Tensile Strength Characteristics in Different Types of Rubber

Tensile strength is one of the most widely specified mechanical properties in elastomer selection. It represents the maximum stress a rubber compound can withstand before rupture, typically measured according to:

• ASTM D412

• ISO 37

However, tensile strength is not purely a polymer property. It depends on:

• Polymer backbone chemistry

• Molecular weight

• Crosslink density and cure system

• Filler type and dispersion

• Processing conditions

Understanding how different elastomers achieve tensile performance is critical for proper material selection.

Natural Rubber (NR)

Typical Tensile Strength: 20–30+ MPa

Natural rubber remains the tensile benchmark among general-purpose elastomers.

Why?

NR exhibits strain-induced crystallisation. When stretched, polymer chains align and form temporary crystalline regions that reinforce the material during deformation.

This results in:

• Very high tensile strength

• Excellent tear resistance

• Outstanding fatigue life

Limitation: Poor ozone and oil resistance without protection.

EPDM (Ethylene Propylene Diene Rubber)

Typical Tensile Strength: 7–21 MPa

EPDM has a saturated backbone, providing:

• Excellent ozone resistance

• Superior weathering stability

• Good heat resistance

However, it lacks strain crystallisation, so tensile performance depends primarily on filler reinforcement.

EPDM is selected for durability, not peak tensile performance.

Nitrile Rubber (NBR)

Typical Tensile Strength: 10–25 MPa

Tensile performance varies with acrylonitrile (ACN) content:

• Higher ACN → Better oil resistance

• Higher ACN → Reduced elongation

• Balanced grades → Good mechanical strength + oil resistance

Common in seals, O-rings, and fuel system components.

Silicone Rubber (VMQ)

Typical Tensile Strength: 6–12 MPa

Silicone rubber relies heavily on silica reinforcement because:

• It does not strain-crystallise

• The Si–O backbone provides flexibility rather than strength

Advantages:

• Wide temperature range (-60°C to +200°C+)

• Excellent aging resistance

• Stable mechanical retention

Often used where thermal stability outweighs tensile requirements.

Fluoroelastomers (FKM)

Typical Tensile Strength: 10–20 MPa

FKM offers:

• Exceptional chemical resistance

• High temperature capability

• Good compression set resistance

Tensile strength is moderate but stable in aggressive environments.

Technical Factors Governing Tensile

Strength

1. Strain-Induced Crystallisation

Present in NR — major contributor to superior tensile and tear strength.

2. Filler Reinforcement

Carbon black and silica dramatically increase tensile strength by:

• Improving stress transfer

• Restricting chain mobility

• Increasing modulus

3. Crosslink Density

• Low crosslink density → Weak network

• Excessive crosslink density → Brittle behaviourOptimisation is critical.

4. Cure System

• Sulphur systems → Better dynamic tensile in diene rubbers

• Peroxide systems → Better heat resistance

• Addition cure (silicone) → Cleaner network and consistency

Comparative Summary

Rubber Type	Typical Tensile (MPa)	Dominant Mechanism
Natural Rubber	20–30+	Strain crystallisation
SBR	15–25	Carbon black reinforcement
EPDM	7–21	Filler reinforcement
NBR	10–25	ACN balance + reinforcement
Silicone	6–12	Silica reinforcement
FKM	10–20	Stable fluorinated backbone

Engineering Consideration

Tensile strength alone should not drive material selection. In many industrial applications, more critical properties include:

• Tear resistance

• Compression set

• Fatigue resistance

• Chemical exposure resistance

• Thermal stability

The correct elastomer choice depends on the complete service environment.