Polymer Mechanical Property Measurement

What are polymers?

Polymers are found in every corner of the modern world. Films, foams, adhesives, coatings, rubbers, composites, textiles and fibres, to name a few, which can take many forms. Their uses are both functional and cosmetic, but wherever and however they are used, they must be fit for purpose. A large part of their research and development and quality control processing revolves around the measurement of mechanical properties. Many polymers are used in the form of engineering materials, but there are also natural polymers in foods (e.g. gluten, pectin or gelatin), wood, paper and other materials, which must be tested to the same extent.

How do polymers behave?

Unlike metals or ceramics, polymers do not follow a formulaic pattern of behaviour, and their properties can consequently be difficult to define. They are viscoelastic, with properties a combination of a viscous liquid and elastic solid, the dominant behaviour dependent on test conditions such as strain rate and temperature as well as the microstructure and composition of the polymer itself. Unlike metals or ceramics, they do not necessarily undergo elastic-plastic deformation, with a defined linear elastic initial load below the yield point, followed by permanent plastic deformation. Additionally, many polymers have a ‘glass transition point’. As the polymer is cooled, this term refers to a temperature range at which a polymer transforms from a soft solid to a rigid glassy material. The polymer’s properties are vastly different below this temperature, fracturing in a brittle manner.

Factors such as the polymer’s temperature relative to its glass transition point, degree of cross-linking, degree of crystallinity, composition and reinforcements all have a large effect on the form of its stress-strain graph. Polymers are typically classified into one of three broad groups: thermoplastics, thermosets and elastomers.

Sketch graphs showing some typical polymer deformation behaviours

How to test polymers

Due to the wide-ranging properties of polymers and their vast range of applications, test types are broader than the traditional tensile and bend tests usually used for engineering materials. Testing can help to determine the optimum material for an application, design parts to withstand forces during use in applications, determine strength or longevity of parts under varying stress conditions and provide the best quality control checks for each application. As with any sample type, there are industry specifications that must be complied with. Many manufacturers, particularly those in consumer, automotive, medical and aerospace industries, choose to test their samples according to standard methods. Product performance is a key concern to manufacturers at every level of the polymer supply chain, from raw material suppliers and manufacturers through to those of the finished product. Polymer raw materials often come in powder form. These can be measured using powder measurements such as Powder Flow Analysis, Unconfined Yield Stress and Powder Vertical Shear.

Tests used to investigate polymer properties include standard configurations such as flexural, compressive and tensile in their pure form, although many tests include a combination of stress states. These include indentation and imitative measurement. Imitative measurements provide the possibility to assess samples in a manner in which they are perhaps subjected to in real-life and are not bound by the restricted form specified in a standard method, thereby providing testing flexibility and potentially more meaningful results. Time-dependent creep and relaxation properties are also of interest to many manufacturers due to the viscoelastic nature of most polymers. These tests are typically carried out using a similar configuration or test setup to standard tests, but rather than a standard load-unload cycle, tests are performed with a hold period at a set stress or strain, or cycles may be performed at a variety of test speeds.

Typical measurements for polymers

Typical measurements include: • Elongation • Compressive Strength • Flexural stiffness/strength • Creep • Proof stress • Breaking Strength • Yield strain/strength • Tensile Strain/Stress/Strength (at break) • Elastic Modulus • Penetration resistance • Deformation to failure • Tensile resilience • Relaxation

Over the years we’ve worked with top scientists and companies in many industries to develop new fixtures to measure specific product properties. When the test solution doesn’t exist we go ahead and develop it and add it to our growing number of Community Registered Designs, testament to our leading innovative approach to finding the right solutions to a customer’s testing needs. The Mini Stickiness System is just one example of this.

We are totally dedicated to the physical/mechanical property measurement of your polymers – but don’t just take our word for it, read an example of what our customers have to say.

A wide range of Polymer test methods (including ASTM and ISO Standards) is built into Exponent Texture Analyser software and will automatically load at the click of a button. We help make your testing quicker to access and the analysis of your product properties already prepared for you.

Options for the measurement of polymers

A selection of special attachments and typical measurements which are commonly used in this application area are shown, although this does not necessarily include the complete range available for the testing of polymers. Test procedures include: compression, puncture / penetration, tension, fracture / bending, extrusion, cutting / shearing. Any of the texture analyser range can be used for the product tests listed.