WO1990002655A1

WO1990002655A1 - Realease assist microcapsules

Info

Publication number: WO1990002655A1
Application number: PCT/US1989/003845
Authority: WO
Inventors: Bruce K. Redding, Jr.; Walter S. Garrison; Arden E. Schmucker
Original assignee: ENCAPSULATION SYSTEMS Inc
Current assignee: ENCAPSULATION SYSTEMS Inc
Priority date: 1988-09-06
Filing date: 1989-09-06
Publication date: 1990-03-22
Anticipated expiration: 1991-03-06
Also published as: EP0464023A1; EP0464023A4

Abstract

The invention is a microcapsule consisting of three components: 1) a shell or wall material layer (4); 2) a core material (3) consiting of an active ingredient; 3) a propellant (2) which is the second core material. The invention is a microcapsule (1) consisting of two core materials, one being an active core ingredient (3) while the other is a propellant (2), both surrounded by an encapsulating shell layer (4). The propellant (2) acts to either explode the capsule (1) or to help accelerate the release of the active core material (3) when the capsule is heated. The capsules have a triggered time release function, caused by heat acting upon the propellant core (2), which then acts to push more of the active ingredient (3) through the wall membrane of the capsule shell, or to explode the capsule from within.

Description

RELEASE ASSIST MICROCAPSDLES Field of Invention

This invention relates to an improved method of effecting the release of an active core from microcapsules.

Additionally this invention relates to encapsulated fungicides.

A further relationship exists to encapsulated

catalysts.

There are four main release mechanisms for

microcapsules available. Conventional microcapsules release their active core material through:

1. Diffusion of the active across the shell membrane

2. Leakage of the active core through pores in the structure of the microcapsule

3. Interaction with another chemical, or solvent

4. The use of a force which causes the capsule to break or dissolve.

A microcapsule may be classified as a either a Break On

Demand or a Time Release product.

Time Release is a term used to describe a slow release over a period of time of the active core material from the confines of the capsule.

There are several problems with conventional Time

Release microcapsules:

1. They generally have a short shelf life period, often releasing the core too soon.

2. Most such capsules generally have very fragile shells, making them unsuitable to many industrial

applications.

3. They often have pre-mature chemical reactions with the environment they are placed in due to early release of the active core material from the capsule. Break on Demand microcapsules are capsules which generally have thicker shells, allowing little or no release of the active core material until a physical force or chemical reaction causes the shell to rupture. Such capsules are virtually inert until an outside force effects the breaking or dissolution of the shell.

The shelf life stability of microencapsulated chemicals has inhibited the application of microencapsulation

technology to many fields. In many cases the Time Release capsules release far too early. Using a Break On Demand capsule system can often result in a capsule which does not release at all. Examples of such problems can be found through an examination of the encapsulation of catalysts and fungicides:

HILDEW SUPPRESSANT FUNGICIDES FOR PAINT USE

In this project a fungicide is mixed into a can of water based paint, the intent being to provide mildew protection after a surface has been painted. The paint is generally used in wet or damp environments. The fungicide is intended to avoid discoloration of the painted surface. Most fungicides available on the market react with the

ingredients in the paint or dissolve while the paint can is stored on the shelf, a period of time which can last past one full year. The shelf life reactivity problem leads to a loss of potency for the fungicide. Encapsulation of the fungicide, to avoid the chemical reaction problem, can produce either a Reservoir capsule or a Micro-sponge type capsule, a capsule with pores. The Micro-Sponge type capsule allows the fungicide to leak from the capsule while stored on the shelf, thereby having little protection for the active material.

The Reservoir construction leads to a capsule with a shell which is so thick as to not allow any leakage of the active or diffusion to occur, avoiding premature reaction during shelf storage, but the shell may also not allow release after the surface has been painted either. In this application, where microencapsulation appears to be of potential benefit the encapsulation effort is defeated through shelf life and release difficulties.

ENCAPSULATED CATALYSTS

In this field microcapsules are intended as a means of delivering a catalyst to a particular location or at a particular time in a chemical process, without chemical reaction along the way. Catalysts are isolated within a capsule until the shell is broken of dissolved. Heat, is often used to melt away the capsule shell to release the active catalyst material. In theory this should enable satisfactory controlled delivery of a catalyst. In practice however catalyst compounds often react with the shell material, crosslinking the polymer. This can increase the melt point of the shell material to a level higher than desired or it can lead to a weakening of the shell itself, producing cracks and lesions in the capsule wall. The resultant capsules often have no longevity or actually do not contain the original quality of catalyst. Using

substitute shell materials in the capsule construction, shells which will not react with the catalyst, often produce undesirable traits in the resulting product. An example would be the encapsulation of Butyl Tuads, a common rubber catalyst, which was desired to be released from the capsule at temperatures below 100° (c ) . The chosen shell mater i al , which had a melt point of 65° (c) reacted with the catalyst and would release only at 180° (c). An attempt was made to use a non reactive second polymer which isolated the

catalyst properly, but had a temperature melt point of 250 (C). The problem was: How to get the catalyst to release in a non-reactive shell at a temperature below 100 (C) ?

Conventional microcapsule release methods did not have a suitable effect in this project.

To solve these problems the use of a secured form of reservoir microcapsule construction is employed, a capsule which does not allow diffusion and does not have pores which would allow core leakage. Normally such Secured

Ilicrocapsules do not have a Time Release Function. Instead these capsules are usually employed in operations whereupon a Break on Demand function is desired. In this instance the Break on Demand capsule is caused to a have a time release function by using a Triggered Release approach.

In this embodiment of the invention a propellant is added to the core material to cause the active core to release under certain conditions. The preferred condition is to employ heat to cause the propellant to accelerate the release of the active material. A propellant is placed along with the active material as the core of the microcapsules. When heat is applied the propellant can act to explode the capsule from within, thus providing a burst release

function, OR the propellant can act to accelerate the diffusion rate of the active by pushing the active material through the pores in the capsule shell.

In the Fungicide example outlined above the use of a Release Assist Approach was attempted whereby a solvent was added to the fungicide core material and a Reservoir type sealed capsule was produced around the combination core. The resultant capsules exhibited a strong resistance to shelf life deteriorate in the paint can. Later after the paint was applied to a painted surface the capsules began to release. In this case the painted surface was the exterior of a house where the heat of sunlight acted to warm the capsules within the paint layer. The heat caused the solvent within the capsule to partially volatilize. The resulting vapors produced a pressure within the capsules which then acted to "push'" the fungicide second core through the shell of the capsule. This push is triggered by heat and acts to

accelerate the release of the capsule. By adjusting the amount of solvent within the core the release rate after the heat trigger can be controlled to take place over a series of days or weeks.

In the catalyst example where the catalyst core reacted with the shell of the capsule the release function

difficulties are solved by using a non-reactive polymer as the capsule shell, together with a solvent as a second core. The catalyst does not react with the new polymer but the capsule normally has a melt point higher than desired. The addition of the solvent solves this problem. By choosing a solvent with a low enough boiling point the capsule can be made to release at the desired temperature. When the capsule is heated to the boiling point of the solvent second core, the solvent volatilizes completely. The resultant gas then exerts a pressure upon the interiorof the capsule, causing the capsule to explode outward. The explosion occurs at a temperature lower than that of the shell polymer, and closer to the boiling point of the solvent. In this manner the solvent acts as a propellant, providing a Burst Release of the active material when the correct temperature level has been obtained.

There are many applications in the prior art where capsules are produced using a wide variety of techniques, capsules which can have an enhanced release profile or function using the Release Assist approaches defined in this invention.

The present invention encompasses encapsulation of substances in liquid, solid or slurry form. These substances may be pharmaceutical agents, catalysts, fungicides,

pesticides or any other chemical. Encapsulating walls may or may not be permeable to core material or other materials to which the completed capsules may later be added. Capsules provided by the present invention may be capable of slow release mechanisms (commonly referred to as "time release") or may be released in a sudden method caused by heat, ultraviolet light, microwaves, ultrasonics or other energy media or may be released through dissolution of the capsule wall. In each instance of capsule release the release function is aided by the use of a propellant material which has been added to the active core material, whereby the propellant acts to accelerate the release of the

active.

Accordingly , a first object of this invention is to provide a microencapsulated product that possess a special release function which may be either a Sudden or a Slow release system which employs the use of a propellant or solvent as an additive to the active core of said

microencapsulated product, whereby the release function of the capsule is enhanced in some manner through the use of the propellant additive.

A second object of this invention is to enable Time Release capsules to become stable during shelf storage, beginning their time release function after that function has been triggered by some effective means.

A third object of this invention is to enable

microencapsulated compounds to have a greater utility by providing an expanded selection of polymers for use as capsule wall materials, by enhancing the release function of the resulting microencapsulated product.

A fourth object of the invention is to provide means of encapsulating difficult or very active chemical compouncs by providing a different release mechanism, which in turn provides a greater selection of encapsulating techniques and technologies.

A fifth object of the invention is to provide a means of fast release at narrow temperature ranges of a confined active core from a microcapsule by providing a propellant additive to the the active core material.

Other objects and advantages of the present invention will e obvious from the following descriptions, drawings and experiments.

Brief Description Of The Drawings

FIG. 1 is an illustration of the major capsule forms available through conventional microencapsulation processes.

FIG. 2 is an illustration of the preferred

embodiment of the invention, a capsule containing a propellant additive to the active core material.

FIGS. 3 and 4 are graphs indicating the release temperatures of a microcapsule containing a catalyst active core material, mixed with a solvent, which acts as the propellant.

FIG. 5 is an illustration of one method usable to form microcapsules, called Coacervation.

TABLE 1 is an chart indicating the release rates of capsules containing encapsulated Folpet compared to

capsules using the Release Assist enhancement as measufed by a per centage of weight loss over a period of time.

TABLE 2 is a comparison of the release temperatures of a catalyst known as Butyl Tuads which has been

encapsulated using a conventional reservoir type

construction vs. a release assist construction, whereupon various solvents have been used as propellants.

TABLE 3 is a list of potential polymers which may be used as capsule shell materials.

TABLE 4 is a chart indicating the bioactivity of capsules containing a fungicide compared to capsules

containinga Release Assist mechanism employing a heat sensitive propellant material as a second core.

Detailed Description Of The Invention

FIG. 1 illustrates several conventional forms of microcapsule construction. Each construction is capable of enhanced release dynamics using this invention. In the first example a Reservoir Capsule construction is indicated by a egg type construction, characterized by a large amount of core material surrounded by a single shell. The Micro-Sponge construction is a capsule with a lower quantity of core material embedded within many shell layers, allowing for a slow release.

The Multi-Wall capsule construction involves a

Reservoir type microcapsules with more than one shell layer. The liulti-Core capsule is essentially a capsule, or several smaller capsules, contained within a larger capsule. Each construction offer unique properties for

particular product applications. The normal release

mechanisms involve:

Slow dissolution of the shell layer

Permeation of the core active through the shell

Degradation of the shell due to heat, ultrasound, microwave energy, pressure, impact, radiation, ultraviolet light, solvent reactions or Ph adjustments

FIG. 2 indicates the preferred embodiment of a Release Assist capsule construction of the present invention 1 using the Reservoir format, whereupon a phase change material or propellant 2 has been added to the core material 3, both contained by a shell layer 4.

The illustrated capsule construction of the present invention is a Reservoir type capsule. However, the present invention may use any of the other capsule forms indicated in FIG. 1.

The embodiment of the invention shown in FIG. 2 is a capsule 1 containing a shell 4 which surrounds a core material combination consisting of a active ingredient 3 which is mixed with a propellant 2. The active 3 can be any solid, liquid or slurry chemical compound. A partial list of potential active's includes:

Pharmaceuticals Pesticides

Catalysts Fungicides

Fragrances Emollients

Lubricants Adhesives

Laundry additives Bleaching agents

Dyes and Colorants Essential Oils

A number of chemical compounds can be used as the propellant 2 including:

Solvents Phase Change Materials

Waxes Peroxides

Propellants Hydrocarbonous solutions Water Alcohols

Fuels Gases The propellant 2 may be any material which will

dissolve or expand, or convert into a gas when heat is applied to the capsule 1. The action of the propellant 2 causes the Release Assist function of the invention to take place, in that the propellant 2 acts to cause either a Burst Release or an Accelerated Release or a Triggered Release function to occur. The propellant may be in either solid, liquid or gaseous form.

BURST RELEASE

The application of heat to the capsule 1 causes the propellant 2 to expand within the capsule. This can cause the shell 4 of the capsule 1 to elongate and expand beyond the elastic limits of the polymer, eventually causing the capsule to explode outward, providing a sudden or burst release effect. The exploding action may be caused by a propellant material 2 which expands under heat or phase changes into a gaseous form during heating. The resulting gas exerts a pressure against the interior of the shell 4 causing it to expand outward, eventually exploding the capsule.

ACCELERATED RELEASE FUNCTION

The propellant 2 expands or phase changes with heat application but does not generate enough force to actually rupture the shell 4. Instead the pressure generated from the expanding propellant 2 acts to accelerate the diffusion rate of the active core 3 across the shell membrane 4. In most capsules using a single shell reservoir type of construction or a sponge type construction the active leaks from the capsule through pores or cracks in the shell layer of the capsule. Additionally the active core can diffuse across the shell layer itself. In this application of the invention the leakage rate or the diffusion rate is accelerated because of a "push" exerted upon the active 3 by the expanding

propellant 2. The force of the expanding propellant or its gaseous decomposition result can act to push the active 3 through pores in the capsule shell 4, at a rate faster than normal. Additionally the propellant under heat can act to accelerate the diffusion rate of the active.

In this case heat acts as the trigger to cause the accelerated release function to take place. By careful choice of the propellant 2 the release function can be controlled so as to only occur when heat is present. When the heat is removed certain propellants 2 will coalesce back into a neutral form, thereby slowing or ceasing the release rate of the active 3, until heat is applied again. The release function is triggered and controlled through the application of heat to the capsule 1.

TRIGGERED RELEASE FUNCTION

Since the propellant acts by heat the capsules can be made to have long shelf life stability. Normal time release capsules frequently employ active diffusion functions or use the sponge approach to allow a generous leakage rate from the capsule. Sponge type capsules generally have very short life spans. Diffusion capsules tend to have very thin shells, thereby affecting their ability to resist stress. Reservoir type capsules generally can be made with thicker shells, allowing them to withstand higher degrees of shear and industrial stress. However these type capsules have very poor time release functions. Using the heat application to a propellant core 2 can cause the release to become triggered at a certain point, enhancing the shelf life stability of the capsule product. Heat is used as the trigger, and can be timed to occur at a specific point in a given process.

FIG. 3 is a temperature curve indicating the release point of a microencapsulated capsule containing a catalyst - solvent combination core. The catalyst is Butyl Tuads mixed at 50% of the total core volume of the capsule .The solvent is cyclohexane and causes the capsule to explode at a point near the boiling point of the solvent. Cyclohexane was mixed at 50% of the total core volume. In this case the shell material was ureaformaldehyde which occupied 20 % of the total volume of the capsule while the combination core materials occupied the remaining 80%. The graph is a

Differential Scanning Calorimetery test using a Dupont Model 2000 DSC device.

FIG. 4 is a graph illustrating the Thermogrimetric Analysis of the same sample described in FIG. 3. Both graphs indicate the point at which the capsule shell explodes, indicated by a sharp peak in the temperature curve profile. Through these graphs it can be seen that the capsules reach a certain point whereupon the boiling solvent, cyclohexane, causes the capsule to explode outward. In both thermal studies the release point is comparable.

FIG. 5 is a illustration of the Coacervation method of encapsulation, which was used in this particular series of experiments. It should be noted that the Release assist function can be duplicated through the use of a multitude of encapsulation methods, systems, techniques and technologies and the scope of this invention is not intended to be limited by the use of just one such encapsulation process. The Coacervation process is provided only as an example of a encapsulation technique employed in the following

experiments.

TABLE 1 is a chart illustrating the release rates of two samples of microencapsulated fungicides. The chart illustrates the release rate of a conventionally

encapsulated fungicide known as Folpet when the capsule has been immersed in water compared to the Release Assist system of an enhanced encapsulated Folpet sample. The

Release Assist sample uses a propellant composed of

cyclohexane, an organic solvent, when heat is applied.

TABLE 2 is a table comparing the release results of a particular test conducted using a conventional encapsulation approach to a release assist approach using a common

chemical catalyst as the active core.

TABLE 3 is a listing of polymers known to be useful as shell materials for microcapsules using the Release Assist approach.

TABLE 4 is a chart describing a test involving various Fungi whereupon a encapsulated fungicide was used in an effort to test boiactivity effects upon the target fungi. In one test capsules containing a given fungicide, known as Folpet, were made by conventional encapsulation techniques as taught by Noren in the article, Investigation of

Microencapsulated Fungicides for use in Exterior Trade Sales Paints, Vol 58, No. 734, March 1986 of the Journal of

Coatings Technology. These capsules, called Batch 1 on the chart, were then compared to capsules made in the same manner, except that a propellant was added to the core material, ascribing to the teachings of this invention, and indicated on the chart as Batch 2.

TABLE 4

FUNGAL RESISTANCE TO AQUEOUS PAINT FILMS CONTAINING ENCAPSULATED FUNGICIDE, WITH SAMPLES BEING SUBJECTED TO ARTIFICIAL "SUN LAMP" HEAT TO 90 (F) FOR 30 DAYS AFTER

SURFACE WAS COATED

SCALE : Growth Rate scale of 10 to 0 where 10

corresponds to complete coverage of the surface of the coated film. Lower number indicates reduced Fungi growth activity , and greater efficiency for the fungicide. The Raw fungicide was used as the standard.

Fungicide Used was known as Folpet, supplied by Nuodex Inc., A standard loading of 40 % in latex paint was used. Samples had been aged for 4 weeks prior to the test.

Results indicate that the encapsulated fungicide does not have a significant effect. The Release Assist capsules, under heat, however did provide decreased bioactivity for the fungi.

In Table 4 it can be seen that the Batch 2 capsules remained stable until heat was applied to them. At this point the release function was triggered as the propellant inside the capsule began to expand, pushing the fungicide through cracks which developed in the expanding shell layer.

EXAMPLE 1

INCREASED SHELF LIFE STABILITY AND ACCELERATED RELEASE THROUGH THE USE OF A RELEASE ASSIST CAPSULE CONSTRUCTION USING A THERMAL TRIGGER TO INITIATE TIME RELEASE FUNCTION

A fungicide known as Folpet, obtained from Nuodex

Corporation, was encapsulated using the Coacervation

procedure using a urea-formaldehyde polymer as the shell layer.

A second microcapsule batch was produced using the same coacervation procedure, the same shell and the Folpet active, but with a second core consisting of a solvent known as Cyclohexane.

The two capsules were then compared for shelf life stability, bio-activity upon release, and release

effectiveness.

The procedure used to produce the capsules was:

A pre-condensate of urea - formaldehyde resin was first formed using 120 grams of urea mixed with 325 grams of 37% aqueous formaldehyde containing 15 % methyl alcohol at room temperature. Triethanolamine was added , one drop at a time, to adjust the pH to 8. After 1 hour of agitation , 600 ml of di stilled water was added to the mixture, at room

temperature. Then, 130.5 grams of the pre-condensate was further diluted with 200 ml of distilled water, producing a final polymeric solution to be used as the shell layer.

Next 10 grams of the above described urea-formaldehyde shell solution was mixed with 40 grams of Folpet fungicide supplied by Nuodex Corporation, in 400 ml of water, for 60 minutes, at a temperature of 25 (c), under rapid agitation.

At the end of the 60 minute period the heat was removed to allow the mixture to cool back to room temperature, while the agitation was maintained. Hydrochloric acid was added to the mixture to adjust the Ph to slightly less than 6. Once the Ph level had been obtained the mixture was allowed to air cool for another 2 hours under moderate agitation.

Capsules were formed during this procedure and hardened by the Ph adjustment into a solidified form. The capsules were then filtered from the liquid mixture and examined. The capsules contained approximately a 25% shell volume, and were of the Reservoir type construction.

The capsules were then allowed to air dry for 5 days, after which 20 grams of the completed capsules were then immersed in a beaker of water containing 400 ml of tap water. The capsules were then observed for release rate activity over a period of 30 days, by testing the amount of Ph change and conductivity change of the tap water over the 30 day period of time, in daily intervals. The capsules were weighed prior to immersion. The weight was then periodically checked to determine the loss rate over the test period. Additionally some of the capsules were subjected to fung i for tests of bioactiv ity .

Another batch of microcapsules were produced using the same ingredients and procedures listed above except that a second core material was added to the reaction. The second material was a solvent known as cyclohexane, which was applied at a ratio of 20 % of the overall core material.

Folpet occupied 80 % of the resulting capsules while cyclohexane occupied approximately 20%. In this batch however 40 grams of urea-formaldehyde resin instead of 10 grams was used in the final capsule mixture.

The second batch of capsules were well formed, having a Reservoir type construction, and size approximating the first batch, from 5 to 30 microns. The second batch was then filtered and tested using the same described

procedures. This batch was the release assist sample, and contained a thicker shell than the first batch.

The capsules released at varying rates. TABLE 1

(A and B) illustrates the release rate over the 30 day period for Batch 1 (coacervated Folpet) and Batch 2

(Coacervated Folpet/Cyclohexane) . It can be seen that Batch 1 releases very quickly compared to Batch 2, which is the Release Assist capsule product form of this invention.

TABLE 1 A

WEIGHT LOSS OF ENCAPSULATED FUNGICIDE SAMPLES, WHEN IMMERSED

IN TAP WATER, OVER 30 DAY PERIOD, AT ROOM TEMPERATURE

BATCH 1 = Encapsulated Folpet fungicide, reservoir type construction using 15% shell (Urea-formaldehyde resin), 85% active core material, encapsulated through coacervation procedure (interfacial polymerization technique), a

conventional encapsulation product.

BATCH 2 = Encapsulated Folpet using a reservoir type capsule construction using 25% shell (ϋrea-formaldehyde resin ) 75% total core of which 50% is Folpet active and 50% is cyclohexane solvent acting as a propellant.

BATCH 2 had a thicker shell and thereby had a greater shelf life.

TABLE 1 B

RELEASE RATE OF ENCAPSULATED FOLPET FUNGICIDE WHEN SAMPLE IS SUBJECTED TO HEAT OVER A THIRTY DAY PERIOD TEMPERATURE LEVEL: 90 (F)

NOTE: BATCH 1 did not release at all during the 30 day trial period. BATCH 2 slowly released as the propellant (Cyclohexane ) phase changed and produced a gas which exerted pressure against the interior surface of the capsule shell. The pressure caused cracks and lesions to form in the capsule shell, through which the active fungicide could escape, yet the release rate was over a prolonged period of time.

Even though the urea-formaldehyde resin was supposed to be a water barrier for the microcapsule the Folpet active managed to diffuse or leak from the capsule. Folpet is a commonly used fungicide for mildew suppression in household paints. This test revealed that the product would have released slowly in the paint can during shelf storage, having no remaining potency after the paint had been applied to its target surface. Indeed further examination of the data corresponded with a test conducted by Desoto Paints on encapsulated Folpet listed in the reference section of this application. The tests conducted by Desoto also indicated poor performance for the encapsulated Folpet.

Batch 2 had virtually no effective release over the 30 day period, loosing less than 1 % of its weight over the period compared with nearly a 92 % loss rate for the conventionally made capsules. Over a year long period the Batch 1 would have leached all of its active from the capsule and would thereby become totally useless by the time it was applied as part of a paint coating to a given surface. Batch 2 would loose less than 12% of its weight by then however, providing approximately 88% active still remaining within the sealed capsule (assuming that none of the shell degraded within the liquid) These tests were conducted with the application of water based paints in mind .

In the bioactivity test the results wereremarkably different. Both capsule batches showed no reaction when exposed to Penicillium Funiculosm; Aspergillus Niger;

Gliocladium Virens fungi and a mixture of the three at room temperature. But when "sunlight" exposures were made, whereupon the capsules were subjected to the target fungi under a heat lamp, which approximates a temperature of 85 (f), the true differences in the batches presented

themselves. Batch 1 showed little or no effective reaction. Batch 2, containing the Release Assist construction, showed a marked ability to kill the fungi. It is suggested by the inventor that the heat of the lamp caused the cyclohexane second core of the capsules of Batch number 2 to volatilize. This generated an internal pressure within the capsules and this then acted to "push" the Folpet active fungicide through pores in the capsule shell which had developed during immersion in the water. This "push" action acted to accelerate the release function, after shelf storage while the capsules were immersed in water, and after the heat trigger was applied by the heat lamp. Batch two was observed for another 30 days to still be effective in destroying the target fungi, indicating that the "push" effect was not a sudden effective release, but a prolonged release over a period of time.

Table 4 indicates the bioactivity results of the above example. Observation of the painted surfaces revealed that niether capsule system had broken. The coacervated sample with no solvent core, Batch 1 had remained intact with no release at all. Batch 2, the Release Assist capsules, had not exploded or completly broken but photographic analysis revealed that there were several cracks and pores in the surface of the capsule, which had not been present when the capsules were first made. The cracks and pores in the shell of the capsule were the result of the heating activity, whereupon the expanding propellant forced the fungicide through the weak spots in the capsule construction.

EXAMPLE 2

SUDDEN ( EXPLOSIVE ) RELEASE THROUGH A THERMAL

TRIGGERING OF A RELEASE ASSIST MICROCAPSULE

A batch of Urea-formaldehyde resin was made according to the procedure outline in Example 1 above. In this

experiment 40 grams of a chemical catalyst known as Butyl Tuads was added to the reaction vessel instead of the Folpet material used in Example 1. The encapsulation procedure was the same. Capsules were produced at approximately 50 - 100 microns with a Reservoir type construction. These capsules were then filtered from the mixture and allowed to air dry for 24 hours.

A second bath was produced using the Release Assist approach whereupon a solvent was added to the capsule as the second core material. The solvent was added at differing quantities as a ratio of the active catalyst to the solvent. The shell layer remained at a constant 20% of the volume of the overall capsule in each formulation tested. Differing solvents were used to obtain a wide variety of Release points.

Each capsule batch produced was subjected to a Heat

Stage device to measure the point at which the capsule shell melted under heat. The rupturing of the capsule was observed visually under a magnifying lens attached to the heat stage device. In each instance where a solvent was employed as a second core, the capsules were observed to explode when heat was applied to the capsule unit. The capsules which

contained no solvent slowly melted as heat was applied whereas the solvent filled capsules tended to expand in size and then explode.

The samples were then subjected to DSC ( differential scanning calorimetry ) and TGA ( thermogravimetric analysis) tests to determine the temperature profiles of each sample. The results are shown in TABLE 2:

TOL. = TOLUENE CYC. = CYCLOHEXANE HEX. = HEXANE

RELEASE TEMP. = THE RELEASE TEMPERATURE OF THE CAPSULE

U/F = UREA-FORMALDEHYDE RESIK SHELL

In sample 2A the capsule was observed to release through a Melting of the shell. The shell began a slow degradation as heat was applied to the capsule.

Sample 2C contained a capsule shell which had not been crosslinked into a hardened form, enabling the vololizing sovent to break the weaker shell. The other samples were crosslinked into a harder shell formation by adjusting the Ph to a lower level during the final polmerization stage of the resin based shell. It is apparent from these tests that the strength of the shell layer is a factor in achieving the desired release temperature. If the capsule shell is very thick the strength of the wall membrane is sufficient to resist the expansion force exerted upon it from within the capsule by the gases emitted from the volotilizing

propellant or solvent second core. Likewise if the shell is hardened through a polymerization or other strenghtening process it may also raise the release temperature of the capsule.

The temperature profiles indicated in the release curves of FIG. 3 and FIG. 4, developed by a Differential Scanning Calorimeter and Thermogravimetric Analysis device manufactured by E.I. Du Pont De Nemours Instruments

Division, Model 2000, indicate the release tempera-ture of Sample 2 E.

In Examples 2B,2C,2D,2E, the capsules exploded when they were heated to the indicated release temperature. The release was sudden and complete, in a burst effect which not only destroyed the capsule shell but actually expelled the active core from the catalyst.

In this example it can be seen that the solvent acts as a propellant to explode the shell of the capsule at

temperatures lower than the normally melt point of the polymer used in the capsules. In each case the solvent acted to lower the release temperature of the capsule.

Additionally the solvent acted to explode the capsules at a very precise release point as opposed to an extended release over a period of time. The solvent provided for a sudden burst release. For each sample the solvents vaporization tended to expel or throw the active material from the capsule once the capsule was broken. Heat is used as the release trigger but the temperature at which the capsule ruptures can be adjusted through the choice of the solvent used and the percentage of solvent used in the capsule as opposed to the percentage of active core.

In the above examples several functions of the

invention have been described. The invention provides for a propellant compound to be added to the active core of a microcapsule whereupon the resultant capsule product will provide the following functions:

1. Enable a microcapsule to have a thicker shell construction so as to provide an extended shelf life for the product. 2. Enable the sudden release of a microcapsule at a specific temperature level.

3. Enable the acceleration of a active core material through pores or lesions in the capsule shell layer after a thermal trigger has been applied to the capsule.

4. Increase the diffusion rate of of an active core across a shell membrane after a thermal trigger has been applied.

5. Enable shell to core incompatibility problems to be solved be allowing the use of compatible shell polymers, with adjusted release points through the use of a propellant as a second core to the capsule.

6. Allow for exploding capsules through the use of a propellant second core, which when heated will cause a rapid rupture of the shell layer and a expulsion of the active core from the confines of the capsule.

In the above examples a batch process was employed using Coacervation to produce Reservoir type microcapsules. The invention may also be practiced using any other form of capsule construction including but not limited to

Micro-sponge or entrapment capsules, multi-wall capsules or multi-core capsules. The invention may also be practiced using any other form of capsule manufacture process

including but not limited to Coacervation, Thermal

Coacervation, Interfacial Polymerization Solvent Evaporation or any mechanical means.

While the invention has been described with respect to specific embodiments, it is understood that variations are possible. The examples given are intended to be

illustrative, and not limiting. These and other variations of the invention should be deemed within the spirit and scope of the following claims:

Claims

What is claimed is:

1. A microcapsule which comprises: a first core material, and a second core material, said second core material having the property of causing the release of said first core material when heat is applied to the capsule.

2. The microcapsule of claim 1, wherein said first and second core materials are encompassed by a shell, said second core material causing said shell to loose its

structural integrity when said heat is applied to the microcapsule, thereby permitting the release of said first core material.

3. The microcapsule of claim 2, wherein the loss of structural integrity of said shell is caused by the

volatilization of said second core material due to said heat.

4. The microcapsule of claim 3, wherein said

volatilization of said second core material causes said first core material to diffuse through said shell.

5. The microcapsule of claim 3, wherein said shell has openings therein and wherein said volatilization of said second core material causes said first core material to be forced through said openings in said shell.

6. The microcapsule of claim 3, wherein said

volatilization of said second core material is rapid, thereby causing said shell to burst.

7. The micfocapsule of claim 1, 2, 3, 4, 5, or 6 wherein said first core material is a biocide selected from the group consisting of fungicides, bactericides, and pesticides.

8. The microcapsule of claim 1, 2, 3, 4, 5, or 6 wherein said first core material is a catalyst.