Description

METHOD FOR IMPROVING COEFFICIENT OF FRICTION IN CURED FLUORORUBBER PARTS

Cross-Reference to Related Application

[0001 ] This application claims priority to earlier filed on 22 November 2019 filed with the European Patent Office with filing number 19210808.2, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

[0002] The present disclosure relates to a method for making fluororubber parts possessing improved sliding properties, and to fluororubber parts obtained therefrom.

Background Art

[0003] Fluoroelastomers are rubber-like materials that are widely used as seals or sealing component in articles exposed to fuels because of their high chemical resistance to these compounds.

[0004] Cured articles obtained from these fluoroelastomers find the most suitable employment as seals (gaskets) in general, both in static and dynamic conditions, in a variety of fields of use, including automotive, aerospace, naval, mechanics, chemical industry, and the like.

[0005] In all these fields of use, surface of the cured fluororubber part is often required to exhibit sliding properties. This may notably occur at the time the fluororubber cured part is placed in its final positions in complex sealing assemblies: during mounting and assembling, fluororubber part has to possess ability to e.g. slide into the groove or encasing whereas it has to play its sealing role, requiring hence low coefficient of friction. Further, in certain fields of use, fluororubber part may be called to seal to moving parts: also in this case, ability to slide and move without stick/slip phenomena is particularly advantageous. Actually, events of “stick-slip” occur when moving surfaces alternate between sticking to each other and sliding over each other, with a corresponding change in the force of friction. In these circumstances, typically, the static friction coefficient between the said two surfaces is larger than the kinetic friction coefficient: so, if an applied force is large enough to overcome the static friction, then the reduction of the friction to the kinetic friction can cause a sudden jump in the velocity of the movement, causing hence such stick/slip phenomena.

[0006] Provision of cured fluororubber parts having improved friction coefficient (CoF), in particular both lower static and dynamic CoF, is hence a desirable attribute since long time.

[0007] In this area, the use of perfluoropolyether (PFPE) derivatives has been already explored for addressing this problem.

[0008] Now, when PFPE oils are blended into fluororubbers or applied onto the surface of cured parts, the lack of miscibility cause the oily additive to exudate, to provide for a wet/oily touch at the surface, and to be bled out and/or to leach out, so that long-term improvement in CoF is not achieved.

[0009] Flence, alternative approaches including chemical anchoring of the PFPE to the fluororubber have been pursued. For instance, EP0805180 describes the use of mono- and/or di-hydroxy-polyfluoroethers having fluoropolyoxyalkylenic repeating of formulae: (C3F6O); (CFXO) wherein X=F, CF3; (CFI2CF2CF2O); (CF ₂ (CF ₂ ) _Z CF ₂ 0) wherein z is an integer equal to 1 or 2; (C2F4O), in curable compositions of fluoroelastomers to improve the coefficient of friction without impairing other properties. In particular, this document teaches the use of hydroxyl-containing polyfluoroethers in ionically curable fluoroelastomer formulations: ionic route curing is taught as involving hydroxyl groups of said derivatives, to anchor the additive to the cured fluororubber matrix. Nevertheless, in this manner, the lubricating oil is present throughout the entire shaped cured article, so affecting the overall properties of the cured part; further, considering the relatively high cost of the said derivatives, the lack of selective application on the surface renders this solution economically not attractive. Finally, chemistry thereof is not compatible with peroxide curing.

[0010] Further, certain documents have already disclosed combinations of functional derivatives of perfluoropolyethers (PFPE) and fluororubbers, to the aim of addressing other problems; for instance, US5266650 teaches curable compositions comprising a fluororubber and a PFPE having a functional group reactive with the said fluororubber, which can be notably a hydroxyl group, a primary amino or allyl functional groups, depending on the curing system used (ionic, peroxide...); US5700861 discloses vinylidene fluoride-based fluororubber coating compositions including a PFPE oil having a functional group which is capable of chemically bonding to the fluororubber, and a vulcanizing agent; said coating compositions are intended to provide for permanent non-tackiness (water- and oil- repellency) to the substrate on which they are applied. The said functional groups of the PFPE may be notably H, Cl, Br, I, amino, tio-, isocyanate, carboxylic, phosphonic, sulfonic, hydroxyl, epoxy.

[0011] There remain a need in the art for providing an improved method of making cured fluoroelastomer parts maintaining all the advantageous attributes typical of fluororubbers (e.g. chemical and thermal resistance characteristics), and possessing short and long-term improved friction properties, in particular lower coefficient of friction, both static and dynamic, through optimized use of lubricating additives.

Summary of invention

[0012] Surprisingly, the Applicant has found that applying to a partially cured fluoroelastomer part, obtained by a first step of peroxide curing at limited temperature, a functional PFPE possessing certain reactive groups, and then finishing curing of the said part through post-cure, it is possible to obtain a cured part possessing stable improved coefficient of friction. [0013] Therefore, there is provided a method of making a cured fluoroelastomer surface-modified part [part (P _C ^SM )], comprising:

(1) a step of shaping a peroxide curable composition [composition (C)] comprising:

- at least one (per)fluoroelastomer comprising iodine and/or bromine cure sites [fluoroelastomer (A)];

- at least one organic peroxide [peroxide (O)];

- at least one polyunsaturated compound [compound (U)]; at a temperature of at most 165°C, so as to obtain a partially cured fluoroelastomer part [part (P _pc )]; and

(2) a step of applying on at least a part of the surface of the said part (P _pc ) a composition (C2) comprising at least one fluoropolyether compound having a (per)fluoropolyoxylakylene chain [chain (R _f )] and comprising at least one of iodine and bromine atoms, so as to obtain a partially cured fluoroelastomer surface-treated part [part (P _P c ^ST )]; and

(3) a step of completing curing the said part (P _c ^ST ) by exposing the same to a temperature of at least 170°C, so as to obtain the target part (P _C ^SM ).

[0014] In another aspect, there is provided a cured fluoroelastomer surface- modified part [part (P _C ^SM )], obtainable from the method described above. Description of embodiments

[0015] For the purposes of this invention, the term “fluoroelastomer” as used for designating fluoroelastomer (A) is intended to designate a fluoropolymer resin serving as a base constituent for obtaining a true elastomer, said fluoropolymer resin comprising more than 10 % wt, preferably more than 30 % wt, of recurring units derived from at least one ethylenically unsaturated monomer comprising at least one fluorine atom (hereafter, (per)fluorinated monomer) and, optionally, recurring units derived from at least one ethylenically unsaturated monomer free from fluorine atom (hereafter, hydrogenated monomer).

[0016] True elastomers are defined by the ASTM, Special Technical Bulletin,

No. 184 standard as materials capable of being stretched, at room temperature, to twice their intrinsic length and which, once they have been released after holding them under tension for 5 minutes, return to within 10 % of their initial length in the same time.

[0017] Fluoroelastomers (A) are in general amorphous products or products having a low degree of crystallinity (crystalline phase less than 20 % by volume) and a glass transition temperature (T _g ) below room temperature. In most cases, the fluoroelastomer (A) has advantageously a T _g below 10° C, preferably below 5°C, more preferably 0°C, even more preferably below -5°C.

[0018] Fluoroelastomer (A) may optionally further comprise recurring units derived from one or more than one monomer free from fluorine (hydrogenated monomer, herein after). Examples of hydrogenated monomers are notably C ₂ -C ₈ non-fluorinated olefins (Ol), in particular C ₂ - Ce non-fluorinated alpha-olefins (Ol), including ethylene, propylene, 1- butene; diene monomers; styrene monomers; C ₂ -C ₈ non-fluorinated alpha- olefins (Ol), and more particularly ethylene and propylene, will be selected for achieving increased resistance to bases.

[0019] Non limitative examples of suitable (per)fluorinated monomers, recurring units derived therefrom being comprised in the fluoroelastomer (A) are notably:

(a) C ₂ -C ₈ perfluoroolefins , such as tetrafluoroethylene (TFE) and hexafluoropropylene (HFP);

(b) hydrogen-containing C ₂ -C ₈ , such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene (TrFE), perfluoroalkyl ethylenes of formula CFI ₂ = CFI-R _f , wherein R _f is a C ₁ -C6 perfluoroalkyl group;

(c) C ₂ -C ₈ chloro and/or bromo and/or iodo-fluoroolefins such as chlorotrifluoroethylene (CTFE);

(d) (per)fluoroalkylvinylethers (PAVE) of formula CF ₂ =CFOR _f , wherein R _f is a C ₁ -C6 (per)fluoroalkyl group, e.g. CF3, C ₂ F5, C3F7;

(e) (per)fluoro-oxy-alkylvinylethers of formula CF ₂ = CFOX, wherein X is a C ₁ -C ₁₂ ((per)fluoro)-oxyalkyl comprising catenary oxygen atoms, e.g. the perfluoro-2-propoxypropyl group;

(f) (per)fluorodioxoles having formula : wherein each of R _f3, R _f4, R _f s _, R _f 6, equal or different each other, is independently a fluorine atom, a C ₁ -C ₆ fluoro- or per(halo)fluoroalkyl, optionally comprising one or more oxygen atom, e.g. -CF3, -C ₂ F5, -C3F7, - OCF _3I -OCF2CF2OCF3.

(g) (per)fluoro-methoxy-vinylethers (MOVE, hereinafter) having formula: CFX ₂ = CX ₂ OCF ₂ OR" _f wherein R" _f is selected among C ₁ -C ₆ (per)fluoroalkyls , linear or branched; C ₅ -C ₆ cyclic (per)fluoroalkyls; and C ₂ -C ₆ (per)fluorooxyalkyls, linear or branched, comprising from 1 to 3 catenary oxygen atoms, and X ₂ = F, H; preferably X ₂ is F and R" _f is -CF ₂ CF ₃ (MOVE1); -CF ₂ CF ₂ OCF ₃ (MOVE2); or -CF ₃ (MOVE3).

[0020] The fluoroelastomer (A) is preferably selected among:

(1) VDF-based copolymers, in which VDF is copolymerized with at least one additional comonomer selected from the group consisting of :

(a) C ₂ -C ₈ perfluoroolefins , such as tetrafluoroethylene (TFE), hexafluoropropylene (HFP);

(b) hydrogen-containing C ₂ -C ₈ olefins, such as vinyl fluoride (VF), trifluoroethylene (TrFE), hexafluoroisobutene (HFIB), perfluoroalkyl ethylenes of formula CFI ₂ = CFI-R _f , wherein R _f is a C ₁ -C6 perfluoroalkyl group;

(c) C ₂ -C ₈ fluoroolefins comprising at least one of iodine, chlorine and bromine, such as chlorotrifluoroethylene (CTFE);

(d) (per)fluoroalkylvinylethers (PAVE) of formula CF ₂ = CFOR _f , wherein R _f is a C ₁ -C6 (per)fluoroalkyl group, preferably CF3, C ₂ F5, C3F7;

(e) (per)fluoro-oxy-alkylvinylethers of formula CF ₂ = CFOX, wherein X is a C ₁ -C ₁₂ ((per)fluoro)-oxyalkyl comprising catenary oxygen atoms, e.g. the perfluoro-2-propoxypropyl group;

(f) (per)fluorodioxoles having formula : wherein each of R _f3 , R _f4 , R _f s, R _f 6, equal to or different from each other, is independently selected from the group consisting of fluorine atom and Ci- C6 (per)fluoroalkyl groups, optionally comprising one or more than one oxygen atom, such as notably -CF3, -C2F5, -C3F7, -OCF3, -OCF2CF2OCF3; preferably, perfluorodioxoles;

(g) (per)fluoro-methoxy-vinylethers (MOVE, hereinafter) having formula: CF ₂ =CFOCF ₂ ORf2 wherein R _f 2is selected from the group consisting of C1-C6 (per)fluoroalkyls; C5-C6 cyclic (per)fluoroalkyls; and C2-C6 (per)fluorooxyalkyls, comprising at least one catenary oxygen atom; R _{f2 i} s preferably -CF ₂ CF ₃ (MOVE1); - CF ₂ CF ₂ OCF3 (MOVE2); or -CF ₃ (MOVE3);

(h) C2-C8 non-fluorinated olefins (Ol), for example ethylene and propylene; and

(2) TFE-based copolymers, in which TFE is copolymerized with at least one additional comonomer selected from the group consisting of (c), (d), (e), (g), (h) and (i) as above detailed.

[0021] Fluoroelastomer (A) is generally selected among TFE-based copolymers, as above detailed.

[0022] Optionally, fluoroelastomer (A) may comprises recurring units derived from at least one bis-olefin [bis-olefin (OF)] having general formula : wherein Ri, R2, R3, R ₄ , Rsand R6, equal or different from each other, are H, a halogen, or a C ₁ -C ₅ optionally halogenated group, possibly comprising one or more oxygen group; Z is a linear or branched C ₁ -C ₁₈ optionally halogenated alkylene or cycloalkylene radical, optionally containing oxygen atoms, ora (per)fluoropolyoxyalkylene radical, e.g. as described in EP 661304 A (AUSIMONT SPA) 5/07/1995 .

[0023] The bis-olefin (OF) is preferably selected from the group consisting of those complying with formulae (OF-1), (OF-2) and (OF-3) :

(OF-1) wherein j is an integer between 2 and 10, preferably between 4 and 8, and R1 , R2, R3, R4, equal or different from each other, are H, F or C1-5 alkyl or (per)fluoroalkyl group;

(OF-2) wherein each of A, equal or different from each other and at each occurrence, is independently selected from F, Cl, and FI; each of B, equal or different from each other and at each occurrence, is independently selected from F, Cl, FI and ORB, wherein RB is a branched or straight chain alkyl radical which can be partially, substantially or completely fluorinated or chlorinated; E is a divalent group having 2 to 10 carbon atom, optionally fluorinated, which may be inserted with ether linkages; preferably E is a -(CF2) _m - group, with m being an integer from 3 to 5; a preferred bis-olefin of (OF-2) type is F ₂ C=CF-0-(CF ₂ )5-0-CF=CF ₂ .

(OF-3) wherein E, A and B have the same meaning as above defined; R5, R6, R7, equal or different from each other, are FI, F or C1-5 alkyl or (per)fluoroalkyl group.

[0024] As said, fluoroelastomer (A) comprises iodine and/or bromine cure sites.

[0025] Generally, the amount of iodine and/or bromine cure site is such that the I and/or Br content is of from 0.04 to 10.0 % wt, with respect to the total weight of fluoroelastomer (A).

[0026] These iodine and/or bromine cure sites might be comprised as pending groups bound to the backbone of the fluoroelastomer (A) polymer chain or might be comprised as terminal groups of said polymer chain. [0027] According to a first embodiment, the iodine and/or bromine cure sites are comprised as pending groups bound to the backbone of the fluoroelastomer (A) polymer chain; the fluoroelastomer (A) according to this embodiment typically comprises recurring units derived from brominated and/or iodinated cure-site comonomers selected from:

- bromo and/or iodo alpha-olefins containing from 2 to 10 carbon atoms such as bromotrifluoroethylene or bromotetrafluorobutene described, for example, in US 4035565 (DU PONT ) 12/07/1977 or other compounds bromo and/or iodo alpha-olefins disclosed in US 4694045 (DU PONT ) 15/09/1987 ;

- iodo and/or bromo fluoroalkyl vinyl ethers (as notably described in patents US 4564662 (MINNESOTA MINING ) 14/01/1986 and EP 199138 A (DAIKIN IND LTD ) 29/10/1986).

[0028] According to a second preferred embodiment, the iodine and/or bromine cure sites (preferably iodine cure sites) are comprised as terminal groups of the fluoroelastomer (A) polymer chain; the fluoroelastomer according to this embodiment is generally obtained by addition to the polymerization medium during fluoroelastomer (A) manufacture of at least one of:

- iodinated and/or brominated chain-transfer agent(s); suitable chain- transfer agents are typically those of formula R _f (l) _x (Br) _y , in which R _f is a (per)fluoroalkyl or a (per)fluorochloroalkyl containing from 1 to 8 carbon atoms, while x and y are integers between 0 and 2, with 1 < x+y < 2 (see, for example, patents US 4243770 (DAIKIN IND LTD ) 6/01/1981 and US 4943622 (NIPPON MEKTRON KK ) 24/07/1990 ); and

- alkali metal or alkaline-earth metal iodides and/or bromides, such as described notably in patent US 5173553 (AUSIMONT SRL) 22/12/1992 .

[0029] Advantageously, for ensuring acceptable reactivity it is generally understood that the content of iodine and/or bromine in the fluoroelastomer (A) should be of at least 0.05 % wt, preferably of at least 0.06 % weight, with respect to the total weight of fluoroelastomer (A).

[0030] On the other side, amounts of iodine and/or bromine not exceeding preferably 7 % wt, more specifically not exceeding 5 % wt, or even not exceeding 4 % wt, with respect to the total weight of fluoroelastomer (A), are those generally selected for avoiding side reactions and/or detrimental effects on thermal stability.

[0031] Among specific compositions of said fluoroelastomer (A), which are suitable for the purpose of the present invention, mention can be made of fluoroelastomers having the following compositions (in mol %):

(i) vinylidene fluoride (VDF) 35-85 %, hexafluoropropene (HFP) IQ-

45 %, tetrafluoroethylene (TFE) 0-30 %, perfluoroalkyl vinyl ethers (PAVE) 0-15 %, bis-olefin (OF) 0-5 %;

(ii) vinylidene fluoride (VDF) 50-80 %, perfluoroalkyl vinyl ethers (PAVE) 5-50 %, tetrafluoroethylene (TFE) 0-20 %, bis-olefin (OF) 0-5 %;

(iii) vinylidene fluoride (VDF) 20-30 %, C2-C8 non-fluorinated olefins (Ol) 10-30 %, hexafluoropropene (FIFP) and/or perfluoroalkyl vinyl ethers (PAVE) 18-27 %, tetrafluoroethylene (TFE) 10-30 %, bis-olefin (OF) 0-5 %;

(iv) tetrafluoroethylene (TFE) 50-80 %, perfluoroalkyl vinyl ethers (PAVE) 20-50 %, bis-olefin (OF) 0-5 %;

(v) tetrafluoroethylene (TFE) 45-65 %, C2-C8 non-fluorinated olefins (Ol) 20-55 %, vinylidene fluoride 0-30 %, bis-olefin (OF) 0-5 %;

(vi) tetrafluoroethylene (TFE) 32-60 % mol %, C2-C8 non-fluorinated olefins (Ol) 10-40 %, perfluoroalkyl vinyl ethers (PAVE) 20-40 %, fluorovinyl ethers (MOVE) 0-30 %, bis-olefin (OF) 0-5 %;

(vii) tetrafluoroethylene (TFE) 33-75 %, perfluoroalkyl vinyl ethers (PAVE) 15-45 %, vinylidene fluoride (VDF) 5-30 %, hexafluoropropene FIFP 0-

30 %, bis-olefin (OF) 0-5 %;

(viii) vinylidene fluoride (VDF) 35-85 %, fluorovinyl ethers (MOVE) 5-40 %, perfluoroalkyl vinyl ethers (PAVE) 0-30 %, tetrafluoroethylene (TFE) 0- 40 %, hexafluoropropene (FIFP) 0-30 %, bis-olefin (OF) 0-5 %;

(ix) tetrafluoroethylene (TFE) 20-70 %, fluorovinyl ethers (MOVE) 30- 80 %, perfluoroalkyl vinyl ethers (PAVE) 0-50 %, bis-olefin (OF) 0-5 %. [0032] Suitable examples of (per)fluoroelastomers are the products sold by SOLVAY SPECIALTY POLYMERS S.p.A. under the trade name Tecnoflon ^® , such as for example Tecnoflon ^® P757.

[0033] The composition (C) further comprises at least an organic peroxide

[peroxide (O)]; the choice of the said peroxide (O) is not particularly critical provided that the same is capable of generating radicals with the assistance of a transition metal catalyst. Among most commonly used peroxides, mention can be made of:

- di(alkyl/alryl) peroxides, including for instance di-tert-butyl peroxide, 2,5- dimethyl-2,5-bis(tert-butylperoxy)hexane, di(t- butylperoxyisopropyl)benzene, dicumyl peroxide;

- diacyl peroxides, including dibenzoyl peroxide, disuccinic acid peroxide, di(4-methylbenzoyl)peroxide, di(2,4-dichlorobenzoyl)peroxide, dilauroyl peroxide, decanoyl peroxide;

- percarboxylic acids and esters, including di-tert-butyl perbenzoate, t- butylperoxy-2-ethylhexanoate, 1,1 ,3,3-tetramethylethylbutyl peroxy-2- ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane;

- peroxycarbonates including notably di(4-t- butylcyclohexyl)peroxydicarbonate, di(2-phenoxyethyl)peroxydicarbonate, bis[1 ,3-dimethyl-3-(tert-butylperoxy)butyl] carbonate, t- hexylperoxyisoproprylcarbonate, t-butylperoxyisopropylcarbonate,

- perketals such as 1 , 1-bis(tert-butylperoxy)cyclohexane and 2, 2- bis(tertbutylperoxy)butane;

- ketone peroxides such as cyclohexanone peroxide and acetyl acetone peroxide;

- organic hydroperoxides such as cumene hydroperoxide, tert-butyl hydroperoxide, methylethylketone peroxide (otherwise referred to as 2-[(2- hydroperoxybutan-2-yl)peroxy]butane-2-peroxol) and pinane hydroperoxide;

- oil-soluble azo initiators such as 2, 2'-azobis (4-methoxy-2. 4-dimethyl valeronitrile), 2, 2'-azobis (2.4-dimethyl valeronitrile), 2,2'- azobis(isobutyronitrile), 2, 2'-azobis(2-cyano-2-butane), dimethyl-2, 2'- azobisdimethyli sobutyrate, dimethyl-2, 2'-azobis(2-methylpropionate), 2,2'- azobis(2-methylbutyronitrile), 1 ,1'-azobis(cyclohexane-l-carbonitrile), 2, 2'- azobis[N-(2-propenyl)-2-methylpropionamide], 1 -[(1 -cyano-1 -methyl ethyl)azo]formamide, 2, 2'-azobis(N-cyclohexy1 -2-methylpropionamide), 2,2'-azobis(i sobutyronitrile), 2,2'-azobis(2-cyano-2-butane), dimethyl-2, 2'- azobisdimethylisobutyrate, 1 ,1'-azobis(cyclohexanecarbonitrile), 2-(t- butylazo)-2-cyanopropane, 2,2'-azobis[2-methyl-N-(1, 1)- bis(hydroxymethyl)-2-hydroxyethyl]propionamide, 2, 2'-azobis[2-methyl-N- hydroxyethyl]-proprionamide, 2, 2'-azobis(N, N'- dimethyleneisobutyramine), 2, 2'-azobis(2-methyl-N-[1 ,1- bis(hydroxymethyl)-2-hydroxyethyl] propionamide), 2,2'-azobis(2-methyl- N-[1 ,1-bis(hydroxymethyl) ethyl] proprionamide), 2, 2'-azobis[2-5 methyl- N-(2-hydroxyethyl) propionamide], 2,2'-azobis(isobutyramide) dihydrate, 2,2'-azobis(2, 2, 4-trimethylpentane), 2, 2'-azobis(2-methylpropane).

[0034] Other suitable peroxide systems are those described, notably, in patent applications EP 136596 A (MONTEDISON SPA ) 10/04/1985 and EP 410351 A (AUSIMONT SRL ) 30/01/1991 , whose content is hereby incorporated by reference.

[0035] The amount of peroxide (O) in the composition (C) is generally of 0.1 to 15 phr, preferably of 0.2 to 12 phr, more preferably of 1.0 to 7.0 phr, relative to 100 weight parts of fluoroelastomer (A).

[0036] The composition (C) comprises one or more than one compound (U), as above detailed.

[0037] Compounds (U) may be selected from compounds comprising two carbon- carbon unsaturations, compounds comprising three carbon-carbon unsaturations and compounds comprising four or more than four carbon- carbon unsaturations.

[0038] Among compounds (U) comprising two carbon-carbon unsaturations, mention can be made of bis-olefins [bis-olefin (OF)], as above detailed, preferably selected from those complying with any of formulae (OF-1), (OF-2) and (OF-3), as above detailed.

[0039] Among compounds (U) comprising three carbon-carbon unsaturations, mention can be made of:

- tri-substuituted cyanurate compounds of general formula: wherein each of R _cy , equal to or different from each other and at each occurrence, is independently selected from H or a group -R _rcy or-OR _rcy , with R _rcy being C ₁ -C5 alkyl, possibly comprising halogen(s), and each of J _cy , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; tri-substuituted cyanurate compounds include notably preferred triallyl cyanurate, trivinyl cyanurate;

- tri-substuituted isocyanurate compounds of general formula: wherein each of Ri _S0Cy , equal to or different from each other and at each occurrence, is independently selected from H or a group -R _ri soc _y or-OR _ri soc _y , with R _ri soc _y being C1-C5 alkyl, possibly comprising halogen(s), and each of Ji _S0Cy , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; tri-substuituted isocyanurate compounds include notably preferred triallyl isocyanurate (otherwise referred to as “TAIC”), trivinyl isocyanurate, with TAIC being the most preferred;

- tri-substituted triazine compounds of general formula: wherein each of R _az , equal to or different from each other and at each occurrence, is independently selected from H or a group -R _{raz O} r-OR _raz , with R _raz being C ₁ -C5 alkyl, possibly comprising halogen(s), and each of J _az , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; tri-substituted triazine compounds include notably compounds disclosed in EP 0860436 A (AUSIMONT SPA)

26/08/1998 and in WO 97/05122 (DU PONT) 13/02/1997 ;

- tri-substituted phosphite compounds of general formula: wherein each of R _Ph , equal to or different from each other and at each occurrence, is independently selected from H or a group -R _rph or-OR _rph , with R _rph being C ₁ -C5 alkyl, possibly comprising halogen(s), and each of J _Ph , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; tri- substituted phosphite compounds include notably preferred tri-allyl phosphite;

- tri-substituted alkyltrisiloxanes of general formula: wherein each of Rsi, equal to or different from each other and at each occurrence, is independently selected from H or a group -R _s\ or-OR _rSi , with R _s\ being C1-C5 alkyl, possibly comprising halogen(s), each of R’ _Si , equal to or different from each other and at each occurrence, is independently selected from C1-C5 alkyl groups, possibly comprising halogen(s), and each of J _Si , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; tri- substituted alkyltrisiloxanes compounds include notably preferred 2,4,6- trivinyl methyltrisiloxane and 2,4,6-trivinyl ethyltrisiloxane;

- N,N-disubstituted acrylamide compounds of general formula: wherein each of R _an , equal to or different from each other and at each occurrence, is independently selected from H or a group — Rran or— ORran, with Rran being C1-C5 alkyl, possibly comprising halogen(s), and each of J _an , equal to or different from each other and at each occurrence, is independently selected from a bond or a divalent hydrocarbon group, optionally comprising heteroatoms; N,N-disubstituted acrylamide compounds include notably preferred N,N-diallylacrylamide. [0040] Among compounds (U) comprising four or more carbon-carbon unsaturations, mention can be made of tris(diallylamine)-s-triazine of formula , hexa-allylphosphoramide,

N,N,N',N'-tetra-allyl terephthalamide, N,N,N',N'-tetra-allyl malonamide.

[0041] It is generally preferred for the compound (U) to be selected from the group consisting of (i) olefins (OF), as above detailed, in particular olefins of (OF-1) type; and (ii) tri-substuituted isocyanurate compounds, as above detailed, in particular TAIC.

[0042] The amount of the compound (U) ranges normally from 0.1 to 20 weight parts per 100 parts by weight (phr) of fluoroelastomer (A), preferably from 1 to 15 weight parts per 100 parts by weight of fluoroelastomer (A), more preferably from 1 to 10 weight parts per 100 parts by weight of fluoroelastomer (A).

[0043] Composition (C) may comprise further at least one basic metal oxide, which is generally selected from the group consisting oxides of divalent metals. Among these latter, mention can be notably made of ZnO, CaO, BaO, MgO, PbO, Na ₂ 0, and their mixtures, with MgO being preferred.

[0044] When used, the amount of the metal oxide is generally of at least 0.5 phr, preferably at least 1 phr, and/or generally at most 25 phr, preferably at most 15 phr, more preferably at most 10 phr, with respect to the fluoroelastomer (A) weight.

[0045] Composition (C) may further additionally comprise one or more than one metal hydroxide. Flydroxides which can be used are generally selected from the group consisting of Ca(OH)2, Sr(OH)2, Ba(OH)2, although their presence is not mandatory. [0046] Also, other conventional additives, such as reinforcing fillers (e.g. carbon black, silica, in particular precipitated silica), thickeners, pigments, antioxidants, stabilizers and the like, may then be added to the composition (C).

[0047] Carbon black is among preferred reinforcing fillers. When used, reinforcing fillers, and more particularly carbon black, may be present in the composition (C) in an amount of at least 3, preferably at least 5 weight parts; and/or at most 50, preferably at most 45, more preferably at most 40 weight parts per 100 weight parts of fluoroelastomer (A).

[0048] Similarly, precipitated silica has been found to be advantageous in formulating certain compositions (C). When used, precipitated silica, may be present in the composition (C) in an amount of at least 3, preferably at least 5, more preferably at least 10 weight parts; and/or at most 50, preferably at most 45, more preferably at most 40 weight parts per 100 weight parts of fluoroelastomer (A).

[0049] As said, step (1) is a step of shaping composition (C): shaping may be obtained through a variety of molding techniques, including injection molding, compression molding, extrusion molding.

[0050] The step (1) of shaping comprises exposing composition (C) at a temperature of at most 165°C. The choice of such temperature is advantageously effective for achieving partial cure and hence consolidation of the composition (C), but yet avoiding complete reaction of radical species which are generated or can be generated.

[0051] Generally, in Step (1) composition (C) is transferred into the cavity of a mold, and heated at a temperature of at most 165°C in said mold, so as to shape the said composition (C) and obtain a part (P _pc ) possessing the desired three-dimensional shape corresponding to the cavity of the mold.

[0052] Injection molding technique is preferred, although other techniques may equally be used.

[0053] As said step (1) is carried out at a temperature of at most 165°C, preferably at most 160°C, more preferably at most 155°C, even more preferably at most 150°C. Lower boundaries for temperature of step (1) are not particularly limited and will be selected by one of ordinary skills in the art considering softening behaviour of the fluoroelastomer (A) and radical generation of the peroxide (O). It is nonetheless generally understood that temperature used in Step (1) will be of at least 10O°C, preferably at least 110°C, more preferably at least 120°C, even more preferably at least 125°C.

[0054] Duration of Step (1) will be adapted, depending on the kinetics of crosslinking, depending notably upon temperature, type of fluoroelastomer (A), peroxide (O) and compound (U) and of size and shape of targeted part (Pp _C ).

[0055] This can be done notably by investigating curing behaviour using MDR technique pursuant to ASTM D5289 standard at the temperature of Step (1), and hence advantageously determining in this manner time required for achieving increases in torque, as representative of progress of crosslinking at the said temperature.

[0056] Once fixed temperature of Step (1), duration of Step (1) is hence generally adjusted (i) to exceed , (ii) not to exceed (tso + 1 minute), and (iii) not to exceed tgo, whereas , tso and tgo are, respectively, the time required at the temperature of Step (1) for an increase of torque up to twice minimum torque (t _S 2) , for an increase of torque up to 50 % of the maximum torque (tso), and for an increase of torque up to 90 % of the maximum torque, at the temperature of Step (1), as determined by ASTM D5289.

[0057] Generally, step (1) includes shaping the composition (C) at the temperature as above detailed for a duration of at least 1 minute, preferably at least 2 minutes, even more preferably at least 5 minutes and/or at most 120 minutes, preferably at most 90 minutes, more preferably at most 60 minutes. Durations ranging from 5 to 25 minutes are generally effective for obtaining suitably part (P _pc ).

[0058] Once step (1) is completed, the result is a part (P _pc ), which is partially cured: this status can be easily demonstrated by the improvement in mechanical properties (tensile strength, for instance) which is achieved by exposing the said part (P _pc ) to a further thermal treatment at a temperature exceeding 165°C. [0059] As said Step (2) is a step of applying on at least a part of the surface of the said part (P _pc ) a composition (C2) comprising at least one l/Br-containing fluoropolyether compound [compound (l/Br-PFPE)], said compound (l/Br- PFPE) comprising a (per)fluoropolyoxylakylene chain [chain (R _f )] and comprising at least one of iodine and bromine atoms.

[0060] The expression l/Br-containing fluoropolyether compound and compound (l/Br-PFPE), are hereby understood to designate one or more than one of said compounds.

[0061] The (per)fluoropolyoxyalkylene chain [chain (R _f )] of compound (l/Br-PFPE) is preferably a chain comprising a plurality of recurring units (Ri), said recurring units having general formula: -(CF ₂ ) _k -CFZ-0-, wherein k is an integer of from 0 to 3 and Z is selected between a fluorine atom and a Ci- C ₆ perfluoro(oxy)alkyl group.

[0062] While compound (l/Br-PFPE) may comprise l/Br atom(s) chemically bound to one or more than one recurring unit (R2) of the chain (R _f ), it is nonetheless generally understood that said l/Br atom(s) is/are generally chemically bound to end-groups of the said chain (R _f ).

[0063] Chain (R _f ) of the compound (l/Br-PFPE) hence preferably complies with formula:

-(CF2CF20)a'(CFY0)b'(CF2CFY0)c'(CF20)d'(CF2(CF2)zCF ₂ 0)e'-, wherein the recurring units are statistically distributed along the (per)fluoropolyoxyalkylene chain, wherein:

- Y is a C ₁ -C ₅ perfluoro(oxy)alkyl group;

- z is 1 or 2;

- a’, b’, c’, d’, e’ are integers > 0.

[0064] Most preferably, chain (R _f ) of compound (l/Br-PFPE) complies with formula:

-(CF2CF20) _a" (CF20) _b" (CF2(CF2) _z CF ₂ 0) _c" -, wherein:

- z is 1 or 2;

- a”, b”, c” are integers > 0.

[0065] Chain (R _f ) is generally selected so as to possess a number averaged molecular weight of 500 to 6000, preferably of 750 to 5000, even more preferably of 1000 to 4500. [0066] Generally, compound (l/Br-PFPE) complies with formula (I):

T ^A -0-R _f -T ^A ’ (I) wherein:

- R _f is (per)fluoropolyoxyalkylene chain [chain (R _f )] comprising recurring units having at least one catenary ether bond and at least one fluorocarbon moiety, as detailed above;

- T ^A and T ^A ’, equal to or different from each other, are selected from the group consisting of:

(i) Ci-C ₆ (hydro)(fluoro)carbon groups, possibly comprising one or more than one of H, O, and Cl; and

(ii) l/Br-containing C1-C6 (hydro)(fluoro)carbon group comprising at least one of iodine and bromine atoms (group T ^l/Br ), with the provisio that at least one of T ^A and T ^A ’ is a group T ^l/Br .

[0067] In particular, compound (l/Br-PFPE) complies with formula (II): T ^B -0-R ^* _f -T ^B ’ (II) wherein:

R ^* _f is a chain (R _f ), as above detailed; each of T ^B and T ^B ’, equal to or different from each other, are selected from

(j) a group of any of formulae -CF3, -CF2CI, -CF2CF3, -CF(CF3)2 _, -CF2FI, - CFH ₂ , -CF2CH3, -CF2CHF2, -CF2CH2F, -CFZ*CH ₂ OH, -CFZ*COOH, - CFZ*COORh and -CFZ*-CFh(OCFl2CFl2)k-OFI, wherein k is ranging from 0 to 10, wherein Z ^* is F or CF3; Rh is a C1-C6 hydrocarbon chain; and

(jj) a group T ^l/Br* , of any of formulae -CFZ ^* (CFl2) _mi -l, -CFZ ^* (CFte) _mb -Br,- CFZ ^* -CH ₂ (OCH ₂ CH ₂ ) _k -l , and -CFZ ^* -CH ₂ (OCH ₂ CH ₂ ) _k -Br, wherein k is ranging from 0 to 10, Z ^* is F or CF3; each of mi and mb are independently zero or one; preferably a group T ^l/Br* , of any of formulae -CF2CF2Br, - CF(CF ₃ )Br, -CF2CF2I and -CF(CF ₃ )I; with the provisio that at least one of T ^B and T ^B ’ is a group T ^l/Br* , as above detailed.

[0068] Compound (l/Br-PFPE) is preferably a iodine-containing compound, and more preferably complies with formula (III):

TC-0-(CF2CF20)a _' (CFY0) _b' (CF2CFY0)c _' (CF20) _d' (CF2(CF2)zCF ₂ 0)e _' -TC’, wherein: - Y is a C ₁ -C ₅ perfluoro(oxy)alkyl group;

- z is 1 or 2;

- a’, b’, c’, d’, e’ are integers > 0;

- each of T ^c and T ^c ’, equal to or different from each other, are selected from

(k) a group of any of formulae -CF3, -CF2CI, -CF2CF3, -CF(CF3)2 _, -CF2FI, - CFH _2I -CF2CH3, -CF2CHF2, -CF2CH2F, -CFZ*CH ₂ OH, and -CFZ*- CFl2(OCFl2CFl2) _k -OFI, wherein k is ranging from 0 to 10, wherein Z ^* is F or CF3; and

(kk) a group T ^1” of any of formulae -CFZ*(CFte)mi-l, and -CFZ*- CFh(OCFl2CFl2)k-l, wherein Z ^* is F or CF3; k is ranging from 0 to 10, and mi being zero or 1 ; preferably a group T ^r of any of formulae -CF2-I and - CF(CF ₃ )-I, with the provisio that at least one of T ^B and T ^B ’ is a group T ^1” , as above detailed.

[0069] Compounds (l/Br-PFPE) as above detailed, may be provided, as a consequence of their synthetic methods and precursors used, as mixtures of compounds comprising different chemical entities differing because of the nature and length of the (per)fluoropolyoxyalkylene chain, may comprise variable fractions of compounds wherein both chain ends are (hydro)(fluoro)carbon groups comprising at least one l/Br atom and of compounds wherein only one chain end is a Br/l-containing (hydro)(fluoro)carbon group, and maybe associated to minor amounts of side products of similar structure, but wherein both of chain ends of the (per)fluoropolyoxyalkylene chain fails to be bound to a Br/l-containing (hydro)(fluoro)carbon group.

[0070] With regards to the proportion of so-called “mono” and “di” functional compounds, it is generally understood that best results have been achieved when the compound (l/Br-PFPE) consisted of a majority of compounds of formula (I) [T ^A -0-R _f -T ^A ’] as above detailed, wherein both T ^A and T ^A ’ were groups T ^l/Br , as above detailed [di-functional (l/Br-PFPE ^d )], and a minor amount of compounds of formula (I) [T ^A -0-R _f -T ^A ’] as above detailed, wherein only one of T ^A and T ^A ’ is a group T ^l/Br , the other group being free from l/Br-containing groups [mono-functional (l/Br-PFPE ^m )].

[0071] While di-functional (l/Br-PFPE ^d ) and mono-functional (l/Br-PFPE ^m ) may be separately and individually used in a fluoroelastomer composition, the compound (l/Br-PFPE) is generally a mixture of di-functional (l/Br-PFPE ^d ) and mono-functional (l/Br-PFPE ^m ). When the compound (l/Br-PFPE) is provided as a mixture of corresponding mono- and difunctional compounds, the amounts of di-functional (l/Br-PFPE ^d ) compounds and mono-functional (l/Br-PFPE ^m ) compounds are generally such that the groups T ^l/Br are representative of at least 80 % mol, preferably at least 85 % moles, more preferably at least 90 % moles of all end groups of compounds (l/Br-PFPE).

[0072] While it may be possible to isolate compounds (l/Br-PFPE) as “pure” compounds, i.e. separated from side products of similar structure, but wherein both of chain ends of the (per)fluoropolyoxyalkylene chain fails to be bound to a Br/l-containing (hydro)(fluoro)carbon group, very minor amounts of the said side products are not detrimental, and to the sake of economy, may be tolerated in admixture with compound (l/Br-PFPE). The amount of those “non-functional” compounds, which may be tolerated in admixture with compound (l/Br-PFPE) is generally such that the overall amount of end groups free from l/Br is of less than10 % moles, preferably less than 7 % moles, more preferably less than 5 % moles, with respect to the total number of end groups of the mixture of compounds (l/Br-PFPE) and non-functional side products.

[0073] Generally, side products of compound (l/Br-PFPE) may comply with formula (IV):

W-O-R _f -W’ (IV) wherein:

R _f is a chain R _f , as above detailed; each of W and W’, equal to or different from each other, are selected from: - a group of any of formulae -CF3, -CF2CI, -CF2CF3, -CF(CF3)2 _, -CF2FI, - CFH ₂ , -CF2CH3, -CF2CHF2, -CF2CH2F, -CFZ*CH ₂ OH, -CFZ*COOH, - CFZ ^* COOR _h and -CFZ ^* -CH ₂ (OCH ₂ CH ₂ ) _k -OH, wherein k is ranging from 0 to 10, wherein Z ^* is F or CF ₃ ; R _h is a hydrocarbon chain.

[0074] Side products more recurrently found may comply with formula (V):

W ^* -0-(CF ₂ CF ₂ 0)a'(CFY0) _b' (CF ₂ CFY0)c'(CF ₂ 0) _d' (CF ₂ (CF ₂ )zCF ₂ 0)e'-W ^* \ wherein:

- Y is a C ₁ -C ₅ perfluoro(oxy)alkyl group;

- z is 1 or 2;

- a’, b’, c’, d’, e’ are integers > 0;

- each of W ^* and W ^* ’, equal to or different from each other, are selected from groups of any of formulae -CF3, -CF ₂ CI, -CF ₂ CF3, -CF(CF3) _{2 ,} -CF ₂ FI, - CFH ₂ , -CF ₂ CH3, -CF ₂ CHF ₂ , -CF ₂ CH ₂ F, -CFZ ^* CH ₂ OH, -CFZ ^* COOH, - CFZ ^* COOR _h and -CFZ ^* -CH ₂ (OCH ₂ CH ₂ ) _k -OH, wherein k is an integer comprised ranging from 0 to 10, wherein Z ^* is F or CF ₃ ; R _h is a hydrocarbon chain.

[0075] Compounds (l/Br-PFPE) can be synthesized applying to available PFPE precursors suitable chemistry so as to introduce l/Br groups in chain ends, according to known synthetic methods.

[0076] In said Step (2), a composition (C2) comprising compound (l/Br-PFPE) may be applied on solely a fraction of the surface of the said part (P _pc ) or may be applied to the whole surface of the said part (P _pc ).

[0077] Methods for applying said composition (C2) are not particularly limited; spray coating composition (C2) and dipping in a bath of composition (C2) are exemplary techniques which can be used.

[0078] Generally, composition (C2) further comprises a liquid medium, which is advantageously selected so as to advantageously provide a liquid composition (C2) possessing appropriate liquid viscosity appropriate for the application technique of choice.

[0079] According to preferred embodiments, composition (C2) includes at least one fluorinated solvent, which may be notably selected from volatile fluorinated solvents having normal boiling points of less than 135°C, including perfluorodecaline, perfluoro(1 ,2- or 1 ,3-dimethylcyclohexane, perfluorokerosene, perfluoro(methyldecalin), perfluoroheptane mixed isomers, and, more generally, (hydro)fluoro(poly)ethers, including low molecular weight PFPE, hydrofluoroethers (HFEs), i.e. ethers comprising partially fluorinated hydrocarbon structure, comprising both hydrogen and fluorine atoms bound to sp ³ -hybridized carbons.

[0080] As said, result of Step (2) is a partially cured fluoroelastomer surface- treated part [part (P _c ^ST )].

[0081] As said, Step (3) is a step of completing curing the said part (P _pc ^ST ) by exposing the same to a temperature of at least 170°C, so as to obtain the target part (P _C ^SM ).

[0082] Step (3) can be carried out notably in an oven, in particular in a ventilated oven bleating in Step (3) may also be useful for eliminating liquid medium which may be used in composition (C2) as above detailed.

[0083] Temperature of Step (3) is generally of at least 175°C, preferably at least 180°C, more preferably at least 185°C, even more preferably 190°C, and/or of at most 260°C, preferably at most 250°C. Duration of the treatment at the afore-mentioned temperature in Step (3) may last several hours, although shorter treatment may also be effective, depending notably of the shape and size of target part (P _C ^SM ). Generally, duration is of at least 30 minutes, preferably at least 45 minutes and/or at most 12 hours, preferably at most 8 hours.

[0084] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0085] The invention will be now be described in connection with the following examples whose purpose is merely illustrative and not intended to limit the scope of the invention.

[0086] EXAMPLES

[0087] Raw materials

[0088] Tecnoflon(R) P457 is a peroxide curable fluoroelastomer comprising iodine cure sites, commercially available from Solvay Specialty Polymers Italy, SpA.

[0089] Luperox® 101XL crosslinking peroxide is neat 2,5 Dimethyl 2,5 Di(tert- butylperoxyl) hexane organic peroxide. [0090] DRIMIX TAIC is triallylisocyanurate polyunsaturated compound.

[0091] Reinforcing filler Carbon black N990MT was obtained from Cancarb (N990 MT, hereinafter).

[0092] Fomblin®Y45 is a non-functional PFPE lubricant oil, commercially available from Solvay Specialty Polymers Italy, SpA.

[0093] Preparative Example 1 synthesis of I-CF2O-PFPE-O-CF2-I starting from HOOC-CF2-O-PFPE-O-CF2-COOH

[0094] a) Acylation of H00C-CF ₂ -0-(CF ₂ CF ₂ 0)n(CF ₂ 0) _m -CF ₂ -C00H (Z-D/AC).

In a glass reactor equipped with a mechanical stirrer, a dripping funnel, a condenser and a N2 inlet valve were placed 983 g of thionyl chloride (8.26 moles) and 68.7 g of anhydrous pyridine (0.87 moles). The system was purged with N2 to create an inert atmosphere. Following purging, the system was maintained in a static inert atmosphere and heated to 75°C with vigorous stirring. Once the internal T reached 70° - 75°C dripping of Z-DIAC (3000 g = 2.53 eq. ; EW = 1184 g/eq. ; m/n= 1,17; Functionality, i.e. averaged number of -COOFI groups per PFPE molecule = 1.88) began at a rate of about 30 g / min. Following complete addition of Z-DIAC, the reaction was let stir at 70°C for an additional 30 minutes. Spectroscopic analyses (FT-IR: formation of -C(=Q)-CI at 1805 cnr ¹ and concomitant disappearance of -C(=Q)-OH at 1780 cnr ¹ ) confirmed complete conversion of the starting Z-DIAC into the acyl chloride derivative. The inhomogeneous solution was let cool and 2 clear-cut phases separated. The lower acylchloride phase was collected, filtered through a PTFE 0.2 micron membrane to remove residual pyridinium chloride and the residual SOCI2 was distilled under vacuum at 70°C for 2 hrs.

Isolated yield of Z-DIA-CI = 97 mol%; MW = 2250 g/mole; EW = 1203 g/eq; n/m= 1.18; Functionality, i.e. averaged number of -COCI groups per PFPE molecule = 1.88.

[0095] b) Decarbony/ation of CI0C-CF ₂ -0-(CF ₂ CF ₂ 0)n(CF ₂ 0) _m -CF ₂ -C0CI (Z-DIA- CI).

Z-DIA-CI (3000 g = 2,493 eq) was placed in the same reactor as described in (a) along with anhydrous Kl (5,81 mole = 964 g). The solvent-free mixture was purged with N2 and once purging is complete, the reaction system was kept under inert atmosphere and heated to 210°C with vigorous stirring. The decarbonylation reaction was complete (100 mole% conversion of the acyl end-groups) after 16 hours, during which time CO liberated was measured (2.493 moles = 59.8 liters). The progress of the reaction was followed both by FT-IR and NMR by observing the decrease of the IR absorportion intensity of the C(=0) _st stretching band at 1805 cnr ¹ and the apparence of the -OCF2I ¹⁹ F-NMR peaks at 8 and 10 ppm with concomitant decrease of the NMR peaks at -76 and -78 ppm of the - CF ₂ C(=0)CI.

Once the conversion is complete, the reaction was cooled to 25 °C. An inert low boiling fluorinated solvent was used to wash the reactor to recover all the product. Trace amounts of I2 formed during the reaction were reduced by adding 2 -3 g of solid sodium thiosulphate under vigorous stirring. The mixture was then filtered through a PTFE membrane (5 microns) to remove the salts and then the solvent was evaporated at reduced pressure at 70°C for 2-3 hours. Flence, 2580 g of product having structure: l-CF2-0-(CF2CF20)n(CF ₂ 0)m-CF2-l were obtained, having MW = 2581 g/mole; EW = 1399 g/eq; m/n= 1.16; Functionality, i.e. averaged number of -CF2I groups per PFPE molecule = 1.85.

[0096] General method for preparation of curable composition

[0097] P457 fluoroelastomer was compounded in an open mill at a temperature of below 40°C for 10 minutes with 4 phr of TAIC, 3 phr of 101X1 and 30 phr of N990MT.

[0098] The slab so obtained was compression molded conditions as below detailed in the examples.

[0099] Comparative method - Application of non-functional PFPE on cured and post-cured parts

[00100] A slab of the curable composition as detailed above was molded in standard condition (10 min at 170°C), and the cured part so obtained was post cured for additional 4 hours at 230°C. Different solution of Fomblin®Y45 in Galden®SV70, prepared with concentrations (%by weight) of 0.1%, 0.5%, 1%, 2.5% and 5%, were applied by spray coating on the so-obtained parts. After evaporation of the solvent in an oven at 80°C for 40 minutes, the static and dynamic coefficients of friction of the so modified surface of cured part has been evaluated according to the ASTM D1894 standard method using the “LF PLS LLOYD Dynamometer”.

[00101] Inventive method - Application iodine-containing PFPE on partially cured parts before post-curing

[00102] Curable composition as detailed above was tested for its curing behaviour by Moving Die Rheometer (MDR) technique according to ASTM D5289 at a temperature of 140°C; Minimum torque (ML), Maximum torque (MH), time required for double torque ( ) and for achieving xx% of MH (t _xx ) were so determined, and results are summarized below.

[00103]

[00104] A specimen of the cured composition was then molded at 140°Cfor a duration not significantly exceeding tso, and largely below tso, so as to obtain a partially cured slab. Said slab was then treated with l-PFPE-l, which was uniformly spread on the said slab with a tongue, and the so treated partially cured slab was post cured in an oven for 4 hours at 230°C. The static and dynamic coefficients of friction of this sample have been evaluated according to the ASTM D1894 standard method using the “LF PLS LLOYD Dynamometer”.

[00105] Results of static and dynamic coefficients of friction determinations, carried out in conditions specified in Table 2, are summarized in the Table 3 below.

[00106] Table 2 - ASTM D1894 test conditions

[00107] Table 3 - Friction results

[00108] In conditions as listed above, CoF for a cured part of P457-based curable recipe with no PFPE additive could not be determined; indeed under these conditions, due to the stick slip phenomena, sample jumps on the aluminium plate during the test, so that no meaningful determination of a static/dynamic CoF is possible.

[00109] In general, stick/slip phenomena can be recognized in a frictional force/displacement graph as spikes, corresponding to sudden increase and decrease of CoF during the trial.

[00110] Once, the rubber specimen of P457 has been superficially treated after molding/post-curing with non-functional PFPE, CoFs (both static and dynamic) are reduced, with more significant improvements at higher load of lubricant, although stick/slip phenomena are still significantly present; moreover, the specimen is oily/wet and PFPE can be easily swiped off. Once specimens were extensively cleaned with a fluorinated solvent able to dissolve the used PFPE, the reduction in CoFs was lost. [00111] Inventive method provided most effective decrease of CoF: in this case, the treated specimen was found to have no oily/wet surface, and to maintain the aforementioned advantageous friction properties, including after extensive cleaning of the same specimen with a fluorinated solvent able to dissolve the l-PFPE-l used.