Q.1 |
What is
the difference between polyurethane and urethane?
Polyurethane and urethane are terms which are used interchangeably.
The term polyurethane came about in the past because the predominant
chemical group present in the polymer was the urethane group. Today
the term polyurethane may be somewhat misleading because it covers
a wide variety of materials which may have similar general characteristics,
but can differ in specific properties. These properties are explained
by different chemical structures existing in polyurethanes.
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| Q.2 |
What is an elastomer?
The word ‘elastomer’ is simply a contraction of ‘elastic
polymers’. An elastomer can be described as a material characterised
by the property of high elasticity. This means it has the ability
to stretch to a great extent under load and recover more or less
completely when the load is released. Solid polyurethanes posses
this property and can be classed as elastomers. |
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| Q.3 |
What are the raw materials
used in manufacture of polyurethane?
The raw materials used in the manufacture of polyurethane are Polyesters,
Polyethers, Diisocyanates and glycols. |
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| Q.4 |
How is urethane molded?
The methods of molding polyurethane differ according to the application.
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Compression molding
is a technique used for production of small parts. Here the
prepolymer is heated to normal mixing temperatures via a batch
system and the curative is added. The system is mixed and then
poured or cast into an open cavity where it is allowed to gel
to the point where sufficient polymer integrity is developed
so that on closing the mold entrained air will be squeezed out.
After the appropriate cure time, the part will have the advantage
of being molded to any size in any configuration. |
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Open casting accounts for a majority
of the manufacture process of applications. Here the prepolymer,
and the curative are heated to the appropriate temperature in
mixing machines and then ‘poured’ from dispensing
head of the machine into an open cavity. There are many techniques
which have evolved over the years to eliminate trapped air,
flow lines etc. |
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One shot systems may utilize either
polyethers or polyesters in the polyol portion and any one of
the icocyanates to co react to form the cured polymer. |
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| Q.5 |
What are the types
of solid polyurethane elastomers?
The types of solid polymers are as follows:
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Linear Polyurethanes:
They were prepared by the addition reaction between aliphatic
glycols and aliphatic diisocyanates and no cross linking occurred,
neither were any trifunctional compounds added to cause branching.
These can be correctly called polyurethanes as the structurally
important group is the urethane group. |
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Castable Polyurethanes:
These can be subdivided into three main groups, unstable prepolymers,
stable polymers and one shot systems, but though the technologies
are different, the chemical basis is similar. A slight excess
of diisocyanate is normally employed and this enables cross
linking to occur at the urethane or urea groups. |
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Millable Polyurethanes:
Using a deficiency of diisocyanate results in a hydroxyl-terminated
polymer which is relatively stable and non cross linked. Provided
the correct molecular weight is chosen this product takes the
form of a plastic gum which can be handles on a rubber mill
in a similar manner to other elastomers. |
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Thermoplastic Polyurethanes:
Chemically similar to cast polyurethanes, the choice of diisocyanate
used is such that the crosslinks ar thermolabile and at the
temperatures encounters during processing through the injection
cylinder the crosslinks are broken around 160 degrees and the
polymer becomes linear. The corsslinks reform upon cooling.
The main advantage of this is the economic manufacture of small
components in large quantities. |
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Cellular Polyurethanes:
The specific gravity of cellular polyurethanes is in the range
of 0.35 to 0.65, making them best regarded as more flexible
and softer versions of solid polyurethanes from which they are
derived. |
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Sprayable Polyurethanes:
The thermoplastic or millable grades can be modified to be more
linear in structure and soluble in common solvents such as methylethyl
ketone, ethyl acetate etc., and the solutions can be sprayed
or brushed on awkwardly shaped surfaces as protective coatings
or used as adhesives. |
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Poromeric Polyurethanes:
They can be described as having a porous, polymeric structure,
an example of which is Du Pont’s CORFAM, a leather like
material based on polyurethane containing randomly dispersed
fibres and possessing a porous structure, allowing it to ‘breathe’
like leather. |
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Spandex Fibres:
These are fibres consisting of long chain synthetic polymers
comprising 85% of a segmented polyurethane. Compared with rubber
thread, spandex fibres are stronger and more resistant to weathering,
enabling finer counts to be used. |
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| Q.6 |
Is it harmful to handle
polyurethane?
Extensive experimentation have been carried out to determine to physiological
effects of polyurethane products. The generally accepted view today
is that no danger is experienced by handling of polyurethane products
under normal conditions.
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| Q.7 |
What are the properties
of polyurethane?
Urethane possesses many physical, mechanical and environmental properties
which are advantageous in many applications.
• Excellent Resistance (high or low
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• Extra Toughness and Durability
• Very high load bearing capacity
• High Tear Strength and Cut Resistance
• Low Compression Set
• Very good impact resistance
• Outstanding Abrasion resistance
• Good machinability
• Oil, solvent, water and ozone resistance.
• Non-marking
• Mold, mildew , fungus resistance
• Flex fatigue resistance
• Good Electrical properties
• Friction (high or low) Mechanical
Properties
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| Q.8 |
What is cross linking?
The molecular structure of elastomers consists of long flexible molecular
chains which are linked at intervals. The links are called crosslinks
and may be in the from of either primary chemical bonds or simple
mechanical entanglement of the chains. The elastic properties are
greatly improved and the material becomes usable for present day applications
only by introduction of some chemical crosslinks.
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| Q.9 |
What is hysteresis?
When stress is applied to an elastomeric material there is a small
but positive time lag before the material takes up the corresponding
strain. This time lag is caused by the need for the intermolecular
attractions to be overcome by vibrational energy of the atoms. The
practical result of this time lag is that the stress – strain
curve in recovery does not follow the same path as when the stress
was applied, consequently causing a loss of energy called hysteresis
which is converted into heat. This loss of energy can be measured
in terms of rebound resilience; since the vibrational energy of the
atoms increases as the temperature rises, the time lag will decrease
as the temperature decreases. |
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| Q.10 |
What is the difference
between urethane, rubber and plastic?
Differences between polyurethane, rubber and plastic are in the manufacturing
methods, properties and resulting applications.
Synthetic rubbers and plastics are generally manufactured by a polymerization
process, either addition polymerization or condensation polymerization.
In addition polymerization an unsaturated monomer polymerises by a
chain reaction via a free radical or ionic mechanism and the reaction
can be controlled only within certain limits.
Condensation polymerization or step polymerization involves the reaction
by the use of heat and/or catalysts between small to medium sized
poly – functional molecules and the formation of water, sodium
chloride or similar low weight compounds.
Polyurethane elastomers do not easily fit into either of these. Low
to medium molecular weight polyfunctional compounds, one of which
is a diisocyanate , are joined together by an addition type process
but by an intermolecular hydrogen transfer and not by free radical
or ionic mechanism. Cross linking of the polymer to form a cured elastomer
is achieved by a similar additive process. The term diisocyanate polyaddition
or polyaddition was coined to describe the process. |
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Die-Thane Tooling 
