PU (Polyurethane) Rubber Technical Guide

1. What Is Polyurethane (PU) Rubber?

Polyurethane elastomers occupy a unique space between conventional rubbers and rigid plastics. Chemically, PU is formed by the reaction of a polyol (polyester, polyether, or polycaprolactone) with an isocyanate (MDI, TDI, or NDI), extended with a short-chain diol or diamine. This produces a segmented block copolymer consisting of alternating hard segments (isocyanate + chain extender) and soft segments (polyol).

The hard segments aggregate into domains that act as physical crosslinks and reinforcing fillers, while the soft segments provide elasticity. This two-phase morphology, visible under transmission electron microscopy, gives PU its extraordinary combination of:

• • Highest abrasion resistance of any elastomer (3–5× that of NBR)
• • Highest tensile strength of any elastomer (30–50 MPa, and up to 60 MPa for NDI-based systems)
• • High load-bearing capacity with minimal compression set

PU is classified under ASTM D1418 as AU (polyester-based) and EU (polyether-based).

2. Types of Polyurethane Elastomers

2.1 By Processing Method

Selection guidance:

• • Large parts, thick sections, or superior physical properties → CPU
• • High-volume, thin-walled, recyclability needed → TPU
• • Processor already equipped for conventional rubber molding → MPU

2.2 By Polyol Backbone

3. Mechanical and Physical Properties

Abrasion benchmark comparison:

4. Chemical and Environmental Resistance

Critical failure mode — Hydrolysis: Polyester-based PU undergoes hydrolytic degradation in hot water (>80°C). The ester linkages are cleaved by water molecules, causing progressive loss of mechanical properties. This is the most common failure mode encountered in field service. Polyether PU is 5–10× more resistant to hydrolysis and must be specified for any water-contact application above 50°C. A simple field test: if the failed part surface feels tacky or sticky, hydrolysis is likely the cause.

Microbial attack: Polyester PU is susceptible to fungal and bacterial degradation in soil burial or warm, humid environments. Biocides (e.g., Vinyzene, 0.1–0.5 phr) can be added to CPU and MPU formulations for such applications.

5. Key Applications

5.1 Mining and Aggregate Processing

PU is the material of choice for high-wear mining components: screen meshes and panels for vibrating screens (tensioned or modular), hydrocyclone liners and apexes, slurry pump impellers and volute liners, flotation cell rotors and stators, and chute liners and impact pads. Polyester CPU with hardness 85–95 Shore A is the standard for most mining wear applications.

5.2 Roller Covers and Industrial Rolls

PU roller covers are used in steel mills (bridle rolls, tension rolls), paper mills (press rolls, wire return rolls), printing (inking rolls, damping rolls), and material handling (conveyor drive and idler rollers). The combination of high coefficient of friction, excellent cut-and-chip resistance, and long wear life makes PU superior to rubber roller covers in dry running and limited-slip applications.

5.3 Hydraulic Seals

PU is the primary material for hydraulic cylinder rod seals and piston seals (U-cups, lip seals, wipers) due to its:

• • High abrasion resistance (critical for reciprocating motion)
• • High modulus (extrusion resistance without backup rings at moderate pressures)
• • Low compression set (maintains sealing force)
• • Compatibility with mineral hydraulic oils

Seal standards include ISO 5597 (hydraulic cylinder seal housing dimensions) and ISO 6195 (wiper seal dimensions).

5.4 Couplings and Flexible Elements

PU flexible couplings (spider/jaw type, pin-and-bush type, tire type) transmit torque while accommodating angular, parallel, and axial misalignment. Key specification: shore hardness selection based on torque rating, with 92–95 Shore A being standard and 55–64 Shore D used for high-torque compact designs.

5.5 Industrial Wheels and Rollers

PU-treaded wheels and castors: forklift load wheels (92–95 Shore A), AGV (Automated Guided Vehicle) drive wheels, escalator step rollers, and skateboard/scooter wheels (TPU).

5.6 Bumpers and Impact Pads

PU's high energy absorption per unit volume makes it ideal for shock absorption applications: dock bumpers, crane buffers (cellular PU meeting AISE standards for steel mills), offshore platform boat landing fenders, and automotive suspension bump stops (TPU, microcellular PU).

6. PU vs Conventional Elastomers

7. Design Considerations

1. Heat buildup: PU's low resilience means dynamic applications generate internal heat. For cyclic loading >5 Hz, part thickness should be limited to avoid heat buildup and thermal degradation.

1. Hydrolysis protection: For any water contact >50°C, specify polyether or polycaprolactone PU.

1. UV stabilization: While PU has inherently good UV resistance, UV stabilizers (hindered amine light stabilizers, carbon black) should be added for outdoor exposure >2 years.

1. Bonding to metal: PU bonds excellently to steel and aluminum with appropriate primers (Chemlok 213/219 or equivalent). Sandblasting to SA 2.5 and solvent degreasing is the standard surface preparation.

1. Post-cure: CPU parts should be post-cured at 100–120°C for 16–24 hours to optimize physical properties and remove residual isocyanate.

8. Standards

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Nanjing Yuhang Rubber Co., Ltd. produces high-performance polyurethane (PU) wear components — screen meshes, hydrocyclone liners, scraper blades, roller covers, hydraulic seals, wheels, and custom castings — for mining, oilfield, steel, and heavy industrial applications. Our in-house PU casting facility formulates polyester, polyether, and polycaprolactone systems from 60 Shore A to 75 Shore D. Full mechanical testing and material certification provided with every order.

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