The Silicas segment is led by Evonik Resource Efficiency GmbH and brings together Evonik’s activities in specialty chemicals for industrial applications. Versatile Raw Materials for Personal Care Formulations AEROSIL® R is a structure modified with hexamethyldisilazane after treated fumed silica. Applica. AEROSIL® R Hydrophobic fumed silica. Evonik Industries AG | Product information AEROSIL® R | Aug Page 1/2. Properties and test methods .
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Rubber World — August It has been more than 60 years since Degussa developed fumed silica ref. InDegussa started to develop a white filler material for the tire industry as aerosip alternative to carbon black filler which could only be produced by oil. During the same period, polysiloxane materials were produced in the early s by a joint venture of Corning Glass and Dow Chemical. Degussa developed different grades of hydrophilic fumed silica in the s, followed by hydrophobic fumed silica treated with dichlorodimethyl silane DDS in the s and hexamethyldisilazane HMDS and polydimethylsiloxane silicone oil in the s.
The development of hydrophobic fumed silica modified with octametylcyclotetrasiloxane D4 was established in the early s. From the s to present, the fumed silica with structure modification has been introduced. The applications of these grades of fumed silica will be discussed in detail in silicone rubbers and MRG rubbers in aerpsil following sections. Production of fumed silica The fumed silica pyrogenic silica process, so called Aerosil process, was first patented at Degussa by Areosil Kloepfer in ref.
The reaction is shown as follows: The byproduct is gaseous hydrogen chloride which is separated from fumed silica solid matter in the process. The primary particles are not present in an isolated state but are formed as chain-like, branched aggregates due to the fusion of spherical primary particles. The agglomerates were formed due to collision of aggregates. The surface of fumed silica consists of the three-dimensional network of siloxane groups -Si-O-Si- and silanol groups -Si-OH located on the surface of silica.
The molecular modeling of fumed silica with Si-OH density of approximately 1. Moisture can be absorbed and interacted with silanol groups; therefore, fumed silica has hydrophilic characteristics and can be wetted by water. Besides the surface modifications, the fumed silica can be structurally modified in the process.
Table 1 shows the various types of fumed silica with surface and structure modifications from Evonik Degussa which can be used in silicone and MRG rubbers.
Their physico-chemical data are also included. All of the fumed silica grades except Aerosil OX50 and Aerosil 90 will be examined throughout this article. The silanol groups of untreated fumed silica hydrophilic figure 2 on the left can form a three-dimensional network due to developed strong hydrogen bonding which often gives undesirable thickening properties and a creep hardening effect in silicone products. To avoid these problems, use of the hydrophobic fumed silica or in-situ treatment of hydrophilic fumed silica with silane is recommended.
The surface of hydrophobic fumed silica figure 2 on the right shows low hydrogen bonding interaction because hydroxyl groups on the hydrophilic fumed silica surface are replaced by dimethylsilyl groups of silane D4 or DDS.
The extent of replacement controls the degree of hydrophobicity of surface treated silica. The water adsorption of hydrophobic fumed silica is greatly reduced compared to that of hydrophilic fumed silica. Fumed silica can also be treated with bifunctional silanes to introduce functional groups such as vinyl, methacryl, epoxy and amino groups onto the surface to enhance silica-polymer interactions. These fumed silica products will not be discussed herein.
The Aerosil fumed silica process is also used for manufacturing other fumed oxides, such as titanium dioxide or aluminum oxide. The details of the fumed titanium dioxide can be found in the literature refs. Fumed silica in silicone rubbers The silicone rubbers are polyorganosiloxanes in which the polymer backbone consists of alternating silicon and oxygen atoms with organic groups attached to silicon. Silicone rubbers can also be classified into three types based on curing mechanism and curing condition: However, silicone rubbers have low vulcanizate tensile strength 1 Mpa when crosslinked and poor hydrocarbon oil and solvent resistance.
To improve mechanical properties such as tensile strength, modulus and tear strength, reinforcing fillers such as fumed silica or precipitated silica are often required in silicone rubbers. Fumed silica is the most common reinforcing filler used in silicone rubbers. Table 2 compares physico-chemical properties of fumed silica and precipitated silica refs.
The moisture content and silanol group density of fumed silica are less than those of precipitated silica. Fumed silica imparts not only reinforcing properties, but Also the necessary rheological thickening control, optical property transparency and electrical insulation property of silicone rubbers. Because of many advantages that silicone rubbers possess, silicone applications are diverse.
HTV and LSR used in the automotive industry include shaft sealing rings, spark plug caps, o-rings, gaskets, ignition cables and coolant and heater hoses for buses and trucks. Silicones are also used for corona-resistant insulating tubing, keyboard and contact mats, and transfusion and dialysis tubing. RTV rubbers are widely used for seals and caulking and for flexible mold making applications ref.
Precipitated silica and carbon black will be compared with fumed silica in the next MRG section. All fillers tested in this article are shown in table 3.
AEROSIL R – MITM
At equal filler loading, the tensile strength is improved as BET surface area increases. This is attributed to the fact that at higher surface area, hydrophilic fumed aeroeil is aeeosil readily dispersed in the silicone matrix. The use of additional processing additive silicone oil terminated with hydroxyl groups can further improve the tensile strength. Silicone oil acts as a hydrophobizing agent to the fumed silica, resulting in better polymer-filler interaction.
Elongation at break and hardness are also affected by the BET surface area figures 4 and 5. Tear resistance increases as a function of BET surface area, as seen in figure 6. To asrosil good tear resistance, higher surface area fumed silica is suggested. In addition to hydrophilic silica, different grades of hydrophobic silica in tables 1 and 3 were also investigated in the HTV formulation containing 6 aerosli processing additive, as shown in table 4 ref.
It can be seen in figure 7 that Williams plasticity, which is a measure of viscosity of uncured silicone compounds, has higher value in the hydrophilic silica containing compound than that in the hydrophobic silica containing compounds. The higher viscosity is attributed to the strong filler-filler interaction from hydrogen bonding of silanol groups on the surface aerosiil hydrophilic fumed silica.
This hydrophilic silica-containing silicone compound becomes stiff within a few days and is not easy to process. The removal of by-products such as hydrochloric acid or other aaerosil products resulting from in-situ treatment of DDS or HMDS is not necessary if hydrophobic fumed silica is used. In figure 8, the tensile strength of both hydrophobic containing silicone compounds is slightly higher than the hydrophilic containing silicone compound. The reduction of filler-filler networking due to hydrophobicity results in lower aaerosil and higher elongation at break.
Fumed silica in LSR Liquid silicone rubbers LSR are made of a two-component, platinum addition cure system that can be injection molded and cured with very fast cycle times, at elevated temperatures. The two components, A containing silcone polymer with vinyl groups, filler, platinum as catalyst and inhibitor and B containing silicone polymer with vinyl groups, filler and crosslinker with silicon hydride groups generally require mixing in a ratio of 1: The vulcanization mechanism of addition cure is based on hydrosilation under catalysis with a very small amount of Pt compound.
There are no by-products released during vulcanization of LSR, unlike peroxide cure. Fillers used in LSR compounds should provide high reinforcement and low rheological properties.
In some LSR applications, aerosill optical property such as transparency is also required. Fumed silica is used to provide both high mechanical properties and transparency. Precipitated silica is used in products such as keypads, whose low compression set is required. The Aerosil R fumed silica is not only hydrophobic or HMDS treated, but it is also structurally modified via an additional processing step. The Aerosil R S fumed silica is hydrophobic or HMDS treated whose trimethylsilyl groups are chemically reacted on the surface of fumed silica.
The Aerosjl formulation tested is a one-component system for easier handling instead of two components, as used in practice. As demonstrated in figure 9, the viscosity increases as the r820 loading increases.
The mechanical properties such as tensile strength and tear strength are exhibited in figures 10 and 11, respectively. As filler loading of both fumed silica grades increases, their tensile strength and tear strength are enhanced. This indiCates that the special structural modification and the lower aeorsil area of Aerosil R allow the Aerosil R compounds to maintain low viscosity at higher loading and still gain higher mechanical properties compared to the Aerosil R S compounds.
Recently, Evonik Degussa has also developed new grades of structure-modified fumed silica to enhance transparency properties of silicone rubbers ref. Through a proprietary process, the structure of fumed silica aggregates and agglomerates is altered without affecting the average particle size. This process leads to easier dispersion of the particles and lower thickening viscosity effect e8200 the silicone rubber systems.
Transparency in silicone rubbers is directly correlated to the surface xerosil of fumed silica. As seen in figure 12, transpar Ency increases as surface area increases. The goal of development of SP fumed silica is to enhance transparency or clarity without increasing surface area. It can be seen that AerosiL SP exhibits higher transparency than Aerosil at similar surface area. The arrosil effect is seen in Aerosil SP versus Aerosil It should be noted that the SP fumed silica can reinforce silicone rubbers similar to the conventional fumed silica.
This is because the type of aerlsil used is moisture cure and the high moisture content in precipitated silica could affect the vulcanization process. In addition to reinforcing properties, other quality characteristics of silicone sealants such as processability, dispersion surface smoothness and shelf life are very important.
Figure 13 demonstrates aeroskl yield point as a function of storage time for different alkoxy-cured silicone sealant compounds. Yield point is the yield stress that is required to cause thixotropic material to flow. Qerosil sag resistance characteristic ability to remain in place until a small force is applied is especially important for sealants that are applied to vertical surfaces or to surfaces where they can sag under their own weight.
However, yield point should not be too high to cause sealant to flow when it is applied on to surfaces. In figure 13, hydrophilic Aerosil sealant compound exhibits the highest yield point compared to other grades of hydrophobic silica at given storage times.
At the longest storage time 56 daysthe yield point of the Aerosil sealant compound was increased at the highest level compared to after one day. However, all grades of the hydrophobic fumed silica show lower yield point even at long storage times, leading to longer shelf life. This can be explained that the ability of moisture Adsorption of hydrophilic fumed silica is much higher than that of hydrophobic fumed silica and causes filler-filler networking during storage, resulting in high yield points.
Aerosl silica in mechanical rubber goods Precipitated silica aeroisl for the majority of silica aersoil consumed in rubber applications which include tires, mechanical rubber goods and footwear.
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They exhibit high mechanical properties and abrasion resistance. Their main applications are the automotive, mechanical engineering and oil industries. The automotive market is the largest consumer for HNBR used for dynamic and static seals, hoses and belts. The investigation of HNBR containing different fillers at 60 phr loading was studied by Friehmelt ref.
The Mooney viscosity of the unmodified experimental precipitated silica and Aerosil VS compounds and is higher than that of the Aerosil R V and N Compounds 95 and Durometer A hardness of the experimental precipitated silica and Aerosil VS is in the range of 80, whereas, durometer A hardness of Aerosil R V and N is about The strong silica-silica network of the experimental precipitated silica and Aerosil VS can attribute to high Mooney and stiffness.