Ink formulation for cooking bags requires careful selection of materials to ensure durability, adhesion, and performance under high-temperature conditions. When it comes to gravure printing, two main types of resins are commonly used: high-temperature resistant polyurethane resins and high-chlorine content ternary chlorinated vinyl resins. These resins are specifically chosen for their ability to withstand the harsh conditions of retort processing.
Polyurethane resins, often abbreviated as PU, are liquid resins created through a reaction between polyurethane monomers and polyether or polyester polyols. They come in both soft and hard varieties depending on the application. For ink formulations, these resins must offer strong adhesion to plastic films, excellent heat and boiling resistance, good solvent release, and high gloss. Most commercially available polyurethane resins are mixed with solvents like isopropyl alcohol, methyl ethyl ketone, and toluene, with a solid content typically ranging from 25% to 35%.
On the other hand, high-chlorine content vinyl resins are copolymers of vinyl chloride and vinyl acetate. These resins are ideal for forming ultra-thin ink films that remain flexible and durable even after high-temperature exposure. They are often used in combination with other resins to enhance the overall performance of the ink, particularly in terms of toughness, flexibility, and resistance to water and permeability.
When selecting pigments for high-temperature retort bag inks, brightness, color strength, and heat resistance are key factors. Pigments must not only provide vibrant colors but also resist fading at temperatures above 120°C. Black pigments typically use channel carbon black, while white pigments often rely on silica-modified titanium dioxide. Color pigments such as phthalocyanine blue and green are preferred for their stability at high temperatures.
Wetting and dispersing agents play a crucial role in ensuring that pigments are evenly distributed within the ink. These agents act as surfactants, reducing surface tension and preventing pigment clumping. Anionic surfactants are commonly used due to their effectiveness in improving dispersion and maintaining ink stability.
Special wax powders are added to enhance the wear and scratch resistance of the ink film. These waxes improve printability, reduce adhesion, and increase ink flow. The addition of wax is carefully controlled, usually around 0.5–1.0% of the total ink weight, to avoid issues like particle deformation during grinding.
Antistatic agents are essential in high-speed printing to prevent static buildup, which can cause smudging, blurring, and even safety hazards. Anionic antistatic agents like ethyl phosphate solutions are commonly used, providing effective static control without compromising ink quality.
To improve adhesion, especially on plastic films, adhesion promoters such as modified chlorinated polypropylene resin and fumed silica (matting agent) are often included. Fumed silica helps prevent pigment settling, improves ink flow, and enhances composite fastness.
Finally, the choice of organic solvents is critical for proper film formation. Solvents like toluene, xylene, ethyl acetate, and methyl ethyl ketone are commonly used, with special solvents like methyl isobutyl ketone and propylene glycol methyl ether helping to balance evaporation rates and ensure smooth ink performance.
By carefully selecting and combining these components, manufacturers can produce high-quality gravure inks that meet the demanding requirements of high-temperature cooking bag applications.
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