Glass fiber composites were fabricated from two different manufacturing techniques, vacuum infusion and vacuum bag, two resin types (epoxy, vinyl ester), as well as and two fiber combinations (S-glass and E-glass). The analyzed properties were tensile modulus, strength as well as compression strength, with relation to sample morphology and fracture surface due to compression. The results showed that all variables and their interaction did not significantly influence tensile modulus, while manufacturing methods was the only significant variable influenced tensile strength. The results showed that vacuum infusion samples had better tensile strength than those produced by vacuum bag due to less resin and bubbles. In term of compression, the change on strength was highly contributed by resin type and fiber used on layer A. Composites samples with epoxy resin showed better strength than those of vinyl ester may be due to better initial resin property. While, imbalance transfer load and fiber density inhibiting resin penetration and causing resin rich samples produced lower compression strength for samples with S-glass applied to layer A than E-glass. Further analysis on fracture surface showed that most samples failed on compression test due to shear, kink and resin break.

Kenaf core/unsaturated polyester (KC/UP) composites are prepared by mechanical stirrer followed by compression molding. Effect of fiber percentage as well as alkali treatment (NaOH concentration and immersion time) on sound damping performance are analyzed and optimized using three factors and three levels of Box–Behnken experimental design. Flexural properties, heat deflection temperature (HDT), density, lignin content, and morphology are investigated on selected samples. The results show that the optimum condition to produce KC/UP composites is predicted at 54.5% of fiber, 8.1% of NaOH used on alkali treatment, and 2 h fiber immersion on NaOH, giving 5.6% of sound damping/mm. Verifying the model results in 5.1% of sound damping/mm. Despite lower value of verification sample than predicted, this is still between 95% of predicting interval (4.1–7.1%), which is still higher than control sample (3.8%), suggesting model capability to predict the result. Further analysis on selected samples with the highest, mid and lowest sound damping property results in the change of flexural modulus at 38%, −9%, and 28%, respectively, and improvement in HDT (23%, 12%, and 22%). However, the flexural strengths of all selected samples are lower than that of control (−37%, −67%, and −44%). Morphology analysis shows that sample with 45% fiber content is more compact than 30% fiber content, which improves sound damping.