Physical Properties of Silk Fiber

The basic properties and performance of silk, including gloss, fineness, length, tensile strength, elasticity, absorbency, thermal and electrical properties, etc.

Cocoon silk has no special luster, even after being reeled into raw silk. However, the refined silk has a bright and elegant luster, similar to pearl luster.

Hand-touched raw silk is smooth, soft and elastic, and has a warm and plump feeling.

Generally expressed in fineness, under the condition of a fixed length of 450 meters, the weight is 0.05 grams as 1 denier, and the length does not change, the heavier the thicker. The fineness of cocoon silk is generally 2 to 4 deniers, and raw silk depends on the target fineness, with central specifications such as 14 deniers, 21 deniers, 28 deniers, 31 deniers, 42 deniers and so on. Generally, when the cocoon filament is 3 denier, the long diameter is about 30 to 32 microns, and the short diameter is 8 to 10 microns.

The cross-sectional area of ​​the cocoon silk is two blunt triangles, and the area of ​​the cocoon silk is generally about 200-400 microns when it is 2 to 4 deniers2. The round shape per denier is calculated to be 80.5 micrometers2, which is actually about 100 micrometers2. The area of ​​raw silk is relatively large and the density is small, about 129 micrometers2.

Generally 700 to 1,500 meters long.

The specific gravity is the weight per unit volume of the measurement, which is usually the ratio of water. When measuring the liquid with water, the raw silk is 1.37 g/cm3, and the refined silk is 1.25 g/cm3.

The strength of cocoon silk is about 10 grams, and each denier is 3.3 to 4.5 grams. The elongation of cocoon silk is 13 to 18%, raw silk is 18 to 23%, and refined silk is 17 to 20%. When wet, the elongation increases and the strength decreases; when dry, the opposite is true. After scouring and degumming raw silk, the strength is reduced by 15-20%, but the fineness is generally reduced by 20-25%, so the strength per denier of scoured silk increases inversely.

Elasticity is the tensile force formed under the action of external force, that is, the degree of resistance and recovery of the fiber within a certain limit. The elongation recovery rate of raw silk at 1% elongation is related to the relative humidity, which is 84% ​​when the relative humidity is 60%, and 78% when the relative humidity is 90%. The modulus of elasticity is the load required for the deformation of the unit cross-sectional area of ​​the sample silk. It generally refers to the external force applied when the raw silk is stretched at 1%. The larger the data, the smaller the deformation during use. The silk is generally 50-100 grams force. /dawn.

Absorbability varies with silkworm species, thread thickness, and glue content, etc. The general entity accounts for 70-75%, and the gap is 25-30%. It can inhale or absorb moisture. Hygroscopicity and water absorption, mainly in three forms: polar adsorption, capillary action and adhesion. Because the diameter of the silk fiber void is below 10-4 cm, in addition to the adhesive water at the surface of the skin, the fiber has only polar adsorption and capillary action. The official moisture regain of raw silk is 11%, which is equivalent to the moisture content of raw silk at 22°C and a relative humidity of 60-65%. The moisture content of silk varies with temperature and humidity, and the influence of humidity is greater than temperature. When the relative humidity is >60%, the moisture content gradually increases, and at 100%, it is equivalent to more than 1/3 of its own weight. When the relative humidity is less than 60%, it will gradually decrease. When immersed in water, raw silk can absorb water 1.01 times, and the silk is slightly less. After absorbing water, sericin swells or swells from the gel state; the volume of silk fibroin increases by about 30-35%, which is limited swelling, while sericin is infinite swelling.

Thermal properties. The heat absorbed or released by 1 gram of silk fiber with a temperature change of 1°C is called the specific heat of the silk, and the unit is calories/g·°C. The specific heat of cocoon silk and raw silk is 0.331 cal/g·℃. The best temperature for silk fiber washing is 30-40℃. Dry heat strongly emits moisture at 100°C. After 72 hours of treatment at 120°C, the silk is milky white. At 130°C, it begins to emit volatile substances such as ammonia, free amino acids, oils, etc., about 0.3% of the dry content, and the sericin is hard. After two hours at 150°C, the silk turns into a thick yellow color, and the tensile strength is reduced by about 10%. After being placed for several hours to absorb moisture and recover, the tensile strength can be restored. When heated at 175°C for 1 hour, the silk is yellowish-brown. It recovers in the atmosphere for a day and night without regaining its weight. Its strength is reduced by about 15%, and its elongation is reduced by about 20%. At 200°C, it becomes light yellow within 5 minutes; if it is exposed for 1 hour and then restored in the atmosphere for 24 hours, the tensile strength will be reduced by about half. At 250°C, it turns dark brown in 15 minutes. When it reaches 280°C, it will smoke within 5 minutes, volatilize its unique odor and become carbonized. The ignition point of the raw silk is 366°C. The thermal conductivity of silk is low, so it is warm in winter and cool in summer. At 17℃, the air content of the silk fabric is 45%, the thermal conductivity is 0.00022 cal/cm·sec·℃, the air content of the braid is 60%, and the thermal conductivity is 0.00018 cal/cm·sec·℃. The thermal conductivity is about 0.04867 kcal/m·hour·℃.

Optical properties. The birefringence refractive index is 1.598% in the axial direction and 1.543% in the transverse direction, with a rate difference of 0.055%. The light resistance is weak and the strength is significantly reduced.

Electrical properties. The dry ribbon is static and has a positive charge after being rubbed. The resistance of the wire is large, the dielectric constant is low, about 4.2, and it is a good insulating material for electricity.