Electrical Conductivity of Urea-Formaldehyde-Cellulose Composites Loaded with Copper

This work is concerned with the preparation and characterization of composite materials prepared by compression molding of mixtures of copper powder and a commercial grade thermosetting resin of urea-formaldehyde filled with alpha-cellulose in powder form. The electrical conductivity of the composites is <10(-12) S/cm, unless the metal content reaches the percolation threshold of 24.0 vol %, beyond which the conductivity increases markedly by as much as 11 orders of magnitude, indicating an insulator-conductor phase transition. The homogeneity of these composites was checked by the morphologies of the constituents (filler and matrix) and the composites characterized by optical microscopy. The density of the composites was measured and compared with values calculated assuming different void levels within the samples to discuss the porosity effect. Finally, the obtained results on electrical conductivity have been well interpreted with the statistical percolation theory. The deduced critical parameters, such as the threshold of percolation, V-f*, the critical exponent, t, and the packing density coefficient, F, were in good accord with earlier studies. In addition, the hardness of samples remained almost constant with the increase of metal concentration. POLYM. COMPOS., 32:193-198, 2011. (C) 2010 Society of Plastics Engineers