Making Plastic Film: The Unavoidable Gel Particle Problem

As is well known, gel particles are a common problem for film manufacturers. For transparent plastic bags, gel particles noticeably affect their appearance. For plastic bags that require higher printing quality, especially large-area plastic bag printing, the "white spot" rejects caused by gel particles account for a relatively large proportion of losses in the plastic bag industry's production process. Therefore, controlling the formation of gel particles and eliminating their causes are important process steps in improving the technical level of the plastic bag industry.

1. What are gel particles

Gel particles are in fact "over-polymerized polymer," that is, the molecular weight of the polymer at the "gel particle" is higher than that of the surrounding polymer of the same type. Because of the high molecular weight, the polymer at the gel particle has a higher melting point, and when melted, the melt has a higher viscosity. During blown film or cast film extrusion, the polymer at the gel particle cannot uniformly disperse and mix with the surrounding polymer of the same type, and after the melt is blown or cast into a film, it solidifies before the surrounding polymer of the same type. Therefore, the solidified mass of "arrow-shaped" or "spherical" over-polymerized polymer is customarily called a "gel particle."

1. Causes of gel particles

(1) Residual catalyst in the polymer;

(2) The polymer melt adheres to the metal surfaces of production equipment (including polymerization equipment and blown film equipment), and at high temperature, residual;

(3) The catalyst continues to catalyze the polymerization of the polymer, thus forming over-polymerized polymer;

Oxygen contained in the polymer can also cause gel particles in the polymer melt (adding a certain amount of antioxidant helps prevent oxygen from generating gel particles).

(1) Gel particles caused by the polymerization process. Different polymerization equipment and processes have different effects on the formation of gel particles. For example, some brand-name raw materials contain few gel particles because of advanced production technology, low residual catalyst content in the polymer, and good equipment structure. However, some raw materials of poorer quality contain more gel particles due to problems in production technology and equipment.

(2) Gel particles caused by the process. For example, films produced on good forming equipment have fewer gel particles, while films produced on poor equipment have more gel particles.

1. Methods to reduce or basically eliminate gel particles

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

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

(2) Add a surface lubricant during production so that the polymer melt being processed cannot adhere to the surface of the blown film equipment.

From the perspective of the production process, although a finer filter screen can filter out most inherent gel particles in the polymer and can also filter out gel particles caused by polymer adhering to the screw surface at the screw and barrel of the equipment, the problem is that the equipment back pressure increases accordingly, which is difficult to achieve for some old forming equipment. Therefore, frequently changing the filter screen can also help control gel particles.

To date, a relatively effective means of controlling gel particles is the use of high-performance fluoropolymer processing aids. Because their polymer molecules are highly polar, high-performance fluoropolymer processing aids can rapidly migrate out of olefin polymer (such as PE and PP) melts and adhere to the metal surfaces of the equipment, thereby eliminating the stagnant layer of polymer melt on the metal surfaces.

As a result, the polymer melt can be pushed forward continuously and at a uniform speed, eliminating prolonged heating of the polymer melt adhering to the metal surface, preventing residual catalyst from continuing to polymerize the polymer, and ultimately preventing gel particles generated during the film forming process.

On the other hand, because fluoropolymer processing aids can also adhere to the surface of the filter screen, the resistance of the polymer melt flowing through the screen is reduced. Therefore, the use of fluoropolymer processing aids provides the process conditions for using finer filter screens. The use of finer filter screens is an important process measure for filtering out inherent gel particles generated during polymer production.