AEROSIL® R 8200 offers excellent reinforcement with low thickening behaviour in silicone formulations

Blog Archive | 10 mintues  | Author: Adam Morgan , Ph.D.

High performance fumed silica fillers in silicone rubber systems

Synthetic fumed silica grades have been used as reinforcing powders in silicone systems since the 1940’s. Fumed silica, in particular, has contributed to the success of silicone rubber. Initially, hydrophilic grades were the only option but then hydrophobized grades became available for more demanding applications.

This article gives an overview of the latest generation fumed silica for silicone rubber – AEROSIL® R 8200. This hydrophobic silica is modified and treated via a series of physical process steps, which result in its characteristically low thickening effect and high level of reinforcement - when compounded into silicone rubber.

Particular focus will be paid to its use in two-component room temperature curing silicone rubber (RTV-2C), RTV-1 silicone sealants and in liquid silicone rubber (LSR) formulations.

 

Synthetic silica powders in silicone rubber

In addition to enhancing the mechanical strength of cured silicone products, silicas often give undesirable thickening properties as well. These can result in the “crepe hardening” effect in Heat Cured Rubber (HCR). Silicone compounds that have not been cross-linked become unworkable within just a few days of storage, making it difficult or impossible to process them further. For RTV systems it is crucial to get the correct balance of thickening and reinforcing effects.

It is possible to mitigate the effect of crepe hardening by hydrophobizing silanol groups on the fumed silica in-situ, during the compounding stage. However, the condensation by-product of water must be adequately removed from the compound, since it would seriously impair the properties of the cured compound. This is a laborious, time consuming and energy-intensive approach that also generates the need to treat the exhaust air generated.

EVONIK have been offering solutions to the above problem for many years – hydrophobically modified fumed silica. AEROSIL® R 972 is a dimethyl- dichlorosilane treated fumed silica, which was introduced in 1962. Since then various new developments have been released for silicone rubber including AEROSIL® R104, R106 and R 812S – all of which posses strongly hydrophobic surface treatments resulting from the modification with trimethylsilyl groups. These grades are highly effective options for the production of peroxide, condensation and additive cross-linked silicone compounds. For more difficult applications, there is AEROSIL® R 8200…

 

Physico-chemical properties of AEROSIL® R 8200

A comparison of the physical properties of AEROSIL® R 8200 with other fumed silica grades used in silicone rubber is given in Table 1.

Table 1. Physico-chemical data of AEROSIL® R 8200 in comparison to AEROSIL® 200, R 974, R 104 and R 812 S.

 

AEROSIL® R 8200 has a particularly high carbon content, which results from the intensive hydrophobization process. Only AEROSIL® R 812 S has a similar level of hydrophobicity.

Another way in which AEROSIL® R 8200 differs from the other grades is in its much higher tamped density. This results in lower dusting and reduces addition time which leads to increased productivity on the plant. Of course, the main difference in application is the much lower level of thickening it produces / unit loading - allowing for much higher loadings to be achieved. This will be explored in greater detail below.

 

Properties of RTV-2C silicone rubber containing AEROSIL® R 8200

Some of the key advantages of using two-component silicone rubber systems are easy processing at room temperature and the ability to precision mould. Low viscosity characteristics of the polymer are therefore important and any filler added must not significantly impact upon this. Fumed silica is necessary however, in order to achieve a high level of mechanical strength in the crosslinked compound. AEROSIL® R 8200 is the only AEROSIL® grade to meet these requirements.

Figure 1 illustrates viscosity build relative to addition levels of two AEROSIL® grades, R 8200 and R 812S. Both are surface treated with trimethylsilyl chemistry. At only 20% loading the viscosity of R 812 S is 10 times higher than that of AEROSIL® R 8200. Hydrophilic AEROSIL® grades, such as AEROSIL® 200, would give a comparative viscosity of several thousand Pascal seconds at this loading.

 

Figure 1. Relationship between the viscosity of RTV-2C silicone formulations and the filler loadings.

 

Tear resistance improves sharply with addition levels above 15%. At this concentration one achieves a “locking effect”. Increasing the loading further sees a continued but more gradual increase (Figure 2).

Figure 2. Rheological and mechanical properties of an RTV-2C formulation containing AEROSIL® R 8200 at different addition levels.

 

A comparison between the properties of a RTV-2C silicone rubber containing AEROSIL® R 8200 vs. one containing AEROSIL® R 812 S is given in Table 2.

Table 2. Properties of RTV-2C formulations based on AEROSIL® R 8200 and AEROSIL® R 812 S respectively, at 20% silica loading.

 

Properties of RTV-1 silicone sealants containing AEROSIL® R 8200

Because of the extremely low thickening effect, AEROSIL® R 8200 can be used to full advantage in RTV-1 silicone sealants for the production of self-levelling rubbers e.g. for coating textile-based building materials. AEROSIL® R 8200 also displays a particularly low yield point in such formulations when compared with other fumed silica grades.

Mechanical properties of the rubber are of the same order however, despite the low thickening effect (Figure 3).

Figure 3. Comparison between the rheological properties of a RTV-1C formulation based on AEROSIL® R 8200 and other types of AEROSIL®. Filler content is 8%.

 

Increasing the filler loading into the range of 14-20% gives sealant formulations with comparable viscosities to commercially available silicone sealant compounds, but with the added benefit of high mechanical strength after the cross-linking process is complete (Table 4).

Table 4. The relationship between the properties of an RTV-1C formulation containing AEROSIL® R 8200 at different filler loading levels.

 

Properties of Liquid Silicone Rubber (LSR) containing AEROSIL® R 8200

As with the other systems, AEROSIL® R 8200’s unique processing allows it to impart a very low thickening effect in LSR. The behaviour of AEROSIL® R 8200 is compared with AEROSIL® R 812 S using flow curves in Figure 4.

Figure 4. Comparison between the flow of viscosity of a LSR formulation containing AEROSIL® R 8200 and one containing AEROSIL® R 812 S.

 

Further detailed information concerning rheological, mechanical and optical properties of AEROSIL® R 8200 and R 812 S can be found in EVONIK’s Technical Information 1253 (8).

 

In conclusion and what to do now if you are interested in evaluating AEROSIL® R 8200

AEROSIL® R 8200 is a high-performance fumed silica for various flexible silicone systems. It offers very low viscosity build with addition level and excellent reinforcement at higher loading - where conventional silica grades simply impart too much viscosity to be workable.

If this article was of interest to you, and you would like to evaluate a sample of AEROSIL® R 8200, then please request one through our online sample request page. Lawrence Industries’ technical sales team are available on 01827 314151, to answer any questions you may have. For any specifics on the experimental details, please get in touch.

Author: Adam Morgan , Ph.D.

Adam studied chemistry at the University of Warwick for 8 years, where he obtained a Ph.D. in the field of polymer and inorganic colloid science. He has been with Lawrence Industries since 2014 as a technical sales manager covering all industry areas. He is now responsible for marketing within the company as well.