Mechanical Measurement of Polymers using a Texture Analyser – Introduction

Polymers are found in every corner of the modern world. Films, foams, adhesives, coatings, rubbers, composites, textiles and fibres, to name a few, 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. 

PROPERTIES 

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. If a polymer is at a temperature well below the glass transition point, or if it has a high degree of molecular alignment, its stress-strain graph may follow the elasticplastic trend of a metal. If the degree of crosslinking is low and the polymer’s temperature is well above the glass transition, it is likely to behave as an elastomer and its stress-strain graph may take the form of an S-shape, or it may show a continuous stress increase over a very large period of extension. Some rubbers can reach an elongation at fracture of around 1000%. 

Consequently, the yield stress of certain polymers cannot be determined. A proof stress is often used instead at 0.1-0.5% strain. The absence of initial linear elastic deformation also means a true Young’s modulus cannot be calculated. A secant or tangent modulus is used instead at some prespecified point.

Thermoplastics 

Thermoplastics are relatively soft and deformable polymers, particularly at elevated temperatures. They can melt at specific temperatures and be shaped and reshaped (via reheating) to fit a mould. They have a wide range of structural applications, including wire and light duty utilities. They are also used as a matrix for natural and synthetic fibres. 

Thermosetting Polymers 

Thermosetting polymers are hard and brittle. They do not melt, but instead harden irreversibly upon heating. Their starting material is a soft solid, and the application of heat allows the formation of covalent bonds and cross-linking of subunits. The temperature-dependence of their properties is negligible compared to that of thermoplastics. They are formed into their final shape through injection moulding, extrusion moulding, compression moulding, or spin casting. Uses include joints, adhesives and protective coatings. 

Elastomers 

Elastomers are viscoelastic polymers with rubber-like elasticity. They possess weak intermolecular forces along with a low elastic modulus and a high failure strain. Uses include foams, sealings and rubber gloves. 

Sketch graphs showing some typical polymer deformation behaviours

There is a Texture Analysis test for virtually any physical property. Contact Stable Micro Systems today to learn more about our full range of solutions.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplusC texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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