Isocynates are very reactive compounds and will react with any compounds like amines, hydroxyl, water, carbolic, phenolic, amide containing an active hydrogen atom. Polyurethanes are the reaction products of poly-functional isocynates and polyhydroxy compounds.
The physical and chemical properties of these coatings depend on type, functionality, and ratio of the reactants and curing conditions. Based on literature survey the choice of isocyanate required to produce a specific property depends on type of isocyanate i.e aliphatic, cycloaliphatic or aromatic. Some isocyanates are chosen for high thermal and fire resistance. The hydroxyl compound can be polyesters, polyethers, epoxies, alkyds, castor oil, hydroxyl, acrylics, partially hydrolysed vinyl copolymers, silicone resins.
% NCO is normally given of material as supplied (Not converted on 100% NVM basis)
For example OH value of Acrylic resin considered to be 110
OH value is normally given on 100% NVM basis.
Theoretical mixing ratio
1 OH equivalent = 1 NCO equivalent preferably 5% excess NCO in compound
Now, OH equivalent is x in 100% basis we can convert it on material of supplied basis in the following manner
Theoretical percentage of hardener ratio (PHR)
- This means 100 qms of solid acrylic resin will be 37.25gms of 100% isocyanate resin to keep the ratio of 1:1 on equivalent basis which is considered as ideal ratio to get optimum performance of polyurethane coatings.
- Higher percentage of isocyanate resin is desirable.
- Higher OH value of Acrylic resin means higher percentage of isocyanate resin which results in better hardness, gloss retention properties.
The isocyanate groups are made to react with known excess of din- butylamine and by back titration of excess amine after reaction with Isocyanate, the iso-cyanate is determined.
Burette, pipette, conical flask
Standard solution of 0.1 normal sulfuric acid, methyl red indicator, mixture 1,4 dioxine & n-butyl amine. (96:4)
a) Weigh required amount of sample in 250ml conical flask. b) Add to it 10ml above prepared mixture of 1,4 dioxane & nbutylamine (96:4) c) Add 20ml of distilled water in conical flask and 3-4 drops of methyl red indicator. d) Titrate above total solution in the conical flask against 0.1 normal standard H SO solution. 2 4 e) End point is yellow to pink, take a burette as a A
Take blank reading as B same as above procedure without taking sample in flask.
Calculate NCO content as follows
Where W = weight of sample taken, N = Normality of std H SO 2 4 solution, B = Blank reading, A = back reading