Powder coatings and defoamers
A too wide molecular weight distribution means that there are resins with too small or too large molecular weights, which cannot be well integrated with other components of the coating. Under such conditions, when baked at high temperature, the miscibility between the coatings is poor, and the surface tension of the coating cannot be balanced. The forces between the particles will produce repulsive or contraction movements, and the melting activity time is limited. There is no sufficient leveling time between them. As the coating solidifies, defects such as shrinkage holes and orange peel will appear on the surface of the coating. Therefore, resins with low melt viscosity, long powder coating gelation time, and high melt level mobility should be selected; however, in terms of the physical and mechanical properties of the coating, the mechanical properties of short leveling time and long cross-linking curing time are better, and both should be taken into account. In addition, polyester powder products are easy to agglomerate during storage, and are prone to sagging, orange peel, pinholes, etc. during spraying. This is because the glass transition temperature (Tg) of the polyester resin is not suitable. In production practice inspection, the Tg of polyester resin is controlled within the range of 50 to 65 degrees Celsius, which can not only ensure the fullness of the coating and good leveling, but also prevent the powder coating from agglomerating when stored below 40 degrees Celsius for four to six months, and has a certain degree of stability.
The commonly used degassing agent in powder coatings is benzoin, which has a very low surface tension in the molten state, and can effectively eliminate the small molecular compounds generated by the powder coating during the curing reaction, as well as the pinholes and pores formed by the volatiles contained in the powder or adsorbed on the surface of the coated workpiece during the baking process. The defoaming mechanism can be briefly described as follows: first, the contact between the defoamer and the bubble, then the spread of the defoamer on the bubble interface, then the defoamer enters the bubble or replaces the bubble membrane, and finally the bubble collapses.
