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SOFT SEGMENTS OF COMPOSITE MATERIALS Ensuring superior composite characterisation

Aug 22, 2022

An application story on the usage of DMA/SDTA 1+ from Mettler-Toledo that delivers superior results, when comparing soft segments of composite materials consisting of hard & soft layers. The instrument’s design, which includes clamps is optimised for shear measurements and enables external sample preparation. This, therefore, reduces the risk of clamping errors relative to tension mode for conventional DMAs.

Dynamic mechanical analysis (DMA) is used to measure the mechanical and viscoelastic properties of a material as a function of temperature, time, and frequency while it is subjected to an oscillating stress.

DMA/SDTA 1+: Features & benefits

  • Unique measurement of both displacement and force – results in a very accurate determination of moduli

  • Wide force ranges from 1 mN to 40 N – allows very soft and very hard samples to be measured

  • Broad frequency ranges from 0.001 to 1,000 Hz – measurements can be performed under both real conditions and more rapidly at higher frequencies

  • DMA experiment wizard – assists in setting up the perfect experiment for the best results

  • Patented SDTA™ technique – enables to calibrate sample temperature and accurately measure thermal effects

  • Extremely wide stiffness range – measurement for a sample can be done from the glassy to the liquid state in a single run in one temperature segment

  • Ergonomic design and touch-screen control – ensures faster setup and optimisation of experiments

Measurements with static forces

Besides the dynamic mode, DMA 1 permits measurements to be performed using static forces (TMA mode). All the deformation modes available for DMA can be used.

The glass transition of the soft parts in composite materials have significant effect on their performance but is hard to analyse by DSC technique, owing to the low proportional fraction of amorphous content. METTLER TOLEDO’s DMA/SDTA 1+ is uniquely suited to analysis of these materials, providing shear measurements that effectively distinguish similar samples to a great extent.

Often, composite materials consist of a hard carrier material coated with a soft thermosetting one. It is essential to utilise the technology to monitor the glass transition of thermoset, which reveals vital information about both processing and anticipated temperature range for the application of composites. However, in such sandwiched composite materials with certain weak glass transitions of amorphous component, it may be challenging to find out the optimal characterisation techniques.

Usually, the quantity of later contained in the material is very small, which makes difficult to find glass transition by conventional differential scanning calorimetry (DSC). In such case, Dynamic Mechanical Analysis (DMA) is the method of choice, as it is the most sensitive thermal analytical method for glass transition measurements. DMA can be number of times more sensitive than DSC to the changes of amorphous fraction in the composite material. Among all the thermal analysers, DMA is probably the most sensitive tool for Tg evaluation. It analyses a material’s response to an oscillatory stress (or strain) and how that response changes with temperature, frequency, or both. The DMA graph can show how the material reacts to temperature changes in a variety of ways.

Herein, we have investigated the ability of several DMA measurement modes to analyse two adhesive tape samples comprising of same PET-based hard segment but different soft adhesive layers.

DMA/SDTA: Operating principle

The operating principles of the DMA/SDTA are in many respects very different to those of the current generation of conventional DMA instruments. The massively built stand results in the system having an intrinsic resonance frequency of about 1,500 Hz, well above the measurement frequencies used. The sample itself is fixed directly to the force sensor so that the force applied to the sample is measured. This technique was developed at the Institute of Dynamic Material Testing, University of Ulm, Germany, where it has been in successful use for several years and has undergone continuous development. The modulus is calculated from the ratio of force to displacement, multiplied by a geometry factor given by the sample dimensions. The modulus can be determined with great accuracy because both force and displacement are measured. The fixed and moving parts can be adjusted via a three-dimensional alignment device so that the force is applied at an angle of exactly 90° to the sample and no errors due to transverse forces occur.

Dynamic mechanical analyser DMA 1

The unparalleled versatility of the DMA allows applications to be performed in the optimum measurement configuration. The DMA is quick and easy to set up, whether for conventional DMA analyses or for experiments using static forces or measurements in liquids.

Measurements at controlled relative humidity: The humidity option consists of a special humidity chamber, a circulating heating bath and a humidity generator. It allows user to perform measurements under optimum conditions in every deformation mode. Special readjustment is not necessary after installing the humidity chamber.

Measurements with static forces: Besides the dynamic mode, the DMA permits measurements to be performed using static forces (TMA mode). All the deformation modes available for DMA can be used.

Measurements in liquids: The Fluid Bath option allows user to perform DMA or TMA experiments in liquids using all the standard deformation modes. The entire sample holder and sample is immersed in the liquid. The Fluid Bath option consists of a special immersion bath and external temperature control using a circulating heating bath or chiller.

DMA measurements

The measurement principle of the DMA/SDTA 1+ ensures high accuracy because it measures both displacement and force, yielding very accurate modulus values. It was designed to measure material behaviours at high frequencies to match real-life conditions. This is enabled by its very stiff stand resulting in an intrinsic resonance frequency of about 1,500 Hz – well above the measurement frequencies used (up to 1,000 Hz).

Force is measured directly by means of a piezoelectric crystal and is not set using a force-current graph as in conventional DMA instruments. The force measured is that which is applied to the sample. Compensation for frictional losses, membrane force and inertia is no longer necessary.

A special temperature resistant LVDT allows measurements to be performed over a large displacement range with nanometer resolution. The LVDT is located close to the sample so that only the deformation of the sample is measured. This eliminates any effect due to deformation of the stand and improves the accuracy of measurement of the phase shift. The reproducibility of the displacement measurement is improved by measuring the temperature of the LVDT sensor and correcting for the deviation.

The frequency range has been extended to the kHz region for the first time ever in a DMA instrument. In the shear mode, six decades are available. The region above 1 Hz is particularly interesting because it means that measuring times can be kept to a minimum.

Sample measurements in DMA can be performed in several measuring modes (shear, liquid shear, tension, dual and single cantilever, 3-point bending, compression), though most of the conventional DMAs on the market are optimised for bending and tension.

The external sample preparation feature of this instrument is extremely beneficial compared to competitor models. In Figure 1 (right), the differences in the shear modulus, G’, & tangent delta of the samples can be observed clearly. In both samples, the glass transition temperatures are almost 20°C apart, and the width of the glass transitions is also different. In the tension mode of DMA measurement, the hard PET component plays the dominant role whereas with shear mode, by contrast, the principal contribution to the measurement comes from the soft segments.

Courtesy: METTLER TOLEDO

Image Gallery

  • The measurement principle of the DMA/SDTA 1+ ensures high accuracy

  • Measurements via DMA / SDTA 1+ in tension (left) and shear (right) modes of adhesive tape samples

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