In the production process of spunbond nonwoven fabric, various factors may affect the physical properties of the product. Analyzing the relationship between these factors and product performance can help to correctly control process conditions and obtain high-quality and widely applicable polypropylene spunbond non-woven fabric products. Here, we will briefly analyze the main influencing factors on the physical properties of spunbond nonwoven fabrics and share them with everyone.
Melt index and molecular weight distribution of polypropylene slices
The main quality indicators of polypropylene slices are molecular weight, molecular weight distribution, isotropy, melt index, and ash content. The molecular weight of PP chips used for spinning is between 100000 and 250000, but practice has shown that the rheological properties of the melt are best when the molecular weight of polypropylene is around 120000, and the maximum allowed spinning speed is also high. The melt index is a parameter that reflects the rheological properties of the melt, and the melt index of polypropylene slices used in spunbond is usually between 10 and 50. In the process of spinning into a web, the filament only receives one draft of air flow, and the draft ratio of the filament is limited by the rheological properties of the melt. The larger the molecular weight, that is, the smaller the melt index, the worse the flowability, and the smaller the draft ratio obtained by the filament. Under the same conditions of melt ejection from the nozzle, the fiber size of the filament obtained is also larger, resulting in a harder hand feel for spunbond nonwoven fabrics. If the melt index is high, the viscosity of the melt decreases, the rheological properties are good, the resistance to stretching decreases, and under the same stretching conditions, the stretching ratio increases. As the orientation degree of macromolecules increases, the fracture strength of spunbond nonwoven fabric will also increase, and the fineness of the filaments will decrease, resulting in a soft hand feel of the fabric. Under the same process, the higher the melt index of polypropylene, the smaller its fineness and the greater its fracture strength.
The molecular weight distribution is often measured by the ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of the polymer (Mw/Mn), known as 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 conducive to improving spinning speed. It also has lower melt elasticity and tensile viscosity, which can reduce spinning stress, make PP easier to stretch and become finer, and obtain finer fibers. Moreover, the uniformity of the network is good, with good hand feel and uniformity.
Spinning temperature
The setting of spinning temperature depends on the melt index of the raw materials and the requirements for the physical properties of the product. The higher the melt index of the raw material, the higher the spinning temperature, and vice versa. The spinning temperature is directly related to the viscosity of the melt, and the temperature is low. The viscosity of the melt is high, making spinning difficult and prone to producing broken, stiff or coarse fibers, which affects the quality of the product. Therefore, in order to reduce the viscosity of the melt and improve its rheological properties, the method of increasing the temperature is generally adopted. The spinning temperature has a significant impact on the structure and properties of fibers. The lower the spinning temperature, the higher the stretching viscosity of the melt, the greater the stretching resistance, and the harder it is to stretch the filament. To obtain fibers of the same fineness, the speed of the stretching airflow needs to be relatively high at low temperatures. Therefore, under the same process conditions, when the spinning temperature is low, the fibers are difficult to stretch. The fiber has a high fineness and low molecular orientation, which is manifested in spunbond nonwoven fabrics with low breaking strength, high elongation at break, and a hard hand feel; When the spinning temperature is high, the fiber stretching is better, the fiber fineness is smaller, and the molecular orientation is higher. This is reflected in the high breaking strength, small breaking elongation, and soft hand feel of spunbond nonwoven fabrics. However, it is worth noting that under certain cooling conditions, if the spinning temperature is too high, the resulting filament will not cool enough in a short period of time, and some fibers may break during the stretching process, which may form defects. In actual production, the spinning temperature should be selected between 220-230 ℃.
Cooling forming conditions
The cooling rate of the filament has a significant impact on the physical properties of spunbond nonwoven fabric during the forming process. If the molten polypropylene can be rapidly and uniformly cooled after coming out of the spinneret, its crystallization rate is slow and the crystallinity is low. The resulting fiber structure is an unstable disc-shaped liquid crystal structure, which may reach a larger stretching ratio during stretching. The orientation of the molecular chains is better, which can further increase the crystallinity, improve the strength of the fiber, and reduce its elongation. This is manifested in spunbond nonwoven fabrics with higher fracture strength and lower elongation; If slowly cooled, the resulting fibers have a stable monoclinic crystal structure, which is not conducive to fiber stretching. This is manifested in spunbond nonwoven fabrics with lower fracture strength and greater elongation. Therefore, in the molding process, increasing the cooling air volume and reducing the temperature of the spinning chamber are usually used to improve the fracture strength and reduce the elongation of spunbond nonwoven fabrics. In addition, the cooling distance of the filament is closely related to its performance. In the production of spunbond nonwoven fabrics, the cooling distance is generally selected between 50-60cm.
Drawing conditions
The orientation of molecular chains in silk strands is an important factor affecting the tensile strength and elongation at break of single filaments. The greater the degree of orientation, the stronger the single filament and the smaller the elongation at break. The degree of orientation can be represented by the birefringence of the filament, and the larger the value, the higher the degree of orientation. The primary fibers formed when polypropylene melt comes out of the spinneret have relatively low crystallinity and orientation, high fiber brittleness, easy fracture, and significant elongation at break. To change the properties of fibers, they must be stretched to varying degrees as needed before forming a web. In spunbond production, the fiber’s tensile strength mainly depends on the size of the cooling air volume and suction air volume. The larger the cooling and suction air volume, the faster the stretching speed, and the fibers will be fully stretched. The molecular orientation will increase, the fineness will become finer, the strength will increase, and the elongation at break will decrease. At a spinning speed of 4000m/min, polypropylene filament reaches its saturation value of birefringence, but in the air flow stretching process of spinning into a web, the actual speed of the filament is generally difficult to exceed 3000m/min. So, in situations where strong demands are high, the stretching speed can be boldly increased. However, under the condition of a constant cooling air volume, if the suction air volume is too large and the cooling of the filament is not enough, the fibers are prone to breakage at the extrusion site of the die, causing damage to the injection head and affecting production and product quality. Therefore, appropriate adjustments should be made in actual production.
The physical properties of spunbond nonwoven fabrics are not only related to the properties of the fibers, but also to the network structure of the fibers. The finer the fibers, the higher the degree of disorder in the arrangement of fibers when laying the net, the more uniform the net is, the more fibers there are per unit area, the smaller the longitudinal and transverse strength ratio of the net, and the greater the breaking strength. So it is possible to improve the uniformity of spunbond non-woven fabric products and enhance their breaking strength by increasing the suction air volume. However, if the suction air volume is too large, it is easy to cause wire breakage, and the stretching is too strong. The orientation of the polymer tends to be complete, and the crystallinity of the polymer is too high, which will reduce the impact strength and elongation at break, increase brittleness, and thus lead to a decrease in the strength and elongation of the non-woven fabric. Based on this, it can be seen that the strength and elongation of spunbond nonwoven fabrics increase and decrease regularly with the increase of suction air volume. In actual production, it is necessary to adjust the process appropriately according to the needs and actual situation in order to obtain high-quality products.
Hot rolling temperature
The fiber web formed by stretching fibers is in a loose state and must be hot-rolled and bonded to become fabric. Hot rolling bonding is a process in which the fibers in the web are partially softened and melted by hot rolling rolls with certain pressure and temperature, and the fibers are bonded together to form a fabric. The key is to control the temperature and pressure well. The function of heating is to soften and melt the fibers. The proportion of softened and melted fibers determines the physical properties of spunbond nonwoven fabrics. At very low temperatures, only a small portion of fibers with lower molecular weight soften and melt, and there are very few fibers bonded together under pressure. Fibers in the fiber web are prone to slip, and non-woven fabrics have lower breaking strength but greater elongation. The product feels soft but is prone to fuzzing; As the hot rolling temperature gradually increases, the amount of softened and melted fibers increases, the fiber web bond becomes tighter, the fibers are less likely to slip, the fracture strength of non-woven fabric increases, and the elongation is still relatively large. Moreover, due to the strong affinity between fibers, the elongation slightly increases; When the temperature rises significantly, most of the fibers at the pressure point melt, and the fibers become melt lumps, starting to become brittle. At this time, the strength of the non-woven fabric begins to decrease, and the elongation also decreases significantly. The hand feel is very hard and brittle, and the tear strength is also low. In addition, different products have different weights and thicknesses, and the temperature setting of the hot rolling mill also varies. For thin products, there are fewer fibers on the hot rolling point, and less heat is required for softening and melting, so the required hot rolling temperature is lower. Correspondingly, for thick products, the hot rolling temperature requirement is higher.
Hot rolling pressure
In the hot rolling bonding process, the role of the hot rolling mill line pressure is to compact the fiber web, causing the fibers in the web to undergo certain deformation heat and fully exert the effect of heat conduction during the hot rolling process, making the softened and melted fibers tightly bonded together, increasing the adhesion force between fibers, and making it difficult for fibers to slip. When the hot rolling line pressure is relatively low, the fiber compaction density at the pressure point in the fiber web is poor, the fiber bonding strength is not high, the holding force between fibers is poor, and the fibers are relatively easy to slip. At this time, the hand feel of the spunbond non-woven fabric is relatively soft, the fracture elongation is relatively large, and the fracture strength is relatively low; On the contrary, when the line pressure is relatively high, the resulting spunbond non-woven fabric has a harder hand feel, lower elongation at break, but greater breaking strength. 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 fiber web is difficult to flow and diffuse, which also reduces the fracture tension of the non-woven fabric. In addition, the setting of line pressure is also closely related to the weight and thickness of the non-woven fabric. In production, appropriate selection should be made according to the needs in order to produce products that meet performance requirements.
In summary, the physical and mechanical properties of polypropylene spunbond non-woven fabric products are not determined by a single factor, but by the combined effects of various factors. In actual production, reasonable process parameters must be selected according to actual needs and production conditions in order to produce high-quality spunbond non-woven fabric products that can meet various needs. In addition, strict standardized management of the production line, careful maintenance of equipment, and improvement of the quality and proficiency of operators are also key factors in improving product quality.
Dongguan Liansheng Non woven Technology Co., Ltd. was established in May 2020. It is a large-scale non-woven fabric production enterprise integrating research and development, production, and sales. It can produce various colors of PP spunbond non-woven fabrics with a width of less than 3.2 meters from 9 grams to 300 grams.
Post time: Nov-29-2024