals it may not be possible to 
achieve the desired physical properties following curing. The physical 
properties of the cured article may vary with time when this ratio exceeds 
about 20 moles of silicon-bonded hydrogen per mole of alkenyl radicals. 
The Platinum-Containing Hvdrosilation Reaction Catalyst (Ingredient C) 
Curing of the present compositions is catalyzed by a hydrosilation catalyst 
that is a metal from the platinum group of the periodic table or a 
compound of such a metal. These metals include platinum, palladium and 
rhodium. Platinum and platinum compounds are preferred based on the high 
activity level of these catalysts in hydrosilation reactions. 
Examples of preferred curing catalysts include but are not limited to 
platinum black, platinum metal on various solid supports, chloroplatinic 
acid, alcohol solutions of chloroplatinic acid, and complexes of 
chloroplatinic acid with liquid ethylenically unsaturated compounds such 
as olefins and organosiloxanes containing ethylenically unsaturated 
hydrocarbon radicals bonded to silicon. Complexes of chloroplatinic acid 
with the aforementioned organosiloxanes containing ethylenically 
unsaturated hydrocarbon radicals are described in U.S. Pat. No. 3,419,593, 
which issued to David N. Willing on Dec. 31, 1968. The relevant portions 
of this patent are incorporated herein by reference as a teaching of 
preferred catalysts. 
The concentration of ingredient C in the present compositions is equivalent 
to a metal concentration of from 0.1 to 500 parts by weight of metal, 
preferably from 1 to 50 parts by weight of metal, per million parts (ppm), 
based on the combined weight of ingredients A and B. 
Curing does not proceed satisfactorily at below 0.1 ppm of metal, while 
using more than 500 ppm results in no appreciable increase in cure rate, 
and is therefore uneconomical. 
Platinum Catalvst Inhibitor 
Mixtures of the aforementioned ingredients A, B and C may begin to cure at 
ambient temperature. To obtain a longer working time or "pot life" for a 
two-part composition or a longer shelf life for a one-part composition, 
the activity of the catalyst under ambient conditions can be retarded or 
suppressed by addition of a suitable inhibitor. 
Known catalyst inhibitors include the acetylenic compounds disclosed in 
U.S. Pat. No. 3,445,420, which issued on May 20, 1969 to Kookootsedes et 
al. Acetylenic alcohols such as 2-methyl-3-butyn-2-ol constitute a 
preferred class of inhibitors that will suppress the activity of a 
metal-containing catalyst at 25.degree. C. Compositions containing these 
catalyst inhibitors typically require heating at temperatures of 
70.degree. C. or above to cure at a practical rate. 
When it is desired to increase the pot life of a curable composition under 
ambient conditions, this can be accomplished using an alkenyl substituted 
siloxane of the type described in U.S. Pat. No. 3,989,667, which issued on 
Nov. 2, 1976 to Lee and Marko. Cyclic methylvinylsiloxanes are preferred. 
Inhibitor concentrations as low as one mole of inhibitor per mole of metal 
will in some instances impart satisfactory storage stability and cure 
rate. In other instances inhibitor concentrations of up to 500 or more 
moles of inhibitor per mole of metal are required. The optimum 
concentration for a given inhibitor in a given composition can readily be 
determined by routine experimentation and does not constitute part of this 
invention. 
When it is desired to prepare compositions that exhibit substantially 
infinite stability under ambient conditions as a one-part composition, but 
will cure rapidly at elevated temperatures, microencapsulated particles of 
a finely divided catalyst in a thermoplastic polymer that is insoluble in 
the curable organosiloxane composition and melts at the desired curing 
temperature of the composition can be used. The encapsulated catalysts are 
preferably used in combination with an acetylenic compound as a catalyst 
inhibitor. 
The Reinforcing Agent 
Resinous organosiloxane copolymers suitable for use as the reinforcing 
agent in the present compositions are solid materials at 25.degree. C. and 
comprise repeating units of the general formulae, R.sup.1.sub.3 
SiO.sub.1/2, R.sup.1.sub.2 R.sup.2 SiO.sub.1/2 and SiO.sub.4/2. Each of 
the R.sup.1 substituents on the two units containing these substituents 
represent identical or different monovalent hydrocarbon radicals and 
R.sup.2 is an alkenyl radical. The term "monovalent hydrocarbon radicals" 
encompasses alkyl, halogenated alkyl, cycloalkyl, aryl, alkaryl, and 
aralkyl radicals. The hydrocarbon radicals represented by R.sup.1 are 
preferably alkyl, most preferably methyl, and the alkenyl radicals are 
vinyl or 5-hexenyl. The molar ratio of the combination of triorganosiloxy 
units to SiO.sub.4/2 units in the resinous copolymer is from 0.7 to 1.2, 
inclusive. The units containing alkenyl radicals constitute at least 2 and 
preferably from 2 to 8 percent of the total weight of the copolymer, which 
preferably contains at least two alkenyl radicals per molecule. 
In preferred embodiments of the copolymer the range for the molar ratio of 
R.sup.1.sub.2 R.sup.2 SiO.sub.1/2 :R.sup.1.sub.3 SiO.sub.1/2 :SiO.sub.4/2 
units is 0.08-0.1:0.06-1:1. 
The resinous copolymers used as reinforcing agents can be prepared as 
described in U.S. Pat. No. 2,676,182, which issued to Daudt and Tyler on 
Apr. 20, 1954 and is hereby incorporated in this specification by 
reference thereto to teach the preparation of and the scope of these 
resins. The copolymers described in this patent contain from 2 to 23 
percent by weight of hydroxyl groups, which is considerably above the 
maximum level of about 0.8 weight percent preferred for precursors of the 
present copolymers. The hydroxyl content of the precursor can be 
conveniently reduced to the desired level by employing a higher 
concentration of triorganosiloxy units than the concentration range taught 
by Daudt et al. 
The concentration range for the resinous copolymer that will provide the 
desired degree of reinforcement without increasing the viscosity of the 
present compositions to the extent that they are difficult to process will 
be determined by the molecular weight and functionality of the resin. The 
concentration range for preferred copolymers is from 5 to 60 weight 
percent, preferably from 10 to 20 weight percent, based on the weight of 
the curable organosiloxane composition. 
The Quartz Filler 
The combination of resinous reinforcing agent and quartz improves the 
machinability of the cured elastomer and is responsible for the resistance 
to erosion exhibited by the present compositions following curing relative 
to compositions prepared using the same curable ingredients but with 
silica as the reinforcing filler. The present curable compositions contain 
from 5 to about 60 weight percent, preferably from 15 to 30 weight percent 
of quartz. The particle size of the quartz is preferably from 0.5 to about 
50 microns, most preferably about 5 microns. 
The Thermally Stable MicrosPheres 
The presence of at least 20 volume percent of thermally resistant 
microspheres in the curable organosiloxane composition is in part 
responsible for the smooth surface that remains when a portion of a cured 
elastomeric coating formed from the composition has been sheared away or 
otherwise abraded by the rotating blade of a turbine or other cutting 
device. 
It should be apparent that the smoothest possible surface at the interface 
between the edge of a compressor blade and the inner surface of the 
compressor wall is required to minimize leakage and the associated loss in 
efficiency. 
In addition to this beneficial contribution of the microspheres, the 
present inventors discovered that the erosion resistance of the cured 
material decreases as the concentration and density of the microspheres 
are increased. To obtain the optimum balance between the positive and 
negative effects of the microspheres, these additives should constitute 
from 30 to about 45 volume percent of the curable composition and the 
density of the microspheres should preferably be from 0.2 to about 0.6 
g/cc. Glass microspheres within this density range are typically hollow. 
At densities below about 0.2 g/cc, using glass microspheres the diameter 
of the microspheres becomes sufficiently large or the walls sufficiently 
thin such that the microspheres are easily broken during processing of 
preferred curable compositions. It should be apparent that the shear 
required to mix a curable composition, which is responsible for breakage 
of the microspheres, decreases as the viscosity of the composition 
decreases. The lower operable limit for the density of the microspheres 
will be therefore determined by the viscosity of the curable composition. 
The microspheres can be formed from any material that will not soften or 
degrade at temperatures that the cured elastomer is exposed to during use. 
Because a preferred intended end use of cured coatings prepared from the 
present compositions is as abradable seals in the intake stages of jet 
engines where the temperature can reach 200.degree. C. or higher, glass is 
the preferred material. If the intended end use of cured composition does 
not require this level of heat resistance, the microspheres can be formed 
from thermosetting organic resins such as epoxide and phenolic resins. 
Preparation of Curable Compositions 
The compositions of this invention can be prepared by combining all of 
ingredients at ambient temperature. Any of the mixing techniques and 
devices described in the prior art can be used for this purpose. The 
particular device used will be determined by the viscosity of the 
ingredients and the final curable composition. Suitable mixers include but 
are not limited to paddle type mixers, kneader type mixers and two- and 
three-roll rubber mills. Cooling of the ingredients during mixing may be 
desirable to avoid premature curing of the composition. 
To maximize storage stability the curable compositions are preferably kept 
in closed containers until used. If greater storage stability is desired, 
the compositions can be packaged in two or more containers with the 
organohydrogensiloxane (ingredient B) and the metal group metal catalyst 
in different containers. 
Two-part compositions cure over a period of several hours under ambient 
conditions, whereas one-part compositions require days to years to cure 
under these conditions. As is true for other compositions that cure by a 
platinum-catalyzed hydrosilation reaction, curing can be accelerated by 
heating. Curing temperatures of about 150.degree. C. are preferred. 
Elastomers prepared using the present curable compositions are 
particularly useful as abradable seals in gas turbine engines.

EXAMPLES 
The following examples describe preferred embodiments of the abradable 
organosiloxane compositions of the present invention, and should not be 
interpreted as limiting the scope of the present invention as defined in 
the accompanying claims. Unless otherwise specified all parts and 
percentages in the examples are by weight and viscosities are measured at 
25.degree. C. 
Compositions of the present invention referred to hereinafter as I and II 
were prepared using the following procedure: 
A curable composition prepared by mixing 126 parts of a mixture consisting 
essentially of 82 weight percent of a dimethylvinylsiloxy-terminated 
polydimethylsiloxane having a viscosity of about 55 Pa.s at 25.degree. C. 
as ingredient A and 18 weight percent of a resinous benzene-soluble 
copolymer containing triorganosiloxy units and SiO.sub.2 units in the mol 
ratio of about 0.7 mol of triorganosiloxy unit per mol of SiO.sub.2 units 
as the resinous reinforcing agent. The triorganosiloxy units were 
trimethylsiloxy and dimethylvinylsiloxy, and the copolymer contained from 
1.4 to 2.2 weight percent of silicon-bonded vinyl radicals; 38 parts of 
quartz exhibiting an average particle size of 5 microns; 
0.13 part of a reaction product of hexachloroplatinic acid and 
sym-tetramethyldivinyldisiloxane that has been diluted with a liquid 
dimethylvinylsiloxy terminated polydimethylsiloxane in an amount 
sufficient to achieve a platinum content of 0.7 weight percent; 
30 parts of a trimethylsiloxy-terminated polydiorganosiloxane containing an 
average of five methylhydrogensiloxane units and three dimethylsiloxane 
units per molecule with a silicon-bonded hydrogen atom content in the 
range from about 0.7 to 0.8 weight percent; and 
1 part of octamethylcyclosiloxane as a platinum catalyst inhibitor. 
To the ingredients of the curable composition were added hollow glass 
microspheres with an average diameter of 55 microns and a density of 0.25 
g/cc. and a pigment composition containing 13 weight percent of zinc 
oxide, 7 weight percent of carbon black and 79 weight percent of a 
dimethylvinylsiloxy-terminated polydimethylsiloxane having a viscosity of 
about 0.4 Pa.s at 25.degree. C. The microspheres are available as 
Eccosphere SI from Emerson and Cuming. 
Composition I contained 14.3 parts, equivalent to 44.4 volume percent, of 
the glass microspheres and 6.9 parts of the pigment composition. 
Composition II contained 9.8 parts, equivalent to 33.3 volume percent, of 
the glass microspheres and 7.6 parts of the pigment composition. 
Composition IIIc was prepared for comparative purposes, and contained 4.8 
parts, equivalent to 16.5 volume percent of the microspheres. 
For comparative purposes three curable organosiloxane compositions 
containing silica as the reinforcing filler were evaluated. The 
comparative compositions (IVc, Vc and VIc) were prepared using the 
following procedure: 
A curable organosiloxane composition was prepared by blending 
148 parts of a dimethylvinylsiloxy terminated polydimethylsiloxane having a 
viscosity of about 2.1 Pa.s at 25 degrees C.; 
33 parts of fume silica that had been treated with hexamethyldisilazane; 
0.1 part of a reaction product of hexachloroplatinic acid and 
sym-tetramethyldivinyldisiloxane that has been diluted with a liquid 
dimethylvinylsiloxy terminated polydimethylsiloxane in an amount 
sufficient to achieve a platinum content of about 0.7 weight percent; 
4.5 part of a dimethylvinylsiloxy terminated 
dimethylsiloxane/methylvinylsiloxane copolymer containing an average of 96 
dimethylsiloxane and 2 methylvinylsiloxane units per molecule; 
1.74 parts of water; 
13 parts of trimethylsiloxy-terminated polydiorganosiloxane containing an 
average of five methylhydrogensiloxane units and three dimethylsiloxane 
units per molecule with a silicon-bonded hydrogen atom content in the 
range from about 0.7 to 0.8 weight percent; and 
0.6 part of octamethylcyclotetrasiloxane as a catalyst inhibitor. 
The ingredients of the curable composition were blended with the following 
ingredients: 
composition IVc--7.1 parts of the pigment composition used in composition I 
and 16.7 parts of the same glass microspheres, composition Vc--7.1 part of 
this pigment composition and 16.7 parts of the microspheres; 
composition VIc--7.5 parts of pigment composition and 11.0 parts of the 
microspheres. 
Each of the six curable compositions were applied as 0.25 inch (6.4 
mm)-thick coatings to separate curved sections formed from a steel plate 
that had previously been coated with primer composition containing 5 parts 
of n-propyl orthosilicate, 5 parts of tetrabutyltitanate, 5 parts of 
methylcellosolve orthosilicate and 85 parts of VM&P naphtha. The primer 
had been allowed to dry for 2 hours under ambient conditions prior to 
application of the curable organosiloxane composition to be evaluated. 
The compositions were cured by heating the coated substrates for 2.5 hours 
at 150.degree. C. followed by a one-hour post-cure at 200.degree. C. 
The erosion rates of the coated substrates were determined by exposing them 
to a stream of 50-70 mesh Ottawa sand. The velocity of the stream was 953 
ft. (286 meters) per second and the angle of impingement with the 
substrate was 20.degree.. The erosion rate was calculated as a function of 
the volume of cured composition removed per kilogram of sand. 
The smoothness of the surfaces of the cured organosiloxane compositions 
following abrasion was determined by contacting the cured coatings with 
the edges of titanium blades rotating at a speed of 700 feet (210 meters) 
per second 
The amounts of curable composition and pigment composition and volume 
percent of microspheres present in the five compositions evaluated 
together with the erosion rate and appearance of the abraded surface of 
each of the coatings are recorded in the following table. 
TABLE 1 
______________________________________ 
Composition 
(Parts) I II IIIc IVc Vc VIc 
______________________________________ 
Curable 88.9 92.6 97.6 -- -- -- 
Composition.sup.1 
Curable -- -- -- 86.2 86.2 91.5 
Composition.sup.2 
Pigment 6.9 7.6 8.2 7.1 7.1 7.5 
Composition 
Microspheres 
44.4 33.3 16.5 44.4 44.4 33.3 
(Volume %) 
Erosion Rate 
14.4 9.4 Not 15.1 16.5 12.4 
(cc/kg) Tested 
Appearance of 
Not Very Fair Rough Rough Not 
Machined Tested Smooth Tested 
Surface 
______________________________________ 
.sup.1 = Present invention (resin reinforcement) 
.sup.2 = Comparative Example (silica reinforcement) 
The data in the foregoing table demonstrate that the erosion rates for 
comparative samples IVc and Vc containing silica as the reinforcing agent 
and 44.4 volume percent of microspheres were substantially higher than for 
sample I, which contained the same concentration of microspheres with a 
resinous organosiloxane copolymer of the present invention as the 
reinforcing agent. The same is true for the relative erosion rates of 
samples II and VIc. The reduction in smoothness of the machined surface on 
Sample IIIc demonstrates that the concentration of microspheres was 
insufficient to provide the desired machinability of the composition.