GB1566297A

GB1566297A - Discrete polyurea microcapsules

Info

Publication number: GB1566297A
Application number: GB42518/76A
Authority: GB
Original assignee: Stauffer Chemical Co
Current assignee: Stauffer Chemical Co
Priority date: 1975-10-30
Filing date: 1976-10-13
Publication date: 1980-04-30
Also published as: PT65761B; DE2648562A1; AU1914676A; NO763696L; NO148703C; PT65761A; JPS5254687A; SU707510A3; CS197280B2; YU262876A; IT1066331B; PH14926A; MX146759A; HU173923B; IN145979B; NL7612022A; FR2329340A1; PL118818B1; CH628525A5; FR2329340B1

Abstract

Polyurea microcapsules having individual capsule walls are produced. The capsule walls are obtained by interfacial polymerisation. Firstly, an organic phase is formed, which contains water-immiscible material to be encapsulated, a solvent which is capable of excluding water from the organic material and/or of controlling the porosity of the polyurea wall in accordance with the solubility parameter for the polyurea microcapsule wall, and a polyisocyanate. This organic phase is dispersed in an aqueous phase which, in addition to water, contains a surfactant and a protective colloid. The individual polymer walls of the capsules are then formed. This process allows the quality, in particular the porosity and thus the permeability of the capsule walls to be controlled.

Description

(54) IMPROVED DISCRETE POLYUREA MICROCAPSULES (71) We, STAUFFER CHEMICAL COMPANY, a corporation organised under the laws of the State of Delaware, United States of America, of Westport, Connecticut 06880, United States of America; do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to improved discrete polyurea microcapsules; more particularly, it relates to encapsulation and to the production of small or minute capsules constituted by a skin or a thin wall of organic polymer composition enclosing a water-immiscible material, such as an organic liquid.

The present invention relates to the production of improved discrete polyurea microcapsules containing various core materials by the addition of solvents to the organic phase. The improvement may be the result of either or both effects as noted by: (1) the formation of distinct polymer walls by excluding water from the organic phase; and (2) selectively controlling wall porosity. The present process is directed to the production of such capsules, wherein microcapsule polymer wall solvents or non-solvents are included in the organic phase prior to the microencapsulation.

Belgian Patent No. 796,746 describes a method for encapsulating various waterimmiscible materials employing an organic isocyanate to form a polyurea capsule enclosure around a water-immiscible material dispersed in an aqueous continuous phase.

Capsules of this nature and description have a variety of uses, such as for containing dyes, inks, chemical reagents, pharmaceuticals, flavouring materials, fertilizers, fungicides, bactericides and pesticides, such as herbicides and insecticides, which substances may be dissolved, suspended or otherwise dispersed in or as the core material to be enclosed by the capsule.

The material to be encapsulated may be employed in the initial dispersion at a temperature above its melting point, or dissolved or dispersed in suitable water-immiscible organic solvent. The nature of the water-immiscible material to be encapsulated may be organic or inorganic in origin. Once encapsulated, the liquid or other form is preserved until it is released by some means of instrumentality that breaks, crushes, melts, dissolves or otherwise removes the capsule skin, or until release by diffusion is effected under suitable conditions.

An important specific aspect of the present invention is the method for selectively controlling wall porosity and formation of distinct polyurea walls by preventing water from entering the organic phase during the polymerization of the isocyanate.

Effective encapsulation by interfacial polymerization by an organic isocyanate may be accomplished in a process which utilizes two substantially immiscible liquids, one termed an aqueous phase and the other termed an organic phase, which comprises establishing a physical dispersion of the organic phase in the aqueous phase. The said organic phase contains the isocyanate for the polyurea capsule skin or enclosure. The interfacial polymerization, as it is termed, involves hydrolysis of an isocyanate to form an amine, which in turn reacts with another isocyanate to form the polyurea enclosure. Capsules formed in this manner may range in size from 0.5 to 100 microns.

Certain organic materials termed "water immiscible" are capable of dissolving an appreciable amount of water. When this takes place during the formation of the polyurea microcapsule wall on the surface of the droplet, there will be an increased proportion of polymer formation throughout the core material. Although water is essential to the system employing organic isocyanates to prepare polyurea walls, inclusion of water in the water-immiscible material in the capsule core is not desirable. The presence and amount of water in the organic or water-immiscible material to be encapsulated will depend upon the nature of the material.

According to the present invention, it is possible to exclude water from the core material or organic phase, thereby providing improved discrete polyurea microcapsules having distinct well-formed walls.

There has for some time been a need for improved microcapsules which may control more closely the loss of the active core material. Earlier methods required the permeability of the microcapsule wall to be controlled by varying its thickness and cross-linking density. Also desirable, and heretofore difficult to accomplish, is the prevention of water from entering the organic phase during the formation of polyurea microcapsules containing certain organic materials which dissolve undesirable amounts of water.

Therefore, it is an object of the present invention to provide a microencapsulating process and products produced thereby which products are characterized by optimized rate of release characteristics by formation of polyurea walls and by selectively controlling wall porosity.

It is another object of the present invention to provide a system of microencapsulation employing a preferred solvent which, by virtue of good polymer solvent characteristics, causes a reduction in the pore size of the microcapsule wall. Conversely, by the use of a poor polymer solvent, the permeability of the capsule wall may be increased by increasing the pore size of the microcapsule wall.

It is a further object of the present invention to provide microcapsules having improved rate-release controlling capabilities. The said rate of release being a consequence of the diffusive permeability of the resulting microcapsule wall.

Another object of the present invention is to provide an improved process for obtaining discrete microcapsules having thin walls which are relatively impervious to the loss of core material until the desired release time and conditions are realized. The said process is highly versatile and affords a controlled means for achieving a large number of variations in the wall thickness, permeability, polymeric compositions and release characteristics of the products produced.

The above objects have been achieved by forming polyurea microcapsules while employing a good polymer solvent in the core together with the active ingredient. Conversely, increased permeability may be achieved using a poor polymer solvent. That is, the porosity of the wall may be controlled by the inclusion of microcapsule polymer wall solvents and non-solvents in the organic phase thereof prior to microencapsulation. Accordingly, the encapsulation process is basically as described in Belgian Patent No. 796,746; however, it has been found that the choice of organic solvent included in the core of the capsules may impart the desired effect of pore size reduction or increase, as well as exclusion of water from the core material, i.e. the active material.

The choice of solvents generally is large, depending upon the particular system that is utilized for the production of the porous polymeric microcapsule wall. The advent should not be such a good solvent for the polymer that it is completely miscible in all proportions, nor should the solvent be a material which is a non-solvent for the monomer. Generally, materials which are completely soluble result in a polymer product which has no apparent pore size and is swollen bv the solvent, while microcapsules having polymeric walls produced by the use of non-solvents have pore sizes that are too large to be of practical utility. Suitable solvents for the practice of the present invention may be an undiluted or unmixed solvent. However, suitable solvents are readily prepared by admixing the solvents and non-solvents or, alternatively. by selecting a suitable solvent having the desired characteristics.

Suitable solvents and mixtures of solvents are readily determined for the preparation of a specific polymer system by use of the relationship: ≈= o + 0.8, wherein ≈represents the solubility parameter tor the solvent system and o represents the solubility parameter for the polymer. Low porosity polymers are obtained when the solubility parameter falls within this range, The solubility parameters are discussed in "Some Factors Affecting the Solubility of Polymers". by P.A. Small, Journal ofApplied Chesty 3,71(1953) and also by Harry Burnell in the ' Interchemical Review". 14, 3-16 31-46 (1955). For mixed solvents the value of is readily calculated by additive averaging of a weight basis.

Particularlv beneficial and advantageious in the practice of the present invention is the utilization of xylene as a solvent. Xylene as a solvent in the core of polyurea microcapsules has been found to be exceedingly beneficial. Loss of core material may be decreased by forming a distinct and tighter. i.e. more dense, compact, shell structure to decrease permeability.

Within the preferred practice of the present invention, core material may be effectively microencapsulated by interfacial polymerization of an organic isocyanate in a process which utilizes two substantially immiscible liquids, one termed an aqueous phase and the other termed an organic phase, and which comprises establishing a physical dispersion of the organic phase in the aqueous phase. The organic phase contains the organic isocyanate for the polyurea capsule skin or enclosure, the active ingredient and polymer solvent. The interfacial polymerization to form the capsular wall involves hydrolysis of an isocyanate to form an amme which in turn reacts with another isocyanate to form the polyurea enclosure. The addition of no other reactant is required once the dispersion establishing droplets of the organic phase within a continuous liquid phase, i.e. aqueous phase, has been accomplished.

Thereafter, preferably with moderate agitation of the dispersion, the formation of the polyurea capsule skin or enclosure around the dispersed organic droplets is brought about by heating the continuous liquid phase or by introducing a catalytic amount of a basic amine or other agent capable of increasing the rate of isocyanate hydrolysis, such as tri-n-butyl tin acetate, optionally in addition adjusting the pH of the dispersion, thereby effecting the desired condensation reaction at the interface between the organic droplets and the continuous phase.

In this fashion, fully satisfactory, discrete microcapsules are formed having a skin or outer wall consisting of the polyurea produced by the reaction and containing the encapsulated core material and polymer solvent. Within the process recording to the present invention the reaction which forms the skin or enclosure for the capsule generally is complete, such that essentially no unreacted isocyanate remains. If a good polymer solvent is used in the core, the pore size of the microcapsule wall will be reduced and therefore there will be a reduction in the permeability of the wall. If a poor polymer solvent is used, the wall will be more porous and the permeability will be increased. It is not necessary to separate the capsules for desired utilization, i.e. the encapsulated material may be directly usable, depending upon the intended utilization. However, such separation prior to utilization may be carried out by any of conventional separation processes involving, for example, settling, filtration or skimming of the collected capsules, washing and, if desired drying. The product is particularly suitable for direct agricultural pesticidal applications and additional agents, such as thickeners, biocides, surfactants and dispersants, may be added to improve storage stability and ease of application. The initial dispersion of the organic phase in the aqueous phase may be assisted by the use of an appropriate emulsifying or dispersing agent and the control of the size and uniformity of the ultimate capsules is readily effected by any convenient method to disperse one liquid into another.

The present invention is illustrated by the following: (Examples I and II illustrate the preparation of the microcapsule and Example III and IV illustrate the properties thereof.) Example I The use of the polymer solvent technique according to present invention was applied to the encapsulation of the herbicide "EPTAM" (EPTC), S-ethyl dipropyl thiocarbamate. The purpose was to decrease the loss of the active ingredient by making the wall less permeable and to eliminate increased water penetration into the core material. Xylene was chosen as a good solvent for aromatic polyurea wall.

Microcapsules typically were prepared as follows: Water (300 cc) containing 2.0 per cent of neutralized poly-(methyl vinyl ether/maleic anhydride) protective colloid and 0.3 per cent linear alcohol ethoxylate emulsifier were placed into an open reactor vessel. In a separate container, 270 g. S-ethyl dipropylthiocarbamate (herbicide 68 g. x lene, 18.2 g. polymethylene polyphenylisocyanate (PAPI) and 9.1 g. tolylene diisocyanate (TDI, 80% 2,4 and 20%2,6) were mixed together. This mixture was then added to the reactor vessel and emulsified using a high shear stirrer. The resulting particle size range was from 5 to 30 ,u . Only mild agitation was required for the remainder of the reaction. The temperature of the reactants was raised to 500C. over a 20 minute period.

The temperature was maintained at 50"C. for 2 hours 40 minutes.

There was dispersed 3.5 g. Attagel 40 (attapulgite clay), 14.0 g. sodium tripolyphosphate and 0.35 g. "Dowcide (Registered Trade Mark) G" (sodium pentachlorophenate) into the microcapsule dispersion using a high shear stirrer. The pH was adjusted to 11.0 using 3.5 ml. of 50 per cent sodium hydroxide.

This formulation disperses very well in water and discrete capsules are observed under a microscope. These capsules have a wall content of about 7.5 per cent.

The prepared materials were bioassayed by determining per cent grass control after 24 hours delayed incorporation. During the 24 hour period, EPTC normally evaporates, thereby providing substantially decreased herbicidal efficacy. This is compared between samples and emulsifiable concentrate as a control check. All materials were applied at an effective rate of 1 Ib/acre a.i. The results of these experiements appear in Table I.

TABLE I Wet Soil Bioassay % Grass Control 1 Ib/Acre Ratio Ratio 24 hr Formulation % Wall in PAPI EPTC Delayed lb. EPTC/Gal. Microcapsule TDI Xylene 0 hr incorporation EPTC 6* 99 42 EPTC 2** 25 2.0 All EPTC 98 80 EPTC 3** 17 2.0 All EPTC 99 35 ETPCa 3** 7.5 2.0 4.0 99 96 EPTC 3** 7.5 2.0 9.0 99 96 EPTC 3** 7.5 2.0 19.0 99 97 EPTC 2** 15 2.0 4.0 99 98 * = Emulsifiable Concentrate (non-microcapsule system) ** = Microcapsule Dispersion a = Prepared above These results show that the volatility loss of EPTC from the microcapsule increases markedly as the per cent wall is reduced from 25 per cent to 17 per cent in the absence of xylene. However, with the xylene present, excellent delayed incorporation wet soil activity was achieved even with the 7.5 per cent wall system. The xylene reduces the pore size in the polyurea wall and therefore reduces its permeability to EPTC.

As the xylene content in the thin wall (7 .5 % wall) microcapsule was reduced a less distinct wall was provided. In the thin wall microcapsules when the xylene content was reduced to zero it was impossible to form a discrete microcapsule having a distinct wall.

Example II In the same manner as Example I, the herbicide "RO-NEET", Cycloate, S-ethyl cyclohexyl ethylthiocarbamate, was microencapsulated. The purpose using the procedure of the present application was to reduce vaporization loss. The following Table II summarizes the formulations and results of the tests.

TABLE II Wet Soil Bioassay Formulation % Grass Control (1 lb/A) SAMPLE lb/Gal. % PAPI RO-NEET Incorporation No. RO-NEET Wall TDI Xylene 0 Hour 24 Hour 1* 4 7.5 2.0 - 51 39 2* 3 7.5 2.0 4.0 17 28 3* 3 15.0 2.0 4.0 4 3 4* 4 15.0 2.0 - 51 49 5** 6E - - - 42 1 * = 1-4- Microcapsule Dispersion *+ = Emulsifiable Concentrate (non-microcapsule system) Formulations 1 and 4 are comparable to RO-NEET 6E in herbicidal activity and display excellent wet soil persistence over the 6E formulation. Samples 2 and 3 contain xylene in the organic phase. This causes the wall to be less porous and therefore less permeable to the herbicide. The low per cent of grass control for samples 2 and 3 upon immediate incorporation demonstrates the effect of the xylene in the organic phase during microencapsulation to produce a less porous and hence a less permeable wall.

Example III Experimental Procedure A 4 gram sample of the R-20458 (4-ethylphenyl geranyl ether epoxide) microcapsule formulation (containing approximately 10% R-20458), was diluted to 100 ml. using deionized water and then 2 ml. of this suspension was diluted to 2 litres, using deionized water, producing a 4 ppm R-20458 solution if all the toxicant was released from the capsules. At 0, 15, 60, and 120 minutes a 250 ml. sample was removed from the agitated suspension, the microcapsules removed by filtration and the filtrate analyzed for R-20458. Two filtration methods were used in two separate tests. The suspension was filtered through a Millipore (Registered Trade Mark) filter, 0.65 micron, using a fresh filter each time, or was filtered through a fresh Celite (Registered Trade Mark) 454 filter cake on a vacuum funnel. Both operations were carried out under a slight vacuum; the filtration of 250 ml. taking only one to two minutes for either method.

TABLE III The R-20458 content of water suspensions of R-20458 microcapsules Sample No. 1 2 3 4 5 Per cent wall 7.5 25 7.5 7.5 7.5 PAPI/TDI 2.0 2 2.0 2.0 2.0 Solvent - - xylene 1,1, 1- Ethylene trichloro- dichloride ethane (Solvent/ (2) (2) (2) R-20458 tech.) Sample ppm R-20458 t = 0 0.2 (0.6)a 0.1 0.1 0.1 0.1 t = 15 min. 1.1(1.2) 0.1 0.1 0.1 0.1 t=60min. 1.5(1.8) 0.1 0.1 0.1 0.1(0.1) t= 120 min. 1.8(2.2) 0.1 0.1 0.1 0.17 (0.2) a = First value from Millipore filtration procedure and second value in ()from Celite filtration procedure. Only one value shown if the values were the same in both procedures.

The insect juvenile hormone readily passed through the microcapsule having a 7.5 per cent wall, but was stopped by the microcapsule having a 25 per cent wall and by the solventcontaining walls formed during the microencapsulation procedures using a solvent present in the organic phase.

Example IV In the same manner as Example I, microcapsule formulations of thiocarbamate herbicide and an antidote therefor, EPTC and N,N-diallyl dichloracetamide, were produced and bioassayed on wet soil. The bioassay was to establish per cent grass control and per cent corn injury for comparison to immediate incorporation and 24 hour delayed incorporation. In accordance with the above, the presence of a solvent, such as xylene, in the microcapsule during wall formation in the encapsulation system reduces the size of the micropores in the polyurea wall and thereby reduces the permeability of organic molecules through the microcapsule wall.

TABLE IV EPTC + N,N-diallyl dichloroacetamide Microcapsule Formulations Wt. Ratio in % %Corn Injury** lb lb in Microcapsule micro- % Grass Control* PAG Corn Sample "Eptam" Antidote ("Eptam" + Antidote) capsule PAPI Incorporation Incorporation No. gal gal ( Xylene ) Wall TDI Immediate 24 Hr Immediate 24 Hr.

1 3.0 0.25 4.0 7.5 2.0 99 98 0 0 0 0 2 3.0 0.25 9.0 7.5 2.0 100 99 0 10 0 5 3 3.0 0.25 19.0 7.5 2.0 100 98 0 0 0 30 4 3.0 0.25 no xylene 7.5 2.0 100 99 0 0 0 30 EPTC + Antidote Alone*** 6.0 0.5 - - 99 26 - - EPTC/6E Alone*** 6.0 - - - - - 70 70 * Rate of Application ** Rate of Application 1 lb EPTC/A 2 lb EPTC/A and 1 lb EPTC/A 1/12 lb. Antidote: N, N-diallyl 1/6 lb Antidote 1/12 lb Antidote dichloroacetamide An additional 4 lb EPTC/A was added just prior to incorporation *** Non-microcapsule Systems This test indicates that reducing the xylene or solvent content within the microcapsule will cause increased volatility loss of the antidote component.

Claims

(Unless indicated otherwise, all percentages referred to herein are calculated on a weight basis.) WHAT WE CLAIM IS:

1. A process for the preparation of improved discrete polyurea microcapsules having distinct polyurea walls formed from the interfacial polymerization of an organic phase containing an isocyanate and an aqueous phase which comprises: (a) adding to an organic phase comprising water-immiscible material to be encapsulated, a solvent capable of excluding water from the organic phase and an isocyanate; (b) forming a dispersion of the organic phase in an aqueous phase comprising water, a surfactant and a protective colloid; and (c) allowing formation of the distinct polyurea polymer walls to take place.

2. A process as claimed in claim 1 in which the solvent in (a) is xylene.

3. A process as claimed in claim 1 substantially as herein described.

4. A process as claimed in claim 1 substantially as herein described with reference to Example I or Example II.

5. A process for the preparation of improved discrete polyurea microcapsules having improved rate-release controlling capabilities by selectively controlling the porosity of the poyurea wall which comprises: (a) adding to an organic phase comprising water-immiscible material to be encapsulated, a good polyurea microcapsule polymer wall solvent and an isocyanate; (b) forming a dispersion of the organic phase in an aqueous phase comprising water, a surfactant and a protective colloid; and (c) allowing formation of the distinct polyurea polymer walls to take place.

6. A process as claimed in claim 5 in which the solvent is xylene.

7. A process as claimed in claim 5 substantially as herein described.

8. A process as claimed in claim 5 substantially as herein described with reference to Example I or Example II.

9. A process for the preparation of improved discrete polyurea microcapsules having improved rate-release controlling capabilities by selectively controlling the porosity of the polyurea wall which comprises: (a) adding to an organic phase comprising water-immiscible material to be encapsulated, a polyurea microcapsule polymer wall non-solvent and an isocyanate; (b) forming a dispersion of the organic phase in a aqueous phase comprising water, a surfactant and a protective colloid; and (c) allowing formation of the distinct polyurea polymer walls to take place.

10. A process as claimed in claim 9 substantially as herein described.

11. A process as claimed in claim 9 substantially as herein described with reference to Example I or Example II.

12. Improved polyurea microcapsules containing a water-immiscible material in the microcapsule core comprising a distinct polyurea polymer wall and a core comprising a water-immiscible material and a solvent capable of excluding water from the microcapsule core.

13. Improved polyurea microcapsules having a pre-selected controlled wall porosity containing a water-immiscible material in the microcapsule core comprising a waterimmiscible material and a polyurea microcapsule wall solvent.

14. Improved polyurea microcapsules having a pre-selected controlled wall porosity containing a water-immiscible material in the microcapsule core comprising a waterimmiscible material and a polyurea microcapsule wall non-solvent.

15. Improved polyurea microcapsules having a preselected controlled wall porosity as described in claim 9 wherein the solvent possesses a suitable solubility parameter.

16. Discrete polyurea microcapsules when prepared by a process as claimed in any of claims 1 to 4.

17. Discrete polyurea microcapsules having improved rate-release controlling capabilities when prepared by a process as claimed in any of claims 5 to 8.

18. Discrete polyurea microcapsules having improved rate-release controlling capabilities when prepared by a process as claimed in any of claims 9 to 11.