How to Determine Powder Coating Curing

Scope

This method describes a field qualitative test to determine the proper degree of cure for a powder coating primer in order to maximize the inter-coat adhesion properties between itself and the final topcoat. As a powder primer approaches and obtains full cure, the chances for optimal inter-coat adhesion properties will diminish. In severe over-cure cases this may lead to de-lamination of the top-coat to the primer.

Solvent testing of the primer may be completed during the coating process to insure the right amount of cure is obtained to promote proper inter-coat adhesion. Chemical resistance and cure schedules will vary with different primer selections. The information contained in this document may be used as a reference for PT-ES49-ES09 ATP AG M Gray.

Pictures are included at the end of this document as a visual reference. However, the best comparative method is to have laboratory test panels as a reference at each manufacturing facility.

Materials

  • 100% MEK solvent or a blend of 10% MEK 90% Xylene.
  • Q-tip cotton swabs

Procedure

Fully immerse a cotton swab in the selected solvent. Immediately apply swab to the surface of the gelled primer and proceed with 50 double-rubs using strokes approximately 3.5" in length. One double-rub would be considered a forward motion followed by a return to the starting point. The test will be completed in one continuous cycle until 50 double-rubs are achieved. Results to be evaluated immediately.

Evaluation

As a powder coating is subjected to heat, its cure proceeds along various stages. Descriptions of stages 2 – 5 are referenced below with the accompanying chemical resistance results in Table 1.

  1. Pre-Melt
    The initial stage will have an appearance similar to freshly applied powder. Powder will be easily scratched off. Application of a low-moderate PSI air hose will not blow powder off in a cloud, but may be able to blow chip sections off. Topcoat may be applied, but not recommended.
  2. Continuous film
    Smoothness will be of a tight orange peel appearance. Typically the film will have a gloss higher than the specification for cured powder. Product will re-flow if exposed to heat. Any type of impact will result in shattering and severe loss of film adhesion.
  3. Gel
    Product has formed a continuous film and enough reaction has taken place in order to advance to a pre-cure state. It will not re-flow if exposed to heat. Impact will typically cause severe cracking with moderate loss of adhesion. Cure advancement is considered 50-70%. This is the optimal "Green Cure" stage.
  4. Pre-Cure
    The coating will have advanced to a stage where impact resistance is starting to develop. Cracking will be apparent with slight loss of adhesion possible. The gloss level will be slightly higher than a fully cured film. Chemical resistance remains poor and easy to differentiate between a fully cured film.
  5. Full Cure
    Optimal film properties will be achieved. In a primer application, cure has advanced to a stage where chemical interaction is minimized thereby decreasing the opportunity to reach full inter-coat adhesion properties.
  6. Over Cure
    The film will start to lose some flexibility, potentially shifting yellow in color and gloss may decrease below the expected full cure reported levels. Inter-coat adhesion is compromised and catastrophic failures likely.
Table 1: Chemical Resistance Ratings - MEK
Cure Stage Chemical Resistance Cure Cycle Topcoat Adhesion
Panel 1 – Continuous Film Penetration to substrate. Severe blistering. Full color transfer. 20 Minutes @ 250F Recommended
Panel 2 – Gel Severe film softening with moderate blistering. Full color transfer 20 Minutes @ 300F Optimal
Panel 3 – Pre-Cure No blistering. Complete loss of gloss. Full color transfer. 20 Minutes @ 325F Recommended
Panel 4 – Full Cure* No blistering. Slight film penetration, gloss loss. Slight color transfer – green pigment primary. 20 Minutes @ 350F Not Recommended

* For the purposes of this specific primer, full cure for top-coating is being defined at this stage. Impact, Chip Resistance, and Solvent Resistance will continue to develop through a second bake stage.

Table 2: Chemical Resistance Ratings – 10% MEK 90% Xylene Blend
Cure Stage Chemical Resistance Cure Cycle Topcoat Adhesion
Panel 1 – Continuous Film Slight blistering and film wrinkling. Full color transfer. 20 Minutes @ 250F Recommended
Panel 2 – Gel Severe film softening with complete loss of gloss. Full color transfer. 20 Minutes @ 300F Optimal
Panel 3 – Pre-Cure Slight film softening with moderate amount of solvent penetration into film. Color transfer of green pigment only. 20 Minutes @ 325F Recommended
Panel 4 – Full Cure* Slight film softening with a small amount of solvent penetration into film. Color transfer of green pigment only. 20 Minutes @ 350F Not Recommended

* For the purposes of this specific primer, full cure for top-coating is being defined at this stage. Impact, Chip Resistance, and Solvent Resistance will continue to develop through a second bake stage.

 
Picture Representation
Panel 1 – Continuous Film Panel 2 - Gel
Picture 3 – Pre-Cure Picture 4 – Full Cure*