CA2070054A1

CA2070054A1 - Silicone rubber composition for coating electric wire or cable

Info

Publication number: CA2070054A1
Application number: CA002070054A
Authority: CA
Inventors: Koji Nakanishi; Takao Matsushita
Original assignee: Individual
Current assignee: DuPont Toray Specialty Materials KK
Priority date: 1991-05-30
Filing date: 1992-05-29
Publication date: 1992-12-01
Also published as: MX9202562A

Abstract

SILICONE RUBBER COMPOSITION FOR COATING
ELECTRIC WIRE OR CABLE

Abstract A silicone rubber composition particularly useful for extrusion into electrical insulation for wire or cable using hot air vulcanization makes use of the combination of alkyl-type organoperoxide and a specific type of platinum containing catalyst in an addition reaction-curing between an alkenyl containing organopolysiloxane gum and an organohydrogenpolysiloxane crosslinking agent. The platinum containing catalyst is a spherical microparticulate catalyst which consists of thermoplastic resin that contains at least 0.01 weight percent, as platinum metal atoms, of a platinum type catalyst, with the provisos that the thermoplastic resin has a softening point of 50 to 200 degrees Centigrade and the catalyst has a particle diameter of 0.01 to 10 micrometers.

Description

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SILIC02iE RUBBER COMPOSITION FOR COATING
ieLECTRIC WIRE OEI~ CABLl~

The present invention relates to a silicone rubber composition for coating elec~ric wire and cable.
Extruiion molding is typically employed for the coating of electric wire or cable with millable silicone rubber compositions. The application in this method of an alkyl-type organoperoxide vulcanizing agent for the silicone rubber composition is burdened by two problems:
molding is impaired by the d~velopment of a vulcanization inhibition thought to be due to atmospheric oxygen, and the slow vulcani2ation rate causes foaming. Acyl-type or~anoperoxides do not suffer from these problems, and this has led to their use. Advantages associated with the use of acyl-type organoperoxides are the easy vulcanization of silicone rubber compositions in hot-air vulcanization ovens, excellent storage stability in the vicinity of room temperature, long use times, and ease oP
handling. However, vulcanization based on acyl-type organoperoxides is associated with a strong odor during vulcanization, and this causes a substantial deterioration in the working environment. Another problem is the corrosion of exposed metals. In the case of such typical acyl type organoperoxides as 2,4-dichlorobenzoyl peroxide and 4-chlorobenzoyl peroxide, the chlorinated benzoic acid-decomposition residue may be dispersed into the atmosphere. In addition, the moldings suffer from blooming as well as the development of surface tack caused by vulcanization inhibition at the surface.
There have also been inves~igations into the coating of electric wire or cable by addition reaction-curing silicone rubber compositions which cure , ' ' ~ ' , ::

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under platinum catalysis. However, this type of addition reaction-curing silicone rubber composition has a poor storage stability in the vicinity of room temperature.
In the specific case of electric wire/cable coatin~, the core wire functions as a heat sink, and it becomes difficult to set or fix the curing conditions and bubbles tend to appear in the cured product. Specifically, molding is made difficult because foaming tends to occur very readily in hot-air vulcanization at above 400 degrees Centigrade. Moreover, the bubbles and the absence of cure in depth due to defective curing cause the molded product to have a reduced thermal resistance, electrical insulation performanceg and voltage resistance.
Japanese Patent Application Laid Open CKokai or VnexaminedJ Number 52-98051 [98,051/77] (U.S. Pat. No.
4,020,014, i~sued Apr. 26, 1977~, discloses a conductive silicone rubber composition which employs both a platinum-type catalyst and an alkyl-type organoperoxide.
This particular curing technology employs carbon black, which has little tendency to cause the foaming which is so frequently a problem with platinum-catalyzed curing.
As a result, one cannot contemplate its application to a system which employs the foaming-prone silica fillers.
U.S. Pat. No. 45293,677, issued Oct. 6, 1981, teaches a composition which obtains shelf life by using microcapsules which contain the organohydrogenpolysiloxane ingredient, thus preventing the curing reaction from taking place until the capsules are ruptured to release it.
Japanese Patent Application Laid Open [Kokai or Unexamined] Number 59-33362 C33,362/84], equivalent to U.S. Pat. No. 4,487,906, issued Dec. 11, 1984 has proposed a platinum-catalyzed addition-reaction-curing silicone rubber composition which ~lso contains ~ ~ 7 ~ ~ 5 L~

organoperoxide. However, this composition is still encumbered by the previously cited problems of strong odor and blooming when 2,4-dichlorobenzoyl peroxide or 4-chlorobenzoyl peroxide is used as the organoperoxide.
Nor does this composition solve the above-mentioned problems of defective curing and foaming which are associated with hot-air vulcanization using alkyl-type organoperoxide.
U.S. Pat. No. 4,766,176, issued Aug. 23, 1~88, teaches a storage stable organosiloxane composition which cures by a platinum-catalyzed hydrosilation reaction in which the platinum catayst is microencapsulated within one or two layers of thermoplastic organic polymers.
U.S. Pat. No. 4,784,879, issued Nov. 15, 1988 teaches a method of preparing microencapsulated compound wherein the compound is a platinum group metal. U.S. Pat. No 5,01S,716, issued May 14, 1991, teaches a microparticle of platinum-containing hydrosilation catalyst and polydiorganosiloxane exhibiting a softening point of rom 50 to 200 C.
None of the above references suggest the unexpected advantage found when a combination of alkyl-type organoperoxide and microparticulate catalyst containing platinum metal and thermoplastic resin is used to catalyze a mixture of organopolysiloxane gum which contains at least 2 silicon-bonded alkenyl groups in each molecule and organohydrogenpolysiloxane.
The combined use of alkyl-type organoperoxide and a specific type of platinum-type catalyst in an addition reaction-curing silicone rubber composition containing reinforcing filler gives a composition which is useful in producing silicone rubber stock for extrusion on wire or cable and curing in hot air. The catalyst is a spherical microparticulate catalyst which , , :

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consists of thermoplastic resin that contains at least 0.01 weight percent, as platinum metal atoms, of a platinum-type catalyst, with the provisos that the thermoplastic resin has a softening point of 50 to 200 degrees Centigrade and the spherical microparticulate catalyst has a particle diameter of 0.01 to 10 micrometers.
The present invention takes as its ob~ect the introduction of a silicone rubber composition for coating electric wire and cable which has the following properties:
prior to curing: long use time;
during curing: no odor evolution, relative insensitivity to variations in curing conditions, no production of bubbles in the molding;
after curing: very-low surface tack, free of blooming, good curing in depth in the cured product, thus affording a silicone rubber covering which has an excellent thermal resistance, electrical insulation performance, and voltage resistance.
This invention relates to a silicone rubber composition for coating electric wire or cable, said composition consisting essentially of (A) 100 weight parts organopolysiloxane gum which contains at least 2 silicon-bonded alkenyl groups in each molecule and which is represented by the following average compositional : :
fo~mula, RaSia(4 a)/2, wherein R is a substituted or unsubstituted monovalent hydrocarbon group and a is a number with a value of 1.8 to 2.3; (B) 10 to 100 weight parts reinforcing filler; (C) 0.1 to 10 weight parts organohydrogenpolysiloxane which contains at least 2 silicon-bonded hydrogen atoms in each molecule; (D3 0.05 ~:
to 10 weight parts alkyl-type organoperoxide; and (E) a :
catalytic quantity of a spherical microparticulate catalyst which consists of thermoplastic resin that :

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contains at least 0.01 weight percent, as platimlm metal atoms, of a platinum-type catalyst, with the provisos that the thermoplastic resin has a sotening point of 50 to ZOO degrees Centigrade and the spherical microparti~ulate catalyst has a particle diameter of 0.01 to 10 micrometers.
In the above described invention, conventional platinum-type catalysts are replaced by a platinum-type catalyst which has been stabilized by isolation in a thermoplastic resin. Because the platinum-type catalyst then does not come into contact with air or reinforcing filler until immediately prior to the initiation of vulcanization, any deterioration in its activity is avoided. This produces a composition which gives a rapid curing reaction, in which secondary reactions are diminished, and in which the possibility of foalning is strongly reduced. The further addition of the alkyl-type organoperoxide serves to provide a certain and reliable development of the curing reaction, and this improves the thermal resistance, electrical insulation performance, and voltage resistance.
To explain the preceding in greater detail, the organopolysiloxane gum comprising the component (A) employed by the present invention is the principal or base component of the composition according to the present invention, and it must contain at least 2 silicon-bonded alkenyl groups in each molecule. The groups R in the preceding formula for this organopolysiloxane gum comprise monovalent hydrocarbon groups as exemplified by alkyl groups such as methyl, ethyl, propyl, butyl, he~yl, and octyl; alkenyl groups such as vinyl, allyl, and hexenyl; aryl groups such as phenyl; and substituted hydrocarbon groups such as 3,3,3-trifluoropropyl. The value of a in the preceding formula is 1.8 to 2.3.

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The molecular structure of this organopolysiloxane gum should be straight chain, but a moderately branched siloxane skeleton is also permissible. Its degree o F polymerization should fall within the range designated for organopolysi~oxane gums in the concerned art. As a general matter, its viscosity at 25 degrees Centigrade should be at least 104 Pa.s (107 centistokes) and its average molecular weight should be at least 25 x 104 and preferably at least 40 x 104.
The reinforcing filler comprising component (B) consists of those reinforcing fillers heretofore used for silicone rubbers. Microparticulate silica is provided as an example. Said microparticulate silica is exemplified by fumed silica, precipitated silica, and so forth.
Among these, ultramicroparticulate silica with a particle si~e not exceeding 50 millimicrons and with a specific surface area of at least 100 m2/g is preferred. Even more preferred is the use of surface-treated silica, that is, microparticulate silica whose surface has been treated with, for e~ample, organosilane, organosilazane, and diorganocyclopolysiloxane.
The organohydrogenpolysiloxane comprising the component (C~ employed by the present in~ention is a crosslinker for the organopolysiloxane comprising component (A). The former must contain at least 2 silicon-bonded hydrogen atoms in each molecule in order for the composition according to the present invention to form a network structure and develp the properties of a silicone rubber. The silicon-bonded organic groups in this case are exemplified as for the R groups in the organopolysiloxane comprising component (A) as above.
The molecule may contain only a single species of organic groups or two or more species may be present in combination. The molecular structure of this :;

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organohydrogenpolysiloxane may be straight chain, branched chain, network, cyclic, or three-dimensional, and homopolymers or copolymers thereof can be used as can mixtures of two or more types of polymers. The viscosity of this organohydrogenpolysiloxane at 25 degrees Centigrade should generally fall within the range of 0.0005 to 50 Pa.s (0.5 to 50,000 centipoise) and preferably falls within the range of 0.001 to 10 Pa.s (1 to 10,000 centipoise). Component (C) should be added at 0.1 to 10 weight parts per 100 weight parts component (A)-The alkyl-type organoperoxide compriising the component (D) employed by the present invention is a curing catalyst of the vulcanization of the silicone rubber composition according to the present invention, and thos~ already known to the art may be employed in the present case.- This alkyl-type organoperoxide is exemplified by dicumyl peroxide, di-tert-butyl peroxide, tert-butyl cumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, and bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane. This component should be added at 0.05 to 10 weight parts per 100 weight parts component (A).
The spherical microparticulate catalyst comprising the component (E) employed by the present invention is the component which characteri2es or distinguishes the present invention.
Component (E) comprises a spherical microparticulate catalyst which is composed of thermoplastic resin containing at least 0.01 weight percent, as platinum metal atoms, of a platinum-type catalyst. The platinum-type catalyst present in this microparticulate catalyst takes the form of a platinum metal or a platinum compound which is catalytically .
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active in the hydrosilylation reaction or a composition whose main component is such a platinum compound. Such platinum-type cataly~ts are exemplified by platinum micropowders, chloroplatinic acid, alcohol-modified chloroplatinic acid, platinum/diketone complexes, chloroplatinic acid/olefin complexes, chloroplatinic acid/alkenylsiloxane complexes, and the preceding supported on a carrier such as alumina, silica, or carbon black. Among these, chloroplatinic acid/alkenylsiloxane complexes are preferred for their high hydrosilylation catalytic activity, and the platinum/alkenylsilo~ane complex disclosed in Japanese Patent Publication Number 42-22924 [22,924/67] (U.S. Pat. No. 3,419,593, issued Dec. 31, 1968) is particularly preferred.
The thermoplastic resin employed by the present invention must have a softening point in the ran~e of 50 to 200 degrees Centigrade. When its softening point is less than 50 degrees Centigrade, the hydrosilylation-curing silicone rubber composition containing the cataly~t will suffer from a substantially reduced storage stability. When its softening point exceeds 200 degrees Centigrade, the temperature which will induce catalytic activity becomes so high that the catalytic function is essentially absent. Based on these consideration~, the thermoplastic resin preferably has a softening point in the range of 70 to 150 degrees Centigrade.
The molecular structure and chemical structure of the thermoplastic resin under consideration are not speci~ically restricted~ but this thermoplastic resin must not be permeable to the platinum-type compound catalyst and must not be soluble in the organopolysiloxane component in the silicone rubber composition.

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Among these thermoplastic resin~, the silicone resins are exemplified by silicone resins with the following average unit formula:
RlaS (4_a)/2 wherein R is the methyl or phenyl group and a is a number with a value of 0.8 to 1.8.
The component (E) employed by the present invention takes the form of a spherical microparticulate catalyst in which a platinum-type catalyst as described hereinbefore is contained in thermoplastic resin as described hereinbefore. The content of the platinum-type catalyst preferably falls within the range of 0.01 to 5 weight percent as platinum metal.
In order to obtain a satisfactory development of the catalytic activity and in order to obtain a stable dispersion upon blending into the silicone rubber composition, component (E) should be spherical and should have an average particle diameter in the ran~e of 0.01 to 10 micrometers.
The spherical microparticulate catalyst under consideration can be prepared, for e~ample, by dissolving both the platinum-type catalyst and thermoplastic resin Ssoftening point of 50 to 200 degrees Centigrade) in a solvent to ~ive a solution and by spraying this solution into a hot gas current in order to evaporate the solvent off while the silicone resin is simultaneously solidified into a microparticulate morphology while in the spray state.
In another preparative method, a solution is prepared from the platinum-type catalyst, thermoplastic resin with softening point of 50 to 200 degrees Centigrade, and solvent miscible with the preceding two components. This solution is then emulsified in an aqueous surfactant solution, and the solvent is dried out ' '`

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of the emulsion to afford a spherical microparticulate catalyst of platinum-type catalyst-containing thermoplastic resin. This product is washed with a solvent which will dissolve the platinum-type catalyst but which will not dissolve the thermoplastic resin.
Methods for producing the sperical microparticulate catalyst are well known in the art and include the methods taught in U.S. Pat. Nos. 4,766,176;

4,784,879; and 5,015,716; which are hereby incorporated by reference to teach methods of making sperical microparticulate-catalyst.
Component (E) should be added in sufficient quantity to induce the curing of the composition according to the present invention. Its addition will generally fall within the range of 0.0000001 to 0.01 weight parts and preferably within the range of 0.000001 to 0.001 weigh~ parts5 in each case as platinum metal atoms per 100 weight parts organopolysiloxane gum comprising component (A).
The present invention's silicone rubber composition for coating electric wire and cable is composed of components (A) through (E) as described above, but it may contain, on an optional basis, a crepe-hardèning inhibitor (e. g., silanol-terminated diorganopolysiloxane, organoalkoxysilane, hexaorganodisilazane) and an addition-reaction inhibitor (e. g., benzotriazole, acetylenic compounds, hydroperoxides) in order to improve the storage stability and curing characteristics (rate) even further. The various ad~itives used heretofore for silicone rubber compositions may be added according to the purpose.
Examples in this regard are nonreinforcing inorganic fillers, pigments, heat stabilizers, and core wire adhesion inhibitors. The nonreinforcing fillers are .

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exemplified by diatomaceous earth, quart~ powder, calcium carbonate, mica, alumin~lm oxide, ~inc oxide, magnesium oxide, and titanium oxide; the pigments are exemplified by carbon black and iron o~ide red; the heat stabilizers are exemplified by rare-earth oxides, rare-earth hydroxides, cerium silanolate, and the fatty acid salts of cerium; and the core wire adhesion inhibitors are exemplified by zinc oxide and the higher fatty acids and their metal salts (e. g., stearic acid, zinc stearate, and calcium stearate).
The electric wire/cable-coating silicone rubber composition according to the present invention can be easily prepared by simply mixing the aforementioned components (A) through (E) to homogeneity. In a particularly preferred mode, components (A) and (B) are first mixed to homogeneity ~ollowed by the admixture of components (C), (D), and (E).
The electric wire/cable-coating silicone rubber composition according to the present invention does not release odor during thermosetting, and, due to its good curing characteristics, does not produce bubbles in the cured product. The curing conditions strongly affect the cure of addition reaction-curing silicone rubber compositions which contain conventional platinum-type catalysts. This results in abnormal curing patterns at temperatures in excess of 400 degrees Centigrade and creates a very strong foaming tendency. On the other hand, the platinum-type catalyst in the electric wire/cable-coating silicone rubber composition according to the present invention is protected by thermoplastic resin. The simultaneous presence of the alkyl-type organoperoxide also suppresses these phenomena. This makes pos~ible molding under a broad range of curing conditions, and abnormalities or defects are not produced .

even during cu~ing at high temperatures. Accordingly, the electric wire/cable-co~ting ~ilico~e rubber composition according to the present invention is very strongly qualified for application as a coating for electric wire and cabl~.
The invention will be explained in greater detail below through reference, illustrative, and comparison examples.
In the examples, parts = weight parts, the viscosity is the value at 25 degrees Centigrade, and cst = cen~istokes. The physical properties of the silicone rubber in the exampl~ refer to physical property values for the silicone rubber coating obtained by extracting the core wire from the silicone rubber-coated cab]e.

Reference Example 1. To prepare a platinum/vinylsiloxane complex composition six grams aqueous chloroplatinic acid solution (platinum content of 33 weight percent) and 16 g 1,3-divinyltetramethyldisiloxane were dissolved in 35 g isopropyl alcohol. Then 10 g sodium bicarbonate was added to this solution, and a reac~ion was run for 30 minutes at 70 to 80 degrees Centigrade while stirring the suspension. The isopropyl alcohol and water were evaporated off at 50 mm Hg and 45 degrees Centigrade.
The solids were filtered off to afford a 1,3-divinyltetramethyldisiloxane solution of a vinylsiloxane-coordinated platinum complex catalyst (platinum content o 9.8 weight percent).

Reference Example 2. To prepare thermoplastic silicone resin hydrolysis wa~ conducted by dripping a solution of 332 g phenyltrichlorosilane, 53 g dimethyldichlorosilane, and 110 g diphenyldichlorosilane diluted with 150 g toluene into a solution o 430 ~ tol~lene, 142 g methyl : ' ~

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ethyl ketone, and 114 g water. The reaction mixture was subsequently washed with water in order to remove the hydrogen chloride, and the organic layer was then separated. The methyl ethyl ketone was removed by heating. Then 0.2 g potassium hydroxide was added, followed by heating and removal of the evolved water, then neutralization with acetic acid and repeated washing with water. Distillation of the solvent afforded a thermoplastic silicone resin. This thermoplastic silicone resin had a glass-transition point of 65 degrees Centigrade and a softening point of 85 degrees Centigrade.

Reference E~ample 3. To prepare a spherical microparticulate catalyst first, 900 ~ of the thermoplastic silicone resin prepared in Reference Example 2, 500 g toluene, and 4,600 g dichloromethane were introduced into a stirrer-equipped glass container and mixed to homogeneity. Then 44.4 g of the platinum/vinylsiloxane complex composition as prepared in Reference Example 1 wa9 added, and a homogeneous solution of the platinum/vinylsiloxane complex and thermoplastic silicone resin was prepared by stirring. This solution was continuously sprayed into a spray drier chamber (Ashizawa-Niro Atomizer Kabushiki Kaisha) using a fluid nozzle and a hot nitrogen current. The temperature of the hot nitrogen current was 95 degrees Centigrade at the spray drier inlet and 4S degrees Centigrade at the spray drier outlet, and the velocity of the hot gas current was 1.3 m3/minute. After operation for 1 hour, 450 g silicone resin microparticulate catalyst containing the Pt/vinylsiloxane complex composition had been collected in a bag filter. The average particle size of this microparticulate catalyst was 1.1 micrometers, and it : ~ , .
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contained 0.5 weight percent microparticulate catalyst with a particle size of 5 micrometers or larger. This microparticulate catalyst was confirmed to have a spherical morphology by scanning electron microscopy.

Reference Example 4. To prepare a P~ catalyst-containing polymethyl methacrylate resin microparticles polymethyl methacrylate (8.0 g, softening point of 110 C, average molecular weight of 931) and Pt/vinylsiloxane complex catalyst (1.0 g) as prepared in Reference Example 1 were dissolved in 165 g methylene chloride. This methylene chloride solution was added with stirring to water containing 7.5 g polyvinyl alcohol (Gosenol GL-05 from Nippon Gosei Kagaku Kogyo Kabushiki Kaisha), and the methylene chloride was subsequently evapo~ated out at 25 to 40 ~C over 40 hours. The solids were separated from the resulting suspension by centrifugal separation. The solids were then washed with water, with a large volume of methyl alcohol, and finally with hexamethyldisiloxane to afford microparticles of Pt catalyst-containing polymethyl methacrylate resin (average particle size of 10 micrometers, platinum content of 0.104 percent).
i Example 1 First 100 parts dimethylvinylsiloxy-terminated organopolysiloxane gum with a degree of polymerization of S,000 (99.6 mole percent dimethylsiloxane units and 0.4 mole percent methylvinylsiloxane units), 8.0 parts silanol-terminated dimethylpolysiloxane with a viscosity of 60 cst, 40 parts dry-method silica with a specific ; surface area of 200 m2/g, and 15 parts diatomaceous earth were introduced into a kneader mixer and mixed to homogeneity while heating. Then 0.5 parts cerium oxide ~:

: ., , , ," ~, , ~5 (heat stabilizer) was added with mixing to homogeneity to give a silicone rubber base. The following were mixed using a two-roll mill to give silicone rubber stocks:
Composition 1, 100 parts of the above rubber base, O.S
parts dimethylhydrogenpoly~iloxane with the following average molecular formula Me3SiO(Me2SiO)3~MeHSiO)5SiMe3, where Me is methyl radical, 0.03 parts spherical microparticulate silicone resin catalyst as prepared in Reference Example 3, 0.002 parts l-ethynyl-l-cyclohexanol (platinum catalyst inhibitor), and 0.25 parts 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane.
Composition 2, for the purposes of comparison, a silicone rubber (Comparison Example 1) was prepared as above, but in the present case omitting the addition of the 0.25 parts 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane used in silicone rubber coMposition 1. Composition ~ (Gomparison Example 2) was also prepared as above, but in the present case adding 0.08 parts of the platinum/vinylsiloxane complex composition (platinum metal content o~ 0.6 weight percent) as prepared in Reference ~xample 1 in place of the spherical microparticulate catalyst used in silicone :~
rubber composition 1. In addition, in the present case 0.005 parts l-ethynyl-l-cyclohexanol was used instead of 0.002 parts.
.
Composition 4 (Comparison Example 3) was also prepared as above, but in the present case addin~ 0.08 parts of the platinum/vin~lsiloxane complex composition (platinum metal content of 0.6 wei~ht percent) as prepared in Reference Example 1 in place of the spherical microparticulate catalys~ used in silicone rubber composition 1. In addition, in the present case 0.0~

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parts l-ethynyl-l-cyclohexanol was used instead of 0.002 parts.
Silicone rubber compositions 1, 2, 3, and 4 were coated on cable using an extruder. Thus, electric cable (outside diameter of core cable was 1.0 mm, 20/0.18 mm) was coated with the silicone rubber composition to a thickness of 0.45 mm, and this was treated in a hot-air oven (400 degrees Centigrade) for 18 seconds in order to thermoset the silicone rubber compo~ition.
The present invention's silicone rubber composition l and the comparison silicone rubber compositions 2, 3, and 4, all o which lacked acyl-type peroxide, in each case could be molded under good environmental conditions without the evolution of strong odor during curing. However, silicone rubber composition 2 (Comparison Example 1) was not cured as well as silicone rubber composition 1 (Example 1) and suffered from a reduced heat resistance. Silicone rubber composition 3 (Comparison Example 2) concerned the suppression of foaming during molding of the cable coating film through a substantial reduction in the quantity of reaction inhibitor. As a consequence, the use time was very short (2 to 3 hours) and the potential arose for scorching d~ring molding. Silicone rubber composition 4 (Comparison Example 3) employed an increased addition of reaction inhibitor in order to extend the use time to 24 hours; however, its cured product suffered from foamin~ and therefore could not develop good properties.
The various measurement results are reported in Table 1.

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Example 2 Silicone rubber composition 1 from Example 1 (now designated as Example 2), silicone rubber composition 2 from Comparison Example 1 (now designated as Comparison Example 4), and silicone rubber composition 3 from Comparison Example 2 (now designated as Comparison Example 5) were molded into cable coatings using an extruder under the conditions specified below.
In this experiment, the silicone rubber composition was molded into a 1.0 mm-thick cable coating on a core cable wi~h outside diameter of 4.9 mm (8~/0.45 mm). The silicone rubber composition was then thermoset for 18 seconds in a hot-air oven at 450 degrees Centigrade.
Silicone rubber composition 1 of Example 2 formed a high-quality cable coating film as in Example 1, but silicone rubber composition 2 (Comparison Example 4) was inadequately vulcanized in depth due to the heat scavenged by the wire and thus did not have satisfactory properties (e. g., tensile strength). Foaming appeared in the case of silicone rubber composition 3 (Comparison Example 5), and a good-quality wire coating film could therefore not be produced. This behavior was thought to be due to a combination of factors: the stronger hea~
take-off effect by the core cable due to its increased size, and, with respect to the platinum catalyst, its strong inactivation by atmospheric oxygen at high temperatures.
The various measurement results are reported in Table 2.

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Example 3 Silicone rubber composition l from Example 1 (now designated as E~ample 3) and silicone rubber composition 3 rom Comparison Example 2 (now designated as Comparison Example 6) were molded into cable coatings using an extruder under the conditions specified below.
In this experiment, the silicone rubber composition was molded into a 5.0 mm-thick cable coating on a core cable with outside dia~eter of 1.2 mm (30/0.18 mm). The silicone rubber composition was then thermoset by treatment for 24 seconds in a hot-air oven at 400 degree~ Centigrade.
The silicone rubber composition 1 of Example 3 formed a high-quality cable coating film as in Example 1, but silicone rubber composition 3 (Comparison Example 6) suffered from foaming, and a good-quality wire coating ilm could therefore not be produced. Thus, in the case of a thick coating as in this experiment, it was confirmed that a curing procedure based on a conventional platinum catalyst suffers from a very strong foaming tendency.
The various measurement results are reported in Table 3.

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Example 4 A silicone rubber composition, composition 5, was prepared as for silicone rubber co~position 1 in Example 1, bu~ in the present case using 0.03 parts of the spherical microparticulate polymethyl me~hacrylate resin catalyst from Reference Example 4 in place of the spherical microparticulate silicone resin catalyst in silicone rubber compo~ition 1 of Example 1. Cable coating was then carried out under the following condition~.
In this experiment, the silicone rubber composition was molded into a 0.45 mm-thick cable coating on a core cable wlth outside diameter of 1.0 mm (20/O.].B
mm~. The silicone rubber composition was then thermoset by treatment for 18 seconds in a hot-air oven at 400 C.
The quality of the cable coating film fabricated using the spherical microparticulate polymethyl methacrylate resin catalyst was as good as in : Example 1.
The various measurement results are reported in ~: Table 4.

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Table 4 Cable Coating Conditions and Measurement Results Example 4 Example 1 use time at 25C3 months 3 months vulcanization conditions vulcanization oven 400 400 temperature (C) : vulcanization 18 18 time (seconds) foaming no no surface tack surface tack? no no talc required? no no blooming no no physical proper~ies of ~he silicone rubbers : tensile strength, MPA 7.16 7.35 elongation (%) 330 320 - haat resistance (220 C/96 hours) residual tensile 96 ~7 ` strength ~%) : residual 68 71 elongation (%) -:~
cable properties : . .
insulation resistance 7200 7500 (Mohms-Km) dielectric breakdown 13 14 voltage (kilovolt~s) ' :-, ~ ~
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Claims

CLAIMS:

1. Silicone rubber composition for coating electric wire or cable, said composition consisting essentially of (A) 100 weight parts organopolysiloxane gum which contains at least 2 silicon-bonded alkenyl groups in each molecule and which is represented by the following average compositional formula R a SIO (4 - a)/2 wherein R is a substituted or unsubstituted monovalent hydrocarbon group and a is a number with a value of 1.8 to 2.3), (B) 10 to 100 weight parts reinforcing filler, (C) 0.1 to 10 weight parts organohydrogenpolysiloxane which contains at least 2 silicon-bonded hydrogen atoms in each molecule, (D) 0.05 to 10 weight parts alkyl-type organoperoxide, and (E) a catalytic quantity of a spherical microparticulate catalyst which consists of thermoplastic resin that contains at least 0.01 weight percent, as platinum metal atoms, of a platinum-type catalyst, with the provisos that the thermoplastic resin has a softening point of 50 to 200 degrees Centigrade and the spherical microparticulate catalyst has a particle diameter of 0.01 to 10 micrometers.