MOISTURE SENSOR AND PROCESS FOR PRODUCING SAME

Abstract: A moisture sensor having the moisture sensitive film com­ prising an inner layer in the form of a plasmapolymerized high polymer layer of a hydrophobic organic compound and a sur­ face layer in the form of a plasma polymerized high polymer layer incorporated hydrophilic group of an ammonium salt type, and a process for producing same.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a moisture sensor and a process for producing the same. More particularly, the invention relates to a moisture sensor which is useful for measuring the water content of various industrial process gases or recycle gases, for measuring the water content of high-pressure gases contained in cylinders and for usual humidity measurements, and also to a process for producing the sensor. Description of the Prior Art Moisture sensors are already known which comprise a solid substrate of insulator, piezoelectric crystal, semiconductor or the like, and a moisture sensitive film formed on the substrate and made of a polyelectrolyte serving as the moisture sensitve substance. Of these, the moisture sensor wherein quartz crystal oscillator or like piezoelectric element is used as the substrate determines the water content based on the variation of oscillation frequency of the piezoelectric element serving as an indicator and resulting from a change in the overall weight of the moisture sensitive film due to its hygroscopic action. Accordingly the sensor is capable of measuring lower water contents of the order of several ppm than those of other type wherein the variation of resistance or capacitance of the moisture sensitive film itself is used as an indicator. Such piezoelectric moisture sensor is produced usually by forming a pair of electrodes on the piezoelectric element and coating the electrodes with a polyelectrolyte. However, because the moisture sensitive film is formed by coating, the film is insufficient in its adhesion to the electrode surface and has poor durability, and the sensor is low in responsiveness and stability when measuring very low water contents of the order of ppm. The moisture sensor further has the problem that it is difficult to control the thickness of the moisture sensitive film itself. The moisture sensor of the resistance type or capacitance type wherein a moisture sensitive film of polyelectrolyte is formed between a pair of electrodes on an insulator or semiconductor similarly has problems in respect of the durability of the film, responsiveness, stability, etc. We have already found that the above problems can be overcome by a piezoelectric moisture sensor which comprises a thin high polymer film formed by plasma polymerization, having hydrophilic groups introduced into its surface layer and serving as the moisture sensitive film (US-A-4562725). The present invention, further developed from the above finding, provides a moisture sensor which comprises a moisture sensitive film consisting essentially of a plasma-polymerized high polymer layer and which has increased durability and improved responsiveness. SUMMARY OF THE INVENTION More specifically, the present invention provides a moisture sensor wherein a moisture sensitive film is formed between, or on at least one of, a pair of electrodes on a solid substrate, the moisture sensor being characterized in that the moisture sensitive film consists essentially of an inner layer in the form of a plasma-polymerized high polymer layer of a hydrophobic organic compound, and a surface layer provided over the inner layer and in the form of an amino-containing high polymer layer of an organic amino compound formed by plasma polymerization, the amino-containing high polymer surface layer having incorporated therein hydrophilic groups of the ammonium salt type converted from amino groups. The present invention further provides a process suited to the fabrication of the above moisture sensor. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are a front view and a side elevation showing a moisture sensor of the present invention for illustrative purposes; Fig. 3 is a view illustrating the construction of the moisture sensor; Fig. 4 is a diagram showing a plamsa polymerization apparatus for use in producing the moisture sensor of the present invention; Figs. 5 and 6 are graphs each showing the moisture responsiveness of the sensor of the invention in comparison with that of a conventional sensor; Fig. 7 is a graph showing the durability of the present sensor in comparison with that of the conventional sensor; Fig. 8 is a graph showing the distribution of nitrogen atoms in the surface layer in the direction of thickness thereof, as determined for the moisture sensitive films of sensors of the invention for illustrative purposes; and Fig. 9 is a graph showing the responsiveness of moisture sensors of the invention wherein different organic amino compounds are used. DESCRIPTION OF THE PREFERRED EMBODIMENT The most distinct feature of the present invention is that the moisture sensitive film comprises a substantially hydrophobic inner layer and a surface layer having hydrophilic groups of the ammonium salt type and that each of these layers is in the form of a high polymer layer which is formed by plasma polymerization. The inner layer of the moisture sensitive film of the present invention is formed by forming a pair of electrodes on a solid substrate for moisture sensors, and thereafter subjecting the electrode bearing substrate to plasma polymerisation conditions in the vapor of a hydrophobic organic compound. Examples of useful hydrophobic organic compounds are various organic compounds having no hydrophilic group, which include vinyl aromatic compounds such as styrene and divinylbenzene, aromatic hydrocarbons such as benzene and toluene, unsaturated hydrocarbons such as ethylene and propylene, etc. These compounds are used singly, or at least two of them are usable in combination. The inner layer can be of multi-layer structure. Of these compounds, it is desirable to use styrene which is amenable to plasma polymerization. Although the conditions for the plasma polymerization are not limited specifically, it is generally desirable to resort to glow discharge at 0.1 to 100 KHz in an atmosphere of the hydrophobic organic compound at 0.1 to 2 torr. It is suitable that the inner layer thus formed be usually 0.01 to 10 µm in thickness. The layer is easily controllable to such a thickness by adjusting the plasma polymerization time. The inner layer of plasma-polymerized high polymer per se is crosslinked to a high degree, has strong adhesion to the underlying electrode or solid substrate and is a substantially hydrophobic film of polymer of the hydrophobic organic compound. Accordingly, the layer serves as the base layer of the moisture sensitive film of the present invention. The surface layer of the present moisture sensitive film is prepared by subjecting the solid substrate coated with the inner layer to plasma polymerization conditions in the vapor of an organic amino compound to thereby form an amino-containing high polymer film over the inner layer first, and subsequently treating the high polymer film chemically to thereby convert amino groups in the film to hydrophilic groups of the ammonium salt type. Examples of useful organic amino compounds are amines having at least one straight-chain substituent, such as diallylamine, diamylamine, diethylallylamine and N,N,N',N'-tetramethylhexanediamine, and cyclic or aromatic amines such as aniline, pyridine and phenethylamine. Also useful are triethylamine, ethylenediamine, diethylenetriamine, triethylenetetramine, etc. At least two of these amines may be used in combination. Of these compounds, it is preferable to use tertiary amines having a straight-chain substituent, such as tetramethylhexanediamine and diethylallyamine. For the plasma polymerization, it is preferred to resort to glow discharge which is generally conducted at 0.1 to 100 KHz in an atmosphere of the organic amino compound at 0.1 to 2 torr. It is suitable that the surface layer have a thickness of 0.1 to 10 µm. The thickness can be controlled by adjusting the plasma polymerization time to adjust the thickness of the amino-containing high polymer film. The high polymer film formed over the inner layer by the plasma polymerization of the organic amino compound is an amino-containing high polymer film having a high crosslinking degree and is united with the hydrophobic high polymer inner layer by being chemically bonded thereto, for example, by radical formation resulting from the glow discharge. All the amino groups of the organic amino monomer do not always remain in the chain of the high polymer film formed by the plasma polymerization. Under the foregoing conditions for the plasma polymerization of organic amino compound, a random polymerization-crosslinking reaction between carbon chains (substituents) appears predominant owing to the formation of may radicals and ions, whereas other reactions such as decomposition of amino groups will also take place. In this connection, we have obtained the interesting finding that the high polymer film resulting from the plasma polymerization of the organic amino compound has a higher amino content toward its front side and a lower amino content toward its rear side. Such a gradient of amino content is distinct when organic amino compounds with a straight-chain substituent are used. It is believed that the gradient is related to the responsiveness of the moisture sensor of the invention, especially to the effect to rapidly adsorb and desorb water. The amino groups present in the amino-containing high polymer film are chemically treated preferably by holding the substrate formed with the film in the vapor of an alkyl halide. Suitable alkyl halides are monohalogenated lower alkyls, which include, for example, methyl chloride, ethyl chloride, methyl iodide, methyl bromide, etc. When the high polymer film is brought into contact with the vapor of such alkyl halide, amino groups present therein are alkylated into hydrophilic groups of the ammonium salt type, such as the quaternary ammonium salt, tertiary amine salt, secondary amine salt type or the like. While the conversion reaction can be accelerated by effecting the contact at an elevated temperature, the reaction can be carried out at room temperature. The alkyl halide vapor is supplied preferably at a flow rate of about 1 liter/minute at atmospheric pressure for contact with the sensor having the amino-containing high polymer film formed thereon. It is suitable that the contact (retention) time be usually about 30 to about 300 minutes, although dependent on the thickness of the surface layer. The solid substrate on which the moisture sensitive film comprising an inner layer and an outer layer is to be formed is selected according to the detection system contemplated. For example, when the variation of electrical resistance or capacitance serves as an indicator for the contemplated moisture sensor, an insulator or semiconductor is used as the solid substrate, and the moisture sensitive film is formed between, and further over, a pair of electrodes, preferably comb-shaped electrodes, provided on one surface of the substrate. Most preferably, the present invention is embodied as a piezoelectric moisture sensor which comprises a piezoelectric element serving as the solid substrate, a pair of electrodes formed on the opposite sides of the element individually, and a moisture sensitive film formed over at least one of the electrodes. Partly because of the measuring system used, the sensor is capable of measuring water contents of as low as several ppm with good stability over a prolonged period of time. Examples of useful piezoelectric elements are quartz crystal oscillator, achroite, Rochelle salt, barium titanate crystal, zinc oxide crystal, etc. Of these, a quartz crystal oscillator in the form of a flat plate is especially preferable. A thin film of gold, silver or like noble metal is used as the electrode on the solid substrate. The electrode is formed by vacuum evaporation. As described above, the moisture sensor of the present invention comprises a moisture sensitive film which is composed of a substantially hydrophobic high polymer film (inner layer) formed by plasma polymerization, and a plasma-polymerized high polymer film (surface layer) formed over the inner layer integrally therewith and having incorporated therein hydrophilic groups of the ammonium salt type. Accordingly, the surface layer only adsorbs and desorbs water, substantially without permitting water to penetrate or migrate into the inner layer. Further because the concentration of hydrophilic groups incorporated in the surface layer is highest at the surface of the layer and decreases toward the inner layer, water can be adsorbed and desorbed smoothly. This minimizes adverse effects such as delayed response due to the present of remaining water in the moisture sensitive film. Accordingly, the moisture sensor of the present invention is outstanding in responsiveness and stability and has improved durability. Example 1 A gold electrode, 8 mm in diameter and about 1 µm in thickness, was formed by vacuum evaporation at the center of a disk-like quartz crystal oscillator 2, 14 mm in diameter and 0.2 mm in thickness, on each surface thereof. The oscillator was about 9.03 MHz in oscillation frequency. The oscillator 2 was set in a plasma polymerization apparatus as shown in Fig. 4. With reference to this diagram, indicated at 7 is an aluminum discharge electrode, at 8 a similar counter-electrode, at 9 a compound supply nozzle member, at 10 an R.F. power supply, at 11 a pipe connected to a vacuum pump, at 12A a container for a hydrophobic organic compound, i.e. styrene monomer, at 12B a container for an organic amino compound, i.e. N,N,N',N'-tetramethylhexanediamine, at 13 a seal, and at 14 a sealed container. The oscillator 2 was masked at its peripheral portion with an aluminum holder plate 7' so that a plasma-polymerized thin film, 12 mm in diameter, was formed over each circular gold electrode provided on the oscillator in intimate contact therewith. Subsequently, the interior of the container 14 was evacuated by the vacuum pump to a vacuum of up to 0.01 torr. With the vapor of styrene monomer introduced into the container to a pressure of 1 torr, plasma polymerization was conducted under the following conditions for 20 seconds, whereby a thin polystyrene film, about 3000 angstroms in thickness, was formed over each gold electrode concentrically with the electrode in intimate contact therewith. Discharge conditions Temperature: 25 o C Discharge frequency: 1 KHz Discharge power: 3.5 W Next, the interior of the container was evacuated to 0.01 torr, the vapor of N,N,N',N'-tetramethylhexanediamine was supplied to a pressure of 1 torr, and plasma polymerization was conducted for 60 seconds under the same conditions as above, whereby an amino-containing high polymer film, about 5000 angstroms in thickness, was formed over the polystyrene film in intimate contact therewith. The quartz crystal oscillator thus coated by plasma polymerization was allowed to stand for 12 hours in a container containing methyl chloride vapor (closed container of atmospheric pressure with 100% methyl chloride gas introduced therein), whereby amino groups present in the amino-containing high polymer film (surface layer) were alkylated into hydrophilic groups of the ammonium salt type. Figs. 1 to 3 show the moisture sensor thus obtained according to the present invention. With reference to these drawings, the sensor 1 comprises gold electrodes 4, 4' formed on the opposite surfaces of the quartz crystal oscillator 2 in intimate contact therewith, and moisture sensitive films 3, 3' formed over the electrodes 4, 4', respectively, in intimate contact therewith. End portions of the gold electrodes 4, 4' are connected via terminals 5, 5' to lead wires 6, 6', which are connected to an oscillator circuit when the sensor is to be used. Each of the moisture sensitive films 3, 3' comprises a plasma-polymerized polystyrene film 3a (3'a) in intimate contact with the gold electrode and with the oscillator, and a plasma-polymerized high polymer film 3b (3'b) containing hydrophilic groups of the ammonium salt type and formed over the polystrene film. When to be used for a gas, the sensor is positioned with the moisture sensitive films 3, 3' exposed to the gas, and the water content of the gas can be measured based on the variation of frequency of the quartz crystal oscillator. The moisture sensor thus prepared was fixedly provided within a container having a specimen gas inlet tube and an outlet for measuring water contents. The results are given below in comparison with those achieved by a conventional moisture sensor. (1) Responsiveness in N₂ having a water concentration of 500 to 10000 ppm The responsiveness of the moisture sensor was evaluated by introducing an N₂ gas containing 10000 ppm of water into the container for 10 minutes, measuring the output of the sensor due to the adsorption of water, thereafter introducing an N₂ gas with a water content of 500 ppm and measuring the resulting change in the output. The result is shown in Fig. 5, in which indicated at A is the change in the output of the moisture sensor of the invention, and at B that of the conventional moisture sensor, which was prepared in the same manner as above except that the moisture sensitive layer consisted singly of a plasma-polymerized polystyrene film having sulfonate groups incorporated in the surface layer of the film and serving as hydrophilic groups. It is seen that the moisture sensor A of the present invention reached a steady state in about 0.5 hour in response to the abrupt moisture content change from 10000 ppm to 500 ppm within the region of relatively high concentrations and is therefore exceedingly superior in responsiveness to the conventional sensor B which required about 4 hours to reach a steady state. (2) Responsiveness to a water concentration of 0 to 5 ppm The responsiveness was similarly evaluated by measuring the variation of output resulting from changes in moisture content (0 → 5 → 0 ppm) in the region of very low concentrations. Fig. 6 shows the result. The graph reveals that the sensor A of the invention has extremely high responsiveness than the conventional sensor B to the moisture content changes in the low concentration region, especially to the reduction of moisture content. (3) Durability of the moisture sensitive film The durability of the moisture sensitive film was evaluated by using the sensor for continuous measurement over a prolonged period of time for N₂ gas containing 1000 ppm of water and tracing the resulting variation of the output. Fig. 7 shows the result. The conventional sensor B initially exhibited a marked reduction in sensitivity and developed a change in the quality of the film in 90 days (indicated in the broken line), whereas the sensor of the invention was small in initial sensitivity reduction and was found free of any change in the film even upon lapse of 120 days. Example 2 A moisture sensitive film of double layer structure was prepared in the same manner as in Example 1 except that methyl iodide was used as the alkyl halide. Another moisture sensitive film was prepared in the same manner as in Example 1 with the exception of using aniline in place of N,N,N',N'-tetramethylhexanediamine and using methyl iodide as the alkyl halide. Measurements were made of the distribution of ammonium group contents in the surface layers of these films by ESCA and Ar etching. The conditions for ESCA analysis were as follows. X-ray source: 9 KV, 30 mA, Mg target Scanning speeds: 1 eV/sec and 0.1 eV/sec Ar etching: 2 KV, 20 mA Fig. 8 shows the relationship between the film thickness and N 1s peak intensity as established by the results of measuremnts. The graph shows that the surface layer contains ammonium groups with a greater gradient from its surface inward when prepared from an amine monomer having a straight-chain substituent, such as tetramethylhexanediamine than when prepared from an aromatic amine, such as aniline, and that the ammonium content is highest at the outermost surface portion of the layer. The use of amines of the former type is therefore desirable to assure rapid adsorption and desorption of water. Example 3 Piezoelectric moisture sensors were prepared in the same manner as in Example 1 using various organic amino compounds (triethylamine , ethylenediamine, diethylenetriamine, triethylenetetramine and diamylamine). The sensors were tested for responsiveness to water contents. Fig. 9 shows the results. As described above, the moisture sensor of the present invention is superior in responsiveness and durability to the moisture sensor in which a plasma-polymerized high polymer film in the form of a single layer serves as the base of the moisture sensitive film. As compared with conventional sensors wherein the film is formed by coating, the present sensor is superior in responsiveness and durability, assures greater ease of film thickness control and more facilitated quality control and, moreover, can be produced by a more convenient and economical procedure without losses because the entire production process is practiced under a dry condition with use of commercial gases each as it is.

Claims: A moisture sensor wherein a moisture sensitive film is formed between, or on at least one of, a pair of electrodes (4,4') on a solid substrate (2), and consists essentially of an inner layer (3a,3'a) in the form of a plasma-polymerized high polymer layer of a hydrophobic organic compound, and a surface layer (3b,3'b) provided over the inner layer and in the form of an amino-containing high polymer layer of an organic amino compound formed by plasma polymerization, the amino-containing high polymer surface layer having incorporated therein hydrophilic groups of the ammonium salt type converted from amino groups. A moisture sensor as defined in claim 1 wherein the amount of hydrophilic groups of the ammonium salt type incorporated in the surface layer (3b,3'b) decreases from the surface of the surface layer toward the inner layer with a gradient in the direction of thickness of the surface layer. A moisture sensor as defined in claim 1 wherein the solid substrate (2) is a piezoelectric element. A process for producing a moisture sensor characterized by forming a pair of electrodes (4,4') on a solid substrate (2) by vacuum evaporation, subjecting the substrate to plasma polymerization in the vapor of a hydrophobic organic compound to form a high polymer film between the electrodes (4,4') or on at least one of the electrodes, subsequently subjecting the substrate to plasma polymerization in the vapor of an organic amino compound to form an amino-containing high polymer film over the high polymer film, and thereafter holding the resulting product in the vapor of an alkyl halide to convert amino groups in the amino-containing high polymer film to hydrophilic groups of the ammonium salt type. A process as defined in claim 4 wherein the organic amino compound is an amine selected from the group consisting of diallylamine, diethylallylamine, N,N,N',N'-tetramethylhexanediamine, aniline, pyridine, phenethylamine, triethylamine, ethylenediamine, diethylenetriamine and triethylenetetramine. A process as defined in claim 4 wherein the alkyl halide is a compound selected from the group consisting of methyl chloride, ethyl chloride, methyl iodide and methyl bromide. A process as defined in claim 4 wherein the hydrophobic organic compound is a vinyl aromatic compound. A process as defined in claim 4 wherein the hydrophobic organic compound and the organic amino compound are subjected to plasma polymerization under the glow discharge conditions of a high vacuum and 0.1 to 100 KHz. A process as defined in claim 4 wherein the solid substrate is a piezoelectric element.

Inventors: KOBAYASHI, JUNYA, OKA, SYOTARO, TAHARA, SHU
Applicants: SHIMADZU CORPORATION
Application year: 1985
Main classification: G01N27
IPC codes: G01N5, G01N27