MANUFACTURING THE SAME

Technical Field

The invention concerns the fields of materials science and technical physics and can be applied in devices transforming mechanical pressure into electrical signal such as pressure sensors, tension meter elements, electric signal commuters, etc.

Background Art

Sensing elements designed for pressure detection and producing a series of signals equivalent to the value of the pressure are widely spread in technical practice. Mainly they are based on piezoelectric crystals or piezoelectric ceramics and, therefore, essentially are solid elements of certain shape.

The polyisoprene rubber and PRTNTEX XE2 nano-structured carbon (producer - Evonik Degussa GmbH, registration No. EINECS 215-609-9) composite is proved [1] to possess considerable positive piezoresistive effect near the percolation threshold (electric resistivity of the composite increases with mechanical pressure), which allows it to be used as a highly sensitive pressure detector/sensor [2, 3]. There are published scientific data on composites of hyper-elastic polymer and nano-structured carbon exhibiting weak negative piezoresistive effect and attempts of their applications in less sensitive pressure indicators [4]-

There is known a piezoresistive sensor [5] in which amorphous carbon is used as the sensitive material. The main disadvantage of the known sensor is its fragility and lack of flexibility.

There is known glued hyper-elastic piezoresistive sensor element [2] consisting of multiple bound components. Its main disadvantages are: the bound structure that fails to ensure sufficient integrity or uniformity, i.e., the electrodes may come off the nano- composite with time, and, secondly, a comparatively high filler concentration at which the percolation transition and, accordingly, the piezoresistive effect occurs.

Disclosure of Invention

The goal of the invention is to eliminate the drawbacks of the prior art by producing a monolithic, completely flexible piezoresistive sensing element. The goal is reached by vulcanizing within a unified structure at pressure of 25-35 atm. and temperature of 120-160 °C a hyper-elastic pressure sensor comprising a hyper- elastic piezoresistive polyisoprene rubber and nanostructured carbon composite layer, hyper-elastic polyisoprene rubber and nanostructured carbon composite electro-conducting layers, the piezoresistive layer being placed between them, and an hyper-elastic electro- insulating coating, where the electro-conductive filler (electro-conducting carbon black of primary particle size smaller than 35 nm, specific surface at least 900 m ² /g, and absorbing ability of dibutylphthalate between 300 and 450 ml/100 g) of the piezoresistive and electro-conducting layers is being dispersed by ultrasonic mixing to reduce the necessary filler concentration thus improving the hyper-elasticity of the composite.

Cross section of the structure of embodiments of the sensor element is shown in Fig. 1 and Fig. 2, where 1 is the elastic piezoresistive layer of polyisoprene rubber and nanostructured carbon black composite; 2 - elastic electro-conducting layers of polyisoprene rubber and nanostructured carbon black composite; 3 - elastic electro- conducting wires, plates or ribbons; 4 - flexible electric wiring or similar means for connecting layers 2 with the resistance reading device; 5 - elastic insulating coating.

The proposed element of pressure sensor comprises:

- hyper-elastic piezoresistive layer 1 of polyisoprene rubber and nanostructured carbon black composite,

- electrodes comprising hyper-elastic electro-conducting layer 2 of polyisoprene rubber and nanostructured carbon black composite adapted to allow to place the layer 1 between them and to allow their electrical connection with the resistance reading device,

- hyper-elastic electro-insulating coating 5,

hereto the sensor element is being monolithic and completely flexible being made by vulcanizing under pressure of 25-35 atm at the temperature of 120 - 160 °C the layer 1, the electrodes 2, and the coating 5 preassembled in the required configuration. The layer 1 contains natural polyisoprene caoutchouc, sulphur, cyclohexylbenzothiazolylsulfenamide, zinc oxide, stearic acid, and electro-conducting carbon black having particle size below 35 nm, specific surface at least 900 m ² /g, and ability of absorbing dibutylphthalate more than 300 and less than 450 ml/lOOg with the following mass % proportions: 85-89 of natural polyisoprene caoutchouc, 2.5-3.5 of sulphur, 0.5-1 of cyclohexylbenzothiazolylsulfenamide, 4-5 zinc oxide, 0.7-1.1 of stearic acid, and 2.5-6 of carbon black. The layer 2 contains natural polyisoprene caoutchouc, sulphur, cyclohexylbenzothiazolylsulfenamide, zinc oxide, stearic acid, and conducting carbon black of particle size below 35 ran, specific surface at least 900 m ² /g, and ability of absorbing dibutylphthalate more than 300 and less than 450 ml/lOOg with the following mass % proportions: 79-86 of natural polyisoprene caoutchouc, 2.5-3.5 of sulphur, 0.5-1 of cyclohexylbenzothiazolylsulfenamide, 3.5-5 zinc oxide, 0.6-1.1 of stearic acid, and 6-12 of carbon black. The coating 5 contains natural polyisoprene caoutchouc, sulphur, cyclohexylbenzothiazolylsulfenarnide, zinc oxide, and stearic acid with the following mass % proportions: 89-93 of natural polyisoprene caoutchouc, 3-3.5 of sulphur, 0.5-1 of cyclohexylbenzothiazolylsulfenamide, 4-5 zinc oxide, and 0.7-1.1 of stearic acid.

The method of manufacturing of the hyper-elastic pressure sensor element comprises the following steps:

(a) obtaining of hyper-elastic piezoresistive polyisoprene rubber and nanostructured carbon black layer 1 and of hyper-elastic polyisoprene rubber and nanostructured carbon black conducting layers 2 comprising:

(i) mixing (preferably on roll mill) of natural caoutchouc with admixtures of vulcanization— sulphur, cyclohexylbenzothiazolylsulfenamide, zinc oxide, and stearic acid,

(ii) grinding of the obtained raw rubber and solving in chloroform in proportion 100 g of rubber in 1000 ml of chloroform,

(iii) dispersion in chloroform and simultaneous ultrasonic homogenization (for example, at specific ultrasound energy 1 W/ml) of the conducting carbon black of primary particle size below 35 ran, specific surface at least 900 m ² /g, and ability of absorbing dibutylphthalate more than 300 and less than 450 ml/lOOg,

(iv) adding the obtained carbon dispersed in chloroform to the solution of raw rubber in chloroform and homogenization by stirring,

(v) discharging the obtained mixture on a substrate allowing chloroform to evaporate,

(vi) homogenization of the obtained film (for example, cold-rolling 5-10 times at possibly smaller distance between rolls);

(b) obtaining the layers of insulating coating 5, which comprises the mentioned steps (i), (ii), (v), and (vi), excluding steps (iii) and (iv);

(c) separate vulcanization of the obtained layer 1 , layers 2, and layers of coating 5 under pressure of 25-35 atm at temperature 115-165 °C, preferably, at 120-160 °C; (d) assembling the layers of coating 5, the conducting layers 2, and the piezoresistive layer 1 into required configuration and providing the ability to connect electrically the layers 2 with the resistance reading device, vulcanization of the assembled preform of the hyper- elastic pressure sensor element under pressure of 25-35 atm at 120-160 °C temperature.

The proportions of mass % of the components of the layers of the sensor element are presented in Table 1.

Table 1

Piezoresistive Conducting Coating

Components of layers layer, layer, layer,

mass % mass % mass %

Natural polyisoprene caoutchouc 85-89 79-86 89-93

Sulphur (S) 2,5-3,5 2,5-3,5 3-3,5

Cyclohexylbenzothiazolylsulfenamide 0,5-1 0,5-1 0,5-1

Zinc oxide (ZnO) 4-5 3,5-5 4-5

Stearic acid 0,7-1,1 0,6-1,1 0,7-1,1

Carbon black* 2,5-6 6-12 0

* electro-conducting carbon black having primary particle size below 35 nm, specific surface of which is at least 900 m ² /g and ability of absorbing dibutylphthalate - more than 300 and less than 450 ml/lOOg.

Thus, mixing of natural caoutchouc with vulcanizing admixtures in step (a)(i) of obtaining the piezoresistive layer 1 of polyisoprene rubber and nanostructured carbon black is being made in the following proportions of mass % of the components: 85-89% of natural polyisoprene caoutchouc, 2.5-3.5% of sulphur, 0.5-1% of cyclohexylbenzothiazolylsulfenamide, 4-5% of zinc oxide, 0.7-1.1% of stearic acid while in step (a) (iii) the amount of carbon black dispersed in chloroform corresponds to 2.5-6 % of the total mass of the components.

The mixing of natural caoutchouc with vulcanizing admixtures in step (a)(i) of obtaining the conducting layers 2 of polyisoprene rubber and nanostructured carbon black is being made in the following proportions of mass % of the components: 79-86% of natural polyisoprene caoutchouc, 2.5-3.5% of sulphur, 0.5-1% of cyclohexylbenzothiazolylsulfenamide, 3.5-5% of zinc oxide, 0.6-1.1% of stearic acid while in step (a)(iii) the amount of carbon black dispersed in chloroform corresponds to 6-12% of the total mass of the components. The mixing of natural caoutchouc with vulcanizing admixtures in step (a)(i) of obtaining the layers of the insulating coating 5 is being made in the following proportions of mass % of the components: 89-93% of natural polyisoprene caoutchouc, 3-3.5% of sulphur, 0.5-1% of cyclohexylbenzothiazolylsulfenamide, 4-5% of zinc oxide, 0.7-1.1% of stearic acid.

In step (d) the layers of coating 5, the conducting layers 2, and the piezoresistive layer 1 are assembled to obtain the required configuration, for example, the coated sandwich type sensor element in which the piezorezistive layer 1 is being put between the conducting layers 2 having considerably lower electrical resistance and the layers 1 and 2 - between the coating layers 5; optionally putting in or putting on the layers 2 elastic wires, plates or ribbons 3 of some elastic electro-conducting material (for example, metal, better brass, providing good electrical contact with the conducting polyisoprene rubber after vulcanization), such that the elastic wires, plates or ribbons 3 would not be in a direct contact with the layer 1 ; and electrically connecting the layers 2 or wires, plates or ribbons 3 to the wiring 4 or similar means to ensure connection of the sensor element with a device taking readings of electric resistance.

Examples of implementation of the invention Example 1

3.5 mass parts (mp) of colloidal sulphur, 0.8 mp of cyclohexylbenzothiazolylsulfenamide, 5 mp of zinc oxide, and 1 mp of stearic acid are being mixed by rolls in 100 mp of Pale Creppe natural polyisoprene caoutchouc. The prepared mixture is rolled 10-15 minutes to obtain raw rubber subsequently being solved during 24 h in 1438 mp of chloroform. After 24 h in a separate vessel 4 mp of electro-conducting carbon black of primary particle size below 35 ran, specific surface of which is at least 900 m /g and ability of absorbing dibutylphthalate - more than 300 and less than 450 ml/lOOg (in this example PRINTEX XE2, producer - Evonik Degussa GmbH, registration Nr. EINECS 215-609-9) are dispersed during 5 minutes by ultrasound in 1483 mp of chloroform. The concentration of the conducting carbon black is being selected in accordance with critical parameters of the percolation threshold of the particular filler. Dispersed carbon black is being mixed with the solution of the raw rubber and stirred for 24 h. The mass obtained after evaporating chloroform is homogenized by cold rolling for 1 min. The result being the hyper-elastic piezoresistive layer 1 of polyisoprene rubber and nanostructured carbon black. Conducting layers 2 of polyisoprene rubber and nanostructured carbon black are being prepared in the same way taking 10 mp of the conducting carbon black. The layers of the insulating coating 5 are being made of the same composition, but without carbon black. The raw rubber for coating 5 is obtained by cold rolling for 10-15 minutes. The obtained raw rubbers prepared for the layers are being hot-pressed at 25 atm and 150 °C for 3-5 minutes to make the elementary layers of the required geometrical configuration. Subsequently the prepared performs are joined in the required order and pressed under the same pressure at the same temperature for 10-15 minutes more to obtain the sensor element.

Example 2

3.5 mass parts (mp) of colloidal sulphur, 0.8 mp of cyclohexylbenzothiazolylsulfenamide, 5 mp of zinc oxide, and 1 mp of stearic acid mix by rolls in 100 mp of Pale Creppe natural polyisoprene caoutchouc. The prepared mixture is rolled 10-15 minutes to obtain raw rubber subsequently being solved during 24 h in 1438 mp of chloroform. After 24 h in a separate vessel 5 mp of conducting carbon black (hereinafter "conducting carbon black" - carbon black of primary particle size below 35 nm, specific surface of which is at least 900 m ² /g and ability of absorbing dibutylphthalate - more than 300 and less than 450 ml/lOOg) are being dispersed during 5 minutes by ultrasound in 1483 mp of chloroform. Dispersed carbon black is being mixed with the solution of the raw rubber and stirred for 24 h. The mass obtained after evaporating chloroform is homogenized by cold rolling for 1 min. the result being the hyper-elastic piezoresistive layer 1 of polyisoprene rubber and nanostructured carbon black. The conducting layers 2 of polyisoprene rubber and nanostructured carbon black are being prepared in the same way taking 11 mp of the conducting carbon black. The layers of the insulating coating 5 are being made of a similar composition, but without carbon black. They are being prepared by cold rolling for 10-15 minutes. The layers performs are being hot-pressed under pressure of 35 atm at temperature of 150 °C to obtain elementary layers of the required size. After that the performs (the layers 1, 2, 5), ribbons 3 and electrical wiring 4 are being joined in the required configuration (see Fig. 1) and pressed under the same pressure at the same temperature for 10-15 minutes more to obtain the sensor element. Example 3

3.5 mp of colloidal sulphur, 0.8 mp of cyclohexylbenzothiazolylsulfenamide, 5 mp of zinc oxide, and 1 mp of stearic acid are mixed by rolling with 100 mp of Pale Creppe natural polyisoprene caoutchouc. The prepared mixture is rolled 10-15 minutes to obtain raw rubber. In a separate vessel 4 mp of conducting carbon black (Printex XE2) is being dispersed by means of ultrasound at specific energy of 1 W/ml. The raw rubber cut in possibly smaller pieces is being added to the dispersed carbon black and solved in chloroform by stirring for 24 h. After the evaporation of the chloroform the product is being homogenized by cold rolling for 1 min to obtain the layer 1. In the same way the layers 2 are being prepared taking 10 mp of carbon black. A similar composition of the components, but without carbon black, is being used to obtain the layers of coating 5. It is being prepared by cold rolling for 10-15 minutes. The elementary layers of required size are obtained from the raw rubbers by hot pressing under 30 atm at 150 °C for 3-5 minutes. To obtain the finished sensor element they are assembled in the required configuration and vulcanized at the same temperature under the same pressure for 10-15 minutes more. The time of vulcanization is being determined in accordance with technological parameters of the rubber. Due to ultrasonic dispersion the obtained sensor element has a lower concentration of the filler and considerably higher conductivity of the layers 2. Example 4

3.5 mp of colloidal sulphur, 0.8 mp of cyclohexylbenzothiazolylsulfenamide, 5 mp of zinc oxide, and 1 mp of stearic acid are mixed by rollers in 100 mp of Pale Creppe natural polyisoprene caoutchouc. The prepared mixture is being rolled 10-15 minutes to obtain raw rubber. In a separate vessel 6 mp of conducting carbon black is being dispersed for 5 minutes by ultrasound in 1483 mp of chloroform. The raw rubber being cut in possibly smaller pieces is being added to the dispersed carbon black and dissolved in chloroform by stirring for 24 h. After the evaporation of the chloroform the product is being homogenized by cold rolling to obtain the layer 1. The layers 2 are being prepared in the same way taking 12 mass parts of carbon black. The coating 5 is being prepared from the same composition, but without carbon black; it is being prepared by cold rolling for 10-15 minutes. The obtained raw rubbers are being hot-pressed at 140 °C under pressure of 30 atm for 6-10 minutes to make the elementary layers of the required size. Assembled in the required configuration they are being pressed at the same temperature under the same pressure for 20-30 minutes more to obtain the sensor element.

Sources of information

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