HYGROSCOPIC COMPOSITE MATERIAL

Abstract: A hygroscopic composite material comprising a humidity absorbing body (1) having hygroscopic fillers in its basic material and a porous body (2) having numberless micropores. This hygroscopic composite material is enabled to retain hygroscopicity for a long period of time by enhancing humidity absorbing function by generating free water in the porous body (2) and by discharging the free water.

Description: This invention relates to a highly hygroscopic composite material suitable for use, for example, as an interior panel material , a wall material or as a hygroscopic component in various structures. Known materials for absorbing humidity in rooms and storehouses, for instance, include timbers, zonotolite system calcium silicate plates and paper impregnated with hygroscopic fillers. However, all of these materials have disadvantages in that they do not have a fully satisfactory regulating effect and require time and energy to fulfil their dehumidification function. Upon taking up humidity in the air as adsorbed water, they eventually reach saturation and cannot absorb humidity any more. The present invention has been made in view of the above-mentioned disadvantages of the conventional materials and seeks to provide a hygroscopic composite material which can retain its hygroscopicity for a longer period of time because the free water that is generated in the material through moisture absorption is then discharged. According to this invention there is provided a hygroscopic composite material comprising a) a porous body composed of a cement hardened body, a plaster hardened body, a calcium silicate hardened body, an inorganic sintered body or like porous substance, and having numerous micropores whose average diameter is less than lOµm, and b) a humidity absorbing body containing hygroscopic fillers. In one embodiment, a humidity absorbing body is laminated to only one side of the porous body. In an alternative embodiment, a humidity absorbing body is laminated to each side of the porous body. In a still further embodiment, the porous body is contained within one or more voids provided in the humidity absorbing body. The porous body may be provided with one or more water discharging holes through which free water generated in the material is discharged. In addition, any exposed surface of the porous body may be provided with a humidity intercepting layer. The presence of this latter feature in the construction of the hygroscopic composite material of the invention ensures that the humidity absorption is oriented such that the absorbed humidity passes from the hygroscopic fillers in the humidity absorbing body through to the porous body. When a hygroscopic composite material composed as described above is left to absorb humidity through the humidity absorbing body under conditions of constant temperature the following effects are seen: (1) The percentage water content on the side of the porous body becomes higher than the percentage of saturated humidity content (the state at which an equilibrium is reached by absorbing humidity) of the porous body itself due to the generation of free water. (2) There are cases where while the percentage water content on the side of the porous body increases, that on the side of the humidity absorbing body does not increase and, in such cases, humidity absorbed moves into the porous body. (3) If humidity absorption continues for a long period of time, water trickles down from the porous body. The above phenomena indicate that humidity in the air is absorbed into the humidity absorbing body and moves to the porous body, in which it turns into water. The mechanism by which free water is generated is not fully understood. It is assumed that under the conditions of a low percentage of equilibrium water content and a high temperature, while humidity which cannot be absorbed at a substantial part (the part other than the void) and is allied to saturated vapour exists in the micropores, the humidity absorbing body is high in the percentage of equilibrium water content and has humidity absorbing power in reserve, unless hygroscopic fillers absorb humidity perfectly, and therefore very small differences in vapour pressure and in temperature are generated at the interface between the two which are incorporated in one, with the result that humidity condenses on the side of the porous body which is almost saturated and water is generated. It has been confirmed by experiments that in such a state as the above, liquefaction is promoted by cohesive power in the case where the diameter of the micropores of the porous body is less than lOµm, especially less than 3µm. If the diameter of the micropores is more than lOµm (for example, filter paper whose average diameter of micropores is about 2Oµm) , the cohesive power is small and humidity becomes hard to turn into waterdrops, with the result that the humidity absorbing effect is reduced. It must be realised that during actual use of the material according to the present invention there will be changes in temperature. When the temperature falls, condensation of humidity in the porous body increases further and water retention in the porous body develops. When the hygroscopic composite material absorbs humidity continuously under high temperature conditions, free water is generated in a large quantity in the porous body. The porous body cannot retain all of the free water generated and thus water trickles outside the porous body. Under normal atmospheric conditions, however, the void in the humidity absorbing body is rarely filled with condensed water and therefore hygroscopic fillers within the body rarely flow out and hygroscopicity can be retained for long periods of time. Furthermore, the generation of free water contributes to effective fire prevention because such water checks the rise of temperature which occurs at the outbreak of a fire. If a humidity intercepting layer is provided at an exposed side of the porous body, humidity absorption into the hygroscopic composite material will then occur only from the side on which the humidity absorbing body is located, due to the existence of the humidity intercepting layer, and thus the difference in vapour pressure at the interface where the humidity absorbing body and the porous body connect with each other becomes easy to take place and condensed water generates readily within the micropores in the porous body. If water discharging holes are provided in the porous body, when the hygroscopic composite material absorbs humidity under high temperature conditions free water is generated in a large quantity and water trickles through the water discharging holes of the porous body. Collection of water is promoted if water is sucked positively from the water discharging holes by using an appropriate suction means. As mentioned above, according to the humidity absorbing panel of the present invention, more water than can be absorbed by the porous body as a simple substance can be taken up as free water and therefore hygroscopicity is improved. In addition, once the retention of free water by the porous body reaches its full capacity, water is discharged outside the porous body. Accordingly, the amount of time and energy required for dehydration can be minimized and continuous humidity absorption from the humidity absorbing body side is made possible. In other words, hygroscopicity can be retained for a longer period of time, the desired humidity regulating function can be displayed and the fire preventative role can be enhanced. Under atmospheric conditions where humidity is generated in a large quantity, water in the porous body can be removed quickly by sucking the porous body using an appropriate suction force. Thus, the material according to the present invention can be used not only as a wall material in structures but also as a dehumidifier. In the case where water discharging holes are made in the porous body, free water in the porous body is discharged through the water discharging holes and so the time and energy required for dehydration can be reduced to the minimum. When a humidity intercepting layer is present on an exposed side of the porous body, humidity absorption into the hygroscopic composite material will be effected only from the side of the humidity absorbing body to the side of the porous body due to the existence of the humidity intercepting layer. Thus, a difference in vapour pressure at the interface between the humidity absorbing body and the porous body more readily occurs and condensed water is generated easily in the micropores of the porous body. The present invention is illustrated in the accompanying drawings. Figure 1 is a perspective view of the most basic embodiment; Figures 2, 3, 4 and 5 are perspective views, each showing a different embodiment. Figure 6 is a cross-sectional view of the embodiment shown in Figure 5. Various embodiments of the present invention are described below with reference to these drawings. Figure 1 shows a humidity absorbing body 1 of flat plate shape. A porous body 2, also of flat plate shape, is fastened to one side of the humidity absorbing body. The humidity absorbing body 1 contains hygroscopic fillers which comprise one or more of the following: deliquescent substances, such as calcium chloride, magnesium chloride and lithium chloride; water-soluble high molecules, such as diethylene glycol , triethylene glycol , sodium polyacrylate and PVA; inorganic humidity absorbing materials, such as sodium silicate, bentonite, sepiolite, zeolite, activated alumina, molecular sieves and zonotolite; and water-insoluble high molecular humidity absorbing materials, such as grafted starch, isobutylene maleic anhydride and polyacrylate. These fillers are built up in to a basic material having many voids communicating with the open air. To be more precise, the humidity absorbing material is obtained by kneading together a hydraulic substance, such as cement or plaster, and the hygroscopic fillers into a body and hardening it; by impregnating, for example, calcium silicate plate, paper or cloth with hygroscopic fillers; by mixing woody fibre or rock wool fibre, for example, with hygroscopic fillers; or by kneading together olefinic resins, or similar substances, and hygroscopic fillers and then foaming such mixture. A humidity absorbing body which is obtained by mixing together an inorganic humidity absorbing material, such as bentonite, and calcium chloride, diethylene glycol or the like, and then kneading such mixture together with cement or plaster, exudes less of the hygroscopic fillers and has a moderate moisture permeability. This type of humidity absorbing body is most preferable. The porous body 2 will contain numerous micropores of irregular sizes, regardless of its actual constituent material, but it is required that the size of the micropores is such that humidity which has entered the micropores from the humidity absorbing body and has been retained in an almost saturated state, condenses by a slight difference in energy. As the result of measurements taken using the technique of injecting pressurized mercury, or by other such methods, it has been found that the average diameter of the pores should be less than 1Oµm, and preferably less than 3µm. For use as the porous body 2 having such micropores there are available various inorganic sintered bodies, such as a plaster hardened body, a cement hardened body, a calcium silicate hardened body, mullite, bricks and unglazed tiles. Alternatively, a porous resin body or a glass foamed body could be used so long as the average diameter of the pores is less than 1Oµm. However, a porous body made of woody fibre, inorganic fibre plate, paper, felt sheet or like substances having voids with an average diameter of more than 2Oµm and good movement of water by capillary phenomenon, would not be suitable because generation of condensed water is slight. As the porous body 2, a cement hydrate hardened body is desirable because it shows only a slight dimensional change and a slight lowering of strength in relation to the water and therefore loses little of its ability to function, even if condensed water is generated. The cement hydrate hardened body is made chiefly of Portland cement, silica cement, alumina cement, calcium silicate or the like and is available as, for example, concrete plate, mortar plate, asbestos cement slate, calcium silicate plate, ALC plate, pulp cement plate or GRC plate. A cement hydrate hardened body made of the above material has micropores with an average diameter of less than 1Oµm at the cement hydrate connecting part. Through experimentation, it has been confirmed that vapour taken in to such micro-voids is easy to condense. To produce the hygroscopic composite material of this invention in which a porous body 2 and a humidity absorbing body 1 are united, the following methods are available. Firstly, the method of laminating a moulded humidity absorbing body 1 and a cement hydrate hardened body in to a body. Secondly, the method of moulding either one of the humidity absorbing body and the cement hydrate hardened body, while making the other body in a paste state, and then coating the paste on to the moulded body in the desired thickness to produce a lamination. It is also possible to laminate the cement hydrate hardened body on to both sides of the humidity absorbing body 1. A bonding layer which does not affect moisture permeability or a water repellent layer, a semi-permeable membrane or the like which prevents movement of hygroscopic fillers and water may be put at the interface of the humidity absorbing body 1 and the porous body 2. Also, paper, cloth or a moisture permeable material , such as a moisture permeable membrane, plywood, plaster board or rock wool sound-absorbing plate, for example, may be laminated to the exposed side of the humidity absorbing body 1. Figures 2 to 6 show various alternative embodiments of the hygroscopic composite material of this invention. Figure 2 shows a hygroscopic composite material in which a humidity absorbing body 1 is provided with a plurality of holes 3 of appropriate diameter which pass vertically therethrough. A porous body 2 is contained within said holes. To manufacture a hygroscopic composite material in this form either the porous body is filled in to said holes in the state of a paste and then left to harden, or the porous body may be first formed in a suitable shape and then placed within said holes. Figure 3 shows a hygroscopic composite material in which the porous body 2 has a plurality of water discharging holes 4 passing vertically therethrough for discharging water from the porous body to the outside. Figure 4 shows a hygroscopic composite material in which the porous body 2 is sandwiched between two humidity absorbing bodies 1, 1 of flat plate shape. The porous body has a plurality of water discharging holes 4, 4 .... 4, 4 passing vertically therethrough. A hygroscopic composite material of such construction can be used as a dehumidifier by decompressing the water discharging holes 4 and dehydrating. Figures 5 and 6 show a hygroscopic composite material in which a humidity intercepting layer 5 is provided at an exposed side of the porous body 2 so as to avoid humidity absorption from that side. The humidity intercepting layer 5 is formed by laminating a resin sheet, such as polypropylene, polyethylene, vinyl chloride, cellophane or the like, a sheet-like substance made by applying metallic evaporation to such sheets as above, a moisture-proof sheet, or a metallic sheet, to the porous body 2, or may be formed by coating a resin liquor, such as silicon, urethane, polyester or the like, on to the surface of the porous body. The membrane or sheet-like body of the humidity intercepting layer typically has a percentage of moisture permeation of less than 1 x 10-⁴g/m.h.mmHg. Specific embodiments of the present invention and some comparative examples are shown below. Embodiment 1 A hygroscopic composite material was prepared using triethylene glycol as the hygroscopic filler, by kneading together 20 parts by weight of triethylene glycol and 100 parts by weight each of plaster and water, forming such kneaded mixture into a flat plate of 15mm thickness and hardening it into a humidity absorbing body, and then laminating a commercially available calcium silicate plate of 9mm thickness (the average diameter of the micropores = 0.6 2 / 3 m) to one side of the humidity absorbing body. Embodiment 2 A humidity absorbing body was formed by kneading together Portland cement : bentonite : CaCl 2 : diethylene glycol : water at a weight ratio of 350 : 700 : 105 : 105 : 500. A hygroscopic composite material was then produced by laminating a calcium silicate plate to the humidity absorbing body. Embodiment 3 A humidity absorbing body was formed by mixing together polyvinyl chloride : sodium polyacrylate : plasticizer and others at a ratio of 36 : 36 : 28 and then heating and foaming such kneaded mixture by the paste calender method. A hygroscopic composite material was produced by laminating a calcium silicate plate to the humidity absorbing body. Embodiment 4 A hygroscopic composite material was obtained by laminating a porous body (the average diameter of the micropores at a substantial part = 2.3µm), which was made by kneading together plaster and water at a ratio of 100 : 50, forming a plurality of water discharging holes which pass vertically therethrough and then allowing to harden, to one side of a humidity absorbing body of the same composition as that described in Embodiment 1. Embodiment 5 A hygroscopic composite material was produced by laminating an extrusion moulded cement calcium silicate plate (the average diameter of the micropores = 0.6µm) to a humidity absorbing body of the type described in Embodiment 1. Embodiment 6 A hygroscopic composite material was produced in which a polypropylene sheet of 20µm thickness is bonded to the surface of a porous body of the type described in Embodiment 4. Embodiment 7 A hygroscopic composite material was formed by laminating a calcium silicate plate (the average diameter of the micropores = 0.6µm) to the humidity absorbing body of Embodiment 1 and then coating urethane resin on the calcium silicate plate at the rate of 1Og/square shaku. Comparative Example 1 A zonotolite calcium silicate plate of 25mm thickness was used as an example of a hygroscopic material currently on the market. Comparative Example 2 A hygroscopic composite material was obtained by laminating filter paper (average diameter of the micropores = 20µm), in place of the porous body used in Embodiment 1, to the humidity absorbing body of Embodiment 1. In each of the above Embodiments 1 to 7 and Comparative Examples 1 and 2, the percentage water content values at the time when the hygroscopic composite materials, and each porous body and humidity absorbing body composing a hygroscopic material, were caused to absorb humidity until they reach equilibrium in a desiccator of 95% RH, were as follows:- Humidity absorbing body : 70% Calcium silicate plate : 15% Plaster hardened body : 5% Extrusion moulded cement calcium silicate plate : 13% Zonotolite calcium silicate plate : 25% Filter paper : 20% Further, each humidity absorbing body and porous body of the above Embodiments 1 to 7 and Comparative Examples 1 and 2 was regulated for humidity by 35% RH and then the respective hygroscopic composite materials were composed and left to absorb humidity for seven days in a desiccator of 95% RH. Each porous body and humidity absorbing body of these hygroscopic composite materials was measured for the percentage water content, with the following results:- Humidity absorbing body : 20 - 25% Plaster hardened body : 15% Calcium silicate plate : 20% Extrusion moulded cement calcium silicate plate : 20% Filter paper : 5% The water adsorbed quantity (increase in weight) of the humidity absorbing body in each of Embodiments 1 to 7 was within the range of 180 - 20OOg/m², as compared with 450g/m² for the hygroscopic material of Comparative Example 1. From the above results it is understood that the porous bodies in Embodiments 1 to 7 show a weight increase which is higher than the percentage of saturated moisture content, which means that free water is generated in the porous body. On the other hand, in the case of Comparative Example 2 it has been confirmed that movement of water from the humidity absorbing body to the filter paper is slight due to the low percentage water content of filter paper, even if filter paper exhibits good capillary action. When the hygroscopic composite materials of Embodiments 1 to 7 and Comparative Example 2 were caused to absorb humidity for 14 days continuously, the hygroscopic composite materials of Embodiments 1 to 7 showed trickling down of water from the porous body. However, no trickling down of water was observed in the material of the Comparative Example.

Claims: A hygroscopic composite material comprising a) a porous body (2) composed of a cement hardened body, a plaster hardened body, a calcium silicate hardened body, an inorganic sintered body or like Porous substance, and having numerous micropores whose average diameter is less than 1Oµm, and b) a humidity absorbing body (1) containing hygroscopic fillers. A hygroscopic composite material as claimed in claim 1, wherein a humidity absorbing body (1) is laminated to one side of the porous body (2). A hygroscopic composite material as claimed in claim 2, wherein there is provided a humidity intercepting layer (5) on the opposite side of the porous body (2) to the humidity absorbing body (1). A hygroscopic composite material as claimed in claim 3, wherein the humidity intercepting layer (5) comprises a sheet-like body or a membrane whose percentage of moisture permeation is less than 1 x 10⁴g/m.h.mmHg. A hygroscopic composite material as claimed in claim 1 , wherein a humidity absorbing body (1) is laminated to either side of the porous body (2). A hygroscopic composite material as claimed in any one of claims 1 to 5, wherein the porous body (2) and the humidity absorbing body (1) are each of flat plate shape. A hygroscopic composite material as claimed in any one of the preceding claims, wherein there are water discharging holes (4) provided within the porous body (2). A hygroscopic composite material as claimed in claim 1, wherein the porous body (2) is contained within one or more voids provided in the humidity absorbing body (1).

Inventors: ASANO, KAZUHIKO, MATSUOKA, AKIRA, YOSHIMI, SATOSHI
Applicants: DAIKEN TRADE & INDUSTRY CO LTD
Application year: 1987
Main classification: E04C2
IPC codes: E04C2