Abstract:
A hardenable molding composition comprising hydratable gypsum, a vinyl monomer, a source of hydrogen sulfite ions and water is disclosed. With this molding composition a hardened product can be obtained having high strength due to the combined effect of the hydraulic reaction of the gypsum and the polymerization reaction of the vinyl monomer.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a novel gypsum-based composition suitable as a molding material. 
     Molding compositions giving the combined effects of the hydrualic reaction of low hydrated gypsum and the curing reaction of a synthetic resin have heretofore been obtained by kneading a mixture of half-hydrated gypsum powder and a synthetic resin such as urea-formaldehyde resin, phenol-formaldehyde resin, acryl resin, polystyrene resin and polyvinyl alcohol resin dissolved in water. However, when a previously prepared resin composition is kneaded, great improvement in strength due to the effect of curing the resin itself cannot be expected, so that there is a limitation to the strength attainable. Also, if it is intended to increase the strength, a very large quantity of resin should be used. 
     It is also known in the art to obtain a hardened product of high strength by the so-called impregnation method, in which gypsum is impregnated with a solution of a monomer, the resultant system being subsequently subjected to a step of thermal polymerization. This method, however, involves a number of steps and requires special equipment. Moreover, it is usually difficult to achieve sufficient impregnation with the monomer liquid, so that the lack of uniformity of strength inevitably results. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a gypsum-based molding composition which contains a small quantity of a monomer component and nevertheless results in hardened products having high strength. 
     It is another object of the present invention to provide a novel molding composition utilizing the hydraulic reaction of gypsum together with the polymerization reaction of a vinyl monomer in the presence of the gypsum and acidic sulfite ions serving as a polymerization initiator. 
     It is a further object of the present invention to provide a method of molding comprising hardening a molding composition of the type in question by bringing gypsum and a vinyl monomer into contact in the presence of water containing hydrogen sulfite ions. 
     The above and further objects, features and advantages of the present invention will appear more fully from the following description. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The various objects of the present invention can be achieved using a mixture of low hydrated gypsum, vinyl monomer, a source of acidic sulfite ions, and water as a molding composition. 
     The term &#34;hydratable gypsum&#34; means gypsum capable of hardening by absorbing water. Such gypsum may, for instance, be dehydrated gypsum, half dehydrated gypsum and partially dehydrated gypsum. Also, gypsum produced as a by-product in the desulfurization of exhaust smoke may be used as well. 
     The term &#34;vinyl monomer&#34; means a compound capable of polymerization into a vinyl polymer in the presence of a radical polymerization initiator, for example, acrylic acid, methacrylic acid, acrylate, methacrylate, acrylic ester, methacrylic ester, acrylamide, N-methylolacrylamide, acrylonitrile, styrene, vinyl chloride, vinyl acetate, divinylbenzene, etc. The quantity of this monomer is not particularly limited, but in case high strength and fire-resistance are aimed at, it is desirable to select an amount within a range of 1 to 15 parts by weight for 100 parts by weight of gypsum. 
     The term &#34;hydrogen sulfite ion source&#34; means a substance capable of provide hydrogen sulfite ions (HSO 3 .sup. - ) in the presence of water. Examples of such substances are sulfur dioxide, sulfurous acid solution, salts of hydrogen sulfite, such as ammonium hydrogen sulfite and sodium hydrogen sulfite and a combination of sulfite and acid. The hydrogen sulfite ion source is incorporated in an amount of 0.01 to 500 parts by weight, preferably 0.1 to 100 parts by weight, calculated in terms of sulfur dioxide, for 100 parts by weight of the monomer. 
     Water in the composition according to the present invention is incorporated in an amount required for hydration of the gypsum, usually in a range of 40 to 80 parts by weight for 100 parts by weight of gypsum. 
     While the composition according to the present invention is essentially a mixture composed of gypsum, vinyl monomer, hydrogen sulfite ion source substance and water, it is possible to incorporate various other additives if desired. For example, it is possible to incorporate customary emulsifier and defoaming agents. Furthermore, other polymers or polymer emulsions may be added to achieve the end result of suppressing volatilization of the monomer. Examples of such additives are such polymers as polyacrylamide and polyacrylic acid and such emulsions are polyvinyl chloride emulsion, polyvinyl acetate emulsion and polybutadiene emulsion. 
     The present composition may be readily prepared by uniformly kneading the mixture of the above components and desired additive components. This means that it is possible to adopt the same method as used in prior art gypsum forming. According to the invention, the hardening proceeds due to the polymerization of the monomer only when both gypsum and hydrogen sulfite ions are present. Accordingly, it is advantageous to add the hydrogen sulfite ion source at the time of molding, but not during storage. 
     In order to obtain the intended effects, the composition must contain the four components, gypsum, vinyl monomer, hydrogen sulfite ion source and water. Contrariwise, sufficient effects cannot be achieved if one of these essential components is lacking. By way of example, a hardened product of high strength cannot be obtained by adding a well-known polymerization initiator capable of causing radical polymerization of the monomer in lieu of the hydrogen sulfite ion source. 
     The composition according to the present invention can be prepared very simply compared to the prior art impregnation method. It can be prepared by merely kneading the requisite components together without the need for any special procedures such as heat treatment to obtain the hardened product or any special equipment. Also, since a hardened product of high strength is obtained with a small polymer content, it is possible to greatly improve the fire resistance thereof. 
     Further, the composition according to the present invention can be used exactly in the same manner as the prior art resin impregnated gpysum. For example, it may be used as construction material, for industrial art work and for daily commodities, sundry goods and furniture. Also, it can find useful application as a base material and as plaster. 
     The following examples are given to illustrate some preferred modes of executing the present invention. 
     EXAMPLE 1 
     2.0 g of methyl methacrylate were suspended in 55 ml of water at room temperature. 2.0 g of sulfur dioxide were then blown into the monomer suspension, and 100 g of half dehydrated gypsum were further added. The resultant mixture was kneaded for 2 minutes into a paste and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed under reduced pressure and the bending strength and compression strength of the resultant product were measured. 
     For comparison, gypsum alone was added to 55 ml of water at room temperature and the paste prepared by kneading the resultant mixture for 2 minutes was quickly charged into the same mold and left there. Subsequently, the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________      Curing Bending   Compression      Time   Strength  Strength      (Days) (Kg/cm.sup.2)                       (Kg/cm.sup.2)______________________________________Composition  1        81        167of           2        80        170Invention    7        87        179Comparison   1        38         81Composition  2        36         79______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 2 
     2.0 g of methyl methacrylate were suspended in 50 ml of water at room temperature, and 6.0 g of sulfur dioxide were blown into and dissolved in the monomer suspension. Then, 80 g of half dehydrated gypsum and 20 g of calcium hydroxide were added to the resultant reaction liquid and the resultant mixture was kneaded for 2 minutes into a paste and quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     For comparison, 80 g of gypsum and 20 g of calcium hydroxide were added to 50 ml of water at room temperature and the paste prepared by kneading the resultant mixture for 2 minutes was quickly charged into the same mold and left there. Subsequently, the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________      Curing Bending   Compression      Time   Strength  Strength      (Days) (Kg/cm.sup.2)                       (Kg/cm.sup.2)______________________________________Composition  1        71        149of           4        74        154Invention    7        77        161Comparison   1        32         80Composition  7        36         77______________________________________ 
    
     The measured values in the table are averages of 5 to 8 mersurements. 
     EXAMPLE 3 
     0.6 g of polyvinylidene chloride and 2 g of polyvinylidene chloride-polyvinyl chloride lates were added to 55 ml of a water solution containing 0.2 g of a non-ionic surface active agent. Then, 2.0 g of methyl methacrylate monomer were added to form an emulsion and 2.0 g of sulfur dioxide were blown into and dissolved in the emulsion, to which were further added 100 g of half dehydrated gypsum. The resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1            79            1604            81            1637            85            165______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 4 
     2 g of the polymer latex used in Example 3, 0.2 g of non-ionic surface active agent and several drops of silicone were added to 55 ml of water. Then, 2.0 g of methyl methacrylate monomer were added to form an emulsion and 1.8 g of sulfur dioxide were blown into and dissolved in the emulsion, to which were further added 100 g of half dehydrated gypsum. The resultant mixture was then kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1            101           1854            116           229______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     Example 6 
     2 g of the polymer latex used in Example 3, 0.2 g of non-ionic surface active agent and several drops of silicone were added to 55 ml of water and then 2.0 g of methyl methacrylate monomer were added to form an emulsion. 0.6 g of sulfur dioxide were added to the emulsion, to which were further added several drops of hydrogen peroxide and 100 g of half dehydrated gypsum. The resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________ 1/12         45            891             71           1503            109           1937            125           246______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 7 
     2.0 g of methyl methacrylate were suspended in 50 ml of water at room temperature and 20 g of sulfur dioxide were blown into and dissolved in the suspension. Further, 80 g of half dehydrated gypsum and 20 g of calcothar were added and the resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     For comparison, 80 g of half dehydrated gypsum and 20 g of calcothar were added to 50 ml of water at room temperature and the resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________      Curing Bending  Compression      Time   Strength Strength      (Days) (Kg/cm.sup.2)                      (Kg/cm.sup.2)______________________________________Composition  1        78        163of           4        81        168Invention    7        83        170Comparison   1        36         79Composition  7        35         78______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 8 
     3.0 g of methyl methacrylate were suspended in 50 ml of water at room temperature and 1.8 g of sulfur dioxide were blown into and dissolved in the suspension. Then, 80 g of half dehydrated gypsum and 20 g of river sand were added at room temperature and the resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     For comparison, 80 g of half dehydrated gypsum and 20 g of river sand were added to 50 ml of water at room temperature and the resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________      Curing Bending   Compression      Time   Strength  Strength      (Days) (Kg/cm.sup.2)                       (Kg/cm.sup.2)______________________________________Composition  1        73        151of           4        75        163Invention    8        78        169Comparison   1        40         84Composition  8        41         85______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 9 
     1.5 g of styrene monomer and methyl methacrylate monomer were suspended in 50 ml of water at room temperature and then 1.3 g of sulfur dioxide were blown into and dissolved in the suspension. To the resultant liquid were further added 100 g of half dehydrated gypsum and the resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed by the pressure reduction method and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1            75            1602            83            1737            85            176______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 10 
     3.0 g of acrylonitrile monomer were suspended in 35 ml of water at room temperature. Then, 15 ml of 1.8 N sulfurous acid solution were added and, further, 100 g of half dehydrated gypsum were added. The resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1            72            1508            79            169______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 11 
     2.5 of styrene monomer were added to 55 ml of a water solution containing 0.1 g of non-ionic surface active at room temperature to form an emulsion. Then, 1.0 g of sulfur dioxide was blown into and dissolved in the emulsion, to which were added 100 g of gypsum. The resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1            75            1588            80            165______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements. 
     EXAMPLE 12 
     10 g of methyl methacrylate and 10 g of methyl acrylate were suspended in 50 ml of water at room temperature. Then, 1.5 g of sulfur dioxide was blown into and dissolved in the suspension, to which was further added 100 g of half dehydrated gypsum. The resultant mixture was kneaded for 2 minutes and then quickly charged into a mold and left there. Subsequently, unreacted monomer was removed and the bending strength and compression strength of the resultant product were measured. 
     
         ______________________________________Curing       Bending       CompressionTime         Strength      Strength(Days)       (Kg/cm.sup.2) (Kg/cm.sup.2)______________________________________1             89           1807            110           220______________________________________ 
    
     The measured values in the table are averages of 5 to 8 measurements.