Cohesive Powders Research Articles

Powder spreading over realistic laser melted surfaces in metal additive manufacturing

Powder recoating is a key step in metal Additive Manufacturing (AM) processes where powder is spread across laser processed surfaces to add material for the next layer. Achieving the desired thin powder layers that are both sufficiently dense and uniform is essential for maintaining the requisite geometric tolerances and final part quality. In this study, we focus on the influence of the substrate surface topography by comparing spreading performance over a set of realistic surfaces. We simulate powder spreading over these surfaces using a calibrated non-spherical particle Discrete Element Model for Ti-6Al-4V that incorporates cohesion and Coulomb friction interactions between particles and surfaces. We identify the four key length scales of the recoating process determined by frictional contacts, powder size distribution, the layer thickness and the melted surface topography. We find that realistic AM surfaces show markedly different powder coverage compared to an idealised flat-plane. Rougher surfaces are found to be recoated with larger amounts of powder than smoother surfaces, as smaller particles get trapped by the grooves and valleys across the surface. Counterintuitively, we find that a finer more cohesive powder can achieve the best layer coverage over realistic surfaces - indicating that powder flowability is an incomplete measure of powder spreading performance on realistic AM surfaces. We also demonstrate how the recoating process can significantly size segregate the feedstock powder, favouring deposition of smaller sized particles on the melted surfaces.

Characterization of powder apparent viscosity: A reapplication of the Poiseuille equation to silo discharges

In the present work, a new method to characterize powder apparent viscosity was proposed by applying the Poiseuille equation to silo discharges. Flow properties of powders were first identified in terms of angle of repose and Hausner ratio, based on which experimental materials were classified as free-flowing and cohesive powders. These powders were further discharged from the pressurized silo, and an exponential increase in the powder discharge rate on the pressure difference was obtained. A model to predict the apparent viscosity was further developed grounded on the Poiseuille equation. The apparent viscosity of both free-flowing and cohesive powders decreases with an increasing flow rate within a specific range, showing the characteristic of shear-thinning. There was a reasonable agreement of powder apparent viscosity characterization between results from the silo discharge and those obtained from the FT4 rheometer, thus, confirming the method's reliability. The new method presented reliable values and had a wide testing range, providing an alternative way to measure powder rheological parameters.

A concise treatise on model-based enhancements of cohesive powder properties via dry particle coating

• Advances in model-guided dry coating of cohesive powder flow enhancement presented. • Chen-NJCEP model explains the mechanism of dry coating induced flow enhancements. • Models offered for the guest amount and type, and granular Bond number estimation. • Dry coating led to drastically enhance flow, packing, agglomeration, dissolution. • Agglomeration, indicator of flowability, dramatically reduced with dry coating. This paper presents a review of our key advances in model-guided dry coating-based enhancements of poor flow and packing of fine cohesive powders. The existing van der Waals force-based particle-contact models are reviewed to elucidate the main mechanism of flow enhancement through silica dry coating. Our multi-asperity model explains the effect of the amount of silica, insufficient flowability enhancements through conventional blending, and the predominant effect of particle surface roughness on cohesion reduction. Models are presented for the determination of the amount and type of guest particles, and estimation of the granular Bond number, used for cohesion nondimensionalization, based on particle size, particle density, asperity size, surface area coverage, and dispersive surface energy. Selection of the processing conditions for LabRAM, a benchmarking device, is presented followed by key examples of enhancements of flow, packing, agglomeration, and dissolution through the dry coating. Powder agglomeration is shown as a screening indicator of powder flowability. The mixing synergy is identified as a cause for enhanced blend flowability with a minor dry coated constituent at silica < 0.01%. The analysis and outcomes presented in this paper are intended to demonstrate the importance of dry coating as an essential tool for industry practitioners.

Smoothed particle hydrodynamics for cohesive dense granular media

• An elastic–viscoplastic constitutive model for cohesive dense granular media is built. • Rheological model considering cohesion for liquid-like phase is put forward. • The solid-like phase and liquid-like phase are both modelled and simulated. • Smoothed particle hydrodynamics was first used for cohesive dense granular media. • Cohesive force plays an important role in granular pile collapse, flow and depositing. Dense granular media with cohesion exist widely in nature. A constitutive model that considers cohesion in terms of solid-like and liquid-like phases in dense granular media was designed in this study. In this new model, a linear elastic–plastic constitutive relationship based on the Drucker–Prager yield criterion affected by the parameters of cohesion was used to describe the solid-like phase. A viscoplastic constitutive model based on rheology, which also considers cohesion, was obtained through formula derivation for the liquid-like phase after plastic yielding. The conversion and transitional relationships from elastic to elastic-micro viscoplastic and then to complete elastic–viscoplastic were built. Smoothed particle hydrodynamics was adopted to solve the new model discretely, and the relationship between smoothed and actual particles and an approach employed to avoid particle aggregation caused by cohesion were illustrated. Several typical cases including the collapse of cohesive granular accumulation, collapse of dry and wet granular column, and flow of cohesive powder in a rotating drum were used for verification. A comparison between cohesive and dry granular flows and among numerical results, experimental results, and those of other numerical methods proved that the proposed model and algorithm are effective for simulating dense granular flow with cohesion; further, they provide a valid approach to describe issues of granular flow with veritable cohesion that exist in nature.

Comparative studies of powder flow predictions using milligrams of powder for identifying powder flow issues

Characterising powder flowability can be challenging when sample quantity is insufficient for a conventional shear cell test, especially in the pharmaceutical industry, where the cost of the active pharmaceutical ingredient (API) used is expensive at an early stage in the drug product development. A previous study demonstrated that powder flowability could be predicted based on powder physical properties and cohesiveness using a small quantity of powder samples (50 mg), but it remained an open question regarding the accuracy of the prediction compared to that measured using industry-standard shear cell testers and its potential to substitute the existing testers.In this study, 16 pharmaceutical powders were selected for a detailed comparative study of the predictive model. The flowability of the powders was predicted using a Bond number and given consolidation stresses, σ1, coupled with the model, where the Bond number represents powder cohesiveness. Compared to the measurements using a Powder Flow Tester (Brookfield) and an FT4 (Freeman Technology) Powder Rheometer shear cell tester, the results showed a good agreement between the predictions and the measurements (<22 % difference) from the two shear cell testers with different consolidation stresses, especially for cohesive materials. The model correctly predicts the class of flowability for 14 and 12 of the 16 powders for the PFT and the FT4, respectively. The study demonstrated that the prediction method of powder flowability using a small sample (50 mg) could substitute a standard shear cell test (>15 g) if the available amount of sample is small.

Effect of Hydrophobic Powders on the Tensile Strength of the Tablet

Zoe Chu1,2, Christopher Windows-Yule¹, Ian Gabbott², Gavin Reynolds², Rachael Shinebaum² and Andy Ingram¹1. School of Chemical Engineering, University of Birmingham, Edgbaston, B15 2TT2. Oral Product Development, Pharmaceutical Technology &amp;amp; Development, Operations, AstraZeneca, Macclesfield, UKEmail: zhc906@student.bham.ac.ukIntroductionA tablet consists of the active pharmaceutical ingredients (API), and the other components, excipients. A tablet needs to withstand coating, packing and shipping stresses whilst maintaining the dissolution time; therefore, the tensile strength is of great importance. Currently, there is a lack of understanding on why excipients improve the bulk powder properties and the bonding mechanisms of the powder during compression. This makes tablet formulation development an inefficient trial-and-error process that is required for every new API (1). Literature has shown that the surface energy of the powders contributes a significant amount towards the tablet tensile strength. Hydrophobic powders have a lower surface energy (2) and are seen to disrupt this tablet strength (3,4).Experimental Aim and MethodThe aim was to investigate the impact of the hydrophobicity of excipient powders on tabletability of microcrystalline cellulose. This was done by mixing powders of varying hydrophobicity with MCC at varying mass fractions. There was focus on a lower mass fraction (0.02-0.1 m/m) with increasing increments of 0.02 m/m of added additive as well as gaining a global picture (0.2-1 m/m) with increments of 0.2 m/m.ResultsIt was found that at low mass fractions (0.02-0.4 m/m) of additive added the hydrophobic powders disrupted the tablet strength more compared to hydrophilic powders which could be attributed to the levels of surface energy of the additive. In contrast, at higher mass fractions (0.6-0.8 m/m), the surface energy is not as important and it is hypothesised that the deformation mechanisms of the additives governs the tablet strength.1. Reynolds GK, Campbell JI, Roberts RJ. A compressibility based model for predicting the tensile strength of directly compressed pharmaceutical powder mixtures. Int J Pharm [Internet]. 2017;531(1):215–24. Available from: http://dx.doi.org/10.1016/j.ijpharm.2017.08.0752. Sunkara D, Capece M. Influence of Material Properties on the Effectiveness of Glidants Used to Improve the Flowability of Cohesive Pharmaceutical Powders. AAPS PharmSciTech. 2018;19(4):1920–30.&amp;nbsp;3. Zuurman K, Maarschalk KVDV, Bolhuis GK. Effect of magnesium stearate on bonding and porosity expansion of tablets produced from materials with different consolidation properties. 1999;179:107–15.&amp;nbsp;4. Mishra SM, Rohera BD. Mechanics of tablet formation: a comparative evaluation of percolation theory with classical concepts. Pharm Dev Technol [Internet]. 2019;24(8):954–66. Available from: https://doi.org/10.1080/10837450.2019.1599913

On the fluidization of cohesive powders: Differences and similarities between micro‐ and nano‐sized particle gas–solid fluidization

AbstractThe fluidization of cohesive powders has been extensively researched over the years. When looking at literature on the fluidization of cohesive particles, one will often find papers concerned with only micro‐ or only nano‐sized powders. It is, however, unclear whether they should be treated differently at all. In this paper, we look at differences and similarities between cohesive powders across the size range of several nanometres to 10s of micrometres. Classification of fluidization behaviour based on particle size was found to be troublesome since cohesive powders form agglomerates and using the properties of these agglomerates introduces new problems. When looking at inter‐particle forces, it is found that van der Waals forces dominate across the entire size range that is considered. Furthermore, when looking into agglomeration and modelling thereof, it was found that there is a fundamental difference between the size ranges in the way they agglomerate. Where the transition between the types of agglomeration is located is, however, unknown. Finally, how models are made and agglomerate sizes are measured is currently insufficient to accurately predict or measure their sizes consistently.

Effects of different nano-agglomerated powders on the microstructures of PS-PVD YSZ coatings

Plasma spray physical vapor deposition (PS-PVD) is a technology that combines the advantages of traditional atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). As the feedstock of the PS-PVD, nano-agglomerated powder is critical on determining the microstructure of the obtained coating. In this study, a method to characterize the cohesion of nano-agglomerated powders was investigated. The nano-agglomerated powders fractured into smaller particles under ultrasonic waves. Their particle size distributions were measured to quantitatively compare their cohesiveness. The change rate in the percentage of powders with particle size less than 5 μm was selected as the value for the cohesion comparison. A high change rate corresponded to a faster fracture and lower powder cohesion. Furthermore, the fracture behavior and heat and mass transfer process of nano-agglomerated powders in the plasma torch were studied by combining 3-D simulation and observation of the microstructures of PS-PVD coatings sprayed with different powders. To obtain a quasi-columnar coating, the nano-agglomerated powder required high cohesion. Finally, a suitable powder was selected and quasi-columnar structure coatings were obtained by optimizing the PS-PVD parameters.

Dry powder coating in additive manufacturing

Dry powder coating is used in many industries to tailor the bulk solid characteristics of cohesive powders. Within this paper, the state of the art of dry coating of feedstock materials for powder based additive manufacturing (AM) processes will be reviewed. The focus is on feedstock materials for powder bed fusion AM processes, such as powder bed fusion of polymers with a laser beam and powder bed fusion of metals with lasers or an electron beam. Powders of several microns to several ten microns in size are used and the feedstock’s bulk solid properties, especially the flowability and packing density are of immanent importance in different process steps in particular for powder dosing and spreading of powder layers onto the building area. All these properties can be tuned by dry particle coating. Moreover, possibilities to improve AM processability and to manipulate the resulting microstructure (c.f. grain refinement, dispersion strengthening) by adhering nanoparticles on the powders will be discussed. The effect of dry coating on the obtained powder properties along the whole AM process chain and the resulting part properties is assessed. Moreover, appropriate characterization methods for bulk solid properties of dry-coated AM powders are critically discussed.

Powder cohesion and energy to break an avalanche: Can we address surface heterogeneity?

• The cohesion was related to the specific surface area. • The dispersive component of the surface energy (van der Waals forces) prevailed. • Surface heterogeneity results confirmed preferable drug-excipient interactions. • Alginic acid showed a better carrier effect than calcium alginate. • The W adh / W coh ratio near to unity confirmed stability of interactive mixtures.

Development of a predictive model for gravimetric powder feeding from an API-rich materials properties library

A model was developed for predicting the feed factor profile of a powder, processed through a gravimetric feeder, as a function of material properties and process parameters. Predictive models proposed in existing literature have often used excipients and active pharmaceutical ingredients (APIs) with good powder flow characteristics in their development. In this work, a material properties library containing a large proportion of APIs, as well as excipients and co-processed blends, was used to build the model and enhance the prediction of feed factor profile for cohesive powders. Gravimetric feeder trials were performed at varying mass flow rates and screw geometries to determine the feed factor profiles. A semi-empirical exponential model, with parameters fmax, fmin, and β, was then used to fit the experimental feed factor profiles. Bayesian optimisation and Support Vector Regression (SVR) modelling techniques were utilised to optimise and predict the exponential model parameters as a function of material properties. The parameters found to strongly influence the model were particle size, bulk density, FFC and FT4 rheometer parameters. Results showed low prediction errors between the estimated and experimental data. The final model produces good estimations of the feed factor profile and requires minimal powder consumption.

Experimental study on agglomeration characteristics and suspension velocity of cohesive adsorbent powders in a conical airflow cylinder

Pneumatic conveying is widely used in the process of injecting adsorbents into furnace. Agglomeration characteristics and suspension velocity of cohesive adsorbent powders are useful for the design and optimization of pneumatic conveying systems. Meanwhile, the agglomerate size significantly affects the suspension velocity of powders. In the present work, a conical airflow cylinder was used to suspend the cohesive adsorbent powders, and some important characteristics during suspension, such as the agglomerate size, suspension velocity, and agglomerate density were systematically studied. The effects of moisture content and particle size on the agglomerate size and suspension velocity were investigated. The results showed that the agglomerate size and suspension velocity increased with increasing moisture content and decreasing particle size. In addition, agglomerate density was measured, and an improved correlation for predicting the suspension velocity of cohesive powders was obtained. The correlation performed well, and the relative errors were better than ±20%. • Effects of water content and particle size on agglomeration behaviors are studied. • The agglomerate size increases with increasing moisture content. • Suspension velocity increases due to the increasing of agglomerate size. • An improved correlation for predicting the suspension velocity is obtained.

Decompaction wave propagation in a vibrated fine-powder bed.

We experimentally study the crack formation and decompaction-wave propagating in a vibrated powder bed consisting of glass beads of 5 μm in diameter. The vibrated powder bed exhibits three distinct phases depending on the vibration conditions: consolidation (CS), static fracture (SF), and dynamic fracture (DF). Particularly, we found an upward wave propagation in the DF regime when the powder bed is strongly vibrated. As a remarkable feature, we found that in fine cohesive powders, the decompaction-wave propagation speed normalized to gravitational speed is independent of the shaking strength. This result implies that the wave propagation speed is governed by the balance between gravity and cohesion effect rather than vibration strength. We also explore the universality of wave propagation phenomenon in coarser and low-density granular powders.

Titania coatings: A mechanical shield for cohesive granular media at high temperatures

Fine granular media are pivotal in thermochemical energy storage technology. Reactors based on granular materials store the heat using reversible reactions at high temperatures. Yet, powders become increasingly cohesive in those conditions. The rise of powder cohesion at high temperatures is one of the most irksome phenomena still limiting the scalability of this technology. We found titania coatings comprise an excellent solution to control cohesion in fine limestone powders at high temperatures. Limestone is the main component in granular flows running solid-based storage circuits based on the calcium looping process. It is also involved in many other industrial applications. Titania layers were used to shape stiffer carbonate surfaces at high temperatures (close to the Tamman point). The experiments conducted in this work investigated the benefits of these layers, examining the powder cohesion as the contact between particles evolved from rigid to plastic surfaces. In doing so, samples were subjected to different temperatures varying from 25 °C to 500 °C and preconsolidations up to 2 kPa. The results revealed that titania coatings shield (mechanically) carbonate particles, making surfaces more resilient to deformation while particles interact. The efficiency of titania layers was compared with samples coated with nanosilica, which is a solution broadly accepted nowadays for limestone powders. The experiments tackled one of the weaknesses of nanosilica coatings, namely their efficiency when particles are barely coated. Interestingly, at high temperatures, samples treated with titania outperformed those layered with nanosilica for surface coverages around 9%. Moreover, despite such a moderate amount of coverage, samples coated with titania reached an easy-flow regime even at high temperatures. However, samples treated with nanosilica fluidized less uniformly, and their flowability fell into a cohesive-flow regime in similar conditions. In conclusion, titania coatings represent an excellent alternative to deal with those flowability issues that still limit the scalability of solid-based storage technology.

An environmentally friendly turnkey method to determine pore volume of powdered catalysts

Determination of intraparticle pore volume of powdered catalysts is of great interest in a large number of applications, such as fluidized catalytic cracking. Established methods, including mercury intrusion porosimetry (MIP) and gas adsorption method can only measure the volume of pores within a limited range of pore diameter. In this paper, a turnkey method to determine the total intraparticle pore volume of powdered catalyst by water adsorption is proposed. The detailed experimental procedure, accuracy, factors influencing the quality of measurement results as well as the scientific basis of this method are discussed. The observed sudden increase of powder cohesion at the titration end point is characterized by storage modulus. The results of the water adsorption method are significantly correlated with the results of the widely accepted methods such as MIP. The relevant ASTM standard (D8393–21) has been granted. Quick, convenient, accurate measurement and environmentally friendly properties are the key industrial incentives of this method.

EFFECT OF GELATIN AS A BINDER ON TURMERIC EXTRACT TABLET FORMULATION

The main components for tablet formulations are the fillers, binders, crushers, thinners and lubricants. Among these components, the binders play a key role on the powder cohesion properties during the formation of tablet granules. In this study, turmeric extract tablets were formulated using 15 g of turmeric extract, 35 g of aerosil, 12.8 g of lactose, 1% magnesium stearate, 200 ml of aquadest and gelatin as a binder. To evaluate the effect of gelatin on the tablet formulation, the gelatin was then prepared in different formulation namely 0.1 g (formulation A), 0.2 g (formulation B) and 0.5 g (formulation C), respectivley. Based on the tablet properties, the turmeric tablet with formulation C met the standard requirements of friability less than 1%, the hardness in the range of 4 -8 kg and disintegration time under 15 minutes.

The physicochemical properties of fruit powders and their residence time on stainless steel surfaces are associated with their ease of removal by brushing

Dry cleaning activities such as brushing, scraping, and vacuuming have been widely used in low-moisture food production. However, the impact that product compositional differences and time between cleaning cycles have on the ease of food particulate removal is unclear. In our study, fruit powders were deposited on stainless-steel coupons, then removed using a hygienic brush. The residence time was defined as the duration between fruit powder deposition and attempted removal. Longer residence time resulted in elevated water activity (aw), which decreased the degree of removal for most fruit powders other than pumpkin powder and apple powder (sieve size: 2.36 mm). For instance, the aw of peach powder increased from 0.59 ± 0.01 on day 1 to 0.64 ± 0.01 on day 7, and residual powder after cleaning increased from 86.12 ± 1.73% to 93.26 ± 2.12% from day 1 to day 7. Peach powder and apple/pear/plum powder were the most difficult to remove, with average residual powder of 92.79 ± 4.97% and 58.41 ± 11.41%, respectively. Fruit powders that were more difficult to remove also exhibited higher cohesion and adhesion. For example, the cohesion and adhesion of peach powder at 76% relative humidity for 3 days were 4479.53 ± 681.86 N/m2 and 1347.53 ± 439.15 N/m2, respectively. By contrast, the cohesion and adhesion of pumpkin powder at 76% relative humidity for 3 days were only 102.34 ± 93.80 N/m2 and 90.64 ± 36.52 N/m2, respectively. These results suggested that aw could significantly influence the brush cleaning outcome in a manner that was dependent on the identity of the fruit powder.

Study on solid-liquid transition behaviors of cohesive powders and predict the critical arch-breaking gas flow rate

Cohesive powders cannot flow freely in the silo and even behave like solid, which brings a great challenge to the powder handling process. Aeration is an effective method used to break the arch structure formed above the silo outlet and to promote the powder flow. However, it is difficult to determine the solid-liquid transition behaviors and to predict the critical arch-breaking gas flow rate in the aerated discharge systems. In this paper, cohesive powders including alumina (Al2O3), pulverized coal (COAL) and calcium carbonate (CaCO3) were used as experimental materials. Powder flow properties were characterized by conducting shear test, permeability test, and aerated discharge experiment. Unconfined yield strength of powders under zero pre-consolidated stress was obtained based on using a linear extrapolation procedure of shear parameters. Aerated discharge characteristics of cohesive powders were determined and, the critical condition corresponding to the transition from solid-like to liquid-state and the stable condition distinguishing the funnel flow pattern and mass flow pattern were defined, respectively. The Brown and Richards model, and the flow/no-flow criterion of Jenike were modified to describe the powder flow state and to predict the solid mass flow rate under the action of aeration by introducing the positive gas pressure gradient. A logical algorithm was proposed to predict the critical arch-breaking gas flow rate by assuming the same gas distribution coefficient under different aeration conditions. As a precondition, the critical solid mass flow rate was calculated by an iterative algorithm giving the same order of magnitude of that from the experiment, which was further related to the stable solid mass flow rate and the powder permeability index. This simplification provided us with satisfactory results for all the powders producing errors generally less than 30%, a more easy-to-operate method to determine the critical arch-breaking gas flow rate.

Decoding Fine API Agglomeration as a Key Indicator of Powder Flowability and Dissolution: Impact of Particle Engineering.

Fine API agglomeration and its mitigation via particle engineering, i.e., dry coating, remains underexplored. The purpose was to investigate agglomeration before and after dry coating of fine cohesive APIs and impact on powder processability, i.e., flowability (FFC), bulk density (BD), and dissolution of BCS Class II drugs. Ibuprofen (three sizes), fenofibrate, and griseofulvin (5-20µm), before and after dry coating with varying amounts of hydrophobic (R972P) or hydrophilic (A200) nano- silica, were assessed for agglomeration, FFC, BD, surface energy, wettability, and dissolution. The granular Bond number (Bog), a dimensionless parameter, evaluated through material-sparing particle-scale measures and particle-contact models, was used to express relative powder cohesion. Significant powder processability improvements after dry coating were observed: FFC increased by multiple flow regimes, BD increased by 25-100%, agglomerate ratio (AR) reduction by over an order of magnitude, and greatly enhanced API dissolution rate even with hydrophobic (R972P) silica coating. Scrutiny of particle-contact models revealed non-triviality in estimating API surface roughness, which was managed through the assessment of measured bulk properties. A power-law correlation was identified between AR and Bog and subsequently, between AR and FFC & bulk density; AR below 5 ensured improved processability and dissolution. Agglomeration, an overlooked material-sparing measure for powder cohesiveness, was a key indicator of powder processability and dissolution. The significant agglomerate reduction was possible via dry coating with either silica type at adequate surface area coverage. Reduced agglomeration after dry coating also countered the adverse impact of increased surface hydrophobicity on dissolution.

Effect of magnetic flux density of bar magnet on removal rate of magnetic material in powder

In this study, in order to improve the iron removal rate of iron remover, the effects of magnetic flux density, number of magnetic poles and powder fluidity on the removal rate of filed SUS304 particles mixed with high-purity spherical silica powders of different particle sizes were investigated. The iron removal tests were performed using an iron removal testing device equipped with a single bar magnet. The silica powders mixed with filed SUS304 particles were sifted, and the weight of the filed SUS304 particles captured by the bar magnet was measured. Three types of bar magnets were prepared in order to investigate the effect of the maximum magnetic flux density and the number of magnetic poles. As a result, the number of magnetic poles was more effective in improving iron removal performance than the maximum magnetic flux density. In addition, the iron removal rate decreased with decreasing the nominal particle size of the silica powder. As a result of the constant load direct shear test, the shear cohesion of silica powders increased with decreasing the average particle size [Formula: see text]. Therefore, it was found that the iron removal performance deteriorates when the powder cohesion increases.