Bubbles, blisters, pinholes: diagnosing moisture-related defects in PU coatings

The industry problem 

Moisture remains the most persistent and least controlled variable in polyurethane (PU) coating application. Even in well-designed systems, ambient humidity, substrate moisture, and residual solvent water can trigger defects that only become visible after cure: 

Bubbles / foaming – gas evolution during film formation 

Blisters – localised pressure pockets beneath the film 

Pinholes – microvoids formed by gas escape through a partially cured coating matrix 

For formulators and applicators, these defects are often treated as processing issues. In reality, they are chemical events coupled with application conditions, particularly in polyisocyanate-based systems. Understanding how and when moisture interacts with the chemistry is critical to eliminating these defects—not just mitigating them.

Why moisture creates defects in polyisocyanate systems 

In conventional PU coatings, isocyanate groups (–NCO) are highly reactive toward water. Even trace moisture initiates side reactions: 

1. Isocyanate + water → unstable carbamic acid 
2. Carbamic acid → amine + CO₂ (gas evolution) 
3. Amine + isocyanate → urea linkage (fast reaction)  

The key issue is CO₂ generation during film formation: 

• If gas evolves before viscosity builds, it escapes → minimal defects 
• If gas evolves during gelation, it becomes trapped → bubbles and pinholes 
• If gas evolves beneath a skinned surface, pressure builds → blisters 

Why application environment matters 

Moisture defects are rarely formulation-only problems. They are governed by: 

Relative humidity (RH): higher RH increases water uptake into the film 

Substrate temperature: cold substrates promote condensation 

Film thickness: thicker films trap gas more easily

Cure speed vs diffusion rate: fast surface cure traps subsurface gas  

The mismatch between reaction kinetics and gas diffusion is what ultimately drives defect formation. 

What was developed: moisture-activated curing control with Aldirez®‑211 

Aldirez®‑211 introduces a different approach to managing moisture in PU systems. 

It is a blocked cycloaliphatic diamine, designed to remain chemically latent during storage and early application, but activate in the presence of moisture. 

Key chemistry (only what matters in application) 

• The active diamine is blocked (latent) → no premature reaction with isocyanate 
• Upon exposure to moisture, it hydrolyses to release the free diamine
• The liberated diamine reacts rapidly with isocyanates to form urea linkages 

This creates a triggered curing mechanism, where moisture does not simply cause side reactions, it initiates a controlled primary reaction pathway. 

Why this changes defect formation 

1. Controlled activation vs uncontrolled side reaction 

In conventional systems: 

• Moisture → uncontrolled side reaction  → CO₂ formation → coating defects 

With Aldirez®‑211: 

• Moisture → predictable activation point → rapid urea formation 

This shifts moisture from a destabilising factor to a controlled curing trigger.  

2. Faster network build at the right moment 

Once hydrolysed, the diamine reacts faster than typical polyol pathways, leading to: 

• Rapid viscosity build after activation 
• Earlier formation of a coherent polymer network 
• Reduced time window where gas can grow and coalesce 

This helps suppress: 

• Bubble growth 
• Gas coalescence into blisters  

3. Improved balance of cure vs gas escape 

Defect formation is governed by a balance between: 

• Gas generation 
• Gas diffusion 
• Polymer Network formation 

Aldirez®‑211 improves this balance by: 

• Delaying primary reactivity until moisture is present 
• Then accelerating cure locally and quickly 
• Preventing gas from accumulating into large voids 

Result: finer morphology, fewer visible defects, more uniform films  

Performance advantages in application  

Formulators using moisture-activated diamine systems typically observe: 

• Reduced bubbling and pinholing, especially under variable humidity 
• Lower blister incidence on challenging substrates 
• Improved film integrity in thicker applications 
• More reproducible finishes across environmental conditions 

Importantly, this is achieved without relying solely on environmental control, which is often impractical in field applications.

Application relevance 

Aldirez®‑211 is particularly valuable in situations where moisture cannot be fully controlled: 

• Floor coatings  (substrate moisture variability) 
• Protective coatings applied in outdoor or semi-controlled environments 
• Thick-film PU systems where gas escape is limited 

It is most effective when used to stabilise coating performance across real-world variability, not just optimise ideal lab conditions. 

Formulation guidance and implications

To maximise performance, consider the following: 

1. Positioning in the formulation 

• Use Aldirez®‑211 within systems where moisture exposure during application is expected 
• Particularly effective in 2K PU systems with polyisocyanate components 

2. Balance reactivity

• Ensure the overall system does not gel too rapidly at the surface, which could still trap gas 
• Pair with appropriate catalyst levels and polyol selection 

3. Film build strategy 

• Allows for higher confidence in thicker films, but optimisation is still required 
• Evaluate wet film thickness vs cure profile 

4. Environmental robustness testing 

Validate under:  

• High RH conditions 
• Cold substrate scenarios 
• Real application workflows 

Practical takeaway 

Moisture in PU coatings cannot be eliminated—but it can be managed chemically. 

By shifting from: 

Uncontrolled moisture side reactions  to  moisture-triggered, controlled curing mechanisms 

Aldirez®‑211 enables more predictable film formation, reducing the root cause of bubbles, blisters, and pinholes rather than only treating symptoms.