With the growing trend in enhancing beauty through healthy means, customers request for apparels and home textiles containing not only their original basic characteristics, such as warmth and comfort, but also ones that carry extra functions, including environmental protection, anti-pollution and most importantly, health and beauty care, in an attempt for a more natural and healthier life.
Owing to the rapid development of novel sciences and technologies, textile materials have also found applications in the cosmetics field in recent years. A new sector of cosmetic textiles is launched and the textile industry is very optimistic that these products will open up new target groups and sustainable markets.
On contact with human body and skin, cosmetic textiles are designed to transfer an active substance for cosmetic purposes. One particular example is the transfer of skin moisturising substances. The principle is achieved by simply imparting the cosmetic and pharmaceutical ingredients into the fabric of the clothing so that with the natural movements of the body, the skin is slowly freshened and revitalised. To achieve these functional effects, microencapsulation technology appears as an alternative way to provide satisfactory performance with increased durability.
Microencapsulation Technology and Its Advantages
Currently, microencapsulation technology is rapidly developing in the field of chemical finishing because of its versatility and flexibility.
One major advantage of using microencapsulation technology is its ability to protect the active ingredients from hazardous environments, i.e. oxidisation, heat, acidity, alkalinity, moisture or evaporation.
It also simultaneously, protects the ingredients from interacting with other compounds in the system, which may result in degradation or polymerisation. Another important advantage of this versatile technology is its controlled release properties that seem to be the best choice for increasing efficiency and minimising environmental damage.
Microencapsulation is actually a micro packaging technique that involves the production of microcapsules which act as barrier walls of solids or liquids. The microcapsules are produced by depositing a thin polymer coating on small solid particles or liquid droplets, or on dispersions of solids in liquids. The core contents are released under controlled conditions to suit a specific purpose.
An active ingredient is the substance that may be in a liquid or solid form. It also refers to the core contents, internal phases, encapsulations, payloads or fillers.
Wall Shell
A polymer coating that surrounds the active ingredients which may also be called the wall, shell, external phase, membrane or matrix. It may be natural, semi-synthetic or synthetic polymer.
The release mechanisms of the core contents vary depending on the selection of wall materials and more importantly, its specific end uses. Table 1 demonstrates the relationship between the textiles end uses and their release mechanisms. The core content may be released by friction, pressure, change of temperature, diffusion through the polymer wall, dissolution of the polymer wall coating, biodegradation etc.
Microencapsulating Methods
Complex Coacervation
This method takes advantage of the abilities of cationic and anionic water soluble polymers to interact with water, forming a liquid, polymer-rich phase called complex coacervation. When the complex coacervate forms, it will be in equilibrium with a dilute solution called the supernatant. The supernatant acts as the continuous phase, whereas the complex coacervate acts as the dispersed phase. As the water insoluble core materials are dispersed in the system, each droplet or particle of dispersed core material is spontaneously coated with a thin film of coacervate. The liquid film is then solidified to make the capsules harvestable. This method has been applied to encapsulate much water immiscible liquids and is used in a variety of products.
Polymer-Polymer Incompatibility
Two chemically different polymers dissolved in a common solvent are incompatible and do not mix in the solution. The essential chemicals repel each other and form two distinct liquid phases. One phase is rich in polymer and designed to act as the capsule shell while the other is rich in incompatible polymer. The incompatible polymer is presented in the system to cause the formation of two phases. It is not designed to be part of the final capsule shell, although a small amount may remain entrapped in the final capsule as an impurity. The process is normally carried out in organic solvents and used to encapsulate solids with a finite degree of water solubility.
Interfacial Polymerisation and in Situ Polymerisation
In interfacial polymerisation, the capsule shell is formed at or on the surface of a droplet or particle by polymerisation of reactive monomers. A multi-functional monomer is dissolved in the liquid core material. The resulting solution is dispersed to a desired drop size in an aqueous phase that contains a dispersing agent. The aqueous coreactant, usually a multifunctional amine, is then added to the aqueous phase. A rapid polymerisation reaction is then produced at the interface which finally generates the
capsule shell. Both the liquid and solid can be encapsulated by interfacial polymerisation reactions, but the polymerisation chemistry is typically different.
For in-situ polymerisation, capsule shell formation occurs because of the polymerisation of monomers that is added to the encapsulation reactor, similar to interfacial polymerisation. However, no reactive agents are added to the core material.
Polymerisation occurs exclusive in the continuous phase and on the continuous phase side of the interface formed by the dispersed core material and continuous phases. Polymerisation of reagents located there produces a relatively low molecular weight prepolymer. As this prepolymer grows in size, it deposits onto the surface of the dispersed core material being encapsulated, where polymerisation with cross linking continues to occur, thereby generating a solid capsule shell.
Spray Drying
Spray drying serves as a microencapsulation technique when an active material is dissolved or suspended in a melt or polymer solution and becomes trapped in the dried particle. In the widely used spray drying process, the dried solid is formed by spraying an aqueous solution of the core material and the film forming wall materials as fine droplets into hot air. The water then evaporates and the dried solid is usually separated by air-separation.
This method has been used to encapsulate labile materials because of the brief contact time in the drier. However, one disadvantage of using the spray drying method is that some low boiling point aromatics can be lost during the drying process. Another disadvantage is that the core material may also form on the surface of the capsule, which allows for oxidation and possible scent changes of the encapsulated product.
Centrifugal Extrusion
In centrifugal extrusion processes, liquids are encapsulated by using a rotating extrusion head with concentric nozzles. The fluid core material is pumped through a central tube while the liquefied wall material is pumped through a surrounding annular space. A membrane of wall material is formed across a circular orifice at the end of the nozzle and the core material flows into the membrane, causing the extrusion of a rod of material. Droplets break away from the rod and hardening takes places on a passage through a heat exchanger. Solid capsules are removed by filtration or mechanical means and the immiscible carried fluid is reheated and recycled after passing through the files.
This process is excellent for forming particles of 400-20001Jm in diameter. Since the drops are formed by the breaking up of a liquid jet, the process is only suitable for liquid or slurry.
Air Suspension Coating
In air suspension coating, the particles are coated by dissolved or molten polymers while suspended in an upward-moving air stream.
During the process, the solid particles to be encapsulated are first placed in a coating chamber where they are suspended in an air stream, which causes the cyclic flow of particles passing through a nozzle at the chamber bottom.
The nozzle sprays a liquid coating phase onto the particle. The freshly coated particles are carried away from the nozzle by air stream and up into the coating chamber. The coating solidifies because of solvent evaporation or cooling of a melt. At the top of the spout, the particles settle back into the bottom of the chamber to repeat the cycle. The cycle is repeated many times during the time frame of a few minutes until the coating has been applied to the desired level of thickness.
Air-suspension coating of particles by solutions or melts generally gives better control and flexibility. However, it is commonly used to encapsulate tablets, granules, crystals and powders. It is not used with liquid unless they are absorbed on a porous solid.
Pan Coating
Widely used in the pharmaceutical industry, this method is a traditional industrial procedure for forming small, coated particles or tablets. During the pan coating process, the particles are tumbled in a rotating pan or other device while the coating material is applied slowly at a controlled temperature profile. Additional coatings of film-forming polymers may be added in successive stages.
Emulsion Hardening Process
Emulsion hardening microencapsulating processes can be achieved when the core compound is highly soluble in the polymer solution (wall). The mixture is emulsified in an immiscible liquid and then the solvent is removed by evaporation, extraction etc. The core compound is solidified inside the polymer solution droplet and thus, forms the microcapsule. One typical example of this process is the production of polylactic acid microcapsule for use in injectable particle systems.
This article was originally published in the December issue of the magazine, New Cloth Market the complete textile magazines from textile technologists."
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