Polyurethane elastomer

Material identity and supplied form

Smooth-On PMC-780 Dry/Wet specifies 2A:1B by weight or volume, 25 min pot life at 23°C and 48 h cure at 23°C. Its 80 Shore A result under ASTM D2240 was measured after seven days at 23°C. Separate handling, demolding and acceptance timing. Smooth-On PMC-780 bulletin

Polyurethane elastomer describes a family rather than one interchangeable rubber. Distinguish reactively cast thermoset polyurethane from thermoplastic polyurethane, TPU. Gallagher's manufacturing guidance describes thermoset systems cast in low-pressure molds and TPU processed by injection molding; it also warns that production-volume rules of thumb do not always determine the best route. Specify the material system and manufacturing route before comparing costs or properties. Gallagher: polyurethane injection molding

Chemistry and environmental selection

Covestro's Desmopan product guide, explicitly marked Edition: 2021, organizes TPU grades by raw-material families including ester, ether, ester-ether, carbonate and aliphatic systems. This range shows why a trade family or hardness alone is insufficient to identify performance. Preserve the full grade suffix when requesting the current datasheet and processing instructions. Covestro: Desmopan TPU product guide

BASF's Elastollan chemical-properties guidance explains that prolonged hot-water, water-vapor or tropical exposure can hydrolyze polyester chains and reduce mechanical properties; its polyether-based TPU is more resistant to that mechanism. This is a chemistry-based selection direction, not a guarantee that any ether polyurethane survives every wet duty. The guide's hydrolysis comparisons use specified tensile-strength or elongation endpoints, so “hydrolysis resistance” needs an acceptance criterion and exposure temperature. BASF: Elastollan chemical properties

For a seal, wheel or scraper, document the actual fluid, concentration, temperature, contact duration and cleaning cycle. Ask for compound-specific evidence after aging under representative strain. Do not extend a favorable water result to fuels, lubricants or detergents, or assume unchanged hardness proves unchanged tear strength and service life.

Hardness, geometry and recovery

Gallagher defines hardness as resistance to indentation under a specified indenter and load. Shore A and Shore D use different indenters and springs and have no linear relationship. Its compression guidance also explains that a part's ability to bulge depends on whether loaded faces are lubricated, dry or bonded. Restricting lateral movement changes the compression response even when the compound and external dimensions remain the same. Gallagher: load-bearing capacity

For a compression pad, obtain a load-deflection curve with the specimen geometry, shape factor and surface restraint stated. For a vibration mount or roller, also request behavior at the relevant frequency, strain amplitude and temperature. A hardness reading is useful for incoming identification and consistency checks, but cannot replace those application tests.

Compression set measures residual deformation after a specified loading and recovery procedure. Gallagher distinguishes ASTM D395 constant-force Method A from constant-deflection Method B, whose percentage uses the applied deflection as its denominator. Preserve the method, temperature, duration and recovery interval when comparing compounds. A set value is not the same as creep while loaded or stress relaxation while held at fixed deformation. Gallagher: compression set

Illustrative Method B arithmetic: a 10 mm specimen compressed to 7.5 mm and recovered to 9.5 mm has set = (10 − 9.5) / (10 − 7.5) × 100 = 20%. This hypothetical result is neither 5% set nor a measured property of a named polyurethane. It says nothing about fatigue life without further evidence.

Bounded cast-system example

Smooth-On's PMC-780 Dry/Wet bulletin, footer code 030724DH, specifies 2A:1B by weight or volume, 25-minute pot life at 23 °C, and 48-hour cure at 23 °C. Properties, including Shore A 80 under ASTM D2240, were measured after seven days at 23 °C. A separate post-cure procedure is provided. Demolding and property-test age are different records. Smooth-On: PMC-780 technical bulletin

For an illustrative 300 g batch at that product's stated mass ratio, use 200 g A and 100 g B. That arithmetic does not authorize the same ratio for another system, and a mass ratio cannot generally be substituted for a volume ratio.

The product page distinguishes Dry, which does not exude oil, from Wet, which contains a built-in release agent for concrete demolding and has greater net shrinkage over time. Those variants are therefore consequential to a contact surface, coating or later bonding operation. Smooth-On: PMC-780 Dry product information

Manufacturing and inspection controls

For cast systems, retain component identities, actual quantities, mixing and dispensing records, mold preparation, demolding time and any post-cure. For TPU, obtain the specific grade's drying and melt-processing instructions; do not reuse a cast-rubber cure schedule. Covestro's grade tables list different injection-molding and extrusion recommendations, alongside mechanical data at stated conditions. A processing temperature is not a continuous service rating. Covestro: grade tables and processing recommendations

The PMC-780 bulletin calls for prompt resealing of moisture-sensitive liquids, clean equipment and premixing Part B. Include storage and dispensing history when investigating defective castings. Smooth-On: preparation and mixing instructions

Qualify the finished component for its duty: inspect voids and dimensions, test insert or substrate adhesion where present, and measure deformation or wear under representative loads. Abrasion results need their test method, counterface, abrasive and conditions; tensile strength is not an allowable repeated working stress. Compare parts made with the intended tooling and cure history, including after environmental aging.

Handling and specification record

Reactive components, cured parts and TPU granules require different handling information. Before dispensing or changing a cure process, obtain the current SDS for each exact component and additive, and follow the supplier's handling instructions. A cured-part material description does not establish precautions for uncured ingredients or heated processing.

A useful specification records:

• Complete compound and components, form, hardness scale, tolerance and test timing.
• Cure, post-cure or thermoplastic-processing history and permitted substitutions.
• Required compression, tear, abrasion, dynamic or bond tests with conditions.
• Fluid, moisture and temperature exposures and acceptable property retention.
• Finished-part dimensions, defects, inspection intervals and replacement criteria.

The Covestro guide supplies an explicit edition; the Smooth-On bulletin supplies a footer code rather than an unambiguous printed date. The cited BASF and Gallagher documents do not establish clear revision dates. Do not turn upload paths or search-engine dates into technical editions, or treat family-level guidance as approval for a particular service.

Related reference

Material selection through production