JPS6158422B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for firing a sintered hydroxyapatite body used as a biomaterial. Biological implant materials that have conventionally been mainly used for bones, tooth roots, etc. include metal materials and polymer materials. All of these have the disadvantage that they are foreign substances to the bones and roots of the living body and lack compatibility with the living body. Recently, ceramic materials, which are stable in vivo, non-toxic, and non-irritating, have been developed as biomaterials aimed at improving this drawback. Biomedical glass, glassy carbon, alumina, hydroxyapatite [Ca _x (PO ₄ ) _Y OH], etc. have been proposed as ceramic-based bioimplant materials. Among these, hydroxyapatite, which has the same composition as the main inorganic component of living organisms and has excellent affinity with living organisms, is attracting attention. Hydroxyapatite is represented by the chemical formula Ca _x (PO ₄ ) _Y OH, and the theoretical hydroxyapatite is X=5, Y=3, and the X/Y ratio is 1.67. However, the actual composition is influenced by the manufacturing method,
It is known that products with an X/Y ratio of about 1.33 to 1.95 are produced. Various theories have been proposed as the cause of this wide range of X/Y ratio phenomena: heterogeneous phase coexistence theory, surface adsorption theory, and ion defect theory.All theories are derived from the crystal structure of hydroxyapatite. There is. In order to use hydroxyapatite as a biological implant material for bones, tooth roots, etc., a method of obtaining a high-density and high-strength sintered body by sintering and using this has been considered. In this case, the hydroxyapatite sintered body used as a biological implant material has no cracks inside or on the surface, is large enough to be used as a biological implant material, has high density,
High strength is required. Further, elements and ions other than Ca and P contained in impurities and additives derived from various starting materials used in the synthesis of hydroxyapatite cause the phenomenon of coloring of the sintered body. When used as a biological implant material, it is preferable to use an uncolored substance due to various factors. Therefore, there is a demand for a hydroxyapatite sintered body that is white or milky white, translucent, and has a color similar to that of living bones and teeth. However, the conventional method for sintering hydroxyapatite compacts involves firing a synthetic hydroxyapatite raw material in the form of a gel or powder at a temperature range of 1000 to 1300°C in air, vacuum, or an inert gas atmosphere. It was hot. Hydroxyapatite is manifested by its characteristic formula Ca ₅ (PO ₄ ) ₃ OH
Naturally produced materials that have OH groups are said to have a stable structure and are not easily decomposed by heating, but when synthetic raw materials are sintered using the above method, they tend to decompose more easily than naturally produced materials. It is believed that a portion of the calcium phosphate is easily converted into tricalcium phosphate Ca ₃ (PO ₄ ) ₂ according to the following formula, and that excess calcium exists as free lime CaO. Ca ₅ (PO ₄ ) ₃ OH→ Ca ₃ (PO ₄ ) ₂ +CaO+H ₂ O This type of decomposition depends on the starting material Ca/PO ₄ molar ratio (hereinafter referred to as Ca/PO ₄ ratio), particle size, and molding. Although it varies depending on the method of firing the body, it starts at around 500 degrees Celsius and decomposition accelerates as the temperature gets higher. Tricalcium phosphate thus produced has a density lower than that of hydroxyapatite Ca ₅ (PO ₄ ) ₃ OH, which has a density of 3.16 g/cm ³ , and is in the β form of the low temperature stable phase and 3.07 g/cm ³ of the high temperature stable phase. The mold weight is 2.87g/ ^cm3 . In this manner, at the time when sintering or almost sintering is completed, a portion of the hydroxyapatite changes into a low-density substance, resulting in structural heterogeneity and expansion stress within the sintered body. Such heterogeneity tends to occur in the surface layer and grain boundaries of the sintered body, resulting in the formation of fine cracks and heterogeneity in internal stress, resulting in a sintered body with low mechanical and thermal strength. , the dimensions are also
Only small pieces of less than 10 mm could be obtained.
For example, if the sintered body has a simple shape and small dimensions, the compressive strength is 1000 to 3000 Kg/ ^cm2 , and the bending strength is 200 to 500 Kg/cm2.
If the size is only about ² cm2 and the shape is complex, or the sintered body is simple but large in size, there will already be visible cracks in the sintered body when it is taken out of the furnace after firing. However, it is not suitable as a biological implant material. If the Ca/PO ₄ ratio of the hydroxyapatite raw material is low, the decomposition of the sintered body, that is, the formation of tricalcium phosphate, is likely to occur, so it is also possible to set the Ca/PO ₄ ratio to the theoretical ratio or higher. However, according to the conventional hydroxyapatite firing method, excess calcium is present in the hydroxyapatite raw material, and the Ca/PO ₄ ratio is the theoretical ratio in hydroxyapatite represented by the chemical formula Ca ₅ (PO ₄ ) ₃ OH.
If it is 1.67 or more, the sinterability of hydroxyapatite is poor, and the ratio of sintered body density/theoretical density of Ca ₅ (PO ₄ ) ₃ OH (hereinafter referred to as sintering rate) is 92% or less.
The drawback was that only a low sintered body could be obtained compared to the sintering rate of about 96% when the Ca/PO ₄ ratio was 1.67 or less. A method of adding small amounts of various different ions and firing them to improve the sinterability of hydroxyapatite or prevent its decomposition has been considered, but there is no sufficient method, and the addition of special elements is not suitable for biomaterials. This is unfavorable in terms of toxicity and other aspects, and the emergence of high-density, high-strength sintered bodies that do not contain impurities has occurred. Furthermore, it is added to the hydroxyapatite raw material to eliminate impurities caused by the starting material or to improve the physical properties of the hydroxyapatite sintered body produced.
Although it may contain impurities such as Fe, Al, Mg, Si, Ti, Zn, Cu, etc., according to the conventional manufacturing method of sintered hydroxyapatite, for example, the purity of the hydroxyapatite raw material is 99.8% or more. Even with high purity, there are dozens of ions that cause coloring, such as iron ions.
When ppm is contained, the disadvantage is that the color of the ions comes out and the sintered body becomes colored. Therefore, in order to obtain a white to milky white and transparent sintered body, it is necessary to use a hydroxyapatite raw material of extremely high purity, and the manufacturing cost becomes extremely high. The present invention improves the above-mentioned drawbacks of the conventional method and provides a method for firing a hydroxyapatite sintered body that has a high sintering rate, high mechanical strength, and does not become colored. The gist of the present invention, therefore, lies in a method for firing a hydroxyapatite sintered body, which comprises firing a hydroxyapatite raw material in a reducing gas containing water vapor. The hydroxyapatite raw materials used in the present invention include synthesized gel-like or powder-like hydroxyapatite, calcined hydroxyapatite obtained by further calcining these, bone ash of animals, etc., or a mixture of two or more of these. However, in any case, one with a Ca/PO ₄ ratio of 1.33 to 1.95 is preferable. If the Ca/PO ₄ ratio is less than 1.33, the density will be low and therefore sufficient mechanical strength will not be obtained, and if it is more than 1.95, free calcium will be produced in the sintered body, which is important for strength or medical reasons. May cause undesirable effects. Ca/ _PO4 ratio is 1.67~
When the value is 1.80, a hydroxyapatite sintered body with particularly excellent physical properties can be obtained. In order to prepare such a hydroxyapatite raw material, for example, slaked lime and phosphoric acid are reacted at pH 9 to 11 in the presence of water, and the resulting hydroxyapatite precipitate is separated from the reaction solution without washing with water.
A hydroxyapatite raw material with a Ca/PO ₄ ratio of 1.67 to 1.80 is obtained. At this time, the pH during synthesis and
Since there is a correlation between the Ca/PO ₄ ratio, the Ca/PO ₄ ratio can be controlled by adjusting the pH during synthesis. Further, if the hydroxyapatite precipitate formed above is washed with water, the calcium content will flow out, so it is preferable not to wash it with water. To adjust the Ca/PO ₄ ratio of the hydroxyapatite raw material, add tricalcium phosphate to lower the Ca/PO ₄ ratio, or add CaO, CaCl ₂ ,
It is also possible to increase the Ca/PO ₄ ratio by adding a calcium component such as CaF ₂ . Furthermore, the hydroxyapatite raw material used in the present invention may contain MgO, Na ₂ O,
2% by weight of additives such as K ₂ O, CaF ₂ , Al ₂ O ₃ , SiO ₂ etc.
It may be added within The hydroxyapatite raw material prepared in this manner may be fired as it is, or if necessary, it may be pressure-molded using a mold or a rubber press and then fired. In order to obtain a sintered body with a higher sintering rate, it is better to perform pressure molding at normal pressure. Furthermore, if the powdered hydroxyapatite raw material is calcined, its bulk density increases, so if it is calcined at about 800°C before use, a sintered body with a higher sintering rate can be obtained. Now, in the present invention, the hydroxyapatite raw material prepared or pretreated as described above is molded under pressure and then heated to a temperature of 1100~
It is unique in that it is fired at a temperature range of 1350℃. In a reducing atmosphere, sintering of the hydroxyapatite compact is promoted and rapid densification occurs, especially
If there is an excess of calcium above the theoretical Ca/PO ₄ ratio, the consequences are significant. As mentioned above, when hydroxyapatite is fired, it decomposes into tricalcium phosphate, but the present inventors discovered that this can be completely prevented by adding water vapor to the reducing gas, and the theoretical density It has become possible to produce an extremely dense and structurally uniform sintered body with a density of more than 99%, resulting in an increase in strength that is incomparable to conventional methods. According to this method, a pure hydroxyapatite sintered body can be obtained without the need for additives to promote sintering, so it is extremely suitable as a method for producing biomaterials. Furthermore, even if it becomes necessary to contain a small amount of a different element due to the requirements as a biomaterial, this method does not cause any hindrance to sintering, and a sintered body with the same high strength as pure apatite can be obtained. Examples of the reducing gas used in the present invention include hydrogen, carbon monoxide, water gas, ammonia decomposition gas, mixed gases thereof, and mixtures of these gases with inert gases. Any other reducing gas can be used; for example, hydrocarbons have reducing properties, but there is a risk that carbon decomposed during firing may adhere to the surface of the sintered body.
Not preferable if coloring is a problem. Next, the amount of water vapor added to the reducing gas is preferably greater than the amount of saturated water vapor at 5°C.
When the amount is less than this, α-tricalcium phosphate is generated at the grain boundaries of the sintered body, resulting in a decrease in mechanical strength.
To add steam to the reducing gas, the reducing gas may be added by passing it through water at 5° C. or higher, or the steam or water may be added directly into the furnace. The firing temperature varies depending on the sintering method, and is 1,100 to 1,000 yen when the gel-like raw material is directly placed in a container and sintered, or when uncalcined or calcined powder raw material is pressure-formed in advance and sintered. It is preferable to sinter at a temperature range of 1350°C, and when pressure molding was performed, a sintered body with a sintering rate of 94 to 99.5% was obtained. When uncalcined or calcined powder raw materials are sintered using a hot press, the temperature is preferably in the range of 700 to 1200°C, and it was possible to obtain a sintered body with a sintering rate of 95 to 99% or more. In this case, there is a correlation between pressure and sintering temperature, even at 700℃.
Applying a pressure of 2000Kg/cm ² results in a sintering rate of over 99%. An economically advantageous pressure range is 100 to 2000 Kg/cm ² . According to the method of the present invention, sintering was difficult with the conventional method and only a low sintering rate of 92% could be obtained.
A sintered body with a high sintering rate can be obtained even in a raw material composition range of hydroxyapatite in which the Ca/PO ₄ ratio is 1.67 or more. As mentioned above, the sinterability of hydroxyapatite can be significantly improved by firing it in a reducing atmosphere, and its decomposition into tricalcium phosphate can be prevented by adding water vapor. be. In particular, the effect is remarkable when the theoretical ratio of Ca/PO ₄ is set to 1.67 or more. However, the hydroxyapatite raw material
The most suitable Ca/PO ₄ ratio is 1.67 to 1.80, and when raw materials in this range are used, a sintered body with a high sintering rate of 98 to 99% can be obtained. When the Ca/PO ₄ ratio is 1.67 or less, crystal grain growth is promoted, and when it is 1.80 or more, crystal grain growth is suppressed too much, and the sintering rate slightly decreases in both cases. Also, the Ca/PO ₄ ratio is 1.67.
It was confirmed that free calcium does not exist in the sintered body within the range of 1.80 to 1.95 as mentioned above, and the hydroxyapatite sintered body produced by the method of the present invention has a different physical property. It was found that the chemical properties did not change at all. or,
In the case of synthetic hydroxyapatite raw materials that contain elements that inhibit sintering, such as Mg, Zn, and Si,
For the same reason, we were able to obtain a sintered body with a sintering rate of 99% or more. The hydroxyapatite sintered body produced by the method of the present invention has a high sintering rate, no cracks, and does not contain expansion stress, so it has a compressive strength of 4000~
It has a high mechanical strength of 7000Kg/cm ² and a bending strength of 1000 to 1600Kg/cm ² , which is 2 to 3 times the mechanical strength of conventional sintered bodies using hydroxyapatite powder, making it an excellent biological implant material. It is extremely excellent. Next, the hydroxyapatite raw material may contain impurities originating from the starting raw material or impurities added to improve the physical properties of the produced hydroxyapatite sintered body. Even when such a hydroxyapatite raw material is used, in the method of the present invention, the sintered body is not colored due to the above-mentioned impurities because it is fired in a reducing gas. For example, in addition to Ca and P, living bones contain Na, K, Mg, Sr,
Because elements such as Fe, Zn, Al, F, and Cl are mixed in, if living bone or bone ash is used as a raw material, or if these are mixed into a synthetic apatite raw material and fired using the conventional method, it will be colored blue or light red. A sintered body was obtained, and when fired by the method of the present invention, a white to milky white transparent sintered body was obtained. In the above case, the amount of impurities was 2% by weight. Similarly, when 2% by weight of impurities such as Mg, Zn, Si, Na, K, Al, Ti, and Cu are mixed into the synthetic apatite raw material and fired by the method of the present invention, a white to milky white transparent sintered body can be obtained. It was done. The method of the present invention will be explained in more detail with reference to Examples below. Example 1 A molded body was produced using a hydroxyapatite raw material that had been calcined and granulated at 800°C in air for 3 hours.
While flowing hydrogen gas at ~1350℃ at 200c.c./min,
Sintering was performed by directly adding 1 g of steam per hour into the furnace to obtain a sintered body of hydroxyapatite. The firing time was selected from 30 minutes to 9 hours to obtain a sintered body with maximum density. The hydroxyapatite used has a Ca/ _PO4 ratio of 1.62~
It was synthesized to have a value of 1.85. For comparison, a molded body was produced in the same manner using the same hydroxyapatite raw material, and a sintered body was produced by firing in air. The results are shown in FIGS. 1 to 3. Figure 1 shows the Ca/PO ₄ ratio of 1.62 to 1.70.
The relationship between firing temperature and sintering rate when firing in air using the conventional method is shown below. Optimal from the diagram
The Ca/PO ₄ ratio is 1.62-1.65 and the firing temperature is 1200-1300.
℃, the sintering rate reaches 94-95%, but Ca/PO ₄
It can be seen that when the ratio is 1.67 or more, that is, 1.70 in Figure 1, the sintering rate is significantly reduced. Second
The figure shows the relationship between firing temperature and sintering rate when the Ca/PO ₄ ratio was varied from 1.65 to 1.85 and firing was performed in a hydrogen gas atmosphere containing water vapor by the method of the present invention.
From the figure, it can be seen that sintered bodies with a Ca/PO ₄ ratio in a wide range of 1.65 to 1.85 are superior to those obtained by conventional air firing. FIG. 3 is a graph showing the relationship between the Ca/PO ₄ ratio and the sintering rate at each firing temperature using the same data as in FIG. 2. It can be seen from FIGS. 2 and 3 that an excellent sintered body with the highest sintering rate can be obtained when the Ca/PO ₄ ratio is 1.67 to 1.80. Figure 4 shows the results of sintered bodies produced by the method of the present invention and by the conventional method when the firing temperature was 1250°C for 1 hour.
The relationship between Ca/PO ₄ ratio and bending strength is shown. From the diagram
Over the entire range of Ca/ _PO4 ratios 1.60 to 1.85,
The sintered body obtained by the method of the present invention has a bending strength 2 to 6 times that of the sintered body obtained by the conventional method,
It can be seen that the bending strength is highest when the Ca/PO ₄ ratio is in the range of 1.67 to 1.80, reaching 1400 to 1600 Kg/cm ² . Furthermore, the sintered body obtained by firing in air according to the conventional method was opaque and colored blue, but the sintered body obtained by firing in hydrogen gas containing water vapor according to the present invention was white to milky white and transparent. It was hot. The synthetic hydroxyapatite raw material used in this example was a special grade reagent and had a purity of 99.8% or more. Example 2 The same hydroxyapatite raw material as used in Example 1 was used, and ammonia decomposition gas, CO, and water gas were used as reducing gases, and these gases were
per hour while flowing at a flow rate of cc/min.
1 g of water vapor was added and sintering was performed at 1250°C for 1 hour to produce a sintered body. As a result, H ₂ of Example 1
The sintering rate and mechanical strength were comparable to those using gas, and the color was white to milky white and transparent. Example 3 The Ca/PO ₄ ratio of synthetic hydroxyapatite is
A molded body using 1.75 powder was fired at 1250°C for 1 hour. For the firing atmosphere, hydrogen gas was passed through water adjusted to various temperatures, and hydrogen gas containing saturated vapor of water at that temperature was used. The results are shown in FIG. In the case of a hydrogen gas atmosphere containing saturated steam at a temperature of 5℃ or less, the sintering rate will decrease, and the sintered body may crack, and its mechanical strength will be 800 to 1000.
The bending strength was as low as Kg/ ^cm2 . But 5℃
The above results showed the same results as Examples 1 and 2. Further, even when the same amount of steam or water as the above conditions was added directly into the furnace, similar results were obtained. Although the flow rate of hydrogen gas was varied from 0.5 cc/min to 500 c.c./min, only similar results were obtained. Example 4 A hydroxyapatite raw material synthesized at pH 9.5 using industrial slaked lime as slaked lime and food additive phosphoric acid as phosphoric acid was sintered by the method of Example 1 using H ₂ gas to produce a sintered body. I got it. Although the synthetic hydroxyapatite raw material contained the impurities shown in Table 1, the obtained sintered body was white to milky white and transparent. 【table】

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between firing temperature and sintering rate in the conventional method, Figure 2 is a graph showing the relationship between firing temperature and sintering rate in the method of the present invention, and Figure 3 is a graph showing the relationship between firing temperature and sintering rate in the method of the present invention. /PO ₄ ratio and sintering rate, Figure 4 is a graph showing the relationship between sintering temperature and bending strength of sintered bodies according to the method of the present invention and the conventional method, and Figure 5 is a graph showing the relationship between the method of the present invention and the sintering rate. 2 is a graph showing the relationship between the temperature of water that provides steam to the reducing gas and the sintering rate of the obtained sintered body.

Claims (1)

[Scope of Claims] 1. A method for firing a hydroxyapatite sintered body, which comprises firing a hydroxyapatite raw material in a reducing gas containing water vapor. 2. A patent claim characterized in that the reducing gas is composed of one or more selected from hydrogen, carbon monoxide, water gas, and ammonia decomposition gas, or a mixed gas of these and an inert gas. Range 1
A method for firing a hydroxyapatite sintered body as described in . 3. The method for firing a hydroxyapatite sintered body according to claim 1 or 2, wherein the amount of water vapor is greater than or equal to the saturated vapor amount at 5°C. 4 Using the gel produced by the wet method as it is as the hydroxyapatite raw material, the gel is
A method for firing a hydroxyapatite sintered body according to claim 1, 2, or 3, characterized in that the firing is performed at a temperature of 1100 to 1350°C. 5. Using an uncalcined powder raw material as the hydroxyapatite raw material, pressure molding the powder raw material.
A method for firing a hydroxyapatite sintered body according to claim 1, 2, or 3, characterized in that the firing is performed at a temperature of 1100 to 1350°C. 6. Claims 1 and 2, characterized in that a calcined powder raw material is used as the hydroxyapatite raw material, and the calcined powder raw material is pressure-molded and fired at a temperature of 1100 to 1350°C. A method for firing a hydroxyapatite sintered body according to item 2 or 3. 7. Claims 1, 2, or 3, characterized in that an uncalcined powder raw material is used as the hydroxyapatite raw material, and the powder raw material is fired at a temperature of 700 to 1200°C by hot pressing. A method for firing a hydroxyapatite sintered body as described in . 8. Claims 1, 2, or 8, characterized in that a calcined powder raw material is used as the hydroxyapatite raw material, and the calcined powder raw material is fired at a temperature of 700 to 1200°C using a hot press. The method for firing a hydroxyapatite sintered body according to item 3. 9 Na in the hydroxyapatite raw material,
Any one of claims 1 to 8, characterized in that one or more of the following elements are mixed: K, Mg, Sr, Si, Fe, Zn, Ti, Al, F, Cu, or Cl. A method for firing a hydroxyapatite sintered body as described in . 10 Ca/ of the hydroxyapatite raw material
The method for firing hydroxyapatite according to any one of claims 1 to 9, characterized in that the molar ratio of PO ₄ is 1.67 to 1.80.