Analysis of a Fiber-Reinforced Composite by TOPEM and DMA

The characterization of the curing reaction and glass transition of a matrix resin is important for guaranteeing the properties and performance of a composite material. In this article, TOPEM^® and DMA are used to investigate a carbon-fiber reinforced epoxy composite.

Sample specimens used for 3-point bending (left) and shear (right) measurements

Introduction

Composite materials are widely used as structural components in the aerospace, automobile and building industries. This has to do with their special properties such as high strength and stiffness-to-density ratio, low coefficient of thermal expansion, and favorable vibration-damping characteristics.

The strength and stiffness of a composite are mainly determined by the type of fiber used in the composite, the volume content of the fiber, and its orientation (e.g. unidirectional or as woven fabric, etc). Thermosetting resins are often used as the matrix component. The resin binds the fibers together in the right orientation to form a structure, transfers the forces between fibers and not least protects the fibers from chemical or physical damage.

It is therefore very important to characterize the matrix resin in order to adapt and optimize its properties to the production process, to control the quality, and analyze possible causes of failure. The glass transition and the curing behavior of the resin can have a major effect on the properties of a composite. These properties include impact resistance, brittleness, creep behavior, and solvent resistance.

Significant deviation of the glass transition temperature from normal values may indicate insufficient curing (undercure) or a reaction that has gone too far (over-cure). This can for example result from an incorrect processing temperature or from temperature gradients in the production piece. Storage conditions (e.g. temperature and moisture) can also have an adverse effect on the matrix resin especially if the material is insufficiently cured. Postcuring during use of the composite can also adversely affect its properties. The glass transition of a pure or a lightly filled resin can be easily determined by DSC by measuring the change in the specific heat capacity (cp). However, in a highly filled composite it may be difficult or even impossible to detect the glass transition because cp is very small due to the dilution effect of the filler. In this case, a more sensitive technique has to be used, for example dynamic mechanical analysis (DMA). This method determines the glass transition by measuring changes in mechanical properties such as the elastic modulus or the damping factor.

Sometimes, changes in the glass transition are masked by other effects such as postcuring or the vaporization of constituents. Furthermore, thermal or mechanical pretreatment of the material can ead to relaxation effects. The measurement curve is then more difficult to interpret and evaluate. The determination of the glass transition temperature of the matrix resin by DSC may even become impossible. In such cases, temperature modulated DSC, for example TOPEM^® [1], can be used to separate the cp changes of overlapping effects and obtain a better understanding of the transition.

In this article, we will show how carbon-fiber-reinforced epoxy resins can be characterized by TOPEM^® and DMA. These techniques can be used both in research and development and in process and quality control.

Experimental Details

The sample was a 0.64-mm thick sheet of a composite consisting of a cured epoxy resin reinforced with carbon fibers.

The TOPEM^® measurements were performed using a METTLER TOLEDO DSC 1 equipped with an IntraCooler cooling system. A small flat piece of material from the sheet weighing about 10 mg was placed in a standard 40-µL crucible and closed with a pierced lid. The temperature program consisted of a heating-cooling-heating cycle performed at an average heating rate of 2 K/min and standard pulse parameters (pulse height ± 0.5 K, pulse width 15–30 s).

The DMA experiments were carried out in both shear and bending modes using a METTLER TOLEDO DMA/SDTA861e. For the 3-point bending mode, a piece measuring 40.0 mm x 12.4 mm x 0.64 mm was cut out from the sheet. A constant force of 0.3 N was applied for predeformation. The maximum force and displacement amplitudes were 0.1 N and 50 µm respectively. The measurement was performed at a frequency of 1 Hz and a heating rate of 2 K/min.

The shear measurements were carried out using pieces cut out from the sheet measuring 1.9 mm x 2.3 mm x 0.64 mm. The surfaces were polished with fine sandpaper to remove the surface texture and ensure good surface contact with the shear sample holder; the shear clamp without texture was used. The following measurement parameters were used: heating rate 2 K/min; modulation frequencies 100, 10 and 1 Hz in sequence; maximum force and displacement amplitudes 15 N and 2 µm.

Results and Discussion

TOPEM^® results

Figure 2 shows the total, reversing, and non-reversing heat flow curves obtained from the first and second DSC heating runs using TOPEM^® . The total heat flow corresponds to the heat flow measured by conventional DSC at the same heating rate of 2 K/min. Based on this curve alone (black curve in Figure 2), it is difficult to identify the glass transition and other effects and to assign temperatures. The separation of the total heat flow into the reversing and non-reversing components facilitates the interpretation and allows the glass transition temperature to be determined.

Conclusions

It is important to characterize the glass transition and postcuring behavior of the polymer matrix resin in order to ensure that the composite exhibits the desired properties and performance. The DSC can be used to measure the properties of a pure polymer matrix or a filled composite, in particular the glass transition, the curing behavior, and a possible postcuring reaction. If individual effects overlap, temperature-modulated DSC techniques such as TOPEM^® can be used to separate the glass transition from other overlapping effects. This makes it easier to interpret the curves and to evaluate the different effects. The original glass transition temperature of a matrix resin can only be determined in the first heating run because the postcuring reaction that occurs immediately after the glass transition shifts it to higher temperature.

Dynamic mechanical analysis (DMA) is a very sensitive method for characterizing highly filled composites. When this technique is used, it is important to use a suitable deformation method and to choose the right direction of measurement. The properties of the soft component (i.e. the polymer matrix) are best measured in a shear measurement parallel to the direction of the fibers. The 3-point bending mode is, however, the method of choice for measuring changes in the elastic modulus of the composite material.

^{Analysis of a Fiber-Reinforced Composite by TOPEM^® and DMA | Thermal Analysis Application No. UC 325 | Application published in METTLER TOLEDO Thermal Analysis UserCom 32}