WO2016182500A1

WO2016182500A1 - Whitening of metals

Info

Publication number: WO2016182500A1
Application number: PCT/SE2016/050428
Authority: WO
Inventors: Lars-Magnus Bjursten; Niklas AXÉN
Original assignee: Tigran Technologies AB
Current assignee: Tigran Technologies AB
Priority date: 2015-05-12
Filing date: 2016-05-12
Publication date: 2016-11-17
Anticipated expiration: 2017-11-12

Abstract

The present invention relates to a method for the production of a whitened surface of an object comprising a metal or a metal alloy. The present invention further relates to an object having a whitened surface, wherein the whitened surface has been produced according to the present invention. The invention also relates to a whitened implantable unit having a whitened surface. The implantable unit may be an implantable dental unit.

Description

WHITENING OF METALS

Technical Field of the Invention

This invention relates to a method for the production of a whitened surface of an object, such as e.g. a metal object, to an object having a whitened surface and a whitened implantable unit having a whitened surface. In relation to the expression "whitened" this implies in this context a colour being more white than the used starting material, but of course includes all different tones between white and yellow. Background

Dental implants are used widely in patients who have lost one or several teeth. There are known existing aesthetical issues with many dental implants. As many of the parts are made of metal, like titanium, they exhibit a dark metal surface that may become directly visible in the mouth of a patient, or visible through the gingiva of a patient.

Today, zircona ceramics are used as a material for providing whitish dental implants. Zircona ceramics have proven to have disadvantages in terms of exhibiting a chemical phase transformation, which is undesirable for a dental implant. Several coating techniques have been described. These include coating using a ceramic like porcelain or application of a white paint. The problems with the painting technique are the degradation and abrasion of such coatings. As the surface loosens its white appearance, the patient has to obtain new white coatings from time to time. It is well known that anodic oxidation will create a coloured titanium oxide surface layer. This layer is however always of spectral colour or black or greyish, never white. It is well known that thermal oxides of titanium are white. However, they only appear after heating to a relatively high temperature. The disadvantage is that high temperatures (above about 500 °C) cause the bulk properties of the titanium base material to change, most often with deterioration of its mechanical properties as a result. We have previously described how the temperature for thermal oxidation may be lowered by coating the implant with a compound or solution prior to the thermal oxidation (WO 2014/104966).

Summary of the Invention

According to a first aspect of the invention, the above and other objects of the invention are achieved, in full or at least in part, by a method as defined by claim 1 . According to this claim the above object is achieved by a method for the production of a whitened surface of an object comprising a metal or a metal alloy, wherein said method comprises thermal oxidation of said surface in the presence of fluoride in gaseous state. The fluoride may be delivered in various ways. It may originate from a fluoride salt which is decomposed at elevated temperatures in the vicinity of the metal object. Suitable fluoride salts are ammonium fluoride and ammonium bi-fluoride or ammonium hydrogen fluoride. It is advantageous if the fluoride salt contains fluoride and hydrogen and has a low decomposition temperature. The fluoride salt may be provided as a dry powder. The fluoride may be provided as a solution. The fluoride may be provided as a paste. Fluoride may also be provided to the object as a gas. An advantage of this method of application of the fluoride is that there is no need to apply a coating of fluoride to the surface prior to elevating the temperature. When applying a coating, it is often difficult to obtain a homogenous coating resulting in colour differences of the oxidized surface. In the present invention the fluoride is applied in gas phase by introducing a fluoride containing compound that releases fluoride into the gas phase around the part that is to be oxidized when heated.

Once again, it should be noted that "whitened surface" not only implies a white surface, but also a whitish surface, an off-white surface, a white to yellow surface or a lightly coloured surface.

The present invention applies to a method for the production of a white or whitish or off-white oxide layer on titanium or on dental implants. However, the present invention is not limited to titanium or to dental implants. There are other applications for low temperature oxidation of metals for the purpose of producing coloured oxides. The process of the present invention also applies to other metals than titanium, primarily also to zirconium and hafnium, as well as to alloys based on these metals.

Description of some specific embodiments

Below some specific embodiments of the present invention are disclosed and discussed.

The method may comprise one ore two steps.

According to one embodiment of the invention, the method may comprise one single step comprising thermal oxidation of a surface of an object in the presence of a fluoride in gaseous state.

According to another embodiment of the invention, the method may comprise a first step of low temperature oxidation of at least a part of a surface of said object, wherein said step of low temperature oxidation is performed by chemical oxidation, by electrochemical oxidation, or by a physical process, and wherein said step of low temperature oxidation is performed prior to said thermal oxidation.

Thus, the method for the production of a whitened surface on an object comprising a metal or a metal alloy, may comprise the steps of

i) performing a low temperature oxidation of at least a part of a surface of said object, wherein said step of low temperature oxidation is performed by chemical oxidation, by electrochemical oxidation, or by a physical process, and

ii) performing a thermal oxidation of said surface in the presence of a fluoride in gaseous state.

Thus, the method may comprise one or two individual steps.

By performing a first step of low temperature oxidation of at least a part of a surface of an object, the treated surface is protected from further oxidation during the following step of thermal oxidation in the presence of a fluoride in gaseous state. This is especially advantageous when the object consists of different functional parts. E.g. a dental abutment has parts that connect mechanically to the implant and to the crown or bridge, respectively, and also parts that are visible. For these devices the geometrical properties of the attachment surfaces are crucial and are preferably left un-oxidised whereas the visible parts should be white from oxidation. The same principle applies to so-called scan bodies, which are cylinders that are used to determine the position of dental implants that have been placed into the jaw bone. For this application the precision of the surfaces attaching to the implant is critical as well as the optical properties of the oxidized cylinder that is used as a target for laser scanning. Here the matte white surface achieved by the oxidation is critical. Thus the first step is used as a masking in order to limit the oxidation in the second step to certain areas where a white or whitish color is desired.

This low temperature masking may be performed directly by an anodization procedure, by ion beam treatment or by exposing the surface to a laser beam, or an oxidizing plasma.

Another way to apply this technology is to do the masking in a two-step procedure: Firstly, the area where the final white oxidation is to appear is shielded by a coating. Examples of such coatings are wax, lacquer, and acrylic paint. In a second step, the object is oxidized at low temperature by any of the above mentioned methods, e.g. by anodization in an electrolyte. The shield is removed, e.g. by a solvent or slight temperature elevation and the object is finally oxidized in a fluoride atmosphere.

Generally, the purpose of a white oxide on a metal obejct would be esthetic. In the case of a dental implant or a dental abutment primarily visible parts of the object should be whitened. Other non-visible parts of the object may deliberately be non-whitened. The purpose may be biocompatibility or mechanical aspects such as keeping a thread non-oxidized or a fitting surface of high surface rougness accuracy.

The step of low temperature oxidation may be performed by a chemical oxidation. One example of a chemical oxidation is treatment of the object with hydrogen peroxide.

The step of low temperature oxidation may be performed by

electrochemical oxidation. Such a process is also called anodization. An example of an electrochemical process is an electroplating process combining an electrolyte with an electrical field. For an extensive review see: A. Aladjem, Anodic oxidation of titanium and its alloys in Journal of Materials Science, May 1973, Volume 8, Issue 5, pp 688-704. For examples see V Zwillinga, M Aucouturierb, E Darque-Cerettia: Anodic oxidation of titanium and TA6V alloy in chromic media. An electrochemical approach,

Electrochimica Acta, Volume 45, Issue 6, 1 December 1999, Pages 921 -929.

The step of low temperature oxidation may be performed by a physical process. One example of such a physical process is treatment with gas plasma. Other examples are using local heating using a high intensity laser beam under atmospheric conditions.

Several techniques to produce thin film titanium oxide coatings for optics have been developed based on the very high refractive index of titanium oxide. Several techniques to produce thin film titanium oxide coatings for self cleaning windows have been developed based on its catalytic properties. Some of these techniques have drawbacks in creating useful optical coatings but may be used for the purpose of protecting the titanium surface from thermal oxidation in the presence of fluoride. These techniques include electron beam evaporation, energetic ion-beam- and plasma-based processes and conventional physical vapor deposition (PVD) of TiO₂.

A two-step process may be advantageous if the objective is to protect a part of the surface from further oxidation during the step of thermal oxidation.

By performing a first step of low temperature oxidation of at least a part of a surface of said object, the treated surface is protected from further oxidation during the following step of thermal oxidation in the presence of a fluoride in gaseous state. The spontaneously formed oxide layer on selected areas of the titanium object is improved by the low temperature oxidation process. In the case of the object comprising titanium, it is believed that the first low-temperature oxidation passivates the intrinsically reactive titanium surface in such a way that it withstands the thermal fluoride oxidation. Only non-passivated areas undergo an additional oxidation in the fluoride enriched environment. A low-temperature process can easily be performed locally, by protecting selected areas of the object, compared to a high temperature process. Areas may be protected as described above with various coatings; plastics, lacquers, etc., providing a tight protection to the metal.

The step of low temperature oxidation may be performed at room temperature or temperatures below 400 °C, preferably below 100°C.

The second step may be performed in the presence of fluoride at a temperature between 400 and 600 °C. If the second step is performed at higher temperatures a pre-treatment according to the first step described above does not protect the surface from oxidation.

According to one embodiment of the invention the fluoride containing compound may be introduced as a powder, together with a metal object, in a heating device and may interact with the metal surface as a gas produced when the powder decomposes due to the elevated temperatures. The fluoride containing compound may also be used in the form of a solution. The heating device may be any kind of heating device capable of generating temperatures above a few hundred degrees, typically 300 or 600°C. Such elevated temperatures are required to decompose and gasify the fluoride containing compound and to increase the chemical reactivity of the metal surface to allow for the formation of a sufficiently thick metal oxide layer/coating.

An example of a heating device is a temperature controlled oven designed for sintering of ceramics. When the fluoride containing compound interacts with the metal surface it catalyses the oxidation to occur at a lower temperature. This is advantageous since the bulk properties of the metal are unaltered.

According to a second embodiment of the invention the fluoride containing compound may be ammonium fluoride (NH₄F). Other fluoride containing salts or compounds may be used provided that fluoride is released to the gas phase when heated. An example of such a compound is NaF (sodium fluoride). A second example is ammonium difluoride.

According to another embodiment, the thermal oxidation may be carried out at a temperature of between 300 and 900°C, preferably between 300 and 800°C, more preferably between 350 and 650°C and most preferably between 400 and 600°C. The thermal oxidation may also be carried out between 300 and 800°C, preferably between 500 and 800°C.

According to another embodiment, the thermal oxidation may be carried out at a temperature below 500°C. This is especially advantageous, since there is minimal change of the bulk properties, especially the

mechanical properties of the metal.

According to yet another embodiment of the present invention, the method may be performed inside a closed container inside of which the fluoride containing compound is placed together with the object to be oxidized and wherein the container allows equalization of the pressure between the inside and outside of the container. This is done in order to increase the concentration of fluoride in the gaseous phase around the metal to be oxidized. This may preferably be done in a closed ceramic container with a small hole allowing pressure adjustment to the outside.

Many titanium alloys contain vanadium or niobium as alloy elements: Thermal oxidation at temperatures above 700°C of such alloys do not create a white oxide, since the vanadium oxide or niobium oxide that is formed makes the surface dark, brown or black. However, the method according to the present invention, where the fluoride is present in the gas phase, creates an environment where the titanium oxide is preferentially formed. The method according to the present invention is especially advantageous, since also the formed thermal oxide of alloyed titanium is white even when the method is carried out at a relatively low temperature.

The invention is not limited to objects of pure titanium or titanium alloys, but also applicable to metal objects of zirconium, aluminium, hafnium, vanadium, niobium and alloys containing these metals.

The object may be an implantable unit.

Furthermore, the implantable unit may be an implantable dental unit. The dental unit may be a so called full-body dental implant unit, such as a screw, abutment, implant part, bridge or crown. A dental implant unit according to the present invention exhibits superior aesthetics as well as mechanical benefits, when being compared to existing alternatives today. Dental units having a white surface are especially advantageous in at least two situations: Firstly, it is more esthetical pleasing than a bare metal surface if the abutment or implant head is exposed in a restoration to the teeth in the front of the mouth. The present invention is directed to all kinds of implants and implant parts, such as abutments, which are visible in the mouth of a patient, e.g. also through the gingiva. Secondly, today dental crowns are manufactured from digital measurements in the mouth. These are accomplished by putting a temporary abutment on top of the implants and record its position by computer tomography or optical laser. The temporary abutment must have a matte, non shiny, but reflective surface and at the same time be radio opaque. A metal with a white thermal oxide formed according to the present invention fulfils these requirements

According to yet another embodiment, the implantable unit may comprise titanium or a titanium alloy. Such implantable units are of

importance since the alloyed titanium has much better mechanical properties than the pure titanium, especially when it comes to flexural and breaking strength. Several different titanium alloys having these advantages are known. One comprises the elements vanadium and aluminium and another niobium.

The object may comprise vanadium.

The object may comprise aluminium.

The object may comprise niobium.

The object may comprise any combination of vanadium, aluminium and niobium.

The object may comprise zirconium.

The object may comprise alloys comprising zirconium.

The object may comprise hafnium.

The object may comprise alloys comprising hafnium.

The object may comprise any combination of zirconium and hafnium. According to a second aspect of the present invention, an object having a whitened surface is also provided, wherein the object comprises a metal or a metal alloy and wherein the whitened surface has been produced according to a method as described above. Such an object may be used for other purposes when a white resistant surface is preferred over the metal surface. Examples of such uses are for targets for lasers or when the visibility of the metal object needs to be enhanced. Another possible goal for this surface treatment may be to increase its chemical resistance or chemical properties in other ways. Also the surface treatment may change other optical properties than solely the colour.

According to a third aspect of the invention a whitened implantable unit having a whitened surface is also provided, wherein the whitened implantable unit comprises a metal or a metal alloy and wherein the whitened surface has been produced according to a method as described above.

According to one embodiment of the invention, the whitened

implantable unit may comprise a core and the core may comprise titanium.

According to another embodiment of the invention, the implantable unit may be a full-body implant comprising a core and the core may comprise titanium.

According to yet another embodiment, the implantable unit may be a particle, grain or granule. The particle, grain or granule may be between 0.1 μιη and 100 μιη. The particle, grain or granule may be spherical, discshaped, oblong, irregular or of any other shape. Such a particle, grain or granule may be used for e.g. bone augmentation, especially in situations where the metal colour may shine through the tissue making the tissue look black or otherwise discoloured. The implantable unit may comprise metallic titanium.

According to another embodiment, the particle, grain or granule may be fully-oxidised. Such a particle, grain or granule has the advantage of having a solid white colour while maintaining the beneficial surface associated effects of a titanium granule. According to another embodiment of the invention, the whitened implantable unit may be an implantable dental unit.

Claims

1 . Method for the production of a whitened surface on an object comprising a metal or a metal alloy, said method comprising thermal oxidation of said surface in the presence of a fluoride in gaseous state.

2. Method according to any of the preceding claims comprising a first step of low temperature oxidation of at least a part of a surface of said object, wherein said step of low temperature oxidation is performed by chemical oxidation, by electrochemical oxidation, or by a physical process, and wherein said step of low temperature oxidation is performed prior to said thermal oxidation.

3. Method according to claim 1 or 2, wherein the fluoride originates from a fluoride containing compound which is introduced as a powder that decomposes in a heating device and interacts with the metal surface as a gas.

4. Method according to any of claims 1 to 3, wherein the fluoride originates from a fluoride containing compound which is ammonium fluoride (NH₄F), ammonium di-fluoride or other fluoride containing salt.

5. Method according to any of claims 1 to 4, wherein the thermal oxidation is carried out at a temperature of between 300 and 900°C, preferably between

300 and 800°C, more preferably between 350 and 650°C and most preferably between 400 and 600°C.

6. Method according to any of claims 1 to 4, wherein the thermal oxidation is carried out at a temperature below 500°C.

7. Method according to any of the previous claims, wherein the method is performed inside a closed container where a fluoride containing compound or fluoride gas is placed together with the object to be oxidized and wherein the container allows equalization of the pressure between the inside and outside of the container.

8. Method according to any of the preceding claims, wherein the object is a metal object.

9. Method according to any of the previous claims, wherein the object is an implantable unit.

10. Method according to claim 8, wherein the implantable unit is an

implantable dental unit.

1 1 . Method according to any of claims 8-10, wherein the metal object or the implantable unit comprises titanium, zirconium, hafnium, aluminium or an alloy thereof.

12. Method according to any of the preceding claims, wherein the object is made of titanium or an alloy thereof.

13. Method according to any of the preceding claims, wherein the object made of alloyed titanium containing any combination of the following elements: aluminium, vanadium and/or niobium.

14. An object having a whitened surface, wherein the object comprises a metal or a metal alloy and wherein the whitened surface has been produced according to any one of claims 1 to 13.

15. The object according to claim 14, comprising a core and wherein the core comprises titanium.

16. The object according to claim 14 or 15, wherein the object is a whitened implantable unit.

17. Whitened implantable unit according to claim 16, wherein the implantable unit is a full-body implant comprising a core and wherein the core comprises titanium.

18. Whitened implantable unit according to claim 16 or 17, wherein the implantable unit is a particle, grain or granule.

19. Whitened implantable unit according to claim 16 or 17, wherein the implantable unit is a particle, grain or granule and wherein the implantable unit comprises metallic titanium.

20. Whitened implantable unit according to claim 18 or 19, wherein the particle, grain or granule is fully-oxidised.

21 . Whitened implant according to any of claims 16 to 20, wherein the whitened implantable unit is an implantable dental unit.