Causes and Countermeasures of Dry Compound Bubbles

In dry composite products, bubbles often appear, not only affect the appearance of the product, but also affect the oxygen barrier, breathability and peel strength of the composite film. There are many reasons that lead to the generation of bubbles, and the reflection forms of bubbles are also varied. This is combined with years of practical experience to analyze and propose solutions.

Effect of adhesives

1. Kind of adhesive

The adhesives for dry compounding are divided into single-component hot-melt adhesives and two-component two-liquid reactive adhesives.

One-component hot melt adhesives have a much higher probability of generating bubbles than two-component two-liquid reactive adhesives, and bubbles in the finished product caused by them are not eliminated. This is because the single-component hot-melt adhesives reach the drying tunnel for drying after being coated by the top roller, and the aging of the finished product is no longer performed, and the diffusion of molecules in the adhesive is relatively weakened. Of course, two-component two-component reactive adhesives, if not properly selected, can also cause bubbles to form. When choosing the two-component two-liquid reactive adhesive, the regular manufacturers should use the adhesive with slightly larger molecular weight, uniform molecular weight distribution, excellent solvent release performance, and good leveling property to effectively reduce the probability of bubble generation.

In general, general-purpose adhesives have better leveling than cooking adhesives, but for cooking products that require cooking, a special cooking-resistant adhesive must be used.

2. The deployment and use of adhesives

Prior to the formal compounding, adhesives should be formulated in strict accordance with the instructions for use. The correct operation method is: first add the main agent, then add ethyl acetate and stir evenly, then add the curing agent, and add while stirring. The formulated adhesive should be clear and transparent. Our company uses PU-1975 adhesive with a solids content of 75%. The blending ratio is: main agent: ethyl acetate: curing agent = 1:2.5:0.2. In actual production, the solid content of the adhesive should be compatible with the coated anilox roller.

Our company's management of adhesives is very strict, first pumping with a vacuum pump, and then pour it into a plastic disc for coating. When the adhesive is poured into the plastic tray, the movement should be gentle to prevent the impact force from being too large, and the air is mixed into the adhesive and bubbles are generated. To this end, our company has developed a gluing device on its own. The specific method is: after processing an iron drum (holding adhesive), connect a plastic flow tube, and set a switch on the flow tube. Used to control the flow of adhesive. When the adhesive is added through the device, the impact force is reduced so that air cannot enter the glue liquid, thereby reducing the probability of bubble generation.

3. Use of thinner

When using a two-component two-liquid reactive adhesive, it is necessary to add a diluent to the adhesive to dilute it in order to ensure uniform coating and facilitate infiltration. Diluents are toluene, ethyl acetate, methyl ethyl ketone and so on. Toluene is gradually being eliminated because it is not conducive to environmental protection and it easily leads to the generation of bubbles. Methyl ethyl ketone is less used due to its high price and poor affinity with adhesives. The currently used diluent is ethyl acetate.

Diluents must not contain water, alcohols, amines, etc. If the water content exceeds 0.2%, the rate of volatilization 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 compounding the BOPP film and the CPP film, the composite product will generate large bubbles and irregularly diffuse outwards. After aging, the peel strength at the large bubbles is extremely poor, like no glue. Therefore, the purity of the diluent should be above 98%.

Effect of residual solvents

After the composite film substrate is coated with a sizing agent and dried in a drying tunnel, it is required that the solvent evaporates as clean as possible. Otherwise, the residual solvent will not only produce odor, but also form fine bubbles on the cooled composite film.

Drying machine drying tunnel is divided into three areas, namely: evaporation area, hardening area, exclusion of odor areas. In actual production, the temperature of these three zones should be adjusted according to the actual composite substrate. In general, the temperature of the three stages is controlled to be 50 to 60°C, 70 to 80°C, and 80 to 100°C, respectively, and the temperature increases in a gradient. It is strictly forbidden to enter the baking mouth and the initial temperature is high, and the temperature of the odor eliminating zone is low. This is because the excessively high baking temperature is not conducive to volatilization of the solvent from inside to outside and cannot be completely dried, but forms a "ring" on the surface of the substrate. "" results in the creation of air bubbles.

Effect of aging time

If the compounded product is not fully matured, it is also easy to produce tiny bubbles. As the curing time increases, the bubbles in the product gradually decrease. When using PU-1975 adhesives, the curing time is generally 24-48 hours, and some products require a longer maturation time.

Effect of anilox roll

The role of the anilox roller is to evenly apply the adhesive on the surface of the composite substrate. The amount of coating is determined by the depth of the anilox roller and the working concentration of the adhesive. Under normal circumstances, the anilox roller can only apply 1/6 ~ 1/4 of the adhesive to the substrate under the pressure of the rubber roller. Therefore, the anilox roller must be kept clean without clogging. Otherwise, the adhesive will occur. Poor adhesive transfer, uneven coating, and finally the composite material generates air bubbles.

It is best to periodically clean the anilox roll and use a special detergent. When the machine is not working, the anilox roller should also be cleaned and the rubber roller and the anilox roller separated.

Environmental impact

The temperature and humidity of the environment also have a great influence on the quality of composite products.

When the ambient temperature is too high, the two-component adhesive tends to become turbid, resulting in poor coating performance and bubble generation. Therefore, the adhesive should be used with the match, and how much with the amount of use. If the ambient temperature is too low, the flowability of the adhesive will deteriorate, and bubbles will also be generated. Especially in winter, it is often found that the adhesive becomes thick and the leveling property becomes worse. In response to this phenomenon, the company first matured the main agent for 1 to 2 hours, and then deployed it. The leveling property is much better, but it needs to be guarded by dedicated personnel. The ambient temperature is generally controlled at 23 ~ 25 °C.

The environment humidity is too high (more than 70%). Condensation may occur at the anilox roll and the rubber roll. Reaction of water and curing agent will produce carbon dioxide, which will eventually lead to bubble generation. The relative humidity control is preferably 50% to 60%.

Effect of mechanical factors

The bubbles produced by the composite film are sometimes very regular, such as repeated air bubbles in the same location. The regular bubbles are often caused by mechanical reasons, such as the uneven surface of the rubber roller or the hot roller, or the low temperature of the hot roller, the pressure is not enough, or the angle of the composite film is too large. Among them, the excessively low temperature of the hot roller is not conducive to the further activation of the adhesive resulting in the generation of bubbles. The temperature of the hot roller should be adjusted within the range of 60-80°C according to the actual situation. The pressure is generally adjusted within the range of 0.15 to 0.4 MPa. The angle of the composite film is too large, and it is easy to bring air into the film and generate bubbles. The angle is generally 35 °C.

There are many reasons for the formation of dry composite bubbles, and we need to carefully summarize and communicate with each other in our usual work.