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Chemical Based Microencapsulation Technologies

Coacervation      Interfacial Polymerisation      In-situ Polymerisation      Liposomes      Inclusion Complexation

 

Coacervation

Sometimes called phase separation this technology is considered as the oldest true encapsulation technology first developed by the National Cash Register Company for carbonless copy-paper. Microencapsulation by coacervation, involves the phase separation of one or more hydrocolloids from the initial solution and the subsequent deposition of the newly formed coacervate phase around the active ingredient suspended or emulsified in the same reaction media. After being hardened, the wall of the microcapsules forms a cross-linked structure, thus the microcapsules have good thermal and moisture-resistant properties and can be used for controlled release applications. Coacervation generally involves a number of steps which are carried out under continuous agitation

1. Disperse the oil phase in a solution of a surface active hydrocolloid.
2. Precipitate the hydrocolloid onto the oil phase by lowering the solubility of the hydrocolloid (add a non-solvent or change pH or temperature)
3. Induce the formation of the polymer-polymer complex by addition of a second complexing hydrocolloid.
4. Cross-link to stabilise the microcapsule
5. Dry the material to form microcapsules with sizes in the range 10-250microns

Although many materials (e.g. alginates, chitosan, starch, and methyl cellulose) can be used the most commonly used are mixtures of proteins and anionic polysaccharides. Perhaps the most well-known example is gelatin and gum arabic.

At pH values above 6 both these materials are miscible, however when the pH is lowered below gelatin’s isoelectric point the net charge on the gelatin becomes negative and it then interacts with the positively charged gum arabic. The active ingredient, e.g. dye dissolved in an oil phase is first emulsified at about 40% in 10%w/v gelatin. The mixture is then added to about 2 parts gum arabic solution, maintaining the pH of the system above 5. As the pH is lowered to 4.5, microcapsules form from coacervate material positively charged gelatin and negatively charged gum arabic depositing around the oil droplets. After wall formation the wall is hardened by cross linking with for example glutaraldehyde or alternatively using transglutaminase.

 
 

Interfacial Polymerisation

This technique is based on the classical technology involved in interfacial polycondensation polymerisation which is widely used to produce synthetic fibres such as polyester, nylon and polyurethane and is characterized by wall formation via the rapid polymerization of monomers at the surface of the droplets or particles of dispersed core material. A multifunctional monomer is dissolved in the core material, and this solution is dispersed in an aqueous phase. A reactant to the monomer is added to the aqueous phase, and polymerization quickly ensues at the surfaces of the core droplets, forming the capsule walls. Interfacial polymerization can be used to prepare bigger microcapsules, but most commercial interfacial polymerization processes produce smaller capsules in the 20-30 micron diameter range, or even smaller 3-6 micron diameter range for carbonless paper ink.

 

In interfacial polymerization, the two reactants in a polycondensation meet at an interface and react rapidly. The basis of this method is the classical Schotten-Baumann reaction between an acid chloride and a compound containing an active hydrogen atom, such as an amine or alcohol, polyesters, polyurea, polyurethane. Under the right conditions, thin flexible walls form rapidly at the interface.

 
 
In-situ polymerisation
 

In situ polymerization is a chemical encapsulation technique very similar to interfacial polymerization. The distinguishing characteristic of in situ polymerization is that no reactants are included in the core material. All polymerization occurs in the continuous phase, rather than on both sides of the interface between the continuous phase and the core material, as in interfacial polymerization. Examples of this method include urea-formaldehyde (UF) and melamine formaldehyde (MF) encapsulation systems. Typically an oil-phase is emulsified in water using water-soluble polymers and high shear mixers yielding a stable emulsion at the required droplet size. A water-soluble melamine resin is added and dispersed. The pH is then reduced by the addition of acid initiating the polycondensation which yields cross-linked resins that deposit at the interface between the oil droplets and the water phase. During hardening of the wall material the microcapsules form and the aqueous dispersion of polymer-encapsulated oil droplets is produced.

 
 
Liposomes

A liposome is a tiny vesicle generally made from phospholipids which spontaneously form when disrupted in water, with diameter ranging from 25nm to 10microns. Both hydrophobic and hydrophilic active ingredients can be entrapped. The vesicles are made of a bilayer similar to that of a cell membrane hence orient themselves so that the inner and outer phase is hydrophilic. The central core can therefore contain water soluble active ingredients with hydrophobic ingredients being trapped within the bilayer. In a similar way micelles can be formed to entrap hydrophobic materials in the central core.

 

 
 
Inclusion Complexation

Inclusion complexation involves the use of cyclodextrins which are a group of structurally related natural products formed during bacterial digestion of cellulose (Martin Del Valle, 2004). These cyclic oligosaccharides consist of (α-1,4)-linked α-D-glucopyranose units and contain a lipophilic central cavity and a hydrophilic outer surface. Cyclodextrin inclusion is a molecular phenomenon in which usually a single guest molecule interacts with the cavity of a cyclodextrin molecule to become entrapped and form a stable association. Molecules or functional groups of molecules which are less hydrophilic than water can be included in the cyclodextrin cavity in the presence of water. In order to become complexed, the "guest molecules" should fit, at least partly, into the cyclodextrin cavity. The cavity size as well as chemical modifications to the cyclodextrin defines the affinity of the various guest molecules. In the case of some low molecular weight molecules, more than one guest molecule may fit into the cavity.

 

 
 
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