Technical Misconceptions and Scientific Approaches in White Carbon Black Selection and General Processes for Silazane Treatment

　　It is well known that the strength of silicone rubber vulcanizates without reinforcing fillers is quite low. In practical applications, most silicone rubbers are used as reinforced composites, typically incorporating either fumed silica or precipitated silica as the reinforcing filler. Within a suitable range, the strength of the cured silicone rubber increases proportionally with the addition of silica filler. Notably, the surface of silica particles contains a high concentration of silanol groups, which form hydrogen bonds with the organic silicon polymer. This interaction leads to thickening of liquid silicone rubber formulations and structuring of semi-solid compounds. As a result, the optimal proportion of silica filler is somewhat limited by these factors.
　　To prevent structural changes in silicone rubber compounds or to maintain the proper fluidity of liquid silicone rubber while maximizing its mechanical performance, organic chlorosilanes, oligomeric organosiloxanes, and hexamethyldisilazane (hereafter referred to as silazane) are commonly used as treating agents for pre-surface modification of precipitated silica fillers. Among these, silazane treatment has proven particularly effective for improving the properties of white carbon black.
　　There are various processes for treating silica with silazanes, ranging from traditional methods that have been developed based on early technical literature and persistently applied over the years—many of which contain significant technical misconceptions—to more recent, innovative approaches that, unfortunately, aren’t always scientifically sound. In this article, we’ll draw on fundamental chemical principles to clarify the critical issue of selecting the right type of silica, evaluate the treatment process conditions, assess the accuracy of the commonly used silazane-based processing techniques, and ultimately present a scientifically proven method that has been validated through years of successful production practice.
　　Analysis of Common Technical Misconceptions in the Selection of Raw Materials for White Carbon Black
　　⑴ "Newly manufactured white carbon black is difficult to use," and "Older, stored white carbon black tends to perform worse."
　　It is inaccurate to broadly assume that "newly manufactured precipitated silica is ineffective" or that "aged, stored precipitated silica will perform worse over time."
　　For newly manufactured gas-phase silica, the silica particles, having undergone high-temperature calcination, initially have very low levels of silanol groups on their surfaces—since they haven’t yet fully come into contact with and absorbed moisture from the air. As a result, their reinforcing effect is weak. If freshly produced gas-phase silica is directly treated with silazane, the treatment will yield minimal improvement, as the particle surfaces lack sufficient silanol groups to react with the silazane. However, after being stored for an appropriate period, the gas-phase silica naturally absorbs moisture from the air, leading to the formation of a moderate amount of silanol groups on its surface. This allows the material to exhibit its inherent reinforcing properties. When subjected to surface modification under these conditions, the desired enhancement effect can finally be achieved.
　　For newly manufactured precipitated silica, since it undergoes only moderate drying temperatures, the surface of the silica particles still retains silanol groups, giving it effective reinforcing properties. When directly applied to silazane treatment, it can achieve the desired processing results as well. However, after years of long-term storage, the reinforcing performance of precipitated silica deteriorates significantly when used to reinforce silicone rubber. As a result, the vulcanized silicone rubber not only exhibits poor mechanical strength but also shows markedly reduced heat resistance and dielectric properties.
　　⑵ "Neutral white carbon black is more suitable for reinforcing silicone rubber."
　　As a standard product, fumed silica—strictly speaking—does not exist as "neutral fumed silica." Primarily composed of silicon dioxide, fumed silica is essentially polymeric silicic acid. The silanol groups on the surface of its particles inherently give rise to an acidic nature in its aqueous suspension. Typically, the pH of gas-phase fumed silica suspensions hovers around 4, while precipitated fumed silica tends to have a pH of approximately 5 to 6. However, some precipitated fumed silica formulations exhibit a pH closer to neutral—this is due to the presence of residual alkaline impurities like sodium ions. Yet, this so-called "neutral" fumed silica is ill-suited for reinforcing silicone rubber, as its inherent alkalinity can severely compromise the material's heat resistance and dielectric properties. Moreover, even though these "neutral" grades may appear pH-neutral, their actual alkalinity becomes pronounced in the presence of moisture, potentially triggering the oxidation of divalent iron ions into trivalent iron within silicone rubber exposed to air. This oxidation process could ultimately lead to yellowing of the silicone rubber’s appearance.
　　⑶ "The finer the particles or the larger the specific surface area of silica, the better the reinforcing effect."
　　For silicone rubber vulcanizates with a fixed silica loading level, the general trend is that the finer the silica particles or the larger their specific surface area, the greater the reinforcing effect. However, the particle size and specific surface area of silica are closely linked to the viscosity and processing properties of the silica-rubber composite—especially in the case of liquid silicone rubber, where fine silica particles exhibit significant thickening effects. At the same time, the reinforcing ability of silica also depends on its filler concentration. To maintain the flowability or processability of the silicone rubber compound, using silica with an excessively high specific surface area often requires reducing the amount of silica added. Yet, applying silica at lower concentrations can diminish its reinforcing performance. As a result, relying on ultra-fine silica particles doesn’t always guarantee the desired level of reinforcement—it may even lead to suboptimal outcomes.
　　To achieve reinforcement effects, it is essential to select the appropriate particle size of silica and the right amount of filler based on specific application requirements. In some cases, combining silica particles of different sizes can yield even better performance.
　　Analysis of Common Technical Misconceptions in the General Process for Treating White Carbon Black with 3-Silazane
　　The typical procedure for treating silica with silazane involves first performing necessary pre-treatment steps before adding the silazane to the silica. Afterward, the silazane is introduced into the silica, and after an appropriate reaction period, the byproduct ammonia is removed, resulting in hydrophobic silica.
　　The general process for treating precipitated silica with silazane involves several steps, but the following sequentially analyzed operational procedures are not entirely scientifically sound or reasonable:
　　⑴ "White carbon black must be pre-wetted with water" and "White carbon black must be pre-heated to remove moisture."
　　The chemical reaction involving the silazane modification of fumed silica surfaces essentially involves silazane reacting with the silanol groups on the fumed silica surface, where trimethylsilyl groups are attached to the particle surface. This transformation converts hydrophilic fumed silica into hydrophobic fumed silica, while simultaneously releasing ammonia as a byproduct.
　　The reaction enabling the silazane modification of fumed silica is facilitated by the presence of an appropriate number of silanol groups on the surface of the silica particles.
　　If the surface of the precipitated silica particles not only contains silanol groups but also has excess surface-bound water, then when silazane is added to the silica, the free surface water will react with the silazane via hydrolysis and condensation—forming hexamethyldisiloxane—before the silanol groups on the silica surface can do so.
　　Both the unqualified statements that "white carbon black must be pre-wetted with water" and "white carbon black must be pre-heated to remove moisture" are inappropriate.
　　If the surface of the precipitated silica particles lacks silanol groups, adding silazane will simply fail to initiate any reaction, rendering the modification ineffective. On the other hand, if the precipitated silica being treated is freshly produced gas-phase silica—whose manufacturing process involves exposure to temperatures around 1000°C—the silanol group content on its particle surface is extremely low. In such cases, before proceeding with silazane treatment, it’s essential to carefully add a controlled amount of water. This step ensures that sufficient silanol groups are formed on the silica particle surface, creating reactive sites capable of interacting with the silazane and ultimately achieving the desired modification effect. Conversely, if the precipitated silica already contains an appropriate level of silanol groups, adding excess water at this point could lead to unintended consequences. The free water would likely react more readily with the silazane through hydrolysis, inadvertently converting the silazane into hexamethyldisiloxane—a compound with significantly lower reactivity. As a result, the modified material may lose some of its intended functionality, ultimately weakening the overall treatment effect.
　　For precipitated silica particles that do not have excessive surface moisture or overly high levels of silanol groups, pre-treating them with heating to remove water before processing will inevitably result in the loss of some essential silanol groups. This, in turn, leaves the silica lacking the reactive sites needed to effectively interact with silazanes, making it difficult—even if a relatively large amount of silazane is added—to achieve the expected modification reaction. On the other hand, precipitated silica that has been stored for extended periods in humid conditions may have absorbed significant amounts of moisture. In such cases, prior to adding silazane, a dehydration treatment via heating becomes absolutely necessary. However, this process must be carefully controlled to avoid completely stripping away the silanol groups from the particle surface.
　　⑵ "Pre-ammonia treatment is required before introducing ammonia into the white carbon black system."
　　Releasing ammonia as a byproduct when silica black reacts with silazane, according to the general principles of chemical equilibrium, removing this reaction byproduct—ammonia—from the system during the process will help drive the forward reaction further. Therefore, deliberately introducing the ammonia product that would otherwise form into the reaction mixture before the main chemical reaction even begins will inevitably hinder the progress of the forward reaction. As a result, pre-feeding ammonia prior to treating the silica black is an ineffective and counterproductive practice.
　　Early technical reports indicated that pre-introducing ammonia into the white carbon black treatment process has a catalytic effect and helps enhance the performance of silazane treatment. In fact, the reaction between silazane and white carbon black occurs quite readily. As for the purported enhancement in treatment efficiency, this is essentially due to ammonia preventing the hydrolysis of silazane—a process that could be more effectively achieved by first removing excess moisture already present in the white carbon black itself, rather than by simply introducing ammonia gas into the system.
　　⑶ "Silicon Nitride Treatment of Silica Under Supercritical Carbon Dioxide Conditions"
　　In recent years, driven by the growing momentum behind supercritical carbon dioxide application technologies, some silicone companies have been exploring new techniques for treating silica using silazanes under supercritical CO₂ conditions. However, based on the fundamental nature of the chemical reaction, the interaction between silazanes and silica is expected to release ammonia—a byproduct with alkaline properties. Meanwhile, carbon dioxide is an acidic oxide, meaning ammonia will inevitably react with CO₂ to form ammonium carbonate salts. Unfortunately, these ammonium carbonate residues persist in the treated silica, making them extremely difficult to remove. As a result, using this method to process silica for silicone product manufacturing would inevitably lead to a deterioration in the heat resistance and dielectric properties of the final silicone products. Therefore, treating silica with silazanes under supercritical CO₂ conditions is neither beneficial nor effective—it’s simply an impractical and harmful process.
　　⑷ "Closed-system reaction after adding white carbon black to silazane"
　　At room temperature, after adding silica to silazane, a condensation reaction immediately occurs, releasing ammonia in the process. If the reaction system is sealed, the byproduct ammonia cannot escape freely, which negatively affects the continuation of the forward reaction.
　　Additionally, the ammonia released during the reaction exists as a gas, and at room temperature, its volume can easily exceed several times that of the original reaction mixture. Moreover, since the reaction is exothermic, the temperature of the reactants rises significantly, further causing the volume of the byproduct ammonia to expand even more—up to several times larger. As a result, large amounts of ammonia accumulate inside the sealed reaction vessel, creating a serious risk of explosion.
　　After adding silica black to silazane, the reaction should not be carried out under sealed conditions; instead, it should be conducted with venting while allowing the silazane to reflux and condense.
　　⑸ "Add silicon nitride after preheating and raising the temperature of the white carbon black, then allow the reaction to proceed under reflux heating."
　　At room temperature, silica nanoparticles react readily with silazanes, accompanied by a noticeable exothermic effect. For exothermic reaction systems that proceed smoothly at room temperature, external heating clearly works against shifting the equilibrium toward the forward reaction.
　　The mixed system of white carbon black and silazane involves a gas-phase/solid-phase reaction. Since silazane has a relatively low boiling point (126°C), the white carbon black is heated first, causing the added silazane to vaporize upon heating. This prevents the silazane from coming into direct contact with the white carbon black, thereby reducing the likelihood of a reaction occurring. However, if the mixture is subjected to intense heating, it could also trigger condensation and dehydration of the active silanol groups on the surface of the white carbon black. The resulting gaseous water might then readily undergo hydrolysis with the remaining gaseous silazane, forming hexamethyldisiloxane. As a result, some of the silazane initially introduced into the reaction system may fail to effectively participate in the desired modification reaction with the white carbon black.
　　Reacting by first heating and raising the temperature of precipitated silica before dripping in silazane, or carrying out the silica modification reaction while maintaining silazane at an elevated reflux temperature, are both energy-intensive processes that also reduce efficiency.
　　⑹ "Quickly add silazane dropwise to the white carbon black for reaction"
　　The byproduct ammonia released during the reaction between precipitated silica and silazane acts as a catalyst for the reaction itself. Therefore, during the initial stage of the reaction, silazane should be added slowly to allow the released ammonia to catalyze the subsequent silazane-modification process of the precipitated silica. Once the reaction stabilizes, the addition rate of silazane can be gradually increased—but care must be taken not to add it too quickly. Especially under conditions where the reaction system doesn’t cool effectively, rapidly increasing the feed rate could cause the silazane to heat up, vaporize, and escape, resulting in significant losses.
　　When adding white carbon black to silazane, it’s crucial to add the silazane slowly during the initial stage. Even after the reaction stabilizes, avoid speeding up the addition rate too much.
　　(7) "After mixing white carbon black with the base compound of condensation-type room-temperature vulcanizing silicone rubber, silazane is added to treat the white carbon black."
　　After kneading and mixing precipitated silica with a condensation-type room-temperature vulcanizing silicone rubber base compound to form a paste-like material, silazane is then added to treat the silica. This process effectively addresses the issue of ammonia-containing dust flying off the treated silica after the reaction. However, the silazane originally intended for surface modification of the silica not only reacts with the surface silanol groups of the silica but also interacts with the silanol groups present in the base silicone rubber polymer, ultimately forming an organosilicon polymer that lacks cross-linking activity.
　　The reaction between silazane and the base silicone rubber polymer not only consumes part of the silazane but also effectively introduces a plasticizer into the silicone rubber, which can negatively impact the mechanical properties of the cured silicone rubber. Moreover, when silazane is used to treat precipitated silica—specifically, silica coated by liquid rubber—even after thorough water washing and vacuum treatment, complete removal of ammonia remains challenging. Residual ammonia in the composite rubber compound can severely compromise the storage stability of the silicone rubber material.
　　⑻ "Thermally vulcanized silicone rubber and addition-cure silicone rubber can first have white carbon black added to the compound, kneaded thoroughly, and then treated with silazane."
　　After thorough kneading and mixing of heat-vulcanized silicone rubber, addition-type reactive-vulcanized silicone rubber, and white carbon black with silazane, the process was designed with the primary goal of enabling a surface-modification reaction between the silazane and white carbon black—aiming to either suppress the structuring effect in the composite material or reduce its viscosity. However, by this point, the white carbon black component within the rubber compound has already been fully coated by the silicone polymer matrix. As a result, the externally added silazane has very limited opportunity to come into contact with and react directly with the white carbon black particles, significantly diminishing the effectiveness of silazane in modifying the white carbon black.
　　⑼ "Silicon nitride can be rapidly heated and subjected to reduced pressure to remove ammonia after being added to fumed silica."
　　Although silazanes can react with fumed silica even at room temperature, it is not advisable to immediately initiate rapid heating and vacuum evaporation of ammonia right after the silazane is added. This is because the reaction between silazanes and fumed silica requires a certain amount of time—especially if the reactor’s mixing efficiency is poor. In such cases, silazanes added too quickly may struggle to achieve adequate contact and complete reaction with the fumed silica. Moreover, the exothermic nature of the silazane-fumed silica reaction could cause some silazanes to heat up and vaporize prematurely, bypassing the reaction altogether. Under these conditions, where significant amounts of unreacted silazane still remain, starting the heating and vacuum process too early would result in the loss of silazanes carried away by escaping ammonia, ultimately leading to unnecessary waste of the material.
　　⑩ "After the reaction between silazane and white carbon black is complete, ammonia can be removed at high temperature."
　　Ammonia is a reducing agent and can burn in oxygen to produce nitrogen gas and water. During the silazane treatment of precipitated silica, ammonia is generated as a byproduct. If significant amounts of residual ammonia remain, it could ignite rapidly at elevated temperatures, potentially leading to an explosion.
　　White carbon black is a fine particulate material with a high specific surface area. After the reaction between silazane and white carbon black is complete, even after thoroughly stirring the reaction mixture, some of the byproduct ammonia remains adsorbed onto the white carbon black. To remove the residual ammonia, transferring this mixture—still containing significant amounts of ammonia—into an oven and rapidly heating it up immediately can often lead to explosive incidents. Under no circumstances should this hazardous procedure be attempted!
　　⑪ "After silazane treatment, the deamination of precipitated silica to pH = 7 indicates that deamination has been completely removed."
　　A convenient method to determine whether the silane treatment has completely removed ammonia from precipitated silica is to use water-moistened pH paper for testing. The pH test relies on measuring the concentration of hydrogen ions. Typically, you dip the water-moistened pH paper into the treated precipitated silica, observe the color change, and compare it against the standard pH paper color scale to assess the material’s acidity or alkalinity. By convention, a pH reading of 7 on the paper indicates neutrality. However, in practice, since the water used to moisten the pH paper isn’t exactly pH 7—often registering around pH 5–6—the treated silica is usually tested with this slightly acidic paper. If the pH paper turns neutral (around pH 7) after being dipped into the treated silica, it actually means the material remains alkaline, signaling that residual ammonia is still present in the white carbon black. For truly complete ammonia removal, the pH paper should show no color change when moistened with the test water.
　　4 Scientific Methods for Treating White Carbon with Silazane
　　⑴ Technical Approach
　　To properly apply silazane to precipitated silica, follow this technical procedure: First, determine the moisture content of the wet precipitated silica and adjust the state of its surface silanol groups. Next, slowly add silazane at room temperature to initiate the surface modification reaction on the silica particles. After the reaction is complete, gradually reduce pressure at room temperature, followed by heating under reduced pressure, and finally complete the process by heating while further reducing pressure to remove ammonia completely.
　　⑵ Handling Methods for Small-Scale Test Samples
　　To prepare a small batch of silica-treated carbon black in the lab, an open reactor equipped with a stirrer and a reflux condenser can be used as the reaction vessel. In the open reactor, first add the carbon black, then slowly drip in the silazane under continuous stirring. Any byproduct ammonia generated during the reaction is continuously removed via the reflux condenser. After the silazane addition is complete, continue stirring for an appropriate period before switching the setup to a distillation apparatus fitted with a filter. The process involves gradually reducing pressure at room temperature, followed by further vacuum heating, and finally purging the system with nitrogen while maintaining reduced pressure. This sequence effectively removes residual ammonia from the treated carbon black, yielding hydrophobic silica-treated carbon black.
　　⑶ Industrial-Scale Silica Treatment Equipment and Processes
　　For handling precipitated silica on an industrial scale, a stainless steel reactor equipped with attachments such as a ribbon-type agitator and a filter should be used as the processing unit.
　　Processing procedure: First, measure the moisture content of the precipitated silica and analyze the silanol group status on its particle surface. Then, use reduced pressure to draw the precipitated silica into the processing reactor, start stirring, and, based on the test results, decide whether to add water for humidification or apply heat to remove excess moisture. After pre-treatment is complete, while continuously stirring, slowly introduce silazane to initiate the surface modification reaction of the precipitated silica. Any byproduct ammonia generated during the reaction is continuously removed via a filter. Once the silazane addition is finished, maintain stirring for an additional period to allow the reaction to proceed fully. Finally, the process involves sequentially reducing pressure at room temperature, followed by heating under reduced pressure, and then introducing nitrogen or air simultaneously with continued vacuum reduction—ultimately eliminating any residual ammonia from the treated precipitated silica, yielding the final hydrophobic product.