See the plastic film problem here

[Chinese Packaging Network News] As we all know, the crystal dot is a common problem for thin film manufacturers. For transparent plastic bags, the crystal point significantly affects the appearance of the transparent plastic bag. For plastic bags requiring higher printing quality, especially for large-area plastic bags, the “white spots” of defective products caused by the crystal dots are a large proportion of losses in the plastic bag manufacturing process. Therefore, controlling the generation of crystal points and eliminating the origin of crystal spots are important process steps to improve the technical level of plastic bags.

1, what is the crystal point

The crystal point is actually an "over-polymer," that is, the molecular weight of the polymer at the "crystal point" is higher than the molecular weight of the surrounding same polymer. Because of its high molecular weight, the polymer at the crystal spot has a higher melting point. #FormatStrongID_2# The melt has a higher viscosity at melting. The polymer at the crystal spot can not be uniformly dispersed and mixed with the surrounding same polymer during blown film or flow delay, and solidified prior to the same polymer in the surrounding melt after being blown or cast into a film. Thus, a solidified body that forms an "arrowheaded" or "spherical" transition polymer is conventionally referred to as a "crystal spot."

2. The reason for the crystal point

Material reason

(1) Residual catalyst in the polymer;

(2) The polymer melt sticks to the metal surface of the production equipment (including polymerization equipment and blown film equipment) and remains at high temperature;

(3) The catalyst continues to catalyze the polymerization of the polymer, thus forming an excessive polymer;

Oxygen contained in the polymer also causes the polymer melt to have a crystal spot (a certain amount of antioxidant is added, and the oxygen is prevented from acting on the crystal point).

Process reason

(1) The crystal point caused by the polymerization process. Different polymerization equipment and processes have different effects on the generation of crystal dots. For example: Some famous plant raw materials, because of the high production technology, the residual content of the catalyst in the polymer is low, the equipment structure is good, therefore, the raw material itself contains less crystals. However, due to production technology and equipment problems, some of the raw materials with poor quality have caused many crystal points contained in the raw materials themselves.

(2) The crystal point caused by the process. For example: Films made from good molding equipment have few crystals. The film made of poor equipment has many crystal points.

3. Methods to reduce or substantially eliminate crystals

In summary, there are two main ways to eliminate crystals in the film blowing process:

(1) Improve the fineness of the filter of the processing equipment (for example: change the filter screen to 120 mesh, or even 150 mesh - for ultra-thin film).

(2) Surface lubricants are added during the production process so that the polymer melt to be processed cannot stick to the surface of the film blowing equipment.

From the point of view of the production process, although a finer filter screen can be used to filter out the primary crystal point in most of the polymers, it is also possible to filter the crystals caused by polymer sticking to the surface of the screws in the screw and barrel parts of the equipment. Point, but the problem is that the back pressure of the equipment is therefore increased, which is difficult for some old molding equipment. Therefore, depending on the filter, it can also help to control the crystal spot.

So far, the more effective crystal point control method is the use of high-performance fluorine process additives. The high-performance fluorine process additive, due to its highly polar nature of the polymer molecule, allows it to quickly overflow from the melt of olefin polymers (eg PE, PP) and adhere to the metal surface of the device. This eliminates the viscous layer of the polymer melt on the metal surface of the device.

As a result, the polymer melt can be driven continuously at a constant speed, eliminating the phenomenon that the polymer melt sticking to the metal surface is heated for a long time, preventing the continued polymerization of the residual catalyst on the polymer, and ultimately preventing the film from being The crystal point produced in the molding process.

On the other hand, since the fluorine process aid can also adhere to the screen surface, the resistance of the polymer melt flowing through the screen is reduced. Therefore, the use of fluorinated process aids provides process conditions for the use of finer mesh screens. The use of finer meshes is an important technological measure to filter out the primary crystals produced during polymer production.