Abstract : The article examines the factors affecting the dispersion of ink through experiments. The results show that the dispersion effect of water-based inks is the key to obtain the desired properties such as gloss, tinting power and hiding power. The quality of ink dispersion mainly depends on the dispersion resin in the system and The characteristics of the pigment itself and the degree of its interaction, in which the pigment ratio affects the dispersing effect of the water-soluble polymer on the pigment to a large extent. The experiment also proved that the wetting and dispersing agent and co-solvent can change the surface tension and wetting property of the system, thereby improving the dispersion performance of the system. In addition, the temperature at which the ink is dispersed is also an important factor influencing the dispersion effect.
Key words: water-based ink; dispersion effect; surface tension; viscosity
0 Preface
For ink manufacturers, in order to adapt to environmental protection requirements and reduce or eliminate organic emissions, a possible consideration is to develop a water-based system. Due to the high surface tension of the ink and the blistering property, the ink dispersion is more difficult than the solvent ink. In actual ink manufacturing, the end point of the dispersion is often terminated by characteristics such as fineness, hiding power, and coloring power. Among them, fineness is generally used as an indicator of control and final control. The literature suggests that in order to obtain a good grinding effect, there are three methods that can be used: 1 Use highly efficient dispersion equipment such as ball mills. 2 choose high performance good pigment or universal color paste. 3 to achieve a predetermined fineness index by extending the grinding time.
1 Experiment
1.1 Raw materials
(1) R1 (acrylic resin). Tg=98° C., w(KOH)=205 mg/g, Mr=6500, used as a dispersion resin.
(2) R2 (acrylic resin). Tg=112° C., w(KOH)=215 mg/g, Mr=10000, used as a dispersion resin.
(3) R3 (acrylic resin). Tg=85° C., w(KOH)=215 mg/g, Mr=8500, used as a dispersion resin.
(4) R4 (acrylic resin). Tg=135° C., w(KOH)=245 mg/g, Mr=7800, used as a dispersion resin.
(5) R5 (acrylic resin). Tg=93° C., w(KOH)=300 mg/g, Mr=2300, used as a dispersion resin.
(6) R6 (acrylic emulsion). Tg=105° C., w(KOH)=25 mg/g, Mr>200000, used as a thinner.
(7) R7 (acrylic emulsion). Tg = 105 °C, w (KOH) = 62 mg/g, Mr > 200000, used as a diluent.
(8) P1 (high pigment carbon black).
(9) A1 (polyoxyethylene octylphenol ether sulfate sodium salt). Used as a dispersant.
(10) A2 (acetylene glycols). Used as a dispersant.
(11) A3 (EO/PO block copolymer). Used as a dispersant.
(12) A4 (polyoxyethylene fatty alcohol condensate). Used as a dispersant.
(13) A5 (Polyacrylate). Used as a dispersant.
1.2 Experimental device
The experiment uses a media mill (sanding) to disperse the small-scale test equipment, and the dispersion is achieved by the medium beads exerting force on the pigment and the disc impeller's high-speed rotating centrifugal force. Glass beads are used as grinding media in the media mill and have a volume of 100% of the volume of the dispersed material.
1.3 Dispersion Performance Test Method
(1) The coloring power, light translation, fineness, and viscosity are all checked according to the provisions in the literature.
(2) The surface tension was measured using a Sensadyne 6000 surface tension meter.
2 Results and Discussion
2.1 Effect of Ink Abrasive Composition on Dispersion Effect
2.1.1 Selection of Carbon Black
The basic properties of the pigment greatly affect its dispersion effect. For carbon black, because of its relatively small primary particle size and large specific surface area, more dispersion medium is needed to “wetâ€. Considering that there are more aggregates per unit mass of carbon black, this will inevitably lead to smaller aggregate distances and greater attraction between aggregates, which also causes carbon black to be difficult to disperse. Therefore, in the formulation design should be considered appropriate to lower the viscosity of the abrasive to facilitate the dispersion of the dispersion medium and the separation of aggregates.
Since the hydrophobicity of carbon black (HLB value of untreated carbon black is 10-12), carbon black that has been post-treated with an oxidizing agent should be considered because of the chemical oxygen complex (ie, carboxyl, sulfhydryl, and lactone). Bases, etc., which are collectively referred to as volatile dispersions in industry, can act as surfactants and can improve their "wetting" properties to a certain degree. According to the characteristics of carbon black, a lower specific surface area and a higher volatile carbon black species were initially selected, ie, P1 was the dispersed object.
2.1.2 Optimization of Dispersion Base Resin
The flow point technique was used to examine the flow of dispersed carbon black P1 in the resin (R1, R2, R3, R4, R5) as shown in FIG. 1 . As a result, it was found that the two different shapes of the flow point curves R1 and R3 have distinct flow points and are suitable for dispersing carbon black P1. R2, R4 and R5 may have good performance, but are not suitable for dispersing carbon black P1, as can be seen from FIG. 1 . The pour point is about 20% of the base material mass fraction.
2.1.3 Effect of Abrasive Composition on Dispersion Effect
Based on the determination of the flow point, R1 is selected as the dispersion resin and P1 is the dispersion target. In order to reduce the interference factors of the dispersion test, especially the positive and negative effects of surfactant on the grinding effect, the polymer defoamer A5 was chosen in the experiment because the polymer defoamer has good miscibility with the system. It will produce stable foam and cause shrinkage and other defects. In addition, using the above pour point method, a pigment base ratio of 3/1 to 2/1 suitable for dispersing carbon black P1 was obtained. Based on the experiment, the design of the comparative experiment formula was examined to investigate the effect of the composition of the abrasive on the dispersion effect (dispersion speed, hiding power, coloring power and gloss at 60°C). For comparative experimental formulations, see Table 1.
Figure 2 reflects the effect of high pigment and low pigment mass fraction formulations on ink dispersion fineness, hiding power, tinting strength and gloss at 60°C. Clearly, the dispersion properties of inks are closely related to the composition of the abrasive.
(1) As shown in Fig. 2a, the dispersion fineness decreases with the extension of dispersion time, and the lower pigment filling amount does not effectively disperse the last "coarse grain" in the grinding material.
(2) As shown in Fig. 2b, with the progress of dispersion, the hiding power of the two formulations after dispersion was significantly improved, but the hiding power of the high-pigment content formulation after dispersion was significantly higher than that of the low pigment content formulation.
(3) The pigmentation power of the high-pigment content formulation is obviously higher than that of the low-pigment content formulation in the dispersion process, but after grinding, the fineness is relatively close to the coloring power. This shows that after the pigment is dispersed to a certain degree of fineness, its coloring power is mainly related to the dispersion resin, as shown in Fig. 2c.
(4) As can be seen from Fig. 2d, the difference between the two effects on gloss is small, which is close to the limit of experimental error and the difference brought about by the small variation of pigment dispersion.