📋 Table of Contents
- Why Rubber Testing is Different from Plastics Testing
- Common Rubber Types and Their Applications
- Test 1: Shore Hardness (Shore A and Shore D)
- Test 2: Tensile Strength and Elongation (ASTM D412)
- Test 3: Compression Set (ASTM D395)
- Test 4: Tear Resistance (ASTM D624)
- Test 5: Specific Gravity / Density (ASTM D792)
- Test 6: Thermal Ageing (ASTM D573 / ISO 188)
- Test 7: Abrasion Resistance (DIN 53516)
- M100 and M300 Modulus — What They Tell You
- Standards Reference: ASTM, ISO, IS 3400
- Typical Property Values for Common Rubber Types
- A Complete Rubber Testing Lab — What You Need
- Tips for Accurate Rubber Testing
- Frequently Asked Questions
Rubber is not plastic — and testing rubber requires a fundamentally different approach. The elastic nature of rubber, its non-linear stress-strain behaviour, its sensitivity to temperature and ageing, and its unique failure mechanisms mean that the test methods, instruments, specimens, and interpretation of results are all different from polymer testing. This guide covers all seven essential rubber tests, the standards that govern them, typical property values for common rubber types, and how to set up a complete rubber QC lab.
India has one of the world's largest rubber processing industries — from natural rubber plantations in Kerala to synthetic rubber compounding and vulcanisation across automotive, industrial, medical, and consumer product sectors. Whether you are producing automotive seals, O-rings, conveyor belts, footwear soles, vibration dampers, or medical gloves, the same seven tests form the foundation of rubber quality control.
Why Rubber Testing is Different from Plastics Testing
Rubber (vulcanised elastomer) differs from thermoplastic polymers in several fundamental ways that affect how it must be tested:
Elastic recovery
Rubber recovers to its original shape after large deformations (up to 700% elongation). Plastics fracture or permanently deform. This unique elasticity defines most rubber applications — and most rubber tests.
Non-linear stress-strain
Rubber shows a J-shaped stress-strain curve (initially soft, then stiffening at high elongation). This is why M100 and M300 modulus (stress at fixed elongation) is used instead of Young's Modulus.
Temperature sensitivity
Rubber properties change dramatically with temperature — it softens at high temperatures and becomes brittle at low temperatures (glass transition). Ageing tests at elevated temperatures are mandatory.
Viscoelasticity
Rubber has both elastic and viscous character. Properties depend on test speed and loading rate — fast deformation gives higher apparent stiffness and strength than slow deformation.
Vulcanisation state
Unlike thermoplastics, rubber must be vulcanised (cross-linked, usually by sulfur at elevated temperature) to develop its elastic properties. Un-vulcanised rubber (gum) has very different properties from vulcanised compound.
Compound complexity
Rubber products contain 10-30+ ingredients: base polymer, vulcanising agent, accelerators, antioxidants, fillers (carbon black, silica), plasticisers, processing aids. Each affects properties differently.
Common Rubber Types and Their Applications
| Rubber Type | Hardness (Shore A) | Tear Resistance | Chemical Resistance | Key Applications |
|---|---|---|---|---|
| NR — Natural Rubber | 20-90 | Excellent | Moderate (not oil resistant) | Tyres, conveyor belts, engine mounts, medical gloves |
| SBR — Styrene-Butadiene | 40-80 | Good | Poor | Tyres, footwear, hoses, conveyor belts |
| NBR — Nitrile Butadiene | 40-90 | Good | Excellent (oil/fuel resistant) | O-rings, oil seals, fuel hoses, gaskets |
| EPDM — Ethylene Propylene | 40-90 | Good | Excellent (weather/UV) | Weatherstripping, roofing, automotive seals, HVAC |
| CR — Neoprene | 40-80 | Good | Good (moderate oil resistance) | Adhesives, wetsuits, gaskets, hoses |
| Silicone (VMQ) | 20-80 | Moderate | Excellent (temp range -60 to 230 degC) | Medical, food contact, high-temp seals |
| FKM — Fluoroelastomer | 60-90 | Good | Excellent (chemical resistance) | Aerospace seals, chemical hoses, high-temp gaskets |
| PU — Polyurethane | 40-95 | Excellent | Good | Tyres, castors, hydraulic seals, rollers |
| Ebonite / Hard Rubber | 80-95 A or 40-80 D | Low | Moderate | Battery cases, bowling balls, musical instruments |
Test 1: Shore Hardness (Shore A and Shore D)
Test 2: Tensile Strength and Elongation (ASTM D412)
The rubber tensile test differs fundamentally from plastic tensile testing in test speed (500 mm/min for rubber vs 50 mm/min for rigid plastics) and modulus reporting — rubber uses M100/M300 rather than Young's Modulus because the stress-strain curve is non-linear from the start. The J-shaped rubber curve means that the initial slope (which would be Young's Modulus for a plastic) is very low and not a meaningful stiffness indicator for rubber applications.
Test 3: Compression Set (ASTM D395)
Compression Set Formula
CS (%) = [(t₀ − tᵢ) / (t₀ − t₁)] × 100
t₀
Original specimen height (mm)
t₁
Spacer height = 0.75 x t₀
tᵢ
Recovered height after 30 min
Test 4: Tear Resistance (ASTM D624)
Test 5: Specific Gravity / Density (ASTM D792)
Test 6: Thermal Ageing (ASTM D573 / ISO 188)
Test 7: Abrasion Resistance (DIN 53516)
M100 and M300 Modulus — What They Tell You
Unlike plastics, rubber does not have a meaningful Young's Modulus because its stress-strain curve is not linear — the slope changes continuously from very low at small strains to progressively higher at large strains (J-shaped curve). Instead, rubber compounds are characterised by the stress at specific elongation levels:
| Parameter | Definition | Physical Meaning | Applications |
|---|---|---|---|
| M50 | Stress at 50% elongation | Early stiffness — relevant for seals that deform only slightly in service | Soft foam, low-compression seals |
| M100 | Stress at 100% elongation | Standard stiffness indicator — most widely reported. Specimen doubled in length. | General rubber compounds, O-rings, hoses |
| M200 | Stress at 200% elongation | Mid-range stiffness — for applications with moderate dynamic deformation | Flexible bellows, boot seals |
| M300 | Stress at 300% elongation | High-deformation stiffness — specimen at 4x original length. Reported alongside M100. | Tyres, conveyor belts, highly elastic seals |
| M400 | Stress at 400% elongation | Very high elongation stiffness — for materials with >400% elongation at break | Natural rubber, EPDM high-elongation grades |
M100 / M300 Comparison for Common Rubber Compounds (approximate values at 23 deg C)
| Rubber Type | Hardness (Shore A) | M100 (MPa) | M300 (MPa) | Elongation at Break (%) |
|---|---|---|---|---|
| NR (natural rubber) compound 60A | 60 | 1.5 - 2.5 | 4 - 8 | 400 - 600 |
| SBR (black filled) 60A | 60 | 2.0 - 3.5 | 6 - 12 | 300 - 500 |
| NBR (oil resistant) 70A | 70 | 2.5 - 4.5 | 8 - 15 | 250 - 450 |
| EPDM (weathering grade) 60A | 60 | 1.5 - 3.0 | 5 - 10 | 300 - 500 |
| Silicone (VMQ) 60A | 60 | 0.8 - 1.5 | 1.5 - 4 | 200 - 500 |
| Neoprene (CR) 65A | 65 | 2.0 - 3.5 | 7 - 13 | 300 - 450 |
Standards Reference: ASTM, ISO, IS 3400
| Test Property | ASTM | ISO | IS 3400 | Key Notes |
|---|---|---|---|---|
| Hardness (Shore A / D) | ASTM D2240 | ISO 7619-1 | IS 3400 Part 3 | Indentation hardness; both A and D scale |
| Tensile / Elongation | ASTM D412 | ISO 37 | IS 3400 Part 1 | Die C or D specimens; 500 mm/min; M100, M300 |
| Compression Set | ASTM D395 | ISO 815-1 | IS 3400 Part 6 | Method B (constant deflection 25%) |
| Tear Resistance | ASTM D624 | ISO 34-1 | IS 3400 Part 9 | Die B, C, T — kN/m |
| Specific Gravity | ASTM D792 | ISO 1183-1 | IS 3400 Part 11 | Archimedes method; g/cm³ |
| Thermal Ageing | ASTM D573 | ISO 188 | IS 3400 Part 20 | Air oven; % change in tensile properties |
| Abrasion Resistance | DIN 53516 | ISO 4649 | IS 3400 Part 16 | DIN abrasion; volume loss mm³ |
| Resilience (Rebound) | ASTM D2632 | ISO 4662 | IS 3400 Part 12 | Vertical rebound test; % resilience |
| Low-temp brittleness | ASTM D2137 | ISO 812 | IS 3400 Part 14 | Temperature at which rubber becomes brittle |
| Adhesion to substrate | ASTM D429 | ISO 813 | — | Peel adhesion rubber-to-metal or rubber-to-fabric |
| Ozone resistance | ASTM D1149 | ISO 1431-1 | IS 3400 Part 17 | Exposure to ozone; crack appearance |
| Fluid resistance | ASTM D471 | ISO 1817 | IS 3400 Part 18 | Volume and mass change after fluid immersion |
Typical Property Values for Common Rubber Types
| Rubber Type | Hardness (Shore A) | Tensile Strength (MPa) | Elong. at Break (%) | Tear Str. (kN/m) | Density (g/cm³) | Compression Set | Abrasion |
|---|---|---|---|---|---|---|---|
| NR (Natural Rubber) | 30-90 | 15-25 | 400-600 | 30-60 | 0.91-0.97 | <15% at 70h/70 deg C | Excellent |
| SBR | 40-80 | 12-20 | 300-500 | 20-40 | 0.94-1.15 | <20% at 70h/100 deg C | Good |
| NBR (Nitrile) | 40-90 | 10-20 | 250-450 | 15-35 | 0.98-1.15 | <25% at 70h/70 deg C | Moderate |
| EPDM | 40-90 | 10-18 | 300-500 | 15-35 | 0.85-0.90 | <20% at 70h/120 deg C | Excellent |
| Neoprene (CR) | 40-80 | 12-20 | 300-450 | 20-40 | 1.15-1.25 | <25% at 70h/70 deg C | Good |
| Silicone (VMQ) | 20-80 | 5-10 | 200-500 | 5-20 | 1.10-1.25 | <10% at 70h/175 deg C | Excellent |
| FKM (Viton) | 60-90 | 10-20 | 150-350 | 15-30 | 1.80-1.95 | <15% at 70h/200 deg C | Excellent |
| Polyurethane (PU) | 40-95 | 20-50 | 300-700 | 40-80 | 1.05-1.25 | <30% at 70h/70 deg C | Outstanding |
A Complete Rubber Testing Lab — What You Need
Phase 1 — Essential (Day 1)
Universal Testing Machine or Tensile Testing Machine
ASTM D412 tensile strength, elongation, M100, M300; ASTM D624 tear resistance. The single most important rubber testing instrument.
View product →Digital Density Apparatus
ASTM D792 specific gravity. Verifies compound formulation consistency. Quick, simple, inexpensive — should be in every rubber lab.
View product →Hot Air Oven
ASTM D573 thermal ageing. Forced-air circulation at 70-200 deg C. Essential for predicting long-term rubber performance in service.
View product →Phase 2 — Complete Lab
Shore Durometer (A and D)
ASTM D2240 hardness testing. Most frequently tested rubber property. Shore A for standard rubber; Shore D for hard rubber and semi-rigid.
View product →Compression Set Fixtures
ASTM D395 Method B. Cylindrical button and flat platen fixtures for the UTM. Essential for seals, O-rings, gaskets. Oven needed for elevated temperature tests.
View product →Izod / Charpy Impact Tester
Notched Izod impact per ASTM D256 — for hard rubber and rubber-modified plastics. Motorised Notch Cutter needed for consistent specimen notching.
View product →🔗 Related Products:
- → Universal Testing Machine (UTM) — ASTM D412 tensile / ISO 37 — primary rubber tensile and tear instrument
- → Tensile Testing Machine — Dedicated tensile — lighter alternative to full UTM for rubber labs
- → Digital Density Apparatus — ASTM D792 — specific gravity of rubber compounds
- → Hot Air Oven — ASTM D573 / ISO 188 — thermal ageing, forced-air circulation
Tips for Accurate Rubber Testing
- 1Always condition rubber specimens at 23 deg C / 50% RH for at least 16 hours before testing. Rubber properties are highly temperature-sensitive. Testing warm specimens (e.g. just removed from the mould or oven) gives significantly different results from conditioned specimens. ASTM D412 and ISO 37 both require 16h minimum conditioning.
- 2Use the correct test speed — 500 mm/min for rubber tensile, NOT 50 mm/min. This is the most common error when testing rubber on a UTM configured for plastic testing. At 50 mm/min, rubber gives much lower apparent strength and higher elongation than at 500 mm/min. Always verify speed before starting a rubber test series.
- 3Mark the gauge length on the specimen with ink before testing. Elongation at break is calculated from the gauge length marks, not from crosshead displacement. Crosshead displacement includes grip zone compliance and specimen thinning outside the gauge — it gives falsely high elongation. Mark a 25.4 mm (1 inch) gauge length on each Die C specimen.
- 4Discard specimens that fail at or near the grips. A failure inside the grip zone means the grip was applying concentrated stress, not the test geometry. Valid results must show fracture within the gauge zone. Report as 'grip failure' and retest. Use smooth, rubber-lined pneumatic grips at the minimum clamping pressure that prevents slipping.
- 5For compression set, measure specimen height to 0.01 mm. Compression set calculation requires accurate measurement of original, compressed (spacer), and recovered heights. A 0.05 mm error in any measurement can shift the compression set result by 3-5%. Use a calibrated digital calliper or height gauge for all measurements.
- 6For Shore hardness, use specimens at least 6 mm thick. Thinner specimens give artificially high hardness readings because the indenter is sensing the hard backing surface. If the product is thinner than 6 mm, stack layers to achieve minimum thickness — but note this in the test report.
- 7For ageing tests, ensure the oven has proper air circulation. ASTM D573 and IS 3400 require forced-air ovens with 3-10 air changes per hour. Static ovens allow oxygen depletion near the specimens, giving falsely optimistic ageing results. The Hot Air Oven from International Equipments meets this circulation requirement.
- 8Always run control specimens alongside aged specimens. For ageing percentage change calculations, the 'original' tensile values must be measured on specimens from the same batch as the aged specimens — tested on the same day. Do not use historical values from previous batches as the reference.
Key Takeaways
- ✓Rubber testing requires different methods, speeds, specimen shapes, and result interpretation compared to plastics testing. The 7 essential tests are: hardness, tensile/elongation, compression set, tear resistance, specific gravity, thermal ageing, and abrasion resistance.
- ✓Shore A (soft rubber) and Shore D (hard rubber) are the most frequently specified rubber properties. Switch from Shore A to Shore D when readings exceed 90 Shore A.
- ✓ASTM D412 tensile testing of rubber uses 500 mm/min test speed (not 50 mm/min for plastics) and reports M100 and M300 (stress at 100% and 300% elongation) rather than Young's Modulus.
- ✓Compression set (ASTM D395 Method B) is critical for seals and O-rings — a high compression set means the rubber will not spring back to seal against a mating surface. Target: <25%.
- ✓Thermal ageing (ASTM D573 / ISO 188) requires a forced-air oven with defined air circulation — not a static oven. Predicts long-term property retention at elevated service temperatures.
- ✓Standards: ASTM D412, D2240, D395, D624, D792, D573; ISO 37, 7619, 815, 34, 1183, 188; IS 3400 (Indian standard series covering all rubber test methods).
- ✓A complete rubber testing lab from International Equipments includes: Tensile/UTM, Digital Density Apparatus, Hot Air Oven — covering ASTM D412, D792, and D573 from a single supplier.
Frequently Asked Questions
Common questions about rubber testing instruments, standards, and typical property values.