Claims:
WE CLAIM:
1. A sound absorbing material 100, comprising:
a base material 110 comprising rubber and one or more additives;
and
plurality of surface treated natural fibres 120 dispersed in the range of 5 to 30% into the base material 110, each surface treated natural fibre 120 of predetermined length.

2. The sound absorbing material as claimed in claim 1, wherein said rubber is Ethylene Propylene Diene-Monomer Rubber (EPDM) based rubber or acrylonitrile butadiene rubber and polyvinyl chloride blend (NBR/PVC) blend rubber.

3. The sound absorbing material as claimed in claim 1, wherein each surface treated natural fibre 120 is alkali treated.

4. The sound absorbing material as claimed in claim 1, wherein the surface treated natural fibre 120 is a surface treated fibre selected from abaca fibre, bagasse fibre, bamboo fibre, coir fibre, cotton fibre, coconut fibre, fique fibre, flax fibre (linen), hemp fibre, jute fibre, kapok fibre, kenaf fibre, piña fibre, pine fibre, raffia fibre, ramie fibre and sisal fibre.
5. The sound absorbing material as claimed in claim 1, wherein said predetermined length of each surface treated natural fibres 120 is 2mm to 5mm.

6. The sound absorbing material as claimed in claim 1, wherein the additives include carbon black, oil, at-least one curative.

7. A process for manufacturing a sound absorbing material, the process comprising the steps of:
? cutting plurality of natural fibres to a pre-determined length;
? soaking each natural fibre in a predetermined amount of an alkali salt for 45 to 90 minutes at room temperature;
? neutralizing surface of said natural fibre with predetermined amount of 4% acetic acid for a time period in the range of 45 to 90 minutes and drying the aforesaid to obtain surface treated natural fibre;
? adding said surface treated natural fibre and carbon black to a rubber in a mixing mill, wherein said surface treated natural fibre in said mixture is in the range of 5 to 30%;
? mixing the rubber, surface treated natural fibre and carbon black in the mixing mill for 3 minutes to obtain a mixture;
? adding predetermined amounts of process oil and at least one of curatives and blending for a time period in the range of 3 to 6 minutes to form a final mixture; and
? sheeting out the final mixture followed by moulding to obtain said sound absorbing material.

8. The process as claimed in claim 7, wherein each fibre is cut to a length of 2mm to 5mm.

9. The process as claimed in claim 7 wherein the surface treated natural fibre 120 is a surface treated fibre selected from abaca fibre, bagasse fibre, bamboo fibre, coir fibre, cotton fibre, coconut fibre, fique fibre, flax fibre (linen), hemp fibre, jute fibre, kapok fibre, kenaf fibre, piña fibre, pine fibre, raffia fibre, ramie fibre and sisal fibre.

10. The process as claimed in claim 7 wherein said alkali salt is sodium hydroxide.

11. The process as claimed in claim 7 wherein said step of drying is carried out at 30 °C for 24 hours or 60 °C for 1 hour.

12. The process as claimed in claim 7 wherein said step of molding comprises the following steps:
- receiving said final mixture after sheeting out in a mold;
- heating said compression molds to a temperature in the range of 150 to 190 °C at a pressure in the range of 200 bar for a time period in the range of 5 to 25 minutes to obtain said sound absorbing material.

13. The process as claimed in claim 12 wherein said mold is selected from compression mold, transfer mold or injection mold.
, Description: FIELD OF THE INVENTION
[001] The invention relates to a sound absorbing material and a process for manufacturing thereof.

BACKGROUND OF THE INVENTION
[002] Sound absorption refers to the process by which a material, structure, or object absorbs sound when encountered as opposed to reflecting the sound. Within the field of sound absorption, absorptive techniques are typically utilized to prevent or reduce sound by using absorptive materials usually combined with a rigid material with an air space separating the two materials. This approach of combining a sound absorptive material with a rigid material and a separating air space increases both the overall weight and thickness of the resulting sound absorbing structure and therefore may not be feasible as such approaches tend to be very dense, and thus heavy.
[003] Conventionally sound absorption can be achieved by using simple barriers. Most barriers, such as those made of fibres or foam, attenuate the sound by forcing the sound waves to change direction repeatedly. With each change of direction a portion of the energy of the sound wave is absorbed. Such materials tend to be relative lightweight and are quite effective at reducing sound.
[004] EP2207701B1 discloses sound absorption using a sound insulation made of a foam layer and a barrier layer. Micro-perforated nonwoven layer, a micro-perforated foil or a micro-perforated foil web is located between the soundproofing foam layer and the barrier layer. Foam has disadvantage of not giving structural stability to the components and thus has limited usage.
[005] Some conventional sound absorbing materials used to absorb sound over different frequencies work well generally if they are of a high weight and thickness and therefore may not be feasible in multiple applications. Other available commercial sound absorptive materials use natural or synthetic fibres. The sound absorbing materials made from natural fibres are less hazardous to human health and more eco-friendly than those made of conventional synthetic fibres. Therefore, a growing concern for human health and safety issues has encouraged the use of alternative materials from natural fibres as a replacement for synthetic fibres. Natural fibres are preferred due to their biodegradable, lightweight, cheaper, nontoxic, and nonabrasive qualities.

SUMMARY OF THE INVENTION
[006] In one aspect, the present invention provides a sound absorbing material comprising: a base material comprising rubber and one or more additives; and plurality of surface treated natural fibres dispersed in the range of 5 to 30% into the base material, each surface treated natural fibre of predetermined length.
[007] In another aspect, the present invention provides a process for manufacturing a sound absorbing material, the process comprising the steps of: cutting plurality of natural fibres to a predetermined length; soaking each natural fibre in a predetermined amount of an alkali salt for 45 to 90 minutes at room temperature; neutralizing surface of said natural fibre with predetermined amount of 4% acetic acid for a time period in the range of 45 to 90 minutes and drying the aforesaid to obtain surface treated natural fibres; adding said surface treated natural fibre and carbon black to a rubber in a mixing mill, wherein said surface treated natural fibre in said mixture is in the range of 5 to 30%; mixing the rubber, surface treated natural fibre and carbon black in the mixing mill for 3 minutes to obtain a mixture; adding predetermined amounts of process oil and at least one of curatives and blending for a time period in the range of 3 to 6 minutes to form a final mixture; and sheeting out the final mixture followed by moulding to obtain said sound absorbing material.

BRIEF DESCRIPTION OF THE DRAWINGS
[008] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1 shows a sectional view of a sound absorbing material in accordance with an embodiment of the invention.
Figure 2 shows a flow diagram for a process for manufacturing a sound absorbing material in accordance with an embodiment of the invention.
Figure 3 shows an isometric view of a molded sound absorbing material in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION
[009] The present invention is directed towards a sound absorbing material having natural fibres blended with a rubber base material. Such a blend of natural fibres with rubber base material effectively absorbs sound in different frequency ranges.
[010] Figure 1 shows a sound absorbing material 100 in accordance with an embodiment of the invention. The sound absorbing material comprises a base material 110 and plurality of natural fibres 120.
[011] The base material 110 comprises of rubber and one or more additive mixed/blended with the rubber. The rubber can be selected either from Ethylene Propylene Diene-Monomer Rubber (EPDM) based rubber or acrylonitrile butadiene rubber and polyvinyl chloride blend (NBR/PVC) blend rubber. The additives include carbon black, paraffinic oil, curatives, anti-oxidants and anti-ozonants. Anti-oxidants are used in rubber to prevent the oxidative degradation of the rubber whereas the anti-ozonants prevent rubber from degradation due to ozonolysis.
[012] The curatives can be selected from any of thiurams, dithiocarbamates, sulphonamides, sulphur and peroxides. Typically, the paraffinic oil is used to modify the viscosity and for easier processing. The curatives are used to create the crosslinking of rubber matrix that gives the rubber the ability to regain deformations.
[013] In an embodiment of the invention, base material 110 with EPDM based rubber comprises ethylene content (60% to 70%), diene content (0.5% to 10%), propylene content (rest) mixed with carbon black (10 to 30%), paraffinic oil (10 to 30%), and curatives (1%).
[014] In an embodiment of the invention, base material 110 with NBR/PVC blend rubber comprises acrylonitrile Butadiene Rubber (30% to 50%) and Polyvinyl Chloride (50 to 70%) mixed with Carbon black (10 to 30%), paraffinic oil (10 to 30%), and curatives (1%). molds.
[015] As shown, the natural fibres 120 are dispersed in the base material 110. The natural fibres 120 can be selected either from abaca fibre, bagasse fibre, bamboo fibre, coir fibre, cotton fibre, coconut fibre, fique fibre, flax fibre (linen), hemp fibre, jute fibre, kapok fibre, kenaf fibre, piña fibre, pine fibre, raffia fibre, ramie fibre and sisal fibre. The natural fibre 120 is preferably coconut fibre. The natural fibres are native to the Indian subcontinent. Typically, the coconut fiber is sourced from Pollachi, Tamilnadu, India. In an embodiment of the invention the natural fibres 120 are surface treated natural fibres 120, whereby the natural fibres 120 are treated with alkali salts. Such surface treated natural fibres 120 dispersed in the base material improves the structural integrity/rigidity and the sound absorbing capacity of the base material. The surface treated natural fibres 120 are dispersed in the range of 5 to 30% into the base material. In an embodiment, each surface treated natural fibre 120 has a pre-defined length, preferably in range of 2mm to 5mm. Quantity and length of surface treated natural fibres 120 dispersed in the base material can be varied depending upon requirement of sound absorption. In an advantageous aspect, natural fibres keep the overall weight of the sound absorbing material low, and are environment/health friendly.
[016] Figure 2 shows a flow diagram for a process for manufacturing a sound absorbing material in accordance with an embodiment of the invention. The process starts at step 2A where plurality of natural fibres are cut to a pre-determined length. Each natural fibre is cut in the range of 2mm to 5mm. Thereafter, at step 2B, each natural fibre is surface treated by soaking the natural fibres in a predetermined amount of an alkali salt for 45 to 90 minutes at room temperature. In an embodiment of the present invention the alkali salt used for soaking the natural fibres is sodium hydroxide.
[017] At step 2C, the surface of said natural fibre is neutralized with predetermined amount of 4% acetic acid for a time period in the range of 45 to 90 minutes. These natural fibres are then dried to obtain plurality of surface treated natural fibre, whereby the natural fibres are subjected to a temperature of 30 °C for 24 hours or 60 °C for 1 hour.
[018] The surface treated natural fibre is thereafter added to rubber in a mixing mill at step 2D, whereby the surface treated natural fibres is added in pre-determined amounts. Further, carbon black is also added while adding the surface treated natural fibres to the rubber. Carbon black improves the mechanical properties of the rubber like tensile strength, elongation and tear resistance. In an embodiment, surface treated natural fibre in the mixture is in the range of 5 to 30%. As soon as the surface treated natural fibres and the carbon black are added to the rubber in the mixing mill, the mixing mill is operated at step 2E whereby the rubber, surface treated natural fibres and carbon black are milled/mixed for 3 minutes to form a mixture. Thereafter, at step 2F, paraffinic oil is added in pre-determined amounts along with at-least one curative to the mixture, and blended for a time period of 3 to 6 minutes to form a final mixture. This final mixture is then sheeted out and undergoes a step of moulding to obtain the sound absorbing material.
[019] The final mixture undergoes a step of moulding. In the step of moulding the final mixture is sheeted out in desired molds. Typically, the molds are compression molds, transfer molds or injection molds. These molds containing the sheeted out final mixture are then heated to a temperature in the range of 150 to 190 °C at a pressure in the range of 200 bar ± 20 bar for a time period in the range of 5 to 25 minutes to obtain the sound absorbing material.
[020] The sound absorbing material discussed hereinabove used to form parts like automotive floor mats, under-hood sound absorption pads, paddings in interiors of vehicles for sound absorption and paddings and sound absorption pads in DG sets. Figure 3 depicts one such application whereby an isometric view of a floor mat is illustrated.

[021] EXAMPLES:
Example 1:
The sound absorbing material of the present invention was prepared using 100 grams of coconut fiber which was cut into lengths of about 2mm – 5mm. This cut coconut fibre was then subjected to alkali treatment in 200 ml of sodium hydroxide solution for 45 minutes. Further 105 gm of these alkali treated fibres were then taken out and neutralized in 4% acetic acid for 45 minutes and then dried for 1 hour at 60 ºC. The resultant fibers were then mixed in a two-roll mixer for 3 minutes along with 500 gm of EPDM Rubber and 150 gm of carbon black to obtain a mixture. 150 ml of paraffinic oil, 50 gm of antioxidants and 10 gm of curatives were added and blended with the mixture for a time period of 4 minutes to form a final mixture. This final mixture was then sheeted out to obtain a sheet.
The desired part for use in vehicles was prepared by compression molding the sheet by heating at 160 ºC at a pressure of 200 bar for a period of 5 minutes.

Example 2
100 gm of the alkali treated and neutralized fibres, as prepared by the method of example 1, were mixed in a two-roll mixer for 3 minutes along with 450 gm of rubber and 150 gm of carbon black to obtain a mixture. 100 ml of oil, 40 gm of antioxidants and 10 gm of curatives were added and blended with the mixture for a time period of 4 minutes to form a final mixture. This final mixture was then sheeted out to obtain a sheet. The sheet was taken and placed in different molds, having pre-determined shapes, at 170 ºC for 10 minutes at a pressure of 200 bar to obtain the sound absorbing material of the present invention.
Another rubber material was prepared using the same parameters as mentioned herein above in example 2 but without the addition of the fibres.

The resultant parts were taken up for subjective noise analysis testing.
Parameter Coconut fiber blended part Rubber without fiber
Subjective noise analysis (Rating – Higher the better) 4 3

Example 3
150 gm of the alkali treated and neutralized fibres, as prepared by the method of example 1, were mixed in a two-roll mixer for 3 minutes along with 500 gm of rubber and 150 gm of carbon black to obtain a mixture. 150 ml of oil, 40 gm of antioxidants and 10 gm of curatives were added and blended with the mixture for a time period of 4 minutes to form a final mixture. This final mixture was then sheeted out to obtain a sheet. The sheet was taken and placed in different molds, having pre-determined shapes, at 170 ºC for 10 minutes at a pressure of 200 bar to obtain the sound absorbing material of the present invention.
The sheet was tested for physical properties using standard test procedures. The results are tabulated below.
Parameter Coconut fiber (One of the natural fiber) reinforced rubber Rubber material without Coconut fiber
Tensile strength 7 MPa 10 MPa
Elongation at break 300% 350%
Deterioration in tensile strength after aging the sample in 70ºC for 70 hours -25% -16%
Deterioration in elongation after aging the sample in 70ºC for 70 hours -28% -22%

[022] While the present invention has been described with respect to certain embodiments, it will be apparent to those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.