Grading
Reference guidance for Ribbed Smoked Sheets (RSS) and Indian Standard Natural Rubber (ISNR) characteristics.
RSS Grading (RSS 1)
Ribbed Smoked Sheets (RSS) are coagulated rubber sheets processed from fresh field latex sourced from well-managed plantations, adopting modern processing methods. Higher grades RSS 1X to RSS 3 are mainly used for manufacture of products for medical, pharmaceutical, and engineering. Lower grades RSS 4 and RSS 5 are generally used for automobile tyres, retreading materials, and other general products. RSS 3 and RSS 4 are preferred raw materials for radial tyres.
Quality is ascertained under Green Book Standards. In India, RSS grades are packed as 50 kg bales in polythene cover and as 110 kg crates as per buyer requirements. RSS is graded according to IS 15361:2003 by visual comparison method, commonly by observing sheets held against light for apparent defects.
Cluster groups among rubber farmers (Rubber Producers Societies, RPS) are involved in producing high quality grades in group processing centres. Domestically produced RSS grades are in demand for tyres and other product manufacturing.
ISNR 20 (Technically Specified Rubber) – Overview
Processing natural rubber into blocks similar to synthetic rubbers using modern methods improved the presentation of dry natural rubber. This aligns with global consumer preference for Technically Specified Rubber due to:
- Technically certified rather than visually graded (as with RSS/crepe).
- Consistency in quality.
- Compact packaging.
- Low contamination in handling and transportation.
BIS formulated specifications for ISNR in 1969 (IS 4588). IS 4588 prescribes 6 grades: ISNR 3CV, ISNR 3L, ISNR 5, ISNR 10, ISNR 20 and ISNR 50.
ISNR Specification Details (IS 4588)
| Characteristic | ISNR 3CV | ISNR 3L | ISNR 5 | ISNR 10 | ISNR 20 | ISNR 50 |
|---|---|---|---|---|---|---|
| Dirt percent by mass (Max) | 0.03 | 0.03 | 0.05 | 0.10 | 0.20 | 0.50 |
| Volatile matter, percent by mass (Max) | 0.80 | 0.80 | 0.80 | 0.80 | 0.80 | 0.80 |
| Ash, percent by mass (Max) | 0.50 | 0.50 | 0.60 | 0.75 | 1.00 | 1.50 |
| Nitrogen, percent by mass (Max) | 0.60 | 0.60 | 0.60 | 0.60 | 0.60 | 0.60 |
| Initial plasticity Po (Min) | As agreed | 30.00 (Min) | 30.00 (Min) | 30.00 (Min) | 30.00 (Min) | 30.00 (Min) |
| Plasticity retention index, PRI (Min) | 60.00 | 60.00 | 60.00 | 50 | 40 | 30 |
| Colour (Lovibond scale) | - | 6.0 | - | - | - | - |
| Mooney viscosity (1+4) at 100°C | 60 ± 5 | - | - | - | - | - |
| Accelerated storage hardening Po (max) | 8.00 | - | - | - | - | - |
| Colour code | Black | Black | Green | Brown | Red | Yellow |
Applications of ISNR grades
- ISNR 3CV: High quality products with superior dynamic properties.
- ISNR 3L: Colored products like injection bottle caps, syringe heads, transparent items.
- ISNR 5: Moulded/extruded items like auto components, bridge bearings, rubber linings, etc.
- ISNR 10: Inner tubes, conveyor belts, footwear, waterproofing materials, hoses and tubes.
- ISNR 20: Automobile tyres, retreading materials, mats and other general rubber products.
- ISNR 50: General rubber products including footwear, mats, etc.
Latex – Processing Notes
Processing latex into concentrate of ~60% dry rubber content is done through centrifugation, separating preserved field latex into: (i) concentrated latex (>60% DRC) and (ii) skim latex (4–6% DRC). Centrifuged latex is available as HA (min 0.6% ammonia) and LA (max 0.3% ammonia).
LA latex may offer advantages such as better quality, savings on preservatives/acid, and lower effluent treatment cost.