Water absorption and stress relaxation behavior of PP/date palm fiber composite materials

In this study, the effects of the fiber loading, the coupling agent content and the amount of the absorbed humidity on the stress relaxation behavior of the PP/date palm fibers composite material were characterized, using a dynamic mechanical analyzer (DMA). The obtained results have indicated that the rate of stress relaxation decreases as the date palm fiber content increases. It is therefore suggested that the presence of the natural fibers in composite materials do not help polymer molecules to recover their original configurations, generally present in the polymer matrix before any mechanical loading. The use of the coupling agent was found to also reduce such rate, due to the improvement of the quality of adhesion between the reinforcing phase and the polymer molecules. By increasing the amount of the absorbed humidity, a higher relaxation rate was obtained. This is an expected result since the presence of water molecules in composite materials encourages the relative movement (sliding) of polymer molecules, reducing therefore their recovering time and these can regain their initial configuration at a higher rate.