Abstract:

Objective In order to expand the application field and reduce the unnecessary resource waste of waste hemp fibers. Waste hemp fibers reinforced sound-absorbing composites have the advantages of green source, no polluion and biodegradability. It can promote China's building materials industry carbon emission reduction work, and promote the achievement of China's carbon peaking and carbon neutrality goals.

Method Waste hemp fibers were used as reinforcement and PLA fibers were used as matrix. The combing networking-hot pressing process was chosen to produce the sound-absorbing composites with waste hemp/PLA fibers. The transfer function method was chosen to test the sound-absorption of the composites using the impedance tube test system. The influences of process conditions on its sound absorption properies were explored. The preparation process of the composites was optimized by a one-factor test with the objective of improving the sound absorption properties of the composites.

Result The PLA fibers were soften and molten under reasonable hot pressing conditions and combined evenly with the waste hemp fibers. The pores were evenly distributed and the porosity size increased. The gas in the pores also increased. When the sound wave was propagated, the vibration of the fibers and air within the composite material increased, and the friction between the both was enhanced, causing an increase in the loss of sound energy and an increase in sound absorption. The increase in waste hemp fibers content would result in an increase in the number of interwoven points between the fibers and an increase in porosity. But the excessive fibers would cause uneven combination with PLA fibers, which would lead to uneven porosity. On the one hand, it could increase the transmitted sound wave. On the other hand, it would weaken the air and fibers vibration, and the friction become reduced. Thus, the sound energy consumption was reduced, and the sound-absorption properties became worse. Increased density within a certain range would result in smaller pores between the fibers and more complex structure. Sound wave was in more curved propagation, and more frequent contact with the fibers, the consumption of sound energy increased. However, over high a density would make it difficult for sound waves to enter the material, reflecting more sound waves and thus reducing sound-absorption properties. The material thickness and rear air layer mainly improved the sound-absorption properties of the material at the low and middle frequencies. This was mainly because the thickness and rear air layer increase the propagation distance of sound wave. Low-frequency sound waves were long and traveled a long distance, and sound waves traveled a long distance in composites. The sound energy could be better consumed during propagation.The process conditions were optimized by one-way analysis of factor. The optimal process parameters for the preparation of the composite materials were obtained as the hot pressing temperature of 140 ℃, the hot pressing time period of 15 min, the hot pressing pressure of 10 MPa, the mass fraction of waste hemp fibers of 50.00%, the thickness of the material of 3.0 cm, the volume density of the material of 0.153 g/cm³, and the thickness of the rear air layer of 4.0 cm. The waste hemp/PLA fibers sound-absorbing composites obtained under the optimal process conditions had a maximum sound absorption coefficient of 0.920, the average sound absorption coefficient of 0.575, the noise reduction coefficient of 0.553, and wide sound absorption frequency band (80-6 300 Hz).

Conclusion The use of waste hemp fibers to prepare sound-absorbing materials solves the problem of resource waste and environmental pollution caused by a large number of waste hemp fibers, and has excellent social benefits. At the same time, it provided experimental and theoretical basis for the development of sound-absorbing composites of waste hemp/PLA fibers, and provided a new idea for the recycling of waste hemp fibers. However, the sound absorption in the mid-frequency region of the material can be further improved.

Key words: waste hemp fiber, composite material, sound-absorbing material, hot pressing process, polylactic acid fiber, waste textiles