Key points
The unwanted transformation of free-flowing bulk particulate solids to include varying levels of agglomeration or hard lump forms (caking) is a common problem for companies that handle powders when storing at elevated temperature, humidity and/or high consolidation stress.
Caking could potentially occur in almost every sector of industry, i.e. pharmaceutical, food and detergents. Caking potentially could increase wastage through poor quality product or even stop production in extreme cases.
It is always desirable that particulate materials retain their desired flow properties during processing, handling and storage as well as through the distribution chain to the final consumer. The reduction in flowability of powders and other quality related issues, can cause significant losses for industry.
The definition of caking
There is not a general/common widely accepted definition for caking. That is mainly due to the fact that caking can occur for some processes when only few lumps are formed in the bulk materials, while for some other processes caking can happen when the permanent fusion of particles leads to the conditions where materials may no longer be acceptable to the process or end user.
For instance, formation of small quantity of lumps inside storage units of a vending machine or inside a printer cartridge could stop the discharge of powders from these units.
On the other hand, the same caked lumps will probably break up inside the larger scale handling systems and hence cause no problem (assuming that the caking is not so severe that discharge from the storage vessel is not impeded or halted).
Common mechanisms responsible for caking
The two most common mechanisms responsible for caking are divided into i) moisture induced caking and ii) plastic flow caking. One way that moisture induced caking could occur is when powder is stored in the presence of moisture which cause the formation of liquid bridges between particles.
Liquid bridges can increase the interparticle capillary forces. The capillary forces are very weak and, hence do not themselves constitute caking forces. However, these forces are the early mechanism that cause consecutive caking in the bulk solids.
There are several ways, i.e. moisture loss from soluble liquid bridges, supersaturation and dissolution crystallisation, in which liquid bridges convert to solid bridges.
These bridges are responsible for continuous strong interactions between neighbouring particles which lead to a formation of strong caked powder.
In moisture loss from soluble liquid bridges, when the relative humidity of the surrounding environment is higher than a critical relative humidity, particles start to dissolve into adsorbed water forming liquid bridges between particles.
If adsorbed water is then evaporated from these capillary bridges, dissolved solids then recrystallise, thereby forming solid bridges between the particles. In the supersaturation mechanism, the liquid bridges can be supersaturated by decreasing the surrounding temperature until crystallisation occurs.
This caking mechanism can be seen in powders stored in closed containers where water evaporation is limited. In dissolution crystallisation, particles are dissolved in liquid bridges until saturated. However, particles with a small radius of curvature can dissolve in a saturated solution and make supersaturated solution.
This supersaturated solution enables small particles to join, making large and harder particles in the powder and thereby increasing the particle size and tensile strength of the powder bed.
It should be noted that the level of moisture in the air that leads to the formation of liquid bridges depend on the nature of powder and particularly on the hydroscopic level of the powder.
Hygroscopicity is based on the quantity and velocity of adsorbed water by particles from storage environment at the fixed relative humidity and temperature.
According to the classification, powders with high hygroscopicity can adsorb water to their particle surfaces when the relative humidity of storage condition is just 50%, such as amorphous lactose. On the other hand, non-hygroscopic powders do not adsorb water to the particle surface in storage conditions below 90%.
The second mechanism responsible for caking is plastic flow caking. It is assumed that the cake strength increase occurs because of an increase in the cross-sectional area at each inter-particle contact throughout the bulk solid, through plastic deformation of the particles (creep) as a function of temperature and/or pressure over a long period of time. This contact area increase will result in an increase in the tensile strength of material.
There are two main methods applied in industry and academia for quantifying caking in the bulk solids; conventional shear testers such as the Schulze unit and Brookfield Powder Flow Tester; and uniaxial compression caking tester.
Caking often takes place over a few days, so conducting caking tests measurements under varying humidity and temperature for varying periods of time would need one shear tester to be tied up for many days. It is mainly for this reason that it is not reasonable to use such expensive shear testers for estimating caking strength of bulks solids.
In addition, another limitation with using a shear tester in regard to measuring powder caking is the fact that shear testers are unable to measure very high level of cake strength.
The practice that has evolved at the Wolfson Centre for Bulk Solids Handling Technology for caking studies, is therefore to use a traditional cylindrical uniaxial test. Because the test cell is very simple, many individual cells can be loaded, different stresses applied and then left in a single climate chamber for varying periods of time.
In this way, many different test conditions can be created simultaneously to simulate storage conditions in industry, and the cells of caked sample then transferred to the uniaxial tester to take the cake strength measurement.
However, the limitations of the effects of wall friction, as well as an un-defined shear plane, and, hence, substantial scatter in the caking measurements results, are still a problem with this tester.
The other problem in using this tester is that a large quantity of material is needed for testing. For a single test, there should be around 100 cc of materials. Some particulate materials are expensive or access to a large quantity is unavailable due to early production stages, i.e. pharmaceutical powders.
In addition, caking magnitude is low for some materials or storage conditions, and, hence the formed consolidated powder cannot hold its own weight and breaks after removing the tester’s moulds. In other words, this tester is not a suitable tester for materials which have undergone low level of caking.
Considering these problems with the conventional caking tester, we developed a simple, easy to use and fast tester with a low height to diameter ratio to get low wall friction, higher exposed surface, and defined shear plane for better accuracy and repeatability of results.
The Greenwich Caking Tester
The Greenwich Caking Tester (Figure 1) has two sizes; the smaller version only requires a few cc of powder to conduct one test. The tester is purposely designed to evaluate caking propensity of powder after storage at high consolidation stress and temperature (plastic flow caking).
The Greenwich Caking Tester (GCT) is suitable for measuring caking strength of powders at fairly low, medium to large magnitude of hardness. Powder is poured into the caking tester and the excess removed.
The lid is then placed on the powder bed and loaded with different dead weights until the desired consolidation stress is achieved, after which the GCT is stored for the desired duration.
After storage, the base is detached from beneath of the tester, the lid taken and then the tester centred below a plug attached to texture analyser to perform a cake strength measurement. The force necessary for the plug to penetrate and push out the plug of the caked sample through the cell hole registers the cake strength.
Case study
The effect of different variables on caking behaviour of a commercial detergent powder was investigated. The variables are temperature (27 °C, 37 °C and 47 °C), consolidation stress (3.9 kPa, 7.5 kPa and 21.3 kPa) and storage duration (2, 4 and 7 days).
The caking results derived from the tester after storing the powder for 7 days is reported in Figure 2a (results of powder storage after 2 and 4 days are not reported here).
Results showed that the developed tester and the measuring technique has the potential to quantify the magnitude of plastic flow caking at different storage conditions. Results also showed that the variation of caking strength is below 10%.
We developed surface response contour plots to better illustrate the caking behaviour of the detergent powder. The contour plots of caking strength as the function of consolidation stress and temperature at the fixed storage duration of 7 days is depicted in Figure 2b.
Plots are divided into different area based on the caking strength. 10 kPa are the weakest caking strength while 100 kPa are the highest. Figure 2b showed that the smallest caking magnitude was estimated to happen in the temperature range of 27 °C to 45 °C and consolidation stress below 7.5 kPa.
In contrast the highest caking strength was predicted to happen when consolidation stress and temperature went above 16 kPa and 41 °C respectively.
It can be seen from the red and black region, where the magnitude of caking is very high, that temperature plays an important role in defining a critical stress that leads to high level of caking.
The surface contour plots are a very useful in quality control and R&D units to predict the consolidation stress, storage time and temperature which leads to different level of caking strength.
In summary
At the Wolfson Centre we have developed a simple and easy to use plastic flow caking tester. The tester produces repeatable caking strength results with low level of standard deviation. We also introduce/apply a statistical method as a way to better predict/estimate the caking magnitude of powders.
