Fluororubber compositions and method of making

Details Fluororubber compositions and method of making Fluororubber compositions and method of making

1999-06-21

2001-10-02

Wilson, D. (Department: 1713)

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

Synthetic resins

From silicon reactant having at least one...

C528S026000, C528S028000, C528S031000, C528S032000, C528S042000, C524S462000, C523S205000

Reexamination Certificate

active

06297339

ABSTRACT:

This invention relates to fluororubber compositions which can be effectively roll milled and cure into products having excellent mechanical properties as well as satisfactory solvent resistance, chemical resistance, heat resistance, and low-temperature properties, and a method for preparing the same.
BACKGROUND OF THE INVENTION
Prior art vinylidene fluoride base fluororubbers are used in a variety of applications mainly in the automobile and mechanical industries since they produce elastomers having good heat resistance and mechanical strength.
These fluororubbers, however, are not satisfactory in chemical resistance. They are readily swollen in polar solvents such as ketone, lower alcohol, carbonyl and organic acid solvents. They are deteriorated by amine-containing chemicals so that their rubber strength and elongation may substantially decline. With respect to low-temperature properties, the fluororubbers lose rubber elasticity at temperatures below −20° C. to such an extent that they cannot be used as sealants. This generally imposes a limit on the use of fluororubbers in cold areas.
In order to eliminate these drawbacks, curable fluorine-containing compositions comprising a perfluoro compound and a fluorinated organohydrogenpolysiloxane as main components were proposed. These compositions, however, are liquid because they are based on liquid perfluoro compounds having a low degree of polymerization. Then the compositions are suitable in the FIPG process and LIMS molding, but less effective in compression molding conventionally employed for rubber molding.
In particular, the curable fluorine-containing compositions often prohibit the use of conventional two-plate molds for rubber because of the difficulty of molding and the frequent occurrence of defects by air entrainment. Steady production is difficult unless a special LIMS mold is newly fabricated. The LIMS mold, however, has the problems that it is generally more expensive than the conventional two-plate molds for rubber, the mounting of the mold in a LIMS molding machine is cumbersome, and the molding machine requires time-consuming adjustment after mold mounting. The LIMS mold does not lend itself to the manufacture of a variety of parts in small quantities.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a fluororubber composition which can be effectively roll milled and compression molded, and cure into products having satisfactory solvent resistance, chemical resistance, heat resistance, and low-temperature properties. Another object is to provide a method for preparing the same.
We have found that using a polymer comprising a perfluoro compound having at least two alkenyl groups in the molecule and a divalent perfluoroalkylene or divalent perfluoropolyether structure in the backbone, with a compound having at least two hydrosilyl groups in the molecule and capable of addition reaction with alkenyl groups being added to some of the alkenyl groups in the perfluoro compound, there is obtained a rubber composition which can be roll milled (that is, millable rubber) and compression molded.
With prior art synthetic techniques, it was difficult to increase the molecular weight of a liquid perfluoro compound to the extent enabling roll milling. By addition reacting a hydrosilyl-bearing compound with some alkenyl groups in the liquid perfluoro compound to induce crosslinking, the perfluoro compound can be increased in molecular weight and converted into a non-flowing gel-like polymer (this is referred to as precure). When a reinforcing filler is further blended in the gel-like polymer, the resulting mixture can be compounded by a two-roll rubber mill. That is, a millable type fluororubber composition is obtained which can be worked by a rubber mill and molded in a mold for compression molding. Additionally, this composition cures into products having satisfactory solvent resistance, chemical resistance, heat resistance, and low-temperature properties.
While the fluororubber composition is millable and compression moldable, it can also be extrusion molded and calendered. Using a twin-roll rubber mill, a crosslinking agent can be added to the fluororubber composition before molding. Since accurate sheeting of the composition is possible, the loss of material is reduced and conventional two-plate molds for rubber are applicable.
Further, when a compound having at least one fluoroalkyl group and silanol groups in the molecule is blended as a surface treating agent for the filler, the filler becomes more dispersible and thus easy to blend whereby the rubber is improved in mechanical properties, especially tensile strength. More particularly, if the filler is blended with the polymer in the absence of the surface treating agent, there is a risk that the addition of the filler would become a cumbersome operation, the dispersion of the filler would become insufficient, and the less interaction between the polymer and the filler would make it difficult to achieve excellent rubber physical properties including a tensile strength of more than 100 kg/cm
2
. When the filler is blended with the polymer along with the surface treating agent, the filler can be readily blended to form a fluororubber composition of the millable type which provides excellent rubber physical properties including a high tensile strength.
Accordingly, the present invention provides a crosslinkable fluororubber composition comprising
(A) 100 parts by weight of a polymer comprising a perfluoro compound having at least two alkenyl groups in the molecule and a divalent perfluoroalkylene or divalent perfluoropolyether structure in the backbone and a compound having at least two hydrosilyl groups in the molecule and capable of addition reaction with alkenyl groups, the hydrosilyl-bearing compound being added to some of the alkenyl groups in the perfluoro compound,
(B) 1 to 100 parts by weight of a reinforcing filler, and
(C) a sufficient amount to cure component (A) of a crosslinking agent having a hydrosilyl group in the molecule and capable of addition reaction or a peroxide crosslinking agent.
In another aspect, the invention provides a method for preparing a fluororubber composition comprising the steps of:
(a) adding a compound having at least two hydrosilyl groups in the molecule and capable of addition reaction with alkenyl groups to a perfluoro compound having at least two alkenyl groups in the molecule and a divalent perfluoroalkylene or divalent perfluoropolyether structure in the backbone in such amounts that the molar ratio of hydrosilyl groups in said hydrosilyl-containing compound to alkenyl groups in said perfluoro compound may range from 0.1/1 to 0.99/1, and effecting addition reaction to form a polymer,
(b) adding a reinforcing filler to the polymer of step (a) in an amount of 1 to 100 parts by weight per 100 parts by weight of said perfluoro compound, to form a base compound, and
(c) adding to the base compound of step (b), a crosslinking agent having a hydrosilyl group in the molecule and capable of addition reaction or a peroxide crosslinking agent in a sufficient amount to react with alkenyl groups in the polymer.
DETAILED DESCRIPTION OF THE INVENTION
Component (A)
Component (A) of the crosslinkable fluororubber composition is a polymer comprising (I) a perfluoro compound having at least two alkenyl groups in the molecule and a divalent perfluoroalkylene or divalent perfluoropolyether structure in the backbone and (II) a compound having at least two hydrosilyl groups in the molecule and capable of addition reaction with alkenyl groups wherein the hydrosilyl-bearing compound is added to some of the alkenyl groups in the perfluoro compound.
The perfluoro compound as component (I) is technically difficult to synthesize into a high molecular weight polymer as typified by a resin or rubber. The perfluoro compound should have at least two alkenyl groups in the molecule and a divalent perfluoroalkylene or divalent perfluoropolyether structure in the backbone. It is preferably a linear perfluoro compound having a visc

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