Ribbed Smoked Sheets (RSS) are coagulated rubber sheets processed from fresh field latex sourced from well managed rubber plantations adopting modern processing methods. The grades available are shown in the table. The higher grades RSS 1x to RSS 3 are mainly used for manufacture of products for medical, pharmaceutical and engineering. The lower grades of RSS 4 and 5 are generally used for the manufacture of automobile tyres, re-treading materials and all other general products. RSS 3 and RSS 4 are the preferred raw material for radial tyres. Quality of Ribbed Smoked Sheets is ascertained as laid down in Green Book Standards.

In India, RSS grades are packed as 50kg bales in polythene cover and as 110kg crates as per the requirements of the buyer. Ribbed Smoked Sheets are graded according to the standards as per IS – 15361-2003 by visual comparison method. Normally this is done by observation of sheets held against light, when the obvious defects become apparent.

Cluster groups formed among rubber farmers in India, popularly known as “Rubber Producers Societies (RPS) are involved in the production of high quality grades in their group processing centres for domestic and world markets. High grade sheets are produced in individual farms as well.

Domestically produced RSS rubber grades are in good demand for tyres and other product manufacture indicating their quality and market acceptability.


Processing of natural rubber into blocks similar to synthetic rubbers adopting cost effective and modern processing methods is a notable improvement in the presentation of dry natural rubber from India. This is in line with the consumer preference in the world rubber market for Technically Specified Rubber, which has the following advantages:

  • Technically certified rather than visually graded as in the case of RSS and crepe grades of natural rubber.
  • Consistency in quality
  • Compact packaging
  • Low contamination in handling and transportation

Bureau of Indian Standards formulated specifications for ISNR in 1969 (IS 4588).IS 4588 prescribes 6 grades viz., ISNR 3 CV, ISNR 3 L, ISNR 5, ISNR 10, ISNR 20 and ISNR 50. Its specification details are given below.

CharacteristicISNR 3CVISNR 3LISNR 5ISNR 10ISNR 20ISNR 50
Dirt percent by mass (Max)
Volatile matter, percent by mass (Max)0.800.800.800.800.800.80
Ash, percent by mass (Max)0.500.500.600.751.001.50
Nitrogen, percent by mass, Max0.600.600.600.600.600.60
Initial plasticity Po (Min)As agreed to between the purchaser and the supplier30.00(Min)30.00(Min)30.00(Min)30.00(Min)30.00(Min)
Plasticity retention index, PRI (Min)60.0060.0060.00504030
Colour (Lovibond scale)-6.0----
Mooney viscosity (1+4) at 100 C60+(-)5-----
Accelerated storage hardening Po (max)8.00-----
Colour code

Applications of ISNR grades

ISNR 3 CV: High quality products with superior dynamic properties.

ISNR 3 L: Colored products like injection bottle caps, syringe heads, transparent items.

ISNR 5: Moulded and extruded items like auto components, bridge bearings,rubber linings etc.

ISNR 10: Inner tubes, conveyor belts, footwear, water proofing materials, hoses and tubes.

ISNR 20: All types of automobile tyres, re-treading materials, mats and other generalrubber products.


Processing of natural rubber latex into high quality latex concentrate of 60% dry rubber content is done through centrifugation. Centrifuging involves the separation of preserved field latex into two fractions, one containing the concentrated latex of more than 60% dry rubber and the other containing 4-6% dry rubber. Centrifuged latex is today available commercially in two different verities with high ammonia (min.0.6% of ammonia) and low ammonia (max. 0.3% of ammonia). The former is preserved solely with ammonia and later contains one or more preservatives besides ammonia. LA latex has several advantages which include better quality, lower cost of production by way of savings in preservatives, acid and low cost of effluent treatment.

Grades and applications

  • High Ammonia (HA) -Foam products, dipped goods, adhesives, elastic thread, household and industrial gloves, balloons, rubber bands and finger caps.
  • Low Ammonia (LATZ) – Preferred for all applications when de ammonia is necessary for product manufacture. Requirement for centrifuged natural rubber latex is given in the table.
Dry rubber content, percent by mass, Min60.060.0
Non-rubber solids percent by mass, Max2.02.0
Coagulum content, percent by mass, Max0.050.05
Sludge content, percent by mass, Max0.100.10
Alkalinity as ammonia, percent by mass of latex0.6 Min0.3 Max
KOH number, Max1.00.80 Max
Mechanical stability, Min475475
Volatile fatty acid number, Max0.150.10 Max
Copper content, ppm of total solids, Max88
Manganese content, ppm of total solids, Max88

HA – High Ammonia,   LA – Low Ammonia

  • Double Centrifuged – High technology dipped products such as condoms, examination gloves, surgical gloves, catheters etc. These products require a more pure form of concentrated latex. Usually this is achieved by a second stage of centrifuging. Ordinary centrifuged latex is diluted to about 30% drc and then centrifuged to 60% drc. Specifications for double centrifuged latex are prescribed in IS 11001:1984, which is given below.

All the latex processing companies in India take care to ensure supply of high quality contamination free product. It is packed in good quality barrels of 200 + or – 5 kg and or in high quality flexy bags, as per demand.

Creamed Latex – The processing of latex into creamed concentrates involves the mixing of a creaming agent such as ammonium alginate or cooked tamarind seed powder with properly preserved field latex and allowing the latex to separate into two layers, an upper layer of concentrated latex and lower layer of serum containing very little rubber. The lower layer is removed, leaving the latex concentrate of 50%-55% drc which is tested, packed and marketed.

Packing Standards

RSS grades are packed in 50 or 111.1 kg bales, ISNR (Block rubber) in 25 or 50 kg bales, and Centrifuged Latex (Cenex) in 194 kg barrels or flexi tanks, Or as agreed to between purchaser and supplier.

Packing standards of RSS grades – The sheets after grading are marked on the bales and marketed. The following may also be noted while packing RSS grades.

RSS 1X/1 – Each bale must be packed free on mould but very slight traces of dry mould on wrappers or bale surfaces adjacent to wrapper found at time of delivery will not be objected to provided there is no penetration of mould inside the bale.

RSS 2/3 – Slight resinous matter (rust) and slight amounts of dry mould on wrappers, bale surfaces and interior sheets, found at time of delivery will not be objected to. Should rust or dry mould in an appreciable extent appear on more than 5% (RSS 2)/10% (RSS 3) of the bales sampled, it shall constitute grounds for objection.

RSS 4/5 – Slight resinous matter (rust and slight amounts of dry mould on wrappers, bale surfaces and interior sheets, found at time of delivery will not be objected to. Should rust or dry mould in appreciable extent appear on more than 20% (RSS 4)/ 30% (RSS 5) of the bales sampled, it shall constitute grounds for objection.

Packing material specifications for RSS – RSS 1x, 1, 2, 3, 4, 5 in LDP cover and for 3,4,5 may be wrapped with sheet rubber of equal or higher quality and painted with one coat of official bale coating solution as specified in IS 15361: 2003.

Packing Standards of ISNR (Block Rubber) – The material shall be wrapped in low density polythene sheets and shall be further wrapped as agreed to between the purchaser and the supplier. The block rubber shall be further packed in pallets or crates, if required, as agreed between the purchaser and the supplier. Material of pallet/crate is wood, steel or any other material as agreed to between the purchaser and the supplier. Net mass of rubber is 1MT/1.2MT/1.26MT or as agreed to between the purchaser and the supplier. Polythene sheets which are easily dispersible in rubber and as agreed to between the purchaser and the supplier having thickness 0.030 to 0.038 mm, relative density 0.92 and melting point 109 C max are generally found suitable for wrapping raw rubber blocks.

Packing Standards of Centrifuged Latex (Cenex) – For latex packing, standard size 194 kg barrels or flexi tanks or other packing material as per the agreement between purchaser and seller can be used. Under the present procedure, each barrel of centrifuged latex may be marked with its gross weight, tare weight and drc. MS barrels with inside paint coated with ammonia resistant bitumen paint are used for cenex filling.

Note: color code as defined in IS 4588:1986