Problem: Thick Skinning / Surface Film Formation in Storage
Definition (Skinning): Formation of a rubbery/leathery film (skin) on the paint surface during storage, caused mainly by surface water loss and premature latex coalescence at the air–paint interface. It leads to lumps, poor leveling, shade variation, nozzle/filter blockage, and customer complaints.
Quick clues (field symptoms)
- Skin present only at the top surface; bulk paint below looks normal.
- More frequent in partially used packs (large headspace).
- Worsens in hot storage or low-humidity environments.
- Often accompanied by poor open-time / fast drying at the surface.
Fast prevention checklist
- Reduce headspace, improve lid sealing/liners.
- Optimize humectant + coalescent balance.
- Check preservative program (bio-stability).
- Store 20–30 °C; avoid heat exposure.
| Cause (What drives skinning) | Mechanism / What happens | Remedy (What to do) | Suitable additive options |
|---|---|---|---|
| Surface water evaporation High headspace, warm storage, dry air |
Water loss concentrates polymer at top; latex particles pack and start forming a continuous film. | Improve sealing; reduce headspace; maintain moderate storage temperature; increase water-retention system. |
Humectants
Propylene glycol
Glycerol (limited)
Butyl diglycol*
*Also improves open-time; use within supplier limits.
|
| Premature coalescence High coalescent / favorable film-forming conditions |
At surface, latex particles fuse (coalesce) earlier than intended, creating a tough skin. | Rebalance coalescent level; select appropriate MFFT; ensure correct thickener system; avoid over-coalescing. |
Coalescent optimization
Lower VOC coalescents
MFFT tuning
Choose based on resin + temperature range.
|
| High solids / high binder Lower free water, faster surface set |
Less free water means surface dries faster; polymer concentration rises quickly, encouraging skin formation. | Optimize solids; adjust PVC/CPVC balance; use water-retention and open-time additives. | Open-time extender Humectant blend Cellulosic/HASE tune |
| Low headspace humidity Partially used packs; repeated opening |
Dry air in the container accelerates evaporation at the surface. | Recommend decanting; supply smaller pack sizes; instruct to close lid immediately; consider inert headspace. | Packaging liner upgrade Decant guidance N₂ blanketing (bulk) |
| Surfactant/protective colloid imbalance Surface depletion, instability |
Top layer becomes less stabilized; particles can pack/aggregate and form a tougher surface film. | Rebalance surfactant package; use compatible wetting/dispersing agents; validate with storage tests. | Wetting agent (nonionic) Dispersant (polymeric) Protective colloid system |
| Microbial activity (in-can) Preservative under-dose, contamination |
Biodegradation of stabilizers and rheology modifiers can change surface behavior and accelerate skinning / lumps. | Strengthen preservative system; improve hygiene; sanitize tanks/lines; verify pH window for preservative efficacy. |
In-can preservative
Biocide (supplier system)
Select per regulations & compatibility; avoid off-odour issues.
|
| Temperature cycling Hot–cool cycles during transport/storage |
Repeated cycles drive evaporation/condensation patterns and can promote surface film growth and thickening. | Avoid heat exposure; improve logistics; use thermal-stable packaging; run accelerated stability tests. | Stability additive review Packaging improvement |
| Incorrect thickener balance Over-structured surface layer |
Some rheology profiles can “structure” the surface, reducing flow-back and encouraging a surface film. | Optimize thickener blend (HEUR/HASE/cellulosics); target balanced low-shear viscosity and flow. | HEUR tuning HASE tuning Cellulosic (HEC) tuning |
| Air entrainment / poor deaeration Foam at top accelerates drying |
Foam increases surface area, promoting faster water loss and film formation at the top. | Improve defoamer selection; reduce foam during filling; allow deaeration before packing. | Defoamer (mineral/silicone) Deaeration step |
Recommended lab checks (quick)
- Storage test: 40 °C (7–14 days) and ambient (30 days); observe skin thickness and re-dispersibility.
- Headspace test: compare full can vs half-filled can to isolate evaporation-driven skinning.
- pH + biostability: verify pH window; check for odour / viscosity drift / microbial growth signs.
- Rheology curve: low-shear viscosity vs high-shear; optimize thickener package for flow-back.
Note: Additive choice and dosage must be finalized based on resin type, VOC target, regulatory limits, and stability tests.