CA1094402A

CA1094402A - Discrete polyurea microcapsules

Info

Publication number: CA1094402A
Application number: CA263,716A
Authority: CA
Inventors: Herbert B. Scher
Original assignee: Stauffer Chemical Co
Current assignee: Stauffer Chemical Co
Priority date: 1975-10-30
Filing date: 1976-10-19
Publication date: 1981-01-27
Also published as: DE2648562A1; IT1066331B; FR2329340A1; HU173923B; IN145979B; BE847878A; NZ182472A; GB1566297A; NO148703B; RO69910A; PH14926A; PT65761A; MX146759A; PT65761B; NL7612022A; YU262876A; NO763696L; GR68264B; AU510355B2; NO148703C

Abstract

IMPROVED DISCRETE POLYUREA MICROCAPSULES

Abstract of the Disclosure Improved discrete polyurea microcapsules and their preparation effected by the addition of microcapsule polymer wall solvents or non-solvents in the organic phase prior to microencapsulation, wherein said improvement is the result of the formation of distinct polymer walls by excluding water from the organic phase or by selectively controlling wall porosity or both effects.

Description

~ 09 44 0 2 Background of t e Invention This invention relates to encapsulation and particu-larly to the method of production of small or minu~e capsules constituted by a skin or a thin wall of organic polymer composi-tion enclosing a water-immiscible material, such as an organic li~uid. More particularly, thi~ invention relates to the pro-. duction of improved discrete polyurea microcapsules containing various core materials by the addition of solvents to the organic phase. T~e improvement can be the resu}t of either or both effects as noted by (l) *he formation of di8tinct polymer walls by e~luding water from the organic phase and (2) selec-tively contxolling wall porosity. The process of this inven~ion is directed to the production of such capsules, wherein micro-capsu~ polymer wall solvents or non-solvents are included in the organic phase prlor to microew apsulat~on.

Reference i8 made to Belgian Patent No. 796~746, a8signed to Stauffer Chemical Company, published September 14, ,.
1973. The sforementioned patent describes a method for encap-sulatlng various water-immiscibIe màterials employing an organic isocyanate intermediate to form a polyurea cap~ule enclosure around~a water-immiscible material dispersed in an aqueou~
cont~nuou~ phase.

Gapsules of this nature and description have a variety of uses~ such as for containing d~es, inks, chemical reagents, ~. ~
25~ ~ pharmac~euticals, flavoring materials, fertili~ers, fungicides, bactericides, pesticides, such as herbicides, insecticides and the like, which substances can be dissolved, suspended or other-wise dispersed in or as the core material to be enclosed by the

-2-capsule. The material to be encapsulated can be employed in the ini.ial dispersion at a temperature above its melting point, or dis~olved o~ dispersed in suitable water-immiscible organic solvent. The nature of the water-immiscible material to be encapsulated can be organic or inorganic in origin. Once encap-sulated, the liquid or other form is pxPserved until it is released by some means or instrumentality that breaks, crushes, melts, dissolve~ or otherwise removes the capsu~ skin, or until release by diffusion is effected under suitable conditions. An important specific aspect of this invention, toge~her with other features and advantages contèmplated by the invention, is the novel method!or selectively controlling wall porosity and formation of distinct polyurea walls by preventing water from entering the organic phase dùring the polymerization involving the reactlon between polyisocyanate monomers.

Efective encapsulation by interfacial polymerization by a`n organic isocyanate intermediate can be accomplished in a proces~ which utilizes two substantially immiscible liquid , one tenmed an aqueous phase and the other termed an organic phase, which compr~ses es~ablishing a physical di~persion of ~20 the organic phase in the aqueous phase. Said organic phase contains the isocyanate intermediate for the pclyurea capsule ~kin or enclosure. The interfacial polymerization, as i~ i~
termed, ~nvol~es hydrolysis of an isocyanate monomer to form an amhne, which in turn reacts with another isocyanate monomer to form the polyurea enclosure. Capsules fo~med in this manner can range in size from about 0.5 microns to about 100 microns.

-3-1094~02 Certain organic materials and material termed '~ater-immiscible", are capable of dissolving an appreciable amount of water. When this takes place during the formation of the poly-urea microcapsule wall on the surface of the droplet, there will be an increased proportion of polymer formation throughout the core material. Whereas water is essential to the system em-ploying organic polyisocyanates to prepare polyurea walls, inclusion of water in the water-immiscible material in the cap-sule core is not desirable. The presence and amount of water in the organic or water-immiscible m~terial to be encapsulated will depend upon the nature of thé material. By this invention, it is possible ~o exclude water from the core material or organic pha~e, thereby provide improved discrete polyurea microcapsules with distinct well-formed walls.

The art has for some time been in need of improved microcapsules which can control more closely the loss of the active core material. Prior methods required the penmeability of the microcapsule wall to be controlled by varying its thick-ness and cross-linking density. Alsv desirable, and heretofore difficult to æcomplish, i8 the prevention of water from entering the orga~ic phase during the formation of polyurea microcapsules containing certain organic materials which d~ssolve undesirable amounts of water.

Summary of the In~ention Therefore, it is an ob3ect of the present invention to provide a novel microencapsulating process and novel products produced thereby which products are characterized by optimized rate of release characteristics by formation of polyurea walls ~09~40Z

and by selectively controlling wall porosity.
It is another object of this invention to provide a novel system of microencapsulation employing a preferred solvent which by virtue of good polymer solvent characteristics, causes a re-duction in the pore size of the microcapsule wall. Conversely, by the use of a poor polymer solvent, the permeability of the capsule wall can be increased by increasing the pore size of the microcapsule wall.
It is a further object of this invention to provide micro-capsules having improved rate-release controlling capabilities.
Said rate of release being a consequence of the diffusive per-meability of the resulting microcapsule wall.
Another object of the invention is to provide an improved process for obtaining discrete microcapsules with thin walls and relatively impervious to the loss of core material until the desired release time and conditions. 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 therewith.
Other objects of this invention will be apparent to those skilled in the art on reading the instant specification.
In accordance with the present teachings, a process is provided for the formation of improved discrete polyurea micro-capsules which have distinct polyurea walls formed from the inter-facial polymeri~ation of a pol~isocyanate and an aqueous phase wherein the addition to the organic phase of a solvent during the microencapsulation comprises the steps of adding to an organic phase comprising water-immiscible material to be encapsulated, a solvent capab]e of excluding water from the organic phase, and D

~.0~440;;~

polyisocyanate, forming a dispersion of the organic phase in the aqueous phase comprising water~ a surfactant and a protective colloid and formation of the distinct polyurea polymer walls.
In accordance with a further embodiment of the present teachings, an improved polyurea microcapsule is provided which contains a water-immiscible material in the microcapsule core which comprises a distinct polyurea polymer wall and a core which comprises a water-immiscible material and a solvent capable of excluding water from the microcapsule core.
The a~ove have been achieved by forming polyurea micro-capsules while employing a good polymer solvent in the core together with the active ingredient. Conversely, increased -Sa-. . .

109 440 ~

permeability can be achieved using a poor p~lymer solvent. ~-That is, the permeability o the microcapsule wall, to control the porosity of the wall7 is by the inclusion o~ microcaps~le polymer wall solvents and non-solvents in the organic pha~e thereof prior to microencapsulatlon. Accordingly, the encap-sulation proces~ is basically as described in Belgian Patent ~o~ 796,746; however, it has bee~ found that the choice of organic sol~ent included in the core o the capsules can impart the desired effect of pore slze reduction or increase, as well o as, exclusion o water from the core material, i.e. ac~ive ~=
materia~

~ The choice of solvents generally ~ large, depending ~r ; upon the particular monomer system that i~ utiLized for the ~ ~ production of the por~us poLymeric mlcrocapsule wall. The sol-L5 vent should not be such a good solvent for the polymer that it is completely miscible in all proportions, nor should the sol- ~-f.~
vent be a material which 1~ a non-solvent for the monomer.
General b~ materials which are complete b soluble result in a polymer product which has no apparent pore size and is swollen by the solvent, whereas microcapsules having polymeric walls psoduced by the use of non-solvents have pore sizes that are r~
too large to be o practical utility. Suitable solvents for ; the practice of thl~ iivention can be an undiluted or unmlxed ~ solvent, hawe~er, suItable solvents are readily prepared by ; 25 admixing the solvents and non-~olvents or alternatively, by seIecting a suitable solvent having the desired characteristics.

Sultable solvents an~ m~xtures of solvents are readily determined for the prepara~ion o a speciic polymer system by . , .

~- .
' ' ~
. . . ~ . : ~

1091~40Z

use of the relationship: = O~ 0.8, wherein is the solu-bility parameter for the solvent system and O is the solu-bility parameter for the polymer. Low porosity polymers are obtained when the solubility paræme~er fal1s within this range. The solubility paræmeters are discussed in "Some Fæ tors Af~ecting the Solubility of Polymers", by P.A. Small, ~ournal of Applied Chemistry 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 on a weight ~asis.

Particularly beneficial and advan~ageous in ~he prac-tice of ~he present invention is the utilization of the solvent, xylene, as a solvent. Xylene as a solvent in the core of poly-urea microcapsules has been found to be exceedingly beneficial.
Loss of core material can be decreased by forming a ~istinct and tighter, i.e. more dense, compact, hell structuresto decrease permeability. --- Within the preferred practice of the present invention, core material can be effectively microencap~ulated by interfacial polymerization of an organic isocyanate intermediate in a process which utilizes two substantially immi~cible liquids, one termed an aqueous phage and the other termed an orgaNc phase, and which comprises establishing a physical dispersion of the organic phase in the aqueou~ phase. Said organic phase contain-25- ing the organic isocyanate intermediate for the polyurea capsule skin or enclosure, the active ingredient and polymer solvent The interfacial pol~meriæation to form the capsular wall involves hydrolysis of an isocyanate monomer to form an amine which in turn reacts with another isocyanate monomer to form the polyurea enclosure. The addition of no other reactant i~ required once the dispersion establishing drop~s of the organic phase within a continuous liquid phase, i.e. aqueous phase, has been accom-plished. Thereafter, and preferably with moderate agitation of the dispersion, the formation of the polyurea capsule skin or enclosure around the dispersed Qrganic droplets is~brought abou~
by heating the continuous liquid phase or by introducing a catalytic amount of a basic amhne or other agent capable of increasing the ra~e of isocyanate hydroly~i~, such as tri-n-butyl tin acetate, ~pt~onally ~n addition adjusting the pH of the dis-persion~ thereby effecting the desired condensation re æ tion at the interface b~ween the organic droplets and the continuous phase.

In this fashion, fully satisfactory, discrete micro-capsules are formed having a skin or outer wall consisting of the polyurea produ~ed by the reaction and containing the encap-sulated core material and polymer solvent. Within the process of the invention the reaction which forms the skin or enclosure for the capsule generally is complete~ such that`essentially no unreacted poly~socyanate 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 permeabil-i~y of the wall. If a poor polymer solvent i8 used, the wall will be more porous and the permeability will be increased. It is not necessary to separate the capsule~ for desired utiliza-tion, i.e. the encapsulated material may be directly usable, depending upon the intended utilizati~n. However, s~ch separa-tion prior to utilization may be carried out by any of the normal ~09440Z

separation processes involving, for example, settling, filtra-tion or skimming of the collect~d capsules, washing and if desired, drying. The product from the process of this invention is particularly suitable for direct agricultural pesticidal applicat~ons, additional agents can bP added such as thickeners, bioc~des, surfac~ants snd dispersants to improve storage stability and ease of application. The initial dispersion of the organic phase in ~he aqueous phase may be assisted with an appropriate emul~ifying or dispersing agent and the control of the size and uniformity of the ultimate capsules is readily effected by any ; convenient method to dispersé one liquid into another Improved microcapsules having polyurea walls were pre-pared in accordance with the method of the present invention~
The i~vention is further illustrated, but not limited, by the following examples.

EXAMPLE I
The use of the polymer solvent technique of the pre-sent invent~on was applied to the encapsulation of the herbi-cide EPT ~ (EPTC), S-ethyl dipropyl thiocarb~mate. The purpose was to decrease the loss of the active ingredlent by making the wall less permeable, and to eliminate inc~eased water penetra-tion into the core material. Xylene was chosen as a good sol-vent 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 _g_ .. .

~og L/~ 4 oz placed into an open reactor ves~el. In a separate container 270 g. S-ethyl dipropylthioca~bamate ~an herbicide), 68 g.
xylene, 18.2 g. polymethylene polyphenyliso yanate (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 with a high shear ~tirrer. The resulting partiole range was about 5 to abnut 30 p. Only mild agitation was required for the balance of the reæ tion. The temperature of the reactants was raised to 50C~ over a 20 minute period.
The temperature was maintained at 50C. for 2 hours 40 mi~utes.
.
There was dispersed 3.5 g. Attagel 40 ~attapulgite clay), 14.0 g. sodium tripolyphosphate and 0.35 g. Dowcide G
(sodium pentachlorophena`te) into the microcapsule dispersion w~th high shear stirrer. The pH wa~ adju~ted to 11.0 with 3.5 ml. 50 per cent sodium hydroxide.

This formulation disperses very well ~n water and discrete capsules are observed under a microscope. These cap-sules h~3:ve a wall content of about 7.5 per cent.

The prepared material~ were bioassayed by determining per cent gra8g control after 24 hour~ delayed incorporat~on.
During the 24 hour period, EPTC normal~y evaporates, thereb~
providing substantially decreased herbicidal efficacy. This is compared be~ween samples and an emulsifiable concentrate as a control check. All materials were appl;ed at an effective: rate Or 1 lb/acre a.~. The results of the~e experiments appear in Table I.

1(19440Z

TABLE I

Wet S~
~r Ratio Ratio 1 lb/Acre Formulation% Wall in PAPI EPTC d 1 d .
crocapsule T~ r~ne 0 hr EPTC 6* 99 42 EPTC 2** 25 2.0 All EPTC98 80 EPTC 3** 17 2.0 All EPTC99 35 EPTCa 3** 7.5 2.0 4.0 99 96 ~PTC 3** 7.5- 2.0 9.0 g9 96 EPTC 3** T 7.5 2,0 19.0 99 97 EPTC 2** 15 2.0 4.0 99 98 * - Emulsifiable Concentrate (non-microcapsule system) ** ~ M~crocapsule Dispersion a - Prepared in illustrat~ve examples These results show that the volatility 1095 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, wi~h the ~ylene present, exc~llent delayed incorporation wet soil activ~ty was achieved even with the 7.5 per ce~t wall system. The xylene reduces the pore size in the polyurea wall and therefore r~du~es its permeability to EPTC.
A5 the xylene content in the thin wall (7.5% wall) micro-capsule was reduced a le~s distinct wall was provided. In the thin wal~ microcap~ules when the xylene content was reduced to zero it was impossible to form a dîscrete microcapsule with a distinct wall.

~09440Z

EXAMPLE II
In the same manner as Example I, the herbicide R0-NEE ~, Cycloate, S-ethyl cyclohexyl ethylthiocarbamate, was microencapsulated. The purpose using the procedure of the present application was to reduce vaporization loss. The fol-lowing T~.ble II summarizes the formulations and results of the tests.

TABLE II

Formu- Wet Soil Bioassay SAM- lat~on % Grass_Cont ~ I lb/A~
PLE lb/Gal~ % PAPI R0-N~E ~ ~ }~
No. R0-NEE~ Wall TDI ~ylene 0 Hour Z4 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 * - 1-4- Microcapsule Dicpersion ** - Emulsifiable Concentrate (non-microcapsule sys~em) Fonmulations 1 and 4 are comparable to R0-NEE ~ 6E
ia herbicidal activity and display excellent wet soil persistence over the 6E formulation. Samples 2 and 3 contain xylene in the organlc phase. This causes the wall to be less porous and terefore less permeable to the herbicide. The low per cent of grass control for samples 2 and 3 upon immediate incorporation demons~rates the effect of the xylene in the organic phase during microencapsulation to produce a less porous and hence a less permeable wall.

~12 109 LJJ~ 40Z
.

EXAMPLE III

E~erimental Procedure . A 4 gram ~ample of ~he R-2o458 (4-ethylphenyl geranyl ether epoxide) microcapsule formulation ~containing approxi-mately 10% R-20458), was d~lute~ to 100 m7. with deionized water, and then 2 ml. of this suspension was diluted to 2 S liters, with 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 agi-tated suspension~ the microcapsules removed by filtration, and the filtratetanalyzed for ~-20458. Two filtration methods were used in ~wo separate tests. The susp2nsion was filtered through a Millipore filter, 0.6S micron, using a fresh filter each ~ime, or was filtered through a fresh Celite 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 ~wo min~te~
for either method.

' -13 `
Yj TABLE III
The R-20458 content of water suspensions of R-20458 microcapsules Samp le 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- Eth~lene tri- dic loride chloro-ethan (Solvent/ (2) (2) ~2) 0 R-20458 tech.) Sample - ppm R-20458 t3 o , 0.2(.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 - 60 min. 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 3 First value from Millipore~filtration procedure and second value in ( ) from Celite filtration proceture. Only one value shown ~ the values were the same in both procedures.

The insect ~uvenile hormone readily passed through the microcapsule with 7.5 per cent wall~ but was stopped by the mi~rocapsule with 25 per cent wall and by the solvent-containing . ~
walls formed duri:ng the micrsencapsulation procedures with a sol-

5~ ~ vent present in the organic phase.

EXAMPLE~I~
In the same manner as Examp~e I, microcapsule formula-tions of thioc arbamate herbicide and antidote therefor~ EPTC
and N~N-diallyl dichloracetamide, were produc~d and bioassayed on wet soil. The bioassay was to establish per cent grass - 14 - ~

~0 ~40 ~

control and per cent corn injury for comparison to immediate in-corporation and 24 hour delayed incorporation. I~ accordance with the previous disclosure hereinabove, the presence of a solvent, such as xylene, in the microcapsule during wall forma-tion in the encapsulation system reduces the size of the micro-pores in the polyurea wall, and thereby reduces the permeability of organic molecules through the microcapsule waLl.

~,:

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. ~ .

lQ9'~40;2 c ~1 ~ o ~ i~ ~
O ~ a~
C C~ ~ ~
C ~ o~ o o ~ ~ ~ o o ~ ~ o o ~--' ~
a 1~ æ
~ 10~l ~
C
~ c ~
~ ~ ~ _~ o = ~ ' ' ;~

", o ~ ~ o ~
~1 ~ ~ 1 ~
~ Ei ~15 3 r~

_~ ,~
,~ ` +

~ . ~ ¦ - i Ç
C ~ o o o o o I ~o : ~C ~ ,,5s,, . o o ~,j~, C ~

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This test indicates that reducing the xylene or solvent eontent within the microcapsule, will cause increased volatility loss of the antidote component.

As is apparent from the foregoing specification, the method of the present invention is susceptible of being embodied with various altera~ions and modification~ which may differ particularly from those that have been descrlbed in the preceding specification and description. For this reason, it is to be fully under~tood that a11 of the foregoing ~s intended to be merely illustrative and is not to be con~trued or ~nterpreted as being res~rictive or otherwise limiting of ~he present inven-tion, excepting as it is set forth and de~ined in the hereto appended claims.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The process for the formation of improved discrete polyurea microcapsules having distinct polyurea walls formed from the interfacial polymerization of a polyisocyanate and an aqueous phase wherein said improvement is the addition to the organic phase of a solvent during the microencapsulation comprising the steps of:
a) adding to an organic phase comprising water-immiscible material to be encapsulated, a solvent capable of excluding water from the organic phase, and polyisocyanate;
b) forming a dispersion of said organic phase in an aqueous phase comprising water, a surfactant and a protective colloid; and c) formation of the distinct polyurea polymer walls.

2. The process of Claim 1 wherein the solvent in step a) is xylene.

3. The process for the formation of improved discrete polyurea microcapsules having improved rate-release controlling capabilities by selectively controlling the porosity of the polyurea wall comprising the steps of:
a) adding to an organic phase comprising water-immiscible material to be encapsulated a good polyurea micro-capsule polymer wall solvent, said solvent being selected so that the polymer solubility parameter approaches the polymer wall solvent solubility parameter and polyisocyanate;
b) forming a dispersion of said organic phase in an aqueous phase comprising water, a surfactant and a protective colloid; and c) formation of the distinct polyurea polymer walls.

4. The process of Claim 3 wherein the solvent is xylene.

5. The process for the formation of improved discrete polyurea microcapsules having a liquid core and solid wall polymer and having improved rate-release controlling capabilities by selectively reducing the porosity of the polyurea wall without addition of a second reactant, comprising the steps of:
a) adding to an organic phase comprising water-immiscible material to be encapsulated a good polyurea micro-capsule polymer wall solvent having a suitable solubility parameter wherein the following relationship exists:
.delta.= .delta.0?0.8 where .delta. is the solubility parameter of the solvent and .delta.0 is the solubility parameter of the polymer, and polyisocyanate;
b) forming a dispersion of said organic phase in an aqueous phase comprising water, a surfactant and a protective colloid; and c) formation of the distinct polyurea polymer walls.

6. The process of Claim 5, wherein the solvent is xylene.

7. 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 said microcapsule core.

8. Improved polyurea microcapsules having a preselected controlled wall porosity containing a water-immiscible material in the microcapsule core comprising a water-immiscible material and a polyurea microcapsule wall solvent, said solvent being selected so that the polymer solubility parameter approaches the polymer wall solvent solubility parameter.

9. Improved polyurea microcapsules having a preselected controlled wall porosity containing a water-immiscible material in the microcapsule core comprising a water-immiscible material and a polyurea microcapsule wall solvent having a suitable solu-bility parameter wherein the following relationship exists:
.delta. = .delta.0?0.8, where .delta. is the solubility parameter of the solvent and .delta.0 is the solubility parameter of the polymer.