Use of silicone compositions crosslinkable by the cationic route under UV radiation and of a photoinitiator of the onium borate type for coatings for sheet gaskets, in particular cylinder head gaskets

Abstract

The invention concerns the use of cross-linkable silicone compositions for fast, economical and simple impregnation and/or varnishing of flat joints (e.g. cylinder head gaskets). More precisely it concerns the use of silicone compositions cross-linkable under UV radiation, by cationic process and in the presence of specific phoinitiators selected among onium borates or organometallic complexes, of which the borate counter-anions contain at least a boron bound to at least a substituted phenyl (Me, F). The silicone liquid precursor is a polydimethylsiloxane (PDMS) substituted by cross-linking functional groups, by cationic process (G f p ) under UV of the epoxy or vinyloxy type. These G f p are present at the rate of 0.15 to 2.0 pr kg of PDMS. The invention also relates to the method for impregnating/varnishing flat joints (e.g. cylinder head gaskets) using the specific PDMS composition+photoinitiator of borate type as well as the treated joints and the composition themselves.

1 . Use for carrying out impregnation(s) and/or for preparing coating(s) which is (are) antiadhesive employed at the engine block/cylinder head interface of engines and applied to sheet gaskets, in particular cylinder head gaskets, of compositions based on at least one polyorganosiloxane (POS) crosslinkable by the cationic route and on an effective catalytic amount of at least one initiator salt (PI), the said use being characterized in that: 
 (i) the initiator salt (PI) is formed by a borate of an onium of an element from groups 15 to 17 of the Periodic Classification [Chem. & Eng. News, Vol. 63, No. 5, 26 of Feb. 4, 1985] or of an organometallic complex of an element from groups 4 to 10 of the Periodic Classification (same reference), 
 □ the cationic entity of the said borate being chosen from:  
   1) onium cations of the formula (I):    [ R   1 ) n   −A− ( R   2 ) m ] +   (I)    in which formula: 
 A represents an element from groups 15 to 17, such as I, S, Se, P or N,  
 R 1  represents a C 6 -C 20  carbocyclic or heterocyclic aryl radical, it being possible for the said heterocyclic radical to comprise nitrogen or sulphur as heteroelements,  
 R 2  represents R 1  or a linear or branched C 1 -C 30  alkyl or alkenyl radical; the said R 1  and R 2  radicals optionally being substituted by a C 1 -C 25  alkoxy, C 1 -C 25  alkyl, nitro, chloro, bromo, cyano carboxy, ester or mercapto group,  
 n is an integer ranging from 1 to v+1, v being the valency of the element a,  
 m is an integer ranging from 0 to v−1, with n+m=v+1;  
   2) oxoisothiochromanium cations having the formula:                          where the R 6  radical represents a linear or branched C 1 -C 20  alkyl radical;    3) sulphonium cations where the cationic entity comprises: 
 → 3.1. at least one polysulphonium species of formula III.1  
                     
 in which:  
 the Ar 1  symbols, which can be identical to or different from one another, each represent a monovalent phenyl or naphthyl radical optionally substituted with one or more radicals chosen from: a linear or branched C 1 -C 12  alkyl radical, a linear or branched C 1 -C 12  alkoxy radical, a halogen atom, an —OH group, a —COOH group, a —COO-alkyl ester group, where the alkyl part is a linear or branched C 1 -C12 residue, and a group of formula —Y 4 —Ar 2,  where the Y 4  and Ar 2  symbols have the meanings given immediately below,  
 the Ar 2  symbols, which can be identical to or different from one another or Ar 1 , each represent a monovalent phenyl or naphthyl radical optionally substituted with one or more radicals chosen from: a linear or branched C1-C 12  alkyl radical, a linear or branched C 1 -C, 12  alkoxy radical, a halogen atom, an —OH group, a —COOH group or a —COO-alkyl ester group, where the alkyl part is a linear or branched C 1 -C 12  residue,  
 the Ar 3  symbols, which can be identical to or different from one another, each represent a divalent phenylene or naphthylene radical optionally substituted with one or more radicals chosen from: a linear or branched C 1 -C 12  alkyl radical, a linear or branched C 1 -C 12  alkoxy radical, a halogen atom, an —OH group, a —COOH group or a —COO-alkyl ester group, where the alkyl part is a linear or branched C 1 -C 12  residue,  
 t is an integer equal to 0 or 1, with the additional conditions according to which:  
   + when t=0, the Y symbol is then a Y 1  monovalent radical representing the group of formula:                          where the Ar 1  and Ar 2  symbols have the meanings given above,    +when t=1: 
 {circle over (10)} on the one hand, the Y symbol is then a divalent radical having the following meanings Y 2  to Y 4 : 
 Y 2 : a group of formula:  
                     
 where the Ar 2  symbol has the meanings given above,  
 Y 3 : a single valency bond,  
 Y 4 : a divalent residue chosen from:  
                     
 a linear or branched C 1 -C 12  alkylene residue and a residue of formula —Si(CH 3 ) 2 O—,  
 
   ★ on the other hand, solely in the case where the Y symbol represents Y 3  or Y 4 , the Ar 1  and Ar 2  (terminal) radicals have, in addition to the meanings given above, the possibility of being connected to one another via the Y′ residue consisting of Y′ 1 , a single valency bond, or of Y′ 2 , a divalent residue chosen from the residues cited with respect to the definition of Y 4 , which is inserted between the carbon atoms, facing each other, situated on each aromatic ring in the ortho position with respect to the carbon atom directly bonded to the S +  cation;    → 3.2. and/or at least one monosulphonium species having a single S +  cationic centre per mole of cation and comprising, in the majority of cases, species of formula:                          in which Ar 1  and Ar 2  have the meanings given above with respect to the formula (III.1), including the possibility of connecting directly between them only one of the Ar 1  radicals to Ar 2  according to the way indicated Above with respect to the definition of the additional condition in force when t=1 in the formula (II) involving the Y' residue;    4) organometallic cations of formula (IV):    (L 1 L 2 L 3 M)q +   (IV)    in which formula: 
 M represents a metal from group 4 to 10 chosen from iron, manganese, chromium or cobalt,  
 L 1  represents a ligand bonded to the metal M via π electrons, which ligand is chosen from η 3 -alkyl, η 5 -cyclopentadienyl and η 7 -cycloheptatrienyl ligands and η 6 -aromatic compounds chosen from optionally substituted η 6 -benzene ligands and compounds having from 2 to 4 condensed rings, each ring being capable of contributing to the valency layer of the metal M via 3 to 8 π electrons,  
 L 2  represents a ligand bonded to the metal M via π electrons, which ligand is chosen from η 7 -cycloheptatrienyl ligands and η 6 -aromatic compounds chosen from optionally substituted η 6 benzene ligands and compounds having from 2 to 4 condensed rings, each ring being capable of contributing to the valency layer of the metal M via 6 or 7 π electrons,  
 L 3  represents from 0 to 3 identical or different ligands bonded to the metal M via σ electrons, which ligand(s) is(are) chosen from CO and NO 2   + ; the total electronic charge q of the complex to which L 1 , L 2  and L 3  and the ionic charge of the metal M contribute being positive and equal to 1 or 2;  
   □ the anionic entity of the said borate having the formula:    [BX a R b ] −     in which formula: 
 a and b are integers ranging from 0 to 3 for a and from 1 to 4 for b, with a+b=4,  
 the X symbols represent:  
 ★ a halogen atom with a=0 to 3,  
 ★ an OH functional group with a=0 to 2,  
 the R symbols are identical or different and represent:  
 a phenyl radical substituted by at least one electron-withdrawing group, such as OCF 3  CF 3 , NO 2  or CN, and/or by at least 2 halogen atoms, this being when the cationic entity is an onium of an element from groups 15 to 17,  
 a phenyl radical substituted by at least one electron-withdrawing element or group, such as a halogen atom, CF 3  OCF 3 , NO 2  or CN, this being when the cationic entity is an organometallic complex of an element from groups 4 to 10,  
 an aryl radical comprising at least two aromatic nuclei, such as biphenyl or naphthyl, which is optionally substituted by at least one electron-withdrawing element or group, such as a halogen atom, OCF 3 , CF 3 , NO 2  or CN, whatever the cationic entity;  
   (2i) the POS comprises at least one monomer and/or one oligomer and/or one polymer selected: 
 from compounds comprising at least one (organo)functional bridging group crosslinkable by the cationic route (G f     p   ) with a heterocyclic nature having one or more electron-donating atoms, such as O, S, N and P,  
 and/or from those comprising at least one ethylenically unsaturated G f     p   group which is substituted by at least one electron-donating atom which increases the basicity of the π system, and  
   (3i) the G f     p   s are present in a proportion (expressed in eq per kg of POS) of at least 0.01.    
     
     
         2 . Use according to  claim 1 , characterized in that the POS employed exhibits G f     p   s of the epoxide and/or vinyl ether type and is selected from the POSs which are: 
 → a either linear or substantially linear and composed of units of formula (V), terminated by units of formula (VI),    → or cyclic and composed of units of formula (V):                          in which formulae:    the R 1  symbols are alike or different and represent: 
 either a linear or branched C 1 -C 6  alkyl radical which is optionally substituted, advantageously by one or more halogens,  
 or an optionally substituted C 5 -C 6  cycloalkyl radical,  
 or an aryl or aralkyl radical which is optionally substituted, in particular by halogens and/or alkoxys,  
   the Z symbols are alike or different and represent: 
 either the R 1  radical,  
 or a G f     p    group corresponding to an epoxide or vinyl ether residue connected to the silicon via a divalent radical comprising from 2 to 20 carbon atoms optionally comprising a heteroatom,  
   at least one of the Z symbols corresponding to a G f     p    group.    
     
     
         3 . Use according to  claim 1  or  2 , characterized in that the POS has a viscosity η (expressed in mPa.s at 25° C.) of between 200 and 3000.  
     
     
         4 . Use according to  claim 1  or  2 , characterized in that the POS has a viscosity η (expressed in mPa.s at 25° C.) of between a value of greater than 3000 and 10,000.  
     
     
         5 . Use according to any one of  claims 1  to  4 , characterized in that the (photo)initiator (PI) comprises a borate anionic entity selected from the following group:  
       [B(C 6 F 5 ) 4 ] − [B(C 6 H 4 CF 3 ) 4 ] − [B(C 6 H 3 (CF 3 ) 2 ) 4 ] − [(C 6 F 5 ) 2 BF 2 ] − [C 6 F 5 BF 3 ] − [B(C 6 H 3 F 2 ) 4 ] − , [B(C 6 F 4 OCF 3 ) 4 ] −   
       and their mixtures.  
     
     
         6 . Use according to any one of  claims 1  to  5 , characterized in that the cationic entity of the PI is of the onium type and is chosen from the following entities:  
       [(Φ) 2 I] + [C 8 H 17 —O-Φ-I-Φ] + [(Φ-CH 3 ) 2 I] + [C 12 H 25 -Φ-I-Φ] + [(C 8 H 17 —O-Φ) 2 I] + [(C 8 H 17 —OΦ-I-Φ)] + [(Φ) 3 S] + [(Φ) 2 -S-Φ-O—C 8 H 17 ] + [(CH 3 -Φ-I-ΦCH(CH 3 ) 2 ] + [Φ-S-Φ-S-(Φ) 2 ] + [(C 12 H 25 -Φ) 2 I] + [(CH 3 -Φ-I-ΦOC 2 H 5 ] +   
     
     
         7 . Use according to any one of  claims 1  to  5 , characterized in that the cationic entity of the PI is an organometallic cation chosen from the following group: 
 (η 5 -cyclopentadienyl) (η 6 -toluene)Fe +   
 (η 5 -cyclopentadienyl) (η 6 -1-methyl-naphthalene)Fe +   
 (η 5 -cyclopentadienyl) (η 6 -cumene) Fe +   
 bis (η 6 -mesitylene) Fe +   
 bis (η-benzene) Cr + .  
 
     
     
         8 . Use according to any one of  claims 1  to  7 , characterized in that the PI is an onium borate and/or a borate of organometallic cations chosen from the following group:  
       [(Φ) 2 I] + [B(C 6 F 5 ) 4 ] − [(C 8 H 17 )—OΦ-I-Φ] + [B(C 6 F 5 ) 4 ] − [C 12 H 25 -Φ-I-Φ] + [B(C 6 F 5 ) 4 ] − [(C 8 H 17 —O-Φ) 2 I] + [B(C 6 F 5 ) 4 ] − [(C 8 H 17 )—O-Φ-I-Φ] + [B(C 6 F 5 ) 4 ] − [(Φ) 3 S] + [B(C 6 F 5 ) 4 ] − [(Φ) 2 S-Φ-O—C 8 H 17 ] + [B(C 6 F 5 ) 4 ] − [(C 12 H 25 -Φ) 2 I] + [B(C 6 F 5 ) 4 ] − [(Φ) 3 S] + [B(C 6 F 4 OCF 3 ) 4 ] − [(Φ-CH 3 ) 2 I] + [B(C 6 F 5 ) 4 ] − [(Φ-CH 3 ) 2 I] + [B(C 6 F 4 OCF 3 ) 4 ] −   (η 5 -cyclopentadienyl)(η 6 -toluene)Fe + [B(C 6 F 5 ) 4 ] −     (η 5 -cyclopentadienyl)(η 6 -1-methylnaphthalene)-Fe 30  [B(C 6 F 5 ) 4 ] −     (η 5 -cylclopentadienyl) (η 6  -cumene)Fe + [B(C 6 F 5 ) 4 ] − .    
     
     
         9 . Use according to any one of  claims 1  to  8 , characterized in that the initiator (PI) is employed in solution in an organic solvent chosen from proton-donating solvents.  
     
     
         10 . Use according to any one of  claims 1  to  9 , characterized in that the support receiving the silicone coating obtained by crosslinking by the cationic route is a sheet gasket, in particular a cylinder head gasket, 
 ∘ which is preimpregnated with at least one material for rendering leaktight formed by at least one crosslinked polymer  
 ∘ and/or which is precoated with at least one layer of at least one elastomer.  
 
     
     
         11 . Use according to any one of  claims 1  to  10 , characterized in that the coating obtained by crosslinking by the cationic route is mono- or multilayer and exhibits a thickness of between 1 and 100 μm.  
     
     
         12 . Process for carrying out impregnation(s) and/or for preparing coating(s) which is (are) antiadhesive at the engine block/cylinder head interface of engines on supports composed of sheet gaskets, in particular cylinder head gaskets, characterized in that it consists essentially in employing a composition which is crosslinkable under the effect of light radiation and optionally under the effect of heat, the said composition and the support being those as defined in any one of  claims 1  to  11  relating to the use.  
     
     
         13 . Sheet gasket, in particular cylinder head gasket, characterized in that it is impregnated and/or coated with a matrix obtained by crosslinking by the cationic route in accordance with the use as defined in  claims 1  to  11  and/or with the process according to  claim 12 .  
     
     
         14 . Silicone composition crosslinkable by the cationic route, characterized in that it is intended for carrying out impregnation(s) and/or for preparing coating(s) which is (are) antiadhesive on sheet gaskets, in particular cylinder head gaskets, and in that it comprises: 
 at least one POS possessing G f     p    groups as defined in any one of  claims 1  to  3 ,    at least one initiator, preferably a photoinitiator (PI), of an onium borate type as defined in any one of claims  1  and  5  to  8 ,    at least one solvent of the PI as defined in  claim 9 ,    and at least one additive chosen from those generally employed in treatments targeted at rendering leaktight and antiadhesive the cylinder head/engine block interface and in particular sheet gaskets, such as, especially, cylinder head gaskets.