WO2019129388A1 - Structured composites useful as low force sensors - Google Patents

Abstract

Composite material comprising a) a porous matrix material comprising a siloxane polymer, comprising a closed porosity volume fraction and, optionally, an open porosity volume fraction, and b) a conductive or semiconductive filler substantially present in said closed porosity volume fraction of said porous matrix material a), films, coated substrates and multilayer systems comprising the composite material and the use thereof in pressure sensing devices.

Description

Structured composites useful as low force sensors

[0001] The present invention relates to composite materials comprising a porous siloxane polymer matrix with a closed porosity volume fraction and a conductive or semiconductive filler substantially present in said closed porosity volume fraction of the matrix.

[0002] Pressure sensors have attracted much attention in the recent past due to their potential for a variety of different applications. Especially the demand for pressure sensors with high sensitivity in low pressure regions is very high, which systems are needed i.a. for healthcare and medical diagnosis systems as well as in electronic systems, in particular so called e-skin systems.

[0003] Pressure sensing devices are typically categorized into three types,

depending on the parameter being used for the sensing. Piezoresistive devices use the change in conductivity upon application of external pressure. Piezoelectric devices use the piezoelectric effect, i.e. the generation of an electric charge in a material upon application of pressure. The sensitivity of a piezoielectric device is limited by the physical properties of the piezoelectric substance at the origin of the effect.

Piezocapacitive sensors on the other hand make use of the capacitance change occuring in reaction to the aaplication of pressure and their sensitivity is not theoretically limited. The change of capacitance can be a consequence of the distance of two electrodes of the system forming a capacitor changing in reaction to the application of pressure or due to the modification of the equivalent dielectric constant of the dielectric material sandwiched between two electrodes under the application of pressure.

[0004] Compared to piezoresistive devices piezocapacitive sensors offer some advantages such as low power consumption and better reproducability. Compared to piezoelectric devices piezocapacitive sensors are easier to process and easier to shape into different forms. Moreover, they do not require a poling or stretching.

[0005] The magnitude of the capacitance change is determined by the change in dielectric constant, the thickness of a dielectric layer and the surface area of an electrode.

[0006] Micro- or nano-structures have been suggested in such devices to improve the sensitivity in particular in the low pressure range. However, this requires usually complex and expensive fabrication processes.

[0007] B.Y. Lee et al, Sensors and Actuators A 240 (2016), 103 to 109 describes low-cost pressure sensors based on dielectric elastomer films with micro- pores. Porous films are prepared by using a siloxane elastomer material and water droplets without any additives. Polydimethylsiloxane is used as a base material and water droplets are selected as dispersion substance.

A solution of PDMS prepolymers , mixed with a curing agent, and water is stirred in a container. Through the stirring process, micro-droplets of water are uniformly dispersed in the PDMS solution due to the insolubility of water. The solution thus obtained is placed between two glass substrates and thereafter the solution is cured. During curing, the water evaporates and a polymerized porous PDMS film having micro-pores where water was initially present is obtained. This film having a thickness of appr. 100 pm forms the dielectric layer of a capacitive type pressure sensor.

[0008] A.J. Gallant, Procedia Chemistry 1 (2009), 568-571 relates to porous

PDMS force sensitive resistors. Elastomeric force sensitive resistors are 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 . Sugar cubes are placed in a dish with PDMS precursors and left for one hour to become saturated with the PDMS. The cubes are then cured, excess PDMS is trimmed away and the cubes are put in a beaker with distilled water to dissolve the sugar. The structure thus obtained is the inverse matrix of the sugar cube in which 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 creating a high concentration of carbon within the open porosity volume fraction of the sponge. Once filled, the sponge is left to dry and a thin layer of PDMS is coated thereon and cured to seal the carbon inside the sponge. In the sponge, the pore walls are lined with carbon. Upon application of pressure, the carbon-black lined pore walls come into contact, thereby increasing the number of carbon-carbon connections and the pores become conducting.

[0009] 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 piezoresistive flexible pulsation sensors obtained by applying a so called additive manufacturing method for printing PDMS with an internal porous structure. The pores are reported to have average pore sizes of appr. 1mm. To obtain conductive sensors, multi walled carbon nanotubes are added during the manufacturing process. The resistivity is said to be non-linear and hysteresis was observed. Both are undesired effects.

[0010] It was an object of the present invention to provide composite materials suitable for use in piezocapactive sensors providing high sensitivity and good reproducability.

[0011] This object is achieved with composite materials in accordance with claim 1. Preferred embodiments of the invention are set forth in the dependent claims and in the detailed specification hereinafter.

[0012] A further object of the present invention are films comprising the

composite material in accordance with claim 1 as well as substrates coated with a film made of the composite material in accordance with the present invention.

[0013] The composite material in accordance with the present invention

comprises

a) a porous matrix material comprising a siloxane polymer, comprising a closed porosity volume fraction, and, optionally, an open porosity volume fraction, and

b) a conductive or semiconductive filler substantially present in said closed porosity volume fraction of said porous matrix material a).

[0014] Porous materials are usually characterized by their porosity. Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is the fraction of the volume of voids over the total volume, between 0 and 1 , or as a percentage between 0% and 100%.

[0015] The apparent porosity or open porosity (oPo) is a fraction of the porosity and is the volume of the open pores, into which a liquid or gas can penetrate, as a percentage of the total volume of the material.

[0016] 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 any kind of closed pores in the material.

[0017] Open porosity and closed porosity sum up to the total porosity of the

material.

[0018] The porous matrix material of the composite materials in accordance with the present invention comprises a closed porosity volume fraction, in which a substantial part of the conductive or semiconductive filler is present.

[0019] In accordance with a preferred embodiment of the present invention, the closed porosity volume fraction is preferably equal to or greater than the open porosity volume fraction of the material, i.e. the volume of the pores which form 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 (the ratio of both pore volume fractions thus preferably is at least 1).

[0020] In accordance with a particularly preferred embodiment, the ratio of the closed porosity volume to the open porosity volume is in the range of from 1 :1 to 100:1 , preferably in the range of from 1.5:1 to 50:1

[0021] The open porosity volume fraction of a porous material can be deternined by gas displacement pycnometry, a technique known to the skilled person. This technique uses the gas displacement method to measure volume accurately. An inert gas, usually He, is used as the displacement gas. A sample of known weight is sealed in a compartment of the measuring device having a known volume. Then He is allowed to flow into the chamber through an inlet valve until equilibrium is reached, i.e. until the pressure is constant. Then the inlet valve is closed and an outlet valve to a second chamber of precisely known volume is opened. The pressures observed upon filling the sample chamber and then upon discharging the gas into the second empty chamber allow the computation of the sample solid phase volume (which equals the volume of gas displaced by the solid part of the sample plus the volume of the pores not accessible to the gas). Helium gas quickly fills even small pores quickly, only the volume part of the sample which cannot be accessed by the He gas displaces the gas. This part of the sample consists of the solid part of the sample plus the volume represented by the closed porosity volume fraction (as same is defined as being not accessible to the gas).

[0022] If the volume displaced by the sample is denoted as V_s, the known volume of the sample cell is denoted as V_c, the volume of the second

compartment into which the gas is displaced is V_r, the pressure after filling the sample cell is P_a and the pressure after expansion into the

compartment cell is P_e, the volume displaced by the sample can be calculated as

[0023] Vs = Vc - Vr (Pe/(Pa-Pe))

[0024] The displaced or pycnometer volume Vs reflects the volume of the solid part of the porous sample (which is referred to herein as as theoretical volume) plus the volume of the closed pores. 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 determined. Subtracting the theoretical volume from the pycnometer volume yields the volume of the closed pores.

[0025] The bulk volume of the porous sample is the geometric volume of the

porous sample, which is the sum of theoretical volume plus the volume of the closed pores plus the volume of the open pores. Accordingly, the volume of the open pores can be obtained by subtracting the theoretical volume and the closed pore volume (obtained as explained above) from the bulk (geometrical) volume of the sample. [0026] 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 an analogous manner. The ratio of both fractions is then obtained by simply dividing the closed pore volume fraction by the open porosity volume fraction.

[0027] The total porosity of a porous sample can also be obtained by dividing the bulk density by the theoretical density and subtracting the value from 1.

[0028] The foregoing may be explained through the following example: A sample having a theoretical volume of of 2 cm³ and a pycnometer volume of 3 cm³ has a closed porosity volume fraction of 1 cm³ (obtained by subtracting the theoretical volume from the pycnometer volume). If the porous sample has a geometric (bulk) volume of 4 cm³, the total porosity, based on the bulk volume, is 2/4 or 0.5. The closed porosity volume fraction, relative to the bulk volume, in this case is 0,25, relative to the total pore volume of the sample, 0.5. This yields a ratio closed porosity volume fraction/open porosity volume fraction of 1.

[0029] If, with the same theoretical volume and bulk volume, the pycnometer volume is 3,5 cm³, then the volume of the closed pores is 1 ,5 cm³ which translates into 37,5 % , based on the bulk volume or 75 %, based on the total pore volume. In this case the ratio closed porosity volume fraction to open porosity volume fraction is 3:1.

[0030] The matrix polymer of the composite material in accordance with the

present invention is a siloxane polymer.

[0031] Siloxane polymers or polysiloxanes, also known as silicones, are polymers that include an inert, synthetic compound made up of repeating units of siloxane, frequently combined with carbon or hydrogen or both. They are typically heat-resistant and rubber-like.

[0032] A siloxane is a functional group in organosilicon chemistry with the -Si-O- Si-linkage. The word siloxane is derived from the words silicon, oxygen and alkane. Siloxane materials may be composed of several types of so called siloxide groups, depending on the number of Si-0 bonds: M-units represented by general structural element R3S1O0_.5, D-units by the general structural element R₂S1O and T-units represented by the general structural element RS1O1.5.

[0033] Siloxane functional groups form the backbone of the silicones.

[0034] More precisely polymerized siloxanes or polysiloxanes, silicones consist of an inorganic silicon-oxygen backbone chain ( -Si-O-Si-O-Si-O-·· ·) with organic side groups attached to the silicon atoms. The side groups are preferably selected from alkyl groups or aryl groups or combinations thereof.

[0035] In some cases, organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions.

[0036] Organic side groups may be alkyl, haloalkyl, aryl, haloaryl, alkoxyl, aralkyl and silacycloalkyl groups as well as more reactive groups such as alkenyl groups such as vinyl, allyl, propenyl, butenyl, pentenyl, hexenyl, heptenyl, octenyl, nonenyl and/or decenyl groups. Polar groups such as acrylate, methacrylate, amino. Imino, hydroxy, epoxy, ester, alkyloxy, isocyanate, phenolic, polyurethane oligomeric, polyamide oligomeric, polyester oligomeric, polyether oligomeric, polyol and carboxypropyl groups may be attached to silicon atoms of the siloxane backbone in any combination.

[0037] Siloxanes may be terminated with any useful group such as alkenyl and/or alkyl groups such as methyl, ethyl, isopropyl, n-propyl or vinyl groups or combinations thereof. Other groups that may be used to terminate a siloxane are acrylate, methacrylate, amino. Imino, hydroxy, epoxy, ester, alkyloxy, isocyanate, phenolic, poly polyurethane oligomeric, polyamide oligomeric, polyester oligomeric, polyether oligomeric, polyol and carboxypropyl groups and halo, e.g. fluoro groups.

[0038] Polydialkylsiloxanes (where the organic groups are alkyl groups) are a preferred group of siloxane polymers suitable for use in the composite materials of the present invention.

[0039] Polydialkylsiloxane polymers may be represented by the following general formula

Aik

Alk- -Si - O - -Si - O- Si - Aik

Aik Alk Aik

n [0040] wherein Aik, which may be the same or different at each occurence, represents a linear, branched or cyclic alkyl group.

[0041] Preferred alkyl groups are linear or branched alkyl groups having 1 to 12, preferably 1 to 8 and more preferably 1 to 4 carbon atoms.

[0042] The best known example of polydialkylsiloxanes is polydimethylsiloxane (where Aik is a methyl group, hereinafter referred to as PDMS), which is also the most preferred polydialkylsiloxane in accordance with the present invention. The term polydimethylsiloxane or PDMS, when used herein, encompasses derivatives thereof such as hydroxy-, vinyl- allyl- etc. end- capped PDMS.

[0043] The composite materials in accordance with the present invention

comprise as component b) a conductive or semiconductive filler

substantially present in the closed porosity volume fraction of the microporous polymer matrix a).

[0044] Substantially present for the purpose of the present invention means that at least 50, preferably at least 60 and even more preferably at least 70 % of the filler is present in the closed porosity volume fraction. Up to 99, preferably up to 95 and even more preferably up to 90% of the total content of the filler can present in the closed porosity volume fraction of the composite material.

[0045] The semiconductive or conductive fillers in the composites of the present invention may be selected from any material which has semiconducting or conducting properties.

[0046] Thus, a suitable conductive filler may be selected from the list consisting of metal particles like copper, silver, gold, and zinc. Preferably, the

conducting metal filler is silver or copper and more preferably is silver.

[0047] Conductive polymer particles are essentially composed or even composed of intrinsically conducting polymers (ICPs). They are organic polymers composed of macromolecules having fully conjugated sequences of double bonds along the chains. Such compounds may have metallic conductivity or can be semiconductors. Examples of intrinsically conducting polymers are polyacetylene, polythiophene, polypyrrole, or polyaniline. Among ICPs, polythiophene and polyaniline are preferably used. Poly(3,4-ethylenedioxythiophene) or PEDOT and, in particular PEDOT-PSS, a polymer blend of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate) are used more preferably.

[0048] Semi-conductive fillers are essentially composed of or composed of a

semi-conductive material. The semi-conductive core comprises generally at least 95 wt. % of a semi-conductive material, preferably at least 97 wt.

% and more preferably at least 99 wt. %.

[0049] Generally, the semi-conductive material is selected from the list consisting of Si, Si-Ge, GaAs, InP, GaN, SiC, ZnS, ZnSe, CdSe, and CdS.

Preferably, the semi-conducting material is selected from the list consisting of GaAs, SiC, ZnS and CdS. More preferably, the semi-conducting material is SiC.

[0050] Preferably, the semi-conducting filler is selected from the list consisting of GaAs, SiC, ZnS and CdS nanoparticles.

[0051] Another group of suitable fillers are metal oxide particles typically

containing a metal and an anion of oxygen in the oxidation state of -2, such as ZnO.

[0052] In accordance with one embodiment, the conductive or semi-conductive fillers suitable for the invention may have an aspect ratio close to 1. When the aspect ratio is close to 1 the particle tends to be spherical.

[0053] In accordance with another embodiment, the conductive or semi- conductive fillers suitable for the invention may have an aspect ratio higher than 1. In this case, the aspect ratio is preferably of at least 5, more preferably of at least 10, even more preferably of at least 15 and the most preferably of at least 20. The particles have usually an aspect ratio of at most 5000, preferably of at most 1000, more preferably of at most 500 and even more preferably of at most 200.

[0054] The aspect ratio is the ratio of length to width of a particle (ISO 13794 :

1999). An average aspect ratio may be determined by the skilled person by image processing of transmission electron microscopy (TEM) or scanning electron microscopy (SEM) pictures.

[0055] In some cases metallic fillers having an aspect ratio higher than one in the form of nanowires have been found advantageous. Particularly preferred nanowires are silver nanowires.

[0056] Another grorup of semiconductive or conductive fillers for use in the

composite materials of the present invention are carbonaceous fillers.

[0057] For the purpose of this invention, the term“carbonaceous filler” denotes fillers comprising more than at least 50 wt% of elemental carbon, preferably at last 75 wt% of elemental carbon, more preferably at least 90 wt% of elemental carbon. Especially preferred carbonaceous fillers comprise 99 wt % or more of elemental carbon or consist of elemental carbon.

[0058] Preferably carbonaceous fillers are selected from carbon nanotubes,

carbon nanohorns, graphite, graphene and carbon black. Particularly preferred for economical reasons is carbon black.

[0059] Graphene itself is usually considered as a one-atom thick planar sheet of sp²-bonded carbon atoms that are densely packed in a honeycomb structure. The name graphene is derived from graphite and the suffix -ene. Graphite itself consists of a high number of graphene sheets stacked together.

[0060] Graphite, carbon nanotubes, fullerenes and graphene in the sense

referred to above share the same basic structural arrangement of their constituent atoms. Each structure begins with six carbon atoms, tightly bound together chemically in the shape of a regular hexagon - an aromatic structure similar to what is generally referred to as benzene.

[0061] Carbon nanohorns is the name for horn-shaped sheath aggregate of

graphene sheets. Single-walled nanohorns ( SWNH) with about 40-50 nm in tubule length and about 2-3 nm in diameter are derived from single walled nanotubes (SWNTs) and ended by a five-pentagon conical cap with a cone opening angle of ~20. SWNHs may associate with each other to form‘dahlia-like' and‘bud-like’ structured aggregates which have an average diameter of about 80-100 nm. The former consists of tubules and graphene sheets protruding from its surface like petals of a dahlia, while the latter is composed of tubules developing inside the particle itself.

[0062] Carbon black (CAS 1333-86-4)is a form of paracrystalline carbon that has a high surface-area-to-volume ratio, albeit lower than that of activated carbon.

[0063] Chemically, carbon black is a colloidal form of elemental carbon consisting of 95 to 99% carbon. It is usually obtained from the partial combustion or thermal decomposition of hydrocarbons, existing as aggregates of aciniform morphology which are composed of spheroidal primary particles, uniformity of primary particle sizes within a given aggregate and

turbostratic layering within the primary particles.

[0064] Suitable carbonaceous fillers as described above are available from a

variety of sources and suppliers and the skilled person will, based on his professional knowledge and the specific application case, select a suitable material for use in the composite material in accordance with the present invention.

[0065] In certain application cases sperical nanoparticulate fillers with an average diameter of 300 nm or less, preferably of 200 nm or less, have been found to provide certain advantages.

[0066] The term average particle diameter of a sperical particle when used herein refers to the D50 median diameter computed on the basis of the intensity weighed particle size distribution as obtained by the so called Contin data inversion algorithm. Generally said, the D50 divides the intensity weighed size distribution into two equal parts, one with sizes smaller than D50 and one with sizes larger than D50.

[0067] In general the average particle diameter as defined above is determined according to the following procedure. First, if needed, the particles are isolated from a medium in which they may be contained (as there are various processes for the manufacture of such particles, the products may be available in different forms, e.g. as neat dry particles or as a

suspension in a suitable dispersion medium. The neat particles are then used for the determination of the particle size distribution preferably by the method of dynamic light scattering. In this regard the method as described in ISO Norm Particles size analysis - Dynamic Light Scattering (DLS), ISO 22412:2008(E) is recommended to be followed. This norm provides i.a. for instructions relating to instrument location (section 8.1.), system

qualification (section 10), sample requirements (section 8.2.),

measurement procedure (section 9 points 1 to 5 and 7) and repeatability (section 11). Measurement temperature is usually at 25 °C and the refractive indices and the viscosity coefficient of the respective dispersion medium used should be known with an accuracy of at least 0.1 %. After appropriate temperature equilibration the cell position should be adjusted for optimal scattered light signal according to the system software. Before starting the collection of the time autocorrelation function the time averaged intensity scattered by the sample is recorded 5 times. In order to eliminate possible signals of dust particles moving fortuitously through the measuring volume an intensity threshold of 1.10 times the average of the five measurements of the average scattered intensity may be set. The primary laser source attenuator is normally adjusted by the system software and preferably adjusted in the range of about 10,000 cps.

Subsequent measurements of the time autocorrelation functions during which the average intensity threshold set as above is exceeded should be disregarded.

[0068] Usually a measurement consists of a suitable number of collections of the autocorrelation function (e.g. a set of 200 collections) of a typical duration of a few seconds each and accepted by the system in accordance with the threshold criterion explained above. Data analysis is then carried out on the whole set of recordings of the time autocorrelation function by use of the Contin algorithm available as a software package, which is normally included in the equipment manufacturer's software package.

[0069] The conducive or semiconductive fillers used in the composite materials accordance with the present invention may deviate form the spherical shape, which is characterized by an aspect ratio of close to 1. [0070] Platy particles are also suitable. Typically, platy particles consist essentially of, or even consist of, particles having the shape of, or resembling to a plate, i.e. the particles are flat or nearly flat and their thickness is small in comparison with the other two dimensions.

[0071] Acicular particles are also suitable. Typically, acicular particles consist essentially of, or even consist of, particles having the shape of, or resembling a needle.

[0072] Finally, fibrous particles are also well known by the skilled in the art.

Typically, fibrous particles consist essentially of, or even consist of, particles having the shape of, or resembling a fibre, i.e. the particles are slender and greatly elongated, and their length is very high in comparison with the other two dimensions. Notably to the purpose of increased reinforcement, the fibrous particles which are advantageously contained in the polymer composition in accordance with the instant invention, have:

- a number average aspect ratio of typically above 5, preferably above 10 and more preferably above 15;

- a number average length of typically at least 50 pm, preferably at least 100 pm and more preferably at least 150 pm; and

- a number average diameter of typically below 25 pm, preferably below 20 pm, and more preferably below 15 pm

[0073] The average pore diameter, determined using image processing of top view SEM images of the composite materials in accordance with the present invention, is preferably in the range from 0.1 to 200 pm. preferably in the range from 0.5 to 100 pm and even more preferably in the range of from 1 to 50 pm. In some cases average pore diameters of from 10 to 30 pm have been found beneficial.

[0074] SEM is well-suited for quantitative analysis of the pore structure, since it allows a wide range of magnification, a high depth of field, and produces digital data fit for image analysis. SEM combines the best aspects of light microscopy and TEM.

[0075] A typical procedure for determining average pore diameter is desribed in more detail as follows: [0076] A gray scale analysis of the pictures using the software package ImageJ was performed to determine the pore size distribution by thresholding the pictures in order to select the internal area of the pores and then using the Particle Analysis package. This procedure allows the identification of the pore area distribution. Assuming that pores have a spherical shape and are cut through their centers therefore exhibiting their equivalent great circle, the pore size D was extracted as the equivalent diameter from the surface area A, ie D=2 sqrt(A/pi). The average and the standard deviation of pore size were obtained via a statistical analysis accumulating pore size distribution over several pictures (e.g., 10 pictures for a given sample).

[0077] In accordance with another preferred embodiment of the present

invention, the composite material has a specific electric conductivity, in the absence of external pressure, in the range of from 10^-5 to 10 ¹² S/m, preferably in the range of from 10⁶ to 10⁹ S/m.

[0078] Electrical conductivity or specific conductance is the reciprocal of electrical resistivity, and measures a material's ability to conduct an electric current.

[0079] For capacitive devices, i.e. devices using a change in capacitance upon application of external pressure, it is desirable to obtain a high dielectric function of the material without the material becoming conductive.

[0080] It is desirable to obtain a large variation of the relative capacitance change AC/Co (Co represents the capacitance without application of external pressure whereas AC represents the change in capacitance upon application of a given pressure) under mechanical compression in order to be considered as good piezocapactive sensor. Increasing the amount of conductive filler increases the variation of AC/C_0.

[0081] Relative permittivity is the ratio of the capacitance of a capacitor using that material as a dielectric, compared with a similar capacitor that has vacuum as its dielectric. Relative permittivity is also commonly known as dielectric constant e. Permittivity is a material property that affects the Coulomb force between two point charges in the material. Relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum. [0082] Relative permittivity is a dimensionless number that is in general complex- valued; its real and imaginary parts are denoted as

[0083] e = e’ - i e”

[0084] where e’ is the real part of the permittivity and e” the imaginary part of the permittivity.

[0085] The relative permittivity is an essential piece of information when

designing capacitors, and in other circumstances where a material might be expected to introduce capacitance into a circuit. If a material with a high relative permittivity is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric.

[0086] Capacitance is the ability of a body to store an electric charge. The

capacitance of a capacitor is a function only of the geometry of the design (e.g. area of the plates and the distance between them) and the

permittivity of the dielectric material between the plates of the capacitor.

[0087] Capacitance can be calculated if the geometry of the conductors and the dielectric properties of the insulator between the conductors are known. The capacitance C is directly propotional to the relative permittivity and inversely proportional to the distance between the plates of the capacitor.

[0088] Upon application of an external pressure, the distance of the pore walls of a pore within the composite material (which constitute the plates of a deemed capacitor) is reduced, thereby increasing the capacitance of the capacitor. This yields the value for AC at a given pressure. The higher the relative permittivity of the material between the plates of the capacitor, the higher AC becomes. Thus, achieving a relative permittivity as high as possible is desired.

[0089] The relative permittivity in accordance with the present invention is

measured as follows: The sample, preferably in the form of a film, is sandwiched between two metallic disc electrodes and the permittivity is measured in the frequency range from 10 to 10⁶ Hz under an applied voltage of 1 V using an impedance analyzer (BioLogic Impedance analyzer MTZ-35). [0090] The permittivity (dielectric constant) of the composite materials in accordance with the present invention (as well as of the films comprising such composite materials) may span over a wide range without being subject to particular limitations. The higher the permittivity, the higher the sensibility for pressure sensing applications. The upper limit of the permittivity is defined by the composite material becoming conductive i.e. having a conductivity exceeding 1CH S/m. Permittivities in the range from 3 to 200, preferably in the range of from 5 to 190 have been achieved.

[0091] It is not desirable, however, that the material becomes conductive.

[0092] Increasing the amount of conductive filler within the pores increases the relative permittivity but once the percolation point is reached, the material becomes conductive which is undesired. Localizing the conductive filler within the closed porosity volume fraction increases the amount of filler needed to reach the percolation point thereby retaining a low conductivity while significantly increasing relative permittivity, which improves the signal when external pressure is applied and the distance between the pore walls (forming the plates of the capacitor) is reduced.

[0093] Overall, this leads to a very good sensitivity of the capacitance sensors made using the composite material of the present invention.

[0094] In accordance with a preferred embodiment of the present invention, the amount of filler is in the range of from 0.1 to 15, preferably in the range of from 0.5 to 12 wt%, based on the entire weight of the composite.

[0095] For an amount of conductive filler close or above the percolation point, an additional layer of a non-conductive material can be coated on top of the composite material in order to turn the overall material into a non- conductive composite having low conductivity.

[0096] The porous microstructure of the composite materials in accordance with the present invention allows achieving materials with equivalent elastic moduli that cannot be achieved in a homogeneous material. The porous structure allows significant deformations of the dielectric layer in comparison to a non-porous dielectric layer. This increased deformability leads to large changes of the capacitance under compression. [0097] Using an insulating layer coated on the composite material reduces the overall conductivity which allows an increase of the amount of conductive fillers above the percolation threshold within the pores thereby increasing the relative permittivity.

[0098] Suitable non-conductive materials are e.g. polydialkylsiloxanes, in

particular polydimethyl siloxane (PDMS) and polyesters, preferably polyethylene terephthalate polymers. Layers of biaxially oriented polyethylene terephthalate films have been found particularly

advantageous in certain application cases. Just by way of example for such films, there may be mentioned Mylar^®, a product commercially available from DuPont or Hostaphan^®, available from Mitsubishi Chemical Corporation.

[0099] Another embodiment of the present invention relates to a process for the manufacture of a composite material in accordance with the present invention, comprising the following steps.

[00100] a) providing a first non-aqueous phase comprising a siloxane polymer precursor and a curing agent and, optionally, a surfactant,

b) providing a second aqueous phase comprising a conductive filler dispersed in water and, optionally, additives to faciliotate and support dispersion of the conductive filler in water,

c) preparing an emulsion by adding aqueous phase b) to the non-aqueous phase a) under stirring,

d) reticulating the product obtained in step c) and , finally,

e) subjecting the product obtained in step d) to a heat treatment to remove the water.

[00101] The composite material in accordance with the process of the present invention is obtained by using an inverse emulsion technology wherein the non-aqueous phase is a mixture of monomer and crosslinker and, optionally, a surfactant, and wherein the aqueous phase is an aqueous solution containing the conductive or semiconductive filler and, optionally, a surfactant. [00102] In step a) of the process of the present invention, a non-aqueous phase is prepared by using a siloxane polymer precursor, a curing agent and, optionally, a surfactant.

[00103] The siloxane polymer precursor may be preferably a two component kit as described hereinafter.

[00104] Two component kits comprising a siloxane precursor polymer and a curing agent are commercially available from a variety of suppliers and the skilled person will select the appropriate precursor products based on his professional knowledge and the needs of the specific application case.

[00105] Just by way of example, the principal constitution of such two component kit is explained in more detail for Sylgard^®184.

[00106] Sylgard 184^® is a silicon elastomer comprising a dimethyl siloxane and an organically modified silica. Sylgard^® 184 is prepared by combining a base (Part A) with a curing agent (Part B). The base includes a siloxane

(dimethyl-vinyl terminated dimethyl siloxane) and a dimethylvinylated and trimethylated silica) in a solvent (ethyl benzene). The curing agent also includes a mixture of siloxanes and silica in a solvent including dimethyl methyl hydrogen siloxane, dimethyl-vinyl terminated dimethyl siloxane, dimethylvinlylated and trimethylated silica, tertramethyl tetravinyl cyclitetra siloxane and ethyl benzene.

[00107] Sylgard^® 527 is a silicone elastomer gel substantially similar to Sylgard

184 but without the silica filler. It is also prepared from a base and a curing agent. A large variety of siloxane compositions are commercially available from various suppliers. The Sylgard^® series of products is just one example for such suitable two component kits which may be used in the process of the present invention in step a) and which are commercially available e.g. from Dow Chemical. Another group of suitable curable siloxane polymer precursors are the Elastosil^® series of products available from Wacker Chemie.

[00108] Exemplary PDMS precursors are vinyl-functional PDMS crosslinkable with hydride-functional crosslinking agents or hydroxyl-functional PDMS crosslinkable with hydride functional crosslinking agents or hydroxyl- functional PDMS crosslinkable in the presence of metal catalysts.

[00109] Sylgard^® 184 is a particularly preferred siloxane polymer precursor which may be used in the process according to the present invention.

[00110] 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 (e.g. Pt complexes for addition curing or Sn complexes for condensation curing) or peroxides (peroxide curing).

[00111] The non-aqueous phase a) may also optionally comprise a surfactant to stabilize the system. Suitable surfactants for this purpose are known to the skilled person and are available in great variety from a multiplicity of commercial suppliers. The skilled person will, based on his professional expertise select a suitable surfactant.

[00112] Just by way of example, silicone alkyl polyethers such as the Silube®

series of products may be mentioned here as suitable surfactants. Silicone alykl polyethers are alkylated silicones co-reacted with polyethers. Such surfactants are effective for emulsifying organic oils and silicones with water respectively aquoeus phases.

[00113] The Silube^® products available from Siltech company are represented by the following structure:

[00114]

[00115] The surfactant may be added to the siloxane precursor composition and is usually present in an amount from 0.5 to 10 wt%, preferably of from 0.75 to 7.5 wt% of the total weight of non-aqueous phase a).

[00116] In step b) of the process of the present invention, an aqueous phase

comprising the conductive or semiconductive filler dispersed therein, is provided. [00117] To obtain the aqueous phase provided in step b), the conductive or semiconductive filler is preferably added to water, preferably deionized water, under stirrring or under the application of ultrasound to disperse the conductive filler. When using ultrasound to support homogeneous dispersion of the filler the system is preferably cooled e.g. with an ice bath to avoid excessive heating-up of the system.

[00118] The solution prior to addition of the conductive or semiconductive filler may comprise additives to facilitate and support the dispersion of the

conductive or semiconductive filler. A preferred surfactant for this purpose, is gum arabic, also known as acacia gum. Acacia gum is a natural gum consisting of the hardened sap of various species of the acacia tree. Gum arabic is a complex mixture of glycoproteins and polysaccharides.

[00119] The skilled person is aware of further additives which facilitate and support the dispersion of conductive and semiconductive fillers in aqueous systems and respective products are commercially available in great variety from a number of different suppliers so that no further details have to be given here. The skilled person will select a suitable dispersion aid based on his professional knowledge and experience.

[00120] To obtain the 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 process.

[00121] Fluid undergoes shear when one area of fluid travels with a different

velocity relative to an adjacent area. A high-shear mixer uses a rotating impeller or high-speed rotor, or a series of such impellers or inline rotors, usually powered by an electric motor, to work the fluid, creating flow and shear. The tip velocity, or speed of the fluid at the outside diameter of the rotor, will be higher than the velocity at the center of the rotor, and it is this velocity difference that creates shear.

[00122] In a preferred embodiment, a high-shear mixer disperses, or transports, the aqueous phase provided in step b) into the main continuous phase provided in step a) with which it would normally be immiscible, thereby creating an emulsion. [00123] The skilled person will select the diameter of the stirrer and its rotational speed (and thereby defining the shear rate applied) in accordance with the needs of the specific application situation and the desired final morphology of the product.

[00124] Through the application of high shear rates it has been surprisingly found that it is possible to uniformly distribute high amounts of the non-aqueous phase in the silicone rubber and to form a stable emulsion, said emulsion being stable over extended periods of time.

[00125] The weight ratio of the non-aqueous phase to the aqueous phase is not subject to particular limitations and is usually within the range of 1 :10 to 10:1 , preferably in the range of 1 :5 to 5:1. Preferably, the non-aqueous phase forms the continuous phase of the system, in which the aqueous phase is dispersed and the amounts of non-. aqueous and aqueos phase are chosen respectively. In such case, the weight of the aqueous phase preferably does not exceed the amount of the non-aqueous phase and is usually in the range of form 30 to 40 wt% of th entire emulsion. In some application cases approximately equal weights of non-aqueous and aqueous phase have been found to provide certain advantages.

[00126] After step c) an emulsion is obtained which has droplets of the water

phase containing the conductive filler dispersed in the non-aqueous phase. The average diameter of these droplets is usually in the range from 0.1 to 300 pm, preferably in the range of from 0.5 to 150 pm and particularly preferred in the range of from 1 to 30 pm. The mean droplet size obtained depends on the viscosity of the continuous phase.

[00127] Solid materials are then obtained in step d) by reticulating (curing) the emulsion obtained in step c) usually at a temperature below the boiling point of water, preferably in the range from 60 to 95°C for a period of time of 0.5 to 12, preferably from 1 to 8 hours. In some cases, curing times of appr. 4 h have been found to be best. The relative humidity in this step is usually close to 100% or is equivalent to 100%. [00128] In one embodiment, curing may take place in the form of addition-based curing, such as by the use of Pt as a catalyst wherein Si-H groups of the crosslinking agent react with vinyl groups of the silicone polymer.

[00129] In accordance with another embodiment, curing may take place in a

condensation based system, such as through the use of a Sn based curing system and a room-temperature vulcanizing silicone rubber wherein an alkoxy-crosslinker experiences a hydrolysis step and is left with a hydroxyl group participating in a condensation reaction with another hydroxyl group attached to the polymer in question.

[00130] In still another embodiment, curing may take place in a peroxide-based system wherein an organic peroxide compound decomposes at elevated temperatures to form reactive radicals that chemically crosslink the polymer chains.

[00131] In the final step, the product obtained in step d) is subjected to a heat

treatment to remove the water. As the siloxane polymer formed after curing is permeable to water vapor, the droplets leave a porous structure with the conductive or semiconductive filler being sunstantially present in the closed porosity volume fraction of the matrix material, preferably with pore walls being coated with the conductive or semiconductive filler, thereby yielding the composite material in accordance with the present invention.

[00132] The conditions of curing in step d) and drying in step e) influence the

morphology of the porous composite material and the skilled person will select the conditions thereof in a suitable manner to obtain the desired morphology.

[00133] Another embodiment of the present invention relates to a film comprising, preferably consisting essentially of, and even more preferably consisting of the composite material in accordance with the present invention.

[00134] In accordance with a preferred embodiment, the thickness of the film is in the range from 1 to 500 pm preferably in the range of from 10 to 250 pm.

[00135] The films in accordance with the present invention can be obtained by forming the emulsion of step c) of the process in accordance with the present invention into a film by pouring same into a mold before applying step d). The film thus obtained is then subjected to steps d) and e) in accordance with the process of the present invention to obtain the final film suitable for use in pressure sensing devices.

[00136] In the final step of the process in accordance with the present invention, the product obtained in step d) is subjected to a heat treatment to remove the water. This heat treatment step is usually carried out at a temperature exceeding the boiling point of water at atmospheric pressure, preferably at a temperature in the range of from 100 to 200°C and for a duration of from 0.1 h to 5 h, preferably of from 0.5 to 5 hours. In some cases temperatures of 130 to 170°C and treatment times of 0.75 to 3 h, particularly of from 1 to 2 h have been found to be suitable.

[00137] After the final step, a microporous composite material, eventually in film form is obtained, which comprises pores in a closed porosity volume fraction with an average diameter preferably in the range from 0.1 to 200 pm and preferably with the pore walls being lined and the pores being filled with the conductive filler to a certain degree.

[00138] A further embodiment of the present invention relates to a substrate

coated with a film according to the present invention.

[00139] The substrate is not subject to particular limitations as structure and

composition are concerned and the skilled person will select the substrate taking into account the needs of the specific application situation.

[00140] The structure of the substrate may be adopted to the specific intended use and the substrate may have the function of a carrier for the deformable film or it may provide increased mechanical stability for the said film.

[00141] The material of the substrate may be metallic or non-metallic respectively insulating or conductive depending on the intended final use of the coated substrate in a pressure sensing device. In some cases, aluminum substrates or substrates comprising aluminum have been found to provide certain advantages.

[00142] The coating of the film onto the substrate may be effected using

conventional coating techniques known to the skilled person which have been described in the literature so that no further details need to be given here.

[00143] Another embodiment of the present invention relates to multilayer systems comprising a first layer of a film in accordance with the present invention, and, adjacent thereto, a second layer which is an insulating layer.

[00144] The films comprising the composite material in accordance with the

present invention exhibit losses once an amount of conductive filler near to or above the percolation limit is used, which is a certain disadvantage.

[00145] The multilayer systems in accordance with the present invention overcome these disadvantages by adding a second layer onto the films in

accordance with the present invention which second layer is an insulating layer. Thereby, the high permittivity of the material is maintained while at the same time the conductivity is significantly reduced.

[00146] The material of the second insulating layer may be any insulating material which may be formed into a film or a suitable coating on the first layer. For economical and processability reasons, insulating layers of thermoplastic polymers are preferred and silicone rubbers or polyesters may be mentioned as examples. Mylar^®films based on polyethylene terephthalate polymers, which are commercially available from a number of suppliers, have been found advantageous in terms of processabity and costs and thus represent a particularly preferred group of insulating materials for the second insulation layer.

[00147] The thickness of the second insultaing layer is not subject to particular limitations and often is in the range from 0.5 to 500 pm, preferably in the range from 1 to 100 pm.

[00148] The insulating layer may be spread onto the films comprising the

composite material in accordance with the present invention. The insulating layer may itself be deposited on a substrate.

[00149] Coating of the insulating layer onto the film comprising the composite

material may be achieved by conventional coating techniques such as spin coating, rotation coating or other coating techniques known to the skilled person and described in the literature. [00150] The composite materials in accordance with the present invention, the films comprising same and the coated substrates or multilayer structures comprising such films are particularly suitable for use in piezcapacitive devices. Due to their high permittivity at low conductivity, the sensitivity of the devices using said materials is high and very low variations in external pressure can be reliably determined.

[00151] When a compressive stress is applied on the microporous composite

material in accordance with the present invention, a large deformation is created as well as a modification of its microstructure. Both effects lead to large variation of the equivalent capacitance and therefore to a large piezocapacitive sensitivity at low external pressures e.g.in the range of from 0.1 kPa to 10kPa. Accordingly, the composites in accordance with the present invention are excellent candidates for capacitive pressure sensing applications, and more specifically for low force pressure sensors commonly needed for bio-signals such as blood pressure and heart rate monitoring.

[00152] Example 1

[00153] A solution comprising 5.0 wt% of arabic gum in water was prepared in a flat bottom flask by mixing 5 g of arabic gum (obtained from Sigma Aldrich) with 95 g of deionized water. Magneitc stirring was applied to

homogeneously dissolve the arabic gum which serves as a surfactant to disperse carbon black poweder (the conductive filler). The carbon black used was purchased from Alfa Aesar under the reference 39724- carbon black and was used as received.

[00154] The dispersion of the carbon black powder was carried out in a flat-bottom flask by mixing the carbon black powder in the desired amounts and arabic gum solution. The mixture was sonicated for one hour to homogenously disperse the carbon black particles while the solution was cooled in an ice bath to avoid an excessive temperature increase as a result of the sonication. The obtained product was used as the aqueous phase.

[00155] As non-aqueous phase, Sylgard 184 was purchased from Dow Coring as a kit consisting of a PDMS base and a curing agent. The relative dielectric permittivity of the PDMS materials was approximately 2. To the mixture of PDMS base and crosslinker, Silube^® J-208-212 was added as a surfactant to reach a concentration of 5 wt% of surfactant.

[00156] The aqueous phase was slowly added to the non-aqueous phase under mechanical stirring with a spatula in order to reach a ratio of aqueous phase to non aqueous phase of 50:50.

[00157] The water-in-oil emulsion thus obtained was poured into a mold having a depth of 500 pm and a diameter of 24 mm and covered. Thereafter the film was reticulated by subjecting the mold to a temperature of 90°C for 4 hours in a water bath (to have 100% humidity).

[00158] In the final step, the reticulated film was removed from the mold and

placed in an oven at a temperature of 150°C for one hour to remove the water.

[00159] The composite material obtained had a microporous structure with pores having an average diameter of from 10 to 30 pm. The average pore size was determined on scanning electron microscopy (SEM) images of the products.

[00160] The carbon black content of the composite material ranged from 4.6 to 10.2 wt%, based on the entire weight of the composite material.

[00161] The permittivity was determined by broadband dielectric spectroscopy using a sandwich geometry with circular brass electrodes. Measurements were performed at room temperature over frequencies f from 10 Hz to 10⁷ Hz. The real part of the permittivity was 3 for a material without any carbon black, 13.5 for a composite material comprising 4.6 wt% of carbon black.

At am amount of 10.2 wt% of carbon black the permittivity was determined to be 4000 but the material was conductive as the concentration exceeded percolation threshhold. If the film obtained was formed into a multilayer structure with an insulating film (Mylar film), the permittivity was 330 but the material remained non-conductive.

[00162] Static sensitivity was measured in compression using circular stainless steel clamps of diameter 25 mm and acting as electrodes. RSA Gil Solids Analyzer was used to maintain a normal pressure on the sample while measuring the complex impedance at 1V and 100Hz using a Keysight Precision LCR Meter. In order to estimate the equivalent capacitance Cp and resistance Rp of the sample it was assumed that the material acts as the combination of a resistor and a capacitor in parallel. The reference capacitance Co was arbitrarily defined when 0.1 kPa is applied on the sample. Measurement was performed over pressures ranging from 0.1 kPa up to 70 kPa.

[00163] For carbon black concentrations of 4.2 wt% without an insulating layer, AC/Co reached values in the range from 0.9 to 1.8 in the pressure range form 2 kPa to 10 kPa. If the carbon black concentration was 10 % and no insulating layer was present, the value for AC/C₀ dropped to values close to 0 at a pressure of 2 kPa. With an insulating layer and a carbon black concentration of 10 wt%, AC/C₀ was approximately 1.8 at a pressure of 2 kPa and exceeded a value of four at a pressure of 10 kPa.

[00164] These results show the excellent sensitivity of the composite materials in accordance with the present invention for use in piezocapacitive sensing devices.

Claims

Claims

1. Composite material comprising

a) a porous matrix material comprising a siloxane polymer, comprising a closed porosity volume fraction, and, optionally, an open porosity volume fraction, and

b) a conductive or semiconductive filler substantially present in said closed porosity volume fraction of said porous matrix material a).

2. The composite material in accordance with claim 1 wherein the ratio of closed porosity volume fraction to open porosity volume fraction is at least 1 :1 , preferably 1 :1 to 100:1.

3. The composite material of claim 1or 2 having an electrical conductivity, in the absence of external pressure, in the range of from 10^-5 to 10 ¹² S/m.

4. The composite material of any of claims claim 1 to 3 wherein the amount of filler is in the range of from 0.1 to 15 wt %, based on the entire weight of the composite.

5. The composite material of any of claims 1 to 4 wherein the siloxane polymer polymer is a polydimethylsiloxane (PDMS).

6. The composite material of any of claims 1 to 5 wherein the conductive or

semiconductive filler is selected from carbon nanotubes, carbon nanohorns, graphite, graphene and carbon black.

7. The composite material in accordance with any of claims 1 to 5 wherein the conductive or semiconductive filler is selected from metal particles like copper, silver, gold, and zinc.

8. The composite material in accordance with any of claims 1 to 5 wherein the conductive or semiconductive filler is selected from intrinsically conducting polymers (ICPs).

9. The composite material in accordance with any of claims 1 to 5 wherein the semiconductive filler is selected from the group consisting of Si, Si-Ge, GaAs, InP, GaN, SiC, ZnS, ZnSe, CdSe, and CdS or matal oxide particles.

10. The composite material of claim 6 wherein the conductive filler is carbon black.

11. Film comprising the composite material of any of claims 1 to 10.

12. The film of claim 11 having a thickness in the range of from 1 to 500 pm.

13. Substrate coated with a film in accordance with any of claims 11 or 12.

14. Multilayer system comprising a first layer of a film in accordance with any of claims 11 or 12 and, adjacent thereto, a second layer which is an insulating layer.

15. Multilayer system in accordance with claim 14 wherein the second layer is a polyester layer, particularly a polyethylene terephthalate layer.

16. A process for the manufacture of a composite material in accordance with any of claims 1 to 10 comprising the following steps:

a) providing a first non-aqueous phase comprising a siloxane polymer precursor and a curing agent and, optionally, a surfactant,

b) providing a second aqueous phase comprising a semiconductive or conductive filler dispersed in water,

c) preparing an emulsion by adding aqueous phase b) to the non-aqueous phase a) under stirring,

d) reticulating the product obtained in step c) and , finally,

d) subjecting the product obtained in step d) to a heat treatment to remove the water.

17. The process of claim 16 for manufacturing a film in accordance with claim 11 or 12 wherein the emulsion obtained in step c) is formed into a film by pouring same into a mold before applying step d).

18. Use of the composite material in accordance with any of claims 1 to 10 or of the film in accordance with claim 11 or 12 or of the substrate in accordance with claim 13 or of the multilayer structure in accordance with claim 14 or 15 for use in pressure sensing devices.