1. Melt index and molecular weight distribution of polypropylene chips
The main quality indicators of polypropylene chips are molecular weight, molecular weight distribution, isotacticity, melt index and ash content. The molecular weight of PP chips used for spinning is between 100,000 and 250,000, but practice has proved that the rheological properties of the melt are the best when the molecular weight of polypropylene is about 120,000, and the maximum allowable spinning speed is also high. . Melt index is a parameter that reflects the quality of melt rheology. The melt index of polypropylene chips for spunbond is usually between 10 and 50. In the process of spinning into a web, the yarn is drawn by airflow only once, and the draft ratio of the yarn is limited by the rheological properties of the melt. The larger the molecular weight, the smaller the melt index, the worse the rheological properties of the yarn. The smaller the draft ratio obtained for the strip, the larger the fineness of the thread obtained under the same conditions of the melt discharge from the spinneret hole, so the spunbond non-woven fabric exhibits a hard hand feeling. If the melt index is larger, the viscosity of the melt will decrease, the rheology will be better, and the resistance of drafting will decrease. Under the same drafting conditions, the drafting ratio will increase. As the orientation degree of the macromolecules increases, the breaking strength of the spunbond nonwoven fabric will increase, and the fineness of the filaments will decrease, and the fabric will appear soft to the touch. Under the same process, the higher the melt index of polypropylene, the smaller its fineness and the greater the breaking strength.
The molecular weight distribution is often measured by the ratio (Mw/Mn) of the weight average molecular weight (Mw) of the polymer to the number average molecular weight (Mn), which is called the molecular weight distribution value. The smaller the molecular weight distribution value, the more stable the rheological properties of the melt, and the more stable the spinning process, which is beneficial to increase the spinning speed, and has lower melt elasticity and elongational viscosity, which can reduce spinning stress. Make PP easier to stretch and thinner, obtain finer denier fibers, and have better web-forming uniformity, with good hand feeling and uniformity.
2. Spinning temperature
The setting of the spinning temperature depends on the melt index of the raw materials and the requirements for the physical properties of the product. The higher the raw material melt index, the higher the spinning temperature correspondingly, 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, spinning is difficult, and it is easy to produce broken filaments, stiff filaments or thick filament clusters, which affect the quality of the product. Therefore, in order to reduce the melt viscosity and improve the rheology of the melt, the method of increasing the temperature is generally adopted. The spinning temperature has a great influence on the structure and properties of the fiber. The lower the spinning temperature, the higher the elongational viscosity of the melt and the greater the drawing resistance. The more difficult it is to draw the thread. To obtain fibers of the same fineness, the drafting airflow speed should be higher when the temperature is low. Therefore, when other process conditions are the same, the fiber is difficult to draft when the spinning temperature is low. The fineness of the fiber is relatively large, and the degree of molecular orientation is relatively low. The performance of the spunbonded non-woven fabric is that the breaking strength is relatively low, the elongation at break is relatively large, and the hand feel is harder; when the spinning temperature is relatively high, the fiber draft is better and the fiber Its fineness is small, and its molecular orientation is high. It shows high breaking strength, low breaking elongation and soft hand feeling on spunbonded non-woven fabrics. However, it is worth noting that under certain cooling conditions, if the spinning temperature is too high, the resulting filaments will not be cooled enough in a short time, and some fibers will break during the drafting process, which may cause defects. In actual production, the spinning temperature should be 220-230℃.
3. Cooling and forming conditions
During the forming process of spunbond nonwoven fabric, the cooling rate of the thread has a great influence on the physical properties of the spunbonded nonwoven fabric. If the molten polypropylene can be cooled quickly and uniformly after it comes out of the spinneret, its crystallization rate will be slow, and the crystallinity will be low. The structure of the resulting fiber is an unstable disc-like liquid crystal structure. Relatively large, the orientation of the molecular chain is better, which can further increase the crystallinity, increase the strength of the fiber, and reduce its elongation. It is shown in the spunbond non-woven fabric that the breaking strength is higher and the elongation is lower; if it is cooled slowly , The obtained fiber has a stable monoclinic crystal structure, which is not conducive to the drafting of the fiber. It is manifested in the spunbond non-woven fabric as the breaking strength is small and the elongation is large. Therefore, in the molding process, the method of increasing the cooling air volume and lowering the temperature of the silk chamber is usually used to increase the breaking strength of the spunbonded non-woven fabric and reduce the elongation. In addition, the cooling distance of the thread is also closely related to its performance. In the production of spunbonded non-woven fabrics, the cooling distance is generally selected between 50 and 60 cm.
4. Drafting conditions
The degree of orientation of the molecular chains in the thread is an important factor that affects the elongation of the monofilament at break. The greater the degree of orientation, the greater the strength of the monofilament and the smaller the elongation at break. The degree of orientation can be expressed by the birefringence of the thread, and the larger the value, the higher the degree of orientation. The nascent fiber formed when the polypropylene melt comes out of the spinneret has relatively low crystallinity and orientation, and the fiber is brittle, easy to break, and has a large elongation at break. To change the properties of the fiber, the fiber must be stretched to different degrees according to the needs before the fiber is formed into a web. In spunbond production, the draft degree of the fiber mainly depends on the cooling air volume and the suction air volume. The larger the amount of cooling air and suction air, the faster the drafting speed, the fiber will be fully drawn, and the molecular orientation will increase, the fineness will become finer, the strength will increase, and the elongation at break will decrease. Polypropylene filament reaches its saturation value of birefringence at a spinning speed of 4000m/min, but it is generally difficult for the actual speed of the filament to exceed 3000m/min in the air drafting process of spinning into a web. Therefore, in the case of high demand for strength, the drafting speed can be increased boldly. However, if the cooling air volume is constant, if the suction air volume is too large, the filaments are not cooled enough, and the fiber is prone to break at the die extruding area, causing the injection head, affecting the production and product quality, and it should be adjusted appropriately in actual production. .
The physical properties of spunbond non-woven fabrics are not only related to the properties of the fibers, but also have a lot to do with the fiber web structure. The finer the fiber, the higher the degree of disorder of the fiber arrangement during web laying, the more uniform the web is formed, the greater the number of fibers per unit area, the smaller the vertical and horizontal strength ratio of the web, and the greater the breaking strength. Therefore, the uniformity of the spunbonded non-woven fabric product can be improved by increasing the suction air volume, and its breaking strength can be improved at the same time. However, if the suction air volume is too large, it is easy to produce broken filaments, and the drafting is too strong, the orientation of the polymer tends to be complete, and the polymer crystallinity is too high, which will reduce the impact strength and elongation at break, and increase the brittleness. The strength and elongation of the woven fabric are reduced. According to this, the strength and elongation of the spunbonded non-woven fabric increase and decrease regularly with the increase of the suction air volume. In actual production, the process must be adjusted appropriately according to the needs and actual conditions to obtain high-quality products.
5. Hot rolling temperature
The fiber web formed after the fiber is stretched is in a loose state, and it must be hot-rolled and bonded to become a cloth. Hot rolling bonding is a process in which the fiber web passes through a hot roll with a certain pressure and temperature, and the fibers in the fiber web at the nip point of the pattern roll are partially softened and melted, so that the fibers are bonded together and consolidated into a cloth. , The key is to control the temperature and pressure. The function of heating is to soften and melt the fiber. The proportion of softened and melted fibers determines the physical properties of the spunbonded non-woven fabric. When the temperature is very low, only a small part of the fibers with lower molecular weight are softened and melted. There are few fibers that are bonded together under pressure. The fibers in the fiber web are easy to slip, and the breaking strength of non-woven fabrics is higher. Small but large elongation, the product feels soft but easy to fluff; when the hot-rolling temperature gradually increases, the amount of softened and melted fibers increases, the fiber web is tightly bonded, the fibers are not easy to slip, and the breaking strength of the non-woven fabric increases and stretches The length is still large, and due to the strong affinity between the fibers, the elongation increases slightly; when the temperature rises significantly, most of the fibers at the pressure point melt, and the fibers become frit and begin to become brittle. The strength of the non-woven fabric began to decrease, the elongation also decreased greatly, the hand feeling was very hard, very brittle, and the tear strength was also low. In addition, different products have different weight and thickness, and the temperature setting of the hot rolling mill is also different. For thin products, there are fewer fibers at the hot rolling point, and less heat is required for softening and melting, and the required hot rolling temperature is lower. Correspondingly, for thick products, the hot rolling temperature is higher.
6. Hot rolling pressure
In the hot rolling and bonding process, the function of the hot rolling mill line pressure is to compact the fiber web, so that the fibers in the fiber web generate a certain amount of deformation heat during the hot rolling process and fully play the role of heat conduction to soften and melt the fiber. Tightly bond together, increase the cohesive force between the fibers, so that the fibers are not easy to slip. When the linear pressure of hot rolling is relatively low, the compaction density of the fibers at the pressure point in the fiber web is poor, the bonding fastness of the fibers is not high, the cohesive force between the fibers is relatively poor, and the fibers are relatively easy to slip. The spunbond non-woven fabric has a softer hand feel, a larger breaking elongation, but a lower breaking strength; on the contrary, when the thread pressure is relatively high, the resulting spunbond non-woven fabric has a harder hand and lower breaking elongation. The breaking strength is greater. However, when the line pressure of the hot rolling mill is too high, the softened and melted polymer at the hot rolling point of the web will be difficult to flow and spread, which will also reduce the breaking tension of the nonwoven fabric. In addition, the setting of the line pressure has a lot to do with the weight and thickness of the non-woven fabric. In the production, it must be selected appropriately according to the needs to produce a product that meets the performance requirements.
All in all, the physical and mechanical properties of polypropylene spunbond non-woven products do not depend on unilateral factors, but the result of the combined effect of various factors. In actual production, reasonable process parameters must be selected according to actual needs and production conditions. Produce high-quality spunbonded non-woven products that can meet various needs. In addition, the strict standardized management of the production line, the careful maintenance of equipment, the improvement of the quality of operators and the improvement of operating proficiency are also the key to improving product quality.