How do I get the most out of AEROSIL® fumed silica in epoxy systems?

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

The importance of rheology in epoxy resins

As a formulator of epoxy resins, you know that rheology control is an important consideration when designing a system. Take proper care over the rheology profile and you will have a product that resists sedimentation, has excellent shelf life stability, is easier and more accurate to blend (2k systems), can be applied with ease and has a high level of anti-sagging performance. Some of these factors are important before, during and after application – making yours and your customers' lives easier.

AEROSIL® fumed silica has been a successful rheological additive in epoxy systems for decades. There is a good chance you will have worked with it at some point. Its success stems from the versatility it gives formulators in building viscosity, thixotropy and in controlling rheology over a wide range of shear and temperature conditions.   

There are many factors that influence the successful use of AEROSIL® fumed silica in epoxy resins. Let's take a look at how fumed silica works, how to choose the right grade for your system, how to use it properly and what to bear in mind about the interactions other components can have on its efficacy.

 

How does fumed silica thicken liquid systems?

First of all, let’s take a look at how AEROSIL® fumed silica works as a thickening agent. Fumed silica is supplied in the form of agglomerates that need to be broken down under high shear into aggregates. The latter would ideally have a particle size in the region of 100-300 nm. To achieve this breakdown of particle size you would incorporate the fumed silica with a dissolver, planetary mixer, triple roll mill or kneader. Propeller stirrers are NOT suitable for this process! Dissolvers and planetary mixers are the most popular due to the ease of continuous incorporation and lower energy demands.

Once the particles have been broken down they will reform as a continuous 3D gel network, imparting a yield value and higher viscosity at rest. This is a reversible process - in that applying shear will break the network down and decrease the viscosity. Once again, the network can reform. This behaviour is what gives rise to the shear thinning and thixotropic behaviour AEROSIL® is so well known for in epoxy resins.

OK, so that’s how it works on a mechanistic level. To understand why some grades work in certain systems, but not in others, we need to look at the interactions between the fumed silica particles. AEROSIL® grades come both untreated (silanol surface groups) and with hydrophobic silane treatments (various options, some silanol groups remain). Interactions between the silanol groups are responsible for the formation of stable networks. In non-polar fluids, this interaction is maximised. When trying to thicken a polar fluid these molecules solvate the AEROSIL® and lead to a destabilisation of the gel network. This is why relatively large amounts of hydrophilic silica are required when thickening water or ethanol*.

 

How to thicken an epoxy resin with fumed silica

It is interesting to note that the presence of a small concentration of polar molecules in a non-polar fluid can have a synergistic effect in the thickening behaviour; through a bridging mechanism. Consider the use of a polyamine curing agent for example. This effect works the other way above a certain critical concentration due to the solvating effect mentioned above.

Hydrophilic fumed silica is often not suitable for use in epoxy resins as many grades are polar in nature, solvating the AEROSIL®. Epoxies also have curing agents added to them which can destabilise the gel network. To demonstrate this, we direct your attention to Figure 1.

Graph demonstrating hydrophobic fumed silica is best for epoxy resins

Figure 1. Viscosity drop of a fumed silica thickened epoxy resin (ARALDIT M, Huntsman), both (left) before and (right) after the addition of curing agent and accelerator. Three grades of AEROSIL® are used at a loading of 5.6% by weight pre-addition and 3.8% post-addition: AEROSIL® R 202 (PDMS surface-treated, most hydrophobic), AEROSIL® R 805 (octylsilane, hydrophobic) and AEROSIL® 300 (highest surface area, hydrophilic).

Notice how the hydrophobic grades increase viscosity in this polar resin to the greatest extent. Addition of polar hardener molecules then massively boosts the viscosity of the AEROSIL® 300 due to the bridging effect mentioned earlier. This effect is short lived however, as the fumed silica particles are increasingly solvated by polymer molecules containing hydroxyl groups that originate during the curing reaction. In conclusion, if you want a stable viscosity profile through cure, then you need to use a hydrophobic AEROSIL® grade such as AEROSIL® R 202, AEROSIL® R 208 or AEROSIL® R 805. These grades have hydrophobic groups on the surface to shield the remaining silanol groups from undesired wetting. Note that both silanol groups and the hydrophobic chains contribute to thickening with these grades.

Evonik™ have found the same trend of thickening performance across all tested epoxy resins. An example of this behaviour is shown in Figure 2.

AEROSIL R 202 and R 805 thicken epoxy resins

Figure 2. Comparison of the thickening effect of different AEROSIL® fumed silica grades at a loading of 4% by weight in a plasticized DGEBA resin at room temperature.

Fumed silica surface area usually has a significant effect on the thickening performance of liquid systems, however - when it comes to epoxy resins - surface chemistry is far more important.

 

How do silanes influence fumed silica rheology in epoxy resins?

Bi-functional silanes are frequently employed in epoxy adhesives. You will likely be familiar with names such as GLYMO and other Dynasylan® products. These molecules can have a huge impact on the rheology of fumed silica thickened epoxy resins - if the wrong grade is used. Silanes are known to decrease the viscosity in pure epoxy resin through a dilution effect. Please see Figure 3, comparing the left-hand side with the right.

Silanes can affect epoxy rheology

Figure 3. Comparison of viscosity changes of an epoxy resin (EPIKOTE 216) thickened with various AEROSIL® fumed silica grades, as a function of storage time at 50°C. (left) Without silane addition. (right) with Gamma-glycidocypropyl-trimethoxy-silane, Dynasylan® GLYMO.

It is clear to see that the addition of silane increases the thickening effect of hydrophilic fumed silica dramatically. Over time this viscosity plummets back down again because of the reaction between silanol groups and the methoxy groups of the silane, resulting in methanol release and the subsequent solvation of the silica; disrupting the gel network.

Hydrophobic grades such as AEROSIL® R202 and R805 are only marginally impacted by the addition of silane. Note the excellent storage stability offered by hydrophobic fumed silica grades, AEROSIL® R 805 in particular.

 

AEROSIL® fumed silica as an anti-settling agent

Fillers are frequently employed in the formulation of epoxy systems, either to improve performance like shrinkage or to reduce cost. These particles are prone to settlement if not properly stabilised. AEROSIL® fumed silica can slow down, or even prevent, the settling of fillers and pigments. After extended storage times, some sediment may be seen but it is often soft and easy to resuspend. This effect is attributable to the increased yield value that fumed silica imparts. Either AEROSIL® R805, R202 or R208 would perform best in this function.

 

Fumed silica for the thickening and thixotropy of hardeners for epoxy resins

By adding fumed silica to the less viscous hardener component, blending mistakes can be avoided and a more homogenous mixture obtained. Note that fillers can be suspended in the hardener component with the proper choice and use of AEROSIL® fumed silica.

Now here is where the recommendation can change from those made so far, depending on the polarity of your hardener (see Figure 4). Non-polar polyaminoamides like VERSAMID 140 are thickened best by hydrophilic fumed silica grades (AEROSIL® 300). In high polarity mercaptan hardeners, like CAPCURE 3-800, hydrophobic grades are the best (R805, R202 and R208). It is worth noting that the hydrophobic grades often have better storage stability though.

Epoxy hardeners can be thickened with fumed silica

Figure 4. Thickening of various epoxy hardeners with AEROSIL® 300, AEROSIL® R202 and AEROSIL® R805 at room temperature.

 

What should you do now?

If you think that one of the samples mentioned above would be interesting to look at then you can submit a sample request on our AEROSIL® fumed silica page. Product brochures and datasheets can also be found on this site. Our sales team is comprised entirely of chemists with decades of industry experience between us so please get in touch if you would like to discuss your project further – we are here to help.

If you would like to read the Evonik™ article in full that this blog is based on then you can find it on our AEROSIL® page linked in the paragraph above. Are you looking to optimise your processing conditions to get the most out of AEROSIL® fumed silica? If so then you will want to read the brochure titled “Successful Use of AEROSIL® Fumed Silica in Liquid Systems”, also found on our AEROSIL® page linked above.

 

* Note that it is possible with a special grade of AEROSIL® called COK 84. Please get in touch if this is of interest

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.