Polymer-based humidity sensing elements

Details Polymer-based humidity sensing elements Polymer-based humidity sensing elements

1997-02-18

2001-06-19

Williams, Hezron (Department: 2856)

Measuring and testing

Gas analysis

Moisture content or vapor pressure

C073S335050

Reexamination Certificate

active

06247349

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved polymer-based humidity sensing element. More specifically, the present invention relates to a poly(2-acrylamido-2-methylpropanesulfonic acid)-based impedance-type humidity sensor with improved response time and reduced hysteresis, and the novel method of making the same.
BACKGROUND OF THE INVENTION
Humidity sensors based on electric signals have become very popular in recent times. A humidity sensor measures the humidity content in the surrounding environment. More recently, polymeric films have been used as a humidity sensing element. Polymer-based humidity sensing elements can generally be classified into two categories: capacitance-type and impedance-type. The former typically involves more complicated circuit design and manufacturing process, and thus is more expensive, than the latter. A impedance-type electric humidity sensing element changes its electrical impedance as the humidity of the surrounding environment changes, and the measured impedance is converted into humidity readings.
The polymer-based sensing elements can also be further classified into two categories: porous (or more specifically, micro-porous) type, and non-porous type. There are numerous examples of porous polymer film based impedance-type humidity sensors. One of the examples is a microporous polyethylene film, in which 2-acrylamido-2-methylpropane sulfonic acid was graft-polymerized by ultraviolet irradiation. There are also numerous examples of non-porous polymer film based humidity sensors. One of the examples is a three-polymer-coated quartz crystal whose frequency varies as a function of the change in humidity. The first example is discussed in an article entitled: “Humidity Sensor Composed of a Microporous Film of Polyethylene-Grafted-Poly-(2-Acrylamido-2-Methylpropane Sulfonate),” by Y. Sakai, et al.,
Polymer Bulletin
, 18, 501-506 (1987). The second example is discussed in another article entitled: “Relative Humidity Measurements Using A Coated Piezoelectric Quartz Crystal Sensor,”
Sensors and Actuators
, 11 319-328 (1987). The three polymer coatings used in the second example include HEM-AMPS copolymer (HEM=2-hydroxyethyhnethyacrylate, AMPS=2-acryl-amido-2-methyl-propane sulphonic acid); cellulose acetate; and a modified epoxy resin. When moisture is absorbed by the polymer coating, the mass of the crystal is changed, causing its oscillating frequency to be changed.
The above two examples are specifically mentioned here because they both involve the use of PAMPS (poly(2-acryl-amido-2-methyl-propane sulphonic acid)) in the sensing element. In these examples, PAMPS itself is non-porous (as in Example 2). To make a porous polymer film, the PAMPS must be graft-polymerized into a micro-pourous polyethlene film. This unavoidably complicates the manufacturing process and increases the production cost.
Non-porous PAMPS can be used in making a impedance-type electric humidity sensing element by forming a PAMPS film on a pair of electrodes. However, the thickness of the non-porous PAMPS film must be carefully controlled within a very narrow range. If the PAMPS film is too thick, several undesirable effects, such as slow response time and significant hysteresis, will be experienced. On the other hand, if the PAMPS film is too thin, the impedance will be too high. Porous polymer-based sensing elements can provide the advantages of quick response time and small hysteresis. However, these advantages, at the present time, cannot be enjoyed by PAMPS-based sensing elements, without the extra step of grafting the PAMPS polymer into another polymer, such as polyethylene.
SUMMARY OF THE INVENTION
The primary object of the present invention is to develop a impedance-type PAMPS-based (poly(2-acryl-amido-2-methyl-propane sulphonic acid)) electric humidity sensor with quick response time and low hysteresis. More specifically, the primary object of the present invention is to develop a method, according to which porous impedance-type PAMPS-based electric humidity sensors can be prepared which possess quick response time and low hysteresis. In the past, PAMPS can only be provided in the form of a non-porous film, which typically exhibits slow response time and undesirable hysteresis effects when used as a humidity sensing element . Moreover, the conventional non-porous PAMPS films are not very stable in high humidity environment; they typically become water-soluble when the humidity exceeds about 70% RH. With the present invention, the porosity of the novel porous PAMPS films allows the water molecules to travel in and out more speedily. The novel porous PAMPS-based humidity sensing elements of the present invention also exhibit substantially improved stability over the conventional elements; they maintain excellent stability at humidities as high as 95% RH without any observable hysteresis and the response time is well within 1-2 minutes.
In the present invention, the humidity sensing element is made using a two-stage process. In the first stage, poly(2-acryl-amido-2-methyl-propane sulphonic acid) is prepared by polymerizing 2-acryl-amido-2-methyl-propane sulphonic acid in the presence of small amounts of an initiator, such as AIBN, and other desirable additives. In the second stage, a film of poly(2-acryl-amido-2-methyl-propane sulphonic acid) with a thickness of about 5-30 &mgr;m is formed on a pair of electrodes that have been formed on a insulative substrate, such as aluminum oxide, and the polymer film is heated at a temperature between about 170-240° C. The poly(2-acryl-amido-2-methyl-propane sulphonic acid)-based humidity sensing element so prepared has a porous structure when observed under a microscope; it also shows a white/light yellow or brownish surface color, and exhibits excellent stability, quick response time, and little or no hysteresis. If the heat treatment temperature is too low during the second stage (i.e., less than 170° C.), it was difficult to form a porous structure. On the other hand, if the temperature is too high (i.e., greater than 240° C.), the polymer molecules are likely to be degraded, resulting in substantially high impedance.
Poly(2-acryl-amido-2-methyl-propane sulphonic acid) is a polymeric electrolyte which can be converted into a protonic electrical conductor (in the form of a Bronsted acid) when subject to an alternating current. Preferably, aqueous solutions containing conductive ions such as lithium hydroxide, sodium hydroxide, potassium hydroxide, or ammonium hydroxide, can be added into the aqueous 2-acryl-amido-2-methyl-propane sulphonic acid solution before or during the polymerization reaction, to form a modified PAMPS, represented as PAMPS•X, wherein X is a cation, such as H
+
, Li
+
, Na
+
, K
+
, ammonium, primary to tertiary arnine ions, or tetra-alkyl ammonium ions, etc. The amount of the cations should be less than the amount of AMPS. The primary to tertiary amine ions can be methylphenylamine, trimethylamine, triethylamine, etc. The primary to tertiary amines, including ammonia, can be converted into ammonium ions by the addition of a hydrogen atom, for example, NH
3
will be converted into NH
4
+
, and HN(C
2
H
4
)
3
will be converted into HN(C
2
H
4
)
3
+
, etc. A higher temperature will be required to achieve the required porosity of the PAMPS film at higher ratios of amines/AMPS. However, different amines may also require different heat treatment temperatures.


REFERENCES:
patent: 4529642 (1985-07-01), Miyoshi et al.
patent: 5356936 (1994-10-01), Howell et al.
patent: 86447 (1983-05-01), None
patent: 2210253 (1990-08-01), None
patent: 4309855 (1992-11-01), None

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