Definition of yarn strength
Yarn strength is an indicator of the tensile strength of the yarn. It has absolute strength and relative strength. When comparing the strength of different linear density yarns, the relative strength is applied.
Absolute strength (breaking strength): the external force that the yarn can withstand when it is stretched to break. There are: ① single yarn strength; ② strand strength; ③ strand strength, etc. Units are Newton, grams, kilograms, or pounds.
Relative strengths include: ① breaking strength; ② specific strength; ③ breaking length; ④ quality index.
Break strength: The external force that the yarn can bear per unit cross-sectional area when the yarn is stretched to break, the unit is N / mm2.
Specific strength: The external force that the unit linear density yarn can withstand when the yarn is stretched to break, the unit is N / tex.
Break length: The length of the yarn when its weight is equal to its breaking strength, the unit is km. In the Turks series, it is equal to the ratio of the strength (gram force) of a single yarn or strand to its Turks, which is the specific strength;
Quality index: An index indicating the relative strength of the strands. When the linear density of yarn is in English, it is equal to the product of strand strength (pound force) and the number of inch of single yarn; when the linear density of yarn is in Tex, it is equal to the strength of strand (kg force) and its single yarn. 1000 times the quotient of quotient.
Yarn strength is a reflection of the intrinsic quality of the yarn, and is a necessary condition for the yarn to have processability and end use. The quality index is currently the main basis for determining pure or blended yarns of cotton yarns or cotton-type chemical fibers, so yarn strength is one of the most important routine inspection items in textile production.
Yarn strength utilization factor
The strength of a single yarn is always less than the sum of the breaking strength of each fiber in its section. The ratio of the two is called the fiber strength utilization coefficient in the yarn. Cotton yarn is usually 0.40 ～ 0.50, and wool yarn is often 0.20 ～ 0.30.
The strength of skein yarn is always less than the sum of the strengths of individual yarns in the yarn. The ratio of the two is called the strand ratio. Generally, the cotton yarn is 0.70 to 0.78, and the wool yarn is 0.40 to 0.82.
The strength of the strands in the reverse twisting of the plied strands is generally higher than the sum of the strengths of the individual yarns. The ratio of the strength of the yarn converted from the strength of the fabric to the strength of the yarn before weaving is called the yarn strength utilization factor in the fabric, and its value is greater than 1. Cotton poplin fabric has a warp direction of about 1.155 and a weft direction of about 1.115.
The main factors affecting yarn strength
1) Fiber properties
The higher the fiber strength, the smaller the linear density and the longer the length, the higher the yarn strength. The natural twist of cotton fiber, the curl of wool and chemical fiber, when the twist of the yarn is not large, will increase the cohesion between the fibers, thus increasing the yarn strength. Staple content has a greater effect on yarn strength. Among cotton yarns, the lint rate below 16 mm increases by 1%, and the strength of cotton yarn decreases by 1% to 2%.
2) Yarn structure
Fibers transfer more inside and outside the yarn radius. When the structure is uniform, the yarn strength can be increased. When the fibers are arranged in layers and there are many folds, buckles, and hooks, the yarn strength decreases. Below the critical twist, the yarn strength increases as the twist increases After exceeding the critical twist, it will decrease as the twist increases. Pure cotton yarn is easy to break at the details and large neps; polyester cotton yarn is easy to break at the inflection points of thick and coarse details.
3) Spinning method
Yarns spun with different spinning methods have different strengths due to different structures. The strength of coarse air-flow yarn is lower than that of ring-spun yarn of the same number by about 5% to 10%, but the density of air-flow yarn is uniform, and the weakest point is not lower than that of ring-spun yarn. Electrospinning strength is similar to air-jet yarn. The same-phase self-twisted yarn has low strength due to the presence of untwisted areas on the yarn; the out-of-phase self-twisted yarn has increased strength due to staggered non-twisted areas on the yarn. The twisted self-twisted yarn strength is further increased due to the reasonable distribution of twist.
4) blending ratio
二 In the two-component blended yarn, when the blending ratio of one component gradually increases from zero, the strength of the blended yarn generally decreases gradually. If the proportion of this component is increased after the lowest value, the strength of the spun yarn gradually increases. The greater the difference in elongation at break between the two mixed fibers, the more pronounced this trough of strength.
5) High temperature and humidity
The temperature rises, the yarn strength decreases; the relative humidity increases, the yarn moisture regain increases, the strength of cotton and hemp yarns increases, and the strength of wool and rayon yarns decreases. Therefore, the strength test should be carried out after equilibration under the specified standard state (temperature is 20 ° C and relative temperature is 65%), otherwise, the measured strength should be corrected for temperature and moisture regain.
Yarn strength test
The strength of the single yarn or strand is measured on a pendulum-type yarn strength tester, a bevel-type yarn strength tester or an electronic strength tester, and the sample length is specified as 500 mm. At present, the new type of yarn strength testing machine can automatically perform the strength test, and print out the average value of strength and elongation and its coefficient of variation.
Strand strength is measured on a pendulum-strand strand strength tester. Cotton or cotton-type chemical fiber pure or blended yarns have a length of 1 m, and each 100 turns is 100 m. The length of wool loops is 1 m. 20 laps, 20 m is a ray, and combed yarn 50 laps, 50 m is a ray
