How to Measure Viscoelasticity
What is Viscoelasticity?
When a force is applied to a material, it causes the material to deform. This deformation can take one of two extremes – pure elastic deformation (e.g. a spring) or pure viscous flow (e.g. oil).
The force causes the elastic material to instantly deform by a set amount, and the viscous material to flow for the whole time the force is applied. When the force is removed, the elastic material instantly returns to its original state, whereas the viscous material does not recover at all.
Elastic behaviour is usually caused by the bonds stretching between atoms (which is instant). Viscosity is caused by atoms or molecules moving past each other, which takes time but is also not easily reversed, unlike elasticity.
The majority of materials are somewhere in between. They show both viscous and elastic behaviour and so are called ‘viscoelastic’. This means that when a force is applied, there is an initial elastic ‘stretch’, and the material will continue to deform in a viscous manner until the force is removed. After unloading, the elastic deformation is reversed but the viscous deformation is not.
The faster a viscoelastic material is deformed, the more it shifts towards elastic behaviour and away from viscous. This is due to the time-dependent mechanisms that allow creep and relaxation to occur in the first place. If the deformation is faster, they do not have time to occur.
Depending on the type of product under consideration, different proportions of elastic and viscous behaviour are desirable. A car tyre should have low viscosity (so heat build-up is less likely), whereas bread dough should have both viscous and elastic qualities (to allow for both mixing and stretching when being kneaded).
How can Viscoelasticity be measured using a Texture Analyser?
As a viscoelastic material shows time-dependent behaviour (it will keep stretching for as long as a force is applied), a measurement of viscoelasticity must include a holding period. This usually involves loading a sample to a set force and observing the change in deformation over a time (a ‘creep’ test), or loading to a set distance and observing the change in force (a ‘relaxation’ test).
Stress and strain are often used in the place of force and distance when the samples can easily be given a constant and measurable cross-section. However, for the majority of quality control tests in industry, it is more convenient to use force and distance (for samples with a uniform height) or strain (for samples with varying height). Strain is simply the deformation distance divided by the starting height.
There are several simple ways of measuring creep and relaxation on a Texture Analyser:
A Compression test involves a sample being placed between two flat plates (usually the instrument base and a flat probe). The probe must be larger than the sample to avoid penetration or cutting. Ideally, samples for this test type will have flat, parallel top and bottom surfaces. However, if testing a finished product, that might not be possible. For example, if a chocolate egg or a tablet is to be tested, the main concern will be to load the sample in the same orientation each time.
Other samples simply cannot be modified to form a sample with convenient geometry. Friction between the sample and the plates can cause errors in the viscoelasticity measurement, although this is often a small effect. To help avoid friction, lubricating the sample or bonding it to the probe and instrument base can be useful. Additionally, very tall samples should be avoided when testing in compression as they can buckle, but samples that are too short can result in inaccuracies due to the effect of testing too close to the base.
Tensile tests are useful for fibrous or elastomeric materials, or samples that are put under tension during use. In a tensile test, the sample is clamped between jaws. In many cases, the pressure exerted by the jaws weakens the material and causes premature failure. This can be prevented by notching (although this is not so easily repeatable) or forming the sample into a dog-bone shape to encourage deformation in the thin central section. There are many variations of tensile grips available from Stable Micro Systems including serrated and rubber coated grips.
Bend tests are also sometimes used for viscoelastic measurements, but they are usually restricted to elongated samples that are not easily clamped for tensile testing or formed into compact shapes for compression testing. They are also useful for brittle materials, as the true material deformation is small compared to the measured deflection.
Indentation testing is not ideal for viscoelastic assessment, as it is difficult to extract fundamental parameters. However, it can be very useful for samples that are not freestanding (that must be contained), such as some gels and soft materials. It is very simple to perform – the most important condition is a flat sample surface. The indentation depth is held for a set time period and the resulting relaxation observed as a drop in force.
The general shape of a relaxation graph shows a force that decreases over time, whereas a creep graph shows an increasing deformation. In both cases, this behaviour is due to internal rearrangements at the molecular level allowing partial relaxation of the force exerted by the sample on the loading arm (or an increase in deformation in response to the force). This dissipates some of the energy stored during the initial loading period. This energy is lost as heat. When the loading arm removes the force, the material undergoes elastic recovery. It is still pushing back on the probe, so it is doing work.
During this process, the remainder of the stored energy is recovered. In an ideal elastic material, there is no lost energy. In an ideal viscous material, there is no elastic recovery. Most materials are somewhere between these behaviours.
With all viscoelastic testing, the results will depend on the test conditions such as test speed, temperature and sample composition and geometry. Unless the conditions are kept identical, the tests are best used to compare samples rather than to find absolute material properties. For example, the compressive behaviour of a cube of mango should be interpreted in the context of compressive behaviour of other mangos of similar intended use, or a target value known to represent a good product.
Temperature can have a particularly large influence on the viscous component. A higher temperature leads to more viscous behaviour as molecules have more thermal energy, allowing them to move past each other more easily. Temperature does not have a noticeable effect on elastic properties unless the temperature change is very large. The effect of temperature on viscoelasticity is studied for many products, particularly around the temperature range of intended use.
The range of Texture Analysers by Stable Micro Systems are the ideal testing solution for semi solid to solid products such as many of those found in the food, cosmetics, pharmaceutical and materials industries. All of the above measurements are made simple with the use of specially-designed probes and fixtures as well as the possibility of temperature control and specialised test sequences. Data analysis is automatic, at the press of a button.
For more industry-specific information on the measurement of Viscoelasticity using a Texture Analyser, read our blog posts:
Viscoelasticity in the Food Industry>>
Viscoelasticity in the Pharmaceutical and Cosmetics Industries>>
Viscoelasticity in the Materials Industry>>
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