Patent Application
20100139413
The invention relates to a sensor as integrated component of an article, in particular an elastic article, where the sensor comprises a polymeric material.
A sensor of the abovementioned type is known from WO 00/65533 A1. Detectable material particles are embedded in a matrix, in particular a matrix composed of plastic, rubber or rubber-like material. These material particles are, in particular, metal pieces, permanent magnets or polymer particles. Magnetizable material, in particular ferrite material, can also be mixed into the matrix. This matrix is, in particular, in strip form. This sensor matrix is part of a device for coding, i.e. identification and addressing, and also for marking of articles and is employed, in particular, for the following elastic articles:
With regard to the devices using sensors, reference is made, in particular, to the following documents: DE 100 25 504 C1, WO 03/076953 A2, WO 2007/109896 A1 and U.S. Pat. No. 5,802,030.
In the context of a further development, it is an object to provide a sensor which firstly retains the standard properties of a polymeric material and secondly is sensitive to strain changes and excitation frequencies to measure the loading state of an article in order to be able to recognize overloading or predict failure during operation.
This object is achieved by the polymeric material being provided with electrically conductive additives and acting as strain sensor by measuring static and dynamic strain in the article in relation to the forces acting on the article and in addition monitoring the changes in the polymeric material over time resulting from the static and dynamic strain in the article.
As regards the polymeric material which can be provided with electrically conductive additives, the following variants in particular are used:
Variant A
The polymeric material is an elastomeric material based on a vulcanized rubber mixture which comprises an unblended rubber component or a rubber component blend, electrically conductive additives and customary mixture ingredients. As rubber components, particular mention may be made of:
ethylene-propylene copolymer (EPM)
ethylene-propylene-diene copolymer (EPDM)
nitrile rubber (NBR)
(partially) hydrogenated nitrile rubber (HNBR)
fluoro rubber (FR)
chloroprene rubber (CR)
natural rubber (NR)
styrene-butadiene rubber (SBR)
isoprene rubber (IR)
butyl rubber (IIR)
bromobutyl rubber (BIIR)
chlorobutyl rubber (CIIR)
butadiene rubber (BR)
chlorinated polyethylene (CM)
chlorosulfonated polyethylene (CSM)
polyepichlorohydrin (ECO)
ethylene-vinyl acetate rubber (ACM)
silicone rubber (MVQ)
fluorinated methylsilicone rubber (FFPM)
perfluorocarbon rubber (FFCR)
polyurethane (PU)
The use of a blend, in particular in combination with one of the abovementioned types of rubber, is also possible.
EPM, EPDM, HNBR, CR, NR, FR or a blend of NR and BR are of particular importance.
The electrically conductive additives are present in an amount of from 3 to 100 phr (parts per hundred of rubber), associated with the following differentiation:
The customary mixture ingredients comprise at least one crosslinker or a crosslinker system (crosslinker and accelerator). Further mixture ingredients are usually a processing aid and/or a plasticizer and/or an aging inhibitor and if appropriate further additives. In this respect, reference may be made to general knowledge in rubber mixture technology.
Variant B
The polymeric material is a thermoplastic elastomer (TPE) which is provided with the electrically conductive additives and comprises customary mixture ingredients (DE 100 04 632 A1).
Thermoplastic elastomers employed are, in particular, thermoplastic elastomers based on styrene (TPE-S), uncrosslinked or partially crosslinked olefin-based thermoplastic elastomers (TPE-O) or fully crosslinked olefin-based thermoplastic elastomers (TPE-V). A blend of a thermoplastic polymer and an at least partially crosslinked rubber (e.g. EPDM) is also possible.
The electrically conductive additives are present in an amount of from 0.5 to 20% by weight, in particular from 1 to 15% by weight, more particularly from 1 to 10% by weight. Amounts of from 1 to 3% by weight are usually sufficient.
Variant C
The polymeric material is a polymer which is provided with the electrically conductive additives.
Use is here made of, in particular, a fluorine-containing polymer, in particular polytetrafluoro-ethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF) or chemical modifications thereof.
As regards the proportion of electrically conductive additives, what has been said in the case of variant B also applies here.
Regardless of the abovementioned polymeric material variants, the polymeric strain sensor is characterized by a filler concept based on electrically conductive additives. These are, in particular, metals and/or metal compounds and/or carbon black and/or carbon fibers and/or graphite and/or nanotubes and/or ionic liquids. These materials are suitable for influencing a dielectric in a targeted manner and optimizing it for the measurement principle which represents the actual basis of the measurement.
The filler-filler interactions are reduced by strain as a function of the dynamic stress (amplitude, frequency, time) and only partly recombine after interruption of the mechanical stress. The material therefore acts as a strain-dependent (variable) resistance. The change in the material can be monitored over the number of loading cycles as a result of the only partial recombination of the filler-filler interaction. In addition, the relaxation time after mechanical deformation has been stopped is greatly dependent on the preceding deformation amplitude. The combination of these two effects opens up the possibility of constructing a strain sensor which measures the mechanical stress and aging state of polymers.
Advantageous specifications of electrically conductive additives are given below:
The use of conductive carbon blacks has been found to be particularly advantageous compared to other solutions since the filler network formed therefrom is able to conduct adequate currents and react very sensitively to strain changes. In addition, when they replace standard carbon blacks in polymer formulations, in particular in rubber mixture formulations, they lead to barely any significant changes in the physical properties.
If carbon fibers are used in addition to a conductive carbon black, the change in the conductivity can be optimized further. The relaxation times are significantly shortened so that rapid dynamic deformations are also readily measured. In addition, this combination reduces the decrease in the conductivity with increasing deformation amplitude, so that relatively large deformation displacements can be measured more easily.
The electrically conductive additives are uniformly distributed within the polymeric material. Furthermore, they can be additionally coated with a low molecular weight compound. They are in this case coated with, in particular, a wax and/or a fat and/or an oil and/or stearic acid and/or an aging inhibitor and/or a short-chain polymer.
The polymeric materials obtained by means of the conductive additives can in terms of their conductivity be classified into:
The moderately to readily conductive materials are suitable for conductivity measurements independently of the frequency (DC conductivity and dielectric properties). The moderately conductive to antistatic materials, on the other hand, are preferably suitable for characterization at relatively high frequencies (dielectric properties).
Three mixture examples based on material variant A are given below.
Mixture Example 1
A rubber mixture based on 100 phr of sulfur-modified chloroprene rubber contains conductive additives in a total amount of 30 phr, namely:
This rubber was crosslinked with the customary mixture ingredients (e.g. waxes, aging inhibitors) with the aid of metal oxides (mixture of MgO and ZnO). This gives a vulcanizate having a hardness of 65 Shore A (in accordance with DIN 53505) and an elongation at break of 750% (in accordance with DIN 53504). The electrical volume resistance in accordance with DIN IEC 93 is 13 Ω.
If the DC conductivity of this material at a potential of 10 volt is measured on a strip having dimensions of 50×12×2 mm (length×width×thickness), the following behavior as a function of the elongation is observed:
80 kΩ at 0% elongation; 20 kΩ at 10% elongation; 200 kΩ at 20% elongation; 300 kΩ at 35% elongation; 550 kΩ at 50% elongation; 1.1 MΩ at 75% elongation; 2.0 MΩ at 100% elongation.
After releasing the stress, the conductivity returns to virtually the initial level in a very short time. This property profile makes the material very well suited as sensor material for monitoring very fast dynamic deformations, for example as load cycle counter.
Mixture Example 2
A rubber mixture based on a blend of 40 phr of natural rubber and 60 phr of butadiene rubber contains 65 phr of a standard carbon black as conductive additive, namely:
This rubber blend was crosslinked with the customary mixture ingredients (e.g. waxes, aging inhibitors) by means of sulfur (sulfenamide-accelerated and ZnO-activated). This gives a vulcanizate having a hardness of 60 Shore A (in accordance with DIN 53505) and an elongation at break of 500% (in accordance with DIN 53504). The electrical volume resistance in accordance with DIN IEC 93 is 1.2 lΩ.
If the polarizability of this material is measured in AC conductivity measurements at 200 kHz, the following results are obtained as a function of the degree of fatigue of the material:
Real part of the relative permittivity (εr′), measured on a specimen geometry of 5 mm diameter and 2 mm thickness, applied potential 500 mV, frequency 200 kHz, measuring instrument Agilent 4294A Precision Impedance Analyzer:
The dynamic fatigue test was carried out by a method based on DIN EN ISO 7854 on a flexural testing machine in accordance with ISO 132 (test specimen: 120×40 mm, bending frequency 5.0 Hz). The maximum number of load cycles is determined by the occurrence of cracks and the associated destruction of the test specimen. In this example it was 5 million bending cycles.
The sensitive monitoring of the dielectric property over a very long time under stress makes this example particularly suitable as sensor material for monitoring long-term fatigue. The state of fatigue of the material and the expected failure can be predicted reliably in this way, for example during operation. In addition, this material can also be utilized for monitoring the number of load cycles.
Mixture Example 3
A rubber mixture based on 100 phr of natural rubber contains conductive additives in a total amount of 25 phr, namely:
This rubber was crosslinked with the customary mixture ingredients (e.g. waxes, aging inhibitors) by means of sulfur (sulfenamide-accelerated and ZnO-activated). This gives a vulcanizate having a hardness of 67 Shore A (in accordance with DIN 53505) and an elongation at break of 400% (in accordance with DIN 53504). The electrical volume resistance in accordance with DIN IEC 93 is 3.6 Ω.
This material is characterized by extremely good DC conductivity combined with a high AC conductivity. In addition, the dielectric properties over a frequency range from 100 Hz to 500 kHz in the elongation range up to 200% have been found to be independent of frequency and elongation (i.e. constant).
These properties make the material particularly suitable as an electrode material for dynamic and especially dielectric measurements during dynamic loading of the elastomer product.
The novel sensor concept presented here has the advantage, as shown in the examples, that the sensor material can in each case be matched very specifically to the application, for example in respect of hardness, conductivity and polymer system.
Another significant advantage is that a strain sensor based on elastomers makes it possible for it to be joined to the article directly during vulcanization, so that no additional defects or adhesive bonds occur.
In addition, it is found that polymeric conductors can also be used as contacts, as shown in mixture example 3, for example by elastomers filled with from 3 to 10 phr of carbon nanotubes. These conductors can likewise be embedded in the article without defects during vulcanization. Instead of a polymeric conductor, reinforcing materials which have good conductivity (e.g. with metal filaments incorporated in, the woven fabric) can also be used as contacts.
The polymeric strain sensor can be a component of:
Apart from the use as strain sensor for monitoring the stress state and the stress history of an article, applications in overload controls, controls for the tightening torque of fastener/screw connections and pressure-sensitive “switching” (for example in optical controls or musical keyboard instruments) are also possible.
Finally, the abovementioned strain sensor in all its material variants is a component of a sensor arrangement, in particular as part of the following component combinations:
In connection with the figure description 2, this sensor arrangement will be described in more detail.
The invention will now be illustrated with the aid of examples with reference to schematic drawings. In the drawings:
The sketch of the resistance-elongation curve of
The elongation of an elastic component, for example a pneumatic spring bellows, breaks the filler-filler interaction which, after interruption of the mechanical stress, recombines only partially. In this way, the material acts as strain-dependent (variable) resistance. The only partial recombination of the filler-filler interaction enables the change in the material over the number of load cycles to be monitored. In addition, the relaxation time after the mechanical deformation is released is strongly dependent on the preceding deformation amplitude. The combination of these two effects makes it possible to construct a strain sensor which measures the mechanical loading and aging states of polymers. This situation is schematically shown here with the aid of the two curves a (R/R0) and b (elongation) in
In a preferred embodiment, the polymeric strain sensor 2 is in the form of a strip. The strip is contacted in a suitable way, for example by means of a polymeric conductor having mixed-in carbon nanotubes, and connected in series with a fixed resistance 3. This series arrangement is provided with current from a controlled current/potential source 7.
In favorable cases, the elastomeric material of the article can itself also be modified so that it can be utilized directly as sensor material.
The strain-dependent resistance modulates the current, as a result of which a variable potential which is proportional to the resistance change in the strain sensor 2 and thus also proportional to the elongation of the strain sensor is obtained over the fixed resistance 3. In view of the only partial recombination of the filler-filler network, a memory for the most recently achieved measured values is advantageous (level memory). The evaluation of the electric signal is carried out by a microcontroller 5 having an analog/digital converter 4 and an integrated memory. The measured data can be transmitted electrically or alternatively by radio or via field coupling to the data processing unit.
The controlled current/potential source 7 makes it possible to generate even complex excitation signals, for example high-frequency AC potentials, in order to be able to monitor the relaxation behavior and/or the dielectric properties.
The energy supply 8 to the total sensor comprising strain sensor 2 and microcontroller 5 is effected either electrically, via field coupling or by means of a dedicated piezoelectric generator unit.
A further advantageous possibility is provision of an additional comparative sensor, for example in the form of a sensor strip, outside the dynamically stressed region which allows continual comparison between stressed and unstressed specimen. This can serve to allow for temperature effects caused by the dynamic load and feed them into the data processing unit.
Purely static aging of the material also leads to a change in the polymer-filler and filler-filler interaction. The comparative sensor enables these effects, too, to be taken into account. In addition, the proposed arrangement can also be utilized for measuring static aging.
As a result of the strain-dependent resistance changes of the sensor material and their relaxation being temperature dependent, the strain sensor 2 can in principle also be utilized for temperature measurement.
In one variant, the total polymeric material of the pneumatic spring bellows 12 can be provided with electrically conductive additives so that the total pneumatic spring bellows forms the strain sensor.
In a further variant, the polymeric material of the article can be provided with electrically conductive additives in at least one segment in the pneumatic spring bellows region Z. Within this segment, the strain sensor is, in particular, in the form of a strip. The strain sensor can in this case be an integral component of the pneumatic spring bellows 12 or be installed as additional component on or especially in the pneumatic spring bellows. When the strain sensor is used as additional component, exchangeability would also be able to be achieved.
(Part of the Description)
1 Sensor arrangement
2 Strain sensor
3 Fixed resistance
4 Analog/digital converter
5 Microcontroller with memory
6 Radio interface
7 Controlled current/potential source
8 Energy supply (rectifiers and energy store)
9 Coupling coil
10 Receiver (radio antenna)
11 Pneumatic spring arrangement
12 Pneumatic spring bellows
13 Lid
14 Rolling piston
15 Volume-elastic air chamber
X Loading cycles
Y Resistance
Z Pneumatic spring bellows region with strain sensor
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 030 680.8 | Jul 2007 | DE | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2008/057833 | Jun 2008 | US |
| Child | 12655497 | US |
