Client context
Alkyd resins are polymers ester resins produced by esterification of phthalic anhydride, fatty acids, and glycerol in a reactor operating above 220 degrees Celsius. They form the base of oil-based, alkyd, and alkyd-acrylic paints.
Quality and repeatability are determined by hitting the esterification endpoint, which is directly governed by two parameters: acid value and viscosity. The acid value decreases as the reaction progresses, and its control is challenging because the kinetics vary depending on the batch of drying oils.
Challenge.
- The acid value was determined offline by titration (ISO 3682), requiring 30–60 minutes, which necessitates stopping the reactor and manually sampling the medium above 220 degrees.
- Variable reaction kinetics make it difficult to predict the end of esterification, and any overshoot of the target requires correction with excess solvent or results in an off-spec batch.
- The medium contains flammable organic solvents (white spirit D40, xylene), so the measurement must operate in an ATEX-rated zone.
Solution
We performed the measurement using a Spectrally™ X1 analyzer with an in-line immersion probe (785 nm laser), without sample extraction and under process-like conditions. The acid value has no single, dedicated spectral band, so we built a chemometric model (CNN network) correlating the entire Raman spectrum with the parameter, based inter alia on the ratio of acid/ester carbonyl group bands.
The study was conducted in two stages: first at-line on samples, then with an immersion probe under process-like conditions. We also honestly noted the limitations, namely the labor-intensive model building and sensitivity to signal stability, which we are addressing before pilot implementation.
Results
| Metric | Result |
|---|---|
| Model vs. laboratory correlation (acid value) | R = 0.982–0.998; MAE error approximately 0.015 mg KOH/g |
| Additional parameter from the same spectrum | viscosity, R = 0.988–0.998 |
| Time to result | real-time, instead of 30–60 min of titration |
Result of a feasibility study (technical proof), not implementation. Client anonymized.
What this means for your process
If you produce alkyd resins or varnishes and control esterification based on titration, the same measurement allows you to move endpoint control directly into the reactor and obtain both acid value and viscosity simultaneously from a single spectrum, also in ATEX zones. This shortens the cycle and reduces the risk of off-spec batches.
The most reliable way to verify on a specific formulation is a short feasibility study on real samples from the given process.
