Dentures and Process for Making the Same

1. Field of Invention

This invention relates to novel dentures and to a method for preparing the same. It is particularly directed to novel dentures in which the teeth are formed of hard polyurethane elastomer and the gums of soft polyurethane elastomer.

2. Prior Art

It has been proposed to provide dentures with a soft layer in contact with the gums and other mouth parts to provide tissue relief. Such soft layers have been composed of acrylics, silicones, and like rubber-like materials. But on aging, such soft layers tend to harden and give off undesirable odors. In addition, some decomposition of the polymer may also occur presumably due to an oxidation process as well as to pH fluctuations within the mouth.

Also, in order to provide a unitary structure in which the teeth are firmly imbedded in the gum portion of the dentures, various types of holding means such as pins, rods, diotorics, that is, undercut channels or holes, are provided so that the teeth become firmly imbedded in the denture material.

It is an object of the invention to overcome difficulties heretofore encountered in the prior art. It is a further object of the invention to provide dentures in which the teeth are chemically bonded into the denture material. It is a further object of the invention to provide a denture in which the teeth are imbedded in a soft polyurethane elastomer. Still other objects are to provide dentures which are inexpensive, trouble free, and easy to keep clean. Further objects will appear as the description proceeds.

The invention consists of a denture comprising teeth consisting essentially of hard non-hydrophilic polyurethane elastomer having a hardness of not less than about Shore D 40, imbedded in and chemically bonded to a gum member consisting essentially of soft non-hydrophilic elastomer having a hardness of not greater than about Shore A 65 and to a process for making the same in which a model assembly comprising a wax replicate denture (artificial teeth imbedded in a wax pattern) mounted on a mouth model as prepared by a dentist or dental technician after a series of dental impressions, bites, and trials with the patient, is coated with release material and imbedded in investment material to form a female mold or investment having a cavity corresponding to the wax replicate denture. Hard non-hydrophilic polyurethane elastomer-forming material is then poured into the mold to a depth above the gum line but below the gum engaging portion of the denture to be formed and caused to set up to form hard non-hydrophilic polyurethane elastomer. The casting is removed and trimmed of its flash down to the gum line to delineate the shape of that portion of the teeth to be imbedded in the gum portion of the denture and replaced in the mold. The wax replicate denture is then removed from the mouth model and the exposed gum engaging portion of the wax model is then coated with release compound. The mouth model and the female mold are then assembled to form a denture mold and soft non-hydrophilic polyurethane elastomer-forming material is introduced thereinto by suitable sprue holes and caused to set up therein to form soft non-hydrophilic polyurethane elastomer gum portion integrally chemically bonded into a unitary mass to the hard non-hydrophilic polyurethane elastomer teeth with which it is in contact. The cast dentures are then removed from the mold and finished as needed, for example, by removing the sprues and polishing.

For forming the female mold, any suitable investment material may be used. It has been found according to the invention, however, that it is of advantage to use a rigid polyurethane foam-forming material because it is simple to mix up such material, inject it into the mold cavity and allow it to set up therein. It has the advantage of providing a light, strong investment which, unlike plaster or investment compound, can be easily broken away from the cast denture and sprues, if necessary, to effect removal of the dentures from the mold. The rigid polyurethane foam also provides a dimensionally stable mold. Any of the so-called one-shot or foam-in-place formulations can be utilized.

Suitable pigments and/or dyes can be incorporated in the polyurethane elastomer-forming material as may be needed to give the desired color to the teeth and gums.

Polyurethane elastomers of varying degrees of hardness and elasticity are known in the art. They include both the simple urethane polymers and the urethane copolymers, such as urethane-urea copolymers. Any of these known polyurethane polymers can be utilized for the purposes of this invention provided the components are not colored or at least the resulting product is not colored in such a way that the desired coloring of the teeth and gums cannot be obtained by the introduction of pigments and dyes. Also, provided that the elastomers are not hydrophilic and provided they have a degree of hardness consonant with the purposes of the invention. The hard non-hydrophilic polyurethane elastomers should have a hardness of at least Shore D 40 and up to about Shore D 70, whereas the soft non-hydrophilic polyurethane elastomer should not have a hardness greater than about Shore A 65 and preferably at least about Shore A 15. Preferred non-hydrophilic elastomers are those formed by isocyanate-terminated prepolymers which are cross-linked or cured by mixing with a cross-linking agent and heating as required to effect curing.

Isocyanate-terminated prepolymers suitable for preparing the hard non-hydrophilic polyurethane elastomers (hard prepolymers) are prepared by the reaction of polyether diols or triols with aliphatic or cycloaliphatic or aralkyl di- or polyisocyanates in proportion to give free NCO groups. The prepolymers are then cured or cross-linked with a diol, polyol, an alkanolamine, a diamine or a tertiary amine containing polyol, or blends thereof. Advantageously, the diol or polyol is a polyether diol or polyol or a hydroxyl-terminated prepolymer.

The polyether diols can be selected from poly-(oxypropylene) glycols, poly-(oxypropylene)poly-(oxyethylene) glycols, poly-(1,4-oxypropylene) glycols, graft copolymers of poly-(oxypropylene)-(polyoxyethylene) glycols with acrylonitrile or mixtures of acrylonitrile and styrene ("Polymer Polyols"). The equivalent weight of these polyether diols may range between 200 to 1000 with a preferred range of 200 to 400. The polyol may consist of simple polyfunctional alcohols such as glycerine, trimethylolpropane, 1,2,6-hexanetriol, or pentaerythritol, or they may consist of polyether triols such as poly(oxypropylene) or poly(oxyethylene) adducts of the above polyols. The equivalent weight of the polyether polyols may range between 100 and 800 with a preferred range of 100 and 500. It is also understood that various combinations of diols and polyols may be used.

Isocyanate-terminated prepolymers suitable for preparing the soft polyurethane elastomers (soft prepolymers) are based on polyether diols alone or combinations of polyether diols or triols, and aliphatic, cycloaliphatic or aralkyl di- or polyisocyanates. The same diols and polyols as described above may be used but the average equivalent weight is significantly higher than that used in the preparation of the hard polymer. The preferred range of equivalent weight of the polyethers (diols or combination of diols and triols) is 450 to 1500. They are cured in the same way as the hard prepolymers.

The diisocyanates used for the preparation of the hard or soft isocyanate-terminated prepolymers may be selected from the following, although they are not necessarily restricted to these examples:

4,4'-Dicyclohexylmethane diisocyanate, isophorone diisocyanate, 2,2,4-trimethyl-1,6-hexane diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, "dimeryl" diisocyanate, methylcyclohexyl diisocyanate and the reaction product of 3 moles of hexamethylene diisocyanate with 1 mole of water (Desmodur N-triisocyanate).

The ratio of NCO to OH in the preparation of the soft isocyanate-terminated prepolymer may range between 1.75 to 2.5 with a preferred range of 2.0 to 2.25, while the NCO/OH of the hard isocyanate-terminated prepolymer may range between 2 to 3. The soft isocyanate-terminated prepolymers should have a free NCO content of about 3.5 to 5.5 percent, preferably, 3.7 to 4.7 percent, and the hard isocyanate-terminated prepolymers, a free NCO content of about 9.5 to 14 percent, preferably, 10 to 13 percent.

For the curing (crosslinking) of the soft or hard prepolymers, preferred polyols are tertiary amine- containing polyols such as poly(oxypropylene) or poly(oxyethylene) adducts of diamines or triamines, such as ethylenediamine, diethylene triamine, tolylenediamine, phenylenediamine, or aniline, or any diols, polyols or their blends. Advantageously, they are polyols of relatively low molecular weight such as are obtained by condensing propylene oxide with ethylenediamine or pentaerythritol to a molecular weight of about 500, or of trimethylolpropane or any other base compound to a molecular weight up to 2500.

Another preferred curing or crosslinking agent is a hydroxyl-terminated prepolymer. These are prepared essentially the same way as the isocyanate-terminated prepolymers but the ratio is such that there are free and un-reacted hydroxyl groups. The same diols and polyol and isocyanates can be used, though it is preferred that the prepolymer have a functionality greater than 2, which can be obtained by using a polyol having a functionality greater than 2 and/or an isocyanate having a functionality greater than 2. Advantageously, the isocyanate is 2,2,4-trimethyl-1,6-hexane diisocyanate, hexamethylene diisocyanate and Desmodur N.

The ratio of OH/NCO in the hydroxyl-terminated prepolymers, advantageously, may be in the same range as the NCO/OH ratio in the isocyanate-terminated prepolymers. It will be understood, however, that inasmuch as the cross-linking agent may consist of one or more diols or polyols (no isocyanate), the ultimate OH/NCO ratio is infinity.

Another preferred curing or crosslinking agent is a prepolymer-polyol blend. Thus, a polyurethane prepolymer, advantageously, one having neither free NCO nor free OH groups, can be mixed with a polyol, advantageously a polyol having a functionality of more than 2, to form a prepolymer-polyol blend. When such a blend is mixed with an isocyanate-terminated prepolymer in a NCO/OH ratio of greater than 1, crosslinking is effected both through an NCO--OH reaction and through NCO-urethane reaction.

The isocyanate-terminated prepolymers and the cross-linking agent should be mixed together in proportions to give an NCO/OH ratio of at least about 1.05 to 1.0 and preferably not greater than 1.1 to 1.0. This excess of NCO groups ensures a crosslinked polymer which is non-hydrophilic and one which is sufficiently reactive so that the soft non-hydrophilic polyurethane elastomers react chemically with the hard non-hydrophilic polyurethane elastomers to form an integral and unitary chemical bond between the two.

In order to accelerate the formation of the prepolymers or the cure of both the hard and soft isocyanate-terminated prepolymers with the crosslinking agents, metal catalysts such as tin catalysts, for example, dibutyltin dilaurate and stannous octanoate can be used.

In the Drawings

FIG. 1 is a side elevation of a denture according to the invention.

FIG. 2 is a plan view of FIG. 1.

FIG. 3 is a cross-section taken along lines 3--3 of FIG. 2.

FIG. 4 is a cross-section taken along lines 4--4 of FIG. 2.

FIG. 5 is a side elevation of the mouth model with replicate denture therein.

FIG. 6 is a plan view of FIG. 5.

FIG. 7 is a cross-section taken along lines 7--7 of FIG. 6.

FIG. 8 is a cross-section of the assembled flask.

FIG. 9 is a cross-section of the lower investment or female mold showing the untrimmed cast teeth therein.

FIG. 10 is a cross-section of the investment of the female mold showing the cast teeth therein.

FIG. 11 is a cross-section of the assembled flask showing the cast denture therein.

As shown in the side elevation of FIG. 1 and in plan view in FIG. 2, 1 represents a denture according to the invention, 2 the artificial teeth, and 3 the artificial gums in which teeth 2 are imbedded. The teeth 2 are composed of hard polyurethane elastomer and the gum 3 of soft polyurethane elastomer. The teeth 2 and the gum 3 are unitarily and integrally united by a chemical bond which results when the gum portion is cast in contact with the cast teeth.

If desired, the teeth may be cast in contact with a re-inforcing element 4 which has the shape of the teeth assembly. The re-inforcing element 4, advantageously, is also made of hard polyurethane elastomer, preferably freshly cast, and the teeth and the gum portions are unitarily and integrally bonded therewith.

FIG. 3 shows a cross-section of the anterior portion and FIG. 4 is a cross-section of the posterior portion.

By the techniques of this invention, the denture 1 has the shape and configuration of the wax replicate denture 10 prepared by the dentist or dental technician. The wax replicate denture 10 includes the wax pattern 14, or gum portion, with the teeth 13 imbedded therein. The wax replicate denture 10 is positioned on the mouth model 12 and the whole constituting the model assembly is mounted in the upper half 11 of a flask 21. This model assembly is prepared in the usual manner after a series of dental impressions, bites, and trials with the patient so that the wax replicate denture represents exactly what the dentist wants the final dentures to be like.

In preparing dentures from the model assembly, there is attached to the inner ends of the posterior portions of the wax pattern 14, wax sprues 15 for the purpose of providing sprue holes into the mold cavity. These wax sprues should lie on, or be partially imbedded in the edges of the mouth model. (By edges of the mouth model is meant the portions extending beyond the actual impression of the gums or palate), and so shaped and so located that when the mouth model is removed for the investment mold, yet to be described, the cast denture and sprues can be easily removed. The entire exposed surface of the model assembly is then coated with a coating 16 of release material and the whole assembly inserted into the bottom half 22 of the flask 21 as shown in FIG. 8. The bottom half of the flask is then filled with investment material 23 which is allowed to or caused to set up. On removal of the model assembly, there is left a female mold or investment having a cavity 24 which conforms to the shape of the wax replicate denture 10.

If it is desired to provide re-inforcing element 4, it is fitted in the cavity in the desired position, for example, along the posterior side as shown in FIG. 9, or it may be imbedded in hard non-hydrophilic polyurethane elastomer-forming material, or simply laid on top of it, before it sets up.

The cavity 24 now forms the mold for the teeth portion which are formed by filling the cavity 24 with hard non-hydrophilic polyurethane elastomer-forming material to the desired depth intermediate the gum line and the gum-engaging portion of the denture to be formed. This material is then either allowed to or caused to set up as shown in FIG. 9. The casting is then withdrawn from the mold and the flash 25a cut off to the dotted lines 25b, i.e., down to the gum line, to delineate the portion of the teeth to be imbedded in the gum portion. The cast teeth 25, thus modified, are reinserted into the cavity 24 as shown in FIG. 10. The wax replicate denture is then removed from the mouth model 12, the exposed portion thereof coated with release material 16a, and the two halves of the flask assembled as shown in FIG. 11. Mold cavity 24 is then filled with soft non-hydrophilic polyurethane elastomer-forming material which is then allowed to or caused to set up to form the soft non-hydrophilic polyurethane elastomer 27 which is the gum portion 3 of denture 1.

It will be understood that the release material 16 and 16a is not actually as thick as shown since it is really nothing more than a coating of latex, silicone, shellac, zinc stearate, or the like.

The investment material, advantageously, is a rigid polyurethane foam such as commonly used for insulation purposes. Two-component systems are readily available on the market which in a short time after mixing, will set up into a rigid blown foam. It is sufficient simply to mix the two components together and to inject a measured quantity into the mold cavity through suitable sprue holes, where in a few minutes or a few seconds, according to the time factor of the particular composition, it will begin to foam and will fill up the bottom half of the flask 21 into close and intimate contact with the model assembly therein. A hole, or holes, may be provided in a wall to the lower half of the flask to vent the flask and to provide for expansion in case too much of the foam-forming material is introduced into the flask cavity. The flask is placed in a suitable clamp (not shown), to assure that any pressure engendered by the blowing of the foam or any of the subsequent operations, will not disrupt the relative position of the upper and lower flasks. The size of the sprue holes and the vents can be adjusted to the amount of material introduced to cause more or less pressure to be exerted as the foam sets up.

In contradistinction, the non-hydrophilic polyurethane elastomer-forming compositions or materials of the invention do not set up spontaneously and do not expand on setting up. Nonetheless, the flask should be tightly clamped to insure that a precise replication is obtained. The isocyanate-terminated polymers used in the invention are liquids or heat-liquefiable materials as are the cross-linking agents. When the two are mixed in the proper proportions at the proper temperature, the resulting liquid can be poured into or injected into the molds as required and there caused to set up by holding at a suitable temperature for a requisite period according to the time constant and cure constant of the particular system used. Suitable such systems are illustrated in the following Formulations and Examples in which the parts and percentages are by weight unless otherwise specified.

Polymeg 1000 and Polymeg 2000 are charged into the reactor and the mixture heated 70.degree. C. It is demoisturized in vacuum for 2-3 hours until the evolution of bubbles ceases.

Afterwards a dry nitrogen blanket is applied and the mixture is cooled to 50.degree. C and Hylene is added. The reaction mixture is stirred at 100-120 rpm for at least 30 minutes and watched, for a slight exothermic reaction may ensue. The temperature of the reactor is maintained at 65.degree.-70.degree. C. The catalyst is added in portions in order to speed up the reaction. After 3 hours have elapsed the NCO content is checked using the n-dibutylamine titration method. The NCO content should be in the range of 4.8%. The variation here and elsewhere may be .+-. 5 percent.

When this level of free NCO is reached, the contents of the reactor are cooled and are packaged into one gallon or one quart lined containers. Dry nitrogen is used to maintain an inert atmosphere in the containers which are then tightly closed.

Preparation procedures are the same as in Formulation 1. The free NCO content of the prepolymer should be 4.56%.

Preparation procedures are the same as in Formulation 1. The free NCO content should be 4.18%.

Poly(oxytetramethylene) glycols, Polymeg 2000 and Polymeg 1000, are charged into a reactor and demoisturized in vacuum for 2-3 hours upon a gentle stirring of 60-120 rpm at 70.degree. C.

The demoisturized glycol mixture is cooled down to 50.degree. C, a dry nitrogen blanket is applied, and diisocyanate (Hylene W) is added. The catalyst is added in portions in order to speed up the reaction.

The charge of the reactor should exotherm. The temperature of the reactants should not be allowed to go over 75.degree. C. After 2-3 hours of the reaction, the NCO content should be checked by the n-dibutylamine titration method. The NCO content should be in the range of 3.3%. If the content of NCO higher than 3.7% is found, the heating should be continued for an additional hour at 70.degree. C after the addition of a small amount (0.005%) of the catalyst.

The above soft isocyanate-terminated prepolymers are essentially linear.

Polymeg 650 and Pluracol TP 440 are charged into the reactor and the mixture is heated to 70.degree. C. It is then demoisturized under vacuum for 2-3 hours until the evolution of bubbles ceases. Afterwards a dry nitrogen blanket is applied, the mixture cooled to 40.degree. C and Hylene W added. The reaction mixture is stirred at 100-200 rpm for at least 30 minutes, taking care to control any exothermic reaction which may occur. The temperature in the reactor is kept at a level of 65.degree.-70.degree. C. The catalyst is added in portions, if necessary, to speed up the reaction.

After two hours have elapsed, the NCO content is checked by means of the n-dibutylamine titration method. The NCO content should be in the range of 13%. When this level of free NCO is reached, the contents of the reactor are cooled and packaged into one gallon or one quart lined containers. The empty space in the containers is filled with dry nitrogen.

Preparation procedure is identical to the Formulation 5 procedure. The free NCO content should be 10.55%.

The above hard isocyanate-terminated prepolymer which is made from a tri-functional polyol is branched and is introduced for crosslinking purposes.

All the pigments are dispersed in 5% of the total polyol, Pluracol 355. For dispersion purposes a ball mill or roller mill or any well-dispersing high speed mill can be employed.

Then all of the remainder of the polyol, Pluracol 355, is stirred in. Afterwards the mixture is degassed and demoisturized by applying a vacuum and gentle heating at 60.degree.-70.degree. C.

The catalyst has to be added before application. The amount of the catalyst depends on the type of isocyanate-terminated prepolymer to be used. Usually 0.15-0.35% of the catalyst is added, based on the total weight of the polymer and on the type of the polymer and the reacting groups.

All the pigments are dispersed in 5% of the polyols; then all the remainder of the polyols is blended with the pigment dispersion. Afterwards the mixture is demoisturized by applying a vacuum and gentle heating at 60.degree.-70.degree. C.

The catalyst has to be added before application. The amount of the catalyst depends on the type of isocyanate-terminated prepolymer to be used.

Usually for the rigid elastomer formulation the amount of the catalyst is in the range of 0.15-0.25%, for the soft elastomer formulation, in the range of 0.30-0.35%.

Preparation procedure is similar to the procedure of Formulation 8.

Preparation procedure is similar to the procedure of Formulation 8.

Poly(oxytetramethylene) glycol is charged into a reactor and demoisturized in vacuum for 2-3 hours upon gentle stirring at 60-120 rpm at 70.degree. C. Then the vacuum is released under dry nitrogen, and the dry nitrogen blanket is retained during the reaction time.

Desmodur N-triisocyanate is stirred in and reacted with the glycol until the NCO content is reduced to zero. Then Pluracol TP 1540 is blended in.

The pigments are dispersed in a small amount of the triol, Pluracol TP 1540, and stirred in with the total content of the prepolymer-polyol blend.

Poly(oxytetramethylene) glycol is charged into a reactor and demoisturized in vacuum for 2-3 hours upon a gentle stirring of 60-120 rpm at 70.degree. C. Then under a dry nitrogen blanket, the triisocyanate is stirred in. The components are reacted until all free NCO disappears. Then Pluracol PeP 650 is blended in. A small portion of Pluracol PeP 650 is employed for dispersion of pigments.

The dispersed pigment base is added to the prepolymer-polyol blend and the contents are stirred properly.

The components A and B are mixed together with the catalyst, degassed and then cast as above described, according to whether the components are selected to produce a hard non-hydrophilic polyurethane elastomer or a soft one. The following examples are illustrative.

Component A, Formulation 5, 100 parts

Component B, Formulation 7, 36 parts

Catalyst, dibutyltin dilaurate, 8 drops

Nco/oh = 1.08 to 1

Components A and B are degassed and demoisturized for at least 1 hour at 60.degree. C and then blended gently with the catalyst and placed in a pre-heated vacuum oven for 1-2 minutes. The mixture is then poured into a pre-heated denture mold treated with a mold release material as previously described and heated at 40.degree. C for at least 1/2-1 hour. A further period of 2 or 3 hours at the same temperature is necessary to achieve a satisfactory cure but this further heating may be effected simultaneously with the curing of the soft elastomer. The two halves of the flask prepared as above described form the complete denture mold, for the casting of the soft-non-hydrophilic polyurethane elastomer onto the cast hard-non-hydrophilic polyurethane elastomer.

The preparation of soft, non-hydrophilic polyurethane elastomers is illustrated in the following example.

Component A, Formulation 1, 100 parts

Component B, Formulation 7, 13.6 parts

Catalyst, stannous octoate, 8 drops

Nco/oh = 1.08 to 1

Components A and B are degassed and demoisturized for at least 1 hour at 60.degree. C and then blended gently with the catalyst and placed in a pre-heated vacuum oven for 1-2 minutes. They are then cast into a pre-heated denture mold containing a previously cast hard-non-hydrophylic polyurethane elastomer as above described and kept in an oven at 90.degree. C for 3 hours. The denture is then removed from the mold and finished by removing the sprues and flash and polishing as necessary.

In place of the hard non-hydrophilic polyurethane elastomer-forming composition of Example 1, there may be substituted the following.

Component A, Formulation 5, 100 parts

Component B, Formulation 9, 36.9 parts

Catalyst, dibutyltin dilaurate, 16 drops

Nco/oh = 1.05 to 1

Component A, Formulation 6, 100 parts

Component B, Formulation 9, 16 parts

Catalyst, dibutyltin dilaurate, 16 drops

Nco/oh = 1.05 to 1

The compositions of Examples 3 and 4 are degassed, demoisturized, blended and otherwise treated as in Example 1.

Component A, Formulation 5, 100 parts

Component B, Formulation 12, 50 parts

Catalyst, stannous octoate, 0.40 parts

Nco/oh = 1.1 to 1

Components A and B should be degassed and demoisturized under vacuum before blending. Then the catalyst should be blended in. The charge should be cast into a preheated mold, treated with a mold release agent. The elastomer should be cured for a half hour at 40.degree. C.

The soft non-hydrophilic polyurethane elastomer-forming compositions of Example 2 may be substituted by the following:

Component A, Formulation 2, 100 parts

Component B, Formulation 8, 7 parts

Catalyst, dibutyltin dilaurate, 12 drops

Nco/oh = 1.05 to 1

Component A, Formulation 3, 100 parts

Component B, Formulation 8, 6.44 parts

Catalyst, dibutyltin dilaurate, 16 drops

Nco/oh = 1.05 to 1.

The compositions of Examples 6 and 7 are degassed, demoisturized, blended, cast, and cured as in Example 2.

Component A, Formulation 4, 100 parts

Component B, Formulation 11, 56.2 parts

Catalyst, stannous octoate, 0.32

Nco/oh = 1.05 to 1.

Components A and B should be heated up to approximately 60.degree. C and degassed and demoisturized under vacuum before blending. Then the catalyst should be added. The blend should be cast into a preheated mold and heated with a mold release agent. The elastomer should be cured in an oven at 95.degree. C for 2 hours.

Dentures may be made up of any combination of hard non-hydrophilic polyurethane elastomer-forming compositions, for example, Examples 1, 3, 4, and 5 with any of the soft hydrophilic polyurethane elastomer-forming compositions, for example, Examples 2, 5, 6, and 8. Particularly good results are obtained by casting Example 8 on Example 5.

It is to be understood that the invention is not to be limited to the exact details of operation or structure shown and described as obvious modifications and equivalents will be apparent to one skilled in the art.