Coating processes – With post-treatment of coating or coating material – Chemical agent applied to treat coating
Reexamination Certificate
1998-06-17
2001-02-13
Derrington, James (Department: 1731)
Coating processes
With post-treatment of coating or coating material
Chemical agent applied to treat coating
C427S230000, C427S430100, C427S297000, C065S017200, C065S395000, C428S149000, C428S116000, C428S117000, C428S331000, C428S446000, C264S136000, C264S257000, C264S287000
Reexamination Certificate
active
06187381
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for preparing silica gel and a process for producing a dehumidifying element. More particularly, it relates to a method for supporting silica gel on an appropriate base and a process for producing a dehumidifying element comprising a honeycomb structure made of inorganic fiber paper having silica gel supported thereon.
BACKGROUND OF THE INVENTION
A dehumidifying element composed of a honeycomb structure with silica gel supported thereon has conventionally been produced by a process comprising immersing a honeycomb structure fabricated of inorganic fiber paper, such as glass paper and ceramic paper, in an aqueous alkali silicate solution, immersing the impregnated honeycomb structure in an aqueous solution of a water-soluble calcium salt or magnesium salt, followed by acid treatment (hereinafter called a first conventional process) or a process comprising repeating the steps of immersing the honeycomb structure in an aqueous alkali silicate solution and drying several times, immersing the impregnated honeycomb structure in an aqueous solution of a water-soluble calcium salt or magnesium salt, followed by acid treatment (hereinafter called a second conventional process).
Silica gel production by acid treatment of an alkali silicate is complicated and involves repetition of steps, and it has been desired to obtain silica gel through a single series of steps. According to the first conventional process, because the pick-up of the alkali silicate per single impregnation operation is limited, the above-described steps must be repeated until a sufficient amount of silica gel is built up for obtaining practical dehumidifying performance. The amount of silica gel produced could be increased by increasing the concentration of the alkali silicate solution. However, since an alkali silicate is highly viscous, the inorganic fiber paper would be clogged in a high concentration alkali silicate solution. Therefore, there is an upper limit of the alkali silicate concentration, and the steps should be repeated of necessity.
Where the second conventional process if followed, when an alkali silicate-impregnated honeycomb structure is again immersed in an alkali silicate aqueous solution, the previously supported alkali silicate partly dissolves in the immersing solution, resulting in poor production efficiency.
In the present situation, it is difficult to obtain a sufficient amount of silica gel for manifestation of practical dehumidifying performance through one series of steps.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for preparing silica gel through a single series of treatments in a markedly higher yield than in conventional processes.
Another object of the present invention is to provide a process for producing a dehumidifying element having a sufficient amount of silica gel for practical use through a single series of treatments.
The above objects are accomplished by:
(1) A process for preparing silica gel comprising the steps of:
immersing a base in silica gel (hereinafter referred to as a first immersion step),
drying the impregnated base,
immersing the dried impregnated base in a mixed solution of an alkali silicate and an alkali hydroxide (hereinafter referred to as a second immersion step),
heating the impregnated base taken out of the mixed solution, and
treating the heated base with an acid.
(2) A process for producing a dehumidifying element comprising the steps of:
processing inorganic fiber paper into a honeycomb structure,
immersing the honeycomb structure in silica sol (first immersion step),
drying the impregnated honeycomb structure,
immersing the dried impregnated honeycomb structure in a mixed solution of an alkali silicate and an alkali hydroxide (second immersion step),
heating the impregnated honeycomb structure taken out of the mixed solution, and
treating the heated honeycomb structure with an acid.
(3) A process for producing a dehumidifying element comprising the steps of:
immersing inorganic fiber paper in silica sol (first immersion step),
drying the impregnated inorganic fiber paper,
immersing the dried impregnated inorganic fiber paper in a mixed solution of an alkali silicate and an alkali hydroxide (second immersion step),
processing the impregnated inorganic fiber paper taken out of the mixed solution into a honeycomb structure,
heating the honeycomb structure, and
treating the heated honeycomb structure with an acid.
According to the present invention, the first immersion step and the drying step provide a base or inorganic fiber paper retaining a large amount of solidified silica sol, which is converted to silica gel through the subsequent series of treatments while being supported on the base or inorganic fiber paper. That is, a large amount of silica gel can be supported on a base or inorganic fiber paper through a single series of treatments.
DETAILED DESCRIPTION OF THE INVENTION
Inorganic fiber paper which has been or will be formed into a honeycomb structure is immersed in silica sol to hold silica sol in the individual fibers.
The inorganic fiber paper can be made of glass fiber or ceramic fiber. In order to prevent deterioration of silica gel, glass fiber with less alkali content such as E glass fiber and aluminosilicate fiber are preferred.
Silica sol is a solution in which silica particles are dispersed in a solvent. The silica particles preferably have a particle size of 50 nm or smaller for easy dissolution in a mixed alkali solution of an alkali silicate and an alkali hydroxide hereinafter described. Such silica sol is available on the market under trade names of Snowtex XS, S, N or O, manufactured by Nissan Chemical Industries, Ltd.
Silica sol easily penetrates into the interstices among the inorganic fiber paper. Therefore, on immersing the inorganic fiber paper in silica sol, a large quantity of silica is retained in the interstices in the form of gel.
The silica sol-impregnated inorganic fiber paper is dried to obtain solidified silica sol. Aiming at removing any residual water content from the inorganic fiber paper and fixing silica to the paper, the drying step is carried out by applying hot air to the inorganic fiber paper or by the use of an electric heater. A suitable drying temperature is 100 to 200° C.
The impregnated and dried inorganic fiber paper is then immersed in a mixed solution of an alkali hydroxide and an alkali silicate to pick up the alkali hydroxide-alkali silicate mixture. The reason of using an alkali hydroxide besides alkali silicate in this second immersion step is that an alkali component should be present in this stage in a high proportion so as to secure satisfactory dehumidifying performance. If the molar ratio of silicon oxide to an alkali oxide component exceeds 10, the resulting silica gel will have a small specific surface area, failing to exhibit satisfactory performance.
The mixed alkali solution is a mixture of an alkali silicate aqueous solution, an alkali hydroxide aqueous solution, and water. The alkali silicate includes sodium silicate and potassium silicate. The alkali silicate aqueous solution preferably has a concentration of 20 to 35 wt % (% by weight). The alkali hydroxide includes sodium hydroxide and potassium hydroxide. The alkali hydroxide aqueous solution preferably has a concentration of 20 to 50 wt %. While the concentrations of the alkali silicate and the alkali hydroxide in the mixed solution are not particularly limited, it should be noted that, if the concentrations of the two solutions are higher than the above respective ranges, they hardly mix together. If the concentrations of the two solutes in the mixed alkali solution are too low, the amount of silica gel obtained through a single series of treatments is reduced.
In using sodium hydroxide as an alkali hydroxide, either solid sodium hydroxide or a 48 wt % aqueous solution can be used in the preparation of the mixed alkali solution.
The immersion time is decided appropriately according to a finally desired amount of silica gel to be
Furuya Yumiko
Tanabe Jun
Derrington James
Nichias Corporation
Pillsbury Madison & Sutro
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