METHOD OF MANUFACTURING MOULDED BODIES FROM HYDROXYPHOSPHATE

Abstract: In dental surgery and orthopaedic surgery, implants of the bio-compatible hydroxyphosphate are used. Moulded bodies or a granulate thereof are obtained by sintering in a moist oxygen atmosphere. These products are of a blue colour. In accordance with the invention, white products are obtained by sintering in an inert atmosphere in which the oxygen concentration is maximally 0.005%.

Description: The invention relates to a method of manufacturing moulded bodies or a granulate from calcium hydroxyphosphate and to the moulded bodies and the granulate thus obtained. Hydroxyphosphate is often used as an implantation body in dental surgery and orthopaedic surgery due to the fact that this com­ pound exhibits an excellent bio-compatibility. A granulate of this com­ position is used in plastic surgery as a filler, for example, for cor­ recting facial asymmetry. A method of manufacturing these bodies is known from an ar­ ticle by G. de With c.s. in J. Mat Sci 16 (1981) 1592-1598. In accordance with the said method a powdered calcium hydroxyphosphate [Ca₅(PO₄)₃OH] is isostatically pressed to form a body which is sintered in a water-vapour containing oxygen atmosphere at a tempera­ ture of 1200-1300°C. However, the body thus obtained has a blue colour, probably caused by impurities present in the starting material. However, there are disadvantages attached to the said co­ lour, in particular, when the application of the material is in sight, such as with dental protheses. However, also in the case of invisible application in the human or animal body there is a psychological ob­ jection to the blue colour which is very different from the colour of the natural bone. It is an object of the invention to provide moulded bodies and a granulate having a white colour. It has been considered to obtain this colour by rigorously purifying the raw materials or hot pressing the material at a much lower temperature then the sintering temperature, yet at high pressure, but these possibilities had to be abandoned beacuse of the complexity and high cost of these operations. In accordance with Japanese Patent Application 80/130 854 which is laid open to public inspection and to which reference is made in Chemical Abstracts 94 (1981), 179 537W, a white colour is obtained by sintering the starting materials at a temperature of 1100°C or hot pressing at a temperature between 700 and 1200°C in a flow of H, CO, watergas and/or cracked NH₃-gas comprising water-vapour having a dew point of 5°C. Heating in an inflammable gas mixture is objectionable, however, due to the fire or explosion hazard. In accordance with the invention, the sintering operation is carried out in an inert atmosphere in which free oxygen is present in a concentration of maximally 0.005% and having a moisture content which, dependent upon the selected sintering temperature, ranges from 1.66 to 72.6 mm Hg. It appeared that the moisture limits were critical for ob­ taining the white colour. Preferably, the inert atmosphere consists of nitrogen. Surprisingly, it appeared that the said sintering method did not reduce the mechanical strength of the material with respect to the material sintered in oxygen. It is a great technological advantage that sintering in a reducing, inflammable atmosphere can be avoided. In accordance with a further elaboration of the invention it appeared that products having a blue colour due to sintering in a water-vapour containing, oxygen containing atmosphere can be converted to white products by post-heating them in an inert atmosphere con­ taining maximally 0.005% of free oxygen. This is particularly important for the manufacture of porous moulded bodies. In this case a relatively large amount of organic material is added to the starting material, which must be combusted be­ fore firing. If the firing operation were to be carried out directly in an inert atmosphere, the organic material would decompose thereby depositing carbon. In accordance with a further embodiment of the method in ac­ cordance with the invention, the shaped material, among which a binder has been dispersed, is first heated in an oxygen containing atmosphere until all organic matter is combusted and/or evaporated without leaving any residue before it is sintered at a temperature between 1200 and 1300°C in an inert, moist atmosphere containing maximally 0.005% of free oxygen and having a moisture content which, dependent upon the selected sintering temperature, ranges from 1.66 to 72.6 mm Hg. The invention will now be described in more detail with reference to three examples of embodiments. Example 1: Using approximately 7% by weight of tylose as a binding agent and approximately 5% by weight of butylstearate as a lubricant, calciummonohydroxyphosphate powder having a grain size smaller than ap­ proximately 0.5 µm was mixed with water to form an extrusible mass. Tubes having an outside diameter of 6 mm and an inside diameter of 2.6 mm were extruded from this mass, and after drying they were sintered for 4 hours at 1200°C in a water-vapour containing nitrogen atmosphere having a moisture content of approximately 15 mm Hg and an oxygen content of 0.005%. The product thus obtained was of a white colour. In dental surgery these tubes are used as substructures for a prothesis. Example 2: Using only water as a binder, calciummonohydroxyphosphate powder having a grain size smaller than approximately 0.5 µm was shaped into granules having a diameter between 0.7 and 1.7 mm. After drying the granulate was sintered for 4 hours at 1250°C in a moist nitrogen atmosphere having a moisture content of approximately 15 mm Hg and an oxygen content of 0.005%. Example 3: For the manufacture of a porous body for use as a bone prothesis, a mass was formed comprising calciummonohydroxyphosphate powder having a grain size smaller than approximately 0.5 µm, approxi­ mately 20% by weight of semolina and 15% by weight of water. The said mass was modelled in a mould and subsequently sintered for 4 hours at 1250°C in moist air having a moisture content of approximately 15 mm Hg. The body obtained was porous without any visible residue, but still had a blue colour. Next, the body was sintered for 4 hours at 1250°C in a moist nitrogen atmosphere having a moisture content of approximately 15 mm Hg and containing 0.005% of oxygen. After the sintering process the product had a white colour. When in the method of the above described examples the oxy­ gen content of the sintering atmosphere is selected to be higher than 0.005% by weight, the colour of the end product gradually changes to blue via greyish blue as the oxygen content becomes higher. At an oxygen content of 0.010% by weight the colour is clearly blue. At an oxygen concentration below 0.005% by weight, the colour of the end product obtained is always white.

Claims: 1. A method of manufacturing moulded bodies or a granulate from calciumhydroxyphosphate by sintering the shaped starting material among which a binder has been dispersed, in a water-vapour containing atmosphere at a temperature between 1200 and 1300°C, characterized in that the sintering operation is carried out in an inert atmosphere in which free oxygen is present in a concentration of maximally 0.005%, and having a moisture content which, dependent upon the selected sintering temperature, ranges from 1.66 to 72.6 mm Hg. 2. A method as claimed in Claim 1, characterized in that nitrogen is used for the sintering operation. 3. A method for the post-treatment of products made of calcium­ hydroxyphosphate, which are sintered in a water-vapour containing, oxygen containing atmosphere and have thereby obtained a blue colour, characterized in that the post-treatment is carried out in an inert at­ mosphere having maximally 0.005% of free oxygen. 4. A method of manufacturing porous bodies as claimed in Claim 1 or 2, characterized in that the shaped starting material, among which an organic binder has been dispersed, is first heated in an oxygen containing atmosphere until all of the organic matter is com­ busted and/or evaporated without leaving any residue, before it is sintered at a temperature between 1200 and 1300°C in an inert water-­ vapour containing atmosphere having a moisture content which, dependent upon the selected sintering temperature, ranges from 1.66 to 72.6 mm Hg and in which maximally 0.005% of oxygen is present. 5. Moulded bodies or a granulate of calciumhydroxyphosphate manufactured as claimed in Claim 1, 2, 3 or 4.

Inventors: SWAANEN, PETRUS HENRICUS, VAN DER MEER, AANT BOUWE DANIEL
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: C04B35
IPC codes: A61L27, A61F2, C04B35, A61C8