Compaction is often employed as a terminal process for manufacturing consumer goods, and it has direct effects on the quality of final products. Mechanical properties of compacts are especially important in post-production processes such as packaging, transportation, and end-use by consumers. For example, medicinal tablets should not only maintain its integrity throughout packaging and subsequent handling but also provide appropriate disintegration characteristics when administered. Likewise, biomass pellets should preserve their shape and size to ensure desired handling characteristics throughout transportation with minimal crumbliness. On the other hand, cosmetic compacts need to have desired friability for consumer use as well as enough strength so that they do not develop aesthetic defects during shipping and handling.

In addition, the compaction process often experiences operational issues, e.g., capping, delamination, air-pockets, inconsistent surfaces, and so on. Producing compacts with minimal operational anomalies and desirable quality metrics is very much an art as manufacturers search for an efficient and sustainable window of operational parameters that will reliably produce compacts characteristics satisfying often conflicting quality metrics. Such balances between operational maneuver, different mechanical quality metrics, and performance characteristics of compacts are usually achieved by tedious trial-and-error approaches due to the lack of adequate scientific understanding of or a systematic approach to the compaction operation. This is partly because fundamental understanding of powder compaction is still elusive and, as a result, experiential knowledge plays an essential role. However, the extent, to which such empiricism can be applied, is limited because of particulate materials’ innate heterogeneity and variability. To address this issue and enable a systematic approach in compaction process and quality control, it is essential to increase our knowledge of the fundamental mechanics of powder compactions and to develop quantitative relationships between feed particulate materials’ properties, densification process parameters, and the mechanical quality metrics of the final products.
   
Regarding quality metrics, many of them are developed and employed reflecting specific usages of particular products. For example, one may use a ‘pass or fail’ test, in which a product is dropped from a certain height to determine breakage. Such tests provide only binary information that may have limited usage in quantifiable quality metrics. Similar tests can be re-designed to measure mass and height, at which the product begins to break. Obviously such quality measurement requires more effort and resources, but it will eventually pay off when one has the means to predict the quality of products based on quantitative relationships between properties of feed materials and operational parameters. Ideally, these types of tests can be replaced with a set of standardized tests that reflect various scenarios of logistical events. For example, durability, friability, hardness, and solubility are all tests that may be relevant, depending on the end use of the compacts.

For properties of particulate feed materials, there are two categories of properties of feed materials that are relevant and meaningful in our context, namely physical properties and mechanical properties. Physical properties include bulk density, tap density, particle density, particle size distribution, particle shape and so on. Even though these are fundamental properties, direct relationships to the final product’s properties are difficult to establish because of the mechanical nature of the compaction process and our limited understanding of how the physical and mechanical properties of the powders affect the densification process and properties of compacts. Therefore, mechanical properties of powder en masse have more bearing to the mechanical properties of compacts, which is the reason to focus on the mechanical properties of powders and quality metrics of compacts.

Quantitative Compact Quality Assessment using Powder Properties
At Penn State, the powder mechanics research group has shown the feasibility of developing predictive relationships between mechanical properties and quality metrics of few types of powders. For example, pharmaceutical tablets are formed using powder ingredients such as filler, binder, disintegrant, and active pharmaceutical ingredient, either by dry blending the ingredients or wet granulation of the powder mix followed by compaction. A previous study (Pandeya and Puri 2012) found that a set of mechanical properties of powder en masse, such as spring-back index, compression index, and bulk modulus, were found most suitable for predicting diametral strength, indentation hardness, and friability of compacts. This was found by developing predictive correlations for tablet quality vs. dry and granulated powder’s mechanical properties that were determined using a medium pressure flexible boundary cubical triaxial tester and mechanical quality metrics of compacts.
    
In another study, similar approach was applied to biomass pelleting. Pelletization is one of most commonly used biomass densification technologies to overcome the issue of bulkiness. It is popular due in part to its simple equipment design and seemingly straightforward operation. However, a major hurdle of pelletization is its inferior operational efficiency and unreliable performance including the clogging of the die and producing poor quality pellets. Mitigation of such operational difficulties currently relies heavily on empirical approaches, both for small- and large–scale commercial pellet mills because of the innate highly variable and non-homogeneous (such as physical, chemical/biochemical, and mechanical) properties of each feedstock, and the lack of fundamental understanding of the mechanism of biomass pelletization. Penn State’s powder mechanics research group is attempting to establish a quantitative science-based understanding of the mechanics of pelletization process and a systematic approach that can replace an empirical approach involving time-consuming trial and error. As a result, Karamchandani, Yi, and Puri (2015) showed that pellet’s quality can be related to mechanical properties of ground biomass (Figure 1). Especially, this study showed that the fundamental mechanical properties at low pressure range of compaction is capable of predicting properties of compacts produced at much higher pressure, which is thought to be due to the importance of early stage of compaction including rearrangement and elastic responses. This poses a significant importance of measuring fundamental mechanical properties at low pressure range as it is much more practical in industrial laboratory than the measurement at higher pressure range, i.e. over 1 MPa.

Figure 1: Diametral tensile strength (DTS) of pellets vs. spring-back index (SI) of ground switchgrass at 95 kPa for four different conditions. The quantitative relationship between the mechanical quality metrics and response of powder en masse at the very early stage of compaction (below 100 kPa), shows a high coefficient of determination (r2) suggesting a robust prediction capability of such statistical models (recreated from Karamchandani, Yi, and Puri 2015).

Looking Ahead: The Role of Microscale Interactions
These two studies demonstrate that the quality of compacts can be rationally predicted based on the characterization of feed materials’ mechanical properties. The key to these studies is employing quantitative approaches both in the measurements of quality metrics and characterization of feed materials. This quantitative approach is the first step toward systematically establishing optimal compaction processes to produce compacts with optimal quality; avoiding overshooting production operation parameters as can be the case when using trial and error. To achieve this ultimate goal, one needs to understand particle properties’ role in and contribution to the mechanical behavior of powder during densification. The ability to predict behavior of powder during compaction based on mechanical characteristics of individual particles and their interactions will provide a means to implement quality control by design that takes the characteristics of the feedstock into account.
    
For example, powders undergoing compaction experience the following stages: 1) Rearrangement of the particles by filling  large pores accompanied with the increase in number of contacts (coordination number); 2) packing of particles resulting in decrease in porosity with the formation of localized agglomeration of particles, namely secondary and higher order particle structures; 3) increase in the contact area between particles accompanying elastic deformation of particles; and 4) contact enlargement through plastic deformation of particles. Even though the these stages have been established for decades, quantitative and fundamental understanding of how these stages contribute to the formation of compacts/tablets/pellets is yet to be fully elucidated. An analytical model, which describes how macroscopic compaction mechanics evolves from mechanics of the underlying scales, is absent largely because of the lack of an adequate method to examine mechanics at the scale where individual particles can be scrutinized. The compaction process evolves towards different scales of scrutiny, i.e., microscale (single particle, particle-particle interactions), mesoscale (secondary and tertiary particle structures), and macro scales (bulk powder system). Accordingly, a research question can be posed as how do a single particle’s properties and particle-particle interactions govern the evolution of compaction in powders (and powder mixtures) at different scales?
    
We believe that the multi-scale approach can be successfully employed in addressing this question. In a multi-scale framework, properties of single particle and particle-particle interaction characterized using Nano or MicroElectroMechanical Systems (NEMS/MEMS) devices form the foundation of a mesoscopic and macroscopic governing principles of powder compaction. Ultimately, such characterization will be linked to fundamental mechanical properties of bulk particulate materials during compaction. This can be done using a fundamental tester such as a true Cubical Triaxial Tester (CTT) that is free of the confounding effect of samples’ boundary conditions. Through this approach, we believe that it is possible to formulate rational principles of compaction of powder en masse based on the governing laws at multiple scales employing the mechanical properties of underlying scales, and to develop and validate a multi-scale computational model of the evolution of compacts. We envision that this approach will enable a systematic approach in optimizing powder compaction processes that can quantitatively predict and control a compact’s mechanical quality properties through the design and characterization of the powders used to produce those compacts.

References
Karamchandani, Apoorva, Hojae Yi, and Virendra M. Puri. 2015. “Fundamental Mechanical Properties of Ground Switchgrass for Quality Assessment of Pellets.” Powder Technology 283: 48-56. (doi:10.1016/j.powtec.2015.04.069).

Pandeya, Anuranjan, and Virendra M. Puri. 2012. “Relationships Between Tablet Physical Quality Parameters and Granulated Powder Properties: Feasibility Study.” Particulate Science and Technology 30 (5): 482–496.

    Hojae Yi, Apoorva Karamchandani, Daniel E. Ciolkosz, and Virendra M. Puri, are with the Department of Agricultural and Biological Engineering, The Pennsylvania State University, University Park, PA.

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