# Polyurethane elastomer

Selecting cast polyurethane and TPU by compound, cure history, deformation and exposure requirements.

Reviewed: 2026-11-01

Tags: materials, polymers, elastomers

## 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](https://www.smooth-on.com/tb/files/PMC-780_Dry-Wet.pdf)

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](https://gallaghercorp.com/wp-content/uploads/2017/03/WP-122-POLYURETHANE-INJECTION-MOLDING-1.pdf)

## 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](https://solutions.covestro.com/-/media/covestro/solution-center/brands/downloads/imported/1670483977.pdf?hash=488DD2B41E9CD2B0B955D2075936F7C9&rev=bf4587ffdcfb44aba6153badfac7b433)

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](https://www.basf.com/dam/jcr%3Ae42483e4-4087-3544-a6a2-f958427c1030/Elastollan_Properties-Chemical-Properties.pdf)

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](https://gallaghercorp.com/polyurethane-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](https://gallaghercorp.com/polyurethane-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](https://www.smooth-on.com/tb/files/PMC-780_Dry-Wet.pdf)

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](https://www.smooth-on.com/products/pmc-780-dry/)

## 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](https://solutions.covestro.com/-/media/covestro/solution-center/brands/downloads/imported/1670483977.pdf?hash=488DD2B41E9CD2B0B955D2075936F7C9&rev=bf4587ffdcfb44aba6153badfac7b433)

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](https://www.smooth-on.com/tb/files/PMC-780_Dry-Wet.pdf)

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](../grades-tempers-and-conditions.md)