In this research, producing a particulate conductive composite is being investigated.

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1 Iranian Plymer Jurnal 13 (2), 24, Strain Sensrs Based n Graphite Fillers Salumeh Mshfegh and Nadereh Glshan Ebrahimi * Department f Plymer Engineering, Tarbiat Mdarres University, P.O. Bx: Tehran, I.R. Iran ABSTRACT Received 18 May 23; accepted 8 December 23 In this research, prducing a particulate cnductive cmpsite is being investigated. The cmpsites are made f ethylene-vinyl acetate () and plyvinyl chlride (PVC) as matrix and graphite as filler. There are tw different prcedures applied: disslving the plymer matrix in a suitable slvent and then add the filler t the slutin, which des nt wrk ut fr PVC because phase separatin takes place between the melt mixing f the matrix and filler. The slvents used were tluene fr and THF fr PVC. The cmpsites were prepared in different vlume fractins. After having the cmpsites prepared they were cmpressin mulded int the shape f dumbbells when tw extreme sides were cvered with a cnductive cating. Electrical resistance f the samples befre and during applying the strain was measured. Then, gauge factr (strain sensitivity factr) fr each sample was calculated and finally the sample with higher gauge factr was chsen. Key Wrds: strain sensr; graphite; piezresistivity; electrical resistance; cnductive cmpsite. INTRODUCTION (*)T whm crrespndence shuld be addressed. ebrahimn@mdares.ac.ir Any device measuring the dimensinal changes f an bject due t mechanical r thermal stresses r cmbinatin f bth is called a strain gauge r strain sensr [1]. Strain sensrs have varius applicatins, such as cntrlling systems f different structures (e.g., bridges, buildings, dams, and cntainers), calibrating the accuracy f the labratry devices which measure mechanical prperties f material, and ttally any ther applicatin in which measuring and cntrlling

2 Strain Sensrs Based n Graphite Fillers Mshfegh S. et al. the strain is cnsidered [2,3]. Strain sensrs are divided int fur grups; ptical fibre sensrs, shape memry allys (SMA), piezelectric ceramics and electrmechanical strain gauges [2]. Plymer cmpsites are in the electr-mechanical strain sensr grup. Piezresistivity is ne f the bases f strain measurement in these sensrs. Generally, any material in which the electrical resistance is a functin f internal strain is called piezresistive. The mst cmmn piezresistive material is cmpsites cntaining a plymer matrix and a cnductive filler [1]. Mstly used fillers include metal pwder, carbn fillers, semi-cnductive metal xides (e.g., V 2 O 3, TiO 2 ) r nn-cnductive pwders with their surface mdified using different methds [3-8]. Amng them carbn fillers are used mstly in rder t achieve cmpsites with desirable mechanical prperties, crrsin resistant, light weight and cnductive. Carbn black (CB), graphite (G) and carbn fibres (F), that differ frm each ther structurally, are in this categry [5,8]. In these sensrs any plymer with suitable elasticity and tughness can be used [3]. As the defrmatin f matrix and fillers is nt in the same scale during the applied stress, the strain induced, increases the distance between the filler particles dispersed in the matrix and results in piezresistive effect [9]. Strain sensitivity r gauge factr is defined as the aspect f electrical resistance value t dimensinal changes: G = (dr / R) /(dl / L) = ( R / R) / ε (1) In these sensrs the resistance changes due t the strain is nn-linear. Besides they d nt shw much sensitivity t small values f strain, therefre, they are nt applicable in such cases [1,3,9,1]. Electrical resistance f a cnductive cmpsite depends n the filler vlume fractin. At a distinct temperature an insulatr-t-cnductr transitin ccurs in the cmpsite. When the filler cntent is very lw, the electrical resistance f the cmpsite is very similar t the matrix s, but in a certain vlume fractin, the resistance reduces suddenly, which is called perclatin threshld [8,9]. After that pint, the electrical resistance reduces until the vlume fractin reaches its critical value, at this pint all the fillers particles are cvered with a very thin layer f matrix, and adding anther filler particle is similar t adding a third part t the system and can cause misleadings in the trend f changes in the prperty f the cmpsite [11]. These als include electrical prperties. This critical value, named critical perclatin vlume cncentratin (CPVC), can be calculated using the il absrptin (OA) value f the filler [11,12]. CPVC = 1 ρ ply 1+ ( OA) (2) Where, ρ ply is the plymer density and OA is defined as the grams f il in 1 g f pwder which can make the paste-like particle [11,12]. Sme f the factrs affecting the electrical resistance f the cmpsite are: shape and dimensins f the filler particles. Fr instance in the spherical fillers, smaller diameter particles cause lwer perclatin threshld, r fr the fillers in which L/D rati is mre than ne, mre extensive distributin values f L/D result in the reductin f perclatin threshld. Wetting is anther affecting parameter n the perclatin threshld, which depends n the difference between surface energies f the matrix and filler. Better wetting f filler particles using plymer matrix, reduces the cnnectin between particles, and causes a reductin in the ttal resistance and als in the perclatin threshld f the cmpsite. That is why a difference in surface energies f the plymer and filler seems desirable [8]. Piezresistive Mdel When the stress is applied t the cmpsite, its electrical resistance changes due t the change f distance between particles. Accrding t the mdel intrduced by Zhang et al. [9,1], assuming that the distance between tw particles changes frm S t S, relative resistance (R/R ) is calculated by: R / R = (S / S ).exp[ γ(s S )] (3) Where, R is the initial resistance and S is the initial distance between particles, and γ is defined as: γ = ( 4π / h) (2mϕ) (4) Where, m is electrn mass; h, the Planks cnstant; and ϕ, the ptential barrier height [9,1]. As the mdulus f plymer is much less than the 114 Iranian Plymer Jurnal / Vlume 13 Number 2 (24)

3 Mshfegh S. et al. Strain Sensrs Based n Graphite Fillers mdulus f the filler, the defrmatin f filler particles under strain can be ignred. S, the changes ccurring in the distance between tw particles acrss the electrical cnductin path can nly be CPVC= due t the defrmatin f plymeric matrix. S the distance S after uniaxial cmpressin wuld be: S= S (1 ε) = S (1 σ / M) (5) 15 mm 3 mm 15 mm 8 mm and if the stress is extensinal, then: S= S (1 + ε) = S (1 + σ / M) (6) where, ε is the matrixs strain, σ is the stress being applied and M is the plymer mdulus. Assuming that the filler particles dispersin has a cubic lattice array, the initial distance between tw particles, S, wuld be: Relative resistance, the mst imprtant parameter in piezresistive effect, will be achieved by replacing eqns (6) and (7) in eqn (3): Using eqn (8), the effect f stress (σ), cmpsite mdulus (M), filler particles diameter (D), filler vlume fractin (θ) and ptential barrier height (ϕ), n the piezresistivity can be investigated [9,1]. EXPERIMENTAL S = D[( π / 6) 1 / 3 θ 1/ 3 1] (7) R / R = (1 + σ / M) exp[ γd[( π / 6) 1 / 3 θ 1 / 3 1] ε] (8) The materials used in this research are as fllwing: ethylene vinyl acetate () with 18 wt% f vinyl acetate, melting pint 88C, and density.9 g/ml, plasticized plyvinyl chlride (PVC), with 5% DOP (plasticizer) and density 1.14 g/ml, graphite pwder with particle size f micrns, and density 1.25 g/ml. In this research, the prductin f cmpsites using bth and PVC matrices by disslving and melt mixing prcedures was desired, but because f the existence f DOP in plasticized PVC, the prepared slutin prepared separated int tw phases and s the slutin mixing prcedure was nt applicable in this case. Figure 1. Dumb-bells mulded frm the cmpsites [3]. In slutin mixing prcedure, was disslved in tluene at 75C s that a 35 wt% cncentratin slutin was btained. Then the calculated amunt f graphite was added, and it was stirred at a rate f 4 rpm rate fr 1 h. Then the mixture was transferred int tefln pans s that their slvent cntent wuld be evaprated after 24 h. After that samples were cmpletely dried in the ven, at the temperature f 115C fr 16 h. The samples shwed n electrical cnductance befre being dried in the ven. Then the dried cmpsites were chpped and cmpressin mulded int the shape, shwn in Figure 1. In the melt mixing prcedure, mixing was dne in a Brabender internal mixer at 15C fr and 19C fr PVC, fr 3 min. Then the mixtures were als cmpressin mulded int the shape f Figure 1. The cmpsitins f the prepared mixtures are described in Tables 1-3. After preparing dumbbells accrding t Figure 1, the specified areas n the Figure, were cvered with a cnductive cating (which culd be a silver paint, r aluminium fil, etc.). It shuld als be mentined here that samples EG1-M and EG1-S were rejected because f n cnductin and sample EG6-S was als rejected because f nt having enugh mechanical strenghth. Table 1. Cmpsitin f samples (slutin mixing). EG1-S EG2-S EG3-S EG4-S EG5-S EG6-S Cmpsitin (/graphite) 65/35 6/4 55/45 5/5 45/55 4/6 Graphite Iranian Plymer Jurnal / Vlume 13 Number 2 (24) 115

4 Strain Sensrs Based n Graphite Fillers Mshfegh S. et al. Table 2. Cmpsitin f samples (melt mixing). EG1-M EG2-M EG3-M EG4-M EG5-M EG6-M Cmpsitin (/graphite) 7/3 65/35 6/4 57.5/ /45 5/5 The electrical resistance f the samples, befre applying the strain and als during that, was measured using a digital multi-meter (KAISE, SK 611). Extensin tests were dne by an Instrn 4466 extensmeter. RESULTS AND DISCUSSION Table 3. Cmpsitin f PVC samples (melt mixing). PG1-M PG2-M PG3-M Cmpsitin (/graphite) 4/ / / Graphite The initial values f the samples are shwn in Figure 2. As it is shwn in Figure 2, the perclatin threshld fr cmpsites (slutin mixed) is arund vlume fractin f 35-4; (melt mixed), arund 3-35; and PVC, arund The ther pint in the graphs f Figure 2 is anther increase in the resistance after perclatin. This increase is in vlume fractin f 5 fr samples and 45 fr PVC. It can be said that the mentined vlume fractins are abut the critical value (CPVC), calculated theretically using eqn (2), after measuring OA f graphite which is 17.9 g. The values fr CPVC f the cmpsites are shwn in Table 4. Cmparing the SEM micrgraphs f EG5-M and Graphite Vlume fractin (%) (a) PVC Vlume fractin (%) (b) Figure 2. Resistance vs. filler vlume fractin in cmpsite (a), slutin (b), and PVC melt. EG6-M (Figures 3 and 4) shws a fluffy structure in EG6-M sample, where EG5-M has mre cntinuus structure. This can further prve the reasn why the resistance increases in vlume fractin f 5. Figure 5 shws the electrical resistance vs. strain graphs in cmpsites. T plt these graphs, three dumbbells f each cmpsitin were tested, and the mst suitable curve was fitted with the data. Figure 6, shws Table 4. CPVC Value fr cmpsites f PVC and matrix and graphite filler. Matrix CPVC (%) PVC Figure 3. SEM Micrgraph f EG6-M,1x. 116 Iranian Plymer Jurnal / Vlume 13 Number 2 (24)

5 Mshfegh S. et al. Strain Sensrs Based n Graphite Fillers Figure 4. SEM micrgraph f EG5-M,1x (a) (b) Figure 5. Resistance vs-strain curves: (a) slutin; (b) melt; and (c) PVC melt. (c) Figure 6. Expnential behaviur f resistancevs-strain curve fr sample EG3- M. The equatin is related t fitted curve is: R = exp (.5885 ε) Vlume cncentratin (%) Figure 7. Gauge factr vs vlume fractin fr cmpsites(melt mixing) Vlume cncentratin (%) Figure 8. Gauge factr vs. vlume fractin fr cmpsites (slutin mixing). all three series f data in additin t the fitted curve fr sample EG3-M. It is bvius in Figure 5, that the general trend f resistance changes due t strain is expnential, that was expected accrding t references [5,17,18, 2]. Figures 7-9 shw the values f gauge factr (G) vs. Iranian Plymer Jurnal / Vlume 13 Number 2 (24) 117

6 Strain Sensrs Based n Graphite Fillers Mshfegh S. et al Vlume cncentratin (%) Figure 9. Gauge factr vs. vlume fractin fr PVC cmpsites(melt mixing) Table 5. Theretical equatins describing piesresistive behaviur. Particle size(mic.) Equatin EG3-M R/R = (1 + ε) exp (3.7 ε) R/R = (1 + ε) exp (39.82 ε) EG4-M R/R = (1 + ε) exp (39.12 ε) R/R = (1 + ε) exp (39.6 ε) EG5-M 25 R/R = (1 + ε) exp (59.7 ε) 38 R/R = (1 + ε) exp (6.11 ε) EG6-M 25 R/R = (1 + ε) exp (8.62 ε) 38 R/R = (1 + ε) exp (6.4 ε) EG2-M 25 R/R = (1 + ε) exp (2.19 ε) 38 R/R = (1 + ε) exp (2.2 ε) EG3-M 25 R/R = (1 + ε) exp (53.24 ε) 38 R/R = (1 + ε) exp (53.22 ε) EG2-S 25 R/R = (1 + ε) exp (43.79 ε) 38 R/R = (1 + ε) exp (42.88 ε) EG3-S 25 R/R = (1 + ε) exp (54.89 ε) 38 R/R = (1 + ε) exp (45.77 ε) EG4-S 25 R/R = (1 + ε) exp (16.89 ε) 38 R/R = (1 + ε) exp (17.67 ε) Figure 1. Piezresistive mdel check fr EG5-M sample. vlume fractin f the filler fr each cmpsite( is the middle pint). The values f (G) were calculated using the data f Figure 5 and eqn (1). Assuming a middle pint at each vlume fractin, shws that EG5-M has the highest value and s this is the ptimized sample. Finally, t check the piezresistive trend f the samples with the mdel explained by eqn (8), γ values where calculated, using the electrical resistance and strain values at each pint fr all data series f each cmpsitin. Then the average values f γ were substituted in eqn (8) and the theretical equatin achieved was pltted fr each sample. It shuld be nticed that as the filler particles sizes change in the range f 25-35µ, the theretical graph was pltted twice fr each sample. Cmparing the theretical graphs with experimental data it was shwn, that sample EG5-M has the best fitting with the mdel. This is shwn in Figure 1. The theretical equatins gained accrding t eqn (8) are shwn in Table 5. CONCLUSION The samples with the vlume fractin f 4 t 6 filler cntent were prepared. The electrical resistance f the samples shwed anther increase, after perclatin vlume fractin, arund 5% fr and 45% fr PVC matrices. The cmpsites prepared by disslving mixing prcedure had much lwer electrical resistance than the samples prepared by melt mixing. This is because the shear applied in melt mixing prcedure results in better dispersin f filler particles in the matrix which reduces the pssibility f cnductive netwrk frmatin. Investigating the graphs f electrical resistance vs. strain apprves an expnential behaviur. Cmparing these graphs with the piezresistive mdel, it can be shwn that the samples EG5-M and PG3-M are best fitted with the mdel. Whereas, the sample EG5-S des nt fit at all. The sample EG5-M was recgnized t be the best cmpsitin t be used as strain sensr, because it has the mst gauge factr (G). REFERENCES 118 Iranian Plymer Jurnal / Vlume 13 Number 2 (24)

7 Mshfegh S. et al. Strain Sensrs Based n Graphite Fillers 1. Chung, Methds and sensrs fr detecting strain and stress, U.S.Patent, N , June 27 (2). 2. Gandhi M.V. and Thmpsn B.S., Smart Materials and Structures, St.Eedmandsburg., Chapman & Hall, 1st. ed., Chap. 2 (1992). 3. Kimura T. and Fujisaki T., U.S. Patent, N , Aug. 21(21). 4. Harsanyi G., Plymer Films in Sensr Applicatins, Technmic Publishing Cmpany, Chaps. 1, 6 & 7 (1995). 5. Zu J.F., Yu Z.Z., Pan Y.X., Fang X.P., and Ou Y.C., J. Plym. Sci., Part B: Plymer Physics, 4, (22). 6. Wang X. and Chung D.D.L., Sensrs and Actuatrs (A), 71, (1998). 7. Su K.P., Chaki T.K., and Khastgir D., Cmpsites, Part A, 29A, (1998). 8. Clingerman M.L., King J.A., Schulz K.H., and Meyers J.D., J. Appl. Ply. Sci., 83, (22). 9. Zhang X.W., Pan Y., Zheng Q., and. Yi X.S., J. Plym. Sci., Part B: Plymer Physics, 38, (2). 1. Zhang X.W., Pan Y., Zheng Q., and Yi X.S., Plym. Int., 5, (21). 11. Patn T.C., Paint Flw and Pigmen Dispersin, 2nd. ed., Chap. 6, Jhn Wiley (1979). 12. Brk T., Grteklaes M., and Mischke P., Eur. Cat. Handbk, Curt R. Vincentz, Verlag, Hannver, 251 (2). Iranian Plymer Jurnal / Vlume 13 Number 2 (24) 119