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Curative Calculation

The document discusses calculating the amount of curative needed for composite propellant mixtures based on the equivalent weights of the compounds. It provides the equivalent weights for common binders, additives, and curatives used in composite propellants. An example calculation is shown to determine the mass of Isonate 143L curative needed for a mixture containing R45HTLO binder, Tepanol, and Castor Oil based on their individual masses and equivalent weights. Adjusting the index ratio or substituting a portion of one curative for another is also discussed.

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Kalyanasundaram Thirugnanasambandam
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3K views2 pages

Curative Calculation

The document discusses calculating the amount of curative needed for composite propellant mixtures based on the equivalent weights of the compounds. It provides the equivalent weights for common binders, additives, and curatives used in composite propellants. An example calculation is shown to determine the mass of Isonate 143L curative needed for a mixture containing R45HTLO binder, Tepanol, and Castor Oil based on their individual masses and equivalent weights. Adjusting the index ratio or substituting a portion of one curative for another is also discussed.

Uploaded by

Kalyanasundaram Thirugnanasambandam
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Composite Propellant Notes

John DeMar, john_demar@hotmail.com


6/1/2003, rev. 11/09/2005

Curative calculation for polymer binder and additives


The function of the curative is to link the OH of the binder (hydroxyl compound)
with a NCO of the curative (isocyanate compound). Other liquid additives in the
composite mixture may have OH equivalents and need to be taken into account
when calculating the amount of curative.
The calculation is based on the equivalent weight (EW) of each compound. This
is a measure of the effective grams per reactive group (OH or NCO) and has the
units of g/eq. Some manufacturers specify this number directly and others give
the percent NCO by weight (isocyanates) or the hydroxyl number in g-OH/g
(polymer resins). Each of these must be converted to an EW to calculate the
curative required for the overall mixture.
Equivalent weight for hydroxyl compounds (HTPB, etc.):
EW =

56,100
1000
1700
=
=
Hydroxyl Number Hydroxyl Value %OH

Equivalent weight for isocyanate compounds (MDI, Isonate, etc.):

EW =

4200
%NCO

Some EWs for common compounds used in composite propellants are:


Compound:
Sartomer HTPB R45M
Sartomer HTPB R45HTLO
Sartomer HTPB R20LM
Tepanol (HX-878)
Castor Oil
Water
Curatives:
TDI (generic)
IPDI (generic)
MDI (generic)
PAPI 94
Mondur MR
Isonate 143L
Desmodur E 744

EW
1370
1190
555
211
164
9

87
111
125
131
133
144
179

Note: each batch of HTPB may have a slightly different equivalent weight; check
with the supplier for the tested hydroxyl value or %OH. Also, the %NCO of
curatives will decrease with age (EW will increase).
The general formula to calculate the amount of curative is as follows:

W
Wcurative = (IR )(EWcurative ) n
n EWn
where :
Wcurative = weight (mass) of curative (grams)
EWcurative = NCO equivalent weight of curative
Wn = weight (mass) of each liquid compound
EWn = OH equivalent weight for each liquid
IR = Index Ratio for NCO to OH (1.0 for ideal linkage)
Example:
Given the following portions of binder and other liquid ingredients, find the
amount of Isonate 143L curative required for a full cure (index ratio of 1.0):
Ingredient
R45HTLO
Tepanol
Castor Oil

grams
10
0.4
0.3

10
Wcurative = (1.0)(144)[1190
+

0.4
211

0.3
] = 1.75
+ 164

grams

Reduce the index number to 0.95 to get a slightly softer propellant. Increase the
index to 1.05 for a (possibly) harder propellant. Too much curative or too little
curative will degrade the hardness by leaving behind unlinked liquids.
To mix two different curatives (for increased pot life, etc.), replace a percentage
of one with the equivalent curing capacity of the other by weight. For example, to
replace 20% of Isonate with IPDI in the above example:

WIsonate = (0.8)(1.75) = 1.4 grams


WIPDI = (0.2)(111/ 144)(1.75) = 0.27

gram

Note the ratio of the substitute EW over the original EW (111/144). It takes less
IPDI to affect the same hardness as Isonate 143L but it will take much longer to
fully cure.
A trace amount of water in the ingredients will require more curative. In the
above example, 0.01% H2O requires 144 * 0.01/9 = 0.16 gram more curative.

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