Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Ion-exchange polymer or process of preparing
Reexamination Certificate
2001-03-19
2002-05-14
Berman, Susan W. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Ion-exchange polymer or process of preparing
C522S085000, C522S079000
Reexamination Certificate
active
06387964
ABSTRACT:
The present invention relates to a process for the grafting of an unsaturated monomer to a polymer, in particular to a fluorinated polymer. In particular the present invention relates to a process for forming monomer-grafted cross-linked polymer membranes which may be used as non-ionic exchange membranes or ion-selective exchange membranes. These membranes can be used in various applications such as electrodialysis, dialysis, Donnan dialysis and energy systems such as batteries, redox cells and fuel cells. The present invention may also be applied to the preparation of grafted microporous cross-linked polymer structures. It may also be used to graft different form factors such as threads, non-woven fabrics, tubes, powders, pellets, sheets and/or films.
Techniques for the grafting of an unsaturated monomer to a polymer are described in U.S. Pat. Nos. 3,481,848, 4,012,303, 4,339,473, 4,605,685 and 5743940, EP Patent No 0526203 and GB Patent No 1237293. The techniques known in the art all involve the steps of activation of the polymer, i.e. the generation of free radicals on the polymer backbone, and reaction between the activated polymer and the unsaturated monomer to produce a monomer-grafted polymer. These steps may be carried out either simultaneously or sequentially. A review of the subject area is presented in a paper written by B. Gupta and G. G. Scherer,
Chimia
48 (1994), 127-137.
U.S. Pat. No. 3,481,848 discloses a method of radiation graft polymerization of vinyl compounds onto cellulose in which cellulose is pre-irradiated in air then reacted with a vinyl monomer in an emulsified system and in which alumina or an alumina-silica mixture is used as a catalyst in order to deactivate OH radicals. Without the catalyst a large quantity of homopolymer is formed. The method is only applied to cellulose membranes and indeed would be unsuitable for many polymers, especially fluorinated polymers, which will undergo degradation and mechanical breakdown if irradiated in air.
U.S. Pat. No. 4,012,303 discloses a process for the simultaneous irradiation grafting of trifluorostyrene to an inert polymeric film base. In this case the polymer is activated by irradiation whilst immersed in an organic solution of the monomer. This process suffers from a number of disadvantages. Firstly, the monomer may undergo significant homopolymerization in preference to being grafted to the polymeric film base since it too is irradiated, and therefore activated, simultaneously with the polymeric film base. Secondly, the process utilises an organic solvent to solubilize the monomer and this is disadvantageous for environmental reasons. Thirdly, the process utilises gamma radiation which, because of its low energy density, requires long radiation times to achieve the required dose. Finally, the process is slow, requiring up to 400 hours to achieve graft levels of about 30%.
U.S. Pat. No. 4,339,473 also discloses a simultaneous radiation grafting process which uses water as the solvent and includes homo- and/or co-polymerisation retardants. Thus, the problem of homopolymerisation is reduced and the use of organic solvents is avoided. However this process is limited to grafting monomers which are hydrophilic, that is monomers which are substantially soluble in water.
U.S. Pat. No. 4,605,685 discloses a sequential process for the irradiation grafting of trifluorostyrene to an inert polymeric film base. This process involves activation of the polymeric film base by irradiation with beta radiation prior to contact with the monomer. If stored at sufficiently low temperatures, the activated polymeric film base may remain stable for up to two months. In this method the monomer solution is not exposed to radiation and this alleviates the problem of homopolymerization of the monomer. However it still retains the disadvantage of the use of organic solvents to solubilize the monomer and, although faster than the simultaneous grafting process of U.S. Pat. No. 4,012,303, it still requires lengthy reaction times for the grafting step, typically about 20 hours at 50 degC. The reaction time may be lowered by the removal of the radical inhibitors which are commonly added to commercially available unsaturated monomers to increase their shelf life. However this is a costly and time-consuming process. Furthermore, the percentage graft, even at such long reaction times, is still only about 50% at most.
U.S. Pat. No. 5,743,940 also discloses a sequential process in which an organic high molecular weight compound is exposed to an ionising radiation and thereafter a polymerisable monomer that either contains ion exchange groups or a polymerisable monomer that can be converted to provide ion-exchange groups is incorporated in the irradiated compound by graft polymerization. The monomer may be incorporated by liquid- or vapour-phase graft polymerization but the former is presumably only possible in aqueous solution when the polymerisable monomer is water soluble.
GB-A-1237293 discloses the use of an emulsion of a monomer in water for grafting to a polymer by thermal polymerization. The process involves simultaneous thermal activation of the polymer and reaction with an emulsion of the monomer in water. Although the disclosure makes no mention of homopolymerization, it is highly likely to be a problem for such a process where the polymer and monomer are subjected to thermal activation at the same time. Presumably, in order to alleviate the problem of homopolymerization the process operates at very low concentrations of monomer resulting in only low levels of grafting.
EP-A-0526203 discloses a separation membrane composed of microporous polyethylene having fine pores substantially filled with a graft polymer. The membrane is obtained by bringing microporous polyethylene having radicals formed by plasma irradiation into contact with an emulsion comprising a water-insoluble monomer, a surface-active agent and water.
It is an object of the present invention to provide a process for cross-linking a polymer and grafting a monomer thereto which does not require the use of an organic solvent and which does not result in significant homopolymerization of the monomer but which provides a uniform graft of a monomer throughout a polymer within a reasonable reaction time at a low reaction temperature.
Cross-linking, also known in the art as “reticulation”, generally improves the physical properties of the polymer. It creates a more tightly structured polymer which has a higher glass transition temperature and in which other physical properties such as tensile strength, refractive index, creep, compression set and stress relaxation are also improved. Improvements in such physical properties are particularly advantageous when the polymer is used to produce a thin film such as is required when forming an ion-exchange membrane. Cross-linking also generally reduces the chemical reactivity of the polymer because the tighter structure decreases the diffusion of chemical species throughout the polymer. It also decreases the ability of the polymer to swell when placed in a solvent which further improves its chemical resistance. Improvements in such chemical properties are particularly advantageous when the polymer is used to produce ion-exchange membranes. Such membranes must have high resistance to the chemical environments in which they are used, for example in electrochemical cells. However the alteration in the physical and chemical characteristics of the polymer, whilst advantageous for the reasons given above, would be expected to make the polymer more resistant to the grafting of other monomer species to the polymer
Grafting processes known in the art do not effect cross-linking of the polymer and would not be expected to achieve the stated aims of a uniform graft of a monomer throughout a cross-linked polymer within a reasonable reaction time at a low reaction temperature because of the nature of such a polymer.
Thus the present invention provides a process for the preparation of a monomer-grafted cross-linked polymer comprising the steps of:
D'Agostino Vincent F.
Newton John Michael
Antonelli Terry Stout & Kraus LLP
Berman Susan W.
International Power PLC
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