Synthesis and application of -bromo-perfluoroalkylvinyl ethers

V.M.Andrushin, V.V.Drozdov

Report V. Study of physical and mechanical properties of copolymers of fluoroolefins with -bromoperfluoroalkylvinyl ethers. 

1. Introduction

In report IV (1) the routes to modify fluoroplasts by copolymerization of tetrafluoroethylene (TFE) and vinilydene fluoride (VDF) with bromo-perfluoroalkylvinyl ethers (BrAVE) of the general formula of Br(CF₂)nOCF=CF₂ where n=2 ( BrAVE-2) and n=4 (BrAVE-4) have been described. The introduction of fluoroalkylvinyl ethers to copolymerization with olefins is known to improve processibility of the copolymers, increases their resistance to low temperatures and their stability in aggressive chemical media.
They are used as the second and third copolymers (2-4)

In particular, a fluoroelastomer based on triple TFE copolymer, perfluoromethylvinyl ether and substituted perfluoroalkylvinyl ether of the general formula of CF₂=CFOR_fX is known.
Bromine-containing molecules allow to synthesize new types of fluoroplasts possessing hydrophilic, conductivity, improved solubility and better processibility. Thus, for example, the introduction of bromotrifluoroethylene to the composition of polytetrafluoroethylene increases amphoterism of the copolymer and reduces its melting temperature (5).
Bromine-containing monomers, for example 1-bromo-2,2-difluoroethylene, have found wide application in processes of copolymerization with TFE, VDF, hexafluoropropylene and other fluoroolefins to produce fluoroelastomers and general rubber goods based on rubbers (6-9). In this case bromine atoms are convenient active centers to form space-network structures (10-16). There are known many polymer compositions of radical solidification based on bromated and iodinated fluoroelastomers (17-21).
It was planned to produce new fluoroplasts and fluoroelastomers with a set of performance characteristics peculiar to copolymers based both on perfluoroalkylvinyl ethers and on bromine-containing fluoromonomers using BrAVE monomers as modifiers of fluoropolymers in reactions of copolymerization with fluoroolefins.
In the present paper we state the results of the determination of general physical and mechanical characteristics of copolymers of BrAVE with the most widespread fluoromonomers, TFE and VDF, and their resistance in different aggressive chemical media.

2.Test procedure

Physical and mechanical characteristics of materials were determined on the basis of the results of tensile tests carried out on a test machine of 2055 P-05 type of "TochPribor" (c. Ivanovo, Russia) made.
Test samples were two-sided scoops manufactured by blanking from films with a special knife. The machine is equipped with a hermetic autoclave of a special design with controllable electrical heating to provide testing the samples in different aggressive chemical media.
Based on the tensile test results the following physical and mechanical characteristics were calculated:

-  breaking stress, MPa

, MPa

where Pp is the load at the sample destruction, N
Fo is the starting surface of the sample cross-section, cm;

-  modulus of elasticity, MPa

, MPa

where P_1,P₂   are the loads chosen at the initial linear part of the tensile diagram, N;
l₁,l₂ - are the sample elongation at the loads of P₁,P₂ respectively, cm;
lo- is the length of the sample test part, cm;

- percent elongation

where l_p is the sample elongation at rupture, cm;

- percentage change of the breaking stress

where _p1 is the breaking stress in a medium, MPa;
_p2  is the breaking stress in air, MPa;

- percentage of the modulus of elasticity

where E_p1 is the modulus of elasticity in a medium, MPa;
E_po is the modulus of elasticity in air, MPa

- change of percent elongation

where _p1 is the percent elongation in a medium, %;
_po is the percent elongation in air,%.
To determine chemical resistance of the copolymers, samples were subjected to aging for 7 days in 20% solutions of sulfuric acid and caustic soda at a temperature of 90^oC and then their physical and mechanical properties were determined at tensile at 20^oC.

2. Study of physical and mechanical characteristics of copolymers of TFE.

The fluoroplast of F-10 type widely used for making flexible pipelines, gasket materials etc. was used as the comparison standard.
The comparison of modificating affect of BrAVE monomer and other fluoromonomers such as -fluorosulfuryl-perfluoroalkylvinyl ether (FS-141), perfluoroallylvinyl ether , perfluoroallylalkylvinyl diether was of an undoubted interest also. Some results of the conducted experiments are given in Table1.
The study has shown that the strength characteristics and modulus of elasticity considerably increase after treatment in alkali for almost all types of copolymers of TFE tested. Some reduction in the modulus of elasticity of copolymers with FS-141 was observed in tests in sulfuric acid against a background of some increase in the strength parameters. Deformation characteristics of the all copolymers of FS-141 tested ( with different content of comonomer) are reduced in aggressive media and to a greater degree in aging in alkali.
As regards to BrAVE monomers, all their physical and mechanical parameters increase both in acid and in alkali media and the modulus of elasticity and breaking stress increase to the greatest extent.
It is seen from Table 1 that effectiveness of BrAVE monomers goes down with the chain length growth. The increase in the strength characteristics of copolymers based on BrAVE after their aging both in acid and in alkali is explained by additional cross-linking of the copolymer which takes place at the contact of the bromine-containing copolymer with an aggressive medium.