Title of Invention	"LOW BURN RATE PROPELLANT COMPOSITION"
Abstract	The present invention relates to a low burn rate propellant composition comprising 1-6.0 % of biuret, 60-70 % of ammonium perchlorate, 10-15 % of metal powder and 20-24 % of binder mixture and a process of preparing the same.

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FIELD OF INVENTION This invention relates to low burning rate propellant compositions for rocket motors applications. BACKGROUND OF INVENTION Composite solid propellants commonly comprise of one or more solid inorganic and organic oxidizer materials uniformly dispersed in the matrix of plastic, resinous or elastomeric material. The matrix, also known as the "binder" provides fuel for combustion of the propellant as well as structural integrity / desired mechanical properties. While the oxidizer material generally constitutes the major component of the composite solid propellant, other ingredients being binder, fuel, (metal powder) and solid and/or liquid additives are added to obtain the desired ballistics and/or physical characteristics of the propellant. In some applications, it is desirable to reduce the burning rate of the composite propellant. For example, the thrust delivered by burning of given amount of propellant can be sustained over a longer period of time with a relatively slow burning propellant than with one which burns at a comparatively rapid rate, i.e. the given amount of propellant will be consumed faster at the high burning rate than at the reduced burning rate. Unfortunately, achieving these reduced burning rates without sacrificing other properties of the composite solid propellant, such as its specific impulse or efficiency of rocket motor, has been difficult if not impossible. Particularly in the case of composite solid propellants which employ ammonium perchlorate (AP) as the oxidizer, attempts to reduce the burning rate of the propellant while maintaining its performance, have met with limited success. Generally, the methods employed for reducing the burning rate of ammonium perchlorate containing composite solid propellants, without compromising energy level have been limited to the use of large particle size ammonium perchlorate. However, it has got limitation to the effect of efficient combustion of the mental powder fuels which are being commonly used in such propellants. The burning rate of the ammonium perchlorate containing composite solid propellants has also been reduced by increasing the binder content of the propellant, but this causes a significant decrease in both propellant density and fuel combustion efficiency. Likewise increasing the metal fuel content reduces the burning rate, but with the penalty on fuel combustion efficiency. Many additives have been investigated to lower the burning rate of ammonium perchlorate containing composite solid propellants, but usually sacrificing specific impulse, fuel combustion efficiency or both. Various techniques have been attempted in the prior art to modify propellant burn rates for a hydroxyl terminated polybutadiene/ammonium perchlorate (HTPB/AP) propellant. There is a continuing need of improved techniques to balance the need for high burn rates for propulsion purposes and for a low burn rate for idle purpose so as to conserve fuel(2). However, in certain application it is desirable to have a propellant with low burn rate capable of sustaining thrust level for a longer period of time. One of the oxamide/AP/HTPB based composition, claim to have a lower burn rates without sacrificing the performance. But the main disadvantages of this composition is high pressure exponent value and low 'percent weight' loss. Yet another formulation based on strontium carbonate/AP/HTPB) claim to have high density with reduction in burn rate, but it emits a reddish flame which may trace out the missile trajectory easily and also increases pressure oscillations at the end of combustion. Still another composition based on nitroguanidine/AP/HTPB claims to have a moderate reduction in burning rate of propellant. But the extent of decrease in burn rates is relatively low in comparison to the other burn rate suppressants. It also has disadvantages of decreasing the density of the propellant. Yet another composition based on urea/AP/HTPB claim to have low burning rates but urea is not compatible with the binder and creates some curing problem of the propellant. The drawbacks of the above prior art give rise to a need for the development of a low burn rate solid propellant composition as described below. OBJECT OF THE INVENTION The principal object of the present invention is to provide a biuret based solid propellant with low burning rate capable of sustaining thrust level for a longer period of time. Still another objective of the present invention is to provide a solid propellant with sustained combustion with acceptable reduction in density and energy. STATEMENT OF THE INVENTION The present invention relates to A low burn rate propellant composition comprising - 1.0-6.0% of biuret, - 60-70 % of ammonium perchlorate, - 10-15 % of metal powder and - 20-24 % of binder mixture The present invention further relates to A process for the preparation of low burn rate composition as claimed in claim 1 comprising the steps of - Preparing the binder mixture by mixing, heating and drying hydroxyl terminated polybutadiene (16-17 %), dioctyl adipate (4-5 %), crosslinking agent (0.01-0.1%) and adduct (0.1-0.5%) to prepare the binder mixture, - drying biuret (1.0-0.6%), metal powder (10-15%) course ammonium perchlorate and fine ammonium perchlorate(60-70%), - adding the metal powder , biuret fine ammonium perchlorate and course ammonium perchlorate to the said binder mixture and mixing the same to prepare a slurry, - mixing the propellant slurry under agitation, - cooling and curing the said slurry with toluene diisocyanate (1-2%), - casting the slurry to obtain the propellant grain and - cooling and extracting the propellant grain and cutting the same. DESCRIPTION The process for the preparation of a low burn rate solid propellant involves the use of the following ingredients Composition of propellant (Table removed) The process of the present invention for preparing biuret/AP/HTPB based low burn rate propellant for long duration application comprises of following steps: Preparation of Binder Mixture The binder containing HTPB and DOA along with cross-linking agent, adduct, catalyst, were mixed in vertical planetary mixer (Capacity: 1 Ltr, Baker-Perkin, Germany) for 15 min. Binder temperature was maintained at 40-50° C by bot water circulation through mixer bowl and deaerated under vacuum of 2-3 mm of Hg for 30-60 min. to reduce the moisture. The solid ingredients like biuret, Al powder and AP were dried in electrically heated water jacketed over at 55-60° C for around 5-8 hours and then sieved through 75 urn (200 B.S.S) sieve. MIXING After preparation of the binder as mentioned above, cross linker was added and again mixed without vaccum for 10 min. The humidity level throughout the entire mixing and casting was maintained between 55+5%. The remaining ingredients were added and mixed in following stages at around 45° C by hot water circulation through a mixer bowl. ADDITION OF BIURET The dried and sieved biuret (20 urn) was added to the mix and mixed for 10-15 min. without vaccum. ADDITION OF ALUMINIUM POWDER The dried and sieved aluminum powder was added to the mix and mixed for 10 min. without vaccum. ADDITION OF AMMOUNIUM PERCHLORATE (60 urn) The dried and sieved AP fine (60 urn) was added to the mix in two installments and mixed for 10 min. after each installment. ADDITION OF AMMOUNIUM PERCHLORATE (300 urn) The dried and sieved AP coarse (300um) was added to the mix in two installments and mix for 10 min. after each installment. After complete addition, the propellant slurry was mixed under agitation for 60 min. without vaccum and 60 min. under vaccum. ADDITION OF TOLUENE DHSOCYANATE The mix temperature was cooled down at 38-40° C and the curative Toluene diisocyanate (TDI), was added to the mix and mixed for 20 to 30 min. without vaccum. CASTING The slurry propellant mix was slowly cast in an aluminium mould with a central cylindrical mandrel under vacuum of 4-5 mm of Hg. CURING: The cast propellant grain was cured at ambient temperature for one day and then accelerated curing was done at 50-60° C in water jacketed over for 5 to 7 days. EXTRACTION The cured propellant grain after cooling to room temperature was extracted from mould and cut/machined for further physico-chemical, mechanical and ballistic properties evaluation. ANALYSIS Mechanical properties were obtained with Instron, (Model TIC-1185, UK) The operating instrumental parameters were always maintain constant; gauge length: 25 mm, Cross head speed: 50 mm/min, recorder speed: 100mm/min. The stress and strain properties were determined using dumbbell shaped specimen. Thermal analysis of propellant was carried out on the STA(Q-600, USA) at the heating rate of 20°C/min under N2 atmosphere (sample mass of ~10mg). Gaseous products of decomposition were analyzed by Fourier transform infrared (FTIR) (Bruker make equinox 55) hyphenated with TG. Strand burning rates of the propellants were determined in the pressure range of 5-9 Mpa by employing an Acoustic Emission Technique. The methodology involve combustion of the strand (ignited by means of a nichrome wire) of dimension 100 mmx6mm in the nitrogen pressurized steel bomb. The acoustic signal generated due to perturbations caused by the propellant deflagration were unidirectional transmitted through the water medium to a piezoelectric transducer (200 kHzO in conduction with an oscilloscope. The burring rates were computed from the time that was recorded for the trial conducted at each pressure for each sample. The standard deviation was of the order of 0.2%. Sensitivity of the propellant compositions to impact stimuli was determined by applying the fall hammer method (2 kg drop weight) as per the Bruceton staircase apparatus and results are given in the terms of statically obtained 50% probability of explosion (h50). Friction sensitivity was measured on Julius Peter apparatus by standard methodology. Cured propellant was characterized for mechanical, physico-chemical, and ballistic properties etc. The invention will now be explained with the help of the following examples. EXAMPLES 1. Composition of propellants at 450 g batch size; AP(Coarse): AP(Fine)::60:40 2. (Table removed) Example-2: Biuret(10g) and AP(290g) Remaining composition same as Exp.1 Example-3: Biuret(20g) and AP(280g) Remaining composition same as Exp.1 Example-4: Biuret(30g) and AP(270g) Remaining composition same as Exp.1 5. The binder consisting of 74% of HTPB and 20g of DOA along with 0.6g of adduct were mixed under agitation in a vertical planetary mixer(Capacity:1 liter) without vacuum for 15 min. at a temperature of 40±5°C and under vacuum for 30 minutes at a temperature more than 45°C. To the above mix, 0.4g of dried and sieved pyrogallol was added above 50°C and mixing was continued for 10 min. without vacuum. After this, biuret (10/20/30g) was added to the mix and mixed for 10 minutes. Then 50g of Al powder was added in one installment and mixed. Subsequently, AP (60 urn) was added in two installments and finally, AP(300 urn) was added I two installments. After completion of addition, mixing continued for 60 minutes without vacuum and under vacuum for another 60 minutes. The mixing temperature was cooled to 40°C and at this temperature 5g of curative i.e. TDI was added to the mix and mixing continued for 20 minutes without vacuum. Then propellant mix was cast under vacuum of 2-5mm of Hg in an aluminium mould slowly. The cast propellant was cured at ambient temperature for 1 day and at 50°C for 5 days in electrically heated water jacketed oven. The cured propellant grain after 48hrs of annealing is extracted through aluminium mould and cut/machined for further mechanical, physico-chemical and ballistic evaluation in the required geometry. The base composition was also simultaneously prepared by adopting same procedure. Characteristics of Biuret/AP/HTPB based propellant (Table removed) We Claim, 1. A low burn rate propellant composition comprising - 1.0-6.0 % of biuret, - 60-70 % of ammonium perchlorate, - 10-15 % of metal powder, - 20-24 % of binder mixture, 2. A low burn rate propellant composition as claimed in claim 1, wherein the pore size of biuret is 10-50um. 3. A low burn rate propellant composition as claimed in claim 1, wherein the metal powder is aluminium powder. 4. A low burn rate propellant composition as claimed in claim 1, wherein the binder mixture comprises of : - 16-17 % of Hydroxyl terminated polybutadiene, - 1 -2 % of toluene diisocynate, - 4-5 % of dioctyl adipate, - 0.1-0.5 % of adduct and - 0.01-0.1 % of crosslinking agent and antioxidant 5. A low burn rate propellant composition as claimed in claim 4, wherein the cross-linking agent and antioxidant is pyragallol. 6. A process for the preparation of low burn rate composition as claimed in claim 1 comprising the steps of : i. Preparing the binder mixture by mixing, heating and drying hydroxyl terminated polybutadiene (16-17 %), dioctyl adipate (4-5 %), crosslinking agent (0.01-0.1%) and adduct (0.1-0.5%) to prepare the binder mixture, ii. drying biuret (1.0-0.6%), metal powder (10-15%) course ammonium perchlorate and fine ammonium perchlorate(60-70%), iii. adding the metal powder , biuret fine ammonium perchlorate and course ammonium perchlorate to the said binder mixture and mixing the same to prepare a slurry, iv. mixing the propellant slurry under agitation, v. cooling and curing the said slurry with toluene diisocyanate (1-2%), vi. casting the slurry to obtain the propellant grain and vii.cooling and extracting the propellant grain and cutting the same. 7. The process for the preparation of a low burn rate propellant composition as claimed in claim 6, wherein the mixing in steps (iii) and (iv) was carried out at45-55°C. 8. The process for the preparation of a low burn rate propellant composition as claimed in claim 6, wherein the fine and course ammonium perchlorate in step (iii) are added in two steps in equal amounts respectively. 9. The process for the preparation of a low burn rate propellant composition as claimed in claim 6, wherein the ammonium perchlorate in step (iii) is added in two steps in equal amount. 10. The process for the preparation of a low burn rate propellant composition as claimed in claim 6, wherein the slurry was mixed under agitation for 1 hour without vacuum followed by mixing said slurry for 1 hour under vacuum. 11. A low burn rate propellant composition substantially as herein described with reference to foregoing examples. 12.A process for the preparation of a low burn rate propellant composition substantially as herein described with reference to foregoing example.

Documents:

1568-del-2008-abstract.pdf

1568-del-2008-Claims-(05-03-2014).pdf

1568-del-2008-Claims-(20-05-2013).pdf

1568-del-2008-claims.pdf

1568-del-2008-Correspondence Others-(20-05-2013).pdf

1568-del-2008-Correspondence-Others-(04-03-2014).pdf

1568-del-2008-Correspondence-Others-(05-03-2014).pdf

1568-del-2008-correspondence-others.pdf

1568-del-2008-description (complete).pdf

1568-del-2008-form-1.pdf

1568-del-2008-form-18.pdf

1568-del-2008-form-2.pdf

1568-del-2008-form-3.pdf

1568-del-2008-GPA-(20-05-2013).pdf