US 2246822 A
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Paienied June 24, 1941 12,24asr22 METHOD OF CONTROLLING THE PHYSICAL DIIWENSIONS F MOLDED HYDROCOLLOID BODIES Walter J. van Rossem, Los Angelcs, Calif., as-
signor to Surgident, Ltd., LosAngeles, Califr, a corporation of California.
No Drawing. Application. April 2'7, .1940,
. Serial No. 332,118
This invention relates to the control of a molded body of an aqueous gel of the type characterized by hydrocolloid dental impression compositions, whereby the physical dimensions of such molded body may be controlled, modified, or preserved within close limits, and pertains particularly to the control of the physical dimensions of molded hydrocolloids having an agar-agar base.
One of the particular objects of the invention is to provide a method whereby the molded gel body may be caused to expand or contract within certain narrow limits, for the purpose of controlling the physical dimensions of such gel body in a model reproducing procedure, so that a resultant model diiiering from the physical dimensions of the original model may be obtained.
A further particular object of the invention is to provide a method whereby a hydrocolloid impression of a given model may be preserved in its original physical dimensions over a relatively extended period, so that a faithful reproduction may be obtained of a given subject.
In the use of hydrocolloid dental impression compositions, considerable difficulty has heretofore been experienced in the proper control of an impression made in the mouth for example, with respect to its physical dimensions, if any significant delay is had between the time of formation of the impression and the time that the reproducing plaster is poured into the impression. It will be appreciated that the average dentist is not in a position to take an impression of the patients mouth and then promptly thereafter pour the plaster into the impression in order to secure the best possible plaster reproduction. It has been common practice therefore to allow the impression to stand for some time between the taking of the impression and the pouring of the plaster, and two diiferent techniques have been employed in an attempt to preserve the impression in its original physical dimensions. One of these techniques has been that of immersing the impression in a body of water; the other has been to store it at room temperature in an atmosphere of 100% relative humidity.
Hydrocolloid dental impression compositions, in gel condition, will expand when immersed in water for any significant length of time, the degree'of expansion being roughly in direct proportion to the proportion of gel-forming material in the hydrocolloid composition and averaging about 1% for agar-agar hydrocolloids containing in the neighborhood of agar-agar. When a hydrocolloid gel material is stored in an atmosproductions are to be made.
phere of humidity, it undergoes what is generally termed synaeresis, by which a significant exudation of fluid from the gel body takes place, with a consequent reduction in the physical dimensions of the gel body. After such contraction one might attempt to restore the gel body to its original physical dimensions by subsequent immersion in water, but the swelling would not be uniform unless the body were allowed to remain in the water until it was thoroughly saturated, and this would result in an expansion of the body to greater physical dimensions than those of its original condition, with no control being available.
According to the present invention, I employ an electrolyte in solution to control the degree of hydration of a molded gel body and thus indirectly, but effectively, control the physical dimensions of the gel body. The hydrocolloid impression composition may comprise an aqueous gel of agar-agar with or without added modifying agents or materials such as fibrous materials, waxy materials, resins, etc., which may be added according to the specific use to which the material is to be put in order to improve its physical strength and consistency characteristics. In its preferred aspects, the invention contemplates the addition of an electrolyte directly to the hydrocolloid material, such electrolyte being added in a proportion such as to produce a contraction of the material upon solidification from sol to gel condition which is equivalent to the expansion which is exhibited by the plaster from which re- For example, a good grade of dental plaster or stone will have a setting expansion of approximately 0.15 per cent (as a specific example, one good commercial plaster is sold under a guarantee that the expansion shall be within the range of +0.10 and +0.15%), and in order to match this expansion I incorporate approximately two per cent of potassium sulphate, for example, which will cause a contraction in the solidified molded gel body which results in a uniform dimension reduction of 0.15 per cent in the impression after it is removed from around the subject.
The molded gel body or impression produced from this composition will therefore be 0.15 per cent smaller than the subject from which the impression was made, and if these dimensions of the impression can be preserved until the plaster is poured into the impression for the making of a model, the subsequent models will be substantially of the same size as the original subject owing to the approximately 0.15 per cent expansion which the plaster undergoes in setting. The actual function of the electrolyte in causing this contraction of the impression composition at the time of setting or subsequently thereafter is not clearly understood, but in practice it appears that the composition first sets about the subject and immediately after setting undergoes a contraction which results in the production of the reduced size impression. This is verified by the experience of those who have had impressions of their mouths taken, inasmuch as they report that there is first the sensation of the solidification of the material from sol to gel condition and immediately thereafter there is the sensation that the impression clamps down about the teeth and gums. In order to preserve the impression in the contracted condition, I may immerse the same in an aqueous solution of the same electrolyte as was present in the composition at a concentration substantially equivalent to the concentration of the electrolyte which was present in the composition when the impression was formed and the impression can be maintained in this solution for a period of 24 to 48 hours without significant loss or gain in dimensions. If the impression is immersed in plain water instead of in the electrolyte solution, it will undergo expansion as the electrolyte diffuses out of the gel body, and in a period of 24; hours it will have expanded to substantially the full dimensions of which it is capable, somewhat within the range above set forth. If the impression is immersed in a more concentrated solution of the electrolyte, a slight further contraction takes place, although the change resulting from an increase in concentration of the electrolyte beyond about 0.1 normal is quite nominal.
To provide a specific illustration of the accuracy of reproduction obtainable through the practice of this invention, I made six successive casts from an impression of a standard die provided with two parallel pins spaced one inch on centers, the casts being made with a stand ard dental stone (having a guaranteed expansion of 0.10 to 0.15%) at intervals of approximately one hour. ihe dental impression material used was of the following approximate composition:
Per cent Agar-agar 15 Balata-like rubber isomer 1 Glycerine 0.3 K2SO4 2 Water 82.7 Preservative, coloring and flavoring qs.
One cast was made immediately after the impression was removed from the die; the molded impression was allowed to stand in the air (thus subject to moisture loss by evaporation) for approximately one-half hour, to allow the stone to set, the cast withdrawn, and the molded impression immersed in a 2% solution of K2604 for the same length of time as it had been exposed to moisture loss during the plaster-setting period. Six successive casts were made in this manner, and the resulting casts were found to vary only $013003" from the original die.
As a further illustration, I prepared a mold of the above composition from the same die, produced a cast therefrom promptly, immersed the molded impression in a 2% K2804 solution for 23 hours and made a new cast at that time. This was repeated at 24 hour intervals until six successive casts were obtained, and the accuracy assesses of reproducing was almost identical with those of the first six casts, indicating that prolonged immersion of the molded impression did not cause a swelling of the gel body.
Electrolytes other than potassium sulphate are useful according to this invention when employed at a concentration in the neighborhood of 0.1 normal, it having been observed that the contraction effect due to the electrolyte is in substantial harmony with the so-called lyotropic series of cations. Substantially all cations are found to exert some influence upon the contraction of the agar-agar gel up to concentrations in the neighborhood of 0.01 normal, and the effect up to that concentration appears to be that of a direct electrostatic effect exerted by the cations on the electro-negative agar gel, depending only on the valency, with the exception oi acid electrolytes which are somewhat stronger in their effect than are trivalent cations. At concentrations in excess of .01 normal, and particularly at concentrations in the neighborhood of 0.1 normal, the lyotropic effect becomes more important and the valency of the ions loses its importance; for example, the alkali cations lithium, sodium, and potassium diverge into the well known lyotropic series at concentrations in the neighborhood of 0.1 normal, with potassium exerting the greatest contracting effect. At the concentrations which I prefer to employ, as given above in my specific example in connection with potassium sulphate, slightly in excess of 0.1 normal is provided; in this range the lyotropic effect is pronounced, and the potassium salts appear to be the most advantageous. The anion associated with the potassium cation is also important, and at concentrations in excess of 0.1 normal the effects of the I-Iofmeister series of anions is noted, with the sulphate having a greater effect than most of the other conventionally employed anions. From the relation of the anions and cations in the Hofmeister and lyotropic series, therefore, the desired contraction of the agar-agar gel may be obtained, within the limits offered by the weakest and strongest acting electrolyte.
Should the dental technician desire to prepare a series of models of diiierent size for a given technique, for example, he may immerse the impression in a two per cent solution of potassium sulphate, take an impression therein, then immerse the impression in a 0.1 normal solution of potassium nitrate for sufficient time to secure a thorough diffusion of the potassium sulphate out of the gel and a replacement thereof by the potassium nitrate, which will cause a slight expansion of the impression. The new model made from the thus expanded impression will be of somewhat greater size than the model made with the potassium sulphate-treated impression. The electrolytes which may be employed are necessarily those which are compatible with agar-agar, however, as will be apparent to those familiar with agar-agar gels, i. e., those which will not cause disintegration of agar-agar gels, such as experienced for example by the presence of a strong mineral acid. In this connection potassium sulphate has been found to be the least detrimental to the agar-agar gel, and this feature, taken with the beneficial behavior of potassium sulphate with respect to gypsum plaster as heretofore has been noted by investigators in the art, makes the use of this electrolyte highly advantageous.
It will further be appreciated that the electrolyte need not be incorporated in the gel composition prior to the occasion of taking the impression, inasmuch as the impression may be immersed in the electrolyte solution which will cause the desired contraction to take place as the electrolyte diffuses into the gel structure. The diffusion time in a 15% agar-agar gel has been determined to be on the order of one-eighth inch per hour, and the impression should be immersed in the electrolyte solution for suflicient time to secure a uniform penetration thereof. For example, an impression having a maximum thickness of three-eighths inch should be immersed for a period of not less than one and one-half hours to secure the diffusion of the electrolyte to the inner portions of the impression.
An additional feature of the invention is to be noted in connection with the re-swelling of agar-agar hydrocolloid impressions which have been subjected to shrinkage by moisture loss, in that such a shrunken impression may be immersed in the desired electrolyte solution for a period of 24 hours and it will imbibe the electro lyte solution and attain substantially the same dimensions as it would attain if it had been immersed in the solution immediately after formation. Similarly, an expanded impression (which has been soaked in plain water, for example) may be returned to its correct dimen-' sions by immersion in a body of electrolyte solution, in the same manner.
1. The method of controlling the physical dimensions of a molded body of dental impression hydrocolloid in gel condition and having an agar-agar base, which comprises maintaining such a molded body immersed in an aqueous solution of an electrolyte which is compatible with agar-agar gels and which will restrict the swell ing of such body to a value less than that to which it will swell in plain water.
2. The method of controlling the physical dimensions of a molded body of dental impression hydrocolloid in gel condition and having an agar-agar base, which comprises maintaining such a body immersed in an aqueous solution containing in the neighborhood of 0.1 normal concentration of an electrolyte which is compatible with agar-agar gels.
3. The method set forth in claim 2, said aqueone solution comprising a solution of potassium sulphate.
4. The method of controlling the physical dimensions of a molded body of electrolyte-containing dental impression hydrocolloid in gel condition and having an agar-agar base, which comprises maintaining such a molded body in an aqueous solution of such electrolyte at a concentration equivalent to the concentration thereof in said hydrocolloid, said electrolyte being one which is compatible with agar-agar gels and which will restrict the swelling of such a gel to a value less than that to which it will swell in plain water.
5. The method of controlling the physical dimensions of a molded gel body of dental impression hydrocolloid containing an agar-agar base and an electrolyte at a concentration in the neighborhood of 0.1 normal, said electrolyte being one which is compatible with. agar-agar gels, which comprises maintaining such a molded body in an aqueous solution of such an electrolyte at a concentration in the neighborhood of 0.1 normal.
6. The method set forth in claim 5, said electrolyte comprising potassium sulphate.
WALTER J. VAN ROSSEM