Humidity sensor and method for making

Details Humidity sensor and method for making Humidity sensor and method for making

2001-03-30

2003-05-27

Dawson, Robert (Department: 1712)

Measuring and testing

Gas analysis

Moisture content or vapor pressure

C073S335020, C073S029010, C073S029020, C427S103000, C427S407100, C427S508000, C428S332000, C428S336000, C428S446000, C428S447000, C428S448000, C428S523000

Reexamination Certificate

active

06568265

ABSTRACT:

TECHNICAL FIELD
This invention relates to a humidity sensor for detecting and determining moisture in the surrounding atmosphere and a method for preparing the same.
BACKGROUND ART
Conventional humidity sensors are designed to detect humidity through changes of electrical properties, typically electric resistance. Known sensors use electrolytes such as lithium chloride, metal oxides, and organic polymers as the humidity sensitive material. However, the humidity sensors using electrolytes such as lithium chloride can measure only a narrow range of humidity and are less resistant to water in that their performance can be altered by dew condensation and wetting. The humidity sensors using metal oxides are resistant to water, but low sensitive. Because of the lack of long-term stability when used alone, they require a heat cleaning circuit which would add to the operating cost. The sensor structure is complex.
Among the humidity sensitive materials, organic polymers, especially polymeric electrolytes having quaternary ammonium salt groups have been widely used in commercial and industrial applications and so appreciated. For example, Japanese Patent Publication (JP-B) No. 61-54176 discloses a humidity sensitive material comprising aggregates of latex particles formed of a copolymer between a hydrophobic monomer and an ionic or non-ionic hydrophilic monomer and having a hydrophilic surface layer. There are exemplified some cationic compounds having primary to quaternary ammonium salts.
JP-B 62-7976 discloses a humidity sensitive material in the form of a polymer which is obtained by polymerizing a compound containing 2-hydroxy-3-methacryloxypropyl-trimethylammonium chloride to a degree of polymerization of 1,000 to 10,000.
JP-B 2-24465 discloses a humidity sensitive thin film of a polymer having the structural formula:
—(N
+
(R
1
)(R
2
)X
−
-A-N
+
(R
3
)(R
4
)X
−
-B)
n
—
wherein R
1
to R
4
are alkyl, X
−
is a halide ion, A and B each are —(CH
2
)
m
— wherein m≧2. The polymer may be blended with another polymer such as polyvinyl pyrrolidone for the purposes of improving substrate adhesion and water resistance. The blend is also effective in forming a humidity sensitive thin film.
Humidity sensors using the polymeric electrolytes exemplified above as the humidity sensitive material, however, are still low in water resistance in that the polymeric electrolytes can be partially leached in a high humidity region, especially in a dew condensing atmosphere. They also suffer from a hysteresis phenomenon that they produce different outputs at the same humidity depending on whether the humidity is increasing or decreasing. In a low humidity region having a relative humidity (RH) of less than 10%, they have so high resistance values that practical humidity measurement is impossible.
U.S. Pat. No. 5,546,802 (corresponding Japanese Patent No. 2,808,255 by TDK Corp.) discloses a humidity sensor comprising a polymer electrolyte having ethylenically unsaturated reactive groups introduced at ends and quaternary ammonium salt groups. This sensor is fully resistant to water, steadily operates in a condensing atmosphere and produces consistent outputs over a wide humidity region.
However, in order that the humidity sensor using a polymer electrolyte as defined above produce consistent outputs in a condensing atmosphere, the thickness of the humidity sensitive film must be restricted below a certain limit. Such a thin film is so sensitive to various gases that the output characteristics largely vary therewith. Therefore, the sensor has the drawback of lacking long-term stability under hot humid conditions. If the thickness of the humidity sensitive film is increased beyond the limit, stripping and failure are likely to occur.
SUMMARY OF THE INVENTION
A primary object of the present invention is to provide a humidity sensor device having a humidity sensitive thin film which is resistant to water, maintains effective, stable performance over a long time even in a dew condensing atmosphere, and produces accurate outputs in a stable manner over a wide humidity region.
In one aspect, the invention provides a humidity sensor comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate and defining a gap therebetween, a silicon-containing undercoat layer lying on at least the gap, and a humidity sensitive thin film lying thereon. The humidity sensitive thin film comprises a crosslinked product of a conductive polymer having ethylenically unsaturated groups and is physically bound to the undercoat layer through an interpenetrating polymer network.
In another aspect, the invention provides a humidity sensor comprising an insulating substrate, a pair of opposed electrodes disposed on the substrate and defining a gap therebetween, a silicon-containing undercoat layer lying on at least the gap, and a humidity sensitive thin film lying thereon. The humidity sensitive thin film comprises a crosslinked product of a polymer of the following formula (I) and is bound to said undercoat layer through covalent bonds having not undergone dehalogenation reaction.
Herein A and B each are a divalent group; Y
1
, Y
2
, Y
3
, Y
4
, Y
5
and Y
6
each are a monovalent group, at least one of Y
1
to Y
6
is a group terminated with an ethylenically unsaturated reactive group; at least two of Y
1
, Y
2
, Y
3
, Y
4
, Y
5
, A and portions thereof adjoining the nitrogen (N) atom or at least two of Y
4
, Y
5
, Y
6
, B and portions thereof adjoining the nitrogen (N) atom, taken together, may form a ring with the nitrogen atom; X
−
is a halide ion; and letter n is a number of 2 to 5,000.
Preferably, the humidity sensitive thin film of the second embodiment is physically bound to said undercoat layer through an interpenetrating polymer network.
Typically, the humidity sensitive thin film has a thickness of 0.1 to 20 &mgr;m.
In a preferred embodiment, the polymer has the following formula (II) or (III).
Herein A and B each are a divalent group; each of R
1
, R
2
, R
3
, and R
4
is an alkyl or alkenyl group; a pair of R
1
and R
2
, R
1
and A or a portion of A, R
2
and A or a portion of A, R
3
and R
4
, R
3
and A or a portion of A, R
4
and A or a portion of A, R
1
and R
3
, R
1
and R
4
, R
2
and R
3
, or R
2
and R
4
, taken together, may form a ring with the nitrogen (N) atom; L is a divalent group; R is hydrogen or an alkyl group; X
−
is a halide ion; n is a number of 2 to 5,000; and R
5
and R
6
each are an alkyl or alkenyl group.
Preferably, the divalent group represented by A is an alkylene, alkenylene or arylene group or a mixture thereof; the divalent group represented by B is an alkylene, alkenylene or arylene group in which at least one of an oxy group (—O—) and a carbonyl group (—CO—) may intervene or a mixture thereof.
The polymer is typically obtained by reacting a diamine compound with a dihalogen compound to form an intermediate polymer and introducing an ethylenically unsaturated reactive group into the intermediate polymer at each end.
In a further aspect, the invention provides a method for preparing a humidity sensor comprising the steps of:
applying a silane compound containing at least a hydrolyzable group and an organic group having an unsaturated bond onto a substrate to form an undercoat layer,
applying a solution containing a conductive polymer having ethylenically unsaturated groups onto the undercoat layer, and
exposing the conductive polymer to radiation for crosslinking the polymer and bonding the polymer with the silane compound to thereby form a humidity sensitive thin film.
Preferably, the bond between the conductive polymer and the silane compound is due to covalent bonds by crosslinking between the ethylenically unsaturated groups and the unsaturated bonds. Most often, the radiation is ultraviolet radiation. Preferably the conductive polymer used herein has the formula (I) defined above.
Herein A and B each are a divalent group; Y
1
, Y
2
, Y
3
, Y
4
, Y
5
and Y
6
each are a monovalent group, at least one of Y
1
to

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