The reason why the reinforcing agent can improve the hardness and mechanical strength of the rubber product is because the surface of the reinforcing agent particles is in physical contact with the rubber macromolecule, and the active surface of the reinforcing agent particle and the rubber molecular chain combine to form a strong chemical bond. The formation of "bonded rubber" has both physical and chemical effects.
The precipitated silica particles contain a plurality of silanol groups. The infrared spectrum shows that there are three types of hydroxyl groups on the surface of silica: siloxy groups, isolated light groups, and hydrogen bonds. The Si-O-bond in the center of the molecular structure of silica has polarity and great binding ability, which makes the white carbon black particles have high surface activity and can interact with rubber molecules. The rubber hydrocarbon base of the polymer during the rubber compounding process The break generates free radicals and interacts with the surface layer of SiO-OH. In addition, the amorphous state of the silica particles is confusing, causing the surface layer to form an electrostatic field, which induces an effect on the unsaturated double bonds of the rubber polymer, thereby promoting the combination of the two.
1. Effect of particle size
The particle size of the precipitated silica is divided into a primary particle size and a secondary structure particle size. In general, the secondary structure of the silica in the rubber compounding process is broken, so the most important factor affecting the performance of the rubber compound is the original particle size. The original particle size of the precipitated silica is 8-110 nm, the particle size is fine, the dispersion performance is good, and the effective area in contact with the rubber is also large, which is obviously advantageous for rubber reinforcement.
From the rubber reinforcing mechanism, the amount of OH in the surface layer of silica is directly related to the reinforcing effect. American chemist R·K·ILer summed up the following rules in the study of the relationship between the size of primary particles of hydrogels and the number of hydroxyl groups. This regularity reveals the effect of the particle size of silica on the strength of rubber compounds (see Table 1). It can be seen that the smaller the d, the more active groups, the stronger the action between the particles and the rubber, and the greater the strength.
Table 1 Primary particle size and hydroxyl number relationship
| n (primary particle silicon number) | 8 | 40 | 100 | 311 | 1438 | 11500 |
| d (primary particle diameter nm) | 0.89 | 1.52 | 2.06 | 3.0 | 5.0 | 10.0 |
| OH/Si | 0.99 | 0.85 | 0.72 | 0.55 | 0.37 | 0.20 |
2. Structural influence
The structure of silica is similar to that of carbon black, which is spherical, and the individual particles are in surface contact with each other and are branched in a chain. This structure is called "secondary structure". The chain structure acts as a hydrogen bond force to form a cluster of aggregate structure. This structure is reversible by external force damage, and its structural size is reflected by the oil absorption value (the oil absorption value refers to the adsorption of 1 gram of white carbon black). The number of milliliters of dibutyl phthalate (DBP). The larger the oil absorption value, the higher the structure. The larger the structure, the more the chain structure in which the white carbon black is aggregated, and the reinforcing effect on the rubber is also good. However, the precipitation method of white carbon black with high oil absorption value in the rubber compound must be broken. This branch chain structure consumes more work and energy, so that it is evenly dispersed in the rubber. Refining time. The white carbon black oil absorption value of 2.0-3.5cm 3 /g can meet the tire requirements.
The specific surface area of ​​the precipitated silica is also an important indicator of the structural properties of the product. Although the current national standards take into account the specific test conditions for the test surface conditions of the various manufacturers, they are divided into six categories: A, B, C, D, E, and F according to the numerical level. From the quality survey of hydrated silica (silica) in the national rubber precipitation method over the years, the comprehensive properties of the rubber compound are best in class B (161-190m 2 /g) white carbon black, but only consider "reinforcement". ", Class A (≥ 190 m 2 /g) with a higher specific surface area is better. However, in the practice of production and use, the high specific surface area of ​​carbon black is often encountered, but the physical properties of the rubber compound are very poor, which is caused by the production of many silicon gels in the production process of silica.
The effect of the specific surface area of ​​the precipitated silica on the physical properties of the rubber compound. From the mechanism, the specific surface area is large, so that the rubber and the accelerator can obtain better penetration opportunities, thereby performing a series of physical and chemical reactions and obtaining better results. Transparency and physical properties. However, the white carbon black with large specific surface area has great adsorption rate and high activity, can adsorb more promoters, accelerates the decomposition of the accelerator, and therefore has a strong retardation effect on the vulcanization of the rubber compound, and thus needs to be appropriately increased. The amount of accelerator is used to increase the rate of vulcanization. In the actual production, it is also common to increase the active agent to reduce the strong adsorption of an OH group on the surface layer of the white carbon black to the promoter molecules. In addition, the precipitated white carbon having a large specific surface area generates high heat during the processing of the rubber compound, and the rubber compound tends to be sticky.
3. Influence of surface properties
( 1 ) Moisture
The moisture of the precipitated silica includes: free water and bound water. The free water on the surface of the silica is easily removed under heat. Too little free water indicates that the surface of the white carbon black is not much light, which reduces the activity of the white carbon black. When the mixing is carried out, the white carbon black is concentrated and it is difficult to be distributed in the rubber. The surface water is less, which not only affects the vulcanization rate, but also reduces the reinforcing effect. The free water is slightly higher, the mixing rubber has good operation performance, the powder feeding speed is fast, the dust flying is less, the wrapping roll is good, and the vulcanization speed is slightly faster. However, the water content is too high, the heat is large, the scorch is easy to burn, the agglomeration, and the dispersion is uneven. The surface of the vulcanized rubber is easy to bubble, and the physical and mechanical properties are also correspondingly decreased. Generally, the surface water at 6 to 8% can best isolate the white carbon black molecules, thereby preventing the particles from re-aggregating, and evenly distributing in the rubber compound during the mixing, which is advantageous for performance improvement and speeding up the vulcanization. The combined water is “inside†of silica, which is relatively stable and can be removed above 400 °C. The general content is 4.0-7.0%. If the combined water is too much, it indicates that the gel content generated during the reaction is high, the activity of the finished product after drying is small, and the reinforcing effect on the rubber is also small. The combination of too little water indicates that the dehydration is too heavy during the reaction, and the tendency of sanding is large, which is also unfavorable for reinforcement.
( 2 ) PH value
The acidity and alkalinity of white carbon black have a great influence on the vulcanization of the rubber compound. The acid white carbon black retards the rubber compound to vulcanize, and the alkalinity promotes the vulcanization, but the excessive alkalinity is unfavorable for reinforcing. PH is slightly acidic white carbon black, which is beneficial to the tensile and abrasion resistance of rubber compounds and products. Therefore, the precipitation value of white carbon black for rubber is generally 6-8, which is close to neutral.
( 3 ) Surface modification treatment
The surface modification treatment of silica refers to the purpose of adding a coupling agent to the silica to achieve its hydrophobicity. After the coupling agent is added, the CH 3 O-(methoxy group) at one end of the coupling agent reacts with the silanol group on the surface layer of the white carbon black, and the other end reacts with the gelatin molecule, that is, the coupling agent is in the white carbon. The “bridge†effect between black and rubber. There are many ways to modify the surface of silica, and the most common one is to treat it with a silane coupling agent (such as Si-69).
( 4 ) Influence of impurities
Precipitated silica is easy to bring in impurities such as Cu, Fe, Mn in the preparation process. Although these trace impurities have little effect on the physical properties of the rubber, in order to make the rubber products have excellent oxidation resistance and aging resistance, Cu is still required. The Fe and Mn contents are reduced to a certain limit. In this regard, GB 10517 specifies the precipitated silica: Cu ≤ 30 mg / kg, Mn ≤ 50 mg / kg, Fe ≤ 1000 mg / kg.
The sodium salt in the production of white carbon black by precipitation is difficult to avoid in the production, but the sodium content is too high to be used as a reinforcing filler for natural or synthetic rubber, and the transparency of the rubber is obviously not good. However, in the production process, the reaction synthesis process is controlled to prevent the formation of excessive Na + gel and the rinsing of the semi-finished slurry. The sodium salt content can be reduced to less than 0.5%. No impact.
in conclusion:
1. Precipitation method The physical and chemical indicators of silica directly affect the various properties of rubber compounds. The reinforcement depends on its particle size, structure, surface properties and the type and content of impurities.
2. The specific surface area and particle size index of GB10517 rubber precipitated hydrated silica (white carbon black) are not affected by the specific requirements. The production enterprises should strictly stabilize the production operation and gradually improve the measurement and control means.
3. Precipitation method In the production process of silica, special attention should be paid to “anti-gelationâ€. Only silica with very low “gel†composition can have good activity and satisfactory physical and chemical indicators.
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