The formation factor of water-borne coating

The formation factor of water-borne coating

2019-09-04 16:27:10
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With the progress of water-based resin technology, water-based paint technology has been recognized by more and more industries and applications. In promoting environmental protection at the same time, water-based paint its special hydrophilic structure may also lead to premature failure of the coating. We usually attribute the cause of failure to resin performance defects, while ignoring the water-based paint and the bonding between the substrate.

The binding force can be understood as a good connection between the coating and the substrate. The experimenter has obtained the connection between the coating and the substrate by various methods, but the coating does not completely wet the substrate to form a solid-liquid interface Before the discussion of mechanical anchoring, van der Waals force or chemical bond and other factors will become unrealistic. Therefore, "wetting spread" has become a prerequisite for the combination of factors.

In everyday applications, it is generally believed that relatively rough substrates are more conducive to the formation of binding forces. Theoretically, the surface of the rough substrate has a larger surface area than the flat substrate, and when the substrate is wetted with the coating, the air is removed and both have a larger contact area to obtain the bonding force. However, for waterborne coatings, the surface tension of 40-50 dyn / cm is higher than that of most substrates, and the contact area of the aqueous coating with the substrate is often reduced due to the inability to wetting, resulting in loss of the binding force. For example, Figure 1 shows that the formation of liquid-solid interface is conducive to the formation of binding force.

The water-based coating is in contact with the substrate. The gas-solid interface SG and the gas-liquid interface SL are replaced by the liquid-solid interface SG. The coating and the substrate begin to establish a part of the connection relationship. When this phenomenon is rapidly diffused to a certain extent, Into the drying curing stage, and finally complete the mechanical anchoring or chemical reaction. The angle of contact is related to the surface tension of the substrate and the coating. The angle is directly related to the degree of wetting and spreading of the aqueous coating to the substrate. When the contact angle θ> 90, it is considered that no wetting occurs, and the contact angle θ <90, the paint can quickly spread and penetrate the pores.

When the ideal liquid-solid interface is formed, the increase of the contact area between the coating and the substrate must promote the formation of the binding force. However, due to the surface pressure difference, the complete wetting may only be achieved in large pore size pores, and for pores with small pore size, the wetting process becomes rather slow and tends to complete wetting before curing. When the tiny pores are sufficiently dense, a thin layer of air film is formed between the surface of the non-wetting substrate and the coating, and the presence of the air film hinders the bonding between the coating and the substrate. In order to solve this problem, it is often necessary to change the surface properties of the substrate to a greater extent or to seek solutions from the aqueous coatings to achieve rapid wetting and spreading. As can be seen from Fig. 3, the degree of wetting is related to the capillary pore size.

At present in the water-based paint can be a lot of ways to solve the problem of wetting. From the emulsion polymerization began to join the surfactant, to the finished product of the co-solvent, no doubt on the surface tension of water-based paint produced a certain degree of impact. However, for capillaries or low surface tension substrates, the most effective way is by adding a substrate wetting agent to the aqueous coating formulation to promote the desired wetting of the waterborne coating in order to facilitate the formation of binding forces. This method is widely used because of the convenience of adding and improving the wetting and spreading ability.

Polysiloxane substrate wetting agent is based on hydrogen-containing polysiloxane and allyl polyether synthesis of small molecule modified polysiloxane. Polysiloxanes have less surface tension and hydrophobicity than most substrates. The introduction of polyether chains in polysiloxanes can effectively enhance the water solubility of polysiloxanes and provide high surface activity. Waterborne coating application in the process of substrate wetting agent tends to form a gas-liquid interface in the adsorption of mononuclear film, which effectively reduction the surface tension of water-based paint to enhance the wetting and spreading efficiency. Especially in recent years, the water-based coating process tends to be thin-coated multi-layer, the coating of the construction time cycle is short, the rapid spread of the coating has become particularly important.

The experimental results show that the addition of polysiloxane substrate wetting agent can effectively enhance the wetting of the aqueous coating on the substrate and the spreading ability . From the micro-catheter penetration of wood can be explained that the addition of substrate wetting agent can help water-based paint on the fine pore size substrate for good wetting and infiltration. When the air in most of the pores of the substrate surface is replaced by liquid, the coating acts as a shield against the external water vapor and forms mechanical anchoring or physical entanglement with the internal voids. In some coating systems, the resin reacts with the reactive groups on the surface of the substrate to further enhance the adhesion of the coating to the substrate. This conclusion can be found in the metal salt spray test results .

However, in practical applications, the surface tension of water-based paint is not as low as possible, but the need to choose according to the nature of the substrate to add a best use of the program. This can cause the coating to effectively wet the substrate to achieve a good combination at the same time, to prevent the use of improper substrate wetting agent to bring the risk.

The polysiloxane wetting agent mentioned in the application of experimental data are derived from the standard R & D and production of xx series of water-based substrate wetting agent products. xx series of waterborne substrate wetting agents are not limited to substrates, but they can act as a surface wetting for a variety of materials. Such as the wetting and dispersing of the pigments and fillers of various structures, the wetting of the pits caused by the residual dust, the solubilization of the incompatible substances in the aqueous system.
Conclusion

In the global environment of environmental protection revolution, compared with mature oil-based paint water-based paint is more potential, supporting the development of a special function of additives products become a trend. The development of coatings and ink auxiliaries with higher market acceptance based on materials science and organic chemistry research is the goal pursued .

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