One of the best and most popular thixotropes is fumed silica.^1,2 It has the advantages of being an effective rheological control agent, which will not undergo swelling and which exhibits chemical inertness. This article gives a brief review of the use of fumed silica in adhesives and sealants as thixotropes with emphasis on three areas: basic rheological properties of adhesives and sealants, physical and chemical nature of fumed silica, and the use of fumed silica as thixotrope in general.

Basic Rheological Concepts in Adhesives and Sealants

The most important and basic concept in this science is viscosity. Simply stated, viscosity, h, is a resistance to flow, which can be best defined with a so-called “deck of cards model” for a liquid under shear as shown in Figure 1. A liquid at rest will not flow or move unless a force acts upon it. This force is a stress when defined as acting over a unit of area in the liquid. The resulting motion generates a shearing action between the moving portions of the liquid and their stationary neighbors.

The force causing the motion in a liquid in rheological terms is called a shear stress and quantifies the magnitude of the force. The resulting motion is called shear rate and quantifies the relative speed of motion. Viscosity can now be quantified as the ratio of the shear stress to the rate of shear.

Some commonly observed types of flow behavior are shown in Figure 2. The plots of shear stress against shear rate are often called flow curves and are used to express the rheological behavior of liquid polymer systems. Ideally viscous systems are shown in this plot as a straight line (line 5) and exhibit so-called Newtonian flow behavior. Since viscosity is equal to the slope of the flow curve, it remains constant for Newtonian systems. Most real-world systems often deviate from this linear behavior and exhibit non-Newtonian behavior. They are shown by curves (lines 1, 3, 4, 6) in Figure 2, including shear thinning or pseudoplastic (with and without yield stress), shear thickening or dilatent (with and without yield stress) and Bingham Newtonian behavior (line 2).

It should be pointed out here that the existence of yield stress is still controversial although it has been widely used. To properly understand a non-Newtonian system, the viscosity must be measured over a wide range of shear rates because it is dependent on shear rate. Many adhesives and sealants systems exhibit pseudoplastic behavior.

Thixotrope is derived from Greek and means to “change by touch.” When a system is at a constant shear rate and the viscosity falls over time, this is known as “thixotropic behavior.” It is more than just shear thinning or pseudoplasticity. Thixotropic behavior allows a system to have a higher viscosity at rest.

When an adhesive or sealant application is made, the shear rate induced by mixing, pumping and application will reduce the viscosity, allowing the material to flow more easily. Simple mixing generates shear rates of from 10 to 100 sec-1, and pumping can be higher depending on the design of the pump. When the application is finished and the shear rate falls off to only that induced by gravity, the viscosity will build up again to help hold the material in place until it has cured.

Thixotropic behavior is generally caused by the breakdown of a loose internal structure in the system by the shear stress. If the stress is reduced or removed, the structure begins to rebuild, causing the viscosity to rise. Thus, the shear history of the system is very important.

If the viscosity of such a system is measured at a particular shear rate, with the internal structure at its maximum, the viscosity will decrease the longer the system is held at that shear rate. Also, if the measurement is repeated before the structure is rebuilt to the original level, a lower value will be obtained. If viscosity is measured at increasing shear rates and then at decreasing shear rates, the viscosity will be higher as the shear rate is increased than when the shear rate is decreased.

An ideal adhesive or sealant should have the rheological properties shown in Figure 3.2 As a force (stress) is applied to the adhesive or sealant system, no flow will occur until a certain stress level (yield stress) is reached. As the stress is increased further, the system begins to flow and will exhibit thixotropic behavior. As the shear stress is lowered, the viscosity of the system increases.

There is a time lag in the recovery of viscosity known as the recovery rate. The ideal adhesive or sealant system should have a high yield stress with high shear-thinning index and a fast recovery rate.

In the adhesives and sealants industry, rheological properties are often evaluated by three parameters: viscosity, shear-thinning index, and sag resistance.1-2 Viscosity is frequently measured using a Brookfield viscometer, where the viscosity is measured at two different spindle speeds with a ratio of 10:1. The ratio of the viscosity measured at the low speed to that at the high spindle speed is termed the shear-thinning index and is used as an index for thixotropic behavior of the systems.

Either the Boeing slump test for sealants or the Leneta anti-sag test for adhesives^6-7 can determine the sag resistance of adhesives and sealants. In sag testing, gravity acting on the materials induces the applied shear stress.

Fumed Silica

Fumed silica is produced by the vapor-phase hydrolysis of silicon chlorides in a hydrogen-oxygen flame as shown in Figure 4.⁸ The combustion process creates silicon dioxide molecules that condense to form particles. The particles collide, attach and sinter together. The result of these processes is a three-dimensional branched-chain aggregate with a length of approximately 0.2 to 0.4 microns. Once the aggregates cool below the fusion point of silica (1,710°C), further collisions result in mechanical entanglement of the chains, termed agglomerates.

Due to the existence of silanol groups on the particle surface (Figure 5), fumed silica prepared by the above process is hydrophilic and therefore has high surface energy. Hydrophilic or “untreated” fumed silica is available in a variety of grades with different surface areas and bulk densities. The important properties for untreated silica are surface area, surface hydroxyl population, bulk density and pH.

Grafting inert functional groups to the silica surface can modify the surface character of silica. These surface-modified fumed silicas are called “treated” silicas. Treated fumed silica is produced by the treatment of hydrophilic fumed silica with silanes such as hexa-methyldisilazane (HMDZ), alkyl-chlorosilanes, and oligomers or polymers such as polydimethylsiloxane (PDMS).² In these processes, some or most of the silanol groups on the surface are replaced with organosilicon groups, changing the high-surface-energy, hydrophilic surface to a surface with low surface energy and hydrophobic nature. Besides surface area, pH, bulk density, surface-carbon content and hydrophobicity (surface energy) are also utilized to characterize treated fumed silica. Three major classes of treated fumed silica are PDMS- treated silica (CAB-O-SIL® TS-720), hexamethyldisilazane-treated silica (CAB-O-SIL® TS-530) and dimeth-yldichlorosilane-treated silica (CAB-O-SIL® TS-610). The treatments reduce the number of surface silanol groups and also shield them from forming hydrogen bonding between aggregates, resulting in decreased BET surface area, increased surface carbon content and decreased surface energy. Among the three treated silicas mentioned above, CAB-O-SIL TS-720 has the highest carbon content and lowest surface energy, whereas CAB-O-SIL TS-610 has the lowest carbon content and highest surface energy.

Use of Fumed Silica in Adhesives and Sealants

In the adhesives and sealants industry, fumed silica is used to increase the product viscosity, improve anti-settling properties during storage, control the extrusion properties during application and impart anti-sag properties during cure. The additive can also act as a reinforcing agent to improve the physical properties, especially tear strength, of the cured sealant.

A thixotrope is a material that can provide both shear thinning and time-dependent viscosity-recovery effects. The surface activity or surface energy of fumed silica has a great influence on particle-network formation in liquid systems, which controls the viscosity and thixotrope.

For hydrophilic silica, hydrogen bonding of the surface silanol groups is the main driving force for the formation of a three-dimensional network. Untreated silica can therefore best develop an effective network in a nonpolar liquid medium where particle–polymer interaction is kept to a minimum.

For example, hydrophilic silicas are widely used to thicken nonpolar liquid silicone adhesives and sealants. Hydrophilic silica generally cannot form an effective network in polar liquid environments because the polar media can interfere with the formation of interaggregate hydrogen bonds.

Treated (hydrophobic) fumed silica is able to form a three-dimensional network in some systems, but the driving forces for this network are more complex. The chemical structure of treating agents and the level of treatment determine the characteristics of the silica surface.

For CAB-O-SIL TS-530, hydrophobic interactions as well as van der Waals forces are the main contributors to the particle-particle interactions. For CAB-O-SIL TS-720, where the treating agent consists of long-chain PDMS molecules, the interpenetration of PDMS layers on the silica surface also contributes significantly to the build-up of a network. These fumed silicas are used as thixotropes for polar adhesives and sealants systems where the formation of a particle–particle network is not affected by the presence of polar liquid media.

For example, CAB-O-SIL TS-720 has been widely used as a thixotrope for epoxy adhesives and sealants systems. CAB-O-SIL TS-610, which is a partially treated silica, can form a network either through hydrogen bonding or hydrophobic interactions and van der Waals forces. However, it cannot develop an effective particle–particle network; therefore, its use as a thixotrope is limited.

Another way to view the thixotropic effects of fumed silica is by considering silica agglomerate structure. Fumed silica agglomerates are clusters of individual aggregate particles formed through particle–particle interactions. These agglomerates or clusters also interact with other agglomerates forming a larger three-dimensional network throughout the medium. During the formation of primary and secondary agglomerates, part of the continuous phase becomes trapped and immobilized within and between these structures (Figure 6). This is known as “occluded volume” and leads to a larger effective phase volume than that of the primary aggregate particles. The medium absorbed into this occluded volume is shielded against deformation under stress and would act as part of the silica volume fraction. When shear is applied, these agglomerates begin to break down, and if the applied stress is high enough, they might break down to the primary aggregate particles. Therefore, by trapping and immobilizing some of the continuous phase, the agglomerated structure has the effect of increasing the apparent volume fraction, thus again giving a higher than expected viscosity.

The degree of dispersion of silica in the medium is another important factor affecting the formation of a silica network. Good dispersion results in not only a strong interparticle network, hence high viscosity, but also good thixotropic properties. Silica agglomerate structures take time to break down and rebuild. The driving force to rebuild the floc is Brownian motion. Since this motion increases with a decrease in particle size, the rate of thixotrope changes as a function of particle size, and one would expect systems with large particles to recover their viscosity slower than systems with smaller particles.

There are also some other factors that will affect the rheological controllability of fumed silica. For example, other additives used in the adhesives and sealants formulations can have a significant impact.

Fumed Silica in Specific Adhesives and Sealants - Epoxies

Hydrophilic silica imparts excellent initial rheological properties to adhesives, but on prolonged storage over several months or under high-temperature cure conditions, they lose their sag resistance. The failure mechanism of the thixotrope is believed to be due to the preferential absorption of the epoxy groups on the surface silanol groups, which reduces the strength of the silica network in the epoxy system. Hydrophobic silica can not only give excellent initial rheological properties but also much improved aging properties. Figure 7 compares the sag properties of epoxy resin sealants after accelerated aging for four weeks at 60°C prior to cure for both CAB-O-SIL M-5 and CAB-O-SIL TS-720. The system with CAB-O-SIL TS-720 still maintains acceptable sag resistance after accelerated aging. It should be mentioned here that the epoxy resin is a polar polymer, so hydrophilic silica would not be expected to offer rheological control in these systems. The presence of curing agent in epoxy is the main reason for the effectiveness of hydrophilic silica in epoxies prior to aging.

Polyurethanes

Even when the silica is predried, the viscosity of the polyurethane system increases on aging. This is probably related to premature crosslinking of the polyurethane polymer by interaction of the isocyanate groups at the end of the prepolymer chain with the surface silanols on the fumed silica surface. The hydrophobic silica CAB-O-SIL TS-720 can impart reasonable sag resistance with moderate viscosity increase without the need of predrying.

Silicone Systems

However, there is a general increase in yield value, viscosity and shear-thinning index with increasing surface area. The extrusion rate decreases with increasing silica surface area. The cured physical properties and clarity increase with silica surface area.

CAB-O-SIL TS-610, which is a partially treated fumed silica, provides a high yield value, giving good sag resistance and good cured physical properties. It is normally used only in silicone RTV-1 systems where there is a strong possibility of moisture pickup by the fumed silica due to either long-term storage or high relative humidity.

Fully treated fumed silica CAB-O-SIL TS-720 and CAB-O-SIL TS-530 give moderate reinforcement and have fairly similar viscosity and extrusion properties; however, they impart much lower yield stresses, resulting in poorer sag resistance.

Summary

Produced by a flame process, untreated fumed silica consists of aggregates of primary particles. They can form a three-dimensional network through hydrogen bonding of surface silanol groups. Through surface modification, treated fumed silica with a hydrophobic surface nature can be produced. These particles can also develop a three-dimensional network through hydrophobic interactions, van der Waals forces and others. They have been widely used as thixotropes in a variety of adhesives and sealants systems with totally different characteristics, including epoxies, silicones, polyurethanes, polyacrylics, polysulfides, polyesters and butyl rubbers.

Acknowledgment

The data and conclusions contained herein are based on studies made in Cabot Corp. laboratories and are believed to be reliable. We do not guarantee that similar results and/or conclusions will be obtained by others. We disclaim any liability resulting from the use of the contents of this article.