A process for carrying out an impregnation on sheet gaskets at the engine block/cylinder head interface of engines, comprising the step of applying to said sheet gaskets, a composition comprising (i) an effective catalytic amount of at least one initiator salt (PI), and (2i) at least one polyorganosiloxane (POS) crosslinkable by a cationic route, said initiator salt (PI) is a borate of an onium of an element from groups 15 to 17 of the Periodic Classification or of an organometallic complex of an element from groups 4 to 10 of the Periodic Classification, —the cationic entity of said borate being selected from the group consisting of
- 1) onium cations of the formula (I):
- [(R1)m=A=(R2)m]+ (I)
- wherein:
- A represents an element from groups 15 to 17,
- R1 represents a C6-C20 carbocyclic aryl radial or a C6-C20 heterocyclic aryl radical, said heterocyclic radical comprising nitrogen or sulphur as heteroelements;
- R2 represents R1, a linear C1-C30 alkyl, a branched C1-C30 alkyl, a linear C1-C30 alkenyl radical or a branched C1-C30 alkenyl;
- the said R1 and R2 radicals optionally being substituted by a C1-C25 alkoxy, C1-C25 alkyl, nitro, chloro, bromo, cyano, carboxy, ester or mercapto group;
- n is an integer ranging from 1 to v+1, v+1, v being the valency of the element a;
- and
- m is an integer ranging from 0 to v+1, with n+m=v+1;
- 2) oxoisothiochromanium cations having the formula:
- wherein the R6 radical represents a linear C1-C20 alkyl radical or a branched C1-C20 alkyl radical;
- 3) sulphonium cations wherein the cationic entry comprises:
- 3.1) at least one polysulphonium species of formula III.1
- wherein
- the Ar1 symbols, which can be identical to or different from one another, each represent a monovalent phenyl or a naphthyl radical, optionally substituted with one or more radicals selected from the group consisting of a linear C1-C12 alkyl radical, a branched C1-C12 alkyl radical, a linear C1-C12 alkoxy radical, a branched C1-C12 alkoxy radical, a halogen atom, an —OH group, a —COOH group, a —COO— C1-C12 alkyl ester group, and a group of formula —Y4—Ar2, wherein:
- the Ar2 symbols, which can be identical to or different from one another or Ar1, each represent a monovalent phenyl or naphthyl radical optionally substituted with one or more radicals selected from the group consisting of a linear or branched C1-C12 alkyl radical, a linear or branched C1-C12 alkoxy radical, a halogen atom, an —OH group, a —COOH group and a —COO— C1-C12 alkyl ester group;
- the Ar3 symbols, which can be identical to or different from one another, each represents a divalent phenylene or naphthylene radical optionally substituted with one or more radicals selected from the group consisting of a linear C1-C12 alkyl radical, a branched C1-C12 alkyl radical, a linear C1-C12 alkoxy radical, a branched C1-C12 alkoxy radical, a halogen atom, an —OH group, a —COOH group and a —COO— C1-C12 alkyl group;
- t is an integer equal to 0 or 1, with the provision that:
- when t=0, the Y symbol is then a Y1 monovalent radical representing the group of formula:
- wherein the Ar1 and Ar2 symbols have the meanings given above,
- where t=1;
- on the one hand, the Y symbol is then a divalent radical having the following meanings Y2 to Y4:
- Y2;
- a group of formula:
- wherein the Ar2 symbol has the meanings given above;
- Y3:
- a single valency bond;
- Y4:
- a divalent residue selected from the group consisting of:
- a linear C1-C12 alkylene residue, a branched C1-C12 alkylene residue and a residue of formula —Si(CH3)2O—;
- on the other hand, solely in the case where the Y symbol represents Y3 or Y4, the Ar1 and Ar2 radicals optionally being connected to one another via a valency bond or —O— 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) or at least one monosulphonium species having a single S+ cationic centre per mole of cation and comprising a species of formula:
- wherein Ar1 and Ar2 have the meanings given with respect to the formula (III.1), and are optionally connected directly between then according to a valency bond or —O—;
- and
- 4) organometallic cations of formula (IV):
- (L1L2L3M)4′ (IV)
- wherein:
- M represents a metal from group 4 to 10;
- L1 represents a ligand bonded to the metal M via p electrons, which ligand is η3-alkyl, η3-cyclopentadienyl, η7-cycloheptatrienyl ligands, optionally substituted η6-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 p electrons;
- L2 represents a ligand bonded to the metal M via p elements, which ligand is chosen from η7-cycloheptatrienyl ligands, optionally substituted η6-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 to 7 p electrons,
- L3 represents from 0 to 3 identical or different ligands bonded to the metal M via a electrons, which ligand is CO or NO2+;
- the total electronic charge q of the complex to which L1, L2 and L3 and the ionic charge of the metal M contribute being positive and equal to 1 or 2;
- and
- the anionic entity of the said borate having the formula:
- [BXaRb]+
- wherein:
- 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;
- or
- 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, or by at least 2 halogen atoms, 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, when the cationic entity is an organometallic complex of an element from groups 4 to 10;
- an aryl radical comprising at least two aromatic nucleic, optionally substituted by at least one electron-withdrawing element or group, whatever the cationic entity;
- and
- (2i) the polyorganosiloxanes comprises at least one monomer, one oligomer or one polymer which is:
- a compound comprising at lest one organofunctional bridging group crosslinkage by the cationic route (Gfp) with a heterocyclic nature having one or more electron-donating atoms;
- or
- a compound comprising at least one ethylenically unsaturated Gfp group substituted by at least one electron-donating atom which increases the basicity of the p system;
- and
- (3i) the Gfp5 are present in a proportion expressed in equivalent per kg of polyorganosiloxane of at least 0.01;
- and
- wherein the sheet gaskets are further preimpregnated with at least one material for rendering leaktight formed by at least one crosslinked polymer, or are precoated with at least one layer of at least one elastomer.