WO2005035637A1

WO2005035637A1 - Improved pib barrier coating composition for permeation reduction of silicone rubber parts

Info

Publication number: WO2005035637A1
Application number: PCT/US2004/019478
Authority: WO
Inventors: Randall Paul Sweet
Original assignee: Dow Corning Corporation
Priority date: 2003-09-12
Filing date: 2004-06-18
Publication date: 2005-04-21

Abstract

A cured silicone rubber article of manufacture has a protective coating layer on a surface. The article comprises a substrate layer formed of cured silicone rubber. The substrate layer has at least one untreated surface. A top layer of a cured polyisobutylene coating is applied to the untreated surface of the cured silicone rubber substrate layer. The polyisobutylene coating contains a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter. The adhesion promoter is a mixture containing a polysiloxane having an average of at least one silicon-bonded hydroxyl radical per molecule and an average of at least one silicon bonded vinyl radicals per molecule, and an epoxy silane. A method for preparing cured silicone rubber articles consists of coating an untreated surface of a substrate layer formed of a cured silicone rubber with a layer of a polyisobutylene coating composition containing a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter as described above, and curing the polyisobutylene coating composition.

Description

Improved PIB Barrier Coating Composition for Permeation Reduction of Silicone Rubber Parts

[0001] This invention relates to coated silicone rubber articles, and more particularly to coated silicone rubber articles comprising a silicone rubber substrate having a cured polyisobutylene (PIB) coating on at least one surface. The invention also relates to methods of preparing coated silicone rubber articles.

[0002] Silicone rubber is useful in a variety of applications because of its unique combination of properties including high thermal stability, good water resistance, excellent flexibility, high durability, and good adhesion to various substrates. For example, silicone rubber is widely used in the automotive, electronic, construction, appliance, and aerospace industries. However, silicone rubber exhibits high permeability to gases, water vapor, and hydrocarbons, compared to organic rubbers such as butyl rubber. Although this characteristic of silicone rubber can be beneficial in some applications such as gas-separation membranes, the high permeability of silicone rubber is undesirable in other applications where gas and/or liquid retention is important. Thus, organic solvents can diffuse into silicone rubber causing it to swell, which significantly decreases the physical properties of the silicone rubber material. [0003] Among the numerous approaches to reducing the permeability of silicone rubbers is the coating of the surface of silicone rubbers with a less permeable material. Coated silicone rubber articles and methods for their preparation are well known in the art, and numerous types of less permeable materials have been applied to surfaces of silicone rubbers. [0004] Although the patent literature describes coated silicone rubber articles having a range of permeabilities, there remains a need for coated silicone rubber articles having superior resistance to gases and liquids. Applicant is unaware of anything in the public domain that describes coatings for silicone rubber of the type disclosed herein. The common assignee's copending US Patent Application Serial No. 10/211,446, filed on August 2, 2002, entitled Coated Silicone Rubber Article and Method of Preparing Same, (hereafter referred to as the '446 application), does describe a similar coating for silicone rubber. [0005] However, to obtain proper adhesion of cured polyisobutylene coating compositions to silicone rubber substrates, the '446 application requires that the surface of the silicone rubber substrate be physically or chemically treated before application of the curable polyisobutylene coating composition. Some examples of surface treatments mentioned are solvent washing, exposure to corona discharge or plasma discharge, the application of a primer, and physical roughening of the surface.

[0006] This additional step of surface preparation is a problem for many manufacturers of coated silicone rubber articles in that it adds another separate step in their manufacturing process, and is time consuming and costly in the modern highly competitive environment. The present invention solves this problem by eliminating the need to physically or chemically treat the surface of the silicone rubber substrate before applying a curable polyisobutylene coating composition. This is accomplished by the addition of a particular adhesion promoter to the polyisobutylene coating composition before it is cured.

[0007] The invention is directed to a cured silicone rubber article of manufacture having a protective coating layer on a surface. The article comprises a substrate layer formed of cured silicone rubber. The substrate layer has at least one untreated surface. A top layer of a cured polyisobutylene coating is applied to the untreated surface of the cured silicone rubber substrate layer. The polyisobutylene coating comprises a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter. The adhesion promoter comprises a mixture containing a polysiloxane having an average of at least one silicon-bonded hydroxyl radical per molecule and an average of at least one silicon bonded vinyl radicals per molecule, and an epoxy silane.

[0008] The invention is also directed to a method for preparing cured silicone rubber articles having a protective coating layer on at least one surface. The method comprises coating an untreated surface of a substrate layer formed of a cured silicone rubber with a layer of a polyisobutylene coating composition containing a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter as described above, and curing the polyisobutylene coating composition.

[0009] The coated silicone rubber article of the invention has reduced permeability to liquids and gases compared with articles absent a cured polyisobutylene coating. Moreover, the desirable physical properties of silicone rubber such as its thermal stability, flexibility, and durability, are maintained in the coated article.

[0010] The method of the invention employs conventional coating techniques and equipment. Furthermore, the method uses readily available commercial grade silicone and polyisobutylene compositions. In addition, the polyisobutylene composition effectively wets the surface of the silicone rubber substrate which facilitates the coating process. [0011] These and other features of the invention will become apparent from a consideration of the detailed description.

DESCRIPTION [0012] A coated silicone rubber article according to the present invention comprises a silicone rubber substrate having a cured polyisobutylene coating on a surface thereof. The silicone rubber substrate can comprise any silicone rubber, filled or unfilled, known in the art. Moreover, the silicone rubber substrate can have any desired shape. As used herein, the term silicone rubber refers to a product prepared by curing (vulcanizing or cross-linking) an organopolysiloxane polymer. The mechanical and chemical properties of the silicone rubber substrate depend on the type of polymer, nature and amount of other components in the formulation, processing technique, and method of cure. For example, the silicone rubber substrate can have a hardness of from 5-90 Shore A, and a consistency ranging from a soft gel to a tough elastomer.

[0013] The silicone rubber substrate can be prepared by converting a curable silicone composition into a desired shape by conventional methods, such as compression molding, injection molding, extrusion, and calendaring; and then curing the composition. As used herein, the term curing means the conversion of a liquid or semisolid composition to a cross- linked product. Examples of curable silicone compositions include, but are not limited to, (i) hydrosilylation-curable silicone compositions, (ii) peroxide curable silicone compositions, (iii) condensation-curable silicone compositions, (iv) epoxy-curable silicone compositions; (v) ultraviolet radiation-curable silicone compositions, and (vi) high-energy radiation-curable silicone compositions.

[0014] Curable silicone compositions and methods for their preparation are well known in the art. For example, a suitable hydrosilylation-curable (i.e., addition curable) silicone composition typically comprises (i) an organopolysiloxane containing an average of at least two silicon-bonded alkenyl groups per molecule, (ii) an organohydrogensiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule in an amount sufficient to cure the composition, and (iii) a hydrosilylation catalyst. The hydrosilylation catalyst can be any of the well known hydrosilylation catalysts comprising a platinum group metal, a compound containing a platinum group metal, or a microencapsulated platinum group metal-containing catalyst. Platinum group metals include platinum, rhodium, ruthenium, palladium, osmium and iridium. Preferably, the platinum group metal is platinum, based on its high activity in hydrosilylation reactions. Reference may be had to US Patent 5,248,715 (September 28, 1993), US Patent 5,989,719 (November 23, 1999), and US Published Application US 2003/0072988 (April 17, 2003), as examples of such hydrosilylation-curable silicone compositions. For all practical purposes, compositions for preparing liquid silicone rubbers (LSR) are typically limited to compositions using hydrosilylation-curable (i.e., addition curable) systems. [0015] The hydrosilylation-curable silicone composition can be a one-part composition or a multi-part composition comprising the components in two or more parts. Room-temperature vulcanizable (RTV) compositions typically comprise two parts, one part containing the organopolysiloxane and catalyst and another part containing the organohydrogensiloxane and any optional ingredients. Hydrosilylation-curable silicone compositions that cure at elevated temperatures can be formulated as one-part or multi-part compositions. For example, liquid silicone rubber (LSR) compositions are typically formulated as two-part systems. One-part compositions typically contain a platinum catalyst inhibitor to ensure adequate shelf life. [0016] A suitable peroxide-curable silicone composition typically comprises (i) an organopolysiloxane and (ii) an organic peroxide. Examples of organic peroxides include, diaroyl peroxides such as dibenzoyl peroxide, di-p-chlorobenzoyl peroxide, and bis-2,4-dichlorobenzoyl peroxide; dialkyl peroxides such as di-t-butyl peroxide and 2,5-dimethyl-2,5-di-(t-butylperoxy)hexane; diaralkyl peroxides such as dicumyl peroxide; alkyl aralkyl peroxides such as t-butyl cumyl peroxide and 1 ,4-bis(t-butylperoxyisopropyl)benzene; and alkyl aroyl peroxides such as t-butyl perbenzoate, t-butyl peracetate, and t-butyl peroctoate. Reference may be had to US Patent 5,945,471 (August 31, 1999), as an example of such peroxide-curable silicone compositions.

[0017] A condensation-curable silicone composition typically comprises (i) an organopolysiloxane containing an average of at least two hydroxy groups per molecule; and (ii) a tri- or tetra-functional silane containing hydrolysable Si-O or Si-N bonds. Examples of silanes include alkoxysilanes such as CH3Si(OCH3)3, CH3Si(OCH2CH3)3,

CH3Si(OCH₂CH₂CH3)3, CH₃Si[O(CH2)3CH3]₃, CH₃CH2Si(OCH₂CH₃)3, C₆H₅Si(OCH3)3, C₆H₅CH₂Si(OCH3)3, C₆H₅Si(OCH₂CH₃)3, CH₂=CHSi(OCH₃)₃,

CH =CHCH₂Si(OCH₃)₃, CF₃CH₂CH₂Si(OCH₃)3, CH₃Si(OCH₂CH₂OCH3)₃,

CF₃CH₂CH₂Si(OCH₂CH₂OCH3)3, CH2=CHSi(OCH₂CH₂OCH3)₃,

CH₂=CHCH Si(OCH₂CH₂OCH₃)3, C₆H₅Si(OCH2CH OCH₃)3, Si(OCH₃)_4; Si(OC₂H5)4, and Si(OC3Hy)4; organoacetoxysilanes such as CH3Si(OCOCH3)3,

CH3CH₂Si(OCOCH3)3, and CH₂=CHSi(OCOCH₃)₃; organoiminooxysilanes such as

CH3Si[O-N=C(CH₃)CH₂CH₃]3, Si[O-N=C(CH₃)CH₂CH₃]₄, and CH₂=CHSi[O-

N=C(CH₃)CH₂CH3]3; organoacetamidosilanes such as CH₃Si[NHC(=O)CH3]3 and

C6H5Si[NHC(=O)CH₃]3; aminosilanes such as CH3Si[NH(s-C4H₉)]₃ and CH3 Si(NHC6H 11 )3 ; and organoaminooxysilanes.

[0018] A condensation-curable silicone composition can also contain a condensation catalyst to initiate and accelerate the condensation reaction. Examples of condensation catalysts include, but are not limited to, amines; and complexes of lead, tin, zinc, and iron with carboxylic acids. Tin (II) octoates, laurates, and oleates, as well as the salts of dibutyl tin, are particularly useful. The condensation-curable silicone composition can be a one-part composition or a multi-part composition comprising the components in two or more parts. For example, room-temperature vulcanizable (RTV) compositions can be formulated as one- part or two-part compositions. In the two-part composition, one of the parts typically includes a small amount of water. Reference may be had to US Patent 5,945,471 (August 31, 1999) and US Patent 6,534,581 (March 18, 2003), as examples of such condensation-curable silicone compositions.

[0019] A suitable epoxy-curable silicone composition typically comprises (i) an organopolysiloxane containing an average of at least two epoxy-functional groups per molecule and (ii) a curing agent. Examples of epoxy-functional groups include 2-glycidoxyethyl, 3-glycidoxypropyl, 4-glycidoxybutyl, 2,(3,4-epoxycyclohexyl)ethyl, 3-(3,4-epoxycyclohexyl)propyl, 2,3-epoxypropyl, 3,4-epoxybutyl, and 4,5-epoxypentyl. [0020] Examples of curing agents include anhydrides such as phthalic anhydride, hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and dodecenylsuccinic anhydride; polyamines such as diethylenetriamine, triethylenetetramine, diethylenepropylamine, N-(2-hydroxyethyl)diethylenetriamine, N,N'-di(2-hydroxyethyl)diethylenetriamine, m-phenylenediamine, methylenedianiline, aminoethyl piperazine, 4,4-diaminodiphenyl sulfone, benzyldimethylamine, dicyandiamide, and 2-methylimidazole, and triethylamine; Lewis acids such as boron trifluoride monoethylamine; polycarboxylic acids; polymercaptans; polyamides; and amidoamines. Reference may be had to US Patent 4,082,719 (April 4, 1978) and US Patent 5,358,983 (October 25, 1994), as examples of epoxy-curable silicone compositions. [0021] A suitable ultraviolet radiation-curable silicone composition typically comprises (i) an organopolysiloxane containing radiation-sensitive functional groups and (ii) a photoinitiator. Examples of radiation-sensitive functional groups include acryloyl, methacryloyl, mercapto, epoxy, and alkenyl ether groups. The type of photoinitiator depends on the nature of the radiation-sensitive groups in the organopolysiloxane. Examples of photoinitiators include diaryliodonium salts, sulfonium salts, acetophenone, benzophenone, and benzoin and its derivatives. Reference may be had to US Patent 5,877,228 (March 2, 1999), as an example of such ultra-violet radiation-curable silicone compositions. [0022] A suitable high-energy radiation-curable silicone composition comprises an organopolysiloxane polymer. Examples of organpolyosiloxane polymers include polydimethylsiloxanes, poly(methylvinylsiloxanes), and organohydrogenpolysiloxanes. Examples of high-energy radiation include γ-rays and electron beams. Reference may be had to US Patent 6,515,041 (February 4, 2003), as an example of such high-energy radiation- curable silicone compositions.

[0023] The curable silicone composition of the present invention can comprise additional ingredients, provided the ingredient does not adversely affect the permeability properties of the coated article, as described above. Examples of additional ingredients include, but are not limited to, adhesion promoters, solvents, inorganic fillers, photosensitizers, antioxidants, stabilizers, pigments, and surfactants.

[0024] Examples of inorganic fillers include, but are not limited to, natural silicas such as crystalline silica, ground crystalline silica, and diatomaceous silica; synthetic silicas such as fused silica, silica gel, pyrogenic silica, and precipitated silica; silicates such as mica, wollastonite, feldspar, and nepheline syenite; metal oxides such as aluminum oxide, titanium dioxide, magnesium oxide, ferric oxide, beryllium oxide, chromium oxide, and zinc oxide; metal nitrides such as boron nitride, silicon nitride, and aluminum nitride, metal carbides such as boron carbide, titanium carbide, and silicon carbide; carbon black; alkaline earth metal carbonates such as calcium carbonate; alkaline earth metal sulfates such as calcium sulfate, magnesium sulfate, and barium sulfate; molybdenum disulfate; zinc sulfate; kaolin; talc; glass fiber; glass beads such as hollow glass microspheres and solid glass microspheres; aluminum trihydrate; asbestos; and metallic powders such as aluminum, copper, nickel, iron, and silver powders.

[0025] The silicone composition can be cured by exposure to ambient temperature, elevated temperature, moisture, or radiation, depending on the particular cure mechanism. For example, one-part hydrosilylation-curable silicone compositions are typically cured at an elevated temperature. Two-part hydrosilylation-curable silicone compositions are typically cured at room temperature or an elevated temperature. One-part condensation-curable silicone compositions are typically cured by exposure to atmospheric moisture at room temperature, although cure can be accelerated by application of heat and/or exposure to high humidity. Two-part condensation-curable silicone compositions are typically cured at room temperature, however, cure can be accelerated by application of heat. Peroxide-curable silicone compositions are typically cured at an elevated temperature. Epoxy-curable silicone compositions are typically cured at room temperature or an elevated temperature. Depending on the particular formulation, radiation-curable silicone compositions are typically cured by exposure to radiation, for example, ultraviolet light, gamma rays, or electron beams. [0026] The cured polyisobutylene coating is a cured product of a curable polyisobutylene composition. The polyisobutylene coating typically has a thickness not more than 25 percent, alternatively not more than 15 percent, alternatively not more than 10 percent, of the maximum thickness of the silicone rubber substrate. When the thickness of the cured polyisobutylene coating is greater than 25 percent of the maximum thickness of the silicone rubber substrate, air may become entrapped in the coating during curing. The cured polyisobutylene coatings can be prepared according to the method of the present invention, described below.

[0027] A method of preparing a coated article according to the present invention, comprises the steps of (a) applying a curable polyisobutylene composition to a surface of a silicone rubber substrate; and (b) curing the polyisobutylene composition. [0028] A curable polyisobutylene composition is applied to a surface of a silicone rubber substrate. The curable polyisobutylene composition of the present invention comprises a polyisobutylene polymer containing an average of at least two functional groups per molecule capable of reacting to cross-link (cure) the polymer; and a curing agent, for example, a catalyst or photoinitiator. As used herein, the term polyisobutylene polymer refers to an oligomer, homopolymer, or copolymer containing monomeric units derived from isobutylene and having the formula -CH₂C(CH3) ₂-. The polyisobutylene polymer typically contains an average of at least 50 mol percent, preferably at least 80 mol percent, and most preferably at least 95 mol percent of these monomeric units per molecule. Monomeric units other than - CH₂C(CH3)₂- units can be derived from olefin monomers. The olefin monomers typically contain from 2-20 carbon atoms, preferably 4-10 carbon atoms. Examples of olefin monomers include, but are not limited to, alkenes such as 1-butene, 2-butene, 2-methyl-l- butene, 3 -methyl- 1-butene, 1-pentene, 4-methyl-l-pentene, 1-hexene, and vinylcyclohexane; alkadienes such as butadiene and isoprene; alkenyl ethers such as methyl vinyl ether, ethyl vinyl ether, and isobutyl vinyl ether; cycloalkenes such as cylcohexene and β-pinene; cycloalkadienes such as cyclopentadiene; and aromatic compounds containing aliphatic carbon-carbon double bonds such as styrene, α-methylstyrene, dimethylstyrene, monochlorostyrene, dichlorostyrene, and indene .

[0029] The functional groups in the polyisobutylene polymer can be located at pendant, terminal, or both pendant and terminal positions in the molecules. Examples of functional groups include, but are not limited to, alkenyl, silicon-bonded hydrolysable groups, epoxy, alkenyl ether groups, acryloyl, and methacryloyl. [0030] The polyisobutylene polymer typically has a number average molecular weight of 1,000-1,000,000, preferably 3,000-100,000, as determined by gas permeation chromatography (GPC) employing a low angle laser light scattering detector. [0031] The type of curing and/or crosslinking agent depends on the nature of the functional groups in the polyisobutylene (PIB) polymer. For example, when the PIB polymer contains alkenyl groups, the curing agent typically comprises an organosilicon compound containing silicon-bonded hydrogen atoms and a hydrosilylation catalyst. When the PIB polymer contains silicon-bonded hydrolysable groups, the curing agent is typically a condensation catalyst. When the PIB polymer contains epoxy groups, the curing agent is typically an amine or acid anhydride. When the PIB polymer contains alkenyl ether, acryloyl or methacryloyl groups, the curing agent is typically a photoinitiator.

[0032] Examples of curable polyisobutylene compositions include, but are not limited to, hydrosilylation-curable polyisobutylene compositions, condensation-curable polyisobutylene compositions, epoxy-curable polyisobutylene compositions, and radiation-curable polyisobutylene compositions.

[0033] Curable polyisobutylene compositions and methods for their preparation are well known in the art. For example, a suitable hydrosilylation-curable polyisobutylene composition typically comprises (i) a polyisobutylene polymer containing alkenyl groups; (ii) an organosilicon compound containing silicon-bonded hydrogen atoms; and (iii) a hydrosilylation catalyst. Examples of alkenyl groups include, but are not limited to, vinyl, allyl, butenyl, and hexenyl. Examples of organosilicon compounds containing silicon-bonded hydrogen atoms include organohydrogensilanes and organohydrogensiloxanes. The hydrosilylation catalyst can be any of the well-known hydrosilylation catalysts or microencapsulated hydrosilylation catalysts comprising a platinum group metal. Platinum- containing catalysts, for example, a platinum complex of 1,3-divinyl-l, 1,3,3- tetramethyldisiloxane, are particularly useful due to their high activity in hydrosilylation reactions. [0034] Methods of preparing polyisobutylene polymers containing alkenyl groups and curable compositions comprising such polymers are well known in the art, as exemplified in US Patent 5,728,768 (March 17, 1998), US Patent 5,753,7431 (May 19, 1998), US Patent 5,952,419 (September 14, 1999), US Patent 6,077,892 (June 20, 2000), US Patent 6,034,179 (March 7, 2000), and in European Patents EP 709403 (May 1, 1996) and EP 984036 (March 8, 2000).

[0035] A suitable condensation-curable polyisobutylene composition typically comprises (i) a polyisobutylene polymer containing silicon-bonded hydrolysable groups and (ii) a condensation catalyst. Examples of hydrolysable groups include alkoxy, phenoxy, acyloxy, amino, amido, aminoxy, mercapto, alkenyloxy, halogen, alkoxyalkoxy, and ketoximo. Examples of condensation catalysts include carboxylates of tin such as dibutyltin diacetate, dibutyltin dilaurate, tin tripropyl acetate, stannous octoate, stannous oxalate, stannous naphthanate, and dibutylbis(2,4-pentanedioate)tin; titanium compounds such as tetrabutyl titanate, titanium diisopropoxy-bis-ethylacetoacetate, and tetraisopropoxy titanate; carboxylates of bismuth; carboxylates of lead; carboxylates of zirconium; amines such as triethylamine, ethylenetriamine, butylamine, octylamine, dibutylamine, monoethanolamine, diethanolamine, triethanolamine, diethylenetriamine, triethylenetetramine, cyclohexylamine, benzylamine, diethylaminopropylamine, xylylenediamine, triethylenediamine, guanidine, diphenylguanidine, and morpholine.

[0036] Methods of preparing polyisobutylene polymers containing hydrolysable groups and curable compositions containing such polymers are well known in the art, as exemplified in US Patent 4,524,187 (June 18, 1985), US Patent 4,904,732 (February 27, 1990), US Patent 6,177,519 (January 23, 2001), US Patent 6,258,878 (July 10, 2001), and US Patent 6,380,316 (April 30, 2002).

[0037] A suitable epoxy-curable polyisobutylene composition typically comprises (i) an epoxy-functional polyisobutylene polymer and (ii) a curing agent. Examples of epoxy groups include epoxyethyl, glycidyl, glycidoxy, 2-glycidoxyethyl, 3-glycidoxypropyl, and 4- glycidoxybutyl. Examples of curing agents include organic amines such as diethylenetriamine, amine-functional silanes, and amine-functional siloxanes. Methods of preparing epoxy-functional polyisobutylene polymers and curable compositions comprising such polymers are well known in the art, as exemplified in US Patent 5,977,255 (November 2, 1999).

[0038] A suitable radiation-curable polyisobutylene composition can comprise (i) an alkenyl ether-functional polyisobutylene polymer and (ii) a cationic photoinitiator. Some examples of alkenyl ether groups include those having the formulae

-SiRa¹ [OR²OC(R³)=CH(R )]₃__a and -R OC(R³)=CHR⁴ , wherein each R¹ is independently hydrocarbyl or alkoxy, R² is a divalent hydrocarbon group having from

2-20 carbon atoms, R³ and R⁴ are independently hydrogen or hydrocarbyl, and a has a value of from 0-2. Examples of cationic photoinitiators including onium salts, diaryliodonim salts of sulfonic acids, triarylsulfonium salts of sulfonic acids, diaryliodonium salts of boronic acids, and triarylsulfonium salts of boronic acids. [0039] Methods of preparing alkenyl ether-functional polyisobutylene polymers and curable compositions comprising such polymers are well known in the art, as exemplified in US Patent 6,054,549 (April 25, 2000), US Patent 6,069,185 (May 30, 2000), US Patent 6,242,058 (June 5, 2001), and PCT publication WO 01/88003 (November 22, 2001). [0040] Alternatively, a suitable radiation-curable polyisobutylene composition can comprise (i) an acryloyl- or methacryloyl-functional polyisobutylene polymer and (ii) a photoinitiator. Examples of photoinitiators include 2-hydroxy-2-methyl-l-phenylpropan-l- one, 2,2-dimethoxy-2-phenylcetophenone, acetophenone, benzophenone, benzoin, and benzil or a derivative thereof. Methods of preparing acryloyl- and methacryloyl-functional polyisobutylene polymers and compositions comprising such polymers are well known in the art, as exemplified by US Patent 5,665,823 (September 9, 1997).

[0041] The curable polyisobutylene composition can comprise additional ingredients, provided the ingredient does not adversely affect the permeability properties of the coated article, as described above. Examples of additional ingredients include, but are not limited to, solvents, reinforcing and extending fillers, photosensitizers, antioxidants, stabilizers, pigments, plasticizers, and surfactants.

[0042] The silicone rubber substrate and methods of preparing the substrate are as described above for the coated article of the present invention. To improve adhesion of the cured polyisobutylene composition to the silicone rubber substrate, the polyisobutylene composition contains an adhesion promoter. [0043] The adhesion promoter is described in detail in the common assignee's US Patent 4,087,585 (May 2, 1978), which is considered incorporated herein by reference. It comprises a mixture containing (i) a hydroxyl end-blocked polymethylvinylsiloxane and (ii) an epoxy silane. Polysiloxane (i) has an average of at least one silicon-bonded hydroxyl radical per molecule and an average of at least one silicon-bonded vinyl radical per molecule. Polysiloxane (i) has its siloxane units bonded through silicon-oxygen-silicon bonds, and the valences of each silicon atom in polysiloxane (i) are satisfied by at least one of a monovalent alkyl radical having less than 7 carbon atoms per radical, a phenyl radical, a vinyl radical, or a hydroxyl radical. Any remaining valences of silicon in polysiloxane (i) are satisfied by divalent oxygen atoms. Preferably, polysiloxane (i) should have an average of less than about 15 silicon atoms per molecule.

[0044] The epoxy silane (ii) is a silane having at least one epoxy-containing organo group and at least one silicon-bonded alkoxy group having less than 5 carbon atoms per group. Any remaining valences on epoxy silane (ii) not satisfied by an epoxy-containing organo group or an alkoxy group being satisfied by a monovalent hydrocarbon radical or a fluorinated alkyl radical, each having less than 7 carbon atoms per radical. An especially preferred epoxy silane representative of this component is 3-glycidoxypropyltrimethoxysilane. [0045] The adhesion promoter is prepared by combining the two components (i) and (ii) such that the mixture contains the hydroxyl end-blocked polymethylvinylsiloxane (i) and the epoxy silane (ii) in a weight ratio of about 0.25-1.8 parts by weight of the epoxy silane (ii) per one part by weight of the hydroxyl end-blocked polymethylvinylsiloxane (i). [0046] As noted above, unlike the coated articles described in the '446 application, it is not necessary to improve the adhesion of the cured polyisobutylene composition to the silicone rubber substrate by physically or chemically treating the surface of the silicone rubber substrate before applying the curable polyisobutylene composition. Addition of the particular adhesion promoter described above, to the polyisobutylene composition effectively eliminates the necessity of surface treating the silicone rubber substrate by solvent washing, corona discharge, plasma discharge, application of a primer, or physical roughening of the surface, in order to obtain adhesion.

[0047] The curable polyisobutylene composition can be applied to the silicone rubber substrate by any conventional method known in the art, such as spin coating, dipping, spraying, brushing, or screen-printing. The coating conditions and viscosity of the curable poloyisobutylene composition can be adjusted so the cured polyisobutylene coating has the desired thickness.

[0048] The curable polyisobutylene composition can be cured by exposure to ambient temperature, elevated temperature, moisture, or radiation, depending on the particular cure mechanism. For example, hydrosilylation-curable polyisobutylene compositions are typically cured at room temperature or an elevated temperature. Condensation-curable polyisobutylene compositions are typically cured by exposure to atmospheric moisture at room temperature, although cure can be accelerated by application of heat and/or exposure to high humidity. Epoxy-curable polyisobutylene compositions are typically cured at room temperature or an elevated temperature. Radiation-curable silicone compositions are typically cured by exposure to ultraviolet radiation.

[0049] The coated silicone rubber article of the present invention exhibits reduced permeability to liquids and gases compared with the same article absent the cured polyisobutylene coating. For example, the coated silicone rubber article typically has a permeability of not more than 10 percent of the permeability of the uncoated silicone rubber. Moreover, the desirable physical properties of the silicone rubber, such as thermal stability, flexibility, and durability, are maintained in the coated article. [0050] The method of the present invention employs conventional coating techniques and equipment. Furthermore, the method uses readily available silicone and polyisobutylene compositions. Also, the polyisobutylene composition effectively wets the surface of the silicone rubber substrate, thus facilitating the coating process. [0051] The coated silicone rubber articles of the invention have numerous uses including gaskets, o-rings, adhesives, insulators, encapsulants, containers, and valves such cone-shaped valves used for dispensing waterless hand soaps.

[0052] The following Examples are set forth in order to illustrate the invention in more detail. Unless otherwise noted, all parts and percentages in the Examples are by weight. The following methods and materials were employed in the Examples.

Preparation of the Silicone Rubber Substrate

[0053] Silicone rubber films and articles were prepared by injection molding a 40 Durometer, 2-part, 1 -to- 1 mixture of a translucent injection molding grade liquid silicone rubber composition manufactured and sold by the Dow Corning Corporation, Midland,

Michigan, under its trademark SILASTIC®, at temperatures in the range of 175-200 °C for about 10-20 seconds. Unlike the composition prepared in the '446 application, none of these film and articles were corona treated.

Preparation of the Coated Silicone Rubber Articles

[0054] The curable polyisobutylene composition was dip-coated or sprayed over the surface of the silicone rubber substrates. The coated substrates were heated and cured in a hot air oven temperatures in the range of 100-125 °C for 10-30 minutes. [0055] The following chemical compositions were used as components in the Examples:

[0056] Polyisobutylene Polymer A - An allyl-terminated polyisobutylene polymer sold under the trademark EPION® 200A by Kaneka Corporation, Osaka, Japan. This copolymer had a number-average molecular weight of about 5,000.

[0057] Polyisobutylene Copolymer B - An alkoxysilane grafted polyisobutylene copolymethystrene copolymer. It is the reaction product of vinylmethyldimethoxysilane and a polymethylstyrene-isobutylene copolymer sold under the name XP-50 by the ExxonMobil

Chemical Company, Baytown, Texas. The XP-50 copolymer is a polyisobutylene-p- methylstyerene copolymer having a number average molecular weight of about 160,400 measured by gel permeation chromatography (GPC), using a PIB calibration in tetrahydrofuran solvent. It contains 4.6 weight percent of p-methylstyerene units. The preparation of XP-50 copolymer is described in US Patent 5,162,445. [0058] SiH Cross-linking Agent A - A trimethylsiloxy-terminated poly(dimethylsiloxane/methylhydrogen-siloxane) having an average of three dimethylsiloxane units and five methylhydrogensiloxane units per molecule and containing about 0.8 percent of silicon-bonded hydrogen atoms. [0059] Inhibitor - 3,5-dimethyl-l-hexyn-3-ol, an acetylenic diol, sold under the trademark SURFONYL® 61 by Air Products & Chemicals Incorporated, Allentown, Pennsylvania. [0060] Platinum Catalyst - A concentrated mixture consisting of 38 percent of l,3-divinyl-l,l,3,3-tetramethyldisiloxane and 62 percent of platinum complex of 1 ,3-divinyl- 1,1,3 ,3-tetramethyldisiloxane. [0061] TEOS - Tetraethylorthosilicate (ethyl silicate), i.e., a tetraalkoxysilane of the formula Si(OC₂H5)₄. This component of the composition is part of the adhesion promoter package, and it functions as a crosslinking agent in that it is capable of reacting with moisture in the presence of a titanate catalyst such as TYZOR® organic titanate catalysts, in order to provide better bonding of the PIB coating to the silicone rubber substrate. [0062] Adhesion Promoter - As noted above, this component of the PIB composition is described in detail in the common assignee's US Patent 4,087,585 (May 2, 1978), and it comprises a mixture containing (i) a hydroxyl end-blocked polymethylvinylsiloxane and (ii) an epoxy silane.

[0063] TYZOR® TBT - An organic titanate catalyst, i.e., tetrabutyl titanate Ti(OC₄H9)₄, sold under the trademark TYZOR® by E.I. du Pont de Nemours & Company, Wilmington, Delaware.

[0064] TMPDE - Trimethylol propane diallyl ether, i.e., C₂H5C(CH₂OCH₂CH=CH₂) CH₂OH. This component of the PIB composition functions as a drying agent for the PIB surface in order to decrease the tackiness of the surface of the coated silicone rubber article. Table 1 - PIB Dip Coating Composition

Table 2 - PIB Spray Coating Composition

Example 1

[0065] A curable polyisobutylene composition was prepared by combining the components shown in Table 1. After the components had been combined, they were mixed to homogeneity with a two-cylinder roller used for mixing paint cans. The silicone rubber articles were then coated with the curable polyisobutylene composition.

Example 2

[0066] Example 1 was repeated except that the components in Table 2 were combined. [0067] While Tables 1 and 2 each show a specific amount of each component of the PIB dip and spray coating compositions, respectively, these compositions can be prepared using the components in more general ranges. For example, the Polyisobutylene polymer A can be used in amounts of 15-55 percent by weight, preferably 20-30 percent by weight. The SiH crosslinking agent A can be used in amounts of 2.5-8 percent by weight, preferably 4-7 percent by weight. The inhibitor can be used in amounts of 0.01-0.10 percent by weight, preferably 0.04-0.06 percent by weight. The platinum catalyst and TBT can each be used in amounts of 0.01-0.3 percent by weight, preferably 0.03-0.05 percent by weight. The polyisobutylene copolymer B can be used in amounts of 1-25 percent by weight, preferably 5-20 percent by weight. Crosslinking agents such as TEOS can be used in amounts of 0.1-3 percent by weight, preferably 0.5-1.7 percent by weight. The adhesion promoter can be used in amounts of 0.1 -3 percent by weight, preferably 1 -2 percent by weight. Drying agents such as TMPDE can be used in amounts of 0.1-10 percent by weight, preferably 0.5-1.5 percent by weight. Solvents such as heptane and octane can be used in amounts of 35-80 percent by weight, preferably 50-60 percent by weight. [0068] The cumulative permeation rate and the steady-state permeation rate through these coated silicone rubber articles was considered equivalent to the corresponding cumulative permeation rate and the steady-state permeation rate of corona treated silicone rubber articles in the '446 application. In particular, these coated silicone rubber articles were sufficient in reducing permeation of isopropanol in a soap solution. The PIB coating compositions of the present invention also provided better adhesion to silicone rubber substrates that had not been corona treated. This permitted improved functional wear testing of articles prepared according to the present invention compared to compositions described in the common assignee's copending application.

[0069] Overall improvements in coating integrity and adhesion were observed. Thus, the rub-off and/or detachment of the PIB coating from the silicone rubber substrate did not occur even after stretching the PIB coated silicone rubber articles. The coated articles prepared herein were also found to possess a much lower surface tack following cure. [0070] Other variations may be made in compounds, compositions, and methods described herein without departing from the essential features of the invention. The embodiments of the invention specifically illustrated herein are exemplary only and not intended as limitations on their scope except as defined in the appended claims.

Claims

CLAIMS 1. A cured silicone rubber article of manufacture having a protective coating layer on a surface thereof comprising (i) a substrate layer formed of cured silicone rubber, the substrate layer having at least one untreated surface; (ii) a top layer of a cured polyisobutylene coating on the untreated surface of the cured silicone rubber substrate layer; the polyisobutylene coating comprising a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter, the adhesion promoter comprising a mixture containing a polysiloxane having an average of at least one silicon-bonded hydroxyl radical per molecule and an average of at least one silicon bonded vinyl radicals per molecule, and an epoxy silane.

2. An article of manufacture according to Claim 1 wherein the silicone rubber substrate is prepared by curing a curable silicone composition selected from the group consisting of hydrosilylation-curable silicone compositions, peroxide curable silicone compositions, condensation-curable silicone compositions, epoxy-curable silicone compositions, ultraviolet radiation-curable silicone compositions, and high-energy radiation-curable silicone compositions.

3. An article of manufacture according to Claim 1 wherein the polyisobutylene coating is prepared by curing a curable polyisobutylene composition selected from the group consisting of hydrosilylation-curable polyisobutylene compositions, condensation-curable polyisobutylene compositions, epoxy-curable polyisobutylene compositions, and radiation- curable polyisobutylene compositions.

4. An article of manufacture according to Claim 1 wherein the polyisobutylene coating further comprises at least one additional component selected from the group consisting of an inhibitor, an alkoxysilane grafted polyisobutylene copolymethystrene copolymer, a tetraalkoxysilane, a drying agent, and a solvent.

5. An article of manufacture according to Claim 1 wherein the untreated surface of the silicone rubber substrate layer is free of physical or chemical alteration prior to application of the polyisobutylene coating layer.

6. An article of manufacture according to Claim 5 wherein the physical or chemical alteration is selected from the group consisting of solvent washing, exposure to corona discharge, exposure to plasma discharge, application of a primer, and physical roughening of the surface.

7. A method for preparing a cured silicone rubber article having a protective coating layer on at least one surface thereof comprising (i) coating an untreated surface of a substrate layer formed of a cured silicone rubber, with a layer of a polyisobutylene coating composition, the polyisobutylene coating composition comprising a polyisobutylene polymer, a crosslinking agent, a catalyst, and an adhesion promoter, the adhesion promoter comprising a mixture containing a polysiloxane having an average of at least one silicon-bonded hydroxyl radical per molecule and an average of at least one silicon bonded vinyl radicals per molecule, and an epoxy silane; and (ii) curing the polyisobutylene coating composition.

8. A method according to Claim 7 wherein the silicone rubber substrate is prepared by curing a curable silicone composition selected from the group consisting of hydrosilylation-curable silicone compositions, peroxide curable silicone compositions, condensation-curable silicone compositions, epoxy-curable silicone compositions, ultraviolet radiation-curable silicone compositions, and high-energy radiation-curable silicone compositions.

9. A method according to Claim 7 wherein the polyisobutylene coating composition is prepared by curing a curable polyisobutylene composition selected from the group consisting of hydrosilylation-curable polyisobutylene compositions, condensation-curable polyisobutylene compositions, epoxy-curable polyisobutylene compositions, and radiation- curable polyisobutylene compositions.

10. A method according to Claim 7 wherein the polyisobutylene coating composition further comprises at least one additional component selected from the group consisting of an inhibitor, an alkoxysilane grafted polyisobutylene copolymethystrene copolymer, a tetraalkoxysilane, a drying agent, and a solvent.

11. A method according to Claim 7 wherein the untreated surface of the silicone rubber substrate layer is free of physical or chemical alteration prior to application of the polyisobutylene coating layer.

12. A method according to Claim 11 wherein the physical or chemical alteration is selected from the group consisting of solvent washing, exposure to corona discharge, exposure to plasma discharge, application of a primer, and physical roughening of the surface.