CA2246575A1 - A method for surface treatment of metal parts for odontologic use and such metal part manufactured by the method - Google Patents

Abstract

According to a first aspect, the present invention relates to a method of surface treating metal elements (2) intended for odontological use, such as brackets, molar tubes and dental arches. The metal elements are coated with a surface layer (23) for imparting a tooth-like colour to the elements and also to prevent the leakage of nickel. According to the inventive method, the surface layer (23) is applied by electrocoating. The invention also relates to such elements produced in accordance with the method.

Description

CA 02246~7~ 1998-08-14 A METHOD FOR SURFACE TREATMENT OF METAL PARTS FOR
ODONTOLOGIC USE AND SUCH METAL PART MANUFACTURED BY THE
- METHOD

The present invention relates to a ~ethod of surface treating metal elements intended for odontological use, particularly the metal components of devices for rectifying teeth irregulariti-es, such as brackets, molar tubes, arches, arch wires and ligatures, and like coated metal elements.

As is well known, orthodontic devices are used to rectify teeth irregularities and have long been used as an important auxiliary in odontology. Such devices include brackets that are intended to be detachably affixed to teeth intended for treatment, with the aid o~ an adhesive. The brackets mostly include slots or notches that are intended to accommodate pre-shaped dental arches and to fit thereinto with great precision, said arches coacting with several irregular teeth via the brackets. The ends of the arches are fixed detachably in position in molar tubes, which are mostly mounted on the molars. The position of each tooth in relation to the dental arch is fixed with the aid of fine ligatures. The magnitude of the force with which the brackets are clamped to the dental arch is dependent, among other things, on the friction between the bracket material and the dental arch, and will permit a certain amoune of movement of the brackets in relation to one another.

In order to achieve the effect intended, it is necessary to wear such teeth rectifying devices, or dental braces, for a long period of time, i.e. months and in certain cases even years. Because a dental brace is intended, in principle, to correct irregularities of growth and position of the teeth, it is fitted on the front side of the teeth and therefore fully r visible when the wearer opens his/her mouth.

From the aspect of mechanical strength, stability and treat-ment, stainless 18/8 steel is the best material that can be CA 02246~7~ 1998-08-14 used, since this material enables the brace elements to be - - given small dimensions and made with precision and also to be functional and strong. This steel has therefore totally dominated the world market in this respect.
Because of its unaesthetic appearance, endeavours have been made to scale down the unaesthetic appearance of the steel from the very time of beginning to use the material, often by attempting to give the steel an aesthetically acceptable colour. Another drawback with this steel resides in the leakage of nickel and other harmful substances, with the risk of allergy developments.

Attempts have been made to solve the aforesaid drawbacks with the use of plastic brackets and by coating the dental arches with different plastic substances. Brackets have also been produced from ceramic materials. When fitted to the lower jaw, however, these brackets have the drawback of being brittle and of grinding against the teeth in the upper jaw.
It can be thus said in summary that, in view of the necessary function of a dental brace, the person skilled in this art has accepted the aesthetic drawbacks and the health hazards that accompany the use of steel brackets. Although practical trials have shown that plastic brackets and plastic-coated dental arches solve the aesthetic problem, the weakness of the plastic and its tendency to discolor as a result of taking up liquid present a problem. Furthermore, both plastic material and ceramic material are excessively brittle. ~ith regard to ceramic brackets, it has been found that the brackets tend to loosen from the dental enamel at the end of the treatment and it is therefore often necessary to break loose the ceramic brackets, resulting in damage to the dental enamel. Because of their weakness, ceramic brackets must be made larger than corresponding steel brackets.

CA 02246~7~ 1998-08-14 WO97/2971Z PCT/S~97/00230 The reason why attempts to coat 18/8 steel successfully with - - a dental colouring, which would also inhibit nickel leakage, is because the colouring layer must be applied with great precision on very small elements of particularly complicated ,5 geometry. The brackets include sharp, protruding corners and grooves ~in which the arch must fit with great precision).

The slope of the grooves ln these brackets varies by about +22~
- -30O according to the shape and position of an individual tooth.

The molar tubes into which the ends of the arch are inserted must also provide an excellent fit and are very small in size, 0.20 mm-0.40 mm, and may be both round and square in shape.
Square tubes are intended for so-called edge arches (square arches). It follows from this that it is very important to be able to apply the colouring layer with great precision, both with respect to the brackets and with respect to the molar tubes. An object of the present invention is to prevent the brace elements from leaking nickel. It is therefore important that molar tubes, for instance, can be coated on the inner surface that is not visible to the eye with a tolerance of 1-2 ~, and also in the corners of the 90~-angles of the square tube. It shall also be possible to coat the ligatures, which have a thickness of 0.10 mm.

It is also known to produce the brackets from titanium, with the intention of avoiding alloys in which nickel is present and therewith also avoid associated allergy problems. From an aesthetic point of view, titanium brackets are the worst conceivable option of all types of brackets, since the colours are much darker than that of 18/8 steel. Titanium is difficult to work and has resulted in much sharper bracket edges and t corners than with other types of bracket materials.
US-A 4,050,156 teaches a me~al coated dental brace having an aesthetical tooth-coloured surface comprising, e.g., Teflon and CA 02246~7~ 1998-08-14 Ekonol and TiO2 as the colouring substance. The object is to - - achieve good properties with respect to wear resistance, temperature stability, low friction, shapability and fastening ability.
US-A 4,722,689 describes metal parts or elements intended for odontological use and coated with a thin layer of tooth-coloured polymer, said layer consisting of nylon, epoxy, polyester, vinyl, rubber or like substances and includes a colouring pigment in an amount corresponding to the colour nuance of the teeth.

US-A 4,107,844 describes dental braces that include a metal core which is coated with epoxy to give an aesthetic appearance and then laminated with a transparent plastic material.

US-A 4,946,387 describes a tooth rectification metal wire which is coated with a first layer of tooth-coloured material, preferably a plastic, such as polyethylene or polyurethane, and a second outer layer of transparent material, preferably a silicone elastomer.

US-A 4,731,018 describes methods for the co-extrusion of teeth rectifying wire comprisiny a metal coated with an aesthetically attractive tooth-coloured plastic material or like material.

US-A 4,180,912 describes a metal dental brace that includes a plastic casing.

EP-A 393 002 describes a metal dental arch that is coated with a thin layer of tooth-coloured material having a low coeffici-ent of friction. The layer ma~erial is polyamide and is applied to a thickness of 0.02 to 0.2 mm.

One problem rhat is difficul_ to overcome when surface coating dental braces or teeth rectifying devices of the kind described in the introàuction and exemplified above resides in achieving CA 02246~7~ 1998-08-14 W~9712971Z PCT/SE97/00230 a uniform layer that will adhere satisfactorily to the metal - - elements and that will cover said elements effectively. The problem arises because these elements are very small and present sharp edges, corners and recesses where a uniform layer thickness is normally difficult to achieve.

None of the known methods in which metal elements of the type concerned are coated with a colouring layer is able to prevent satisfactorily the leakage of nickel and other harmful substances. This is because the outer layer tends to exhibit micropores to a greater or lesser extent. It is therefore an established truth in this particular field that the layer of colouring substance cannot function as a nickel barrier.

Accordingly, the object of the present invention is to provide a method of applying an outer layer that has very small layer thickness tolerances (1-2 ~) even on sharp corners and edges, and to prevent the leakage of nickel and to provide an aesthetically attractive surface of low friction. Another object of the invention is to provide metal elements that have these properties.

These objects are achieved in accordance with the invention with a method of the kind defined in the preamble of Claim l that includes the particular steps set forth in the charac-terizing clause of said Claim, and with a metal element intended for odontological use of the kind defined in the preamble of Claim 15 and having the characteristic features set forth in the characterizing clause of the Claim.
Because the surface coating is applied by an electrocoating process to form the colouring layer, it has been possible to achieve a very even layer thickness even on corners of razor blade sharpness, both negative and positive corners, that occur on metal elements of the kinds concerned, and also within the square-section molar tubes of 0.2-0.4 mm in size. The layer also obtains a very narrow thickness tolerance of about l-2 ~, CA 02246~7~ 1998-08-14 WO97/29712 PCTtSE97/00230 even on its critical sharp sections, so as to be impervious to - - nic~el ions, even at internal and external corners of the tube.

This has been possible to achieve, because with electrocoating processes, the colouring substance is carried by an electric current to that place where the electric resistance is lowest.
When a micropore is present, the resistance becomes lower in the pore and the colouring substance is conveyed thereto. The resistance is also lower where the layer is thinnest, and consequently colouring substance is constantly moved to those places where the layer is thinnest.

The colouring solution with which the metal elements are electrocoated is preferably a polyamide/polyurethane based solution.

In one preferred embodiment of the invention, the colouring solution includes one or more of the following components: self cross-linking polyacrylate, amino resins, epoxy esters, epoxy resins and isocyanate pre-polymers.

In another preferred embodiment of the invention, there is first applied an inner electrocoated layer whereafter an outer thin layer is applied using a vacuum coating techni~ue. This outer layer will preferably have a thickness of Q.2-5 ~ and consists of carbon or metal oxides.

The electrocoated layer will preferably have a thickness of about lO-30 ~.
In still another preferred embodiment of the invention, there is applied to the colouring layer by means of a vacuum surface coating process a layer tha~ preferably consists of carbon or metal oxide. This results in a very harG protective outer layer.

CA 02246~7~ 1998-08-14 WO97/29712 PCT/S~97/00230 According to another preferred embodiment, an inner layer of - - titanium or titanium oxide is applied by means of a vacuum surface coating process prior to applying the colouring sub-stance. This enhances adhesion of the colouring substances, ,5 which may be especially necessary when the metal element is made of stainless steel. Furthermore, the titanium/titanium oxide layer also prevents the leakage of nickel should the colouring layer be damaged.

The aforedescribed embodiments and other advantageous em-bodiments of the inventive method and the inventive device will be evident from the dependent Claims.

The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings, in which Fig. 1 is a perspective view of a dental brace;

Fig. 2 is a cross-sectional view of one part of the brace shown in Fig. 1;

Fig. 3 illustrates schematically one step in the production process;
Fig. 4 is a cross-sectional view of a part similar to Fig. 2 but in a further enlarged view and illustrating an alternative embodiment o- the invention; and Fig. 5 illustrates schematically an additional treatment stage in the production process according to said alternative embodiment.

Fig. 1 illustrates one type of dental brace to which the present invention relates. A dental arch 2 that includes brackets 3 is disposed on a row of teeth 1. The brackets 3 are affixed to each tooth 7 wi~:~ the aid of cement or some like CA 02246~7~ 1998-08-14 adhesive. One of the brackets 3 is configured as a molar tube - - 6. One end 4 of the arch 2 is removably fastened to the molar tube 6, which is, in turn, fastened to a molar. Although not shown, the other end of the arch 2 is similarly attached removably to a similar molar tube.

The dental arch 2 is shown in cross-section in Fig. 2. The arch is comprised of a metal core part 22, for instance comprised of 18/8 steel. The surface of the arch 2 is coated with a plastic material that prevents the leakage of nickel and is coloured with a colouring substance that matches the colour of the teeth 7.

The outer layer 23 is an electrocoated layer applied in accor-dance with a method illustrated schematically in Fig. 3. The metal arch 2 is lowered into a bath l0 that contains an appropriate colour solution. A fixed electrode ll is disposed in the bath and the arch 2 forms a counterelectrode. A DC
voltage is applied between the two electrodes 2, ll and pigment particles move towards the arch 2, so-called electrophoreses.
The arch 2 can either constitute the anode or the cathode o~
the cell, this latter alternative (so-called cataphoreses) being preferred. The electrophoretic transportation of pigment particles to the arch 2, functioning as the cathode, and the precipitation of these particles on the arch result in a colouring layer of uniform thickness on the arch. The electrophoreses is continued until a layer of desired thickness has been built-up, about 20-30 ~.

The binding agent used in the bath l0 will have a composition that will ensure that the binder has no negative effect on human tissue when cured. It shall also be ensured that the pigments and other components of the colouring substance will not induce negative reactions with living tissue. P
Binding agen~s of different chemical composition can be chosen on the basis of these r~equirements. For instance, there may be CA 02246~7~ 1998-08-14 WO97/29712 PCT~SE97/002~

used self cross-linking polyacrylates, particular aminoplastic - - resins, epoxy esters, combinations of epoxy resins and iso-cyanate pre-polymers, etc. There is pre~erably used a com-bination that comprises diglycidyl ether of bisphenol A having a molecular weight suitable for the purpose, and a pre-polymer of toluene diisocyanate, where the isocyanate groups have been bloc~ed chemically with appropriate hydroxyl-containing chemical compounds. Pigments used in the a~oredescribed binding agent combination will preferably be inert pigments, which may be inorganic pigments, e.g. titanium dioxide, other oxide pigments, carbon black and the like. Inert substances, such as kaolin, are also chosen as filler.

There will also preferably be used a small catalyst addition, e.g. dibutyltin oxide, in order to achieve a sufficiently high reaction rate in a subsequent oven drying process.

The coating is allowed to harden on the dental elements subse-quent to the electrocoating process.
Fig. 4 is a cross-sectional view of part of an arch 2 according to another embodiment of the invention. In this embodiment, a further layer is ap~lied on the electrocoated layer 23. This outer layer 24 is applied to a thickness of about 0.2-lO ~ with the aid of a vacuum surface treatment process and consists of transparent carbon.

Either a so-called physical vacuum surface treatment (PVD) or a chemical vacuum surface treatment (CVD) process may be used in this regard. The carbon layer is very thin and obtains-good adhesion and will be transparent or have a very weak in-terference colour. The layer obtains a structure of so-called diamond-like carbon (DLC). A metal oxide, such as aluminium oxide, can be applied in the aforesaid manner as an alternative to carbon. A vacuum surface coating process will be described briefly hereinafter in accordance wlth the PVD method in which carbon is used as the coatina material.

CA 02246~7~ 1998-08-l4 In this process, the surface-coated arch 2 is placed in a - - vacuum chamber 12. The material 13 to be applied to the arch 2 is disposed in a holder element 14 in the chamber 12. The chamber 12 is connected to a vacuum pump 15 and a vacuum of about 10-2 to 1o-4 pascal is generated in the container. The material 13, which is initially in a solid state, is therewith heated and vapourized. The vapourized material is transported directly to the arch 2, or via atom/molecule impacts, and condenses on the arch. Condensation of the material can be promoted by applying an electric potential to the arch 2. The arch 2 is mounted on a shaft 16 which passes sealingly through the chamber wall and which is rotated to achieve uniform condensation on the arch 2 and therewith an even surface coating. The material used is carbon and the coating process is continued until the required thickness of 0.2-10 ~ has been obtained. The result is a surface layer of extreme hardness, about 6,000 Vickers, and high wear resistance. The chamber is placed under a powerful vacuum with the intention of avoiding air molecules present from inhibiting free mobility of the carbon atoms in the container and of avoiding chemical reactions between the carbon atoms and the air molecules. The material can be vapourized with the aid o~ an electron gun.

Alternatively, atoms can be loosened from the carbon 13 in the holder element by sputtering, with the aid of gas ion bombard-ment, preferably argon ion bombardment. The carbon atoms are therewith stripped from the carbon 13 and released for movement towards the arch 2.

Electrode coating and vacuum surface coating are both well known and established techniques and do not in themselves constitute a subject of the present invention. On the other hand, neithe~ of these techniques has earlier been applied in the fabrication of elements intended for odontological use.

Claims (25)

1. A method of surface treating metal elements (2) intended for odontological use, in which the metal elements (2) are coated with an outer surface structure that includes the application of a colouring layer (23), characterized by applying the surface layer structure by electrocoating in a colour solution (10).

2. A method according to Claim 1, wherein the metal elements are electrocoated in a polyurethane-based colour solution.

3. A method according to Claim 1 or 2, wherein the metal elements are electrocoated in a colour solution that includes at least one of the components: self cross-linking polyacrylate, amino resin, epoxy ester, epoxy resin and isocyanate pre-polymer.

4. A method according to any one of Claims 1-3, wherein the metal elements are electrocoated in a colour solution that includes a diglycidyl ether of bisphenol A and a pre-polymer of toluene diisocyanate.

5. A method according to Claim 4, wherein the isocyanate groups in said toluene diisocyanate are blocked with hydroxyl-containing compounds.

6. A method according to any one of Claims 1-5, wherein the metal elements are electrocoated in a colour solution in the presence of a catalyst.

7. A method according to Claim 6, wherein the catalyst is dibutyltin oxide.

8. A method according to any one of CIaims 1-7, wherein the metal elements are vacuum coated with an outer transparent layer on said colouring layer subsequent to the electrocoating process.

9. A method according to Claim 8, wherein the metal elements are vacuum coated with carbon.

10. A method according to Claim 8, wherein the metal elements are vacuum coated with metal oxide.

11. A method according to any one of Claims 8-10, wherein the metal elements are electrocoated to an electrocoating thickness of 20 to 30 µ.

12. A method according to any one of Claims 8-11, wherein the metal elements are vacuum coated to a vacuum coating thickness of 0.2 to 10 µ.

13. A method according to any one of Claims 1-12, wherein the metal elements are coated with a layer of titanium or titanium oxide prior to applying the colouring layer.

14. A method according to Claim 13, wherein the titanium layer or the titanium oxide layer is applied by a vacuum surface coating process.

15. A metal element (2) intended for odontological use and having a surface coating that includes at least one colouring layer (23), characterized in that said surface coating includes a colouring layer (23) that has been applied by an electrocoating process.

16. A metal element according to Claim 15, wherein the surface coating includes a layer (23) of polyurethane-based material that has been applied by an electrocoating process.

17. A metal element according to Claim 15 or 16, wherein the surface coating includes a colouring layer (23) that has been applied by an electrocoating process and that includes at least one of the components: self cross-linking polyacrylate, amino resin, epoxy ester, epoxy resin and isocyanate pre-polymer.

18. A metal element according to any one of Claims 15-17, wherein the surface coating includes a colouring layer (23) that has been applied by an electrocoating process and that includes a diglycidyl ether of bisphenol A and a pre-polymer of toluene diisocyanate.

19. A metal element according to Claim 18, wherein the isocyanate groups of the toluene diisocyanate are blocked with hydroxyl-containing compounds.

20. A metal element according to any one of Claims 15-19, wherein the surface coating includes an inner layer (23) that has been applied by an electrocoating process, and an outer transparent layer (24) that has been applied by a vacuum evaporation process.

21. A metal element according to Claim 20, wherein the layer (24) applied by said vacuum evaporation process includes carbon.

22. A metal element according to Claim 20, wherein the layer (24) that has been applied by said vacuum evaporation process includes metal oxide.

23. A metal element according to any one of Claims 20-22, comprising an electrocoated layer (23) that has a thickness of 10 to 30 µ.

24. A metal element according to any one of Claims 20-23, comprising a layer (24) that has been applied by a vacuum surface coating process to a thickness of 0.2 to 10 µ.

25. A metal element according to any one of Claims 15-24, comprising a titanium layer or titanium oxide layer inwardly of the colouring layer.