Fumed Silica Filler – Why so Popular?
SpecialChem | Edward M Petrie - Jun 20, 2011
Introduction
Fumed silica is a popular filler that is used in many adhesive and sealant formulations. Even though fumed silica is not a very good extender for minimizing cost, modifying thermal expansion coefficients, or significantly improving cured strength, it is one of the most widely used fillers in a variety of applications from structural adhesives to pressure sensitive adhesives. Fumed silica, an amorphous silicon dioxide, is a versatile, efficient additive used in adhesive and sealant formulations primarily for flow control and thixotropy. Fumed silica has long been the dominant thixotrope employed in the adhesive and sealant industry.
This article will explore why fumed silica is so widely used and valued by the formulator. The origin and various types of fumed silica will also be described. The property modifications that are possible will be identified with the focus being on thixotropy and rheological properties. Starting formulations will be provided for several adhesive and sealant products.
Origin and Characteristics
Silica is an abundant mineral found in crystalline form (quartz) and amorphous form (diatomaceous silica). Diatomaceous silica is used more extensively than quartz because it is a softer material providing less machining and abrasive problems. There is also concern over respiratory problems possibly associated with inhalation of finely divided quartz. Although it is an excellent additive for increasing viscosity, diatomaceous silica has a low oil adsorption and very large surface area so that it is not a particularly good extender since it cannot be easily incorporated into the formulation in high concentrations.
Fumed silica is produced by the vapor-phase hydrolysis of silicon chlorides in a hydrogen-oxygen flame or an electric arc furnace. Fumed silica is sometimes also referred to as pyrogenic silica. The high temperature creates silicon dioxide molecules which then condense to form discrete particles which can attach themselves to one another due to the high temperature of the process. The result is a three-dimensional branched aggregate.
Fumed silica is naturally hydrophilic due to the silanol groups on the particle surface. This accounts for its high surface energy and good wetting properties. However, fumed silica particle can be "treated" to provide a number of hydrophobic grades. In these processes the silanol groups on the surface are generally replaced with organosilicon groups.
The surface chemistry of fumed silica is extremely important because of its influence on the rheological behavior of the formulation. Three types of chemical groups can be formed on the surface of the particle depending on the processing procedures:
1.Isolated hydroxyl,
2.Hydrogen bonded hydroxyl, and
3.Siloxane
The isolated and the hydrogen bonded hydroxyl groups are hydrophilic sites, whereas the siloxane is a hydrophobic site. Thus, fumed silica grades are generally characterized by their surface area and whether they are hydrophilic (standard grade) or hydrophobic. The hydrophilic silica is most effective in nonpolar and medium polar media. The hydrophobicity of treated fumed silica results in lower adsorbed moisture on the silica. This makes it ideal for use in systems where moisture sensitivity is important and must be maintained low (e.g., moisture cure urethanes and silicones). The performance of hydrophilic fumed silica is often improved by adding a polar substance such as ethylene glycol, glycerin, or some secondary amines to the formulation. In medium polar to polar media, hydrophobic fumed silica is a more efficient thickening agent and generally
preferred. Comparative sag resistance properties of various commercial types of fumed silica in a liquid epoxy adhesive system are shown in Table 1.
Cab-O-Sil PTG Hydrophilic 60 60 45 26
Aerosil 200/Aluminum Oxide C Hydrophilic 60 40 25 13
Aerosil COK 84 Hydrophilic 60 60 40 16
Cab-O-Sil N70-TS Hydrophobic 60 60 60 60
Aerosil R972 Hydrophobic 60 30 25 16
Table 1: Comparative Sag Resistance Properties of Fumed Silica in Liquid Epoxy 1
Fumed silica is typically available with sizes in the 7-40 nanometer range and surface areas ranging from 50 to 380 m2/g. Unlike precipitated silica, fumed silica has no internal surface area. The specific gravity of fumed silica is approximately 2.2. Because of its high surface area to weight ratio, formulations generally require only a little fumed silica (1% to 5% by weight) to achieve thixotropic properties.
Thixotropy
Although most fillers provide adhesive and sealant systems with viscosities that are unaffected by shear rate, certain fillers can provide thixotropy which results in an adhesive that will not flow under low levels of stress (e.g., under its own weight when applied to vertical surfaces). Yet the compound will exhibit lower viscosities when under higher levels of stress such as when being dispensed or applied to a substrate.
Thixotropy provides a shear thinning effect; that is, viscosity decreases as shear rate increases, and vice versa. This not only allows easy pumping, dispensing, and mixing of the adhesive, but also provides sag resistance once the adhesive is applied. The thixotropic fillers work by forming a temporary "structure" in the mixture, which can be broken down at high rates of shear. This structure is generally the result of van der Waals forces between molecules. Viscosity decrease occurs when this structure breaks down due to shearing stress and the resistance to flow decreases. (See Figure 1)
Figure 1: Thixotropic structures
Thixotropy can be obtained at fairly low loading concentrations with colloidal silica, bentonite, metallic leafing powders, and hydrated magnesium aluminum silicates. If required, thixotropic adhesive pastes may be formulated which will not flow during cure even at elevated temperatures
and which are useful for bonding loose fitting joints.
The addition of asbestos fibers at one time provided excellent thixotropic adhesive formulations, especially at elevated temperatures. However, health and environmental regulations have severely limited the use of this material. Today, fumed silica, precipitated calcium carbonate, certain clays, and cellulose and other fibers offer thixotropic properties at relatively low levels of loading.
The thixotropic characteristics provided by fumed silica are due to its ability to develop a loosely woven, lattice-like network by hydrogen bonding between particles. This network raises the apparent viscosity of the system, increases the cohesive forces, and contributes to the suspension of the solid. Because the hydrogen bonds themselves are relatively weak, they are easily disrupted through the action of an applied stress or shearing force and quickly reformed when the stress or shearing force is removed.
Formulation
Fumed silica is generally incorporated at concentrations of less than 10 pph. Adhesive and sealant systems based on lower viscosity resins generally tend to hold thixotroping action better at elevated temperature than systems based on the higher viscosity resins.
Parameters that are important to the performance of fumed silica systems include: •the nature of the system (polarity)
•concentration of the silica
•grade of silica used (particle size, surface area, density, surface chemistry, etc.)
•degree of dispersion
•presence of additives in the formulation other than the fumed silica.
At times the results of the formulation are less than expected because these factors are not considered or understood relative to the final rheological properties. For example, proper dispersion can maximize the efficiency of both hydrophilic and hydrophobic fumed silica. To ensure proper dispers
ion, addition of silica to the formulation in the right sequence, as well as effectiveness of the dispersion equipment becomes very important.
High shear mixing equipment will improve the efficiency of fumed silica in the formulation. In most cases, silica should be added to the resin directly or to the more viscous part of the formulation with as little solvent or diluents as possible. Incorporating the silica before adding any fillers or pigments assures homogeneous distribution. Dispersing the silica in a concentrated base, or master batching, provides optimum efficiency and stability.
The fumed silica also raises the effective viscosity of the base resin to prevent other components from settling while the extrudability or spreadability is unaffected. It also should be noted that fumed silica provides a surface that is free of texture. This is important in architectural grade paints and sealants.
Fumed silica is often used in 100% solids, liquid polymers. Fumed silica is often found in adhesive formulations containing base polymers of epoxy, urethane, or silicone. 2With epoxy adhesives and sealants only a few percent by weight of the additive will eliminate common problems such as slumping and separation. Table 2 shows a starting formulation for several thixotropic two-component, room temperature curing epoxy adhesives. Note that only about 2% of fumed silica is required to make both components thixotropic.
68 50.5
Part A DGEBA epoxy
(EEW=190)
Reactive diluent    7 Talc
29.5 40 Fumed silica thixotrope
2.5    2.5 Part B Cycloaliphatic amine
40  Polyamide
53 Aluminum powder
24 20 Talc
34 25 Fumed silica thixotrope    2    2 Cure
reactive diluentCure schedule
30 min at 150°C 30 min at 120°C Mix ratio 2A:1B by weight 1A:1B by
weight
Properties on Aluminum Lap shear @ 25°C 1734 psi 1640 psi Lap shear @ 120°C 396 psi 175 psi T-peel @ 25°C    4 pli 14 pli
Table 2: Starting Formulation for a Rigid and a Flexible 2K Thixotropic Epoxy
Adhesive
The fumed silica also raises the effective viscosity of the base resin to prevent other components from settling while the extrudability or spreadability is unaffected. It also should be noted that fumed silica provides a surface that is free of texture. This is important in architectural grade paints and sealants.
Fumed silica has also been effective in a variety of sealant systems including butyl, silicone, urethane, and MS Polymer (silyl-terminated polyether) sealants. Table 3 provides an MS Polymer sealant formulation that uses fumed silica as a thixotrope.
MS Polymer
100 Plasticizers
50 Filler (calcium carbonate)
120 Pigment
20 Fumed silica
5 Dehydration agent (organosilane) 2 Adhesion promoter
3 Hardening catalyst (organo-tin)    1.5
Total
301.5 Table 3: MS Polymer Sealant Formulation With polyurethane systems, the thixotropic gels formed with ordinary fumed silica are at times unstable when shipped and stored due to reaction of the surface hydroxyls with the isocyanate groups. However, this problem can be eliminated by using silane treated fumed silica or adding small percentages of polyoxypropylene prepolymer to the formulation.
Fumed silica has also been used in pressure sensitive and hot melt adhesives to change the rheologic
al properties and to enhance physical properties. In hot melt systems, fumed silica has been incorporated into resins such as polystyrene blocked copolymer and butyl rubber. Five percent of added fumed silica (provided that it is dispersed properly) provides an increase in shore hardness, tensile strength, and elongation without the loss of peel strength.
In waterborne pressure sensitive adhesives, the use of fumed silica dispersion is generally better than using the equivalent fumed silica powder. Both products will make a significant impact in the improvement of shear strength while having moderate or little impact on other pressure sensitive adhesive tests. 3
Table 4 illustrates the enhanced properties obtained with the use of fumed silica in a Kraton (styrene butadiene block copolymer) based pressure sensitive adhesive without the loss of adhesion or tack. The temperature resistance, tensile, and elongation values improve with as little as three percent addition of the silica.
Film thickness, mils    1.5    1.5
Specific gravity 0.93 0.93
Rolling ball tack, cm 0.5 0.5
Probe tack, kg    2.0    2.1
Shear adhesive failure temperature, deg F 190 190
Tensile strength, psi 700 780
Elongation at break, % 660 680
Hardness, Shore A 61 68
Peel temperature, deg F 150 190
Creep: 240 deg F hold, mins 100 180
Table 4: Properties of Pressure Sensitive Adhesive with and without Fumed Silica. (Adhesive based on styrene butadiene copolymer.)
References
1.Katz, H.S. and Milewski, eds., Handbook of Fillers for Plastics, van Nostrand Reinhold,
New York, 1987, p. 180.
2.Wen, J. "Fumed Silica Control Rheology of Adhesives and Sealants", Adhesives and
Sealants Industry, October 2000.
3.Conn, R., "Fumed Silica Use in Pressure Sensitive Adhesives", PSTC Technical Seminar,
2009.

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