Generally speaking, after adding carbon black or white carbon black as a filler to rubber for mixing and crosslinking, its tensile strength and wear resistance can be greatly improved. Although this phenomenon has been explained from a chemical point of view, there is still no conclusion so far, so everyone does not have a very intuitive understanding of it. Therefore, after the researchers have demonstrated through a large number of experiments, everyone can more intuitively understand the chemical mechanism of rubber reinforcement. Below, the editor of Carbon Black Industry Network will take you to understand the principle of rubber reinforcement through various data, hoping to help you have a more accurate and clear understanding of the reinforcing effect of rubber fillers.
People often use carbon black, silica, calcium carbonate and clay as rubber fillers to increase their tensile strength and wear resistance. From the point of view of large amount of main rubber products and good reinforcement, the carbon black and white carbon black will be explained. The relationship between particle size and reinforcement of various fillers is as follows: The reinforcement of fillers is governed by particle size.
The particle size of N880 carbon black is as large as 150nm, and the reinforcement is not high. The particle size of N330 carbon black is as small as 30nm, and its reinforcement is very good, so it is used in tires and other products that require high performance.
The picture above is a comparison of the characteristics and physical properties of N330 carbon black and white carbon black, and more of them are the characteristics of representative varieties of the two chemicals. Carbon black is basically composed of "carbon", while silica is composed of "silicon dioxide and water". White carbon black has a slightly larger specific gravity and a smaller average particle size. Therefore, white carbon black has a large specific surface area and a large oil absorption value. In terms of physical properties, white carbon black is superior in coloring, tear resistance, viscosity and flex fatigue resistance, while carbon black is superior in wear resistance. , Resilience, weather resistance and light resistance are better. Comparing the surface functional groups of the two, the difference in physical properties becomes apparent.
There are a very small amount of oxygen-containing functional groups on the surface of carbon black, including acidic groups such as carboxyl and phenolic hydroxyl groups, basic groups such as oxygen naphthalene structure, carboxyl and quinone groups and other neutral groups. However, due to the low number of functional groups, carbon black is hydrophobic (as shown in the figure below). The representative carbon black N330 (HAF) contains 0.7% oxygen, 0.4% fish and 97.9% carbon, and there seem to be many types of functional groups. But its quantity is very small, so various functional groups are difficult to quantify. Carbon black is classified according to nitrogen adsorption specific surface area and vulcanization rate, the type of carbon black.
The meanings of the abbreviations in the picture above: MT—medium particle thermal black, FT—fine particle thermal black, SBF—semi-reinforcing furnace black, GPF—general purpose furnace black, HMF—high modulus Furnace black, FEF——Fast Extrusion Furnace Black, FF——Fine Particle Furnace Black, HAF——High Wear-resistant Furnace Black, ISAF——Super Wear-resistant Furnace Black, SAF——Super Wear-resistant Furnace Black.
Carbon black is obtained by incomplete combustion of various organic compounds. According to the production method, it is divided into furnace black, channel black, thermal black and lamp black. Furnace black is produced by incomplete combustion of hydrocarbons in a furnace. Channel black is produced by contacting channel steel with a natural gas flame. Thermal carbon black is obtained by pyrolysis of natural gas. Lamp soot is made by burning hydrocarbons in an open shallow pot.
The basic structure of the silica surface is shown in the figure below
Let's take a look at the changes in rubber filled with carbon black. As shown in the figure below, the tensile strength of carbon black-filled rubber and pure rubber without carbon black is compared. Because natural rubber will crystallize after stretching, it has the property of enhancing durability and abrasion resistance, so even if it is filled with carbon black, its reinforcing effect is not so outstanding. Therefore, after SBR or NBR is filled with carbon black, the reinforcement can be increased by 5-10 times.
There are many theories about the reasons for the reinforcement of carbon black. However, it is mostly believed that the bonded rubber is formed when rubber is mixed with carbon black. There is a view that the bonded rubber is a bound component formed by interacting with the carbon black surface, contained in the colloidal phase (carbon black gel phase) in the pores of the aggregate formed by the fusion of carbon black particles, and the rubber phase during the mixing process. The molecular chains are cut and then connected to form a rubber phase (rubber gel orange). Although it seems too complicated to explain the main formula formed. However, by extracting and kneading unvulcanized rubber compound with carbon black by solvent, the amount of bonded rubber can be measured as the remaining rubber compound without extraction. In the following, specific examples will be used to deduce the combination of rubber and carbon black and the dispersion of carbon black. Tin-modified SBR is known as SBR having a functional group that easily reacts with carbon black. Therefore, the four kinds of SBR synthesized by synthesizing the various SBRs shown in the figure below are: unmodified SBR, SBR with the same structure but with sn C bonds at the end, unmodified SBR and low molecular weight tin-modified SBR. Blends and branched sBR modified with tin compounds.
After these kinds of sBR were filled with carbon black for kneading, the unvulcanized rubber was extracted with a solvent to measure the bonded rubber. After these rubbers were vulcanized, the 50°C was measured with a viscoelasticity testing machine. tanS value (tangent value of loss angle at 50~C, which is the tangent value of the phase difference between vibration stress and vibration deformation, which is equal to the ratio of loss modulus to elastic modulus). tanS(50~C) is an indicator of tire rolling resistance. The smaller the value, the greater the rolling resistance. The result is shown in the figure below. Taking the unmodified SBR as the standard, after the increase of the combined rubber, it is divided into a group in which tan~i (50~C) decreases and a group in which tanS (50~C) remains basically unchanged. This shows that it is necessary to reduce the tire To reduce fuel consumption, it is not enough to form a network with carbon black.
Next, look at the dispersion of carbon black. Several tens of nl carbon black particles are fused together to form a very high-strength aggregate. Many of these aggregates aggregate together to form agglomerates that are visible to the naked eye. SBR is an insulator. Carbon black is conductive. After filling 50 parts of carbon black in 0 parts of SBR and kneading, if the carbon black is well dispersed, the resistance value (logf]) will increase. Conversely, if the dispersion is poor, the resistance value decreases.
Therefore, the resistance value of the unvulcanized compound in the figure below was measured. The relationship between the resistance value of vulcanizate and tanS(50*C) is shown in the figure below. A good correlation was obtained for tan~i (50 "C) to decrease with the increase of the resistance value (that is, good dispersion). From this result, it can be considered that the tin-modified SBR is not only bound to carbon black, but also has a great influence on the dispersion. also works.
How does the dispersion process of carbon black occur? This problem can also be considered as shown in the figure below. In the case of large molecular weight modified SBR, carbon black reacts with SBR. In the mixing process, the SBR reacted with carbon black is also combined with other SBR that cannot react with carbon black to break up the carbon black agglomerates and disperse the carbon black.
However, as shown in the figure below, in the case of small molecular weight modified SBR and unmodified SBR, although the small molecular weight modified SBR reacts with carbon black, because the molecular weight is too small, the combination with other SBR is small during the refining process. Carbon black cannot be dispersed.
In addition, in the case of branched modified SBR in the above figure, it can be considered that although SBR can react with carbon black, it cannot contain carbon black and cannot produce the effect of breaking up agglomerates. It can be seen from the above description that in order to give full play to the filling and reinforcing effect of carbon black, the reactivity with carbon black and the dispersibility of carbon black are crucial. Here, a tin compound is exemplified as the functional group, but aminobenzophenone-based compounds, isocyanate-based compounds, and the like are known as functional groups having the same effect.
Reinforcement Mechanism of Silica
For SBR, carbon black is hydrophobic and is an easy-to-mix filler; while silica is hydrophilic and is a difficult-to-mix filler. This shortcoming can be overcome by treating the silanol group of white carbon black with a silane coupling agent to make it hydrophobic. Recently, in order to improve the skid resistance of tires on wet roads and reduce rolling resistance, silica is often used instead of carbon black. The silane coupling agent used at this time is bis(3-triethoxysilylpropyl)tetrasulfide (TESPT) of the sulfide system, and its action process is shown in the figure below. TESPT has two functional groups, an alkoxysilyl group (blue Si-OR) that can react with white carbon black and a sulfide bond (⋯SS Ss-s-) that can react with SBR. First, knead SBR, silica and TESPT at about 140°C where sulfide bonds do not react. Alkoxysilyl groups react with silica at this temperature. Results TESPT was bound to the silica surface. At this point the sulfide bond has not yet reacted. Second, vulcanize with a vulcanizing agent and a vulcanization accelerator. At a temperature above about 140~C, the sulfide bond of TESPT reacts and attaches to SBR to obtain vulcanized SBR rubber filled with white carbon black. As such, TESPT exhibits a reinforcing effect by reacting with the silanol groups of silica at a lower temperature and crosslinking with SBR at a higher temperature.
Imitating the above example, through living anion polymerization, the modifying agent containing alkoxymethyl and silyl groups is reacted at the end of SBR, and after introducing a functional group that can react with silica at the end of SBR, the reinforcing effect of silica is as follows As shown in the figure.
This SBR was filled with silica and carbon black, respectively, and the amount of bonded rubber was measured. In the case of unmodified SBR, filled with silica and filled with carbon black, the amount of bonded rubber produced is not too much. When the SBR with alkoxysilyl group is filled with carbon black, the amount of bonded rubber is similar to that of unmodified SBa, but after being filled with white carbon black, the amount of bonded rubber is more than doubled, up to about 50%. After vulcanization of this alkoxysilyl-containing SBR filled with silica, its tensile strength and wear resistance are improved.
When filling with silica, unlike when filling with carbon black, the alkoxysilyl group that can react with the silanol group is very important for the reinforcement.
Focusing on carbon black and silica from the chemical mechanism of filler reinforcement, the functional groups that can react with carbon black and the functional groups that can react with silica, as well as the changes in the physical properties of fillers after dispersion, etc. are analyzed. . However, there are still many problems in this field that have not been clarified, and I believe that further discussion and research will be made on this in the future.