EFFECTS OF DIFFERENT CARBON BLACK CONCENTRATIONS IN EPOXY ADHESIVE ON THE JOINT STRENGTH OF MICROWAVE ASSISTED CURING BETWEEN TWO POLYCARBONATE SHEETS

摘要

The significance of adding carbon black dopant to epoxy adhesive is to facilitate the fast heating of the epoxy adhesive. Without the addition of carbon black, heating of the epoxy adhesive under microwave radiation will be either difficult or impossible to achieve. Carbon black, like polyaniline, can enhance the absorbance of the microwave energy in the epoxy adhesive due to its high dielectric loss properties which in turn makes the adhesive conductive allowing interaction with the microwave energy. A polymerisation reaction between the adhesive and the polycarbonate substrate in the the area of the joint will take place on the surface of the pre-treated sheets. The doped epoxy adhesives under microwave radiation results in the hardening of the adhesive joint. The objective of this work is to study the effect of adding various carbon black concentrations into the epoxy adhesive mixture on the polycarbonate joint strength when subjected to microwaves. The joints were mechanically tested using an Instron universal testing machine. A single mode 6.0 kW maximum power level microwave unit was used to assist fast curing. The microwave energy exposure was arbitrarily chosen at the input power of 1000 Watt for 30 seconds for all samples. Two types of carbon black were employed in the experiments, i.e. Vulcan 66 (high microwave activity) and Sterling (low microwave activity). It was assumed that the carbon black particles had uniform dimensions after being finely ground using a mortar and pestle. Carbon black has high loss dielectric properties thus it will couple well with the microwave energy. It can store and convert the microwave energy into heat. In a material-microwave interactions, the material properties of greatest importance in microwave processing is called permittivity complex, consisting of the dielectric constant, ε', dielectric loss, ε", and the loss tangent, tan δ which is defined as the ratio of ε" and ε'.The relation is expressed as follows; ε~* = ?' - j.ε" where by definition; ε~* : permittivity complex of the doped epoxy adhesive ε' : dielectric constant of doped epoxy adhesive ε" : dielectric loss of doped epoxy adhesive j : coefficient of the imaginary part of a complex number. The amount of power absorbed (P) by the adhesive greatly influences its curing and resulting bond strength and is dependent on both the permittivity and the loss tangent of the adhesives as follows; P= 1/2 x ε_0 x ε' x tan δ x E~2. (Watt.m~3) Where: ε_0, is the dielectric permittivity of free space, 8.85x10~(-11) farad/meter, tan δ is the loss tangent, defined by the ratio of ε" (dielectric loss) over ε'(dielectric constant) and E is the rms electric field strength.