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Reinforcing filler for silicone rubber
Release Date:
2020-06-15
Silicone rubber is a type of specialty synthetic rubber—a linear polysiloxane polymer with a Si–O–Si backbone—exhibiting outstanding properties such as heat resistance, cold resistance, ozone resistance, radiation resistance, electrical insulation, and biocompatibility. It has found widespread applications across numerous industries, including aerospace, electrical and electronic engineering, chemical instrumentation, automotive, and mechanical engineering, as well as in medical and healthcare fields and everyday life. Due to the high molecular flexibility of raw silicone rubber and weak intermolecular interactions, its mechanical strength is very low (not exceeding 0.4 MPa), rendering it practically useless without reinforcement. Adding reinforcing fillers is therefore the primary approach to enhancing its physical and mechanical properties. After reinforcement, the crosslinking structure of silicone rubber becomes more complex, leading to a significant increase in strength. The most commonly used reinforcing filler for silicone rubber is precipitated silica (5102), which also delivers the best reinforcement effect. Today, there are over thirty commercially available grades of precipitated silica specifically formulated for dimethyl‑based liquid silicone rubbers, with manufacturers including Degussa, Wacker, CABOT, and Shenyang Chemical Plant. Based on their preparation methods, precipitated silicas can be broadly classified into gas‑phase silica and precipitation‑method silica. However, regardless of the type, compatibility issues persist: the addition of these fillers often causes a sharp rise in compound viscosity. When high flowability and transparency are required, precipitated silica proves inadequate as a filler. While precipitated silica markedly improves the mechanical properties of rubber, silicone compounds reinforced with this filler tend to exhibit reduced transparency. Moreover, as the amount of precipitated silica increases, the compound’s viscosity rises sharply, complicating processing and adversely affecting subsequent manufacturing steps, thus failing to meet the demands of products that require both high strength and good fluidity. In such cases, using MQ resin as a reinforcing agent offers a superior alternative. MQ resin demonstrates excellent dispersibility and compatibility within silicone rubber and can achieve reinforcement through chemical bonding with the polymer matrix. Compared with precipitated silica, MQ resin possesses the following advantages: (1) Composed of both organic and inorganic components, MQ resin closely resembles the basic structural framework of silicone rubber. This unique composition ensures exceptional compatibility; incorporation of MQ resin does not significantly alter the compound’s viscosity. Wang Xinxin conducted a comparative study of MQ resin versus precipitated silica in reinforcing room‑temperature vulcanizing (RTV) liquid silicone rubber, demonstrating that, at comparable mechanical strengths, products reinforced with MQ resin exhibited superior viscosity control and adhesion performance compared to those reinforced with precipitated silica. (2) MQ resin consists of M‑units and Q‑units; by tailoring the substituents attached to the M‑units, MQ resin can be readily modified to suit specific applications. As a reinforcing material, MQ resin can be used alone or blended with other fillers. Factors influencing its reinforcing effectiveness include the M/Q ratio and the types of functional groups present. The M/Q ratio determines the compatibility between MQ resin and silicone rubber, while the nature of the functional groups affects the strength of the chemical bonds formed between them, thereby impacting the overall mechanical performance of the silicone rubber.
Silicone rubber is a type of specialty synthetic rubber—a linear polysiloxane elastomer with a Si–O–Si backbone. It exhibits excellent properties such as heat resistance, cold resistance, ozone resistance, radiation resistance, electrical insulation, and biocompatibility, and has found widespread applications in industries including aerospace, electrical and electronic engineering, chemical instrumentation, automotive, and mechanical engineering, as well as in medical and healthcare fields and everyday life. Due to the high molecular flexibility of raw silicone rubber and the weak intermolecular interactions among its chains, its tensile strength is very low (not exceeding 0.4 MPa), rendering it practically useless without reinforcement; therefore, the addition of reinforcing fillers is the primary approach for enhancing its physical and mechanical properties. After reinforcement, the crosslinking structure of silicone rubber becomes more diverse, leading to a substantial improvement in its strength.
The most commonly used reinforcing filler for silicone rubber is precipitated silica (5102), which also delivers the best reinforcement performance. Today, more than thirty commercial grades of reinforcing precipitated silica are available for dimethyl‑based liquid silicone rubbers, with manufacturers including Degussa, Wacker, CABOT, and the Shenyang Chemical Plant. Depending on the preparation method, precipitated silica can be classified into two main types: gas‑phase silica and precipitation‑method silica. However, regardless of the type, compatibility issues with silicone rubber persist—specifically, the addition of the filler leads to a sharp increase in compound viscosity. When high flowability and transparency are required, using precipitated silica as a filler proves inadequate.
White carbon black, when used as a reinforcing filler, can significantly improve the mechanical properties of rubber. However, silicone rubber reinforced with white carbon black exhibits somewhat reduced transparency, and as the loading of white carbon black increases, the compound’s viscosity rises sharply, leading to processing difficulties and adversely affecting subsequent application processes. Consequently, such materials cannot meet the requirements of products that demand both high strength and excellent flowability. In this context, employing MQ resin as a reinforcing agent for silicone rubber represents a superior alternative. MQ resin demonstrates excellent dispersibility and compatibility within silicone rubber and can achieve reinforcement through chemical bonding with the polymer matrix.
Compared with precipitated silica, MQ resin offers the following advantages:
(1) MQ resin is composed of both organic and inorganic components and has a structure very similar to that of the base constituents of silicone rubber. This unique structural feature endows it with excellent compatibility with silicone rubber, and its addition does not significantly affect the viscosity of the system. Wang Xinxin compared the reinforcing effects of MQ resin and precipitated silica on room‑temperature vulcanizing (RTV) liquid silicone rubber; the results showed that, at comparable mechanical strength, products reinforced with MQ resin exhibited superior viscosity and adhesion performance relative to those reinforced with precipitated silica.
(2) MQ resin consists of M and Q units, and its properties can be readily tailored by varying the substituents on the M units.
MQ resin, as a reinforcing material, can be used either alone or in combination with other reinforcing agents. Factors influencing the reinforcement provided by MQ resin include the M/Q ratio and the type of functional groups. The M/Q ratio determines the compatibility between MQ resin and silicone rubber, while the nature of the functional groups affects the strength of the chemical bonds formed between them, thereby impacting the mechanical properties of the silicone rubber.
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