Dry Powder Coating

Microencapsulation entails the formation of new particles that have a certain material as the core,

with a surrounding coat composed of a distinctly different material. The core can be solid, liquid or

gas, with the coat usually formed of a solid (Singh et al., 2010). Dry particle coating / hybrid mixing is

an emerging process whereby fine guest particles are strongly adhered to the surface of a coarser host

particle, forming new functionalised particles in the solid state (Dahmash, 2016). This form of mixing

utilises the cohesive nature of the fine particles and attracts them to the surface of the host particle,

as long as the difference in size is at least a magnitude of two. The coating of the fine particles is due

to the cohesive forces that are present between the coarse particle and the fine particle, with surface

energies also playing a part, and the fine particles not weighing a substantial amount. The weight plays

a huge role in the adhesion of the fines as the coating only occurs if the weight of the fine particle is

lower than the forces of attraction present between the guest and the host. The low weight of the

guest causes the fine particle to be attracted and adhere to the surface of the coarse particle,

producing a mechanically strong coating, leading to newly formed functionalised particles. This is very

useful for fine particles, which usually would be difficult to process due to problems with

agglomeration/segregation, as during dry particle coating they would be evenly dispersed upon the

surface of the host particle, allowing a uniform blend to be produced. (Dahmash and Mohammed,

2015, Ishizaka et al., 1989, Honda et al., 1994, Alonso et al., 1990). During the dry particle coating

mixing process the agglomerates of the fine powder would be uniformly dispersed prior to coating,

therefore promoting attraction to the surface of the coarse particles and leading to production of

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Figure 1.6. The level of coating would depend on the amounts of each excipient present within the

blend, the particle size and true density of both and the levels of attractive forces generated between

the two materials (Yang et al., 2005). Dry particle coating has several advantages over traditional

mixing methods especially for powders utilised in direct compression. This processing method

produces very flowable powders from excipients that are very fine and cohesive in nature, allowing

the powder to flow freely during the tableting process (Han et al., 2013, Yang et al., 2005, Dahmash,

2016). The utilisation of two excipients to produce new functionalised particles is also advantageous,

as the beneficial aspects of each of the excipients are maximised whilst reducing the negative aspects.

This can lead to production of particles with not only an increased flowability but also for example,

enhanced dissolution behaviour, a sustained release, an improved palatability or an increased

wettability (Han et al., 2013, Dahmash, 2016). Dry particle coating is also a one-step, solvent free

method of processing powders, which utilises widely available excipients to produce new

functionalised particles, thereby providing not only an environmentally friendly mixing method, but

also a very cost effective manufacturing process for new materials (Dahmash, 2016). As it is a relatively

Figure 1.6: Figure showing the dry particle coating mixing process, whereby agglomerates of fine particles are

dispersed prior to attraction and coating on to a coarse particle, levels of coating can vary between a discrete coat, where an element of the coarse particle surface is covered with the guest or a continuous coating where the whole host particle is encapsulated with particles of the fine guest particle.

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new mixing concept, a multitude of dry coating devices have been developed over recent decades;

with devices utilising high shear forces in a mechanofusion dry coating method (Yokoyama et al.,

1987); devices utilising high shear in a hybridizer mixing method (Ishizaka et al., 1988); dry coating

devices that utilise magnetic assistance (Beach, 2011) and dry particle coating using a fluidised energy

mill (Han et al., 2013). With these methods particles may undergo some forms of chemical alteration

and will often suffer attrition and damage during processing, therefore leading to changes in particle

size and potential alterations in crystallinity and stability of the materials being processed. Also high

levels of friction are generated in these processes which leads to a heating effect, meaning that heat

sensitive materials may not be suitable for processing. With certain methods particles are required to

be of a certain size, otherwise they cannot be processed, which does limit the materials that can be

utilised for these particular dry coating methods.

A novel dry particle coating equipment developed by our research group was designed to overcome

the limitations associated with previous devices (Dahmash, 2016). This dry particle coater provides a

one-step, environmentally friendly process of producing functionalised particles, which utilises no

solvents or generates no heat, with the particles not suffering any form of attrition during the mixing

process. The device was able to produce materials with enhanced flowability, very high content

uniformities, and able to produce particles with new functionalities such as modified release APIs

(Dahmash, 2016), showing that the disadvantages associated with previous dry coating technologies

could be minimised whilst enhancing the advantages of the process. The manufactured device was

not only able to produce particles with new functionalities, but was able to enhance powder

properties, with content uniformity and powder flow of blends produced in the mixer being superior

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