The powder coating industry occupies an important position in the modern coating industry, from common thermosetting and thermoplastic powder coatings to special types with special functions, heavy anti-corrosion and special powder coatings for aluminum profiles, etc., which are widely used in people's daily life and high-tech science and technology fields. Adhesion, as the basic index of mechanical properties of powder coatings, has been developed for many years, but its theory still needs to be further scientifically explained. This paper will combine practical experience and experiments to comprehensively discuss the adhesion of powder coatings.
Powder coating film-forming and adhesion mechanism
Film-forming process
Powder coatings are usually attached to the surface of the workpiece in a powder state through electrostatic coating, and then go through three key stages of aggregation, leveling, and curing to finally form a coating film. Specifically, the first is the softening process, in which the powder absorbs heat, and the resin in it becomes sticky and softens when heated; Then it enters the melting process, the resin absorbs enough heat, the active chemical bonds open, and the powder appears in a molten state to cover the surface of the product, which is extremely short; Then there is the leveling process, in which the molten powder flows from the originally uneven continuous surface and gradually forms a smooth and flat surface; Finally, the curing process is the cross-linking reaction of the active chemical bonds of the powder coating after melting, the molecular weight increases rapidly, the viscosity continues to increase, and finally solidifies into a hard coating film.
Attachment mechanism
The adhesion mechanism of powder coating film mainly includes mechanical adhesion and chemical adhesion. The amount of mechanical adhesion depends on the characteristics of the substrate, such as roughness and porosity, but also on the strength of the coating film formed. The roughness and porous structure of the substrate surface can provide more anchoring points for the coating film, so that the mechanical bond between the coating film and the substrate is similar to "anchoring", and the adhesion effect is enhanced. Chemical adhesion involves a variety of forces at the interface between the coating film and the substrate, including electrostatic force, van der Waals attraction, hydrogen bonding, and chemical adhesion. These forces together determine the adhesion of the coating film on the surface of the coated object, for example, the electrostatic force can make the oppositely charged coating film and the substrate attract each other, the van der Waals force provides a weak but widespread attraction at the molecular level, and the hydrogen bond and chemical binding force significantly improve the bonding strength of the coating film and the substrate through the formation of chemical bonds or special interactions between molecules.
adhesion
meaning
Adhesion usually refers to the work required to separate the interface between the coating film and the substrate, the smooth substrate relies on the interface suction, and the actual microscopic roughness of the substrate increases the contact area, and penetrates into the capillary like a liquid, and the rate is related to the surface tension of the coating and the roughness of the substrate. The viscosity of powder film formation first decreases and then increases, the low viscosity stage needs enough time to penetrate the pores of the substrate, and the thick film (>25-75μm) needs to consider gravity, and full penetration is the key to improve adhesion.
Influencing factors
viscosity
In general, the melt viscosity of resins increases with the increase of molecular weight. All else being equal, the use of lower molecular weight resins tends to confer better adhesion to the coating after crosslinking, and this has been confirmed in practice. Low-molecular-weight resins may also have the advantage of being able to penetrate smaller crevices than high-molecular-weight resin molecules, resulting in a tighter bond on the substrate surface and enhanced adhesion.
Wetting effect and surface tension
The adhesion of the coating film is due to the attraction of the polar group on the surface of the coating and the metal, and its strength depends on the wetting ability of the coating to the substrate (related to the surface tension): when the surface tension is lower than the free energy of the substrate, the liquid spreads spontaneously, and the smaller the contact angle, the better the adhesion. In addition, polymer polarity and molecular mobility also affect adhesion - high molecular weight polyester has low adhesion due to difficult macromolecule movement and insufficient polar group adsorption, while low molecular weight epoxy (such as small molecular weight curing agent system) polar group is easy to adsorb and has better adhesion.
Internal stress
The force resulting from the cohesion between the molecules of the same substance is known as internal stress. The internal stresses in the coating counteract some of the adhesion, resulting in less external force required to break the bond. Internal stresses are usually caused by the inability of the coating to shrink freely when a film is formed on a rigid substrate. Reducing the thickness of the coating can reduce internal stress, and in addition, adding the right amount of pigment can also reduce internal stress. Therefore, in general, basecoats have better adhesion than clearcoats, because the presence of pigments in basecoats helps to disperse and reduce internal stresses, thus improving the adhesion between the coating and the substrate.
other
The surface treatment and material of the substrate significantly affect the adhesion: grinding makes the surface rough, and increasing the effective adhesion area can enhance the adhesion; Chemically reactive materials such as metals are easy to bond with coating films, while inert surfaces such as plastics require special treatment or special coatings to improve adhesion.
Adhesion testing
Due to the complexity of the phenomenon, there is no good method for adhesion detection, and it is especially difficult to quantify small changes. Commonly used scribing method (ISO standard): Use a 30° single knife to draw a 1mm spacing square on the sample (cut through the coating film), brush off the debris and grade according to the shedding area (no shedding in grade 0, more than 65% in grade 5). In CHINA, the tape method is mostly combined, and the degree of falling off is compared after sticking and tearing. This method is easily disturbed by factors such as scribing speed, tape stickiness, and peel-off angle, and the results have limitations, and there is still a lack of accurate quantitative benchmarks.
summary
Adhesion is one of the key indicators to evaluate the performance of powder coating films, and it is of great significance to have an in-depth understanding of adhesion and achieve better application on this basis. Only when the powder coating film has a certain adhesion, can it be firmly attached to the surface of the coated object, and then give full play to the high decorative performance and protection of the powder coating, and finally achieve the application purpose of the powder coating. In the future research and practice, it is necessary to further explore the theoretical basis of adhesion and improve the detection method of adhesion, so as to promote the powder coating industry to make greater progress in improving the adhesion of coating film and meet the needs of the developing industry and life.
