Three structural axes govern every polyurethane elastomer grade on a spec sheet: the soft-segment polyol family, the hard-segment isocyanate, and the manufacturing route that converts prepolymer into finished part. The polyol backbone alone shifts tensile strength from 25.3 to 36.7 MPa on identical MDI-BDO formulations, as tabulated in a polypropylene-carbonate vs polyether vs polybutylene-adipate comparison [S1].
Selection is therefore less about "polyurethane" as a single material and more about matching a backbone + isocyanate + process triplet to the operating envelope: hydrolysis, cold-flex, dynamic cut-growth, and any food-contact or in-vivo regulatory rule that constrains the formulation.
Soft-Segment Polyol Families and Their Hard Numbers
Polyester polyols (polybutylene adipate, MW 3000) deliver the highest elongation at break in published prepolymer work, with 558.2% measured against a 1.05 isocyanate index and 120 °C / 3–4 h cure, paired with tensile strength of 36.4 MPa and Shore A 82 [S1]. Polyether polyols (N220, MW ~2000) trade peak tensile for cold-flex and hydrolytic stability, landing at 25.3 MPa tensile, 294.7% elongation, and a lower Shore A 70 on the same protocol [S1].
Polycarbonate and polycaprolactone polyols sit in the upper bracket for both hydrolytic resistance and mechanical strength; polypropylene-carbonate polyol (MW 3000) returned 36.7 MPa tensile, 268.9% elongation, and a markedly higher 92 Shore A, with tear strength at 133.9 kN/m and 100% modulus at 15.2 MPa [S1]. Where the application forces a hard-rubber replacement — wheels, high-durometer rollers, mining screens — polycarbonate grades are the practical substitute, not standard polyether. A direct four-criteria comparison:
Polyol family — Tensile (MPa) — Elongation (%) — Shore A — Primary weakness<br>Polyester (PBA, MW 3000) — 36.4 — 558 — 82 — hydrolysis in hot/wet service<br>Polyether (N220) — 25.3 — 295 — 70 — lower cut/abrasion<br>Polycarbonate (PPC, MW 3000) — 36.7 — 269 — 92 — cost, processing window<br>Polycaprolactone (PCL) — high — high — mid — middle-cost balance grade
All four data points above draw from the S1 prepolymer protocol (MDI, 11% NCO target prepolymer, BDO chain extension at index 1.05); cross-grade differences therefore reflect polyol behaviour, not formulation drift.
Isocyanate Choice: Aromatic vs Aliphatic Hard Segments
Aromatic isocyanates — MDI (4,4'-diphenylmethane diisocyanate) and TDI (toluene diisocyanate) — dominate CPU and TPU production because the rigid aromatic ring stiffens the hard segment and raises modulus per unit mass. The S1 prepolymer procedure is built on MDI: 55 °C flask charge, 20-minute polyol addition, 2.5 hours at 80 °C to reach ~11% residual NCO before BDO chain extension [S1].
Aliphatic isocyanates — HDI, IPDI, HMDI — are mandated where UV colour stability matters: lightfast aqueous polyurethane dispersions for leather finishing are formulated from aliphatic isocyanate specifically to suppress aromatic yellowing [S2]. Where the part is an outdoor seal, a clear topcoat, or an automotive interior skin, the formulator trades raw mechanicals for non-yellowing aliphatic hard segments; where the part is hidden, abrasion-loaded, or cost-driven, MDI/TDI remains the default. The decision is rarely about strength and almost always about light, regulatory, or aesthetic exposure.
Processing Routes: CPU, MPU, TPU, and PUD
Cast polyurethane elastomer (CPU) is the prepolymer-plus-chain-extender route documented in S1: prepolymer degassed 8–10 min under vacuum, poured on release-coated glass at ~1 mm thickness, cured 3–4 h at 120 °C, then demoulded for property testing. CPU delivers the widest hardness window (Shore A 70 to well past 90 in the S1 set) and the largest part geometries, at the cost of pot-life discipline. [S1]
Millable polyurethane (MPU) is a gum-stock rubber that can be processed on conventional rubber equipment; a documented "raw rubber for millable polyether polyurethane elastomer" has been prepared for extrusion and calendering [S2]. MPU is the right call when a plant already runs rubber mills and wants to drop PU into existing calendars or extruders rather than install a casting line. Thermoplastic polyurethane (TPU) is melt-processable — injection moulded, extruded, blow-formed — and is the form behind most PU transmission belts, shoe soles, and wire-and-cable jackets; Megadyne-class manufacturers run TPU as their base stock for power-transmission and rubber belting [S2].
Aqueous polyurethane dispersion (PUD) is the waterborne branch; the cited leather-finishing PUD uses aliphatic isocyanate as the hard-segment monomer specifically for lightfast, low-VOC coatings [S2]. Reaction-injection moulding (RIM) and spray-grade systems complete the route map but are not documented in the cited material — treat them as a fifth process family when scoping, not as fact for this article.
Selection Criteria vs Application Fit
Hydrolysis resistance in hot/wet service is the single largest application gate. Polyester PBA in the S1 data set is mechanically best-in-class, yet polyesters are known to hydrolyse via the ester linkage, so specifying polyester-PU into a 60 °C aqueous or steam-clean environment is a service-life risk; polyether or polycarbonate backbones are the conservative pick. Nitrile-rubber (NBR) is the more common hydrocarbon/fuel elastomer — the NBR spec map covers fuel-side selection when the medium is petroleum rather than water. [S1]
Dynamic load and abrasion point to polyester or polycarbonate CPU; the S1 data shows polycarbonate-PU lifting 100% modulus to 15.2 MPa vs polyether's 7.4 MPa at otherwise identical formulation [S1]. Low-temperature flexibility still belongs to polyether; EPDM and other hydrocarbon rubbers cover the very-low-TCR range, and the EPDM TCO model is the reference for 20-year spend planning in roofing and sealing where temperature swing dominates. For foam-cored or insulation-laminated assemblies, the polyurethane insulation reference is the right cross-check on thermal performance, since rigid PU foam and flexible PU elastomer share an isocyanate chemistry but not a property map.
Standards, Sourcing Signals, and What to Verify on the Datasheet
No single ISO or ASTM standard "covers polyurethane elastomer" in the way a flange standard covers a flange. Relevant call-outs that engineers actually check: ISO 4649 (DIN abrasion) for wear-rated parts, ASTM D2240 for Shore A/D hardness, ASTM D412 for tensile/elongation, ASTM D624 for tear, ASTM D429 for rubber-to-substrate adhesion, and ISO 37 / ISO 34 for the same tensile and tear figures on a global test basis. For food-contact and potable-water grades, FDA 21 CFR 177.1680 and EU Regulation 1935/2004 are the regulator-anchored frameworks; for in-vivo and skin-contact, USP Class VI and ISO 10993 are the named references — but only invoke them when the certificate on the specific grade is in hand. [S1]
Trackable sourcing signals for the next buying cycle: (a) the polypropylene-carbonate polyol route delivers a Shore A 92 part on standard MDI-BDO lab equipment, so any "polycarbonate-PU at 70–80 Shore A" offer is a soft-segment dilution, not a backbone change [S1]; (b) any "aliphatic PU" claim without naming the isocyanate (HDI/IPDI/HMDI) is a soft claim and should be backed by a UV-aged ΔE colour-shift test; (c) cast-PU hardness and abrasion ratings should be quoted from a CPU prepolymer route, since TPU and MPU spec sheets can quote Shore A values from different test geometries. Two comparison anchors that fit the broader elastomer landscape: industrial valve seats and flow-meter liners increasingly use cast PU as a wear part, which is where the polyol-family decision above directly drives the part number on a datasheet.