Title of Invention

AN INHIBITOR COMPOSITION FOR SOLID PROPELLANTS AND A PROCESS FOR PREPARATION THEREOF

Abstract

This invention relates to an inhibitor composition and a process for preparation thereof. The process for preparation of an inhibitor composition for solid propellants comprising of the step of: preparing a binder mixture by mixing hydroxy terminated polybutadine (58-64%), dioctyl adipate (4-8%), n-butanediol (1-4%) and cardanol (1-4%) and deaerating the binder mixture under vacuum at <5 torr; heating the binder mixture to 50-60° C and then mixing pyrogallol 0.1-0.3 part by weight as antioxidant cum cross-linking agent along with ferric acetyl acetonate (0.1-0.4 part) by weight as curing and catalyst and capolyte 0.5-1.0 part by weight as tackifier in the binder mixture obtained in step (a); cooling the mixture obtained in step (b) to room temperature then mixing the fillers i.e. carbon black 8-12% and antimony trioxide 18-22% pridried at 100-110°, followed by mixing under vacumm at <5 torr; mixing 4,4 methylen bis cyclohexyldiisocyanate (8-12%) in the slurry obtained in step (c), by continous stirring at room temperature and maintaining the humidity at 55±5%; curing the slurry obtained in step (d), by degassing under vacuum, pouring in a tray and leaving at room temperature.

Full Text

FIELD OF INVENTION The present invention relates to an inhibition formulation for solid propeilants, particularly for composite modified double base (CMDB) propeilants and a process for preparation thereof. PRIOR ART A definite pattern of propellant burning for specific applications is achieved by applying inert polymeric materials called inhibitors to restrict propellant combustion. Slurry cast composite modified double base (CMDB) propeilants are comparatively more soft than conventional double base propeilants and may develop stresses at propellant-inhibitor interface during thermal cycling if rigid inhibitors • commonly used for double base (DB) propeilants are applied. Another problem with CMDB propeilants from inhibition point of view is their high flame temperature than DB propeilants which is due to the presence of oxygen rich ammonium perchlorate (AP) and metallic fuel aluminium in DB matrix. Therefore, conventionally used inhibitors for DB propeilants such as cellulose acetate (CA) or ethyl cellulose (EC) cannot restrict combustion of CMDB propeilants. The main disadvantage of cellulose acetate or ethyl cellulose is that over prolonged periods of time, due to migration of plasticizer especially nitroglycerine, the coating gets softened and gets debonded from the propellant charge losing its effectiveness. Another inhibition formulation for nitramine propeilants known in the art is styrene butadiene and acrylonitrile-butadiene rubber blend filled with asbestos, silica and carbon black. The disadvantage of these inhibition formulation is that the asbestos is very stiff. Another disadvantage is that the asbestos is highly toxic and banned in many countries. Unsaturated polyester resin is another inhibition formulation for nitramine propeilants known in the art. The main disadvantage of using unsaturated polyester resin is that it is very stiff due to which the inhibitor-propellant interface debonds during thermal cycling or storage of the propellant. Another disadvantage is that it has high cure exotherm. Another inhibition formulation known in the art with very good bond strength for double base propellants consists of polyester resin containing polyvinyl chloride powder and titanium dioxide as fillers. The main disadvantage of this inhibition formulation is nitroglycerine migration. Systems comprising of epoxy resin and polyamine hardener using antimony trioxide as filler are also known in the art as inhibitor formulation for CMDB propellants. The main disadvantage is that it is very stiff which results in debonds at the inhibitor-propellant interface during thermal cycling or storage of the propellant Another disadvantage of using epoxy resins is that they are incompatible with propellants containing DB matrix. Flexible polyester resin using two barrier coats of Desmodur R and fast setting polyester resin and alumina as filler is another inhibitor formulation known in the art. The main disadvantage of polyester resin as inhibitor is that it has limited shelf life as it gets delaminated after a period of six months to one year. Another disadvantage is that it exhibits high cure exotherm and high cure shrinkage. Another inhibitor formulation known in the art comprises of an isocyanate containing polyether, a diamine hardener and an aliphatic plasticizer with a coolent filler. The major disadvantage of amine type of hardeners is. that the amine compounds are not compatible with double base propellant matrix. Another disadvantage is that these inhibitors require triisocyanate precoat to prevent plasticizer migration which is a moisture sensitive compound and has to be protected from surrounding moisture, otherwise chances of debonding between propellant and inhibitor increases. NEED FOR PRESENT INVENTION There is a need for an inhibitor formulation which will have superior mechanical properties, excellent adhesion characteristics, curing at room temperature and with all these properties remaining unaffected during storage of the propellant. OBJECTS OF THE INVENTION The primary object of the present invention is to provide an inhibitor formulation for CMDB propellants and a process for preparation thereof which has high bond strength of inhibitor-propellant interface. Another object of the present invention is to provide an inhibitor formulation for CMDB propellants and a process for preparation thereof which has higher tensile strength and percentage elongation then that of the propellant, so that inhibitor is not damaged during handling of large sized free standing propellant grains. Yet another object of the present invention is to provide an inhibitor formulation for CMDB propellants which can be cured at room temperature. Still another object of the present invention is to provide an inhibitor formulation for CMDB propellants which has better shelf life. Further object of the present invention is to provide an inhibitor formulation for CMDB propellants in which the inhibitor propellant bond is not adversely affected during thermal cycling over a wide range of temperature from -30°C to +55°C. Yet further object of the present invention is to provide an inhibitor formulation for CMDB propellants which has better compatibility with the propellant. Still further object of the present invention is to provide an inhibitor formulation for CMDB propellants which has low glass transition temperature which improves the low temperature properties of the propellant. Even further object of the present invention is to provide an inhibitor formulation for CMDB propellants which has low cure exotherm and minimum cure shrinkage. STATEMENT OF THE INVENTION According to the present invention, there is provided an inhibition formulation for CMDB propellants and a process for preparation thereof which results in high bond strength of inhibitor-propellant interface. The inhibitor formulation of the present invention is prepared from hydroxy terminated polybutadiene (HTPB) based polyurethane filled with antimony trioxide and carbon black. The tensile strength of the inhibitor formulation of the present invention is slightly higher and percentage elongation is very high as compared to CMDB propeilant. In the present inhibitor formulation, the inhibitor-propellant interface remains stable during thermal cycling of the propeilant over a wide range of temperature from -30°C to +55°C and the shelf life of the inhibited propeilant is also very high. The inhibitor formulation is room temperature cured and has adequate pot life thereby facilitating the application of inhibitor slurry over the propeilant and easy handling of the large free standing grains. The glass transition temperature of the inhibition formulation is very low thereby making it useful for low temperature applications. The proposed inhibitor formulation, when applied to the propeilant surface, does not result in the significant migration of the plasticizer to the propeilant surface over a period of time thus improving the storability of the propeilant. DESCRIPTION OF THE PROCESS According to present invention the process for preparing inhibitor formulation for CMDB Propellants comprises of following steps: a) Preparation of Binder The binder material viz. hydroxy terminated polybutadiene (HTPB, 58-64%); plasticizer material, dioctyl adipate (DOA, 4-8%); chain extender, n-butanediol (nBD, 1-4%) and cardanol (1-4%) are weighed individually and taken in a mixer bowl. The mixture is deaerated under vacuum (at b) Addition of Additives In the binder slurry prepared in step(a), antioxidant cum crossliriking agent, pyrogallol, 0.1-0.3 part; curing catalyst ferric acetyl acetonate (FeAA, 0.1-0.4 part) and a tackifier, capolyte, 0.5-1.0 part are dissolved. The solution is then cooled to room temperature. c) Addition of Filler Material The solid fillers (C-black & Sb2O3) are dried in an electric oven at 100 -110°C for a period of minimum 4 hours. Carbon black (N 774, 8-12%) is added to the mix obtained in step(b) with continuous stirring and mixing is carried out for about 15 minutes, followed by addition of antimony trioxide (Sb2O3,18- 22%) and further mixing for about 10 minutes. It is then mixed under vacuum for about 30 min (at d) Addition of Isocyanate To the slurry obtained in step(c), isocyanate 4,4'-methylene bis cyclohexyldiisocyanate (H12MDI) is added with continuous stirring at room temperature while room humidity is maintained at 55 + 5%. e) Curing of Inhibitor Formulation Finally, the slurry obtained from step(d) is degassed under vacuum and poured in a tray to obtain 2.5-3 mm thick sheets. These sheets are cured at room temperature for 48 hrs. At the end of the processing these sheets are characterised for tensile strength (20 - 30 kgf/cm2) and percent elongation (150-250%) using Instron Universal Testing Machine while the decomposition pattern is studied by carrying out Differential Thermal analysis in air. The glass transition temperature (-70 to'-80°C) of the cured formulation is determined by using Differential Scanning Calorimeter. The rate of erosion (0.1 - 0.5 mm/sec) is measured by Plasma Jet Method. Similarly, plasticizer migration (0 -10%) and bond strength (10 -20 kgf/cm2) of the inhibitor propellant interface are also determined. The CMDB propellants are inhibited using necessary moulds and fixtures to get desired geometry and dimensions. This invention will now be illustrated with a working example which is intended to be typical example to explain the technique of the present invention and is not intended to be taken restrictively to imply any limitation to the scope of the present invention. Working example 300g of binder material, hydroxy terminated polybutadiene (HTPB), 30 g plasticizer material, dioctyl adipate (DOA); chain extender, 10 g n-butanediol and 10 g cardanol were taken in a mixer bowl. The mixture was deaerated under vacuum (at of antimony trioxide (Sb2 03) and mixing was continued for 15 minutes. The contents were further deaerated for 30 minutes under vacuum (at Inhibition of propeilant grain was done by casting technique. Various formers and accessories were used to get the desired geometry after inhibition. The slurry containing isocyanate was poured over the propeilant to get the desired thickness of the inhibitor. The inhibitor was cured at room temperature for 48 hours. The former and accessories were removed and the propeilant grain was x-rayed to check for defects in the inhibitor. The mechanical properties and other performance characteristics of the sheets are summarized in the following table:- (Table Removed) Static evaluation Inhibited propellants using the formulation given above Were subjected to static evaluation at ambient, + 55°C as well as -20°C from the point of view of establishing practical value of inhibition system over the range of propeilant operating temperatures. A smooth and neutral pressure-time profile was obtained during the burning of propeilant grains, establishing effective bonding between propeilant and inhibitor and proving the efficacy of the inhibition system. Inhibited propellants kept at 50°C for accelerated ageing for 24 months showed no debonds and no change in bond strength. Inhibited grains have performed successfully in flight trials of the missile system. It is to be understood that the process of the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention which is further set forth under the following claims:- WE CLAIM: 1. A process for preparation of an inhibitor composition comprising of the steps of: a. preparing a binder mixture by mixing hydroxy terminated polybutadiene (58-64%) dioctyl adipate (4-8%), n-butanedio (1-4%) and cardanol (1-4%) and deaerating the binder mixture under vacuum at b. heating the binder mixture to 50-65°C and then mixing pyrogallol 0.1-0.3 part by weight as antioxidant cum cross-linking agent along with ferric acetyl acetonate 0.1-0.4 part by weight as curing catalyst and capolyte 0.5- 1.0 part by weight as tackifier in the binder mixture obtained in step (a); c. cooling the mixture obtained in step (b) to room temperature then mixing the fillers, carbon black 8-12% and antimony trioxide 18-22% followed by mixing under vacuum at d. mixing isocyanate in the slurry obtained in step (c), by continuous stirring at room temperature and maintaining the humidity at 55±5%; e. curing the slurry obtained in step (d), by degassing under vacuum, pouring in a tray and leaving at room temperature. 2 A process as claimed in claim 1 wherein the binder mixture is prepared in a mixer bowl, the binder mixture being then heated by circulating hot water in outer jacket of the mixer bowl 3. An inhibitor composition comprising of hydroxy terminated polybutadiene (58-64%), dioctyl adipate (4-8%), n-butanedio (1-4%), cardanol (1-4%), pyrogallol 0.1-0.3 part by weight, ferric acetyl acetonate 0.1-0.4 part by weight, capolyte 0.5-1.0 part by weight, carbon black 8-12%, antimony trioxide 18-22% and isocyanate 4. An inhibitor composition and a process for preparing the same substantially as described and illustrated herein

Documents:

856-del-2003-abstract.pdf

856-del-2003-claims.pdf

856-del-2003-complete specification (granted).pdf

856-del-2003-correspondence-others.pdf

856-del-2003-correspondence-po.pdf

856-del-2003-description (complete).pdf

856-del-2003-form-1.pdf

856-DEL-2003-Form-15-(19-08-2010).pdf

856-del-2003-form-19.pdf

856-del-2003-form-2.pdf

856-del-2003-form-3.pdf

856-del-2003-pa.pdf