it is believed that the concepts of the present invention are applicable to many types of microcapsules and that certain beneficial results will flow from the use thereof . the concepts of the invention are especially applicable to microcapsules of the type obtained from the teachings of ruus in u . s . pat . no . 3 , 429 , 827 , of baxter in u . s . pat . no . 3 , 578 , 605 and of macaulay in u . s . pat . no . 3 , 016 , 308 the disclosures of which are hereby incorporated into this disclosure by reference . the method as taught by ruus is an interfacial polycondensation type method of producing microcapsules . this method includes the steps of producing an aqueous dispersion of a water - immiscible organic liquid containing one or more reactants , and then adding a second reactant of group of reactants to the dispersion whereupon the reactants form a polymer wall at the interface between the dispersion and continuous phase . as an example , the organic dispersed phase may contain compounds such as diacid chloride , mixtures of diacid chloride and disulfonyl chloride , and the aqueous continuous phase may contain compounds such as hexamethylenediamine , ethylenediamine , diethylenetriamine , triethylenetetramine , tetraethylenepentamine , or mixtures of a polyamine and polyol such as bisphenol a , thus forming microcapsules having a polyamide or copolyamide wall . the method of making microcapsules as taught by baxter may be generally termed a dual - wall method . in this method , microcapsules are prepared by forming an oil in gelatin - gum arabic aqueous emulsion , the oil containing a small amount of diacid chloride . the diacid chloride of the oil reacts with the gelatin molecules of the emulsion to form an inner wall for the microcapsule and releases hydrogen chloride which in turn induces coacervation between the gelatin and gum arabic and thereby forms an outer wall on the microcapsule . optionally , the microcapsules may be hardened by treating with a formaldehyde solution . a method of forming microcapsules as taught by macaulay may be generally termed an urea - formaldehyde condensation method . this method includes the steps of forming a low molecular weight urea - formaldehyde precondensate followed by mixing the precondensate with a fill material , dispersing the fill material into microdroplets and then , by adjusting the ph of the mixture to acidity , forming a high molecular weight urea - formaldehyde condensate over the microdroplets which thereby forms the walls of the microcapsules . according to the concepts of the present invention , once microcapsules have been formed , a methylolated epoxy compound is reacted with the outer surfaces of the microcapsules to thereby yield improved microcapsules . the term methylolated epoxy compound as used herein may be defined as an epoxy type compound or resin having one or more methylol groups and includes methylolated epoxy compounds of the structure formula ; ## str1 ## where r 1 , r 2 , r 3 and r 4 are selected from -- h or -- ch 2 oh provided that at least one of the r groups of the compound is ch 2 oh and where n is an integer from 0 to 3 . preferably n is 0 or 1 since when n is a higher integer , the epoxy compound tends to become a solid , especially when n is 4 or more , and is therefore more difficult to be made into a useful emulsion . generally , methylolated epoxy compounds having an epoxide equivalent within the range of about 200 to 750 perform reasonably well for the purposes of the present invention , preferably , the epoxide equivalent is from 200 to about 380 . the methylol groups of the epoxy compound enhance the reactivity of the compound so that the compound is able to provide the desired improved mirocapsules . it has been found that the use of general purpose types of epoxy emulsions such as the emulsion made by emulsifying genepoxy 190 ( general mills , inc .) and the emulsions made by emulsifying epon 828 , 834 and 836 ( sheel chemicals ) do not seem to provide the desired improved microcapsules when brought into reactive contact . the epoxy compound is preferably in emulsion form which helps the compound to react with the individual microcapsules . if the epoxy compound is added directly to a microcapsule slurry , the benefits of the present invention may not be obtained . one method of reacting the methylolated epoxy compound with the microcapsules , which is particularly applicable to microcapsules formed in a liquid vehicle , is to simply add the epoxy compound dispersed as an emulsion in the same type liquid vehicle to the vehicle containing microcapsules after formation of the microcapsules . in such a method , preferably , the amount of epoxy compound which is added may vary from about 2 to about 20 weight percent of the dry weight of the microcapsules contained therein . preferably , the amount of epoxy compound added is about 2 to 10 weight percent , most preferably , about 5 weight percent , of the dry weight of the microcapsules . the ph of the microcapsule slurry prior to the addition of epoxy compound is not believed to be critical for the reaction , in practice , the ph is generally in the range of about 5 to 9 . in addition , the epoxy compound may be added to the microcapsules at any time after about 2 hours of reaction time in the preparation of the initial microcapsule wall . as mentioned previously , the microcapsules of the present invention are preferably utilized in carbonless copying systems so as to reduce or substantially eliminate discoloration type problems which are encountered in carbonless copying systems which utilize conventional microcapsules . while it is not entirely understood why microcapsules including the reaction product of a methylolated epoxy compound and the outermost surfaces of the wall tend to produce these improved results , it is believed that the epoxy and hydroxymethyl groups of the methylolated epoxy compound react with the free amino groups of the wall of the microcapsules and thus tend to produce a more cross - linked and less permeable wall structure for the microcapsules . discontinuities in the initial microcapsule wall also tend to be closed by the reaction of the outermost surface of the wall with epoxy compound . the microcapsule walls may also become tougher due to the balanced hardness and flexibility characteristics of the epoxy compounds . if such is the case , the toughened capsules are less likely to rupture under non - marking pressure such as the pressures encountered during handling , coating and other associated processes and thus discoloration is less likely to result . the present invention is particularly adaptable to carbonless copying systems which utilize a colorless dye precursor of the general structure ; ## str2 ## when r is a hydroxy group , the precursor is generally known as michler &# 39 ; s hydrol . various other derivatives of the above compound are known as the methyl ether , the benzyl ether p - nitrobenzyl ether , the bis ( 1 , 4 - benzene dimethanol ) ether , the acetophenone oxime ether , the 2 - benzothiazole thio ether , p - toluene sulfinate , and the morpholine derivative , all of michler &# 39 ; s hydrol , and as n -( m - trifluorophenyl ) leucoauramine . conventional microcapsules containing these precursors , when used in a carbonless copying system , tend to produce significant cf discoloration and exhibit a bluish color and back print on acid type paper . the use of microcapsules according to the present invention tends to reduce or even eliminate these undesireable characteristics . although the above mentioned dye precursors are the preferred precursors to be used with microcapsules of the present invention , it should be understood that the concepts of the invention are equally applicable to microcapsules containing other types of colorless dye precursors known in the art of carbonless copying systems . the concepts of the present invention are more clearly illustrated in the following examples where various types of microcapsules are prepared using the methods of the present invention , the resultant microcapsules then being tested as a component of a carbonless copying system . it should be understood that the examples are given for the purposes of illustration only and the examples do not limit the invention as has heretofore been described . microcapsules are prepared generally by a method as taught in the patent to ruus mentioned previously and then an emulsion containing a methylolated epoxy compound is reacted with the formed walls of the microcapsules to yield improved microcapsules . to prepare the initial microcapsules , about 3 . 04 g of terephthaloyl chloride ( tpc ) and about 1 . 00 g of p - toluene sulfinate of michler &# 39 ; s hydrol ( ptsmh ) are dissolved in about 30 g of dibutyl phthalate ( dbp ) and then this mixture emulsified with a 2 % elvanol 50 - 42 ( a partially hydrolyzed polyvinyl alcohol from the dupont company ) for about 2 minutes . a mixture of about 1 . 54 g of diethylene triamine ( det ), about 1 . 15 g sodium carbonate and about 40 ml of water is prepared and then added to the emulsion . the entire mixture is stirred for about 20 hours when a stable ph of about 7 is observed . the resultant microcapsules have a core containing ptsmh dissolved in dbp and encapsulating wall of the polyamide polyterephthalamide . a batch containing the above microcapsules is then coated on a 13 pound neutral base continuous bond paper sheet with a coating rate of approximately 2 . 30 to about 3 . 00 g per square meter . the coated sheet is then oven dried at a temperature of about 105 ° c . for about 45 seconds . the resultant coated paper containing microcapsules is designated as the reference sheet and is utilized for comparison purposes . to a second batch of the above initial microcapsules , about 4 . 0 g of apogen 401 ( a 50 percent aqueous dispersion of a methylolated epichlorohydrin / bisphenol a type resin sold by schaefer chemical company ) is added and then stirred for about 3 hours . the modified resin of apogen 401 has the structural formula of ## str3 ## the resulting microcapsules are then coated on a neutral base paper sheet in the same manner as the reference sheet was coated . the sheet containing microcapsules from the second batch is designated as the modified sheet . the modified sheet and the reference sheet are each combined with a developer cf sheet having an acid - leached bentonite - type clay coating to form a carbonless copying paper system . from a visual observation of each copying system , the system containing the modified sheet shows noticably less blush and offset then the system containing the reference sheet . the image intensity of the system containing the modified sheet is not significantly different than the image intensity of the system containing the reference sheet . thus it may be concluded that the system containing the modified sheet having microcapsules according to the present invention yields an improved copying paper system . microcapsules are prepared as in example i except that a copolyamide material is used for the initial microcapsule walls instead of a homopolyamide material and the resulting microcapsules are tested in a carbonless copying paper system . again , two batches of microcapsules are prepared . in each batch , about 2 . 03 g of tpc , about 1 . 38 g of m - benzenedisulfonyl chloride and about 1 . 0 g of ptsmh are dissolved in about 30 g of dbp and the mix is emulsified with about 100 g of 2 percent elvanol 50 - 42 solution for about two minutes . a second mixture of about 1 . 54 g det , about 1 . 20 g sodium carbonate and about 40 ml water is then added to each emulsion . each batch was stirred for about 20 hours to form microcapsules having a copolyamide encapsulating wall . the microcapsules from one batch are then coated on a paper sheet as in example i to be used as a reference sheet . to the second batch , about 4 . 0 g of apogen 401 is added and then stirred for about 3 hours to yield the microcapsules according to the present invention . these microcapsules are then coated on a paper sheet to yield a modified sheet . each sheet is then combined with a recording cf sheet to yield a carbonless copying system . again , as in example i , the system having the modified sheet shows a reduced amount of blush and ghosting from an visual observation than the copying system containing the reference sheet . microcapsules are prepared similarly to example ii except that a different type of copolyamide material is used to form a microcapsule wall . the resulting microcapsules are then utilized in a carbonless copying paper system . two batches of microcapsules are prepared . in each batch , about 1 . 52 g of tpc , about 1 . 52 g of isophthaloyl chloride and about 1 . 0 g of ptsmh are dissolved in about 30 g of dbp to yield a mixture which is subsequently emulsified in 100 ml of a 2 percent elvanol 50 - 42 solution for about two minutes . to each batch is then added about 0 . 77 g of det , about 0 . 6 g sodium carbonate in about 20 cc of water and a solution of about 1 . 71 g bisphenol a and about 0 . 60 g sodium hydroxide in about 20 cc water . each batch is then stirred for about 20 hours to produce microcapsules having a copolyamide wall . the microcapsules of one batch are coated upon a paper sheet in the manner set forth in example i to yield a reference sheet . to the second batch , about 4 . 0 g of apogen is added and then stirred for about three hours to yield microcapsules according to the present invention . these microcapsules are then coated on a paper sheet in a manner similar to that of the reference sheet to yield a modified sheet . as in the above examples , the modified sheet , when used in a carbonless copying paper system with a developer cf sheet , shows upon visual observation much less blush and ghosting than a similar system utilizing the reference sheet . the microcapsules according to the present invention are prepared utilizing microcapsules made by a dual - wall type method as taught by the patent to baxter previously mentioned and the resulting microcapsules incorporated into a carbonless copying system to thereby compare the properties of these microcapsules with microcapsules not utilizing the concepts of the present invention . two batches of microcapsules are prepared by the dual - wall type method . to prepare each batch , two solutions are formulated , the first by dissolving about 5 . 0 g of gum arabic in 150 g of water and the second by dissolving 5 . 0 g of gelatin of pig skin , ( isoelectric point of 8 ) in about 150 g of water at about 50 ° c . the two solutions are then mixed in a blender and then a solution containing about 21 g of dbp , about 0 . 5 g of ptsmh , and about 0 . 02 g of tpc is added with agitation at moderate shear for about two minutes . about four drops of octyl alcohol are added to reduce foaming and then about 2 ml of a 40 percent formaldehyde solution is added . the resultant mixture is then allowed to cool to about 10 ° c . in a period of about 30 minutes to thereby form microcapsules . the microcapsule walls are then hardened by adjusting the ph of the mixture to about 9 with a ten percent sodium hydroxide solution . the microcapsules of one batch are coated on a paper sheet in the manner as set forth in example i to produce a reference sheet . to the second batch about 6 . 0 g of apogen 401 is then added and stirred for about two hours . the resultant microcapsules according to the present invention are then coated onto a paper sheet in similar fashion to produce a modified sheet . a portion of each of the modified sheet and the reference sheet are then combined with a developer cf sheet to produce a carbonless copying paper system . upon application of a marking pressure , the formed image has an image intensity of about 47 . 2 in the system containing the reference sheet , and an image intensity of about 52 . 5 in the system containing the modified sheet . the remaining portions of the sheets are then placed on an accelerated blush test . in an accelerated blush test , the microcapsules are intentionally ruptured under a controlled pressure so as to thereby free the colorless dye precursor from the microcapsules . in one form of the test , the coated side of each sheet is placed against a conventional piece of paper and then passed through a manually operated test device which applies a gradually increasing pressure . the pieces of paper are removed and the coated test sheets then placed against a developer cf sheet . the two sheets are then placed in an oven at about 50 ° c . for about 1 week under a weight of about 2 psi . discoloration on the developer cf sheet is then measured using a photo volt reflection meter . a higher reflection value indicates less discoloration . upon completion of the accelerated blush test , the above mentioned portions of the reference sheet and the modified sheet had a reflectance value of about 71 . 5 and about 96 . 1 respectively . the higher value realized for the modified sheet containing microcapsules according to the present invention indicates a significant improvement over the reference sheet . microcapsules are prepared by a urea - formaldehyde condensation method as disclosed by the patent to mccaulay mentioned previously and a portion of these microcapsules are modified in accordance with the concepts of the present invention . the microcapsules are then used in carbonless copying paper systems and compared with systems containing microcapsules not in accordance with the present invention . two batches of microcapsules are prepared , one batch formed by conventional processes and the other according to the present invention . each batch is prepared by forming a mixture of about 117 . 2 grams of 37 % formaldehyde , about 444 grams of urea and about 7 grams of triethanolamine and then heating to about 70 ° c . for about 2 and one half hours to form a solution . to this solution is then added a solution of about 368 grams of sodium chloride and about 2 , 008 grams of water to yield a precondensate solution . two portions of about 180 grams of the precondensate solution are used to form each batch . while stirring rapidly , a solution of about 1 . 2 grams ptsmh and 36 grams of dbp is dispersed into each batch . about 9 grams of cab - o - sil m5 ( fumed silica from cabot corp ., boston , mass .) was also added to each batch as an emulsification aid . the ph of each batch is then reduced to about 2 . 5 using a 20 % formic acid solution . the temperature is then raised to about 60 ° c . and mixing is continued for about 2 hours at this temperature . the temperature is then lowered to about room temperature and the ph is raised to about 7 . 5 using a 10 % sodium hydroxide solution to yield the microcapsules . the microcapsules are then filtered , washed with water and then diluted with water to form a slurry . about 20 ml of a 10 % polyvinylalcohol solution is then added to the slurry as a binder . one batch of the resultant microcapsules is then coated on a paper sheet to form a reference sheet . to the second batch of microcapsules , about 6 . 0 g of apogen 401 is added while stirring for about 3 hours and then coated on another paper sheet to yield a modified sheet . portions of each of the modified sheet and the reference sheet are then tested for accelerated blush and image intensity as in example iv . the results are as follows : ______________________________________ accelerated blush image intensity______________________________________reference sheet 63 . 4 50 . 0modified sheet 87 . 8 53 . 3______________________________________ as the data in the above table indicates , the modified sheet containing microcapsules according to the present invention produced less discoloration in the accelerated blush test while the intensity of the formed image when used in a carbonless copying system was not significantly affected . microcapsules and sheets having coatings containing the microcapsules are prepared as in example i , the only difference being that the acetophenone oxime ether of michler &# 39 ; s hydrol is used as the dye precursor instead of ptsmh and the carrier for the precursor is r - 300 instead of dbp . r - 300 is the trade name of a commercial solvent of kureha corporation of america and is a mixture of isomeric diisopropylnaphthalenes . in the manner as set forth in example i , a batch of microcapsules is prepared having a ph of about 7 . 0 and portion of the batch is coated upon a paper sheet as a reference sheet . to about an 80 gram second portion of the batch , about 2 . 0 g of apogen 401 is added and to another 80 gram portion of the batch , about 3 . 0 g of apogen 401 is added . the two portions are then stirred for about 4 hours to produce microcapsules according to the present invention and then each portion is coated on a separate sheet to produce two modified sheets . when tested as set forth in the previous examples , the following results are obtained : ______________________________________ accelerated blush image intensity______________________________________reference sheet 85 . 6 35 . 02 . 0 g modified sheet 98 . 0 39 . 93 . 0 g modified sheet 98 . 6 39 . 0______________________________________ the above method of this example is then repeated so as to produce another reference sheet and two additional modified sheets . the only difference is that the amount of sodium carbonate used in preparing each batch is about 1 . 05 grams so that the resultant ph of the initial microcapsule slurry is about 6 . 5 . the following test results are obtained : ______________________________________ accelerated blush image intensity______________________________________reference sheet 86 . 5 39 . 02 . 0 g modified sheet 96 . 8 38 . 83 . 0 g modified sheet 99 . 0 38 . 5______________________________________ the above method for making microcapsules is again repeated and another reference sheet and two modified sheets are prepared . in this instance , about 1 . 47 g of sodium carbonate are included in the initial mixture so as to yield a ph of about 8 . the results are set forth below : ______________________________________ accelerated blush image intensity______________________________________reference sheet 82 . 9 38 . 02 . 0 g modified sheet 98 . 4 41 . 53 . 0 g modified sheet 99 . 1 40 . 3______________________________________ as the above test data indicates , the modified sheets containing microcapsules according to the present invention yield a higher reflectance value than the reference sheets on an accelerated blush test while the image intensity is not significantly affected . the data also illustrate that the ph of the microcapsule slurry prior to modification with epoxy resin is not critical . a number of batches of microcapsules are made according to an interfacial polycondensation method , each batch having a different colorless dye precursor included in the fill material . some of the microcapsules of each batch are modified according to the present invention and then coated upon two types of paper sheets for comparative testing . a number of batches of microcapsules are prepared by the method set forth in example i except that about 1 . 0 g of various colorless dye precursors are included in the fill material rather than ptsmh . a portion of each batch of microcapsules is modified with apogen 401 material as set forth in the example and then each type of microcapsule is coated upon appleton neutral base paper and also an oxford acid base paper . acid based paper is normally difficult to use as a substrate for cb sheets in carbonless copying systems as the acid nature of the paper tends to react with free dye precursor from the microcapsules and thereby cause significant discoloration of the sheet . the colorless dye precursors used in the various batches are selected from bis ( 1 , 4 - benzene dimethanol ) ether of michler &# 39 ; s hydrol ( bdmh ), p - nitrobenzyl ether of michler &# 39 ; s hydrol ( nbemh ) n -( m - trifluorophenyl ) leucoauramine ( tfl ) and 2 - benzothiozole thio ether of michler &# 39 ; s hydrol ( btmh ). the results of an accelerated blush , image intensity , backprint , and cb color tests in terms of reflectance values are shown in the following table for both the reference sheet ( r ) and the modified sheet ( m ) of each batch containing the various dye precursors . in the table , &# 34 ; a &# 34 ; type paper is oxford acid base paper and &# 34 ; n &# 34 ; type paper is appleton neutral base paper . __________________________________________________________________________ accelerated image blush intensity black print cb colorprecursor paper r m r m r m r m__________________________________________________________________________bdmh n 70 . 2 99 . 0 45 . 9 46 . 8 no no white whitembemh n 72 . 5 96 . 8 48 . 0 52 . 9 no no white whitetfl n 62 . 3 93 . 6 54 . 0 54 . 0 no no white whitebtmh n 62 . 1 95 . 0 46 . 0 45 . 1 no no white whitebdmh a 71 . 7 99 . 0 45 . 6 50 . 5 88 . 8 93 . 7 92 . 0 97 . 8nbemh a 75 . 6 98 . 5 51 . 8 58 . 3 87 . 9 92 . 2 91 . 3 96 . 1tfl a 70 . 8 96 . 3 60 . 0 60 . 3 88 . 2 91 . 1 87 . 5 95 . 5btmh a 65 . 2 96 . 2 47 . 1 54 . 0 83 . 3 88 . 6 88 . 1 91 . 6__________________________________________________________________________ the backprint test either measures qualitatively or quantitatively the amount of discoloration on the sheet containing the microcapsules . back print may be defined as self - imaging on a cb type sheet when the microcapsules thereon are intentionally or unintentionally impacted . the backprint on the acid paper was measured with a reflectance meter after one week using the same sheet which was used for accelerated blush test . as the above results indicate , copying paper systems utilizing the modified sheets containing microcapsules according to the present invention have less discoloration on an accelerated blush test and have approximately the same image intensity as a carbonless copying system containing the reference sheets . in addition , the cb sheet of acid base paper using microcapsules according to the present invention experiences less discoloration than the reference cb sheet . microcapsules are produced according to the present invention and used in carbonless copying systems to investigate the effect on the cf side of a roll of cfb acid paper upon aging . to prepare the microcapsules , about 32 . 6 parts of tpc , about 20 parts of epon 1002 ( an epoxy resin sold by the shell chemical company ) and about 10 parts of ptsmh are dissolved in about 300 parts of dbp . the solution is then emulsified in about 650 parts of a 3 % elvanol 50 - 42 until microdroplets of about 4 . 5 microns are obtained . to the emulsion is then added about 18 . 6 parts of det , 9 . 6 parts of sodium carbonate in 110 parts of water . after stirring for about 12 hours , about 100 parts of the resultant slurry containing microcapsules is mixed with about 3 . 6 parts of apogen 401 and allowed to react for about 3 hours to yield modified microcapsules . about 100 parts of the original slurry and 100 parts of the modified slurry are then compounded with about 7 . 5 parts of starch granules , about 13 . 2 parts of 10 % vinol 325 solution ( a fully hydrolyzed polyvinylalcohol sold by air products and chemical corporation ), and about 5 . 7 parts of water . each formulation of microcapsules is then coated on an opposite side of a roll of cf oxford acid base bond paper . each coated sheet then is wound into a roll and aged for about 3 months . after aging , the coated sheet containing unmodified microcapsules exhibits distinct cf discoloration from the cb side , while the coated sheet containing modified microcapsules according to the present invention has considerably less discoloration . while the present invention has been described with reference to particular embodiments thereof , it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the spirit and scope of the invention as defined in the appended claims .