JPH0543317A - Production of orthodontic component - Google Patents

Abstract

PURPOSE:To obtain orthodontic components of complicated shape excellent in mechanical strength and appearance characteristics by injection molding process. CONSTITUTION:A powder mixture, 0.5-8mol% in yttrium oxide content, of yttrium oxide and zirconium oxide is mixed with polyvinyl alcohol, and the resulting mixture is then incorporated with a binder comprising (A) wax 50-90 deg.C in softening point and (B) a thermoplastic polymer 100-300g/10min in MFR to prepare a kneaded product for injection molding, which is, in turn, put to injection molding to effect binder removal followed by sintering, thus obtaining the objective components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an orthodontic component made of ceramics.

[0002]

2. Description of the Related Art Conventionally, a translucent aluminum oxide product has been used as a ceramic orthodontic part that is used by being adhered to the surface of a tooth because it has a good appearance and excellent appearance characteristics. ..

By the way, since the orthodontic component has a three-dimensionally complicated shape, the conventional ceramic orthodontic component is generally formed by press-molding ceramic powder, followed by sintering, and then cutting. It is manufactured by doing. In this case, since aluminum oxide alone has no translucency, a small amount of magnesium oxide is added as a sintering aid. The role of magnesium oxide is
In the final stage of sintering, grain growth of crystal grains is suppressed and the disappearance of pores contained inside the sintered body is promoted.

However, in the above-mentioned conventional aluminum oxide products, there are large variations in the crystal grain size, the size of the unavoidable residual pores is large, and they do not have sufficient translucency and mechanical strength. I was running out. It is considered that the cause of these is that the conventional aluminum oxide products were molded by the press molding method. That is, in press molding, the pressure is not uniformly applied to the compact, and the packing density of the metal oxide powder varies among the parts of the compact, and as a result, the density (pores) in the sintered final product (sinter) is reduced. , And the grain size vary depending on the part. Further, in press molding, since it is difficult to obtain a three-dimensionally complicated shape, cutting is performed after sintering, but during the cutting, fine cracks called hair cracks are formed on the surface of the sintered body. Occurrence, which reduced the mechanical strength of the orthodontic component and also caused its failure during use.

Therefore, there has been proposed a method of manufacturing an orthodontic component by an injection molding method without using the press molding method. In this method, a mixture of ceramic powder and an organic binder is injection-molded in a mold of a predetermined shape, the resulting injection-molded body is debindered, and then sintered to complete the final product. According to this method, an orthodontic component having a three-dimensionally complicated shape is manufactured without machining.

[0006]

However, in the above injection molding method, it has been impossible to manufacture an orthodontic part having excellent mechanical strength, although it is satisfactory in terms of translucency. The cause is that the strength of the alumina-magnesia-based material is about 40 kg / mm ² at the maximum in bending strength, and the strength of the material itself is insufficient.

[0007] Therefore, the present invention is an orthodontic treatment in which the mechanical strength is greatly improved without impairing the appearance characteristic (that is, good appearance) which is an important characteristic for use as an orthodontic component. It is an object to provide a method by which parts can be manufactured.

[0008]

According to the present invention, polyvinyl alcohol is added to a metal oxide mixed powder of yttrium oxide and zirconium oxide prepared to have an yttrium oxide content of 0.5 to 8 mol%. 1 to 10% by weight, a step of uniformly dispersing the yttrium oxide, the mixed powder in which the yttrium oxide powder is uniformly dispersed, the softening point (JIS K 7206) wax of 50 ~ 90 ℃ And Melt Flow Rate (ASTM D 1236, 230 ℃)
A step of adding and kneading a binder material composed of 100 to 300 g / 10 min of thermoplastic resin so that the total amount of the binder material and the polyvinyl alcohol is 35 to 70% by volume of the whole, Injecting the kneaded product into a molded product of a predetermined shape by injection molding, removing the polyvinyl alcohol and the binder material from the molded product, and removing the polyvinyl alcohol and the binder material from the molded product in an oxidizing atmosphere. In the above, there is provided a method for manufacturing an orthodontic component, which comprises a step of sintering at 1300 to 1700 ° C.

Metal Oxide Mixed Powder In the present invention, it is important to use, as a starting material, a metal oxide mixed powder in which yttrium oxide powder is 0.5 to 8 mol% and the balance is zirconium oxide powder.

For example, in order to exhibit excellent mechanical strength, it is necessary to set the relative density of the sintered body obtained by the sintering described below to at least 99%. As described above, yttrium oxide suppresses the growth of crystal grains in the final stage of sintering, thereby promoting the disappearance of pores inside the grain boundaries and improving the sintering density, and the size of the crystal grains increases. It acts to align the height. Therefore, yttrium oxide is
It plays a major role in improving mechanical strength. When the content of the yttrium oxide powder in the metal oxide mixed powder is less than 0.5 mol%, such an effect is not sufficiently exhibited, the density of the sintered body is not sufficiently high, and it is used as an orthodontic part. Cannot obtain sufficient mechanical strength. When the content of yttrium oxide powder is more than 8 mol%, yttrium oxide or a composite oxide of yttrium oxide and zirconium oxide, which causes destruction, is formed at the crystal grain boundaries of the sintered body, and The strength decreases, and the color of the sintered body becomes yellow, which is unsatisfactory as an orthodontic component from the aspect of the appearance.

In the present invention, as the yttrium oxide powder and the zirconium oxide powder used for preparing this mixture, commercially available products can be used, respectively.
It is preferable to use a highly pure one in order to exhibit excellent mechanical strength. It also improves the sintering density,
In order to exhibit good mechanical strength, it is desirable to use those having an average particle size in the range of 0.01 to 1 μm. If a powder having an average particle size of more than 1 μm is used, it becomes difficult to increase the density of the sintered product as a final product, and mechanical strength may become unsatisfactory. If a powder smaller than 0.01 μm is used, the amount of binder used for injection molding described later will increase, and as a result, the initial packing density of the ceramic powder will decrease, and a sintered body with a high density can be obtained. May be difficult.

Preparation of Kneaded Product for Injection Molding In the present invention, polyvinyl alcohol is added to the above-mentioned metal oxide mixed powder to form a uniform mixture, and then a binder material is added to carry out kneading to obtain a mixture for injection molding. The kneaded product is prepared. The polyvinyl alcohol used here has a surfactant-like action, and by mixing this polyvinyl alcohol prior to kneading with the binder material, yttrium oxide powder is added to the zirconium oxide powder. The advantage of being evenly distributed is achieved. This polyvinyl alcohol has a total amount of 1 to 10 based on the total amount of the metal oxide powder.
It is used in proportions by weight, in particular 2-6% by weight. When the amount of polyvinyl alcohol used is less than 1% by weight or more than 10% by weight, yttrium oxide is not uniformly dispersed in the zirconium oxide powder, and as a result, the mechanical strength of the final product (sintered body) is partly affected. It depends on

As the polyvinyl alcohol,
Those having an average degree of polymerization of 500 to 2500 and an average degree of saponification of 66% or more are preferably used. Mixing of this polyvinyl alcohol and the above-mentioned metal oxide mixed powder can be carried out by various methods known per se, for example, a method of producing granule powder by a spray dryer often used in fine ceramics is most preferable. Is.

A polyvinyl alcohol is mixed and yttrium oxide is uniformly dispersed in the mixture, and a binder material is added to the mixture to prepare a kneaded product for injection molding. The binder material used here is used for effectively performing the injection molding described below, and has a softening point (JIS K 720
6) Waxes in the range of 50 to 90 ° C and melt flow rate (MFR, ASTM D 1236, 230 ° C) of 100 to 300 g /
It consists of thermoplastics in the range of 10 minutes.

The waxes contribute to improving the fluidity at the time of injection molding, and examples of such waxes include paraffin wax, microcrystalline wax, low molecular weight polyethylene wax, modified wax, atactic polypropylene and the like. You can If the softening point is lower than 50 ° C, the cycle time of injection molding becomes long, and the softening point is likely to be softened, resulting in a decrease in workability during molding. The softening point is 90
When the temperature is higher than 0 ° C, the fluidity is lowered and the thermal decomposition point is further raised, so that the debinding property is lowered.

Further, the thermoplastic resin contributes to maintaining the shape of the molded product after injection molding, and examples thereof include polyethylene, polypropylene, ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer, polystyrene and the like. It can be illustrated. MFR is 100g /
If it is lower than 10 minutes, the injection moldability will be unsatisfactory, and if it is higher than 300 g / 10 minutes, the strength and shape retention of the injection molded product will be low.

These waxes and thermoplastics differ depending on their types, but they are generally used in a weight ratio of 4: 1 to 1: 4, and particularly 3: 1 to 1: 3. It is suitable.

In the present invention, it is necessary that the above-mentioned binder material is used in a total amount of 35 to 70% by volume based on the kneaded material in the total amount with the above-mentioned polyvinyl alcohol. If it is less than 35% by volume, the fluidity of the kneaded product will be reduced, making injection molding difficult, and if it is more than 70% by volume, debinding will be very difficult and the transparency of the final product will decrease. It causes inconvenience. The kneading with the binder material can be easily performed using a kneading device such as a kneader or various mixers.

Injection Molding Next, the kneaded material prepared in the above step is injected into a mold having a desired mold to obtain a molded product having a target mold. This injection molding can be carried out by directly using the equipment and devices used in ordinary plastic injection molding. The injection conditions are generally heating temperature of 80 ℃ ~ 200
It is desirable that the temperature and the injection pressure be 500 to 2000 kg / cm ² . In the case of press molding, stress is not evenly applied to the molded body, and the packing density of the ceramic powder shows an uneven distribution, resulting in an unsatisfactory mechanical strength of the sintered body. Since the present invention adopts injection molding, it is possible to effectively avoid such a problem.

The debinder then performed to remove the binder for the shaped body, to remove the polyvinyl alcohol and the binder material contained in the molded body. This debinding can be easily carried out usually by heating in the air or in an inert atmosphere such as nitrogen gas to the above-mentioned thermal decomposition temperature of polyvinyl alcohol and the binder material.

Sintering Treatment Following the above debinding, a sintering treatment is performed.
In the present invention, it is necessary to perform this sintering in an oxidizing atmosphere such as the air or an oxygen atmosphere, and the sintered body obtained by this exhibits a white color and has a desirable appearance as an orthodontic part. Becomes Discoloration occurs in a reducing atmosphere or an inert atmosphere such as argon gas,
Does not appear white.

The sintering temperature is 1300 to 1700 ° C., preferably
It is set in the range of 1400 to 1600 ℃. At a temperature lower than 1300 ° C, the relative density of the obtained sintered body is not sufficiently improved, resulting in a decrease in mechanical strength. As a result, yttrium evaporates from the surface and the relative density after sintering does not improve.

In the method of the present invention, the crystal grain size of the sintered body is affected by the sintering temperature and time. For example, when the sintering is performed at a temperature of 1300 to 1700 ° C. according to the present invention, the sintering time is preferably set to 4 to 7 hours, not exceeding 8 hours. If the sintering time is short, the crystal grain size will not be uniform, and if it is too long, abnormal grain growth will occur and the crystal grain size will not be uniform.

Orthodontic parts The orthodontic parts thus obtained have a composition in which the yttrium oxide content is 0.5 to 8 mol% and the balance is zirconium oxide, the crystal grain size is 5 to 40 μm, and the relative density is It is composed of 99% or more of the sintered body, and has excellent mechanical strength and appearance characteristics in relation to such composition and crystal grain size.

[0025]

Examples 1 to 5 Using yttrium oxide powder having an average particle size of 0.15 μm and zirconium oxide powder having an average particle size of 0.15 μm, the yttrium oxide content was variously changed as shown in Table 1. A powder was prepared. Next, polyvinyl alcohol (average degree of polymerization: 1700, average degree of saponification: 88%) was added to each of the above mixed powders at a ratio of 5% by weight based on the whole, and the mixture was granulated using a centrifugal spray dryer and uniformly dispersed. The thing was prepared. Paraffin wax (Nihonsei Wax 125, softening point 50 ° C) and low density polyethylene (Sumitomo Chemical Sumikasen G808, MFR; 150g / 10min) were added to each of these uniform dispersions in a weight ratio of 3: 1. The binder material containing is added so that the total amount with the polyvinyl alcohol is 40% by volume,
The kneaded material was prepared with a planetary mixer.

Using this kneaded product, an injection molding machine was used to perform injection molding under the conditions of an injection temperature of 90 ° C. and an injection pressure of 600 kg / cm ² to form a plate having a thickness of 3 mm, a width of 4 mm and a length of 25 mm. The body was molded. Next, debinding was performed in an air stream while heating the molded body (heating temperature: 500 ° C.), and further, sintering was performed at a temperature of 1450 ° C. for 4 hours in an oxygen stream.

Table 1 shows the distribution and color of the crystal grain size of the obtained sintered body. Bending strength was measured as the mechanical strength of the sintered body, and the results are also shown in Table 1. For the measurement of bending strength, the dimensions of the sintered body are the thickness of 2 mm, the width of 4 mm and the length.
It was measured with 20 mm aligned.

Comparative Examples 1 to 5 Instead of injection molding, press molding (press pressure; 1 ton / c)
m ²⁾ except that performed has created a sintered body in the same manner as in Example 1 (Comparative Example 1). Further, a sintered body was prepared in the same manner as in Example 1 except that metal oxide mixed powders having yttrium oxide contents of 2 mol% and 0.25 mol% were used (Comparative Examples 2 and 3). Furthermore, in Example 1, metal oxide mixed powders having different yttrium oxide contents were used, and the sintering temperature was 1450.
℃, sintering time 10 hours, sintering temperature 1750 ℃,
Sintered bodies were prepared in the same manner as in Example 1 except that the sintering time was 6 hours (Comparative Examples 4 and 5).

The same measurements as in Example 1 were performed on these sintered bodies, and the results are shown in Table 1.

[0030]

[Table 1]

Next, the above Examples 1-4 and Comparative Examples 1-5.
By changing the mold, a sintered body having the shape of an orthodontic part that is actually used is prepared. This shape is shown in FIGS. 1 and 2 (FIG. 1 is a perspective view of the above shape, and FIG. 2 is a sectional view taken along line AA of FIG. 1). The mechanical strength of this sintered body was evaluated, and the results are shown in Table 2. The mechanical strength was evaluated by applying a force at a constant speed in the direction of the arrow in FIG. 2 and evaluating the maximum bending strength at that time.

[0032]

[Table 2]

[0033]

According to the present invention, it is possible to manufacture an orthodontic component having a complicated shape excellent in mechanical strength and appearance characteristics by the injection molding method.

[Brief description of drawings]

FIG. 1 is a perspective view showing an orthodontic component created in an example.

2 is a cross-sectional view of the orthodontic component shown in FIG. 1 taken along the plane AA.

Claims (4)

[Claims] 1. The yttrium oxide content is 0.5 to 8 mol%.
In a metal oxide mixed powder of yttrium oxide and zirconium oxide prepared to be 1 to 10 based on the total amount of polyvinyl alcohol and the metal oxide mixed powder.
Mixing so that the yttrium oxide is evenly dispersed in the mixture so that the yttrium oxide powder is uniformly dispersed, the mixed powder in which the yttrium oxide powder is uniformly dispersed is mixed with waxes having a softening point (JIS K 7206) of 50 to 90 ° C. Flow rate (ASTM D 1236, 230 ° C) 100-300
a step of adding and kneading a binder material composed of g / 10 minutes of a thermoplastic resin so that the total amount of the binder material and the polyvinyl alcohol is 35 to 70% by volume of the whole; A step of injection molding into a shaped molded article, a step of removing the polyvinyl alcohol and the binder material from the molded article, and a molded article from which the polyvinyl alcohol and the binder material have been removed, in an oxidizing atmosphere, 1300 to A method of manufacturing an orthodontic part, which comprises a step of sintering at 1700 ° C. 2. The manufacturing method according to claim 1, wherein the waxes and the thermoplastic are used in a ratio of 4: 1 to 1: 4 on a weight basis. 3. The method according to claim 1, wherein at least one of paraffin wax, microcrystalline wax, low molecular weight polyethylene wax, modified wax, and atactic polypropylene is used as the wax. 4. The production method according to claim 1, wherein at least one of polyethylene, polypropylene, an ethylene-vinyl acetate copolymer, an ethylene-acrylate copolymer and polystyrene is used as the thermoplastic.