The editor of this article will introduce to you the reinforcement of carbon black. Carbon black can be used as a reinforcing agent in many ways. In this article, we will start with the basic concepts of reinforcing agents and fillers and the basic classification of fillers. Learn about this aspect.
Reinforcing agent: refers to the filler that can improve the physical and mechanical properties of rubber products; filler: refers to the filler that can increase the volume of rubber products, reduce the rubber content and reduce the cost.
The classification of fillers can be divided from function, color, source and shape, according to function: reinforcing agent, filler; according to color: black filler, non-black filler; according to source: organic filler, inorganic filler; according to shape : Granular filler, resin filler, fiber filler. After understanding the basic concepts of reinforcing agents and fillers, the editor first sorted out the varieties and classifications of carbon black for everyone.
1. Types and classification of carbon black
There are more than 40 varieties of carbon black. According to the role: hard carbon black (below 40nm particle size, high reinforcement); soft carbon black (above 40nm particle size, low reinforcement). According to the production method: channel carbon black, furnace carbon black, thermal cracking carbon black, new process carbon black.
1. Contact method carbon black (channel black, acidic, highly reinforcing, belonging to hard carbon black), which uses cooled metal or non-metal as the contact surface, and cools and collects the flue gas from the flame of the raw material gas. Main varieties: natural gas tank black (with natural gas as raw material) with low pH. Easy-to-mix tank black EPC, ASTM label is S300, mixable tank black MPC, ASTM label is S301; gas-mixed carbon black; roller carbon black; conductive channel carbon black CC, coarse particles, small resistance. Basic properties: finer particles (with a particle size range of about 30nm) have a great reinforcing effect on NR, and have good tensile properties, tear resistance and cut resistance; high oxygen content (with more oxygen-containing groups) ) is acidic and delays vulcanization, because the acidic additive has an adsorption effect on the accelerator; it is mainly used in NR to make tire tread rubber, and it is rarely used in synthetic rubber.
2. Oil-based furnace black (alkaline, highly reinforcing, hard carbon black), also known as oil furnace black or furnace black, is made of raw material oil. Combustion in a special furnace, and then the flue gas is cooled by spraying water, (alkaline due to the absorption of salts in the water), main varieties: super wear-resistant carbon black SAF (supper abrasion furnace black), ASTM The label is N110, the particles are fine, the reinforcing effect is good, but it is not easy to disperse, and the process performance is poor. ISAF (Intermediate super abrasion furnace black), ASTM label is N220, high structure medium super abrasion furnace black HS-ISAF, ASTM label is N242, low structure medium super abrasion furnace black LS-ISAF, ASTM label is N219 , High wear-resistant furnace black HAF (High abrasion furnace black), ASTM label is N330, high-structure high wear-resistant furnace black HS-HAF, ASTM label is N347, low-structure high wear-resistant furnace black LS-HAF, ASTM label is N326 . Basic properties: fine particles, high reinforcing hardness carbon black, very suitable for synthetic rubber (also suitable for NR), low water content and oxygen, alkaline, can promote vulcanization, widely used in wear-resistant Products with good properties, such as tire tread rubber.
3. Gas furnace method carbon black (gas furnace method carbon black, less reinforcing, called soft carbon black), obtained by incomplete combustion of natural gas and a certain proportion of air in the furnace. Fast Extruding Furnace Black FEF (Fast Extruding Furnace black), ASTM label is N550, the particles are thicker than before, different from the mixing extrusion process, used for inner tube rubber. Fine Furnace black FF (Fine Furnace black), ASTM label N440, has a low structure and can improve the elasticity and flex resistance of the compound. General purpose furnace black GPF (General purpose furnace black), ASTM grade N660, large particles, good processing performance, high elongation, high elasticity, low heat generation and so on. High modulus furnace black HMF (High modulus furnace black), ASTM label N601, increases the modulus stress of the compound, and its reinforcement is higher than SRF, but lower than GPF. Semi-reinforcing furnace black SRF (Semi-reinforcing furnace black), ASTM labels are N754 (low structure), N765 (high structure), large particles, low heat generation during processing, endow vulcanized rubber with high elasticity and high elongation, Good flex resistance and adhesion performance. When used for inner tubes and rubber hoses, the filling volume can be larger.
4. Thermal cracking and other types of carbon black (soft carbon black, basically no reinforcing effect) fine particle thermal cracking furnace black FT (Fine thermal black), ASTM label N880, suitable for NR and synthetic rubber, often used in inner tube. Medium thermal black MT (Medium thermal black), ASTM designation N990, suitable for NR and synthetic rubber, mostly used for molded products, especially for oil-resistant products. The above two kinds of particle sizes are relatively large, and have almost no reinforcing effect. They are mostly used as cheap fillers, and are suitable for extruded products and products that do not require high mechanical strength. Conductive furnace black CF (Conductive furnace black), ASTM label N293. Acetylene black ACET (Acetylene black) has a medium particle size and the highest structure among all carbon blacks. The new process carbon black is based on the furnace method and is obtained by improving the process performance. MPF (Multi-purpose furnace black) APF (All-purpose furnace black) Super processing furnace black SPF (Super processing furnace black). It is characterized by small particle size, narrow particle size distribution, smooth surface, good processing performance, and can be used to reinforce tire side inner tubes and other industrial products.
2. What kind of carbon black should be used in the preparation of synthetic rubber tread
1. Requirements for tread rubber: high mechanical strength, good wear resistance, good elasticity, certain heat resistance and aging resistance, and poor self-reinforcing property of synthetic rubber, so carbon black with high reinforcing property (hard carbon black) should be selected ), the SAF particles in the furnace black are too fine, not easy to disperse, and the processing performance is not good, while the ISAF and HAF materials are suitable, and the groove black has little reinforcement for synthetic rubber.
2. For NR treads, EPC, MPC, ISAF, HAF, and SAF are optional because of the strong reinforcement of MPC and EPC in the grooves.
3. If NR/synthetic rubber is used together, and NR is the main one, use slot furnace black together
4. If synthetic rubber/NR are used together, and mainly synthetic rubber, furnace black should be selected.
3. Basic properties of carbon black
Carbon black has an excellent reinforcing effect on rubber, which is closely related to the structure and properties of carbon black.
1. Basic structure of carbon black
Hydrocarbons are produced from C and H compounds (oil or natural gas) through high-temperature cracking.
The difference between various carbon blacks lies in: the regularity of the microcrystal packing process, that is, the regularity of the arrangement of microcrystals in the particles varies with various carbon blacks.
Regularity: Thermal Black>Gas Furnace Black>Channel Black>Furnace Black
2. Chemical composition of carbon black
The analysis results show that, in addition to most of the C elements, the composition of carbon black also contains a small amount of H, S, O and other elements, as well as other impurities and moisture, and its content varies with various carbon black varieties.
3. Basic properties of carbon black
Surface properties and surface chemical activity of carbon black particles. (the first factor affecting the reinforcement performance, strength factor), surface properties: the unsaturation of the oxygen-containing group microcrystals on the particle surface. Chemical activity: refers to the ability of carbon black particles to bond with rubber. Experiments have proved that carbon black with high chemical activity has a large binding ability with rubber and a strong reinforcing effect. The chemical activity comes from the unsaturation of the microcrystalline structure in its particles; the oxygen-containing groups on the surface of the particles. They directly affect the chemical properties of carbon black; at the same time, they affect the pH value of carbon black and the vulcanization speed of rubber.
The size of carbon black particles, (the second factor that affects the reinforcement performance, the breadth factor), generally speaking, the greater the reinforcement effect is, the specific surface of carbon black is >50m/g. The structure of carbon black (the third factor affecting the reinforcement performance, shape factor)
The primary structure of carbon black (basic aggregate, polymelt, permanent structure). During the manufacturing process of carbon black, the chain branch or grape-like aggregate formed by the fusion between particles is called the primary structure. It is combined by chemical bonds and is not damaged during the rubber mixing process. It is the smallest dispersible unit of carbon black in the rubber compound.
The secondary structure (secondary aggregate) of carbon black refers to two or more loose associations formed by van der Waals force (physical adsorption). The combination is weak, some are destroyed during granulation, and most are destroyed during rubber mixing. Structure refers to the aggregation tendency of carbon black particles in a three-dimensional space formed by connecting long chains and fusing together. The more particles stick together, the higher the structure. Structural representation method, shape factor (early use, observed by electron microscope), form factor (used in recent years)
Oil absorption value - refers to the number of milliliters of oil absorbed per gram of carbon black. It is often measured by DBP (dibutyl phthalate), which is called DBP oil absorption value. The larger the DBP value, the greater the oil absorption and the higher the structure of carbon black.
In short, carbon black with high activity, small particles and high structure has high reinforcing performance, good tensile strength, constant elongation stress, hardness, tear strength, wear resistance and fatigue resistance under constant load of vulcanized rubber; The elongation and resilience are small, and the fatigue heat generation is large under constant deformation.
4. The amount of carbon black
Generally, it is 40-50 parts. If it is too small, it will not have a good reinforcing effect and is not conducive to reducing costs; if it is too much, it will have a hardening effect, but it is not limited to thermal black.
The effect of the amount of carbon black on the reinforcement is manifested in: hardness, modulus and heat generation, etc., monotonously increase with the increase of carbon black; resilience, elongation, etc., monotonically decrease; tensile strength (40- 50 parts), tear strength and wear resistance (50-60 parts), etc. appear maximum with the increase of carbon black. For self-reinforcing rubber, the amount is slightly lower, and for non-reinforcing rubber, the amount is slightly higher.
5. Mechanism of carbon black's reinforcing effect on rubber
First, stress softening is introduced: the phenomenon in which rubber is stretched and the tensile force used is gradually reduced.
1. Volume effect
Carbon black does not deform under stress, so in carbon black compounds, the deformation of rubber macromolecules is greater than the deformation of appearance. This is called the volume magnification effect. Mullins and Tobin believe that the stress softening of carbon black rubber is the same as that of pure vulcanizate, the difference is that there is greater stress softening and loss in carbon black rubber due to the volume effect.
2. Weak bond and strong bond theory
This theory was put forward in the 1950s. Stress softening is the result of weak bonds of physical adsorption detaching from the surface of carbon black under the action of external force. Only strong bonds are left at breakage, and the strong number of minor chemisorptions is most important for the reinforcement of carbon black, such as tensile strength, tear resistance, and abrasion resistance, in relation to the rubber's ability to resist eventual rupture. . Therefore, for those with high requirements for reinforcements, there should be more strong bonds, that is, carbon black with high activity and large specific surface area should be used.
3. Bueche's theory of finite elongation of carbon black particles and rubber chains
This theory only considers the strong bonds formed by the carbon black particles and the rubber chains, and when the rubber chains are stretched under stress to approach their maximum length between the particles, a high modulus is obtained. When this length is exceeded, it will break away from the carbon black surface or break. When stretching beyond the length of the shortest chain, it breaks first, and then breaks in order of length; when it is stretched for the second time, it lacks the support of these chains, and the stress decreases, that is, stress softening. Stress recovery is the redistribution of rubber chains between carbon black particles in a relaxed state, and the detached chains are replaced by new chains. When there is no carbon black, after the rubber chain breaks, its stress is borne by the adjacent chains, and it is easy to break one after another; when there is carbon black, there are many rubber chains between the particles, and one chain is broken, and the stress is shared by other chains. Carbon black acts as a uniform stress and slows down the overall fracture. When the elongation is large, carbon black will also move, and this movement also plays a role in relieving stress. Uniform and relaxing stress is the reason for reinforcement.
4. Shell Model Theory
Nuclear magnetic resonance research has confirmed that there is an adsorption layer composed of two kinds of rubber macromolecules in motion on the surface of carbon black. The inner layer of about 0.5nm (equivalent to the diameter of macromolecules) close to the surface of carbon black is in a glassy state; the rubber within the range of about 0.5-5nm away from the surface of carbon black has mobility and is in a sub-glassy state. This layer is called the outer layer. layer. These two layers constitute the double shell on the surface of the carbon black. The binding energy in the interfacial layer of the double shell must decrease continuously from the inside to the outside, that is, the bondage of the carbon black surface to the mobility of macromolecules decreases continuously. Finally to the free state where the rubber molecules are not bound.
The explanation for the reinforcing effect of the shell layer is that the double shell layer acts as a skeleton, forming an overall network of rubber macromolecules and fillers, which changes the structure of the vulcanized rubber, thus providing the physical and mechanical properties of the vulcanized rubber.
5. The sliding theory of rubber macromolecular chains
This is a relatively new, comprehensive and comprehensive theory of carbon black reinforcement. The core of this theory is that rubber macromolecules can slide on the surface of carbon black, which explains the reinforcement phenomenon. The activity on the surface of carbon black particles is not uniform, there are a few strong active points and a series of adsorption points with different energies. The rubber chains adsorbed on the surface of carbon black can have various binding energies, ranging from mostly weak van der Waals adsorption to least a few strong chemical adsorption. Adsorbed rubber segments slide and elongate under stress.
The basic concept of macromolecular sliding theory is represented by schematic diagram.
Indicates the original state of the rubber compound, and rubber molecular chains of varying lengths are adsorbed on the surface of carbon black particles. When elongated, the shortest chain does not break but slides along the surface of carbon black. The length of adsorption in the original state is marked with dots, and the length of the slip can be seen. At this time, the stress is borne by most stretched chains, which acts as a uniform stress, and the relief of less stress set is the first important factor for reinforcement. When the elongation increases again, the chain slides again, so that the rubber chain is highly oriented, bears a large stress, and has a high modulus, which is the second important factor for reinforcement. Due to the friction of sliding, the rubber compound has hysteresis loss. The loss will eliminate a part of the external work and turn it into heat, so that the rubber will not be damaged, which is the third factor of reinforcement. The condition of the rubber compound after shrinkage indicates the stress softening effect when it is stretched again. The length of the rubber chain between the carbon black particles is almost the same after the rubber is retracted. If it is stretched again, it does not need to slide again, and the required stress decreases. Under suitable conditions (expansion), after a long time, due to the thermal movement of the rubber chain, the dynamic balance of adsorption and desorption, and the redistribution of the length of the molecular chain between particles, the rubber will return to its original state. However, if the deformation of the initial elongation is large, the recovery usually does not exceed 50%.