1、 Flammability of rubber
Most rubbers have varying degrees of flammability. And the rubber containing halogen in the molecule, such as neoprene, fluororubber, etc., has a certain flame resistance. Therefore, chloroprene rubber and chlorosulfonated polyethylene containing chlorine atoms are difficult to burn even after removing the external flame, while fluororubber is completely self extinguishing. The flame retardancy of the compound can be improved by adding flame retardants (such as phosphate or halogen containing substances).
2、 Gas permeability (gas tightness) of rubber
The gas permeability of rubber is the product of gas solubility and diffusion in rubber. The gas solubility decreases with the increase of the solubility parameter of rubber, and the gas diffusion speed in rubber depends on the number of side chain groups in rubber molecules. The gas transmission speed in various rubbers is very different. Polyether rubber and butyl rubber have lower gas permeability in rubber, and butyl rubber has only 1/20 gas permeability of natural rubber. Silicone rubber has the highest air permeability. The air permeability of rubber increases rapidly with the increase of temperature. For products using carbon black as filler, its variety and filling amount have little effect on the air permeability. However, the amount of softener has a great impact on the air permeability of vulcanizate. For rubber products with high air permeability requirements, it is better to reduce the amount of softener as much as possible.
3、 Electrical properties of rubber
General rubber is an excellent electrical insulator. Natural rubber, butyl rubber, ethylene propylene rubber and styrene butadiene rubber all have good dielectric properties, so they are widely used in insulated cables. Nitrile butadiene rubber and chloroprene rubber have poor dielectric properties due to the existence of polar atoms or atomic groups in their molecules. On the other hand, adding conductive fillers such as conductive carbon black or metal powder into rubber will make it have enough conductivity to disperse static charges, or even become a conductor.
4、 Thermal properties of rubber
① Thermal conductive rubber is a poor conductor of heat, and its thermal conductivity coefficient is about 2.2~6.28 W/m 2.0K when the thickness is 25 mm. It is an excellent heat insulation material. If the rubber is made into microporous or spongy state, its heat insulation effect will be further improved, and the thermal conductivity will be reduced to 0.4~2.0 watts. Any rubber parts may generate heat due to hysteresis loss in use, so heat dissipation shall be paid attention to.
② Thermal expansion Because there is a large free volume between rubber molecular chains, when the temperature rises, the internal rotation of its chain segments becomes easier, which will make its volume larger. The linear expansion coefficient of rubber is about 20 times that of steel. This must be considered in the vulcanization model design of rubber products, because the linear size of rubber products will be 1.2~3.5% smaller than the model. For the same type of rubber, the hardness of the rubber compound and the content of raw rubber also have a greater impact on the shrinkage of the rubber compound. The shrinkage is inversely proportional to the hardness and is proportional to the rubber content. The order of theoretical shrinkage of various rubbers is:
Viton rubber>silicone rubber>butyl rubber>nitrile rubber>neoprene rubber>styrene butadiene rubber>natural rubber
When rubber products are used at low temperature, special attention should be paid to the influence of volume shrinkage. For example, oil seals will leak due to shrinkage, and rubber and metal bonded products will cause excessive stress due to shrinkage, leading to early damage.
5、 Relationship between rubber deformation and temperature, deformation speed and time
The deformation movement of rubber molecules cannot be completed instantaneously, because the intermolecular attraction must be overcome by the vibration energy of atoms. If the temperature decreases, these vibrations become less active, and the intermolecular attraction cannot be destroyed quickly, so the deformation is slow. At very low temperatures, vibration is not enough to overcome attraction, and rubber will become a hard solid.
If the temperature is fixed and the deformation speed increases, the same effect can be produced as lowering the temperature. In the case of extremely high deformation speed, rubber molecules have no time to rearrange, and will behave as hard solids.
The molecular chain of rubber material will be destroyed slowly under the action of stress, resulting in “creep”, that is, the deformation gradually increases. When the deformation force is removed, this creep will form small irreversible deformation, which is called “permanent deformation”.
6、 Stress-strain properties of rubber
The stress-strain curve is a typical curve of elongation crystalline rubber, and its main component is the entropy change caused by the system becoming orderly. As the molecules are gradually straightened, the isolation effect of the branch chains on the molecular chain disappears, and the intermolecular attraction becomes significant, which helps to resist further deformation. Therefore, the rubber will show higher tensile strength when fully stretched
The stress of rubber under constant strain is a function of temperature. The stress of rubber will increase proportionally with the increase of temperature.
The dependence of rubber stress on temperature is called Joule effect, which can explain the fundamental difference between metal elasticity and rubber elasticity. In metals, each atom is kept in a strict lattice by the interatomic force. The work done to deform the metal is used to change the distance between atoms, causing changes in internal energy. Therefore, its elasticity is called “energy elasticity”. The range of elastic deformation is much smaller than that of “entropy elasticity” in rubber, which is mainly caused by the change of entropy in the system.
In the general scope of use, the stress-strain curve of rubber is nonlinear, so the elastic behavior of rubber cannot be simply determined by Young’s modulus.
7、 Molecular characteristics of rubber — characteristics of rubber elastomer:
① A long chain molecule whose molecules are composed of repeating units (links). The molecular chain is soft and its segments have high mobility, and the glass transition temperature (Tg) is lower than room temperature;
② Its intermolecular attraction (van der Waals force) is small, and it is amorphous under normal (no stress) conditions, so the molecules are easy to move relative to each other;
③ Some parts of its molecules can be connected by chemical crosslinking or physical entanglement to form a three-dimensional network molecular structure to limit the large range of activity of the entire macromolecular chain.
From the microscopic point of view, the atoms and chain segments of the long chain molecules that make up rubber are in constant motion due to thermal vibration, making the whole molecule take on an extremely irregular coil shape, and the distance between the two ends of the molecule is much less than the straight length. An unstretched piece of rubber is like a tangle of coiled linear molecules. When rubber is not affected by external force, the entropy value of undeformed state is maximum. When the rubber is stretched, its molecules are arranged in rows in different degrees in the stretching direction. In order to maintain this directional arrangement, work needs to be done on it, so the rubber is resistant to stretching. When the external force is removed, the rubber will shrink back to the state of maximum entropy. Therefore, the elasticity of rubber is mainly derived from the “entropy elasticity” of the change of entropy in the system.