Title of Invention	A CURABLE SILICONE RUBBER COMPOSITION FOR MAKING VIBRATION ISOLATORS
Abstract	THE RAW MATERIAL SELECTION SHOULD BE BASED ON SIMPLE PAREMETERS

Full Text

This invention relates to a silicone rubber composition for making vibration isolators, particularly, but not exclusively for avionic applications such as launch vehicles. The invention also includes a method of making vibration isolators and vibration isolators made thereby. Installation of aerospace equipments is subject to shock and vibration excitation from a number of sources. The vibrations may be periodic or random in nature. In rockets and launch vehicles, a high level of acoustic noise output and the resultant structural vibration of the launch vehicle are experienced. Lift-off acoustic, separation and jettisoning shocks etc. generate considerable vibration inputs. Critical electronic equipments cannot function without protection from such transient shocks and vibration inputs. They are invariably mounted on elastomeric vibration isolators for this purpose. All sensitive electronic equipments used in avionic applications are supported over rubber based elastomeric vibration isolators to protect them from shock and vibration during the flight. Silicone rubber is found to possess superior characteristics over all the other elastomers in providing effective vibration isolation and environmental resistance. Elastomeric vibration isolators used in avionic applications must possess the following characteristics for better performance and for cost effectiveness. 1. The raw material selection should be based on simple parameters. 2. The processing and compounding of rubber should be simple and easy to adopt. 3. The elastomer should have stable ageing characteristics. 4. The dynamic and environmental properties of the elastomer should meet the requirements of relevant standards like MIL-5400-T, MIL-172-C, MIL-STD-810. 5. The design of the rubber part (configuration) should be such as to suppress or avoid the effects of nonlinearity over the operating range of the component. 6, The elastomeric mount should have dynamic spring constants which give excellent isolation of high frequency disturbance while still affording reasonable stability of the system. 7. The elastomer should be bondable to metallic inserts with good adhesive strength, so as to provide consistent load-deflection characteristics in fail-safe construction of an isolator. Both synthetic and natural rubber elastomers find wide applications in the manufacture of vibration mounts in view of their moldability into desired shapes and their stiffness. Mounts made by elastomers possess more internal damping than metal springs and occupy minimum space and weight. Further, metallic inserts may be bonded to mounts made with elastomers as per the desired requirements. Unique mechanical properties such as low moduli and inherent damping make these elastomers as excellent medium for vibration isolator. Furthermore, these properties may be modified by the addition of other components. Natural rubber embodies the most favourable combination of mechanical properties for making vibration mounts. However, its limited resistance to deterioration under the influence of hydrocarbon ozone, high temperatures is one of the major drawbacks in using natural rubber for vibration mounts. High transmissibility at resonance is yet another factor against natural rubber, Neoprene rubber has a usable temperature range of 233 K to 366 K under actual service conditions. Further, both neoprene and natural rubber tend to crystallise under high strain. Their physical properties also tend to vary with time, temperature and environment. Among synthetic rubbers, butyl rubber offers a good compromise between damping and isolation but has poor thermal resistance. Neoprene rubbers though possessing superior fluid resistance and low transroissibility at resonance exhibit poor isolation characteristics. Silicone rubber is found to possess ideal mechanical properties for vibration isolation. They are stable and offer effective isolation over a wide temperature range. Their dynamic characteristics do not vary with amplitude, frequency and temperature and as such they are considered superior over the other elastomers described hereinabove. Silicone rubbers are silica filled polydimethyl siloxane premix. The object of this invention is to develop a curable silicone rubber composition containing a mixture of silicone rubbers of different grades. The silicone polymers have high molecular weight, polydimethyl siloxane backbone end capped with 0,2% vinyl methyl siloxane units. Silicone rubbers expressed by general purpose grade shore A 40 and shore A 80 are found ideal for obtaining the desired result. The proportion of these two grades of silicone rubber may be varied as desired. The composition also contains dicumyl peroxide and colouring pigments. Such a composition is found to exhibit excellent vibration damping properties. The silicone rubber requirements are identified by a simple procedure using Yersley mechanical oscillograph equipment. Compounding of various proportion of silicone rubber of different grades wil1 depend on the requirement. The curable silicone rubber composition for making vibration isolators according to this invention comprises 20 to 80 parts per 100 parts by weight of silicone rubber of grade shore A40, 80 to 20 parts per 100 parts by weight of silicone rubber of grade shore A 80, and 0.5 to 2.0 parts by weight of dicumyl peroxide per 100 parts by weight of said silicone rubber composition. This curable composition may be compression moulded by known techiques to any desired shape to produce vibration isolators. The vibration mounts obtained by this method shows good ageing characteristics, and dimensional stability for a minimum period of 5 years. In another embodiment vibration isolators may be produced by in situ bonding of metal with the curable silicone rubber composition. Metal components may be inserted with the help of a thin amino alkoxy silane primer coating thereon and is allowed to stand at 298 K for an hour prior to bonding with the silicone rubber composition blank, Silicone rubber composition weighing about 6,0 gm is made into a blank in a mold assembly. A metal insert coated with a thin aminoalkoxysilane primer is inserted and this assembly is introduced in the mould and pressed preferably using a hydraulic press. The temperature of the assembly is regulated at 443 K and maintained for a residence time of 15 minutes. The assembly is then demoulded under running water and the bonded vibration mount is subjected to post curing in an air oven at 473 K for two hours. This vibration mount may also be provided with bushes which may be rivetted to the metal insert. Vibration isolators made from the silicone rubber composition may be differently coloured to indicate its strength and mechnaical properties. The method of making a vibration isolators according to this invention comprises the step of compression moulding a curable silicone rubber composition of 20 - 80 parts per 100 parts by weight fo silicone rubber of grade shore A 40, 80-20 parts per 100 parts by weight of silicone rubber of grade shore A 80 and 0.5 to 2.0 parts by weight of dicumyl peroxide per 100 parts by weight of said rubber composition, and subsequently demoulding the vibration isolators from said mould. The rubber composition may be mixed in a double roll mill for about 30 mts at 298 to 313 K prior to moulding. Compression moulding may be carried out at 423 to 453 K for about 8-15 mts. Postcuring of the article may be carried out for about 2 to 10 hours at 473 K. This invention also includes a vibration mount made from curable silicone rubber composition described hereinabove. Vibration isolators provide adequate protection to electronic devices and equipments by effectively isolating vibration during flights of launch vehicles. Though this invention has been described hereinabove with specific embodiments, obvious alteration and modifications known to persons skilled in the art are to be considered within its the scope and that of the appended claims. WE CLAIM: 1. A curable silicone rubber composition for making vibration isolators comprising 20 to 80 parts per 100 parts by weight of silicone rubber of grade shore A 40, 80 to 20 parts by 100 parts by weight of silicone rubber of grade shore A 80 and 0.5 to 2 parts by weight of dicumyl peroxide per 100 parts by weight of said rubber composition, 2. The composition as claimed in claim 1, wherein said silicone rubber is a silicone polymer having polydimethyl siloxane backbone end capped with 0.2% of vinyl methyl siloxane units. 3. The composition as claimed in claims 1 and 2, wherein composition has a colourant added thereto. 4. A method of making a vibration isolator comprising the step of compression moulding a curable silicone rubber composition of 20 - 80 parts per 100 parts by weight of silicone rubber of grade shore A 40, 80 to 20 parts per 100 parts by weight of silicone rubber of grade shore A 80 and 0.5 to 2 parts by weight of dicumyl peroxide per 100 parts by weight of said rubber composition and subsequently demoulding the vibration mount from said mould. 5. The method as claimed in claim 4, wherein a metal insert is introduced to said rubber composition prior to compression moulding. 6. The method as claimed in claims 4 and 5, wherein said curable rubber composition is mixed in a double roll mill for about 30 mts at 298 to 313 K prior to compression moulding. 7. The method as claimed in claims 4-6 wherein said rubber composition is compression moulded at 423 to 453K for 8 to 15 mts. 8. The method as claimed in claims 4 to 7, wherein said mount is demoulded under running water. 9. The method as claimed in claims 4-8, wherein said mount is postcured in an air oven at 473 K. 10. The method as claimed in claims 4-9, wherein metal inserts are introduced prior to compression moulding said rubber composition. 11. A curable silicone rubber composition for making vibration isolators substantially as herein described, 12. A method of making a vibration isolator substantially as herein described.

Documents:

1167-mas-1999-abstract.pdf

1167-mas-1999-claims filed.pdf

1167-mas-1999-claims granted.pdf

1167-mas-1999-correspondnece-others.pdf

1167-mas-1999-correspondnece-po.pdf

1167-mas-1999-description(complete)filed.pdf

1167-mas-1999-description(complete)granted.pdf

1167-mas-1999-form 1.pdf

1167-mas-1999-form 19.pdf

1167-mas-1999-form 26.pdf

1167-mas-1999-form 3.pdf