Polyurethane 101

Want Versatility? Reliability? Durability? Manufacturability?

One material does it all: cast polyurethane.

• Cast Urethanes
• Physical Engineering & Environmental Properties
• Typical Cast Polyurethane Applications
• Consumer Information
• Cast Polyurethane

Cast Urethanes 

All engineering plastics and rubbers have combinations of properties which make them useful in industry. The castable polyurethane elastomers have such a broad range of property combinations that they far excel all other engineering materials in versatility. While you may not combine all of the properties below into one article, experienced urethane processors can help you choose those properties needed to produce the most cost-effective product.

• Resilience (high or low)
• Excellent abrasion resistance
• Light weight relative to metals
• Impact and shock resistance
• Bonding to metals and plastics
• Compression modulus (high to low)
• Cut and tear resistance
• Flex fatigue resistance
• Friction (high or low)
• Good electrical properties
• Oil, solvent, water, ozone resistance
• Age resistance
• Mold, mildew, fungus resistance
• Non-marking characteristic
• High load capacity
• Chemical resistanceMachinability

Physical Engineering & Environmental Properties

Urethane rubber will deflect under compressive or impact stress and recover its original shape when the stress is removed. The graph above shows the huge difference between available work from cast polyurethane and that from conventional rubbers at the same hardness and original shape. The area under the curve represents the useful work that the elastomer can do. This graph shows the comparison at 75-80A durometer; the difference becomes much more dramatic as the hardness levels increase. No plastic has these dynamic capabilities.

Progress has been made in finite element analysis as a predictive design tool. Computer calculations are more complex because elastomers have many variables when compared to metals. Also, the failure mechanism in metal is usually in shear while urethane elastomers tend to fail in tension. The photos above are of the top and bottom view of an auger. The red, pink, and white areas pinpoint the high strain area in service, which careful design kept within acceptable limits.

Typical Cast Polyurethane Applications

Wheels can be designed to handle any load under the widest possible environmental conditions.

 

No other elastomer or plastic wheel can withstand the demanding requirements of the ubiquitous skateboard.

Flexible couplings transmit power without shock. They reduce misalignment forces which shorten bearing life.

 

The high compression forces required to deflect urethanes are used in metal-forming operations. Forming costs are reduced because the need for matched punches and dies is eliminated. Urethanes don’t scratch metal.

 

The ease of fabrication and long wear with this pneumatic valve drastically reduce maintenance costs. Castable urethanes outlast metal valves more than two times.

 

Snowplow blades prevent road damage while resisting abrasion and providing impact resistance at very low temperatures.

 

Long-wearing role coverings provide high pressure transfer in many industrial applications.

 

Delicate instruments are protected from the abuse of handling, weather, oils, chemicals and solvents. Colorability allows easy identification of instrument types.

 

The castable nature of urethanes permits precision cast locators and fixtures to be used in automated assembly operations. While holding dimensional tolerances, urethanes resist wear and impact.

 

Resistance to wear and flex fatigue under extreme weather conditions makes urethane tracks reliable in snowmobiles.

 

Mining applications such as paddle wheels provide long life in abrasive slurries.

 

This snowblower manufacturer selected a urethane elastomer auger for this new lightweight unit. The urethane elastomer provided abrasion resistance and eliminated the corrosion problems of the steel augers it replaced.

 

Many types of cogs, star wheels, sprockets and gears can employ the high-modulus urethanes to reduce noise while transmitting power.

 

Today’s top-of-the-line bowling balls are cast urethane. They offer a more consistent level of performance, long life, and less deflection when hitting pins, which translates into higher scores. Good bowlers find the balls can be made to “hook” more for higher scores.

 

Industrial wheels can be designed for high cut and wear resistance while achieving enhanced impact absorption using the bending beam principle. This design is an example of the application of finite element analysis to achieve buckling above a pre-described operation deflection.

 

Intricate shapes – such as these components for filtering presses – can be easily molded.

 

Silk-screening applications require durability for the straight edges, flexibility, and solvent resistance without damaging the screen itself.

 

The availability of a large variety of sheet, rods and blocks provides engineers with a quick way to evaluate urethanes for suitability. Even prototype shapes can be machined with metal-working tools.

 

The castable nature of urethanes – which don’t require high pressure like typical rubber and plastic – permits quick, low-cost molds and fast response time in many cases.

 

Top-cast surfaces can be faced off in a secondary operation to achieve precise dimensions. The liquid urethane containing all the mixed ingredients “cures” to a rubbery solid at elevated temperatures.

 

In the paper converting industry, the cost of the longer-lasting urethane log pusher (right) is only 75% the cost of the steel pusher (left) it replaced. An added bonus: if the large saw blade accidentally contacts the urethane pusher, you must replace the pusher – but the $300 saw blade is not damaged.

 

Urethanes are currently used in the manufacture of everything from skate and roller-coaster wheels to conveyor drive rollers. Weatherability, shock absorption and abrasion resistance are the principal reasons, but high load-bearing properties, resilience, non-marking characteristics, and strong bonding to metal hubs are also important.

 

Short lead times and low-cost tooling, as well as increased durability, often make urethane components the most economical choice for limited production.

 

High load-bearing capacity (two to four times that of conventional rubber), long wear (three to six times that of conventional rubber, cut resistance, low rolling resistance and non-marking of concrete floors are principal reasons why urethanes are preferred.

 

Sanitary waste facilities create harsh environments but urethanes outperform other elastomers in these guide wheels.

 

Mining separator screens are another example in which superior abrasion, impact and cut resistance combine with resilience to provide long service life.

 

No other elastomer or plastic wheel can withstand the demanding requirements of the ubiquitous skateboard.

 

Consumer Information

A polyurethane (PUR and PU) is polymer composed of a chain of organic units joined by carbamate (urethane) links. Polyurethane polymers are formed by combining two bi- or higher functional monomers. One contains two or moreisocyanate functionalgroups (with formula –N=C=O) and the other contains two or more hydroxyl groups (with formula –OH). The alcohol and the isocyanate groups combine to form a urethane linkage:

ROH + R'NCO → ROC(O)N(H)R' (R and R' are alkyl or aryl groups)

This combining process, sometimes called condensation, typically requires the presence of a catalyst. More complicated monomers are also used.

Polyurethanes are used in the manufacture of flexible, high-resilience foam seating; rigid foam insulation panels; microcellular foam seals and gaskets; durable elastomeric wheels and tires; automotive suspension bushings; electrical potting compounds; high performance adhesives; surface coatings and surface sealants; synthetic fibers (e.g., Spandex); carpet underlay; and hard-plastic parts (e.g., for electronic instruments). Polyurethane is also used for the manufacture of hoses and skateboard wheels as it combines the best properties of both rubber and plastic.

Read the full article.

Cast Polyurethane

For many applications, there's a viable alternative to plastic, metal and rubber—one that many OEMs aren't even aware of. It's called cast polyurethane, and it's among the most versatile, durable materials available in manufacturing.

Originally developed by Otto Bayer early in World War II, polyurethane gained traction with numerous applications in the 1950s. Today, with its comparatively low tooling costs, castable polyurethane elastomers are a perfect alternative to more common materials in a variety of applications.

But many product designers may be surprised to learn of polyurethane's physical versatility. Capable of being as flexible as a rubber band or as rigid as some metals, cast polyurethane is especially appropriate for parts that require toughness and durability, including:

• Gears
• Cogs
• Sprockets
• Solid wheels of all types

Here are some of the advantages cast polyurethane has over other materials in low-volume applications.