Abstract:

Objective Bast fiber reinforced composites with lightweight quality, low cost, biodegradability, reusability and designability have been widely used in numerous applications in recent years. The length of extracted bast fibers, unlike synthetic fibers capable of continuous production, is limited, which is not beneficial for composite manufacturing and stress transfer. Staple spinning, a traditional method in the textile industry for acquiring continuous short fibers, breaks through the limitation of bast fiber length. Staple yarns are the foundations of various fabric reinforced composites and yarn pultrusion composites. One of the most important factors in determining the properties of staple yarn-based composites is the yarn structure. The effects of yarn count and twist on breaking properties of yarns are well known in the textile industry. However, the impacts of them on yarn reinforced composites have been rarely reported in the literature. Based on the relationship between staple yarn and staple yarn reinforced composites, the influences of the above structure factors on tensile properties of bast fiber staple yarns and their composites are investigated.

Method The ramie fibers were used in the spinning process to prepare staple yarns with various yarn counts and twist factors. The ramie fiber staple yarn reinforced unsaturated polyester resin composite was fabricated via immersing and squeezing process. The breaking properties of ramie fiber staple yarns and tensile properties of ramie fiber staple yarn reinforced composites were tested and analyzed, respectively. Morphologies of ramie staple yarn and fracture section of ramie staple yarn reinforced composite were observed by using scanning electron microscope. The internal defects of ramie fiber staple yarn reinforced composites with different yarn counts or twist factors were detected by water immersion ultrasonic C-scan defect detector.

Results The results showed that as the yarn count increased, the yarn diameter and breaking force of ramie staple yarn increased. When the yarn count was 80 tex, the tensile strengths of ramie staple yarn reinforced composite reached maximum in both short and long gauge tensile test. In the long gauge tensile test, the tensile strength of ramie staple yarn reinforced composite increased with increasing twist factor, whereas in the short gauge test, the maximal tensile strength was found to be 528.39 MPa at 360 critical twist factor. In the short gauge test, the tensile properties were subject to the balance of mechanical interlocking and twisting force components. However, mechanical interlocking failed to perform its corresponding function for the tensile properties in the long gauge test due to the discontinuity of the fibers in the yarn. In ultrasonic detecting, the looser ramie fiber staple yarn, the more interfacial and unimpregnated defects in ramie staple yarn reinforced composites.

Conclusion In conclusion, ramie staple yarn reinforced composite with 80 tex and 360 twist factor is recommended for bast fiber reinforced composite industry. Moderate yarn count and twist factor are conducive to yarn infiltration in staple yarn reinforced composite. Ramie fibers are easily pulled-out by resin in coarser yarn reinforced composite in the constant twist factor, resulting in the decrease of tensile strength of ramie fiber staple yarn reinforced composite after exceeding 80 tex. Because of the larger arrangement angle of fibers in the yarn with the increasing twist, ramie fibers are more difficult to pull out from resin due to the mechanical locking interface effect. Under the short gauge testing, the load bearing effect of the fibers in the ramie fiber staple yarn reinforced composites is fully exerted. Therefore, the tensile strength of ramie fiber staple yarn reinforced composite in the short distance between clamps test is higher than in the long gauge tensile test. The selection principle of twist factor is to balance the positive effect of the fiber inclination angle on the interface and the negative effect on the load-carrying efficiency. There is no doubt that the yarn structure possessed significant potential for promoting the development of bast fiber staple yarn reinforced composite.

Key words: ramie fiber staple yarn, composite, yarn structure, critical twist factor, distance between clamps in tensile test