The thermal conductivity of rubber
Rubber is a poor conductor of heat, but it is required to have certain thermal conductivity to meet the requirements of use in practical application. Rubber products with thermal conductivity are widely used, and the improvement of thermal conductivity has a great impact on the performance of rubber products. For example, the temperature of the rubber material used for insulating and heat conducting components of electronic components rise per 2 ℃, its reliability may be dropped by 10%. Therefore, rubber composites with high thermal conductivity can effectively transfer heat, which is of great significance for the densification, miniaturization and reliability improvement of electronic products.
In addition, the rubber material used in tire should have the characteristics of low heat generation and high heat conduction. On the one hand, it can improve the heat transfer performance of rubber, increase the vulcanization efficiency and reduce energy consumption; On the other hand, the high thermal conductivity can help timely derive the heat generated by the tire in the process of high-speed driving, reduce the temperature of the tire body, so as to reduce the performance degradation caused by excessive temperature.
When the car is running at high speed, the friction heat and deformation heat will increase significantly as the wheel speed increases. If the heat is not released quickly enough, it will cause the temperature of the tire to rise and the air pressure inside the tire to rise accordingly, which can accelerate the rubber aging and cause tires to explode, leading to accidents.
Thermal mechanism of thermal rubber
Thermal rubber is divided into intrinsic thermal rubber and filled thermal rubber. The synthesis process of intrinsic thermal conductivity rubber is complex and costly, while the price of filled thermal conductivity rubber is low and easy to process. Therefore, thermal conductivity rubber is generally prepared by filling a filler with high thermal conductivity. The thermal conductivity of the filled thermal conductive rubber mainly depends on the rubber substrate, thermally conductive filler and their common interface.
Thermal conductivity filler is the main thermal conductivity carrier, its own thermal conductivity is much greater than the matrix material whether in the form of particles or fibers. When the thermal conductivity of the filler is small, the filler can be evenly dispersed in the system, however, no contact and interaction was formed between them. At this time, the contribution of the filler to the thermal conductivity of the whole system was not significant. However, when the filling amount reaches a critical point, the fillers begin to contact and interact with each other, forming a structure similar to a chain and a network in the system, which is called the heat conduction network chain. When the orientation of the heat conduction network chain is parallel to the heat flow direction, the thermal conductivity of the system will be improved to a great extent.
Types of heat conducting filler
According to the electrical properties of rubber materials, they can be classified into insulation type and non-insulation type. Thermal insulation rubber is mainly used in aerospace, weapons, and equipment used in the power tube, integrated block, heat pipe and other equipment, as well as microelectronics, communication equipment, motor, and electrical equipment need insulation thermal conductivity parts. The main thermal insulation fillers used are nitrides, carbides, and metal oxides.
Thermal non-insulating rubber is mainly used in the fields of the heat exchanger, solar water heater, battery cooler, etc., such as chemical production and wastewater treatment. Commonly used fillers are mainly metal powder, carbon fiber, graphite and carbon black, etc.
So the most common rubber products -- tires, are insulation or non-insulation rubber materials? The answer is that truck tires are insulated, whereas airplane tires are required to conduct electricity. During the flight, the aircraft will produce a large amount of electric charge attached to the fuselage by friction with the atmosphere. Aircraft have discharge brush to release static electricity, but it cannot ensure that the complete release so that electrostatic discharge will cause when passengers off the plane, which is extremely dangerous. If the aircraft tires conduct electricity, it can maximize the release of a large amount of static accumulated during the flight, thus reducing accidents.
Thermally conductive fillers for "thermally conductive insulating rubber"
* Filling of nitrides and carbides
Nitride and carbide with good insulation and high thermal conductivity mainly include aluminum nitride, boron nitride, silicon nitride, silicon carbide, boron carbide, and titanium carbide. By filling these inorganic ceramic fillers into the rubber matrix, the heat-conducting insulating rubber with good comprehensive properties can be prepared.
* Filling of metal oxide
Metal oxides such as beryllium oxide, aluminum oxide, magnesium oxide, and silicon oxide have a relatively high thermal conductivity, which can be filled into rubber to give it thermal conductivity and insulation, and make it have good physical and mechanical properties.
For example, silicon rubber filled with alumina can be used to make the thermal conductivity layer of electronic components. When the amount of alumina is three times that of silicon rubber, the thermal conductivity of the material can reach 2.72W/(m•K).
* Composite filler
Sometimes, the use of a single filler cannot meet the needs of the application, it is necessary to use the composite filler. For example, when silicone rubber is filled with refined silica powder and carbon black with a thermal conductivity greater than 15 W/(m•K), thermal insulation composite rubber with the coefficient of thermal conductivity is greater than 0.4 W/(m•K) and resistivity is greater than 1012Ω·cm can be made.
Thermally conductive fillers for "Thermally conductive non-insulating rubber"
* Filling of metal powder
Filling aluminum powder into rubber can not only greatly improve the thermal conductivity of rubber, but also produce rubber products with an excellent comprehensive performance. Silicon rubber with v-0 (UL 94) and 1.09 W/(m•K) thermal conductivity can be prepared by adding metallic aluminum powder and aluminum hydroxide powder treated with stearic acid.
* Filling of carbon black, graphite and carbon fiber
Carbon black and graphite have a high thermal conductivity, and some carbon fibers have thermal conductivity up to 1200 W/(m•K), so they can be filled into rubber to produce thermal non-insulating rubber.
* Composite filler
The thermal conductivity of rubber will change when barium titanate is added to the butyl rubber filled with 50 parts of lamp black. When 20 barium titanate powders were added, the thermal conductivity of butyl rubber reached the maximum. Silastic materials with both flame retardant properties and thermal conductivity can also be prepared by filling silver powder and boron nitride and platinum-based flame retardants. At a certain ratio, the material can have a thermal conductivity of 14 W/(m•K) and a flame retardant grade of v-1 (UL 94).
Generally, the thermal conductivity of the rubber matrix is small, which has little influence on the thermal conductivity of composite materials. Therefore, the main influencing factors of thermal conductivity are the variety, filling amount, particle size and shape of thermal conductivity filler, as well as the interface thermal resistance between matrix and filler.
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About The Author
Cathie Montanez is the Project Scientist in Stanford Advanced Materials (SAM). She once served as a research professor at university's school of materials science and engineering, and now is responsible for the performance testing and technical guidance of SAM's products such as refractory metals, ceramics, laboratory crucibles and grinding bars, etc.