Novel Conductive Paste based on Cellulose Acetate Butyrate

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1 Printed in the Repulic of Kore w r p ½ k x* rf r ( td) Novel Conductive Pste sed on Cellulose Acette Butyrte Te-Hyun Kim* Estmn Chemicl Compny, Kingsport, TN 37662, USA Deprtment of Chemistry, University of Incheon, Dowh-dong 177, Nm-gu, Incheon , Kore (Received Novemer 4, 2006). š k w r p»» w m œw. w r p š. û w v ƒ l p k p w r p w. d, ³ w gq ùkü. : s rf, of, s, r r ABSTRACT. Polymer-sed electriclly conductive pstes hve een used to mke the conductive pths etween voltge sources nd devices. The pstes used for these pplictions consist of two min components: polymer inder nd conductive filler. Hving oth low viscosity nd good metl-encpsulting properties, cellulose cette utyrte (CAB) ws regrded to e good cndidte s inder for the conductive pste. We hve prepred formultion for novel conductive pste sed on CAB. Preliminry studies showed tht this conductive pste reveled stle conductivity, together with uniform coting nd flexiility. Keywords: Conductive Pste, Cellulose Acette Butyrte, Conductivity, Percoltion Point INTRODUCTION Conductive pths etween voltge sources nd devices re the key for constructing electronic circuits. While trditionl rigid nd flex circuits use solid metls, minly copper, nd solid metl solders to ttch components to the conductive pths, polymer-sed electriclly conductive pstes re eing used to mke the conductive pths. 1,2 Polymer-sed conductive pstes offer distinct dvntges over conventionl solid metls or solder lloys, including reduced pckge size nd thickness, improved processiility, nd lower ssemly temperture. The pstes used for these pplictions consist of two min components: polymer inder nd conductive filler. The inder is required to hold the conductive filler prticles together nd to mke the filler stick to the sustrte. In conductive pstes, the inder is usully thermoplstic mteril, which cn e dissolved in solvent. The mjority of conductive pstes use silver s the conductive metl filler. Much less expensive, ut with similr conductivity, metls such s copper experience n incresed resistnce gretly over the time due to the formtion of copper oxide, upon exposure to oxygen, which is not very conductive. The higher resis- 171

2 172 ½ k x tnce in conductive pstes s compred to pure metls is scried to the fct tht electron encounters tremendous resistnce when it hs to jump from one metl prticle to nother. In order to minimize the resistnce in conductive pstes, it is essentil to optimize the mount of surfce contct re etween metl prticles. Most significnt phenomenon which contriutes to the conductivity of polymer-sed conductive systems is the percoltion point, where some of the metl prticles egin to contct one nother, creting conductive pth. After this point, ddition of smll mounts of metl filler will produce rpid drop in resistnce. Continued ddition of metl filler will eventully rech point where the resistnce levels off, nd not much improvement in the conductivity will e otined even with incresed loding of the metl filler. Therefore, good formultions of conductive pstes will use n optiml metl configurtion, preferly flke-shped, t loding towrd the ottom of the percoltion point curve. It is the key in this field of study to lower this percoltion point (PP), due to the high cost of silver, nd to chieve stle conductivity. Cellulose esters re nontoxic mterils widely used s coting lyers, fiers, films nd dditives. 3-6 Recent study to prepre ultrthin films using cellulose for iomedicl ppliction ws lso reported. 7 We were interested in using cellulose cette utyrte (CAB) 1 s inder to prepre new conductive pste. Due to the good film-forming properties, together with its low viscosity, CAB ws regrded s good cndidte for the conductive pste. Focused on the CAB effect s inder, this pper lso rodly covers other components for conductive pste. Structurel 1. EXPERIMENTAL Smple Preprtion Polymeric inder of choice, ws dissolved in nlyticl grde methyl ethyl ketone (MEK) (unless otherwise mentioned) contining the dditives (lefing gent nd plsticizer), if required, t the desired concentrtion. The solutions were left to shke vigorously for 5 min t r.t. efore the pproprite metl filler ws dded. The heterogeneous solutions were gin llowed to e well-mixed under the shker for t lest 5 min to ssure uniform consistency. The solutions were then pplied to the polyester sustrte y drwdown technique (wet film deposit of c. 50 µm) nd dried t r.t. for 5 min, followed y oven-dry t 90 o C for 90 min. The inder/filler rtio is the weight % rtio etween the polymer inder nd the metl (silver) filler. All the non-voltiles including dditives were considered s inder. The silver metl fillers were otined from Metlor in three different forms; flke, prticle nd powder. The silver flke hs the size distriution etween 1 nd 8 µm, silver powder hs the size distriution etween 0.4 nd 4 µm with the men vlue of 1.2 µm, nd silver prticle hs the sizes of c. 50% with less thn 7.3 µm nd 90% with less thn 16.3 µm. Cellulose cette utyrte (CAB) , nd were commercilly ville products from Estmn chemicl compny. Diutyl phthlte nd zinc sterte were otined from Aldrich. Poly(methyl methcrylte) (PMMA) with moleculr weight (M w ) of 40,000 nd glss trnsition temperture (T g ) of 105 o C ws otined from Dinl Americ (rnd nme: BR- 83). Poly(utyl methcrylte) (PBMA) with moleculr weight (M w ) of 337,000 nd glss trnsition temperture (T g ) of 15 o C ws otined from Aldrich. Poly(utyl methcrylte-co-methcrylte) [P(BMA-co-MMA)] with moleculr weight (M w ) of 100,000 nd glss trnsition temperture (T g ) of 64 o C ws otined from Aldrich. Film deposition, drwdown speed, drying time nd conductivity mesurement for ll the smples were crried out under the sme conditions.

3 w r p 173 Electric Mesurement The electricl conductivity ws investigted y mesuring the surfce resistnce (Ω/cm 2 ) of the films ccording to the conventionl four-terminl method using luminum foil electrodes. 8 The resistnce cross the top lyer of the prepred films ws mesured y plcing two electrodes (positive nd negtive) connected to n ohmmeter with fixed distnce on the surfce of conductive film. The mesurement ws crried out t three different plces with the sme distnce nd the verge ws tken s surfce resistnce of the prepred conductive film. It should e noted tht for most cses the film thickness of the conductive film ws not considered ecuse the sme Meyer r ws used for the film formtion nd the resistnce of the films within the experimentl rnge of thickness (25 to 75 µm of wet film deposit) reveled no difference with the vrition of the film thickness. The percoltion point ws regrded s the point of metl filler/inder rtio where the surfce resistnce (Ω/cm 2 ) of the mesured dried film surpssed 1. For exmple, if the resistnce ws mesured s 1 for the smple with filler/inder rtio of 40/60, the percoltion point ws tken s 40. RESULTS AND DISCUSSION Tle 1. Conductivity of the films prepred from three different types of silver t the filler/inder rtio of 66/34 using CAB s inder Silver Type Silver Content (wt.%) Resistnce (Ω/cm 2 ) Flke Prticle 66 > 1 Powder Silver Type It is generlly known tht the preferred silver filler size for the conductive pste ppliction is etween 1 nd 30 µm: elow 1, electricl properties re dversely ffected nd ove 30, the composition will e too difficult to e pplied onto the sustrte. 9 All the metl fillers used for our experiments hd the verge size distriution within this desired rnge. Smples with different silver forms (flke, prticle nd powder) were prepred t the fixed silver/inder rtio of 66/34 (wt.%) using CAB s inder nd MEK s solvent, nd their resistnce ws mesured (Tle 1). As the films prepred using silver flke nd powder showed much etter conductivity (lower resistnce) thn those from silver prticle t this initil silver filler/inder rtio, further electric mesurement for oth silver flke nd powder ws crried out t vrious silver/inder rtios using CAB s inder. The concentrtion of the totl solids (CAB nd silver) ws set to 72.4 wt.%. As shown in Tle 2 nd Tle 3 (nd lso in Fig. 1), etter conductivity (lower resistnce), with lower percoltion point (60 for silver powder nd 56 for silver flke), ws otined for silver flke. This ws consistent with the ssumption tht flttened metl flke configurtion would increse the re of metl contct, resulting in more conduction pths compred to the contct point generted from either metl powder or prticle. 10 From the ove results, it ws concluded tht metl configurtion hs sig- Tle 2. Conductivity of the films prepred from silver powder/cab in MEK solution CAB (g) Silver Powder [g (wt.%)] Resistnce (Ω/cm 2 ) (78) (69) (66) (64) (63) (60) > 1 Tle 3. Conductivity of the films prepred from silver flke/ CAB in MEK solution CAB (g) Silver flke [g (wt.%)] Resistnce (Ω/cm 2 ) (78) (69) (66) (64) (60) (58) (56) > 1

4 174 ½ k x Fig. 1. Percoltion curve of the conductivity for the CABsed conductive pste contining the silver powder (dotted line) nd silver flke (solid line). nificnt effect on the conductivity of the films prepred, nd flke is superior to oth prticle nd powder shpes. Binder Type In order to investigte the inder effects on the conductivity, silver flke ws dded to MEK solution of vrious inders t different filler/inder rtios under the sme conditions nd the dried films, prepred in the sme wy s previously descried, were monitored for their resistnce. The results re summrized in Tle 4. CAB showed etter conductivity (lower resistnce) thn the nd versions nd not much difference in conductivity ws oserved etween CAB nd over the rnges of filler/inder rtios. This indictes tht the viscosity is n importnt fctor which contriutes to chieve n optiml silver flke configurtion since the mjor difference etween CAB nd the other CAB versions is their viscosity (Tle 5). It ws thought tht inder with higher viscosity reduces the ility of the metl fillers to orient t the surfce, nd thus the optiml orienttion of the fillers to generte the conductive pth is impeded. To further investigte the viscosity effect, the smple with CAB hving the silver content of 56 wt.% ws diluted from 50 wt.% to 40 wt.% nd 33 wt.% of the totl solid (CAB + silver) concentrtion, nd their resistnce ws compred (Tle 6). Tle 5. Physicl dt for vrious CABs used CAB T M Type n viscosity g utyryl content ( o C) (%) , , , (%) is the verge vlue for the utyryl composition out of cetyl, utyryl nd hydroxyl sustituents in 1. Tle 4. Conductivity of the films prepred from silver flke with vrious types of inders in MEK solution. No dditives were dded nd the concentrtion of the pste ws set to 72.4 wt.%. PBMA P(BMA-co-MMA) PMMA CAB CAB CAB Silver (wt.%) Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 )Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 ) 78 Poor Film Poor Film Poor Film Poor Film Poor Film Poor Film Poor Film Poor Film > > 1 > > 1 > > 1 wt.% = [(silver(g) + inder(g)) / (silver(g) + inder (g) + solvent(g)] 100%.

5 w r p 175 Tle 6. The dilution effect on the conductivity of the smples prepred with Ag/CAB in MEK solution CAB Silver Concn Resistnce (g) [g (wt.%)] (%) (Ω/cm 2 ) (56) 50 > (56) (56) Concentrtion(%) = [(silver(g) + inder(g)) / (silver(g) + inder(g) + solvent(g))] 100%. As expected, etter conductivity (lower resistnce) ws otined s the smple ws more diluted. As the viscosity of the solution is closely relted to its concentrtion, this result confirmed tht the viscosity is the key fctor for determining the conductivity of the smples. The CAB ws then compred with other types of crylic inders; PMMA, P(BMA-co-MMA) nd PBMA (Tle 5). The resistnce for the Ag flke/pmma system, hving the lowest viscosity mong three crylic inders, ws lower thn tht of Ag/P(BMAco-MMA), which then reveled much lower resistnce thn tht of Ag/PBMA, ut the films otined from oth PMMA nd P(BMA-co-MMA) were of worse qulity. The films were in fct too poor to mesure conductivity t the lower loding of these inders. Better films were formed s the loding of the PMMA or P(BMA-co-MMA) ws incresed. It ws thought tht PBMA nd P(BMA-co-MMA) hving more hydrophoic longer lkyl chins (utyl group) etter encpsultes the silver pigment, forming etter coting, ut this encpsultion of the pigment lso cused higher resistnce for the PBMA nd P(BMA-co-MMA) system. Further studies involving vrious other crylic inders hving different moleculr weights re, however, required for this ssessment. The film s integrity lso seemed to e dversely ffected when the inder s loding ws too low especilly for the less viscous inder, such s PMMA. Interesting point from the results in Tle 4 ws tht CAB offered much etter conductivity (lower resistnce), together with lower percoltion point thn PMMA nd P(BMAco-MMA), two most widely used inders for the conductive ink system. In ddition, ll the CAB inders offered much etter qulity of the film thn the crylic inders even t low lodings. The etter conductivity (lower resistnce) of the film prepred using CAB ws scried to its low viscosity, nd the high utyl content in CAB (see Tle 5) seemed to offer good film-forming property. Considering the lnce etween conductivity nd film qulity, CAB ws regrded s the est inder mong those investigted, nd this ws used for further studies. The studies, so fr, hve een minly focused on the conductivity, nd it ws necessry to look into other properties such s flexiility. Plsticizer Plsticizer is mteril of low moleculr weight dded to polymer to seprte the moleculr chins. Tle 7. Plsticizer effect on the conductivity of the films prepred from silver flke/cab system. The concentrtion of the pste ws 72.4 wt.% CAB (g) DBP (g) Silver [g (wt.%)] Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 ) (60) (56) (52) CAB (g) DBP (g) Silver Content [g (wt.%)] Resistnce (Ω/cm 2 ) Resistnce (Ω/cm 2 ) (60) (56) (52) (wt.%) = [ silver(g) / (silver(g) + inder(g) + DBP(g))] 100%. Resistnce vlue when no plsticizer ws used t the sme lodings of silver.

6 176 ½ k x This results in depression of the glss trnsition temperture, reduced stiffness nd rittleness, nd improved processility. Diutyl phthlte (DBP) ws used in 5 nd 10 wt.% reltive to CAB nd the resistnce of the dried films t three filler/inder rtios (60/40, 56/44 nd 52/48) ws mesured. As shown in Tle 7, the resistnce vlue for the smples contining the plsticizer ws higher compred with their originl silver/ inder system, nd the difference ecme lrger s the silver/inder rtio gets closer to the percoltion point. When enzyl utyl phthlte ws used s plsticizer insted of diutyl phthlte, much higher increse in resistnce ws oserved. The preferred glss trnsition temperture of the inder for the conductive ppliction is etween 45 nd 105 o C. Under 45 o C, locking occurs, wheres films ecome frgile ove 105 o C. The DSC studies for the smples contining CAB/diutyl phthlte in 95/5 nd 90/10 wt.% were ttempted ut did not give ny relile dt. As the presence of the plsticizer seems to hve n dverse effect on the conductivity, especilly t the lower silver/inder rtio, the mximum loding of the plsticizer ws set to 5 wt.% reltive to the inder. The films contining this level of plsticizer pper to offer good flexiility. From the ove results, it ws found tht the plsticizer not only provided enefits to the prepred films such s flexiility nd uniformity, ut they lso dversely ffected the conductivity of the Ag/CAB system. Therefore, it ws concluded tht the mount (nd the type) of the plsticizer hs to e crefully controlled. Finlly, it hs to e dded tht the effect of the cosolvent ws lso investigted to control the drying rte using utyl cette, nd the results showed tht the presence of the co-solvent significntly incresed the resistnce for the conductive pste. It ws thought tht the more viscous utyl cette thn MEK incresed the overll viscosity of the solution, ffecting its conductivity. The use of co-solvent ws, therefore, not recommended s no significnt improvement in the film s properties ws otined when the co-solvent ws used. Tle 8. Conductivity for CAB-sed conductive pste Silver (wt.%) Resistnce (Ω/cm 2 ) > 1 (wt.%)=[silver(g)/(silver(g)+inder(g)+dbp(g))] 100%. Fig. 2. Percoltion curve of the conductivity for the CABsed conductive pste. Developing CAB-sed conductive pste Bsed on the knowledge we hve gined so fr, the formultion for our conductive pste ws suggested: silver flke s metl filler, CAB s inder, diutyl phthlte s plsticizer nd MEK s solvent. The plsticizer/cab rtio ws 95/5 y weight nd the totl solid content [(silver(g) + CAB(g) + DBP(g)) / (silver(g) + CAB(g) + DBP(g) + MEK(g))] ws 55 wt.%. The films prepred from this conductive pste showed surfce resistnce of Ω/cm 2 nd percoltion point of 50 (Tle 8 nd Fig. 2). A steep percoltion curve nd stle conductivity up to 60 wt.% of the silver lodings were otined, implying tht relile conductivity cn e produced regrdless of the sustrtes pplied onto nd films thickness. In ddition, the films prepred from the CAB-sed conductive pste offered uniform coting nd good flexiility.

7 w r p 177 CONCLUSION In summry, we hve investigted the possiilities to use CAB s inder for the conductive pste. Hving oth low viscosity nd good metlencpsulting properties hence good film-forming properties, CAB ws concluded to e good cndidte s inder for the conductive pste. We hve lso prepred formultion for the conductive pste, which showed promising results. Finlly, nother very importnt fctor to e considered for the choice of conductive pste is its drying rte: the current levels of conductivity, in fct, re suitle for the commercil ppliction, ut most of the conductive pstes dry too slowly t the thicknesses needed to llow for the higher speeds of printing. Therefore, conductive pste which could deliver comprle conductivity with fster drying (nd with lower cost, of course) could e the winner of ll. This will e the focus of our future studies, together with the rheology of the conductive pste prepred. Acknowledgment. Prt of this work ws generously supported y the reserch fund of Incheon University in REFERENCES 1. Gentile, C.; Wllce, M.; Avlon, T.; Goodmn, S.; Fuller, R.; Oks, S.; Hll, T. US Ptent 5,086,785 (1992). 2. Jurgens, J.; Ptterson, P. Med. Eng. Phys. 1997, 19, Young, C. R.; Koleng, J. J.; McGinity, J. W. Int, J. Phrm. 2002, 242, Gilert, R. D.; Venditti, R. A.; Zhng, C.; Koelling, K. W. J. Appl. Polym. Sci. 2000, 77, Qiu, W.; Zhng, F.; Endo, T.; Hirotsu, T. J. Appl. Polym. Sci. 2003, 87, Choi, S. H.; Cho, M. S.; Kim, D.; Kim, J.-H.; Lee, D. H.; Shim, S. J.; Nm, J.-D.; Lee, Y. Polymer(kore), 2005, 29, Kosk, P. M.; Kwno, Y.; Slvdori, M. C.; Petri, D. F. S. Cellulose, 2005, 12, Higshi, F.; Cho, C. S.; Kkinoki, H.; Sumit, O. J. Polym. Sci. Polym. Chem. Ed. 1977, 15, Buckley, M. S.; Bowns, R. E. US Ptent 5,286,415 (1994). 10. It ws lso found tht the conductivity of the silver flke/cab smples ws slightly incresed with the ddition of zinc sterte (ZS) s n dditive, ut this improvement in conductivity ws not oserved for the silver powder/cab/zs system. It ws thought tht zinc sterte cted s lefing gent to ring the silver flke to the surfce re, ut this ws not effective for the silver powder.