Polyurethanes are in the class
of compounds called reaction polymers, which include epoxies, unsaturated
polyesters, and phenolic. Polyurethanes are produced by reacting an isocyanate
containing two or more isocyanate groups per molecule (R−(N=C=O)n ≥ 2) with a
polyol containing on average two or more hydroxyl groups per molecule (R'−(OH)n
≥ 2) in the presence of a catalyst or by activation with ultraviolet light.
Polyurethane is a polymer composed of
organic units joined by carbamate (urethane) links. While most polyurethanes
are thermosetting polymers that do not melt when heated, thermoplastic polyurethanes are also
available.
Polyurethane is a polymer composed of
organic units joined by carbamate (urethane) links. While most polyurethanes
are thermosetting polymers that do not melt when heated, thermoplastic
polyurethanes are also available.
Thermoplastic polyurethane (TPU) was
discovered in the late 1930s as part of Germany's World War II research. It is
thermoplastic elastomers (TPE), which combines the mechanical and physical
properties of rubber with the advantages of thermo plasticity and process
ability. Other examples of TPEs include polyethylene and polypropylene.
Noted for its high performance and
general overall toughness, urethane rapidly became the material of choice for a
wide range of critical and "can't fail" applications. Urethane's unique
characteristics make it an extremely versatile material that outperforms many
other thermoplastics. For example, it retains its flexibility even at low
temperatures, where polyvinyl chloride (PVC) becomes brittle.
On a molecular level, urethane is comprised
of the four most common elements in the world: carbon, hydrogen, nitrogen, and
oxygen. To be a urethane, it must contain the molecular urethane linkage
(NHCO2).
The actual chemistry consists of a
series of block copolymers with alternating hard and soft phases. A block
copolymer is a string of chemically different molecules in repeated sequences.
The ratio and molecular structure of these segments determines the specific
characteristics of the resin, which can be varied by modifying its chemical backbone.
Depending upon its chemical makeup, the
urethane soft phase is generally either Polyether or Polyester. These two
categories are further segmented into Aliphatic or Aromatic hard segments.
a)
Polyether urethane provides a softer “feel” or drape than polyester,
with better moisture vapour transmission rates and superior low temperature
properties. It is also inherently stable when exposed to high humidity, and
polyethers are naturally more fungus resistant.
b) Polyester urethane offers greater
toughness (i.e. abrasion resistance, tensile/tear strength) at a given
durometer than polyether. It is more resistant to fuels, and offers better
aging (oxidation) resistance. However, polyester urethanes will eventually
break down when exposed to conditions of high humidity.
c) Aliphatic urethane is inherently light
stable, resistant to ultraviolet light degradation, and provides excellent
optical clarity. It is used as an optical interlayer, providing strength as a
lamination adhesive in encapsulated glass and security glazing, to name a few
applications. It is often a more expensive product.
d) Aromatic urethane is a strong,
general-purpose resin originally developed as synthetic rubber. It is generally
less expensive than aliphatic urethanes, but it is susceptible to UV light
degradation, which tends to yellow the polymer without affecting physical
properties.
Properties:
Polyurethane combines the best
properties of rubber and plastic, without the weaknesses inherent in
plasticized vinyl films. Because it contains no plasticizers, it is not subject
to the brittleness and other problems caused when they leach to the surface.
Urethane is easy to work with, and is readily modified to suit a particular
application through the addition of fillers, colours, stabilizers, and
lubricants, as well as other additives. Urethane compounds possess a combination
of properties which are not available in any other thermoplastic material,
offering significant design flexibility.
· ·
Thermosetting
mechanisms: Paints that cure by polymerization are
generally one- or two-package coatings that polymerize by way of a chemical
reaction, and cure into a crosslinked film. Depending on composition they may
need to dry first, by evaporation of solvent. Classic two-package epoxies or
polyurethanes would fall into this category. The drying oils,
counter-intuitively, actually cure by a cross-linking reaction even if they are
not put through an oven cycle and seem to simply dry in air. The film formation
mechanism of the simplest examples involve first evaporation of solvents
followed by reaction with oxygen from the environment over a period of days,
weeks and even months to create a cross-linked network
·
Durometer (Hardness). Urethane film and sheet can be
produced in a range of durometer (Shore A 75 - Shore D 55) from a very stiff
material to a very thin film having an extremely soft, non-plastic
"feel" or "hand". The latter is ideal for use in products that
come in contact with the skin.
·
Low temperature flexibility Urethanes feature superior flexibility over a
wide durometer range, even at temperatures as low as -60°F.
·
Tensile strength. A tensile strength range from 4,000 to
10,000 psi assures reliability and durability over the lifespan of the end
product. Because urethanes are tough, they can be used in thinner gauges when
compared to vinyl.
·
Elongation
Urethane film and sheet can elongate as much as
800% and return to its original dimension without significant loss of "memory."
·
Abrasion resistance Urethane provides excellent abrasion
resistance. For example, it is used in profile form as a conveyor drive belt,
and to protect elevator cables and pulleys from excessive wear while cushioning
the ride.
·
Chemical and environmental resistance Excellent resistance
to hydrocarbons, chemicals, ozone, bacteria, fungus, and moisture, as well as
skin oils Polyether or polyester urethanes offer specific characteristics that
provide durability and long life to products that must survive and perform in
harsh industrial environments.
Typical Applications
Because of its toughness and
versatility, as well as a generally higher price tag than other TPEs, urethane
is typically used for parts requiring a high level of performance. These
applications frequently demand a flexible material with a high degree of flex
resistance, wear ability, and durability. Particularly at lower temperatures,
other materials do not provide this combination of properties. These are some
of the reasons it makes an ideal protection for automotive upholstery (finished
leather)
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