1. Basic concepts
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.
2. Classification of filler
According to action: reinforcing agent and filler
By color: black filler, non black filler
By source: organic filler, inorganic filler
According to shape: granular filler, Resin filler, fiber filler
Section 1 Varieties and Classification of Carbon Black
There are more than 40 kinds of carbon black.
According to action: hard carbon black (particle size below 40nm, high reinforcement); Soft carbon black (particle size above 40nm, low reinforcement). According to the preparation method, it is divided into tank carbon black, furnace carbon black, hot cracking carbon black and new process carbon black
1、 Contact carbon black (tank black, acidic, strong, hard carbon black)
The cooled metal to nonmetal is taken as the contact surface, and the flue gas is cooled and collected from the flame of feed gas.
Main varieties:
(1) Natural gas tank black (with natural gas as raw material) PH is low.
Easy mixing tank black EPC, ASTM mark S300
Mixed tank black MPC, ASTM mark S301
(2) Air mixed carbon black
(3) Roller carbon black
(4) Carbon black CC produced by conductive bath method has coarse particles and small resistance.
Basic nature:
(1) The fine particles (with a particle size range of about 30nm) have a great reinforcing effect on NR, and have good tensile strength, tear resistance and cutting resistance,
(2) High oxygen content (with more oxygen containing groups) is acidic and has delayed vulcanization effect. Because acidic additives have adsorption effect on accelerator,
(3) It is mainly used in NR to make tread rubber, and seldom used in synthetic rubber.
2、 Oil based furnace carbon black (alkaline, high reinforcement, hard carbon black)
Also known as oil furnace black or furnace black, it takes raw oil as raw material. It is obtained by burning in a special furnace, and then cooling the flue gas with water spray (it is alkaline because it absorbs salts in water)
Main varieties:
(1) Super abrasion furnace black (SAF), ASTM grade N110
Fine particles, good reinforcement effect, but not easy to disperse, poor process performance.
(2) Intermediate super abrasion furnace black (ISAF), ASTM mark N220
High structure medium super wear-resistant furnace black HS-ISAF, ASTM mark N242
Low structure medium super wear-resistant furnace black LS-ISAF, ASTM grade N219
(3) High abrasion furnace black (HAF), ASTM N330
High structure high wear-resistant furnace black HS-HAF, ASTM grade N347
Low structure high wear-resistant furnace black LS-HAF, ASTM grade N326
Basic properties
Fine particles, high reinforcing hardness carbon black,
Suitable for synthetic rubber (also for NR)
The water content and oxygen content are low and alkaline, which plays a role in promoting vulcanization
Widely used for products requiring good wear resistance, such as tire tread
3、 Gas based furnace carbon black (gas furnace carbon black, less reinforcing, called soft carbon black)
It is obtained by incomplete combustion of natural gas and a certain proportion of air in the furnace.
1. Fast Extruding Furnace black (ASTM N550)
The particle is thicker than the front, which is different from the mixing and pressing process. It is used for inner tube rubber
2. Fine Furnaceblack FF (ASTM mark N440)
Low structure, which can improve the elasticity and bending resistance of the rubber.
3. General purpose furnace black (GPF), ASTM mark N660
The particles are large, with good processing performance, high elongation, high elasticity, small heat generation and other characteristics.
4. High modulus furnace black (HMF), ASTM mark N601
Improve the constant elongation stress of rubber compound, and the reinforcement is higher than SRF, but lower than GPF
5. Semi reinforcing furnace black (SRF), ASTM grade N754 (low structure), N765 (high structure)
The particles are large, and the heat generated during processing is small, which endows the vulcanizate with high elasticity, large elongation, good flexibility and adhesion. When it is used for inner tube and rubber tube, the filling amount can be larger.
4、 Hot cracking method and other types of carbon black (soft carbon black, basically without reinforcement)
1. Fine thermal black, ASTM N880
Applicable to NR and synthetic rubber, commonly used for inner tube.
2. Medium thermal black (ASTM mark N990)
It is suitable for NR and synthetic rubber, mostly used for molded products, especially for oil resistant products.
The above two kinds of particles are large in size and almost have no reinforcement effect. They are mostly used as cheap fillers, and are suitable for pressed products and products with low mechanical strength requirements.
3. Conductive furnace black (CF), ASTM mark N293
4. Acetylene black ACET
The particle size is medium and the structure is the highest among all carbon black.
5. New process carbon black
It is prepared on the basis of furnace production by improving the process performance.
Multi purpose furnace black
All purpose furnace black
Super processing furnace black.
It is characterized by small particle size, narrow particle size distribution, smooth surface, good processing performance, and reinforcement of tire inner tube and other industrial products.
Example analysis: What carbon black should be used when preparing synthetic rubber tread, and why?
(1) Requirements for tread rubber: high mechanical strength, good abrasion resistance, good elasticity, a certain degree of heat resistance and aging resistance, and poor self reinforcement of synthetic rubber. Carbon black (hard carbon black) with high reinforcement should be selected. SAF particles in furnace black are too fine to be dispersed, and the processing performance is poor. ISAF and HAF materials are appropriate, and slot black has little reinforcement for synthetic rubber.
(2) For NR tread, EPC, MPC, ISAF, HAF and SAF can be selected due to the great reinforcement of MPC and EPC in the groove.
(3) If NR/synthetic rubber is used together, and NR is mainly used, tank furnace black is used together
(4) If synthetic rubber/NR is used together and mainly synthetic rubber is used, furnace black shall be used.
Section II Basic Properties of Carbon Black
Carbon black has 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 formed by pyrolysis of C and H compounds (oil or natural gas) at high temperature.
The difference between various carbon blacks lies in the regularity in the process of microcrystalline stacking, that is, the regularity of the arrangement of microcrystals in particles varies with various carbon blacks.
Regularity: hot cracking carbon black>gas based furnace carbon black>tank black>furnace black
2、 Chemical composition of carbon black
The analysis results show that the carbon black is mainly composed of C elements, but also contains a small amount of H, S, O and other impurities and moisture. Its content varies with various carbon black varieties.
3、 Basic properties of carbon black
1. Surface properties and surface chemical activity of carbon black particles. (The first factor affecting reinforcement performance, strength factor)
(1) Surface property: the unsaturation of micro crystals containing oxygen groups on the particle surface.
(2) Chemical activity: refers to the ability of carbon black particles to combine with rubber. The experiment proved that the carbon black with high chemical activity has great binding capacity with rubber and great reinforcement effect.
The chemical activity comes from the unsaturation of microcrystalline structure in its particles; The oxygen-containing group on the particle surface.
They directly affect the chemical properties of carbon black; At the same time, it affects the PH value of carbon black and the curing speed of rubber compound.
2. Size of carbon black particles, (the second factor affecting reinforcement performance, the breadth factor)
Generally speaking, when the specific surface of carbon black is more than 50m/g, the reinforcement effect is greater.
3. Structure of carbon black (the third factor affecting reinforcement performance, shape factor)
Basic concepts
Primary structure of carbon black (basic aggregate, polymelt, permanent structure)
In the manufacturing process of carbon black, the chain like or grape like aggregates formed by the mutual fusion of particles are called primary structures. It is combined by chemical bonds, and will not be damaged in the rubber mixing process. It is the smallest dispersible unit of carbon black in the rubber compound.
Secondary structure of carbon black (secondary aggregate)
It refers to two or more loose associated compounds formed by van der Waals force (physical adsorption). The binding is weak. Some are destroyed during granulation, and most are destroyed during rubber mixing.
Structural
It refers to the aggregation tendency of carbon black particles in three dimensions formed by connecting long chains and melting together. The more particles adhere, the higher the structure.
Structural representation
Shape factor (early use, observed by electron microscope)
Morphological factors (used in recent years)
Oil absorption value
Oil absorption value refers to milliliters of oil absorption per gram of carbon black. DBP (dibutyl phthalate) is commonly used for measurement, which is called DBP oil absorption value. The larger the DBP value is, the greater the oil absorption of carbon black is, and the higher the structure is.
In a word, carbon black with large activity, small particles, high structure, large reinforcement performance, tensile strength, constant elongation stress, hardness, tear strength, wear resistance, and good fatigue resistance under constant load; Elongation and resilience are small, and fatigue heat generation under constant deformation is large.
4、 Amount of carbon black
It is generally 40~50, too little can not play a very good role in reinforcement, and is not conducive to reducing costs; Too much will cause hardening effect, but it is not limited to hot cracking carbon black.
The effects of carbon black content on the reinforcement are as follows: hardness, elongation stress and heat generation, which increase monotonously with carbon black increment; Resilience and elongation decrease monotonously; The tensile strength (40-50 phr), tear strength and wear resistance (50-60 phr) have the maximum value with the carbon black increment. For self reinforcing rubber, the dosage is slightly lower, while the dosage of non reinforcing rubber is slightly higher.
5、 Reinforcing mechanism of carbon black on rubber
First of all, stress softening: the phenomenon that the tension used in rubber stretching gradually decreases.
（1） Volume effect
Carbon black will not deform under stress, so the deformation of rubber macromolecules in carbon black compound is greater than the appearance deformation. This is called the volumetric amplification effect. Mullins and Tobin believed that the stress softening of carbon black compound was the same as that of pure vulcanizate, but the difference was that there was greater stress softening and loss in carbon black compound due to volume effect.
（2） Weak bond and strong bond theory
This theory was put forward in the 1950s. Stress softening is the result of the rubber chain breaking away from the carbon black surface under the action of external force due to the physically adsorbed weak bond. Only strong bonds are left at the time of fracture. For the reinforcement of carbon black, such as tensile strength, tear resistance and wear resistance, the number of strong chemical adsorption is the most important for the ability of rubber to resist the final fracture. Therefore, those requiring high reinforcement should have more strong bonds for several days, that is, carbon black with high activity and large specific surface area should be used.
（3） Bueche's Theory of Limited Elongation of Carbon Black Particles and Rubber Chains
This theory only considers the strong bond between carbon black particles and rubber chain. When the rubber chain is stretched to the maximum length between particles under stress, high modulus is obtained. When it exceeds this length, it will break away from the carbon black surface or break. When the tension exceeds the length of the shortest chain, it breaks first, and then breaks in turn according to the length; The second extension is timely. Without the support of these chains, the stress drops, that is, the stress softens. The stress recovery is the redistribution of rubber chains between carbon black particles in the 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 adjacent chains, and it is easy to break successively; In the presence of carbon black, there are multiple rubber chains between particles. One chain is broken, and the stress is shared by other chains. Carbon black acts as a uniform stress, slowing down the overall fracture. When the elongation is large, the carbon black will also move, which also plays a role in easing the stress. Uniform and moderate stress is the reason for reinforcement.
(IV) Shell model theory
Nuclear magnetic resonance studies have confirmed that there is an adsorption layer on the surface of carbon black, which is composed of two kinds of rubber macromolecules in motion. In the inner layer of about 0.5 nm (equivalent to the diameter of macromolecule) adjacent to the surface of carbon black, it is in a glassy state; The rubber in the range of 0.5-5nm away from the carbon black surface has mobility and is in a sub glass state. This layer is called the outer layer. These two layers form a double shell on the surface of carbon black. The binding energy in the interface layer of the double shell must continuously decrease from the inside to the outside, that is, the binding of the carbon black surface to the mobility of macromolecules is constantly decreasing. Finally, the free state of rubber molecules is reached.
The explanation of shell reinforcement is that the double shell acts as a skeleton, forming an integral network of rubber macromolecules and fillers, changing the structure of the vulcanizate, thus providing the physical and mechanical properties of the vulcanizate.
（5） Sliding theory of rubber macromolecular chains
This is a relatively new and comprehensive carbon black reinforcement theory. The core of this theory is that rubber macromolecules can slide on the surface of carbon black, which explains the reinforcement phenomenon. The surface activity of carbon black particles is uneven, with a few strong active points and a series of adsorption points with different energy. The rubber wave chains adsorbed on the surface of carbon black can have different binding energies, from most weak van der Waals forces to a small amount of strong chemical adsorption. The adsorbed rubber chain segments will slide and elongate under stress.
(1) It indicates the original state of the rubber compound. Rubber molecular chains with different lengths are adsorbed on the surface of carbon black particles.
(2) When elongation occurs, the shortest chain does not break but slides along the carbon black surface. The length of the original adsorption is marked with a dot to show the length of the slip. At this time, the stress is borne by most straight chains, playing the role of stress uniformity. The first important factor for reinforcement is to reduce the stress set.
(3) When the elongation increases again, the chain slides again, making the rubber chain highly oriented, bearing large stress and high modulus, which is the second important factor for reinforcement. The rubber material has hysteresis loss due to sliding friction. The third factor for reinforcement is that the loss will eliminate part of the external force work and turn it into heat, so that the rubber will not be damaged.
(4) It refers to the condition of the shrunk rubber compound, indicating the stress softening effect during re elongation. The length of rubber chains between carbon black particles after rubber family shrinkage is almost the same, so it is unnecessary to slide again after re elongation, and the required stress is reduced. Under appropriate 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 molecular chain length between particles, the rubber compound returns to the original state. However, if the deformation of the initial elongation is large, the recovery usually does not exceed 50%.
