Polyurethanes (MDI, TDI, polyols): Characteristics
How NCO content sets the mixing ratio
What this tab covers The physical and chemical properties that set its grade, price and uses, with the commercial grades and the standards that define them.Isocyanates are sold on NCO content, functionality and isomer mix; polyols on hydroxyl value, molecular weight and functionality. Together these set foam density, hardness and cell structure. Water content and reactivity matter because isocyanates react with moisture.
Properties that matter commercially
| Property | Value / specification | Why it matters | Source |
|---|---|---|---|
| NCO content | polymeric MDI about 31%; TDI 80:20 about 48% (indicative) | Sets mixing ratio with polyol | indicative, producer data sheets |
| Isomer ratio | TDI 80:20 or 65:35 2,4-/2,6-isomers | Controls reactivity in slabstock foam | indicative |
| Polyol hydroxyl value | flexible about 28-56 mg KOH/g; rigid about 300-500 (indicative) | Low OH gives soft foam; high OH gives rigid crosslinked foam | indicative |
| Thermal conductivity of rigid foam | about 0.022-0.028 W/m K (indicative) | Main selling point for insulation | indicative |
Commercial grades
- Polymeric MDI (rigid foam, binders)
- Pure (monomeric) MDI (elastomers, TPU, spandex, adhesives)
- Modified MDI prepolymers
- TDI 80:20 (flexible slabstock)
- Flexible, high-resilience and rigid polyether polyols
- Polyester polyols