The prepolymers are prepared by reacting an isocyanate and a polyol of various types. Almost all commercial-grade prepolymers available are based on two different isocyanates: TDI (toluenediisocyanate) and MDI (methylenebisdiphenyl diisocyanate). Both of these isocyanates give different properties to the prepolymer and require varying types of processing systems. The other reactant within the prepolymer, the polyol, is available in three basic types: PTMEG (polytetramethylene ether glycol), PPG (polypropylene ether glycol) and polyester.
The prepolymer is then blended with a diamine, diol or triol curative. As with the prepolymers, the type of curative used can effect the overall physical properties of the final product. The most commonly used diamine curative is methylene-bis-orthochloroaniline (MOCA), primarily for use with TDI based prepolymers. The main diol curatives, Butanediol (BDO) and Hydroquinone Di-(beta-hydroxyethel)-Ether (HQEE), are primarily used with MDI based prepolymers. Triols can be used in combination with diols in MDI systems or in TDI systems to offer a wide variety of physical properties.
Additives can also be incorporated into a compound by the processor. The most commonly used is a pigment. Other additives could include plasticizers, fillers, UV and hydrolysis stabilizers.
Below are some of the general properties of polyurethane.
Specific Gravity 1.27
Shrinkage, in/in, 1/8 in. thick 0.0025
Shrinkage, in/in, 1/4 in. thick 0.0050
Water Absorption, % 24 hrs 0.150
MECHANICAL
Impact, Izod, Notched (Ft-Lb/In) 2.00
Impact, Izod, Unnotched (Ft-Lb/In) 7.00
Tensile Strength (Psi) 12,000
Tensile Elongation (%) 6.600
Tensile Modulus (Psi x E+6) 0.50
Flexural Strength (Psi) 16,000
Flexural Modulus (Psi x E+6) 0.50
Compressive Strength (Psi) NA
Hardness (Rockwell R) 119.0
ELECTRICAL
Dielectric Strength (V/Mil) 510
Dielectric Constant (@ 1 MC dry) 3.40
Dissipation Factor (@ 1 MC dry) 0.012
Arc Resistance (sec) NA
Volume Resistivity (ohm-cm) 10E## NA
THERMAL
Heat Deflection Temp 264 psi (F) 175
Heat Deflection Temp 66 psi (F) NA
Flammability HB
Thermal Expansion (In/In/F) xE-5 NA
Thermal Conductivity NA
WEAR
Wear Factor NA
U L yellow card NO
Because polyurethane is available in a very broad hardness range, it
allows the engineer to replace rubber, plastic and metal with the ultimate
in abrasion resistance and physical properties. Polyurethane can reduce
plant maintenance and OEM product cost. Many applications using
polyurethane have cut downtime, maintenance time and cost of parts to a
fraction of the previous figures.
There are many advantages to using polyurethane. It has already been
mentioned that polyurethanes offer a huge range of hardness, but the
advantages don't stop there. Polyurethanes have better abrasion and tear
resistance than rubbers, while offering higher load bearing capacity.
Compared to plastics urethanes offer superior impact resistance while
offering excellent wear properties and elastic memory. Polyurethanes have
replaced metals in sleeve bearings, wear plates, sprockets, rollers and
various other parts, with benefits such as weight reduction, noise
abatement and wear improvements being realized. Parts made of
polyurethane will often outwear other materials by a margin of 5 to 50/one
when severe abrasion is a factor. Polyurethane has been proven to be
vastly superior to rubber plastics and metal in many applications. In
addition, polyurethane has excellent resistance to oils, solvents, fats,
greases and gasoline, and has a higher load-bearing capacity than any
conventional rubber. Because of this characteristic, it is an ideal
material for load wheels, heavy-duty couplings, metal-forming pads, shock
pads, expansion joints and machine mounts.
Tear-strength ranges
between
500-1000 Ibs./linear inch, which is far superior to rubbers. As a result,
polyurethane is often used as drive belts, diaphragms, roll covers,
cutting pads, gaskets and chute liners. Polyurethane also has a high
resistance to oxygen, ozone, sunlight and general weather conditions. The
hard polyurethanes are now being used as gears in products where engineers
desire sound reduction. The soft polyurethanes are used to replace rubbers
for improved sound/vibration dampening. Most polyurethane products offer
extremely high flex-life and can be expected to outlast other elastomer
materials where this feature is an important requirement. Dust boots,
bellows, diaphragms, belts, couplings and similar products are made from
urethane for this reason. Polyurethane has excellent electrical
insulating properties and is used successfully in many molded wire and
cable harness assemblies.
While polyurethane does have a large number of advantages, users of these
products must be aware of a few caveats. Continuous use above 225F is not
recommended nor is urethane recommended in hot water over 175F. At low
temperatures, polyurethane will remain flexible down to -90F. A gradual
stiffening will occur at 0F, but will not become pronounced until much
lower temperatures are obtained.
The most important consideration when choosing polyurethanes for a
particular application is the physical properties. There are a wide
variety of prepolymer and curative systems available, each with their own
set of physical properties. Choosing a casting system can be a difficult
job. Typically, that is why most processors will choose more than one
material type to test for a new application.
Esters are best suited for applications that require higher tear
strength.
Although tensile strength is usually not a determining factor when
choosing a urethane compound it is good to note that they excel in this
area. Ester compounds are the choice for resistance to oil and heat aging.
Where abrasion resistance is involved esters have a tendency to wear
better in applications that involve sliding abrasion.
Ethers, on the other hand, perform better where impingement abrasion is a
factor. Ethers are the choice for applications where heat buildup is a
consideration. Since the ethers have a higher resilience this gives them
the capability of not taking on as much heat in dynamic applications.
Finally, ethers fair much better than esters in moist environments.
Both TDI and MDI compounds have similarities to them. It is useful to
note, however, that MDI Ethers can perform much better in applications
that demand a high resilience, hydrolysis resistance and low temperature
properties. TDI compounds do withstand the higher temperatures better and
have the best compression set properties.
The second factor to note when choosing a polyurethane compound is the
ease of processing. TDI systems are the easiest of both systems to
process. Perhaps 80% or more of the processors use the TDI system for this
reason.
Bibliography
http://www.elastech.com/mis/polyfaq.htm
http://www.sdplastics.com/polyuret.html
http://www.plasticsusa.com/pur.html