ITMI20131007A1 - DENTAL PROSTHESES - Google Patents

Description

The present invention relates to dental prostheses and the process for their preparation.

More specifically, the invention relates to dental prostheses that are made starting from a predefined model which has the characteristic of being able to be used for the preparation of any type of dental prosthesis.

It is well known that numerous long and complex steps and procedures are required for the production of dental prostheses. The traditional technique used to obtain dental prostheses requires numerous operations that involve heavy time and cost commitments both for the patient and for the medical and dental technicians. For example, we usually proceed with a first appointment with the patient in the dentist's office, in which the dental arches are taken using a preparation consisting of alginate and / or an elastomer using aseptic wrapped metal spoons. We then proceed to choose the color of the teeth to be inserted in the prosthesis, making sure that it is identical to that of the patient's teeth. The obtained impressions are then sent to the dental laboratory where plaster models of the upper and lower arch are prepared. Once the plaster has hardened, the models are mounted on an articulator. At this point, a wax prosthesis is built with the teeth of the prescribed shape and color. The product thus obtained is melted, for example in a flask, to obtain a rough prosthesis. The dental technician roughs out the dental prosthesis, finishes it, polishes it and tries it on the articulator, taking care to fit the prosthesis for chewing. The dentist makes the necessary changes to the dental prosthesis thus obtained, using for example small bench drills, to adapt the dental prosthesis to the patient's mouth. If the changes are more substantial, it is generally preferred to repeat the procedure by preparing a prosthesis from scratch. In conclusion, the traditional technique requires quite a long time. In addition, each prosthesis must be prepared case by case by the dental technician.

Patent US 5,304,063 is known in the art, which describes a denture adapted to be inserted on a toothless gum and on the surfaces of the tissues surrounding it, and a method for producing and inserting complete dentures which include a thermally deformable base. The denture support has an inner surface and an outer surface and is obtained by a molding process from a mixture of pasty consistency formed by a plasticized liquid monomer and a methyl methacrylate polymer powder in a 1: 3 ratio. -3.5. When heated to a temperature above approximately 135 ° F (~ 57 ° C) the denture holder becomes malleable and can be molded into the patient's mouth to adhere the denture holder to the gum surface. The denture holder can then be assembled into a complete denture by crosslinking an uncured resin liner on the inside surface of the holder, then inserting the teeth on its outside surface. In particular, the patent claims a method for producing and adapting a modular prosthesis to the patient's mouth, comprising the following steps:

preparation of the prosthesis support as defined above;

heating the denture support to temperatures above 135 ° F (57 ° C) to make the support malleable; insertion of the prosthesis holder into the patient's mouth; applying pressure to adapt the support to the toothless gum and surrounding tissues of the patient's mouth;

removal of the prosthesis holder from the patient's mouth;

adaptation of an uncured resin liner to the internal surface of the prosthesis support; inserting the prosthesis holder into the patient's mouth by applying pressure to create a final detailed impression of the toothless gingiva and surrounding tissues of the patient's mouth;

removal of the denture holder from the patient's mouth;

crosslinking of the resin liner;

insertion of teeth into the outer surface of the denture holder.

Heating to a temperature of 57 ° C can be obtained by immersion in a warm water bath. Curing of the liner can be achieved by curing by curing by various methods, including light, heat or chemical reagents. The bond between the liner and the reticulated support of the prosthesis is such as to stabilize the support so that it is not deformed when the patient ingests hot food or liquids. The description states that the teeth can be affixed to the holder before or after the prosthesis holder has been adapted to the patient's mouth.

This patent requires the preparation of a support adaptable to the toothless gingiva and the need to have a liner to attach the prosthesis to the support.

US Patent 5,775,900 describes a transparent stent for diagnostic or surgical use, thermally deformable, which can be inserted on a partially or totally toothless gum and surrounding tissues, and a method for producing and adapting stents using their characteristics. of heat deformability. The patent also relates to a kit comprising a pair of stents having the characteristics defined above, one of which is the footprint of the other, and a temporary prosthesis, heat-deformable like the stent, and a method for producing and adapting the temporary prosthesis using its heat deformability characteristics. The stent and the prosthesis are prepared starting from a pasty mixture having the same composition as that used in US patent 5,304,063. When the mixture is heated to temperatures above 120 ° F (49 ° C) the stent and prosthesis become malleable and can be molded in the patient's mouth or on a corresponding model to adapt to tissue surfaces. More particularly, the invention refers to the field of prosthodontic implants in which the teeth are reinserted into their place by means of a fixed or removable implant. To make these implants, a surgical and / or radiographic stent of transparent acrylic resin is used, which in the molded form maintains a high degree of accuracy. Furthermore, using the kit, the dental technician (dental practitioner) can prepare a surgical and / or radiographic stent and an interim or temporary prosthesis that is perfectly aligned and functioning, to take impressions of both people with and without teeth in order to prepare prostheses. permanent. The purpose of this patent is to eliminate the various steps in the clinic and in the laboratory in the process of preparing surgical / radiographic stents for implants and for temporary prostheses. This is accomplished by preparing several stents, in which the base size is small, medium or large, respectively, with different tooth arrangements on the base. The stent resin can be immersed in water at 120 ° -160 ° F (49 ° -71 ° C) to make it moldable. Once formed and cooled, the base remains stable. The position of the teeth and the esthetics can be verified and corrected if necessary, because the resin stent can be heated, formed and adjusted many times. The clear acrylic resin base can be colored to make it similar to the gum tissue. Interim or temporary prostheses, such as stents, can be immersed in water at 49-71 ° C to make them formable. The denture can then be formed to the correct anatomical configuration of the patient. The teeth are also made of acrylic material and mounted on the base. The kit described in this invention is used for diagnostic purposes, see the block diagram of Fig. 4A of the patent, step 8 which refers to Fig. 4B. As mentioned above, temporary prostheses can also be prepared for diagnostic use to correctly position the teeth of the upper jaw in correspondence with those of the lower jaw and check if they close well, i.e. check their height. Another diagnostic use is that of surgical guides to determine where to insert the implants, to insert a new tooth when the natural one has been removed. Another diagnostic use is for radiological purposes in which a marker is contained in the base of the stent, to place the implants. In conclusion, this patent does not describe permanent prostheses suitable for chewing but only serves to prepare surgical guides for the precise insertion of the implants. On the other hand, these kits are well known to the expert in the field.

Patent application US 2004/0248065 relates to embodiments comprising either a lower denture, or an upper one without palate, or a set of dentures containing both the lower and upper dentures. The denture comprises a layer of a reline material. These realizations are adapted to the mouth of the single individual by immersion in lukewarm water to soften the reline material. Generally, the temperature of lukewarm water is higher than the ambient temperature (18 ° -21 ° C, corresponding to 66-71 ° F) but lower than 100 ° C (212 ° F); or the water temperature can vary from 45 ° C (112 ° F) to 80 ° C (176 ° F) or from 51 ° C (125 ° F) up to 58 ° C (135 ° F). The immersion times of the material in water, in order to soften the material of the reline, vary from 2 to 10 minutes. Generally the water temperature can vary from 38 ° C (100 ° F) to 95 ° C (204 ° F) The thickness of the reline material is in the range from about 1 to about 5 mm, preferably from 1.5 to 3 mm. When the reline material has softened, the prosthesis fits inside the mouth and a force is exerted on the dentures for a few minutes. In this time interval, the reline material conforms to the mouth and gum of the individual, thus avoiding the use of impressions and having to resort to the dentist for the adaptations of the prosthesis. This patent application relates to materials or reline products well known in the market, such as those sold under the brand name Hydrocast®, which are inserted into the internal part of the prosthesis to form an adhesive pad to hold the prosthesis in place. This patent application therefore describes a reline material for making a prosthesis adhere to the gingiva so as to make it fixed.

The need was felt to have available dental prostheses adaptable to casts of gums overcoming the disadvantages of the known art and that they could be obtained through a simplified process, which substantially reduced the number of rework necessary to adapt the prostheses to the different casts and it also allowed to avoid the formation of scraps in the event that the dental prosthesis prepared by the dental technician is not suitable for dental use.

The Applicant has unexpectedly and surprisingly found the solution to the technical problem indicated above as specified below.

An object of the invention is dental prostheses obtainable starting from a model prosthesis, obtainable by molding a polymer starting from a cast of a dental arch, and subjecting the model prosthesis to heat treatment at temperatures between 105 ° C and 250 ° C ° C at atmospheric pressure, the model prosthesis being such as to satisfy the following test:

a strip of the polymeric material that constitutes the model prosthesis, having dimensions 6X6X80 mm, takes the external shape of a metal cylinder having a diameter of 30 mm and having a concavity with a radius of curvature 12 mm and a center of curvature with Cartesian coordinates (15 mm, 15 mm), and a convexity with a radius of curvature 8 mm and a center of curvature with Cartesian coordinates (8 mm, 8 mm) (Figs.1a and 1b), after having been subjected to the heat treatment indicated above, the strip being made to adhere to the cylinder and reproduce its external surface without showing fractures or cracks (Figs. 2a and 2b) by applying a force of up to 1 kg weight,

the prosthesis obtainable after heat treatment, modeled on the cast and cooled to room temperature, maintains the shape of the cast;

the prosthesis after heat treatment being subjected to further cycles of heat treatment, modeling and subsequent cooling for at least 20 times to obtain further prostheses.

The strip of polymeric material that constitutes the model prosthesis can be obtained by molding in a suitable mold having suitable dimensions.

After the test has been carried out, the strip, as mentioned, must not show cracks or cracks, in other words the strip after it has been made to adhere to the cylinder must maintain the same characteristics as the initial one.

If the polymer strip of the model prosthesis passes the test, that is, it is able to mimic the concavity and convexity of the cylinder on which it is adhered, the model prosthesis can be used as a dental prosthesis of the invention after having subjected it to the treatment. thermal, shaping and cooling as indicated above.

Generally the force exerted to apply the strip to the cylinder varies according to the temperature and the duration of the heat treatment. Generally a force between 0.01 and 1 kg weight is sufficient to carry out the test. This type of force can generally also be applied using the hands.

The heat treatment time is established according to the temperature, the higher the temperature, the shorter the treatment time. The heat treatment is carried out for a time from 0.5 to 10 minutes, preferably from 1 to 5 minutes, more preferably from 2 to 4 minutes.

Preferably the temperatures are comprised between 110 ° and 180 ° C, more preferably 120 ° and 170 ° C for a time preferably from 1 to 5 minutes, more preferably from 2 to 4 minutes.

The heat treatment at the indicated temperatures can preferably be carried out in an oven. The treatment can also be carried out in water. In this case we work under pressure to reach temperatures within the ranges indicated above.

By modeling the prosthesis on the cast we mean the operation with which the model prosthesis after heat treatment for the required time and temperature, is adapted to the cast by applying a force preferably not exceeding 1 kg weight, as indicated in the test described above for the strip. Force is preferably applied with the hands. Generally, the modeling is carried out as soon as the heat treatment of the model prosthesis is finished. Generally, modeling is generally started within 1 minute, preferably within 30 seconds of taking it out of the oven. Generally the modeling is carried out in a time not exceeding 5 minutes, preferably in 2-5 minutes.

By model prosthesis, as mentioned, we mean any prosthesis obtainable by molding a polymer that satisfies the test indicated above starting from a cast of gums. It is well known that any prosthesis can be obtained preferably with the following procedure:

the gum is taken with a spoon containing a formable material made up preferably of silicone, also known as female;

in the laboratory, a plaster cast is obtained from the female, which reproduces the gum. Generally, plaster is poured into the impression. This constitutes the plaster cast;

the wax is mounted on the cast and the prepared teeth are fixed, obtaining the wax prosthesis;

from the wax prosthesis the polymer prosthesis is obtained by immersing the wax prosthesis in a liquid silicone rubber, which is hardened; at this point the wax is detached and the polymer in the liquid state is poured, which replaces the wax. In this way, after the polymer has hardened, the model prosthesis is obtained.

The model prosthesis can be obtained with any other process known in the art.

Once the silicone mold is obtained, inserting the teeth into their seat in the mold, the polymer resin is poured in a liquid state, it is left to harden and copies of the prosthesis are obtained.

As mentioned, once the prosthesis has been subjected to heat treatment, modeled according to the shape of the cast and cooled, it maintains the shape of the cast. It has been surprisingly and unexpectedly found that the resulting prosthesis shows neither retraction nor dilation. This is totally unexpected and surprising as the prosthesis is made up of thinner sections and thicker sections. It is also unexpected and surprising that even the teeth of the model prosthesis do not undergo alterations and do not detach from the polymer of the prosthesis during the phases of heat treatment and shaping of the prosthesis on the cast.

As mentioned, the prostheses of the invention can also be subjected to 20 or more cycles, even up to 50 of heat treatment, modeling on cast and subsequent cooling after each treatment, to obtain further prostheses having different shapes using different casts, in turn resulting from different footprints.

Therefore the prostheses made according to the invention can be adapted to different casts. This represents an advantage as the dental arches of an individual, and therefore also the relative casts, change over time and moreover each person has a different cast from another.

Generally the plaster cast can be obtained from an impression that reproduces the shape of a gum as described above, or it can be prepared from scratch in the laboratory starting from a pre-existing cast and making a silicone mold of the type described above.

All the prostheses made according to the invention are new compared to those of the known art, as the casts obtained from the dental impressions are different from each other, as the casts used as a support for the modeling are obtained from unique and different footprints.

The model prosthesis could also be made by duplicating an existing prosthesis by making a silicone mold of the type described above.

It has been surprisingly and unexpectedly found by the Applicant that a model prosthesis immersed in hot water having a temperature up to 100 ° C, or in an oven under the same temperature conditions, does not allow to obtain the prostheses of the invention as a The polymer strip of the model prosthesis is unable to reproduce the outer surface of the cylinder used in the above test.

Here is a description of the attached figures. Fig. 1a represents a perspective view of the metal cylinder used in the test, having a diameter of 30 mm and having on the side surface respectively a concavity with a radius of curvature 12 mm and a center of curvature with Cartesian coordinates (15 mm, 15 mm), and a convexity with a radius of curvature 8 mm and a center of curvature with Cartesian coordinates (8 mm, 8 mm).

Fig. 1b represents a section view perpendicular to the longitudinal axis of the cylinder used in the test of Fig. 1a.

Fig. 2a represents a perspective view of the cylinder of Fig. 1a with the polymer strip of the prosthesis of the invention reproducing the surface of the cylinder including the convexity and concavity.

Fig. 2b represents a section view perpendicular to the longitudinal axis of the cylinder with the polymer strip of the prosthesis of the invention.

Fig. 3 is a model prosthesis as it is applied to a plaster cast of a dental arch. It can be seen that the model prosthesis does not reproduce the shape of the cast.

Fig. 4 shows the prosthesis of the invention applied to the plaster cast of Fig. 3. It can be seen that in this case, after the heat treatment, the shaping of the prosthesis on the cast and cooling to room temperature, there is a perfect coupling between the cast and prosthesis.

Generally, the model prosthesis basically consists of one or more polymers. Preferably, one or more thermoplastic polymers are used provided they satisfy the test indicated above. Optionally, said one or more polymers, preferably thermoplastic, can be partially cross-linked with the condition that a strip of said polymers is such as to satisfy the test indicated above.

As cross-linking agents, the ones reported further on can be used.

Additives can be added to said polymers, such as fillers, crosslinkers, reinforcing agents, plasticizers, lubricants, stabilizers, pigments, anti-adhesives, antistatic agents, anti UV agents, antioxidants, polymerization inhibitors, etc.

The polymers of the present invention can be preferably selected from the classes of thermoplastic polymers indicated below by way of example. Thermoplastic polymers belong to the class of synthetic resins and derive from the union of several molecules of one or more chemical substances. The polymers can be obtained by addition polymerization or by polycondensation. By addition polymerization we mean the reaction in which a more or less high number of molecules of one or more monomers, the same or different, bind together to form a polymer. A necessary condition for this addition reaction to take place is that the monomer has only one unsaturation of the ethylene type according to the present invention.

By polycondensation we mean the reaction in which several molecules of one or more monomers, with two reactive functional groups, bind together to form a macromolecule with the elimination of other substances, such as water.

Copolymers are polymers in which the monomers are different from each other and the monomer units follow each other, in the polymer chain, more or less randomly or with a certain order. Considering the case of polymers obtained from two comonomers, it is possible to have the following types of copolymers: copolymers with random succession when the two monomer units follow each other at random, ie without an ordered distribution; block copolymers when there are blocks of rather long sequences of one monomer that follow one another. Copolymers with alternating succession are known when the two monomer units succeed each other alternately. This class of polymers is formed in polycondensation reactions, but it can also be obtained from polymerization reactions, at least with long sequences in alternating succession.

Grafted or graft copolymers are polymers consisting of a long chain of a polymer onto which polymer chains formed by another monomer are grafted. In polycondensation the polymer is formed gradually, starting from two bifunctional monomers it can be admitted that first a simple condensation reaction takes place between them with the formation of a dimer which, having two functional groups, respectively arranged at the head and tail, can yet to react with another monomer molecule forming a trimer, then with yet another forming a tetramer, and so on, continuously increasing the molecular weight. If high molecular weight products are to be obtained, polycondensation must proceed efficiently and that there are no foreign substances or impurities that could produce secondary reactions capable of interrupting the elongation of the chain.

Polycondensation is carried out by mixing the monomers with the possible catalyst in reactors well known for this type of polymerization. It operates in controlled conditions of temperature and environment, sometimes even pressure.

The reaction can be carried out in an aqueous medium, in solvent or in mass, that is, keeping the substances in the molten state. When the degree of polycondensation has reached the desired value, the reaction is stopped, generally by simple cooling.

The polymerizations are chain reactions and have the following general course: a molecule of the monomer M is activated by a catalyst (activation phase); the activated molecule, which becomes very reactive, combines with another monomer molecule to form a dimer which retains the activation and is therefore able to react with another monomer molecule forming a trimer, and so on ¬ via (propagation phase). The process continues until a cause intervenes that deactivates the molecule (termination phase).

This chain development proceeds very quickly so that a situation different from that of polycondensation occurs in the polymerization. In the latter, in fact, the reaction proceeds with all the molecules of the monomers; therefore, if the process is stopped at a certain moment, there are many polymeric molecules having a slightly different molecular weight. On the other hand, in the polymerization the reaction mixture contains polymers together with a certain amount of monomer molecules that have not yet reacted.

The active center that triggers the initial phase can be an ion (cation or anion) or a radical. In fact, there are radical type polymerizations and ionic type polymerizations (cationic and anionic).

Radical polymerization occurs when the active center is made up of a radical, that is, a group with a free valence, containing a valence electron or unpaired electron. In radical polymerization there are three stages, respectively initiation, propagation and termination, as indicated below:

The initiation or activation phase occurs in the presence of a substance (catalyst) capable of giving rise to a radical. For example, organic peroxides and hydroperoxides have this property which, by heating, decompose, originating a radical. The radical, being unsaturated, reacts rapidly with a monomer molecule, forming a radical with a molecular weight greater than the previous one. The reaction proceeds with the shift of the unpaired electron so that the radical active center moves to the external carbon atom.

In the propagation phase, the radical thus formed reacts with another monomer molecule forming a longer radical, and so on. The reaction proceeds with rapid development of the chain. There is always the slip of the unpaired electron, that is the electron of the active center, on the carbon atom at the extremity of the polymer chain.

The lengthening of the polymer chain does not proceed indefinitely because different causes always intervene which interrupt the radical succession. These are above all the combination of two radicals, in which there is the coupling of the two terminal unpaired electrons of the two chains; or the combination of a radical of the initiator with the chain. In the latter case, the coupling of the unpaired electron of the propagating chain with the unpaired electron of the initiating radical takes place; or the transfer of a hydrogen atom, or disproportionation. The termination phase then takes place.

Radical polymerizations can also occur cold, by operating with initiators that decompose under the action of energy sources other than heat. For example azodiisobutyronitrile can be used as photoinitiator as it has the property of decomposing into radicals even at low temperatures by irradiation with ultraviolet light; redox initiators can be used, ie redox systems, with which the decomposition of the peroxide is caused by a reducing agent. A redox system used in emulsion polymerizations is for example made up of hydrogen peroxide and ferrous ions. In the presence of a monomer, the ∠™ H radicals deriving from hydrogen peroxide initiate polymerization. Another example of a redox initiator is the persulfate ion in the presence of a reducing agent such as thiosulfate or bisulfite. The ∠™ SO4- radical can in turn react with water providing ∠™ OH and ∠™ HSO4- radicals, which act as polymerization initiators.

In cationic polymerizations the active center is constituted by a positive electric charge, that is, by a group with a cation character. Also in this case the reaction proceeds through the three stages indicated above.

In the initiation phase, cationic polymerizations take place in the presence of initiators (catalysts) capable of easily releasing a cation. Such are the so-called Lewis acids such as BF3, SnCl4; AlCl3, AlBr3, TiCl4 and some strong acids such as H2SO4 and HF. To start the polymerization Lewis acids require the presence of a cocatalyst such as water, alcohol, acetic acid.

In the propagation phase the formed ion reacts with another monomer molecule giving a dimer cation and so on. The reaction proceeds rapidly by addition of successive monomer molecules to obtain a polymeric ion.

The termination phase occurs because various types of factors intervene that produce the arrest, especially the following: the transfer of a polymer proton to a monomer molecule with corresponding start of a new polymer chain or the transfer of a proton from the polymer to the anion of the catalyst.

Unlike the radical process, the cation process allows very fast polymerizations to be carried out even at very low temperatures, obtaining a molecular weight of several million in a few seconds.

In anionic polymerizations, various monomers can be polymerized in the presence of substances capable of giving rise to an initiator anion, such as for example sodiumamide NaNH2, potassiumamide KNH2 or metal alkyls. The reaction begins with the formation of a negative ion by the catalyst, then the anion reacts with a first molecule of monomer and then there is the propagation phase.

The polymerization takes place in the homogeneous phase or in the heterogeneous phase: in the homogeneous phase according to the mass and solution methods, in the heterogeneous phase according to the emulsion and suspension methods.

In mass polymerization, the gaseous monomers and liquid monomers (for example methyl methacrylate, styrene) polymerize without the addition of solvents and operating under the appropriate conditions of temperature and pressure and in the presence of the catalyst.

During solution polymerization, the monomer is dissolved in a solvent and the catalyst is added to the solution. Eventually the polymer can be separated by evaporating the solvent and uncured monomer, or by precipitating the polymer by adding a suitable precipitating agent.

In emulsion polymerization, water-insoluble monomers such as vinyl monomers, styrene, etc. they are polymerized in aqueous emulsion in the presence of suitable emulsifying agents. The monomer is mixed with water and the emulsifying agent is added so that an emulsion of the monomer is formed in the aqueous mass. Peroxides or redox initiators are added as initiators. At the end of the polymerization an emulsion of the polymer is obtained, called latex, from which the polymer can be obtained by coagulation by adding an electrolyte. In this system the reaction proceeds in a practically isothermal way because it operates in the presence of sufficient quantities of water to disperse the heat of reaction; furthermore, high polymerization speeds, high and constant molecular weights are achieved.

The polymerization in suspension or in pearls (powder) takes place when the emulsifying agent is in small quantity or is absent. In these cases an aqueous suspension is obtained. The monomer collects in beads (pearls) with a larger diameter than in the case of the emulsion. The dimensions of the pearls can also be of the order of a few millimeters, and are held in suspension by mechanical agitation. To prevent the pearls from joining together, small quantities of stabilizers are added such as gelatin, starch, bentonite.

At the end of the polymerization, a product is obtained having the shape of regular spheres, easily separable from the aqueous suspension. The reaction proceeds with less speed than the emulsion polymerization, but there is the advantage of obtaining high purity polymers, since in the emulsion polymerization the product remains more or less polluted due to the numerous auxiliary substances introduced.

The polymerization process has a statistical nature as not all monomer molecules polymerize with the same speed and the polymer chains do not have the same length. For this reason, the resins obtained are never made up of a single chemical compound but of a mixture of polymeric chains having different degrees of polymerization. Since many physical and chemical properties depend on the degree of polymerization, it is sometimes necessary to proceed to a separation or fractionation, to divide the product obtained into fractions having each molecular weight included in a narrow range, or to isolate the fraction having the molecular weight desired. For fractionation, the principle is usually applied that generally the solubility in a given solvent decreases as the molecular weight increases. The method consists of a fractional precipitation from a dilute solution of the polymer, using a non-solvent substance as a precipitant. In this way, the less soluble fractions, which are those with the highest molecular weight, precipitate preferably.

Another method for fractioning polymers consists in treating the product obtained from the polymerization with a solvent in order to favor the solubilization of the low molecular weight fractions. This method is called fractional extraction.

Even if the various synthetic resins have in common, for example, the magnitude of the molecular weight, they nevertheless have a complex of properties that allow them to be differentiated from each other and to direct them towards different and specific sectors of use.

Plastics are produced in the form of powders or granules, in tablets, beads, cylinders, etc. For the preparation of the various plastic objects (molding or shaping) they are mixed with some auxiliary ingredients, then formed using different machines depending on the type and the object to be prepared.

Auxiliary ingredients are substances that are added and mixed with resins prior to molding, and are sometimes already incorporated into molding powders. Auxiliary ingredients include the additives indicated above and defined here in detail.

Fillers or fillers are used to improve mechanical and electrical properties. They are used in percentages up to 50% by weight of the formulation. The fillers are of both organic and inorganic origin. Among the former we can mention wood flour, cotton, jute or cellulose fibers; among the inorganic fillers, diatomaceous earth, mica, graphite, glass. Organic fillers improve some mechanical properties, in particular resilience, while they slightly worsen the tensile and flexural strengths and do not change the heat resistance and electrical properties. Inorganic fillers, with the exception of those having a fibrous structure, worsen the mechanical properties more markedly but improve the electrical and thermal properties. Fillers are rarely used in thermoplastic resin technology.

Reinforcing agents are materials with a fibrous structure, such as cotton, but especially glass fibers, felt, short fibers, fabric, etc. The so-called reinforced plastic materials and plastic laminates are prepared with the reinforcing agents, in which these additives act as a support for the resin and significantly increase its mechanical properties.

Plasticizers are used in thermoplastic polymers to decrease stiffness and increase flexibility. The most commonly used plasticizers are for example the phosphates of higher alcohols, such as trioctyl phosphate and tricresyl phosphate, or the glycerin esters such as diacetate and the corresponding triacetate, the esters of polyethylene glycols, oleates, stearates, ricinoleates. Blends of plasticizers are generally used.

Lubricants are used in order to improve the sliding characteristics of thermoplastic polymers and therefore facilitate their processing. Metal soaps are used as lubricants, for example stearates and oleaates of calcium, zinc, magnesium, or amyl and butyl stearates. The percentages of these additives are generally lower than 1% by weight.

Stabilizers are additives that have the purpose of preventing or limiting the degradation of the resin by agents such as light, heat, oxygen. Metal soaps are generally used, in which organic acid is usually chosen from the following: stearic, lauric, ricinoleic, benzyl and phenol acids. The cation is generally a metal chosen from the following: barium, cadmium, calcium, lead, tin, zinc. Organic phosphites are added to maintain transparency. It is also possible to use stabilizers other than soaps and which are completely organic, in particular substances containing epoxy groups, such as epoxidates and liquid epoxy resins.

Most plastics are colored with pigments or organic dyes that dissolve or disperse in the plastic mass during processing. The dye must possess certain requisites, such as easy dispersibility in the resin mass, thermal stability, light stability, absence of toxicity, especially for articles of particular use.

Other additives that can be used are anti-adhesives, which are applied directly by brush or spray on the surface of the molds to facilitate the detachment of the piece after forming. Examples of anti-adhesives are silicone oils and cellulosic resins, which are applied as paints in films or in thin layers.

Curing agents are the catalysts that are added when curing is to occur or be completed at the site of application.

Cross-linking agents are generally cross-linking comonomers which are added in polymerization. These comonomers, containing at least two double bonds, are added in quantities ranging from 0 to 2%, preferably from 0 to 1% by weight with respect to the total amount of monomers. Examples of cross-linking comonomers are (meth) acrylate ethylene glycol, di-, tri-, tetraethylene glycol di (meth) acrylate, 1,3- 1,4-butylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate , 1,9-nonandiol di (meth) acrylate, divinylbenzene, trivinylbenzene, (meth) allyl acrylate, diallyl maleate, diallyl fumarate, triallyl cyanurate, pentaerythritol triacrylate, trimethylolpropane tri (meth) acrylate etc. In the case of crosslinking of acrylic polymers, the following monomers can also be used as crosslinking agents: (meth) acrylic acid, glycidol (meth) acrylate, (meth) acrylamide.

The forming of plastic objects is commonly referred to as molding. It is performed on the resin previously mixed with one or more of the auxiliary components described above to form a homogeneous mixture.

The processing of thermoplastic molding powders is carried out by transferring them into the mold and heating it so that the powders melt and become fluid to the point of filling every cavity in the mold. Before being discharged from the mold, the resin is cooled and solidified. In injection molding, the resin is loaded from the hopper; the piston compresses it causing it to heat up. The softened resin passes into the mold and acquires its shape as it cools.

Among the plastics obtained by polycondensation, the following can be mentioned, by way of example.

Polyester resins derive from the polycondensation of acids with alcohols. There are different types of polyester resins. For example, in general, saturated polyesters, obtained from the condensation of saturated dicarboxylic acids with a divalent alcohol. The acid can be chosen, for example, from the following: saturated acids, for example phthalic acid and isophthalic acid; the alcohol can be chosen from ethylene glycol, propylene glycol or butylene glycol.

Long chain aliphatic saturated acids can also be used, for example azelaic, sebacic acid, etc. Longer chain glycols, such as dipropylene glycol and triethylene glycol, can also be used as alcohols. Epoxy resins are prepared by polyaddition of an epoxide, usually epichlolourhydrin with a phenol. The epoxy groups still react with phenol and epichlorohydrin and then the reaction proceeds until the formation of polymers having a linear structure and high molecular weight. By varying the operating conditions, it is possible to adjust the molecular weight and obtain products with a low degree of polymerization (fluid resins) or products with a high degree of polymerization (solid resins). Epoxy resins are therefore linear and thermoplastic; however, their chains contain a large number of very reactive groups, the hydroxyl groups -OH and the epoxy groups, which are equivalent to two hydroxyls. The chains can therefore react with each other or with another bifunctional compound and join in three-dimensional complexes with a high molecular weight, which do not melt, are highly hard and insoluble. This is achieved by adding suitable substances, called hardeners, which can be of two types: catalytic hardeners, which catalyze only the reaction between two epoxy groups, and reactive hardeners which instead take an active part in the process. Catalytic hardeners are for example tertiary amines and boron fluoride. These substances are capable of inducing ionic polymerization between the epoxy groups. Reactive hardeners are mainly primary and secondary amines, organic acids, ie substances capable of reacting both with hydroxyl groups and with epoxy groups.

Other resins derived from polycondensation processes are for example polyurethane resins.

Polyurethane resins are prepared from alkyl di-isocyanates by reaction with glycols, obtaining linear thermoplastic polyurethanes. An example of usable glycol is butylene glycol. The most used isocyanates are for example phenylene diisocyanate and hexamethylene diisocyanate. The plastics obtained by polymerization reaction preferably comprise acrylic resins, obtained from the polymerization of vinyl monomers.

With the terms acrylic resins, polyacrylic resins, acrylic (co) polymers, we mean the polymers obtained using as monomers the derivatives of acrylic and methacrylic acids, such as for example the (meth) alkyl acrylates, acrylic nitriles, etc., individually or in mixture between them.

The polymerization of acrylic esters is carried out in bulk, in suspension or in aqueous emulsion. Organic peroxides or potassium or ammonium persulfate and hydrogen peroxide are used as catalysts.

The mass polymerization is carried out by heating the monomer in the presence, for example, of benzoyl peroxide. The mass is then poured into the molds in which the polymerization is completed. After cooling, the product is discharged.

To obtain the molding powders, it is polymerized in aqueous suspension in the presence of peroxides, for example of benzoyl peroxide; the pearls are separated by centrifugation or filtration, then mixed with the additives and ground.

Emulsion polymerization can be used to obtain acrylic resins in the form of molding powders.

The artifacts based on polymethacrylic resins have high transparency, considerable mechanical and chemical resistance, and can be colored with the most varied dyes.

More particularly, the acrylic (co) polymers derive from polymerizable monomers and can be obtained, as mentioned, from suspensions of said monomers. The polymerizable monomers are alkyl esters of acrylic or methacrylic acid in which the alkyl group contains from 1 to 8 carbon atoms, preferably from 1 to 6 carbon atoms, or the relative amides, and relative mixtures. Examples of such esters are methyl (meth) acrylate (MMA), ethyl (meth) acrylate, propyl (meth) acrylate, butyl (meth) acrylate, tert-butyl (meth) acrylate, sec-butyl (meth) acrylate, hydroxyethyl ( meth) acrylate, hydroxypropyl (meth) acrylate, (meth) acrylamide.

To the monomer or to the mixture of acrylic monomers, another monomer can optionally be added that has only a double bond that can be polymerized by radical means, in an amount generally not exceeding 80%, preferably not exceeding 50% by weight, such as, for example, styrene, alpha-amethyl-styrene, (meth) acrylonitrile, N-alkyl or N-aryl-maleimides, having respectively the alkyl from 1 to 10 carbon atoms and the aryl from 6 to 12 carbon atoms.

The preferred acrylic (co) polymers are those containing methyl methacrylate for at least 70% by weight and the copolymers of methyl methacrylate with esters of acrylic acid, preferably ethyl or methyl acrylate.

Particularly preferred is methyl methacrylate which can be used alone to obtain polymethyl methacrylate or mixed with other vinyl monomers. As vinyl monomers, styrene, alpha-methyl-styrene, etc. can be mentioned.

Mineral fillers can be used in the composition, such as inorganic substances having hydrophilic surfaces characterized by the presence of polar groups, preferably of the hydroxyl type, capable of reacting with a silanizing agent present on the filler. As inorganic fillers we can mention amorphous and crystalline silica (quartz, cristobalite, etc.), glass, alumina, aluminum trihydrate, calcium carbonate, mineral silicates, aluminum silicates (mica, talc, wollastonite, etc.) mineral oxides such as Fe2O3, TiO2, etc. It is preferred that the filler particles be granular or spheroidal in shape. The average diameter of the particles is generally between 0, l and 50 µm and the surface area between 0.5 and 10 m <2> / g. Generally, the quantity of mineral filler to be used in the compositions depends on the desired characteristics in the finished articles and on the fluid characteristics of the suspension.

Generally the quantity of fillers, expressed as percent by weight, varies from 30 to 80% of the total suspension. The remaining part is made up of polymerizable monomers.

The silanizing agent is an organic derivative of silicon containing hydrolyzable groups.

The silanes that can be used are, for example, the following: methyltrimethoxysilane, methyltrimethoxysilane, vinyl triethoxysilane, γ-methacryloxypropyltrimethoxysilane, γmethacryloxypropyltris {2-methoxyethoxy) si1ano, γ -Îidoxy propyl-trimethoxysilane, γ -Îidoxy propyl-trimethoxy1 aminoethyl) γ-amino propyltrimethoxysilane, etc.

The silane hydrolysis catalyst is selected from the ammonium salts of organic acids having the general formula R4COO- [NR5R6R7R8] <+>

In which R4 represents a hydrogen atom or a C1-C20alkyl, cycloalkyl, alkylaryl, optionally unsaturated radical, R5, R6, R7, RS, the same or different from each other, they can be hydrogen or C1-C20 alkyl or cycloalkyl radicals. Examples of catalysts are isopropylammonium acetate, isopropylammonium methacrylate, ammonium oleate, ammonium stereate, ammonium methacrylate, n-propylammonium methacrylate, dimethylammonium butyrate, isopropylammonium oleate, ethyl ammonium benzoate, etc.

The silane is used at concentrations ranging from 0.01% to 2% by weight with respect to the feed and the catalyst between 0.01% and 1% by weight of the total composition.

Surfactants soluble in the monomer and having sufficient affinity with the mineral filler can also be added to the suspensions. Generally, surfactants containing ethoxy or propoxyl groups are used such as ethoxylated alkylphenols, sulfonated agents, alkyl sulfates, alkylammonium phosphates and phosphonates, in which the alkyl group has up to 20 carbon atoms, phosphoric esters, etc. Examples of surfactants are nonylphenol ethoxylate containing from 1 to 15 molecules of ethylene oxide, octylphenol ethoxylate containing from 1 to 15 molecules of ethylene oxide, ammonium dodecylbenzense1phonate, sodium 2-ethylhexylsulfosuccinate, double ammonium sulphate and 1-3 molecules of lauryl alcohol ethoxylate , phosphates of C4-C12 alcohols, acid phosphates of stearamidopropyldimethyl-β-hydroxy ethylammonium, etc. The amount of surfactant is between 0 and 0.5% by weight on the total mass, preferably between 0.05 and 0.2%.

Other additives can be added to the suspension, in particular inorganic and / or organic pigments and / or dyes, in quantities between 0 and 5% by weight of the total; agents to promote mold release, antistatic agents, plasticizers, anti UV agents, antioxidants, polymerization inhibitors, etc. in quantities between 0 and 2% by weight of the total. See for example those mentioned above.

The viscosity of the suspension can be increased and brought to the optimum level for injection molding or for mass polymerization by adding viscosity modifiers, such as homo and / or methacrylic or vinyl copolymers. Generally, compounds having a high molecular weight are used; for example polymers or copolymers with average molecular weight (Mw) between 20,000 and 1,000,000, and preferably between 50,000 and 500,000. Examples of viscosity modifying polymers are polyimethyl methacrylate, methyl methacrylate methylacrylate copolymers, methyl methacrylate-styrene copolymers, methyl methacrylate-butylacrylate copolymers, polyethyl methacrylate, methyl methacrylate-methacryloxypropyltrimers copolymers.

In general, the viscosity modifiers are added in quantities by weight not exceeding 20% with respect to the monomer, preferably between 0.5 and 10%.

The transformation into manufactured articles of the compositions can be carried out by molding by applying a pressure between 1 and 4 atmospheres and at temperatures between 70 ° and 100 ° C for closed molds, after adding a catalyst, preferably peroxide, in quantities generally ranging from 0, 1 to 2% by weight. Examples of these catalysts are benzoyl peroxide or t-butylcyclohexylperoxycarbonate. Optionally, agents can be added to promote release from the molds, such as for example stearic acid, glycerin monostearate, etc.

Before molding, it is advisable to deaerate the suspension under stirring at reduced pressure.

Using milder polymerization conditions and adequate thermal cycling it is possible to polymerize the suspension with techniques similar to those used for the production of cast metacryl plates.

Acrylic (co) polymers with good mechanical characteristics can also be used as polymers for preparing the prostheses of the present invention. These polymers are prepared using compositions based on acrylic (co) polymers having impact-resistant properties, as for example described in EP 270,865, USP 3,985,703. These flexible materials, in particular those described in the EP patent, are obtained by mixing the acrylic (co) polymers with an impact-resistant additive in a quantity equal to 20% by weight or even higher. Examples of substances used as impact additives are for example core-shell emulsions having a resin core, an intermediate layer of acrylic rubber and an outer layer of (meth) acrylic resin. The core can be constituted for example by a cross-linked acrylic polymer and the intermediate layer is formed by a cross-linked elastomeric copolymer having a Tg lower than 25 ° C, preferably lower than -10 ° C; the outer layer is made up of a (meth) acrylic resin crimped to the rubber.

A typical composition of impact resistant acrylic (co) polymers includes:

- 40 - 95% by weight of a thermoplastic resin formed from acrylic (co) polymers,

- 60 - 5% by weight of a polymer with a multilayer structure comprising:

5 - 60% by weight of a core of thermoplastic acrylic resin as defined,

20 - 50% by weight of a first layer, which surrounds the central core, consisting of a cross-linked elastomer formed by butyl acrylate / styrene 85/15.

13 - 35% by weight of an acrylic resin which constitutes the external layer.

Very low Tg elastomers can be used as shock-absorbing additives for acrylic (co) polymers. In this way, in the compounding phase it is possible to use quantities of additive lower than the 20% indicated above.

Acrylic (co) polymers can also be used as described above having abrasion resistance comparable to that of acrylic (co) polymers, using the following compositions:

- from 70% to 99.5% by weight, preferably from 80% to 99%, even more preferably from 90 to 98% by weight, of a thermoplastic resin based on homopolymers or acrylic copolymers as defined above in which at least 20 % by weight, preferably at least 50% by weight consists of (meth) acrylic monomers,

- from 0.5% to 30% by weight, preferably from 1 to 20%, even more preferably from 2 to 10% by weight of an elastomer, preferably cross-linked, having a glass transition temperature Tg on-set (ASTM D 3418 -75) lower than 0 ° C, preferably lower than -5 ° C, even more preferably lower than -10 ° C,

the elastomeric component being dispersed in the acrylic resin in the form of spherical particles and / or elongated particles having a diameter, measured by electron microscopy (TEM transmission electron microscopy), between about 10 nm and about 2,000 nm, the particles of the elastomeric component being able include particles of the acrylic resin.

The preferred acrylic (co) polymers of the thermoplastic resin are those containing methyl methacrylate for at least 70% by weight, such as PMMA and the copolymers of methylethacrylate with (meth) acrylic acids or their esters, preferably ethyl, methyl, butyl or acid (methyl) acrylate )acrylic.

The elastomers can be obtained for example by polymerizing one or more (co) monomers selected from the following groups:

- esters of acrylic acid in which the alkyl group has from 1 to 16 carbon atoms, preferably from 2 to 12 carbon atoms, such as ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, etc.,

- alkoxy-alkyl acrylates, wherein the number of total carbon atoms between the alkyl group and the alkoxy group is between 2 and 16, preferably between 3 and 15; such as 2-methoxyethyl acrylate,

- monomers having a double ethylenic unsaturation e.g. butadiene or substituted butadiene such as e.g. isoprene, chloroprene, 2-3 dimethylbutadiene,

- vinyl monomers, e.g. styrene and its derivatives, such as for example methyl- and ethyl-styrene, in which the alkyl group is in the ortho or para position; Î ± -methylstyrenes; mono-, di-, tri-, tetra-, penta-halogenostyrenes, in which halogen à © Cl, F; said monomers in quantities not exceeding 40% by weight, preferably not exceeding 30% by weight of the total monomers of the elastomeric component.

The preferred elastomer is the butyl or 2-ethylhexyl or octyl acrylate copolymer containing styrene in quantities ranging from 5 to 30% by weight, preferably from 10 to 20%.

The compositions in the form of beads can be prepared by a suspension polymerization process comprising the following steps:

1) preparation of the elastomer beads by means of a suspension polymerization process of the monomers, optionally in the presence of at least one cross-linking monomer;

2) polymerization, in the same polymerization suspension containing the beads formed of the elastomer obtained in step 1), of the (co) monomers that make up the acrylic polymer, said (co) monomers being selected from those indicated above.

A preferred process for the polymerization in suspension, preferably aqueous suspension, of the indicated monomers is carried out in the presence of a radical initiator soluble in the monomers and of a suspending agent for stabilizing the suspension. For example, inorganic suspenders and organic suspenders can be mentioned. Among the latter we can mention the polymeric organic compounds such as polyvinyl alcohol, acrylic copolymers containing a (meth) acrylic acid, carboxymethylcellulose, etc.

The following are mentioned as preferred suspendants:

- homopolymers of a compound of formula

R2

CH2 <C> CO AC CH2 <SO> 3 <M>

R1 R3

wherein R1 = H or CH3; R2 and R3, the same or different, are H or C1-C8 alkyls, possibly branched when possible; M is an alkaline or alkaline earth metal or ammonium and A is NH, oxygen or NCH3,

- copolymers of the compound of formula (I) with acrylic monomers in a quantity not exceeding 40% by weight.

Generally the quantity of the suspending agent is comprised between 0.1 and 1.5%, preferably 0.2 and 1% by weight, referred to the total weight of the aqueous phase.

In the polymerization in aqueous suspension for the preparation of the elastomer (step 1) of the process) the ratios between aqueous phase and monomers generally comprised between 1.5: 1 and 20: 1 by weight, preferably from 2: 1 and 10: 1 by weight, in the presence of a radical polymerization initiator that is soluble in the monomer. It can be operated without a molecular weight regulator (chain transfer agent). The reaction temperatures are those at which the initiator decomposition takes place, and are generally between 50 ° C and 120 ° C.

In the polymerization in aqueous suspension for the preparation of the acrylic (co) polymers (step 2) of the process) the ratios between aqueous phase and monomers generally comprised between 1: 1 and 10: 1 by weight, preferably between 1.4: 1 and 6: 1 by weight, in the presence of a molecular weight regulator and a radical polymerization initiator, both selected from those soluble in the monomer. The reaction temperatures are those at which the decomposition of the initiator takes place, and are generally between 50 ° C and 120 ° C.

As radical initiators, peroxides such as dibenzoyl peroxide, t-butyl peroxy diethyl acetate or unstable azo compounds, such as azodiisobutyronitrile for example, can be mentioned.

As chain transfer agents, the alkylthiols with the linear or branched alkyl group from C3-C20, preferably C4-C12, such as, for example, n-butantiol, n-octanethiol, n-dodecantiol, tert-dodecantiol, cyclohexantiol, pinantiol.

The preferred suspending agents of formula (I) or their copolymers with acrylic monomers are described in patent application EP 457.356 incorporated herein by reference. In particular, the suspending agents of formula (I) can be, for example, sodium 2- (meth) acrylamido-2-methylpropane sulfonate, sodium 2-acrylamidopropanesulfonate, sodium 2-acrylamido-2-ethanesulfonate.

The acrylic monomers which can be copolymerizable can be, for example, (meth) acrylamide, alkaline or alkaline earth salts of (meth) acrylic acid, esters of (meth) acrylic acid with a C1-C4 aliphatic alcohol, acrylonitrile.

For the crosslinking of these polymers, crosslinking comonomers containing at least two double bonds can be added in the polymerization, in the amounts indicated above. Examples of cross-linking comonomers are ethylene glycol di- (meth) acrylate, di-, tri-, tetraethylene glycol di (meth) acrylate, 1,3- 1,4-butylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol di (meth) acrylate, pentaerythritol triacrylate, trimethylolpropane tri (meth) acrylate etc. Cross-linking monomers containing a polar functional group can also be used, in percentage from 0 to 2% by weight of the total monomers; examples of these monomers are (meth) acrylic acid, glycidyl (meth) acrylate, (meth) acrylamide.

In the case of acrylic (co) polymers, crosslinking can also be carried out without the addition of crosslinking monomer if acrylic esters are used as comonomers in which the alkyl has a number of carbon atoms greater than or equal to 4, preferably C4-C10, to for example butyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate.

The polymers used to prepare the prostheses of the present invention meet the requirements established by the health authority for their clinical use, primarily the absence of toxicity.

The prostheses of the present invention preferably comprise, or consist of, acrylic (co) polymers.

The beads obtained with the suspension polymerization process described above, after washing with water and drying, are compounded (see above), preferably by extrusion, to obtain granules.

A preferred process for preparing the prostheses of the present invention is carried out by mixing an acrylic (co) polymer in powder form, for example polymethyl methacrylate (PMMA) alone or in a mixture with other acrylic polymers, with a liquid acrylic monomer, for example methyl methacrylate ( MMA).

The powder contains, in addition to the polymer, a plasticizer other than phthalate, initiators such as peroxides, and pigments.

The liquid, in addition to MMA, contains a crosslinker, for example ethylene glycol dimethacrylate and a catalyst, for example a peroxide.

A commercially available composition has the following composition (% by weight percent)

Powder: polymethyl methacrylate 95.5, plasticizer (not phthalate) 3.8, initiators 0.6, pigments 0.1.

Liquid: 95.9 methyl methacrylate, 4.0 crosslinking agent, 0.1 catalyst.

By mixing the two components, first a swelling occurs and then a malleable homogeneous mass is formed which, depending on the quantity of the components and the mixing time, can also take on the consistency of a liquid. The mixing is carried out in a time not exceeding 30 seconds, preferably less than 10 seconds.

Preferably, one operates at a temperature ranging from 70 ° C to 40 ° C or lower and, if desired, also up to room temperature. Preferably one operates under pressure, generally not higher than 5 atm and not lower than 2 atm, preferably between 4 and 2 atm. The powder: liquid weight ratios are generally from 1.0 (powder): 1.0 (liquid), up to 4.0: 1.0, preferably 1.0: 1.0 to 2.0: 1.0.

The curing times depend on the temperature and pressure used. Generally they are less than an hour, preferably not more than 30 minutes. For example, operating at 4 atm at 40 ° C, the polymerization time is about 15 minutes.

A further object of the present invention is a process for preparing dental prostheses comprising the following steps:

a) heating a model prosthesis as defined above, at a temperature between 105 ° C and 250 ° C, at atmospheric pressure,

b) transfer of the prosthesis on a plaster cast of a dental arch,

c) modeling of the prosthesis on the cast, adapting it to the shape of the cast by applying a force preferably less than 1 kg weight, preferably manually,

d) cooling to room temperature.

The obtained prosthesis can be subjected to subsequent processes including steps a) -d), using casts of different dental arches, at least 20 times, to obtain other prostheses of different shapes.

Ambient temperature means a temperature between 15 ° and 25 ° C.

In step a) the treatment time generally ranges from 0.5 to 10 minutes, preferably from 1 to 5 minutes, more preferably from 2 to 4 minutes.

In step a) the treatment temperature is preferably comprised between 110 ° and 180 ° C, more preferably 120 ° and 170 ° C; the treatment time for a treatment time preferably from 1 to 5 minutes, more preferably from 2 to 4 minutes.

In step b) the transfer of the prosthesis on the plaster cast of a dental arch is carried out immediately after the treatment, preferably in a time not exceeding 1 minute, more preferably in a time not exceeding 30 seconds.

In step c) the force applied is preferably between 0.1 and 1 kg weight.

In step d) the prosthesis is preferably left to cool in the air to room temperature.

A further object of the present invention is the use of the dental prostheses of the invention obtainable from model prostheses according to the process described above, to obtain other prostheses by subjecting the prostheses of the invention to steps a) -d) of the process described above. but using in step c) a different cast each time.

The following examples are for illustrative and not limitative purposes of the present invention.

EXAMPLES

Polymer material test of the model prosthesis

The strip of polymeric material that constitutes the model prosthesis, having dimensions 6X6X80 mm, takes the external shape of a metal cylinder having a diameter of 30 mm and having a concavity with a radius of curvature 12 mm and a center of curvature with Cartesian coordinates (15 mm, 15 mm), and a convexity with a radius of curvature 8 mm and a center of curvature with Cartesian coordinates (8 mm, 8 mm) (Figs. 1a and 1b), after having been subjected to heat treatment at temperatures between 105 ° C and 250 ° C at atmospheric pressure, the strip being made to adhere to the cylinder and reproducing its external surface without showing fractures or cracks by applying a force of up to 1 kg weight,

EXAMPLE 1

A composition for prosthetic bases is used, consisting of a powder, essentially consisting of an acrylic polymer and a liquid, essentially consisting of an acrylic monomer, marketed under the brand name Sintodent®.

1.1

Preparation of the strip for the evaluation of the polymer for use in the preparation of the prosthesis: Test

The two components are mixed in the ratio g solid component / g liquid component of about 1.4. It is observed that the mixture swells and a malleable mass similar to a paste is obtained. It is mixed with a spatula for 5 seconds. The mold that reproduces the shape of a strip having dimensions 6X6X80 mm is filled with a spatula. The mold is heated for 15 minutes at 40 ° C with a pressure of 4 atmospheres. It is left to cool to room temperature and the product is removed from the mold.

The sample obtained is subjected to heat treatment at 140 ° C for 4 minutes at atmospheric pressure in an oven and is then used for the test described above.

Within 30 seconds from the heat treatment the strip is applied to the cylinder. It can be observed that it adapts perfectly to the surface of the cylinder, shown in Figs. 1a and 1b, applying light pressure with the hands of about 0.4 kg weight (4 Newton). The results are illustrated in Figs. 2a and 2b.

The test is then passed and the polymer is suitable for preparing the dental prosthesis.

1.2

Preparation of the dental prosthesis

30 g of powder (acrylic polymer) and 22 g of liquid (methylmethacrylate monomer) of the commercial product Sintodent® are mixed for 5 seconds until a homogeneous mass but with a liquid consistency is obtained. It is poured into a filled muffle, to obtain the counter-mold, with gelatin or silicone. The liquid resin is poured into the muffle through a special opening until the mixture comes out of the hole for venting the air. It polymerizes in a pressure cooker for 15 minutes at 40 ° C with a pressure of 4 atm.

It is allowed to cool to room temperature and the prosthesis is removed from the flask.

This prosthesis is used for a cast of a different dental arch. Fig. 3 shows the prosthesis resting on the cast before the heat treatment. By subjecting the prosthesis to the same heat treatment in the oven as for the strip, the prosthesis can be modeled on the different cast. The recessed part of the prosthesis exactly reproduces the shape of the cast. Fig. 4 shows the prosthesis of the invention modeled on the cast.

The prosthesis that can be obtained after air-cooled heat treatment does not show any retraction or dilation with respect to the cast, as illustrated in Fig. 4.

1.3

Use of the prosthesis obtained in Example 1.2 (the one shown in Fig. 4) subjected to a further heat treatment and modeling on a cast other than that of Fig. 4.

The prosthesis obtained in example 1.2 was subjected to a further heat treatment at 140 ° C for 4 minutes at atmospheric pressure and modeled on a cast obtained from a different dental arch, then cooled to room temperature.

The prosthesis of the invention thus obtained reproduces the shape of the cast perfectly and does not show any retraction or dilatation with respect to the cast.

1.4

Example 1.3 was repeated on different casts deriving from different footprints 20 times. The results obtained are of the same type as those obtained in example 1.3: the prosthesis fits exactly to the cast in all the tests carried out and does not show any retraction or dilation with respect to the cast.

EXAMPLE 2

The polymer of example 1 is used and example 1.2 is repeated to prepare the prosthesis but using a heat treatment at 180 ° C and for a time of three minutes at atmospheric pressure.

To model the prosthesis on the cast, a slight pressure was applied with the hands of about 0.4 kg weight. It has been found that the prosthesis thus obtained adapts perfectly to the cast used in example 1.2 and does not show any retraction or dilation with respect to the cast.

2.1

Example 2 was repeated but using another cast different from the previous one.

It has been found that the prosthesis thus obtained adapts perfectly to the new cast and does not show any retraction or dilation compared to the new cast.

Example 3 comparison

Example 1 was repeated but subjecting the strip to heat treatment in an oven at a temperature of 90 ° C at atmospheric pressure. The treatment time is 9 minutes.

The strip is subjected to the test, applying a force of about 1 kg with your hands using your hands to model the strip on the cylinder.

It is observed that the strip retains its initial shape, does not bend and therefore cannot be adapted to the surface of the cylinder. So the test fails.

From this example it can be deduced that the polymer used, subjected to the indicated heat treatment, does not allow to obtain the prosthesis of the invention.

Example 4 comparison

Example 3 was repeated but applying a force of 10 kg weight to shape the strip to the cylinder.

It has been observed that the strip retains its initial shape, does not bend and therefore cannot be adapted to the surface of the cylinder. In addition, cracks begin to form in the strip. To avoid breaking the specimen, the test is stopped.

From this example it can be deduced that the polymer used, subjected to the indicated heat treatment, even by increasing the force applied to the strip by about 10 times with respect to example 3 comparison, does not allow to obtain the prosthesis of the invention.

Claims (15)

CLAIMS 1. Dental prostheses obtainable starting from a model prosthesis, obtainable by molding a polymer starting from a cast, and subjecting the model prosthesis to heat treatment at temperatures between 105 ° C and 250 ° C at atmospheric pressure, the model prosthesis being such to satisfy the following test: a strip of the polymeric material that constitutes the model prosthesis, having dimensions 6X6X80 mm, takes the external shape of a metal cylinder having a diameter of 30 mm and having a concavity with a radius of curvature 12 mm and a center of curvature with Cartesian coordinates (15 mm, 15 mm), and a convexity with a radius of curvature 8 mm and a center of curvature with Cartesian coordinates (8 mm, 8 mm), after having been subjected to the heat treatment indicated above, the strip being made to adhere to the cylinder and reproduces it the external surface without showing fractures or cracks by applying a force up to 1 kg weight, the prosthesis obtainable after heat treatment, modeled on the cast and cooled to room temperature, maintains the shape of the cast, since the prosthesis after heat treatment can be subjected to further heat treatment cycles for at least 20 times to obtain further prostheses. 2. Dental prostheses according to rev. 1, in which in the test the applied force is between 0.01 and 1 kg weight. 3. Dental prostheses according to rev. 1-2, in which the heat treatment is carried out for a time from 0.5 to 10 minutes. 4. Dental prostheses according to rev. 3, in which the heat treatment is carried out for a time from 1 to 5 minutes. 5. Dental prostheses according to rivv. 1-4 in which the temperature of the heat treatment is between 110 ° and 180 ° C for a time from 1 to 5 minutes. 6. Dental prostheses according to rev. 5 in which the temperature of the heat treatment is between 120 ° and 170 ° C for a time from 1 to 5 minutes. 7. Dental prostheses according to rivv. 1-5 in which the model prosthesis consists of one or more polymers. 8. Dental prostheses according to rev. 7 in which the polymers are thermoplastic. 9. Dental prostheses according to rivv. 7-8 in which said one or more polymers are partially cross-linked with the condition that a strip of said one or more polymers obtained by molding satisfies the test indicated above. 10. Dental prostheses according to rev. 7-9 wherein the polymers constituting the prostheses include acrylic (co) polymers. 11. Dental prostheses according to rev. 7-10 in which the monomers which are polymerized to obtain the polyacrylic resins are alkyl or hydroxyalkyl esters of acrylic or methacrylic acid in which the alkyl group contains from 1 to 8 carbon atoms, or the relative amides and relative mixtures. 12. Dental prostheses according to rev. 7-11 in which the cross-linking agents are cross-linking comonomers selected from the following: ethylene glycol of (meth) acrylate, di-, tri-, tetraethylene glycol of (meth) acrylate, 1,3- 1,4-butylene glycol of (meth) acrylate , 1,6-hexanediol di (meth) acrylate, 1,9-nonanediol di (meth) acrylate, divinylbenzene, trivinylbenzene, (meth) allyl acrylate, diallyl maleate, diallyl fumarate, triallyl cyanurate, pentaerythritol triacrylate, tri (meth) trimethylolpropane ) acrylate. 13. Dental prostheses according to rivv. 1-12 where the model prosthesis is obtained by mixing a powdered acrylic (co) polymer with a liquid acrylic monomer. 14. Process for preparing dental prostheses according to rivv. 1-13 comprising the following steps: a) heating a model prosthesis, as defined above, at a temperature between 105 ° C and 250 ° C, at atmospheric pressure, b) transfer of the prosthesis on a plaster cast of a dental arch, c) modeling of the prosthesis on the cast, adapting it to the shape of the cast, d) cooling to room temperature. 15. Use of dental prostheses according to rivv. 1-13 to obtain different prostheses, in which the prostheses of rivv. 1-13 are submitted to the process of rev. 14 using in step c) a different cast each time.