GB2165252A - Method for producing artificial marble - Google Patents

Abstract

A method for producing artificial marble is disclosed, in which methyl methacrylate partial polymer syrup, beta -calcium silicate powder and organic peroxide are uniformly mixed together, and the resulting mixture is defoamed and polymerized under heating conditions to provide a sheet-like moulding.

Description

SPECIFICATION Method for producing artificial marble This invention relates to a method for producing artificial marble which is used as building material, top plates of kitchens, drains of bathrooms and the like in general buildings and, more particularly, to a method for producing artificial marble useful for the above purposes which consists of a synthetic resin product essentially comprising calcium silicate as the filler and acrylic resin syrup as the binder. The artificial marble produced by the method of the present invention presents surface and external appearance features similar to those of natural marble.

Generally, natural marble has iight transmission characteristic due to a proper degree of transparency and complex appearance characteristic provided by colored or transparent crystalline calcite or the like embedded in the marble. Thus, it has been very difficult to reproduce such light transmission and complex appearance characteristics inherent in natural marble for artificial marbles comprising synthetic resin products containing fillers. Therefore, in order to produce artificial marble having complex appearance similar to that of natural marble, it is essential and very important that synthetic resin itself used in the artificial marble as the binder has transparency and is whitish to give such complex appearance to the surface of the artificial marble.If the synthetic resin used in the production of the artificial marble does not possess such essential properties, the produced artificial marble does not look like marble.

Further, it is necessary that artificial marble also has a certain degree of hardness because, if the hardness of the artificial marble is insufficient, abrasion occurs easily on the surface of the artificial marble which scarcely occurs on natural marble and thus, such artificial marble will lack aesthetic appearance.

Synthetic resins used as binders hitherto for the production of artificial marble include unsaturated polyester resin, epoxy resin, polystyrene resin and acrylic resin. Among the above-listed synthetic resins, acrylic resin is superior to the other resins with respect to transparency, weather resistance, hardness and heat resisting characteristics.

Prior art artificial marbles used for the above-mentioned applications comprise in most cases acrylic resin syrup as binder and Six2, CaCO3 or Al(OH), as filler. However, artificial marbles containing these fillers have been found to have the following individually inherent drawbacks in the course of tests thereof: Although SiO2 has high hardness, such resin is difficult to be processed by cutting.

CaCO3 and Al(OH)3 cannot be added in a great amount to acrylic resin syrup. Especially, Al(OH)3 transforms into boehmite at 180"C to produce water to thereby impair aesthetic appearance on the surface of the obtained artificial marble. And when the amount of the resin is greater than that of the filler in an artificial marble, the ratio of the resin to the filler becomes higher in the artificial marble to such a degree that a substantial contraction occurs in the composition in the course of polymerization thereof and thus, the product prepared from such polymer would have the irregular surface.

One example of the prior art which is pertinent to artificial marble produced by the method of the present invention is illustrated in Japanese Patent Appln. Publication No. 43422/1980 which is directed to synthetic marble. The prior art synthetic marble comprises a mixture of acrylic resin, Al(OH)3 particles of average particle size of 3-15 m and Al(OH)3 particles of average particle size of 30-100 /tm.

Therefore, the present invention is to eliminate the drawbacks inherent in the prior art artificial marbles referred to hereinabove. For this purpose, the present invention is to provide a novel and improved method for producing artificial marble which essentially comprises the steps of mixing acrylic resin syrup as the binder and calcium silicate in fine powder form as the filler together, and polymerizing the resulting mixture in the presence of organic peroxide. According to the present invention, since calcium silicate is employed as the filler, the obtained polymer not only can be easily moulded, but has low water and oil absorption coefficient.Furthermore, according to the present invention, since the filler can be employed in a greater amount than the binder and, thus, the percentage of the resin is lower than that of the filler in the resulting polymer, the contraction rate of the resin is reduced and a moulding formed of such polymer presents an external appearance quite similar to that of natural marble.

Still furthermore, since calcium silicate has a needle-like structure of crystals (whisker), although the filler is employed in a greater amount than the binder in the polymer, physical characteristics of the calcium silicate after setting thereof will not be impaired when subjected to impact, and the filler adheres to the resin satisfactorily and exhibits excellent heat resistance within the temperature range to which the artificial marble is exposed. Furthermore, calcium silicate has low solubility in water and hot water. Thus, products formed of such artificial marble have low expansion by hydration coefficient.

In order to make commercial products from the above-mentioned artificial marble, the marble always has to be processed. When such artificial marble contains SiO2 of Mohs hardness 7-8, it is difficult to shear or cut the artificial marble containing SiO2 by the use of an ordinary cutting tool and the marble has to be cut by a diamond tipped cutting tool. However, since the artificial marble uses calcium silicate having Mohs hardness 3.5-4.5 as the filler according to the present invention, the artificial marble can be easily cut by the use of any conventional carbide tip saw to thereby produce satisfactorily processed products.

In the present invention, calcium silicate is employed in the amount of at least 20 weight parts and preferably about 75 weight parts.

Calcium silicate has a low moisture separation rate below 0.1%. And it has been believed that hydrate fillers such as Al, Mg and CaSO4 and hydrates act to accelerate the polymerization of acrylic resin syrup, but test results have shown that the greater the amount of such hydrate fillers or hydrates is, the greater adhesion power to metallic molds for moulding they have.

Thus, it has been found that for mouldings comprising polymers formed of acrylic resin syrup and filler, anhydrides and inert fillers are suitable as the filler. For this reason, calcium silicate used in the present invention is most suitable as the filler.

Acrylic resin syrups suitably employed as the binder in the present invention are partial polymers syrups comprising principally methyl methacrylate and having a viscosity in the range from 300-2000 cp and preferably in the range from 500-1500 cp. Such partial polymer syrups are copolymers of methyl methacrylate having at least one styrene unsaturated bond and at least one monomer copolymerizable with such methyl methacrylate in the amount of 1-30% by weight and preferably 1-25% by weight based on the amount of the latter.Monomers useful in the present invention are, for example, at least one member selected from the group consisting of styrene methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, 2-hydroxyethyl methacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylol propane trimethacrylate, aryl methacrylate, diaryl phthalate, acryl diglycol carbonate, acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and lauryl acrylate.

The methyl methacrylate and one or more monomers as described above are prepolymerized in the presence of one of catalysts such as azobisisobutyronitrile, benzoyl peroxide, lauryl peroxide, t-butyl peroxyneodecanoate and one of chain carriers such as n-dodecylmelcaptan and t-dodecylmelcaptan, until the prepolymer will have a viscosity in the range from 500-1500 cp or a previously prepared methyl methacrylate polymer is dissolved in a methyl methacrylate monomer to provide a monomer syrup copolymerizable with methyl methacrylate as mentioned hereinabove.

20-60 weight parts and preferably, 25-35 weight parts of the thus obtained acrylic resin syrup comprising methyl methacrylate as its principal component, 20-80 weight parts and, preferably, 65-75 weight parts of fl-type calcium silicate over 99.9% of which passes 325 mesh and which has the average particle size of 8 im and needle or elongated pillar-shaped structure of crystals and a small amount or 0.2-2 weight parts of organic peroxide, for example, one or more selected from the group consisting of benzoyJ peroxide, lauryl peroxide, t-butyl peroxyneodecanoate, t-tubyl peroxy-2-ethylhexanoate are uniformly mixed together.The obtained mixture is defoamed, poured into a square mold, polymerized at a temperature in the range of 50-110"C for 2-5 hours and left as it is to allow the polymer to set to thereby obtain a moulding.

B-type calcium silicate is a low temperature natural crystalline structure in which its principal composition CaO-SiO2 exhibits a superior chemical characteristic (high acid resisting) by its contact denaturation action to (type synthetic calcium in which its components CaO and SiO2 individually exhibit a chemical characteristic (low acid resisting).

As mentioned hereinabove, according to the present invention, artificial marble is prepared by uniformly admixing calcium silicate powder with acrylic resin syrup, defoaming the resultant mixture and pouring the defoamed mixture into a square mold to polymerize them to thereby provide a moulding having flat surfaces. The moulding is then cut into desired end products which present an excellent appearance without concaves and convexes. This eliminates the drawbacks that products having SiO2 added thereto cannot be easily cut, that CaCO3 exhibits low acid resistance, that Al(OH)3 exhibits low heat resisting surface characteristic and that the filler cannot be added to resin syrup in a great amount whereby products which can be easily moulded can be provided. Even when the artificial marble is used as top plates of kitchens and drains of bath rooms which are frequently exposed to acids, the marble can maintain external apperance similar to that of natural marble.

The present invention will be now described by way of specific examples of the same which illustrate, but do not limit the invention.

Example 1 30 kg of methyl methacrylate partial polymer syrup (10 poise viscosity), 70 kg of calcium silicate powder and 0.3 kg of benzoyl peroxide are uniformly mixed together. The liquid mixture is defoamed, poured into a square mold and polymerized therein at 100"C for 3 hours to obtain a sheet moulding having the smooth surface and thickness of 16 mm.

The moulding has the hardness of 72 (Bacol 934-1) sufficient for artificial marble.

Example 2 30 kg of methyl methacrylate partial polymer syrup (10 poise viscosity), 35 kg of calcium silicate powder, 35 kg of SiO2 and 0.3 kg of benzoyl peroxide are uniformly mixed together. The liquid mixture is defoamed, poured into a square mold and polymerized therein at 100"C for 3 hours to obtain a sheet moulding having the smooth surface and thickness of 16 mm.

The surface of the moulding presents transparency and is soft to touch and the moulding has a hardness of 74 (Bacol hardness 934-1).

The properties of moulding obtained by the method of the present invention were examined.

-(1) Oil Absorption Amount The oil absorption amounts of these fillers were measured and the measurement results are given in Table 1.

Table 1

Type of Filler Oil Absorption Amount: (ml/100 g) CaSiO2 25 SiO2 20 - 25 CaCO3 42 - 45 Al (OH)3 38 - 45 Mg(OH)2 50 (2) Viscosity When the particle size of the filler is reduced in order to enhance adhesion between the resin or binder and filler, although the amount of CaSiO, and SiO2 which absorb oil in relatively small amounts can be increased in the composition of artificial marble, the amount of CaCO3, Al(OH)3, and Mg(OH)2 which absorb oil in relatively large amounts cannot be increased in the composition.

After 30 kg of methyl methacrylate partial polymer syrup (10 poise viscosity), 60 kg of calcium silicate powder and 10 kg of SiO2 were uniformly mixed together and the mixture was defoamed, the viscosity of the mixture was measured to find out that the mixture had the viscosity of 600 poise quite suitable for moulding. Mixtures were prepared under substantially the same conditions as those described just above, except for the use of CaCO3, Al(OH)3 and Mg(OH)2, respectively, as the filler instead of calcium silicate, and the viscosity of the mixtures was measured as in the range of 2000-3000 which show that the CaCO3, Al(OH)3 or Mg(OH)2 filled mixture is inferior to the CaSiO2-filled mixture with respect to mouldability.

(3) Abrasion Mouldings prepared using calcium silicate, SiO2 and Al(OH)3, respectively, as the filler under the conditions as described in connection with Example 1, were subjected to abrasion tests and the test results are given in Table 2.

Table 2

Type of Filler [ Taper Abrasion Test Procedure ) SiO2 0.12 * JIS-K CaSiO2 0.46 6902-29 Al(OH)3 0.46 XA test piece of 100 mmX100 mm is attached to a tapered abraser having S-33 sand-paper, the abraser is rotated 500 times under load of 500 g and the loss in volume of the test piece is determined as the abrasion amount of the test piece.

From the above-given test results it is apparent that the calcium silicate filler is more effective than the two other fillers with respect to cutting efficiency of resin mouldings in which the fillers are contained.

(4) Heat Resisting, Boiling Water Resisting and Chemical Resisting Properties, and Impact Strength Mouldings containing calcium silicate, SiO2 and Al(OH)3, repsectively, as the filler were subjected to tests for heat resistance, boiling water resistance, chemical resistance and impact strength.

In heat resisting tests, each of the mouldings was placed on a metal block heated to 200 C, left as it was for 5 minutes and checked for any change in the surface of the moulding, assuming that a heated frying pan, pan or like was carelessly placed on the artificial marble top plate of a kitchen formed of such moulding. In boiling water resisting tests, each of the mouldings was placed in boiling water maintained at 100"C for 5 hours and checked for any change in the surface of the artificial marble moulding as a water leakage acceleration experiment, because the marble moulding is employed in most cases in places where a great amount of water is used.

In chemical resisting tests, each of the mouldings was left as it was in a room maintained at 200"C for 24 hours with the surface of the moulding in contact with chemicals and dirt and thereafter checked for any change in the surface of the moulding. In impact tests, the mouldings were compared with each other for their Charpy strengths varying depending upon the type of filler used therein with resin syrup, filler and organic peroxide employed in corresponding amounts under corresponding conditions. The mixing ratio is 39 weight parts of resin syrup, 75 weight parts of filler and 1 weight part of benzoyl peroxide, and the components are polymerized at 100"C for 3 hours.

The test results are shown in Tables 3-6.

Table 3 (Heat Resisting)

Type of Filler Surface Condition CaSiO3 No change SiO2 Luster disappearing Al(OH)3 Becoming white and swelling Table 4 (Boiling Water Resisting)

Type of Filler Surface Condition CaSiO3 No-change SiO2 Luster disappearing Al(OH)3 Becoming white Table 5 (Chemical Resisting)

Type of Chemical and dirt CaSiO3 SiO2 Al(OH)3 109 citric acid 0 0 0 10% hydrochloric 0 0 0 acid 1% sodium hydroxide 0 0 0 solution 10% aznmonia water O 0 0 2% mercurochrome 0 A A solution Table 6 (Impact Strength)

Tye of Filler Impact Strength (Kg-cm/cm2) CaSiO3 4.3 Al (OH)3 3.4 CaCO3 2.8 SiO2 2.8 (By use of impact tester) The U-F impact tester is a pendulum type impact tester designated as JIS-K-7114-4. The weighing of the tester is 5g f cm having the center striking speed of 2.9 m/sec., hammer rising angle 150 , hammer blade angle of 30+ and circular blade edge radius of 2 mm.

From the test results, it is apparent that the moulding containing calcium silicate as the filler is superior to the mouldings containing SiO2 and Al(OH)3, respectively, as the filler with respect to heat resistance, boiling water resistance, chemical resistance and impact strength, and the calcium silicate containing moulding exhibits excellent physical properties for use as artificial marble products.

While only two examples of the invention have been described in detail, it will be understood that they are for illustration purpose only and not to be taken as a definition of the invention, reference being had for the purpose to the appended claims.

Claims (8)

1. A method for producing artificial marble, comprising the steps of uniformly mixing 20-60 weight parts of acrylic resin syrup and 20-80 weight parts of fl-type calcium silicate in fine powder form, and polymerizing the resulting mixture in the presence of at least one organic peroxide.

2. The method for producing artificial marble, as claimed in Claim 1, in which said acrylic resin is a monomer selected from the group consisting of styrene methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, 2-hydroxyethyl methacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, trimethylol propane trimethacrylate, aryl methacrylate, diaryl phthalate, acryl diglycol carbonate, acrylic acid, methyl acrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and lauryl acrylate.

3. The method for producing artificial marble, as claimed in Claim 1 or 2, in which said organic peroxide is selected from the group consisting of azobisisobutyronitrile, benzoyl peroxide, lauryl peroxide, t-butyl peroxyneodecanoate and t-butyl peroxy-2-ethylhexanoate.

4. The method for producing artificial marble, as claimed in any preceding Claim, further including the step of deforming the mixture after said mixing step and before said polymerizing step.

5. The method for producing artificial marble, as claimed in any preceding Claim, in which said polymerizing step is carried out at 100"C for 3 hours.

6. A method for producing artificial marble, substantially as hereinbefore described with reference to the Examples of the invention.

7. Artificial marble, whenever produced by a method as claimed in any preceding Claim.

8. The features herein described, or their equivalents, in any patentably novel selection.