JPH0380405B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] Molded products based on polymethyl methacrylate (hereinafter abbreviated as PMMA) containing fillers have a deep appearance, a luxurious feel,
Due to its excellent weather resistance, the use of artificial marble as an interior material for countertops, wallboards, furniture, etc. has been expanding in recent years, and various manufacturing methods have been proposed. The present invention contains fillers.
The present invention relates to a method for manufacturing PMMA-based molded products, and more specifically to a technology for casting a mixture of a filler and a polymerizable monomer containing methyl methacrylate (hereinafter abbreviated as MMA) as a main component.

[Conventional technology]

Manufacturing methods for PMMA-based molded products containing fillers can be roughly divided into two. One is thermoplastic
This is a method of kneading a filler into PMMA resin and obtaining a product by extrusion molding or injection molding.The other method is typified by cast molding.
A slurry of MMA syrup and filler is filled into a mold and polymerized and hardened. MMA
has a volume shrinkage rate of about 21% due to polymerization,
During curing, the slurry partially separates from the mold surface, resulting in defects called sink marks.
In cast molding of simple shapes such as flat plates, 2
A mold with a soft gasket sandwiched between two glass plates is used, and as the glass plates shrink due to polymerization, the glass plates compress the soft gasket with the adhesive force of the cast product and are drawn together.
The polymerization shrinkage of MMA is absorbed to obtain cast molded products with no sink marks. However, in the case of complex shapes, it is not possible to solve the problem by simply moving the mold wall. For cast molding of complex shapes, we use a special mold with a fitting mechanism similar to the relationship between a piston and a cylinder on the parting line of the mold to compensate for curing shrinkage (Modern Plastics International). 1979 (November), p. 20), and in order to concentrate sink marks only on the unused side and take advantage of the used side, only the parts that make up the used side of the mold are heated and harden faster than the unused side. technology (PLASTICS DESIGN &
PROCESSING. Volume 15, 1975 (November), 19-21
(described in JP-A-51-5383 and JP-A-59-1999), a technique of applying a release film to the entire surface of the mold constituting the unused surface and curing it under pressure.
232813) and the like are known.

[Problem that the invention seeks to solve]

However, it requires a special mold, moving the mold wall in only one direction cannot prevent sinking on the surface parallel to the direction of movement, and requires a special device to heat only one side. It requires expensive pressure equipment and a strong casting mold that can withstand pressure, and it also has disadvantages such as the complexity and difficulty of applying film to the entire surface of a complex mold, especially if there are recesses. have.

[Means for solving problems]

The inventors have developed a simple and inexpensive casting mold that avoids the drawbacks mentioned above, namely, the need for expensive or specialized equipment, and allows for the difficult application of a film along complex mold surfaces, especially along recesses. As a result of intensive research into a cast molding method that does not require complicated operations, the present invention was developed. In other words, when polymerizing a mixture of a filler and a polymerizable monomer mainly composed of MMA in a casting mold, the use surface of the molded product is The mold releasability of the part facing the concave part of the mold surface is
It has been found that the above-mentioned problems can be solved by using a casting mold that has better releasability than the recesses.

The polymerizable monomer mainly composed of MMA in the present invention is defined as MMA alone or less than 50% by weight of MMA substituted with another polymerizable monomer. Examples of other polymerizable monomers include acrylic acid,
Methacrylic acid, ester of monohydric alcohol with 1 to 18 carbon atoms and acrylic acid, ester of monohydric alcohol with 2 to 18 carbon atoms and methacrylic acid,
Monofunctional monomers such as olefinic nitriles such as acrylonitrile and methacrylonitrile, aromatic olefins such as styrene and α-methylstyrene, vinyl esters such as vinyl acetate and vinyl benzoate, and vinylamides such as acrylamide and methacrylamide. and ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-
Hexanediol di(meth)acrylate, trimethylolethane tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethanetetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate Examples include, but are not limited to, polyfunctional monomers such as acrylate and divinylbenzene. Two or more of these polymerizable monomers other than MMA may be used in combination. Note that the polymerizable monomer containing MMA as a main component may be partially substituted with a polymer soluble therein. In other words, it may be a syrup in which a polymer is dissolved in a polymerizable monomer whose main component is MMA. Such a syrup can be made by dissolving a polymer in a polymerizable monomer containing MMA as a main component or by partially polymerizing a polymerizable monomer containing MMA as a main component.

The filler used in the present invention is not particularly limited. Examples of fillers include aluminum hydroxide, aluminum oxide, calcium carbonate, calcium silicate, calcium aluminate, calcium sulfate, magnesium hydroxide, which are commonly used as fillers.
Examples include, but are not limited to, powders of titanium oxide, silica, mica, talc, clay, hydroxyapatite, wood, and the like.
Two or more types of fillers may be used in combination.
The filling rate of these fillers is not particularly limited, but is usually 10 to 90% by weight based on the total amount of the mixture of filler and polymerizable monomer mainly composed of MMA.
is within the range of If the filling rate is less than 10% by weight, the filling effect will be poor, and if it exceeds 90% by weight, the mixture of the filler and the polymerizable monomer mainly composed of MMA will lose its fluidity, making the casting process difficult. becomes difficult.

When a mixture of a polymerizable monomer mainly composed of MMA, fillers, and other substances is injected into a complex-shaped casting mold and polymerized and cured, sink marks occur as described above. As a result of careful observation of the areas where the molded product is cast, it was found that sink marks occur on the convex surface of the cast molded product. The reason for this is that volumetric contraction due to polymerization occurs centripetally, so a force that tries to separate from the mold surface acts on the convex surface, whereas a centripetal force acts on the concave surface that acts to pinch the mold. Therefore, mold release does not occur and therefore no sink marks occur. The present invention thus provides a method for preventing sink marks from occurring on the convex surface of a cast molded product, and can prevent sink marks from occurring on a cast molded product without using expensive or special equipment. By actively releasing the back side of the convex part, the convex part remains in close contact with the mold surface, and prevents the convex part from releasing from the mold during polymerization and causing sink marks. It is possible. To this end, it is sufficient to simply perform a simple operation in which the mold surface corresponding to the rear side of the convex portion has better mold releasability than the mold surface corresponding to the convex portion. The present invention is most effective when applied to a plate-shaped artificial marble cast molded product, and
The releasability of the mold surface, which constitutes the unused surface, facing the concave portion of the mold surface, which constitutes the unused surface, should be made better than the releasability of the concave portion of the mold surface, which constitutes the used surface of the cast molded product. There are ways to partially improve the mold release properties of the mold surface, such as applying wax, which is known as an external mold release agent, to the part, or applying a PVA aqueous solution and drying it. Although methods generally known as mold release methods such as a method of coating the part with a fluororesin can be employed, the method of pasting a mold release film on the part is most preferable. The reason for this is that the present invention uses the difference in mold releasability between the convex surface on the use side and the unused side on the back side to keep the product in close contact with the mold on the use side. This means that the larger the difference in releasability, the better. Therefore, it is desirable to have as good a mold releasability as possible on the unused surface side, and for this purpose, it is most suitable to use a mold release film with low mold release resistance. As an opposite approach, there is a method of increasing the adhesion on the convex surface portion of the use surface, but this is not preferable because it may result in poor mold release, resulting in cracking of the product or chipping of the use surface. Note that the release film is usually supplied in a flat form in many cases, and it is particularly difficult to apply it to a concave surface. Artificial marble cast molded products are usually plate-shaped, so if the surface used is convex, the back surface is often concave or flat.
The mold surfaces constituting it are either convex or flat. If the convex surface is flat, the work of applying the film is not so difficult. However, if you apply it to the entire unused surface, it is not only complicated as the area is large, but also
If there is a concave surface, the difficult task of pasting a flat film there must also be carried out. There are no particular limitations on the release film as long as it does not interfere with the polymerizability of the polymerizable monomer containing MMA as a main component. Examples of such materials include polyvinyl alcohol film, ethylene-vinyl alcohol copolymer film, polyester film,
Examples include, but are not limited to, cellophane film, polyamide film, and polyethylene film.

In the present invention, a recess on the mold surface is a line segment connecting a point on the mold surface to a point 5 mm apart in all directions on the mold surface, and does not touch the mold surface or pass through the inside of the mold wall. It refers to a part where at least one object exists. Sink marks are also likely to occur around the convex surface of a molded product, so the area of the mold surface that constitutes the non-use surface of the molded product is more likely to be affected than the area that faces the concave portion of the mold surface that constitutes the use surface of the molded product. It is also desirable that the part extending at least twice the wall thickness of the molded article at the boundary between the parts has better mold releasability than the facing mold surface. In some cases, the releasability of only part of the mold surface that constitutes the unused surface of the molded product that corresponds to the concave portion of the mold surface that constitutes the used surface of the molded product may be affected. It is also possible to use a casting mold that has better releasability than the facing recesses.

There are no particular limitations on the material of the casting mold used in the present invention. Examples of the material include commonly used glass, metal, resin, plaster, etc., but are not limited to these. There are no particular restrictions on the structure of the casting mold. There is no need for special equipment such as a pressurizing mechanism or a fitting mechanism along the parting line, and the mold surface that constitutes the working surface and the mold surface that constitutes the unused surface are simply joined together via a soft gasket. A structure with a similar structure is sufficient.

There are no particular limitations on the polymerization curing method of the present invention. Any method generally employed in cast molding, such as a method using a radical polymerization initiator, a method using heating, a method using ultraviolet rays or radiation, can be employed.

EXAMPLES The present invention will be explained in more detail with reference to Examples below, but the present invention is not limited to these Examples in any way.

〔Example〕

Example 1 In order to make a kitchen counter as shown in Fig. 1, both ends of a 3 mm thick chrome-plated stainless steel plate were bent 90 degrees in opposite directions and cast as shown in Fig. 2. Assembled the mold. In addition, gaskets are placed on both left and right ends and the bottom end of the mold to prevent liquid leakage, and polyethylene terephthalate is placed on the part of the upper mold that faces the concave part of the lower mold that forms the use surface of the counter, as shown in Figure 3. A film was applied and fixed with cellophane tape.

To 38 parts by weight of methyl methacrylate syrup obtained by partial polymerization (viscosity at 25°C: 3 poise), 2 parts by weight of ethylene glycol dimethacrylate, 0.01 part by weight of 2,2'-azobisisobutyronitrile, and aluminum hydroxide. Powder (Hygilite H
-310, manufactured by Showa Light Metal Co., Ltd.) and 45 parts by weight of silica powder (Crystallite AA, manufactured by Ryumori Co., Ltd.) were added, mixed uniformly, and poured into the mold from the injection part shown in Figure 2. injected into. The entire mold was kept in a hot air oven at 60℃ overnight to confirm hardening, and then kept in a hot air oven at 120℃ for 2 hours to complete polymerization and hardening.
I got a 12mm counter. No sink marks occurred on the used surface of the obtained counter.

Comparative Example 1 A counter was obtained in the same manner as in Example 1, except that the polyethylene terephthalate film was not applied to the mold. Sink marks were observed in and around the convex portions of the used surface of the obtained counter.

Example 2 For the purpose of making a bathroom vanity as shown in FIG. 4, a casting mold as shown in FIG. 5 was assembled using chrome-plated copper plates. A gasket was placed around the steel plate to prevent liquid leakage, and a vinylon film was placed on the part of the upper mold that faces the concave part of the lower mold, which constitutes the use surface of the vanity, as shown in Figure 5. I applied it and fixed it with cellophane tape.

A syrup (viscosity at 25°C
1.7 poise), 40 parts by weight of aluminum hydroxide powder (Hygilite H-310, manufactured by Showa Light Metal Co., Ltd.), 60% by weight, N,N-di(2-hydroxypropyl)-P
-Toluidine 0.2 parts by weight, benzoyl peroxide 0.12
Add parts by weight, mix uniformly, pour into the mold shown in Figure 5, and leave the entire mold in a room at 23°C to polymerize and harden to obtain a washstand with a sink wall thickness of 15 mm. . No sink marks were observed on the use surface of the obtained bathroom vanity.

Comparative Example 2 A washstand was obtained in the same manner as in Example 2, except that the vinylon film was not applied to the mold. Sink marks were observed in the convex portions and the vicinity of the convex portions of the used surface of the obtained washstand.

Example 3 The same procedure as in Example 1 was carried out except that instead of applying polyethylene terephthalate film to the mold in Example 1, that part was coated with Teflon.
I got a counter that is as good as it gets.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a kitchen counter showing an embodiment of the cast molded product of the present invention. FIG. 2 is a perspective view showing the casting mold, and FIG. 3 is a cross-sectional view of the kitchen counter casting mold, which is assumed to be cut along a plane perpendicular to the direction A in FIG. FIG. 4 is a perspective view showing half of a washstand showing an embodiment of the cast-molded product of the present invention. FIG. 5 is a sectional view showing the casting mold. Note that the scale of each part is arbitrary. 1...Kitchen counter, 2...Upper mold constituting the unused surface of the kitchen counter, 3...Lower mold constituting the usable surface of the kitchen counter, 4...Gasket, 5...Bathroom vanity, 6... Upper mold constituting the unused surface of the washstand, 7... Lower mold constituting the usable surface of the washstand, 8... Release film, B...
Inlet.

Claims (1)

[Claims] 1. When polymerizing and curing a mixture of methyl methacrylate, a polymerizable monomer mainly composed of methyl methacrylate, and a filler in a casting mold, on the surface of the mold constituting the non-use surface of the molded product, Filler-containing polymethyl methacrylate characterized by using a casting mold in which the part of the mold surface facing the recess that constitutes the use surface of the molded product has better mold releasability than the releasability of the recess. A method for manufacturing cast molded products. 2. Claim 1 using a casting mold in which the part of the mold surface that constitutes the unused surface of the molded product that faces the concave portion of the mold surface that constitutes the used surface of the molded product is covered with a release film. The method described in section.