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The white spots and bubbles in recyclable laminated packaging are the most common, frequent, and headache inducing problems in laminated packaging. There are many reasons for their occurrence. These issues will be discussed and researched in detail in the article on laminated packaging in the future. Today, we will first talk about the problem of white spots and bubbles caused by printing in recyclable laminated packaging.
There are not many reasons why printing can cause lamination white spots. In summary, there are roughly five conditions: printed film roughness, poor affinity with glue, solvent residue, uneven film, and moisture absorption of the printing film. As these five conditions flow to the lamination process, they may have an impact on laminating white spots and bubbles. We will analyze and explore them one by one:
The roughness of the ink film refers to the surface roughness of the ink film formed after the ink is dried. If the roughness of the ink film is large, a large amount of glue is needed to fill and wrap the rough surface of the ink film during the coating of the composite adhesive. Once the glue cannot fill and level the surface of the ink film, the areas not fully coated with glue will produce composite white spots.
It is estimated that most people simply understand the roughness of ink film as the fineness of ink or the roughness of ink pigment particles. The formation of ink film is naturally related to the fineness of pigment or ink, but the roughness of ink film is not only determined by the fineness of pigment particles, but also by the roughness of resin and the overall flow and film-forming performance of ink. Even if the fineness of the ink is high, the ink’s leveling and film-forming performance is poor, and the ink cannot form a good film on the surface of the printed film, which will also make the surface of the ink film rough.
The rougher the ink film, the more uneven its surface and the more gaps it has. When compounding, the first step is to fill the uneven surface of the composite substrate (film and ink film surface) with glue, and apply a certain thickness of glue layer on the surface of the composite film in order to firmly bond different films together. The rough ink film surface is mixed between the film and the adhesive. Due to the rough surface, it is necessary to increase the amount of adhesive to fully fill it and avoid the formation of white spots.
The occurrence of white spots due to rough ink film follows a certain pattern:
By using the above points to preliminarily determine whether the problem is caused by the roughness of the ink film, solving the problem of white spots caused by roughness is simple. Basically, as long as the following points are paid attention to, solving the problem of ink film roughness is relatively easy:
It should be noted that when paying attention to the roughness of the ink film, attention should also be paid to the amount of glue applied and the film-forming and leveling properties of the glue itself. The insufficient amount of glue applied is the main cause of white spots and bubbles. In light colored ink printing with low coverage, careful observation is more obvious. Therefore, blindly pursuing cost control and reducing the amount of glue applied is not advisable. Too little glue application makes it extremely difficult for the glue solution to be evenly coated to form a complete glue film. Once encountering rough ink or film, it poses a great risk of white spots or bubbles.
The surface tension value of the thin film ink film surface is too low, or the affinity with the adhesive is poor, resulting in the coating of the adhesive liquid on the ink film surface being unable to wet and level, causing the liquid surface to shrink and unable to form a complete adhesive film, thereby producing white spots.
The reasons for white spots caused by poor affinity with glue are as follows:
The white spots generated due to the poor affinity between the ink film surface and the adhesive are usually concentrated in a certain color. Upon careful observation, the formed white spots exhibit irregular shrinkage resembling pinholes, especially during the application of the adhesive. This can be basically judged as the problem. When encountering such situations, first check the corona value of the film, and detect the surface tension value of the ink film that produces shrinkage with 37 dynes of corona water. Ink films that do not reach the corona value can be basically determined to have a problem with their surface tension value.
The problem of poor affinity with glue can be solved by fully considering the following issues:
For those who have already encountered this problem, as the printing has been completed, measures such as increasing the amount of glue applied and raising the processing temperature can only be taken to alleviate the problem as much as possible.
Due to the incomplete drying of printing ink, solvent residues exist, causing solvent molecules to be sandwiched between the two layers of film after lamination. The slow escape of solvent leads to the generation of bubbles.
Whether it is printing or lamination, if the solvent is not completely evaporated, the residual solvent will gradually overflow from the ink film or adhesive film, forming uniform white spots or small bubbles in the lamination film.
The white spots and bubbles generated due to excessive printing residue often appear in areas with large areas of multi-color overlay printing. The position and size of the bubbles do not have a clear pattern, and there may not be many when the composite is just removed from the machine. With aging and time, the white spots and bubbles not only do not decrease, but gradually increase, forming large bubbles or dense white spots.
Due to the generation of bubbles caused by residual solvents, in addition to reducing the printing speed, improving the ink drying performance, and increasing the drying capacity of the oven at the printing end to ensure thorough drying of the ink, the drying temperature of the oven at the composite end should be appropriately increased to reduce the composite speed, and the exhaust air volume should be increased to fully evaporate the solvent. After detecting excessive residual solution in the already printed film, it can be re opened for secondary drying to ensure that residual solution does not exceed the standard before composite processing.
Due to poor winding, the surface of the film after unfolding was uneven. After the composite unwinding, the film was not completely flattened. When entering the composite coating, the unevenness of the film caused white spots on the adhesive due to uneven coating.
The phenomenon of white spots caused by uneven film is mostly due to horizontal wrinkles caused by poor winding. When entering the glue application area, the film cannot be fully stretched, resulting in insufficient glue application and white spots.
The white spots generated by the unevenness of the film are highly regular, often appearing as horizontal regular white spots. The center position of the film wrinkles is significantly more, and the two ends gradually decrease. Upon careful observation, there are still some traces of dark wrinkles on the film.
To solve such problems, it is necessary to keep the printing roll in good condition, and not roll it too tightly or too loosely. Tightness can cause wrinkles in the film, while looseness can result in unstable tension during rolling. Additionally, the weight of the film during rolling can cause it to shake, resulting in deformation of the loose film due to stacking. For films with poor winding quality, rewinding can be carried out in advance to flatten the film before lamination to ensure coating stability.
The moisture absorption performance of the film can also produce composite white spots. The reason is that the moisture content of the film is too high after it absorbs moisture. After compounding, water molecules react chemically with isocyanate groups (- NCO) in the adhesive curing agent, generating carbon dioxide gas. The carbon dioxide gas is trapped in the middle of the composite layer of the film and cannot penetrate, forming white spots.
The white spots caused by the moisture absorption of the film usually do not appear when the composite is just removed from the machine. With the aging and extension of time, the white spots gradually appear and mostly appear at the two ends of the film roll, with no obvious regularity in their position and size.
To solve the problem of carbon dioxide gas and white spots caused by moisture absorption in printing, it is necessary to control the humidity of the printing and composite environment. When printing on thin films that are prone to moisture absorption, if there are excess vacant color groups, they can be heated and baked through the film to reduce the moisture absorption of the film. When printing the film under the machine, it should be quickly wrapped with moisture-proof film such as aluminum foil, and the composite should also avoid the exposure time of the film in the air as much as possible, and avoid the film from getting damp during storage and transportation. If conditions permit, the film can be placed in an oven for low-temperature baking and dehumidification before processing.
Longdapac as the leading recyclable mono-material PE pouches factory in China. We are working hard on the packaging quality. The lamination of recyclable package is difficult, but we are able to make it perfect with the efforts from our engineers and experienced workers.