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What is polyurethane?
The scientific answer is that polyurethane is a polymer (a material composed of many repeating chemical subunits) which has urethane as a repeating unit. Precisely, a urethane chemical group is a collection of 5 specific atoms (two oxygen, one nitrogen, one carbon, and one hydrogen) arranged in certain way. The urethane chemical group (chopped out of the middle of the polymer) looks like this:

What is the difference between polyurethane and urethane?
As shown above, polyurethane is just "many urethanes", so, technically, urethane is just the chemical group shown above and polyurethane is a material than incorporates many urethane groups.

What are the major reasons to use polyurethane?
Polyurethane can be formulated to possess superior properties in the following categories:

Toughness

Abrasion Resistance

Cut Resistance

Long term stability in outdoor environments (no cracking or hardening over time)

Soft materials that can stretch up to ten or more times their length.

Hard materials that are not brittle--even at temperatures of 40 below zero.

High or low friction formulations

High or low energy absorption formulations (i.e. bouncy or dead)

Custom formulations for many widely diverse applications

Economical prototyping in liquid poured systems to test formulations and design.

Of all elastomers, polyurethane has the best resistance to gamma-ray radiation.

Is there a benefit over rubber?
Many advantages can be found over rubber. Depending on the formulation, urethane has a higher load-bearing capacity, greater tear strength, better compression set, greater abrasion resistance, tolerant to greases, oils and ozone and urethanes molded in a number of different processes.

What's the difference between rubber and urethane?
Rubber is the sap of trees found mostly in tropical climates. The sap is altered by mixtures of carbon (why it's black) and mineral oils and various fillers. Polyurethane is completely chemical or man-made. Because rubber is a biodegradable product it is affected by ozone and will over time dry rot and degrade, while urethane will keep going and going.

What are the service temperature limitations of polyurethane?
Most polyurethanes are limited to continuous service temperatures to about 185°F; however, special hybrid formulations can extend that to about 400°F in many cases.

What is durometer or Shore hardness and how is it measured?

The hardness testing of plastics is most commonly measured by the Shore (Durometer) test or Rockwell hardness test. Both methods measure the resistance of the plastic toward indentation. Both scales provide an empirical hardness value that doesn't correlate to other properties or fundamental characteristics. Shore Hardness, using either the Shore A or Shore D scale, is the preferred method for rubbers/elastomers and is also commonly used for 'softer' plastics such as polyolefins, fluoropolymers, and urethanes. The Shore A scale is used for 'softer' rubbers while the Shore D scale is used for 'harder' ones. The shore A Hardness is the relative hardness of elastic materials such as rubber or soft plastics can be determined with an instrument called a Shore A durometer. If the indenter completely penetrates the sample, a reading of 0 is obtained, and if no penetration occurs, a reading of 100 results. The reading is dimensionless.

The Shore hardness is measured with an apparatus known as a Durometer and consequently is also known as 'Durometer hardness'. The hardness value is determined by the penetration of the Durometer indenter foot into the sample. Because of the resilience of rubbers and plastics, the hardness reading my change over time - so the indentation time is sometimes reported along with the hardness number.

What are environmental concerns with using polyurethane materials?
Polyurethane’s do not break down easily, and their lifetime of service is often quite long. When disposed of they will biodegrade extremely slowly (like polyethylene), but it is unlikely that they pose a toxic risk in a normal disposal manner. Some of the raw materials that go into polyurethane’s are toxic prior to the reaction forming polyurethane.

How does polyurethane compare in cost to other materials?
Polyurethane costs more per pound than the high volume utility plastic materials such as polyethylene or polypropylene, but it offers far higher properties in many areas. It doesn’t make sense to use polyurethane in instances where it provides no performance advantages over the alternate materials listed above.

Will polyurethane harden or crack with age?
They shouldn’t. Polyurethanes which are properly formulated for outdoor exposure should last many years.

Is there a difference between colors?
No. In most cases the color of the part has no relation to the hardness or other physical characteristic. The pigment used to color our urethane components is a paste-like product that is mixed into the urethane in quantities of about 2-4%.

What is the difference between Polyester Systems vs. Polyether Systems?

Polyester Urethane	Polyether Urethane
Oil/solvent resistance Resistance to weak acids/basics Abrasion resistance Excellent mechanical properties Fungus Resistance Excellent Vibratory Dampening	Low temperature flexibility Excellent hydrolytic stability Food Grade Applications Temperature resistance Excellent mechanical properties Weather (UV) resistance
They are not recommended for use in high humidity or exposure to water, as volume swell and reduction of properties may result.	Because of inherent advantages in low heat build-up, polyether-based urethanes are recommended for applications undergoing medium to high stress.

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