Abstract:

Objective As a new functional knitted product, compression socks exert pressure on human legs through deformation, which can help lower limb vascular reflux and reduce tissue edema, thus improving lower limb venous pressure and promoting blood circulation. In order to improve the comfort of conventional compression socks, shape memory polyurethane filament (SMPU), whose glass transition temperature is close to the surface temperature of human body, was used to prepare and design SMPU compression socks with three fleecy ratios.

Method In this paper, SMPU/polyamide(SMPU/PA) covered yarn and cotton yarn were used to knit the stocking part of the compression sock by a two-thread fleecy stitch. The glass transition temperature, shape memory properties, tensile strength and elastic recovery properties of SMPU and SMPU/PA covered yarn were evaluated. The influence of the fleecy ratio and the feeding tension of the fleecy yarn of SMPU compression socks on the pressure at the thinnest pressure test point B of human ankle was investigated.

Results The DSC curves showed that the transition temperature of SMPU was 32.15 ℃(Fig. 5), which was close to the human body surface temperature of 34 ℃. While the shape fixity ratio and shape recovery ratio of SMPU were 90% and 91%, the shape fixity ratio and shape recovery ratio of SMPU/PA covered yarn were 85% and 89%, respectively. The elastic recovery rate and breaking strength of SMPU/PA covered yarn were increased by 24.76% and decreased by 18.56% while the pressure of the stocking part of the SMPU compression sock increased by 16.26% when the test temperature was above glass transition temperature compared with that below glass transition temperature. The pressure test data showed that at a feeding tension of 6 cN, the compression sock with a fleecy ratio of 1∶3 had a maximum pressure increase of 11% compared to that with a fleecy ratio of 1∶1(Fig. 6). A long float stitch allowed more room for the internal stresses stored in the SMPU to be released, and the SMPU could reach the maximum variation in length, resulting in the largest pressure. At a fleecy ratio of 1∶3, the pressure of the SMPU compression sock with a feeding tension of 8 cN was increased by 30% compared to that of 4 cN. When the feeding tension was increased, the content of SMPU/PA covered yarn fed into the sock was risen, and thus the pressure of the compression sock increased. When the fleecy ratio was 1∶3 and the yarn feeding tension was 8 cN, the maximum pressure of SMPU compression socks reached 2 841 Pa.

Conclusion The shape memory performance of SMPU can be triggered by body surface temperature. Both SMPU and SMPU/PA covered yarn have good shape memory performance that are suitable for fabrics and clothing. The SMPU compression socks are easier to wear because SMPU can fix large deformation and the deformation can recover by internal stress release after the temperature rise. When the fleecy ratio of the two-thread fleecy in the stocking part is 1∶3 and the feeding tension of the fleecy yarn is 8 cN, the SMPU/PA content is the highest, and the float length is the longest, and the pressure of the socks knitting parameter will be the highest. The pressure of the SMPU compression socks in this study meets the requirements of FZ/T 73031—2009 for the thinnest pressure test point B of the ankle circumference.

Key words: polyurethane/polyamide covered yarn, compression sock, glass transition temperature, plain backing structure, fleecy ratio, yarn feeding tension