What issues should be paid attention to when producing PVC calendered biaxially oriented film?

Shrinkage too large

Excessive shrinkage involves two meanings: the film shrinks too much during the stretching process and does not reach the required width of the product; the film is retracted too hot during the secondary compounding process, exceeding the standard requirements and failing to meet the requirements of the product. width.

Question 1:

It can be explained by the malleability of plastics. PVC is an amorphous polymer. When subjected to a tensile force greater than the yield strength in a certain temperature range, plastic elongation occurs. During the deformation process, the polymer structural units (stripe, macromolecule and crystallite) start due to stretching. orientation. As the degree of orientation increases, the force between the macromolecules increases, causing the viscosity of the polymer to increase and tend to harden, and the deformation tends to be stable and no longer develops. This phenomenon is called "stress hardening". When the temperature is appropriately increased, the ductility of the plastic is further increased, the draw ratio can be increased, and even some polymers having poor ductility can be stretched. The deformation near room temperature to Tg (minimum processing temperature) is generally referred to as "cold stretching", and after the stress hardening of the polymer during stretching, the flow of the polymer molecules is restricted, thereby preventing further increase in the draw ratio. The film in which "cold stretching" occurs is restored to its original state after the tensile force is released, resulting in a large retraction.

In fact, the design of the stretch film workshop should consider the warmth and air convection of the workshop, because the heat loss of the film during the stretching process is very fast. The thinner the film and the wider the stretch, the faster the heat dissipation. The author measured that the surface temperature of the film from the stretch inlet to the stretched outlet was reduced from 175 ° C to 135 ° C for 3 m, so the temperature in the production workshop should not be lower than 20 ° C. The workshop is designed to leave only the exhaust window at the top and the window below. In addition, in order to prevent cold stretching, a heat insulating chamber can be formed in the transverse stretching portion to ensure the ambient temperature of the stretched film. By appropriately reducing the speed ratio of each machine, the stretching of the film can also be reduced.

Question 2

Generally, the lower the stretching temperature (above Tg), the larger the stretching ratio, the faster the stretching speed and the cooling rate, and the higher the degree of orientation, the more the polymer after the stretching and orientation cooling is heated to a temperature higher than Tg. When the shrinkage occurs remarkably, the shrink film is made according to this principle. In order not to cause heat shrinkage of the article, the article can be treated at the highest temperature for a certain period of time while maintaining the stretch, and then cooled to room temperature, so that the article thus obtained has good stability. This can add a heating device below the rear half of the stretching machine to maintain the stretching temperature. In addition, the warming water can be supplied to the No. 2 warming wheel to slowly cool the stretched film, so that the stretched film can be effectively reduced. Heat shrinkage.

Thin film thickness

The thickness unevenness of the stretched film is mainly caused by two cases: the film itself from the calender is thin and uneven, and finally the thickness of the stretched product is different; during the stretching process, the transverse temperature of the film is different due to air convection. The film thickness after stretching is different, and finally the product roll has different thickness in the transverse direction.

It can be solved by adjusting the four-roll calender. Specifically, the shaft cross device of the middle roller and the reverse bending device of the lower wheel can be adjusted. In addition, the storage between the rollers should be controlled to prevent the cold material from entering the uniformity of the film stretching. Sex. The diameter of the front wheel and the upper wheel is not more than 250mm, the upper and middle wheels are stored in 15~30mm, and the middle and lower wheels are stored in 10~15mm. The situation is due to the difference in air convection between the transverse directions, resulting in different temperatures at various points of the film, so that the transverse thickness of the stretched film is inconsistent.

According to practical experience, the ends of the film are generally thicker, because the air convection on both sides is faster than the middle, so the temperature on both sides is lower. Therefore, the film can also be used to cover both sides of the widening machine. If the above method does not solve the problem, you can install a jet tube to make up. Placed in the horizontal direction at a distance of 150 mm from the film, the nozzle is required to be a duckbill type, and a microvalve is used to adjust the displacement, so that the airflow emitted is relatively soft. The exhaust valve can be opened appropriately in the thin film, and the exhaust valve is closed in the rest.

Perforation problem

There are three main cases of perforation: the material is too dirty, the filter mesh is not fine enough; the four-roll calender has the cold material mixed; the filling material and the plasticizer.

Situation 1

It can be solved by increasing the fineness of the filter and changing the frequency of the filter; if there is still a perforation, consider replacing the material.

Situation 2

It can be solved by controlling the storage between the rollers; in addition, the material should be plasticized as much as possible to avoid the incorporation of unplasticized raw materials, and the speed ratio of each machine should be controlled to minimize the resulting holes. problem.

The perforation of the stretched film is much more obvious than that of the ordinary calendered film, because the tensile force of the ordinary calendered film is unidirectional, smaller than that of the uniaxially stretched film, and can be compensated to some extent by the calendering device. . The tensile force of the biaxially stretched film is two-way, much larger than that of biaxial stretching. If the film of 2 m width is stretched to 5 m, the hole having a stretch ratio of 215.1 mm is stretched to become 215 mm. In fact, the hole is larger than 215mm, because the stress around the hole is concentrated and easily damaged. Therefore, it is very important to solve the problem of perforation of the stretched film.

Situation 3

The filling material mainly uses light calcium carbonate, and the proper amount of calcium carbonate can increase the strength of the film, but when the amount of calcium carbonate reaches 50-60 parts (relative to 100 parts of PVC), the tear strength of the film is obviously reduced, and then the film is stretched. The number of perforations has increased significantly. Therefore, controlling the amount of calcium carbonate is also an important factor in reducing the pores of the stretched film. When the amount of the plasticizer is increased, the pores are reduced due to the softening of the film and the elongation at break, and conversely, the pores are increased.

Precipitation problem

In the process of producing a stretched film, there is sometimes a phenomenon of precipitation, that is, a layer of hoarfrost on the surface of the film or on the calender roll (or traction roll), which seriously affects the use of the film and even scraps the film (such as a light box). The film may affect the ink absorbing properties of the light box cloth due to surface precipitation. From the production situation, the precipitated materials mainly include lubricants, powder stabilizers and calcium carbonate. In the process of producing PVC calendered film, in order to make the surface of the product smooth and beautiful, and also to improve the flow properties of the plastic melt, reduce and avoid the adhesion and friction to the equipment, a certain amount of lubricant is added to the plastic. However, if the lubricant is used improperly, it will easily precipitate on the surface of the product when it exceeds the compatibility limit. This is commonly referred to as the blooming, which affects the appearance of the product, and also affects the ink absorption of the inkjet advertising cloth; the amount of lubricant is too small. The lubricant is not used, and the amount of lubricant is generally less than 1%. Commonly used lubricants are stearic acid, polyethylene wax, and the like.

The processing temperature of PVC is very close to its decomposition temperature. When processed at 160~200 °C, severe thermal degradation will occur, and the product will change color and performance will deteriorate. Therefore, in the processing of PVC, it is necessary to add a heat stabilizer. Many heat stabilizers not only have a stabilizing effect, but also have a certain lubricating effect, such as (powder) barium stearate, cadmium stearate. If the compatibility of these materials is poor, frosting, migration, etc. may occur. In order to improve the mechanical properties of the product, the cost and the unit consumption of the resin are reduced and an appropriate amount of filler material is added. Adding an appropriate amount of additives reduces the linear expansion coefficient of the plastic and the molding shrinkage of the product, and improves the heat resistance, flame retardancy and strength of the plastic. The amount of the additive should be appropriate, and too much will precipitate from the surface of the film. Increasing the fineness of the additive can increase the amount of the additive, such as the use of ultrafine calcium carbonate more than ordinary calcium carbonate. It is also possible to treat the calcium carbonate with a coupling agent to increase the tightness of the combination of calcium carbonate and PVC material.