Each week, CPCA’s Technical Committee publishes a practical tip drawn from member questions, field observations, and technical discussions. This week’s tip addresses ratio drift — one of the most insidious equipment problems in polyurea application because it can produce a coating that looks acceptable but performs poorly.
What Is Ratio Drift and Why Does It Matter?
Polyurea systems are formulated to react at a precise volume ratio of A-side (isocyanate) to B-side (resin blend) — typically 1:1 by volume. When your spray equipment delivers components at the wrong ratio, the chemistry doesn’t complete properly. Off-ratio polyurea can exhibit poor adhesion, reduced elongation, tacky surface, reduced chemical resistance, and in severe cases, failure to gel at all.
The problem is that mild ratio drift often produces a coating that passes casual visual inspection. It’s only when the coating is tested — or when it fails in service — that the problem becomes apparent. This is especially concerning for high-stakes applications like secondary containment or potable water applications where performance is not negotiable.
How to Check Your Ratio
The simplest ratio check uses a graduated catch container: collect output from each side simultaneously for a fixed time (say 15 seconds), then compare volumes. For a 1:1 system, volumes should be within 2% of each other. Many modern proportioners have built-in ratio monitoring, but these should be verified against manual checks periodically.
For more precision, gravimetric testing (weighing the output and calculating by density) is more accurate than volumetric testing and should be performed when troubleshooting suspected ratio issues.
Common Causes of Ratio Drift
Based on the field experience of CPCA members and equipment service technicians, the most common root causes of ratio drift are:
- Worn pump packings — As packing wears, pump efficiency drops unevenly. Because A-side and B-side pump components wear at different rates, ratio drift develops gradually. Regular packing replacement per manufacturer schedule (or more frequently with high daily shot volumes) prevents this.
- Heat exchanger fouling — Reduced heat transfer changes material viscosity and therefore flow characteristics. Monthly solvent flushing of heat exchangers is good preventive maintenance.
- Check valve sticking — Crystallized isocyanate on A-side check valves is a common cause of partial blockage. Maintaining proper A-side system purge procedures after use prevents buildup.
- Filter restriction — Partially clogged material filters create back pressure that affects pump output. Check and replace filters per schedule.
- Hydraulic pressure imbalance — If the hydraulic system delivering power to A and B pumps isn’t balanced, output will be unequal. This is a mechanical issue requiring service.
When to Call for Service
If ratio adjustment procedures don’t resolve the drift, or if it returns quickly after adjustment, the equipment needs professional service. Don’t continue spraying with out-of-ratio material to “use up” product or meet a deadline — the costs of coating failure dramatically exceed any short-term scheduling savings.
For members looking to build a robust equipment maintenance program, the recent BC Chapter technical seminar on spray equipment maintenance provides excellent foundational guidance.
Have a technical question you’d like addressed in a future Weekly Tech Tip? Submit it to the CPCA Technical Committee through the Contact page. Questions from members are prioritized.