Common Problems in Isocyanate Drum Storage After Purchase

This article reviews four key isocyanate storage issues: viscosity increase, crystallization, discoloration, and NCO value reduction, helping production, purchasing, and quality-control teams manage the main risks before use.

For Production Managers and QC Problem-focused guide Practical for MDI and TDI
April 21, 2026 About 6 min read Practical Knowledge Hub

How Can Isocyanate Quality Loss Be Prevented in Storage?

Isocyanates are sensitive materials, and improper storage can negatively affect reactivity, viscosity, color, and final foam behavior. This page helps teams identify warning signs quickly and review practical methods for controlling storage conditions.

What Will You Learn in This Article?

1. Viscosity Increase (Thickening)

One of the most common isocyanate storage problems is increased viscosity, which directly affects pumping, mixing, and final foam quality.

Viscosity Increase (Thickening)
NCO + H₂O ⟶ Urea + CO₂
  • Cause:Moisture absorption from air and the reaction of isocyanate with water, leading to polyurea formation and higher molecular weight.
  • Quality impact:Harder pumping and mixing, lower free NCO, and reduced reactivity with polyol.
  • Foam consequence:Higher density, lower flexibility, and reduced mechanical properties.
Practical solution:Store drums in a dry environment with relative humidity below 50%, keep lids fully sealed, and prevent material overheating in the warehouse.

2. Crystallization, Especially in MDI

Pure MDI and some polymeric MDI grades are prone to crystallization at low or fluctuating temperatures. This can create handling and usage problems on the production line.

Crystallization, Especially in MDI
  • Cause:Temperature drop below the stable range and the presence of certain isomers with higher crystallization sensitivity.
  • Quality impact:Non-uniformity, transfer-line blockage, and altered material reactivity.
  • Important range:Pure MDI is commonly considered prone to crystallization below about 15 to 18 °C.
Practical solution:Keep the temperature stable at about 20 to 25 °C. If crystals form, warm the entire drum gradually and uniformly to about 40 to 50 °C, then homogenize the material.

3. Discoloration (Yellowing or Darkening)

Discoloration in isocyanates does not always have the same significance; its importance depends on the isocyanate type and final application.

Aromatic IsocyanatesTDI and MDI may turn yellow to dark brown when exposed to air and light.
Aliphatic IsocyanatesDiscoloration is usually limited in this group, so noticeable color change should be treated more seriously.

3-1. In Aromatic Isocyanates

Color change is usually caused by oxidation and UV exposure affecting the aromatic structure. In mild to moderate cases, severe physical-performance loss does not necessarily occur, but strong darkening may indicate heat exposure, metal contamination, or more serious structural changes.

3-2. In Aliphatic Isocyanates

This group is inherently more resistant to yellowing, so any noticeable discoloration should be reviewed more carefully, especially in color-sensitive applications.

Practical solution:Dark storage, dry nitrogen after sampling, temperature control, and preventing contact with metals that catalyze oxidation are among the most important preventive measures.

4. NCO Value Reduction (Activity Loss)

Even if the material looks unchanged, gradual loss of active NCO groups can disrupt the formulation ratio and change production-line results.

NCO Value Reduction (Activity Loss)
  • Cause:Gradual NCO reaction with moisture or the formation of structures such as dimers, allophanates, and urea at higher temperatures.
  • Quality impact:Disruption of the isocyanate-to-polyol ratio and the appearance of soft orunder-cured.
  • Operational risk:Using the material without checking the NCO value can lead to incorrect formulation adjustment and quality instability.
Practical solution:Keep storage time short, apply FIFO, and check the NCO value by titration before use.

5. Quick Isocyanate Storage Control Checklist

If you are responsible for purchasing, warehousing, or production, control these six items routinely.

  • Keep warehouse relative humidity low and controlled.
  • After opening, protect drums with complete sealing and, if needed, dry nitrogen.
  • Keep warehouse temperature stable within the recommended range for the material grade.
  • Do not expose materials to direct light or heat sources.
  • Apply inventory rotation based on FIFO.
  • Before use, check appearance, viscosity, and NCO value when needed.
Proper isocyanate storage is not just a warehouse issue; it directly affects foam quality, process stability, and scrap levels.
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