Powdered rubber moisture absorbents tend to agglomerate after absorbing moisture, leading to uneven dispersion and, in turn, affecting the physical properties and processing stability of rubber products. To address this issue, comprehensive improvements are needed across multiple dimensions, including material selection, process optimization, equipment improvement, and auxiliary methods, to enhance dispersion uniformity.
Selecting highly dispersible fillers and compounding agents is essential. Hygroscopic powdered rubber moisture absorbents are susceptible to hydrogen bonding or capillary forces due to surface moisture adsorption, leading to enhanced cohesion between particles. Surface-modified fillers, such as silica treated with silane coupling agents or highly active zinc oxide, can be used in this situation. Their surface hydrophobic groups can reduce interaction with water and reduce agglomeration. Furthermore, avoid using highly hygroscopic inorganic release agents. Organic release agents or soaps are preferred to minimize the risk of structural damage after moisture absorption.
Optimizing formulation design is key. The interactions among the components in the formulation directly impact dispersion effectiveness. For example, when zinc oxide is used as an active agent, dispersion is difficult due to repulsion between its surface charge and the rubber. This can be treated with surfactants (such as low-order fatty acids like propionic acid) to reduce its surface energy and enhance compatibility with the rubber. Furthermore, a judicious combination of lubricants and additives, such as stearic acid or process oil, can improve the wettability of the filler within the rubber matrix and promote uniform dispersion. For highly hygroscopic systems, a small amount of a less hygroscopic reinforcing agent, such as calcium carbonate, can be added to balance the overall hygroscopic properties.
Increasing the mixing temperature and speed can enhance dispersion dynamics. Increasing the temperature reduces rubber viscosity, reducing entanglement between molecular chains and making the filler more susceptible to shear forces. Furthermore, high temperatures accelerate water evaporation and reduce liquid bridging forces on the surface of the powdered rubber moisture absorbent. However, it is important to note that excessively high temperatures may cause rubber degradation or damage the filler structure. Therefore, the temperature range should be adjusted based on the rubber type (for example, EPDM rubber has good high-temperature resistance). High-shear equipment (such as internal mixers or extruders) effectively breaks up agglomerates through intense mechanical action. Their shear rate and energy input are significantly higher than those of open mixers, making them particularly suitable for powdered rubber systems that harden after moisture absorption.
Controlling mixing time and pressure is crucial for ensuring uniform dispersion. Insufficient mixing time results in incomplete filler dispersion, while excessive mixing time can cause excessive shear heating, exacerbating rubber degradation or filler reagglomeration. Therefore, mixing time should be adjusted dynamically based on the rubber's Mooney viscosity and filler type. For example, highly filled systems require longer mixing times to ensure thorough dispersion. Increasing mixing pressure appropriately can increase shear stress and promote agglomerate breakup, but avoid excessive pressure that can cause equipment wear or rubber overheating.
Using cyclical shearing and multiple mixing cycles can further enhance dispersion. Cyclic shearing, by varying the shear direction of the rubber between rollers (e.g., staggered shearing), can break up localized high-concentration agglomerates and achieve more uniform dispersion. For highly hygroscopic systems, a two-stage mixing process can be employed: the first stage involves preliminary mixing at low temperature to prevent premature moisture absorption by the powdered rubber moisture absorbent; the second stage involves high-shear mixing at high temperature to ensure thorough filler dispersion. Furthermore, thin-pass mixing after mixing can further eliminate residual agglomerates and improve uniformity.
Auxiliary agents such as surfactants and pre-dispersion techniques can enhance the dispersion process. Surfactants reduce the filler's surface tension, enhancing its compatibility with the rubber and reducing agglomeration after moisture absorption. For example, dispersants such as ammonium oleate form a protective layer on the filler surface, preventing moisture absorption. Pre-dispersion techniques, by pre-mixing the powdered rubber with a portion of the liquid compounding agent (such as process oil) to form a uniform dispersion before adding it to the main rubber compound for mixing, can significantly reduce agglomeration.
Controlling the production environment is an often overlooked yet crucial step. High humidity can accelerate moisture absorption by the powdered rubber moisture absorbent, leading to some agglomeration before mixing. Therefore, strict humidity control in the production workshop is essential. Raw materials should be dried when necessary (e.g., in a vacuum oven). Sealed packaging and rapid transportation procedures should be employed to minimize moisture absorption time. Furthermore, regular cleaning of mixing equipment prevents residual fillers from contaminating the new material, ensuring consistent dispersion quality.
