It is well known that no vulcanized rubber can fully rebound; there will always be some degree of damping, either large or small. This damping or viscosity property will reduce the resilience of the vulcanized rubber and increase hysteresis or heat generation during periodic deformation. Generally, the better the resilience of the rubber compound, the lower the hysteresis. However, sometimes, due to differences in temperature or deformation rate, such a completely opposite relationship does not always exist.
1.Fillers
Avoid overfilling the reinforcing filler, as the energy loss due to internal friction in the filler-polymer interaction and the fracture and reconstruction of the filler-polymer network can lead to increased hysteresis. Reducing the amount and specific surface area of the filler (increasing the particle size) and increasing the surface activity of the filler will usually reduce the hysteresis and heat generation of the binder.
2. Carbon Black
Reduce the amount of carbon black, reduce the specific surface area of carbon black (increase particle size) can increase the resilience of the rubber and reduce hysteresis.Selection of carbon black with high surface activity can reduce hysteresis and increase resilience.The higher the surface activity of carbon black.The higher the surface activity of the carbon black, the tighter the bond with the matrix rubber, the less slip between the carbon black and the rubber chain, and the lower the hysteresis.To reduce hysteresis and improve resilience, consider replacing the original carbon black with a high structural carbon black with a low filler content and adjusting the compound to the same hardness with a suitable processing oil.At constant strain, finer particle size carbon blacks can significantly increase the hysteresis of the vulcanized compound, but at constant stress the effect of this finer particle size is not as pronounced.At constant strain, a more structured carbon black increases the hysteresis of the compound, but at constant stress, a higher structured carbon black actually decreases the hysteresis of the compound.In some cases, a more uniform dispersion of the carbon black in the binder will reduce the hysteresis.On the other hand, low structural carbon blacks behave like discrete coarse particles in the binder and reduce the hysteresis of the binder.
The use of low filler, high structure "L" carbon blacks (Long Linkage carbon blacks) instead of regular high structure carbon blacks results in lower rolling resistance of the tread compound.Research has shown that high structural carbon blacks are easily crushed during mixing and therefore have to be replaced by normal carbon blacks.This high structure "L carbon black" has high crush resistance.
By replacing ordinary reinforcing carbon blacks with high structural carbon blacks with low filler content, the rubber can have lower hysteresis.This special high structured carbon black (e.g. Columbia CD-2038) is a highly branched primary structured carbon black aggregate with many voids in it, which adsorbs a lot of rubber and oil.The wide particle size distribution of the primary structure of the carbon black reduces the contact between the primary structure aggregates, reduces hysteresis and improves resilience, and at the same time reduces the rolling resistance of the tire.
3.Carbon Black Surface Activity
A high surface activity of carbon black results in a low hysteresis of the adhesive.The surface activity of carbon black is related to the degree of defects and disorder in the structure of the graphite layer on the surface.The higher the degree of disorder in the graphite layer structure, the higher the surface activity of carbon black.Generally, carbon blacks with a high degree of disorder in the graphite layer structure have a shorter thermal history during preparation in a furnace reactor.A high surface activity of the carbon black means a strong interaction with the polymer and a weak contact between the particles of the carbon black and therefore a low hysteresis.
The use of high surface roughness and activity carbon blacks instead of regular carbon blacks results in low hysteresis over a wide range of strain in the binder.
4. Chemical Improvers for Carbon Black
The use of carbon black-rubber coupling agents (or chemical modifiers) in carbon black-filled compounds can increase the resilience and modulus of the compound, while also reducing wear losses.In the past, coupling agents such as N-(2-methyl-2-nitropropyl)-4-nitroaniline, N-4-dinitroso-N-methylaniline, p-nitrosodiphenylamine and p-nitro-N-N-dimethylaniline were used.Nowadays, these nitroso compounds are no longer used because they release a nitrosamine carcinogen. So, people started to experiment with different coupling agents, such as p-amino-benzenesulfonyl azide (or amine-BSA), which has been newly studied to improve the resilience of adhesive. It has been reported that a new laboratory-grade chemical improver, Benzofuran (BFO), can be used to improve the bonding between carbon black and rubber, thus reducing the hysteresis of the rubber, but the mixing temperature of the rubber is generally required to be over 160°C. The rubber can also be used to improve the elasticity of the rubber. However, an aromatic by-product, benzofuran, is produced, but researchers have found that nickel salts can be used to effectively inhibit the production of this by-product.
5. Mixing Degree
Increasing the dispersion of carbon black improves the elasticity of the rubber, especially when the dispersion of carbon black is increased from 95% to more than 99% (which results in a good microdispersion and a weakening of the filler network), which is the case with the addition of N330 carbon black in SBR/BR and NR/BR blends. However, if the NR/BR blends are filled with N326, the situation may be reversed, where the mixing degree is increased but the hysteresis is increased. This opposite result may be due to the fact that N326 is a slightly less structured carbon black, which forms "hard" carbon black agglomerates. This corresponds to the presence of large smooth particles in the binder, which are broken up during mixing, and the space between the agglomerated particles becomes smaller, thus increasing the hysteresis and decreasing the resilience. It has been reported that the resilience of carbon black N347 increases and then decreases with increasing processing time and mixing degree. Therefore, the influence of the degree of mixing on the resilience of the compound depends to a large extent on the type of carbon black and the matrix elastomer. In the case of SBR, SBR/BR and NR/BR rubbers, increasing the dispersion of the carbon black usually reduces the heat generation and hysteresis of the rubber. However, the opposite result was observed for butyl rubber filled with high structural carbon black, probably due to the breakage of the butyl rubber chain.
6.Mixing Order and Heat Treatment
When mixing, carbon black should be added early, avoid adding with oil, stearic acid or other polar components such as antioxidants, because these components will be adsorbed to the surface of carbon black, interfering with the adsorption of carbon black to the polymer. Therefore, when carbon black is added with oil or other components, it will affect the formation of rubber bonding between carbon black and rubber, which will increase the lag of rubber. Therefore, it is better to add carbon black before other components so that the lag of the rubber will be lower.
For dienophilic rubbers such as SBR or BR, which have better mechanical oxygenation stability, increasing the mixing strength and mixing time is equivalent to "heat treatment" of the rubber, which can promote the formation of the binding rubber and better dispersion of the carbon black, thus improving the abrasion resistance of the rubber and lowering the hysteresis.
7. Phase Mixing
Sometimes the unbalanced distribution of carbon black in different rubber phases results in low lagging of the compound. If the concentration of carbon black in a rubber phase is high, a dispersed phase is easily formed and the lagging tends to be low.
For SBR/NR blends, an effective way to reduce lagging and heat generation is through phase mixing, where the NR phase contains approximately 75% carbon black (mass fraction). This also applies to BR/NR, where a higher content of carbon black in the NR phase through phase mixing technology results in a lower lagging of the rubber. For BR/SBR hybrids, however, the effect of phase mixing on hysteresis is not as sensitive. The premise of these studies is that the carbon blacks used are pure reinforcing carbon blacks. In fact, the distribution of carbon black in different rubber phases is also affected by the degree of structure.
8. Carbon Black - Silica Biphasic Filler
It has been reported that the choice of carbon black - silica biphasic filler and the use of silane coupling agents such as Silicone - 69 can reduce the hysteresis of the rubber by 30% while maintaining the same abrasion resistance.This biphasic filler is prepared by combining carbon black and silica through the co-gas phase method.
9. Plasticizers
Plasticizers will reduce the glass transition temperature of the adhesive.
10. Low Viscosity Oils
Choosing low viscosity processing oil can make the resilience of the adhesive good, because high viscosity processing oil will make the resilience of the adhesive worse.To improve the resilience of oxybutadiene rubber, canola oil can be used, because of its low viscosity, can reduce the hysteresis, on the other hand, its volatility is low, can make the rubber aging resistance is good.
11.Sulfur Vulcanization
For the vulcanization of rubber with sulfur yellow and a small amount of sulfonamide accelerator, the amount of zinc oxide can be increased to reduce the heat generation of the rubber.
12. Vulcanization Temperature
For NR/BR blends, by adjusting the vulcanization temperature, the distribution of cross-linking density between the NR and BR phases can be controlled, thus adjusting the resilience of the rubber.
13. Crosslinking Density.
Increasing the crosslinking density of a rubber improves resilience and reduces heat generation.
14. Glass transition of Rubber
Selecting a rubber with a low glass transition temperature as the base rubber will result in a low hysteresis of the adhesive.
A large difference in the glass transition temperatures of the two rubbers in a compound will have a significant effect on the glass transition temperature and hysteresis of the compound. If the two rubbers are highly compatible, then the blend can have a relatively wide glass transition peak. However, if the two adhesives are incompatible, then the blend has two separate glass transition peaks.In incompatible compounds, it is usually the rubber with the higher glass transition temperature that gives the compound a higher hysteresis value than in compatible compounds.
15.NR
Natural rubber adhesives usually have low hysteresis and good resilience.
For NR adhesives, a sulfide system with a high sulfur content is usually used to provide low hysteresis and high resilience. Avoid the use of epoxidized natural adhesives such as ENR-20 and ENR-50, which have a higher glass transition temperature than natural adhesives.
16. Avoid butyl or halobutyl adhesives because of their high damping factor, high hysteresis and low resilience.
17.NR/BR Blends
For NR/BR blends, the tan8 of the blend decreases significantly with increasing BR content.Therefore, by increasing the BR content, the resilience of NR/BR blends can be improved and the hysteresis can be reduced.
18.NBR
NBR is not normally considered in formulations requiring low heat generation and flex resistance, but if it is necessary, the use of large particle size carbon black and ester plasticizers with high crosslinking density can be considered so that the resilience of the compound will be improved.
Using NBR with low acrylonitrile content, the resilience of the rubber will be better.
High temperature polymerized NBR has lower hysteresis and better resilience than low temperature polymerized NBR.
19. NBR with Narrow Molecular Weight Distribution
NBR with narrower molecular weight distribution has better resilience.
20.NR/NBR Blends
For NR/NBR blends, lowering the content of NBR improves the resilience and reduces the hysteresis of the blends.
21.AEM
To obtain high-resilience rubber compounds, the use of AEM should be avoided as it is a type with low resilience and is very difficult to achieve.
An elastomer with high damping over a wide temperature range, and its effective damping temperature range is even wider than that of butyl rubber. For porous rubber, the closed-cell structure can endow the rubber compound with higher resilience than the open-cell structure.
22. Molecular Weight
Increasing the molecular weight of the base rubber results in a lower hysteresis heat generation of the compound.
23.Double Crosslinking Network
It is well known that plastics are subjected to stretching during processing, which causes orientation and anisotropy.
The anisotropic properties are improved when the plastic is cooled to below the melt temperature or glass transition temperature.Rubber, however, is quite different in that the various orientations induced during processing usually decay and disappear at the end of the process.However, the "double crosslinking network" during vulcanization is one way to introduce hydrophobicity into the rubber.The first layer of the network is formed by light or partial vulcanization.This lightly vulcanized compound is stretched to an elongation of α. After a second vulcanization, the vulcanized compound is released, during which the second crosslinked network prevents the retraction of the first network, which results in a residual elongation α, of the vulcanized rubber.In some cases, the double crosslinked network may attenuate the Payme effect, reducing hysteresis at low stresses and weakening the filler-filler interaction, although this has not been tested.
