the following description and examples illustrate a preferred embodiment of the present invention in detail . those of skill in the art will recognize that there are numerous variations and modifications of this invention encompassed by its scope . accordingly , the description of a preferred embodiment should not be deemed to limit the scope of the present invention . for preparing the adhesive composition of the present invention , a thickening agent , i . e . the polymer of the second cyanoacrylate monomer , should be made first . and then the thickening agent , in an amount depending on the demand , is added into the adhesive substrate containing the first cyanoacrylate monomer . after mixing , the resulting adhesive composition with the desired viscosity can be achieved . the first cyanoacrylate monomer or the second cyanoacrylate monomer may be selected from the group consisting of alkyl 2 - cyanoacrylate , cycloalkyl - 2 - cyanoacrylate , fluoroalkyl - 2 - cyanoacrylate , fluorocycloalkyl - 2 - cyanoacrylate , alkoxyalkyl - 2 - cyanoacrylate , alkoxycycloalkyl - 2 - cyanoacrylate , fluoroalkoxyalkyl - 2 - cyanoacrylate , and mixtures of two or more thereof . in order to obtain the optimum compatibility , e . g ., dissolving the polymer of the second cyanoacrylate monomer well in the first cyanoacrylate monomer , the first cyanoacrylate monomer and the second cyanoacrylate monomer are prefer to , but not limited to , the same monomer . further , the thickening agent ( i . e . the polymer of the second cyanoacrylate monomer ) may be poly alkyl 2 - cyanoacrylates , poly cycloalkyl - 2 - cyanoacrylates , poly fluoroalkyl - 2 - cyanoacrylates , poly fluorocycloalkyl - 2 - cyanoacrylates , poly alkoxyalkyl - 2 - cyanoacrylates , poly alkoxycycloalkyl - 2 - cyanoacrylates , or poly fluoroalkoxyalkyl - 2 - cyanoacrylates . the preferred weight average molecular weight of the polymers is from about 5 , 000 to about 4 , 000 , 000 ; more preferably from about 5 , 000 to about 1 , 000 , 000 . for the preparation of the thickening agent , an accelerator / initiator with a low boiling point , e . g ., ammonium hydroxide , alcohol , but not limited to , can be added to start the polymerization . utilizing the property of their relatively low boiling points , ammonium hydroxide or alcohol can evaporate or be easily removed in the following heating procedures . the alcohol can be selected from the group consisting of methanol , ethanol , n - propanol , butanol , and mixtures thereof . the alcohol having a lower boiling point such as methanol , ethanol , n - propanol or mixtures thereof is preferred . the amount of thickening agent ( polycyanoacrylate ) intended to be added to the adhesive substrate ( cyanoacrylate monomers ) is dependent upon the molecular weight of the polymers and the desired viscosity for the adhesive composition . the thickening agent typically may be about 0 . 5 - 25 % by weight based on the total weight of the adhesive composition , preferably about 1 - 10 % by weight , more preferably about 1 - 5 % by weight . in addition , a plasticizer can also be added to the mixture of the first cyanoacrylate monomer and the thickening agent for desired viscosity and elasticity . the plasticizer may be selected from , but not limited to , the group consisting of citric esters , glycerol esters , sebacic esters , fatty acid esters , cellulose esters , polyethylene glycol ethers and mixtures of two or more thereof . preferably , the plasticizer can be selected for example from the group consisting of glycerol triacetate , glycerol tripropionate , glycerol tributyrate , tricaproin , trivalerin , tricaprin , tributyl 2 - acetylcitrate , isobutyl myristate , ethyl myristate , ethyl stearate , methyl sebacate , ethyl sebacate , ethylcellulose , polyethylene glycol diethers , and mixtures thereof . tributyl 2 - acetylcitrate is particularly more preferred . for the preparation of the accelerator , 1800 ml of deionized water was first placed in a 2000 ml beaker , and then 0 . 7 g ( 0 . 04 % by weight ) of nh 4 oh ( showa chemical co ., lot . number 111024 ) was added with a pipette and mixed by stirring for about 5 minutes . then the second cyanoacrylate monomer , 32 ml of n - butyl cyanoacrylate monomer ( nbca , chemence co . ), was added into the nh 4 oh solution drop by drop and mixed by continuous stirring for about 0 . 5 hour . the resulting nbca polymer , namely thickening agent a , was decanted and dried in the vacuum oven at 65 ° c . for 8 hours . the molecular weight of nbca polymer was measured with a gel permeation chromatography ( gpc ). consequently , the molecular weight of nbca polymer , i . e . thickening agent a , is 57 , 300 . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent a prepared as described in example 1 was then added slowly into the flask . then they were mixed at 100 ° c . for 60 minutes to form adhesive a . the viscosity of the adhesive a was measured with a capillary viscometer . the viscosity of the adhesive a is 43 cp at 20 ° c . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent a prepared as described in example 1 was then added slowly into the flask . 15 g of tributyl 2 - acetylcitrate ( atbc , safc , lot number mkbg8107v ) was also added slowly subsequently , and then all were mixed at 100 ° c . for 60 minutes to form adhesive b . the viscosity of the adhesive b was measured with a capillary viscometer . the viscosity of the adhesive b is 39 cp at 20 ° c . for the preparation of the accelerator , 1800 ml of deionized water was first placed in a 2000 ml beaker , and then 1 g ( 0 . 06 % by weight ) of nh 4 oh ( showa chemical co ., lot . number 111024 ) was added with a pipette and mixed by stirring for about 5 minutes . afterwards the second cyanoacrylate monomer , 32 ml of n - butyl cyanoacrylate monomer ( nbca , chemence co . ), was added into the nh 4 oh solution drop by drop and mixed by continuous stirring for about 0 . 5 hour . the resulting nbca polymer , namely thickening agent b , was decanted and dried in the vacuum oven at 65 ° c . for 8 hours . the molecular weight of nbca polymer was measured with a gel permeation chromatography ( gpc ). consequently , the molecular weight of nbca polymer , i . e . thickening agent b is 41 , 200 . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent b prepared as described in example 4 was then added slowly into the flask . after this , they were mixed at 100 ° c . for 60 minutes to form adhesive c . the viscosity of the adhesive c was measured with a capillary viscometer . the viscosity of the adhesive c is 31 cp at 20 ° c . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent b prepared as described in example 4 was then added slowly into the flask . 15 g of tributyl 2 - acetylcitrate was also added slowly subsequently , and then all were mixed at 100 ° c . for 60 minutes to form adhesive d . the viscosity of the adhesive d was measured with a capillary viscometer . the viscosity of the adhesive d is 48 cp at 20 ° c . 0 . 8 ml of adhesive b sample was placed in each aluminum tube and sealed . a total of 300 adhesive b samples were prepared for the execution of the testing . those adhesive b samples were randomly separated into 5 groups ( group 1 - 5 ) consisting of 60 samples / each . those samples were aged in an environmental chamber at 60 ° c . to simulate various aging time at 20 ° c . the results are shown in table 1 . after aging test ( example 7 ), the samples were tested for their wound closure strength according to astm f2458 wound closure strength test . in this test , a thin layer of adhesive sample was applied on a piece of porcine skin specimen with a cut in the middle to simulate wound . then , the force to pull the wound apart was measured . sufficient adhesive b was first applied uniformly on a 2 . 5 cm × 1 . 0 cm area adjacent to the cut to bond 2 pieces of porcine skins together . 10 sets of specimens were prepared . they were then placed in a sealed plastic bag and conditioned in 30 ± 1 ° c . for 1 hr ± 15 min . once the adhesive was cured , the specimens were allowed to cool to room temperature and were clamped between the upper and lower jaws of a universal test machine . the specimens were tested at a crosshead speed of 250 mm / min until failure . both failure mode and peak load were recorded . the results of the wound closure strength test are shown in table 1 . as seen from table 1 , the viscosity of adhesive b increased as time goes by , but the wound closure strength thereof reduced slightly and possesses 8 . 1n after 24 months &# 39 ; aging . thus , the adhesive provided by the present invention can prevent cyanoacrylate monomer from premature polymerization and has a desirable viscosity for convenient application . in addition , it also maintains appropriate wound closure strength for a long period of time . for preparation of the accelerator , 1800 ml of deionized water was first placed in a 2000 ml beaker , and then 5 g ( 0 . 3 % by weight ) ethanol ( echo chemical co .) was added and mixed by stifling for about 5 minutes . afterwards the second cyanoacrylate monomer , 32 ml of n - butyl cyanoacrylate monomer ( nbca , chemence co . ), was added into the ethanol solution drop by drop and mixed by continuous stirring for about 0 . 5 hour . the resulting nbca polymer , namely thickening agent c , was decanted and dried in the vacuum oven at 65 ° c . for 8 hours . the molecular weight of nbca polymer was measured with a gel permeation chromatography ( gpc ). consequently , the molecular weight of nbca polymer , i . e . thickening agent c , is 64 , 300 . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent c prepared in example 9 was then added slowly into the flask . after this , they were mixed at 100 ° c . for 60 minutes to form adhesive e . the viscosity of the adhesive e was measured with a capillary viscometer . the viscosity of the adhesive e is 47 cp at 20 ° c . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent c prepared in example 9 was then added slowly into the flask . 15 g of tributyl 2 - acetylcitrate ( atbc , safc , lot number mkbg8107v ) was also added slowly subsequently . then they were mixed at 100 ° c . for 60 minutes to form adhesive f . the viscosity of the adhesive f was measured with a capillary viscometer . the viscosity of the adhesive f is 43 cp at 20 ° c . for preparation of the accelerator , 1800 ml of deionized water was first placed in a 2000 ml beaker , and then 1 . 8 g ( 0 . 1 % by weight ) sodium bicarbonate ( sigma - aldrich co .) was added with a pipette and mixed by stirring for about 3 minutes . afterwards the second cyanoacrylate monomer , 32 ml of n - butyl cyanoacrylate monomer ( nbca , chemence co . ), was added into the sodium bicarbonate solution drop by drop and mixed by continuous stirring for about 0 . 5 hour . the resulting nbca polymer was decanted and rinsed several times with deionized water and then decanted again . the bicarbonate therein was neutralized with 0 . 1n hcl and then nbca polymer was rinsed again with deionized water . the resulting product was dried in the vacuum oven at 65 ° c . for 8 hours to form thickening agent d . the molecular weight of the thickening agent d is 43 , 100 . the first cyanoacrylate monomer , 285 g of 2 - octyl cyanoacrylate monomer ( 2 - oca , the viscosity thereof is 6 cp at 20 ° c . ), was poured into a 1 liter round bottom flask on a heater / mixer and stirred at 200 rpm . 15 g of thickening agent d prepared in example 12 was then added slowly into the flask . 15 g of tributyl 2 - acetylcitrate ( atbc , safc , lot number mkbg8107v ) was also added slowly subsequently ; and then all were mixed at 100 ° c . for 60 minutes to form adhesive g . the viscosity of the adhesive g was measured with a capillary viscometer . the viscosity of the adhesive g is 31 cp at 20 ° c . 0 . 8 ml of adhesive g sample was placed in each aluminum tube and sealed . a total of 300 adhesive g samples were prepared for the execution of the test . the adhesive g samples were randomly separated into 5 groups ( group 6 - 10 ) consisting of 60 each . they were accelerately aged in an environmental chamber at 60 ° c . to simulate various aging time at 20 ° c . the results are shown in table 2 . after aging test ( example 14 ), the samples were tested for their wound closure strength according to astm f2458 wound closure strength test as described in example 8 . sufficient adhesive g was first applied uniformly on a 2 . 5 cm × 1 . 0 cm area adjacent to the cut to bond 2 pieces of porcine skins together . 10 sets of specimens were prepared . they were then placed in a sealed plastic bag and conditioned in 30 ± 1 ° c . for 1 hr ± 15 min . once the adhesive was cured , the specimens were allowed to cool to room temperature and were clamped between the upper and lower jaws of a universal test machine . the specimens were tested at a crosshead speed of 250 mm / min until failure . both failure mode and peak load were recorded . the results of the wound closure strength test are shown in table 2 . referring to the results shown in tables 1 and 2 , the wound closure strength of adhesive g has been reduced to 7 . 5n after 12 months &# 39 ; aging , and 4 . 2n after 24 months &# 39 ; aging . the adhesive thickened by polycyanoacrylate , which was prepared by conventional accelerators exhibits premature polymerization in a cyanoacrylate monomer adhesive due to its residual accelerators . owing to this premature polymerization of adhesive substrate , either the wound bonding strength is reduced or the shelf life is shortened . however , with the thickening agent prepared by the ammonium hydroxide or alcohol provided by the present invention , the premature polymerization can be reduced and the adhesive having excellent properties can be used for various medical applications .
the present invention provides a method for making an adhesive composition . an adhesive substrate comprising at least a first cyanoacrylate monomer is provided and then mixed with a thickening agent containing polycyanoacrylate prepared by polymerization of a second cyanoacrylate monomer initiated with an aqueous solution of ammonium hydroxide or alcohol . in light of low boiling point of ammonium hydroxide and alcohol , they can be easily removed by heating at low temperature . as such , conventional premature polymerization of adhesive substrate owing to addition of a thickening agent containing residual accelerators can be overall improved .