US2733157A

US2733157A - Impression material and method of -

Info

Publication number: US2733157A
Authority: US
Publication date: 1956-01-31
Anticipated expiration: 1973-01-31

Description

United States Patent 2,733,157 Patented Jan. 31, 1956 IMPRESSION MATERIAL AND lVIETHOD OF MAKING SAME John A. Cornell, Upper Merion Township, Montgomery County, and Harry E. Kemrnler, Elkins Park, Pa., assignors to H. D. .lusti & Son, Inc., Phiia'iieiphia, Pih, a corporation of Pennsylvania N Drawing. Application September 22, 1953, Serial No. 381,736

12 Claims. (Cl. 1ll638.5)

This invention relates to impression materials for taking impressions, pertaining particularly to materials for taking impressions of parts of human beings, such as of the mouth for dental purposes and of the outer ear for aural purposes, and constitutes a continuation-in-part of application Serial No. 283,984, filed April 23, 1952.

In taking dental impressions, for instance, according to well-established practices, the impression material mass is mixed to its attained consistency, it is applied to a support, such as a tray, is inserted in the patients mouth in forced intimate contact with the teeth and gums to be reproduced, is held in such contact until set-up of the mixture in the imparted form is completed, the tray-mounted mass is then withdrawn to form an integral mold from which requiredly accurate plaster-of- Paris or artificial stone models can be made.

The fact that the impression material is to be used for human dental and like impression purposes imposes certain critical and minimum requirements for an impression material to meet all of the factors encountered with patients in dentistry. These requirements of the ideal impression material are as follows: it must be bland and neither toxic nor caustic in its reaction with human tissue; it must be usable and settable at substantially mouth temperatures to minimize thermal shocks before, during and after setting; it must set rapidly in the mouth to a gel of sutlicient dimensional stability adequate to prevent tearing and with high elasticity and of fine texture, from which very accurate reproductions of the contacted structure of the mouth can be made; its dimensional stability must be such as to minimize early shrinkage and contraction; it must have a closely controlled rate of gelation so that the mixture will not set after mixing and while preparing the tray, and yet will set in a predictably short period of time, as a result of which there is a minimal discomfort to the patient, and with the time rate of gelation substantially constant for each successive batch or mix, so that the dentist will quickly recognize and allow for the time factors necessary in successful use of the material. A satisfactory impression material is inert and non-reactive with dental stone or plaster; and it must have immunity to deterioration during storage, regardless of incident temperature and humidity conditions, so that it has satisfactory shelf life. The foregoing requirements are found from prior experience with known impression materials to involve operating factors which may be mutually antagonistic, so that an improvement in one factor will work detriment to another factor. Some of the desired characteristics such as the setting rate and factors affecting comfort, cannot be controlled within thewide range required for different types of patients and different kinds of dental manipulations. The order of importance of the requirements may vary with dilferent situations and the foregoing listing should not be considered as one in order of importance.

It is an object of the invention to provide an impres sion material which more nearly approximates an ideal impression material and which will better satisfy the requirements of this ideal material as indicated above, said impression material being available at reasonable cost.

Many efforts have been made to improve impression materials for dentistry. However, each of these eflforts has failed to meet one or more of the recited essential attributes of the dental impression material stated above. These efforts, so far as known, are characterized by certain disadvantageous features militating against the completely satisfactory use thereof, despite the fact that, perforce, in the absence of better material, a certain degree of commercial utilization has been effected.

In general, so far as actually produced and sold dental impression materials are concerned, in the main they have been of two types or groups. One group (a) is a reversible type of impression material, typified, for instance, by an agar hydrocolloid compound, and the other group (b) is an irreversible type of impression material, typified, for instance, by an alginic acid compound. These are unsatisfactory within the concept for the requirements of the ideal impression material as recited above for'several reasons.

For instance, the (a) type ofcompounds require extension manipulation, must be heated to be softened to a degree making them usable but without raising the temperature too high to complicate the manipulative operations before using the material, and thus require accurate temperature control. The heated wax has a shocking and adverse reaction to the human tissues. Further, in order to make the compounds set, it has been customary, in the interest of speed, to spray the compound with chilled Water, requiring bulky cooling equipment, and again with definitely adverse reactions from the patient. Finally, even when set, the material has small elasticity and inadequate form-retention so that undercutsand the like tear the impression and militate against its fidelity at points Where it is or may be most important.

The type (b) alginate compounds are fairly satisfactory as regards equipment and handling, as well as from the standpoint of the patient and his comfort, but are inherently characterized by other unsatisfactory characteristics. As is well known, the type (b) compounds involve the gelling of a soluble potassium or sodium alginate in aqueous solution by the gradual dissolution of a slightly soluble calcium salt, producing calcium ions, 1

which form a gel with the alginate. One of the primary disadvantages of this type of material is that, although the formed gel is elastic, in the sense that ittends to return by its own resilience to its acquired or imparted form after deformation from that form, as in the removal of formed gel from contact with the teeth or edentulous ridges of a patient, it is not possessed of very accurate dimensional stability, and the strength thereof is frequently insufiicient or inadequate to prevent the breaking off of thin undercuts or ridges. This type of material is also one that has an unfavorable reaction with dental stone or plaster with which it comes into contact, with consequent lack of fidelity of the molded form to the impression. Additionally, this material has a short shelf life, especially in hot and humid climates, and its dimensional stability suffers by reason of an early contraction of the gel.

The structure of a carboxy-methylcellulose unit has a considerable similarity to that of an alginate, although there are some diiferences therefrom. The main difference is in the repeat distance and the dimethylne ether linkage in the cellulosic material. The composition of carboxy-methylcellulose can be closely controlled so that the variability of composition and properties thereof is less than that of the alginate, which is produced from seaweed. A composition incorporating carboxy-methylcellulose is more uniform than one utilizing an alginate. One of the advantages of carboxy-methylcellulose lies in the fact that the alkali metal and ammonium salts of carboxy-methylcellulose are water soluble. However, ammonium salts measurabily affect the setting of artificial stone or plaster-of-Paris. The alkaline earth salts have limited solubility, are somewhat thixotropic in character and may behave with the unsubstituted hydroxyl groups to form additional compounds which in turn affect the setting of artificial stone or plaster.

The degree of substitution is important, since it has been found that a lower carboxyl substitution in the recurring anhydro glucose units tends to proportionately inhibit the setting of artificial stone. It has further been found that the highest substitution (1.3 per glucose unit) causes but little interference in respect of the setting of such artificial stone. This is a factor which prior researchers have apparently ignored with consequent deficiencies in the performance of impression materials incorporating carboxy-methylcellulose.

As a result of extensive tests, it has been discovered that the salts most likely to precipitate and form a strong gel with carboxy-methylcellulose alginic acid are those of the amphoteric multivalent metal elements, such as alummum, chromium, iron, lead, cadmium, zirconium, cerium, and the like. However, there are few salts of these metalswhich can be practically used because the precipitat1ng ion is not readily available and, too, their use must assure an acceptable end product. Lead salts are not acceptable as gel-forming agents for the specific purposes of this invention, even though effecting the formation of an excellent gel. This is because (1) of their toxicity; and (2) the solubility product of lead carboxymethylcellulose, versus calcium sulfate supplied by the artificial stone or plaster upon casting, allows the free calcium to migrate to the gel, and to react undesirably as previously noted. The use of basic aluminum acetate is also limited, since the solubility of this salt is dependent upon the acetate content in both neutral and acid solution, and since long storage causes it to lose acetate in the form of acetic acid, thus becoming more difiicultly soluble; its use is unwarranted, and the availability of the precipitating aluminum 1011 throughout a homogeneous admixture of the carboxylated gel forming material is therefore limited, because of an essential lack of control of the amount of precipitatmg ion due to the inherent instability of the aluminum salt mentioned above.

It is an object of this invention to improve impression materials; to provide impression materials of high elasticity with dimensional stability; to provide impression materials which can be closely controlled as to their time of setting from the initial mixing to the final set; to provide at low cost a highly efiicient impression material for dental and aural purposes which is possessed of a minimal adverse relation to or upon human tissues; to provide impression materials, which, while attaining each of the other stated objects of the invention is substantially inert andpossessed of a controlled rate of gelation by the mere addit on of water to the composition.

This and other objects are accomplished by utilizing a source or reservoir of ions from a first material which is normally non-reactive with polycarboxy-polyhydroxy carboxymethyl cellulose or alginic acid material, and a water soluble salt forming ions which are reactive with the first material at a timed rate to liberate from said reservoir of non-reactive ions, ions which will precipitate and be reactive with the polycarboxy polyhydroxy material to form a gel; by utilizing a water-soluble metal chelate which is substantially non-reactive with the p lyhy r y polycarboxy material, the precipitating ion from said chelate may be liberated by displacing said precipitating ion with a salt of a displacing ion which is more tightly bound to the chelate than the precipitating ion and thereby causes the precipitating ion to react with polycarboxypolyhydroxy material to form a gel at a controlled rate.

Still further objects and advantages of the present invention will appear from the more detailed description set forth below, it being understood that such more detailed description is given by Way of illustration and explanation only, and not by way of limitation, since various changes therein may be made by those skilled in the art without departing from the scope and spirit of the present invention.

In carrying out the invention in an illustrative embodiment, there are provided a composition and a method of forming and of using the composition to form gels of adjustable, superior mechanical and reproducing qualities, such as are essential in the critical field of impression materials to which the invention pertains.

The composition contains in homogeneous admixture the water-soluble gelforming polyhydroxylic polyacid or its salts, such as the alginate or carboxymethyl cellulose water soluble alkali metal salts and a water-soluble chelate of a coagulating ion for the gel-forming material, said coagulating ion releasable from said chelate at a beneficial rate under minimum constant conditions of hydration to produce uniform strength and elongation in the coagulum after release by the action of a displacing ion more tightly bound to said chelate than said coagulating ion.

While the ability of many polycarboxylates, or their derivatives, to gel, has been recognized heretofore in a broad sense, the specific adaptation of these materials to the critical requirements for dental impression materials has not met with the desired success.

The invention herein thus recognizes that these materials, i. e. the polycarboxylates will not give all of the necessary properties as impression compounds, nor furnish the high fidelity of reproduction essential for good dental impressions, unless attention is given to:

(a) The number and distribution of carboxy groups and hydroxyl groups on the chain of the polymer. This ratio or relation basically controls gel strength and syneresis i. e. contraction of the gel expressing water, and the rate of gelation.

(b) The molecular weight of the polymer.

(0) The type of the gelling ions and the amount used.

(d) The actual rate of gelation and its control to the desire of the ultimate user.

(e) The use of particular salts to retain solvation of hydroxyls, or the use of hygroscopic salts to balance syneresis.

(f) The use of salts and ions which do not interfere with setting of plaster-of-Paris or artificial stones. (Certain ions and compositions may change plaster crystal habit, absorb components from the plaster, or migrate into the plaster, forming irregular or soft plaster or stone surfaces, or completely preventing setting thereof.)

(g) The mixability and wettability of the impressionpowder mix.

(h) The capability of long time storage stability before mixing. (The so-called shelf life) In carrying out this invention, in a one-phase or anization, that is one that requires only the addition of water to form the complete composition, with careful observance of the foregoing factors, a highly effective impression material is produced.

The basic constituent of the impression material is a natural polymeric material such as alginate or modified natural polymeric material (carboxymethyl cellulose) which contains free hydroxyl groups and free carboxyl groups. Illustratively, this may comprise a modified cellulosic material produced by carboxylating cellulose, and which material is known in the art and in its commercial forms as alkali; neutral or acid," carboxymethyl cellulose, referred to hereinafter as CMC." This material is marketed in various grades, based on molecular weight and the number of substitutions of carboxyls per glucose unit; from 1 to 1.3 carboxyl groups per recurring anhydro-glucose unit provides satisfactory dis- E solving mixing and setting characteristics. These polyhydroxyl-polycarboxy materials with the just recited characteristics will precipitate to form fair (but inadequate) gels under certain conditions with a number of divalent or higher valent ions. However, they coagulate to form a strong gel, with certain specific ions described hereinafter.

While the use of calcium, lead and aluminum salts have been previously mentioned in the art of formulating algimate and carboxymethylcellulose impression material, many of these formulations require the use of an excess of the precipitating ions to allow a sufficiently rapid reaction by mass action, and hence the completed gel tends to be too highly bound and shows strong syneresis. In contrast to this, the invention controls the rate and amount of precipitation or gelation in a unique double decomposition replacement system.

The controlled release of the divalent or higher valent ions is achieved by the use of a water soluble primary metal chelate. The metal in a chelate is so complexed as not to be susceptible to precipitation by many of the common precipitating agents or by the polyhydroxy-polycarboxy acids used for the gel formation. The complexed metal in the chelate, however, may be displaced from the chelate by a metallic ion from a salt which will not strongly gel CMC, but which is capable of forming a stronger secondary chelate compound by virtue of the solubility product relationship. The original metal from the complex or reservoir is defined as the gelling ion, and the metal which displaces the gelling ion in the original or primary chelate to form a new secondary chelate is defined as the displacing ion. synchronously with the formation of the new secondary chelate, the gelling ion is freed to react with the CMC.

The rate of release of the gelling ion, and thus the rate of gelation itself, can be controlled by the type and concentration of the displacing ion. The displacement reaction is governed by the difference in dissociation constant of the primary and secondary chelates, and by their relative amounts. If the displacing ion forms a much more strongly bound chelate than the displaced gelling ion, the rate of displacement will be high. If the displacing ion is at the same time present in amounts smaller than the displaced ion, then a part release will occur up to the amount of the displacing ion, and then the reaction will stop. .As the concentration of the displacing ion is increased, eventually the total of the displaced ion will leave the chelate, and then the reaction stops. The same gelation at slower speeds can be obtained with the displacing ion forming a secondary chelate of higher dissociation constant which is not as firmly bound as in ti e foregoing example, but is still morestrongly'bound than the primary chelate of the ion to be displaced. A further control is then obtainable by using larger amounts of this weaker displacing ion.

In brief, by selecting the appropriate dissociation constants and concentrations, this system of ion supply from the reservoir of the chelates permits a more sensitive control over rate and extent of release of the gelling ion than obtained heretofore, and presents a novel and important improvement in the art of gel formation for dental irnpression materials.

Previous experimenters in the art have sought to effect the control by using particular ions selected 'withspecial regard to the solubility of the salt used. The selected salt, however, has had other undesirable characteristics such as poor stability, high toxicity, or reactions with stone or plaster surfaces. The mechanisms for the release of these selected ions often produces excessive density of crosslinks and hence high syneresis. Altogether the previous efforts to effect gelation have not been found to be satisfactory.

Following are features of selection and control of the composition and process variables in accordance with the invention.

(1) Predetermining the nature of chelating compound.

(2-) Predetermining the degree of saturation of the primary chelate with regard to its metal.

(3) Predetermining the percentage of the primary chelate employed.

(4) Regulation of the pH of the solution by suitable buffers.

(5 Addition of a salt which will compete for either the gelling ion or the displacing ion, the union of which would retard the reaction. The addition of a borate or an acetate, for example, retards the reaction, and as the pH of the mixture is gradually lowered, this activity is gradually lost, and the ion which has been affected is freed-toreact.

(6) Addition of one or more types of retarding agents, such as a citrate, tartrate, oxalate, or a phosphate. The etficiency of the polyphosphates varies widely depending upon what materials are present as precipitating agents. The polyphosphates may exert a threshold effect which allows these materials to temporarily inhibit the precipitation of a metal ion even though there is not enough of the phosphate to completely complex the metals present.

(7) Employment of replacement ions in the form of salts, according to their dissociation constants and the of the resultant mixture. For example, with ethylene diamine tetra acetic acid, a pH range of 5.007.50, the decreasing order of the chelating strength is ferric ion ion (Zn++.), and aluminum ion (Al+++).

(8) Variation of the types and amounts of fillers.

(9) Variation of the dilution by water, and the temperature thereof.

According to an illustrative example of the invention, the mixture is a dry mass comprising carboxymethyl cellulose as polymeric material containing hydroxyl groups and carboxyl groups which may be any of a group comprised of either a natural polyhydroxypolycarboxy material such as alginate or pectin, a modified natural material such as carboxymethyl cellulose or any other carboxyalkyl cellulose, oxidized cellulose or phosphorylated cellulose or synthetic polyhydroxypolycarboxy polymers such as hydrolyzed copolymer of methacrylic acid, vinyl acetate so that free acid groups and free hydroxyl groups are formed, a soluble chelate salt formed with a trivalent metal such as the cerium compound formed by reacting cerium sulfate with potassium ethylene diamine tetra acetate, a filler, a retarding agent, a soluble displacing metal salt other than an alkali metal salt or alkaline earth salt, such, illustratively, as nickel sulfate. This mass reacts to form a gel upon the addition .of water, and establishes a controlled 'gelation rate and a gel of adequate dimensional strength and stability in accordance with theinvention.

A typical and illustrative composition, according to the above .procedure of a one-phase mixture, may comprise the following in the proportions shown by weight.

Percent l. CMC 1.20 carboxyl substitutions per glucose unit, high molecular weightfl 2.09 2. Cerium potassium ethylene diamine tetra acetate 5.01 3. Potassium ethylene diamine tetra acetate 2.25 4. Diatomaceous earth 17.62 5. Nickel sulfate 2.54 6. Water 70.49

nickel ion (Ni++), cobalt ion (Co++), zinc be understood, is representative of a large number that may be used.

Item 4 is an illustrative filler and can be replaced by others with appropriately changed proportions, as is well known in the art.

Item 5 is an illustrative displacing salt, of which several can be used, a few illustrative cations of salts being mentioned elsewhere in this application.

The mixture of the above composition with water provides a mass with timeand temperature-controlled gelation, adequate for proper mixing of the mass, application thereto to a support, insertion into the patients mouth, forming the mixture about the teeth or gums or both, setting-up of the mixture into an elastic gel of substantial dimensional stability, and removing the formed gel as an integral imparted-form-retaining mass, from which an accurate plaster-of-Paris or artificial stone model can be obtained.

In the foregoing example, carboxy methyl cellulose,

molecular weight 243 at 1.2 carboxyl substitution, provides a molecular equivalent of 0.01. Cerium potassium ethylene diamine tetra acetate, molecular weight 467, has a divalent molecular equivalent of 0.02 and a trivalent molecular equivalent of 0.03 whereas the displacing ions of nickel from nickel sulfate, molecular weight 263 furnishes a molecular equivalent of 0.02. Thereby there is displaced by the nickel ions an equivalent amount of cerium ions for gelling the CMC.

The illustrative composition contains about 70% water, and on a dry basis about 5 parts of the soluble cerium chelate mixed with about 2 parts of restrainer and about 2 /2 parts of nickel sulfate displacing salt provide the gelling ions for about 2 parts of CMC which is extended with about 18 parts of diatomaceous earth as a filler.

According to a second organization of a one-stage mixture, further illustrative of the invention, a dry mass is formed of the polymeric material, such as CMC or alginate, containing hydroxyl groups and carboxyl groups, a soluble chelate formed with a higher valent metal such as the zirconium compound formed by reacting a zirconium salt with potassium ethylene diamine tetra acetate, a filler, a retarding agent, and a soluble metallic salt other than an alkali, to wit, for illustrative purposes, zinc sulfate used in appropriate amounts to provide a strong resilient gel. This mixture also establishes the desired controlled gelation rate of a gel which is adequate and proper for dental impression according to the recited requirements thereof.

Percent l. CMC 4.009 2. Ferrous oxalate 0.501 3. Starch 4.009 4. Cotton flock 1.002 5. Sodium citrate 0.301 6. Water 80.178

Of the above, item 1, as noted, is a polymeric material which contains in it free hydroxyl groups and free carboxyl groups. Any other material conforming with relatively high molecular weight within this definition can be used in substitution for the CMC.

Item 2 can be replaced with any other salt containing an ion which is readily oxidized to a higher valency level at which it possesses gelling power.

Items 3 and 4 represent illustrative fillers and can be replaced by others such as are well known in the art.

Item 5 is an illustrative butfer, of many that may be used, as will be understood.

It will be recognized that the given percentages of phase or stage 1 do not total 100%, and the deficiency is supplied by the proportions of the triggering solution of phase or stage 2, which follows, in an illustrative mixture:

Percent Chromic acid 0.500 Water 9.500

Mixture of phases one and two provides a mass of timeand degree-controlled gelation, adequate for mixing of the mass, application thereof to a support, insertion in the patients mouth, forming the mixture about the teeth and gums, setting-up of the mixture into an elastic gel of substantial dimensional stability, and removing the formed gel as an integral imparted form-retaining mass.

Although all of the organizations of impression material described and their reasonable equivalents in constituents, according to the Well known bases for chemical equivalence are available and highly operative for the intended purposes of the invention, it will be understood that for general utility and convenience it is easier to obtain and mix water with a given composition of cooperative ingredients than it is to package and open two related packages as in the last described two-stage form of the invention, and for general commercial purposes the onestage type of invention is presently preferred.

Having thus described the invention, what is claimed is:

l. A method of producing a water-containing gel from an aqueous solution of a gel-forming polyhydroxypolycarboxy material selected from the group consisting of carboxy-rnethyl cellulose, an alginate and a pectate, comprising uniformly incorporating in said solution a soluble multi-valent metal salt of ethylene diamine tetra-acetic acid, the multi-valent salt being chelated by said ethylene diamine tetra-acetic to prevent gelling of said gel-forming material, and incorporating a water-soluble divalent metal salt which releases a divalent metal ion having a higher chelating strength for said ethylene diamine tetra acetic acid than said multi-valen: metal salt of ethylene diamine tetra-acetic acid whereby said multi-valent metal salt ion is displaced by said divalent metal ion and precipitates the gel forming material.

2. A method as claimed in claim 1 wherein said multivalent salt is a cerium potassium salt of ethylene diamine tetra-acetic acid and said displacing divalent metal salt is nickel sulfate.

3. A method as claimed in claim 1 wherein said multivalent salt is an aluminum salt of ethylene diamine tetraacetic acid and said divalent salt is zinc sulfate.

4. A method as claimed in claim 1 wherein said multivalent salt is a ferric salt of ethylene diamine tetraacetic acid and said divalent salt is nickel sulfate.

5. A process as claimed in claim 1 in which there is present an inert filler.

6. A process as claimed in claim 1 in which there is present an alkali metal salt of ethylene diamine tetraacetic acid as a restrainer.

7. An impression material consisting essentially on a dry weight basis of about 2 parts of a gel-forming polyhydroxy-polycarboxy material selected from the group consisting of carboxy methyl cellulose, an alginate and a pectate, about 5 parts of a water-soluble multi-valent metal salt of ethylene diamine tetra-acetic acid, the multivalent salt being chelated by said ethylene diamine tetraacetic acid to prevent gelling of said gel-forming material, about 2 parts of a water-soluble divalent metal salt which releases a divalent metal ion having a higher chelating strength for said ethylene diamine tetra-acetic acid than said multi-valent metal salt of ethylene diamine tetraacetic acid whereby said multivalent metal salt ion is displaced by said divalent metal ion from the ethylene diamine tetra-acetic acid chelate to precipitate the gel-forming material, and about 18 parts of an inert filler.

8. An impression material as claimed in claim 7 wherein said multi-valent salt is a cerium potassium salt of ethylene diamine tetra-acetic acid and said displacing divalent metal salt is a nickel sulfate.

9. An impression material as claimed in claim 7 wherein said multivalent salt is a zirconium salt of ethylene diamine tetra-acetic acid and said divalent salt is zinc sulfate.

10. An impression material as claimed in claim 7 wherein said multivalent salt is a ferric salt of ethylene diamine tetra-acetic acid.

9 10 11. An expression material as claimed in claim 7 References Cited inthe file of this patent wherein said multivalent salt is an aluminum salt of ethylene diamine tetra-acetic acid and in which there is TE STATES PATENTS present an alkali metal salt of ethylene diamine tetra 2249'694 wlldufg July 15, 1941 acetic as a restrain 2,657,971 Lochridge Nov. 3, 1953 12. An impression material as claimed in claim 7 wherein there is present a polyphosphate as a retarding agent.

Claims

7. AN IMPRESSION MATERIAL CONSISTING ESSENTIALLY ON A DRY WEIGHT BASIS OF ABOUT 2 PARTS OF A GEL-FORMING POLYHYDROX-POLYCARBOXY MATERIAL SELECTED FROM THE GROUP CONSISTING OF CARBOXY METHYL CELLULOSE, AN ALGINATE AND A PECTATE, ABOUT 5 PARTS OF A WATER-SOLUBLE MULTI-VALENT METAL SALT OF ETHYLENE DIAMINE TETRA-ACETIC ACID, THE MULTIVALENT SALT BEING CHELATED BY SAID ETHYLENE DIAMINE TETRAACETIC ACID TO PREVENT GELLING OF SAID GEL-FORMING MATERIAL ABOUT 2 PARTS OF A WATER-SOLUBLE DIVALENT METAL SALT WHICH RELEASES A DIVALENT METAL ION HAVING A HIGHER CHELATING STRENGTH FOR SAID ETHYLENE DIAMINE TETRA-ACETIC ACID THAN SAID MULTI-VALENT METAL SALT OF ETHYLENE DIAMINE TETRAACETIC ACID WHEREBY SAID MULTIVALENT METAL SALT ION IS DISPLACED BY SAID DIVALENT METAL ION FROM THE ETHYLENE DIAMINE TETRA-ACETIC ACID CHELATE TO PRECIPITATE THE GEL-FORMING MATERIAL, AND ABOUT 18 PARTS OF AN INERT FILLER.