It is well known that crystal lattice is a common problem for film manufacturers. For clear plastic bags, the crystal dots significantly affect the appearance of the transparent plastic bag. For plastic bags that require high printing quality, especially for large-area plastic bag printing, the “white spot” waste products caused by crystal dots are a large proportion of the loss of the plastic bag industrial production process. Therefore, controlling the generation of crystal points and eliminating the cause of crystal points is an important process for improving the technical level of the plastic bag industry.

1. What is a crystal point?

The crystal point is actually an "excessive polymer", ie the molecular weight of the polymer at the "crystal point" site is higher than the molecular weight of the surrounding homopolymer. Due to the high molecular weight, the polymer at the crystal point has a higher melting point. #FormatStrongID_2# The melt has a higher viscosity when melted. The polymer at the crystal point is blown or delayed in flow, and cannot be uniformly dispersed and mixed with the surrounding polymer, and is solidified before the melt is blown or cast into a film before the surrounding polymer. Therefore, a solidified body which forms an "arrow-like" or "spherical" excessive polymer is conventionally referred to as a "crystal point".

2. Reasons for crystal point generation

Material reason

(1) a residual catalyst in the polymer;

(2) The polymer melt is stuck 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 undergo catalytic polymerization of the polymer, thereby forming an excessive polymer;

The oxygen contained in the polymer also causes the polymer melt to produce a crystal point (a certain amount of antioxidant is added to prevent oxygen from acting on the crystal point).

Process cause

(1) A crystal point caused by a polymerization process. Different polymerization equipment and processes have different effects on the formation of crystal points. For example, some famous raw materials, because of high production technology, low residual catalyst content in the polymer, and good equipment structure, therefore, the raw material itself contains less crystal points. However, some of the poor quality raw materials, due to production technology and equipment problems, cause the raw materials themselves to contain more crystal points.

(2) The crystal point caused by the process. For example, a film prepared by a good molding apparatus has few crystal points. The film made by the poor equipment has many crystal points.

3. Method of reducing or substantially eliminating crystal points

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

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

(2) The surface lubricant is added during the production process so that the processed polymer melt cannot be stuck to the surface of the blown film equipment.

In terms of production process, although a finer filter screen can be used to filter out the native crystal points in most of the polymer, it can also filter out the crystals caused by the polymer sticking to the surface of the screw. Point, but the problem is that the back pressure of the equipment is increased, which is difficult for some old molding equipment. Therefore, relying on the diligent change of the filter can also help control the crystal point.

So far, the more effective crystal point control method is to use high-performance fluorine process aids. High-performance fluorine process additive, due to its high polarity, allows it to quickly escape from the melt of olefinic polymers (eg PE, PP) and adhere to the metal surface of the equipment. This eliminates the viscous layer of the polymer melt on the metal surface of the device.

Therefore, the polymer melt can be continuously pushed forward at a constant speed, eliminating the phenomenon that the polymer melt stuck to the metal surface is heated for a long time, preventing the residual polymerization of the polymer to the polymer, and finally preventing the film from being formed. The crystal point produced in the molding process.

On the other hand, since the fluorine process aid can also adhere to the surface of the screen, the resistance of the polymer melt flowing through the screen is lowered. Therefore, the use of fluorine process auxiliaries provides process conditions for the use of finer screens. The use of finer filters is an important process for filtering out the native crystal spots produced by the polymer.

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