New Generation Aqueous Ink Base Material for Packaging Film and Paper (1)

Foreword

Over the past 30 years, polymer membranes made from latex dispersions have been extensively studied. In particular, the transition from a solvent-based to a water-based type has been driven by environmental protection requirements in the last 10 years. A large number of basic researches have been conducted in this field. Emulsion polymerization is the main synthetic route for latex dispersions, and emulsion polymers play an important role in certain fields, especially in the decorative materials for construction and printing inks. Most of these latex dispersions are produced in a batch process in industrial production, and the monomer composition is an acrylate or styrene/acrylate with a single particle size distribution. Previous studies on emulsion polymers have focused on (reaction) kinetics, particle size, film formation, surfactant adsorption and distribution, dryness, gel particle shape, and final coating film properties.

The requisite characteristics of a printing ink are reversibility, ie re-wetting, re-dissolving or redispersing of the ink. That is, the ability of the dried polymer to be redispersible with the same polymer in the wet state is required. The most important of this reversibility is that it has good printing properties for inks, especially in flexible or gravure printing. Because the ink is not allowed to dry at a high speed in a gravure cylinder bath. In general, solvent-based polymers have good reversibility because the dry film obtained after the solvent is completely volatilized can still be dissolved in the solvent. However, in the aqueous emulsion polymer, the film formation process is completely different. The method, therefore, has an adverse effect on reversibility. In general, this process is divided into 6 stages and can be summarized into 3 main stages: flocculation, coalescence, and auto-adhesion.

During the flocculation process, the polymer film is still reversible, because all the particles are still in the separated state, but they will lose their reversibility when they start to coalesce because the coalesced polymer film is insoluble. Water.

The first-generation aqueous ink polymer was completely alkali-soluble, meaning that at high pH, ​​the polymer particles were separated and the molecular segments that make up the particles were completely dissolved in the aqueous phase (see Figure 1). . This is the so-called alkaline solution or varnish. Its film formation process is very similar to that of a conventional solvent-type polymer because the film is in a dissolved state or is very sensitive to a solvent (in this case, water), so that it maintains a long-term reversibility. In addition, such oligomers exhibit excellent ink characteristics such as good pigment wettability, good miscibility, and low MFFT. However, it has some major drawbacks such as poor water resistance. This is apparently due mainly to hydrophilic polymer backbones, and slow drying is due to the low solids content (20% to 30%) in such systems. The poor resistance to surface impermeability and resistance to back tack (the ability to prevent two layers of coating from sticking together after a certain load) is poor.

In order to overcome these deficiencies, new polymers for inks have been developed, mostly using aqueous polymers. They are made up of a mixture of oligomers and polymer particles in which the oligomer acts as a surfactant, stabilizing the polymer particles during polymerization (see Figure 2). Polymer particles are more hydrophobic than oligomers and have a higher relative molecular mass, which can improve performance. In general, such polymers also have better resistance to back tackiness. However, in order to obtain a good co-filming property, it is usually necessary to add a co-solvent in such a polymer, but this does not meet the requirements of environmental protection. Reversibility is maintained in this system, and due to better mass/volume ratio relative to free molecular chain polymer particles in solution, this oligomer/polymer system can be higher (40% ~50%) Solids content, which reduces drying time compared to pure oligomer systems. However, the drying time of such oligomers/polymers is not short enough, partly due to the water affinity phenomenon of hydrophilic oligomers, especially in nonabsorptive heat-sensitive substrates such as polyethylene or polypropylene. On the substrate, the drying speed is still too slow, which limits the printing speed and substrate range. Addition of salt can partially improve dryness, but it has an adverse effect on water resistance.

There have been studies in the past on the effect of particle size distribution (PSD) of two sizes on film formation, rheology, and dryness in emulsion polymers. It was found that by using two types of particle size distributions, a higher solid content (optimum filling ratio) can be obtained compared to using a single particle size distribution system. Due to the higher solids content, the drying time of these systems is significantly shorter than that of conventional (single size distribution) systems. The polymer used in that document was a single dispersion and therefore was irreversible.

This article will discuss the use of third-generation aqueous polymers for inks, using a mixture of oligomers/polymers with two size distributions (PSD). By mixing the dispersions of the two PSDs with the oligomers, polymers with very high solids content, fast drying and good reversibility can be obtained. Reversibility, rheology, drying speed, film formation, and adhesion were investigated. The rheology was investigated by measuring the relationship between the low shear viscosity of the polymer and the filling ratio or the solid content. The drying time is determined by measuring the mass of the wet film that decreases over time. The use of atomic force microscopy (AFM) to measure MFFT for film formation studies has an important relationship in coatings and printing inks, namely MFFT/resilience-resilience balance. In fact tending to or can get a low MFFT, no need to add a co-solvent that has a good film-forming properties, or a good anti-tackiness, but it is difficult to get good performance at the same time.

The influence of the ratio of oligomer/polymer, the change of Tg of the oligomer/polymer, and the change of the composition of the polymer obtained by mixing the large and small particles in different mass ratios was studied.

Experimental part

Preparation of latex

All polymers were synthesized using continuous emulsion polymerization. For all the recipes, see Table 1. The ratio of BA/MMA in the oligomers was varied to give different Tg. After the polymerization, the pH of the latex was adjusted to 8.5 with ammonia and then filtered. Table 2 shows more detailed parameters for the various polymers provided. All polymer particles are diluted to the same solid content (39.7%) and mixed in different mass ratios of particle latex (mixing ratio L/S refers to large particles/ Small particles) and then add 10% (mass ratio, the same below) or 30% of the oligomer to the above mixture.