STAIN-RESISTANT MICA COMPOSITIONS AND ARTICLES THEREOF IN PARTICULAR DENTAL CONSTRUCTS

Abstract: A thermally crystallizable glass consisting essentially in weight percent as calculated from the batch of 45-70%, SiO₂, 8-20% MgO, 8-15% MgF₂ 5-35% R₂O+ RO, wherein RO ranges from 0-25% and consists of one or more oxides selected in the indicated proportions from the group consist­ ing of 0-20% K₂O, 0-23% Rb₂O and 0-25% Cs₂O and wherein RO ranges from 0-20% and consists of oxides selected from the group consisting of SrO, BaO and CdO, 0-7% ZrO₂, 0-2% Al₂O₃ and 1-9% total of ZrO₂ + Al₂O₃ and 0-10% total of glass colourants crystallizable to a tetrasilicic fluorimica glass-­ ceramic material exhibiting excellent stain resistance. Articles having reduced stain resistance, in particular dental constructs made a glass-ceramic of the above com­ position are particularly useful.

Description: TITLE STAIN-RESISTANT MICA COMPOSITIONS AND ARTICLES THEREOF IN PARTICULAR DENTAL CONSTRUCTS The present invention relates to new materials useful for dental products and processes or the like, more specifically to new mica-based compositions for dental applications which exhibit improved resistance to staining in use. A major purpose of the dental profession is to replace or correct damaged or deformed tooth structure by fabricating and installing dental constructs. Among the dental constructs frequently employed are dental appliances, e.g., denture plates, bridges or orthodontic brackets, attachments for such appliances, and dental prosthetic devices, e.g., inlays,onlays, partial or full dentures, crowns or caps. All such dental constructs should be inert in the oral environment, be able to resist the forces of mastication, be capable of assuming a desired anatomical configuration, and exhibit aesthetic qualities similar to those of natural teeth. Few presently available materials simultaneously satisfy all of these requirements. Present dental constructs are customarily composed of metal alloys, porcelain, amalgam, acrylic polymers or combinations thereof. Metal alloys and amalgams are undesirable in situations where aesthetics are important because they sharply differ from teeth invappearance. Porecelain and acrylic polymers are too weak or brittle to resist masticatory forces in many locations. Composite constructs comprising, for example, a metallic substructure and a porcelain superstructure for appearance are extremely technique sensitive, and are too bulky in many situations. Thus prior dental constructs have offered at best a compromise among the various requirements. A recent and promising development in the field of dental treatment relates the use of mica glass-ceramics for the fabrication of dental constructs, as described by C.H. Pameijer et al. in Physical Properties of a Castable Ceramic Dental Restorative Material", AADR Abstract 327 J. Dent. Revs., 59 (march 1980) page 474. Glass-ceramics are semi-crystalline materials produced by the controlled crystallization in situ of glasses through an appropriate heat treatment, such materials having been first described in U.S. Patent No. 2,920,971. Because glass-ceramics are predominantly crystalline (more than 50% crystalline and frequently more than 90% crystalline by volume), they exhibit physical properties more closely akin to those of the predominating crystal phase than to those of the parent or precursor glass from which they are made. Thus glass-ceramics may vary widely in properties depending upon the particular crystal phases present therein. Among the recently developed glass-ceramic materials are the mica-containing glass-ceramics, first described in U.S. Patent No. 3,669,293. Mica-containing glass-ceramics demonstrate a relatively unique property which renders them particularly desirable for the fabrication of dental tools and constructs, in that they deviate from brittle behaviour in a manner permitting them to withstand point impact without fracture propagation. Thus mica glassceramic bodies can be indented in a point hardness test procedure wherein conventional porcelains are fractured. This behaviour is attributed to the fact that the mica crystals of the predominating crystal phase can flow plastically to some extent through transitional gliding along the basal or cleavage plane. The glass-ceramics of U.S. Patent No. 3,689,293, have been referred to as machinable glass-ceramics because they can be cut and shaped using conventional metal-working tools and procedures. They comprise a predominant crystal phase denominated fluorphlogopite solid solution (e.g. comprising fluorphlogopite (KMg3ALSi3010F2) and/or boron fluorphlogopite [EMg3BSi301oF2] crystals). Notwithstanding the excellent machinability of the above-described materials, C.ll. Pameijer et al. suggest in their aforementioned publication the use of tetrasilicic mica glass-ceramic compositions to produce dental restorative materials. This family of glass-ceramics, described in U.S. Patent No. 3,732,087, characteristically includes tetrasilicic fluormica (e.g., KMg25Si4O10F2) crystals as the predominant crystal phase. A further description of the use of these materials for dental applications is found in published European Patent Application 22,655. Tetrasilicic fluormica glass-ceramics differ from glass-ceramics containing fluorphlogopite and boron fluorphlogopite fluormica solid solutions in that both A1+3 and B+3 are eliminated from the crystal structure in favor of an added silicon (Si+4) constituent. By virtue of this replacement, tetrasilicic glass-ceramic materials exhibit somewhat superior chemical durability and mechanical strength. U.S. Patent Nos. 3,689,293 and 3,732,087 are expressly incorporated herein by reference and may be consulted for a complete description of these known types of mica glass-ceramic materials. Summarv of the Invention The present invention is concerned with an improvement in tetrasilicic mica glass-ceramics for dental construct fabrication and other uses, wherein a considerable enhancement in stain resistance is obtained. Stain resistance is obtained. Stain resistance is of prime importance for dental prostheses and the like, but is also desirable for applications such as culinary ware, wherein contact with staining media is anticipated to be frequent. In accordance with the invention, thermally crystallizable glasses for tetrasilicic fluormica glass-ceramic exhibiting greatly enhanced stain resistance are provided by adding minor controlled amounts of Al2 and/or Zero2 3 and/or ZrO2 to selected tetrasilicic fluormica compositions. Thus the invention comprises a thermally crystallizable glass and a stain-resistant tetrasilicic fluormica article, such as a dental construct, productd by the crystallization in situ of that glass. The glass and glass-ceramic articles of the invention desirably have a composition which includes in weight percent as calculated from the hatch for the glass, about 45-70% SiO2, 8-20% MgO, 8-1; MgF2, 5-35% R20+RO wherein R20 ranges from 5-25% and consists of one or more oxides selected in the indicated proportion from the group consisting of 0-20% E2 0.25% Rb20 and 0-25* Cs20 and wherein RO ranges from 0-20% and consists of oxides selected from the groups consisting of SrO, BaO and CdO, 0-7% ZrO2, 0-2* A1203 and 1-9Q total of ZrO2 + A1203. Optional constituents which are advantageously employed for dental applications include 0-7% Ti02 and 0-10% of conventional glass colourants. Of course, these compositions may additionally comprise other constituents known to be compatible with tetrasilicic fluormica compositions, including but not being limited to many of the Group II metal oxides and oxides of the metallic transition elements. Glasses within the above-described composition range may be crystallized in situ to glass-ceramic articles by a nucleation and crystallization heat treatment of the kind conventionally used for glass-ceramic manufacture. A suitable treatment comprises heating the glass to a nucleation temperature in the range of about 650-8500C for a time in the range of 0.25-10 hours, and to a crystallization temperature in the range of 800-12000C for a time in the range of about 1-100 hours. The invention may be further understood by reference to the single figure of the accompanying drawing which demonstrates the highly stain resistant character of glass-ceramic materials provided in accordance with the invention. An objective technique for evaluating the colour of teeth and candidate materials ior dental constructs has recently been described by D.G. Grossman et. al. in "Evaluation of the Color of a Cast ceramic Restorative Material", AADR Abstract 1094, J. Dent. Res., 59 (March 1980) page 542. That technique involves the colour analysis of light reflected fom a material sample utilizing a photometer such as the Chromascan shade scanner (commercially available from the Sterndent Corporation, Stamford, Conn). In the operation of this scanner, the intensity of light reflected from a material to be evaluated is measured in red, green and blue regions of the spectrum and the resulting values are used to calculate the hue (predominant colour), chroma (colour saturation) and value (overall lightness on a white-to-black scale) of the reflected light. The stain resistance of a candidate material can be objectively measured by carrying out the colour evaluation before and after subjecting the material to a staining treatment. Using the technique of the above-referenced Grossman et al. publication it was found that, within the normal colouration range established in human teeth, hue ranged from about 0.2-0.8, chroma from about 20-180 and value from about 350-800. It is desirable in developing a dental restorative material to provide reflected colour characteristics within these limits, and this can ordinarily be accomplished by adding colourant to the material. What is more important is that the material not exhibit large colour changes in use, hence the importance of developing a base material for construct fabrication exhibiting excellent stain resistance. Coffee is one staining medium which is of particular concern in developing dental restorative materials, being categorized as one of the more severe staining media insofar as dental and/or culinary application are concerned. One staining test which is presently relied upon to evaluate and compare the stain resistance of candidate materials is a coffee stain test wherein a sample of a selected material is exposed to coffee for a seven-day exposure interval at 800C. This test produces measurable changes in hue, value and chroma in many ceramic materials. The best stain resistance is accordance with the invention is provided with compositions containing both A120, and ZrO. The effect of small alumina and zirconia additions upon the stain resistance of glass-ceramic restorative materials as measured by tile coffee stain test was demonstrated by comparing the performan(e of six candidate glass-ceramic materials in tic test. The compositions of these materials are reported in Table I below, in parts by weight as calcuit'tid from the latch. The base constituents SiO2, EoO, MgO and MgF" total 100 parts so that their reported values correspond to weight percent of the base glass. The remaining constituents can be considered additions in excess of 1()0% to tlic base glass composition. It is noted that Examples i-b correspond, respectively, to Examples 1-3 except for the inclusion of a small amount of A1203 therein. TABLE I 1 2 3 4 - 6 Si02 60.5 64.0 61.1 60.5 64.0 61.1 K20 15.5 14.4 15.7 15.5 14.4 15.7 MgF2 10.5 9.7 10.4 10.5 9.7 10.4 MgO 13.5 11.9 12.8 13.5 11.9 12.8 Zr02 2.0 2.0 1.0 2.0 2.0 1.0 2 - - 3.0 - - 3.0 2 3 - - 0.5 0.5 0.5 Colourants - - 0.65 - - 0.65 All of the materials reported in Table I are tetrasilicic fluormica glass-ceramic compositions, containing SiO2, E20, MgF2 and MgO as essential crystal-forming constituents and in some cases TiO2 and colourants to control reflected colour. Glass-ceramics of these compositions were produced by forming molten glass of each specified composition into glass patties and converting the glass patties to highly crystalline glass-ceramics through heat treatment. Peak crystallization temperatures in the range of 107510900C and crystallization times of h hours were utilized. Plates approximately 0.25 inches in thickness were cut from the glass-ceramic patties and polished prior to testing. In conducting the stain test, each sample was first scanned for colour, then immersed in coffee at 800C for seven days, thereafter removed, rinsed in deionized water and re-scanned for colour, and finally colourscanned a third time after brushing for one minute with a commercially available dentifrice. The data from these tests are reported in Table II below. Included in Table II are hue (H), value (V), and chroma (C) results for each of the six samples, obtained prior to staining (initial) after staining and rinsing (stained/rinsed), and after brushing of the stained and rinsed samples (stained/brushed). TABLE II Sample Initial Colour Stained/Rinsed Stained/Brushed Colour Colour No. H V C H V C H V C 1 0.83 440 22 0.55 380 65 0.60 400 61 2 0.57 500 22 0.53 430 55 0.58 450 46 3 0.33 380 24 0.55 330 116 0.58 350 123 4 o.67 390 20 0.57 370 41 0.63 370 36 5 02 520 26 0.59 490 46 0.62 500 39 6 0.48 360 37 0.52 300 79 0.57 320 80 From a study of the data reported above in Table II, and of similar data recorded for glass-ceramic articles having te compositions of Table I but produced using alternative heat treatments and/or having different surface finishes, it appears teat although the stain resistance of compositions 1-5 containing ZrO2 alone would be acceptable in some cases alumina additions such as employed with compositions M-6 above further improve stain-resistance of the thermal history of tlic gla-('eramic material or surface finish thereof. Although, as suggests by the data in Table II, such additions appear to be most effective in reducing value and/or chroma shiits, stabilization is also observed in many cases. The colour stabilizing effect of these additions is diagrammatically shown in the drawing, which presents selected colour data from Table II relating to compositions 1 and 4, and also additional coffee stain test colour data for these same two compositions, but after crystallization at peak temperature for 12 hours rather than () hours. The graph plots hue, value and chroma data on the vertical axis against stain test stage on the horizontal axis, the test stages shown being the initial (1), stained/rinsed (S/R) and ;tained/bruslled (S/ll) Stages of the test. For examples 1 and 4 of Table I after crystallization in situ for six hours (G-hr. HT) and twelve hours (12-hr. IIT). Although compositions containing both A1203 and ZrO2 provide the best stain resistance in accordance with the invention, significant improvements in resistance to staining can be achieved using ZrO2 alone. A first composition consisting of about 605k SiO2 13.5% g20 14.5% MgO and 11.5% M8F2 by weight was subjected to the coffee stain test along with a second composition containing the same constituents in the same proportions +2.5% by weight excess ZrO2 both compositions having received an identical heat treatment. Whereas the first composition exhibited light staining in the stain test, the second composition exhibited no staining. This occurred not withstanding the fact that the acid durability of HTE second composition, as measured by sample weight loss after 24 hours at 95by. in a 5% HCC solution, was slightly lower than that of the first composition. Based in part on data such as reported above, a preferred range of compositions for thermally crystallizable glasses and stain-resistant tetrasilicic mica glassceramic articles has been defined, based on mica compositions particularly suitable for dental constructs and comprising ZrO2 and preferably A1203 for stain resistance. These compositions consist essentally, in weight percent as calculated from the batch, of about 45-70% SiO2, 8-20% MgO, -15k MgF2, 5-20k K20, 0-2% and preferably 0.05-2% A1203, 0.5-7% Zero2, 1-9% total of ZrO2 + A1203 0-7% TiO2, and 0-10% total of conventional glass colourants. Such colourants can include manganese oxide, iron oxide, nickel oxide or any of the other compounds or elements having known utility for colouring glass. The presence of zirconia in these compositions is particularly important not only to obtain enhanced acid durability, but also to control the translucency of the glass-ceramic product and to inhibit recrystalization in the glass-ceramic at high crystallization temperatures. Although the mechanism by which coffee and other staining media impart permanent stains to tetrasilicic mica glass-ceramics is not well understood, it is believed that factors other than composition, including the thermal history of the glass-ceramic and the nature and extent of crystallization thereof, may also play some role in governing the ultimate stain resistance of these materials. Nevertheless, the effectiveness of small zirconia and alumina additions to enhance durability in a given composition, these other factors remaining constant is believed to constitute a significant improvement in tetrasilicic fluormica compositions for dental construct and other applications.

Claims: CLAIMS: 1. A thermally crystallizable glass consisting essentially in weight percent as calculated from the batch of 45-70% SiO2, 8-20% MgO, 8-15% MgF2 5¯35% R20 + RO, wherein RO ranges from 0-25% and consists of one or more oxides selected in the indicated proportions from the group consisting of 0-20% E20, 0-23 Rb20 and 0-25% Cs2O and wherein RO ranges from 0-20* and consists of oxides selected from the group consisting of SrO, BaO and CdO, 0-7% Zr02, 0-2% A1203 and 1-9% total of Zero, + A1203 and 0-10% total of glass colourants crystallizable to a tetrasilicic fluormica glass-ceramic material exhibiting excellent stain resistance. 2. A stain resistant tetrasilicic fluormica glassceramic article having a composition as specified in claim 1. 3. An article as claimed in claim 2 in the form of a dental construct.

Inventors: GROSSMAN, DAVID GARY
Applicants: CORNING GLASS WORKS
Application year: 1982
Main classification: C03C10
IPC codes: C03C10, A61K6