Polyurethane (PU/PUR) is a segmented copolymer formed by polyaddition of a diisocyanate or polyisocyanate with a polyol, frequently chain-extended with 1,4-butanediol. The hard segments derived from isocyanate and chain extender contribute modulus, tear strength, and load-bearing capacity; the soft segments derived from polyester, polyether, polycaprolactone, or polycarbonate polyols control elongation, low-temperature flexibility, and dynamic recovery. Product designations are supplier-specific but typically encode isocyanate type, polyol backbone, hardness, and cure system. A polyester-MDI prepolymer supplied for cast elastomers may carry a nominal Shore hardness of 90A, a free NCO content of 5.8–6.5%, a Brookfield viscosity of 1,200–2,500 mPa·s at 85°C, and a curative mix ratio of 100:11.5–12.8 by weight. Solid PU elastomers are produced across Shore 60A to 75D, with tensile strength of 25–55 MPa under ISO 37 or ASTM D412, elongation at break of 300–700%, and density of 1.02–1.25 g/cm³ under ISO 1183-1. Cellular grades range from 30 kg/m³ rigid foam to 900 kg/m³ microcellular structural foam. The main processing forms are thermoplastic polyurethane granules, liquid two-component or three-component cast systems, millable gums, and reactive spray coatings; each form imposes a different drying, metering, and tooling window.

What Distinguishes Solid PU from Vulcanised Rubber, Epoxy, and Flexible PVC?

Solid PU differs from vulcanised rubber by its combination of high abrasion resistance, tear strength, and load-bearing capacity without sulfur cure. In a Shore 90A cast elastomer, DIN ISO 4649 abrasion loss is commonly reported between 20 and 45 mm³, while a natural rubber compound of equivalent hardness may lose 100–150 mm³ under the same method. The difference reflects the urethane/urea hard-domain network rather than carbon-black-reinforced polyisoprene. Against epoxy, solid PU offers elongation at break of 300–700% and split tear strength of 60–140 kN/m under ISO 34-1, whereas an unfilled epoxy typically exhibits elongation below 5% and brittle impact behaviour under ISO 179-1. Against plasticised PVC, PU retains low-temperature flexibility without migratory plasticisers; PVC-P relies on plasticiser distribution and embrittles at -20 to -40°C as plasticiser mobility decreases, while solid PU grades based on polyether or polyester can remain flexible below -40°C depending on formulation. The comparisons are not universal because PU formulations span a wide property envelope; they are valid only when the test method, hardness, and conditioning are stated.

Processing method also defines the boundary between PU and rubber. Vulcanised natural rubber requires compounding, a two-roll mill, and press cure at 140–170°C for 4–10 min, whereas cast PU is degassed and cured at 100–120°C for 16–24 h without high-pressure presses. TPU injection moulding in thin sections can reach cycle times of 20–50 s, but moisture control and compression set remain the limiting constraints. This asymmetry in cure time and machinery is why PU is selected for large low-pressure cast parts or high-speed thermoplastic moulding rather than for short-cycle compression moulding of rubber.

On a co-rotating twin-screw compounding line with an L/D of 40:1 and intermeshing screws, TPU shows shear heating that can raise melt temperature 15–30°C above the barrel set point at screw speeds of 200–400 rpm. Barrel profiles are typically maintained at 180–230°C, with the melt temperature not exceeding 230°C for MDI-based ester grades because hard-segment degradation increases free MDI and causes surface roughness. Pre-drying at 80–110°C to a residual moisture below 0.03% is mandatory for extruded TPU sections and injection moulded parts; residual water above 0.05% produces porosity and reduces tensile strength by more than 10% in thin-walled mouldings. In injection moulding, a screw with 20:1–25:1 L/D, compression ratio 2:1–3:1, and shot capacity limited to 30–70% of barrel volume reduces residence time and thermal risk. Mould temperatures of 20–50°C are used for TPU, and clamp force requirements are generally lower than those for glass-filled engineering thermoplastics because the molten TPU remains compressible during packing. Two-component cast PU machines meter prepolymer and curative with gear pumps to a mix ratio tolerance of ±0.5%; vacuum degassing at 2–5 mbar and cure at 100–120°C for 16–24 h are standard for solid cast parts.

Rheological data further narrow the selection. TPU melt flow index under ISO 1133-1:2022 at 190°C/21.6 kg typically ranges from 5 to 40 g/10 min for injection grades, while extrusion grades are lower. For cast systems, gel time and viscosity increase during processing depend on curative stoichiometry; at 80°C, the initial mix viscosity may be 1,000–2,000 mPa·s, rising to 5,000–10,000 mPa·s before gel. This viscosity window determines whether a gear pump or static mixer can fill a mould without dewetting or air entrapment. Screw recovery speed and backpressure interact with melt compressibility; backpressure of 0.5–2.0 MPa is sufficient to densify the melt without excessive shear heating. Higher backpressure above 4.0 MPa can cause prolonged residence and yellowing at the screw tip.

Specifying hardness, abrasion, and residual NCO in solid cast grades

A purchase specification for a Shore 90A polyester-MDI cast elastomer may include hardness 90 ± 3 Shore A under ASTM D2240-15, tensile strength not less than 45 MPa under ISO 37, elongation at break not less than 450%, tear strength not less than 100 kN/m under ISO 34-1 B, and DIN ISO 4649 abrasion loss not greater than 30 mm³. The free NCO content is controlled to 5.8–6.5% by titration because a shift of ±0.2 percentage points changes stoichiometry sufficiently to alter hardness by 2–5 Shore A points and compression set by 5–10 percentage points. Pot life at 80°C is specified between 5 and 9 min for hand casting; automated low-pressure mixing shortens dosing but does not eliminate the viscosity limit imposed by chain extension. The table below summarises backbone selection, not as a substitution for a full specification, but as a grading filter for wet service, low-temperature impact, and dynamic loading.

For MDI-polyester cast systems with aromatic diamine curatives, gel time at 80°C is 4–12 min depending on curative level and catalyst. The demould time is set by a Shore A hardness of 40–60A at demould; below this, parts tear on removal. Full crosslink density develops over 7 days at 25°C, and exotherm during thick-section casting can locally exceed 120°C, causing bubbles if the mixing head pressure drops below the vapour pressure of dissolved gases. Pre-polymer storage below 15°C risks hard-segment crystallisation and viscosity drift. When ambient relative humidity exceeds 60%, open polyol and curative reservoirs require dry nitrogen blanketing or vacuum drying before use because moisture ingress above 0.05% in the polyol or curative generates surface pinholing and reduces tear strength.

When a Shore 90A polyester-MDI casting replaces a rubber-lined pump stator

In progressing cavity pump stators and slurry pump wear components, a Shore 90A polyester-MDI cast elastomer is evaluated against DIN ISO 4649 abrasion loss, ASTM D471 volume change in service fluid, and ISO 34-1 tear retention after immersion. The casting process meters prepolymer at 70–85°C and an aromatic diamine curative at 100–120°C through a low-pressure gear pump into a mould preheated to 100°C. Demoulding occurs after 16–24 h at 100–120°C; full property development requires a post-cure period of 7 days at 22–25°C. The PU stator typically gives lower sliding abrasion losses than a natural rubber stator, but it also increases hysteresis heat build-up if cyclic shear strain exceeds 10% and reduces tolerance to coarse-particle impingement because of lower rebound resilience under ISO 8307. Published field data for this specific configuration is limited; component producers therefore verify service performance by instrumented wear ring tests and by measuring surface temperature rise under intermittent dry running, not solely by supplier tabulated abrasion numbers.

In mining screen panels and hydrocyclone liners, moulded PU is specified at Shore 80A–95A with DIN ISO 4649 abrasion loss below 30 mm³ and ISO 34-1 tear strength above 80 kN/m. Caster and forklift wheels use Shore 80A–95A solid PU for load capacity and resistance to floor debris, with compression set below 35% after 22 h at 70°C under ASTM D395 Method B. Rotary shaft seals require compression set below 30% and volume change in ASTM oil No. 1 at 100°C below 10% under ASTM D471. Industrial rollers require grinding after casting; a Shore 80A–95A PU roller is ground on a cylindrical grinder to a surface roughness of 0.4–0.8 µm Ra. This demands a formulation with a hardness tolerance of ±2 Shore A because variation across the roller face alters grinding forces and surface finish. In footwear midsoles, expanded TPU with density of 250–700 kg/m³ is tested for rebound resilience under ISO 8307; values of 35–55% are typical, and the foam must be processed with nitrogen or chemical blowing agents at a melt temperature below 230°C to avoid cell coalescence. Automotive suspension bushings and CVJ boots use PU for flex fatigue and grease resistance, with ISO 188 accelerated ageing at 100°C for 72 h used to check tensile retention above 70%.

Hydrolysis, UV stability, and amine incompatibility boundaries

Polyester PU is susceptible to acid-catalysed and base-catalysed hydrolysis. In distilled water at 80°C, a conventional polyester-MDI Shore 90A cast elastomer may lose more than 50% of initial tensile strength within 14–30 days under ISO 527-2, whereas a polyether or polycarbonate grade typically retains a higher fraction of tensile strength over the same interval. Aromatic isocyanate-based PU yellows and microcracks under UV exposure; outdoor colour stability requires aliphatic HDI or IPDI topcoats or compounding with carbon black at 2–4%. Formulators must avoid uncontrolled addition of amine-based additives to isocyanate-containing prepolymers because the resulting fast urea reaction increases viscosity, shortens pot life, and can trap bubbles before degassing is complete. When oil resistance is required, NBR is preferred if volume change in IRM 901 oil must remain below 5% at 100°C for 70 h; conventional PU can swell 5–20%. EPDM and silicone are selected when continuous service temperature exceeds 120°C or when steam sterilisation is used, because standard solid PU is limited to 70–80°C in water and 110–120°C dry heat. PU generally resists mineral oils, greases, and aliphatic hydrocarbons but is swollen by ketones, chlorinated solvents, and aromatic hydrocarbons; a standard 24 h immersion at 23°C in toluene may produce volume change above 50%, making PU unsuitable for seals in those fluids. For medical or food-contact use, only grades validated to ISO 10993-5, ISO 10993-10, or USP Class VI are appropriate; industrial PU grades are not automatically suitable for those boundary conditions.