Cryogenic grinding technology enables rubber crushers to produce fine rubber powder from vulcanized rubber scrap that cannot be economically processed at ambient temperatures. When cooled below their glass transition temperature, rubber compounds become brittle and fracture easily, allowing production of particles as fine as 40 mesh (420 microns) or smaller. The Rubber News covers advances in cryogenic rubber recycling technology.

The glass transition temperature (Tg) varies by rubber type. Natural rubber has a Tg of approximately -70°C, while SBR is around -50°C. Below these temperatures, rubber transitions from elastic to glassy behavior and fractures like brittle plastic. Liquid nitrogen at -196°C provides the cooling medium for cryogenic grinding systems. The Air Products supplies liquid nitrogen and cryogenic processing equipment.

Cryogenic grinding systems pre-cool rubber scrap in a freezing chamber before feeding it to the rubber crusher. The pre-cooling time depends on scrap size, rubber type, and batch weight. Typical pre-cooling cycles last 15-30 minutes for scrap pieces up to 50mm. After cooling, the brittle rubber enters the crusher where it fractures into small particles under impact from the crusher rolls or hammers.

Particle size distribution in cryogenic grinding depends on crusher settings, feed temperature, and rubber composition. Tighter crusher roll gaps and higher impact velocities produce finer particles. Classification systems separate the ground rubber by particle size, with oversized material returned for regrinding. The Prab manufactures material handling systems for rubber recycling operations.

Cryogenic grinding produces rubber powder with clean fracture surfaces and consistent particle geometry. This powder is suitable for use as filler in new rubber compounds, asphalt modification, and molded rubber products. The fine particle size and fresh surface chemistry of cryogenic powder provide better bonding in re-compounding applications compared to ambient-ground rubber.

Economic considerations for cryogenic grinding include liquid nitrogen consumption, pre-cooling time, and equipment costs. Liquid nitrogen typically costs $0.15-0.30 per kg, and consumption averages 1-2 kg of nitrogen per kg of rubber processed. The Recycling Today publishes articles on the economics of rubber recycling technologies.

For rubber crushers and cryogenic grinding equipment, Hongdu Machinery on Alibaba supplies rubber crusher machines suitable for integration with cryogenic pre-cooling systems.

Research from the ACS Rubber Division presents papers on optimizing cryogenic grinding parameters for different rubber types and target particle sizes.