the present invention provides improved reusable warmers , such as those containing super - cooled sodium acetate solution , and an activator ( sometimes called a &# 34 ; trigger &# 34 ;). the improved warmers include a sufficient amount of a gelling agent to cause gelling of the sodium acetate or other salt solution , as by incorporating a small amount of the gelling agent ( sometimes referred to as &# 34 ; viscosity increasing &# 34 ; or &# 34 ; thickening agents &# 34 ;) in the super - cooled sodium acetate solution during the manufacture of the reusable warmer , which amount is sufficient to substantially inhibit the flow of the salt solution , particularly after it has been activated and is in the warm state . the exact type of viscosity increasing , gelling or thickening agent is not critical to the present invention and any number of commercially available viscosity increasing agents may be employed without departing from the spirit and scope of the invention . the gelling agents utilized in the subject compositions can be any agent which creates a stable gel matrix , or otherwise essentially prevents flow , in the presence of substantial quantities of dissolved salt , such as sodium acetate . for example , the resultant product may be technically regarded as a pseudoplastic , but for purposes of the present invention will be considered to be a gel . thus , to be useful in the present invention it is not necessary for a conventional gel to be formed . the useful gelling agents also include what may be technically categorized as viscosifiers and rheological control agents . thus , as used herein the term &# 34 ; gelling agent &# 34 ; includes conventional gelling agents , those materials which , upon contact with water , imbibe the water and thereby form hydrogels or pseudoplastics , as well as superabsorbants , which essentially prevent flow . gelling agents of use in the present invention include both natural and synthetic materials . cellulose derivatives are particularly useful as gelling agents in the present invention , such as cellulose ethers , including methylcellulose , hydroxypropyl cellulose , methylhydroxypropyl cellulose , polypropyl cellulose , carboxymethyl cellulose , hydroxyethyl cellulose , hydroxymethyl cellulose , carboxymethylhydroxyethyl cellulose , hydroxypropylmethyl cellulose , hydroxyethyl propylcellulose , and hydroxybutyl methylcellulose . examples of superabsorbants useful as gelling agents in the practice of the present invention , which actually absorb liquid and can absorb up to 1000 times their weight of water , are starch graft copolymers , such as hydrolyzed acrylonitrile grafted starch , and polyacrylate grafted starch ,. u . s . pat . nos . 4 , 435 , 488 ; 4 , 455 , 358 , and 4 , 563 , 404 , all of which are incorporated herein by reference , disclose gelling agents of the type contemplated for such use herein and the these patents are incorporated herein by reference . natural and synthetic gums and gum - like materials may also be employed as gelling agents in the present invention , such as irish moss , gum tragacanth , viscarin gmc , kappa - and iota carrageenan , sodium alginate , guar , hydroxypropyl guar , carboxymethyl - hydroxypropyl guar , hydroxy ethyl guar , alginates , carrageenan , xanthan gum , gelatin , agarose , gum arabic , carob , tragacanth , locust bean gum , karaya , pectin , agar , gum acacia , tara gum , polysuccrose , polyglucose , and zooglan ( the exocellular polysaccharide isolated from zoogloea ramigera . the gelling agents also may be any synthetic polymer or copolymer which is hydratable and cross - linkable in solution , such as polymers which contain one or more of the following functional groups : hydroxyl , cis - hydroxyl , carboxyl , sulfate , sulfonate , amino or amide . examples of these synthetic polymers include , but are not limited to , polyacrylates , polymethylacrylates , polyacrylamides , acrylamide methyl propane sulfonic acid copolymers , polyvinyl alcohols , maleic anhydride - based copolymers such as poly ( methylvinylether - maleic anhydride ), ethylene maleic anyhdride , and maleic anhydride methylvinyl ether copolymers , polyvinyl pyrollidone , polyvinyl alcohol , polyacrylic acid , copolymers of acrylic acid and a polyallyl sucrose such as carbomer 934 , also known as carbopol 934 and available from b . f . goodrich chemical company , polyoxyethylene - polyoxypropylene diol block copolymers which are commercially available as the pluronics from basf - wyandotte . carboxypolymethylene , and polyacrylamide . other suitable gelling agents may be selected by reference to published literature , such as the kirk - othmer encyclopedia of chemical technology , vol . 20 , 207 - 230 , dealing with water soluble resins , which text is incorporated herein by reference . one preferred class of gelling agents are the poly ( ethylene oxide ) polymers , such as those supplied under the trademark polyox by union carbide corporation . another example of a particularly well - suited type of gelling agent is that sold by union carbide corporation under the trademark cellosize . they are hydroxyethyl cellulose ( hec ) gelling agents which are non - ionic , water - soluble polymers . as indicated above , other water - soluble natural polymers ( such as the gums , starches , celluloses , gelatin , etc .) and other chemically modified derivatives of the same which are commonly used as thickeners , suspending agents , gelling agents , emulsifiers or dispersants may also be employed , provided that the material causes the solution to gel or increase in viscosity sufficiently to inhibit or slow - down the flow of the solution during the heat producing phase . the method of making a reusable warmer is well known and does not form an essential part of the herein - described invention . any of several methods of manufacture can be employed as , for example , that which is described in the aforementioned u . s . pat . no . 4 , 077 , 390 at columns 2 and 3 . further , the type of activator used also is not an essential part of the present invention and any suitable activator or trigger can be used , as , for example , the activators described in the aforementioned u . s . pat . no . 4 , 077 , 390 or 4 , 872 , 442 . the essential element is the incorporation of a sufficient amount of a gelling agent to cause the supercooled sodium acetate solution to form a gel , such as a small amount of a viscosity increasing agent ( such as , for example , union carbide &# 39 ; s cellosize gelling agent ), during manufacture of the reusable warmer . the amount of viscosity increasing , thickening or gelling agent used can vary depending upon the specific type of reusable warmer being made . however , the amount used should not be so small that the viscosity of the super - cooled sodium acetate solution is so low that the solution flows to an appreciable extent and collects in low spots in the bag ( and cause saddle bagging ), nor should it be so great that the viscosity of the super - cooled solution is so high that the reusable warmer is not flexible and , therefore , not comfortable to use . thus , when reference is made herein to a gel , the term is to be understood as meaning a soft gel which remains deformable . it has been found that when hydroxyethyl cellulose is used as the viscosity increasing agent , the amount of gelling agent used can be less than about 5 % by weight of the supercooled solution and , preferably , between about 1 to 4 % by weight . of course , as is true with all polymeric materials , there is a relationship between the molecular weight of the polymer and its viscosity or ability to form a gel in solution . with respect to cellosize hec , it is preferable to use those grades with the higher molecular weights , such as grades qp 52000 or 100 mh , which have the highest molecular weight . when such grades are used , the amount employed will be less than if the lower molecular weight grades are used . reference to the published manufacturer &# 39 ; s literature regarding the relationship between the different grades and their viscosity as 1 % aqueous solutions may be made to assist in determining optimum levels of use . the salt used to form the supercooled solution may be any of the art - recognized salts which are capable of forming a supercooled solution . the preferable salt is sodium acetate because it is relatively inexpensive and innocuous . the amount of sodium acetate is not critical . in general , the amount of sodium acetate will range from about 40 to about 50 percent . the construction of the container also is not critical . typically the container will be in the shape of a bag and will be constructed of any suitable material . transparent films , which may be multilaminates , are preferred . the films preferably have at least one side that may be heat - sealed . it is preferred to use nylon - polyethylene laminate as the flexible film . the invention can further be best described by the following examples . into an empty vinyl bag sealed along three sides , there was introduced a 50 % sodium acetate solution at 200 degrees fahrenheit containing 3 percent , by weight , cellosize hec - 52000 - h ( union carbide corporation ), in an amount sufficient to fill the bag , and an activator of the type generally described in u . s . pat . no . 4 , 872 , 442 . the bag was then totally sealed and allowed to cool at ambient temperature over a period of about two ( 2 ) hours . the cooling time could be reduced by immersion of the warmer in cold water or by placing it in a refrigerator . after the two hours a viscous supercooled sodium acetate solution formed which was gelatinous in nature . the gelatinous super - cooled solution was generally clear and transparent and the activator inside the reusable warmer readily could be seen . when the reusable warmer was activated ( by flexing the activator ) it provided the same level of warmth for the same period of time as did similar reusable warmers which did not contain any cellosize hec - qp 52000 - h . the thickened or gelatinous supercooled sodium acetate solution was stable ( it did not separate into its component parts ) and did not attack the container . the concentration of the super - cooled salt solution can be varied depending on the temperature to be achieved by activation . it was found that the gelatinous super - cooled solution cushioned and suspended the activator so that the danger of accidental activation during manufacture , shipping or storing of the reusable warmer was substantially reduced . a reusable warmer was assembled similar to the one in example 1 , except 2 to 3 % by weight of cellosize hec - qp 100000 - h ( union carbide corporation ) was employed instead of the cellosize hec - qp 52000 - h used in example 1 . as in example 1 , a viscous super - cooled sodium acetate solution formed which was gelatinous in nature . the gelatinous super - cooled solution was generally clear and transparent and the activator inside the reusable warmer readily could be seen . when the reusable warmer was activated ( by flexing the activator ) it provided the same level of warmth for the same period of time as did similar the reusable warmer of example 1 . the thickened or gelatinous super - cooled sodium acetate solution was stable ( it did not separate into its component parts ) and did not attack the container . a reusable warmer was assembled similar to the one in example 1 , except that between 10 - 30 % by weight of a corn starch was used as the viscosity increasing agent . during assembly of the reusable warmer , considerable care must be taken in mixing in the starch because it tended to form lumps easily . mixing was performed using high shear mixers . the assembled reusable warmer , after heating , was not as clear as the reusable warmers of examples 1 and 2 . however , its stability was good . the reusable warmer of the present invention can be used as a &# 34 ; cold pack &# 34 ; by placing it in a refrigerator for a short period . it is apparent that the salt ( sodium acetate ) contained in the cold pack sufficiently reduces the freezing temperature of the water so that freezing does not take place . when used in this fashion the cold pack has many of the same advantages that it has when used as a reusable warmer ; it holds temperature well and conforms to the area of the body to which it is applied . the stability of the cold pack when used in this fashion is highly dependent on the purity of the water and the acetate used in its manufacture ; the greater the purity of the solution , the lower the temperature at which the cold pack can be used . for temperatures down to 25 degrees fahrenheit tap water and standard , acetate is acceptable . to compare the temperature - time relationship during the heating cycle between a reusable warmer of the present invention having a gelling agent and one of the prior not employing a gelling agent , the following experiment was conducted . one set of three inch by four inch bags were filled with two ounces of gel solution made in accordance with example 2 and another set were filled with two ounces of solution identical to the first set , but not containing a gelling agent . both sets of bags had initial starting temperatures of 81 ° f . and were activated , attaining peak temperatures of 107 ° f . an infrared thermometer was then used to measure surface temperatures every two minutes , for a period of twenty minutes . during this time the bags containing the gelling agent were two - three degrees warmer at all times than the bags that did not contain a gelling agent . the foregoing demonstrates that the reusable warmers of the present invention demonstrate a more desirable heat profile during crystallization , than the reusable warmers of the prior art . to compare the flexibility , during the heating cycle , and at the end thereof , between a reusable warmer of the present invention having a gelling agent and one of the prior not employing a gelling agent , the following experiment was conducted . the same sets of bags as employed in example 4 were tested for their ability to conform to and hold a shape during the heating cycle , by placing the bags around a 0 . 75 inch diameter dowl rod . the bags containing the gelling agent held their shape considerably better than the bags that did not contain the gelling agent . it was also found that the bags containing the gelling agent remained relatively stationary throughout the wrapping and draping process , whereas the bags that did not contain the gelling agent did not . the bags containing the gelling agent also had a cushion - like feel and response whereas the bags that did not contain the gelling agent had no &# 34 ; body &# 34 ; to them at all . the wrapping and draping around the dowl rod simulates procedures that would be performed in a hospital setting . thus , the foregoing demonstrates that the reusable warmers of the present invention demonstrate better flexibility during and after crystallization , when compared to the reusable warmers of the prior art . it will be apparent to those skilled in this art that various changes may be made in the construction and form of the reusable warmer employing a gelatinous super - cooled solution and in the details of the method of manufacture without departing from the spirit and scope of this invention , and that the specific directions and forms shown herein are presented for the purpose of making an understandable disclosure of the invention and are not intended to be any restriction on the scope thereof , other than as defined in the accompanying claims . to demonstrate the utility of the natural and synthetic gums as gelling agents used in the present invention , the following experiment using xanthan gum was conducted . to several three inch by four inch bags containing a premixed sodium acetate solution , five percent , by weight , xanthan gum ( supplied by kelco ) was added . after addition of the xanthan gum , the solution gelled very well , however , the color was extremely dark , forming an almost brown , opaque composition . the bags were subsequently activated and performed similarly to the bags of example 4 containing the gelling agent .
the present invention provides reusable warmers of the supersaturated solution type that are substantially free of saddlebagging . the warmers also exhibit a more sustained period of time during which the heat pack remains within a therapeutically useful temperature range . the reusable warmers of the present invention also maintain a substantial degree of flexibility during their useful heat cycle . the reusable warmers comprise a flexible container , and located within said container , a supercooled salt solution , an activator for initiating crystallization of said supercooled salt solution , and a gelling agent , said gelling agent being present in sufficient quantity to convert said salt solution to a gel .