Mingyi silicone（anhui）co,.ltd
Hits: 377 img
Abstract: Hydrophobic modification is a pivotal technology for enhancing the compatibility and performance of fumed silica in polymer matrices. This article focuses on three mainstream organosilicon treatment agents—Dimethyldichlorosilane (DDS), Hexamethyldisilazane (HMDS), and Polydimethylsiloxane (PDMS)—systematically elaborating on their mechanisms of action, product characteristics, and typical applications, providing a scientific basis for material selection and process development.
Keywords: Fumed Silica; Hydrophobic Modification; DDS; HMDS; PDMS; Interfacial Engineering
Fumed silica, a nano-sized powder with high specific surface area and surface activity, is an indispensable functional filler in fields such as rubber, coatings, and adhesives. However, its surface, rich in silanol groups (Si-OH), is highly hydrophilic, leading to agglomeration in organic media and limiting its effectiveness. Surface hydrophobic treatment can significantly improve its dispersibility, rheological control capability, and interfacial bonding with the matrix. Among various treatment agents, DDS, HMDS, and PDMS are three representative types that impart distinct properties through different chemical pathways.
Mechanism: As a representative chlorosilane, the Si-Cl bonds of DDS undergo a silanization reaction with the Si-OH groups on the fumed silica surface. This covalently grafts dimethylsilyl groups (-Si(CH₃)₂H) onto the surface, with HCl as a byproduct. This process results in a complete chemical modification, forming a stable organic monolayer.
Key Properties: The modified silica exhibits an extremely low surface energy and a large hydrophobic contact angle, showing excellent compatibility with non-polar or low-polarity systems like silicone rubber and silicone oil. It effectively reduces system viscosity while providing fundamental reinforcement.
Typical Applications: Primarily used as a reinforcing filler in High-Temperature Vulcanized (HTV) silicone rubber and in specific sealants or coatings requiring special electrical properties or chemical resistance.
Mechanism: HMDS, a silazane, reacts with surface Si-OH groups via aminolysis condensation. It "caps" the silanols with trimethylsiloxy groups (-OSi(CH₃)₃), releasing NH₃. This reaction is highly efficient and thorough, maximizing the elimination of hydrophilic sites.
Key Properties: The product offers excellent hydrophobicity and high purity. The grafted short trimethyl groups introduce minimal steric hindrance, largely preserving the native aggregate structure of the silica. Consequently, it maintains good thickening and thixotropic properties in formulations and yields products with high transparency.
Typical Applications: The most commonly used hydrophobic fumed silica in Room-Temperature Vulcanized (RTV) silicone rubber, Liquid Silicone Rubber (LSR), and high-end electronic potting compounds, perfectly balancing hydrophobicity, rheological control, and optical clarity.
Mechanism: PDMS (silicone oil), a polymeric macromolecule, modifies the surface primarily through physical adsorption and entanglement. Its long chains adsorb onto the particle surface and within the pores of aggregates via intermolecular forces, forming a relatively thick, flexible coating.
Key Properties: PDMS-treated products excel in thickening and thixotropy. Their long-chain structure facilitates entanglement within the system, building a robust three-dimensional network that imparts pronounced shear-thinning behavior. Additionally, they often provide superior reinforcement in polar matrices like epoxy resins compared to small-molecule treated grades.
Typical Applications: Widely used as rheology modifiers and anti-settling agents in epoxy resin systems, polyurethanes, paste paints, and cosmetics (e.g., sunscreens, foundations).
DDS, HMDS, and PDMS represent three distinct surface engineering strategies: "chemical bonding and anchoring," "small-molecule capping," and "macromolecular physical coating."
For low viscosity and high dispersion in non-polar systems, choose DDS-treated grades.
For silicone rubber applications requiring a balance of hydrophobicity, thickening, and transparency, HMDS-treated grades are the classic choice.
For polar systems demanding strong thickening, thixotropy, and reinforcement, PDMS-treated grades offer clear advantages.
Future trends will focus on developing composite treatment technologies (e.g., HMDS capping followed by PDMS coating) and functional treatment agents (e.g., introducing reactive groups like amino or epoxy) to meet the stringent demands of emerging fields such as electric vehicles, new energy, and flexible electronics for high-performance composite materials.
