Composite materials

The melting and crystallisation behaviour of the thermoplastic matrix is characterised by means of differential scanning analysis (DSC). Rheology studies the flow behaviour of the thermoplastic during melting. These analyses allow us to predict the impregnation quality of the composites. Several tests are useful for this purpose, including Melt Flow Index (MFI), rotational and capillary viscometry, viscometry according to Ubbelohde, etc. The (M)DSC method is used to map the curing reactions of thermoset matrices.

The volumetric fraction of fibres of a composite is evaluated in several ways. The methods are either destructive or non-destructive. Please note that the results of destructive methods are more precise. Centexbel usually carries out destructive test methods, where the matrix is chemically or thermally removed. The matrix is burnt by TGA (thermogravimetric analysis) or by the so-called “Resin bur-off test” in the case of ceramic fibre reinforced samples.

In the case of the TGA technique, the samples are very small resulting into a relatively large margin of error because of local differences in the composite. Anyway, the TGA technique has the advantage that it can be used for glass fibre and carbon fibre reinforced composites. In the latter case, the TGA test is performed under an azotic atmosphere.  The Resin Burn-off test is used for larger samples allowing for local differences.

Thermo- mechanical measuring techniques offer the opportunity to study the influence of temperature on mechanical properties. These tests are very useful for materials that will be applied in hot environments. The simplest analysis is the Heat Deflection temperature (HDT) that measures the temperatures at which the material deforms when submitted to a specific load. The test indicates the resistance to deformation at high temperature. It is also possible to measure the Vicat temperature. During this test, the material is being locally deformed by means of a needle at a constantly rising temperature. Thermo-mechanical analyses (TMA) and dynamic thermo-mechanical analyses (DTMA) are more precise and accurate methods. TMA measures the dimensional changes under a rising temperature or under a constant high temperature. TMA also performs a precise control of the (static) force applied on the sample. Thanks to the different modes of this technique, TMA determines several characteristics, including free crimp or crimp force, the relationship between tension-deformation, shear temperature under load, thermal dilatation coefficient, etc.

In the case of DTMA, an oscillating force is applied by moving clamps during a heating and a cooling phase. This technique determines the viscoelastic properties of the sample in relation to temperature, i.e. the elasticity and viscosity module and the damping factor in relation to temperature, frequency and time. The DTMA method determines small transition domains, such as bridging glass transitions that cannot be detected by the DSC method. DTMA also determines in an easy way the relaxation behaviour, the flow and the maximum processing temperature. Centexbel and VKC-Centexbel dispose of a single- or dual-cantilever clamp, three-point flexion and traction modus.