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Understanding and Addressing Ink Dissolution in Flexible Packaging Printing and Lamination


What is the “ink dissolution phenomenon”?

The so-called “ink dissolution phenomenon” refers to the occurrence where ink (typically white ink) on the surface of a flexibly packaged printed product, after lamination processing, undergoes dissolution, displacement, or disappearance in localized areas. The typical visual manifestation is the darkening, coloration, or blurring of contours of portions of the white ink layer, as well as colored or black printed patterns and text.

 


Table of Contents: 

Definition

I. Ink Dissolution Phenomenon Associated with Alcohol-Based Adhesives:  Cause & Countermeasure

II. Ink Dissolution Phenomenon Associated with Ester-Based Adhesives:  Cause & Countermeasure

III. Ink Dissolution Phenomenon Associated with Solvent-Free Adhesives

     Cause:  1. Resolubility of Adhesives in Ink 

                   2. Influence of External Factors The external factors leading to ink dissolution

     Countermeasures.


 

I. Ink Dissolution Phenomenon Associated with Alcohol-Based Adhesives

    Cause: Poor drying, stemming from inadequate drying of both the ink layer and the adhesive layer. 

    Countermeasure: Enhance the drying conditions during printing and composite processes (speed, air volume, air temperature, etc.).

 

II. Ink Dissolution Phenomenon Associated with Ester-Based Adhesives

    Cause: The composite processing speed is too slow, or the “resolvability” of the ink in ester-based solvents is excessively strong, leading to partial dissolution of the white ink.

    Countermeasure: Increase the composite processing speed.

 

III. Ink Dissolution Phenomenon Associated with Solvent-Free Adhesives

    Causes:

     1. Resolubility of Adhesives in Ink (see analysis above).

     2. Influence of External Factors The external factors leading to ink dissolution are primarily analyzed from the following aspects:

  • Firstly, during the crosslinking curing of solvent-free adhesives, a certain amount of shrinkage stress is generated. The greater the adhesive application amount and the higher the curing temperature, the more pronounced the adhesive’s shrinkage stress after curing.
  • Secondly, due to the irregularity of print pattern designs and differences in substrate thickness, the surface tightness of the substrate after solvent-free lamination can vary. Under the influence of roll-up pressure, the solvent-free adhesive layer tends to experience extrusion flow.
  • Thirdly, as most substrates tend to shrink when heated, corresponding shrinkage stress is generated during heating and curing. For instance, in the case of polyethylene films, the shrinking tendency is particularly evident in loosely wound areas. Moreover, the higher the curing temperature, the greater the substrate’s thermal shrinkage stress.
  • Fourthly, if the tension of the two layers of substrate in the composite process doesn’t match, there will be a tendency for relative sliding and displacement between the substrates after winding.
  • Lastly, additives are commonly incorporated during the production of substrate films. Low surface tension additives tend to migrate to the film’s surface, and additives in the printing ink layer can also to varying degrees migrate to the surface of the printing ink layer. This affects the wettability of the solvent-free adhesive on the film’s (or printing ink layer’s) surface. In areas where the substrate film has poor localized wetting, there will be wetting-induced shrinkage stress after the adhesive cures. Under the influence of various stresses, ink pigment particles in liquid flow state will partially rearrange. The resulting appearance of the cured composite product ultimately exhibits the ink dissolution phenomenon.

In summary, the ink dissolution phenomenon in solvent-free lamination occurs during the stage when the solvent-free adhesive layer is still in a liquid state. Once the adhesive layer crosslinks and cures to the point of losing its fluidity (usually within 2 to 4 hours), the ink dissolution phenomenon will no longer occur.


    Countermeasures:

    1. Selection of Adhesives

        Under the condition that the ink remains constant, a compatibility test between adhesives and inks should be performed before production. Only after ensuring a proper match, bulk production should proceed.

    2. Ink Selection

To address the ink dissolution issue, considering the ink’s tolerance to the adhesive is more crucial than the adhesive itself. When choosing inks, the compatibility of resin systems is crucial, and suitable printing and laminating inks should be chosen through pre-production compatibility tests.

    3. Residual Solvents in Printing Film

If drying during printing is inadequate or if excessive solvent residues remain on the printed film’s surface during lamination, it can lead to ink dissolution. It’s recommended to control the level of solvent residue on the printed film (usually required to be <5.0 mg/m²). During summer production, both the type and volatility of solvents should be considered to prevent ink from appearing dry, which can result in excessive solvent retention.

    4. Control of External Factors

Recommendations for control should stem from meticulous management of the lamination process, which includes:

    • Regulating adhesive application amounts
    • Controlling curing temperatures (adopting ambient temperature curing, or considering initial curing at room temperature for around 2 hours followed by further curing at 40-45°C)
    • Managing substrate stress
    • Avoiding substrates prone to “curling”
    • Minimizing joints, ensuring flat and smooth joints where present
    • Suitably reducing the tension in the laminated film rolls.

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