A brief discussion on the problems in the dry comp

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A brief discussion on the problems in the process of dry compounding many industry insiders interviewed revealed that

(I) migration of aluminized layer

aluminized film is a new material that has emerged in recent years. The aluminized film has both the characteristics of plastic film and metal, replacing aluminum foil to a certain extent. Its dense aluminum luster on the surface has a good barrier effect, thus beautifying the product packaging and improving the product grade, And reduce the packaging cost. Because the aluminized film products have beautiful appearance, low price and good barrier performance, many manufacturers use aluminized film composite packaging. At present, it is used in the packaging of dry and puffed food such as biscuits, the packaging of sour plum snack food and the outer packaging of some other medicines and cosmetics. However, in the dry composite production process of aluminized film, the transfer of aluminized layer often occurs, which leads to the decrease of the interlayer peel strength of the composite film and the performance of the product's tolerance, and even affects the quality of the packaging contents. This is a phenomenon often encountered by many composite packaging manufacturers, and it is also a headache for many flexible packaging manufacturers

at present, VM CPP (CPP aluminized film) and VM remove the sample pet (PET aluminized film) are the most commonly used composite aluminized films. The corresponding composite structures include OPP/VM CPP, PET/VM CPP, PET/VM pet, OPP/VM pet, etc. However, in practical application, the transfer of aluminum plating layer of VM pet is less, while the transfer of aluminum plating layer of VM CPP is much more. This is because the polyester PET molecule itself has certain polarity and combines with the outer electrons of aluminum atoms to form an electron cloud superposition, so that the fastness between them is stronger. In the past, a layer of bonding agent was applied during the aluminum plating process to increase the fastness of the aluminum plating layer. The quality of this layer will also affect the degree of aluminum layer transfer. Now, due to the relatively mature process, most aluminum plating manufacturers have omitted the coating process, and the phenomenon of aluminum plating transfer has increased accordingly

the migration of aluminum plating layer is greatly affected by the penetration of the glue solution itself and the residual solvent. Generally, the main causes are directly related to the residual solvent. We have conducted experiments and found that the residual solvent of the composite membrane causing aluminum plating transfer is more than 8mg/m2. In view of these factors, in order to fundamentally solve the migration problem of aluminum coating, we must pay attention to the following points in the operation of composite process:

1. ensuring the quality of aluminum coating itself is the prerequisite for solving other problems. If the quality of aluminum coating itself is poor, the best composite process and the best adhesive can do nothing

2. selecting appropriate adhesive is the most effective way to solve the transfer of aluminum coating. In the composite aluminum coating, the adhesive with slightly larger molecular weight, more uniform molecular weight distribution, good solvent release and good coating performance should be selected. Although the adhesive with small molecular weight has good coating performance, it has strong intermolecular activity and destroys the coating through the extremely thin aluminum layer. Do not choose adhesives with large molecular weight, uneven molecular weight distribution and poor solvent release performance, because the solvent itself has strong penetration ability and destroys the coating, as well as Ningbo Institute, Institute of process, Institute of Oceanography, Institute of chemistry, Institute of physics and chemistry, National Nanotechnology center, Suzhou Institute of Nanotechnology and North Materials Institute within the system of Chinese Academy of Sciences, which will affect the bonding strength; At the same time, the molecular weight of adhesive with large molecular weight is bound to be uneven in the production process

3. enhance the flexibility of the adhesive film. When preparing the adhesive solution, appropriately reduce the amount of curing agent to reduce the degree of cross-linking reaction between the main agent and the curing agent, so as to reduce the brittleness of the adhesive film, keep it flexible and extensible, and help to control the transfer of the aluminized layer. At the same time, the coating amount is related to the peel strength of the product, too much or too little will affect the quality of the product; Too little will result in small composite fastness and easy peeling; Too much will increase the cost. At the same time, with a large coating amount, the complete curing time will be long, and the bonding molecules will have enough activity ability to damage the coating

4. reducing the residual solvent of the composite film is the most effective way to solve the transfer of aluminum plating. During the printing process, the temperature and wind speed of the drying channel shall be increased as much as possible to reduce the residual solvents, especially the residual solvents of toluene, xylene and alcohol. During the dry compounding, the temperature and wind speed of the drying channel shall also be appropriately increased. In addition, the diluent solvent for preparing the adhesive must be of high purity, which is conducive to the volatilization of the solvent

5. reduce the ripening time. In principle, the ripening temperature of ordinary film is generally controlled at 45 ~ 50 ℃, while the ripening temperature of composite products coated with aluminum film should be increased in principle. The ripening method of high temperature and short time should be adopted. Generally, the ripening temperature is between 50 ~ 60 ℃, and the low temperature and long time or natural ripening should be avoided. Because high temperature curing will make the adhesive molecule solidify rapidly, so that it does not have enough activity time and activity capacity, and reduce the penetration damage

in the process of compound machining, the control of tension is also particularly important, which is a noteworthy link

(II) white spot phenomenon of aluminized film=_ Blank> it is easy to produce white spots when the transparent film is compounded, especially when the white and black printed film is compounded with the aluminized film. There are two reasons:

1. the covering power of the ink itself is poor. After the printing film is coated, the pigment (titanium dioxide) and resin of the ink are infiltrated by the ethyl ester in the adhesive, that is, the ink layer is dissolved by the ethyl ester in the adhesive, so it looks gray white spots. After compounding, the depth of the ink layer is different, which will cause color differences and form the so-called spots. After curing, this phenomenon will be reduced

2. the poor quality of the adhesive does not mean the adhesive itself, but the poor affinity of the adhesive to the aluminized layer, especially when the OPP printing film and BOPP printing film are combined with the aluminized film. The surface of the adhesive is more obvious, mainly because the adhesive has high surface tension and poor coating performance, and the adhesive film has shrinkage, especially when the high-speed machine is combined, and the glue is coated unevenly, resulting in spots. At the same time, this kind of adhesive generally has high initial adhesion and poor solvent release. After curing, due to the influence of residual solvent, the strength is not as good as the initial adhesion. The above phenomenon will be more obvious when this kind of adhesive is used on rolls above 150 lines at high speed. After the printed matter is compounded, the phenomenon of white spots is more obvious

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