Polymeric Delivery Systems - American Chemical Society

Chapter 9

Centrifugal Suspension—Separation Coating of Particles and Droplets

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R. E . Sparks, I. C . Jacobs, and N. S. Mason Microencapsulation and Granulation Laboratory, Department of Chemical Engineering, Washington University, One Brookings Drive, St. Louis, M O 63130

A new process has been developed for coating particles and droplets, based on forming a suspension of the core particles in the coating, then using a rotating disk to remove the excess coating liquid in the form of small droplets, while a residual coating remains around the core particles. All particles are then solidified by cooling or drying. The fine pure-coating particles are then separated from the larger coated particles and recycled. Core size can be from about 30 micrometers to several millimeters. Many wall materials are suitable and the coating can be applied from a solution or from a melted material. The process is fast, has high production capacity and is inexpensive. There are perhaps thirty processes for coating small particles which might be termed "microencapsulation" processes, depending upon the criteria employed (1,2,3). Most of these processes can be placed in three basic categories. The first category could be called "spray processes," in which the coating liquid is sprayed directly onto the particles as they are being tumbled, mixed or fluidized. A second category would be "wall deposition from solution", in which the core particles are first suspended in a solution containing all or part of the components needed to form the wall. The wall material is then caused to come out of solution by reaction, phase separation, etc., after which the separated wall phase deposits onto the core particles as a coating. A third category might be called "chemical reaction", in which the wall is formed directly on the core particles by chemical reaction of the wall-forming components. In the practice of these methods, a number of problems often occur which make them difficult or impractical to use for a particular application. Among these difficulties are Poor wetting of core particles by the wall material Aggregation of core particles by the wall formation or hardening 0097-6156/93/0520-0145$06.00/0 © 1993 American Chemical Society

In Polymeric Delivery Systems; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

146

POLYMERIC DELIVERY SYSTEMS

Limited

choice of

Tedious

control

Solvent

handling

Limited

production

rate

Cost

processing

(typically

of

wall

of

materials

process

and

steps

removal

from

product

$1.50

-

$20

per

kilogram)

A New Approach t o t h e Problem


The

operational

concept

starts

with

a particle

around

it.

Some o f

avoided,

and

sulation

might

conceived

core

particles

For The

be

liquid

imagine

the

are

separating With thought

the

methods to

these

thinking

the

is

place

that

a

one

coating

p r o c e s s e s might about

problem of

is

a

be

microencap-

coating

a

then

coating,

unwanted new

excess

a

designed to

liquid

new

a

particle

immersing

remove in

excess the

Figure

1.

Coated

process

from

the

suspension.

than

coated

liquid

residual

the

direction,

the

coarse suspension.

in

producing a

rather

by

a

leaving

illustrated

by d e f a u l t

for

liquid

forming

particles,

as

excess

concept

process begins

liquid,

suspended core

as

the

of

particle,

the

f o c u s i n g on methods o n how t o

place

a

of

coating

particles.

Centrifugal A rotating

Over

suspension core

Suspension-Separation disk

process.

over

(4).

separated

is

The

ciably

smaller

easily

separated the

apparatus

is

a

effective

within

the be

should which core

rotating

all

from

a

the by

the

centrifugal

cyclones or Figure

apprebe The

where

simple

needed

in

particles,

concentration flows.

the

are

sieves. disk

variables,

coated

the is

but

is

applications.)

not

fine

of

being

them t o

2,

the

the

This

of is

the the

final

coating

suspension

core

particles

for

often

20-35% o f

the

on

the

it

Disk.

As i n d i c a t e d

poured

onto

begins

force.

process to

to

The be

the

in

spread outward

importance

called

Figure

central of

region

under

the

"Centrifugal

the

2,

the

of

the

influence

centrifugal

force

Suspension-Sepa-

(CSS).

The outward

is

which

permitting

(Such a

is

such that

droplets

operating

most

liquid

operated

in

for

passing

volume.

where

caused the

for

which

suspension i s

disk

ration"

choice

equipment

separation

excess

into

with

of

r e c y c l e d as

Suspension Behavior core/coating

be

disk.

range

highest

of

conditions,

the

illustrated

rotating

liquid

and

piece

particles,

product

suspension s t i l l

particles

rotating

the

must

converted

coated

narrow of

while

disk

is

useful

operating

causes simultaneous

process i s

the

contain the

the

simple

removed

of

disk

liquid

apparatus

Since must

range

each other

than

of

not

particularly

such a from

excess

operation

a

a wide

particles

removed

has

that

made v i r t u a l l y

this the

of

in

if

coating

processes take

removing

of

difficulties

stimulated,

in

on them

particles

available

d e v i s e s ways

differently.

example,

between

the

then

directions

p r o c e s s i n g equipment

from

on

the

some new

were

behind and

liquid over

the

film

becomes t h i n n e r

increasing

area

of

as the

the

suspension spreads

disk.

It

is

critical


that

Centrifugal Suspension-Separation


9. SPARKS ET AL.

Figure

1.

coated

particles.

Figure

General

2.

The

method

of

process of

separating

centrifugal

a

suspension to

147

obtain

suspension-separation.


148


the

disk

coating

be at

diameter As

of

edge

the

the

is

them

is

into

coating

Under original thin

fine as

only

process.

outward

on the

large

ever,

covering

As pulls ing

the

away with

cores

through from are

the

the

leave

the

a

action than

coating

"de-wet" off

the

from

of

then

the

the

coated

This

KC1, which

several In molten which

700

with

microns

further near

in

test, an

70°C.

the This

if

coating

spreading

so r a p i d l y

liquid.

This

separation first

by t h e not

forces is

over

where

the

sprayed

it

if

the

is

must

spread

over

strong

determinant

they

the

residual film

droplets an

atom-

thrown

as

relative-

film.

How-

coating

of

the

coated the

gives

the

has

possibility poorly

coating The

KC1

tin

sufficiently

does

with

separation.

coated

surface

p l a c e d on the

surface

often

coated

and

except

come

particles

particles

easily were

to

polar

suspension

high

would

particles

by

However,

anything

product which

liquid.

molecules,

lead,

par-

particle

final

the

them

tether-

particles,

core

been

a

core

core

particles

particles

is

surrounding forming

mixture.

have

the is

in

liquid not

liquid

core

with

cadmium energy,

other

low

metals

clean).

However,

potassium

chloride

is

coating

sufficient film

poorly

advantage

to

for

the

liquid,

coating

poorly

liquid

to

pull

away

core

they

wetted.

On t h e

away.

However,

sprayed onto

the

core

particles,

the

core

particle

whether

In

this

case,

a good c o a t i n g

the the

from

by t h e

collide

with

on for

suspension particles

are

surrounded

disk, in the

there

is

processes droplets

surface

wettability

c a n be

the

coating

centrifugal

Since the

are

film time

pull

wetted

processes.

instantaneously. of

wet

of

cause the

when

immersed

for

coating

coating

disk

liquid

tested

bismuth,

there

o c c a s i o n a l l y an

even

enough t i m e

up i n t o

as

method.

spray-coating

totally

liquid

that

to

even

core

waxy

easily

new

surface

droplets,

excess coating

first

surface

was

the

particles,

are

the

to

by t h e

core

present

as

disk,

This

centrifugal

of

Apparently, weak

a

uncoated

material

only

occurs

is

disk

the

c o n c e r n was

same c o r e

then,

disk

of

the

two

are

around

of

diameter

(and

particles

under

particles

coating

coatings.

alloy

Wood's metal

was

by

tendency

of

covering gradually

for

some a d s o r b e d w a t e r

poor

even

have

the

a melted

and a

coating

a

disk

thin

leaving

leaving

viscosity a

the

the

microencapsulated

wetted

hydrophobic

Wood's m e t a l , melts

to

on t h e

An i n i t i a l

which

contains

with

the

droplets

hydrophobic coatings a

coating

which

the

breaking

edge

tension.

possibility

potassium chloride

approximately

liquid,

the from

the

components of

disk

leaves

core

layer

or

particles,

the

well

the

the

breaks,

atomized

of

wet

of

c o l l a p s e s back

surface

the

of not

liquid

the

particles.

fed

still

disk,

liquid

which

liquid

disk.

of

layer

leaving

coating

liquid,

near

where

liquid

film,

core

been

upward

the

Influence of W e t t a b i l i t y . that

of has

same t i m e ,

thin

"tail"

process,

larger

sheet

particles

The t h r e a d a

film than

them.

from

thread.

ticle

the

and

particles,

appearing

core

the core

periphery

liquid

protruding

large

the less

region

excess coating

core

the

disk,

of

the

the

The

thin

At

"rocks"

these

liquid

on t o p

coating

the

diameter,

surrounding thin

the

a

of

appreciably

into

the

conditions

or

the

ization ly

the

between

thickness

is

move

than

conditions.

film

filaments

if

less layer

these

the

disk

particles.

suspension leave

different a

the

particles

thickness pulled

such that of

core

core

film

thinning


operated the

and is

formed.


a

of

9.

SPARKS ET AL.

Subsequent coated

Particle

core

solidified

to the

or

of fat,

sion.

permit

and

or

by

in

which

the

After

droplets

liquid,

drying

Alternatively,

bath

the

subsequent

coating

the

the

it

the

coating

is

coating,

is

leaving of

pure

handling. if

coated

coating

149

Suspension-Separation

Processing.

particles

cooling wax

Centrifugal

This

a

if

is

particles

hardened

by

is

melted it

a

can

the

disk,

both

must

the

be

accomplished

by

material

such as

solution

or

be

caught

various

a

suspen-

in

a

liquid

treatments

or

reactions. If


dling

air

is

spray-drying unique

to

exchange

regions

to

is

also

centrifugal

rotating

disk

produce

tons CSS

per

20

year

chambers,

designs

such as

must

be

air-han-

that

matched

the

centrifugal

suspension separation

differences

in

particle

behavior.

makes

possible to

an

it

samples

to

radically

indication are

tons/hour

of

comparable

of

the

product.

for

use

move

different

of

This

basis.

is

well-behaved

modified

from

the with

no

equipment.

in

The

heat

production

potential used

such

rapidly

Standard

routinely

three-shift

The

in

the coating

distribution,

large-scale

suspension-separation. atomizers

air

used

to

the

test

on a

s h o u l d be

The

then

equipment

of

and

and

medium,

for

laboratory

design

This

to

of

cooling

employed.

equipment

preparation

the

particle-handling

c a n be

account

commercial

of

and

requirements

process

need

used as

equipment

production spray

the

capacity

driers

ceramics

equivalent

production

with industry

to

120,000

capacity

of

systems.

D i f f e r e n c e s From S p r a y - D r y i n g The

suspension-separation

relative drying

to

spray-drying

chambers

are

process and

used,

s h o u l d be

placed

spray-chilling,

and

these

are

in

since

perspective

rotating

traditionally

disks

associated

and with

spray-drying. In in

the

spray-drying form

to

flow

at

the

of

over edge

tion.

All

particles product

the

separation, size

for

In tions

e.g.

by

contrast,

change

only

a

in in

CSS a the

but

uct

changing

(only

has

size

distribution

size

and As

size

singly a

a of the

liquid,

the

atomized

and

another

the

not

purpose

to

size for

solid are

the

distri-

pure

Any

of

separate

atomizing atomiza-

droplets

particles for

fine

is

suspension

liquid

distinct

is

solid

the

coating

subsequent

meeting unused

the

pure

CSS. a

in

the

wall

the

size

in

the

normal result

of

pure

effect

thickness of

change

core

size

slightly.) are

atomization which

conditions

coating

on the

product

the

in

product

atomization small

coated

distribution in

contain

these

spray-cooling,

of

the

simple simple

two

negligible the

practical

are

a

in

permits

suspension separation.

change

size

were

formed

and

product,

in

or

change

recycled,

coated

as

it

suspension, the This

not

cyclones,

recycle

to

core

on the

a

same a s

are

There

coated

spray-drying

leads

which

centrifugal

specification

coating

By

in

if

ingredient,

process.

for

as

as

much t h e

droplets

of

particles.

disk

disk

active

one

material,

spray-cooling

entire

the

the

of

butions,

the of

of

or

suspended f i n e

of

the

In

CSS,

since

condi-

formed.

causes

particles

determined

particles,

is

a

to

be

coated size

largely they

prodand by

the

are

operation. of

this

difference

in

the

physics of


the

150


two is

processes,

usually

spray-drying chilling

and

are

result

coated,

but

and

Core

to

made

Thickness

and

fraction

microns,

particles

coated

particle

the

size

in

conveniently

product

The

droplets size

of

the

CSS, since

from

30

several

by CSS

typical

from

small

both

large

microns

of

sprayrecycled

sets

up t o

millimeters

in

of

pure

particles

2 mm c a n

diameter

particles commercially

usually

have

often

been

requires

equipment. Wall

Capsule Payload.

of

a micron

depending

coating

up t o

such

expensive

a

a

are

of

atomization.

coating

from

the

closely

which

Particles

Wall

of

size

spray-chilling.

of

Size.

coated.

large

particles

the

Characteristics

Particle be

of t h e

than

c o m p a r e s more droplets

the

size

larger

coating

Product


the

much

on the

liquid

(for

size

thickness

solutions)

up t o

of

the

core

and whether

the

coating

c a n be

a

few

particles, is

a

varied

hundred

the

viscosity

melted

material

or

solution. The

capsule

particle) active high

material

fraction

Acceptable particle is

The even

of be

a

for

such is

of

perhaps

of

Since

has

for

be

in

very

the

low

particle

is

coated

if

the

containing

a

of

the

core

coating

hydrophilic,

process,

CSS

hydrophobic,

often

requires

droplets

considerable

of

p o s s i b l e to

aqueous coat

liquids

droplets

of

applications.

material

is

c a n be high

seconds, been

the

etc.

are

thermally

the

on

are

droplets

materials

enzymes

wettability

However,

many

which

have

material

core

effect

which

core

coating

2-20

a

the

little

and i t

exposed to

enzymes

into

particles

wall

as

core

can a l s o

material.

process.

liquids

melted

of

but

formed

liquid

the

material time,

90%,

multi-component, of

speed with

molecules

number

core

coated

suited

if

been

coating

porous,

usually

well

(fraction

over

inert

coating

hydrophobic it

of

for

modification can

has

the

effective

granular,

well

Core M a t e r i a l s .

by

The

payload

c a n be

handled

labile

employed a t safely

coating and

in

the

115°C,

coated

For

highly

because

temperature

little

process

materials.

for

degradation

successfully coated

the

with

makes

example, unstable core

short

periods

can o c c u r .

A

negligible

degradation.

Acceptable the

Wall

process,

solvent

must

operating fats,

be

costs.

glycerides, electron

microns)

coated

a

because

removed,

scanning is

through

a

no

particles

of

low

micrograph

the

have

been

used,

including

particle. or

useful

are

granules

a multi-component The gross wall

of

wax

a

has

a

the

solid

waxes,

3 is

(mean d i a m e t e r of

smaller

are

has than

those

to

form

the

Figure

4

cross-section

appearance,

containing excess

coated

a

700

porosity.

materials

no

lowest

Figure

based c o a t i n g .

portion

coating

CSS p r o c e s s o n l y separably

glycol.

in

Since

gives

porous

micrograph

well

also

melts

of

handle

viscosity.

melts

and p o l y e t h y l e n e

Since the which

use

materials

have

of

layering

Particularly polymers.

the

coating

often

stéarates

with

coated

they

A variety

scanning electron

exhibiting

Melted

Materials.

partly

high

coating

particles,



9. SPARKS E T AL.

Figure

3.

wax-based

Centrifugal

Porous granules coating

Figure

4.

produced

Suspension-Separation

(mean d i a m e t e r

700

by

suspension-separation.

centrifugal

C r o s s - s e c t i o n of

the

coating

micrometers)

in

Figure

with

3.


152


there

is

no

requirement

spray-coating easily up t o

handled. 5000

aqueous


of

melted

have

solutions

Particle in

particle

will

particle

has

be

the

thin

coatings

or

valleys over

where

process are granulation

not

acceptable.

ends

rather

of

final

can

in

be

viscosities

smooth,

the

best

but

mechanisms

of

the

the

coated

core

coating leave

This core

they

pan-granulation,

the

When

and

and

materials.

process.

of

protrusions. The

coating

soluble

shape

tension.

acceptable.

needed

solvents

the

description

thin.

and

by

often

in

protrusions

the

is

round

made

are

coatings

the

as

organic

protrusions,

the

are

having

employed in

other

surface

irregular

coating

Granules

bed

by

been

that

as

viscosity

by C S S .

gums a n d

From the

between

the

the

as

apparent

determined

corners

f i l l

spherical.

is

also

well

Shape.

CSS, i t

will

protection

behave

higher

formulations

polymers

food

droplets

of

applied

of

gelatin,

latexes

small

coating

solutions of

such

coatings

been

solutions

involved

the

of

form

Hence, have

including

Water-based Effect

Many

centipoise

A number liquids,

to

methods.

liquid

relatively

gives

poor

particles

need

not

be

spray-drying

or

Crushed materials

are

for fluidusually

Advantages o f CSS Experience cations

in

has

It

applying

shown

is

a

A wide

it

simple, variety

Meltable

the

to

new

have

core

can be

is

avoided.

The

is

very

Thermally

sensitive

There

no

The

process

High The

is

range

of

a

wide

variety

of

appli-

practical

advantages :

materials

c a n be u s e d .

fast

well.

(seconds).

control

c a n be

easily

coated.

problems.

continuous.

production process

coating

handled

materials

tedious

of

process.

and

Aggregation

are

number

one-step of

coatings

process

process to

a

is

rates

are

possible.

inexpensive

(operating

$0.25-2.50/kg,

including

costs

the

typically

coating

in

the

material).

D i f f i c u l t i e s F o r CSS Experience for

some

with

the

new

process

has

also

highlighted

the

difficulties

applications.

The

lower

limit

coatings. The

less

which Solvent 100

microns

a

irregular in

in

the

diameter

3 0-50 is

the

the

microns

for

most

determining

smaller

the

factor.

particle

easily.

to

obtain

diameter. thin

coatings

is

viscosity

coating,

sufficiently

sufficiently uniform

the

coated

required

viscosity Thin

core

coating

viscous

c a n be is

in

The

to

high This

loading is

spread

in

required the

particles to

coating

below

decrease liquid

the

into

film. cannot

particles.

be

There

applied are

to

always

tablets thin,

and

leaky

highly spots

coating.


9.

Centrifugal

SPARKS ET AL.

Droplets

are

sometimes

viscosity "solids" forces Coatings to

must on

to

the

be

having

use.

Insoluble,

be

more

They

difficult

increased

disk,

upon the a

153

Suspension—Separation

long

to

enough

allowing coating

coat,

for

the

them

major

because to

their

behave

effect

of

as

the

liquid.

solidification

time

stick

together

at

the

non-melting

coatings

cannot

are

bottom be

impractical of

the

tower.

applied.


Conclusions A

method

of

coating

offers

the

coated

particles.

applying

melt

coatings, coat

the

economies to

Literature

These

safe

of

low

scale,

liquids the

extensions the

the

labile

below

than

that

when

coupled with

operating

been

of

a

other

of

developed practical

from

coating

materials

150

engineer of

has

fields

c o u l d come

possibility of

particles

capacity

and

extending

handling

process offers

production lead

of

coatings,

inexpensively The

particles

possibility

microns

with

or in

the the

the

of

very

viscous

ability

much

processes.

ability

of

ease

to

diameter.

process with coating

which uses

to

coat

higher These with

melts

costs.

Cited

1. Sparks, R. E. in Kirk-Othmer Encyclopedia of Chemical Technology, Third Edition; John Wiley and Sons, Inc.: New York, NY, 1981; Vol. 15; pp. 470-493. 2. Deasy, P. Microencapsulation and Other Drug-Related Processes; Marcel Dekker: New York, NY, 1984. 3. Sparks, R. E. In Encyclopedia of Chemical Process Technology; McKetta, J . , Ed.; Marcel Dekker, Inc.: New York, NY, 1989. 4. Sparks, R. E . ; Mason, N. S. Method and Apparatus for Coating Particles and Liquid Droplets; U.S. Patent No. 4,675,140 (1987). R E C E I V E D October 9, 1992


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