Development of a Magnetic Compound Fluid Rubber Stability Sensor and a Novel Production Technique via Combination of Natural, Chloroprene and Silicone Rubbers

Abstract

Expanding on our previous report, we investigate the stability of a magnetic compound fluid (MCF) rubber sensor that was developed for a variety of engineering applications. To stabilize this sensor, we proposed a novel combination technique that facilitates the addition of dimethylpolysiloxane (PDMS) to natural rubber (NR)-latex or chloroprene rubber (CR)-latex using polyvinyl alcohol (PVA) by experimentally and theoretically investigating issues related to instability. This technique is one of several other novel combinations of diene and non-diene rubbers. Silicone oil or rubber with PDMS can be combined with NR-latex and CR-latex because of PVA’s emulsion polymerization behavior. In addition, owing to electrolytic polymerization based on the combination of PDMS and PVA, MCF rubber is highly porous and can be infiltrated in any liquid. Hence, the fabrication of novel intelligent rubbers using any intelligent fluid is feasible. By assembling infiltrated MCF rubber sheets and by conducting electrolytic polymerization of MCF rubber liquid with a hydrate using the adhesive technique as presented in a previous paper, it is possible to stabilize the MCF rubber sensor. This sensor is resistant to cold or hot water as well as γ-irradiation as shown in the previous report.

1. Introduction

Rubber sensors are valuable elements in a wide variety of engineering applications that affect many aspects of our daily life. The elasticity and flexibility of rubber have resulted in significant progress in several applications. To advance this field, we have investigated the mechanical, electrical and photovoltaic characteristics of a proposed magnetically responsive fluid called MCF involving rubber latex, the so-called magnetic compound fluid (MCF) rubber. This material exhibits hybrid properties during electrolytic polymerization under an applied magnetic field [1,2,3,4,5,6,7]. MCF rubber is a soft rubber that can be easily made using the latex of natural rubber (NR), isoprene rubber (IR), chloroprene rubber (CR), and butadiene rubber (BR) based on our proposed electrolytic polymerization procedure [1]. These rubbers contain C=C bonds so that they can be electrolyzed for vulcanization [1,4]. However, given that nitrile rubber (NBR) or styrene-butadiene rubber (SBR) potentially has a high viscosity, electrolytic polymerization is difficult. In addition, the MCF rubber can be used as a sensor by exploiting our proposed novel adhesion technique [8] whereby the MCF rubber is electrolytically polymerized in conjunction with a metal complex hydrate so that the metal electrodes strictly adhere to the MCF rubber. Based on the reaction of the MCF rubber with electromagnetic waves, a variety of engineering applications can be developed. In a previous report [9], the effects of γ-rays, infrared and microwaves on the MCF rubber sensor were evaluated, establishing the feasibility of using the MCF rubber sensor for sensing various solar and thermal sources, etc., related to energy harvesting. In particular, it is suitable for use as a sensor installed in a robot that operates in a nuclear reactor building.

However, one issue is the stability of the MCF rubber sensor. The stability of the changes of electrical properties with time is a significant problem. In the present report, we investigate the causes of the instability and deterioration in MCF rubber. Next, we resolve these problems by introducing a combination of non-diene rubbers. The non-diene rubbers include butyl rubber (IIR), ethylene-propylene rubber (EPM or EPDM), urethane rubber (UR), silicone rubber (Q), fluoro rubber (FKM), etc., which do not or seldom contain C=C bonds. Non-diene rubbers show superior weather, ozone, chemical, and oxidation resistance, less deterioration after years of usage under a variety of environmental conditions, and they can be used at any temperature. In addition, in terms of the sensitivity of a robot operated in a nuclear building, we also investigate in the present report the feasibility of an underwater MCF rubber sensor. To date, few studies have attempted to develop rubber-type water sensors in contrast with typical sensors including acoustic ones, etc. [10,11].

Regarding the combination of any material into a rubber, there have been many studies on the combination of fillers such as carbon, metal, etc. particles in NR [12], CR [13], NBR [14], SBR [15], Q [16,17] rubbers and in the combinations of NR and SBR [18,19,20,21], NR and BR [21], NR and EPDM [22]. However, in terms of the latter combination, vulcanization with sulfur has been used. As observed in the results for γ-irradiation effect on the MCF rubber sensor in the previous report, our electrolytic polymerization for rubber vulcanization without using sulfur results in some typical characteristics, for example, the enhancement of softness during the elongation. Therefore, the utilization of electrolytic polymerization in the combination of several rubbers is expected to be effective. In addition, given that non-diene rubber offers excellent resistance as previously mentioned, the combination of diene and non-diene rubbers is also suitable. However, fabrication of this combination is akin to mixing water and oil because each rubber is generally similar to water and oil, respectively. This means that the rubbers behave as hydrophilic and hydrophobic groups [23]. Therefore, we can apply the principle of emulsion polymerization to the combination as used in the mayonnaise production process. In the present report, we address and propose a novel rubber production process via the combination of NR and CR, and Q.

2. Stability 2.1. Factor That Influence Instability

Firstly, we will begin by discussing the factors that influence the instability of an MCF rubber sensor. In the previous study, we evaluated the secular stability of electrolytically polymerized MCF rubber sheets fabricated by sandwiching between opposing magnets [1]. However, it is typically necessary to measure the electric properties during sandwiching of the MCF rubber between the opposing electrodes. This electric property has secular stability. However, when the electrodes are separated, the stability deteriorates with time. Instability can be divided into two phases: (a) temporary instability during sensing that fluctuates with time; (b) instability over a long span of time leading to aging degradation. However, MCF rubber sheets must be considered because they are fundamental components of MCF rubber sensors. From this perspective, examples of factors that influence instability include: (1) the evaporation of water from the MCF rubber because water is substantially involved in the fabrication process; (2) the vulcanization of the MCF rubber occurs during the electrolytic polymerization process with surpassing this point, in contrast to electrolytic polymerization in our previous studies; (3) the deformation of the MCF rubber by the Mullins effect. Factors (1–3) are related to (a) and (b).

In the case of (1), water is involved and is trapped in the inner MCF rubber, even if electrolytic polarization is performed, because the NR and CR latexes contain water. Therefore, due to the evaporation of water from the electrolytically polymerized MCF rubber, the MCF rubber sensor can exhibit a secular change of its electrical and mechanical properties. This will be discussed in the following section.

In the case of (2), the MCF rubber liquid should be taken into account because it is fundamental in the production of MCF rubber sensors. In the previous study, we experimentally investigated the influence of the aggregation of magnetic particles and rubber molecules of MCF rubber liquid on their electrical characteristics when a magnetic field was applied [24]. The application of a magnetic field to a container immersed in the MCF rubber liquid resulted in the aggregation of particles and molecules to sediment. In the present report, we will introduce the result that the MCF rubber in the middle of vulcanization induces temporal instability by using an experimental apparatus with large electrodes gap as shown in Figure A1 in the Appendix A. The MCF rubber liquid consisted of 12 g Ni powder with particles on the order of microns and bumps on the surface (No. 123 by Yamaishi Co. Ltd., Noda, Japan), 3 g water-based MF with 50 wt% Fe₃O₄ (M-300, Sigma Hi-Chemical Co. Ltd., Tsutsujigasaki, Japan), 16 g NR-latex (Ulacol, Rejitex Co. Ltd., Atsugi, Japan) and 31 g of water. Using electrolytic polymerization, the MCF rubber is vulcanized as shown in Figure A2 in the Appendix A. The vulcanized MCF rubber grows from the anode surface towards the direction of the cathode as a crystal. The thickness of the vulcanized MCF rubber increases with time as shown in Figure A3 in the Appendix A. Isoprene molecules and magnetic clusters are aligned along the same direction as the electric and magnetic field lines, as explained in previous studies [1,4]. The structure can be considered to be approximately quasi-regular crystalline, but not a crystalline lattice such as in hexagonal close-packed, body-centered cubic, and face-centered cubic structures. The density of the aggregation at 0 mm was larger than that at 15 mm such that the thickness of the vulcanize MCF rubber at 0 mm was larger than that at 15 mm. As shown in Figure A4 in the Appendix A, as the thickness of the vulcanized MCF rubber increases, the electrical conductivity decreases. Therefore, if the vulcanization of the MCF rubber occurs during the electrolytic polymerization process without surpassing this point, the electrical signal from the MCF rubber sensor becomes unstable when the piezoresistivity of MCF rubber sensor is examined. This refers to usage whereby a generated electric current is measured via the application of a voltage with any electric source.

In the case of (3), we must consider the change of the electric current flowing in the MCF rubber based on its deformation. The change of the electric current of the MCF rubber by compression has been elucidated in previous studies [1,2,3]. In addition, this has been theoretically explained in other studies [5,24]. The relationship between the transmitted probability T, which represents the electric current flowing in the MCF rubber and the thickness b, is shown in Figure A5a in the Appendix A. The relationship between the dimensionless capacitance C* and the thickness b is shown in Figure A5b in the Appendix. T and C* increase by enhancing b. T is considered to be re-expression of the electric current I, C* applies to the capacitance C, and b applies to the deformation quantity. The motion of electrons described by the tunnel theory is categorized as electron transfer in the field of complex chemistry. The mechanism of the electron transfer is divided into two types based on the distance among the particles of Fe₃O₄ and Ni, and the molecules of polyisoprene: outer-sphere electron transfer reaction (OSETR) and inter-sphere electron transfer reaction (ISETR), as shown in a previous study [5]. Whether OSETR or ISETR is generated depends on the probability of the distance among the particles and molecules—OSETR takes place in the case of long distances and ISETR occurs for shorter distances. Decreasing b represents a contraction of the MCF rubber. As b becomes smaller, ISETR is dominant in the change of the electrical property of the MCF rubber due to the electric-chemical reaction. In contrast, as b gets larger, OSETR is dominant. As the distance among the particles and molecules is reduced by deformation of the MCF rubber, I and C increase as shown in Figure A5. The enhancement of I and C are presented as ΔI and ΔC, respectively. In the case of ΔI and ΔC, we can obtain the results as represented by Equations (1)–(3), together with Equation (A3) in the Appendix. The notion is that I represents the amount of electron transfer and that C represents the amount of counter ion created by an anionic acceptor A⁻ and cationic donor D⁺. A and D are generated by particles and molecules with semiconducting roles by mixing and electrolytic polymerization as shown in Equations (A4)–(A7) in the Appendix, which has been presented in a previous study [5]. However, the reaction occurs from the right-hand-side to the left-hand-side by irradiation of electromagnetic waves of γ-rays, microwave, light, etc. In the case of OSETR, which implies that the distance among the particles and molecules is large, the net reaction from the right-hand-side to the left-hand-side does not occur practically. In the case of ISETR, which implies that the distance among the particles and molecules is small, the net reaction from the right-hand-side to the left-hand-side occurs practically.

In the case of Equation (1), the voltage V increases with time. The amount of A⁻ and D⁺ is small because ISETR is dominant in the reaction as shown in Equations (A4)–(A7), due to the small distance among the particles and molecules. However, the amount of electron transfer is larger than that of the counter ion:

|ΔI|>|ΔC|

In the case of Equation (2), the voltage V with time is constant. The amount of electron transfer is the same as that of counter ion:

|ΔI|=|ΔC|

In the case of Equation (3), the voltage V with time decreases toward zero. The amount of A⁻ and D⁺ is large because of OSETR is dominant in the reaction as shown in Equations (A4)–(A7) due to the large distance among the particles and molecules. However, the amount of electron transfer is smaller than that of the counter ion:

|ΔI|<|ΔC|

Mullins effect is induced by the inner deformation among the particles and molecules of the MCF rubber. MCF rubber was determined to exhibit the Mullins effect in previous studies [1,24]. Therefore, the instability related to the aforementioned (a) and (b) occurs, such as the alternation of increasing and decreasing voltage and electric current of the MCF rubber according to Equations (1)–(3). As a result, the instability based on the deformation of the MCF rubber by Mullins effect also occurs.

Moreover, the instability of the MCF rubber is significant when utilized as a sensor. To prevent the aforementioned instability: (1) a possible idea is to deposit some material on the MCF rubber sensor. However, for the MCF rubber or MCF rubber sensor, this is useless because of the attempt to prevent the aforementioned instability (2) as follows. At first, we can clarify that by comparing the electrical property between the covered and non-covered MCF rubber as shown in Figure A6, the electrical sensitivity is reduced compared to the silicone oil rubber that covers the MCF rubber. Secondly, based on another experiment on coating using a liquid-rubber coating spray, the covering can be determined to be useless. In the case of the MCF rubber sensor produced using the procedure as shown in Figure A1 in the previous report [9], the MCF rubber sensor sprayed with liquid rubber-coating spray, which is an ordinary commercial liquid-rubber spray (LC-311SP, Jefcom, Co. Ltd., Osaka, Japan) exhibits the change that the 11.2 mV induced voltage at the production date becomes zero after 1 month. The MCF rubber sensor was consisted with the MCF rubber liquid with a hydrate as 1 g Ni, 0.75 g MF with 40 wt% Fe₃O₄ (W-40, Ichinen-Chemicals Co., Ltd., Shibaura, Japan), 3 g NR-latex (Ulacol), 3 g CR-latex (671A, Showa Denko Co. Ltd., Tokyo, Japan), 0.5 g TiO₂ (Anatase type, Fujifilm Wako Pure Chemical Co., Ltd., Osaka, Japan), and 0.5 g hydrates Na₂WO₄·2H₂O (Fujifilm Wako Pure Chemical Co., Ltd., Osaka, Japan), and MCF rubber without hydrate as 3 g Ni powder, 0.75 g MF (W-40), 3 g NR-latex (Ulacol), 3g CR-latex (671A), and 0.5 g TiO₂. Irrespective of the material used to coat the MCF rubber or MCF rubber sensor, the instability of the aforementioned (b) is still present in the MCF rubber so that the construction in the MCF rubber sensor must be improved.

Incidentally, the present MCF rubber sensor is sensitive to both normal and shear forces, which has been presented in detail in our previous study [1,2]: the electric current passed between electrodes is changed alternatingly according to the deformation of the rubber by the application of the forces, whose passing phenomena is created by tunnel effect as shown in Figure A5a. For example, regarding the normal force, the electric resistance decreases abruptly by the application of minimal force as shown in Figure A6. This change is different by kinds of dopant involved in the MCF rubber as shown in the reference [2]. In addition, the electric resistivity of commercial pressure-sensitive electrically conductive rubbers (PSECRs) made of NR-latex (NR-latex), CR rubber (CR rubber) and silicon oil rubber has been presented, comparing with that of MCF rubber [1].

2.2. Combination of NR and CR, and Q

To resolve the instability highlighted in the previous section, the resistance of the non-diene rubber to water, heat, etc. must be considered. Moreover, we attempt to combine the non-diene rubber to diene rubber. In the present report, we deal with Q as a non-diene rubber. It is structured as the basis of dimethylpolysiloxane (PDMS) as shown in Equation A8 in the Appendix, which is similar to oil. On the contrary, NR and CR are similar to water. Therefore, when PDMS and NR or CR are combined, polyvinyl alcohol (PVA) is used, which is anionic as shown in Equation (4). PVA is an emulsifier, then NR-latex or CR-latex and PDMS can be combined by emulsion polymerization as shown in Figure 1.

[Image omitted. See PDF.]

Figure 1.Schematic diagram of hydrophilic and hydrophobic group of PVA, and emulsion polymerization between PDMS and NR- or CR-latex.

[Image omitted. See PDF.]

Figure 2.Images of liquid before electrolytic polymerization; (a) KF96 (1000 cSt) + NR-latex without PVA; (b) KF96 (1000 cSt) + NR-latex (Ulacol) with PVA; (c) KE1400 + CR-latex (671A) with PVA; (d) KE1400 + CR-latex (671A) + MCF with PVA.

[Image omitted. See PDF.]

Figure 3.Photographs of surface on cathode-side electrode after electrolytic polymerization with PVA under a magnetic field; (a) KF96 (1000 cSt) + NR-latex (Ulacol); (b) KF96 (100 cSt) + NR-latex (Ulacol); (c) KF96 (50 cSt) + NR-latex (Ulacol); (d) KF96 (1 cSt) + NR-latex (Ulacol); (e) KF96 (1000 cSt) + CR-latex (671A); (f) KE1400 + NR-latex (Ulacol); (g) KE1400 + CR-latex(671A); (h) KE1300T + NR-latex (Ulacol); (i) KE1300T + CR-latex (671A); (j) KE1300T + NR-latex (Ulacol) + CR-latex (671A); (k) KE1300T + NR-latex (Ulacol) + CR-latex (671A) + TiO2.

[Image omitted. See PDF.]

Figure 4.Relation between stress strain and stress in tension; as for KE1300T, 3 g KE1300T + 3 g PVA; as for Non-PDMS, without KE1300T or KF96 + 3 g, and PVA; as for KF96, 3 g KF96 + 3 g PVA; all has 3 g NR-latex (Ulacol) + 3 g CR-latex (671A) + 0.75 g MF(W-40) + 3 g Ni.

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Figure 5.Procedure of the production of an MCF rubber sensor by combining PDMS and PVA.

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Figure 6.Physical model of infiltrated MCF rubber with a liquid.

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Figure 7.Images of surface of metal on cathode-side electrode after electrolytic polymerization with PVA, Na2WO4·2H2O, KF96 (1000 cSt) under an applied magnetic field; (a) aluminum; (b) stainless steel; (c) nickel; (d) zinc; (e) lead; (f) brass; (g) iron; (h) copper; (i) glass coated by TiO2.

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Figure 8.Image of electrical wires adhered to MCF rubber with hydrate inner MCF rubber sensor.

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Figure 9.Procedure for the production of MCF rubber sensor with multi layers.

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Figure 10.Schematic diagram of internal structure of MCF rubber sensor.

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Figure 11.Change of the induced voltage with elapsed time.

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Figure 12.Images of single MCF rubber immersed in water; (a) NR-latex (Ulacol) by drying without a magnetic field; (b) NR-latex (Ulacol) by drying with a magnetic field; (c) NR-latex (Ulacol) by electrolytic polymerization with a magnetic field; (d) NR-latex (Ulacol) + CR-latex (671A) by electrolytic polymerization with a magnetic field; (e) NR-latex (Ulacol) + CR-latex (671A) + TiO2 by electrolytic polymerization with a magnetic field; (f) NR-latex (Ulacol) + CR-latex (671A) + KE1300T + PVA by electrolytic polymerization with a magnetic field; (g) NR-latex (Ulacol) + CR-latex (671A) + KE1300T + PVA + TiO2 by electrolytic polymerization with a magnetic field; all rubbers include MF (W-40) and Ni.

[Image omitted. See PDF.]

Figure 13.Images of the MCF rubber sensor immersed in water; (a) with TiO2 and without PDMS and PVA; (b) without TiO2, and with PDMS and PVA; (c) with TiO2, PDMS and PVA; (d) without TiO2, with PDMS and PVA, and infiltrated with glycerin; all rubbers include MF (W-40) + Ni + NR-latex (Ulacol) + CR-latex (671A) with an applied magnetic field.

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Figure 14.Schematic diagram of sensitive MCF rubber sensor in water under compression.

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Figure 15.Corresponding image of the schematic shown in Figure 14.

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Figure 16.Change of induced voltage when in contact with water.

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Figure 17.Schematic diagram for MCF rubber sensor in contact with water: (a) at time of sensor’s touching to water; (b) after sensor’s touching and then insertion to water.

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Figure 18.Induced voltage of MCF rubber sensor with single layer produced by combination of PDMS and PVA in water by compression: (a) to a hard body (b) to a soft body.

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Figure 19.Comparison of induced voltage in water by compression between MCF rubber sensor with single layer produced by a combination of PDMS and PVA and that a double layer: (a) to a hard body (b) to a soft body.

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Figure 20.Induced voltage in salt water with compression.

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Table 1.Change of the mass of MCF rubber sheet and ratio of water evaporation.

Mass at 0 Day [g] Mass After 2 Days [g] Ratio of Water Evaporation (RWE) (%)

KE1300T 0.1573 0.1538 2.225

KF96 0.0678 0.0648 2.234

Non-PDMS 0.2372 0.2319 4.424

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Table 2.Constitute of MCF rubber sensor in Figure 11 with 3 g NR-latex (Ulacol), 3 g CR-latex (671A), 0.75 g MF (W-40), and 3 g Ni for (f) or (g), 1 g Ni for (d) in Figure 9.

Number of Layers Filtration Combination of PDMS and PVA TiO2 Sulfur in NR-latex

a 2 yes yes yes yes

b 1 yes yes yes no

c 1 no yes yes no

d 1 no yes no no

e 1 no no yes no

f 1 no no no no

Author Contributions

For this research article, K.S. conceived, designed the experiments, performed the experiments, analyzed the data and wrote the paper; H.K., H.T. and R.I. contributed to argue the data for suitable suggestion.

Funding

This work was supported in part by JSPS KAKENHI Grant Number JP 18K04040 and we are very grateful for this support.

Conflicts of Interest

The founding sponsors had no role in the design of the study. The authors declare no conflict of interest.

Appendix A

In the previous study, we experimentally investigated the influence of the aggregation of magnetic particles and rubber molecules of MCF rubber liquid on the electrical characteristics under an applied magnetic field [24]. In the present report, we used the same experimental apparatus as the [24], as shown in Figure A1. The electrodes were inserted into the MCF rubber liquid, which was poured into a nonmagnetic rectangular container and a dc voltage was applied to the electrodes. The electrodes were vertically inserted in the MCF rubber liquid. Then, the electric current and resistance of the liquid were measured. A permanent magnet was positioned on the outside of the container. with 10 mm × 15 mm in size and 5 mm in thickness, and a magnetic field intensity of approximately 300 mT at its surface.

Figure A1.Schematic of experimental apparatus to investigate the effect of particle aggregation via the application of a magnetic field to the MCF rubber liquid.

Figure A1.Schematic of experimental apparatus to investigate the effect of particle aggregation via the application of a magnetic field to the MCF rubber liquid.

The MCF rubber liquid was poured on one metal plate and sandwiched using another. Two permanent magnets were applied to the outer side of each metal plate. The permanent magnets were rectangular, 10 mm × 15 mm in size, and 5 mm thick.

Figure A2.Images of vulcanized MCF rubber on anode by electrolytic polymerization: (a) stainless electrodes; (b) copper electrodes; right is the cathode and the left is the anode.

Figure A2.Images of vulcanized MCF rubber on anode by electrolytic polymerization: (a) stainless electrodes; (b) copper electrodes; right is the cathode and the left is the anode.

Given that the size of the vulcanized MCF rubber depends on that of the permanent magnet, the obtained rubber samples were 19 mm × 24 mm rectangles. We applied 6 V and an electric current of 2.7 A between the plates with 1 mm separation of the electrodes for a 10 min period.

Figure A3 shows the changes in the thickness of the cohesion of particles and molecules of the MCF rubber liquid on an anode electrode with time, after the application of a 10 V electric field. The anode made of stainless steel and copper was settled from the bottom of the container at 0 mm and 15 mm as shown in Figure A1. In the case of 0 mm, the magnetic field intensity was approximately 240 mT underneath the edge of the anode. At 15 mm, the magnetic field intensity was approximately 35 mT, as seen from the previous study [24].

Figure A3.Change of the thickness of vulcanized MCF rubber via electrolytic polymerization with time.

Figure A3.Change of the thickness of vulcanized MCF rubber via electrolytic polymerization with time.

Regarding Figure A3, the change in the electric current flowing in the inner MCF rubber liquid with the applied voltage is shown in Figure A4.

Figure A4.Relation between electric current and voltage of MCF rubber liquid under electrolytic polymerization: (a) with stainless electrodes; (b) with copper electrodes.

Figure A4.Relation between electric current and voltage of MCF rubber liquid under electrolytic polymerization: (a) with stainless electrodes; (b) with copper electrodes.

From the tunnel theory presented in previous studies [5,24], the relationship between the transmitted probability T and the thickness of nonconductive rubber among the metal and Fe3O4 particles b is as shown in Figure A5a, and the relationship between the dimensionless capacitance C* of the MCF rubber liquid and the thickness b is shown in Figure A5b without the applied voltage V0.

Figure A5.Theoretical results based on tunnel theory from previous studies [5,24]; (a) transmitted probability; (b) dimensionless capacitance.

Figure A5.Theoretical results based on tunnel theory from previous studies [5,24]; (a) transmitted probability; (b) dimensionless capacitance.

From the relations presented in Equations (A1)–(A2), Equation (A3) can be obtained, where Q is the electric charge, I is the electric current, t represents time, and C is the capacitance:

Q=It

Q=CV

Vt=IC

In the case of mixing liquids and powders, the reaction can be represented by Equations (A4)–(A5), which is called chemical doping. In general, there are two types of doping: chemical doping and electrochemical doping. In the doping process, p- and n-type semiconductors are ionized. The former is an acceptor, A, which is charged negatively by accepting an electron (denoted as A −); the latter is a donor, D, which achieves a positive charge by releasing an electron (denoted as D+). The MCF rubber has A− and D+ ions by mixing its constituent and using the electrolytic polymerization process. A− and D+ are created from particles and molecules of the MCF rubber liquid, as shown in a previous study [5]. For example, A− is the negatively ionized polyisoprene P, which is an anion that participates in a radical reaction in the normal state of the NR-latex. In the case of electrolytic polymerization, the reaction occurs as shown in Equations (A6)–(A7), which is referred to as electrochemical doping:

_Px+xyA→_{[^Py+ Ay−]x}

_Px+xyD→_[Dy+^Py−]x

_Px+xy^A−→_{[^Py+ Ay−]x}+xy^e−

_Px+xy^D++xy^e−→_{[^{D+ Py−}]x}

Figure A6 shows the results for the piezoresistivity of the MCF rubber sheet with 3 g Ni, 0.75 g MF (W-40) and 3 g NR-latex (Ulacol) for electrolytic polymerization at 6 V, 2.7 A and 5 s, with a 1 mm electrode gap for an applied magnetic field of 188 mT, which is designated as “MCF rubber without covering” in the figure. The MCF rubber was covered using silicone oil rubber (KE1300T with using the curing agent to solidify) with two thin stainless steel plates that touched both sides of the MCF rubber sheet, which is designated as “MCF rubber with covering”. By applying 10 V voltage to the MCF rubber, the electric resistance can be determined by measuring the electric current flowing through the inner MCF rubber under compression, using the same compact commercial tensile testing machine shown in Figure 4. Therefore, this experiment is categorized in as part of the piezoresistivity analysis [3]. The MCF rubber was sandwiched by compression between the electrodes with a size of 7 mm square, which is the same experimental method as a previous study [1].

Figure A6.Comparison of piezoresisivity of MCF rubber for the cases of with and without covering.

Figure A6.Comparison of piezoresisivity of MCF rubber for the cases of with and without covering.

The chemical structure of PDMS is as follows:

Figure A7 shows the change of temperature, voltage, and electric current with time during electrolytic polymerization. The temperature was measured using a thermocouple inserted between the electrodes using the same method in a previous study [28,29]. The mass of each ingredient and electrolytic polymerization conditions are the same as those shown in Figure 3, wherein the case of KE1300T and KF96, 3 g PVA was used. In the case that included Na2WO4·2H2O, 1 g Ni and 0.5 g Na2WO4·2H2O were used for the adhesion of metal electrodes as a sensor using MCF rubber with a metal complex hydrate. In the case of S-500, S-500 (Rejitex Co. Ltd.) is the NR-latex including sulfur and 3 g was used. In each MCF rubber liquid, the time when each rising is the fastest in the figures represents the start of electrolytic polymerization.

Figure A7.Changes of temperature, voltage and electric current with time via electrolytic polymerization; (a) temperature; (b) voltage; (c) electric current.

Figure A7.Changes of temperature, voltage and electric current with time via electrolytic polymerization; (a) temperature; (b) voltage; (c) electric current.

Figure A8 shows the voltage of the MCF rubber after the application of 10 V and the relation between the strain and stress during tension and compression respectively. Figure A8a is not a case of piezoelectricity but a case of piezoresistivity. The difference has been presented in a previous study [3]. Our used NR-latex with sulfur (S-500, Rejitex Co. Ltd.) is different from the NR-latex without sulfur (Ulacol). The MCF rubber consisted on 3 g NR-latex, 0.75 g MF (W-40), 3 g Ni, and was fabricated by the application of a magnetic field of 188 mT, 6 V and 2.7 A, and 30 min. electric field with 1 mm electrode gap. The arrows in the figure represent the cyclic compression and tension.

Figure A8.Comparison of piezoresistivity of NR-latex with and without sulfur; (a) voltage of MCF rubber sheet by compression; (b) relationship between strain and stress.

Figure A8.Comparison of piezoresistivity of NR-latex with and without sulfur; (a) voltage of MCF rubber sheet by compression; (b) relationship between strain and stress.

The relationship between the displacement and force of the MCF rubber sensors when touched with hard and soft bodies in water at 25 °C and 50 °C were measured using the apparatus shown in Figure 14, as shown in Figure A9. The MCF rubber sensors have a single layer due to the combination of PDMS and PVA. The arrows in the figure represent cyclic compression.

Figure A9.Relation between displacement and force during compression.

Figure A9.Relation between displacement and force during compression.

References

1. Shimada, K.; Saga, N. Mechanical enhancement of sensitivity in natural rubber using electrolytic polymerization aided by a magnetic field and MCF for application in haptic sensors. Sensors 2016, 16, 1521.

2. Shimada, K.; Mochizuki, O.; Kubota, Y. The effect of particles on electrolytically polymerized thin natural MCF rubber for soft sensors installed in artificial skin. Sensors 2017, 17, 896.

3. Shimada, K.; Saga, N. Development of a hybrid piezo natural rubber piezoelectricity and piezoresistivity sensor with magnetic clusters made by electric and magnetic field assistance and filling with magnetic compound fluid. Sensors 2017, 17, 1521.

4. Shimada, K. Enhancement of MCF rubber utilizing electric and magnetic fields, and clarification of electrolytic polymerization. Sensors 2017, 17, 767.

5. Shimada, K. Elastic MCF rubber with photovoltaics and sensing for use as artificial or hybrid skin (H-Skin): 1st report on dry-type solar cell rubber with piezoelectricity for compressive sensing. Sensors 2018, 18, 1841.

6. Shimada, K. Elastic MCF rubber with photovoltaics and sensing on hybrid skin (H-Skin) for artificial skin by utilizing natural rubber: 2nd report on effect of tension and compression on properties of hybrid photo- and piezo-electricity in wet-type solar cell rubber. Sensors 2018, 18, 1848.

7. Shimada, K. MCF rubber with photovoltaics and sensing for use as artificial or hybrid skin (H-Skin): Third report on electric charge and storage under tension and compression. Sensors 2018, 18, 1853.

8. Shimada, K.; Kikura, H.; Takahashi, H.; Ikeda, R. Novel adhesion technique using metallic or non-metallic hydrous oxide of metal complexes involving magnetic compound fluid rubber under electrolytic polymerization and magnetic field for producing sensors. Sensors 2019, 19, 689.

9. Shimada, K.; Ikeda, R.; Kikura, H.; Takahashi, H.; Shimada, K. Development of MCF rubber sensor for practical usage on γ-irradiation and energy harvesting for broad-band electro-magnetic waves. World J. Mech. 2019. to be published.

10. Muggleton, J.M.; Brennan, M.J.; Pinnington, R.J.; Gao, Y. A novel sensor for measuring the acoustic pressure in buried plastic water pipes. J. Sound Vib. 2006, 295, 1085–1098.

11. Malzahn, K.; Windmiller, J.R.; Ramirez, G.V.; Schoning, M.J.; Wang, J. Wearable electrochemical sensors for in situanalysis in marine environments. Analyst 2011, 137, 2912–2917.

12. Gonzalez, N.; Custal, M.A.; lalaouna, S.; Riba, J.R.; Armelin, E. Improvement of dielectric properties of natural rubber by adding perovskite nanoparticles. Eur. Polym. J. 2016, 75, 210–222.

13. Ogura, K.; Ogawa, M.; Ohya, K.; Banno, H. Receiving characteristics of D(31)-zero piezo-rubber hydrophone. Jpn. J. Appl. Phys. Part 1 1993, 32, 2304–2306. (In Japanese)

14. Mahmoud, W.E.; El-Lawindy, A.W.Y.; El Eraki, M.H.; Hassan, H.H. Butadiene acrylonitrile rubber loaded fast extrusion furnace black as a compressive strain and pressure sensors. Sensors Actuators A Phys. 2007, 136, 229–233.

15. Ruiz, M.M.; Marchi, M.C.; Perez, O.E.; Jorge, G.E.; Fascio, M.; D’Accors, N.; Negri, R.M. Structured elastomeric submillimeter films displaying magneto and piezo resistivity. J. Polym. Sci. Part B 2015, 53, 574–586.

16. Mamada, S.; Yaguchi, N.; Hansaka, M.; Yamato, M.; Yoshida, H. Matrix influence on the piezoelectric properties of piezoelectric ceramic/polymer composite exhibiting particle alignment. J. Appl. Polym. Sci. 2015, 132, 41817.

17. Liu, P.; Liu, C.X.; Huang, Y.; Wang, W.H.; Fang, D.; Zhang, Y.G.; Ge, Y.J. Transfer function and working principle of a pressure/temperature sensor based on carbon black/silicone rubber composites. J. Appl. Polym. Sci. 2016, 133, 42979.

18. Ismail, H.; Hairunezam, H.M. The effect of a compatibilizer on curing characteristics, mechanical properties and oil resistance of styrene butadiene rubber/epoxidized natural rubber blends. Eur. Polym. J. 2001, 37, 39–44.

19. Berridi, M.J.F.; Gonzalez, N.; Mujiza, A.; Bernicot, C. Pyrolysis-FTIR and TGA techniques as tools in the characterization of blends of natural rubber and SBR. Thermochim. Acta 2006, 444, 65–70.

20. Thermal properties in cured natural rubber/styrene butadiene rubber blends. Eur. Polym. J. 2008, 44, 1525–1534.

21. Chakraborty, S.; Bandyopadhyay, S.; Ameta, R.; Mukhopadhyay, R.; Deuri, A.S. Application of FTIR in characterization of acrylonitrile-butadiene rubber (nitrile rubber). Polym. Test. 2007, 26, 38–41.

22. Gunasekaran, S.; Natarajan, R.K.; Kala, A. FTIR spectra and mechanical strength analysis of some selected rubber derivatives. Spectrochim. Acta Part A 2007, 68, 323–330.

23. Tan, J.; Ma, D.; Feng, S.; Zhang, C. Effect of headgroups on the aggregation behavior of cationic silicone surfactants in aqueous solution. Colloids Surf. A Physicochem. Eng. Aspects 2013, 417, 146–153.

24. Shimada, K. Effect of magnetic field and aggregation on electrical characteristics of magnetically responsive suspensions for novel hybrid liquid capacitor. Magnetochemistry 2019, 5, 38.

25. Li, D.; Zhang, J.; Yuan, J.; Guo, F.; Wang, H. Simulation and measurement of fuel injection quantity based on TB4P. J. Robot. Mechatron. 2014, 26, 34–39.

26. Xu, L.; Luo, C.; Shi, S.; He, L.; Wu, X. Performance evaluation of and research on anti-gas channeling cement slugs. Chem. Technol. Fuels Oils 2019, 55, 183–188.

27. Okosun, F.; Cahill, P.; Hazra, B.; Pakrashi, V. Vibration-based leak detection and monitoring of water pipes using output-only piezoelectric sensors. Eur. Phys. J. Spec. Top. 2019, 228, 1659–1675.

28. Shimada, K.; Saga, N. Detailed mechanism and engineering applicability of electrolytic polymerization aided by a magnetic field in natural rubber by mechanical approach for sensing (Part 1): The effect of experimental conditions on electrolytic polymerization. World J. Mech. 2016, 6, 357–378.

29. Shimada, K.; Saga, N. Detailed mechanism and engineering applicability of electrolytic polymerization aided by a magnetic field in natural rubber by mechanical approach for sensing (Part 2): Other and intrinsic effects on MCF rubber property. World J. Mech. 2016, 6, 379–395.