关键词: 纤网模压成型, 黄麻纤维/聚乳酸短纤复合材料, 针刺频率, 力学性能, 生物降解性能, 汽车内饰复合材料

Abstract:

Objective In order to use biomass degradable composite materials to replace conventional petroleum-based materials in the field of automotive interiors, jute fiber reinforced polylactic acid (JF/PLA) composite sheets were prepared by web molding technique, where needling reinforcement was believed beneficial to the infiltration and encapsulation of PLA matrix on jute fiber during the web forming process. The needling frequency has a certain influence on the structure and properties of the final obtained JF/PLA composite sheets, which has rarely been reported. This paper explores the influence of needling frequency on the structure and performance of composite sheets.

Method PLA short fiber of 60% mass fraction and jute fiber of 40% mass fraction were used for preparing JF/PLA composite sheets by web forming technique, where the needling frequency was selected at 280, 300 and 320 times/min. The volume densities of the JF/PLA composite sheets were 2.50, 2.81, and 2.92 g/cm³ corresponding to the 280, 300, and 320 times/min needling frequencies, respectively. Scanning electron microscope, universal testing machine, impact testing machine, water-absorbency tests, combustibility tests and biodegradability tests were used for exploring the influence of the needling reinforcement frequency on properties of PLA/JF composite sheets.

Results The mechanical properties of JF/PLA composite sheets increased first and then decreased with the increase of needling frequency. When the needling frequency was 300 times/min, the mechanical properties of the JF/PLA composite sheets were optimal, and the vertical tensile strength, the flexural strength and notched impact strength were 14.54 MPa, 33.02 MPa and 9.54 kJ/m², respectively. However, the high needling frequency seemed to cause decline in the mechanical properties of JF/PLA composite sheets due to the fracture of part of the jute fibers. The water-absorbency of JF/PLA composite sheets gradually decreased with the increase of needling frequency (Fig. 6), because the increase of needling frequency caused the further entanglement between the jute fibers and PLA matrix, which made the internal structure of the composite sheets denser after hot-pressing. However, the microporous structure was uniformly distributed to a great extent, making the water absorption rate decrease. The horizontal burning test results of JF/PLA composite sheets with different needling frequencies showed a gradually decreasing as the needling frequency increases (Tab. 1). This may be because the higher needling frequency made the internal structure of the composite sheets more compact with reduced orifice size was, and the flame retardant effect was better. After 4 months of soil burying, the mass loss rate of PLA/JF composite sheets showed a gradual decrease with the increasing needling frequency. This was due to the tighter network structure of the jute fiber in the composite sheets caused by the high needling frequency and by the increased bonding ability between jute fiber and PLA matrix. The natural degradation process made the composite weaker and the mass loss rate reduced.

Conclusion The needling frequency, which is an important parameter in the preparation of composite materials by web forming technique, has not been much reported. In this work, three different needling frequencies of JF/PLA composite sheets were prepared. The results demonstrated that the mechanical properties were optimal at a needling frequency of 300 times/min, and the excessive needling frequency was not favorable to the mechanical properties of the composite sheets. Moreover, the increase of needling frequency facilitates reducing the water-absorbency and biodegradability and to improve the flame retardancy of the composite sheets. This study provides a theoretical basis and application guidance for controlling the mechanical properties, water-absorbency, biodegradability and flame retardancy of JF/PLA composite sheets by needling frequency, and expands the application possibilities of JF/PLA composite sheets used as automotive interior materials.

Key words: web forming technique, jute fiber/polylactic acid composite, needling frequency, mechanical property, biodegradability, automotive interior composite material