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Polyurethane adhesive has excellent bonding performance and is suitable for bonding various flexible packaging film materials, such as transparent and printed plastic films, various aluminum plated films, aluminum foil, paper, etc. Solvent free polyurethane adhesives are increasingly being used in flexible packaging composites due to their solvent-free nature, no VOC emissions during the compounding process, environmental safety, high efficiency, and energy saving.
However, some solvent-free polyurethane adhesives exhibit a mismatch with ink during the compounding process, such as ink dissolution. The consequence of ink melting phenomenon is that the composite flexible packaging film can be directly scrapped and cannot be reused, causing serious losses to packaging and printing enterprises.
Dissolved ink refers to the phenomenon where the printed surface becomes blurred and the printed pattern becomes blurred after the adhesive is compounded into a flexible packaging composite film. When composite aluminum plating and aluminum foil structures are used, ink dissolution not only causes the printing surface to become blurred, but also manifests as aluminum penetration and leakage.
In principle, the problem of ink dissolution in solvent-free composites is caused by the re dissolution of printing ink by solvent-free polyurethane adhesives. Some types of solvent-free polyurethane adhesives are coated on thin films, composite into rolls, and enter the curing chamber for curing. During the compounding process and the initial curing time, the solvent-free adhesive is still in a liquid state, in close contact with the printing ink and has a certain degree of solubility in the ink, resulting in ink dissolution phenomenon.
The theory of “similar compatibility” can explain the phenomenon of ink dissolution, that is, adhesives and ink formulations have similar chemical structures and can dissolve in each other. Under certain external conditions such as temperature, composite products are prone to ink dissolution.
Dissolved ink is closely related to printing ink, including factors such as solvent residue in printing ink and the formula of the ink itself. The components of ink generally include ink resin, pigment, auxiliary resin, solvent, etc. The ink resin of polyurethane integrated ink is polyurethane resin, commonly used pigments include titanium dioxide, and commonly used solvents include ethyl acetate, isopropanol, etc. Polyurethane integrated ink itself requires a certain degree of solvent solubility due to the requirements of printing technology.
Residual solvents and water in ink wash
If the solvent residue of printing ink is high, there will still be a lot of solvent components in the printing ink layer, and the ink will be in a false dry state. After compounding with solvent-free adhesives, the ink pigment particles in the false dry state will be locally rearranged, which is very easy to cause ink dissolution.
If there is a high residual moisture in water-based ink, the same problem can also occur.
Ink resin and auxiliary resin
The formula of ink dissolution and ink itself is also directly related. Different ink resins and auxiliary resins have different solubility in solvents due to their chemical structures. When matched with certain types of solvent-free adhesives, their solubility may vary. Once the ink resin, ink auxiliary resin, and solvent-free adhesive have similar and compatible structures, different degrees of ink solubility will occur.
The chemical structure of ink determines whether and to what extent it dissolves.
Ink pigments
Dissolved ink is related to the pigments in the ink, such as peach red ink and transparent yellow ink, which are very prone to soluble ink. Even when compounded with solvent based polyurethane adhesives, they are very easy to dissolve ink.
Solvent-free polyurethane adhesive
The chemical structure and composite process of solvent soluble and solvent-free polyurethane adhesives are also closely related.
Different solvent-free adhesive formulas have different ink solubility if they do not match the ink. After analysis and research, it was found that some types of polyester based solvent-free polyurethane adhesives have a higher probability of ink dissolution when matched with polyurethane integrated ink; When polyether solvent-free polyurethane adhesive is matched with polyurethane integrated ink, there will be no ink dissolution or the probability of ink dissolution is very small.
Solvent-free composite process
The temperature, pressure, and amount of glue applied in solvent-free composites have an impact on the degree of ink dissolution. The higher the coating composite temperature, the greater the composite and winding pressure, and the larger the amount of glue applied, the more severe the ink dissolution occurs. This can also be explained by the theory of “similar compatibility”.
Ink transfer
The bonding strength between solvent-free polyurethane adhesive and polyurethane ink is very high, far higher than the bonding strength between polyurethane ink and some substrate films such as BOPP. Therefore, during peeling, it is easy to cause ink transfer to the adhesive layer, that is, ink transfer.
The mainstream ink in the current market is polyurethane ink, commonly used solvents include ethyl acetate, isopropanol, etc. Alcohol solvents contain active hydrogen and are prone to react with the A component (NCO group) of solvent-free polyurethane adhesives; The water in water-based ink will also react with component A of solvent-free adhesive, while alcohol or water are weak points in the printing ink layer. As the solvent-free adhesive solidifies, ink transfer is more likely to occur at these weak points, resulting in ink leakage.
The above phenomenon is actually the result of multiple factors such as solvent residue, ink transfer, and adhesive curing, which are consistent with the phenomenon of ink dissolution.
Increasing the molecular weight of ink resin is beneficial for improving ink dissolution phenomenon. The larger the molecular weight of the polymer material, the less likely it is to dissolve into other substances, and the lower the chance of ink dissolution.
Improve the dryness of ink and minimize solvent residue, especially alcohol and water residue, to prevent ink from drying out.
Choose the appropriate formula system. The probability of ink dissolution varies greatly among different formulations, for example, when polyether solvent-free polyurethane adhesive is matched with polyurethane ink, there is basically no ink dissolution.
Improve the molecular weight of polyester based solvent-free adhesives. The increase in molecular weight of composite adhesives greatly reduces their ability to dissolve ink resins and significantly reduces the risk of ink dissolution.
In addition, research has found that designing composite adhesives with different structures can achieve good ink compatibility.
4、 Gaomeng solvent-free adhesive
Gaomeng Company has conducted comprehensive research on solvent-free adhesives of different structures and inks of different manufacturers, types, and structures, and has developed multiple solvent-free adhesives that are insoluble in ink, which can be widely used and matched with inks from multiple manufacturers in the market. SA7591A/SA7591B not only has the advantage of good ink matching, but also has the advantages of low friction coefficient, low odor, and universal aluminum foil.
Longdapac is the leading recyclable packaging manufacturer in China. We are very experienced in the film lamination which enables us to provide the best quality of recyclable flexible packaging in the market.