Fluoroelastomer compositions

Details Fluoroelastomer compositions Fluoroelastomer compositions

2000-02-23

2001-10-30

Teskin, Fred (Department: 1713)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S199000, C525S194000, C525S195000, C525S326200, C525S326300, C525S370000, C525S379000, C525S387000, C524S502000, C524S520000, C524S805000

Reexamination Certificate

active

06310142

ABSTRACT:

The present invention relates to fluoropolymers essentially formed of a mixture of a fluoroelastomer and a semicrystalline fluoropolymer having improved mechanical properties and usable for sealing manufactured articles in the electronic, optical and pharmaceutical industry.
More specifically the present invention relates to fluoropolymers formed of a mixture of a fluoroelastomer and a semicrystalline fluoropolymer containing iodine and/or bromine, characterized by improved mechanical properties combined with good properties of elastic retention (lower compression set) and very good surface appearance without roughness. It is well known that one of the fluoroelastomer uses is the preparation of O-rings for seals: for this application it is essential that the O-ring surface is smooth.
The use of fluoroelastomers containing polytetrafluoroethylene (PTFE) particles to improve the properties of abrasion-resistance and of hot tearing of the obtained manufactured articles is known in the prior art. As described in Japanese patent 57-107,336, the fluoroelastomer abrasion-resistance is improved by physically mixing solid curable fluoroelastomers with PTFE powders having a low molecular weight, in the range 500-200,000 as average molecular weight by number (M
n
). Said PTFE is prepared by thermal decomposition at a temperature between 450° C. and 600° C. for prolonged times or by irradiation with ionic radiation of high molecular weight PTFE. An alternative method for obtaining PTFE having a low molecular weight is that to polymerize TFE in the presence of chain transfer agents. The fluoroelastomer and the PTFE powders are mixed in Banbury or in calender.
In U.S. Pat. Nos. 4,879,362 and 4,904,726 mixtures of fluoroelastomers with resins of PTFE modified with the addition of comonomers such as hexafluoropropene (HFP), perfluoropropylvinylether (PPVE), etc., are used, in order to avoid PTFE fibrillation problems without losing the reinforcement properties that the PTFE gives to the obtained fluoroelastomers. The comonomer results much more present on the polymer particle surface, so as to allow an uniform distribution in the fluoroelastomer without the formation of visible agglomerates. These latter should be the cause of fibrillation phenomena.
In EP 708,797 fluoroelastomer compositions formed of a fluoroelastomer and by a semicrystalline fluorinated filler in the form of micropowder which are obtained in curing compounds not containing metal species, are described. Said compositions give a low release of metal species under conditions where an high purity is required, but they show poor mechanical properties. Tests carried out by the Applicant (see the comparative Examples), have shown that the surface of the manufactured articles obtained from said fluoroelastomer compositions shows roughness. It is well known that in the O-ring preparation, typical fluoroelastomer application, surfaces having a low roughness in order to obtain good sealing properties, are required. The semicrystalline fluorinated filler is based on PTFE or PTFE modified with a comonomer and obtained by emulsion or suspension polymerization. The high molecular weight PTFE is subjected to irradiation, as above said, in order to reduce the molecular weight. This makes easier the PTFE milling produced by a suspension process; it eliminates the fibrillation and reduces the PTFE agglomeration obtained by an emulsion process.
The need was felt to have available fluoroelastomer compositions comprising a semicrystalline fluorinated filler having improved properties compared with those of the prior art and specifically with the following property combination:
improved mechanical properties
good elastic retention properties (lower compression set—very good seal)
very good surface appearance roughness free.
The Applicant has unexpectedly and surprisingly found that it is possible to obtain the combination of the above mentioned properties, by incorporating in the fluoroelastomer matrix PTFE particles or its copolymers having well defined sizes as specified hereinafter.
It is therefore an object of the present invention to provide fluoropolymers comprising a fluoroelastomer matrix incorporating therein particles of a semicrystalline fluoropolymer latex containing iodine and/or bromine and formed of tetrafluoroethylene (TFE) homopolymers, or TFE copolymers with one or more monomers containing at least one ethylene unsaturation in amounts ranging from 0.01% to 10% by moles, preferably from 0.05% to 5% by moles, wherein the average particle sizes of the semicrystalline fluoropolymer latex range from 10 to 100 nm, preferably from 10 to 60 nm. Also semicrystalline fluoropolymers wherein the latex particle sizes have the above mentioned value for at least 60% by weight, preferably 70% by weight of the semicrystalline fluoropolymer, can be used. The invention compositions are obtainable by mixing the semicrystalline fluoropolymer latex containing iodine and/or bromine with the fluoroelastomer latex and subsequent coagulation. Alternatively the invention compositions can be polymerized in the same reactor in two subsequent steps: in a first step the semicrystalline fluoropolymer with the mentioned nanometric sizes and containing iodine is polymerized and in a second step the fluoroelastomer is polymerized. By operating in this way the fluoroelastomer should cover the semicrystalline fluoropolymer latex particles, allowing to obtain a very good dispersion of the latter in the fluoroelastomer.
The semicrystalline fluoropolymer amount inside the fluoroelastomer matrix is in the range 2%-40% by weight, preferably 5-30% by weight, more preferably 10-20% by weight on the total of the polymer mixture.
The semicrystalline fluoropolymer particles having the above mentioned sizes are obtainable for example by a polymerization process in an aqueous microemulsion of perfluoropolyoxyalkylenes as described for example in the European patent application EP 99112083.3 in the name of the Applicant, herein incorporated by reference. Microemulsion polymerization methods can also be used, wherein the oil phase is formed of polymerizable unsaturated monomers, as described in U.S. Pat. Nos. 5,523,346 and in 5,616,648.
The semicrystalline fluoropolymers of the present invention contain in the end group of the macromolecules and/or in the chain iodine and/or bromine atoms. The introduction of said iodine and/or bromine atoms can be carried out by addition, in the reaction mixture, of brominated and/or iodinated comonomers as described for example in U.S. Pat. Nos. 4,035,565, 4,694,045, 4,745,165, 4,564,662 and EP 199,138, in such amounts wherefore the comonomer content in the final product is generally in the range 0.05-2 moles for 100 moles of the other base monomer units.
Alternatively or also in association with these comonomers, it is possible to introduce iodine and/or bromine end atoms by addition to the reaction mixture of iodinated and/or brominated chain transfer agents (see for example U.S. Pat. Nos. 4,243,770 and 4,943,622, 5,173,553) in such amounts whereby the iodine content by weight in the semicrystalline fluoropolymer is in the range 0.01-5, preferably 0.05-1% by weight.
The brominated and iodinated compounds are those mentioned hereinunder in the fluoroelastomer preparation.
The fluoroelastomers can be prepared by copolymerization of the monomers in aqueous emulsion, according to well known methods in the prior art, in the presence of radical initiators (for example alkaline or ammonium persulphates, perphosphates, perborates or percarbonates), optionally in combination with ferrous, cuprous or silver salts, or of other easily oxidizable metals. In the reaction medium also surfactants of various kind, among which the fluorinated surfactants are particularly preferred, are usually present.
Alternatively the fluoroelastomers can be prepared in bulk or in suspension, in an organic liquid in which a suitable radical initiator is present, according to well known techniques.
The polymerization reaction is generally carried out at temperatures in the

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