Title of Invention

A LOW DENSITY CURABLE COATING COMPOSITION AND A PROCESS FOR PREPARING THE SAME

Abstract

This invention relates to low density curbal coating composition useful in providing thermal protection for aerospace vehicles.It consists of a two component system of a premix desperson and a curing component.The premix consists of a mixture of hydroxyterminated polymethlsiloxane silica,ferric oxide,and zinc oxide blended with phenolic microballoons. This is dispersed in an organic solvent. The curing compent consists of alkyloximinosilane,aminoalkyltrialkoxysilane,and an or5ganotin compound. This invention also includes a process for preparing the above composition.

Full Text

This invention relates to a low density curable coating composition and a process of preparing the same. Coating composition of the subject invention is particularly but not exclusively applicable to provide thermal protection to aerospace vehicles. High velocity of aerospace vehicles generate very high temperature while in motion. This results in critical material failure, deterioration due to chemical attack, thermal shock* erosion and differences in thermal expansion coefficients. Heat generated from the propulsion system or the heat transferred from the hot skin of the vehicles may cause strenuous temperature conditions to develop affecting the vehicles' performance. The skin of the structure of the launch vehicles are therefore to be isolated from the hypercondition of the environment which is conventionally effected with the help of thermal protection system. Conventional thermal protection systems are based on polyurethanes, polyesters and epoxies. The systems based on polyurethanes have low temperature stability and their properties deteriorate at high temperatures. The systems based on polyesters and epoxies have processing problems and exhibit unsatisfactory performance at high temperatures. Further > they show poor ageing properties. Thermal protection systems based on polysiloxane exhibit the required thermophysical characteristics and good ageing properties. However a composition having lower density than polysiloxane is preferred in thermal protection system as it helps in reducing the total mass thereby improving the payload mass. The main objective of this invention is to develop a process for preparing a curable composition useful in aerospace thermal protection system having low density and low thermal diffusivity to protect aerospace vehicle structures from thermal shock and chemical attack during its ascend. The skin of the aerospace vehicle is known to experience a temperature upto 450°C during this period. Since the heat shield of the launch vehicle is made of aluminium alloy which starts deteriorating from a temperature of 120°C onwards it is necessary to maintain the skin temperature below 120*0C. Low density thermal protection system having low thermal diffusibility achieves this object. The presently used thermal protection system based on polysiloxane exhibit excellent thermo phycial characteristics and good ageing properties, but possesses relatively high thermal conductivity and low specific heat. Low thermal diffusibility helps in reducing the total mass of the thermal protection system thereby improving the payload mass. The object of this invention is to develop a curable coating composition that possesses low density suitable for aerospace thermal protection system. It consists of a premix and a curing component. The premix is prepared from a linear hydroxy terminated poly dimethyl siloxane preferably of molecular weight between 10000 to 500000, The mechanical properties of the hydroxyterminated polydimethylsiloxane is improved by adding fillers like precipitated silica and fumed silica. The premix may optionally contain finely powdered oxides of iron and sine. This is found to enhance the thermal stability and adhesive strength of the final product. Density of the premix is lowered by the incorproation of phenolic micro balloons. It* is observed that addition of phenolic micro balloons not only lowers the density and thermal conductivity but also enhances the specific heat resulting in reducing the thermal diffusivity of the premix. Linear hydroxy terminated polydimethyl siloxane may be -produced by anionic equilibration of the hydrolysate obtained by the hydrolysis of dimethyldichlorosilane. The curing component is prepared by admixing alkyl oximinosilane, aminoalkyltrialkoxy silane and organotin compounds such as dibutyltin dilaurate or stanouss octoate. Curing takes place at ambient temperature when the premix dispersion is mixed with the curing component. Crosslinking of the hydroxy terminated poly dimethyl siloxane takes place by the reaction of Si-OH with oximinosilane in the presence of atmospheric moisture. Oximinosilane of the curing component has R', and R" may be alkyl groups having 1-10 carbon atoms. Easily hydrolysable Si-O-N linkage of these compounds break up to form Si-OH in the presence of atmospheric moisture which then condenses with Si-O-Si linkage to effect cross linking. incorporated in the curing component improve the adhesive characteristics of the final product R, R', R" , and R'tM are alkyl groups having 1-4 carbon atoms. The highly polar-NH2 group improves the bond strength of the composition with the substrate on which it is applied to through polar and hydrogen bondings. The alkoxy group-OR' cross links with hydroxy terminated polysiloxane. Organotin compounds act as a catalyst to enhance the rate of cross linking reaction. In general the premix is prepared from 1000 parts of hydroxy terminated poly dimethyl siloxane of molecular weight 50000 to 100000. Pretreated fumed silica, ferric oxide, zinc oxide and phenolic microballoons may be added in the range of 60-80 parts, 10 to 20 parts, 5 to 15 parts and 300 to 400 Parts by weight respectively. This premix is dispersed in a solvent such as toluene, xylene, n-hexane or petroleum ether till a sprayable viscosity is obtained by controlling the quantity of solvent used . The curing component consisting of alkyl tris al^ylketoxime . silane, 3-aminoalkyltrialkoxysilane and dibutyltin dilaurate in the weight ratio of 100 : 10 : 1 to 100 : 40 : 3 is prepared V separately. The range may be controlled according to the required tack free and cure time of the final product 8-12 parts by weight of the curing component is added to the solution of the premix to obtain the curable composition of the invention and may be sprayed on the substrate to be protected. Curing takes place at ambient conditions within a minimum period of one hour when the solvent is evaporated and in the presence of atmospheric moisture and is completed within 24 hours of application. This invention relates to a low density curable coating composition which comprises a blend of premix dispersion and a curing component, wherein said premix consists of a mixture of hydroxy terminated polydimethylsiloxane, silica, ferric oxide and 2inc oxide blended with phenolic microballoons and dispersed in an organic solvent and said curing component consists of a mixture of alkyloximinosilane, aminoaikyltrialkoxysilane, and an organotin compound. This invention also relates to a process i or preparing a low density curable coating composition which comprises the steps of (a) preparing a premix by blending hydroxyterminated polydimethyl siloxane, silica, ferric oxide, and zinc oxide and adding phenolic microballoons thereto and subsequently dispersing said premix in an organic solvent, (b) preparing curing component by mixing alkyloximinosilane, aminoalkyltrialkoxysilane and an organotin compound and (c) admixing said premix dispersion with said curing component * to produce the low density curable composition. The following example particularly describes a specific embodiment of the process according to this invention. 1000 parts of hydroxyterminated polydimethylsiloxane of molecular weight 50000 is blended with 75 parts of fumed silica filler, 15 parts of ferric oxide and 7.5 parts of sine oxide in a triple roll mill. To this mixture 350 parts of phenolic microballoons are added and the mixture is kneaded in n kneader to obtain the premix. 200 parts of this premix is dispersed in 250 parts of toluene. The curing component is prepared by mixing methyltris 1 methylethylketoximesilane, 3~amintopropyltriethoxy silane and dibutyl tin dilaurate in the ratio of 100 : 20 : 2. 10 parts of the curing component is added to the premix dispersion and mixed thoroughly. This composition is sprayed on a substrate using a pressure gun. After one■hour, the solvent toluene gets evaporated and the composition gets cured and is found tack- free. Subsequent coatings may be effected after a minimum interval of one hour. The low density curing composition has low thermal conductivity and high specific heat which in toto impafts a lower thermal diffusivity of 8.0cm /sec. Atleast 55% weight saving is acheived by using this coating composition when compared with the coating system currently in use. Substantial improvement in payload mass is ensured by using this low density coating composition . Phenolic microballoons as herein referred are commercially available. These microballoons consist of light weight and mechanically strong microspheres of phenol formaldehyde resins. They are reddish brown in appearance having a bulk density of 0.095 to 0.105 g/cc. Though this invention has been described hereinabove with reference to a specific embodiment, equivalents and modifications known to persons skilled in the art are not excluded from the scope of the appended claims. WE CLAIM: 1. A low density curable coating composition comprising a blend of premix dispersion and a curing component, wherein said premix consists of a mixture of hydroxyterminated polydimethylsiloxane, silica, ferric oxide and zinc oxide blended with phenolic micro balloons and dispersed in an organic solvent and said curing component consists of a mixture of alkyloximinosilane, aminoalkyltrialkoxysilane, and an organotin compound. 2. The low denisty curable composition as claimed in claim 1, wherein the hydroxy terminated polydimethylsiloxane has a molecular weight in the range of 50000 to 100000. 3. The low density curable composition as claimed in claim 1 and 2 wherein said premix consists of 1000 parts by weight of said polydimethylsilane, 80 to 80 parts by weight of silica, 10 to 20 parts by weight of ferric oxide, 5 to 15 parts by weight of zinc oxide and 300 to 400 parts by weight of phenolic micro balloons. 4. The low density curable composition as claimed in claim 3, wherein the organic solvent is selected from tolune, xylene, n-hexane or petroleum ether. 5. The low denisty curable composition as claimed in claim 1, wherein said curing component consists of alkyloximinosilane, aminoalkyltrialkoxysilane and dibutyl tin dilaurate in the weight ratio of 100:10:1 to 100:40:3 respectively . 6. The low density curable coating composition as claimed in claims 1 to 5, wherein 200 parts by weight of said premix is dispersed in 250 parts by weight of toluene. 7. The low density curable coating composition as claimed in claims 1 to 6> wherein said curing comoponent consists of alkyloximinosilane, aminoalkyltrialkoxysilane and dibutyltin dilaurate in the range of 100:20:2. 8. The low density curable coating composition as claimed in claim 6, wherein said 200 parts by weight of the premix dispersed in 250 parts by weight of toluene is mixed with 10 parts by weight of said curing component. 9. A process for preparing a low density curable coating composition comprising the steps of (a) preparing a premix by blending hydroxyterminated polydimethylsiloxane, silica, ferric oxide, and zinc oxide and adding phenolic micro balloons thereto and subsequently dispersing said premix in an organic solvent, aminoalkyltrialkoxysilane and an organotin compound and (c) admixing said premix dispersion with said curing component to produce the low density curable composition. 10. The process for preparing low density curable coating composition as claimed in claim 9, wherein said premix components other than phenolic micro balloons are blended in a triple roll (Rill and is subsequently mixed with phenolic microballoon and kneaded to obtain the premix. 11. The process as claimed in claims 9 and 10 wherein the said hydroxyterminated polydimethylsiloxane has a molecular weight in the range of 50000 to 100000. 12. The process as claimed in claims 9 to 11, wherein 1000 parts by weight of said polydimethylsilane, 80 to 80 parts by weight of silica, 10 to 20 parts by weight of ferric oxide and 5 to 15 parts by weight of 2inc oxide are blended together and 300 to 400 parts by weight of phenolic micro balloons are added thereto and kneaded. 13\ The process as claimed in claim 9, wherein said curing component is prepared by admixing alkyloximinosilane, aminoalkyltrialkoxysilane and dibutyl tin dilaurate in the weight ratio of 100:10:1 to 100:40:3 respectively. 14. The process as claimed in claims 9 to 13, wherein the premix is dispersed in an organic solvent selected from toluene, xylene, n-hexane or petroleum ether. 15. A low density curable coating composition substantially as herein described. 16. A process for preparing a low density curable coating composition substantially as herein described.

Documents:

mas-1999-1113-abstract.pdf

mas-1999-1113-claims duplicate.pdf

mas-1999-1113-claims original.pdf

mas-1999-1113-correspondance others.pdf

mas-1999-1113-correspondance po.pdf

mas-1999-1113-description complete duplicate.pdf

mas-1999-1113-description complete original.pdf

mas-1999-1113-form 1.pdf

mas-1999-1113-form 19.pdf

mas-1999-1113-form 26.pdf

mas-1999-1113-form 3.pdf