Unstable Adhesion in Baked Coatings: What to Check Beyond Substrate Treatment

Normal initial adhesion after application and drying does not mean the coating keeps the same state after long use or other tests. After soaking in water at room temperature, salt-spray testing or repeated wiping, adhesion may change. In multi-coat systems, bonding between primer and topcoat may also be insufficient. Real performance cannot be judged from a single adhesion test. Besides substrate cleanliness and surface treatment, resin system, film crosslinking state and interlayer bonding deserve a place in formulation analysis. For coatings with several performance requirements, an adhesion promoter is one adjustment option.

Substrate adhesion is the bond between the film and metal, plastic or another coated material; interlayer adhesion is the bond between different coating layers. The interfaces differ, and so do the influencing factors. Even if adhesion on the substrate meets requirements, bonding between a later coat and the layer beneath may still be insufficient. In multi-coat systems, examine the two separately and do not judge the whole system from one test result.

Water may enter through the film or the coating edge and affect the interface between coating and substrate. The degree of change depends on film structure, degree of cure, substrate treatment and coating composition. If the coating must withstand soaking or boiling-water tests, look beyond initial adhesion and observe the film before and after treatment under the relevant test conditions. To improve this, first check the curing process and coating structure, then evaluate whether the resin and adhesion promoter are well matched.

In use, a coating may be stretched, bent, impacted or wiped repeatedly. If the elongation and mechanical properties of the film do not match the service conditions, cracks or local peeling may follow even when initial adhesion was acceptable. Formulation design should therefore consider adhesion, elasticity, tensile strength and wipe resistance together, not adjust a single index.

Check substrate treatment and application. Oil contamination, an unsuitable surface state, unreasonable recoat intervals or insufficient curing conditions can all affect final adhesion.

Check resin system and coating structure. For amino baking enamels, two-component polyurethane and similar systems, evaluate the formulation against resin composition, curing conditions and actual service requirements.

Add an adhesion promoter and examine its effect on substrate adhesion, interlayer bonding and film mechanical properties. If water, salt-spray or wipe resistance is also needed, arrange the corresponding tests so that only initial adhesion is not improved while later service conditions are ignored.

DH-7326N is a polyether-modified siloxane copolymer. According to the product data, it does not hydrolyze, can form good bonding with the contact surface, helps raise film crosslink density, and does not affect storage stability of the system.

It can be used to improve substrate adhesion and interlayer adhesion, and to help raise tensile strength, elasticity, mechanical properties and wipe resistance of the film. Coating systems that undergo room-temperature water soaking, salt-spray or boiling-water tests can also be evaluated with it, depending on the formulation. It suits amino baking enamels and two-component polyurethane (2K PU). For two-component systems, the product data also notes that it helps extend pot life; the actual change needs verification in the specific formulation.

Set up a blank and control samples with different addition levels in the existing formulation, and test initial adhesion, interlayer adhesion, water soaking, salt-spray and wipe resistance. For two-component systems, also record usable time after mixing and observe application state and final film performance. An adhesion promoter cannot replace proper substrate treatment and curing process, and the specific addition level should be set according to resin type, coating structure and test results.

Does normal initial adhesion guarantee later performance?
No. After water soaking, salt-spray testing or repeated wiping, adhesion may change, so real performance cannot be judged from one test.

Why test substrate and interlayer adhesion separately?
Their interfaces and influencing factors differ, and good substrate adhesion does not rule out insufficient bonding between layers.

Which systems does DH-7326N suit?
According to the product data, amino baking enamels and two-component polyurethane (2K PU), at a suggested 0.5%–1.5% of total coating.

Can an adhesion promoter replace substrate treatment?
No. It cannot replace proper substrate treatment and curing process.