Process for preparing microcapsules having gelatin walls crosslinked with quinone

A process for conveniently producing microcapsules containing a gelatin wall crosslinked with quinone and a core of an active compound such as a fouling reducing agent, particularly a tributyl tin chloride, involves use of a simple or complex coacervation technique. The quinone crosslinking provides microcapsules of excellent strength, storage stability, and resistance to aqueous exposure, such that the rate of release of the fouling reducing agent can be controlled with precision.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a process for forming microcapsules and to 
the microcapsules so produced and to the use of such microcapsules in 
paint compositions. More particularly, the present invention relates to 
microcapsules having a wall containing gelatin crosslinked with quinone, 
wherein the core contains a fouling reducing agent such as a tributyl tin 
compound. 
Description of Related Art 
Microencapsulation involves the application of a coating around a 
microscopic phase of a liquid or solid core material. The first 
applications of microencapsulation were in carbonless copying papers and 
in the controlled release of drugs. Many other applications have since 
been explored such as described by Bayless, R., "Microencapsulation in New 
Areas," Chemical Engineering News, Vol. 52, p.16, August 1974. 
The use of microencapsulated compounds has many advantages over the use of 
unencapsulated compounds. In particular, the microencapsulation separates 
the core material from its environment and provides a controlled release 
rate. The release rate of the core material and the diffusion of the core 
material through the capsule wall can be controlled by varying the wall 
composition and/or the degree of crosslinking of the walls. Furthermore, 
if a material is encapsulated, its useful life may be significantly 
extended. Also, if a material is toxic and hence difficult to handle, 
encapsulation of the material may reduce the threat of acute exposure and 
allow for easier handling. 
Since many fouling reducing agents are extremely toxic and there is a 
desire to have these compounds act over extended periods of time, 
encapsulation of such compounds is desirable. Such encapsulation should 
serve to control the release rate of the agent and to avoid an initial 
high release of agent which could be environmentally unacceptable. In 
particular, tributyl tin compounds, such as tributyl tin chloride (TBTCl), 
are extremely toxic and thus excellent candidates for encapsulation. 
Work by Noren et al. ("Investigation of Microencapsulated Fungicides for 
Use in Exterior Trade Sales Paints," Journal of Coatings Technology, 
58:724 (1986)) and Porter et al. ("Extended Control of Marine Fouling," 
Applied Biochemistry and Biotechnology, 9:439-445, (1984)) indicate that 
two different coating formulations have been tested which contain 
bioactive microcapsules. Noren et al. have formulated an exterior paint 
containing microcapsules with urea-formaldehyde treated gelatin walls 
surrounding fungicidal compounds. The encapsulation of the fungicides 
allows for control of both the release and volatility of the active 
ingredient. 
Porter et al. describe formulations of a vinyl antifouling coating 
containing microcapsules having gelatin and gum arabic walls crosslinked 
with glutaraldehyde, wherein the core is a tributyl tin chloride 
antifoulant. These microcapsules degrade in an aqueous environment and 
hence are not very useful in aqueous environments. 
Haslbeck et al. in: Proceeding of the 16th International Symposium on 
controlled Release of Bioactive Materials, pages 273-274 (1989), describe 
crosslinking a gelatin/polyphosphate or gelatin/gum arabic microcapsule 
with glutaraldehyde and quinone. That document does not describe a method 
of conveniently producing such microcapsule. 
Accordingly, there has been a need to find an improved microcapsule which 
can be conveniently manufactured and is useful for encapsulating active 
compounds, such as fouling reducing agents, in particular tributyl tin 
compounds, wherein the microcapsules allow for excellent control of the 
release rate of the active core material. Furthermore, there is a need to 
provide an improved method of encapsulating TBTC1 which, though being 
highly toxic, is one of the most effective fouling reducing agents known. 
There is a need to provide a method of producing microcapsules which 
results in a microcapsule exhibiting a controllable release rate and 
allowing for a reduced initial TBTC1 release rate so as to continue its 
safe use in, for example, antifoulant coating compositions. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved 
microencapsulating process for encapsulating compounds, such as fouling 
reducing agents, with a capsule wall comprising gelatin, wherein the 
microcapsules are capable of being reduced to a powder and dispersed into 
a coating. 
It is a further object of the present invention to develop a microcapsule 
particularly useful in antifouling paints which offers excellent control 
of the release of the core material from the capsule. 
It is also an object of the present invention to provide an antifouling 
coating composition which contains a fouling reducing agent encapsulated 
in a shell, wherein the fouling reducing agent is released over time so as 
to impart excellent antifoulant characteristics to the coating over time. 
In accomplishing the foregoing objectives, there has been provided, in 
accordance with one aspect of the present invention, a process of forming 
a microcapsule comprising the steps of: 
a) emulsifying a core material in a solution of gelatin at about 50.degree. 
C. so as to produce particles having a diameter of about 30 to about 100 
microns, 
b) adding a polyanion to the emulsion, 
c) adjusting the pH of the emulsion to between about 4 and about 5 so as to 
allow coacervation, 
d) cooling the coacervate to room temperature so as to allow the coacervate 
to gel around the core material thus forming microcapsules with a wall 
comprising gelatin, and 
e) crosslinking said wall with a quinone. 
In accordance with another aspect of the present invention, there is 
provided a method of producing microcapsules comprising the steps of: 
a) emulsifying a core material with a solution of gelatin, 
b) adding a water-miscible alcohol or a salt to the gelatin solution to 
induce phase separation and coacervation, 
c) cooling the solution to gel a wall of gelatin around the core forming a 
microcapsule, and 
d) crosslinking the wall with a quinone. 
In accordance with a further object of the present invention, there has 
been provided a microcapsule produced by each of these processes. 
In accordance with another object of the present invention there has been 
provided a coating composition comprising a binder and a microcapsule 
which has a wall comprising gelatin which has been crosslinked with 
quinone, wherein said wall encapsulates an active material, preferably a 
fouling reducing agent, wherein the microcapsule has been produced by one 
of the above methods. 
Further objects, features, and advantages of the present invention will 
become apparent from the detailed description of preferred embodiments 
which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The microcapsules of the present invention contain a core of an active 
material. The active material may be any material which has utility in an 
encapsulated form due to the desire to control the release of the compound 
and extend the useful life of the compound and/or due to the desire to 
avoid direct handling of the compound. 
The active compound is preferably a fouling reducing agent. Any fouling 
reducing agent may be encapsulated, with particular preference being given 
to tributyl tin compounds, most particularly to tributyl tin chlorides. 
Mixtures of fouling reducing agents may also be encapsulated and mixtures 
of fouling reducing agents with other components, such as herbicides, can 
be encapsulated according to the invention. The microcapsules generally 
contain about 10 to about 20% by weight of wall material and about 80 to 
about 90% by weight of core material. 
The walls of the capsule are formed from a polymeric material containing an 
amino group, preferably the polymeric material comprises gelatin. Any type 
of gelatin can be used. The gelatin is optionally associated with a 
polyanion, such as gum arabic or polyphosphate. As discussed below, the 
polyanion is present if a complex coacervation process is used to form the 
shell. 
An important part of the present invention is that the polymeric walls be 
crosslinked with a quinone. Quinone itself and/or compounds containing a 
quinone moiety can be used to crosslink the gelatin. The quinone may be 
the sole crosslinker, that is quinone can be used alone without other 
crosslinkers such as aldehydes. Alternatively, the quinone crosslinking 
can be used in combination with other crosslinkers. In particular, use of 
quinone crosslinking following crosslinking with an aldehyde such as 
glutaraldehyde or formaldehyde has been found to be particularly useful. 
Other aldehydes can also be used which will crosslink the gelatin. The use 
of quinone as a crosslinker results in a microcapsule which is more stable 
in an aqueous environment than when an aldehyde crosslinker is used alone, 
and provides capsules walls with greater strength, allowing for excellent 
storage stability. 
Quinone crosslinking of proteins has been used in the leather tanning 
industry as described in Spurr, K., Thesis, Cornell University, Ithaca, 
N.Y. (1958); Gustavon K. H., The Chemistry of the Tanning Process. 
Academic Press, N.Y. (1965); and Green R. W. JACS 75: 2729, (1953) as 
cited by G. Loeb, Ph.D Thesis, Cornell University, June 1960. 
An important part of the present invention is the production of the 
microcapsules. It is preferred to use simple or complex coacervation 
microencapsulation techniques. Although these techniques are known per se, 
they are not known to form microcapsules as described herein. In 
particular, the ability to use coacervation processes depends upon 
numerous parameters including the properties of the core and how the wall 
material interacts with the core. Hence, much experimentation is needed to 
determine the processing parameters necessary for the coacervation. 
Simple coacervation is a phase separation phenomena caused by decreasing 
the solubility of a hydrocolloid in a solvent. According to the present 
invention, the following process has been found useful. A gelatin 
solution, preferably.an aqueous solution, at about 40.degree. to about 
60.degree. C. is emulsified with the core material. To the emulsion is 
added a water-miscible alcohol, such as ethanol or isopropanol, and/or a 
salt, such as sodium sulfate or ammonium sulfate, to induce phase 
separation. The emulsion is cooled to gel the wall around the core and 
then the wall is crosslinked as described above. If too much alcohol or 
salt is used, the gelatin will precipitate. However, if the amount of a 
coacervating agent is carefully controlled, a liquid polymer-rich phase 
can be produced which forms microcapsules. 
Complex coacervation is a spontaneous liquid/liquid phase separation that 
can occur when oppositely charged polyelectrolytes are mixed in aqueous 
media. This phenomenon is limited to mixtures of polyelectrolytes that 
have a suitable ionic charge density and chain length. Accordingly, not 
all mixtures will form complex coacervates. 
Coacervation processes are described by H. G. Bungenberg de Jong, Colloid 
Science II, H. R. Kruyt, ed., Elsevier Publishing Co., New York, N.Y., 
1949, pp 232-480 and in U.S. Pat. Nos. 3,697,437 and 2,800,457. 
Both simple and complex coacervation process can be used, however complex 
coacervation is the preferred method. In particular, after much 
experimentation with both simple and complex coacervation, the following 
complex coacervation process was found to give excellent results, with 
regards to the formed microcapsule. Complex coacervation as used in the 
present invention comprises emulsifying a gelatin, preferably an aqueous 
gelatin solution at a temperature of about 40.degree. to about 50.degree. 
C. with the core material, and then mixing the appropriate combinations of 
oppositely charged polyelectrolytes into the emulsion. Useful electrolytes 
include gum arabic, polyphosphates, alginate, carboxymethylcellulose, 
carrageenan, and/or ethylene/maleic anhydride copolymers. Coacervation and 
deposition of the wall around the core occurs upon proper adjustment of 
the pH and cooling. The resulting coacervate solution is characterized by 
a polymer rich coacervate phase and a polymer-poor phase. The coacervate 
is adsorbed by the core material and, on cooling, forms the gelled capsule 
wall. This process is described in more detail in the examples which 
follow. 
The microcapsules of the present invention can be used in any desirable 
manner, and are particularly useful as an additive to an antifouling paint 
or coating system. The microcapsules are generally used in amounts which 
will impart the desired fouling reducing affect to the coating. Such 
amount is generally in the range of about 1 to 10% by weight of 
microcapsules based on the total weight of the coating composition. Paints 
based on organic films are particularly useful. Such paints include those 
containing a rosin and vinyl chloride/vinyl acetate copolymer binder. 
Coatings and paints containing the inventive microcapSule additionally may 
contain further ingredients conventionally used in antifouling coating 
systems. 
In addition to the microcapsules described above, other types of 
microcapsules, such as those having a core of other fouling reducing 
agents or herbicides, can be added to the coating material. For instance, 
encapsulated herbicides, such as simazine, can be added to the coating so 
as to further control algal microfouling. 
The invention will now be illustrated with reference to the following 
examples without being limited thereby. 
EXAMPLE 1 
This example illustrates the use of simple coacervation to form 
microcapsules. The core material used was a mineral oil, which was used to 
simulate the encapsulation of TBTCl oil. The procedure used was as 
follows: 
1. 25 grams of gelatin were completely dissolved in 200 ml distilled water 
while placed in a water bath maintained at 55.degree. C. 
2. To the dissolved gelatin, 50 ml of heavymineral oil was added and 
allowed to equilibrate at 55.degree. C. 
3. Concurrently, a solution of 7% isopropyl alcohol was also heated in the 
water bath to 55.degree. C. 
4. The gelatin-oil mixture was then placed in a blender and mixed at the 
highest speed for 1 min. This step emulsified the oil in the gelatin 
mixture, resulting in oil spheres of approximately 50 to 100 .mu. diameter 
dispersed throughout the gelatin solution. 
5. The blender speed was then reduced and the heated 7% alcohol solution 
added slowly to facilitate the phase separation. Discrete microcapsules 
form with the oil as the core material and the gelatin as the wall 
material. 
6. The solution containing the gelatin-oil coacervate was then slowly 
poured into 2 liters of 7% isopropyl alcohol solution at about 10.degree. 
C., with constant stirring, and maintained at 10.degree. C. for 2 hours. 
The rapid cooling of the microcapsules serves to gel the gelatin 
coacervate around the oil drops. The mixture is allowed to stand for 
several hours whereupon the formed microcapsules float to the top with 
free coacervate/unreacted gelatin sinking to the bottom. The formed 
microcapsules are then rinsed to remove any unused gelatin. 
7. Although at this point the gelatin capsules have gelled and remain 
discrete, they are not thermostable, and the capsules will not maintain 
their integrity when separated and dried. For this reason the gelatin 
capsules, while maintained at 10.degree. C., were crosslinked with 
formaldehyde for 4 hours. The formaldehyde crosslinking serves to render 
the capsules heat stable by binding the amino sites of neighboring gelatin 
molecules. 
8. The microcapsules may be further hardened by following the aldehyde step 
with an additional 4 hours of quinone treatment. 
The first dried microcapsules using the formaldehyde crosslinking agent 
alone resulted in the capsules sticking together and being soft. These are 
not suitable for dispersion into an antifouling coating system. Increasing 
the amount of formaldehyde crosslink and the crosslinking time did not 
improve capsule integrity. It was also observed that the capsules could be 
easily broken, causing the oil to be released. To provide greater capsule 
wall integrity, the formaldehyde crosslinking step may be followed by 
additional crosslinking with quinone. It is believed that the addition of 
the quinone will greatly improve the integrity of the capsule wall. 
EXAMPLE 2 
This example illustrates the use of complex coacervation to form 
microcapsules. Initial experimentation involved encapsulating model oils 
(mineral oil or extra virgin olive oil) with gelatin/polyphosphate and 
gelatin/gum arabic to simulate the encapsulation of tributyltin chloride 
oil. Acid precursor gelatins were used with bloom strengths of either 160 
or 280. The polyanions used were gum arabic or polyphosphate. A lipophilic 
blue dye (Oil blue-N) was dissolved in the oil to aid in monitoring 
microcapsule formation. 
The procedures used were as follows: 
1. A water bath was used to heat the following solutions and material to 
50.degree. C.: 
______________________________________ 
Gelatin/Polyphosphate (G/P) 
Gelatin/Gum Arabic (G/GA) 
______________________________________ 
11% gelatin solution 
11% gelatin solution 
5% polyphosphate solution 
11% gum arabic solution 
distilled water distilled water 
oil oil 
______________________________________ 
2. To 800 milliliter (ml) beakers were added: 
______________________________________ 
90.9 ml gelatin solution, 
60 ml gelatin solution 
29.1 ml distilled H.sub.2 O, and 
67 ml distilled H.sub.2 O, and 
5-10 drops n-octanol 
5-10 drops n-octanol 
(defoaming agent) (defoaming agent) 
3. A solution of 90 ml oil with 
A solution of 60 ml oil with 
&lt;1 g Oil blue-N dye was 
&lt;1 g Oil blue-N dye was 
added slowly to the gelatin 
added slowly to the gelatin 
solution and emulsified by 
solution and emulsified by 
rapid impeller stirring to 
rapid impeller stirring to 
desired droplet size of 30- 
the desired droplet size of 
100 microns. 30 to 100 microns. 
______________________________________ 
An even distribution of droplets was achieved by stirring the above 
solution at high speeds for from 5-10 minutes. 
______________________________________ 
4. 120 ml distilled H.sub.2 O and 20 ml 
200 ml distilled H.sub.2 O and 67 
polyphosphate solution were 
ml gum arabic solution were 
added. added. 
______________________________________ 
5. Adjustment to a pH of about 4.0 to about 5.0, preferably 4.1 to 4.6, 
with acetic acid and/or sodium hydroxide, caused coacervate to form. 
6. The mixture was cooled slowly at about 1.degree. C. every ten minutes, 
to room temperature (about 23.degree. C.) with minimal stirring. This 
allowed the coacervate phase to gel around oil droplets and form the 
walls. At this point these were embryo microcapsules which were not 
thermostable. 
7. The microcapsules were rinsed with distilled water to remove excess 
coacervate which would cause agglomeration if allowed to remain. 
8. The mixture was cooled to 5.degree.-10.degree. C. for 30 minutes in an 
ice bath; 5 ml 25% glutaraldehyde solution was added. This was allowed to 
come to room temperature,(about 23.degree. C.) and the desired length of 
time to harden the microcapsule walls, making the microcapsules durable 
and less water-sensitive, and rendering them thermostable. 
9. After about thirty to sixty minutes of crosslinking with the aldehyde, 
the microcapsules were crosslinked further with a saturated quinone 
solution for the desired length of time to produce microcapsules of 
excellent strength and minimal water-sensitivity, generally for 18 to 36 
hours at room temperature. The quinone gives microcapsules of added 
strength and even less water-sensitivity than achieved with aldehyde 
crosslinking alone. 
10. The microcapsules were rinsed and filtered. 
11. Silica drying agent was used to aid in drying microcapsules to a 
free-flowing powder. 
This procedure was used to encapsulate both mineral oil alone and 
combinations of TBTCl oil (94% pure as in Example 2) and mineral oil. 
Microcapsules with 16% and 0.27% TBTCl in mineral oil were successfully 
produced. These microcapsules were then tested for compatibility with 
paint systems. 
Both types of TBTCl microcapsules, 16% TBTCl and 0.27% TBTCl, and mineral 
oil microcapsules for controls, were formulated into a coating system 
having the following composition: 
Formula for approximately 11/2 gallon: 
______________________________________ 
Rosin (grade WW) 1600 g 
Vinyl chloride/vinyl acetate copolymer 
460 g 
TCP (Tri-cresyl phosphate) 
420 g 
MIBK (Methyl iso-butyl Ketone) 
1380 g 
Xylene 960 g 
Bentone 38 70 g 
Propylene Carbonate 22 g 
______________________________________ 
The experimental paints contained concentrations by weight of ten, six, and 
two percent of each type of microcapsules. These coatings were then 
applied to 3".times.5" sand blasted G-10 epoxy fiberglass panels. The 
compatibility of the microcapsules with the paint system was evaluated by 
observing capsule integrity and clumping after mixing. Observations were 
made using a microscope at 15X total magnification. 
The quinone crosslinking greatly increases the capsule wall strength as 
compared to aldehyde crosslinking and results in a capsule having greater 
resistance to aqueous environments, that is, there is not a premature 
breakdown in aqueous environments. 
The TBT-containing capsules dispersed very well in the clear resin. The 
lack of pigment allowed observation of the capsules throughout the film 
thickness. Visual inspection verified that the capsules were compatible 
with the coating system since they did not rupture, even when the coating 
dried. 
Results and Discussion 
Using the complex coacervation techniques, a dry, free-flowing powder of 
microcapsules was obtained in each case which could readily be dispersed 
in the coating system. The size range of capsules in the slurry before 
drying was about 50-125 .mu.m. The dried capsules tended to clump, and 
ranged in size from 65-300 .mu.m. Small microcapsules (20-30 .mu.m) of a 
more uniform size distribution can also be prepared. 
The TBTCl microcapsules were of the same quality as the pure mineral oil 
capsules. That is, they appeared similar under 10X magnification, having 
the same relative shape and wall thickness. They also formed a comparable 
final dried powder. 
Aldehyde cross-linking alone generally did not provide sufficient wall 
integrity and has inadequate stability in aqueous environments. Therefore, 
to provide greater strength and stability in aqueous environments, the 
aldehyde cross-linking step was followed by additional cross-linking with 
quinone. The quinone bonds are more stable in an aqueous environment. 
Dried capsules were left in jars at room temperature for several months 
and retained wall integrity. The dried capsules are characterized by 
non-leaky uniform walls. 
Testing was conducted on the compatibility of the microcapsules with the 
pigment-free coating system described above. Both mineral oil and TBT 
capsules dispersed very well in the coating system. Visual inspection 
verified that the capsules were compatible with the coating system since 
they did not rupture or deform, even when the coating dried. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.