With the addition of hollow glass microsphere, tensile strength and modulus increased at varying percentage volume fractions, highest tensile strength was at EPT60-5 (66.7 MPa), which is an increase of about 65% compared to the neat epoxy resin and highest tensile modulus at EPT60-4 (4.5 GPa).
Where tensile strength of the Polypropylene increased upon the inclusion of hollow glass microsphere as a result of improved inter-facial adhesion with the matrix before it declined at EPT60-6 due to increased void content caused by the agglomeration and reduced resin content resulting in low bonding and load transfer stability between the filler and the resin. The interfacial strength between the microsphere and the matrix is very important for the syntactic foam composites as it affects its overall tensile strength.
Although, some previous studies reported a decline in the tensile strength by 60-80% as compared to neat epoxy. They stated that the increase in hollow glass microsphere volume fraction reduced the tensile strength because as the volume fraction of the epoxy resin in the material decreased, the strength of the composites also decreased due to higher range of microballoons in the composites structure.
