As we all know, crystal point is a common problem in film manufacturers. For the transparent plastic bag, the crystal point obviously affects the appearance of the transparent plastic bag. For plastic bags with high printing quality, especially for large-scale plastic bag printing, the "white dot" waste caused by crystal point is a large proportion of loss in the production process of plastic bag industry. Therefore, controlling the formation of crystal point and eliminating the cause of crystal point is an important process to improve the technical level of plastic bag industry.
1. What is a crystal point
In fact, the molecular weight of the polymer at the "Crystal Point" is higher than that of the surrounding polymer. Due to the high molecular weight, the polymer at the crystal point has a high melting point ා formatstrongid_ 2. The melt has high viscosity during melting. In the process of blowing film or tape casting, the polymer at the crystal point can't disperse and mix with the same kind of polymer around it. After the melt is blown or cast into film, it solidifies before the same polymer around. Therefore, the formation of "arrow shaped" or "spherical" solidification of excessive polymer is conventionally called "Crystal Point".
2. The reason of crystal point
Material reason
(1) There is residual catalyst in the polymer;
(2) The polymer melt adheres to the metal surface of production equipment (including polymerization equipment and film blowing equipment), and remains at high temperature;
(3) The catalyst continues to catalyze the polymerization of the polymer, thus forming an excessive polymer;
The oxygen contained in the polymer can also make the polymer melt produce crystal point (adding a certain amount of antioxidant can prevent the formation of oxygen on the crystal point).
Process reason
(1) Crystal point caused by polymerization process. Different polymerization equipment and process have different effects on the formation of crystal point. For example, the raw materials of some famous factories have less crystal point because of their high production technology, low residual catalyst content in polymer and good equipment structure. However, some poor quality raw materials have more crystal points due to production technology and equipment problems.
(2) Crystal point caused by process. For example: the film prepared by good molding equipment has less crystal point. The film made by poor equipment has many crystal points.
3. Methods to reduce or eliminate the crystal point
In conclusion, there are two ways to eliminate the crystal point in the film blowing process
(1) Improve the fineness of filter screen of processing equipment (e.g. change the filter screen to 120 mesh, or even 150 mesh - for ultra-thin membrane).
(2) In the production process, the surface lubricant is added to prevent the polymer melt being processed from sticking to the surface of the film blowing equipment.
In terms of production process, although the use of a finer filter screen can filter out most of the primary crystal point in the polymer, and also can filter out the crystal point caused by the polymer sticking on the screw surface of the screw rod and barrel of the equipment, but the problem is that the back pressure of the equipment is increased, which is difficult for some old molding equipment. Therefore, it can also help to control the crystal point by changing the filter screen frequently.
So far, the more effective way to control the crystal point is to use high-performance fluorine process additives. Due to the high polarity of its polymer molecules, high-performance fluorine process additive can quickly overflow from olefin polymer (such as PE, PP) melt and adhere to the metal surface of equipment, thus eliminating the viscous layer of polymer melt on the metal surface of equipment.
Therefore, the polymer melt can be continuously and uniformly pushed forward, which eliminates the phenomenon that the polymer melt adhering to the metal surface is heated for a long time, prevents the continuous polymerization of the polymer by the residual catalyst, and finally prevents the crystal point produced in the film forming process.
On the other hand, the fluorine process aids can also adhere to the surface of the filter screen, so that the resistance of polymer melt flowing through the filter screen is reduced. Therefore, the use of fluorine process additives provides technological conditions for the use of finer filter screen. The use of finer filter screen is an important process measure to filter out the primary crystal point produced by polymer production.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service.
The company produces composite bags, roll film, fruit bags, aluminum foil bags, suction mouth bags, self-supporting bags, zipper bags, anti-static bags, special-shaped bags, etc., which are suitable for the internal and external packaging of food, daily chemicals, electronics, toys, medicine, cosmetics, industrial products and other industries.
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