Reinforcing agents can improve the hardness and mechanical strength of rubber products because the particles of the reinforcing agent come into contact with the large rubber molecules, producing a physical adsorption effect. Meanwhile, the active surface of the reinforcing agent particles combines with the rubber molecular chains to form a strong chemical bond, generating "bound rubber", which has both physical and chemical effects.
The precipitated silica particles of the reinforcing agent contain various silanol groups. Infrared spectroscopy shows that there are three types of hydroxyl groups on the surface of the silica particles: silanol groups, isolated light groups, and hydrogen-bonded groups. The polar Si-O bond in the center of the silica molecule structure has a large binding ability, which makes the surface of the silica particles highly active and able to interact with rubber molecules. During the rubber compounding process, the polymer's rubber hydrocarbon chain breaks to generate free radicals, which interact with the OH group on the surface layer of the silica particles. In addition, the amorphous state and disordered crystal structure of the silica particles create an electrostatic field on the surface layer, which induces the unsaturated double bonds of the rubber polymer, promoting the bonding between the two.
1. The influence of particle size
The precipitated silica particles of the reinforcing agent are divided into primary particle size and secondary structure particle size. Generally, the primary particle size of the precipitated silica in the rubber compounding process is more important in affecting the properties of the rubber compound since the secondary structure is broken. The primary particle size of the precipitated silica ranges from 8-110 nm, and a smaller particle size leads to better dispersion performance and a larger effective contact area with the rubber, which obviously benefits the reinforcement of rubber.
From the perspective of the mechanism of the reinforcement effect on rubber, the amount of OH groups on the surface layer of the silica particles is directly related to the reinforcement effect. American chemist R.K. Iler summarized the following regularities in his research on the relationship between the size of primary particles and the number of hydroxyl groups in a water gel, which reveals the influence of the particle size of precipitated silica on the strength of rubber compounds (see Table 1). It can be seen that the smaller the d value, the more active groups there are, and the stronger the interaction between the particles and the rubber, resulting in greater strength.
Relationship between the size of primary particles and the number of hydroxyl groups
n (number of silicon atoms in primary particles) 840 1003 111 438 1150
d (diameter of primary particles in 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 Effects
The structure of precipitated silica (white carbon black) is similar to that of carbon black, with a spherical shape and interlocking branches connecting individual particles, forming a "secondary structure." The branching structure is formed by hydrogen bonding, and it forms clusters of aggregates that can be reversibly disrupted by external forces. The structural size is reflected by the oil absorption value (the number of milliliters of dibutyl phthalate absorbed by 1 gram of white carbon black). The higher the oil absorption value, the higher the structural integrity. A larger structure indicates a greater proportion of clustered branching structures, which improve the reinforcing effect on rubber. However, for precipitated silica with a high oil absorption value, breaking down this branching structure during mixing with rubber requires more energy and effort, which extends the mixing time. A white carbon black with an oil absorption value of 2.0-3.5 cm3/g can meet the requirements for tire production.
The specific surface area of precipitated silica is also an important indicator of product structure. Although the current national standard does not provide specific requirements for this index based on different production testing conditions, it is classified into six categories (A-F) based on the magnitude of the value. According to the results of the national survey of precipitated silica used in rubber production over the years, the best comprehensive properties of rubber materials are obtained with class B white carbon black (161-190m2/g). However, if only "reinforcement" is considered, class A with a higher specific surface area (≥190m2/g) is better. However, in practice, highly specific surface area white carbon black can also have poor physical properties in rubber, as a result of the production process that generates a lot of silica gel.
From a mechanistic point of view, the larger the specific surface area of precipitated silica, the better the penetration opportunities for rubber materials and accelerators, which can lead to a series of physical and chemical reactions, resulting in better transparency and physical properties. However, highly specific surface area white carbon black also has a high adsorption rate and high activity, which can absorb a large amount of accelerator and accelerate its decomposition, resulting in a strong delay effect on rubber vulcanization. Therefore, it is necessary to appropriately increase the amount of accelerator to improve the vulcanization rate. In actual production, it is also common to add surfactants to reduce the strong adsorption effect of an OH group on the surface layer of highly specific surface area precipitated silica. In addition, highly specific surface area precipitated silica generates more heat during rubber processing, making it more likely for the rubber to become sticky.
Influence of Surface Properties
(1) Moisture
The moisture in precipitated silica includes free water and bound water. The free water on the surface of the silica can be easily removed under heating conditions. If the free water content is too low, it indicates that there are fewer active sites on the surface of the silica, which reduces its activity and makes it difficult to disperse evenly in the rubber during mixing, resulting in agglomeration. A low surface water content not only affects the vulcanization rate but also reduces the reinforcement effect. When the free water content is slightly higher, the mixing performance of the rubber is improved, the powder consumption is reduced, and the dust is less. The rolling resistance is also better, and the vulcanization rate is slightly faster. However, if the water content is too high, it will generate a lot of heat, which can cause burning, caking, uneven dispersion, bubbles on the surface of the vulcanized rubber, and a corresponding decrease in physical and mechanical properties. Generally, a surface water content of 6 to 8% can provide the best isolation effect on the silica molecules, which prevents the particles from agglomerating again, and can be evenly distributed in the rubber during mixing, which is conducive to improving performance and accelerating the vulcanization rate. Bound water is located inside the silica particles and is relatively stable. It can only be removed at a temperature higher than 400℃, with a content generally ranging from 4.0 to 7.0%. If the bound water content is too high, it indicates that the gel content generated during the reaction is high, and the activity of the final product after drying is low, resulting in a small reinforcement effect on rubber. If the bound water content is too low, it indicates that the dehydration during the reaction is too heavy, which may lead to sanding and is also detrimental to reinforcement.
(2) pH Value
The acidity and alkalinity of the silica have a significant impact on the vulcanization of the rubber. Acidic silica delays the vulcanization process, while alkaline silica promotes it. However, excessively alkaline silica is not conducive to reinforcement. Silica with a slightly acidic pH value is more advantageous for rubber's tensile strength and wear resistance. Therefore, the pH value of precipitated silica used in rubber is generally around 6 to 8, close to neutral.
(3) Surface Modification Treatment
Surface modification treatment of silica refers to adding coupling agents to achieve its hydrophobicity. After adding a coupling agent, the CH3O (methoxy group) at one end of the coupling agent can easily react with the silanol group on the surface of the silica, while the other end can react with the rubber molecules. That is, the coupling agent acts as a "bridge" between the silica and the rubber. There are many ways to modify the surface of the silica, and the most commonly used method is to use silane coupling agents (such as Si-69) for treatment.
(4) Effects of impurities
In the process of preparing precipitated silica, impurities such as Cu, Fe, and Mn are easily introduced. Although these trace impurities have little effect on the physical properties of the rubber, in order to make rubber products have excellent antioxidant and aging resistance, the content of Cu, Fe, and Mn must be reduced to a certain limit. Regarding this, GB10517 specifies that the content of Cu should be ≤30 mg/kg, Mn should be ≤50 mg/kg, and Fe should be ≤1000 mg/kg for precipitated silica produced by precipitation method.
Sodium salt is difficult to avoid in the production of precipitated silica by precipitation method, but excessive sodium content is not suitable as a reinforcing filler for natural or synthetic rubber, and the transparency of the rubber material is significantly reduced. However, by controlling the reaction synthesis process during production, preventing the formation of gels that contain too much Na+, and strengthening the rinsing of semi-finished pulp, the sodium salt content can be reduced to below 0.5%. Such precipitated silica has little effect on the performance of rubber materials.
Conclusion
The physicochemical indicators of precipitated silica directly affect the various properties of rubber materials, and its reinforcing effect depends on its particle size, structure, surface properties, and type and content of impurities.
The specific surface area and particle size indicators, which are not specifically regulated in the technical conditions for precipitated silica for rubber use in GB10517, have a significant impact on the physical properties of rubber materials. Production enterprises should strictly stabilize production operations and gradually improve measurement and control methods.
In the production process of precipitated silica by precipitation method, special attention should be paid to "anti-gelation". Only precipitated silica with very low gel component can have good activity and satisfactory physicochemical indicators.