Waterborne self-crosslinking acrylic dispersion ink (top)

Over the past two decades, inks with low VOCs have been developed for environmental and health reasons. New Waterborne Self-Crosslinking and Storage Stabilizing Polymers Environmentally friendly printing inks and varnishes have been developed by Akzo Nobel Resins.

In traditional aqueous printing inks, the volatilization and absorption of alkali is the reason for the quick drying of aqueous inks. For non-absorbent substrates (NAS), the situation is completely different. One of the better ways to obtain excellent construction performance is to select normal temperature curing water-based inks. Achieved long-term storage stability and good film properties such as scratch resistance, water resistance and adhesion of water-based single-pack room temperature curing ink, a huge technical challenge. Recently, technologies that can balance various properties have been successfully developed. This technology involves a polymer with an active monomer, acetylacetone acrylamide (DAAA), as shown in FIG. 1 .

Figure 1 Acetylacetone acrylamide

Comparing epoxy, aziridine, or isocyanate curing systems, the important feature of this cross-linking monomer is its low toxicity, which makes sense for the preparation of high-performance, environmentally-friendly, non-polluting coatings.

Chemical principle

In the room temperature curing printing ink of two or more functional compounds can meet the long-term storage stability and high quality printing inks and coating oil applications when the need for continuous improvement of coating performance requirements.
When water is volatilized from the ink, the reaction between the ketone group and the dihydrazide occurs rapidly at room temperature, and this reaction has been widely used in high-performance aqueous coating systems. The main role of the cross-linking reaction is to increase the elastic modulus of the polymer. In Figure 3, the dynamic analysis (DMA) of a film formed from a crosslinked and non-crosslinked acrylic dispersion (Tg 25°C) clearly illustrates this. Due to the formation of a three-dimensional polymer network structure, the chemical resistance is also improved.

Figure 2 Reaction between ketone and hydrazide

Fig. 3 Dynamic analysis (DMA) of acrylic film dispersion with (□) or without (Δ) carboxyl-hydrazide cross-linked acrylic dispersion

Figure 4 Film Formation and Cross-Linking Cases a and b

Research section

The main problem in the development of self-crosslinking acrylic dispersions is to coordinate the physical properties of the cross-linking chemistry and the film formation process. Figure 4 illustrates the film formation of an acrylic dispersion. During printing, water volatilizes early, ie before the diffusion of polymer particles in the vicinity of the polymer particles takes place. If a cross-linking reaction occurs at this time, cross-linking will begin at the particle interface (case a), which will increase the Tg of the particle interface, thus preventing the cross-linker from continuing to diffuse into the particle. As a result, the coated film has higher hardness than the non-crosslinked comparative sample, but the chemical resistance and water resistance are not good, and the mechanical properties of the obtained ink are not satisfactory.

In order to obtain a homogeneous polymer network, the cross-linking reaction should not occur before sufficient in-diffusion of the polymer chains (case b). This occurs only when the particles are crosslinked between and inside. To achieve this goal, several ways are suggested to improve particle morphology. Modern computer simulation techniques can help researchers design the desired polymer components. In a previous article, some preliminary results have been reported on the relationship between particle morphology and the membrane properties formed by these dispersions. Through many model studies, it has been found that there are continuous gradients of polymer particles in the polymer composition. The dispersoids perform better than phase separation (core-shell) or homogeneous particles. It has also been found that the keto group is preferably enriched in the low Tg region of the polymer particles. These gradient acrylic dispersions are prepared using a proprietary emulsion polymerization technique. With these self-crosslinked gradient acrylic dispersions, printing inks with high gloss, excellent adhesion and excellent chemical resistance can be formulated without the disadvantages of conventional thermoplastic acrylic dispersions, and good blocking resistance and low temperature can be obtained. Flexibility. The effect of gradient technology on mechanical properties is illustrated in FIGS. 5 and 6 . These mechanical properties can be associated with high and low temperature flexibility, and it is especially important that the size change occurs when the printed substrate undergoes size changes such as shrinkage (slow deformation) or friction (rapid deformation). Another advantage of the combination with gradient technology is that the acrylic dispersion can form a good film without or with a very small amount of coalescent.

Figure 5 Self-crosslinked acrylic dispersion film tensile-stress curves with (D) or without (Δ) gradient at slow deformation rate (1.2 mm/min)

Fig. 6 Tensile-stress curves of self-crosslinked acrylic dispersion films with (â–¡) or DMA-free (â–³) gradient at slow deformation rate (120 mm/min)

Figure 7 Average Tg 25°C room temperature aged 7 d acrylic dispersion film (△) thermoplastic, (○) gradient self-crosslinking acrylic dispersion, (□) gradient-free self-crosslinking acrylic dispersion)