Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the production of microcapsule~
of which the ou-ter shell is o~ a particular material.
Various types of microcapsules and their production ~re
known. A variety of different polymers may be used ~or the
shell material, the particular choice being governed by the
chemical nature of the core material to be encapsulated. If
the core material is hydrophilic for example, the ~hell-
forming polymers have to be as hydrophobic as possible. I~,
on the other hand, the core material is hydrophobic, the
shell-~orming polymers have to be as hydrophilic as possible.
In addition to these requirements, the relea~e characteristic
or permeability of the shell with respect to the material to
be encapsulated is another cri~ical factor in the choice o~
the shell materials. In this respect too, the general rule
is that the core material and the shell-~orming polymer
should have opposite solubility parameter~ (for example
hydrophobic shell polymers are less permeable to hydrophilic
than to hydrophobic encapsulated materials). Eowever, there
`i are numerou~ borderline cases where a suitable she~-~orming
polymer is not aYailable for a given core matsrial. In
cases such as these 9 it is occasionally possible to produce
two polymer shells of different polymers one over the other.
However, it is not possible, even in this way, to obtain
any required combination of properties.
Polymers, polycondensates and polyaddition products may
be used as the polymer~ with shell-forming properties.
Suitable polymers are, for ex~mple, the u~ual homopolymer~
I and copolymers of ethylene, propylene, vinyl chloride,
vinylidene chloride, vinylacetate, acrylonitrile; ~tyrene,
acrylic acid alkyl esters and methacrylic acid alkyl esters;
suitable polycondensates areJ for example, polyamides, polysulphonamides,
polyesters and polycarbonates, ~hilst suitable polyaddi~ion products are,
for exa~ple, polyure~hanes and polyureas~
It has now been found that film forming polycarbodiimides may
also be used for producing the shells of microcapsules.
According to the present invention, there is provided a microcapsule
comprising a central core and a shell comprising a film-forming polycarbodi-
imide, the core material being solid or liquid and compatible with the
polycarbodiimide~
In another aspect7 ~he invention provides a method of making a
mic~ocapsule deined above, wherein a polycarbodiimide with terminal NCO- ~-groups is dissolved in an inert, water~immiscible solvent, a compatible COTe
material is dispersed into the resulting solution and wherein the resulting
dispersion is introduced in a shear gradient into an immiscible liquid phase
in which an isocyanate~reactive polyamine is present or which is added there~o.
Conventional encapsulation techniques may be used for the pro-
duction of microcapsules with polycarbodiimides. Conventional encapsulation
techn}ques are essentially physical and chemical processes. The physical
p~ocesses comprise coasing the core materials in the ~orm of droplets or
partlcles with polymers that aTe immiscible with them, the encapsulation
process ~eing physirally induced at an early s~age. In the chemical processes,
` it is standard practice to prepare dispersions o~ the core material or solution
; o the core material in an immiscible dispersion medium and then to deposit or
produce the shell-~orming polymer at the phase interface in such a way tha~ it
surrounds the core material in the form of a film. The polymer may be formed
either from the inner phase or fIom the outer phase, depending upon the
particular method of pToduction selected.
Chemical encapsulation processes may roughly be divided into
processes ~or phase sepa~ation and processes for interfacial polymerisation.
.' ,. :: . .:
: :: .
2- ~
:.. ... ,. . . ., .. . . , .. ... . .. , .. .: :
The ollowing are exampl0s o typical chemical encapsulation
` techniques:
1) The coacervation OT complex coacervation process~ By adjusting
the cor~ect temperature and the correct pH-value, a polymer
coacervate is deposited at the phase interface and may
subsequently be hardened. One typical
~2a-
example is the gelatin~gum arabic system which may be
hardened with formaldehydeO
2) The reacti~e process. In this process, two components
dissolve~ separately in the outer phase and in the inner phase
of the dispersion react with one another a-t the phase
interface to form the polymer, ~or example a polyconden-
sate or a polyaddition product
3) The evaporation process. The core material is encapsula-
ted by deposition of the polymer by evaporating a
solven-t for the polymers frQm the dispersion.
4~ The precipitation process. The polymer is deposited by
precipitation from a polymer solution with a non-solvent.
The individual microencapsulation techniques are
described in more detail in J.E. Vandegaer~s work
entitle~ "Microencapsulation, Processes and Applications" 7
Plenum Press, New York 1974.
The present invention relates to the production of
micro¢apsules by the reactive process, the evaporation process
and the precipitation process using film-forming polycarbodi-
imides as shell material. Suitable film-forming polycarbodi
imides contain in the molecule the repeating unit:
- R - N - C = N - R -
; where R represents alkylene, cycloalkylene or arylene groups
:, which may be ~urther substituted, and may contain NC0-groups
a~ terminal groups~
The use of these polymers for microencapsulation a*fords
a number of surprising advantages.
Thus~ film-~orming carbodiimides may be applied in
dissolved form by the evaporation process and by the precip--
itation process. The reactive process may be used with
s~
polycarbodiimides containing ~ree isocy~nate groups.
Microencapsulation may be carried out by several pro-
cesses using one and the same polymer.
By virtue of their chemical nature, further chemical
reactions may be carried out on the polycarbodiimide shells,
to modify the properties of the shells. For example, car-
boxyl groups or amino groups may be added. Thus, it is
possible additionally to crosslink the linear polycarbodiimide
chains, Yor example by reac~ion with dicarboxylic acids,
such as adipic acid, or ~o add a second shell chemically
attached to the first by reaction with the amino and carboxyl
groups of gelatin ~or analogous hydrophilic polymers) by the
methods of coacervation or complex coacervation. The basic-
ally hydrophobic polycarbodiimide can also be made more
hydrophilic by reaction with low molecular weight reactants,
The properties of the polycarbodiimide shells may thus
largely be adapted to any core material.
Accordingly, any organophilic, liquid or solid substances
may in principle be encapsulated in ~ilm-forming polycarbodi-
imides.
Suitable polymeric carbodiimides ~re aromatic, aliphatic,
cycloaliphatic and aliphatic-aromatic polycarbodiimides and
mixtures thereof.
Polycarbodiimides may be obtained from the corre~ponding
isocyanates, Yor example from 2,4- and 2,6-dii~ocyanatotoluene
and their ~someric mixtures, especially an i~omeric mixture
consisting o~ 80~ o~ 2,4- and 20% o~ 2~6-diisocyanatotoluene;
~rom 4,4~-diisocyanatodiphenyl methane; from the phosgenation
products of acid-catalysed aniline-~ormaldehyde condensa~es;
3a from l~3-diisocyana$obenzene, l,3~5-trimethyl- and l,3,5-tri-
i
~ - 4 -
~'7~
ispropyl benzene-2,4-diisocyanate; from l,6-diisocyanatohexane
and from l-isocyanato-3,3,5~trimethyl-5-isocyanatomethyl
cyclohexane. However, the polycarbodiimides suitable for
use in the described process are derived not only from the
pure i~ocyanates, because it is also possible to use their
undistilled precursors and also reaction products o~ these
polyisocyanates with monoalcohols or polyalcohols with an
NCO:OH ratio of greater than l, and modification products o~
these polyisocyanates, such as polyisocyanates which addition
ally contain biuret3 allophanate, isocyanurate and carbodi-
imide groups
For microencapsulation by the evaporation process, pre-
~ cipitation process and reactive process, it is easential
that the polycarbodiimides used should be soluble in water- immiscible solvents~
For the reactive process, these solvents also have .-
to be inert with respect to isocyanate groups.
. For the evaporation proce~s, their boiling points must be
below that of water, or alternatively, the solvents must be
2~ able to be removed ~rom the disper~ion in the ~orm o~ an
azeotrope with water and/or another solvent.
Polycarbodiimides suitable for use in accordance with the
lnvention preferably contain free terminal isocyanate groups
i.e. have the idealised structure
OCN-[R-N=C=N]xR~NCO
in which R represents alkylene, cycloalkylene and ~rylene,
alld X i9 an integer ~rom 2 to 40. R is preferably a
C2~C6 alkylene radical, a C5 C7-cycloalkylene radical or a
C6-Cl2-aryle~e radical
~ . .
~ - 5 -
:
Some of the carbodiimide groups m~y also be converted
with isocyanate into uretone-imine groups thus giving polymers
which contain in the molecule the repeating unit:
- R - N = C - N - R -
- N - C = 0
The evaporation process and precipitation process may
pre~erably be carried out with carbodiimides which contain
phosphonio or ~P - N - structural units, for example with the
idealised structure
ocN~-R-N=c=N-R7x-N=pt~ ~ R'
~ ,"
in which x and R have the same meanings as above whilst R'
represents alkyl and cycloalkyl groups, R' is preferably alkyl
with 1 to 6 carbon atoms and cycloalkyl with 5 to 7 carbon
15 atoms and R" represents alkyl and aryl pre~erably methyl 9 ethyl
' and phenyl.
: Some o~ the carbodiimide groups may also be con~erted
with phosph~line oxides or phospholane oxides to structural
units o~ the type
2Q - R ~ N - C - N - R -
I I ~ R'
- N - P\
R"
in which R, R' and R" have the same meaning as above.
, 25 The production o~ polycarbodiimides o~ this kind is known
and is described for example in "Encyclopedia o~ Polymer
Science and Technology", Vol, 7, pages 751 - 754. In the most
~imple oase3 the polycarbodiimides are obtalned by adding phos-
pholine oxides or phospholane oxides to th~ isocyanates and
6ize-reducing the foam-like material obtained~
6 -
`'' '
.
~o~
According to the in~ention, it is possible to encapsulate
solid and liquid substances. Th~ liquid substances must be com-
patible with the p~lymer solution. Examples o~ suitable core
materials ar~ mineral oils, ~atty oils, trichloroethyl phos-
phate, thiophosphoric acid esters, ethoxylated alkyl phenol~,
perfumes, aromatic and aliphatic hydrocarbons and chlorinated
hydrocarbons as well as mixtures thereof~ ink solutions, tita-
nium dioxide, met~ylene blue, crystal violet and carbon black.
The individual microencapsulation techniques are carried
out, for example~ a~ follows:
1) For the reactive process, the polycarbodiimide is ini-
tially dissolved in an inert solvent and a compatible core ma-
terial is mixed with the resulting solution.
In a shear gradient, p~e~erably produoed by intensive
mixing with small mixers or mixing machines, this mixture is
introduced into an immiscible liquid phase, for example water,
which contains an isocyanate-reactive polyamine. The amine may
also be subsequently added.
Suitable polyamines are for example, 1,2 ethylene diamine9
1,4-diaminobutane, bis-~3-aminopropyl)~amine, hydrazino-2-
ethanol 9 bis-(2-methyl-aminoethyl)-methylamine~ 1,4-diamino-
benzene, 4,4'-diamino~diphe~ylmethane, 1,4-diaminocyclohexane~
3 amino~1-methylamino-propane~ N-hydroxyethyl ethylene diamine 9
N-methyl-bis-(3-amlnopropyl)~amine, hy~razine and 1,2-ethylene
diamine-N-ethanesulfonic acid (Na-salt).
2) For the evaporation process~ the polycarbodiimide is ini-
tially dissolv~d in a solvent which has a boiling point below
100C or which ~orms an azeotrope ~oiling at a temperature be
low 100C. A compatible core material i~q then mixed with the
resulting solution. This mixture is then dispersed with pre-
. ~
~ - 7 -
.
, ' ,
.. . ..
~erably vigorous stirring into a liquid immiscible with the
p~lymer solv~nt, for example, water, followed by gradual heating
to temperatures above the boiling point of the polymer solvent
or azeotrope. The solvent evaporates of~ and the polycarbodiimide
encapsulates th~ core material forming the inner phase at the
phase interface. Emulsification aids or emulsi~iers are best
added to the aqueous phase to obtain better emulsification and
to stabilise the dispersion. ~xamples of such products acting
as protective colloids are carboxymethyl cellulose, gelatin and
polyvinyl alcohol~ Examples of emulsifiers are ethoxylated 3-
benzyl-4-hydroxybiphenyl and reaction products o~ nonyl phenol
with di~ferent quantities o~ ethylene oxide.
3) For the precipitation process, the polycarbodiimide is
initially dissolved, the core material is subsequent~y given to
the resulting solution and a precipitant for the polymer mis-
cible with the polymer solvent is added with stirringO E~ective
solvents for the polycarbodiimide are, ~or example chlorinated
aliphatic and aromatic hydrocarbons such as methylene chloride
and chloroform, aromatic hydrocarbons such as toluene and ben-
zene, esters such as ethylacetate and cyclic ethers such as
tetrahydrofuran or dioxane. E~ective solvents ~or the film-
forming polycarbodiimides are also aprotic ~olvents~ such as
N,N-dimethyl formamide, dimethyl sulphoxide, N,N-dimethyl acet-
amide, N,N-di-n-butyl formamide, N-methyl pyrrolidone and N,N-
di-n-butyl acetamide~
Which,~ver process is used, the polycarbodiimide shell
`l may be additionally modi~ied. For example, compounds which
react with the carbodiimide groups may be added to the micro-
capsule dispersion. Examples of such compounds are polyfunc-
tional carboxylic acids; such as adipic acid9 polyacrylic acid
:
- 8 -
,
and their copolymers, also poly~unctional amines, such as
2,5-diaminobenzene sulp~onic acid, 4,4l~diaminobenzene and
the amino-comp~unds mentioned for the reacti~e process. The
polycarbodiimide shell may be hardened in this way.
The hardening agents may be added either be~ore or during
production of the dispersion to the outer phase. However, the
hardeners may also be added after formation of the microcapsu-
les in the form o~ a solution in a solvent compatible with the
outer phase.
Continuous and batch operation is possible. The degree
o~ turbulence during mixing determines the diameter o~ the
microcapsules obtained. The diameter of the microcapsules may
amount to between about 5 and 5000/u, depending upon the mixing
conditions,
The ratio by weight o~ core material to shell material
in the completed microcap~ules is normally 50 to 90 : 50 to 10.
The microcapsules obtained may containt ~or example,
pesticides, ~lameproofing agents, ink solutions, plasticisers,
catalysts, oils, per~umes, pigments and dyes which are already
being commercially used in encapsulated form.
. I .
~ _ g _
,
EXAMPLE 1
a) Production of the polymer
139 g of a mixture of 80% by weight of 2,4-diisocyanato-
toluene and 20% by weight of 256-diisocyanatotoluen~ were
mixed with stirring at room temperature with 2 g of l-methyl
phospholine-l-oxide.
The mixture ~oamed slowly and after about 12 hours gave
a readily pulverised polycarbodiimide foam which was soluble
in such solvents as methylene chloride, chloroform, chloro-
benzene, o-dichlorohenzene, toluene, tetrahydrofuran, N-methyl
pyrrolidone and dimethyl formamide. The softening range o~
the reaction product was above 200Co It is advisable to
store the polycarbodiimide at temperatures below 5C in order
as far as possible to prevent it from reacting any further.
b) Encapsulation
lg of the polycarbodiimide produced in accordance with
a) was dissolved in 3 g of chloroform and the resulting
solution was added to 22 g of a polychlorinated diphenyl
(Clophen A 30~.
The homogenous mlxture was -then stirred into 300 ml of
water containing 1.5 g of polyvinyl alcohol (Moviol 70/98) as
emulsification aid. This re~ulted in formation of the
dispersion.
It was found to *e suf~icient to use a simple laboratory
stirrer o~ the Lenart Rapid type turning at 500 rpm). A 1
litre glass beaker was u~ed as the reaetion vessel. A solu-
- tion of 14 g o~ ethylene diamine in 56 ml of water was then
added to the resulting dispersionO
~he mixture was heated rapidly with continuous stirring
to 60C and left at that temperature for about 1 hour,
~tr~e ~,
~ - 10 -
resulting in formation o~ the microcap~ules.
The capsules were filtered off and had a diameter of up
to about 2 mm. By changing the dispersion conditions, it was
possible to produce capsules in the order o~ magnitude of mi~ro-
capsules, i.e. with a diame~er of ~rom about 5 to 100
EXAMPLE 2
~ ;. _ ~
The encapsulation o~ 25 g of chlorobenzene as core mate-
rial was carried out in the same way as described in 1b) with
the ~ollowing changes: 2 g o~ the polycarbodiimide prepared in
accordance with 1a) were dissolved in the chlorobenzene without
the addition of chloroformD Under analogous conditions 30 g o~
bis-(3-aminopropyl)-methylamine were added as reactant in the
outer aqueous phase.
., ~
a) ~
228 g of 1,3,5-triisopropyl benzene-2,4-diisocyanate were
mixed with 2 g o~ 1-methylphospholine-1-oxide and the resulting
I mixture was kept at about 110C ~or 5 to 6 hours. A solid was
~ ~ormed with gradual e~olution of carbon dioxide, The solid
~ormed had a softening range o~ from 90 to 110C and was ~ound
to be ~olublc in such solvents as methylene chloride, chloro~orm,
chlorobenzene, N-methyl pyrrolidone, toluene, a mixture of aro-
matic hydrocarbons (Solvesso 100~9 Clophen A 30~ xylene,eth~-
lene chloride, 1,3-dichloropropane, light petrol9 ben~ene, tetra-
hydrofuran, acetone, methylethyl ketone and die~hyl ether. The
polycarbodiimide may readily be ~ize-reduced and should al o be
stored at temperatures below 5C9
b) ~59~ La~
2 g of the polycarbodiimide based on 1,3,5-triisoprop~l
~enzene-2,4-diisocyanate, produced in accordance with Example
~ t~ r~ ~
;' : ,. : ' : -
3a), were dissolved in 6 g of methylene chloride and the
resulting solution was added to ~0 g of tri-n-butyl pho~phate.
The homogeneous mixture was dispersed by means of a simple
laboratory stirrer in the same way as described in lb~,
followed by the addition of 14 g of ethylene diamine in 56 ml
of wa$er. Working up was carried out in the same way as in
Example lb).
EX~MPLE
a) ~195~b~ 'LLJ b~_c~lYC~
13 134 g of hexamethylene-1,6-diisocyanate were mixed with
2 ~ of l-methyl phospholine-l-oxide and the resulting mixture
was heated for 15 hours to 50C. An extremely viscous product
was f~æd with gradual evolution of carbon dioxide. The
; product was soluble in the following solvents: methylene
chloride, chloroform, chlorobenzene, toluene solventnaphtha (mix-
ture o~ aromatic hydrocarbons; BV Aral), chlophen A 30, tri-n-
butyl phosphate, tris-chloroethyl~phosphate, ethylene chloride
1,3-dichloropropane; cyclohexane, light pe-trol, methylethyl ke-
tone, acetone, ethylacetate, pyrrolidone, N-methyl pyrrolidone~
dimethyl formamide, benzene, dioxane and tetrahydrofuran~ The
polycarbodiimide ~hould b~ stored at tempera~ure~ below 5C~
b) ~
Example I: 2 to 5 g of the polycarbodiimide produced in
accordance with 4a) were dissolved in 25 g of chlorobenzene
and disper~ed in 300 ml of water by means of a laboratory
stirrer of the Lenart-Rapid type turning at 500 rpm. 14 g
of e$hylene diamine dissolved in 5~ ml of water were added
to the resulting mixture.
E~ample II: For the encapsulation of 25 g of tri-n-but~l
phosphate, 2 g of the polycarbodiimide of hexamethylene-1,6-
.
diisocyanate were dissol~ed in the phosphate and further
processed in the same way as described in Example I.
Example III: 2 5 g of the polycarbodiimide produced in
accordance with 4a) were dissolved in 25 g of solvent naphtha
and further processed in the same way as desoribed in Example I.
In this case, 2 g of the polycarbodiimide represent the
lower limit.
Mix~ures I and III were worked up in the same way as
described in lb) Another remarkable feature common to all
three mixtures is that there is no need for any increase in
temperature during working up, despite which the capsules
are not ad~ersely affected. It i~ e~en possible to produce
microcapsules without any need for prolonged aiter-~tirring.
In -this connection, however, tests carried out with only
2 g of the polycarbodiimide a~e problematical becau~e the
capsule membranes obtained are less stable. B~v establishing
~, suitable dispersi~n condition~itwas found to be possible in
all tests to produce microcapsules ranging from 5 to lO0
in diameter.
2 ~
O
lO g of the polycarbodiimide based on tolylene diiso
cyanate produced in accordance with la) were dissolved in
90 g of chloroform. 40 g o~ a mixture of aromatic hydrocar-
bons (cumene, ~yleneg toluene, naphthe~ic oils - solvent-
naphtha as produced by BV Aral) were then added and the
homogeneou~ mixture was dispersed in a solution of 5 g o~
polyvinyl alcohol (Moviol 70/98) and 2.5 g of hydrazino~
ethanol in 500 g of water. 2.5 g of gelatin or 2.5 g o~
-I earboxymethyl cellulose (so~ium salt) may also ~e used as
emulsifi¢ation aid. A l litre glass beaker was used ~s the
reaction vessel. The di~persion was heated to 60C and the
~3 -
"~ '
., .
.~ ,. . .
polymer solvent was distilled of~ slowly over a period of
about 4 hours. It was found sufficient to use a simple
laboratory stirrer of the Lenart-Rapid type ~or dispersîo~.
The capsules have an average diameter of about 85 ~ ~or a
stirrer speed of 1750 rpm, and an average diameter of about
150 ~ for a stirrer speed of 700 rpm. It was ~ound that the
hydrazinoethan~l used for hardening the polycarbodiimide
shells may also be added with equal effect after dispersion
or after most of the polymer solvent ,has been distilled ofL.
The resulting capsules were filtered off and dried.
EXAMPLE 6
10 g of the polycarbodiimide based on tolylene diisocyan-
ate produced in accordance with la) were dissolved in 90 g of
methylene chloride and processed in the same way as in
Example 5 with the following changes: 40 g of a heating
bath oil based on diphenyl (Marlotherm, a product of Huls/
Marl) were added to the polymer solution as the core material.
2.5 g of carboxymethyl cellulose (sodium salt) and 2.5 g of
an emulsifier based on nonyl phenol and ethylene oxide
(Emulgator NP 7, a product of Bayer AG~ were used as
emulsification aids for the homogeneous disperse phase. The
dispersion was heated to only 40 - 45C. 5 g o~ adipic acid
were added to the aqueous phase as reagent for hardening the
capsule shell. The resulting capsules were filtered off and
,', 25 dried. As described in Example 5, it was found to be poss-
ible to use gela-tin or polyvinyl alcohol (Mo~iol 70/98) in-
stead of carboxymsthyl cellulo~e as emulsification aid.
'' ~.:Z
a)
To prepare a polycarbodiimide ~rom l-isocyanato-3,5,5
+~d~
- 14
, .
~7~
trimethyl-5-isocyanatomethyl cyclohexane, 177 g of the
diisocyanate were stirred thorou~hly with 2 g of l-methyl
phospholine-l-oxide, ~ollowed by storage for about 12 hours
at a temperature of 100 to 110C. A highly viscous product
was thus obtained. It was soluble in such solvents as
methylene chloride, chloroform, chlorobenzene, toluene,
Solvesso 100, tri-n-butyl phosphate~ ethylene chloride,
1,3-dichloropropane, trichloroethylene, methylethyl ketone,
acetone, tetrahydrofuran, dioxane and benzene.
b) Enca~sulation
Example I: 2 5 g o~ polycarbodiimide produced in
accordance with Example 7a) were dissolved in 25 g o~ chloro-
benzene or Solvesso 100, dispersed in 300 ml of water at
500 rpm, ~ollowed by the addition of 14 g of ethylene diamine
dissolved in 56 ml of water. A simple laboratory stirrer of
the Lenart-~apid type was used as the stirrer. In contrast
to encapsulations with other polycarbodiimides, the best
results in this case were obtained by stirring for 1 hour at
room temperature, i.e. without heating. The cap~ules obtained
were then filtered of~ and dried in air.
~ Example II: 5 g of the polycarbodiimide of Example 7a)
; were di~solved in 10 g of chlorobenzene and the resulting
solution added to 20 g oi Chlophen A 30. Thi~ solution was
dispersed in 300 ml o~ water and further processed in the
same way as described in Example I.
EXAMPLE 8
4 g of the polycarbodiimide o~ 1,3~5-triisopropylbenzene-
2,4-dii~ocyanate produced in accordance with 3a) were dissolved
in 196 g o~ methylene chloride and the resulting solution
wa~ mixed with 20 g of finely powdered medicinal carbon using
~ - ~5 -
~ .
.
a Lenart-Rapid laboratory stirrer turning at 200 rpm.
The resulting dispersion was maintained at about 25C
followed by the ad~tion over a period o~ l hour with contin-
uous stirring of 250 ml of acetone. The polycarbodiimide was
quantitatively precipitated in fine form with the activated
carbon included.
The ef~ect of the included activat~d carbon on a~ueous
methylene blue solution (in analogy to the standardi~ation
according to DAB 6) was distinctly reduced, the transition
to a ratio of core to shell o~ 50 : 50 additionally produced
a decrease in the activity of the included activated carbon.
~; '
; ~ - 16 -
, ,
