In this research, the flexible interphase concept was introduced to enhance the poor mechanical properties of jute fiber reinforced unsaturated polyester matrix composites. The jute cloth reinforcement was obtained from recycled coffee bags. These jute cloths after washing by water and drying were soaked in mixture of Polybutadiene Epoxydied as flexible resin and acetone for 10 seconds. Several mixtures consist of 0, 2, 3.5, 5 and 8 wt% of Polybutadiene Epoxydied and 100, 98, 96.5, 95 and 92 wt% of acetone, respectively, to form flexible interface around the jute fibers. Jute cloth reinforced unsaturated polyester matrix composites with different flexible interphase incremental weight ((Wa-Wb)/Wb) ratios were fabricated by hand lay-up method and examined by a series of mechanical tests. The mechanical testing including tensile, bending, Izod strength impact and drop impact was carried out for these composites to evaluate the effect of the flexible interphase and acetone on the jute cloth composites. The flexible interphase succeed to control the mechanical properties of jute fiber reinforced unsaturated polyester matrix composites. Inserting flexible interphase between unsaturated polyester matrix and jute fibers leads to smooth fluctuation, less matrix cracking, in the second part after the knee point of each stress-strain curve as exhibited in composites with higher flexible interphase incremental weight ratio. This means not only the brittle matrix but also interface/interphase dominates the multiple matrix cracking behavior in jute cloth reinforced unsaturated polyester matrix composites. Inserting flexible interphase between unsaturated polyester matrix and jute fibers leads to less number of multiple cracking as shown in the second portion of flexural stress-displacement curve. This means the number of multiple cracking are dominated by flexible interphase. The impact strength of jute cloth reinforced unsaturated polyester matrix composites with flexible interphase incremental weight ratio of 1.2% is higher than that of jute cloth reinforced unsaturated polyester matrix composites without flexible interphase by about 45%. The impact energy after maximum load has increased significantly with all flexible interphase incremental weight ratios.

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