Flow aids and anticaking agents based on fumed and precipitated silica

 

Why is the flowability of powder important?

Powdered ingredients are increasingly being used across a diverse range of industries. These powders must have good flowability so that they can be easily discharged from storage and accurately dosed. This can be troublesome, as many powders are highly cohesive and so do not flow well on their own. Many powders tend to cake upon prolonged storage or while being transported due to climatic conditions and pressure.

Table 1 shows a selection of powders that have already been successfully treated with EVONIK’s speciality silica, to improve flow and anti-caking performance. Addition levels are low and the products recommended vary on the nature of the host powder and processing conditions. Continue reading to learn more about improving the flowability and anti-caking properties of powder.

 

Table 1: Application areas of speciality silica for improving flow and anti-caking properties.

 

Measuring flowability of powders

There are several methodologies available to characterise the flow behaviour of powders. One such method is the “angle of repose”, in which the powder in question passes through a sieve and falls on the top of a metal cylinder; causing the powder to form a cone. When particles fall onto the cone they either stick or flow off – depending on the angle of the cone and the attractive forces between the particles. As more particles stick, the cone grows steeper - until gravity overcomes these attractive forces. Measuring the height and angle of the slope gives the angle of repose. Stickier particles will, therefore, have a higher cone height and angle (Figure 1, left). A lower angle of repose equates to better flow properties (Figure 1, right).

 

Figure 1: Angle of repose measurement for powder flowability showing (left) sticky particles with poor flow behaviour and (right) free-flowing powder behaviour with a low angle of repose.

 

Another method is to use a series of glass funnels with different outlet diameters. One determines at which diameter the powder stops flowing. Alternatively, the time a powder needs to flow through a specific funnel can be measured.

A more sensitive method is to use a nest of sieves. The powder is poured into the sieve with the largest mesh at the top of the nest. Controlled vibration is applied to the nest for a given time and the powder falls down through the nest of sieves. The more flowable the powder, the more powder will get through the nest of sieves and onto the sieve pan. After vibration has stopped each sieve is weighed for the difference. This mass is then multiplied by a given factor for each sieve and the results are added together. The final result yields the measurement of flowability.

 

Fundamentals of flow aids

All particles stick together when in close contact due to Van der Waals forces. For small particles, these forces dominate over the gravitational forces that are responsible for pulling them apart and allowing the powder to flow. Therefore fine powders do not usually flow well on their own (unless highly electrostatically charged but then static issues can arise).

Flow aids are very fine powders that cover the surface of a host powder and create a surface roughness that helps to space the bulk of the particles from one another. As Van der Waals forces are heavily dependent on density and distance, having a light separator in between helps tremendously. Figure 2 illustrates this concept.

 

Figure 2: Influence of surface roughness on the attraction between two powder particles.

 

AEROSIL® fumed silica and SIPERNAT® precipitated silica are perfectly suited to roughen the surface of particles; keeping them apart and thus reducing attraction forces. This is why these products are so widely used as flow aids and anticaking agents. Caking is used as a time-related decrease in flowability, which in extreme cases leads to the formation of one solid “cake” after prolonged storage.

 

Flow aids for dry, hard powders

To get the optimum result in flow improvement, the flow aid must be dispersed finely on the host powder’s surface. Suitable mixers include Ploughshare®, paddle or ribbon mixers. An example of this can be seen in Figure 3, where SIPERNAT® 320 DS (left) has been dispersed onto a corn starch particle (right).

 

Figure 3: SEM images of pure SIPERNAT® 320 DS (left) and a corn starch particle covered with SIPERNAT® 320 DS (right).

 

Some SIPERNAT® and AEROSIL® grades are easier to disperse than others. This was investigated by Prof. Zimmerman at the University of Wurzburg. Several different grades were investigated alongside tricalcium phosphate, another flow aid which is frequently used in the industry. Corn starch was used as the model host powder and it was mixed with different types of AEROSIL® and SIPERNAT® at different mixing times in a Turbula tumble mixer.

Zimmerman has developed a method for using tensile strength to quantify the cohesive forces and thus powder flowability. In this measurement, a probe which bears a thin Vaseline film touches the flat surface of a powder and then is lifted up again. The force that is needed to separate the upper layer of powder from the bottom layers is recorded by a sensitive tensile strength tester.

Figure 4 shows some of the results of this investigation. Tensile strength was measured at different mixing times. High values for tensile strength refer to bad flowability and low values show good flowability. One can see that after a short mixing time the flowability improved with all of the used flow aids. Some show better results than others, SIPERNAT® D 17 worked particularly well.

 

Figure 4: Tensile strength of corn starch with 0.2% flow aid, according to mixing time.

 

It is important to note that with a long mixing time, the flowability deteriorates again. This could be caused by a loss in surface roughness due to the extremely fine dispersing of the flow aid on the surface of the corn starch.

 

Flow aids for wet powders

Wet powders flow badly due to the liquid film at the surface of the powder particles, glueing them together. This can be aqueous or non-aqueous. Flow aids can improve flowability by also absorbing the liquid film (Figure 5).

 

Figure 5: Silica is able to absorb thin liquid films from a wet powder’s surface.

 

For absorbing the liquid, a flow aid with a high internal and external porosity is required. That is one of the reasons why SIPERNAT® precipitated silica make for excellent flow aids in wet powders.

Part of the porosity of the flow aid can be lost from certain mixing conditions. Breaking down silica agglomerates into the submicron range contributes to a loss in absorption capacity. A very good dispersion of flow aid, which is beneficial for the efficiency in dry powders, may reduce the efficiency in wet powders. For wet powders, highly porous and mechanically robust silica grades are advantageous. Lawrence Industries’ technical sales team will be able to advise the correct SIPERNAT® or AEROSIL® grade for your application and give guidance on how best to mix it into a given host powder.

Figure 6 gives a comparison between SIPERNAT® 50S and SIPERNAT® 22S being used as a flow aid in a wet salt mixture. This mixture itself shows poor flowability of flow grade 7 (measured via the flow funnel methodology). Following the addition of 0.6% SIPERNAT® 22S or SIPERNAT® 50S, the flow improves within 1 minute of mixing to flow grade 2. After a longer mixing time the silica is too finely dispersed and this causes a drop in pore capacity, which deteriorates the flow properties again. Depending on the SIPERNAT® grade chosen, this overmixing effect can happen relatively quickly or take a much longer period to occur.

 

Figure 6: Resistance against overmixing for SIPERNAT® 22 S and SIPERNAT® 50 S at 400 rpm.

 

By tuning the shear regime, this overmixing phenomenon can also be controlled (Figure 7).

 

Figure 7: Resistance of SIPERNAT® 22 S against overmixing depending on mixer speed.

 

Liquids that make a powder sticky can be either aqueous or non-aqueous. In the case that water is the problem, there is another possibility to improve flowability besides the absorption of liquid. Hydrophobic silica has proven to be a very effective class of flow aid for hygroscopic powders. In this case, the silica floats on top of the liquid film present on the powder surface (Figure 8). They improve flowability at lower addition levels because their efficiency is not limited to the absorption capacity. Once again, this system is sensitive to being oversheared as the silica can be wetted out by the water under intense shear. However, the more hydrophobic the silica, the less sensitive it is to overmixing.

 

Figure 8: Hydrophobic silica separate water films on hygroscopic substances.

 

Figure 9 shows the effect of the hydrophobic silicas: SIPERNAT® D10 and SIPERNAT® D17, on a wet salt mixture. Flowability of the salt is improved immediately by the addition of 0.4% silica. At longer periods of mixing the flowability deteriorates as the silica is wetted out. SIPERNAT® D10 is more resistant to this overmixing effect.

 

Figure 9: Sensitivity of hydrophobic SIPERNAT® types to overmixing.

 

Flow aids for soft powders

Powders of soft materials such as fats, waxes or emulsifiers are especially challenging to handle and transport and can cake heavily over time. This problem is compounded when the material is exposed to changing temperatures, for example in a shipping container. An efficient anti-caking agent is, therefore, a prerequisite when shipping these powders long distances. Soft or thermoplastic powders are deformed at elevated temperatures, or when pressure is applied (Figure 10).

 

Figure 10: Soft or thermoplastic particles stick together as they are deformed under pressure and/or elevated temperature.

 

Silica can cover the soft powders’ surface and prevent the particles from sticking together. Higher levels of silica are needed than with dry hard powders, however, especially when a long-lasting anti-caking effect is required. Usually, addition levels of silica in soft powders can be up to 5%, whereas in dry hard powders < 1% is often effective. The reason for this is that part of the anticaking agent may penetrate into the soft powder’s surface on storage, thus losing efficiency. When an adequate amount of anti-caking agent is added, enough will stay on the surface to preserve efficiency (Figure 11).

 

Figure 11: Soft powders capture part of the anticaking aid meaning that more is required to achieve long-lasting performance.

 

Mixing technology

As we have demonstrated in this article, the mixing process is critical in obtaining a good result. Tumble mixers provide a very gentle mixing and so can they be used for very soft powders. Cone mixers like the Nauta® mixer are also very gentle but require longer mixing times.

Paddle mixers are very gentle and at the same time homogenize the mixture very well on a macroscopic level. They are a very good choice for all soft powders and hygroscopic powders where the porosity of the silica needs to be preserved.

Ploughshare® mixers apply more mixing energy but are gentle enough not to press the flow aid into the soft powder’s surface. They can be used for a wide range of powders.

Generally, Ploughshare® or paddle mixers need shorter mixing times. Depending on the individual needs, mixing can be adapted to be shorter for hygroscopic powders and longer for dry, hard powders.

Ribbon blenders are even more intense and are suitable for dry, hard powders.

A special case is the flow improvement of spray-dried products. The addition of silica directly into the spray drier, separately from the slurry, results in a fine dispersion of silica at the particles’ surface. Figure 12 shows possible addition points for silica in a spray drying process, of which points 1 and 2 are the most effective ones.

 

Figure 12: Addition of silica during spray drying.

 

Summary and what to do now

Deagglomeration of the flow aid during mixing with the host powder leads to better coverage of the host powder, which increases the efficiency of silica as a flow aid in dry powders. It reduces the porosity of silica as well and so reduces the efficiency of silica as a flow aid in wet powders. Depending on the type of powder, different mixing procedures and silica types are recommended. See Table 2 for the recommendation matrix.

 

Table 2: Recommendation matrix for flow aid/anticaking aids in different types of powders.

 

Real systems often see combinations of powder types and so require individual solutions. Our technical sales team are available to discuss your requirements, so call us today on 01827 314151 to see how we can help you with your next formulation challenge.

This article has been adapted from two pieces of EVONIK literature: TI 1351 and TI 1213.