Curable fluoroelastomer compositions

Details Curable fluoroelastomer compositions Curable fluoroelastomer compositions

2000-01-12

2002-09-10

Mulcahy, Peter D. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

Reexamination Certificate

active

06448319

ABSTRACT:

The present invention relates to additives to improve the friction coefficient of curable fluoroelastomer compositions without lowering the adhesion with bonding agents to metal supports and having a very good resistance to the curing alkaline medium.
Specifically it relates to fluoroelastomers able to give unsaturations, i.e. subject to nucleophilic attack. Preferably the fluoroelastomers are vinylidenfluoride-based curable compositions.
Cured elastomers based on vinylidenfluoride copolymers, wherein the vinylidenfluoride is copolymerized with one or more monomers containing an ethylene unsaturation and at least one fluorine atom, are well known and are widely used in a variety of applications fields wherein an exceptional chemical resistance to solvents, lubricants, fuels, acids and similar products, is required.
The cured articles obtained by these elastomer products find the most suitable use as sealing gaskets in general, both in static and dynamic conditions, in the motor, aeronautic, missile, naval, mechanical, chemical field, in protective impermeabilizations of various supports, as protective clothes for the contact with aggressive chemical agents.
The fluoroelastomer curable compositions based on vinylidenfluoride can be cured by ionic and/or by radical way. Generally in both kinds of curing working coadjuvants are used to improve the product processability.
The use of curing coadjuvants to improve the blend extrudibility characteristics and to lower the adhesion phenomena to molds and/or the dirting of the same, is well known in the prior art. Vegetable waxes, polyurethanes having low molecular weight, stearates, polyesters, fluorosilicone oils, etc., can for example be mentioned.
The technical problem that the present invention intends to solve is that to have available cured fluoroelastomer compositions combining to the Theological, processability, chemical and thermal resistance characteristics of the known fluoroelastomers, also a lower dynamic friction coefficient, while preserving the capability to adhere to metal surfaces pretreated with bonding agents.
An improved friction coefficient is particularly desired by users, since it leads for example to a longer duration of the manufactured article and the application of smaller forces during the running stage.
In patent application EP 805,180 in the name of the Applicant, the use as additives of monofunctional and bifunctional perfluoropolyethers having end groups —C(X)FCH
2
OH wherein X=F, CF
3
able to give cured fluoroelastomer compositions endowed by a lower friction coefficient, is described.
Tests carried out by the Applicant (see the comparative Examples) have shown that when said perfluoropolyether additives have number average molecular weight lower than 2,500, for example equal to 2,000, there is a reduction of the friction coefficient which is lower to the case wherein the same perfluoropolyethers having a high molecular weight, for example higher than 4,000, are used. This behaviour is difficult to explain from a theoretic point of view. A non binding hypothesis could be that in the case of low molecular weights, the excessive presence of hydroxyl groups in the curing mass would hinder the additive surface action.
The fact that the additives having a low molecular weight are not equally effective in improving the friction coefficient, is a big limitation from the commercial point of view to the diffusion of the additives of the above mentioned patent. Indeed from the industrial point of view fractions having a molecular weight 1,000-2,000, are more available, while fractions having a higher molecular weight, for example 4,000, are available in a limited amount. In order to obtain high molecular weight fractions, expensive enriching processes are required.
The need was felt to have available additives for cured fluoroelastomer compositions able to give a still improved friction coefficient to obtain end manufactured articles with an higher use time and to further minimize in the running phase the wear-out due to the forces of the mechanical parts in relative motion, said additives being obtainable without further enriching processes.
The Applicant has unexpectedly and surprisingly found new additives formed by polyfluorooxyalkylene units sequences bound each other with hydrogenated acetal bridges, stable at the curing alkaline ambient. Such additives are industrially available with the desired molecular weights and allow to obtain cured fluoroelastomer compositions charaterized by a further decrease of the friction coefficient in comparison with the known additives, without lowering the adhesion with bonding agents to metal supports.
An object of the present invention are therefore additives for fluoroelastomer curable compositions having polyacetal structure of general formula:
HOCH
2
—[R
f
—CH
2
O(CH
2
O)
c
CH
2
]
t
—R
f
—CH
2
OH  (I)
wherein:
c=1-10, preferably 1-3; t=0.5-20.
R
f
=poly-fluorooxyalkylene in, having number average molecualr weight ({overscore (M)}
n
) in the range 500-2,500, preferably 1,000-2,000.
The number average molecular weight of the formula (I) additive is in the range 3,000-15,000, preferably 3,000-10,000.
The poly-fluorooxyalkylene chain R
f
preferably comprises monomeric units having at least one of the following structures, statistically distributed along the chain:
(—CF
2
CF
2
O—), (—CF
2
CF
2
CF
2
O—), (—CH
2
CF
2
CF
2
O—),
wherein X=F, CF
3
Specifically, in such formula (I) the R
f
chain can preferably have one of the following structures:
1) —CF
2
O—(CF
2
O)
a
—(CF
2
CF
2
O)
b
—CF
2
—
with b/a in the range 0.5-2, including the extremes;
2) —CF
2
O—(CF
2
—CF
2
O)
b
—CF
2
—
3) —CF
2
O—(C
3
F
6
O)
r
—(C
2
F
4
O)
b
—(CFXO)
t
—CF
2
—,
with r/b=0.5-2.0 (r+b)/t=10-30
4) —CFX—(OC
3
F
6
)
z
—OCF
2
(R′f)
y
—CF
2
O—(C
3
F
6
O)
z
—CFX—
5) —CF
2
CH
2
(OCF
2
CF
2
CH
2
)
q
—OCF
2
(R′f)
y
—O—(CH
2
CF
2
CF
2
O)
s
CH
2
CF
2
—
wherein:
X=F, —CF
3
;
—(C
3
F
6
O)— can represent units of formula:
 —(CF(CF3)CF
2
O)—
and/or
—(CF
2
—CF(CF3)O)—
a, b, g, r, s, t, z are positive numbers in the range 0-25, the sum of which is such that R
f
shows {overscore (M)}
n
values in the range 500-2,500 about, and preferably 1,000-2,000.
R′f=perfluoroalkylene group containing from 1 to 4 carbon atoms;
y=0 or 1.
The preparation of the additives of formula (I) is carried out by reacting, at temperature in the range 0-100° C. and in the presence of an acid medium, oligomers or polymers of the formaldehyde with a diol having a polyfluorooxyalkylene chain of general formula:
HOCH
2
R
f
CH
2
OH  (II)
wherein R
f
has the above mentioned meaning and has preferably the structure:
1) —CF
2
O—(CF
2
O)
a
—(CF
2
CF
2
O)
b
—CF
2
—
with b/a in the range 0.5-2, including the extremes.
As formaldeyde derivatives used in said process, paraformaldehyde and trioxymethylene can be used.
Even though the sulphuric acid is preferred as reaction acid medium, for its double function as decomposition agent of the formaldehyde oligomers or polymers and dehydration agent in the polycondensation reaction, it is understood that other organic and/or inorganic acids, or mixtures thereof, having the same functions, can be used in the present preparation in substitution of, or together with, the sulphuric acid. For example, trifluoroacetic, p-toluensuphonic, canphosulphonic, methansulphonic acids can be used.
The polycondensation reaction is carried out under stirring, to have a good reactant dispersion, and can be stopped by neutralization of the remaining acid with a base, for example ammonia.
By suitable stoichiometric ratios among formaldehyde, fluorinated diol and acid it is possible to control the molecular weight of the polycondensation product. For example, by operating with sulphuric acid and formaldehyde, molar ratios diol/formaldehyde in the range 0.25-4 and formaldehyde/sulphuric acid in the range 0.1-4 allow to obtain the invention additives having the desired number average molecular weight, pr

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