Understanding Rubber Specifications

How Material Properties Affect Performance

Unlike steel alloys, rubber compounds are not universally defined, which often leads to misconceptions about their performance. Standardised testing methods also vary between regions and frequently do not correlate well. As a result, the importance of material specifications is often underestimated.

Too often, buyers and engineers default to a generic specification such as “60 Shore A, black.” While simple, this approach can have serious consequences, including increased downtime and higher associated costs.

Moreover, a compound suitable for a chute liner may not be appropriate for pulley lagging. Recognising these differences is critical when selecting the right material for the application.

The guide below outlines key rubber properties in simple terms. Please contact us for expert guidance tailored to your specific requirements.

Base Polymer

The base polymer is the ideal starting point for evaluating a compound’s suitability for a given application, though it can be complex. Below are some basic examples:

• Butyl – Excellent for acid resistance.
• Natural Rubber – Well-suited to applications with cyclical stresses, such as car tyres and pulley lagging.
• EPDM – Ideal for applications with high UV exposure.

Hardness

Hardness simply defines how hard the material is. It is the next step in evaluating whether a particular compound is suitable. There are different scales, but the most commonly used scale is the "Shore A" scale. Below are some basic examples:

• 60 Shore A – Pulley Lagging and Vehicle Tyres.
• 40 Shore A – "Cushioning" applications such as chute liners.
• 80 Shore A – Drive rollers in printing presses or textile machinery

Tensile Strength

Tensile strength is generally a good indicator of rubber quality: a higher value often suggests longer service life (though there are exceptions).

Unless the tensile strength is particularly low, it is rarely the limiting factor in a product’s lifespan. Compounds with only marginal differences in tensile strength should instead be evaluated based on other properties.

Elongation at Break

Elongation at Break is expressed as a percentage, comparing the stretched length of a rubber sample to its original, unstretched length. A higher elongation indicates a more flexible rubber.

For applications such as pulley lagging, higher elongation is beneficial, as it allows the lagging to distribute load more evenly around the pulley face, reducing the likelihood of slippage. Conversely, applications like high-pressure gaskets may require lower elongation to maintain dimensional stability under load.

Abrasion Loss

Abrasion loss is an important property, but it is often over-emphasised, and testing methods can be somewhat subjective.

Certain applications, such as slurry pipe-lining, definitely benefit from lower abrasion loss, as it directly reduces wear. Pulley lagging also gains from lower abrasion loss, but its performance depends more on higher resilience and flexibility of the rubber compound.

Resilience

In simple terms, resilience measures how “bouncy” a rubber compound is. A higher resilience generally correlates to a longer service life, as the rubber rebounds more efficiently and converts less kinetic energy into heat; heat being a major factor in rubber ageing.

However, higher resilience is not always ideal. For example, chute lining applications benefit from lower resilience, as this helps to absorb and dampen the kinetic energy of the ore, reducing wear and impact forces. Pulley Lagging and vehicle tyres, however, benefit greatly from a higher resilience.

Tear Strength

Tear strength is a measure of a rubber compound’s resistance to the growth of cuts, nicks, or tears once they’ve started. It is usually expressed in Newtons per millimeter (N/mm).

A higher tear strength indicates that the rubber can better resist damage spreading under stress, which is especially important in dynamic applications where small cuts or sharp material contact is likely. For example, pulley lagging benefits from good tear strength to prevent minor surface damage from propagating into full failures.

On the other hand, in some static applications where tearing forces are minimal, tear strength may not be the primary design consideration compared to other properties like hardness, resilience, or chemical resistance.

Cure Characteristics

Cure characteristics may seem highly technical, but they often play a greater role in service life than expected. A compound can appear to meet all physical property requirements, yet if it does not achieve a stable cure (common with cheaper formulations or inappropriate polymers) its performance and lifespan will be compromised.

Cure behaviour is typically assessed using a rheometer trace (rheotrace), which shows how the rubber cross-links during vulcanisation. For compounds used in hot environments or under dynamic loading, a stable cure is essential, one where the trace plateaus after reaching full cure. Below is an example of two relatively stable cures, but one showing to be more stable than the other:

Problematic cures generally fall into two categories: 

• Early reversion – the rubber begins softening after reaching cure, reducing strength and heat resistance.
• Marching modulus – the trace climbs steeply after cure, leading to excessive hardening and brittleness.

In dynamic applications, combining poor cure behaviour with low resilience will dramatically shorten service life.

Other Properties

Many additional properties may influence performance in specific applications. For example, compression set is important for gaskets to ensure sealing under long-term load, while cut and gouge resistance is critical in impact applications such as chute liners or high-impact wear areas.

Not all of these properties are easily measured with standardised tests. However, properties like tear strength and tensile strength can sometimes provide indirect insight into how a compound might perform under cut and gouge conditions, though specialised testing is always preferable when available.

Fire Retardency, Anti-Static and Toxicity

These are safety compliance requirements, and their exact standards vary significantly between regions and applications. They are usually assessed as pass/fail tests, and it is critical to ensure compliance before a product is approved for use.

For example, vehicle tyres and pulley lagging must be at least slightly conductive. Without conductivity, the dynamic friction involved in these applications creates a high risk of arcing or sparks, potentially leading to fire.