The mechanism and development trend of desiccant for water-based coatings (2)

2 desulbant action mechanism
de-foaming agent is a kind of damage around the bubble surfactant double electrolytic layer of additives, so that the bubble wall destabilise and rupture, thereby releasing the wrapped gas. Typical desobbists are mainly composed of carrier oil, active solid grains and emulsifying agents. Carrier oil can be mineral oil, silicon oil, natural oil, white oil, etc. , can quickly transport active solid particles such as hydrophobic silica, paraffin or metal soap to the bubble film to play a role. Emulsizers are used to regulate the compatibleity of desmoticizers with the main body of the coating. The principle of choosing the best desticant is to find a balance between its debouling efficiency and compatibleity with the system. The role of highly incompatible defystants in the system is very efficient, but because it cannot be integrated into the system and migrated to the gas-liquid interface, it is very easy to produce surface defects. However, highly compatible desiccants are quickly fused into the coating system and are not sufficient to provide efficient debouling efficiency. For the role of desiccant in water-based coating machine
system, can be divided into bridge-dehumidification, bridge-stretching and fluid spreading clip belt 3 categories. Regardless of the way the defystant works, the first thing is that the desmoulizer can enter the thin layer of the bubble film, thermodynamics to the seepage coefficient E to indicate the ease of the desmodulation agent into the bubble thin layer, its expression is as (2):of which, the aW, the anti-OW, the OA represents the liquid surface tension, liquid and de-foaming agent interface tension and the surface tension of the de bubble agent. When E-0, it is indicated that the desmoulizer can enter the thin layer of the bubble and connect with its double-layer membrane to form a bridge, and the stability of this bridge effect also affects the efficiency of the anti-bubble molecule, in thermodynamics with the bridge coefficient B, its expression as (3):if B-0 indicates that the desmodulizer and bubble double-layer membrane formation bridge
instability, but can further play a de bubble-defying effect. When B is 0, it becomes a stable bridge, and the bubbles reach stability and are not easily cracked. When E is 0, it indicates that the desiccant cannot enter the bubble double film, but is excluded to the adjacent Plato channel. It is not until the gravity drainage of the bubble causes the capillary pressure to gradually increase, making the Plato channel membrane narrower that the desiccant can be forced into the bubble film for spreading. At this time, the debogator efficiency is closely related to the spread coefficient of carrier oil S, expression such as (4):
S - AW - OW - OA (4)
the study shows that the spread coefficient S higher carrier oil deblistering efficiency is significantly higher than non-spread carrier oil deblistering agent. Therefore, the osmosis coefficient E, bridge system number B and spread coefficient S play a decisive role in the deblistering process. Figure 3 shows a diagram of the workings of deboulants under different conditions.2.1 Bridge connection - dehumidification effect
When the osmosis coefficient of the desmoulizer is E-0, Figure 3 (a) and Figure 3 (b), the de-foaming agent enters the bubble film and, in the case of the bridge coefficient B-0, the de-foaming agent Active solid particles and bubble double-layer film form a bridge, and by the solid particle surface strong hydrophobic, the film layer of liquid dehumidification, as shown in Figure 3 (c) and Figure 3 (d), resulting in the bubble membrane punctured and bubbles cracked. Similarly, de-humidification agent carrier oil with hydrophobic surface also has the function of dehumidification. Unlike solid particles, oil droplets have the ability to decompose and deform, when entering the bubble film, de-deformation of the deflegant oil droplets into prisms, and no longer occur obvious deformation, as shown in Figure 3 (e) and Figure 3 (f), at this time dehumidification occurs. When the debug agent part contains carrier oil and hydrophobic solid particles at the same time, its synergistic effect makes the debunking agent more efficient, which is due to the presence of solid particles make the bubble membrane puncture more effective, that is, has a higher permeability coefficient, making it easier for oil droplets to enter the bubble film layer to play a role.
2.2 Bridge connection-stretching effect
When the oil droplets enter the bubble film layer to form a bridge effect, when the carrier oil spread coefficient of S-0, the spread of the oil droplets diffusion causes it in the oil-water interface and gas-water interface to form a different curvature of the oil droplet surface, as shown in Figure 3 (g). At this point, due to unbalanced capillary pressure, the oil droplets gradually stretch thin as shown in Figure 3 (h) until the break causes the bubble to break. Silicone deblisters benefit from the high spread coefficient of silicone oil, mainly through the bridge-stretching process. When the bridge coefficient B is 0, stable bubbles are formed in Figures 3 (i) and 3 (j).
2.3 Fluid clamping action
As mentioned earlier, when the osmosis coefficient B is 0, the desmoticizer is rejected to the platonic channel adjacent to the bubble film, as shown in Figure 3 (k), and enters the bubble film under unbalanced capillary pressure, as shown in Figure 3 (m) and Figure 3 (n). When the desiccant molecule reaches the second layer of the bubble double film, the surfactant is gradually replaced by adsorption by its strong spreading ability. With the occurrence of Malangni fluid movement, the carrier oil clip with bubble film movement, resulting in local bubble film gradually thinning and eventually rupture. The prerequisite for the fluid clamping process is that the carrier oil has a good ability to spread, that is, S-0. Some deboulants that do not contain hydrophobic solid particles mostly rely on this debobbling process for their action.3 Desculger Classification
3.1 Mineral Oil Deblants
Typical Mineral Oil Desticulants are composed as shown in Table 1.Among them, mineral oil as a de-foaming agent carrier, mainly to aromatic or adipose mineral oil-based, and aromatic mineral oil easy to make the paint yellowing risk, and harmful to human physiology, has been rarely used. The hydrophobic particles are mainly silicon dioxide, paraffin, metal soap or polysaccharine. A small amount of emulsifying agent in desabling agent can disperse hydrophobic particles well in carrier oil, and can also improve the compatible of desiccant with the system. For environmental and health reasons, traditional APEO emulsification agents have been replaced by line-type or branch-chained fatty alcohol compounds. Mineral oil deblisters are mainly used in matte and semi-gloss latex paints. For high-quality water-based industrial coatings, the introduction of mineral oil deblisters can easily lead to the risk of surface oil separation and reduced gloss. The main mechanism of action of mineral oil dispersants is the action of fluid clamping.
composition of 3.2 silicone
and silicone deblisters is shown in Table 2.silicone oil as a defoulant carrier, the main components are polysilica or polymethane. Si-O bonds in polysilica polymers are flexible, sioxygen skeletons provide a high spread coefficient, and methyl groups provide hydrophobicity and low surface stress. These properties make polysilica defoiling agents very effective. And polysilicae can also be modified to improve its compatible, such as the use of polyether chain to change can improve the hydrophthalpability of polydymixylsiloxane, and therefore improve its compatible in the polar system. Because it contains silicones, these desiccants are more expensive than mineral oils and are commonly used in high-end coatings. Silicone deblisters can also improve the dispersion and deblistering properties of silicone oils by combining with hydrophobic particles such as polycep and silicone dioxide. Compared with mineral oils, the main advantage of silicone deblisters is that they do not cause the gloss of the highlight system to decrease, nor do they affect the color paste compatibleness of the system. The chemical structure of polysilica has a better effect of reducing surface stress than non-silicon defoulants, as this is due to its better penetration and spread coefficients. The action of silicone deblister is mainly the bridge-stretching system.
3.3 Molecular deboulant
molecular debobbing agent is essentially a non-ionized surfactant, which competes with the surface active agent of the stable bubble on the thin layer of the bubble to absorb and replace. The wetting dispersants, emulsifiers and so on in the coating formula mainly use interional forces, hydrogen bonds and Vanderwar force to stabilize bubbles, while sub-desiccants destroy these forces at the molecular level and achieve de-foaming effect. Conventional deboulants are mainly used to achieve deblistering properties through incompatibility with the system, but also have side effects such as surface defects, poor recoilability, and reduced performance after storage. As a surface active agent, molecular-grade deboist is compatible with most systems and provides a wetting effect. In general, molecular deboulants achieve a good balance in debobbling efficiency and compatible, which is very efficient for controlling microboules and macro bubbles. Molecular deboist is suitable for low viscosity, high gloss and low PVC color paint and varnish.4 Direction of the development of water-based paint deblisters
With the growing scale of water-based coatings, the demand for high-efficiency desiccants is also gradually increasing. China's research on desiccant has a history of nearly 20 years. The first generation of desiccants mainly used animal and vegetable oils as carriers. In the 1960s, a second generation of defoulants was born based on the ethylene oxide insertion of the polyether chain. The representative of the third generation defoulant is polydymethane as the main active substance, and is currently the most widely used defoulant.
Based on the current application status of desiccants in the coating industry, the future development of desiccants will be mainly concentrated in the following four areas:
(1) to improve the mechanical stability of existing active components and storage stability, to ensure that the coating system to maintain efficiency and durability. Chemical changes to existing active ingredients can be passed or new active compositions can be explored to balance compatibleity with deblistering efficiency.
(2) for different coating systems, the development of special types of desiccants, from the traditional general-purpose to dedicated development, to maximize the use of customized desiccant functions.
(3) to replace the traditional single-part, less cost-effective products, the development of a complex collaborative type of high-efficiency desiccant products.
(4) From the point of view of environmental resources, the development of multi-functional new molecular-grade deblisters, such as wetting function, reducing the minimum film temperature of coatings, reducing or discarding the use of film additives, reducing VOC emissions.5 Conclusion
As a necessary additive in water-based coatings, the effectiveness of desiccants is not only affected by other ingredients in the coating formulation, but also by the physical and chemical properties of the desiccant's own composition, such as surface hydrophobicity, interface strain, fluidization properties, etc. A deep understanding of the mechanism of the role of desobbists is conducive to the selection of the best desticant solutions in practical applications, and has laid a theoretical foundation for the development of new high-efficiency desticants. Benefit from the vigorous development of water-based coatings, desiccant market prospects will certainly be more broad. .