Microencapsulation.pdf

Encyclopedia of Polymer Sceince and Technology

MICROENCAPSULATION

Introduction

Microencapsulation is the coating of small solid particles, liquid droplets, or gas

bubbles with a thin ﬁlm of coating or shell material. In this article, the term

microcapsule is used to describe particles with diameters between 1 and 1000

m are called nanoparticles ; particles greater than

m can be called microgranules or macrocapsules .

Many terms have been used to describe the contents of a microcapsule: active

agent, actives, core material, ﬁll, internal phase (IP), nucleus, and payload. Many

terms have also been used to describe the material from which the capsule is

formed: carrier, coating, membrane, shell, or wall. In this article the material being

encapsulated is called the core material ; the material from which the capsule is

formed is called the shell material .

Although virtually any coating material, conceptually, is a candidate micro-

capsule shell material, most commercial microcapsules produced to date utilize a

relatively small number of different shell materials. Table 1 lists representative

examples of these materials along with typical applications. Microcapsule shell

material selection for a speciﬁc application is determined by a number of factors

including cost, availability, processing ease, and inherent barrier properties. Shell

materials for pharmaceutical, food, and personal care products are limited to ma-

terials that are approved by regulatory agencies responsible for such products.

Deﬁning an optimal shell material for a given application can be complex, since

many interacting parameters determine success of a given capsule shell material.

Fortunately, many suppliers of candidate shell materials provide valuable infor-

mation concerning the use of their products in various encapsulation processes.

Microcapsules can have a wide range of geometries and structures. Figure

1 illustrates three possible capsule structures. Parameters used to characterize

microcapsules include particle size, size distribution, geometry, actives content,

storage stability, and core material release rate. Characterization studies provide

valuable insight into the nature of various types of capsules and how they function

in speciﬁc applications.

Throughout the world, research and development activity dedicated to ad-

vancing microcapsulation technology is producing a steadily increasing number of

commercially successful products that utilize microcapsules. Many established en-

capsulation processes like spray drying and ﬂuidized bed coating are being reﬁned

and improved while new processes continue to be developed. Various reviews of

this subject exist (1–6). Pan coating, a well-established means of producing coated

m. Particles smaller than 1

1000

Table 1. Shell Materials Used to Produce Commercially Signiﬁcant Microcapsules

Shell material

Regulatory status

Chemical class

Encapsulation process

Applications

Gum arabic

Edible

Polysaccharide

Spray drying

Food ﬂavors

Gelatin

Edible

Protein

Spray drying

Vitamins

Gelatin–gumarabic a

Nonedible b

Protein–polysaccharide

complex

Complex coacervation

Carbonless paper

Ethylcellulose

Edible

Cellulose either

Wurster process or

polymer–polymer

incompatibility

Oral pharmaceuticals

Polyurea or polyamide

Nonedible

Cross-linked polymer

Interfacial

polymerization in situ

polymerization

Agrochemicals and

carbonless paper

Aminoplasts

Nonedible

Cross-linked polymer

Carbonless paper,

fragrances, and

adhesives

Maltodextrins

Edible

Low molecular weight

carbohydrate

Spray drying and

desolvation

Food ﬂavors

Hydrogenated vegetable oils Edible

Glycerides

Fluidized bed

Assorted food ingredients

a Treated with glutaraldehyde.

b For intended application, ie, carbonless paper.

MICROENCAPSULATION

Fig. 1. Schematic diagrams of several possible capsule structures: ( a ) continuous

core/shell microcapsule in which a single continuous shell surrounds a continuous region

of core material; ( b ) multinuclear microcapsule in which a number of small domains of core

material are distributed uniformly throughout a matrix of shell material; and ( c ) continu-

ous core capsule with two different shells.

particles for the pharmaceutical industry, is not discussed because it produces

particles larger than 1000

Encapsulation Processes

Because there are so many encapsulation processes, various schemes have been

created in order to classify them. Some schemes classify encapsulation processes

as chemical or physical even though so-called chemical processes may be based ex-

clusively on physical phenomena, while physical processes may involve chemical

reactions. This author prefers to simply classify encapsulation processes as type

A or type B. Type A processes are deﬁned as those in which capsule formation oc-

curs entirely in a liquid-ﬁlled stirred tank or tubular reactor. Type B processes are

processes in which capsule formation occurs because a coating is sprayed or de-

posited in some manner onto the surface of a liquid or solid core material dispersed

in a gas phase or vacuum. This latter category includes processes in which liquid

droplets containing core material are sprayed into a gas phase and subsequently

solidiﬁed to produce microcapsules. Emulsion and dispersion stabilization play a

key role in the success of both type A and B processes.

Differences in some type A and type B processes can be subtle, but signiﬁ-

cant. For example, solvent evaporation is a key step in spray dry encapsulation

protocols (type B), and protocols involving solvent evaporation from an emulsion

(type A). The difference in these protocols is that evaporation in the former case

occurs directly from a liquid to a gas phase, whereas in the latter case evaporation

involves transfer of a volatile liquid from a dispersed phase to a continuous liquid

phase from which it is subsequently evaporated. Another example is encapsula-

tion by gelation. In type A gelation processes, the droplets that are gelled and

become microcapsules are formed by dispersion in a liquid phase and are gelled in

this phase. In type B gelation processes, droplets formed by atomization or extru-

sion into a gas phase are subsequently gelled either in the gas phase or a liquid

gelling bath.

Representative examples of both types of processes follow. Type B processes

tend to be promoted by organizations that sell and service equipment for producing

MICROENCAPSULATION

microcapsules. Most type A processes are developed and used in-house by organi-

zations that produce microcapsules.

Type A processes

Type B processes

Complex coacervation

Spray drying

Polymer–polymer incompatibility

Fluidized bed

Interfacial polymerization at liquid–liquid

and solid–liquid interfaces

Interfacial polymerization at solid–gas or

liquid–gas interfaces

In situ polymerization

Centrifugal extrusion

Solvent evaporation or in-liquid drying

Extrusion or spraying into a desolvation

bath

Submerged nozzle extrusion

Rotational suspension separation

(spinning disk)

Type A Processes

Complex Coacervation. This process occurs in aqueous media and is

used primarily to encapsulate water-immiscible liquids or water-insoluble solids

(4,5). In the complex coacervation of gelatin with gum arabic (Fig. 2), a water-

insoluble core material is dispersed to a desired drop size in a warm gelatin solu-

tion. After gum arabic and water are added to this emulsion, pH of the aqueous

phase is typically adjusted to 4.0–4.5. This causes a liquid complex coacervate of

gelatin, gum arabic, and water to form. When the coacervate adsorbs on the sur-

face of the core material, a liquid complex coacervate ﬁlm surrounds the dispersed

Fig. 2. Flow diagram of a typical encapsulation process based on the complex coacervation

of gelatin with gum arabic.

MICROENCAPSULATION

m, but capsules outside this range can be prepared.

Although core contents are often 80–95 wt%, capsules with lower loadings can

be made. Complex coacervation processes are adversely affected by active agents

that have ﬁnite water solubility, are surface-active, or are unstable at pH values

of 4.0–5.0. The shell of dry complex coacervate capsules is sensitive to variations

in atmospheric moisture content and becomes plasticized at elevated humidities.

Simple Coacervation. Aqueous solutions of water-soluble polymers are

phase-separated in aqueous media when sufﬁcient salt is added to such solutions.

This phenomenon is called simple coacervation . As long as phase separation pro-

duces a liquid polymer-rich phase, simple coacervation can be used to produce

microcapsules (5). Microcapsules with a gelatin or poly(vinyl alcohol) shell have

been formed in this manner. The use of poly(vinyl alcohol) as a capsule shell ma-

terial is of great interest in various applications because it is a widely available

synthetic polymer with excellent oxygen and oil barrier properties (see B ARRIER

P OLYMERS ;V INYL A LCOHOL P OLYMERS ).

Polymer–polymer Incompatibility. Two chemically different polymers

dissolved in a common solvent are usually incompatible. That is, they sponta-

neously separate into two liquid phases, with each phase containing predomi-

nately one polymer. When a core material insoluble in the solvent is dispersed in

such systems, it is spontaneously coated by a thin ﬁlm of the liquid phase that

contains the polymer designed to be the capsule shell material; this polymer must

be preferentially adsorbed by the core material which is not difﬁcult to arrange.

Microcapsules are harvested by desolvating this coating either by chemical cross-

linking or addition of a nonsolvent (Fig. 3) (4,5). In the latter case, the embryonic

capsule as slurry is often added to a very large excess of nonsolvent in order to

minimize capsule agglomeration during isolation.

Polymer–polymer incompatibility encapsulation processes can be carried out

in aqueous or nonaqueous media but, thus far, have primarily been carried out in

core material, thereby forming embryo microcapsules. The system is subsequently

cooled, often below 10 ◦ C, thereby gelling the liquid coacervate shell. After gelation,

the coacervate gel typically is chemically cross-linked. For many years, glutaralde-

hyde was the preferred cross-linking agent, but an enzyme, transglutaminase, has

now emerged as a candidate cross-linking agent, particularly when the capsules

are for food or cosmetic applications.

Many pairs of oppositely charged Polyelectrolytes (qv) are able to form a

liquid complex coacervate suitable for microcapsule formation. Normally, gelatin

is the positively charged polyion, because it is readily available and forms suitable

complex coacervates with a wide range of polyanions. Polyanions typically used

include gum arabic, polyphosphate, poly(acrylic acid), and alginate.

Chemically cross-linked complex coacervate capsule shells generally are

highly water-swollen, but can be dried in a spray dryer or ﬂuidized bed dryer.

If such capsule shells are treated with urea and formaldehyde under acidic con-

ditions, these materials polymerize within the water-swollen complex coacervate

capsule shell, thereby greatly, increasing the degree of chemical cross-linking, re-

ducing water content of the shell, and enhancing ease of drying to a free-ﬂow

powder.

A wide variety of capsules loaded with water-immiscible or water-insoluble

materials have been prepared by complex coacervation. Capsule size typically

ranges from 20 to 1000

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