Controlling Foam During the Preparation of Aqueous Pigment Dispersions

Aqueous pigment dispersions are created by stably dispersing pigment particles in water through processes like dispersion and grinding. These dispersions typically consist of pigments, dispersants, wetting agents, defoamers, and the dispersing medium (water and co-solvents) [1]. The process involves breaking down agglomerated pigment particles into tiny particles, with dispersants adsorbing onto their surfaces to prevent reaggregation.

Dispersants used in this process are usually polymer-based with hydrophilic and hydrophobic segments, exhibiting some surface activity in the aqueous phase, akin to surfactants. During grinding, mechanical forces introduce gas into the aqueous phase, and dispersants, acting as surfactants, reduce the surface tension of these gas bubbles. This stabilization entraps the gas within the liquid phase, forming stable foam.

In many cases, foam can be detrimental, leading to material overflow, environmental hazards, decreased production efficiency, and more. During the preparation of aqueous pigment dispersions, grinding can cause volume expansion, potentially leading to overflow. Additionally, the reduced grinding efficiency due to increased volume may result in subpar dispersion fineness and mass transfer issues. Therefore, it is crucial to promptly eliminate foam during grinding to enhance the contact area between grinding media and pigment particles, improving shear efficiency [2], reducing grinding time, and promoting better adsorption of pigments onto dispersants. This article explores defoaming agents suitable for various pigments, offering a foam control solution for the aqueous pigment dispersion process.

Experimental Section

1. Main Materials and Equipment

Dispersants: DS-194H (40% effective content), DS-195L (40% effective content), DS-192L, Foaming agents: DF-220S, Foamic-024, Foamic-021, Foamic-028 (Tianjin Saifei Chemical Technology Development Co., Ltd); TEGO-760W, TEGO-755W, FOAMEX 810 (Degussa); BYK-190, BYK-2012, BYK-011 (BYK-Chemie); Dispersogen PL 30 (Clariant); High-pigment carbon black FW-200 (Orion); Red iron oxide pigment (Lansheng); Titanium dioxide pigment (Longmang); JSF-550 versatile disperser (Shanghai Yusheng Electromechanical Equipment Co., Ltd); Zirconia beads (particle size 0.8-1.5 mm, specific surface area 2.5 m2/g); Flanged toughened glass reaction vessel (Nantong Puretech Instrument Co., Ltd); Scraper fineness gauge (Tianjin Yongli Testing Machine Factory).

1.2 Experimental Procedure

1.2.1 Experimental Equipment Modification

The laboratory's basket-type multifunctional disperser comprises a sanding disc, motor, and a 1 L steel sanding drum. To facilitate observation and heat exchange during grinding, the steel sanding drum (Figure 1a) was replaced with a 2 L flanged toughened glass reaction vessel (Figure 1b), and a water bath was used for heat exchange during the grinding process. Prior to use, the volume of the flanged toughened glass reaction vessel was calibrated, and volume unit markings were made on the vessel's exterior.

1.2.2 Experimental Process

As per Table 1, measured quantities of water, dispersants, and defoamers were added to the glass reaction vessel, mixed thoroughly, followed by the addition of measured pigment. Subsequently, a predetermined amount of grinding media (zirconia beads, 500 g, fully wetted, with a volume between 560 and 630 mL) was added to the reaction vessel. Grinding was carried out at 3,000 rpm for 120 minutes, and grinding was immediately halted if the liquid level rose to the flange of the glass reaction vessel. Throughout the experiment, the liquid level was continuously monitored, and the unit volume grinding area was calculated. Samples were taken and the dispersion fineness was measured using a scraper fineness gauge, while the viscosity of the pigment dispersion was visually assessed inside the reaction vessel.

1.3 Evaluation Methods

1.3.1 Unit Volume Grinding Area (SV)

The unit volume grinding area refers to the ratio of the total surface area of the grinding media to the volume of the working liquid and can be calculated using Formula (1).

SV = (m / S) / V

Where:SV - Unit volume grinding aream - Mass of grinding mediaS - Specific surface area of the grinding mediaV - Total volume of the working liquid containing the grinding media

A smaller SV indicates the presence of more gas in the system, resulting in reduced grinding efficiency.

1.3.2 Duration of Grinding Process

The duration of the grinding process refers to the time taken for the liquid level in the reaction vessel to rise to the flange. A shorter time indicates more severe foaming during grinding.

1.3.3 Dispersion Fineness

The dispersion fineness was measured using a scraper fineness gauge, with a smaller numerical value indicating finer dispersion.

1.3.4 Visual Viscosity Assessment

The flow behavior of the liquid inside the glass reaction vessel was visually assessed, with better flowability indicating lower viscosity.

Results and Discussion

2.1 Mechanism of Defoaming Agents [3]

The action of defoaming agents involves two processes: defoaming and foam breaking. The defoaming agents used in this experiment are all non-silicone, non-oil, or non-fat defoaming agents. Their principle of action relies on exploiting surface tension differences for defoaming. For gas bubbles inside the liquid, the defoaming process is illustrated in Figure 2, with the Laplace equation as shown in Formula (2).

As depicted in Figure 2, assuming no viscosity effects, two gas bubbles of different sizes within the liquid will exert different pressures within the bubbles. Since P1 > P2, smaller bubbles merge into larger ones, leading to the formation of larger bubbles. Due to buoyancy, these larger bubbles move upwards, increasing in volume, and their ascent rate accelerates, eventually escaping from the liquid's interior. This process is referred to as defoaming. However, if the bubbles formed within the liquid system are of similar size, i.e., P1 ≈ P2, it becomes challenging for the bubbles to merge, and the defoaming process is hindered. The gas remains trapped within the liquid, causing rapid volume expansion. One key factor contributing to uniform bubble size is the action of dispersants or emulsifiers. Defoaming agents, on the other hand, exploit surface tension to achieve defoaming. Relative to dispersants and emulsifiers, defoaming agents have different migration capabilities and are used in smaller quantities. Consequently, not all gas bubbles in the system have equal opportunities to encounter defoaming agents or dispersants, resulting in varying surface tension on different bubble surfaces. As a result, the sizes of these bubbles differ, triggering the defoaming process and promoting an increase in liquid volume. This leads to an increase in upward speed and escape from the liquid's interior, accomplishing bubble breaking for surface bubbles, as shown in Figure 3, where red and white represent molecular defoaming agents.

Defoaming agents alter the structure of the surfaces inside and outside the bubbles, changing the surface tension of the bubble membrane. This initiates surface liquid movement and the appearance of weak spots. Under the action of gravity and drainage, the liquid membrane is torn apart, releasing gas from the bubbles and achieving bubble breaking.

2.2 Impact of Defoaming Agents on Grinding State of Pigment Dispersions

Without foam control measures, air quickly disperses into the liquid phase of the reaction vessel, causing rapid liquid level rise. The characterization results of various systems during dispersion are shown in Tables 2-4.

The introduction of air into the preparation system will inevitably dilute the volumes of pigment and grinding media. Since the total surface area of the grinding media is constant, volume dilution results in a smaller SV, a reduction in effective grinding area, and a noticeable decrease in grinding efficiency. Additionally, pigment particles are diluted with air, reducing their chances of contact with the grinding media, which also leads to decreased grinding effectiveness.

Tables 2-4 reveal that the presence of foam has a significant impact on SV-20, fineness, and the operational time of dispersion for the carbon black system, with TEGO-760W exhibiting severe foaming. In the case of iron oxide dispersion, TEGO-755W also displayed pronounced foaming behavior, while in the titanium dioxide dispersion, both TEGO-755W and BYK-2012 exhibited severe foaming. Experiments were conducted with TEGO-760W, TEGO-755W, and BYK-2012 as reference dispersants for foam control.

2.3 Foam Control Effects of Different Defoaming Agents

Upon introducing defoaming agents into the grinding system, they promptly remove the entrapped air, forming stable foam, reducing liquid level rise, and resulting in minimal changes in liquid phase volume. Thus, SV is maintained, and grinding efficiency remains high. The effects of adding defoaming agents to various pigment dispersions are presented in Tables 5-7.

Tables 5-7 demonstrate that the addition of defoaming agents significantly extends the grinding operation time for carbon black, red iron oxide, and titanium dioxide dispersions, which were initially challenging to grind. Furthermore, they require 20percent less grinding media. The results indicate improved grinding efficiency and extended operational times.