Study MOPP production process of the film: Changzhou Industry Technical College Liu Min Yu Sun Source: Ringer uniaxially stretched polypropylene film (MOPP films for short), a lightweight, transparent, non-toxic, moisture-proof, high mechanical strength, is widely used It is used as a substrate for composite films for packaging films, film, smoke film, etc. The production of MOPP film by uniaxial stretching method is a processing method with high technical requirements in plastic molding processing. In addition to the need for good processing equipment, it is more important to require the production staff to deeply grasp the polymer. The performance and processing conditions have an impact on product performance, and in the actual production process, the use of advanced control methods, the correct and flexible use of professional knowledge, the development of fair process conditions, and timely solve the problems in production.
Production process
At present, the production methods of MOPP film mainly include tube film method and flat film method. The tubular membrane method has the advantages of simple equipment, low investment, small footprint, loss of material and simple operation, but has the disadvantages of low production efficiency and large product thickness tolerance; the product obtained by the flat membrane method has superior performance, and the equipment is mature and produced. High efficiency, suitable for mass production, and widely used by enterprises.
Extrusion-casting production process
Melt and homogenize the uniform resin melt through the extruder, squeeze it through the filter to the machine head, and distribute the melt evenly to the lips by means of the internal flow restrictor (coat hanger type die), and extrude to form a melt. The body film is then cooled by a chill roll to form a film for use, and the structure of the cast piece is shown in FIG. The extrusion-casting process conditions are: the temperature of the extruder conveying section is 200~220°C, the temperature of the melt plasticizing section is 220~260°C, the temperature of the homogenizing section is 240~260°C, and the temperature of the filter (net) is 250~265°C. The melt line temperature is 240~250°C, and the chill roll temperature is 20~30°C.
Through production practice and experiment, the quality requirements of the diaphragm are: 1 the cross section of the diaphragm is symmetrical, meet the requirements; 2 no longitudinal stripes and lateral "water ripples"; 3 crystallinity less than 3%, the lower the better, and evenly distributed ; 4 has a certain amount of pre-stretching to ensure better toughness in the longitudinal direction; 5 no bubbles, pitting and other defects, good finish.
Longitudinal stretching process
Stretching process
The combination of MOPP longitudinal stretching mainly includes vertical combined stretching and horizontal combined stretching. At present, the main commonly used is vertical combined stretching, and its structural schematic is shown in FIG. The membrane first enters the heating zone and the membrane is raised to a certain temperature on a heated roller. Requirements: Each roller should be controlled separately. The surface of the roller should be chrome-plated. The diameter of the roller is generally 300~400mm. The double-wall spiral flow channel is connected to the heat-conducting oil. The surface temperature of the roller is ±1°C. The temperature of each roller is controlled separately. Then proceed to the stretching zone for stretching. At present, there are two main methods for longitudinal stretching of MOPP film: single-point small gap stretching and single-point large-gap stretching. The small gap stretching interval is less than 2.5 mm, and the large gap stretching interval is generally 5 to 50 mm. Requirement: The drawing speed can be steplessly regulated. Each roller is driven separately. The surface of the roller should be chrome-plated, and the surface temperature of the roller should be uniform. After stretching, the film enters the shaping zone and heats the film to make the polymer after orientation. The chain has proper relaxation to hold the orientation state. The requirements for the sizing rolls are basically the same as before; after the sizing is completed, the cooling zone is entered and the film is cooled to room temperature. Requirement: Pass the cooling water, the spiral double-wall flow channel inside the roller, and the surface temperature is even.
Figure 3, the longitudinal stretch composition chart
Influencing factor
There are many parameters for longitudinal stretching, including the number and temperature of the preheating rolls, the heating form of the preheating rolls and the stretching rolls, the stretching ratio of the film, the setting time and temperature of the film, and the temperature of the chill roll. The author discusses the factors that have a greater impact on production—stretch ratio and temperature.
Stretch ratio
The draw ratio is an important process parameter, and the longitudinal draw ratio has a significant impact on the physical and mechanical properties of the MOPP film. At a certain temperature, the larger the draw ratio, the greater the degree of orientation of the PP molecular chain. That is, the mechanical strength of the film is improved, the modulus is increased, the elongation at break is decreased, and the impact strength and folding endurance are improved. After longitudinal stretching of the MOPP film, the polymer chain is uniaxially oriented, which greatly improves the longitudinal mechanical properties of the film, while the lateral properties deteriorate and exhibit anisotropy. Generally, the longitudinal stretching ratio of the film is 6-8 times or even higher.
temperature
The temperature distribution of each zone is a key factor affecting the orientation and crystallization of the MOPP film. The temperature affects the orientation process by the action of polymer viscosity and relaxation time. When the temperature rises, the viscosity of the polymer decreases. Under the action of constant stress, the high elastic deformation and the viscous deformation increase, the increase of the high elastic deformation is limited, and the viscous deformation develops rapidly, which is beneficial to the polymer orientation. Practice has confirmed that the use of relatively low preheating, stretching temperature or cooling immediately after stretching is an effective method to improve the orientation of MOPP film and reduce crystallinity. If the preheating temperature is too high, PP will form spherulites, and the transparency of the film will decrease. When the stretching temperature is too high, the PP segment will be easily de-oriented, which greatly reduces the physical and mechanical properties of the film.
Common in production
In the actual production process, due to various reasons, MOPP film has various problems, making production targets difficult to achieve. The author proposes the following solutions to the common problems in production.
Film thickness deviation
There are several types of variations in film thickness during production that vary in thickness:
1 short-term or periodic longitudinal thickness deviation of the film;
2 long-term longitudinal thickness deviation of the film;
3 film lateral thickness deviation.
The short-term or periodic longitudinal thickness deviation of the film is mainly caused by mechanical properties such as unstable screw speed of the extruder, and it is impossible to adjust during the film manufacturing process. The long-term longitudinal thickness deviation of the film is caused by changes in the filter and raw materials, environmental sanitation conditions, etc., and the adjustment is relatively easy to solve. In addition to the conditions of the die and the adjustment mode, the lateral temperature distribution of the die, the uniformity of the melt viscosity of the extruded material, and the cooling rate of the cast piece, the process conditions during stretching are also important factors. The solution is to adjust the equipment and control the melting temperature of the resin.
Vertical stripes
During the production process, it is sometimes seen that there are continuous longitudinal stripes at the extrusion-casting portion and at the fixed position. If such a film is used to produce an MOPP film, the longitudinal thickness of the film will be uneven; the appearance of the film being wound and slit will have obvious protrusions (bursts) or vertical stripes. The formation of longitudinal stripes is due to: 1 foreign matter in the lip of the mold hinders melt activity. The molten material separated by the foreign matter will rejoin after flowing through the foreign matter, but in a short time before flowing to the cooling drum, the surface tension is not leveled, so that streaks are formed. 2 The lip of the mold is stained, and the stripe is dragged on the surface of the melt film. This stripe is thin and is a single stripe. The material has a lot of volatiles, and the angle between the surface of the melt film and the surface of the die lip is small, which is prone to occur.
The measures usually taken to eliminate longitudinal stripes are as follows: (1) Use a reasonable-quality, good-quality die to ensure that the lips are smooth and free from any mechanical damage. 2 strengthen the melt filtration. 3 Clear the debris on the lip in time to do the maintenance of the nose. 4 improve the uniformity of air knife blowing. 5 fair control of the temperature of each section. 6 Adjust the position of the machine head relative to the chill roll.
Warping on both sides
This phenomenon is mainly caused by the poor effect of the film attached sheet and the excessive temperature difference between the two sides during the casting process. The warpage of the cast piece will affect the flatness of the film. In the case of PP, since the cast piece is unevenly cooled, the crystal is not uniform, which directly affects the film formability of the film. The edge of the cast piece is usually warped toward the lower temperature side. Therefore, in the production process, the temperature of the quenching roll surface and the cooling water can be determined according to the warpage of the cast piece, and the solution can be considered.
Longitudinal hole (film)
Longitudinal holes (membranes) are most common in the production process, mainly due to significant changes in production conditions during extrusion-casting or longitudinal drawing, large variations in longitudinal thickness of the film, or large defects in the cast sheet. The partial tensile stress of the diaphragm during the stretching process exceeds the allowable stress of the material, resulting in a longitudinal hole (film). If there is a vertical hole (film), it can be analyzed from the following aspects: 1 Raw materials contain impurities (low molecular substances, oil stains, etc.) with large differences in performance. 2 There are obvious bubbles on the diaphragm. 3 The filter is damaged and the diaphragm has a high impurity content. 4 Extrusion, longitudinal stretching temperature setting is improper. 5 head leakage. 6 equipment scratched film.
Conclusion
The uniaxially stretched MOPP film is processed and integrated into one, which reduces the process of biaxial stretching, but the product has the performance of two-way stretching, and can save raw materials and reduce costs. However, due to the current maturity of MOPP film technology, it is a topic of concern for MOPP film manufacturers to solve problems in time and strive to improve product quality and quality.
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