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Intrinsic and extrinsic factors of powders
Powders behavior can be impacted by a multitude of factors. Nevertheless, they are influenced by some intrinsic and extrinsic factors. The intrinsic factors are used to denote the specific properties of powders: morphology, porosity, size, size distribution, chemical surface composition, etc. While the extrinsic factors are linked to the process conditions which are mainly temperature and humidity. These extrinsic factors can influence particle generation process or the characteristics of the generated particles. The temperature has an effect on the molecular properties of the powder that can modify the structure of particles, especially their surface [13]. Also mechanical stresses applied to the powder, interactions of the powder with the equipment are extrinsic factors that influence powders behaviors.
Particle morphology
Particle morphology plays an important role in powder behavior such as filtration of suspensions, flowability of powders, compression, etc. It also influences the measured values of the particle size. Particles rarely come in a simple geometric form such as a sphere or a cube. A common way of classifying particles according to their physical appearance is through standardized descriptive terms presented in Table 2.1.
By considering that spherical particle with a smooth surface has fewer contact points and promote good flow behavior [14]. However, a non-spherical particle such as irregular and fibrous shape has a lower flow behavior and it is difficult to interpret their behavior. Meaning that more a particle is far from sphericity more its surface specific and complexity increases. In non-spherical particles, more inter-particle interaction come to play role in governing their behavior since it has more surface contac [15].
Intrinsic and extrinsic factors of powders
In considering powder behavior, surface roughness is an important factors which can has substantial influence in inter-particle interaction. As an instance, in the particles with surface roughness the contact area of particles decrease as the result of increasing distance in contacting particles which it can lead to reduction of the attractive inter-particle interactions. The role of surface roughness has been studied extensively in the literature [16–20].
In overall, increasing surface roughness results in decreasing cohesion consequently powder flowability increases. However, other studies reported the incense of powder flow as a result of smoothing the rough surface of particles. Smoothing particles surface decreases the mechanical interlocking of particles which reduces the inter-particle friction [21–25]. Finally, a study conducted by Raula et al. indicated that the scale of surface roughness is an important factor to determine their effect on inter-particle interaction [26]. Whereby, they reported a decrease in the emission and dispersion due to the mechanical interlocking with increase in surface roughness size.
Particle porosity
A particle with a smooth or porous characteristic may greatly affect the collective properties of the granular medium. When a medium consists of porous particles the void fraction is much greater than if it consists of solid particles. In general, if the medium is dry, the air traps in void fraction corresponds to the inter-particle porosity. In wet medium like as suspension medium, the void fractions are filled with the liquid. With regard to an assembly of particles, the porosity (ε) is an important characteristic and defined as the ratio between the volume of the inter-particle void particles and the total volume occupied by the particles in a bed. It is independent of the absolute size of the particles [27]. ε = Vpores = Vpores (2.1).
However, the void of a granular medium can be divided into inter-granular (like as powder bed porosity) and intra-granular pores (such as powder surface porosity). In a random homogeneous medium, the surface porosity of the system is equal to the volume porosity. The density of the granular medium can therefore be calculated by the following expression: ρb = (1 − ε)ρp + ερf (2.2).
while: ρb is the apparent density of the granular medium (kg/m3), ρp is the real density of the particle (kg/m3), ρf is the density of the fluid that fills the pores (kg/m3).
Whereby, the dry granular medium is a set of particles, powders or grains with pores filled with a gaseous fluid. The components of this environment are always subject to the action of gravity but there are also interactions between them which are not negligible. Powders with a high porosity have high tendency to trap air and flow with difficulty, since the ability to release air is low [28].
Particle size and size distribution
The particle size has been reported extensively in the literature as a factor that influence the bulk behavior of powders [29–34]. Farley and Valentin [34] reported an empirical correlation between bulk cohesion and contact surface area of particle for inorganic materials. They have reported that cohesion increases with decreasing particle size and increasing surface area. Also, a study on milk powders conducted by Fitzpatrick et al. [35] showed that powder flow function increases as particle size increases. Therefore, particle size has a major influence on powder flow behavior. A powder is considered as having a good flowability with a particle size larger than 200 m; between 200 and 100 m it is a transition region and under 100 m the flowability gets worse. One may not notice a major change in flowability as size is reduced from 80 to 60 m while a noticeable decrease in flowability would be expected if the powder is reduced in size by an order of magnitude, (i.e. from 100 to 10 m). This reduction in flowability at smaller particle size is due to the increased surface area of the powder. Meaning that in smaller particle size, the inter-particle force plays a significant role in comparing with gravity force.
The influence of particle size and size distributions on the flow behavior of a powder or a granular materials have been widely studied experimentally and numerically [36,37]. The flowability of powders is influenced by size distribution, particularly by the presence of fine particles. The presence of fine particles tend to block the flow. However, The numerical study, further validated with experimental findings, revealed that a fine size fraction could significantly affect positively the negatively during powder flow through a hopper [38]. In other words, the presence of fine particles into the size distribution may help to the flow behavior of powders. Meaning that the fine particles can fill the hollows at the surface of the particle, it decreases the surface friction force by increasing the inter-particle distance, and reduces the inter-particle forces where the powder flow behavior increases.
Particle surface chemical composition
In the literature, many studies reported the efficiency of surface chemical composition of powders in flow behavior. Different techniques were used for the surface composition of powders but the results were indicative of improved flowability in most of studies. A study conducted by Ehlers et al. [40] presented a thin-coating on pharmaceutics powder with improved flowability without affecting the particle size, shape and size by implementing fluidized bed coating technique. In another study by Genina et al. [22] the surface composition on drug particles reported a thin coating layer which was enough to decrease cohesion forces and to improve flow behavior of powders. In this case, the ultrasound-assisted fine polymeric mist deposition technique is utilized.
In an alternative research, the spray drying method is implemented for surface modification of particles, which result in reducing cohesion and improving bulk behavior of pharmaceutical powders [24,41]. Also, sufficient amount of lipid coating on particles presented an strong efficiency in flow behavior of powders in drug delivery to the lunges, the coating reduced friction between particles by addition of a layer of lipid to particle surface [42]. In a study on food powders, using lecithin coating additive for milk powders improved their wettability [43]. Also it has been reported that adding small amounts of surface active additives to a lactose solution prior to spray drying can increase flowability of lactose powders [44]. Moreover, a study reported that a coating can be helpful in reducing the electrostatic charging of micronized powders [45]. Therefore, modifying the surface properties may significantly influence positively the flowability of powders.
Environmental conditions
Humidity is one of the major extrinsic factors that effects powder flow behavior by modifying inter-particle interaction and causes irregular flow behaviors. This phenomenon is function of the quantity
of water absorbed at the particle surface and is independent of the original water content of the powder. The water content is different from powder to powder and is influenced by factors like storage, temperature, relative humidity and/or exposed surface. Powder water absorption is therefore a problem of physico-chemical properties of the surface [46]. Based on the quantity of the water absorbed by the surface and on the particles structure, different results can appear as follow [14]:
(a) If the particle has capability of water intake, then absorption of moisture soften the surface and increases adhesion between particles.
(b) If the particle does not has water absorption ability, the moisture covers in a multi-molecular thick layer on the surface of particle.
Depending on the amount of air that remains between particles, at very low humidity, particles adhere to each other by electrostatic charge. Increasing humidity until a mono-molecular layer forms decreases in adhesion by removing electrostatic charges and increase the flow behavior of powder. While, too high relative humidity increases the surface area and the number of contact points between particles by appearing the capillary condensation. Therefore, there will be an increase in adhesion and cohesion between particles, consequently it lowers the flow behavior of the powder [47].
Table of contents :
1 General introduction
2 Literature Review
2.1 Introduction
2.2 General information on granular material
2.3 Intrinsic and extrinsic factors of powders
2.3.1 Particle morphology
2.3.2 Particle surface roughness
2.3.3 Particle porosity
2.3.4 Particle size and size distribution
2.3.5 Particle surface chemical composition
2.3.6 Environmental conditions
2.3.7 Processing parameters
2.4 Particle interactions in granular media
2.5 Inter-particle forces between solid particles
2.5.1 Van der Waals forces
2.5.2 Capillary force
2.5.3 Electrostatic force
2.5.4 Gravity force
2.5.5 Relative importance of different forces
2.6 Techniques to measure powder flowability
2.7 The repose angle
2.8 Dynamic angle
2.9 Powder settlement and compressibility tests
2.10 Flow measurements through a fluidized bed
2.11 Flow measurement through an orifice
2.12 Flow measurement in shear cell
2.13 Rheological measurements
2.14 Conclusion
3 Influence of surface formulation and particles size on powder flowability
3.1 Introduction
3.2 Powder surface formulations
3.3 Powder flowability based on different particle sizes and surface treatement
3.3.1 Introduction
3.3.2 Materials and methods
3.3.3 Results and discussion
3.3.4 Supplemental data
3.3.5 Conclusion
3.4 Surface hardness and elasticity of formulated powders
3.5 Concluding diagram
4 Comparison of different flow measurement techniques
4.1 Introduction
4.2 Powder characterisation with Granutools equipement
4.2.1 Granutools methodology
4.2.2 Granutools measurement results and discussions
4.3 Powder characterization with the Discovery-HR3 Rheometer
4.3.1 Methodology
4.3.2 Newtonian and Frictional regimes
4.4 Powders behavior at a controlled temperature and humidity conditions
4.4.1 The powders flow behavior after 50 % humid control
4.4.2 Dynamic water sorption (DVS) of formulated powders
4.5 Conclusion .
5 Influence of humidity and water addition on powder behavior
5.1 Introduction
5.2 The powder preparation and rheology protocol
5.3 Influence of a wide range of humidity on powder behavior
5.3.1 Comparison of influence of humidity on flowability of control glass bead with two different sizes
5.3.2 Influence of humidity on flowablity of hydrophobic glass beads
5.4 Powder flow behaviors governed by the surface properties of particles
5.4.1 Introduction
5.4.2 Materials and Methods
5.4.3 Results .
5.4.4 Discussion
5.4.5 Conclusion
5.5 Influence of water addition on powder behavior
5.6 Conclusion of the chapter
General conclusion and perspectives
Résumé étendu en français
References
