Fluoroelastomeric compositions

Details Fluoroelastomeric compositions Fluoroelastomeric compositions

2002-05-22

2004-06-15

Nutter, Nathan M. (Department: 1711)

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

Synthetic resins

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

C524S520000

Reexamination Certificate

active

06750295

ABSTRACT:

The present invention relates to fluoroelastomeric compositions consisiting essentially of a VDF-based fluoroelastomer and a semi-crystalline fluoropolymer, having improved mechanical properties, combined with high chemical and thermal resistance, good sealing properties (improved compression set) and usable for sealing manufactured articles, pipes, in particular for food industry applications.
Specifically, the present invention relates to fluoroelastomeric compositions comprising a VDF-based fluoroelastomeric matrix which incorporates semi-crystalline fluoropolymer particles, characterized by improved mechanical and elastomeric (sealing) properties and improved surface aspect, roughnesses free. Said combination of properties makes available fluoroelastomers usable in the food field wherein the fluoroelastomer manufactured articles must show a low release of polluting agents.
It is known to increase the fluoroelastomer mechanical properties, reinforcing fillers can be added to the fluoroelastomeric matrix. In this way improved mechanical properties are obtained, but the release of polluting agents is too high, when the fluoroelastomer is to be used in the preparation of manufactured articles for the food industry.
Preferably in food industry one does not wish to use carbon black as reinforcing agent to avoid the polluting agent release, and manufactured articles without fillers are preferred to make easier the control of the equipment cleaning.
By trying to replace the carbon black filler with a semi-crystalline fluoropolymer, for example polytetrafluoroethylene (PTFE) powder, there are many difficulties in its incorporation, without the possibility to prepare a homogeneous compound between the semi-crystalline fluoropolymer and the fluoroelastomer, in particular when the semicrytalline fluoropolymer amount is high. Generally, to obtain a satisfactory homogeneization of the semi-crystalline fluoropolymer in the fluoroelastomer, it is necessary to use complicated technologies, using open mixers with complicated incorporation cycles, for example by heating the rolls and using several steps. Even operating with these extremely complicated technologies, the incorporation of the semi-crystalline fluoropolymer in the fluoroelastomer is not uniform and the manufactured article shows poor mechanical properties (in particular the stress/elongation ratio) and poor reproducibility of the results, whereby there is poor reliability of the preparation process and a high number of pieces to be discarded. Besides, the final manufactured article is not homogeneous due to the poor disperdibility of the semi-crystalline fluoropolymer.
From the European patent application EP 1,031,606 it is known to incorporate under the latex form a semi-crystalline fluoropolymer containing iodine and/or bromine atoms, in a fluoroelastomer to improve the mechanical properties with respect to a fluoroelastomer filled with PTFE powder. The particles of the semi-crystalline fluoropolymer latex have sizes comprised between 10 and 100 nm. In said patent application it is stated that the use of a semi-crystalline fluoropolymer latex containing iodine and/or bromine allows to improve the mechanical properties with respect to the case of absence of iodine and/or bromine. In the Examples the introduction of iodine atoms by using iodinated transfer agents is reported. However, for the applications in food industry the mechanical properties are not suitable. In said patent application it is stated that iodine and/or bromine deriving from comonomers in the preparation of the semi-crystalline fluoropolymer can be used. The comonomer amounts are such that the comonomer content in the final compound results lower than 2 moles per 100 moles of the other monomeric units of the semi-crystalline fluoropolymer. No Examples are reported with these iodinated and/or brominated comonomers in the polymer chain. Tests carried out by the Applicant have shown that even operating in above way no fluoroelastomeric compositions suitable for food industry applications are obtained since endowed with poor mechanical and sealing properties (see the comparative Examples).
The need was felt to have available fluoroelastomeric compositions having the following combination of properties:
improved mechanical properties; in particular improved stress/elongation at break ratio;
high chemical and thermal resistance;
excellent surface aspect, roughnesses free;
working easiness in equipments conventionally used in the fluoroelastomer field, without using complicated technologies for incorporation of fluoropolymer.
improved sealing properties (improved compression set).
The Applicant has unexpectedly and surprisingly found that it is possible to obtain the combination of the above properties by using the specific compositions defined hereunder.
An object of the present invention are therefore fluoroelastomeric compositions comprising:
A) a fluoroelastomeric matrix based on vinylidene fluoride (VDF);
B) a semi-crystalline fluoropolymer, in an amount from 1 to 40% by weight based on the total of A)+B), preferably from 5 to 35% by weight, comprising a semi-crystalline fluoropolymer core coated by a semi-crystalline fluoropolymer shell containing bromine and/or iodine in the polymer chain, the amount of brominated and/or iodinated comonomer being from >2% to 10% by moles per 100 moles of the other basic monomeric units of the semi-crystalline fluoropolymer B) core+shell;
the fluoroelastomer A) incorporates the fluoropolymer B), the fluoropolymer B) being formed by tetrafluoroethylene (TFE) homopolymers, or by TFE copolymers with one or more monomers containing at least one unsaturation of ethylene type, in an amount from 0.01% to 10% by moles, preferably from 0.05% to 7% by moles; the core and the shell of the fluoropolymer B) can be of a different composition between each other, the average sizes of the semi-crystalline fluoropolymer B) particles being from 10 to 100 nm, preferably from 10 to 60 nm.
By semi-crystalline fluoropolymer a fluoropolymer is meant which shows, besides the glass transition temperature Tg, at least one melting temperature.
The fluoroelastomeric compositions of the invention are obtainable by mixing the latex of the semi-crystalline fluoropolymer B) with the latex of the fluoroelastomer A) and subsequent coagulation.
Alternatively to the latex mixing, the fluoroelastomeric compositions of the invention can be obtained in the same reactor, using two subsequent steps: in the first step the semi-crystalline fluoropolymer B) having the above nanometric sizes is polymerized, while in the second step the fluoroelastomer A) is polymerized. By operating in this way the fluoroelastomer A) covers the latex particles of semi-crystalline fluoropolymer B) allowing to obtain a very good disperdibility of the latter in the fluoroelastomer.
The semi-crystalline fluoropolymer B) particles having the above nanometric sizes are obtainable for example by polymerization in aqueous microemulsion of perfluoropolyoxyalkylenes as described for example in European patent application 969,027 in the name of the Applicant, herein incorporated by reference. Polymerization techniques in microemulsion wherein the oil phase is formed by polymerizable unsaturated monomers can also be used, as described in U.S. Pat. No. 5,523,346 and U.S. Pat. No. 5,616,648.
The Applicant has found that to obtain the results of the invention it is essential that the semi-crystalline fluoropolymer B) has the mentioned nanometric sizes, while the size of the fluoroelastomer A) particles to be mixed with the fluoropolymer is not critical.
The semi-crystalline fluoropolymers of the present invention comprise a semi-crystalline fluoropolymer core shelled by a semi-crystalline fluoropolymer containing in the polymer chain bromine and/or iodine atoms. The introduction of said bromine and/or iodine atoms can be carried out by addition in the polymerization mixture of the (co)monomers forming the fluoropolymer B), of brominated and/or iodinated comonomers, such as bromo-olefins ha

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