Duromer matrix

Table 9.2. Increase of Young s modulus and tensile strength of a duromer matrix (polyester resin) by addition of glass fibres with a volume fraction of 65% to 70% [77]...

The influence of the fibre direction on the mechanical properties can be seen from table 9.2 for the example of a glass-fibre reinforced duromer matrix. Young s modulus is strongly increased even when irregularly oriented fibres are added. Directing the fibres further increases the stiffness. Using continuous instead of directed short fibres has no significant effect. 

A polymer matrix composite is made from a duromer matrix containing continuous carbon fibres aligned in the loading direction. Young s modulus is 3 GPa in the matrix and 350 GPa in the carbon fibres, the tensile strengths are 60 MPa and 4900 MPa. The volume fraction of the carbon fibres is 55%. 

Long fibres or fabrics have to be positioned correctly within the manufactured component. This can be achieved most easily if the melting point of the matrix material is well below that of the fibres, as in polymer matrix composites. If duromers are used as matrix, the fibres can be laid down in an uncured mixture of resin and hardener. On curing, the resin hardens to form the duromer. 

Polymer matrix composites (PMC) are used to increase the rather low stiffness or strength of polymers by adding stiffer or stronger fibres. If thermoplastics are used as matrix material, short fibres can be added to the granulate material that is subsequently softened by heating and processed, for example in injection moulding. If thermoset resins (duromers) are used, the fibres can be placed into the liquid resin before curing. 

The failure strain of the fibre may also be larger than that of the matrix in some cases, for example in carbon-fibre reinforced duromers or in ceramic matrix composites. After the strain has exceeded the failure strain of the matrix, the complete load has to be borne by the fibres. Similar to the previous case, the maximum stress in the composite depends on the volume fraction of the fibres. If it is sufEciently large, the fibres do not break but can take a load of /tat. If the volume fraction is too small, the maximum stress is again determined by the isostrain rule of mixtures, equation (9.7), but now taking am as failure stress of the matrix and at as the stress in the fibre at the failure strain in the matrix. In ceramic matrix composites, the matrix frequently does not fail completely, but forms many small cracks bridged by the fibres. The stress-strain curve for this case will be discussed in section 9.3.3. 

A further important point is that the fracture strain of high-strength carbon fibres is about 2% although they deform only elastically. Considering that the strains in the polymer matrix locally exceeds that of the fibre (see figure 9.4), we see that the fracture strain in the matrix has to be rather large. To avoid crack formation in the matrix, its fracture strain should be about twice that of the fibre i. e., 4% to 5%. Currently available duromers do not... 

Depending on the application, different matrix materials are used. Among the duromers, most common are polyester and epoxy resins. Thermoplastic matrix materials are polyethylene (pe) and polypropylene (pp), but the use of thermoplastics with aromatic rings on the chain and thus with increased temperature stability also grows. One example is polyetheretherketone (peek), characterised by high toughness and a glass temperature of about 150°C. 

The selection of polymeric matrix depends on the final application of electrically conductive composites. For a specific purpose, an optimal balance among mechanical, optical, thermal, ecological, and other properties must be maintained. Various classes of polymers have been used for composite preparation, such as soft elastomers and rubbers, linear or branched thermoplastics, or hard duromers as epoxies. All these materials cover a wide range of properties. The main physical and structural parameters that influence the final electrical behavior of multiphase materials are the polymer surface chemistry, degree of crystallinity, and the thermal and mechanical behavior. 

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