How to test the quality of rubber

Have you ever had the o-ring on your garden hose split and leak everywhere? Annoying, isn’t it? If your first thought is that it’s due to a poor quality of rubber, think again. Many o-rings are made from NBR (nitrile), because the large majority of o-rings are used in applications to seal against oil – which NBR excels at. Unfortunately, it also degrades quickly in sunlight. Not a poor rubber quality, then – just the wrong material for the hose application. We take a more detailed look at what we mean by rubber quality and how to test the quality of rubber.

What do we mean by rubber quality?

Before we look at how to test the quality of rubber, we need a clear understanding of the term ‘rubber quality’. Ensuring that you are working with the right quality of rubber might best be described as the process of making sure that the product or material that you choose is most suitable for the application in which you need it to perform. In other words, does it do the right job when it is brand new, and will it continue to perform at an acceptable level for as long as you want it to? 

This analysis can be broken down into two areas: firstly, the question of whether the mechanical design is correct, ensuring the product is functional and reliable and, secondly, whether the rubber type and grade is suitable for the conditions it will operate in. 

If the design of the product is sound and proven, which we can assess even before manufacture using non-linear finite element analysis, then we can really begin to focus on whether the rubber quality is right for your product. To answer this question, we would need to know about the operating environment of the rubber part, as this affects the choices you should make in terms of the polymer type and the grade of that polymer to provide the performance and service life that you need, within a specific budget. 

One of the most important points to remember about ‘rubber quality’ is that it isn’t necessarily true that the highest grade, most expensive material available will be the best solution; instead, we recommend that you aim to use the most cost effective and sustainable solution for the application – and it can often be the case that a lower cost, lower grade of material will perform perfectly well in meeting the design requirement and service life needed. 

What is the difference between a commercial rubber and a specified grade of rubber?

Within the rubber family there are many different polymer types, ranging from the natural rubber that is tapped from rubber trees, through to synthetic rubbers that are derived from crude oil and other chemicals. They all have different strengths and weaknesses and there is not one single ‘magic rubber’ that does everything optimally, so it is vital to understand what the best polymer type is for any given application. 

Once the polymer type has been identified, the required operational parameters may dictate whether its performance needs to be formally defined as part of a specification. For example, if a particular level of chemical or temperature resistance is needed, or tensile strength value must be met, this aspect of the ‘rubber quality’ will be designed to meet those targets and, if tested against those requirements, it becomes known as a ‘specification grade’. Such a rubber spec may be unique to a customer, or it may meet an industry or international standard such as ISO, DIN, ASTM, etc.

Alternatively, a general purpose – or commercial grade – of the polymer choice may be suitable for the application. In this respect, properties such as chemical resistance, compression set and tensile strength will be lower – but in many cases will provide a suitable rubber quality for many products. 

How do you know what quality of rubber is most suitable for your product/application?

Rubber technology is a wide-ranging topic with many subtle aspects to it; a small change in the constituents in a rubber compound can dramatically affect the properties of the rubber products made from that grade, either to enhance or detract from performance. 

General guidance on the typical strengths and weaknesses of the family of rubber types is readily available but it is really important to fully understand the environment that a product will be working in; using the garden hose o-ring example, most o-rings on general sale are commercial grade NBR, rather than EPDM, which is the material best suited to the application due to its operating environment. 

The level to which a rubber product is required to perform will then dictate whether a commercial grade of rubber compound will be suitable, or whether a higher grade of material, to a known and provable specification, is needed. That way, the rubber quality will be suited to your application and product needs. 

How to test the quality of rubber 

We are sometimes contacted by businesses who are receiving rubber components from other sources that are failing or underperforming where they were previously acceptable. This could be caused either by a change in the operating conditions or environment, or by a reduced rubber quality due a change in the rubber composition or the manufacturing process used for the part.

It can be difficult to test the quality of rubber on a range of finished parts, or analyse why components begin to fail when historically they have performed well, but there are steps that you can take: 

• Physical properties such as hardness and tensile strength can be checked if samples of regulation size and shape can be created.
• Soak testing can be done to check fluid resistance.
• A TGA process can be used to destructively test the components to identify the chemical constituents.
• Gas Chromatography can, if needed, provide further insights into the chemical constituents. 

These laboratory tests on the quality of rubber are not routinely done unless a comprehensive failure mode analysis is required once a full review of the traceability with the supply chain has been done, along with a series of process checks and validations. 

Unfortunately, it is all too easy within a global supply chain to provide documentation that does not reflect reality, and, for safety-critical components, reliable, trustworthy suppliers and high-confidence processes are essential to ensure the product supplied is as it should be, and continues to meet your specification. Rubber moulding compound can be impacted where the supply chain has changed the composition to include higher proportions of lower cost materials, to ostensibly produce the same rubber quality. 

Tests that we can offer at Martin’s Rubber include:

• Hardness (Shore A and IRHD)
• Compression Set
• Load deflection
• Rheology (MDR)
• TGA
• Tensile stress-strain
• Peel
• Fluid Swell
• Vibration analysis
• Thermal Conductivity
• Radiation Resistance

How can you ensure consistent rubber quality?

To combat the issue of changes within compound make-up and to ensure that you have the confidence that there is adequate control over supply chains, choose a rubber manufacturer who only buys materials from credible, long-term suppliers and who can demonstrate robust quality systems and proven laboratory facilities to continuously monitor the adherence to specification of the materials used. 

Whether our rubber moulding compounds are specification or commercial grade, we only use highly competent and trustworthy suppliers who are based in western Europe, where we have the confidence and track record to assure us that what we receive and process meets expectation. We have in-house testing of the cure profile of any material batch via our MDR, and we undertake hardness and compression set testing routinely to ensure that our finished products do indeed meet the rubber quality standard required of any given material or product.

If you are concerned about the quality of rubber for your future projects, or would like to discuss how to test the quality of rubber parts you have had manufactured elsewhere, get in touch with our expert team and we will be able to guide and support you in finding the most suitable way forward.