Patent Application: US-4745008-A

Abstract:
the present invention provides a nanocapsule of a blocked isocyanate encapsulated within a polymeric nanosphere . in one aspect , the polymeric nanosphere is functionalized . free isocyanate functionality is released upon thermal annealing or uv exposure of the nanospheres containing the blocked isocyanate . this present invention also provides a novel method for encapsulating isocyanates in aqueous media . in one aspect , the method comprises miniemulsion polymerization . the thermally or uv deblocked isocyanate can be used as an active functional group for many potential applications .

Description:
in a preferred embodiment , blocked isocyanates are encapsulated into polystyrene nanospheres . in a more preferred embodiment , the nanocapsules are prepared via miniemulsion polymerization . in one embodiment , the blocked isocyanate is based on hexamethylene diisocyanate . other isocyanates for use in this invention include toluene diisocyanate , methane diphenyl diisocyanate ( mdi ), hexamethylene diisocyanate , isophorone diisocyanate , hydrogenated mdi , trimethyl hexamethylene diisocyanate , tetramethyl xylylene isocyanate , tetramethyl xylylene diisocyanate , xylylene diisocyanate , isocyanate dimers , isocyanate trimers , polyisocyanates , polydiisocyanates , etc . the blocking agents attached to the isocyanate can be selected from a myriad of chemical groups : methanol , 1 , 1 , 1 , 3 , 3 , 3 - hexafluoro - 2 - propanol , 2 - methyl - 2 - propanol , ethanol , butanol , cyclohexanol , and t - amyl alcohol , 1 - pentanol , isopropanol , 2 - ethylhexanol , 2 - n , n - dimethylaminoethanol , 2 - ethylhexanol , 2 - ethoxyethanol , hydroxyethyl ( meth ) acrylate , etc . methyl ethyl ketoxime , benzophenone oxime , acetone oxime , cyclohexanone oxime , cyclopentanone oxime , acetophenone oxime , methyl isobutyl ketone oxime , etc . malonic esters , such as diethyl malonate , acetoacetic esters , such as ethyl acetoacetate , 2 , 4 - pentaedione , acetyl acetone , malononitrile , etc . blocked isocyanates as may be employed are described , for example , in organischer metallschutz : entwicklung und anwendung von beschichtungsstoffen [ organic protection of metals : development and application of coating materials ], page 159 - 160 , vincentz verlag , hannover ( 1993 ). these are compounds in which the highly reactive nco group is “ blocked ” by reaction with specific radicals , such as primary alcohols , phenol , acetoacetates , . epsilon .- caprolactam , phthalimide , imidazole , oxime or amine . the blocked isocyanate is stable in liquid systems and also in the presence of hydroxyl groups . on heating , the blocking agents are eliminated and the nco group is exposed . blocked isocyanates can be formed by reacting any of the above mentioned blocking agents with any of the isocyanates mentioned earlier . blocked isocyanates are incorporated into the polymeric nanospheres during the miniemulsion process that is the substance of this patent application . essentially , the nanospheres can be made from a wide variety of free radically polymerizable comonomers . in another preferred embodiment , blocked isocyanates are encapsulated within functionalized nanospheres . in one embodiment , the nanosphere shell is functionalized with hydroxyl or amine . other examples of functionalities include epoxy , amide , and carboxyl groups . in another embodiment , the nanocapsule shell is comprised of polystyrene or polystyrene copolymerized with a multifunctional monomer such as divinyl benzene . in another embodiment , the nanocapsule shell is formed via copolymerization of styrene , 2 - hydroxyethyl methacrylate , and 2 -( tert - butylamino ) ethyl methacrylate using divinylbenzene as the crosslinker . in another embodiment , the blocked isocyanate is based on hexamethylene diisocyanate . in another embodiment , the nanocapsules are prepared via miniemulsion polymerization . monomers selected from the wide range of vinyl and ( meth ) acrylate monomers known to those skilled in the art of free radical polymerization can be employed to form the nanocapsule shell . use of comonomers such as glycidyl methacrylate , n - isopropylacrylamide , and ( meth ) acrylic acid will yield functionalized nanocapsules that contain epoxy , amide , and carboxyl functionality , respectively . in another aspect , the blocked isocyanate encapsulated within the nanocapsules are deblocked by freeze - drying the nanocapsules and treating them thermally to release the deblocking agent . in another aspect , the deblocking thermal treatment is performed isothermally . alternatively , the deblocking can be accomplished by exposure to uv light . a detailed review of blocked isocyanates , blocking agents , and their deblocking temperatures can be found in references 13 and 14 . desmodur ® bl3175a [ blocked isocyanate based on hexamethylene diisocyanate and methyl ethyl ketoxime ( meko ), deblocking temperature ˜ 135 ° c . ], was chosen for encapsulation into polystyrene nanoparticles . the bnco nanocapsules , prepared via miniemulsion polymerization are summarized in table 1 . desmodur n3300a ( polymeric hexamethylene diisocyanate ) was employed as the control for ftir characterization . the miniemulsion process yields nanocapsules that contain the blocked isocyanate encapsulated within the polystyrene nanocapsules . pre - emulsion preparation . the pre - emulsion for nanocapsule m - 6 was synthesized as follows : part i [ 40 g styrene , 0 . 8 g divinylbenzene ( dvb ), 0 . 82 g 2 , 2 ′- azobisisobutyronitrile ( aibn ), and 40 g desmodur bl3175a )], and part ii [ 1 . 71 g sodium dodecyl sulfate ( sds ), 1 . 63 g igepal ® co - 887 ( nonyl phenol 30 mole ethoxylate ), and 220 g water )] were blended magnetically in separate containers for 10 minutes at 25 ° c . part ii was poured into part i under mechanical agitation , and the contents were stirred for 30 minutes at 1 , 800 rpm . the pre - emulsion was cooled to & lt ; 5 ° c . before sonification using a misonix ® sonicator 3000 ( until the particle size was & lt ; 250 nm ). emulsion polymerization . the pre - emulsion was transferred to a three - neck round bottom flask equipped with a mechanical stirrer , reflux condenser , and a nitrogen inlet , and degassed for 30 minutes . the temperature was raised to 70 ° c . and maintained for 8 hours to complete the polymerization . characterization . the latex was freeze dried ( lyophilization ) for 48 hours in the freezemobile 35el . glass transition temperatures ( t g s ) were recorded on a differential scanning calorimeter ( dsc q1000 , ta instruments , new castle , del .). for deblocking studies , the freeze dried samples were treated thermally in the dsc under a nitrogen atmosphere as summarized in table 2 and characterized via ftir . ftir analysis showed characteristic peaks at 1682 ( carbonyl ) and 2270 cm − 1 ( free isocyanate ) for desmodur n3300a , and at 1682 and 1725 cm − 1 for desmodur bl3175a ( fig1 a and 1 b ). the characteristic bnco peaks were visible in the ftir spectrum of the nanocapsule , m - 6 ( fig1 e ) but absent from the ftir spectrum of the hollow nanosphere , m - 1 ( fig1 d ), confirming the inclusion of bnco in the nanocapsules . nanocapsule m - 6 was treated with acetone to extract the encapsulated bnco . the ftir spectrum of m - 6 after acetone extraction ( fig1 f ) shows complete disappearance of the characteristic bnco peaks indicating that the blocked isocyanate was extracted from the nanocapsules by acetone . for deblocking studies , nanocapsules m - 6 were freeze dried and treated thermally in the dsc to release the deblocking agent . fig1 c displays the effect of deblocking treatment on bl3175a . while the presence of the isocyanate peak at 2270 cm − 1 indicates deblocking , the peak at 1725 cm − 1 indicates that all the bnco was not deblocked by the thermal treatment . the bnco nanocapsule m - 6 that was subjected to deblocking treatment also shows the characteristic isocyanate peak ( 2270 cm − 1 ) ( fig1 g ). the deblocking appeared to be a function of time . therefore , bl3175a and nanocapsule m - 7 ( 2 % dvb , 60 % bnco ) were heated isothermally at 140 ° c . for varying lengths of time ( fig2 ). in fig2 a , the first curve for bl3175a ( 0 min ) reveals a small amount of deblocked isocyanate , which indicates initiation of the deblocking process at 140 ° c . subsequent spectra show the effects of isothermal heating of bl3175a for varying lengths of time . the close - up profile ( fig2 a , area a ) shows increasing intensity of the isocyanate peak with time . even after 70 minutes at 140 ° c ., a small peak is visible at 1725 cm − 1 ( peak b ) suggesting the presence of residual blocked isocyanate . since the atr can only probe the top 2 microns of the sample ( znse crystal ), the deblocking of bl3175a was also analyzed via transmission ftir between sodium chloride plates ( fig2 b ). to show the time - dependent deblocking property of bnco , the transmission ftir data were replotted after normalization at 1682 cm − 1 . it is evident that the peak a height increases with time while that of peak b decreases . peak b disappears after 50 minutes at 140 ° c . a similar heating profile for the m - 7 bnco nanocapsule summarizes the effect of deblocking by the appearance and continued presence of deblocked isocyanate . by 120 minutes , the 1725 cm − 1 peak has almost disappeared indicating complete deblocking of the original bnco . lee suggested that the blocking and deblocking reaction of diisocyanates with methyl ethyl ketoxime is a reversible reaction ( 12 ). in these nanocapsules , the polystyrene shell functions as a “ shield ” for the deblocked isocyanate by preventing the liberated blocking agent from re - entering the nanocapsule . varying the degree of crosslinking of the polystyrene shell further exemplifies the “ shield effect ” ( fig3 , the ftir spectra of n3300a , bl3175a , and bl3175a after deblocking treatment are replotted for comparison ). subjecting the nanocapsule m - 9 ( no dvb crosslinker in the shell ) to the deblocking treatment shows a small shoulder peak at 1725 cm − 1 indicating the presence of residual bnco ( fig3 f ). the strong peak at 1725 cm − 1 seen upon deblocking the nanosphere m - 1 and bl3175a blend indicates the effect of the polystyrene shell on the reaction between the isocyanate and the liberated blocking agent . increasing the proportion of dvb from 2 % ( m - 5 ) to 6 % ( m - 11 ) results in shorter peaks at 1725 cm − 1 ( fig3 h and 3 i ). it is evident therefore that increased crosslinking of the polystyrene shell enhances the “ shield effect ”. the dsc profile of bl3175a exhibits a glass transition temperature ( t g ) of − 11 . 26 ° c . in the second heating cycle before the isocyanate deblocks at 136 . 59 ° c . the t g shifted to − 2 . 51 ° c . in the next heating cycle and no deblocking peaks (˜ 135 ° c .) were evident . however , this does not signify the complete deblocking of the blocked diisocyanates . for instance , the dsc profile of the thermally treated bl3175a ( fig1 c ), does not have a deblocking thermal peak in the heating cycle following isothermal treatment at 160 ° c . for 40 minutes but its ir spectrum shows the presence of residual blocked diisocyanates . two distinct t g s are discernible in the dsc profile of nanocapsule m - 7 . the first t g of ˜− 5 ° c . is attributed to the bnco , and the second t g of ˜ 105 ° c . is attributed to the presence of partially crosslinked polystyrene . the distinct and separate t g s confirm different phases in the nanocapsules . the effects of time and temperature on deblocking bl3175a were also noticed in the dsc thermograms of nanocapsules containing varying amounts of bnco . nanocapsules containing & gt ; 20 % bnco exhibited exothermic responses after the second heating cycle , while nanocapsules containing & lt ; 20 % bnco did not show similar responses . additionally , higher degrees of crosslinking in the shell ( dvb increased from 2 wt % to 6 wt %) also resulted in increased deblocking times . higher crosslinking of the shell promotes the “ shield effect ”. dsc analysis was conducted with open samples ( no lid ) and closed samples ( pan capped with a small hole in the center ). the thermal behaviors of both processes were similar except for the lack of exothermic responses in the open samples . this example discusses the synthesis of novel nanocapsules containing active isocyanates within hydroxyl or amine functionalized nanocapsule shells synthesized via miniemulsion polymerization . desmodur ® bl3175a was chosen as the core material for the nanocapsules . 2 - hydroxyethyl methacrylate ( hema ) and 2 -( tert - butylamino ) ethyl methacrylate ( tbaema ) were copolymerized with styrene using divinylbenzene ( dvb ) as the crosslinker to form the functionalized shell materials . the bnco nanocapsules prepared via miniemulsion polymerization are summarized in table 3 . desmodur n3300a was employed as the control for ftir characterization . pre - emulsion preparation . the pre - emulsion for nanocapsule m - 5 was synthesized as follows : part 1 ( 40 g styrene , 1 g dvb , 0 . 51 g aibn , 10 g hema and 15 . 3 g desmodur bl3175a ), and part ii ( 1 . 32 g sds , 0 . 95 g igepal co - 887 , and 220 g water ) were blended magnetically in separate containers for 10 min at 25 ° c . part ii was poured into part i under mechanical agitation , and the contents were stirred for 30 min at 1 , 800 rpm . the pre - emulsion was cooled to & lt ; 5 ° c . before sonification using a misonix ® sonicator 3000 ( until the particle size was & lt ; 250 nm ). emulsion polymerization . the pre - emulsion was transferred to a three - neck round bottom flask equipped with a mechanical stirrer , reflux condenser , and a nitrogen inlet , and degassed for 30 min . the temperature was raised to 70 ° c . and maintained for 8 hours to complete the polymerization . characterization . the latex was freeze dried ( lyophilization ) for 48 hours in the freezemobile 35el . particle sizes were determined using mirotrac ® upa 250 . latex particle sizes were recorded on a capillary hydrodynamic fractionator ( chdf 2000 , matec applied sciences , northborough , mass .). for deblocking studies , the freeze dried samples were treated thermally in the dsc under a nitrogen atmosphere as summarized in table 2 and characterized via ftir . isocyanate encapsulation was confirmed via ftir characterization . fig4 a shows two characteristic peaks at 1682 cm − 1 ( carbonyl ) and 2270 cm − 1 ( free nco ) of desmodur n 3300a , while the bnco shows distinct peaks at 1682 and 1725 cm − 1 ( fig4 b ). the nanocapsule m - 5 ( fig4 e ) shows a characteristic bnco peak at 1682 cm − 1 that is not seen in the ftir spectrum of the empty nanosphere , m - 4 ( fig4 d ). the peak at 1725 cm − 1 overlaps with the hema ester carbonyl of m - 4 . the ir spectrum of m - 5 treated with acetone to extract the encapsulated bnco ( fig4 f ) shows the total disappearance of the bnco peak at 1682 cm − 1 indicating that the blocked isocyanate was extracted from the nanocapsules by acetone , and suggesting the core ( isocyanate )- shell structure of the nanocapsules . for deblocking studies , nanocapsules m - 6 were freeze dried and treated thermally in the dsc to release the blocking agent . fig4 c , 4 g , and 4 h show the effects of deblocking treatment on bl3175a , m - 4 — bl3175a blend , and m - 5 , respectively . while the presence of the free isocyanate peak at 2270 cm − 1 indicates deblocking of bl3175a , the peak at 1725 cm − 1 suggests the presence of residual bnco . ir spectra recorded via transmission technique on nacl plate showed total disappearance of the 1725 cm &# 39 ; 11 peak . 14 after the deblocking treatment , nanocapsule m - 5 showed the characteristic isocyanate peak at 2270 cm − 1 indicating the presence of free isocyanate functional groups . the functionalized nanocapsules are expected to have the hydroxyl / amine groups on the outside of the shell due to their hydrophilicity . when the blend of hydroxyl - functional empty nanosphere m - 4 and bl3175a was subjected to the deblocking treatment ( fig4 g ), no peak was seen at 2270 cm − 1 suggesting that the isocyanate generated upon deblocking reacted with the hydroxyl groups present on the nanosphere . however , the isocyanate generated upon deblocking in nanocapsule m - 5 is retained in the core of the nanosphere without reacting with the hydroxyl groups on the outside of the shell . the effect of the degree of crosslinking on the shell is also demonstrated in fig1 . unlike nanocapsule m - 5 ( 2 % dvb in the shell ), the nanocapsule m - 6 ( no crosslinker in the shell ) does not display the isocyanate peak at 2270 cm − 1 after deblocking thermal treatment ( fig4 i ). during deblocking treatment , the deblocked isocyanate flows from the nanocapsules with an uncrosslinked shell , and reacts with the hydroxyl groups in the shell . it is obvious therefore that dvb plays an important role as a crosslinker in confining the bnco to the core of the nanocapsule . the nanocapsule m - 7 ( fig5 e ) shows a characteristic bnco peak at 1682 cm − 1 that is not observed in the ftir spectrum of the empty nanosphere m - 8 ( fig5 d ). the peak at 1725 cm − 1 overlaps with the tbaema ester carbonyl of m - 8 . the ir spectrum of m - 7 treated with acetone to extract the encapsulated bnco ( fig5 f ) shows the total disappearance of the bnco peak at 1682 cm − 1 indicating that the blocked isocyanate was extracted from the nanocapsules by acetone , and suggesting the core ( isocyanate )- shell structure of the nanocapsules . fig5 c , 5 g , and 5 h summarize the effect of deblocking treatment on bl3175a , m - 8 — bl3175a blend , and m - 7 , respectively ( the ftir spectra of n3300a and bl3175a are replotted for comparison ). after deblocking treatment , nanocapsule m - 7 exhibits the characteristic isocyanate peak at 2270 cm − 1 indicating the presence of free isocyanate functional group . this peak is not seen in the similarly treated blend of empty nanospheres m - 8 and bl3175a ( fig5 g ) as the isocyanate generated upon deblocking reacts with the amine groups of tbaema present on the nanocapsule shell . however , the isocyanate generated upon deblocking in nanocapsule m - 7 does not react with amine groups , and remains in the core as the dissociated free isocyanate . the nanocapsule m - 9 ( fig5 i ) without dvb crosslinker in the shell , does not depict an isocyanate peak at 2270 cm − 1 after deblocking treatment . as discussed earlier , it is believed that the deblocked isocyanate is able to flow from the uncrosslinked shell and react with the amine groups on the shell periphery . since isocyanate deblocking appeared to be a function of time , nanocapsule m - 7 was heated isothermally at 140 ° c . for varying lengths of time ( fig6 ). the characteristic bnco peak at 1725 cm − 1 decreased steadily with time . at 170 minutes , a small shoulder is seen which could represent the tbaema ester carbonyl peak . the isocyanate peak at 2270 cm − 1 confirms that the deblocked isocyanates were retained in the core of the nanocapsule and not consumed by the amine groups on the nanocapsule shell . 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