Some Polyurethane Calculation Formulas

To support learning and understanding, this article introduces several key calculation formulas and basic concepts that professionals in polyurethane technology and sponge making machine operations must master. These formulas are not just theoretical—they play a direct role in determining foam quality and improving production efficiency.

1. Hydroxyl Groups

Hydroxyl Equivalent: The weight of resin equivalent to one hydroxyl group, calculated as:

E(OH)=Weight of Resin/Number of Hydroxyl Groups in the Resin Molecule

Hydroxyl Content: The weight percentage of hydroxyl groups in every 100 grams of resin, calculated as:

OH% =(N*17/ Weight of Resin)*100%

where N is the number of hydroxyl groups.

Hydroxyl Value: The number of milligrams of potassium hydroxide equivalent to the hydroxyl content per gram of sample, calculated as:

Hydroxyl Content=1700 / Hydroxyl Equivalent

Hydroxyl Value=56100 / Hydroxyl Content= Hydroxyl Equivalent*33

2. Isocyanate Index

When using sponge making machines to adjust foam formulations, isocyanate index directly affects properties such as cell structure, hardness, and rebound resilience. 

TDI Index: The ratio of the actual amount of TDI used to the theoretical amount of TDI, calculated as:

TDI Index=Actual TDI Amount Used / Theoretical TDI Amount

Equivalent: Calculated as:

Equivalent=Molecular Weight / Functionality

For TDI:

TDI Equivalent=4200 / NCO%

TDI Amount: Calculated as:​

TDI Amount=(TDI Index / 100)*TDI Equivalent*(100 / Polyol Equivalent+Water Content / Water Equivalent)

where Polyol Equivalent=56100 / Hydroxyl Value, Water Equivalent=9.

For MDI, the formulas are the same, substituting TDI with MDI.

NCO Content: The content of isocyanate groups (NCO), usually expressed as a percentage.

3. Reaction Rate

Reaction rate refers to the change in concentration of substances in a chemical system over time. In polyurethane foaming, parameters such as temperature, catalysts, and concentrations significantly influence reaction time and quality, which directly impacts sponge making machine cycle settings and efficiency.

• The average rate is commonly expressed in mol/(dm³·s)

• Influencing factors: pressure, temperature, catalyst, concentration, and solvent

4. Tensile Strength

Tensile strength refers to the stress at which a material exhibits maximum uniform plastic deformation. In tensile testing, the maximum tensile stress a sample withstands until it breaks is the tensile strength, expressed in MPa. It is also referred to as tensile strength or tensile resistance.

When testing tensile strength with instruments, data such as tensile fracture stress, tensile yield stress, and elongation at break can also be obtained.

Tensile Strength: Calculated as:​

Tensile Strength=Maximum Load / (Sample Width*Sample Thickness)

Elongation at Break: Calculated as:

Elongation at Break=(Length at Break−Initial Length) / Initial Length*100%

Peel Strength: The maximum destructive load per unit bonding area, representing the force required to peel the bonded surfaces per unit width of the sample. It is expressed in N/cm, N/m, or kN/m.

Understanding the formulas is only the first step. Achieving stable and high-quality foam production requires reliable polyurethane equipment. We specialize in delivering advanced sponge making machines and custom foaming solutions to help you precisely control the reaction process, reduce waste, and boost production consistency.

Contact us today for tailored equipment suggestions, pricing, and technical support!