How to Precisely Calculate Isocyanate (ISO) Amount in High-Resilience Polyurethane Formulations?

In the process of designing and adjusting formulations for high-resilience (HR) polyurethane foam, the Isocyanate Index (ISOCYANATE INDEX) is a critical parameter. It directly affects final product hardness, density, resilience, and the open-cell or closed-cell condition of the foam. To precisely control product quality, formulators must calculate and master the theoretical ISO demand of the combined polyether (POLYOL BLEND).

1. Definition of the Isocyanate Index

The isocyanate reaction index is defined as the ratio of the

ISO mass added to the theoretically required ISO mass, usually expressed as a percentage.

INDEX = (ISO actual mass / ISO theoretical mass) × 100%

Where:

• ISO theoretical mass: The mass of ISO required for complete reaction with all active-hydrogen components in the polyol blend at a 1:1 stoichiometric molar ratio.

• ISO actual mass: The ISO mass actually used in the formulation.

In HR foam production, the reaction index is usually set between 100 and 115 to ensure full reaction and obtain the required performance.

2. Core Parameter: Theoretical ISO Demand per Unit Polyol Blend (RATIO)

To simplify the work of formulators, we introduce a relative parameter called RATIO. It represents the theoretical ISO mass consumed per unit mass of the polyol blend.

RATIO = ISO theoretical mass / total weight of the polyol blend

Note: “Total weight of the polyol blend” refers to the sum of all active components in the formulation.

3. Precise Calculation Procedure for RATIO and ISOCYANATE INDEX

The calculation of RATIO and INDEX is based on the 1:1 chemical stoichiometry between active hydrogens (ACTIVE H) and NCO groups.

Step 1: Identify all active-hydrogen components

Reactive materials include: polyether types (base polyether, POP, cell-opener polyols) and low-molecular components (water, crosslinkers).

Important reminder: All water must be counted, including added water and moisture contained in raw materials.

Step 2: Calculate the molar amount of active hydrogens (N_H)

Convert the weight of each material into moles of active hydrogen.

For polyether types (based on hydroxyl value OHV):
N_H = (material weight in g × OHV in mgKOH/g) / 56100
(Note: 56100 equals 56.1 × 1000, the mg KOH amount corresponding to 1 mol OH.)

For low-molecular materials (water, crosslinkers), based on molecular weight M and functionality F:
N_H = (material weight in g × F) / M

Step 3: Calculate total theoretical NCO molar amount (N_NCO)

Add up the active-hydrogen moles from Step 2 to obtain the total active-hydrogen moles (sum N_H).
By 1:1 stoichiometry:
N_NCO = sum N_H

Step 4: Calculate theoretical ISO mass (ISO_THEORY)

Convert the theoretical NCO molar amount into ISO raw material mass using the NCO group molar mass 42 g/mol and the NCO% of the ISO raw material.

ISO_THEORY (g) = (N_NCO × 42) / NCO%

Step 5: Calculate RATIO

Divide the ISO_THEORY from Step 4 by the total weight of all active components (the polyol blend) in the formulation.

RATIO = ISO_THEORY / total weight of the polyol blend

Step 6: Calculate the Isocyanate Index (INDEX)

Using the actual ISO mass used in the formulation:

INDEX = (ISO_ACTUAL / ISO_THEORY) × 100%

Accurate calculation of RATIO and INDEX is the essential first step in successful polyurethane formulation design. By mastering these stoichiometric relationships, formulators can effectively predict and adjust foam hardness, density, and resilience, ensuring optimized product performance and stable, economical production. Only on the basis of a precise theoretical ISO demand can one fine-tune properties through controlled adjustment of the actual ISO charge (the Index).