Polyurethane(PU)
Introduction
Polyurethane fiber (Spandex® or Elastane®) is a synthetic
fiber known for its exceptional elasticity (or stretchability)
Spandex® is stronger, lighter, and more versatile and
durable than rubber.
Spandex® fibers can be stretched to almost 500% of
their length.
Polyurethane(PU)
Components of polyurethane
Polyisocyanate n(NCO-R-NCO)
As crosslinking agents, polyisocyanates play a key role with
respect to processing, curing and the resulting coating properties
Polyols n(HO-R’-OH)
Urethane
If the functionalities of the reactants are three or more, branched or
cross-linked polymers are formed
By varying R and R’segments of the polyaddition reaction and
other ingredients, a range of products can be formed (foam, fibers,
coating, soft and hard elastomers, flexible
Polyurethane(PU)
Important polyisocyanates
Hexamethylene diisocyanate Isophorone diisocyanate
Diphenyl methane diisocyanate
Toluene diisocyanate
Polyurethane(PU)
Polyisocyanates
Aromatic isocyanates yield polyurethanes that turn
yellow with exposure to light
Various aliphatic and cycloaliphatic diisocyanates are
used in the industry to produce polyurethanes, which do
not turn yellow upon light exposure
The most important among the aliphatics is
hexamethylene diisocyanate (HMDI).
It is used extensively for coating.
Polyurethane(PU)
Polyols
The second component is polyols
Polyester polyols
Polyether polyols
Polyurethane(PU)
Structure of polyurethane
Polyurethane(PU)
The characteristic of polyurethane coating material
depends on the chain length of isocyanates and
polyols
If they have short chains, the polymer will be hard
and low solubility due to hydrogen bonding between
C=O and N-H.
If the chains are long, the polymer will be very soft
Mostly polyurethane are prepared from three basic
ingredients
Long chain polyols
Isocyanates
Chain extenders and crosslinkers
Polyurethane(PU)
Structure of PU
Polyurethane(PU)
Chain extenders and crosslinkers are low molecular weight poly
functional alcohols and amines
The difunctional amines and alcohols are chain extenders
The compounds with functionality greater than two are cross-linkers
What are these compounds?
Ethylene glycol, 1,4-butanediol, glycerol and trimethylol propane
Derivatives of diaminophenylmethane and m-phenylenediamines
