Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2000-06-14
2002-09-10
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S402200, C428S402210, C428S404000, C428S405000, C523S214000, C523S216000, C523S217000
Reexamination Certificate
active
06447907
ABSTRACT:
The present invention relates to inorganic or possibly organically modified particles which can be subjected to targeted leaching out of certain cations and can thus be used as inorganic components in glass ionomer cements. The invention further relates to novel ways of producing such particles, known as ionomers, and to the cements which can be produced therewith.
For the purposes of the present invention, the term “ionomer particle” refers to inorganic particles which in combination with a preferably acid-containing matrix can be used in a very versatile way as cements (self-curing, light-curable, etc.). For a cement formation reaction to be able to take place at all, these particles have to have a defined or targeted instability, i.e. when combined with water in the presence of a partner with which they are to combine they have to release metal ions which lead to a curing reaction in the partner substance. The composition ranges in which this targeted instability occurs are known to those skilled in the art or can easily be determined, see, for example, the phase diagrams of many systems in “Alumosilicate Glasses for Polyelectrolyte Cements”, A. D. Wilson et al., Ind. Eng. Chem. Prod. Res. Dev. 1980, (19) 263-270, or “Glass-Ionomer Cements”, A. D. Wilson et al., 1988, Quintessence Publishing Co., Inc., Chapter 2 (p. 21 ff.).
The basic makeup of the usually vitreous ionomers usually comprises the ternary system silicon dioxide-aluminum oxide-calcium oxide. Melting these components together gives particles which undergo a two-stage reaction in the presence of, for example, polyalkenoic acids. Here, calcium ions are firstly leached from the glass composite by the attack of protons of the polyalkenoic acids and are complexed by the carboxylate groups of the polyalkenoic acids in an unstable phase or primary curing. Secondary curing then leads to a stable phase in which aluminum cations now also migrate from the glass ionomer. Hydration of the polysalts occurs and aluminum polyalkenoates are formed. At the same time, the outer shell of the aluminum silicate glasses is dissolved by proton attack to form orthosilicic acid. During the further course of the reaction, this orthosilicic acid condenses to form silica gel; a gel layer results.
Some of the reactions described are very slow; however, the reaction can be accelerated by addition of fluoride, for example in the form of fluorspar. Hydroxycarboxylic acids, e.g. tartaric acid, can be added as regulators; they lengthen the processing time and shorten the curing time. Further additives such as aluminum phosphate, cryolite or aluminum trifluoride are known as optimizing processing aids.
The known ionomer particles are obtained by melting together the respective starting compounds (mainly oxides) . Their microstructure is complex. The milling process to which the fused glass ionomer is subjected in order to obtain the desired particles promotes the formation of sharp-edged, nonspherical particles. This makes the resulting abrasion resistance of the ionomer unsatisfactory. The particles formed have a broad particle size distribution and are relatively large; they usually have a diameter far above three microns. They have to be subjected to a complicated classification process in order to be obtained in a size distribution which is still only acceptable to a degree. Apart from the high expenditure of effort, this means a high loss of material and thus extremely poor yields (even when using a plurality of screening steps or air classification steps, a distribution over less than at least one power of ten is not achieved, if only because of the unfavorable geometry) . As a result of the fusion procedure, the aluminum silicate matrix is frequently not homogeneous. Thus, for example, fluorides are embedded in the form of droplets rich in calcium fluoride.
Classical glass ionomer cements having purely inorganic curing, light-curable glass ionomer cements (with additional organic polymerizable components) and compomers (the term is derived from the contraction of the expressions composite and ionomer and is used to refer to cements in which the carboxyl group used is bound to the same molecule which also bears a crosslinkable double bond, see, for example, “Glasionomers, The next Generation”, Proc. of the 2nd Int. Symp. on Glass Ionomers, 1994, p. 13 ff.), are frequently used as filling material, especially in dentistry. Partners employed for curing (cement formation) are usually the polyalkenoic acids mentioned. Advantages of these materials are: little or no shrinkage through to an expansion caused by the ionomer reaction as a result of water uptake, presence of fluorides and phosphates desired, good bond with the tooth tissue due to the acid groups in the matrix. However, use in dentistry would also require excellent mechanical properties, a favorable abrasion performance (e.g. during chewing) and good polishability of the fillings. These requirements are not, however, met by the cements mentioned owing to the size and shape of the ionomer particles. Thus, the sharp edges, the size and asymmetry of the particles result in those in the positions near the surface being torn from the cement composite during chewing or polishing, which increases abrasion and makes it virtually impossible to achieve a smooth surface. Additional disadvantages are the lack of ability to be adapted to special problems such as X-ray opacity or specific requirements in respect of transparency, e.g. index of refraction or the like.
It is therefore an object of the present invention to provide ionomer particles of the type mentioned at the outset which do not have the abovementioned disadvantages and in combination with any, preferably acid-modified matrix systems lead to cements which are mechanically more stable than the known particles.
This object is achieved by the provision of ionomer particles which have a spherical or approximately spherical shape.
The ionomer particles are either purely inorganic particles, or they can be organically modified.
The novel ionomer particles preferably have a diameter smaller than that which is presently customary. The particle size can be set, for example, in the range from 5 nm to 50 &mgr;m. This can be achieved using various, simple methods, which is explained in more detail further below.
The spherical ionomer particles of the present invention have an inner region and also an outer region which comprises silicon ions and whose cations comprise (a) at least one element which in siliceous compounds can occupy lattice sites of silicon to produce a negative charge excess and (b) at least one element which can compensate the negative charge excess and is selected from among elements of main groups I and II and elements which can occur in divalent form. The cations of group (b) serve to effect the primary curing in the unstable phase, while those of the group (a) serve to effect secondary curing and the formation of the stable phase.
The expressions “silicon ions” and “cations” indicate that the elements concerned are present in bound form, but are not intended to rule out their incorporation in structures having some degree of covalent bonding.
The particles can further comprise appropriate additives, for example phosphate (e.g. as aluminum or calcium phosphate) or fluoride (e.g. as NaF, CaF
2
or AlF
3
).
The spherical ionomer particles preferably contain cluster-like, silicate-containing regions. In a further preferred embodiment of the invention, the ionomer particles are entirely homogeneous. In a third preferred embodiment, they consist of an inner region or core which can have a composition different from that of the outer region (the “shell”), in which case the shell can consist of one or more layers. However, it is in each case essential for the outer region, i.e. at least the outermost layer, to have the abovementioned composition. If the structure with an inner region different from the outer region is chosen, this can, if desired, be inert toward leaching and serve as carrier for further properties.
In addition, th
Gellermann Carsten
Wolter Herbert
Foley & Lardner
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Kiliman Leszek
LandOfFree
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