The formation factor of water-borne coating
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...
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