Shock-absorbing rubber is a general term for various rubber shock absorbers. That is, the characteristics of rubber to eliminate mechanical vibration are used to achieve shock absorption, silencer, and reduce the harm caused by impact.
Shock absorbing approach
Shock-absorbing rubber is used to prevent the transmission of vibrations and shocks or to cushion the intensity of vibrations and shocks. Shock-absorbing rubber is widely used in various motor vehicles, equipment and instruments, automated office facilities, and household appliances. In recent years, some large buildings, bridges, computer rooms, etc. have also used laminated rubber pads to isolate earthquakes to support the buildings to reduce the seismic response of the buildings.
Usually, vibration is controlled through the following three ways:
① Reduce the excitation force of the earthquake source;
② Separate the vibration from the source of excitation (seismic isolation);
③Reducing the vibration of the vibrating body.
Shock-absorbing rubber is mainly used for the latter two.
Shock absorption index
From the shock-absorbing principle of shock-absorbing rubber and the design calculation of the shock absorber, it can be known that after the structural shape of the rubber shock absorber is determined, the main performance indicators of the shock-absorbing rubber are as follows:
①The static stiffness (K) of the vulcanized rubber, that is, the elastic modulus of the vulcanized rubber. Because the natural frequency (w0) of the shock-absorbing rubber changes with the stiffness K, when the mass M of the machine is known, the total stiffness of the shock-absorbing rubber K=Mw0
② Damping performance of vulcanized rubber. As a shock absorber, the main function of shock-absorbing rubber is to absorb the vibration energy emitted by the earthquake source, especially to prevent synchronous vibration caused by the resonance effect of vibration waves. Its shock-absorbing effect is closely related to the damping performance of the rubber. The damping of rubber comes from the internal friction of macromolecule motion, which is a manifestation of the relaxation phenomenon of polymer mechanics and is one of the main parameters of the dynamic mechanical properties of rubber materials.
The damping performance of rubber is usually characterized by the damping coefficient tanδ. In order to obtain a better shock absorption effect, it is hoped that tanδ can meet the following two requirements:
First, in the frequency range and temperature range where the product is used, the tanδ value is larger;
Second, the tan δ peak is wider to ensure a better damping effect within a larger range and reduce its sensitivity to temperature and frequency. As the frequency increases, the dynamic modulus increases, and the loss angle reaches a peak value. This peak occurs because the material changes to a glassy state, and increasing the frequency is equivalent to lowering the temperature. The damping coefficient of rubber is one of the important indicators of shock-absorbing rubber. Generally, larger damping is beneficial to shock-absorbing. However, the damping will cause the rubber to generate a lot of heat under dynamic conditions, which will affect the aging performance of the product. Therefore, in order to take into account the shock absorption and heat generation properties of rubber, the damping coefficient tanδ of rubber must be appropriately adjusted and controlled.
Dynamic modulus
Classified according to the direction of the main load, the shapes of shock-absorbing rubber include compression type, shear type, and composite type. The reason why the product has these shapes is to adapt the spring constants of the shock-absorbing rubber in three directions (lateral, longitudinal, and vertical) to a wide range of requirements. Different shock-absorbing products also have different requirements for dynamic modulus. According to the relationship between polymer molecular structure and dynamic mechanical properties, it can be seen that the molecular structure of shock-absorbing rubber is characterized by moderately rigid and flexible molecular chains, because molecules that are too flexible will relax too quickly and cannot fully reflect their viscous behavior. . In addition to considering the above key performance indicators, the formula design of shock-absorbing rubber should also consider fatigue, creep, heat resistance, and bonding strength with metal based on the type and use conditions of the shock absorber.
Shock-absorbing rubber selection
(elastic modulus) of shock-absorbing rubber is mainly achieved by adjusting fillers and plasticizers, and is less affected by the type of rubber. Its damping performance mainly depends on the molecular structure of the rubber. For example, introducing side groups on the molecular chain or increasing the volume of side groups can hinder the movement of rubber macromolecules and increase the internal friction between molecules. Increase the damping coefficient tanδ. The presence of crystals will also reduce the damping characteristics of the system. For example, if crystallized isoprene rubber is mixed into chlorobutyl rubber with a better shock absorption effect, the damping coefficient of the system will decrease as the isoprene rubber content increases.
Among general rubbers, butyl rubber and nitrile rubber have larger damping coefficients; styrene-butadiene rubber, chloroprene rubber, silicone rubber, polyurethane rubber, and ethylene-propylene rubber have medium damping coefficients; natural rubber and butadiene rubber have the smallest damping coefficients. Although natural rubber has a small damping coefficient, it has the best overall performance, good fatigue resistance, low heat generation, small creep, and good adhesion to metal parts. Therefore, natural rubber is widely used as shock-absorbing rubber. If low-temperature resistance is required, it can be used together with butadiene rubber; if weather aging resistance is required, chloroprene rubber can be used; if oil resistance is required, nitrile butadiene rubber with low acrylonitrile content can be used; shock-absorbing rubber with strict low-temperature dynamic performance requirements, Silicone rubber is often used. Generally, when low damping is required, natural rubber is used; when high damping is required, butyl rubber can be used. Choosing a rubber blend with certain compatibility and co-vulcanization is an effective way to widen the damping peak width, which is beneficial to improving damping characteristics and improving other properties.
The vulcanization system has a great influence on the stiffness, damping coefficient, heat resistance, and fatigue resistance of shock-absorbing rubber. Generally, in the network structure of vulcanized rubber, the fewer sulfur atoms and free sulfur in the cross-linked bonds, the stronger the cross-linking, the greater the elastic modulus of the vulcanized rubber, and the smaller the damping coefficient. Using the traditional vulcanization system and appropriately increasing the degree of cross-linking is beneficial to shock absorption and dynamic fatigue resistance, but the heat resistance is not enough. For example, when natural rubber uses an effective vulcanization system or a semi-effective vulcanization system, although the heat resistance is improved, the fatigue resistance and the adhesion of metal parts tend to decline. Therefore, these properties must be properly balanced.
