WO2013049485A1

WO2013049485A1 - Dielectric elastomers having a two-dimensionally structured surface, and electromechanical converter comprising such dielectric elastomers

Info

Publication number: WO2013049485A1
Application number: PCT/US2012/057790
Authority: WO
Inventors: Torsten Feller; Joachim Wagner; Silmon James Biggs; Werner Jenninger
Original assignee: Bayer Material Science Ag
Priority date: 2011-09-29
Filing date: 2012-09-28
Publication date: 2013-04-04

Abstract

The present invention relates to a dielectric elastomer layer having a first surface and a second surface, wherein at least one of the first surface and the second surface has a two-dimensional structuring. The present invention also relates to a transducer made from the inventive dielectric elastomer layer. Transducers made from the inventive dielectric elastomer layer may find use in sensors, actuators and/or generators.

Description

ELECTROMECHANICAL CONVERTER COMPRISING SUCH

This application claims the benefit, under 35 USC § 119(e), of United States provisional patent application numbers: 61/540,698, filed September 29, 201 1, entitled "DIELECTRIC ELASTOMERS HAVING A TWO- DIMENSIONALLY STRUCTURED SURFACE, AND

ELECTROMECHANICAL CONVERTER COMPRISING SUCH DIELECTRIC ELASTOMERS"; the entire disclosure of which is hereby incorporated by reference.

The present invention relates in general to polymers, and more specifically, to electroactive polymers having a two-dimensional iy structured surface. The invention further relates to transducers made with such dielectric elastomers.

Transducers and electromechanical converters play an important role in the conversion of electrical energy into mechanical energy and vice versa. Electromechanical converters can therefore be used as sensors, actuators and/or generators. Electromechanical converters may include a dielectric elastomer layer, for example in the form of an elastomer film, which is provided on both sides, at least in sections, with a conductive coating as the electrode, to produce a capacitor structure. If a voltage is applied to the electrodes, an electrostatic attraction occurs between the electrode layers, and as a result, the thickness of the dielectric elastomer layer is reduced and at the same time, because of the invariance of the dielectric elastomer layer in terms of volume, longitudinal expansion occurs. When the dielectric elastomer expands, electrodes in contact with the elastomer must be able to follow that expansion. In the case of flat elastomer layers, this requirement generally calls for expandable electrode structures or electrode materials. Structured surfaces of the elastomers have been proposed as a way of overcoming this limitation, e.g. U.S. Pat. Nos. 7,199,501; 8,042,264; EP1848046; and U.S. Published Patent Application No. 2011/0154641.

Methods are known in the art for fabricating films in which the inadequate expandability of the electrode materials is compensated for by a wavy surface of the dielectric elastomer layer, specifically for silicone elastomer films, e.g. U.S. Pat. Nos. 7,573,064; and 7,518,284.

Unfortunately, the expandability of many electrode materials is too lo w for numerous applications, especially in combination with dielectric elastomers. Furthermore, so-called electrel materials also have insufficient expandab lit to be useful in many applications. Electret materials allow electric charges to be introduced permanently into a film. Therefore, a need continues to exist in the art for electroactive olymers having a two-dimensionally structured surface.

The present invention improves upon the purely one-dimensional expandability of the materials known in the art by providing electroactive polymers having a two-dimensionally structured surface. The inventive transducers and electromechanical converters made from the inventive dielectric elastomer layers may find use in sensors, actuators and/or generators.

These and other advantages and benefits of the present invention will be apparent from the Detailed Description of the invention herein below. BRIEF DESCRIPTION. OF THE FIGURES

The present invention will now be described for purposes of illustration and not limitation in conjunction with the figures, wherein:

Figure 1 shows an example of a two-dimensional corrugation in the form of a concentric wave pattern which has the form of the propagation of a wave on a liquid surface following punctual excitation (so called "water drop"); and

Figure 2 illustrates a side view of the wave pattern of Fig, 1 ; and Fig. 3 shows a corrugated surface profile with a linewise offset of half a wavelength.

The present invention will now be described for purposes of illustration and not limitation. Except in the operating examples, or where otherwise indicated, all numbers expressing quantities, percentages, and so forth in the specification are to be understood as being modified in all instances by the term "about,"

Dielectric elastomers within the scope of the present invention are elastomers which can change shape by application of an electric field, in the case of elastomer films, the thickness can be reduced, for example, while at the same time there is a longitudinal expansion of the film in the surface direction. Pre- stretching allows the direction of the expansion to be largely fixed in a desired direction.

Examples of electroactive polymer devices and their applications are described, for example, in U.S. Pat. Nos. 7,394,282; 7,378,783; 7,368,862;

7,362,032; 7,320,457; 7,259,503; 7,233,097; 7,224,106; 7,21 1,937; 7,199,501 ;

7,166,953; 7,064,472; 7,062,055; 7,052,594; 7,049,732; 7,034,432; 6,940,221; 6,91 1,764; 6,891,317; 6,882,086; 6,876,135; 6,812,624; 6,809,462; 6,806,621 ;

6,781,284; 6,768,246; 6,707,236; 6,664,718; 6,628,040; 6,586,859; 6,583,533;

6,545,384; 6,543,110; 6,376,971 ; 6,343,129; 7,952,261 ; 7,911,761 ; 7,492,076;

7,761,981 ; 7,521 ,847; 7,608,989; 7,626,319; 7,915,789; 7,750,532; 7,436,099;

7, 199,501; 7,521,840; 7,595,580; and 7,567,681, and in U.S. Patent Application Publication Nos. 2009/0154053; 2008/01 16764; 2007/0230222; 2007/0200457;

2010/0109486; and 201 1/128239, and PCT Publication No. WO2010/054014, the entireties of which are incorporated herein by reference.

The present invention provides a dielectric elastomer layer having a first surface and a second surface, wherein at least one of the first surface and the second surface has a two-dimensional structuring.

The present invention also provides a transducer, including at least one dielectric elastomer layer having a first surface and a second surface, a conductive first layer which covers the first surface at least in sections, and a conductive second layer which covers the second surface at least in sections, wherein the dielectric elastomer layer has a two-dimensional structuring.

The present invention further provides a method for fabricating an electroactive polymer layer involving forming a dielectric polymer film, curing it until adequate green strength is present, and embossing the film with a structure or pattern.

The present, invention lies in the production of a preferably periodic structuring of the surface ("corrugation"), for example a wave profile, in two independent directions, to provide increased expandability of the surface. A two- dimensional structuring within the scope of the present invention may be distinguished from a one-dimensional structuring, as shown for example in U.S. Pat. No. 7,518,284, b the fact that a structuring is provided in both dimensions of the plane of the dielectric elastomer.

A disadvantage of two-dimensional wave profiles is that non-structured and accordingly non-expandable regions may be present in the valleys of the structure where patterns emanating from different center points intersect. The present inventors recommend attention should be paid to these flat regions. To overcome this problem, the present inventors recommend the production of an "offset" corragation. In this "offset" corragation, hills and valleys are arranged in lines offset by half a wavelength, A corrugated or wavy surface of a dielectric elastomer film (in 1 or 2 directions) is preferably produced by embossing techniques known in the art.

Thermoplastic materials e.g., thermoplastic elastomers (TPE), may be useful as the dielectric elastomenc layer of the present mvention. Such materials include styrenic block copolymers, polyolefin blends, elastomeric alloys, polyurethanes, polyesters, polycarbonates, polyamides, and copolymers of these polymers. Thermoplastic polyurethanes (TPU) are particularly preferred in the present invention. Such materials are readily available in commerce and well known to those of ordinary skill in the art. Thermoplastic materials allow the corrugated or wavy surface to be produced in the context of the present invention by heating until the material softens, embossing and cooling. This is not readily possible in crosslinked systems (e.g. conventional elastomers). However, thermoplastic systems may show unacceptable levels of creep under stress which may be inappropriate for some applications.

One embodiment for actuator applications involves the use of crosslinkable or partially cross-linked materials in which the material is cast onto an appropriately shaped form and crosslink the material in situ. Thermal curing or irradiation curing may be used to crosslink the material. Another embodiment involves preparing a partially cross-linked film, stretching it over an appropriately shaped form, exposing the film to additional heat or irradiation to more fully crosslink it, and then releasing the film from the form. Yet another embodiment involves preparing a crosslinked or partially crosslinked film, embossing or stretching it over an appropriately shaped form, depositing one or more electrodes onto the stretched film, and then releasing the film from the form. Partially crosslinked films may be prepared by exposure to minimal thermal or irradiative treatments which leaves a fraction of functional groups unreacted. Alternatively, partially crosslinked films may be prepared using dual-chemistry systems, e.g. a combination of irradiative curing and thermal curing. For example, the film may be first crosslinked by UV exposure, stretched over an appropriately shaped form, and then fully cured with a subsequent themal treatment. Alternatively, the film may be formed by thermal curing and subsequently cured further while stretched over an appropriately shaped form using irradiation. Any of these methods may be carried out as a batch or continuous process.

Embossing techniques may also be used, even with crosslinkable materials. A surface iexture may be imposed on a film that is exposed to heat and pressure in an embossing step. It is known in the art that the bonds of many polymers including silicones, polyesters, and polyurethanes, can open with thermal exposure and then reform or reconfigure upon cooling. For example, some polyurethane bonds begin to open at temperatures above about 150°C, and these polyurethanes therefore soften and can be embossed in that range. On cooling, the polyurethane bonds close again rearranging the backbone chain bonds and locking the embossed texture in place. Embossing techniques in the present invention may be carried out in a batch process or a roll-to-roll process as known in the art.

A preferred method for embossing includes: casting of an electroactive polymer film, partial curing until adequate green strength is present (optionally with dual cure systems as catalysts), then embossing of the structure/pattern and complete curing. The structure/pattern cars be either one or two-dimensional .

Various patterns may be embossed in the practice of the present invention as shown in Figure 1 which is an example of a two-dimensional corrugation, in the form of a concentric wave pattern which has the form of the propagation of a wave on a liquid surface following punctual excitation (a so called "water drop" pattern).

Figure 2 shows a side view of the wave pattern of Fig. 1 , and Figure 3 shows a corrugated surface profile with a linewise offset of half a wavelength.

Using the methods described herein to produce dielectric elastomer layers used in electroactive polymer actuators, it is possible, in actuator mode, on application of a small external electric voltage, to produce quadratically higher actuator effects (force, expansion), analogously in generator technology.

The present inventors contemplate electromechanical converters and transducers made irom the inventive dielectric elastomer layers may find use in sensors, actuators and/or generators.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims

WHAT IS CLAIMED IS:

1. A dielectric elastomer layer having a first surface and a second surface, wherein at least one of the first surface and Ihe second surface has a two- dimensional structuring.

2. The dielectric elastomer layer according to Claim 1, wherein the two- dimensional structuring is two-dimensional periodic.

3. The dielectric elastomer layer according to any one of Claims 1 or 2, wherein the two-dimensional periodic structuring is wave-shaped.

4. The dielectric elastomer layer according to any one of Claims 1 to 3, wherein the two-dimensional structuring produces a concentric wave pattern.

5, The dielectric elastomer layer according to any one of Claims 1 to 4, wherein the two-dimensional structuring produces an offset pattern.

6. The dielectric elastomer layer according to any one of Claims 1 to 5, wherein the two-dimensional structuring produces a pattern of a iinewise offset of half a wavelength.

7. The dielectric elastomer layer according to any one of Claims 1 to 6, wherein the two-dimensional structuring is produced by embossing.

8. The dielectric elastomer layer according to any one of Clairas 1 to 6, wherein the two-dimensional structuring is produced by casting.

9. The dielectric elastomer layer according to any one of Claims 1 to 8, wherein the layer comprises a thermoplastic elastomer.

10. The dielectric elastomer layer according to any one of Claims 1 to 9, wherein the layer comprises a crosslinked elastomer.

11. The dielectric elastomer layer according to any of Claims 1 to 10, wherein the layer comprises one of styrenie block copolymers, polyolefin blends, e!astomeric alloys, polyurethanes, polyesters, polycarbonates, polyamides, silicones, and copolymers of these polymers.

12. A transducer, compri sing :

at least one dielectric elastomer layer according to any one of Claims 1 to 11 having a first surface and a second surface,

a conductive first layer which covers the first surface at least in sections, and a conductive second layer which covers the second surface at least in sections,

wherein the dielectric elastomer layer has a two-dimensional structuring.

13. The transducer according to Claim 12, wherein the two-dimensional structuring is two-dimensional periodic.

14. The transducer according to any one of Claims 12 and 13, wherein the two-dimensional periodic structuring is wave-shaped.

15. One of a sensor, actuator and generator including the dielectric elastomer layer according to any one of Claims 1 to 1 1.

16. One of a sensor, actuator and generator including the transducer according to any one of Claims 12 to 14.

17. A method for fabricating an electroactive polymer layer comprising forming a dielectric polymer film, curing it until adequate green strength is present, and embossing the film with, a structure or pattern. ] 8. The method according to Claim 17, wherein the film is iurther cured after embossing.