first a hydrated solution of preferably sodium acetate of a preferred purity of was prepared in ordinary water by mixing 500 - 7 - 00 gms . in 0 . 1 to 0 . 3 liters of water . the solution was prepared by thoroughly mixing sodium acetate in water . to this was added a polyhydric alcohol and preferably ethylene glycol . this mixture was thoroughly mixed and to this was then added another additive and preferably aniline and the resultant mixture was thoroughly mixed . the addition of polyhydric alcohol is to provide smoothness to the recrystallised supercooled saturated solution of sodium acetate . this is achieved by controlled increase in viscosity of the resulting mixture after recrystallisation due to triggering action by a mechanical device . the use of aniline in the mixture of the heat pack increases the shelf life of the heat pack and also to give the unique property of recyclability of the heat pack . after the mixture has been prepared the same is poured into a plastic bag of preferred dimension of 100 mm 2 and made of thin sheet of polyvinyl chloride ( pvc ). the use of aniline as an additive gives the desired property of preventing the leaching of the plastic material and thereby enhancing the shelf life . a stainless steel crocodile clip is inserted in the heat pack and then finally sealed from the open end . the heat pack is now ready having typical contents , but not limited to , of hydrated salt of sodium acetate 50 wt %, ethylene glycol 1 wt %, aniline 20 wt % and water 29 wt %. the recrystallisation activity was initiated by softly pressing the crocodile clip and instantaneously releasing the same . the re crystallization took place and instantaneously heat was generated over the entire heat pack . this heat pack could be easily contoured over a body part and the part warmed thereby . the temperature of the 25 pack was 45 ° c . above ambient and maintained for about two to three hours depending on the size of the heat pack . in a typical therapeutic application , after energizing the pack the same is wrapped in a towel and applied on the patient . the cooling of the pack to ambient took place in about four hours time . the pack was reactivated by putting the same in hot water resulting in melting of the crystallized content of the pack and then taking it out from hot water and cooled to ambient temperature . this way the heat pack was ready for use again . the principle underlying the invention is that the formation of the hydrated salt results in an endothermic reaction absorbing heat of crystallization . when the crystallization is effected by the triggering action the heat of crystallization is released to the container thereby heating the exterior surface of the container . this renders the device useful for any application . the temperature attainable with the heat pack can be fixed by the amount of the supercooled supersaturated solution of the hydrated salt . accordingly the shelf life of the device will also be adjusted by correspondingly increasing the quantity of the aniline or any of the other additives like hydroxides or carbonates . the novelty of the present invention lies in the heat pack being reusable and having a long shelf life and being capable of recyclable . this novelty of the heat pack is due to the inventive step of adding hydroxides , carbonates or amines to the recrystallizable mixture . in the embodiment of the examples the heat pack is manufactured by first dissolving hydrated salt of sodium acetate in water . ethylene glycol was added to this . this was mixed thoroughly and to this aniline was added again the mixture was thoroughly mixed and then poured in a plastic bag of pvc . the trigger used is a stainless steel crocodile clip and is put inside the bag and then the whole assembly was sealed . the following examples are given by way of illustration only and should not be construed to limit the scope of the invention . heat pack contained hydrated salt of sodium acetate 50 wt %, ethylene glycol 1 wt %, aniline 20 wt % and water 29 wt %. triggering action was done by pressing the clip three times . the crystallization resulted in instantaneous temperature rise to 75 ° c . the cooling of the pack to reach to ambient temperature took place in about four hours time . heat pack contained hydrated salt of sodium acetate 60 wt %, ethylene glycol 4wt %, aniline 15 wt % and water 21 wt %. the triggering action was done by pressing the clip three times . the crystallization resulted in instantaneous temperature rise to 65 ° c . the cooling of the pack to reach to ambient temperature took place in about four hours time . heat pack contained hydrated salt of sodium acetate 70 wt %, ethylene glycol 6wt %, aniline 10 wt % and water 14 wt %. the triggering action was done by pressing the clip three times . the crystallization resulted in instantaneous temperature rise to 75 ° c . the cooling of the pack to reach to ambient temperature took place in about four hours time . heat pack contained hydrated salt of sodium acetate 70 wt %, ethylene glycol 10 wt %, aniline 10 wt % and water 10 wt %. the triggering action was done by pressing the clip three times . the crystallization resulted in instantaneous temperature rise to 70 ° c . the cooling of the pack to reach to ambient temperature took place in about four hours time . table 1 shows the consolidated results of the examples .
the invention discloses a reusable heat pack comprising a soft sealed plastic bag containing a mixture of supercooled supersaturated recrystalizable solution of inorganic substance , alcohol , chemical compound additives , and means to trigger recrystallization of supercooled supersaturated recrystalizable solution of inorganic substance .