Benzoin in Powder Coatings: How a Degassing Agent Removes Pinholes
Category: Powder Coatings
Pinholes in a cured powder coating often come from gas trapped in the melting film. Benzoin, the standard degassing agent, keeps the molten surface open long enough for that gas to escape — this article explains how it works, how much to use, and where it cannot help.
1. Where Pinholes Come From
When a powder coating melts in the oven, gas has to get out of the film before the film sets. That gas comes from three places: air trapped between the powder particles, moisture carried in with the powder or the substrate, and small volatile molecules released by the crosslinking reaction itself. If the surface of the molten film skins over before the gas escapes, the gas breaks through the surface instead — and leaves a pinhole.
That is why pinholes are most obvious in high-gloss films at normal thickness (about 60–90 µm), and why they get worse as the film gets thicker: there is more gas to release, and a longer way for it to travel. A degassing agent does not change the chemistry of the cure; it buys the gas time to escape.
2. What Benzoin Is
Benzoin (2-hydroxy-2-phenylacetophenone; CAS 119-53-9) is the standard degassing agent for powder coatings. Deping’s benzoin is a white or light yellowish powder, melting range 133–137 °C, boiling point 344 °C, volatiles ≤ 1%. It is added during the premixing stage, preblended with the other dry raw materials and extruded together with them.
Deping lists benzoin for epoxy, hybrid, polyester/TGIC, and matte or textured powder systems. It is supplied in 20 kg PE-lined kraft paper bags and stored in a dry, cool place, away from sunshine and fire.
3. How Benzoin Works
The precise mechanism is still debated, but the effect most often described is this: benzoin keeps the surface of the molten film open — fluid and permeable — for longer, so trapped gas can escape before the film crosslinks and sets. In effect it delays the moment the surface skins over, which is exactly the moment that turns escaping gas into a pinhole.
That is also why the dose matters in both directions. Too little, and the surface sets before the gas is out. Too much, and the excess benzoin itself becomes a problem — see section 6 on yellowing. The right dose is the lowest one that clears the pinholes on your own test panels.
4. How Much to Use: Starting Values
Deping’s recommended dosage is 0.3–1% by weight of the total formulation, added at premixing; the exact amount is determined by laboratory testing. Published guidance for benzoin in general is in the same range (about 0.3–1.5% of the total formulation), with about 0.5% as a common starting point.
| System | Starting dosage (total formulation) | Note |
|---|---|---|
| General powder coatings (Deping recommendation) | 0.3–1.0% | Preblend at premixing; confirm by test panels |
| Polyester/HAA systems, light colours | About 0.3% | Keep the dose as low as pinhole control allows, to limit yellowing |
| Thick films (>80 µm) | Within 0.3–1.0%, but do not exceed it | More benzoin will not fix thick-film pinholes — see section 5 |
Test in a ladder — for example 0.3%, 0.5%, 0.8% and 1.0% — on the same substrate, film thickness and cure schedule, and judge two things together: pinholes and yellowing. All figures here are starting values; confirm them on test panels for your own powder and part.
5. Where Benzoin Cannot Help
Benzoin is very effective against the pinholes a formulation makes for itself, but it cannot solve every pinhole. The cases below need a different fix:
- Thick films (above about 120 µm): even a large dose of benzoin usually leaves visible pinholes. Thick-film pinholing needs other types of degassing agents used together with benzoin, or a thinner film.
- Substrate outgassing: cast iron, cast aluminium, hot-dip galvanised steel and hot-rolled steel can release gas from pores in the substrate itself. Pre-bake the part above the cure temperature before coating, and improve pretreatment; benzoin alone will not stop it.
- Back ionization from spraying: too-high voltage or too-close gun distance can also make pinholes (see article #3, Electrostatic Spraying). Fix the spray settings first.
- Contamination and craters: airborne contamination and oil or silicone on the part more often give craters than pinholes; degassing does not remove them.
- Low-temperature curing systems: benzoin cannot fully clear pinholes in some low-temperature-cure powders; other degassing agents may be needed.
In matte systems, pinholes are usually less visible than in high-gloss films, but the same gas is still there — judge a matte film by its surface under good light, not by gloss alone.
6. The Trade-Off: Yellowing
Benzoin’s main drawback is that it decomposes on heating and can yellow the film — most noticeably in white and light colours, and at high cure temperatures. The more benzoin is used, the stronger the effect. That is the real reason to keep the dose low, and the reason polyester/HAA systems, where yellowing is already a concern (see article #2, TGIC vs HAA), start at about 0.3%.
For whites and pale colours that must stay clean, modified benzoin and wax-based degassing agents that yellow less on baking have been developed; they are typically used at about 1% of the total formulation. Whichever agent is used, the choice is still made on test panels: pinholes on one side, yellowing on the other.
7. Adding Benzoin and Checking the Result
Preblend benzoin with the other dry ingredients before extrusion, so it is distributed evenly through the powder; uneven distribution shows up as patchy pinholing. Keep the powder dry in storage — moisture in the powder is itself a source of the gas benzoin is meant to release.
A practical check on a new formulation: spray the dose ladder from section 4 on identical panels, cure on the normal schedule, and look at the films side by side under the same light. Pick the lowest dose with no visible pinholes, then check the same panels for yellowing against a benzoin-free control before approving the dose.
8. Frequently Asked Questions
Q1: Does more benzoin mean fewer pinholes?
No. Above the recommended range it yellows the film, especially in light colours, and it will not fix thick-film or substrate pinholes at all. Find the lowest dose that clears pinholes on test panels.
Q2: Benzoin did not fix my pinholes — what next?
Check the cause first: substrate outgassing (pre-bake castings and galvanised parts), film above about 120 µm, back ionization from too-high voltage (see article #3), or contamination. Benzoin only clears gas from the melting film itself.
Q3: Can benzoin be used in every powder system?
Deping lists it for epoxy, hybrid, polyester/TGIC and matte or textured systems. Start at 0.3–1% of the total formulation and confirm the dose on test panels for each system.
There is no universal benzoin dose — only "starting values plus test-panel verification." Use the lowest dose that clears the pinholes, check it against yellowing on the same panels, and record the dose on the formulation card so the next batch repeats it.