A study of water‐induced elementary hemp fiber swelling and the reinforced polypropylene composite expansion

Abstract To deeply understand the swelling mechanism, the water swelling ratio and direction of elementary hemp fiber and the reinforced polypropylene (PP) composites were analyzed by both experimental methods and numerical simulation. The fiber swelling force was also calculated based on Flory–Rehner theory. The results showed that the simulated fiber radial swelling ratio of 6.5% is slightly higher than the measured one of 5.6%. For the reinforced composites, the simulated cross‐sectional expansion of 3.7% is very close to the measured one of 3.5%. This indicates that fiber mainly swells along its radial direction and the composite expansion is affected by not only fiber swelling ratio and direction but also fiber volume fraction and orientation. The proposed simulation model could easily estimate the swelling ratio and rate of plant fiber and the reinforced composites from their weight gain data. The generated swelling force of fiber is almost 850 MPa at the beginning and decreases to zero until achieving saturation state, which is much higher than the constraint stress of PP matrix and drives composite water expansion behavior. It is concluded that reducing the mixing entropy of plant fiber with water should be an effective approach to inhibit composite water expansion and improve its dimensional stability. The obtained results are beneficial for constructing a water absorption and service life prediction model of shortcut hemp fiber composites.