During the production of spunbond non woven fabric, various factors may affect the physical performance of the product.
The main quality indexes of polypropylene sheets are molecular weight, molecular weight distribution, equivalent specification, melting index and ash content. The molecular weight of PP sheet used for spinning is between 100000 and 250000, but practice has proved that when the molecular weight of polypropylene is about 120000, the rheological property of the melt is the best, and the maximum allowable spinning speed is also very high. Melt index is a parameter reflecting the rheological properties of melt. The melting index of polypropylene sheets used for spinning and bonding is usually between 10 and 50. If the melt index of the spinning is large, the viscosity of the melt decreases, the rheology is good, and the tensile resistance decreases. Under the same stretching condition, the stretching multiple increases. With the improvement of macromolecular directivity, the breaking strength of spunbond non woven fabric will increase, the fiber degree of silk will decrease, and the fabric will feel soft. Under the same process, the higher the melt index of polypropylene, the smaller the fiber size and the greater the fracture strength.
The molecular weight distribution is usually measured by the ratio of weight average molecular weight (MW) to number average molecular weight (MN) of the polymer, which is called the molecular weight distribution value. The smaller the molecular weight distribution value, the more stable the rheological properties of the melt, the more stable the spinning process, which is conducive to improving the spinning speed. The lower the melt elasticity and tensile viscosity, the lower the spinning stress, making PP non woven easier to stretch and thin, and obtain finer fibers. The better the uniformity of the web, the better the handle and uniformity
The setting of spinning temperature depends on the melting index of raw materials and the requirements for physical properties of products. The higher the melting index of raw materials, the higher the spinning temperature, and vice versa. The spinning temperature is directly related to the melt viscosity, and the temperature is low. The viscosity of the melt is high, so it is difficult to spin, and it is easy to produce broken wires, hard wires or coarse wires, which will affect the product quality. The lower the spinning temperature of spunbond non woven fabric, the higher the tensile viscosity of the melt and the greater the tensile resistance, the more difficult it is to stretch the yarn. In order to obtain the same fiber, when the temperature is low, the velocity of the drawing air flow is relatively high.
During the forming process, the cooling rate of the wire has a great impact on the physical properties of the spunbond non woven fabric. If the molten polypropylene can be cooled rapidly and uniformly from the nozzle, its crystallization speed is slow and its crystallinity is low. The obtained fiber structure is an unstable disc liquid crystal structure, the possible drafting multiple is relatively large, and the molecular chain has better directivity, which can further improve the crystallinity, improve the fiber strength and reduce its elongation. If the cooling is slow, the obtained fiber has a stable monoclinic structure, which is not conducive to the fiber drafting, and shows small breaking strength and large elongation. The cooling distance is generally between 50~60 cm.
Analyzing the relationship between these factors and product performance is helpful to correctly control the process conditions and obtain products with good quality and wide applicability.