Simple discussion of problems in the process of dry compounding

(I) Aluminizing layer migration problem

Aluminized film is a new material that has appeared in recent years. The aluminized film not only has the characteristics of plastic film, but also has metal characteristics. It has replaced aluminum foil to a certain extent, and its surface has a dense "aluminum luster", which has a good barrier effect. In order to beautify the product packaging, improve product quality, and reduce packaging costs. Because of the beautiful appearance, low cost, and good barrier properties of the aluminized products, many manufacturers use aluminized composite packaging. At present, it is applied to the packaging of dried and puffed foods such as biscuits, packaging of sour snack foods, and packaging of other medicines and cosmetics. However, in the process of dry composite production of aluminized films, the transfer of the aluminized layer often occurs, which results in a decrease in the interlayer peel strength of the composite film and a decline in the performance of the product resistant contents, and even in the case of serious problems, the quality of the contents of the package is affected. . This is a phenomenon that many composite packaging companies often encounter, and it is also a headache for many flexible packaging manufacturers.

At present, the most widely used aluminized films are VM-CPP (CPP aluminized film) and VM-PET (PET aluminized film). The corresponding composite structures include OPP/VM-CPP, PET/VM-CPP, and PET. / VM-PET, OPP / VM-PET, and several other. However, in practical applications, VM-PET's aluminizing layer transfer phenomenon is relatively small, and VM-CPP aluminizing transfer phenomenon is much more, because polyester PET molecule itself has a certain polarity and aluminum outer layer The combination of electrons forms a superposition of electron clouds so that the fastness between them is stronger. In the past, during the aluminizing process, a layer of linking agent was applied to increase the fastness of the aluminized layer. The quality of this layer of coating will also affect the degree of transfer of the aluminum layer. Now that the process is relatively mature, most aluminum manufacturers have eliminated the coating process, and the corresponding aluminum plating transfer phenomenon has also increased.

The migration of the aluminized layer has a great influence on the permeation of the adhesive itself and residual solvents. The main reason for this is generally a direct factor with the residual solvent. We have tested and produced a composite film with aluminum transfer. The residual solvents exceeded 8 mg/m2. In response to these factors, we must fundamentally solve the problem of “migration” of the aluminum plating layer. In the operation of the composite process, we must pay attention to the following points:

1. To ensure the quality of the aluminized film itself is a prerequisite for solving other problems. If the quality of the aluminized film itself is poor, then the best composite technology and the best adhesives are also ineffective.

2. Use the right adhesive, which is the most effective way to solve the transfer of aluminum plating. In the composite aluminum plating film, an adhesive with a slightly larger molecular weight, a relatively uniform molecular weight distribution, good solvent release, and good coating performance should be selected. Although a binder with a small molecular weight has good coating performance, it has strong intermolecular mobility and penetrates an extremely thin aluminum layer to break the coating. Do not use a binder with a large molecular weight, a non-uniform molecular weight distribution, and poor solvent release, because the solvent itself has a strong ability to penetrate, destroy the coating, and also affect the bonding strength; at the same time, the binder with a large molecular weight is in the production process. The molecular weight is also necessarily non-uniform.

3. To increase the flexibility of the film, when the glue is prepared, the amount of the curing agent is appropriately reduced so that the degree of cross-linking reaction between the main agent and the curing agent is reduced, thereby reducing the brittleness of the film and maintaining good flexibility. Extensibility is conducive to controlling the transfer of the aluminized layer. At the same time, the amount of coating applied when gluing is related to the peel strength of the product. Too much or too little will affect the quality of the product; if too little, it will result in less complex fastness and easy stripping; if too much, it will increase the cost and at the same time Large amount of time, complete curing time is long, adhesive molecules have enough activity to destroy the coating.

4. Reducing the residual solvent of the composite film is the most effective way to solve the aluminum plating transfer. During the printing process, the temperature and speed of the drying tunnel should be increased as much as possible to reduce the residual solvent, especially the residual solvents of toluene, xylene, and alcohols. In dry compounding, the temperature and speed of the drying tunnel should be appropriately increased, and the dilution of the formulated adhesive should also be diluted. The solvent must be of high purity, which is conducive to the volatilization of the solvent.

5. Reduce maturation time. In principle, the aging temperature of ordinary films is generally controlled at 45-50°C, and the compounded products of the aluminized film should in principle increase the curing temperature, adopt a short-term maturation method at a high temperature, and generally have a maturation temperature of 50-60°C. Low temperature for a long time or natural aging. Because the curing at high temperature causes the adhesive molecules to solidify quickly, it does not have enough activity time and activity capacity to reduce the penetration damage.

In the composite processing process, the tension control is also very important and it is worth noting.

(II) Whitening phenomenon of aluminized film

When the aluminized film is combined with the packaging machine'target=_blank>transparent film, it is easy to produce white spots, especially when the white and black printed films are composited with the aluminized film. There are two reasons for this:

1. The hiding power of the ink itself is poor. After the printing film is coated, the ink pigment (titanium dioxide) and the resin are impregnated with the ethyl ester in the adhesive, that is, the ink layer is dissolved by the ethyl ester in the adhesive, and thus looks like Gray and white spots, after the composite ink layer shades, will cause differences in color, to form the so-called spots. After curing, this phenomenon will be alleviated.

2. Adhesive quality is poor. This is not to say that the adhesive itself, but this kind of adhesive has a poor affinity for the aluminized layer, especially OPP printing film and BOPP printing film and aluminized film composite, this glue surface More obvious, mainly because of the high surface tension of the adhesive, poor coating performance, film shrinkage phenomenon occurs, especially high-speed machine composite, uneven coating of glue, causing "spot" phenomenon. At the same time, this kind of glue generally has high initial viscosity and poor solvent release. After aging, due to residual solvents, the strength is not as good as the initial adhesion. The above phenomenon will be more obvious when such adhesives are used in high-speed composites on rolls with more than 150 lines. After the composite of printed matter, the phenomenon of white spots is more obvious.