Electrets

Details Electrets Electrets

2000-02-28

2002-12-03

Wu, David W. (Department: 1713)

Stock material or miscellaneous articles

Composite

Of metal

C526S281000, C526S308000, C526S348100, C428S500000, C525S210000, C525S211000, C525S240000, C307S400000, C365S146000, C029S886000

Reexamination Certificate

active

06489033

ABSTRACT:

The present invention relates to electrets, a process for their preparation and their use as films.
Electrets are materials with a permanent electrical polarity, similar to the permanent magnetic polarity in ferromagnetic materials. However, the term is also used, for example, for materials which have opposing charges on their surfaces, as in the case of films; cf. R. C. Brown et al.; J. Aerosol. Sci., Vol. 25, No. 1, page 149,1994.
The ability to store charges in the interior or on the surface over relatively long periods of time is of great interest, especially in the particle filtration sector. Especially for small particles, the efficiency of electrets is markedly higher than that of uncharged filter media when separating particles from a medium to be filtered, such as a gas or liquid. Materials particularly suitable for preparing electrets are those which have insulating properties. Polymers are therefore particularly good for preparing electrets, due to their high electrical resistances and their good processability.
The performance of electrets depends on the nature of the polymer used. U.S. Pat. No. 4,288,584 describes the preparation of electrets based on polymers such as polycarbonate and polyester. However, the initially high surface potential of electrets made from these polymers reduces again relatively rapidly. This applies especially when the humidity is high. Electrets based on simple polyolefins are also described. The surface potentials of electrets based on these polymers is described as not very stable, especially at relatively high temperatures.
JP-A-08 41260 describes improvement in the stability of charges on electrets via the addition of polymers with polar side groups. However, the electrets of that invention, based on copolymers with cyclic olefins, were studied only over very short periods. In addition, even in these short periods of up to 24 hours some quite marked reductions of from 10 to 15% in surface potentials can be seen.
According to J. van Thournhout, the electret properties of polyolefins, such as polypropylene, cannot be improved by the addition of polar polymers or of polymers with polar side groups, cf. Conference Proceedings of the Dielectrics and Electrical Insulation Society, J. van Thournhout et al.; pp. 961-966 (1994).
There is therefore still a need for materials whose electret properties are retained even at very high temperatures and at very high humidity. Only fluoropolymers, such as polytetrafluorine ethylene (PTFE) have hitherto satisfactorily fulfilled these requirements. However, these polymers are expensive and require specific processing conditions, and this currently restricts their use for many electret applications. There remains therefore a high level of interest in materials for electrets which are simple to process and which, even at high temperatures and humidity, show no, or only slight, reduction in their charges.
The object of the present invention is to provide electrets which show no, or only slight, reduction in their charges at high temperature and high humidity, and also to provide a cost-effective and environmentally friendly process for preparing electrets.
The object of the present invention is achieved by electrets which comprise at least one cycloolefin polymer. The electrets preferably comprise at least one cycloolefin copolymer or a mixture with at least one cycloolefin copolymer and with at least one other polyolefin, whose proportions may be up to 40% (w/w).
The novel electrets preferably comprise at least one cycloolefin copolymer selected from the group consisting of polymers containing from 0.1 to 100% by weight, preferably from 0.1 to 99.9% by weight, based on the total weight of the cycloolefin polymer, of polymerized units of at least one cyclic olefin of the formulae I, II, II′, III, IV, V or VI.
where R
1
, R
2
, R
3
, R
4
, R
5
, R
6
, R
7
and R
8
are identical or different and are a hydrogen atom or a C
1
-C
20
-hydrocarbon radical, such as a linear or branched C
1
-C
8
-alkyl radical, C
6
-C
18
-aryl radical or C
7
-C
20
-alkylenearyl radical,
or a cyclic or acyclic C
2
-C
20
-alkenyl radical, or form a saturated, unsaturated or aromatic ring, where the same radicals R
1
to R
8
in the various formulae I to VI may have a different meaning, and where n may be from 0 to 5, and
from 0 to 99 mol %, based on the total makeup of the cycloolefin copolymer, of polymerized units which derive from one or more acyclic olefins of the formula VII.
where R
9
, R
10
, R
11
and R
12
are identical or different and are a hydrogen atom, a linear, branched, saturated or unsaturated C
1
-C
20
-hydrocarbon radical, such as a C
1
-C
8
-alkyl radical or a C
6
-C
18
-aryl radical.
The cycloolefin polymers may also be obtained by ring-opening polymerization of at least one of the monomers with the formulae I to VI followed by hydrogenation of the resultant products.
The electret contains from 0 to 45 mol % based on the total makeup of a cycloolefin copolymer, a polymerized units which derive from one or more monocyclic olefins of the formula (VIII):
where n is from 2 to 10.
The proportion of the polymerized units which derive from cyclic, in particular polycyclic, olefins is preferably from 3 to 75 mol %, based on the total composition of the cycloolefin copolymer. The proportion of the polymerized units which derive from acyclic olefins is preferably from 5 to 80 mol %, based on the total composition of the cycloolefin copolymer.
The cycloolefin copolymers are preferably composed of polymerized units which derive from one or more polycyclic olefins, in particular polycyclic olefins of the formula I or III and polymerized units which derive from one or more acyclic olefins of the formula VII, in particular &agr;-olefins having from 2 to 20 carbon atoms. Particular preference is given to cycloolefin copolymers which are composed of polymerized units which derive from a polycyclic olefin of the formula I or III and from an acyclic olefin of the formula VII. Preference is also given to terpolymers which are composed of polymerized units which derive from a polycyclic monoolefin of the formula I or II, from an acyclic monoolefin of the formula VII and from a cyclic or acyclic olefin which contains two double bonds (polyene), in particular cyclic, preferably polycyclic, dienes, such as norbornadiene, or cyclic, particularly preferably polycyclic, alkenes which have a C
2
-C
20
-alkenyl radical, such as vinylnorbornene.
The novel cycloolefin polymers preferably contain olefins with a fundamental norbornene structure, particularly preferably norbornene, tetracyclododecene and, if desired, vinylnorbornene or norbornadiene. Preference is also given to cycloolefin copolymers which contain polymerized units which derive from acyclic olefins with terminal double bonds, for example &agr;-olefins having from 2 to 20 carbon atoms, particularly preferably ethylene or propylene. Particular preference is given to norbornene/ethylene copolymers and tetracyclododecene/ethylene copolymers.
Among the terpolymers, particular preference is given to norbornene-vinylnorbornene-ethylene terpolymers, norbornene-norbornadiene-ethylene terpolymers, tetracyclododecene-vinylnorbornene-ethylene terpolymers and tetracyclododecene-vinyltetracyclododecene-ethylene terpolymers. The proportion of the polymerized units which derive from a polyene, preferably vinylnorbornene or norbornadiene, is from 0.1 to 50 mol %, preferably from 0.1 to 20 mol %, and the proportion of the acyclic monoolefin of the formula VII is from 0 to 99.9 mol %, preferably from 5 to 80 mol %, based on the total composition of the cycloolefin copolymer. For the terpolymers, the proportion of the polycyclic monoolefin is from 0.1 to 99.9 mol %, preferably from 3 to 75 mol %, based on the total composition of the cycloolefin copolymer.
The cycloolefin copolymer of the invention preferably contains at least one cycloolefin copolymer which contains polymerized units which derive from polycyclic olefins of the formula I and contains polymerized units which derive from a

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