Abstract:

Objective Polyamide /cotton blended yarns not only possess outstanding wear resistance, elasticity, fatigue resistance and impact properties of polyamide fiber, but also moisture absorption and permeability, soft and warm characteristics of cotton fiber. In order to further improve its quality, the relationship between twist and each quality index of a polyamide/cotton blended yarn was analyzed to predict the blended yarn quality based on the yarn twist. The study provides theoretical basis for twist design and rapid adjustment in spinning production.
Method The polyamide/cotton blended yarns with linear density of 27.9 tex and blending ratio of 50:50 was spun from polyamide 66 with linear density of 1.67 dtex and length of 38 mm and Xinjiang cotton fiber 229 with a linear density of 1.64 dtex by conventional ring spinning. 16 polyamide/cotton blended yarns were spun on FA506 spinning frame by changing the twist gear and setting 16 twist levels. In the spinning process, the sliver blending was adopted, where polyamide sliver and cotton sliver were produced by blowing-carding process respectively, three blending, roving and spinning.
Results The relationship between twist and quality indexes of polyamide /cotton blended yarns was analyzed by the SPSS software. Twist was significantly correlated with 6 quality indexes, i.e., breaking strength, breaking strength CV, breaking elongation, thick places, thin places and hairiness. However, it has little correlation with elongation CV, evenness and neps index of blended yarn (Tab.2). By using the SPSS software, the scatter plot of twist and the above-mentioned six quality indexes was drawn, and the regression curve was plotted. The regression curve shows that the six quality indexes of polyamide/cotton blended yarn do not show linear distribution against twist variation trend (Fig.1, Fig.2). Therefore, the best fitting equation between twist and these six quality indexes was obtained by the curve estimation and fitting analysis method provided by the SPSS software (Tab.3).
The breaking strength of polyamide /cotton blended yarns increases first and then decreases with the increase of twist (Fig.1, Fig.2 and Tab.3). The fitted model judgment coefficient is 0.919, the critical twist is 1 228 twist/m, and the corresponding maximum breaking strength is 8.1 cN/tex. The fitting curve of the breaking strength CV of the blended yarn shows a trend of rising first and then falling, the fitted model judgment coefficient is 0.653, and at the twist of 1 307 twist/m the curve demonstrates a maximum value. The breaking elongation of blended yarns increases with the increase of twist, and the fitted model judgment coefficient of regression equation is 0.627.
The thick places and thin places of polyamide /cotton blended yarn decrease with the increase of twist, and increase with the increase of twist when it decreases to a certain extent. The judging coefficients of the cubic fitting curve are 0.790 and 0.748, when the minimum value of fitting curve appears, the twist is 1 259 and 1 407 twist/m, respectively. The hairiness of blended yarn decreases with the increase of twist, and then tends to level off. The judging coefficient of the cubic fitting curve is 0.971. In order to further verify the validity of the fitting equation model, the residuals between the test data and the predicted values of 6 quality indexes of blended yarn are calculated, the absolute value of the residual value between the test group data and the predicted value was within 3 standard deviations of the residual value, and there is no abnormal point (Tab.4).
Conclusion In this paper, a regression model presenting the relationship between breaking strength and elongation, thick places, thin places, hairiness and twist of polyamide /cotton blended yarn was established using SPSS software, and the validity of the model was verified by the residual error between the test group data and the predicted value with good forecasting capability. It shows that twist can reasonably reflect the tensile properties, thick places, thin places and hairiness of polyamide/cotton blended yarn.
The significance of this model is that it can predict and control the tensile properties, thick places,thin places and hairiness of polyamide/cotton blended yarn based on the twist of process parameters during spinning. The twist of the yarn could be designed and adjusted according to customer requirements to reduce the test cost and to achieve high efficiency and energy saving.

Key words: twist, polyamide/cotton, blended yarn, spinning quality, quality index, regression analysis, prediction model