Polyolefin packaging film appearance effect on printing effect and solution
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Common types of polyolefin packaging films include biaxially oriented polypropylene (BOPP), cast polypropylene (CPP), cast polyethylene (CPE), polyolefin shrink film (POF), and polyethylene blown film (IPE). Etc. There are many factors that affect the printing effect of polyolefin packaging films, including inks, equipment, environment (temperature, humidity), materials, and so on. Among them, due to the selection of raw and auxiliary materials, manufacturing process settings, environmental cleanliness and other factors, the surface/surface of polyolefin packaging film is prone to appearance defects, and some appearance defects have a greater impact on its printing effect.
The appearance of packaging films is divided into many categories. GB/T 2350 "Plastic Terminology and Definitions" defines some of the appearances. The causes of different kinds of appearances, the effects on the printing effect of packaging films, and the solutions are not the same. In this paper, the author mainly looks at the appearance of polyolefin packaging film.
The influence of printing effects and solutions are introduced, and I hope to learn from soft packaging companies.
Crystal points and carbides
The color of the crystal dots on the film is a transparent color of the polymer, and the shape is a circular or elliptical bump. The crystal point is also called an excessive polymer. It is generally believed that the reason is that there is a very small amount of residual catalyst in the polymerization process of the petrochemical enterprise. In the melt extrusion process of the film factory, the residual catalyst continues to catalyze the polymerization of the polymer at a high temperature to form an excessive polymer. The molecular weight of the polymer at the crystal point is higher than the molecular weight of the surrounding polymer, has a higher melting point, and has a higher viscosity in the molten state. The polymer at the crystal point site cannot be uniformly dispersed and fused with the surrounding polymer, and is extruded and cooled by the die, and the surface of the crystal point is rapidly cooled and shrunk to form an appearance defect of the convex surface. Figure 1 shows the appearance of a CPE packaging film. When printing in shallow screens, larger crystal dots may affect the printing effect, causing printing defects (white spots).
It is important to note that some online materials refer to crystal points or carbides as gel points. The 2.42 gel point in GB/T 2350 defines the initial colloidal solid phase that occurs during resin formation. Therefore, crystal points, carbides, and gel points are not only different in cause, definition of terms, and appearance, but also in the film manufacturing process.
The production of carbide is mainly in the extrusion process, the polymer melt sticks to the metal surface of the equipment, and is heated repeatedly for a long time, so that the polymer degrades and carbonizes to form yellow or black defects. The appearance of the CPE packaging film is as shown in the figure. 2 is shown.
In the film production process, the following two methods are generally used to reduce the crystal point and carbide on the surface of the film.
1. Appropriately increase the number of filter meshes or filter layers
Properly increasing the mesh number or the number of filter layers will block some of the larger size carbides from passing through the filter. However, the crystal point is a high polymer which is deformable under heating and shearing conditions. Field management experience has shown that even with increasing mesh counts, most of the crystal points may still deform through the filter while heated. At the same time, it should be noted that appropriately increasing the mesh number or the number of filter layers will increase the back pressure of the extruder and increase the shearing force. For polymers with high shear sensitivity, the melt flow will be affected. Denaturation may also cause degradation of shear sensitive materials and low molecular materials, increasing the probability of carbonization.
2. Add fluoropolymer processing aid
Fluoropolymer processing aids with high polarity, incompatible with olefinic polymers (eg PE, PP), overflowing from polyolefin melts in extruders, in polymer melts and metals A lubricating layer (non-stick layer) is formed on the surface to reduce the viscosity of the polymer melt on the metal surface, and to prevent the polymer melt from being heated for a long time due to sticking to the metal surface, thereby preventing the residual catalyst to the polymer at a high temperature. Continuous polymerization, which reduces the crystal point generated during the film extrusion process, can also effectively reduce the carbonization probability of the polymer.
Anti-stick masterbatch reunion point
Anti-adhesive masterbatch is also known as anti-blocking masterbatch, and some enterprises in the blown film industry also call it open masterbatch.
The anti-stick masterbatch is usually produced by using an inorganic auxiliary agent such as silica or diatomaceous earth, and kneading with a carrier such as PP or PE in a multi-screw extrusion and extruding and granulating. The literature shows that the smallest size that can be seen by the human eye is about 50 microns without the aid of an instrument. Taking a CPP or CPE cast film and a PE blown film as an example, a common anti-stick masterbatch has an average particle diameter of 5 μm, 10 μm, or the like. The 5 10 micron anti-adhesive particles agglomerate to 50 microns, far exceeding the thickness of the film heat seal layer. The human eye can observe the film and see the white point (agglomeration point), as shown in Figure 3. The serious appearance of reunion is caused by printing and printing (white spots).
In order to prevent the occurrence of anti-adhesive masterbatch agglomeration on the surface of the film, the anti-adhesive masterbatch manufacturer and the film manufacturer should make the following adjustments.
1. Anti-stick masterbatch manufacturer
Adhesive additives with suitable particle size and narrow particle size distribution should be selected, suitable extrusion equipment and screw combination, and the extrusion process should strengthen the shear mixing to avoid the agglomeration of the anti-adhesive agent; at the same time increase the mesh number or filter properly. The number of mesh layers blocks large agglomeration points from passing through the filter.
2. Film manufacturer
Choose a suitable anti-stick masterbatch, choose the appropriate filter type, increase the mesh number or the number of filter layers, and block large agglomeration points from passing through the filter.
Frosting, defrosting
The definition of frosting in 2.432 of GB/T 2350: a defect similar to the micro-crystal light scattering surface. Related definitions include defrosting: exudates or powders visible on the surface of plastic articles.
Film blooming usually means that there is a thin layer of visible frost (powder) on the surface of the film, which is foggy to the light observation film. The value of the film haze (some of the early literature translated as "turbidity") is increased by the instrument. The transparency is reduced, the sharpness is lowered, the gloss is lowered, and there is a trace after wiping off. The BOPP film appearance is shown in Figure 4. One of the causes of film blooming is that a large amount of a small molecular weight auxiliary having poor compatibility with a polymer polymer diffuses and migrates to the surface of the film.
Corona treatment is to accelerate the electrons away from the electrode at high voltage. The film is placed in the electric field to become a medium for blocking conduction. Under the action of the electric field, high energy is obtained, and a polar radical group is formed on the surface of the film to generate hydroxyl-OH. A polar group such as a carboxyl group-COOH or a carbonyl group > C=O.
The diffusion of the small molecular weight auxiliary agent is diffused from the high concentration region (layer) to the low concentration region (layer). Therefore, the small molecular weight auxiliary agent can diffuse and migrate to the surface of the thin film heat seal layer, and can also migrate to the surface of the thin film corona layer. Experience and test results show that the probability of migration of small molecular weight additives onto the surface of the corona layer (polar layer) will be greater, which will cause the surface tension to decay rapidly, affecting the printing and composite effects, resulting in printing leakage (white spots). Even large areas cannot be printed. Fig. 5 shows the results of infrared spectrum test of PE packaging film. It is not difficult to find that the amide characteristic peak which is usually used as a smooth masterbatch appears on the surface of the corona layer.
It is worth noting that the film heat seal layer is severely frosted, which not only affects the appearance, but also reduces the heat seal strength of the film. In severe cases, the film may lose the heat seal effect.
In order to prevent the film surface from blooming, the masterbatch manufacturer and the film manufacturer should make the following adjustments.
Masterbatch manufacturer
Production of migration masterbatch, especially antistatic, smooth, anti-fog and other masterbatch, should choose the appropriate additives and ratio.
2. Film manufacturer
Appropriate antistatic, smooth, anti-fog and other migration masterbatch should be selected. According to the total thickness of the film, the thickness of the film layer, the characteristics of the film manufacturing equipment, and the aging time, the ambient temperature and humidity of the downstream customers use the film, and the downstream color printing composite method. Under such conditions, a reasonable proportion of masterbatch is designed to minimize or avoid serious frosting before printing.
Foreign body adhesion
During the film manufacturing process, the melt is still in a high-temperature uncooled setting state after being extruded from the die lip, and it is easy to adhere to the floating fine particles and dust; in the preheating stage, when the metal or the press roller has foreign matter, it is easy to press the foreign matter into the film. Film/sheet surface. Most adhering foreign matter affects the printing effect, so high requirements are placed on the cleanliness of the film production workshop. Figure 6 shows the adhesion of the foreign body of the CPE.
Bandage and edging
Due to the large deviation of the thickness of the film, the thicker points are concentrated in one place or the unwinding tension is not properly arranged, and the unevenness of the distribution on the width and width is higher than that of other parts of the film roll, which is usually called a film. After the aging, the film will appear stress relaxation phenomenon, and permanent deformation will occur at the ribs. When the film is unfolded, it will appear loose and tight. The gluten and the side will affect the printing effect.
Mould line
A lip line generally refers to a fine continuous line of melt that is observed to be incompletely cooled at the lip of the die. One of the reasons for the lip line is that carbide or foreign matter accumulates on the inner wall of the die, a new melt flows out, passes through the carbide accumulation point, marks on the surface of the melt, and after extrusion cooling, it forms on the surface of the film. Continuous linear appearance. Severe lip lines accumulate on the surface of the film roll to form ribs, which affect the printing effect.
The lip line is the most common appearance defect in CPE film production. At present, a better solution is to use a linear low-density polyethylene resin with a large melt index to reduce the temperature of the extruder, the joint pipe, the die, and reduce the carbonization probability of the resin.
As mentioned above, the causes of different types of appearance defects, the effects on the printing effect of packaging films and the solutions are different. Therefore, comprehensive judgments should be made based on the actual conditions such as raw materials and process conditions by means of instruments such as perspective magnifiers. Deal with solutions to quickly eliminate defects and reduce corporate losses.