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More Than You Ever Wanted To Know About Fumed Silica

Progressive Epoxy Polymers, Inc. www.epoxyproducts.com

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Paul Oman, MS, MBA - Progressive Epoxy Polymers, Inc.

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Note: Cab-o-sil fumed silica is a registered trademark of Cabot Corporation

Note: Aerosil fumed silica is a registered trademark of Degussa Corporation

Progressive Epoxy Polymers, Inc. Sells only Aerosil R202 from Degussa Corporation

Hydrophobic Aerosil® in a Highly-Filled Epoxy System
Degussa Corp. Technical Library Document GP-89

Epoxy resin systems are used in a wide variety of applications including industrial flooring, solders, foams, high way surfacing and patching materials, and adhesives because it combines toughness, flexibility, adhesion, and chemical resistance. Many of these systems contain various amounts of fillers and extenders which add weight and volume to the formulation. Fumed silica is a thixotropic additive which when dispersed into the epoxy resin increases viscosity, imparts thixotropic behavior and adds anti-sag and anti-settling characteristics before and during the potlife of a epoxy-amine adhesive.


Aerosil® 300 fumed silica is a fluffy white powder made up of nanometer—sized particles with silanol groups on the surface. Hydrogen bonding resulting from hydroxyl interaction on the surface of adjacent Aerosil® 300 particles is the mechanism creating thickening and thixotropy. However, in an epoxy—amine system, this mechanism is destabilized by the generation of hydroxyl groups during the cross-linking reaction. This interferes with the hydrogen bonding and results in large decreases in viscosity during pot-life. More recently, Degussa s hydrophobic fumed silicas, Aerosil® P202 and Aerosil® R805, have been replacing hydrophilic fumed silicas in this application.


Degussa Corporation produces hydrophobic fumed silica by treating the surface of hydrophilic Aerosil® to render it water repellent. Degussa s Aerosil® P202 is treated with polymethylsiloxane (silicone oil) and Aerosil® R805 is treated with trimethoxyoctylsilane. Degussa Corporation produces fumed silica using a continuous process. This patented process produces a more consistent hydrophilic product and a uniformly treated hydrophobic product. Both hydrophilic and hydrophobic grades are amorphous. Aerosil® contains no crystalline components. This statement is confirmed by x—ray analysis, electron diffraction, and luminescence measurements.

Degussa Corporation s Applied Technology Center has designed an experiment to evaluate our Aerosil® line of fumed silica in a highly filled two part epoxy—amine adhesive system. Using a basic epoxy—amine system of:


Part A Parts by Weight
Reichhold EPOTUFE® poxy resin 37—100 100
100% Non—Volatile Resin

Vanderbilt Nytal® 99 30
Hydrous Calcium Magnesium Silicate
weight percent of Part A
Aerosil® R2O2 4, 2, 1
Aerosil® R8O5 4, 2, 1
Aerosil® 300 4, 2, 1

Part B Parts by Weight
EPOTUF® Epoxy resin hardener 37—612 45
Reactive polyamide - 100% non—volatile

EPOTUF® Epoxy resin hardener 37—633 5
Modified polyamide - 100% non—volatile

Vanderbilt Nytal® 99 20
Hydrous Calcium Magnesium Silicate


Various levels of Aerosil® P202, Aerosil® R805 and Aerosil® 300 were dispersed directly into the epoxy resin using a Ross Powermix which is a combined Cowles dissolver and planetary mixer. After initial wet out, each Aerosil® was dispersed with the Cowles blade rotating at a speed of 3000 rpm and the planetary blade revolving at 70 rpm for 5 minutes. These conditions are optimum for the dispersion of Aerosil®. However, if a PowerMix—type instrument is not available, a Cowles dissolver rotating at 3000—5000 rpm for 5-10 minutes is usually sufficient shear to disperse Aerosil®. Also be aware that with increased dispersion time there will also be an increase in heat generated. This will reduce the viscosity of the resin and eventually decrease the shear generated in the system. Therefore, it is more favorable to generate high shear for a short period of time, than to generate low shear for a longer period of time. All mixtures were then immediately deaerated under 25 In. Hg vacuum pressure. Each mixture was then stored at 6O~C for 4 weeks. Aerosil® P 202 was observed to have excellent suspension characteristics because of the silicone oil surface treatment. This surface treatment does not adversely effect the length of the potlife. Aerosil® R805 and Aerosil® 300 (at 4%) also exhibited excellent viscosity control over the four (4) week test.

Within a week, hard settling was observed in samples containing 1% and 2% Aerosil® 300 and 1% Aerosil® P805. No hard settling was observed in any sample containing Aerosil® P202.

Degussa Corporation recommends the addition of Aerosil® R202 directly into the epoxy portion of the two—part adhesive. Usually, a starting concentration of 2%, by weight of the total formulation, is sufficient to generate the desired viscosity. However, some systems may require 4% Aerosil® R2O2. If a hydrophilic fumed silica is presently being used, you can maintain your present viscosity range using 3/4 as much Aerosil® R202. This will increase the viscosity, thixotropic index, and improve suspension characteristics of the formulation.

Degussa Corporation also recommends Aerosil® R805 as very efficient thickener and thixotrope in a highly-filled epoxy system. While beginning recommendation for a loading level are similar to that of Aerosil® R202, more Aerosil® P805 will be needed to generate the same viscosity.

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