US3294087A

US3294087A - Synergistic decomposable set-inhibitors for plaster of paris

Info

Publication number: US3294087A
Application number: US546572A
Authority: US
Inventors: David F Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-05-02
Filing date: 1966-05-02
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27

Description

United States Patent 3 294,087 SYNERGISTHC DECOR HQSABLE SET-INHIBITORS FOR PLASTElR 0F PARIS David F. Smith, 120 Grove St., Bay Head, NJ. 08742 No Drawing. Filed May 2, 1966, Ser. No. 546,572 6 Claims. (Cl. 1128-91} This invention relates to materials which show a synergistic action in delaying the set of plaster of Paris in contact with water and which are removable by heating. Such materials are particularly useful in making the so-called plaster of Paris bandages used in orthopedics for forming casts over parts of the human or animal body where immobilization or support is required.

The modern type of such bandage, called hard-coated, is usually made by spreading upon a gauze, crinoline, or other flexible, inert carrier, a slurry of powdered plaster of Paris in an inert non-aqueous liquid, or water containing a set-inhibitor which is removed or destroyed when the coated carrier is subsequently heated to remove the slurry liquid. For economic and other reasons, a water slurry is desirable in which case the set-inhibitor described in US. Patent No. 2,557,083 by Eberl may be used. This ammonium borate set-inhibitor is largely decomposed when the slurry is heated, leaving boric acid or an ammonium borate of low ammonia content which does not largely delay the set of the final dry plaster. However, in normal operation I have discovered that at least traces of the set-inhibitor do apparently remain in the dry product. Since minor amounts (very small concentrations in the slurry liquid) of borate can delay the set for many hours or even days, even traces of borate can delay the set of the dry plaster for at least several minutes and such a delay is important to avoid since the plaster bandage is required to set in 2 to 8 minutes when subsequently wet in water preparatory to use in making a cast. For example, a plaster bandage made according to the Eberl method, using about 1% K 80 set-accelerator based on the weight of plaster, sets in 3 /2 minutes while a similar bandage made using 82% aqueous methanol sets in 2 /3 to 2 /2 minutes; the setting times being determined under strictly comparable conditions. In the latter case there is no possibility of any set-inhibitor remaining in the dry bandage. Such a difference in setting-time is important since it can represent the diiference between extra-fast setting (1-3 minutes), and fast setting bandage (3-8 minutes) as defined in federal specification GG-B-10ld; June 2, 1959. As a matter of fact, widely used bandages of the fast and extra-fast type ditfer in setting-time under some practical conditions of use, by less than 1 minute.

The delay of the set of the dry bandage, of course, depends upon the ratio of the weight of set-inhibitor left in the product to the weight of plaster; and the greater the proportion of set-inhibitor to plaster in the slurry, the greater will normally be this proportion in the dry product. One can, of course, reduce the proportion of set-inhibitor to plaster by using less set-inhibitor, but the Eberl method has a definite lower limit in proportion of set-inhibitor to plaster since the life of the slurry thereby decreases to a practical lower limit.

My invention resides in my discovery of a practical method of overcoming this diificulty with the Eberl method. In addition to ammonium borate in the slurry, I use an excess of ammonia which further delays the set of the slurry without causing any detectable delay of the set of the final product since the NH is readily and completely volatilized upon heating and drying of the slurry. More particularly, my invention resides in the discovery that there is a marked synergestic action between ammonium borate and ammonia in delaying the set of plaster in the slurry, as illustrated in the following examples.

Example 1 A solution was made containing the following materials: Grams Water 200 Halo, 1.15 29.4 wt. percent aqua NH 4 K 4.5

The above solution was mixed with 350 grams powdered plaster of Paris which did not set until about 4 hours after it was mixed with the solution. (Within practical limits the amount of plaster relative to solution is immaterial from the standpoint of life of the slurry since a given solution will cause setting of a smal amount of plaster in about the same time as with a large amount of plaster, the composition of the solution being determining. The amount of plaster added is controlled by the thickness of slurry required for the coating method used.

Example 2 A solution was made using the following materials: Grams Water 176.7

H3BO3 1.5 29.4 wt. percent aqua NH 27.3 K SO, 4.5

The above solution was mixed with 350 grams of another part of the same plaster of Paris used in Example 1. The plaster did not set until about 5 hours.

Example 3 A solution was made containing the following materials:

' Grams Water 200 H BO 0.35 29.4 wt. percent aqua NH 4.0 K 80 4.5

After the above solution was mixed with 350 grams of another part of the same plaster of Paris used in Example 1, the plaster set in 7 minutes.

Example 4 A solution was made containing the following materials: Grams Water 136 H BO 0.35 29.4 wt. percent aqua NH 68 K 50 4.5

After the above solution was mixed with another part of the same plaster of Paris used in Example 1, the plaster set in 24 minutes.

Example 5 A solution was made containing the following materials: Grams Water 79 29.4 wt. percent aqua NH K 50 4.5

After the above solution was mixed with 350 grams of another part of the same plaster of Paris used in Example 1, the plaster set in 2% hours.

Example 6 A solution was made containing the following materials: Grams Water 79 H BO 0.35 29.4 wt. percent aqua NH 125 K 80 4.5

by Eberl.

After mixing the above solution with 350 grams of another part of the same plaster of Paris used in Example l, the plaster set in 19 hours.

Example 1 (0.58% NH or 2% aqua ammonia) is illustrative of the Eberl process although usually he uses higher concentrations of borate in order to obtain longer slurry life. Example 2 shows a higher ammonia concentration (3.9% NH or 13% aqua ammonia) than used The slurry life of Example 2 is somewhat longer than that of Example 1 but only slightly longer. Example 3 illustrates the very short slurry life with reduced borate concentration. Example 4 shows a measurable increase in slurry life over that of Example 3 at the same borate concentration, but even when the NH, concentration is increased to 9.8% NH (33% aqua ammonia) the slurry life is only 24 minutes. Example 5 shows a moderate slurry life when the NH concentration is increased to 18% (61% aqua ammonia), without borate. However, in Example 6 where the NH concentration is the same as in Example 5, but the borate (rboric acid) concentration is the same in Examples 3 and 4, a very long slurry life is obtained. Thus Example 3 (or even Example 4) shows the very short slurry life due to ammonium borate at a low concentration and Example 5 shows the moderate slurry life due to 18% NH;, without borate while Example 6 shows a slurry life much greater than can be explained by the sum of the set delay of the borate alone (Example 3) and the NH alone (Example 5). Thus there is marked synergism in the set delay of :borate and NH in the same solution. Thus the use of NH permits the use of much lower borate concentration in the slurry (and thus a lower ratio of borate to plaster) while still obtaining sufiiciently long slurry life.

The slurry of Example 1 when spread upon crinoline and dried at 190 to 250 F. gives a bandage setting in about 3 /2 minutes, whereas, similar use of the slurry of Example 6 gives a setting time of 2% minutes under similar conditions of test.

I may use boric acid in amount from 0.1 to about 1 percent by weight of the plaster of Paris and a concentration of NH in the slurry liquid from about 8 percent NH to about 2 3 percent NH based on weight of slurry liquid (H O plus NH Rather than using a high concentration of NH alone, there is advantage in using at least 0.05 percent H BO based on the weight of plaster so that as the slurry liquid is evaporated, the NH largely lost and the borate thus concentrated, if the partially dry slurry is accidentally subjected to a temperature below about 190 F. (below which plaster of Paris alone will hydrate with water vapor at about atmospheric pressure) the plaster is protected from setting.

My slurry liquid may contain from about 0.5 to 2.5 percent K 50 set-accelerator based on the weight of plaster and from 0.2 to 2 percent based on the weight of plaster of a bonding agent comprising cooked starch, dextrin, polyvinyl acetate emulsion, methyl cellulose, hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose and mixtures of these.

Having thus described my invention, what I claim is:

1. A settable plaster of Paris mix comprising major proportions of powdered plaster of Paris and water and minor proportions of (1) boric acid in amount from about 0.05 to about 1 percent of the weight of plaster of Paris and (2) NH in amount from about 8 percent to about 23 percent of the weight of water plus NH 2. The product of claim 1 to which is added from about 0.2 to about 2% of the weight of said plaster of Paris, of a bonding agent selected from the class consisting of cooked starch, dextrin, polyvinyl aceateemulsion, methyl cellulose, hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose and mixture thereof.

3. The product of claim 2 to which there is further added from 0.5 to 2.5% of the weight of said plaster of Paris, of K 4. A method of making a plaster of Paris bandage which comprises the steps of (1) making a slurry of powdered plaster of Pari in water containing boric acid in amount from about 0.05 to about 1% of the weight of plaster of Paris and NH in amount from about 8% to about 23% of the weight of water plus NH while the plaster of Paris remains unset; (2) coating the product of step (1) upon a flexible inert carrier; and (3) drying the product of step (2) at a temperature of from about F. to about 250 F. to yield a dry, settable plaster of Paris product.

5. The method of claim 4 wherein to the slurry of step (1) is added from 0.2 to 2% of the weight of said plaster of Paris of bonding material selected from the class consisting of cooked starch, dextrin, polyvinyl acetate emulsion, methyl cellulose, hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose and mixtures thereof.

6. The method of claim 5 wherein to the slurry of step (1) is further added from 0.5 to 2.5% of the weight of said plaster of Paris of K 50 No references cited.

HELEN M. MCCARTHY, Acting Primary Examiner. S. E. MO'IT, Assitant Examiner.

Claims

1. A SETTABLE PLASTER OF PARIS MIX COMPRISING MAJOR PROPORTIONS OF POWDERED PLASTER OF PARIS AND WATER AND MINOR PROPORTIONS OF (1) BORIC ACID IN AMOUNT FROM ABOUT 0.05 TO ABOUT 1 PERCENT OF THE WEIGHT OF PLASTER OF PARIS AND (2) NH3 IN AMOUNT FROM ABOUT 8 PERCENT TO ABOUT 23 PRECENT OF THE WEIGHT OF WATER PLUS NH3.