Microencapsulation process

In a microencapsulation process involving an interfacial polymerization reaction between a compound containing amine groups and a polybasic acid chloride or anhydride, the polymerization being carried out in an emulsion of an aqueous solution in a hydrophobic organic liquid, the reaction is terminated by introducing water into the hydrophobic phase of the emulsion where it reacts with and destroys the residual acid chloride or anhydride.

The present invention relates to microencapsulation processes and more 
especially to processes for encapsulating fine droplets of a hydrophilic 
liquid in a hydrophobic continuous phase. 
It has been proposed in the prior art to produce microcapsules containing 
aqueous amine solutions by emulsifying the aqueous solution in an organic 
liquid with the aid of a suitable surfactant and forming a polymer 
membrane around the droplets by interfacial polymerisation. For example, 
it has been proposed to add sebacoyl, succinoyl, adipoyl, phthaloyl, 
terephthaloyl or citroyl chloride or succinic anhydride to the continuous 
phase to produce a polyamide membrane. Once the microcapsules have been 
produced it is frequently desirable to change the continuous phase from a 
hydrophobic liquid to a hydrophilic liquid, especially water. This is 
normally effected by decanting or centrifuging off the majority of the 
hydrophobic liquid, and washing and redispersing the microcapsules several 
times in a hydrophilic liquid containing large amounts of surfactant. 
In the processes proposed in the prior art, certain precautions have to be 
taken to avoid aggregation of microcapsules, especially at the 
phase-change stage of the process where the capsules are close together 
and, for instance, free acid chloride groups in the polymer membrane of 
one capsule may condense with, for instance, residual free amino groups in 
the polymer membrane of a second capsule thereby forming chemical bonds 
between the capsules, or for example, residual acid chloride or anhydride 
in the reaction medium may condense with residual free amino groups on two 
or more adjacent capsules. 
For many applications of the capsules aggregation or clumping of the 
capsules is highly undesirable and efforts have been made to ensure that 
the polymerisation reaction goes to completion thereby avoiding any 
possibility of aggregation of the capsules. 
U.K. Application No. 2040863 discloses a process for the production of 
microcapsules containing an aqueous solution of a hydrophilic protein 
containing a plurality of free amine groups, which comprises forming an 
emulsion of the aqueous protein solution as disperse phase in a 
substantially non-polar solvent as the continuous phase and adding to the 
emulsion a solution of a compound containing a plurality of groups capable 
of reacting with amine groups to form a polymer, especially to form a 
polyamide. 
It is possible when operating in accordance with the instructions of patent 
specification No. 2040863 to obtain batches of microcapsules in which 
little or no aggregation has occurred but great care has to be taken in 
carrying out the process and attempts have therefore been made to find a 
satisfactory and consistent way of preventing or controlling aggregations 
of the capsules. 
This invention provides a process for microencapsulation of an aqueous 
amine solution by emulsifying the aqueous solution in an organic liquid 
and forming a polymer membrane around the droplets by interfacial 
polymerisation by reaction with a poly basic acid chloride or anhydride, 
wherein, when the reaction has proceeded to the desired extent the 
reaction is terminated by introducing water into the reaction medium to 
react with and destroy residual acid chloride or anhydride. 
Water can be introduced into the reaction medium by any method that ensures 
intimate contact of the water with the acid chloride or anhydride 
dissolved in the hydrophobic phase of the emulsion or with acid chloride 
or anhydride groups remaining in the polymer membrane of the capsules. 
The water is preferably introduced into the reaction medium by adding to 
the reaction medium a solution of water in an organic liquid, which 
solution is substantially miscible with the hydrophobic phase of the 
emulsion. 
The organic liquid serves as a carrier vehicle to introduce water into the 
hydrophobic phase of the emulsion where it substantially immediately 
reacts with the acid chloride or anhydride thereby rendering it incapable 
of taking part in further reaction. Accordingly, when the capsules are 
brought into close proximity, as for example when the hydrophobic liquid 
is decanted off, there are no residual acid chloride or anhydride groups 
available to take part in further reaction with the amine groups. 
The organic liquid is preferably a lower aliphatic alcohol containing up to 
5 carbon atoms but it may be any liquid that is capable of dissolving 
water in sufficient quantity to neutralise all the acid chloride or 
anhydride groups to produce a solution which is miscible with the 
hydrophobic phase of the emulsion. Preferably, the solvent should dissolve 
water in an amount of at least 1% and desirably in an amount of at least 
3%. 
When an alcohol is used as the carrier solvent it has been found in some 
instances that the walls of the capsules are somewhat stronger than those 
produced when the reaction is not terminated by the method of the 
invention and it is thought that this may be due to a certain amount of 
ester formation by reaction between the alcohol and the free acid chloride 
or anhydride groups. 
Alcohols preferred for use in the process are methanol, ethanol, 
n-propanol, isopropanol, n-butanol, isobutanol and n-amyl alcohol. 
Other liquids that can be used in the process are, for example, the various 
solvent mixtures that are conventionally used in such operations as 
fluorometric analysis or assay to introduce water into organic solvents. 
Among such solvent mixtures there may be mentioned those materials sold 
under the trade names "Pico-fluor 30" and "Instagel". Pico-fluor 30 is 
believed to be based on pseudocemene and Instagel is based on a mixture of 
dioxane and naphthalene. 
As an alternative to introducing water in admixture with an organic liquid 
it is possible to introduce water in admixture with a gas provided that 
the gas can be introduced into the liquid in sufficiently finely divided 
form. Moist air can for example be introduced into the liquid through an 
air stone or through a stirrer having means for the introduction of air. 
Following termination of the reaction it has been found that there is no 
need to use complicated procedures to separate the capsules from the 
hydrophobic phase. All that is required is to decant off the solvent phase 
and, for example, to sieve the capsules on a screen. Following this, if 
desired, the capsules can be dried simply by passing a current of air 
through the capsules on the screen although other methods such as freeze 
drying may be used if desired. During this drying process any occluded 
hydrophobic liquid can be removed from the capsules. Drying may also be 
carried out to remove the water from within the capsules thereby reducing 
the density of the capsules. When capsules dried in this way are placed in 
water they rapidly rehydrate to be ready for use. Following drying of the 
capsules there may be some caking caused by physical bonding between 
capsules and this can be broken by agitating the capsules but there is no 
aggregation of the capsules caused by chemical bonding unless a certain 
amount of aggregation is desired for some uses of the capsules. 
It will be appreciated that since the reaction is terminated substantially 
immediately it is possible to modify the process of the invention by 
reducing the volume of the reacting mass so that the capsules are brought 
closer together to a stage in which a desired degree of aggregation will 
occur and terminating the reaction at that stage. Thus for example if it 
is desired to produce clumps of aggregated capsules of a given diameter 
termination of the polymerisation reaction can be effected to achieve such 
a goal. 
Although the process of the invention is applicable to any 
microencapsulation process involving an interfacial polymerisation 
reaction between an amine and an acid chloride or anhydride, the process 
is particularly applicable to the process described in U.K. patent 
specification No. 2040863 where the microcapsules are primarily intended 
for fish food or for carrying pharmaceuticals or the like since it is then 
possible to ensure that single capsules of the desired size can be 
obtained in a very simple manner and without extensive precautions against 
aggregation. 
Microcapsules produced by terminating the process of U.K. patent 
specification No. 2040863 using the process of this invention have all the 
advantages of the capsules of the earlier application but have various 
additional advantages. Thus, there is no longer any need to have a 
specific phase change operation with its repetitive washing operations 
although some washing may be desirable to remove traces of surfactant; 
because of the absence of a phase change operation there is little risk of 
capsule rupture due to changes in osmotic pressure and there is no need to 
remove any surfactant used in the phase change operation; and because of 
the reduced chance of aggregation it is possible to reduce the amount of 
hydrophobic liquid used in the process, the ratio of hydrophobic liquid to 
aqueous phase can be reduced to as little as 2:1 or even lower.

The following examples illustrate the invention. 
In Examples 1 to 6 a diet mix comprising homogenised cod roe, water soluble 
fish meal, spray dried egg white and glycerol in a ratio of 10:1:1:1 was 
used. 
EXAMPLE 1 
13.5 grams of egg lecithin were dissolved in 2,200 mls of cyclohexane while 
stirring with a high speed stirrer. Stirring was continued while 700 grams 
of diet were added and the mixture was homogenised by further mixing for 6 
minutes to produce an emulsion. 10 mls of succinoyl dichloride in 800 mls 
of cyclohexane was added and the reaction was allowed to continue for 10 
minutes. A solution of 1 gram cholesterol and 4 grams egg lecithin in 210 
mls of cyclohexane was then added and stirring was continued for 3 
minutes. 200 mls of a 3% solution of water in ethanol was then added and 
stirring continued for a further 2 minutes. The solvent was decanted off 
and the capsules were subjected to a freeze drying operation. Good quality 
dried capsules with no aggregation in a yield of 270 grams were obtained. 
The dried capsules rehydrated in sea water or fresh water without rupture. 
EXAMPLE 2 
Example 1 was repeated except that the cholesterol/egg lecithin solution 
contained 1 gram cholesterol and 2.66 grams of lecithin in 190 mls 
cyclohexane and was added in an amount of 190 mls. 
Similarly good capsules were obtained. 
EXAMPLE 3 
Example 1 was repeated except that emulsification was stopped after 2 
minutes and restarted at faster speed with a smaller bladed rotor. 
Similarly good capsules were obtained but they were smaller and less dense 
than the capsules obtained from Example 1. This is probably due to 
occluded cyclohexane resulting from the changed emulsification conditions 
being removed during the freeze drying operation. 
EXAMPLE 4 
Example 1 was repeated except that the emulsification period was reduced to 
5 minutes using a faster speed and a smaller rotor. 
Similarly good capsules of a slightly smaller size than those obtained in 
Example 1 were obtained. 
EXAMPLE 5 
Example 1 was repeated except that the amount of cyclohexane used in 
preparing the emulsion was reduced to 1500 mls, the amount of cyclohexane 
introduced with the acid was reduced to 500 mls and the reaction time was 
increased to 9 minutes. 
Similarly good capsules were obtained. 
EXAMPLE 6 
1 gram of cholesterol and 4 grams of egg lecithin were dissolved in 2,010 
mls of cyclohexane in a 5 liter beaker with stirring using a high speed 
mixer. 700 grams of diet were added and mixing was continued for 5 minutes 
to homogenise the mix and form an emulsion. 10 mls of succinoyl chloride 
in 800 mls of cyclohexane was then added and reaction was continued for 8 
minutes. The reaction was terminated by adding 200 mls of a 3% water in 
ethanol solution and mixing for 2 minutes. The capsules were allowed to 
settle and the cyclohexane was decanted off. The resulting capsules could 
be satisfactorily freeze dried and were of the same good quality as those 
produced in Example 1. 
EXAMPLE 7 
Example 1 was repeated using 1000 grams of a diet mix consisting of capelin 
oil, spray dried egg whites and spray dried haemoglobin in water in a 
ratio of 4:1:1:14. The diet was made up by forming the oil, egg white and 
haemoglobin into a paste which was then dispersed in the water. 
300 grams of good quality capsules were obtained. 
EXAMPLE 8 
Example 7 was repeated except that in the diet the spray dried hemoglobin 
was replaced by spray dried whole blood. 
Similarly good capsules were obtained. 
EXAMPLE 9 
Example 1 was repeated using 700 grams of a 20% aqueous solution of bovine 
haemoglobin. 
Good capsules were obtained. 
EXAMPLE 10 
20 mls of a 6.25% w/w solution of egg lecithin in cyclohexane were added to 
200 mls of cyclohexane with stirring and while stirring was continued 70 
grams of a 20% w/w solution of egg albumin in water was added. Stirring 
was continued for 5 minutes to produce an emulsion. 
1 ml of succinoyl dichloride dissolved in 80 mls of cyclohexane was added 
and stirring was continued for a further 5 minutes. 
20 mls of n-butanol containing 0.4 mls of water were then added and after a 
further 2 minutes stirring the solvent was decanted off and the capsules 
were subjected to a freeze drying operation. 
Good quality dried capsules were obtained, the capsules being rehydratable 
in sea water or fresh water without rupture. 
EXAMPLE 11 
Example 10 was repeated using 20 mls of isopropanol containing 0.2 mls of 
water in place of n-butanol. Similar results were achieved. 
EXAMPLE 12 
Example 10 was repeated using 20 mls of n-propanol containing 0.6 mls of 
water in place of n-butanol. Similarly good results were obtained. 
EXAMPLE 13 
Example 10 was repeated using 20 mls of n-amyl alcohol containing 0.2 mls 
of water in place of n-butanol. Similarly good results were obtained. 
EXAMPLE 14 
Example 10 was repeated using 20 mls of ethanol containing 0.6 mls of water 
in place of n-butanol. Similarly good results were obtained. 
EXAMPLE 15 
Example 10 was repeated except that 0.6 mls of water in 20 mls of 
Pico-fluor 30 was used in place of the water in butanol solution. 
Good capsules were obtained. 
EXAMPLE 16 
Example 10 was repeated except that instead of pure egg albumin there was 
used a mixture of egg albumin and bovine hemoglobin in a ratio of 1:1. In 
addition instead of the solution of water in butanol there was used a 
solution of 0.6 mls of water in 20 mls of Instagel. Good quality capsules 
were obtained.