Sandwich composite approach for EMI shielding structures

A sandwich composite approach has been employed to develop epoxy based polymer matrix composite materials to realize a class of lightweight structures capable of shielding electromagnetic interference (EMI). Hollow glass microspheres embedded syntactic foams (thickness ap 2 mm; density ap 0.6 g/cc) of two different varieties have been used as core materials in these sandwich constructions (total thickness ap 2.6 mm; effective density ap 0.75 g/cc). The choice of the exact polymer system was guided by the desired temperature withstandability, whereas the choice of the reinforcements was a function of the EMI shielding, structural and other requirements. These sandwich composites were characterized for their shielding effectiveness (SE) in the frequency band of 1 MHz to 100 MHz by coaxial jig method. The materials were also evaluated for their structural capabilities by 3 point bend flexural testing whereas the viscoelastic properties and glass transition temperature (T_g) were measured by dynamic mechanical analysis (DMA). The results amply demonstrated the scope of tailoring such lightweight materials for specific end-use requirements by suitable selection of matrix - core material - skin combinations. Some specific application sectors considered in the present paper include (i) high strength (>250 MPa) /modulus (>15 GPa) - moderate SE (>35 dB) - room temperature applications; (ii) low strength (ap100 MPa) / modulus (ap5 GPa) - high SE (>70 dB) - room temperature applications; (iii) optimized strength (>140 MPa) / modulus (>10 GPa), SE (>65 dB) - elevated temperature applications (les150degC) etc.