Do you understand the difference between thermoset urethane and thermoplastic urethane?

 
Either type is useful for OEM applications. But, certain applications are better suited for one over the other.
 

Thermoplastic Polyurethane (TPU)

  • Thermoplastic urethane is a polymer. TPUs can have various combinations of flexible and rigid segments. This allows TPUs to range from soft and flexible elastomers to rigid plastics.
  • TPU is generally more durable than thermosetting urethane.
  • Thermoplastic urethane can be melted and reformed. It is elastic and flexible, making it a versatile material suitable for use in a wide range of industries.
  • TPUs are often produced in pellets. Once melted, they are then pressed or injection molded into a final product.
  • Load Bearing Capacity: Thermoplastic urethane holds a low weight per square inch.

Thermoset Urethane

  • Thermoset urethane is a polymer that originates as a soft solid or thick liquid prepolymer or resin. Curing changes the resin into an insoluble polymer network. Curing is induced by heat or by mixing with a catalyst.
  • Thermoset urethanes are generally liquid before cured. Often, the process involves molding into finished parts. Once hardened, thermoset urethanes cannot be melted and reformed.
  • Load Bearing Capacity: Thermoset urethane holds a high weight per square inch.

Polyurethane Properties

Aside from the excellent load bearing capacity and abrasion resistance of molded polyurethane; the polyurethane stands up very well in a large variety of environments. The polyurethane parts made here at Gallagher Corporation are shipped all over the world and withstand a wide variety of environmental conditions they encounter. This part of the engineering guide is provided to you as a snapshot of some of the different operating conditions where molded polyurethanes are typically used and how polyurethane properties excel.

Have an application not listed? Feel free to contact us to discuss polyurethane properties specific to your needs and find out what we can provide to exceed your expectations.

The hardness of polyurethane is measured based on a durometer scale. This infographic shows the hardness range of polyurethane properties.

durometer hardness scale

The one true Achilles Heel of molded polyurethane is heat. Because of this, it’s not recommended to use molded polyurethane for applications above 250°F (121°C). Recent advancements in PPDI urethane technology have pushed the upper limits to 300°F (150°C) for certain urethane parts. For applications in environments between 212°F and 300°F (100°C and 150°C), it’s best to individually evaluate the part and conduct some application testing beforehand.

Key Performance Characteristics

Here are some of the advantages and benefits of polyurethane properties.

Low-Temperature Performance

One particular area where molded polyurethane excels is it’s resistance to cold temperatures.

Molded polyurethane remains flexible at low temperatures and possesses outstanding resistance to thermal shock. Many of the polyurethanes used here at Gallagher do not become brittle at temperatures below -62°C (-80°F) although stiffening gradually increases as the temperature is reduced below -18°C (0°F). Special compositions can be made which retain some flexibility at temperatures as low as -87°C (-125°F).

High-Temperature Performance

Molded polyurethane can be formulated for many continuous service applications at temperatures from 85°C to 121°C (185°F to 250°F).

As you can see in Table 1, holding a typical compound for one week at 250°F (121°C) causes only a minor decrease in physical properties when tested at 24°C (75°F).

Table: Effect of Exposure to Elevated Temperature on Properties of 90A TDI Polyether
and Properties of 90A TDI Polyether at Normal and Elevated Temperatures

Test 100% Modulus, MPa (psi)
(ASTM D142-61T)
Tensile Strength, MPa (psi)
(ASTM D142-61T)
Elongation at Break, %
(ASTM D142-61T)
Hardness, Durometer A
(ASTM D142-61T)
Original 7.6
(1,100)
31.3
(4,500)
450 90
After 1 wk @ 100°C (212°F)
measured at room temp
7.4
(1,070)
31.3
(4,500)
520 90
After 3 wk @ 100°C (212°F)
measured at room temp
6.6
(950)
39.5
(4,300)
610 89
After 1 wk @ 121°C (250°F)
measured at room temp
6.2
(900)
18.9
(2,750)
780 88
Measured @ 84°C (75°F) 6.7
(975)
27.6
(4,000)
440 89
Measured @ 100°C (212°F) 5.3
(775)
19.3
(2,800)
480 88
Measured @ 121°C (250°F) 5.3
(775)
10.3
(1,500)
350 88

The one true Achilles Heel of molded polyurethane is heat. Because of this, it’s not recommended to use molded polyurethane for applications above 250°F (121°C). Recent advancements in PPDI urethane technology have pushed the upper limits to 300°F (150°C) for certain urethane parts. For applications in environments between 212°F and 300°F (100°C and 150°C), it’s best to individually evaluate the part and conduct some application testing beforehand.

Key Performance Characteristics

Here are some of the advantages and benefits of polyurethane properties.

Water Resistance

Gallagher Corp polyurethane is resistant to swelling and deteriorating effects of immersion in water and has excellent long-term stability in water at temperatures as high at 122°F (50°C). We have some formulations that can withstand higher temperatures. The chart below shows that after immersion in water for as long as one year at 122°F (50°C), a typical compound retained 76 percent if it’s original strength. The same degree of water resistance is exhibited in exposures where water is present in a mixture or an emulsion with oil.

Outgassing

Products made from polyurethane are very stable under high-vacuum conditions. They exhibit extremely low weight losses in standard outgassing tests. If you have an application where outgassing carries the potential for problems, we can easily provide compounds without additives of any type.

Resistance to Aging

Gallagher Corp processes several different types of polyurethane elastomers. Some of these elastomers are mildly susceptible to breaking down with age and others are extremely resistant to aging. Gallagher Corp polyurethane can be blended such that the passage of time under static, room temperature conditions has little effect. It’s always a good idea to let the team at Gallagher know whether a part will be stored prior to use and what the desired service life should be. The polyurethane compounds that do deteriorate with time make up for this in other areas. This is a relationship that should be taken into account when making material selections.

The introduction of other factors can accelerate deterioration. It’s well known that increasing temperature will promote chemical changes in organic materials, in polyurethane; this influence is reflected in a progressive loss in physical properties. Heat aging effects representative of Gallagher Corp polyurethane are shown in Table 1.

Dynamic forces or chemical agents acting upon a product over a period of time can also intensify the aging process. However, in such multiple attacks, the effects strictly due to aging may be so masked as to become unrecognizable.

Mold, Mildew, Fungus Resistance

Most high-performance polyurethane elastomers do not support fungus growth and are generally resistant to fungus attack and mildew growth. Certain compounds that could possibly be susceptible to fungus attack can be protected by the addition of commercial fungicides.

Oxygen and Ozone Resistance

Oxygen and Ozone in atmospheric concentrations have no observable effect on molded polyurethane parts. Samples exposed to 300 pphm ozone under static strain conditions for 500 hours, for example, show no signs of surface cracking or crazing. Stress-strain properties of polyurethane are little changed after oxygen bomb exposure for four weeks at 158°C (70°C) under 300 psi (2.1 MPa) oxygen pressure.

Flame Resistance

Parts made by molding polyurethane have limited resistance to burning. A modest improvement in this characteristic can be achieved by special compounding. However, in an actual fire situation, even those products will ignite.

Whether or not to use polyurethane where fire safety considerations influence design is a matter of practical evaluation of the potential hazards involved in each specific case.

Weatherability and Color Stability

Polyurethane elastomers will darken and their physical properties will be reduced by prolonged exposure to ultraviolet light. They can be protected against the effects of weathering by pigmentation or by using ultraviolet screening agents. Always be sure to alert the team at Gallagher if your part will be used in direct sunlight so that we can adjust the compound accordingly with UV inhibitors.

Oil, Chemical and Solvent Resistance

Polyurethane has excellent oil which makes its products particularly suited for service in lubricating oils. Aromatic hydrocarbons and polar solvents have a moderate to severe effect on polyurethanes. Typical chemical resistance information is shown below.

Radiation Resistance

Polyurethanes typically perform quite well when subjected to radiation. Following an independent test, it was concluded molded polyether polyurethane products appear capable of giving satisfactory service even when exposed to relatively large gamma ray exposure of 1 x  109Roentgens. Polyurethane is more resistant than other elastomers to stress cracking and retains a greater amount of flexibility and physical toughness while exposed to gamma radiation.