GB2027035A - Dental Impression Material - Google Patents

Abstract

A two-part dental impression material comprises a first part containing a product of the reaction of an alpha , beta -ethylenically unsaturated dicarboxylic acid anhydride and a liquid polydiene resin, and a second part containing a setting agent for said reaction product, one or both said parts of the material optionally comprising one or more catalysts for an anhydride ring-opening reaction, plasticisers, fillers, pigments and diluents and having a pasty consistency. Preferably, both parts of the dental impression material are in the form of pastes which are mixed prior to use to provide an impression material which can set within a relatively short period. The setting agent is suitably a compound having at least two groups capable of reacting with a carboxylic anhydride group, e.g. amino, hydroxy, amido or epoxy groups, alkanolamines (or salts thereof) being particularly useful.

Description

SPECIFICATION Dental Impression Material This invention relates to a composition useful as a dental impression material, a method of preparing the composition, and a process for obtaining dental impressions.

In the practice of dentistry, paste-like impression materials are used to take impressions of the teeth and jaws of patients so that a model of those parts can be made and the dental prosthetis prepared. Such a model must be of the highest accuracy and therefore the impression taken must retain the greatest accuracy of detail and overall dimensions from the time the impression is taken until the model is made.

Furthermore since some prostheses such as crown and bridge prostheses involve intricate preparations, often in sub-gingival areas, the impression material must be strong in thin sections, and sufficiently elastic to be withdrawn from undercuts without permanent distortion.

Reversible hydro-colloidal gels have been used for this purpose but suffer from their low tear strength which makes them extremely difficult to work with, and from their extreme lack of dimensional stability in conditions of variable atmospheric humidity which necessitates casting the model immediately after taking the impression.

Certain thermoplastic materials have also been used but these are inconvenient in that they have to be applied hot, and set to a hard rigid material which renders them unsuitable for impressions with undercuts.

Previously used elatomeric materials include: polysulphide rubbers which have been described as "fairly" slow setting, odorous, and dirty to handle" (Braden, M., Causton B., and Clarke R. L., "A Polyether Impression Rubber", Journal of Dental Research 51, 4, July/August 1972): and silicone rubbers which have been described as "less dimensional!y stable than polysulphides", and of which "the catalyst system has a limited shelf life" (op. cit). Materials have also been proposed which are based upon a polyether rubber with imine group termination to the polymer chains, which is susceptible to dimensional change with change of humidity and has too low a tear strength and is also rather too hard to be ideal. In addition since the preparation of the imine-terminated polyether necessitates handling highly toxic materials, its manufacture is expensive and dangerous.

It has also been proposed to blend a polythioether material with a multifunctional acryiate with which it cures: such materials will be objectionable on the grounds of the sensitivity of most patients to skin contact with such acrylate monomers.

We have now found that an impression material of good dimensional stability, and adequate strength, can be prepared from certain relatively inexpensive, easily obtainable materials based on resins derived from liquid polydienes, and employing curing agents that are neither odoriferous nor toxic, and which set at room temperature in less than 10 minutes.

Accordingly, bne aspect of the present invention provides a two-part dental impression material comprising a first part containing a product of the reaction of an fr, 13-ethylenically unsaturated dicarboxylic acid anhydride and a liquid polydiene resin, and a second part containing a setting agent for said reaction product one or both said parts of the material optionally comprising one or more catalysts for an anhydride ring-opening reaction, plasticisers, fillers, pigments and diluents and having a pasty consistency. Preferably, both parts of the dental impression material are in the form of pastes which are mixed prior to use to provide an impression material which can set within a relatively short period.

According to another aspect of the present invention, there is provided a process for producing a dental impression material, wherein a liquid polydiene resin is reacted with an ez, p- ethylenically unsaturated dicarboxylic acid anhydride, and the product of the reaction is optionally blended with one or more catalysts for an anhydride ring-opening reaction, plasticisers, fillers, pigments and diluents to form a first part of the material, and wherein a second part of the dental impression material comprises a setting agent for said reaction product, and, optionally, one or more catalysts for an anhydride ringopening reaction, plasticisers, filler, pigments and diluents, the dental impression material being a paste obtained by mixing said first and second parts.

Yet another aspect of the present invention provides a process for taking dental impressions, using a two-part dental impression material, in which a first part is mixed with a second part to obtain a pasty mixture which is placed in a suitable mould cavity and the mould so filled brought into contact with a patient's teeth or jaws or a dental model or prosthesis, held in contact until the mixture is set and thereafter removed leaving an impression in the filled mould, wherein the two-part dental impression material is the impression material just indicated.

The liquid polydiene resin used as the starting material for making the present dental impression material may be a homopolymer of a conjugated diene prepared by polymerisation techniques known per se which result in a liquid polymer.

Alternatively, the liquid polydiene resin may be a copolymer of a conjugated diene with one or more other monomers containing ethylenic unsaturation and copolymerisable with the diene, the copolymer comprising a major proportion, that is at least 50% by weight, of diene and a minor proportion of copolymerisable monomer.

The preferred diene is butadiene and a preferred copolymerisable ethylenically unsaturated monomer is styrene.

The liquid polydiene resins used in preparing the aforesaid starting materials are therefore preferably polybutadienes, which may contain a proportion of copolymerised momomer such as styrene particularly where a hard final impression material is required.

The molecular weight of the liquid polydiene resin may conveniently be as low as 500 and the upper limit will be consistent with the requirement that the material derived from the resin be liquid, but in any case will not be higher than about 50,000, and preferably is not higher than about 10,000.

In preparing the aforesaid starting materials, the selected liquid polydiene resin is first reacted with an fr, ss-ethylenically unsaturated dicarboxylic acid anhydride such as maleic anhydride, and this step is referred to in this specification as maleinisation, and the product as a maieinised polydiene resin.

The maleinisation reaction is well known in the preparation of resins, and although the chemistry of the reaction is uncertain, it results in the random addition of maleic anhydride units to the polymer, the anhydride ring structure being retained intact so long as ringopening catalysts are excluded from the reaction.

The conditions of the maleinisation step are conventional, and, for example, involve heating the reacting materials together at elevated temperature around 1 900C. for a period of from 3 to 6 hours in the absence of atmospheric oxygen.

The choice of conditions is governed by considerations well known in the art, viz the necessity of achieving a sufficient degree of reaction of the materials being reacted whilst avoiding gellation of the product or excessive discoloration. In this connection it is usually desirable to incorporate a small proportion, say less than 1% weight, of for example a hindered phenol or other inhibitor to obviate cross-linking, and catalysts for the maleinisation may also be included. It may also be convenient, if it is desired to use a diluent or plasticiser with the maleinised polydiene resin, to incorporate this with the polydiene resin before the maleinisation step, as long as such materials are inert towards the anhydride groups incorporated into the polydiene resin.

The proportion of maleic anhydride reacted during the maleinisation step is selected according to the degree of reactivity required in the impression material paste and the degree of flexibility or rubberiness of the final impression material. Generally enough maleic anhydride is reacted to provide from 1 to 6 anhydride groups per molecule of polydiene resin and preferably from 1 to 3.

After the maleinisation reaction, the product may be stripped by heating under vacuum to remove any traces of volatile material, and to this end a volatile solvent or diluent may be added prior to this operation to assist in this step.

Similarly although such a step will not normaliy be necessary it may be desirable to wash the product of maleinisation by disperion in a solvent with which it is immiscible, such as water, followed by separation of the product from the solvent, drying the product, etc. in order to remove traces of soluble material from the product. in any case it is desirable to provide a liquid maleinised polydiene resin free from such contaminants as would be undesirable in the final impression material.

In the preparation of the first part of the twopart impression material of the present invention, the maleinised polydiene resin obtained in the manner described above, may then be blended with diluents or plasticisers to form a paste if it is desired to control the viscosity of the product or the plasticity of the final impression material in this way, but it is preferable to achieve the control by selection of the molecular weight of the polydiene resin, its reactivity in terms of the proportion of reactive anhydride groups in terms of the proportion of reactive anhydride groups, and the proportion of setting agent used, as will be described below.In this way, the inclusion of materials which are capable of being lost to the final impression material either by evaporation, leaching, or migration is avoided with the consequent undesirable effects on the dimensional stability of the impression material.

In the preparation of the first part in the form of a paste, it is usually desirable to blend the maleinised polydiene resin with one or more inert fillers in order to confer opacity and/or toughness on the final impression material, and to a lesser extent to slow down the rate of setting of the paste, and to control the viscosity of the paste.

The viscosity is selected so that the two parts of the impression material may be easily mixed by hand usually with a spatula or similar instrument, and with this object the paste must not be so fluid as to flow away from the mixing area, nor so stiff as to render efficient mixing difficult. Thus, the proportion of filler chosen will usually be from zero to 500 parts by weight for each 100 parts of liquid maleinised polydiene resin, and preferably from 10 to 300 parts by weight.

The filler used may be any of the materials well known in the fields of rubber or plastics technology and may be of organic or inorganic nature. Preferably the filler will be one which does not impart a strong colour to the impression material and will preferably be of a relatively fine particle size, that is below 500 microns, such that the fineness of detail of the final impression is not impaired. Examples of fillers which are suitable include China Clay, whiting, ground limestone, barytes, lithopone and silica.

The filler may be used in conjunction with one or more pigments whose function is to enhance opacity and to colour the impression material so as to present an attractive appearance. It will be understood that since the maleinised liquid polydiene resin is not of itself strongly coloured, only moderate proportions of opacifying and tinting pigments are necessary to achieve an attractive colouration in the final impression material.

There may also be present in the first paste a catalyst for the setting process, which may be one of the materials described later.

The fillers, pigments, plasticisers, diluents and catalyst may be blended with the maleinised liquid polydiene by grinding, milling, stirring, or any other method well known in the art, to form the first paste of the two-paste impression material.

The second part of the impression material contains one or more setting agents for the maleinised liquid polydiene resin optionally together with one or more catalysts for the reaction of the anhydride groups in the maleinised liquid polydiene in the first paste with the setting agent or agents in the second part which may also be in the form of a paste.

The setting agent is a material which contains at least two reactive hydrogen atoms per molecule and which is capable of forming a crosslink between molecules of the maleinised polydiene. The hydrogen atoms are preferably contained in groups attached to different carbon atoms in the molecule of the setting agent for the sake of superior reactivity, but may be contained in the same group or in groups attached to the same carbon atom.

Suitable setting agents include amines, especially polyamines; polyols; amides especially polyamides; alkanolamines and alkanolamides.

Alternatively, a polyepoxide may be used, but in such a case it is necessary that an epoxide ringopening or carboxylic anhydride ring-opening compound should be present for the sake of enhanced reaction rate.

Catalysts for the setting process may as stated above be present in any case, and these will be substances which enhance the carboxylic anhydride ring opening reaction. Examples of suitable materials include amines whether primary, secondary or tertiary, and their salts, and quaternary ammonium compounds. It will be appreciated from the above description that substances such as alkanolamines or their salts are both setting agents and catalysts for the anhydride ring-opening reaction. It will further be appreciated that such materials may not satisfactorily be incorporated in the first paste as catalysts since this would render the first paste unstable to storage.

Preferred setting agents are the alkanolamines or their salts with carboxylic acids, used either on their own or in admixture with polyols. A particularly preferred alkanolamine is triethanolamine. Preferred salts of triethanolamine are its soaps with oleic acid, stearic acid, and other carboxylic acids.

In general the choice of setting agent and/or catalyst for incorporation in the second paste is dictated by the requirement that the first and second pastes when mixed should set at ordinary room temperatures within a short time such as a few minutes. Another requirement is that the composition of the second paste should not be toxic or noxious either to the person carrying out the mixing of the two pastes, or the patient whose teeth or jaw will come into contact with the impression material formed by the mixture of the two pastes. It is therefore a particularly useful feature of the present invention that those materials may conveniently be employed in the second paste as setting agents and/or catalysts which are of known low toxicity and irritancy, such as the soaps of alkanolamines mentioned above, and which have no objectionable odour or taste.

The second paste, besides containing the substances described may also contain fillers, diluents, plasticisers, pigments and the like. In particular an inert filler may be useful for the same reasons that apply to the first paste, and the same sorts of illers may be used as are used in the first paste. The same considerations apply to the incorporation of plasticisers and diluents in the second paste as apply to their incorporation in the first paste. A pigment or colourant may be particularly useful in the second paste as providing the person carrying out the mixing of the two pastes with a visual check of the completeness of mixing. The constituents of the second paste may be mixed by the same processes as those used to prepare the first paste.

The proportion of the second paste to be used with the first paste will depend on a number of factors, principally the content of setting agent in the second paste and of maleinides liquid polydiene in the first paste, the speed of set required, and the degree of cross-linking required in the final impression material. In general, the greater the degree of cross-linking the harder will be the final impression material and the lower, the greater its flexibility and elongation at break. In general the proportions of second paste to first paste will be chosen so that enough setting agent is mixed with maleinised liquid polydiene to react with from 20% to 100% of the anhydride groups present.

The process for preparing dental impressions of the present invention comprises the steps of (1) measuring the requisite porportions of the first paste and the second paste, the compositions of which are as described above, on to a flat smooth surface on which the mixing is to take place, which measurement may be most accurately made by weighing but will be sufficiently accurately achieved by placing appropriate lengths of extrudate of each of the two pastes extruded from collapsible containers on to the mixing surface, (2) mixing two pastes by hand using a spatula or similar instrument until the mixture is uniform, (3) filling a dental impression tray with the mixed pastes, (40 applying the tray to the patient's teeth or jaw so that the parts of which an impression is required are immersed in the paste, (5) holding the tray in position until the impression material is set, and (6) removing the tray containing the impression carefully from the teeth or jaw.

The time required for set will depend on a number of factors, described hereinbefore, but is conveniently chosen to be from 4 minutes to 10 minutes at ordinary room temperatures.

The impression so obtained is accurate as to fine surface detail and dimensions without any change in the latter greater that 0.1% over a period of seven days which figure is not significantly altered by alteration in the climatic conditions of storage. Furthermore the impression material is strong enough to enable it to be removed from the teeth or jaw without tearing, elastic enough to recover from any temporary distortion necessary for its removal, i.e. in undercut areas, and strong enough in thin sections to reproduce fine sections of sub-gingival areas.

The invention will now be illustrated by the following non-limiting Examples.

Examples 1 to 3 are examples of the preparation of maleinised liquid polybutadiene.

Example 1 2500 grams of a liquid polybutadiene of molecular weight approximately 5000, a viscosity at 250C. of approximately 50 poise and a proportion of vinyl 1,2 linkages comprising approximately 20% of the butadiene linkages in the polymer chain, are charged to a 5 litre reaction vessel with 12.5 grams of an alkaylated aryl phosphire antioxidant and 250 grams of xylene, and the mixture is heated to 1 200C. under a nitrogen blanket with efficient stirring.

At this temperature, 125 grams of maleic anhydride are added, the reaction mixture is then heated to 1 900C. and held at this temperature for 3 hours. The product is then cooled to 1200 C.

and the pressure reduced to Smm.Hg, stirring being maintained for 3 hours while the volatile materials are removed and condensed, whereafter the product is allowed to cool. The resultant maleinised liquid polybytadiene adduct is a clear amber liquid having a viscosity at 250C. of about 300 poise and an acid value of 46 mg.KOH/gram.

Example 2 An adduct is prepared in the manner described in Example 1, but starting with 2375 grams of the polybutadiene to which is added 125 grams of Soya bean oil.

In this case, the maleinised liquid polybutadiene adduct is a clear amber liquid with a viscosity at 250 C. of 160 poise and an acid value of 40 mg.KOH/gram.

Example 3 An adduct is prepared in the manner described in Example 1, but starting with 2000 grams of a polybutadiene of molecular weight approximately 900, a viscosity at 250C. of approximately 10 poise and of which 45% of the butadiene units in the polymer are present as vinyl 1,2 units, the polybutadiene being reacted with 380 grams of maleic anhydride at 1 900C. for 4 hours. The maleinised adduct is an amber liquid with a viscosity at 250C of greater than 1000 poise and an acid value of 1 60 mg./KOH gram. Examples 4 to 6 illustrate the preparation of two-paste dental - impression materials.

Example 4 A two-paste material was prepared. A first paste was prepared by dispersing 800 grams of China clay in 1000 grams of maleinised liquid polybutadiene as prepared in Example 1, using a high-shear mechanical stirrer to obtain a smooth paste. A second paste was prepared by mixing 60 grams of titanium dioxide pigment, with 30 grams os triethanolamine in a planterary mixer to obtain a smooth paste. An impression material was prepared by mixing on a glass plate 100 grams of the first paste and 5 grams of the second paste using a spatula. Mixing took about T a minute to obtain a smooth uniform paste, as evidenced by the uniformity of the colour of the paste.

The impression material was placed in an impression tray for the lower jaw and an impression taken by holding the material in contact with a prosthetic plate of a lower jaw and teeth for 4 minutes. The impression tray was removed easily and was found to have yielded an accurate rubbery impression of the teeth of the prosthesis. The impression exhibited no appreciable change in dimensions over 7 days storage at 250C. and 68% relative humidity.

Example 5 A first paste was prepared by the procedure described in Example 4 but starting with 1000 grams of the maleinised polybutadiene as prepared in Example 2, and dispersing in this 800 grams of China clay. A second paste was prepared from 100 grams triethanolamine oleate soap and 5 grams of finely-divided red pigment powder.

An impression material was prepared by mixing 100 grams of the first paste and 5 grams of the second paste in the manner described above and, in this case, the material took 6 minutes to set sufficiently to be removed from the teeth.

Example A first paste was prepared by the method described in Example 4, but starting with 500 grams of maleinised polybutadiene as prepared in Example 2 blended with 500 grams of maleinised polybutadiene as prepared in Example 3. Into this blend were dispersed 500 grams of China clay. A second paste was prepared as in Example 5. An impression material was prepared by mixing 100 grams of the first paste with 12 grams of the second paste. An impression material was obtained which set within a period of 5 minutes and which was harder than the material obtained in Example 5.

Example 7 A maleinised polybutadiene was prepared by the method described in Example 1 but starting with a blend of 2375 grams of a liquid polybutadiene of molecular weight approximataley 3400, a viscosity at 250 C. of approximately 80 poise, and a proportion of vinyl 1,2 linkages comprising about 20% of the butadiene linkages in the polymer chain, with 125 grams of soya bean oil, and in which the heating time was 4 hours and the temperature 1 850C.

The maleinised polybutadiene so obtained had a viscosity at 250C. of approximately 380 poise and an acid value of 38 mg. KOH/gram.

A first paste was prepared as described in Example 4 but using the maleinised polybutadiene obtained in this Example. The second paste was prepared, and an impression material obtained, as described, in Example 5. The impression material was noticeably stronger and more resilient than that obtained in any other Example.

Claims (25)

Claims

1. A two-part dental impression material comprising a first part containing a product of the reaction of an se, p-ethylenically unsaturated dicarboxylic acid anhydride and a liquid polydiene resin, and a second part containing a setting agent for said reaction product, one or both parts of the material optionally comprising one or more plasticiser, fillers and pigments and having a pasty consistency, and the second part optionally containing a catalyst for an anhydride ringopening reaction.

2. A material as claimed in claim 1, wherein both parts of the material are in the form of pastes.

3. A material as claimed in claim 1 or 2, wherein the liquid polydiene resin is a homopolymer of a conjugated diene, or a copolymer comprising a major proportion of a conjugated diene and a minor proportion of one or more other monomers containing ethylenic unsaturation and copolymerisable with the diene.

4. A material as claimed in anyone of claims 1 to 3, wherein the liquid polydiene comprises polybutadiene.

5. A material as claimed in claims 3 and 4, wherein the liquid polydiene comprises a copolymer of butadiene and styrene as said other monomer.

6. A material as claimed in anyone of claims 1 to 5, wherein the liquid polydiene resin has a molecular weight of from 500 to 50,000.

7. A material as claimed in anyone of claims 1 to 6, wherein the liquid polydiene resin has a molecular weight of from 500 to 10,000.

8. A material as claimed in anyone of claims 1 to 7, wherein the tor, fx"B-ethylenically unsaturated acid is maleic acid.

9. A material as claimed in claim 8, wherein said reaction product contains from 1 to 6 anhydride groups per molecule of polydiene resin.

10. A material as claimed in claim 8 or 9, wherein said reaction product contains from 1 to 3 anhydride groups per molecule of polydiene resin.

11. A material as claimed in anyone of claims 1 to 10, wherein one or both parts of the material comprise up to 500 parts by weight of filler for each 100 parts by weight of reaction product of tr,-ethylenically unsaturated dicarboxylic acid anhydride with liquid polydiene.

12. A material as claimed in anyone of claims 1 to 11, wherein one or both parts of the material comprise from 10 to 300 parts by weight of filler for each 100 parts by weight of reaction product of ru,l-ethylenically unsaturated dicarboxylic acid anhydride with liquid polydiene.

13. A material as claimed in any one of claims 1 to 12, wherein the filler has a particle size below 500 microns.

14. A material as claimed in any one of claims 1 to 13, wherein the filler is selected from China clay, whiting, ground limestone, barytes, lithopone and silica.

1 5. A material as claimed in any one of claims 1 to 14, wherein the setting agents is a substance which contains at least two reactive hydrogen atoms per molecule and which is capable of forming a cross-link between molecules of said reaction product.

1 6. A material as claimed in claim 15, wherein the setting agent is an amine selected from monoarnines, .pelyamics and alkanola mines.

1 7. A material as claimed in any one of claims 1 to 14, wherein the setting agent is a polyepoxide in conjunction with an epoxide ring opening or carboxylic anhydride ring-opening compound.

18. A material as claimed in any one of claims 1 to 17, wherein the catalyst is an amine, an alkanolamine, an amine salt or a quaternary ammonium compound.

1 9. A two-part dental impression material substantially as hereinbefore described in any one of Examples 4 to 7 of the foregoing Examples.

20. A process for producing a dental impression material, wherein the two parts of the dental impression material claimed in any one of the claims 1 to 1 9 are mixed to form a paste.

21. A process as claimed in claim 20, wherein the proportion of second part to first part is such as to provide sufficient setting agent to react with from 20% to 100% of the anhydride groups present in said reaction -product.

22. A process for producing a dental impression material substantially as hereinbefore described in any one of Examples 4 to 7 of the foregoing Examples.

23. A dental impression material produced by the process claimed in any one of claims 20 to 22.

24. A method of taking a dental impression, wherein the -pasty dental impression material claimed in claim 23 is placed in a cavity in a dental impression mould, and the mould is brought into contact with a patient's teeth or jaws or with a dental model or prosthesis, and wherein the mould is held in such contact until the dental impression material is set and is thereafter removed to leave an impression in the material in the mould cavity.

25. A method of taking a dental impression substantially as hereinbefore described in any one of Examples 4 to 7 for the foregoing Examples.