JP2021513580A - Structured composition useful as a low force sensor - Google Patents

Abstract

A composite material, a) a porous matrix material containing a siloxane polymer, comprising a closed porosity volume fraction and optionally an open porosity volume fraction, and b) the porous matrix material a). Composites containing conductive or semi-conductive fillers substantially present in the closed porosity volume fraction, films containing the composites, coating substrates and multilayer systems and their use in pressure sensing devices. [Selection diagram] None

Description

The present invention relates to a composite material comprising a porous siloxane polymer matrix having a closed porosity volume fraction and a conductive or semi-conductive filler substantially present in the closed porosity volume fraction of the matrix. ..

Pressure sensors have recently received a lot of attention due to their various different applicability. Especially in the low pressure region, the demand for high sensitivity pressure sensors is very high, and those systems are needed especially for healthcare and medical diagnostic systems, and in electronic systems, especially so-called e-skin systems.

Pressure sensing devices are typically classified into three types, depending on the parameters used for detection. Piezoresistive devices take advantage of changes in conductivity when external pressure is applied. Piezoelectric devices utilize the piezoelectric effect, that is, the generation of potential changes in the material during pressurization. The sensitivity of the piezoelectric device is limited by the physical properties of the piezoelectric material at the origin of the effect. On the other hand, piezo capacitance sensors utilize capacitance changes that occur in response to pressurization, and their sensitivity is theoretically unrestricted. The change in capacitance changes in response to pressurization or due to a change in the equivalent dielectric constant of the dielectric material sandwiched between the two electrodes under pressurization, the distance between the two electrodes of the system forming the capacitor. Can be the result of.

Compared to piezoresistive devices, piezo capacity sensors offer several advantages such as low power consumption and better reproducibility. Compared to piezoelectric devices, piezo capacitance sensors are easier to process and easier to shape into different forms. Moreover, they do not require polling or stretching.

The magnitude of the capacitance change is determined by the change in the dielectric constant, the thickness of the dielectric layer and the surface area of the electrode.

Micro- or nano-structures have been proposed in such devices to improve sensitivity, especially in the low pressure range. However, this usually requires a complex and costly manufacturing process.

B. Y. Lee et al., Sensors and Actuators A 240 (2016), 103-109 describe a low cost pressure sensor based on a dielectric elastomer film with micropores. Porous films are prepared by using a siloxane elastomer material and water droplets without any additives. Polydimethylsiloxane is used as the base material and water droplets are selected as the dispersant. A solution of PDMS prepolymer mixed with a curing agent, and water are stirred in the vessel. The stirring process uniformly disperses fine droplets of water in the PDMS solution due to the insolubility of water. The solution thus obtained is placed between the two glass substrates and then the solution is cured. During curing, the water evaporates, resulting in a polymerized porous PDMS film with micropores where the water was originally present. This film, which has a thickness of approximately 100 μm, forms the dielectric layer of the capacitive type pressure sensor.

A. J. Gallant, Processia Chemistry 1 (2009), 568-571, relates to a force-sensitive porous PDMS register. The force-sensitive elastomer register is made from a porous matrix of PDMS filled with carbon black. The PDMS matrix has the form of a sponge and is obtained using a sugar scaffold. The sugar cube is placed in a dish containing a PDMS precursor and left for 1 hour to saturate with PDMS. The sugar cube is then cured, excess PDMS is removed, and the sugar cube is placed in a beaker containing distilled water to dissolve the sugar. The structure thus obtained is an inverse matrix of sugar cubes in which the voids are distributed and oriented in a random configuration. To introduce the carbon black particles, a suspension of carbon black in water is added dropwise to the water saturated sponge, thereby producing a high concentration of carbon within the open porosity volume fraction of the sponge. As soon as it is filled, the sponge is left to dry and a thin layer of PDMS is coated over it and cured to seal the carbon inside the sponge. In the sponge, the pore walls are lined with carbon. Upon pressurization, the carbon black-lined pore walls come into contact, thereby increasing the number of carbon-carbon connections and making the pores conductive.

S. J. A. Majerus, "Flexible, structured MWCNT / PDMS sensors for chronic vascular access monitoring", IEEE Sensors Book Series: IEEE sensors, published 2016-Conference 15 th IEE Sensors conference Orlando, FL Oct. 30-Nov. 03, 2016 relates to a piezoresistive flexible pulsation sensor obtained by applying a so-called laminated manufacturing method for printing PDMS having an internal porous structure. The pores are reported to have an average pore diameter of approximately 1 mm. Multiwalled carbon nanotubes are added during the manufacturing process to obtain a conductive sensor. The intrinsic resistance is said to be non-linear, and hysteresis was observed. Both are undesired effects.

It has been an object of the present invention to provide a composite material suitable for use in a piezo capacitance sensor that provides high sensitivity and good reproducibility.

This object is achieved with a composite material according to claim 1. Preferred embodiments of the present invention are set forth in the dependent claims and in the specification detailed below.

A further object of the present invention is a film containing a composite material according to claim 1 and a substrate coated with a film made of the composite material according to the present invention.

Composite materials according to the present invention
a) A porous matrix material containing a siloxane polymer, comprising a closed porosity volume fraction and optionally an open porosity volume fraction.
b) Includes a conductive or semi-conductive filler that is substantially present in the closed porosity volume fraction of the porous matrix material a).

Porous materials are usually characterized by their porosity. Porosity or void ratio is a measure of void (ie, "empty") space in a material and is a fraction of the volume of the void divided by the total volume, 0 to 1, or 0% to 100% as a percentage. is there.

Apparent porosity or open porosity (oPo) is the volume of open porosity that is part of the porosity and through which a liquid or gas can penetrate, as a percentage of the total volume of the material.

Non-interconnected voids trapped in the solid phase are not part of the open porosity volume fraction; they are part of the closed porosity volume fraction. This fraction also includes all kinds of closed pores in the material.

The open porosity and the closed porosity add up to the total porosity of the material.

A porous matrix material of a composite material according to the present invention comprises a closed porosity volume fraction in which a substantial portion of the conductive or semi-conductive filler is present.

According to a preferred embodiment of the present invention, the closed porosity volume fraction is preferably greater than or equal to the open porosity volume fraction of the material, i.e., the volume of pores forming the closed porosity volume fraction is preferably. , At least equal to or greater than the volume of the pores forming the open porosity volume fraction (thus preferably the ratio of both pore volume fractions is at least 1).

According to a particularly preferred embodiment, the ratio of closed porosity volume to open porosity volume is in the range 1: 1-100: 1, preferably in the range 1.5: 1-50: 1.

The open porosity volume fraction of the porous material can be measured by gas replacement pycnometry, a technique known to those of skill in the art. This technique uses gas replacement methods to accurately measure volume. An inert gas, usually He, is used as the replacement gas. A sample of known weight is sealed in a compartment of a measuring instrument with a known volume. He is then allowed to flow through the inlet valve into the chamber until equilibrium is reached, i.e., until the pressure is constant. The inlet valve is then closed and the outlet valve to the second chamber of exactly the known volume is opened. The pressure observed when the sample chamber is filled and then when the gas is discharged into the second empty chamber is the sample solid volume (that is, the volume of gas replaced by the solid portion of the sample plus the gas). Allows the calculation of (equal to the volume of inaccessible pores). The helium gas quickly fills even the small pores, and only the volume portion of the sample that is inaccessible to the He gas pushes the gas away. This portion of the sample consists of the solid portion of the sample plus the volume represented by the closed porosity volume fraction (similar to defined as inaccessible to gas).

The volume pushed away by the sample is _{shown as V s} , the known volume of the sample cell _{is shown as V c} , the volume of the second compartment where the gas is pushed into _{it is V r} , and the pressure after filling the sample cell. There is P _a, and when the pressure after the expansion into the compartment cell is P _e, the volume that is displaced by the sample,
V _s = V _c- V _r (P _e / (P _a- P _e ))
Can be calculated as.

The displaced volume or the specific gravity bottle volume Vs reflects the volume of the solid portion of the porous sample (which is referred to herein as the theoretical volume) plus the volume of the closed pores. The theoretical volume can be obtained from the theoretical density of a solid sample without pores, which is usually known for most materials or can be easily measured. Subtracting the theoretical volume from the specific gravity bottle volume yields the volume of the closed pores.

The bulk volume of the porous sample is the geometric volume of the porous sample, which is the sum of the theoretical volume plus the volume of the closed pores plus the volume of the open pores. Therefore, the volume of open pores can be obtained by subtracting the theoretical volume and the closed pore volume (obtained as described above) from the bulk (geometric) volume of the sample.

The open porosity volume fraction is obtained by dividing the volume of the closed pores by the bulk volume. The open porosity volume fraction can be obtained in a similar manner. The ratio of both fractions is then obtained by simply dividing the closed porosity volume fraction by the open porosity volume fraction.

The overall porosity of the porous sample can also be obtained by dividing the bulk density by the theoretical density and subtracting that value from 1.

Above may be the following illustrated by: sample with a theoretical volume and 3 cm ³ pycnometer volume of 2 cm ³ is 1 cm 3 enclosed porosity volume fraction ^(obtained by subtracting the theoretical volume specific gravity bottle volume) Has. If the porous sample has ^{a geometric (bulk) volume of 4 cm 3} , the overall porosity, relative to the bulk volume, is 2/4 or 0.5. The closed porosity volume fraction is 0.25 in this case in relation to the bulk volume and 0.5 in relation to the total pore volume of the sample. This results in a closed porosity volume fraction / open porosity volume fraction of 1.

With the same theoretical volume and bulk volume, if the specific gravity bottle volume is 3.5 cm ³ , the volume of the closed pores is converted to 37.5% based on the bulk volume or 75% based on the total pore volume. It is 1.5 cm ³ to be made. In this case, the closed porosity volume fraction to the open porosity volume fraction is 3: 1.

The composite matrix polymer according to the present invention is a siloxane polymer.

A siloxane polymer, or polysiloxane, also known as silicone, is a polymer containing an inert, synthetic compound composed of repeating units of siloxane, frequently bonded to carbon, hydrogen, or both. They are typically heat resistant and rubbery.

Siloxane is a functional group in organosilicon chemistry with a -Si-O-Si- bond. The word siloxane is derived from the words silicon, oxygen and alkanes. The siloxane material is represented by several types of so-called siloxide groups: general structural element R ₃ SiO _0.5 , M units, general structural element R ₂ SiO, depending on the number of Si—O bonds. It can consist of D units and T units represented by the _{general structural element RSiO 1.5.}

The siloxane functional group forms the main chain of silicone.

More precisely, polymerized siloxanes, or polysiloxanes, silicones consist of an inorganic silicon-oxygen skeleton chain (...- Si-O-Si-O-Si-O-...) with organic side groups bonded to silicon atoms. .. The side group is preferably selected from an alkyl group, an aryl group, or a combination thereof.

In some cases, organic side groups can be used to link two or more of these -Si-O-main chains together. By varying the −Si—O− chain length, side groups, and crosslinks, silicones with a wide variety of properties and compositions can be synthesized.

Organic side groups include alkyl, haloalkyl, aryl, haloaryl, alkoxyl, aralkyl and silacycloalkyl groups and alkenyl groups such as vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl and / or decenyl groups. It can be a more reactive group. Polar groups such as acrylate, methacrylate, amino, imino, hydroxy, epoxy, ester, alkyloxy, isocyanate, phenol group, polyurethane oligomer group, polyamide oligomer group, polyester oligomer group, polyether oligomer group, polyol and carboxypropyl group Any combination can be attached to the silicon atom of the siloxane main chain.

The siloxane can be terminated with any useful group such as an alkenyl and / or alkyl group, such as a methyl, ethyl, isopropyl, n-propyl or vinyl group or a combination thereof. Other groups that can be used to terminate siloxane are acrylate, methacrylate, amino, imino, hydroxy, epoxy, ester, alkyloxy, isocyanate, phenol group, polypolyurethane oligomer group, polyamide oligomer group, polyester oligomer group, poly. Ether oligomer groups, polyols and carboxypropyl groups and halos such as fluoro groups.

Polydialkylsiloxane (where the organic group is an alkyl group) is the preferred group of siloxane polymers suitable for use in the composites of the present invention.

（式中、出現ごとに同じものであっても異なってもよい、Ａｌｋは、線状、分岐状又は環状アルキル基を表す）
で表され得る。 The polydialkylsiloxane polymer has the following general formula:

(In the formula, it may be the same or different for each appearance, Alk represents a linear, branched or cyclic alkyl group)
Can be represented by.

Preferred alkyl groups are linear or branched alkyl groups having 1-12, preferably 1-8, more preferably 1-4 carbon atoms.

The best known example of polydialkylsiloxane is polydimethylsiloxane, which is also the most preferred polydialkylsiloxane according to the present invention (Alk is a methyl group in the formula, hereinafter referred to as PDMS herein). .. The term polydimethylsiloxane or PDMS, as used herein, includes derivatives thereof such as PDMS endcapped with hydroxy-, vinyl-allyl-etc.

The composite material according to the present invention includes a conductive or semi-conductive filler substantially present in the closed porosity volume fraction of the microporous polymer matrix a) as component b).

"Substantially present" means that at least 50%, preferably at least 60%, even more preferably at least 70% of the filler is present in the closed porosity volume fraction for the purposes of the present invention. Means. 99% or less, preferably 95% or less, even more preferably 90% or less of the total content of the filler can be present in the closed porosity volume fraction of the composite.

The semi-conductive or conductive filler in the composition of the present invention can be selected from any material having semi-conductive or conductive properties.

Therefore, a suitable conductive filler can be selected from a list of metal particles such as copper, silver, gold, and zinc. Preferably, the conductive metal filler is silver or copper, more preferably silver.

Conductive polymer particles are essentially composed of, or are composed of, essentially conductive polymers (ICPs). They are organic polymers consisting of macromolecules with a fully conjugated sequence of double bonds along the chain. Such compounds can have metallic conductivity or can be semiconductors. Examples of essentially conductive polymers are polyacetylene, polythiophene, polypyrrole, or polyaniline. Among the ICPs, polythiophene and polyaniline are preferably used. More preferably, a polymer blend of poly (3,4-ethylenedioxythiophene) or PEDOT and, in particular PEDOT-PSS, poly (3,4-ethylenedioxythiophene) and poly (styrene sulfonate) is used.

The semi-conductive filler essentially consists of or consists of a semi-conductive material. The semi-conductive core generally comprises at least 95% by weight, preferably at least 97% by weight, more preferably at least 99% by weight of the semi-conductive material.

Generally, the semi-conductive material is selected from a list consisting of Si, Si-Ge, GaAs, InP, GaN, SiC, ZnS, ZnSe, CdSe, and CdS. Preferably, the semi-conductive material is selected from the list consisting of GaAs, SiC, ZnS and CdS. More preferably, the semi-conductive material is SiC.

Preferably, the semi-conductive filler is selected from a list consisting of GaAs, SiC, ZnS and CdS nanoparticles.

Another group of suitable fillers are metal oxide particles, such as ZnO, that typically contain a metal and an anion of oxygen in the oxidized state of -2.

According to one embodiment, a conductive or semi-conductive filler suitable for the present invention may have an aspect ratio close to 1. When the aspect ratio is close to 1, the particles tend to be spherical.

According to another embodiment, a conductive or semi-conductive filler suitable for the present invention may have an aspect ratio of greater than one. In this case, the aspect ratio is preferably at least 5, more preferably at least 10, even more preferably at least 15, most preferably at least 20. The particles usually have an aspect ratio of up to 5000, preferably up to 1000, more preferably up to 500, and even more preferably up to 200.

The aspect ratio is the ratio of particle length to width (ISO 13794: 1999). The average aspect ratio can be measured by a person skilled in the art by image processing of transmission electron microscopy (TEM) or scanning electron microscopy (SEM) photographs.

In some cases, metal fillers with an aspect ratio higher than 1 in the form of nanowires have been found to be advantageous. Particularly preferred nanowires are silver nanowires.

Another group of semi-conductive or conductive fillers for use in composites of the present invention are carbonaceous fillers.

For the purposes of the present invention, the term "carbonic filler" refers to at least 50% by weight elemental carbon, preferably at least 75% by weight elemental carbon, more preferably at least 90% by weight elemental carbon. Means a filler containing. A particularly preferred carbonaceous filler contains or consists of elemental carbon in an amount of 99% by weight or more.

Preferably, the carbonaceous filler is selected from carbon nanotubes, carbon nanohorns, graphite, graphene and carbon black. Carbon black is particularly preferred for economic reasons.

Graphene itself is usually thought of as a one-atom-thick flat sheet of ^{sp 2} -bonded carbon atoms that is tightly packed in a honeycomb structure. The name graphene is derived from graphite and the suffix ene. Graphite itself consists of a large number of graphene sheets stacked together.

Graphite, carbon nanotubes, fullerenes and graphene in the sense mentioned above share the same basic structural arrangement of their constituent atoms. Each structure begins with 6 carbon atoms and is chemically tightly bonded in a regular hexagonal shape (an aromatic structure similar to what is commonly referred to as benzene).

Carbon nanohorn is the name for the horn-shaped sheath aggregate of graphene sheet. Single-walled nanohorns (SWNHs) with tube lengths of about 40-50 nm and diameters of about 2-3 nm are derived from single-walled nanotubes (SWNTs) and end with five pentagonal conical caps with a conical opening angle of about 20. SWNHs can relate to each other to form "dahlia-like" and "bud-like" structured aggregates with an average diameter of about 80-100 nm. The former consists of a canal and a graphene sheet protruding from its surface like a dahlia petal, while the latter consists of a canal that grows inside the particle itself.

Carbon black (CAS 1333-86-4) is a form of paracrystalline carbon with a high surface area to volume ratio, albeit lower than the surface area to volume ratio of activated carbon.

Chemically, carbon black is a colloidal form of elemental carbon consisting of 95-99% carbon. It is usually obtained from partial combustion or pyrolysis of hydrocarbons and is a tufted morphology consisting of spheroidal primary particles, primary particle size uniformity within a given aggregate and a stratified layer within the primary particles. It exists as an agglomerate of chemicals.

Suitable carbonaceous fillers as described above are available from a variety of sources and suppliers, and those skilled in the art will follow the invention based on their expertise and specific application examples. A material suitable for use in composite materials will be selected.

In certain application cases, spherical nanoparticle fillers with an average diameter of 300 nm or less, preferably 200 nm or less, have been found to provide certain advantages.

The term average particle diameter of the spherical particles, as used herein, means a D ₅₀ median diameter calculated based on the intensity weighted size distribution as obtained by the so-called Contin data inversion algorithm. Generally speaking, D ₅₀ is, two equal parts the intensity weighted size distribution is divided into those of a larger size than and D ₅₀ of the size smaller than D _50.

Generally, the average particle size as defined above is measured according to the following procedure. First, if necessary, the particles are separated from the medium in which they can be contained (since there are various methods of making such particles, the products are dried in different forms, eg, well-formed. It may be available as particles or as a suspension in a suitable dispersion medium. Well-shaped particles are then preferably used for measuring the particle size distribution by a method of dynamic light scattering. Relatedly, it is recommended to follow the method as described in ISO Nom Particles size analysis-Dynamic Light Scattering (DLS), ISO 22412: 2008 (E). This standard is, among other things, instrument position (Section 8. Instructions are provided for 1.), system eligibility (Section 10), sample requirements (Section 8.2.), Measurement procedures (Sections 9.1-5 and 7) and reproducibility (Section 11). The measurement temperature is usually 25 ° C. and the refractive index and viscosity coefficient of each dispersion medium used should be known with an accuracy of at least 0.1%. After proper temperature equilibrium, the cell position will be system soft. It should be adjusted for the optimal scattered light signal according to the wear. The time average intensity scattered by the sample is recorded 5 times before starting the collection of the time automatic correlation function. Accidental through the measurement volume. An intensity threshold of 1.10 times the average of the five measurements of average scattering intensity may be set to eliminate possible signals of moving dust particles. , Appropriately adjusted by the system software, preferably in the range of about 10,000 cps. Subsequent measurements of the time automatic correlation function above which the mean intensity threshold set above is exceeded in the meantime are ignored. It should be.

A measurement usually consists of a typical duration of a few seconds each, and a suitable number of collections of automatic correlation functions allowed by the system according to the threshold criteria described above (eg, a set of 200 collections). Data analysis is then performed on the recording of the entire set of time-automatic correlation functions by using the Contin algorithm, which is available as a software package, usually included in the equipment manufacturer's software package.

The conductive or semi-conductive filler used in composites according to the present invention can deviate from the spherical shape, which is characterized by an aspect ratio close to 1.

Plate particles are also suitable. Typically, the plate-like particles have the shape of a plate or are essentially made up of particles that resemble a plate, or consist exactly of them, i.e. the particles are flat or nearly flat. , Their thickness is small compared to the other two dimensions.

Needle particles are also suitable. Typically, needle-like particles either have the shape of a needle, or essentially consist of particles that resemble needles, or consist of exactly those.

Finally, fibrous particles are also well known to those of skill in the art. Typically, fibrous particles have the shape of fibers or are essentially made up of particles that resemble fibers, or consist exactly of them, i.e. the particles are fine and very elongated and their length. The size is very large compared to the other two dimensions. Fibrous particles advantageously contained in polymeric compositions according to the invention, especially for the purpose of increased strengthening.
-Typically above 5, preferably above 10, and more preferably above 15 number average aspect ratios;
-Typically at least 50 μm, preferably at least 10 μm, more preferably at least 150 μm number average length; and-typically below 25 μm, preferably below 20 μm, more preferably below 15 μm. It has a number average diameter.

The average pore diameter, measured using image processing of the top view SEM image of the composite material according to the present invention, is preferably in the range of 0.1 to 200 μm, preferably in the range of 0.5 to 100 μm, and further. More preferably, it is in the range of 1 to 50 μm. In some cases, an average pore size of 10-30 μm has been found to be beneficial.

SEM is suitable for quantitative analysis of pore structure because it allows a wide range of magnifications, high depth of field, and produces digital data suitable for image analysis. SEM combines the best aspects of optical microscopy with TEM.

A typical procedure for measuring the average pore size is described in more detail as follows.

A grayscale analysis of the photo using the software package ImageJ is performed to threshold the photo to select the inner area of the pores and then measure the pore size distribution by using the Particle Analysis package. It was. This procedure allows identification of pore area distribution. Assuming that the pores have a spherical shape and are cut through their centers and therefore show their great circles, the pore diameter D is from the surface area A to the equivalent diameter, i.e. D = 2 square roots (A). / Pi) was obtained. The mean and standard deviation of the pore size was obtained by statistical analysis accumulating the pore size distribution over several photographs (eg, 10 photographs for a given sample).

According to another preferred embodiment of the invention, the composite material is in ^{the range of 10-5} to ^10-12 S / m, preferably in the range of ^10-6 to ^10-9 S / m, without external pressure, specific conductivity. Have a rate.

Conductivity or specific conductivity is the reciprocal of the electrical intrinsic resistance and measures the ability of a material to guide an electric current.

For a capacitive element, that is, an element that utilizes a change in capacitance when an external pressure is applied, it is desirable to obtain a high dielectric function of the material without making the material conductive.

To be considered satisfactory piezo capacitive sensor, relative change in capacitance under mechanical compression ΔC / C o _(Co represents the capacitance without the application of external pressure, whereas, [Delta] C is a given pressure It is desirable to obtain a large variation (representing the change in capacitance when applied). Increase in the amount of conductive filler increases the variation in [Delta] C / C _o.

The relative permittivity is the ratio of the capacitance of a capacitor that uses the material as the dielectric as compared to a similar capacitor that has a vacuum as its dielectric. The relative permittivity is also commonly known as the dielectric constant ε. Permittivity is a material property that affects the Coulomb force between two point charges in a material. Relative permittivity is a factor that lowers the electric field between charges as compared to vacuum.

Relative permittivity is a dimensionless number that is generally complexized; its real and imaginary parts are
ε ＝ ε'−iε''
(Here, ε'is the real part of the permittivity and ε'' is the imaginary part of the permittivity)
It is shown as.

Relative permittivity is essential information when designing capacitors and in other situations where the material can be expected to introduce capacitance into the circuit. If a material with a high relative permittivity is placed in an electric field, the magnitude of the electric field will be noticeably reduced within the volume of the dielectric.

Capacitance is the ability of an object to store an electric charge. Capacitance of a capacitor is only a function of the geometry of the design (eg, the area of the plates and the distance between them) and the dielectric constant of the dielectric material between the plates of the capacitor.

Capacitance can be calculated if the geometry of the conductor and the dielectric properties of the insulator between the conductors are known. The capacitance C is directly proportional to the relative permittivity and inversely proportional to the distance between the plates of the capacitor.

When an external pressure is applied, the distance between the pore walls of the pores in the composite (which constitutes the plate of the deemed capacitor) decreases, thereby increasing the capacitance of the capacitor. This gives a value for ΔC at a given pressure. The higher the relative permittivity of the material between the plates of the capacitor, the higher the ΔC. Therefore, it is desirable to achieve the highest relative permittivity possible.

The relative permittivity according to the present invention is measured as follows: the sample, preferably in the form of a film, is sandwiched between two metal disk electrodes and the permittivity is determined by a BioLogic Impedance analyzer. It is measured using MTZ-35) in the frequency range of ^{10 to 10 to 6} Hz under an applied voltage of 1 V.

The permittivity (dielectric constant) of a composite material (as well as a film containing such a composite material) according to the present invention can be widespread without any particular limitation. The higher the permittivity, the more sensitive it is to pressure sensing applications. The upper limit of the dielectric constant is clearly defined by the composite material ^{which becomes conductive, i.e. has a conductivity of greater than 10-4 S / m.} Dielectric constants in the range of 3 to 200, preferably in the range of 5 to 190 have been achieved.

However, it is not desirable for the material to be conductive.

Increasing the amount of conductive filler in the pores increases the relative permittivity, but once the percolation point is reached, the material becomes undesired conductive. Localization of the conductive filler within the closed porosity volume fraction increases the amount of filler required to reach the permeation point, thereby significantly increasing the relative permittivity while reducing the conductivity. It retains the signal when external pressure is applied and the distance between the pore walls (which form the plate of the capacitor) decreases.

In general, this results in very good sensitivity for capacitive sensors manufactured using the composites of the invention.

According to a preferred embodiment of the invention, the amount of filler is in the range 0.1-15% by weight, preferably 0.5-12% by weight, based on the total weight of the composite.

For an amount of conductive filler near or above the infiltration point, an additional non-conductive material is added on top of the composite to turn the whole material into a non-conductive composite with low conductivity. Layers can be coated.

The porous microstructure of composites according to the present invention makes it possible to achieve materials with equivalent elastic moduli that cannot be achieved with uniform materials. The porous structure allows significant deformation of the dielectric layer compared to the non-porous dielectric layer. This increased deformability results in a large change in capacitance under compression.

The use of an insulating layer coated on the composite reduces the overall conductivity, which allows an increase in the amount of conductive filler above the permeation threshold in the pores, thereby the relative permittivity. To increase.

Suitable non-conductive materials are, for example, polydialkylsiloxanes, in particular polydimethylsiloxane (PDMS) and polyesters, preferably polyethylene terephthalate polymers. A layer of biaxially oriented polyethylene terephthalate film has been found to be particularly advantageous in certain application cases. A simple example of such a film may be Mylar®, a product commercially available from DuPont or Hostaphan® available from Mitsubishi Chemical Corporation.

Another embodiment of the present invention includes the following steps:
a) A step of providing a first non-aqueous phase containing a siloxane polymer precursor, a curing agent, and optionally a surfactant.
b) A step of providing a second aqueous phase containing an additive dispersed in water and optionally an additive that facilitates and supports the dispersion of the conductive filler in water.
c) A step of preparing an emulsion by adding the aqueous phase b) to the non-aqueous phase a) under stirring.
d) The step of reticulating the product obtained in step c), and finally,
e) The present invention relates to a method for producing a composite material according to the present invention, which comprises removing water by heat-treating the product obtained in step d).

Composites according to the methods of the invention have a non-aqueous phase that is a mixture of a monomer, a cross-linking agent and, optionally a surfactant, and an aqueous phase of a conductive or semi-conductive filler and. , Arbitrarily obtained by using the reverse emulsion technique, which is an aqueous solution containing a surfactant.

In step a) of the method of the invention, the non-aqueous phase is prepared by using a siloxane polymer precursor, a curing agent and optionally a surfactant.

The siloxane polymer precursor can preferably be a two-component kit as described herein below.

Two-component kits containing siloxane precursor polymers and hardeners are commercially available from a variety of suppliers and will be appreciated by those skilled in the art based on their expertise and the needs of specific application cases. Will choose.

By way of example only, the main configuration of such a two-component kit will be described in more detail for Sylgard® 184.

Sylgard 184® is a silicone elastomer containing dimethylsiloxane and organically modified silica. Sylgard® 184 is prepared by combining a base material (Part A) with a curing agent (Part B). The basic material contains siloxane (dimethyl-vinyl-terminated dimethylsiloxane) and dimethylvinylized and trimethylated silica in a solvent (ethylbenzene). The curing agent also includes a mixture of siloxane and silica in a solvent, including dimethylmethylhydrochloride, dimethyl-vinyl-terminated dimethylsiloxane, dimethylvinylized and trimethylated silica, tetramethyltetravinylcyclitetetrasiloxane and ethylbenzene. included.

Sylgard® 527 is a silicone elastomer gel that is substantially similar to Sylgard 184, but without a silica filler. It is also prepared from the base material and hardener. A wide variety of siloxane compositions are commercially available from various suppliers. Products in the Sylgard® series are merely examples of such suitable two-component kits that can be used in the methods of the invention in step a) and are commercially available, for example, from Dow Chemical. Another group of suitable curable siloxane polymer precursors is the Elastosil® series of products available from Wacker Chemie.

An exemplary PDMS precursor is a vinyl-functional PDMS that can be crosslinked with a hydride-functional cross-linking agent in the presence of a metal catalyst or a hydroxyl-functional PDMS or a cross-linking hydroxyl-functional PDMS that can be cross-linked with a hydride-functional cross-linking agent. ..

Sylgard® 184 is a particularly preferred siloxane polymer precursor that can be used in methods according to the present invention.

The siloxane precursor may contain one or more excipients selected from the group of catalysts, inhibitors, flow agents, silicone oils, solvents and fillers. In one embodiment, the excipient is selected from the group of catalysts (eg, Pt complexes for addition curing or Sn complexes for condensation curing) or peroxides (peroxide curing).

The non-aqueous phase a) may also optionally contain a surfactant to stabilize the system. Surfactants suitable for this purpose are known to those of skill in the art and are widely available from a variety of commercial suppliers. Those skilled in the art will select suitable surfactants based on their expertise.

Simply as an example, silicone alkylpolyethers such as Sirube® series products may be mentioned herein as suitable surfactants. Silicone alkyl polyethers are alkylated silicones that have reacted with the polyether. Such surfactants are effective for emulsifying organic oils and silicones with water, respectively, into the aqueous phase.

で表される。 Sirube®, available from the Silk Company, has the following structure:

It is represented by.

The surfactant may be added to the siloxane precursor composition and is usually in an amount of 0.5-10% by weight, preferably 0.75-7.5% by weight of the total weight of the non-aqueous phase a). Exists.

In step b) of the method of the present invention, an aqueous phase containing a conductive or semi-conductive filler dispersed therein is provided.

To obtain the aqueous phase provided in step b), the conductive or semi-conductive filler is water, preferably deionized, under stirring or applying ultrasonic waves to disperse the conductive filler. Added to water. When ultrasonic waves are used to support a uniform dispersion of filler, the system is preferably cooled, for example, in an ice bath to avoid excessive heat-up of the system.

The solution prior to the addition of the conductive or semi-conductive filler may contain additives to facilitate and support the dispersion of the conductive or semi-conductive filler. A preferred surfactant for this purpose is gum arabic, also known as acacia gum. Acacia rubber is a natural rubber consisting of hardened sap of various species of acacia tree. Gum arabic is a complex mixture of glycoproteins and polysaccharides.

One of ordinary skill in the art facilitates and supports the dispersion of conductive and semi-conductive fillers in aqueous systems and products, and further additives that are widely and commercially available from a number of different suppliers. As we know, no further details need to be given here. Those skilled in the art will select suitable dispersion aids based on their expertise and experience.

To obtain an emulsion, the aqueous phase provided in step b) is slowly added to the non-aqueous phase provided in step a) under mechanical stirring in step c) of the method.

A fluid is sheared when one area of the fluid moves at different speeds with respect to adjacent areas. High shear mixers use a rotary impeller or high speed rotor, or a series of such impellers or in-line rotors, usually powered by a motor, to move the fluid to produce flow and shear. The velocity at the tip of the rotor, or the velocity of the fluid at the outer diameter, will be higher than the velocity at the center of the rotor, and it is this velocity difference that produces shear.

In a preferred embodiment, the high shear mixer disperses or carries the aqueous phase provided in step b) into the primary continuous phase provided in step a) where the aqueous phase would normally not mix with it. It produces an emulsion.

One of ordinary skill in the art will select the diameter of the stirrer and its rotation speed (clearly defining the shear rate applied thereby) according to the specific application situation and the desired final morphology needs of the product.

By applying a high shear rate, it is possible to uniformly distribute a large amount of non-aqueous phase in the silicone rubber and to form a stable emulsion, which is surprisingly stable over a long period of time. I also understood.

The weight ratio of the non-aqueous relative aqueous phase is not particularly limited, and is usually in the range of 1:10 to 10: 1, preferably in the range of 1: 5 to 5: 1. Preferably, the non-aqueous phase forms a continuous phase of the system, the aqueous phase is dispersed in that phase, and the amounts of the non-aqueous phase and the aqueous phase are selected, respectively. In such cases, the weight of the aqueous phase preferably does not exceed the amount of the non-aqueous phase and is usually in the range of 30-40% by weight of the total emulsion. In some application cases, approximately equal weights of non-aqueous and aqueous phases have been found to provide certain benefits.

After step c), an emulsion having aqueous phase droplets containing a conductive filler dispersed in a non-aqueous phase is obtained. The average diameter of these droplets is usually in the range of 0.1 to 300 μm, preferably in the range of 0.5 to 150 μm, particularly preferably in the range of 1 to 30 μm. The average droplet size obtained depends on the viscosity of the continuous phase.

Next, the emulsion obtained in step c) is reticulated for a period of 0.5 to 12 hours, preferably 1 to 8 hours, at a temperature below the boiling point of normal water, preferably in the range of 60 to 95 ° C. A solid material is obtained in step d). In some cases, a cure time of approximately 4 hours was found to be the best. Relative humidity in this step is usually close to or equal to 100%.

In one embodiment, the curing may be an addition-based curing, such as by the use of Pt as a catalyst, in the form of curing in which the Si—H group of the crosslinker reacts with the vinyl group of the silicone polymer.

According to another embodiment, the curing is a condensation-based system, such as by using a Sn-based curing system and the use of room temperature vulcanized silicone rubber, where the alkoxy crosslinker undergoes a hydrolysis step and the hydroxyl group is the problem polymer. It can be done in a system that is left untouched to be involved in a condensation reaction with another hydroxyl group attached to.

In yet another embodiment, curing is a peroxide-based system in which the organic peroxide compound decomposes at high temperatures to form highly reactive radicals, which chemically crosslink the polymer chains. Can be done in the system.

In the final step, the product obtained in step d) is heat treated to remove water. Since the siloxane polymer formed after curing is permeable to water vapor, the conductive or semi-conductive filler is preferably provided with the pore walls coated with the conductive or semi-conductive filler. The droplets are separated from the porous structure that is substantially present in the closed porosity volume fraction of the matrix material, thereby resulting in a composite material according to the present invention.

The conditions of curing in step d) and drying in step e) affect the morphology of the porous composite and one of ordinary skill in the art will select those conditions in a manner suitable to obtain the desired morphology. ..

Another embodiment of the invention relates to a film comprising a composite material according to the invention, preferably made essentially of it, and even more preferably made of it.

According to a preferred embodiment, the film thickness is in the range of 1-500 μm, preferably in the range of 10-250 μm.

A film according to the present invention can be obtained by forming a film by pouring the emulsion of step c) of the method according to the present invention into a mold before applying step d). The film thus obtained is then subjected to steps d) and e) according to the method of the present invention to obtain a final film suitable for use in a pressure sensing device.

In the final step of the method according to the invention, the product obtained in step d) is heat treated to remove water. This heat treatment step is usually at a temperature above the boiling point of water at atmospheric pressure, preferably at a temperature in the range of 100 to 200 ° C. and for a duration of 0.1 h to 5 h, preferably 0.5 to 5 hours. Will be implemented. In some cases, temperatures of 130-170 ° C. and treatment times of 0.75-3 h, especially 1-2 h, have been found to be suitable.

After the final step, a microporous composite is finally obtained in film form, the composite having an average diameter in the range of preferably 0.1-200 μm, and preferably fine during the closed porosity volume fractionation. Includes pores in a state where the pore walls are lined and the pores are filled with a conductive filler to some extent.

A further embodiment of the present invention relates to a film-coated substrate according to the present invention.

The base material is not particularly limited as far as the structure and composition are concerned, and those skilled in the art will select the base material in consideration of the needs of the specific application situation.

The structure of the substrate can be adopted for specific intended use, and the substrate can have the function of a carrier for a deformable film, or it has increased mechanical stability to the film. Can provide sex.

The material of the substrate can be an insulating or conductive metal or non-metal, respectively, depending on the intended end use of the coated substrate in the pressure sensing device. In some cases, aluminum substrates or substrates containing aluminum have been found to provide certain advantages.

Coating a film onto a substrate is a conventional coating technique known to those of skill in the art and is described in the literature and can be achieved using techniques that do not require further details to be presented herein.

Another embodiment of the present invention relates to a multilayer system including a first layer of a film according to the present invention and an adjacent second layer which is an insulating layer.

Films containing composites according to the present invention show a loss once an amount of conductive filler near or above the permeation limit is used, which is a certain disadvantage.

A multilayer system according to the present invention overcomes these disadvantages by adding a second layer (which second layer is an insulating layer) on the film according to the present invention. Thereby, the high dielectric constant of the material is maintained while at the same time significantly reducing the conductivity.

The material of the second insulating layer can be any insulating material that can be molded into a film or suitable coating on the first layer. For economic and processability reasons, a thermoplastic polymer insulating layer is preferred, and silicone rubber or polyester may be mentioned as an example. The polyethylene terephthalate polymer-based Mylar® film, which is commercially available from a large number of suppliers, has been found to be advantageous in terms of processability and cost, and thus insulation for the second insulating layer. Represents a particularly preferred group of materials.

The thickness of the second insulating layer is not particularly limited, and is often in the range of 0.5 to 500 μm, preferably in the range of 1 to 100 μm.

The insulating layer can be spread on a film containing a composite material according to the present invention. The insulating layer can itself be deposited on the substrate.

Coating of the insulating layer on a film containing a composite material can be achieved by conventional coating techniques such as spin coating, rotary coating or other coating techniques known to those of skill in the art and described in the literature.

Composite materials according to the present invention, films containing them and coating substrates or multilayer structures containing such films are particularly suitable for use in piezo capacitive elements. Due to their high permittivity at low conductivity, devices using said materials are highly sensitive and can reliably measure very low fluctuations in external pressure.

When compressive stresses are applied to microporous composites according to the present invention, large deformations as well as changes in their microstructure are produced. Both effects result in large variations in equivalent capacitance and therefore large piezo capacitance sensitivities at low external pressures, for example in the range of 0.1 kPa to 10 kPa. Therefore, composites according to the present invention are excellent candidates for low force pressure sensors commonly required for capacitive pressure sensing applications, more specifically for biological signals such as blood pressure and heart rate monitoring. is there.

Example 1
A solution containing 5.0% by weight gum arabic in water was prepared in a flask by mixing 5 g of gum arabic (obtained from Sigma Aldrich) with 95 g of deionized water. Magnetic agitation was applied to uniformly dissolve Arabic rubber, which acts as a surfactant to disperse the carbon black powder (conductive filler). The carbon black used was purchased from Alfa Aesar at Reference 39724-Carbon Black and used as received.

Dispersion of the carbon black powder was carried out in a flat bottom flask by mixing a desired amount of carbon black powder and gum arabic solution. The mixture was sonicated for 1 hour to evenly disperse the carbon black particles while cooling the solution in an ice bath to avoid excessive temperature rise as a result of sonication. The resulting product was used as the aqueous phase.

As a non-aqueous phase, Cylgard 184 was purchased from Dow Coring as a kit consisting of a PDMS base material and a curing agent. The relative permittivity of the PDMS material was approximately 2. Sirube® J-208-212 was added as a surfactant to the mixture of the PDMS basic material and the cross-linking agent to reach a surfactant concentration of 5% by weight.

The aqueous phase was slowly added to the non-aqueous phase under mechanical agitation with a spatula to reach a 50:50 ratio of water relative non-aqueous phase.

The water-in-oil emulsion thus obtained was poured into a mold having a depth of 500 μm and a diameter of 24 mm and covered. The film was then reticulated by exposing the mold to a temperature of 90 ° C. for 4 hours in a water bath (to have 100% humidity).

In the final step, the reticulated film was removed from the mold and placed in an oven at a temperature of 150 ° C. for 1 hour to remove water.

The resulting composite material had a microporous structure with pores having an average diameter of 10-30 μm. The average pore size was measured by scanning electron microscopy (SEM) images of the product.

The carbon black content of the composite material was in the range of 4.6 to 10.2% by weight based on the total weight of the composite material.

The permittivity was measured by broadband dielectric spectroscopy using sandwich geometry with circular brass electrodes. Measurements were taken at room temperature over frequencies of ^{10 Hz to 7 Hz.} The real part of the dielectric constant was 3 for materials without any carbon black and 13.5 for composites containing 4.6% by weight carbon black. With an amount of 10.2 wt% carbon black, the permittivity was measured to be 4000, but the material was conductive as the concentration exceeded the permeation threshold. When the obtained film was formed into a multilayer structure with an insulating film (Mylar film), the dielectric constant was 330, but the material remained non-conductive.

Electrostatic sensitivity was measured in a compressed state using a circular stainless steel clamp with a diameter of 25 mm and acting as an electrode. The standard pressure on the sample was maintained using the RSA GII Solids Analyzer while measuring the complex impedance at 1 V and 100 Hz using the Keysight Precision LCR Meter. To estimate the equivalent capacitance Cp and resistance Rp of the sample, it was assumed that the material simultaneously acted as a combination of a register and a capacitor. The reference capacitance C ₀ was arbitrarily defined as when 0.1 kPa was added to the sample. Measurements were made over pressures in the range of 0.1 kPa to 70 kPa.

For carbon black concentration of 4.2% by weight without the insulating layer, [Delta] C / _{C o} is reached to a value in the range of 0.9 to 1.8 in the pressure range of 2KPa～10kPa. Carbon black concentration was 10%, and when the insulating layer is not present, the value for [Delta] C / C _o is decreased to a value of near zero at a pressure of 2 kPa. Carbon black concentration of there insulating layer and 10 wt%, [Delta] C / _{C o} is approximately 1.8 at a pressure of 2 kPa, exceeds the value of 4 at a pressure of 10 kPa.

These results show the excellent sensitivity of composites according to the invention for use in piezo capacitive sensing devices.

Claims (18)

a) A porous matrix material containing a siloxane polymer, comprising a closed porosity volume fraction and optionally an open porosity volume fraction.
b) A composite material containing a conductive or semi-conductive filler substantially present in the closed porosity volume fraction of the porous matrix material a). The composite material according to claim 1, wherein the ratio of the closed porosity volume fraction to the open porosity volume fraction is at least 1: 1 and preferably 1: 1 to 100: 1. The composite material according to claim 1 or 2, which has a conductivity in the range of ^10-5 to ^10-12 S / m in the absence of external pressure. The composite material according to any one of claims 1 to 3, wherein the amount of the filler is in the range of 0.1 to 15% by weight based on the total weight of the composite material. The composite material according to any one of claims 1 to 4, wherein the siloxane polymer polymer is polydimethylsiloxane (PDMS). The composite material according to any one of claims 1 to 5, wherein the conductive or semi-conductive filler is selected from carbon nanotubes, carbon nanohorns, graphite, graphene and carbon black. The composite material according to any one of claims 1 to 5, wherein the conductive or semi-conductive filler is selected from metal particles such as copper, silver, gold, and zinc. The composite material according to any one of claims 1 to 5, wherein the conductive or semi-conductive filler is selected from an essentially conductive polymer (ICP). Any of claims 1 to 5, wherein the semi-conductive filler is selected from the group consisting of Si, Si-Ge, GaAs, InP, GaN, SiC, ZnS, ZnSe, CdSe, and CdS or metal oxide particles. The composite material according to one item. The composite material according to claim 6, wherein the conductive filler is carbon black. A film containing the composite material according to any one of claims 1 to 10. The film according to claim 11, which has a thickness in the range of 1 to 500 μm. A substrate coated with the film according to any one of claims 11 or 12. A multilayer system including a first layer of the film according to any one of claims 11 or 12 and a second layer which is an insulating layer adjacent thereto. The multilayer system according to claim 14, wherein the second layer is a polyester layer, particularly a polyethylene terephthalate layer. The method for producing a composite material according to any one of claims 1 to 10, wherein the following steps:
a) A step of providing a first non-aqueous phase containing a siloxane polymer precursor, a curing agent, and optionally a surfactant.
b) A step of providing a second aqueous phase containing a semi-conductive or conductive filler dispersed in water, and
c) A step of preparing an emulsion by adding the aqueous phase b) to the non-aqueous phase a) under stirring.
d) The step of reticulating the product obtained in step c), and finally,
d) A method including a step of removing the water by subjecting the product obtained in step d) to a heat treatment. The method of claim 16 for producing the film of claim 11 or 12, wherein the emulsion obtained in step c) is poured into a mold before applying step d). The method by which it is molded into a film. The composite material according to any one of claims 1 to 10, the film according to claim 11 or 12, the substrate according to claim 13, or the multilayer structure according to claim 14 or 15. , Use for use in pressure sensing devices.