Definition of yarn strength
Yarn strength is an indicator of the tensile force a yarn can withstand. There are absolute strength and relative strength. When comparing the strength of yarns with different linear densities, relative strength should be used.
Absolute strength (breaking strength): the external force that the yarn can withstand when stretched to break. There are: ① single yarn strength; ② strand strength; ③ strand yarn strength, etc. The unit is Newton, gram force, kilogram force or pound force.
Relative strength includes: ① breaking strength; ② specific strength; ③ breaking length; ④ quality index.
Breaking strength: The external force that the yarn can withstand per unit cross-sectional area when the yarn is stretched to break, in 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.
Breaking length: The length of the yarn when its weight is equal to its breaking strength, unit is km. In the TEX series, it is equal to the ratio of the strength (gram force) of a single yarn or strand to its TEX, that is, the specific strength;
Quality index: indicates the relative strength of the yarn Indicator of strength. When the yarn linear density is measured in imperial systems, it is equal to the product of the yarn strength (pound force) and the imperial number of its single yarn; when the yarn linear density is measured in Tex, it is equal to the yarn strength (kg force) and its single yarn characteristics. 1,000 times the Max Quotient.
Yarn strength is a reflection of the inherent quality of the yarn and is a necessary condition for the yarn to have processing properties and end use. Quality indicators are currently the main basis for determining pure or blended yarn products of cotton yarn or cotton-type chemical fiber, so yarn strength is one of the most important routine inspection items in textile production.
Yarn strength utilization coefficient
Single yarn strength is always less than The sum of the breaking strengths of each fiber in the cross section, and the ratio between the two is called the fiber strength utilization coefficient in the yarn. Cotton yarn is usually 0.40 to 0.50, and wool yarn is usually 0.20 to 0.30.
The strength of a strand of yarn is always less than the sum of the strengths of individual yarns in the strand of yarn. The ratio between the two is called the strand ratio. Generally, the cotton yarn is 0.70~0.78, and the wool yarn is 0.40. ~0.82.
The strength of the strands when combined and reversely twisted is generally higher than the sum of the strengths of the single yarns of each strand, and the ratio of double-strand cotton yarn is 0.95 to 1.35. The ratio of the yarn strength converted from the fabric strength to the yarn strength before weaving is called the yarn strength utilization coefficient in the fabric, and its value is greater than 1. Cotton poplin fabric is approximately 1.155 in the warp direction and 1.115 in the weft direction.
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 fibers and the curl of wool and chemical fibers will increase the cohesion between fibers when the yarn twist is not large, thereby increasing the yarn strength. Short lint content has a great influence on yarn strength. Among cotton yarns, the short staple rate below 16mm increases by 1%, and the strength of cotton yarn decreases by 1% to 2%.
2) Yarn structure
The fiber transfers more in and out of the yarn radius direction, and when the structure is uniform, the yarn strength can be improved; 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, and after exceeding the critical twist, the yarn strength decreases as the twist increases. Pure cotton yarn is prone to breakage at details and large neps; polyester-cotton yarn is prone to breakage at thick spots and inflection points of thick details.
3) Spinning method
The strength of yarn spun by different spinning methods is obviously different due to different structures. The strength of thick air-spun yarn is about 5% to 10% lower than that of ring-spun yarn of the same size, but the density of air-spun yarn is uniform and the weakest node is not lower than that of ring-spun yarn. The strength of electrospinning yarn is similar to that of air-spun yarn. In-phase self-twisted yarn has low strength due to the existence of untwisted areas on the yarn; out-of-phase self-twisted yarn has increased strength due to the staggered untwisted areas on the yarn. The strength of twisted self-twisted yarn is further improved due to the reasonable distribution of twist.
4) Blending ratio
In two-component blended yarn, when the blending ratio of one component gradually increases from zero , the strength of blended yarn generally decreases gradually. After reaching the lowest value, if you continue to increase the proportion of this component, the strength of spun yarn gradually increases. The greater the difference in elongation at break between the two blended fibers, the more obvious this dip in strength will be.
5) Atmospheric temperature and humidity
As the temperature increases, the yarn strength decreases; as the relative humidity increases, the yarn moisture regain increases , the strength of cotton and linen yarns increases, while the strength of wool and man-made fiber yarns decreases. Therefore, the strength test should be carried out after equilibrium under the specified standard state (temperature is 20°C, relative temperature is 65%), otherwise the measured strength must be corrected for temperature and moisture regain.
Yarn strength test
Strength of single yarn or strands Measured on a pendulum yarn strength testing machine, inclined plane yarn strength testing machine or electronic strength testing machine, the sample length is specified as 500mm. At present, new yarn strength testing machines can automatically conduct strength tests and print out the average values of strength and elongation and their coefficients of variation.
The yarn strength is measured on a pendulum type yarn strength testing machine. The loop length of cotton yarn or cotton-type chemical fiber pure or blended yarn is 1m, and every 100 loops is 100m. The length of the wool yarn loop is 1m, 20 loops of carded wool yarn is 20m, and 50 loops of combed wool yarn is 50m. </p
