Introduction to Millable PU Rubber
Millable PU rubber is produced by reacting polyester or polyether polyols with diisocyanates (e.g., MDI or TDI) and chain extenders, resulting in an under-indexed polymer (NCO:OH ratio <1.0) that requires vulcanization with sulfur, peroxides, or UV systems. Developed for applications needing superior durability, it was designed to bridge the gap between traditional rubbers and high-end elastomers. Brands like Millathane® exemplify this material, available in bales or premilled sheets. It is compounded with fillers (e.g., carbon black, silica), plasticizers, and curing agents. Unlike castable PU, millable PU allows for standard rubber processing, providing consistency and versatility. It is more expensive than general-purpose rubbers like SBR but offers exceptional value in abrasive or oil-exposed environments.
Characteristics of Millable PU Rubber
Millable PU rubber combines outstanding abrasion resistance with mechanical toughness, oil resistance, and environmental stability. Here are its key characteristics:
- Mechanical Properties: Exceptional abrasion resistance (DIN abrasion as low as 30-40 mm³ loss), high tensile strength (>5,000 psi or 35 MPa), elongation at break (200-600%), and excellent tear strength. It has low compression set and superior load-bearing capacity, making it ideal for dynamic applications. Hardness ranges from 25 to 99 Shore A (up to 60 Shore D). It also provides good vibration damping and friction characteristics, though it may require reinforcement for extreme loads.
- Chemical Resistance: Excellent resistance to oils, fuels, greases, oxygen, ozone, and weathering. It performs well against dilute acids and bases but has moderate resistance to strong solvents, ketones, and esters. This makes it superior to many organic rubbers in oil-contact environments.
- Thermal Properties: Operates effectively from -40°C to +120°C, with good low-temperature flexibility and heat aging resistance. It maintains properties without significant softening or embrittlement, though prolonged high-heat exposure may require stabilizers.
- Durability and Aging: Superior toughness and long-term stability in harsh conditions, with excellent resistance to oxidation and UV. It has low gas permeability and can be formulated for sponge (foamed) versions with densities as low as 0.4 g/cc. Compression set is low, ensuring reliable sealing over time.
- Processing and Cost: Easily processed on Banbury mixers, rubber mills, extrusion, and molding equipment like conventional rubbers. It has good green strength and can be vulcanized quickly. While more costly than NR or SBR, its durability reduces replacement needs in demanding uses.
- Other Properties: Low specific gravity (1.0-1.2), good electrical insulation, and biocompatibility in some grades. It can be colored easily and offers self-lubricating traits in frictional applications. However, it may hydrolyze in humid, high-heat conditions without proper formulation.
In formulations, millable PU is often blended with other rubbers for balanced properties and requires careful curing to avoid scorching.
Applications of Millable PU Rubber
Millable PU's abrasion and oil resistance make it ideal for high-wear and industrial environments:
- Automotive Industry: Used in seals, O-rings, bushings, and vibration mounts for their damping and durability in engines and suspensions.
- Industrial Rollers and Tires: Rubber-covered rollers for printing/textiles and solid tires for forklifts, leveraging abrasion and friction properties.
- Hydraulic and Sealing Systems: Seals in high-pressure hydraulics, handling shock loads and contamination.
- Mining and Material Handling: Conveyor components, wheels, and pads for load-bearing and wear resistance.
- Other Uses: Foamed versions for insulation or cushions; protective coatings, gaskets, and specialty parts in aerospace or electronics.
Millable PU is produced globally, with advancements in bio-based variants for sustainability. It's a premium choice for applications where failure costs are high.