Poor adhesion

 

 

 

Mechanisms: There is no chemical cross-linking between single-liquid primer and UV topcoat. Basically, the intermolecular forces (including van der Waals force and hydrogen bond force) between primer and topcoat, as well as the mutual dissolution between primer and topcoat and the physical anchoring effect caused by penetration, require the matching between primer and topcoat. Primers are too hard to bite, UV topcoats are too weak to form good interlayer bonding, primers are too soft to bite, blossom and fog, and the same topcoats are too thick, too large volume shrinkage during free radical polymerization has an impact on adhesion.

 

 

 

Possible causes affecting adhesion:

 

 

 

1. High hardness of primer:

 

 

 

Too long time after coating and too thorough drying, and too high content of aluminium powder in aluminium powder paint are important factors affecting interlayer adhesion.

 

 

 

2. Reasons for UV topcoat:

 

 

 

The adhesion of resin and monomer itself is not high; or the volume shrinkage of free radical polymerization is too large due to the high functionality of formulation design (especially when the film is too thick), or the UV curing is not complete and the molecular chain is not formed long enough. Solvents are weak or volatilize too quickly, which leads to insufficient secondary solubility of primer and plastic substrates. The surface tension is too high to wet and level the primer sufficiently, and the formulation design is unreasonable.

 

 

 

Solution:

 

 

 

1. Adjust the hardness of primer, increase the baking temperature and prolong the baking time. The content of aluminium powder should be controlled in a reasonable range, and the primer system matched with UV should be used.

 

 

 

2. Controlling the volume shrinkage of UV film, adjusting the solvent solubility and volatilization rate, adjusting the film thickness, initiator dosage and lamp power, re-selecting raw materials and adjusting the formula.

 

 

 

Influencing factors and solutions of hardness:

 

 

 

Definition of hardness: Coating hardness generally refers to the pencil test hardness of the coating surface. The test results mainly depend on the hardness of the coating itself, but also related to the toughness and surface smoothness of the coating.

 

 

 

Possible reasons for hardness:

 

 

 

Primer hardness and coating thickness; the structure and function of the selected main UV resin, the higher the monomer functionality, the larger the volume shrinkage, the worse the adhesion when thick coating; cross-linking density of double bonds of propionic acid in coating; UV infrared leveling temperature and time, UV exposure energy and dry film thickness.

 

 

 

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