Glossary of terms used in the polyurethanes business

Courtesy of Smithers-Rapra & Cromar Solutions Ltd, UK

 

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Hard Segment A short part of a polymer chain composed of identical units. A hard segment has a high softening or melting point and is rigid at room temperature.

HDI An abbreviation for hexamethylene diisocyanate and 1,6-diisocyanato-hexane.

HMDI An abbreviation for hydrogenated MDI and 4,4'-diisocyanate-dicyclohexylmethane.

Hydrogen bonding Points of attraction between molecules, caused by polarised links involving hydrogen atoms.

Hydroxyl An alcoholic group (-O-H). The reactive group in polyols. The hydroxyl value or number is a quantitative measure of the concentration of hydroxyl groups, usually stated as mgKOH/g, i.e. the number of mg. of potassium hydroxide equivalent to the hydroxyl groups in 1 gm. of substance. A primary hydroxyl group has the structure -CH2-O-H, and is more reactive to isocyanates than a secondary hydroxyl group, which has the structure -CH-O-H.