Polyurethane Elastomer

What is a Polyurethane Elastomer

A polyurethane elastomer is a synthetic rubber formed by reacting a di- or poly-isocyanate with a hydroxyl-terminated polyol and a chain extender. The defining chemical feature is the urethane linkage, but the engineering behavior comes from microphase separation: rigid, polar hard segments (formed from the isocyanate and the short-chain extender) cluster and act as physical crosslinks and reinforcing domains, while flexible soft segments (the long polyol chains) provide rubbery extensibility. Because the ratio and chemistry of these two segment types can be tuned almost continuously, a single material family covers hardnesses from a soft Shore 60A gasket to a Shore 75D structural part, a range no single conventional rubber can span.

The chemistry was pioneered by Otto Bayer and co-workers at I.G. Farben (later Bayer, today part of Covestro) in 1937, when the diisocyanate polyaddition reaction was first demonstrated. Cast millable and castable polyurethane elastomers were commercialized in the 1950s; in 1952 Bayer introduced Vulkollan, the naphthalene-diisocyanate cast system still regarded as the benchmark for dynamic load capacity. Thermoplastic polyurethane (TPU) followed in the same era and matured into injection-moldable grades sold today by BASF, Lubrizol, Covestro and Huntsman. The history matters for selection because the legacy NDI cast grades and the modern MDI thermoplastics occupy different ends of the performance and cost spectrum.

Three building blocks define every grade and should be read off the datasheet before anything else. The isocyanate is usually MDI (diphenylmethane diisocyanate), TDI (toluene diisocyanate) or NDI (naphthalene diisocyanate); MDI is the workhorse for TPU, TDI dominates cast prepolymers, and NDI delivers the highest dynamic performance. The polyol is either a polyester or a polyether (most often PTMEG, polytetramethylene ether glycol), or a polycaprolactone; this choice governs hydrolysis resistance and abrasion. The chain extender, commonly 1,4-butanediol for TPU or the aromatic diamine MOCA for high-performance cast systems, sets cure behavior and the upper hardness reachable.

In industrial use, polyurethane elastomers sit between rubber and rigid plastic and are chosen specifically when an application needs the elasticity of rubber together with the wear life and load capacity of a tough plastic. Typical duty includes forklift and caster wheels, scraper and squeegee blades, screen and chute liners in mining, hydraulic rod and piston seals, drive and conveyor rollers, vibration mounts, and shock pads. The global market is large and growing: industry analyses put total polyurethane consumption in the millions of tonnes per year, with cast and thermoplastic elastomers a high-value subset concentrated in wear and dynamic-load applications.

The recurring lesson is that "polyurethane" on a drawing is not a specification. Two parts both labeled "85A polyurethane" can differ by an order of magnitude in service life if one is a polyester grade run wet and the other a polyether grade, or if one is cast NDI and the other a low-cost TPU. The chapters that follow break the family into the decisions that actually determine performance: processing form, base chemistry, comparison against competing elastomers, and the spec-sheet numbers that separate grades.

Processing Types: Cast, TPU, Millable

The same polyurethane chemistry is supplied in three processing forms, and the form is often the first branch in a selection decision because it dictates tooling, part geometry, minimum run size and the ceiling on mechanical performance. The three forms are cast (liquid thermoset), thermoplastic (TPU), and millable gum. The table below summarizes how they differ in process, crosslinking, and where each one wins.

Cast polyurethane is a thermoset. A liquid isocyanate-terminated prepolymer is mixed with a curative (a diol such as 1,4-butanediol or a diamine such as MOCA) in roughly stoichiometric ratio, poured into an open or closed mold, and heat-cured to build a crosslinked network. Because the network is chemical, the finished part cannot be remelted, and cast grades reach the highest tensile strength, tear strength and dynamic load capacity in the family. Cast is the right choice for thick cross sections, large wheels and rollers, and short-to-medium production runs where the soft tooling cost is low. The NDI polyester cast system marketed as Vulkollan is the recognized benchmark for the most demanding dynamic load wheels.

Thermoplastic polyurethane (TPU) is a linear, segmented block copolymer with no chemical crosslinks. Heating softens it so it can be injection molded, extruded or blow molded, and scrap can be reground and reused, which is why TPU dominates high-volume thin-wall parts such as cable and hose jackets, films, tubing and consumer components. TPU divides into polyester-based grades, which lead on abrasion and oil resistance, and polyether-based grades, which lead on hydrolysis resistance and low-temperature flexibility. TPU is supplied as named grades by hardness, for example BASF Elastollan 1180A, 1190A and 1195A, so the buyer orders a specific pellet rather than mixing chemistry on site.

Millable polyurethane gum is supplied as a high-molecular-weight gum that can be processed on conventional rubber two-roll mills and internal mixers, then crosslinked with sulfur or peroxide and compression-molded like ordinary rubber. Its mechanical performance generally sits below cast polyurethane, but it lets a rubber fabricator make polyurethane parts on existing equipment without liquid-casting or injection-molding lines, which is its main commercial reason to exist. For comparable O-ring and seal duty, cast polyurethane outperforms millable polyurethane on abrasion and tensile strength.

The practical takeaway: let geometry and volume pick the form first. A single large heavy-duty wheel points to cast; a million small molded clips point to TPU; a rubber shop adding a polyurethane sheet line without new capital points to millable. Only after the form is fixed do the polyol and hardness decisions in the next chapter come into play.

Chemistry and Hardness Grades

Within any processing form, three chemistry choices set the property envelope: the isocyanate (MDI, TDI or NDI), the polyol soft segment (polyester, polyether or polycaprolactone), and the chain extender (a diol such as 1,4-butanediol, or an aromatic diamine such as MOCA). The most consequential is the polyol, because it decides hydrolysis resistance, the failure mode that ends more polyurethane parts than any other. The table below maps common cast chemistry combinations to their approximate hardness window and signature properties.

Polyester versus polyether is the decision that most often gets specified wrong. Polyester polyols deliver higher tensile and tear strength, better abrasion resistance and better resistance to oils, fuels and aliphatic solvents, which makes them the default for dry abrasive duty such as chute liners and scraper blades. Their weakness is hydrolysis: in warm water, steam or humid heat the ester linkages hydrolyze, the surface turns tacky and the part loses strength, often within months above about 50 degrees Celsius in wet service. Polyether polyols (PTMEG) give up a little abrasion and oil resistance in exchange for excellent hydrolysis resistance, fungus resistance and low-temperature flexibility, so they own wet, humid and sub-zero applications. Polycaprolactone polyols are a premium middle path: near polyester mechanicals with markedly better hydrolysis resistance.

Isocyanate choice shifts the dynamic and thermal ceiling. MDI is the workhorse for thermoplastic TPU and many cast grades, giving a good balance of properties and the cleanest processing. TDI prepolymers cured with MOCA produce tough, abrasion-resistant cast elastomers and dominate the general cast market. NDI (the Vulkollan family) builds the most ordered hard-segment structure and therefore the highest rebound and dynamic load capacity, which is why it is specified for the most heavily loaded wheels and dynamic components despite higher cost and more demanding processing. Note that MOCA is a regulated substance (a suspected carcinogen) controlled under schemes such as EU REACH authorization, so some buyers deliberately move to non-MOCA or thermoplastic systems.

Hardness is the headline number on every grade and is measured per ASTM D2240 or ISO 7619 with a durometer at 23 degrees Celsius. Polyurethane covers roughly Shore 60A to Shore 75D, but the A and D scales overlap at the top: about 95A corresponds to about 45D to 50D, so hardness above 90A should be quoted on the D scale to avoid ambiguity. Hardness is a stiffness indicator, not a strength rating: a higher durometer raises load capacity and cut resistance but lowers elongation, rebound and conformability. A shock-absorbing pad and a maximum-load wheel made from the same chemistry will be specified at very different hardnesses.

Reading these three choices together is the core skill. A datasheet that says "92A, PTMEG polyether, MDI" tells an engineer the part will be hard, resilient, hydrolysis-resistant and good in the cold, while "85A polyester TDI" promises better abrasion and oil resistance but a wet-service liability. Hardness and color reveal nothing about hydrolysis resistance, so the polyol type must always be confirmed explicitly.

Polyurethane vs Other Elastomers

Polyurethane is rarely the only candidate; it competes with nitrile (NBR), natural rubber, EPDM, silicone and fluoroelastomers, and it does not win every contest. The honest summary is that polyurethane leads decisively on abrasion, tensile strength, tear strength and load capacity, is competitive on oil and ozone resistance, and is weak on high temperature, hydrolysis (polyester grades), strong acids and bases, and hot polar solvents. The table below positions polyurethane against the common alternatives on the properties that usually drive a material switch.

Where polyurethane wins is wear-driven and load-driven applications. Cast and thermoplastic polyurethanes commonly show DIN abrasion volume loss in the range of about 20 to 60 cubic millimeters tested per ISO 4649 or ASTM D5963, several times lower than natural rubber or SBR at similar hardness, and tensile strength reaching the tens of megapascals where common rubbers sit far lower. This is why polyurethane displaces rubber on slurry pump liners, hydrocyclone parts, screen panels, scraper blades and forklift wheels: parts last longer between change-outs even though the raw material costs more per kilogram. The advantage is largest in sliding and impingement wear; against a sharp cutting edge, the margin over a well-compounded rubber narrows.

Where polyurethane loses is heat and aggressive chemistry. Continuous service above roughly 120 degrees Celsius pushes most polyurethanes out and silicone, fluoroelastomer or EPDM in. Hot water and steam destroy polyester grades by hydrolysis, and while polyether grades survive, EPDM and silicone are simply more tolerant of steam and many polar fluids. Strong acids, strong bases, hot ketones, esters and chlorinated solvents attack polyurethane, so chemical-process seal duty often goes to fluoroelastomer or PTFE-based materials instead. Polyurethane also has limited resistance to concentrated UV without stabilizers, though carbon-black and HALS-stabilized grades hold up well outdoors.

A useful rule of thumb: pick polyurethane when the failure mode is wear, cutting, tearing or load-bearing fatigue, and look elsewhere when the failure mode is high heat, steam, strong chemical attack, or the need for the very softest, most rebound-elastic seal. Many real selections end with a polyether polyurethane precisely because it keeps most of polyurethane's mechanical edge while removing the hydrolysis liability that disqualifies polyester grades.

Key Specification Parameters

A polyurethane datasheet typically lists ten to twenty values, but only a handful drive a selection decision. The parameters below, with their governing test methods, are the ones a procurement engineer should compare across quotes. Each is defined so a number on one supplier's sheet means the same thing on another's, since differing test methods are a common source of false comparison.

Hardness (ASTM D2240 / ISO 7619) is read in seconds with a durometer and anchors the whole spec. It correlates loosely with modulus and load capacity but not with strength or wear life, so it must never be used alone. Because the Shore A and Shore D scales overlap above 90A, always confirm which scale a quoted number uses.

Tensile strength and elongation at break (ASTM D412 / ISO 37) are measured by pulling a dumbbell specimen to failure. Polyurethane elastomers typically reach tens of megapascals tensile with elongation of several hundred percent, far above common rubbers. High tensile with high elongation signals a tough grade that resists both overload and tearing.

Tear strength (ASTM D624 Die C / ISO 34) measures resistance to crack propagation and is often the parameter that actually predicts field life in cut-and-gouge service such as liners and scrapers. Polyurethane's high tear strength, frequently tens to low hundreds of kilonewtons per meter, is a core reason it outlasts rubber in abrasive duty.

Abrasion resistance (ISO 4649 DIN / ASTM D5963, or Taber per ASTM D1044) quantifies wear. DIN abrasion reports volume loss in cubic millimeters; lower is better, and good polyurethane grades sit well below natural rubber at equal hardness. Confirm the method, because DIN and Taber numbers are not interchangeable and load and wheel choices change the result.

Compression set (ASTM D395 / ISO 815) is the permanent deformation remaining after a specimen is held compressed at temperature, reported as a percentage; it is critical for seals and gaskets that must keep sealing force. Lower compression set means the part recovers and keeps sealing; high set at temperature predicts leakage. Rebound resilience (ISO 4662 / ASTM D2632) indicates how much energy returns elastically, important for dynamic wheels and shock parts, where NDI cast grades lead.

One discipline ties these together: compare like with like. A polyester grade may show higher tensile and lower abrasion loss than a polyether grade on paper, yet fail first in wet service because the datasheet does not put hydrolysis in the headline numbers. Always read the polyol type and the service-temperature rating alongside the mechanical values before declaring one grade superior.

Selection Decision Factors

To turn the preceding chapters into a specific grade, work through the decision sequence below. The most common selection mistakes are choosing the polyol type after the hardness instead of before, and ignoring service temperature and moisture until the part fails in the field. These eight steps can serve as a fixed RFQ template.

1. Processing form and geometry: Decide cast, TPU or millable first, driven by part size, wall thickness and production volume. Thick heavy wheels and short runs favor cast; high-volume thin parts favor TPU; existing rubber tooling favors millable.
2. Polyol type versus environment: If the part sees water, steam, high humidity or wet chemicals above about 50 degrees Celsius, specify a polyether or polycaprolactone grade. Reserve polyester grades for dry abrasive and oil-contact duty.
3. Hardness: Set the durometer from the function, not from habit. Soft (60A to 80A) for shock absorption, conformance and grip; medium (85A to 95A) for general wheels and rollers; hard (50D to 75D) for structural load and minimal deflection. Quote D-scale above 90A.
4. Mechanical targets: Specify the tensile, tear, abrasion and compression-set values the application demands, with their test standards (ASTM D412, D624, ISO 4649, D395). For seals, lead with compression set; for liners and scrapers, lead with tear and abrasion.
5. Temperature window: Confirm continuous and peak service temperature. Above about 120 degrees Celsius continuous, move to silicone, fluoroelastomer or EPDM; below minus 40 degrees Celsius, specify PTMEG polyether or polycaprolactone for low-temperature flexibility.
6. Chemical and media compatibility: Check the actual contact fluids against a manufacturer compatibility chart. Polyurethane handles many oils and fuels well but is attacked by strong acids and bases, hot ketones, esters and chlorinated solvents.
7. Certifications and regulatory: For food and medical contact, specify FDA, USP Class VI or EU food-contact grades. Note MOCA-cured cast grades are subject to REACH and similar controls; non-MOCA or thermoplastic systems may be required by the buyer's compliance policy.
8. Total cost of ownership (TCO): Polyurethane costs more per kilogram than rubber but often wins on change-out interval and downtime in wear service. Weigh part life and replacement labor, not just unit price, and confirm the supplier can hold the same grade over the program's life.

One last commonly overlooked dimension is manufacturer serviceability and grade continuity: whether the supplier holds the named grade in stock, can reproduce the same hardness and polyol chemistry batch to batch, and can document the polyol type and certifications. Major suppliers each anchor a part of the market: Covestro Vulkollan for highest-load cast wheels, Lanxess Adiprene and Vibrathane and Covestro Baytec and Desmodur for general cast prepolymers, and BASF Elastollan, Lubrizol Estane and Pellethane, Covestro Texin and Desmopan, and Huntsman Irogran for thermoplastic TPU. Matching a specific grade rather than ordering generic polyurethane is what makes a selection repeatable five years into a production program.