37078922 urethane calculations

Equivalent weightThe equivalent weight eq.. tells you how many grams of a product you need to have for one equivalent of reactive groups.. For an isocyanate, the reactive group is -N=C=O

Equivalent weight

The equivalent weight (eq wt.) tells you how many grams of a product you need to have for one equivalent of reactive groups For an isocyanate, the reactive group is -N=C=O (NCO), and its concentration is measured by weight percent NCO.

Isocyanate equivalent weight = 4,200

g/eq NCO

Example:

Isocyanate eq wt of Suprasec  5025 = 4,200

= 136 g/eq

For a polyol, the reactive group is -O-H (OH) OH concentration is measured by the OH value (mg KOH/g sample):

Equivalent weight of a polyol = 56,100

g/eq

OH Value

Example:

Eq wt of ethylene glycol = 56,100

= 31.0 g/eq

Molecular weight of a polyol

The molecular weight (mol wt.) of a polyol is approximated by the polyol equivalent weight (eq wt.) multiplied by the nominal functionality (fn).

Polyol mol wt = (eq.wt.)(fn) = 56,100

(fn) g/mole

OH value

Mol wt of any diol = 56,100

(2) g/mole

Mol wt of any triol = 56,100

(3) g/mole

Example:

Mol wt of ethylene glycol = 56,100

(2) = 62.0 g/mole

1,810

Total weight of MDI required for reaction

MDI = Diphenylmethane Diisocyanate

When reacting an isocyanate with one or more polyols to form a polyurethane, one NCO group reacts with one OH group When the number of NCO groups equals the number

of OH groups, you have a stoichiometric NCO : OH ratio of 1.0 This ratio is commonly referred to as the index To determine the amount of MDI required to react with a given polyol blend, you must know the desired index (often 1.0), the MDI equivalent weight (MDI eq wt.), and the weight fractions (pbw) and equivalent weights of the polyols and any water present in the blend.

Total weight of MDI required =

(index)(MDI eq wt.) pbw polyol A + pbw polyol B +………+ pbw polyol N + pbw H

2 O

Example formulation:

Say you have the following polyol blend and would like to determine the amount of Suprasec  5025 to add to achieve a fully reacted polyurethane with a slight excess of isocyanate (target index = 1.05)

Butane diol (BDO); (eq wt = 45) 20.00

Suprasec  5025 (% NCO = 31.0%) X

X = pbw MDI = (index)(MDI eq wt.) 100 + 20 + 0.15

eq wt polyol A eq wt BDO eq wt H2O

X = (1.05) 4,200 100 + 20 + 0.15 = 190 pbw MDI

31.0 56,100 45 9

490

Calculation for formulating a prepolymer

to a specific percent NCO (or equivalent weight)

To calculate an MDI prepolymer,

Let N = desired NCO of the prepolymer (expressed as a fraction)

Let X = equivalent weight of the diisocyanate

Let Y = equivalent weight of the polyol (or average equivalent weight of the polyol blend)

Total amount of MDI needed = X + N (X+Y)

42 - N X

Example:

Say we want a 10.0% NCO prepolymer of Suprasec  1306 (%NCO = 33.5%) with a polyol blend consisting of 95% polyol B (OH value 28) and 5% ethylene glycol

(OH value 1,810) by equivalent weight:

N = 0.10

X = 4,200

= 125 g/eq

33.5

Y = 56,100

(0.95) + 56,100

(0.05) = 1,905 g/eq

28 1,810

Total amount of MDI needed = 125 + 0.10 (125+1,905)

= 985 pbw Suprasec  1306

42 - 0.10 125

So 1,903 pbw polyol B, 1.55 pbw ethylene glycol, and 985 pbw Suprasec  1306 would

be used to produce this prepolymer at 10.0% NCO Converting to weight percents, Amount of Suprasec  1306 = 34.09%

Amount of polyol B = 65.86%

Amount of ethylene glycol = 0.05%