Description :

Field of the Invention

The present invention refers to a method of producing a layer of a vulcanized silicone rubber composition on a substrate surface having an improved adhesion to the substrate surface, said vulcanized silicone rubber being selected from high temperature vulcanized (HTV) and room temperature vulcanized (RTV) silicone rubbers, especially from cured room temperature vulcanizing silicone rubbers, being selected from ultraviolet radiation (UV) cured room temperature vulcanizing silicone rubbers, including cured liquid ultraviolet radiation room temperature vulcanizing silicone rubbers.

The silicone primer composition according to the present invention is a conventional silicone primer composition to which at least one UV-sensitive crosslinking catalyst has been added in any desired sequence, i.e. before, during or after hardening of said silicone primer composition.

By first adding a hardened silicone primer composition to the substrate surface, to which at least one UV-sensitive

crosslinking catalyst has been added in any desired sequence before, during or after hardening the hardenable silicone primer composition, a tight and strong adhesion is obtained between the cured silicone rubber and the substrate surface. For room temperature vulcanizing silicone rubbers, such as UV- cured room temperature vulcanizing silicone rubbers, no additional thermal curing step is required. State of the Art

Silicone rubbers are widely used in the electrotechnical industry as electrical insulation for indoor and outdoor applications, such as for example hollow core insulators, cable accessories, medium and high voltage surge arresters or vacuum interrupters. Both high temperature vulcanizing silicone rubbers (HTV-SR) and Liquid Silicone Rubbers (LSR) are used for these applications . In the case of LSR, the material generally is composed of two different components A and B, which are mixed together and afterwards thermally cured. Curing time plays an important role in the total production time of these products and varies between 15 minutes for small insulators up to 40 minutes or more for large insulators.

A much faster way of producing electrical insulations is to use room temperature vulcanizing (RTV) silicone rubbers, including room temperature vulcanizing liquid silicone rubbers (RTV-LSR) , which cure in the presence of ultraviolet radiation (UV) . The UV-curing of such UV-vulcanizable RTV silicone rubber compositions requires no heating step and occurs within seconds to minutes. The use of UV vulcanizable RTV silicone rubber compositions, including room temperature vulcanizing liquid silicone rubbers, allows reducing significantly production times and therewith also the production costs .

With regard to electrical applications of such UV-curable RTV silicone rubber compositions, it is essential that there exists a strong adhesion between the cured silicone rubber composition and the substrate. If the adhesion is low, electric discharges can occur on the interfaces, which finally can lead to an electrical breakdown of the insulation or device.

For thermally cured silicones there exist numerous commercially available primer compositions to guarantee good adhesion of the thermally cured silicone rubber to the substrate . Depending on the substrate, individual primers have to be selected. Such primer compositions are normally used in the form of liquid solutions containing compounds selected for example from hydro- lysable silicates, hydrolysable titanates, reactive silanes and/or siloxanes, optionally together with an organic solvent. When the primer is applied to a substrate surface and is exposed to moist atmosphere at ambient or elevated temperature a thin solid film is formed on the substrate surface.

In the case of UV-cured silicones, however, it has been shown, that known commercial primer compositions do not provide sufficient adhesion to the substrate and, therefore, do not work properly, although these primers give good result when applied together with thermally cured silicones. This is true for substrates such as metals, ceramics, glass, rubber/glass composites as well as plastic materials. This is a crucial disadvantage for introducing UV-curing silicones into production, because it makes mandatory to either develop new primer compositions or to introduce an additional heat curing step into the production. The latter is a severe disadvantage for the industrial use of UV-cured silicones and strongly limits the time and cost benefits of using UV-curing silicones.

Summary of Invention

It has now been found that known silicone primer compositions, which are combined before, during or after hardening, with at least one UV-sensitive crosslinking catalyst, surprisingly, do not have the disadvantages as described herein above. A UV- sensitive crosslinking catalyst is a UV-active crosslinking initiator compound which initiates and promotes crosslinking and vulcanization of curable silicone rubber compositions.

Using a silicone primer composition, which is combined before, during or after hardening, with at least one UV-sensitive crosslinking catalyst, surprisingly yields a product wherein an excellent adhesion of the cured silicone rubber, especially of a UV-cured silicone rubber, to the substrate surface is obtained without requiring an additional thermal curing step. This is surprising, especially as UV-cured silicone rubbers always already contain in the curable state a sufficient quantity of at least one UV-sensitive crosslinking catalyst.

The expression "silicone primer composition" as used herein means that said "primer composition" is specifically suitable to be used together with a curable silicone rubber composition, preferably with a UV-curable silicone rubber composition, whereby said curable silicone rubber composition is applied onto the cured primer as an additional layer. The expression "silicone primer composition" does not mean that the primer composition needs to contain any silicone compound. The expressions "curable", "vulcanizable" and "crosslinking" as used herein are equivalent.

Description of the Invention

The present invention is defined in the claims . Specifically, the present invention refers to a method of producing a layer of a UV-cured silicone rubber composition on a substrate surface, said UV-cured silicone rubber composition having an improved adhesion to the substrate surface, said method comprising applying a silicone primer composition to the substrate surface and hardening said silicone primer composition followed by applying thereupon a UV-curable silicone rubber composition, and UV-curing said curable silicone rubber composition, whereby the electrical insulator is obtained, characterized in that at least one UV-sensitive crosslinking catalyst selected from compounds which initiate and promote curing, resp. crosslinking, of UV-curable silicone rubber compositions, is added to the silicone primer composition in any desired sequence before, during or after hardening of said silicone primer composition. Surprisingly, when a UV-curable silicone rubber composition is subsequently added onto said hardened silicone primer composition no additional thermal curing step is required when curing said composition.

Said UV-curable silicone rubber composition applied to the surface of the hardened silicon primer composition is preferably selected from room temperature UV-curable silicone rubber compositions, including room temperature UV-curable liquid silicone rubber compositions. On curing these compositions with UV-radiation yield the cured/vulcanized composition.

The present invention further refers to a silicone primer composition, suitable for improving the adhesion of UV- vulcanized silicone rubber compositions to various substrate surfaces, characterized in that said silicone primer composition comprises at least one UV-sensitive crosslinking catalyst, said at least one UV-sensitive crosslinking catalyst being selected from compounds which initiate and promote curing, resp. crosslinking, of UV-curable silicone rubber compositions .

The layer of the vulcanized silicone rubber composition on the substrate surface preferably is an electrical insulator, prefe rably a high voltage electrical insulator. In this sense, the present invention refers to a method of producing an electrica insulator being made as a layer of a vulcanized silicone rubbe composition on a substrate surface, said vulcanized silicone rubber composition having an improved adhesion to the substrat surface, said method comprising applying a silicone primer composition to a substrate surface and hardening said silicone primer composition followed by applying thereupon a UV-curable silicone rubber composition, and curing said curable silicone rubber composition by UV-curing, whereby the electrical insula tor is obtained, characterized in that at least one UV-sensi- tive crosslinking catalyst selected from compounds which initiate and promote curing, resp. crosslinking, of UV-curable silicone rubber compositions, is added to the silicone primer composition in any desired sequence before, during or after hardening of said silicone primer composition.

The present invention further refers to electrical articles comprising an insulator made according to the present

invention .

Silicone primer compositions as used according to the present invention for example are made from hydrolysable silicates, hydrolysable titanates, reactive, resp. hydrolysable, silanes and/or siloxanes, optionally together with an organic solvent

Hydrolysable silicates to be used in said primer composition are e.g. tetra [ (Ci-C ₈ ) alkyl] -orthosilicates , such as tetraethyl- silicate, tetrapropyl-orthosilicate or tetrabutyl-ortho- silicate; or (Ci-C ₈ ) alkoxy- (Ci-C ₈ ) alkyl-orthosilicates , such as butoxy-ethyl-orthosilicates , and similar related compounds.

Hydrolysable titanates to be used in said primer composition are e.g. tetra [ (Ci-C ₈ ) alkoxy] -titanates , such as tetra [propoxy] - titanate or tetra- [n-butoxy] -titanate; or (Ci-C ₈ ) alkoxy- (Ci-C ₈ ) - alkyl-titanates , such as butoxy-ethyl-titanates , and similar related compounds .

Reactive silanes to be used in said primer composition are e.g. tetraalkoxysilanes , trialkoxyalkylsilanes or dialkoxyalkyl- silanes of the formula: [ (Ci-C ₄ ) alkoxy] _x Si [ (Ci-C ₄ ) alkyl] ₄ _ _x , wherein x is 2, 3 or 4, preferably 3 or 4; e.g. tetraalkoxysilanes such as tetraethoxysilane, tetrapropoxysilane ; or trialkoxy-mono- alkylsilane such as triethoxymethylsilane or tripropoxymethyl- silane, and similar related compounds. Examples of reactive siloxanes to be used in said primer composition are liquid and low viscosity polydimethylsiloxanes , such as polydimethylsiloxanes containing at least one reactive group per molecule, such as an alkoxy group or a hydroxy group or a vinyl group or an epoxy group or another known reactive group, and preferably at least two such groups per molecule or more, preferably selected from ethoxy, propoxy or butoxy groups. Such reactive siloxanes may be linear, branched or cyclic and preferably contain 2-8 siloxy groups per molecule. Such alkoxy- siloxanes optionally may additionally contain one or more vinyl groups or one or more hydrosilane groups .

Examples of solvents to be used in said primer composition are aliphatic hydrocarbons, especially linear (C ₄ -C ₈ ) -alkanes, (C ₄ -C ₈ ) -isoalkanes; ketones such as acetone, or benzene, toluene, xylene, ethylbenzene and related compounds.

There exist different silicone primer compositions on the market. A commercial silicone primer composition for example from Dow Corning contains tetrapropyl-orthosilicate, butoxy- ethyl-orthosilicate , tetrabutyl-titanate and octamethyltri- siloxane. Other silicone primers contain reactive silanes and silicones in solvents such as acetone and isoalkanes. Another commercial silicone primer comprises tetrapropyl-orthosilicate, tetra- [n-butoxy] -titanate; tetrakis ( 2-methoxyethyl ) -orthosili- cate, and solvents such as xylene, 2-methoxyethanol and ethyl- benzene .

The silicone primer composition according to the present invention comprises at least one UV-sensitive crosslinking catalyst which is added to the silicone primer composition in any desired sequence, before, during or after hardening. Such UV-sensitive crosslinking catalysts are known in the art. Such UV-sensitive catalysts for effecting the addition of silicone hydride compounds to vinyl-substituted silanes and siloxanes, generally referred to as hydrosilylation of 2-part silicone rubber compositions, are preferably palladium and/or platinum catalysts .

UV-sensitive crosslinking catalysts can be made from metals of group VIII of the periodic system of elements, preferably from ruthenium (Ru) , osmium (Os), cobalt (Co), rhodium (Rh) , iridium (Ir), nickel (Ni), palladium (Pd) and platinum (Pt) , and represent heterogeneous forms as well as chemical compounds and complexes therefrom. Preferred are such forms made from rhodium, palladium and platinum, especially from palladium and platinum, in heterogeneous forms or as chemical compounds or complexes thereof.

Preferred UV-sensitive crosslinking catalysts are heterogeneous forms of these metals, by using the metal per se or compositions wherein the metal is deposited on a carrier material, such as deposited on charcoal, silica or alumina, preferably on alumina and especially on gamma-alumina. As starting materials, for example H ₂ PtCl ₆ , PdCl ₆ or RhCl ₆ are used and reduced to the metal, so that corresponding metal particles are deposited on the carrier material, with an average particle size which is preferably in nano-size range. Preferred are such heterogeneous platinum catalysts. Such catalysts are known in the art.

Preferred UV-sensitive crosslinking catalysts further are chemical compounds and complexes of metals of group VIII of the periodic system of elements, as mentioned above, used as homogeneous catalysts. Preferred are platinum (0) and platinum (II) compounds, e.g. in the form of complexes.

Preferred platinum containing catalysts are chloroplatinic acid (H ₂ PtCl ₆ ) in anhydrous form or as hexahydrate, or platinum Pt(0) or Pt(II) complexes which have ligands selected from halides, preferably chlorine; (Ci-C ₈ )alkyl radicals; (Ci-C ₈ ) aliphatic unsaturated radicals, e.g. 1, 5-cyclooctadiene (C ₈ Hi ₂ ) , such (C ₈ Hi ₂ ) ₂ PtCl ₂ , or styrene, yielding ( styrene ) ₂ PtCl ₂ .

Preferred are also platinum complexes which are obtained by reacting chloroplatinic acid and complexes thereof with an aliphatic unsaturated organosilane or an aliphatic unsaturated organopolysiloxane compound, such as a vinylsilane or vinyl groups containing organopolysiloxane. Such hydrosilylation catalysts are described for example in US 4,681,963,

US 4,705,765, or US 5,106,939, the contents of which are incorporated herein by reference .

Examples for compounds to be used as UV-sensitive crosslinking catalysts on the basis of palladium (Pd) are Pd ( 0 ) -compounds such tetrakis ( triphenylphosphino ) palladium and the corresponding complexes with the ligands tri- (2-tolyl) phosphine, tris-

( 2-furyl ) phosphine , tris ( tert . -butyl ) phosphine, or the two- valent ligands dppm [1, 1-bis (diphenylphosphinomethan) ] or dppe [1, 2-bis- (diphenylphosphino) ethane] . Pd ( II ) -compounds , such as PdCl ₂ , Pd(dppe)Cl ₂ , Pd(OAc) ₂ , Pd (dppe) (OAc) ₂ , π-allyl-Pd- complex, can also be used. Further complexes to be used are: di-acetylacetonate complexes of Pd, Pt or Ni, and trimethyl-

( cyclopentadienyl ) complexes of Pt(IV), optionally with

different side groups. Such analogous complexes made from metals of group VIII of the periodic system of elements, as mentioned above, preferably are made from ruthenium (Ru) , osmium (Os), cobalt (Co), rhodium (Rh) , iridium (Ir), nickel

(Ni) , palladium (Pd) and platinum (Pt) , preferably made from palladium and platinum, and as analogous compounds made from platinum. Such compounds are known in the art and can be used within the scope of the present invention. Further UV-sensitive crosslinking catalysts are known in the art, such as published in DE 10 2004 036 571 Al or DE 10 2008 000 156 Al , the contents of which are incorporated herein by reference, these UV-sensi- tive crosslinking catalysts can be used according to the present invention.

At least one of such UV-sensitive hydrosilylation crosslinking catalyst as defined herein above is added to the silicone primer composition in the hardenable or hardened state, so that the silicone primer composition according to the present invention comprises said UV-sensitive hydrosilylation crosslinking catalyst in amounts within the range of 1 ppm to 5000 ppm, preferably within the range of 5 ppm to 1000 ppm, preferably 50 ppm to 500 ppm, and preferably about 300 ppm, calculated to the weight of metal contained in the catalyst and the weight of the silicone primer composition, calculated without solvent (s). These weight ratios, however, are not critical, and higher amounts of UV-sensitive crosslinking catalyst may be added if this is indicated.

The silicone primer composition is added to the degreased and cleaned substrate surface . The method of cleaning is known to the expert. The solvent, optionally contained in the silicone primer composition evaporates and the reactive components react with atmospheric humidity to form a uniform dry film. The film is allowed to completely dry and to form a dry hardened film. This film, comprising at least one UV-sensitive hydrosilylation crosslinking catalyst as defined herein above, is then coated with the vulcanizable silicone rubber composition, preferably with a UV vulcanizable silicone rubber composition.

High temperature and low temperature vulcanizable silicone rubber compositions are known per se. UV-curable silicone rubber compositions are also known and generally comprise a two-component system, i.e. at least an organopolysiloxane with at least one alkenyl group (-CH=CH ₂ ) per molecule and at least an organohydrogenpolysiloxane with at least one ≡SiH-group per molecule. In order to activate the crosslinking reaction, i.e. the addition reaction between the alkenyl group and the ≡SiH- groups, the composition comprises a UV-sensitive crosslinking catalysts .

Preferred UV-curable silicone rubber compositions are liquid to pasty and preferably contain (a) at least an organopolysiloxane with an average of at least two alkenyl groups per molecule, said polysiloxane being linear, branched or cyclic or a mixture of such polysiloxanes ; (b) at least one organohydrogenpoly- siloxane, preferably with an average of at least two ≡SiH- groups per molecule, said polysiloxane being linear, branched or cyclic or a mixture of such polysiloxanes, and (c) a catalytic amount of at least one UV-sensitive crosslinking catalyst as defined herein above (as a component of the primer composition) , optionally dissolved in a suitable solvent such as toluene, (C ₄ -C ₈ ) -isoalkanes and similar compounds.

The molar ratio of the of the ≡SiH-groups to the alkenyl groups present in the UV vulcanizable silicone rubber composition is within the range of about 1,5 : 4,5; preferably of 1,8 : 2,5; and preferably is at least 2.0.

The UV vulcanizable hardenable silicone rubber compositions according to the present invention may contain further

additives such as fillers, stabilizers, silicon oils, as known in the art.

Organopolysiloxane with an average of at least two alkenyl groups per molecule [component (a) as mentioned above] , are known per se. Preferred such organopolysiloxanes correspond compounds of formula (I) resp. to a mixture of compounds of formula ( I ) :

R 1 R R 1 R 1

R 1— S i - O— I— S i - O— I— I— S i— O— I— S i— R 1

I I L i I J m L )

I J n i (I

I

R 1 R A R 1

I

C H = C H . wherein

R independent of each other is (Ci-C ₈ ) -alkyl or phenyl,

preferably (Ci-C ₄ ) -alkyl, preferably methyl;

Ri independent of each other has one of the meanings of R or is a residue of formula -A-CH=CH ₂ ;

A is a residue -(C _s H _2s ) _p - , preferably -[(CH ₂ ) _s ] _p - , wherein s is 1 to 6, preferably 1;

p is zero or one;

m is zero to 5000, preferably an average value of 20 to

5000, preferably 50 to 1500;

n is zero to 100, preferably an average number of 2 to 100, preferably 2 to 20;

wherein the compound of formula (I), resp. the mixture of compounds of formula (I), has an average of at least two alkenyl groups of the formula -A-CH=CH ₂ per molecule, and the groups of the formulae -[Si (R) (R) O]- and -[Si (Ri) (A-CH=CH ₂ ) O]- are arranged in an arbitrary sequence.

Compounds of formula (I) generally are a mixture of compounds of formula (I) with different molecular weights. This is known to the expert in the art. The alkenyl group [-A-CH=CH ₂ ] can be for example vinyl, allyl, 3-butenyl, 4-pentenyl, and preferably vinyl (i.e. p = zero) . The terminal -[Si (Ri) ₃ ]-groups, preferably independent of each other, stand for dimethylvinylsilyl , wherein n preferably is zero.

The organopolysiloxanes of component (a) , and preferably of formula (I), preferably have a viscosity within the range of 0.5 to 30' 000 Pas, preferably within the range of 1 to 500 Pas, and preferably within the range of 1 to 100 Pas, measured according to DIN 53 019 at 20°C. The sum of [m + n] is

preferably in the average range of 20 to 5000, preferably within the average range of 50 to 1500.

Organopolysiloxane with an average of at least two ≡SiH-groups per molecule, [component (b) as mentioned above] , are known per se. Preferably component (b) comprises at least a linear, branched or cyclic organohydrogenpolysiloxane with an average of at least two ≡SiH-groups per molecule and preferably corresponds to compound of formula (II) or a mixture of compounds of formula (II) :

R2 R R2 R2

¹ r ¹ -| r I -i I

R2— S i- O—— S i— O— — S i-O— |^-S i- R2 (I I) R2 R H R2 wherein

R independent of each other is (Ci-C ₈ ) -alkyl or phenyl,

preferably (Ci-C ₄ ) -alkyl, preferably methyl;

R ₂ independent of each other has one of the meanings of R or is hydrogen;

p is zero to 5000, preferably an average value of 20 to

5000, preferably 50 to 1500;

q is zero to 60, preferably an average number of 2 to 60, preferably 2 to 30; wherein the compound of formula (II), resp. the mixture of compounds of formula (II) has an average of at least two ≡SiH- groups per molecule, and the groups of the formulae

-[Si (R) (R)O]- and -[SiH(R ₂ )0]- are arranged in an arbitrary sequence .

Compounds of formula (II) generally are a mixture of compounds of formula (II) with different molecular weights. This is known to the expert in the art. The terminal -[Si ( R ₂ ) ₃ ]-groups prefe- rably independent of each other stand for dimethylhydrogen- silyl, whereby q preferably is zero. The organopolysiloxanes of component (b) and of formula (II) preferably have a viscosity within the range of 0.1 to 5 Pas, measured according to DIN 53 019 at 20°C.

Component (b) may represent or comprise a cyclic organo- hydrogenpolysiloxane . In this case, two terminal substituents R ₂ of two different terminals of the molecule form together an oxygen atom. The molecule of compound (II) in this case is composed of -[Si (R) (R)0]-units and of -[SiH ( R ₂ ) 0]-units or only of -[SiH ( R ₂ ) 0]-units , which form the ring structure. Such a cyclic molecule is composed preferably 4 to 8 of such units, preferably of an average of 4 of such units corresponding to the formula [-SiH (R ₂ ) 0-] ₄ .

The UV-sensitive crosslinking catalyst added to the UV-curable silicone rubber compositions is independent from the UV-sensitive crosslinking catalysts added to the silicone primer composition. It is, however, preferred that the same UV-sensitive crosslinking catalyst is added to both compositions.

The UV vulcanizable silicone rubber composition contains the UV-sensitive crosslinking catalyst in amounts within the range of 1 ppm to 5000 ppm, preferably within the range of 5 ppm to 1000 ppm, preferably 50 ppm to 500 ppm, and preferably about 300 ppm, calculated to the weight of metal contained in the catalyst and the sum of the weight of component (a) and component (b) , calculated without solvent (s).

The handling, treatment, mixing and application of the components of the UV hardenable silicone rubber composition is known to the expert in the art and needs no further explanation. The silicone primer composition according to the present invention can also be used together with other known UV- vulcanizable silicone rubber compositions such as curable release coatings which are based on polydimethylsiloxane rubbers that have been functionalized with polymerizable acrylate or epoxy groups or which contain such polymerizable monomeric or oligomeric forms dissolved in the polysiloxane rubber. Such compositions are known to the expert.

For curing the UV-curable silicone rubber composition applied onto the solidified silicone primer composition as defined herein, the UV-curable silicone rubber composition is exposed to UV-irradiation, preferably UVA and/or UVB light, within the range of about 200 nm to about 800 nm and with an irradiation energy of about 1200 mJ/cm ² . As a UV source commercially available lamps can be used, such as Aetek Model QC 1202/N UV processor containing two 200 Watt/inch medium pressure mercury vapor lamps or a fusion system irradiator containing one 300 Watt/inch-bulb .

A special embodiment of the present invention is the combined use of UV-irradiation and IR-irradiation, optionally together with ultrasound, for vulcanizing the UV-curable silicone rubber composition applied onto the hardened silicone primer composition. In this respect, mercury vapor lamps are suitable to combine UV-irradiation with IR-irradiation, as mercury vapor lamps partially irradiate also in the IR range. However, the application of a separate IR source is useful with reference to the effect of improved cure and bonding of the silicone primer composition. Optionally, the IR source can be applied before adding the UV-curable silicone rubber composition to the hardened primer and before applying UV-irradiation.

As a substrate, surfaces of metals, glass, ceramics, or syn- thetic polymers can be used, preferred are surfaces of metals, such as aluminum, aluminum alloys, e.g. known AlMgSi-alloys , chromium/nickel-steels or messing. The following examples illustrate the invention without limiting the scope thereof.

General

In the following, the performed experiments and the obtained results are described.

Materials and sample preparation

The following substrates were used to test the adhesion of the vulcanized UV-vulcanizable silicone rubber composition:

aluminum, stainless steel and ceramic (glass coated) . The substrates were pre-conditioned by grinding with a scotch bright to roughen the surface . Afterwards they were cleaned with acetone for ten minutes in an ultrasonic bath. After rinsing with fresh acetone they were dried and stored in closed boxes until usage.

Used materials

For the evaluation there were used commercially available pri- mers and an UV-curable silicone rubber composition sold by the company Momentive Ltd. Table 1 gives an overview about the trade names of these materials.

Table 1 : List of used materials

Materials :

UV curing silicone rubber Commercial Product, TP 3754,

sold by Momentive Ltd.

Primer Commercial Product, G 790,

sold by Wacker Chemie GmbH

Primer Commercial Product, OSi 1200,

sold by Dow Corning Ltd.

Primer Commercial Product, HV 1860/120, sold by Dow Corning Ltd.

Primer Commercial Product, Primer No. 3, sold by ACC Silicone Ltd.

Primer Commercial Product, (SP-270, SP-120,

SP-135), sold by NuSil Primer Ltd. TP 3754: A UV vulcanizable two-component organopolysiloxane composition with an organovinylpolysiloxane component and an organohydrogenpolysiloxane component, containing about equal molar amounts of vinyl groups and ≡SiH-groups ; containing a platinum based UV-sensitive crosslinking catalyst.

G 790: Main components: C ₇ -Ci ₀ isoalkanes (>75%), titanium

tetrabutanolate (<10%), toluene (<5%), tetraethyl silicate (<5%)

OSi 1200: aliphatic solvents (CAS-No. 64742-89-8) : 85%;

tetrapropylorthosilicate : 5.0%; titaniumtetrabutanoleate : 4.9%;

tetrakis ( 2-methoxy) orthosilicate : 4.9%; other solvents: ad 100% b.w.

HV 1860/120: Naphta (85%), Xylol (6.8%), Tetrapropylorthosilicate (5.0%), Titantetrabutanolat (4.9%), Tetrakis ( 2-methoxyethyl ) - orthosilicate (4.9%) . 2-Methoxyethanol (<=3.0%)

Primer No. 3: C ₉ -Ci ₂ Isoalkanes (60-100%), Tetraaethyl silicate (10-30%), Tetra-n-butoxy titanate (5-10%) .

SP-270, SP-120, SP-135: Main components: Naphta 60-80%, other components below reportable levels.

Example 1

Primer preparation and application

For the performed experiments there were used commercial primers from different suppliers (Table 1) . To these primers a UV activator of the family of platinum and/or palladium based compound [e.g. heterogeneous platinum catalyst deposited on gamma-alumina; trimethyl- ( cyclopentadienyl ) complex of Pt(IV); Platinum ( II ) acetylacetonate (from Fluka AG); or H ₂ PtCl ₆ ] was added in an amount of 3 % by weight of UV-activator added, calculated to the weight of the primer, excluding the solvent. The primer mixtures thus obtained were kept in the dark until usage .

The primers each time were applied manually with a brush to the substrate according manufacturer's instructions. A uniform thin single layer of primer was applied with a clean brush. Afterwards they were stored for 1-2 hours at room temperature under shaded conditions and at a relative humidity of 30-60% (no direct sun exposure) for activation. Curing

A UV-curing silicone rubber layer (TP 3754, of Momentive Ltd.) with a thickness of 3-5 mm was applied to the primered

substrate and immediately subjected to UV light for 3 minutes.

The light source used was a Dymax Light Welder® 5000-EC, with light shield and zip shutter. It had a 400W power supply and an iron doped Mercury bulb ("D-bulb") , providing a UV spectra in the range of 260nm to 460nm.

Before performing the adhesion test, the samples were kept under room conditions for at least 24 hours.

Evaluating the adhesion

The adhesion was assessed by a manual peel test of the applie silicone. The testing was performed as follows: With the use a razor blade, a stripe of silicone was cut off along the interface "substrate-silicone" so that a butt strap ("Lasche" of a few centimeters was obtained. Afterwards there was pulle on the butt strap, parallel to the substrate surface and in direction of the still adherent silicone. Depending on the failure mode, the adhesion was categorized into 3 different levels, as follows: zero = no adhesion, the pulled off silicone stripe can be removed without breaking and without visual residuals on the substrates .

1 = partial adhesion, the pulled off silicone stripes can be partially pulled off, but with some visual silicone residuals on the substrate.

2 = good adhesion, the pulled off silicones stripes breaks in cohesive failure. The adhesion force is stronger than the silicone strength. Results

The adhesion was evaluated by a manual peel test as described above. Different commercial primers have been used as listed in Table 1. All of them are suitable for thermal cured silicones. Some of them are especially tailored to bond well to aluminum substrates, the others are tailored to bond to steel and ceramic. In Table 2 the adhesion results are shown for the primers which are suitable for aluminum surfaces and in

Table 3 there are shown the results for the primers suitable for steel and ceramic substrates. As a comparison, the results obtained with the same primer, but modified according to the present invention by adding an UV activator thereto, are shown.

The results show, that a significant improvement of the adhesion of the silicone to the substrate surface is achieved, when the primer contains a UV activator. These results indicate that adding an UV curing activator to commercial primers surprisingly improves significantly the adhesion. Further and surprisingly, these important benefits are achieved without the need of a thermal heating step.

Table 2 : Adhesion levels to aluminum substrates without and with the addition of UV activator.

Aluminum

Primer no UV catalyst in the addition of UV

primer catalyst to the

primer

G 790 zero 1

OSi 1200 zero 2

HV 1860/120 zero 2 Table 3 : Adhesion levels to steel und ceramic substrates without and with the addition of UV activator.

Additional experiments with IR free UV light ("cold"-UV light) There were carried out additional experiments by the usage of a "cold"-UV-light source, using a LED lamp (Wavelength: 365nm, "starfire", supplied by Phoseon) . In difference to a mercury lamp, the LED does not irradiate IR light and therefore the sample is only slightly heated up. Two Primers were used as given in Table 4 :

Table 4 : List of used materials for experiments with the LED

ester (10-20%) , 1 , 2 , 4-Trimethylbenzol (1-2.5%), 1,3,5- Trimethylbenzol (0.3-2-5%), Naphthalene (0.1-0.3%) .

There were added to the primer 300 ppm of Pt(II) in the form of a di-acetylacetonate complex d spersed in dichloromethane . The sample preparation, curing and evaluation was performed in the same way as described above. The obtained results are shown in Table 5. It can be seen, that it is possible to obtain also with cold UV light same positive effect on adhesion as with a mercury lamp.

Table 5 : Adhesion levels to steel und ceramic substrates without and with the addition of UV activator and curing with LED lamp.

Improvements and benefits

- Achieving an improved adhesion of UV-cured silicones on s strates which is clearly better than the adhesion obtaine with commercial primers at room temperature.

- No thermal curing step for the primer is required. This yields a decrease of processing steps and therewith an increase of productivity.

- Enabling the use of UV cured silicones for electric

applications where inserts are required.

Example 2

Example 2 illustrates the method of first applying a primer composition (not containing a UV-catalyst) onto the surface and hardening said primer composition. Then the UV-catalyst is added with subsequent application of the UV-curable silicone layer .

Materials and sample preparation

The following substrates were used to test the adhesion of the vulcanized UV-vulcanizable silicone rubber composition: stainless steel and ceramic (glass coated) . The substrates were preconditioned by grinding with a scotch bright to roughen the surface . Afterwards they were cleaned with acetone for ten minutes in an ultrasonic bath. After rinsing with fresh acetone they were dried and stored in closed boxes until usage.

Used materials

For the evaluation commercially available primers and a UV- curable silicone resin compositions were used. Table 6 gives an overview about the trade names of the materials.

Table 6: List of used materials

Primer preparation and application

Step I : For the performed experiments commercial primers from two different suppliers and a UV-curable silicone composition (Table 6) were used. The primers were applied manually to the cleaned surface with a brush according manufacturers instructions. Afterwards they were let dry for 30 to 60 minutes at ambient temperature .

Step II: In a second step a solution of Platinum ( II ) acetyl- acetonate (from Fluka AG) , dissolved in dichloromethane was applied with a brush. The concentration of the Platinum ( II ) acetylacetonate in dichloromethane was 0.6 wt% .

Curing: After the substrate treatment according to step I and step II, a silicone layer of 3-5 mm was applied to the primed substrates. The substrates were then immediately subjected to LED UV-light for 3 minutes (emitted wave length: 365 nm) .

Description of Samples

Three sets of experiments were carried out using in each case steel and ceramic substrates, but varying the preparation method : In Experiment I the primer was applied as described in step I. Afterwards the substrate was heated up to 90 °C and the UV- curing silicone was applied.

In Experiment II the primer was applied as described in step I. Afterwards the UV-curing silicone was applied at room temperature, without heating-up of the substrate.

In Experiment III the primer was applied as described in step I and step II. Afterwards the UV-curing silicone was applied at room temperature, without heating up of the substrate. The substrates were then subjected to LED UV-light for 3 minutes (emitted wave length: 365 nm) .

Evaluating the adhesion

The adhesion was assessed by a manual peel test, as defined herein above under "Evaluating the adhesion".

Results

The two different commercial primers used are suitable for thermal cured silicones and are made to bond to steal and ceramic substrates. In Table 7 and Table 8 the adhesion results on steel substrates and on ceramic substrates are shown.

Table 7 : Adhesion levels to steel substrates

Primer Experiment I Experiment II Experiment III

Preparation according Preparation Preparation accStep I and cured at according Step I ording step I & 90°C and cured at room step II and UV- temperature cured at room

temperature

TP 3980 2 0 2

Primer No. 3 2 0 1-2 Table 8 : Adhesion levels to ceramic Substrates

From the results the following conclusions can be drawn:

1. If heating-up the substrates, both primer work well with silicone (Experiment I) .

2. If not heating-up the substrates, the primers poorly wor with silicone (Experiment II) .

3. If applying an additional layer of UV-activator on top o the primer, both primers give good adhesion to the silicone without heating-up the substrate (Experiment III).

Experiment III shows, that there can be achieved a significant improvement of the adhesion to the silicone when adding a thin layer of UV activator on top of the primers. These results lead to the conclusion, that a simple way has been found to obtain strong adhesion to UV-curing silicones without the need of introducing a thermal heating step and without the need of developing tailor made primers for UV-curing silicones.