text: the method of the present invention requires four steps : a ) dispersing the active material in the coating material at a temperature sufficient to melt the coating material ; b ) forming droplets of active material interspersed with the coating material ; c ) cooling the droplets to solidify the coating material ; and d ) contacting the droplets with a solvent that dissolves the active material but does not dissolve the coating material , so as to remove active material from the surface of the coating material . the following is a detailed description of each of the four steps . in a first step of the method of encapsulating an active material in a coating material , a heterogeneous mixture of the active material and a coating material is formed at temperatures sufficiently above ambient temperature to melt the coating material without decomposing or volatilizing either the coating material or the active material . the active material and coating material are selected to be insoluble ( or immiscible ) with each other . the coating material preferably has a melting point in the range of about 40 ° c . to about 200 ° c . the coating material is more preferably a petroleum - derived alkane hydrocarbon wax , a polyethylene wax , a polyethylene - alkene copolymer , an oxidized hydrocarbon wax containing hydroxyl or carboxyl groups , a polyester , a polyamide , or a combination thereof . most preferably , the coating material is a petroleum - derived alkane hydrocarbon wax , a polyethylene - alkene copolymer , or a polyethylene wax . most preferably , the coating material is a polyethylene wax . preferred polyethylene waxes preferably have a weight average molecular weight in the range of about 500 , more preferably from about 1000 , to about 3000 , more preferably about 2000 daltons . these waxes are exemplified by polywax ™ 500 , polywax ™ 1000 , and polywax ™ 2000 , or blends thereof , more preferably a 75 : 25 blend of polywax ™ 1000 and polywax ™ 2000 . ( polywax is a trademark of petrolite corporation .) the active material may either be a liquid or a solid at room temperature , but it is preferably a solid at room temperature . more preferably , the active material has a melting point above ambient temperature , but below a temperature of decomposition or volatilization of the coating material . thus , the melting point of the active material may be greater than , less than , or equal to the melting point of the coating material . in any case , it is preferred in the first step of the method , that the active material is dispersed in the coating material at a temperature above the melting point of both the coating material and the active material without decomposing either the coating material or the active material . the active material may be any material that forms a heterogeneous slurry with the coating material and that dissolves in a solvent that does not dissolve the coating material . for example , the active material may be a drug used for sustained - release applications , an insecticide , a herbicide , a flavoring compound , a dye , a catalyst , or a curative . the active material is preferably a curative , more preferably a hardening accelerator having a melting point or a glass transition temperature ( t g ) from about 70 ° c . to about 200 ° c . the hardening accelerator is preferably a urea or an imidazole . preferred ureas include 3 - phenyl - 1 , 1 - dimethylurea ; 3 -( 4 - chlorophenyl )- 1 , 1 - dimethylurea ; 3 -( 3 , 4 - dichlorophenyl )- 1 , 1 - dimethylurea ; 1 , 1 &# 39 ;-( 4 - methyl - m - phenylene ) bis ( 3 , 3 &# 39 ;- dimethylurea ); 3 - isomethyldimethylurea - 3 , 5 , 5 - trimethylcyclohexyldimethylurea ; or 4 , 4 &# 39 ;- methylenebis ( phenyldimethylurea ) the more preferred urea is 3 - phenyl - 1 , 1 - dimethylurea ( pdmu ). preferred imidazoles include alkyl - or arylimidazoles , such as 2 - methylimidazole , 2 - undecylimidazole , 2 - heptadecylimidazole , 2 - phenylimidazole , 2 - ethylimidazole , 2 - isopropylimidazole , and 2 - phenyl - 4 - methylimidazole ; 1 - cyanoethyl derivatives , such as 1 - cyanoethyl - 2 - methylimidazole , 1 - cyanoethyl - 2 - phenylimidazole , 1 - cyanoethyl - 2 - undecylimidazole , and 1 - cyanoethyl - 2 - isopropylimidazole ; and carboxylic salts , such as 1 - cyanoethyl - 2 - ethyl - 4 - methylimidazole - trimellitate . the more preferred imidazole is 2 - methylimidazole . the hardening accelerators may also be a urea - imidazole conjugate such as 2 - methyl - n - phenyl - 1h - imidazole - 1 - carboxamide , which can be prepared by the reaction of imidazoles with organic isocyanates . as stated above , the heterogeneous slurry is formed at an elevated temperature . for the purposes of this specification , an elevated temperature is a temperature sufficiently above room temperature to melt the coating material , more preferably to melt the coating material and the active material , but sufficiently low to prevent thermal decomposition or volatilization of either the coating material or the active material . the concentration of active material is preferably from about 1 weight percent , more preferably from about 10 weight percent , most preferably from about 25 weight percent , to preferably about 60 weight percent , more preferably to about 45 weight percent , and most preferably to about 35 weight percent , based on the total weight of active material and coating material . the second step of the method of the present invention requires forming droplets . though droplets of any size can be employed , the invention is particularly useful for microspheres of active material interspersed with the coating material . for the purposes of this invention , microspheres are spherical particles having a diameter of 500 microns or smaller . preferred particle sizes will vary depending on the application but are preferably from about 300 , more preferably from about 200 microns , and most preferably from about 150 microns in diameter ; to about 10 , more preferably 30 , and most preferably 50 microns in diameter . similarly , the terms &# 34 ; microencapsulate &# 34 ; and &# 34 ; microencapsulation &# 34 ; are used to describe the enclosure of active material by coating material into microspheres . as noted in the background of the invention section , forming microspheres of active material interspersed with coating material can be accomplished in a variety of ways . in a preferred method of forming microspheres of active material interspersed with coating material , the heterogeneous slurry from the first step is poured at elevated temperatures sufficient to melt the coating material , more preferably at temperatures sufficient to melt the coating material and the active material , onto a rotating disk . the rotation causes the coated material / active material interspersion to be thrown off the disk as microspheres , which solidify by the cooling effect of air ( the third step ). the velocity of the rotating disk , the temperature of the disk , the rate at which the slurry is poured onto the disk , and the type of apparatus used determines the size of the microspheres formed . in the fourth step , the solidified droplets , preferably the solidified microspheres of coating material interspersed with active material , are collected and contacted with a solvent that dissolves the active material , but does not dissolve the coating material . preferably , the solvent is polar , since the preferred coating materials are non - polar polymer waxes , and the preferred active materials are comparatively polar . more preferably , the solvent is volatile , that is to say , is easily removed by evaporation . most preferably , the solvent has a boiling point of less than 100 ° c . examples of preferred solvents include water , alcohols , such as methanol , ethanol , and isopropanol ; ketones , such as acetone and methylethyl ketone ; chlorinated hydrocarbons , such as methylene chloride ; and polar aprotic solvents such as acetonitrile . more preferred solvents are water , methanol , ethanol , isopropanol , and acetone . most preferred solvents are acetone and methanol . the amount of active material encapsulated in the coating material after solvent washing can be determined , for example , by thermogravimetric analysis ( tga ), provided the active material and coating material volatilize at different temperatures . in the tga method , a sample is placed in a platinum pan attached to a microbalance , then heated to volatilize the active material and the coating material . since the active material and the coating material volatilize at different temperatures , the composition can be readily determined by measuring weight loss as a function of temperature . the method of the present invention provides droplets of active material encapsulated in a coating material , preferably microspheres of active material encapsulated in a coating material , which exhibit long - term stability in adhesive formulations , yet which also exhibit high reactivity (&# 34 ; cure - on - demand &# 34 ;) under moderate thermal conditions . in a preferred application , ureas or imidazoles microencapsulated in polyethylene based waxes can be stored at 40 ° c . or lower for months in a composition containing an epoxy resin and a dicyandiamide . when the composition is heated to a sufficiently high temperature to melt the wax , the hardening accelerator is released , thereby promoting the reaction between the epoxy resin and the dicyandiamide . ( see for example japanese public patent disclosure bulletin no . 2 - 2923324 , dec . 3 , 1990 .) preparation of solvent - rinsed microencapsulated beads of pdmu coated with a low molecular weight polyethylene wax 3 - phenyl - 1 , 1 - dimethylurea ( pdmu ) ( 300 g ) is dispersed in molten 75 / 25 polywax ™ 1000 / polywax ™ 2000 ( a 75 : 25 weight percent mixture of polywax ™ 1000 and polywax ™ 2000 ) ( 700 g ) the mixture is then heated until the pdmu is molten , then poured at the rate of 500 g per minute onto the center of a disk maintained at 140 ° c . and rotating at 6500 rpm . microspheres of pdmu interspersed with the polywax ™ are flung off the rotating disk and into a collection cone . the resultant solid microspheres ( 1000 g ), which have a diameter in the range of 50 to 300 microns , are soaked in acetone ( 1000 ml ) for 5 minutes , then rinsed 4 additional times with acetone ( 500 ml each time ) to remove pdmu from the surface of the polywax ™. the beads are air - dried , then stored until they need to be used to formulate the adhesive . the solvent - washed microspheres from example 1 are blended into d . e . r .™ 331 ( trademark of the dow chemical company ) liquid epoxy resin at 2 parts active material ( pdmu ) per 100 parts epoxy resin and stored at 110 ° f . the formulation remains fluid for six months . by comparison , a formulation containing non - encapsulated pdmu gels within 10 days . an adhesive formulation is prepared as follows : tactics ™ 123 liquid epoxy resin ( 247 . 5 g ); d . e . r .™ 755 liquid epoxy resin ( 247 . 5 g ); an acrylic rubber - modified epoxy resin ( 495 g ); aluminum powder ( 310 g , reynolds a - 200 ), cab - o - sil ™ m - 5 fumed silica ( 50 g ); byk ™ r - 605 polycarboxylic acid amide ( 15 g ); dicyandiamide ( 50 g ), and the solvent - washed microspheres prepared as in example 1 ( 70 g ) are blended in a heavy duty mixer to form a paste adhesive . the adhesive is applied to 32 - mil thick , cold - rolled steel test coupons with a 5 - mil bond thickness , and cured for 30 minutes at 177 ° c . the adhesive is found to exhibit greater than 2000 psi tensile lap shear strength and greater than 20 pounds per linear inch t - peel strength . ( note : tactics is a trademark of the dow chemical company ; cab - o - sil is a trademark of cabot corporation ; byk is a trademark of byk chemie .)