This document discusses stoichiometric calculations for NCO/OH systems. It defines key values needed like resin percentage and solids, resin OH value, and equivalent weights of NCO and OH. It explains that the NCO to OH ratio should be 1:1 for proper formulation. It also defines acid number and hydroxyl number, providing equations to calculate hydroxyl number from %OH and polyol equivalent weight from %OH number. The document states the amounts of NCO and resin can be derived based on hardener NCO value, resin OH value, and equivalent weights of NCO and OH.

1. NCO / OH STOICHIOMETRIC CALCULATION:
Values required to find NCO Part
 Resin Percentage and Solids
 Resin OH Value
 Equivalent weight of NCO and OH
 NCO Value.
Before formulating any 2KPU system, we should know some definition which is useful to
understand the concept of Stoichiometry. (NCO=OH Should be 1:1 ratio)
Acid Number.
The amount of residual acidic material in a Polyol.It is reported in the same as hydroxyl
number.
Hydroxyl Number:
It is milligrams of KOH equivalent to the hydroxyl content of one gram of Polyol.
OH Number = 56.1×1000
Eq.wt
Where 56.1 is Atomic weight of KOH.
1000 is No of milligram in one gram of Polyol.
Sometimes Polyols quoted by weight percentage of hydroxyl groups.Normally supplier will
give OH number on supply either on Solids.
OH number = 33* % of OH
Where, 33 obtained by reduction of constants.
Polyol Eq.Wt = 17*100 / (% of Hydroxyl number)
NCO Eq.wt = 42*100 / ( % NCO)
Therefore we could derive the formulation based on the above theoretical considerations
Amount of NCO = Amount of Hardener × NCO Value / Eq .wt of NCO
Amount of resin×%NVM×OH value of resin/ Eq .wt of OH