Dry lamination is a process in which two or more films are bonded together with an adhesive using dry lamination equipment. In plastic composite flexible packaging , dry lamination is one of the most commonly used production methods. Due to its wide choice of substrates, high composite strength, convenient printing and fast production speed, it occupies an important position in the packaging industry. . However, in dry composite products, air bubbles often appear, which not only affects the aesthetics of the product, but also affects the oxygen barrier, breathability and peel strength of the composite film. This problem has long plagued flexible packaging manufacturers. There are many reasons that lead to the generation of bubbles, and there are factors that cause bubbles to appear in almost every link in production. The author analyzes this in detail in combination with years of production experience and proposes corresponding solutions.
The process of dry composite process is roughly as follows: raw material inspection → compounding → first unwinding → gluing → drying → second unwinding → compounding → rewinding → curing → delivery. The author analyzes the causes and countermeasures of bubbles from the production environment and the entire process flow.
I. Impact of the production environment
When the ambient temperature is too high, the two-component adhesive is easy to be cloudy, sticky, and poor in coating performance, resulting in air bubbles. The low temperature results in poor dispersion of the adhesive, poor wetting and adhesion to the film, and the generation of air bubbles. Generally, the workshop temperature needs to be controlled at 18 ~ 25 ° C.
The ambient humidity is too high (more than 70%), condensation will occur at the anilox roller and rubber roller, and the reaction between moisture and the curing agent will generate carbon dioxide, which will eventually cause air bubbles. In addition, plastic films are prone to absorb moisture in the air, especially plastic films with high hygroscopicity, such as nylon and cellophane, which cause bubbles to form after lamination. It is advisable to keep the relative humidity between 60% and 70%.
The air contains a large number of particles such as dust, impurities, etc. These particles may be adsorbed on the plastic film, or fall on the coating roller, composite roller, or glue solution, which may cause the generation of air bubbles. Therefore, it is necessary to keep the workshop clean and hygienic, reduce the dust in the air, and clean the drying tunnel frequently, and try to keep the workshop in a good sealed state.
The film surface of the material has dust adsorption, particle impurities, poor plasticization and other crystal points sandwiched in the film.
Moisture absorption on the film surface of the material and precipitation of additives in the material can cause compound bubbles.
The surface tension of the film is too small, the wettability of the adhesive on its surface is relatively poor, which may lead to insufficient coating amount, and air bubbles will be generated in places with little or no glue.
The thickness of the film should be as uniform as possible. If the distribution is uneven, it is easy to cause the imbalance of the compound pressure, and there will be gaps in the place where the pressure is small, thereby forming bubbles.
The way to avoid these effects is to do a good job of raw material inspection, and it is best to increase the online corona while producing.
Third, the impact of adhesive
During the preparation of the glue solution, a large amount of air was mixed due to stirring and circulation, and it could not be discharged in time when it was still degassed, and the air bubbles were formed in the composite film. The correct method is to formulate the adhesive strictly according to the instruction manual, first add the main agent, then add ethyl acetate and stir evenly, then add the curing agent, and add while stirring to make the curing agent full of activity and uniformly dispersed. It is good for affinity with the printing substrate and reduces air bubbles. In addition, the prepared glue solution must be fully still and degassed, and then poured into a plastic tray, and it should be slowly poured along the edge of the plastic tray when pouring , Do not vigorously agitate the glue in the rubber plate, so as to avoid the formation of air bubbles in the composite film due to the mixing of air.
The viscosity of the adhesive has a very important influence on the fluidity, wettability, and uniformity of the adhesive. In dry compounding, the viscosity value of the adhesive should be controlled within an appropriate range. If the viscosity of the adhesive is too high, the dispersibility and leveling may be poor, affecting its wettability on the surface of the composite substrate. Failures such as missing coating and air bubbles may occur during the gluing process.
When using a two-component two-liquid reactive adhesive, in order to ensure uniform coating and facilitate wetting, a diluent needs to be added to the adhesive to dilute it. Diluents include toluene, ethyl acetate, and methyl ethyl ketone. Toluene is being phased out because it is not good for the environment and it can easily lead to bubble generation. Methyl ethyl ketone is used less because of its high price and its poor affinity with adhesives. The most commonly used diluent is ethyl acetate. The diluent must not contain water, alcohol, amines, etc. If the water content exceeds 0.2%, because the volatilization rate of water is much lower than the volatilization rate of ethyl acetate, the volatilization rate of the solvent will be greatly reduced, and the spreading and hardening of the adhesive will be affected. After the BOPP film and the CPP film are compounded, the composite product will generate large bubbles and irregularly diffuse outward. After curing, the peeling strength at the large bubbles is extremely poor, as if there is no sizing. Therefore, the purity of the diluent should be above 98%.
Fourth, the impact of coating
A small amount of glue may also cause air bubbles. If the amount of glue is insufficient, small bubbles will be trapped in the gap. The sizing amount is related to the shape, depth, line number and other parameters of the anilox roll. If there is dry liquid glue at the deepest point of the anilox roll, the volume of the mesh will become less and less, and the sizing amount will be much less After being worn for a long time, the anilox will become shallower and lighter, and the amount of glue will decrease. Therefore, the anilox roller should be cleaned after a period of time. If it is used for too long, it needs to be replaced. In addition, the sizing amount is related to the concentration of the glue solution, the softness and hardness of the coating roller, the pressure, the angle of the doctor blade, and the pressure. It should be adjusted according to the actual situation.
Five, the impact of the drying process
Drying is an important factor in dry compounding. The drying temperature is too low, the adhesive is not sufficiently dried, and the solvent is not completely volatilized, which will produce small air bubbles. If the drying temperature is too high, the adhesive will foam, or the skin of the adhesive layer will condense, which will affect the volatilization of the solvent inside the coating layer, and also cause the residue of the solvent and the formation of fine bubbles. Generally, the drying tunnel of the dry compound machine is divided into 3 regions, namely: an evaporation region, a hardening region, and an odor elimination region. In actual production, the temperature of these three sections should be adjusted accordingly according to the actual composite substrate. Generally speaking, the temperature of the three stages is controlled to 50 ~ 60 ° C, 70 ~ 80 ° C, 80 ~ 100 ° C, respectively, and the temperature increases gradually. It is strictly forbidden to enter the drying tunnel mouth with a high initial temperature, and the temperature in the odor-removing zone is low. This is because an excessively high initial drying temperature is not conducive to the solvent volatilizing from the inside to the outside, and cannot be completely and completely dried. Substances ", resulting in the generation of air bubbles.
Impact of recombination
If the composite pressure is too small, the adhesion between the composite substrates is not tight, which will cause the generation of small bubbles. Generally, the composite pressure is controlled in the range of 0.15 to 0.4 MPa.
If the composite angle is too large, air will be easily trapped between the two layers of the composite film, which will cause bubbles and tunneling, especially when the film material is thicker and harder. Bubbles are more likely to occur. In this case, the smaller the composite angle The better, generally 35 ° is appropriate.
The unevenness of the compound pressure roller will also lead to the generation of air bubbles. Such air bubbles are periodic. As long as the foreign material is cleaned or the uneven rubber roller is ground or replaced, it can be solved. The requirement is that the rubber roller is clean, free of foreign materials, and smooth.
If the temperature of the compound heating roller is not enough, air bubbles will also be generated. The purpose of heating is to melt and flow the dry adhesive, eliminate small holes and small gaps, exhaust air under pressure, and tightly iron the two films. Generally, the laminating substrate of the composite equipment is close to the heat roller, and the heat is transferred from the adhesive layer to CPP or PE. This type of equipment has a preheating roller between the second unwinding substrate guide roller, but most customers do not use it. That is, without heating. When the temperature is low and the PE or CPP is thick, although the temperature of the composite heat roller is not low, the heat of the thicker PE or CPP is large. The short time of the heat roller lamination process cannot make the temperature of the substrate rise quickly. When one side is cold, the fluidity of the adhesive in the middle is greatly reduced, so that air bubbles are easily generated.
Impact on curing
In actual production, sometimes the drying is sufficient and the sizing amount is quite large, but the composite film just after the machine still has air bubbles. In fact, it is unlikely that the freshly compounded product has no air bubbles. Small air bubbles can be eliminated by aging. Because the polyurethane adhesive that does not contain solvent but is not fully cured has a certain fluidity, the higher the temperature, the greater the fluidity, the larger the molecular weight, and the smaller the fluidity. The essence of curing is the process of increasing molecular weight. Film, adhesive slowly leveling under the action of temperature, large bubbles shrink, small bubbles disappear, until the molecular weight of the adhesive increases until the glue can not flow. Sometimes the temperature is not enough, and the time is not enough, so even the small bubbles cannot be eliminated.
There are many reasons that lead to the production of dry composite bubbles. As long as we conscientiously sum up experience in practice, study hard, and analyze the nature of raw and auxiliary materials and field equipment conditions to solve these problems.