Planetary Ball Mills Pulverization with high energy input

Planetary ball mills - function principle

In a planetary ball mill, each jar represents a “planet”. This planet is located on a circular platform, the so-called sun wheel. When the sun wheel turns, the jar rotates around its own axis, but in the opposite direction. Thus, centrifugal and Coriolis forces are activated, leading to a rapid acceleration of the grinding balls. The result is very high pulverization energy required to produce very fine particles. The enormous acceleration of the grinding balls from one wall of the jar to the other produces a strong impact effect on the sample material and leads to additional size reduction effects through friction.

For colloidal milling and most other applications, the ratio between the speed of the sun wheel and the speed of the grinding jar is 1: -2. This means that during one rotation of the sun wheel, the grinding jar rotates twice in the opposite direction. This speed ratio is very common for Planetary Ball Mills in general. Planetary ball mills with higher energy input and a speed ratio of 1:-2.5 or even 1:-3 are mainly used for mechanochemical applications.

Planetary Ball mills - Grinding jar and ball materials

How to choose the most suitable material

If, for example, a sample is analyzed for its heavy-metal content, the abrasion of steel grinding jar and balls might introduce chromium into the sample which would lead to falsified analysis results. Therefore, a metal-free material like zirconium oxide should be selected. Another point to consider is the influence of the tool on the grinding efficiency. Here two aspects are important:

• Energy input (related to the density of the material)
• Hardness of the material

Energy input

The energy input grows with increasing density of a material. If the material of the grinding jars and balls has a high density, like tungsten carbide, acceleration of the grinding balls is higher at a given speed compared to materials of lower density. This means the energy input is higher when the ball hits the sample and, consequently, the crushing effect is higher with dense materials. This effect is beneficial for pulverizing hard-brittle samples.

For soft sample materials, on the other hand, too much energy input can prevent effective crushing. In such cases, the sample is not really pulverized into a fine powder but rather forms a layer that sticks to the jar walls and covers the grinding balls. Homogenization is not possible that way and sample recovery is difficult. For soft sample materials, other mill types, for example rotor mills, are better suited.

Hardness

To find a jar and ball material with suitable hardness, the consideration is simple: The material must be harder than the sample. If the material is less hard, the grinding balls could be ground by the particles of the sample material.

Grinding tools of different materials

It is not recommendable to use tools of different materials, e. g. a jar made of steel used with balls made of zirconium oxide. First, abrasion from both materials will influence the analytical result, and second, wear of the tools is increased.

Planetary ball mills - recommended jar fillings

For dry grinding

For dry grinding, the best results are usually obtained with the so-called one-third-rule. This means, that approximately one third of the jar volume should be filled with balls. Following this rule, the smaller the balls are, the more must be taken to fill a third of the jar. Another third of the jar volume should be filled with sample material. The remaining third is free space to allow the ball movement inside to achieve the required comminution energy for fast pulverization of the sample.

Following this rule, the required crushing energy is provided while at the same time sufficient sample material is in the jars to prevent wear. 

<sup>1. One third free space
2. One third sample
3. One third grinding balls</sup>

For fibrous samples

For fibrous materials, or materials which lose their volume drastically when pulverized, a higher sample filling level is advisable. Sufficient material needs to be in the jar to minimize wear. If necessary, it is possible to add more material after some minutes to maintain the minimum required volume. 

<sup>1. Two third sample
2. One third grinding balls</sup>

For wet grinding

To produce particle sizes down to 100 nm or less, wet grinding and friction is required rather than impact. This is achieved by using many small balls with a large surface and many friction points. Consequently, the one third filling level, which is recommended for dry milling processes, is exchanged by the 60 % rule, meaning that 60 % of the jar are filled with small balls. The sample amount should be approx. 30 %. First, the small balls are added to the jars (by weight!) and then the material is added and mixed. Finally, the dispersant liquid is mixed carefully.

<sup>1. One sixth to one third sample + liquid
2. Two thirds grinding balls</sup>

How to choose the correct grinding ball size

Another rule of thumb is, that the grinding balls should be at least three times bigger than the largest sample piece. In this way it is ensured, that the balls can pulverize the sample quickly.

To find the suitable ball size for the desired final fineness, usually a factor of approximately 1000 can be applied. If a grind size of 30 µm (D90) is the objective, the most suitable ball size would be between 20 mm and 30 mm. If smaller particles are required, the balls must be removed and replaced by smaller ones for a second process step.

As larger balls could crush smaller ones, it is not advisable to combine different ball sizes in one milling process. 

Wet and nanoscale grinding in planetary ball mills

Nanotechnology deals with particles in a range from 1 to 100 nm. These particles possess special properties due to their size, as their surface is greatly enlarged in relation to their volume (so-called “size-induced functionalities”). Ultrafine particles are, for example, harder and more break-resistant than larger particles.

With dry grinding the particle size of a sample can only be reduced to a certain extent as small particles tend to get charged on their surfaces and agglomerate. Therefore, liquid or dispersant is used to keep the particles separated. Salt solutions are used to neutralize the surfaces charges. Long chain molecules in the liquid can keep the particles separated thanks to steric hindrance.  

Due to their significantly enlarged surface in relation to the volume, small particles are drawn to each other by their electrostatic charges. Neutralization of surface charges is only possible by adding a buffer (electrostatic stabilization, left) or by adding long-chained molecules (steric stabilization, right).

Use of planetary ball mills for co-crystal screening

Co-crystals are solid materials composed of two or more molecular components. Co-crystal screening is the process of identifying suitable co-formers that form stable and desirable co-crystals with a target molecule. Co-crystal screening can be used to improve the physicochemical properties of, e.g., pharmaceuticals or agrochemicals such as solubility or stability. With a special adapter, co-crystal screening can be carried out in a planetary ball mill, using disposable vials such as 1.5 ml GC glass vials. Typically, a few 3 mm or 4 mm steel balls are used to mix the substances at low to moderate speed. If required, a few µl solvent are added. The process is usually finished in 30-120 min.

The adapter features 24 positions arranged in an outer ring with 16 positions and an inner ring with 8 positions. The outer ring accepts up to 16 vials, allowing for screening up to 64 samples simultaneously when using the Planetary Ball Mill PM 400. The 8 positions of the inner ring are suitable to perform trials with different energy input, e.g. for mechanosynthesis research.

As the vials are made of glass, the speed of the mill should be selected carefully, we recommend a maximum of 500 rpm in the PM 300 and 550 rpm in the PM 100. The maximum speed of 400 rpm in the PM 400 is not critical.

For co-crystal screening high energy input generated by high speed is disadvantageous as this might lead to alterations of the chemical compounds of the substances. Consequently, optimum results are obtained at low and moderate speed.