Influence of the hottest temperature on BOPP produ

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Influence of temperature on BOPP production

in the production process of BOPP, temperature is a very important production process parameter, and it is a key factor in the production process. It can be said that good temperature control is the basic guarantee for producing high-quality films. The temperature control of each processing section in the production process has its different ways and characteristics. Next, the influence of temperature on BOPP production is discussed from the aspects of cold roll temperature demand, rework fulfillment, longitudinal and transverse stretching temperature control

control of cold roll temperature

after the polymer leaves the machine head, with the help of the external force of the attachment device, it is quickly pasted on the cold roll surface with low temperature and high smoothness. The high-temperature melt exchanges heat with the cold roll surface to make the melt cool quickly. For the crystalline polymer of the base material, the key of casting sheet is to control the crystallinity of the sheet. Too high crystallinity will cause the film to break during stretching, or the haze of the stretched film will increase or the surface roughness of the film will increase. However, too low crystallinity will also lead to the decline of mechanical properties and stiffness of the film. Generally speaking, the crystallization of homopolymer PP with high isotacticity is closely related to the cooling rate. The lower the surface temperature of the cold roll, the faster the heat conduction, or the tighter the surface of the sheet attached to the roll, the faster the melt cooling speed. At this time, the crystallinity of the sheet is small, which is conducive to stretching. However, too low temperature will also cause the extruded sheet to slide or warp on the surface of the cold roll, so appropriate improvement under the condition of low roll surface temperature is also conducive to the adhesion surface of the extruded sheet, which is useful for eliminating gas and preventing surface defects such as bubbles and corrugations

the adjustment of cold roll temperature needs to consider many factors, such as the type of raw materials, film attachment equipment, sheet thickness and so on. In general, in the production of BOPP, within a certain range, the cold roll temperature increases, the tensile strength decreases, and the haze increases. Therefore, cooling at a lower temperature will reduce the crystallinity of the sheet, which is beneficial to the future stretching

longitudinal drawing temperature

in the longitudinal drawing area, the sheet is stretched longitudinally under the action of the longitudinal drawing roller, and the polymer molecular segments are oriented longitudinally. Similarly, in the longitudinal drawing process, it is very important to well control the crystallization of polymers. Under the action of stress, crystalline polymer will accelerate crystallization, because the stress makes the polymer oriented and induces nucleation. At this time, macromolecules stretch along the direction of stress and form ordered regions, becoming primary nuclei, which greatly shortens the time of crystallization induction and accelerates the crystallization speed. In addition, temperature is also an important factor affecting crystallinity. The fastest crystallization temperature of crystalline polymer is the maximum crystallization temperature (Tmax). Generally speaking, the maximum crystallization temperature (Tmax) of crystalline polymers has the following relationship with the melting point temperature (TM): Tmax = (0.80-0.85) TM, that is, the maximum rate of crystal growth is about 0 of the melting point temperature 8-0.85 times. In the Tmax range, the polymer has fast crystallization speed and sharp increase in crystallinity

polymers have high crystallinity and are easy to break the film when stretched. Therefore, the crystallization speed of polymers should be reduced as much as possible during the stretching process, and the low crystallinity should be controlled to ensure the smooth stretching. It is inevitable to accelerate the crystallization rate of polymer due to orientation, but the crystallization rate can be effectively reduced by avoiding stretching in the Tmax range. In addition, it is proved by practice that lower preheating, drawing temperature and immediate cooling are beneficial to improve the orientation of the film, enhance the mechanical properties of the film and reduce the crystallinity. When the temperature is too high, the polymer will form spherulites, which will reduce the transparency of the film. In serious cases, the film will appear white fog

for films with thermal seal, the selection of stretching temperature is particularly important. Generally, the TM of thermal seal is about 135 ℃, so too high longitudinal stretching temperature will also cause the thermal seal to stick to the roll

transverse stretching temperature

the temperature of each transverse stretching zone is a key factor affecting the mechanical strength, film-forming property, thickness uniformity and other properties of the film. Generally, controlling a lower temperature within the range for stretching is conducive to improving the mechanical strength of the film and increasing the thermal shrinkage of the film. However, if the temperature is too high, the film will be broken due to large thickness tolerance and film haze. Of course, too low temperature will also cause problems such as membrane breakage and unclamping

the stretching temperature mainly depends on the conditions of stretching processing (stretching ratio, stretching rate), product thickness and the material being stretched. The faster the drawing speed is, the shorter the heating time of the film in each zone is, and the drawing temperature needs to be increased accordingly; The thicker the film, the higher the temperature. Since the sheet has undergone longitudinal stretching before stretching, and the molecules have been oriented longitudinally, the temperature in the transverse stretching zone should be about 15-25 ℃ higher than that in the longitudinal stretching zone. In addition, the temperature distribution in the transverse stretching zone is also very important, and the temperature in each zone should be stable and uniform. Although the polymer will generate heat during stretching, the viscosity will also increase during the stretching process. To achieve continuous stretching, the temperature in the transverse stretching zone is generally increasing

generally speaking, there are many factors that affect the temperature of the transverse tensile zone, including material brand, film thickness, processing conditions and so on. But generally speaking, in the process of transverse stretching, higher stretching temperature makes the mechanical properties of the film decline, and the thermal shrinkage decreases sharply with the increase of temperature. In the heat setting stage, the light passing through the film has a great impact on the mechanical properties of new materials. Too high temperature reduces the gloss and increases the haze of the film

from the above analysis, it is concluded that in the production process of BOPP, in order to improve the film-forming performance, good temperature control is very critical. The growth of crystal is controlled by quench casting; During the stretching process, control the temperature to avoid stretching within the Tmax range, so as to ensure that the crystallization speed is slow, which is conducive to the orientation of molecular segments; In the process of longitudinal and transverse stretching, using a lower temperature within the allowable temperature range for stretching is a condition to improve the mechanical properties of the film, improve the optical properties of the film, and increase the shrinkage of the film

source: plastic channel

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