CH638972A5 - Process for the manufacture of dental porcelains which have a reinforced mechanical strength - Google Patents

Description

The invention relates to a method for manufacturing dental porcelain having reinforced mechanical strength. Dental porcelains are made up, for the most part, of an aluminous core covered with a vitreous enamel of a rather significant thickness having convex parts with small radius of curvature, which makes it even more fragile. The main defect of glass being its low tensile strength (or bending) and fractures always leaving the surface, it is possible, by putting it in compression, to increase the tensile strength by the same amount from the whole. For this, there are several well-known methods in the glass industry.

Thermal quenching consists of rapidly cooling the surface of the article from the softening point to a temperature below the minimum stress relaxation temperature. The heart of the object then puts the surface in compression while cooling. This method is difficult to apply to dental porcelain which are composite materials with walls of very uneven and often thin thicknesses.

An enamel with a lower coefficient of expansion than that of the core can be deposited on the surface; this enamel is compressed by the heart during cooling. In the case of dental ceramics. this compression could be carried out by the core if it had a coefficient of expansion significantly higher than that of the enamel.

This has two drawbacks: the tensioning of the already fragile core and the risk of splintering on the convex spokes.

The process for manufacturing dental porcelain is characterized in that a dental prosthesis blank is taken which is covered with an enamel of composition suitable for ion exchange, and that the coated prosthesis is soaked in a bath of salt melted at a temperature between 200 C and the transformation point of the enamel so as to cause the exchange of alkaline ions close to the surface of the prosthesis for larger alkaline ions from the bath. The molten salt bath can be K.N03, a K2SO4 / KC1 mixture, or a NaN03 / NaSO4 mixture. The enamel can be an alumino-silicate containing a percentage by weight on the oxides of 5 to 20% of Al 2 O 3 and / or 5 to 15% of ZrO 2 and 5 to 20% of a light alkaline oxide. The method applies to the reinforcement of a

dental prosthesis element chosen from the group comprising a Jacket crown, a ceramic-metallic crown or bridge, a platinum-clad veneer and a porcelain-clipped tooth.

The mechanical resistance of dental porcelain was increased in a first step by firing under vacuum. This technique allows, in addition to improving the aesthetic qualities, a 20% increase in tensile strength.

More recently, the manufacture of core mass containing inclusions of alumina in increasing quantity (up to 20% vol.) Has enabled a 10% increase in the tensile strength thereof. This technique of adding particles intended to reduce the length of the average Griffith fault in the material cannot be applied to the enamel for aesthetic reasons. Because fractures in glassy materials usually start from the surface, it was important to improve the tensile strength of the surface without changing its appearance.

The object of the invention is to improve the tensile strength (and therefore flexion) of the surface layer of dental ceramics by chemical quenching.

Applied to an enamel of composition specific to ion exchange, the chemical hardening makes it possible to increase in great proportions the tensile strength of the latter without altering either the forms or the aesthetic qualities of the prosthesis.

The ease of treatment of the prosthesis, its speed and its low cost are also sought-after goals of this invention.

The improvement in tensile strength provided by the invention makes it possible to increase the reliability of ceramic crowns and ceramic-metallic crowns and bridges, veneers with platinum studs and teeth with porcelain studs.

The invention allows in particular the extension of the use of ceramic crowns to the detriment of metal-ceramic whose aesthetics are less good and whose implementation is longer and more expensive.

The invention will be better understood on reading the examples described with reference to the appended drawings, in which:

K

fig. 1 represents the evolution of the ratio - as a function of time,

If fig. 2 represents the evolution of the tensile strength as a function of time.

The process for manufacturing dental porcelain according to the present invention by chemical quenching can be easily carried out in the following manner: the prosthesis manufactured according to the usual techniques will be covered with an enamel of composition suitable for ion exchange, that is to say -to say an alumino-silicate containing between 5 and 20% by weight of Na20 or LizO intended to be exchanged and 5 to 20% by weight A1203 and / or 5 to 15% of Zr02 intended to facilitate the exchange. Once completed, the prosthesis will be immersed, after preheating, in a bath of molten salt for the time necessary for the exchange to lead to optimal compression, then removed and washed with water after cooling in air.

The salt bath which may be contained in a stainless steel basket will, as the case may be, be KN03, a K2SO4 / KCl mixture or an NaN03 / Na2SO4 mixture.

The bath temperature will be chosen sufficiently below the softening point of the enamel to avoid relaxation of the stresses created by the exchange. Temperature and treatment time depend on the viscosity of the glass; when the temperature increases, the viscosity decreases, leading to an increase in the exchange rate, but also an increase in the rate of stress relaxation. The temperatures usually used are between the strain point and the glass transformation point, an area in which the viscosity is between 3 · 1014 and 1013 Po. The usual treatment times are between 5 min and 5 h.

The following example, which is not limiting, will better understand the advantage of the invention.

A weight decomposition enamel: Si02 66% / Ai203 15% / CaO 1% / Na20 10% / B203 8% is melted in a crucible of

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638,972

platinum at around 1450 C for 3 h from the following raw materials: silica: Si02 - hydrated alumina: Al (OH) 3 - calcite: CaC03 - sodium carbonate: Na2C03 - orthoboric acid:

h3bo3.

Glass with the following dilatometric characteristics:

coefficient of expansion 20-400 C = 6-10 ~ 6 mm / C transformation point (Tg) = 560 C softening point (Pr) = 630 C is poured into water, dried and ground to a size less than 150 (i. io

The enamel obtained is moistened and pressed into bars under 200 kg / mm2 in a parallelepiped mold in the dimensions L x 1 x h = 50 x 6 x 8 mm. The bars are steamed under vacuum overnight at 500 C, cooked under vacuum at 950 C for 1 h, then cooled in the oven at a rate of 50 C / h. They are then machined using a diamond grinding machine to the shape necessary for bending tests (parallelepipeds of size LxWxH = 40x5x3 mm). Each side is then abraded with carborandum 800 and then with gray 600 American sandpaper. The bars placed flat on talcum silica are then glazed under vacuum for 3 min at 940 ° C., annealed for 1 hour at 600 ° C. and cooled in the oven in order to avoid thermal stresses.

A bath of molten potassium nitrate contained in a stainless steel basket is maintained at 480 ° C. in a vertical tube furnace. The bars placed in sets of five in a nacelle made of fine platinum wires are first preheated above the bath 25

for 10 min, then soaked in the molten salt for the desired time (5% or 10 or 15 or 20 min). At the end of the treatment, they are cooled in air and then rinsed with water. The efficiency of the Na-K exchange is measured in X-ray fluorescence by the evolution of the K / Si ratio as a function of the treatment time. In fig. 1, which represents the evolution of this ratio, it can be seen that, at the temperature considered, the exchange is practically complete after 20 min.

The bending strength of the bars is measured on an Instron 3-point bending test machine. The lower knives are 30 mm apart and the machine speed is 0.1 mm / min. The tensile stress at break on the external fiber is calculated by the formula:

3 PI

a kg / mm2 = ———

2 b h2

P = breaking load in kg 1 = distance between the knives (30 mm) b = width of the bar (mm)

h = bar height (mm)

We can see in fig. 2, which represents the evolution of the tensile strength as a function of the treatment time, that it increases first rapidly, passes through a maximum of 13 kg / mm 2 for a time of 5 min, then decreases slowly when the processing time increases.

The exchange rate at 5 min being only 65% for the considered temperature of 480 ° C, it is possible, at a lower temperature, to further increase the maximum tensile strength by increasing this exchange rate .

The enamel formula, the reinforcement of which is described in this example, has made it possible to produce crowns whose shape and aesthetic characteristics comparable to those obtained with conventional enamels have in no way been altered by chemical quenching.

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1 sheet of drawings

Claims (6)

638,972 1. A method of manufacturing dental porcelain, having a reinforced mechanical resistance, characterized in that one takes a blank of dental prosthesis which one covers with an enamel of composition suitable for an ion exchange, and which one quenchs the prosthesis coated in a bath of molten salt at a temperature between 200 C and the point of transformation of the enamel, so as to cause the exchange of alkaline ions close to the surface of the prosthesis for alkaline ions of larger size from the bath. '' 2. Method according to claim 1, characterized in that the molten salt bath is KN03 or a K2S04 / KC1 mixture. 2 3. Method according to claim 1, characterized in that the molten salt bath is a NaN03 / Na2S04 mixture. 4. Method according to claim 1, characterized in that the enamel is an aluminosilicate containing a percentage by weight on the oxides of 5 to 20% of AI203 and / or 5 to 15% of Zr02 and 5 to 20% of a light alkaline oxide. 5. Method according to claim 4, characterized in that the light alkaline oxide is Li20 or Na20. 6. Application of the method according to claim 1 to the reinforcement of a dental prosthesis element chosen from the group comprising a jacket crown, a ceramic or metallic crown or bridge. a facet with platinum studs and a porcelain tooth with studs.