ELECTRICALLY CONDUCTIVE STRUCTURAL ADHESIVES BASED ON BUCKYPAPERS

Transcription

1 18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS ELECTRICALLY CONDUCTIVE STRUCTURAL ADHESIVES BASED ON BUCKYPAPERS I.D. Rosc, S.V Ho* Mechnicl nd Industril Engineering, Concordi University, Montrel, Cnd * Corresponding uthor (hosuon@lcor.concordi.c) Keywords: Structurl dhesives; Cron nnotue; Electricl conductivity 1 Introduction Structurl dhesives re extensively used to uild lightweight structures in erospce nd utomotive industries. Electricl continuity nd electrosttic dissiption cpilities re usully requested for these structures. Since ll of these dhesives re electricl insultors, the structures must e grounded y time intensive opertions like silver rzing or strpping. Recently, cron nnotues (CNTs) were intensively investigted s efficient fillers for electriclly conductive composites [1-2]. However, homogeneous nnotue dispersions require time nd energy intensive opertions nd the resulting dhesive displys high viscosity nd insufficient electricl conductivity. These inconveniences cn e ddressed y using uckypper (BP) technology. 2. Mterils nd methods 2.1. Buckypper preprtion Two types of uckypres were prepred: one mde of single wll cron nnotues (SWCNTs) nmed sbp nd one mde of mixture of SWCNTs nd multi wlled cron nnoues (MWCNTs) clled hyrid uckypper (hbp) For sbp preprtion 0.5 g of SWCNTs from Nikkiso Co. were dispersed in N, N dimethylformmide y horn sonictor for 30 min. For hbp mixture of g (25%) of SWCNTs nd g (75%) of MWCNTs from Nnol Inc, were dispersed in N, N dimethylformmide y horn sonictor for 30 min. Next the CNT suspension ws filtered on nylon memrne-filter with pore size of 45 micron. After filtrtion the uckypper nd the memrne were plced etween severl filter-ppers nd lightly pressed etween two luminum pltes to sor the excess solvent. The wet uckypper ws then seprted from the filter memrne nd dried t 130 C for 12 hours to form sheet of 140x140 mm 2 nd 50 m thick Impregnted BP preprtion The BPs were impregnted with resin using direct impregntion (DI) or solvent impregntion (SI). For direct impregntion ptches of BPs were immersed in mixture of epoxy resin (Epon 862) nd hrdener (26.4 wt% Epikure W) nd plced in vcuum oven t 80 C for 30 min. For solvent impregntion ptches of BPs were immersed for 1 hour t room temperture in n cetone solution (25 %vol) of the resin nd curing gent mixture. Next the impregnted ptches were plced in vcuum oven t 80 C for 40 min to remove the cetone. The third wy to produce impregnted BPs clled one step impregntion (OSI) consists of dispersing CNTs directly in the cetone solution of the resin nd curing gent followed y filtrtion nd solvent evportion Lp joint preprtion Aluminum (2024 lloy T3) were cut to dimensions s shown in Fig. 1, degresed in cetone nd etched in chromic cid solution for 30 min t 65 C. Ptches with desired dimensions were cut out from uckypper sheet nd impregnted with resin s descried in prgrph 2.2. The overlp re of oth dherents ws coted with thin lyer of dhesive. Next, the impregnted ptches were plced on one dherent nd the lp joint ws tightened using C- clmp. The lp-joints were cured in n oven t 175 C for 4 hours.

2 2.4. Mesurements The resistnce of the onded joint ws mesured y four-wire method using current source (Keithley 6220 DC) nd nnovoltmeter (Keithley 2182A). The resistivity of the s produced BPs nd the tht of impregnted BPs were mesured y vn der Puw setup [1] Single-lp specimen for the tension-tension ftigue test nd sher strength is shown in Fig.1. The sher strength of the simple lp joints prepred ccording to ASTM D ws mesured on n MTS 100kN testing mchine t 1.3 mm/min strin rte. In order to expedite the ftigue tests the mximum loding ws 50 % of the verge sher strength nd the lod rtio ws 0.1. The ftigue tests were crried out on n MTS 100kN testing mchine t 10 Hz Fig1. Single-lp joint for tensile testing; dimensions in mm. 3. Results nd discussion Typicl SEM microgrphs of sbp nd hbp re presented in Fig. 2. There re severl resons for producing hyrid BPs: (i) BPs mde of SWCNTs re strong nd highly conductive ut too expensive; (ii) BPs mde of MWCNTs re too frgile to hndle nd low disply conductivity ut re chep. Using less thn 0.25 weight frction of SWCNTs it is possile to prepre strong BPs with good electricl conductivity nd price. The electricl conductivity of the s-produced nd impregnted BPs re presented in Tle 1. During the impregntion process the uckypper swells, nd its thickness increses significntly (Tle 1). Anlyzing the three impregntion methods it is cler tht direct impregntion is the most convenient s it voids the difficulties relted to solvent processing. Furthermore, direct impregntion results in the highest conductivity nd swelling. Fig 2 SEM microgrph of s produced sbp () nd hbp (); scle r 500 nm. Tle 1 Electricl conductivity of the s produced nd impregnted BPs BP Type sbp hbp Conductivity, S/cm PM TIF BI AI DI SI OSI NA 34 NA DI SI PM- BP production method; BI, AI efore nd fter impregntion; TIF- thickness increse fter impregntion

3 Electriclly conductive structurl dhesives sed on uckyppers However, the cross section of BPs impregnted using the DI process is not homogeneous i.e. resin rich lyers re interclted etween CNT rich lyers, s shown in Fig. 3 (regions mrked with white circles). Furthermore, the cross section of impregnted sbps is fr more inhomogeneous thn tht of hbps (Fig. 3). Two fctors contriute to this: the considerle viscosity of the resin system nd the BP morphology. In sbps the CNTs re ligned nd closely pcked to form strtified morphology with higher density (density of 700 kg/m 3, Fig 2) thn hbps in which CNTs re more loosely pcked into n entngled 3D structure (density 230 kg/m 3, Fig 2) tht llow more uniform impregntion. Becuse of its lyered structure, sbp is only in-plne entngled thus the viscous resin rther peels SWCNT lyers thn penetrtes smll pores. The min dvntge of using BPs in dhesive onding is reduction y 11 orders of mgnitude of the electricl resistnce compred to the net resin, nd 10 to 100 times compred to clssicl nnotue dispersions (Tle 2). As the CNT content in the BP dhesive is high (over 20 wt%) we expected improved mechnicl properties, ut when the whole overlp re ws covered y sbp or hbp (Fig. 6) the sher strength decresed y 57% nd 20 % respectively, compred to the net resin (Tle 2). In first instnce we ttriuted the lrge decrese in sher strength in the cse of sbp to the inhomogeneous impregntion. In consequence we hve opted for solvent impregntion, s much lower viscosity is expected to led to more uniform impregntion. Indeed, the cross section of sbp nd hbp is uniformly impregnted s shown in Fig. 4. Tle 2 Mechnicl end electricl properties of lp joints BSC CM PM SS MP FL cycles R NA Net resin NA 20.7± MWCNT 120 dispersion 2% 19.3± ±10 [3] 100% DI 9.1± ±0.4 (Fig. sbp SI 9.1± ±0.2 6) OSI 8.2± ±0.5 hbp DI 16.5± ±0.5 SI 15.9± ±0.2 50% sbp DI 20.6± ±0.4 (Fig. 6) hbp DI 19.4± ±0.4 Fig. 3 SEM imges of sbp () nd hbp () cross sections produced y DI; circles mrk resin rich regions; scle r 50 m. BSC-ond surfce coverge y the conductive medium; CM-conductive medium; P-MWCNT loding in wt % or BP production method; SSpprent sher strength; FL-ftigue life; R-electricl resistnce 3

4 The most uniform impregntion ws chieved y OSI (Fig. 5), s SWCNT undles wrpped in resin re stcked during the filtrtion forming in one step the impregnted BP. The trdeoff of very uniform impregntion is significntly lower conductivity compred to the BPs produced y DI nd SI procedures (Tle 1). Furthermore, frctured lp joint presented in Fig. 7 evidences the strtified morphology of the sbp (lyer edges highlighted y the dshed lines), while in the cse of hbp the frcture surfce evidences the 3D entnglement (Fig 7) Unexpectedly, the sher strength of lp joints mde with sbps impregnted y SI nd OSI remined the sme s for DI (Tle 2). This suggests tht not the uniformity of the resin impregntion is the determinnt fctor ut BP morphology. BPs mde of SWCNTs disply strtified structure with low strength in the norml direction ecuse of the lck of the CNT entnglement in the norml direction. Hyrid BP displys 3D entnglement tht explins their higher sher strength. Fig. 5 SEM microgrph of sbp cross section produced y OSI; scle r 30 m It is well known tht for lp joints the sher nd peel stresses re mximum t the joint ends, hence the presence of medium with low norml strength t the joint ends will clerly ffect the sher strength. c 2 cm Fig. 6 Frctured lp joints with different BP ptterns Fig. 4 SEM imges of sbp () nd hbp () cross sections produced y SI; scle r 50 m The solution for this prolem is to cover prtilly the overlp re with BP s shown in Fig 6 nd c, therefore the middle prt of the onded re will e the functionl prt of the ond, nd the resin-only strips t the end of overlp re will provide strength. The pprent sher strength of lp joints mde with BP-strips tht cover only 50% of the overlp re is very close to tht of the net resin while the ond

5 Electriclly conductive structurl dhesives sed on uckyppers resistnce is close to tht otined t 100 % surfce coverge (Tle 2). Fig. 7 SEM microgrphs of the frctured lp joint mde of sbp () nd hbp (); scle r 20 m Beside the sher strength, ftigue resistnce is eqully importnt for onded joints. Ftigue test reveled tht joints mde with sbp hve 5 times shorter ftigue life thn those mde with hbp (Tle 2). With high proility the low norml strength of the sbp could e held responsile for this effect. In conclusion hyrid uckypper tht covers frction of the overlp re constitutes the est cndidte for high strength nd highly conductive dhesive with n extended ftigue life. 4. Conclusions A new method, sed on uckyppers, ws developed for otining highly conductive onded joints. Buckyppers mde solely of SWCNTs nd uckyppers mde of mixture of SWCNTs (25 %) nd MWCNTs (75%) were investigted. Direct, solvent nd one step impregntion were used to otin impregnted uckyppers. While direct impregntion is the most economicl it results in inhomogeneous impregntion. However, the results showed tht impregntion homogeneity hve no effect on the sher strength. The results strongly suggest tht the BP morphology is the key prmeter tht determines the sher strength. Becuse of the strtified nture of the sbp the norml strength is much lower thn in the cse of hbp were CNTs form highly entngled 3D structure. The sher strength of the lp joints mde BP tht covers 100 % of the overlp re ws significntly lower tht of the net resin. By optimizing the surfce-frction covered y the BP it is possile to minimize the ond resistnce nd restore the mechnicl performnce. HBps tht covers frction of the overlp re ws identified s the est cndidte for high strength nd highly conductive dhesive with n extended ftigue life. Furthermore, using BPs it is possile to develop dhesive tpes in which the BP ct s conductive nd reinforcing medium s well s spcer tht llows mintining constnt ond-line thickness. References [1] I.D. Rosc, S.V Ho Highly conductive multiwll cron nnotue nd epoxy composites produced y three-roll milling Cron, 2009, 47, [2] I.D. Rosc, S.V Ho Conductive nnocomposites sed on mss produced cron nnotues nd epoxy resin Proceedings ASC 24thTech Conf, Newrk, DE, 2009, CD ROM. [3] I.D. Rosc, S.V Ho Method for reducing contct resistivity of cron nnotue-contining epoxy dhesives for erospce pplictions Composites Science nd Technology, 2011, 71, 95. 5