Electrical, mechanical and adhesive properties of ethylene-vinylacetate copolymer (EVA) filled with wollastonite fibers coated by silver

European Polymer Journal 44 (2008) 3827–3834

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Electrical, mechanical and adhesive properties of ethylene-vinylacetate copolymer (EVA) filled with wollastonite fibers coated by silver V. Cecen a, A. Boudenne b, L. Ibos b, I. Novák c, Z. Nógellová c, J. Prokeš d, I. Krupa c,* a

Mechanical Engineering Department, Dokuz Eylul University, 35100 Bornova/Izmir, Turkey CERTES EA 3481 – Centre d’Etude et de Recherche en Thermique, Environnement et Systèmes, Université Paris 12 Val de Marne, 61 Av. du Général de Gaulle, 94010 Créteil cedex, France c Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 8242 36 Bratislava, Slovakia d Faculty of Mathematics and Physics, Charles University Prague, V Holešovicˇkách 2, 182 00 Prague 8, Czech Republic b

a r t i c l e

i n f o

Article history: Received 27 May 2008 Received in revised form 3 July 2008 Accepted 28 July 2008 Available online 12 August 2008

Keywords: Electro-conductive composites Silver-coated filler Wollastonite Mechanical properties Adhesion Ethylene-vinylacetate copolymer (EVA)

a b s t r a c t New type of electrically conductive polymeric composites was prepared using ethylenevinylacetate (EVA) matrix filled with silver-coated wollastonite (W-Ag) fibers. The electrical, mechanical and adhesive properties of the composites are reported in this paper. The electrical percolation threshold was found about 8 vol.% and the highly electrical conductivity value (1.8  105 S m1) is reached for 29 vol.% of filler fraction. The mechanical and adhesive properties of these composites were also discussed and correlated with some models. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction In order to obtain materials with the desired level of electrical conductivity, polymers are frequently blended with different kinds of fillers, because of the insulating character of most industrial plastics [1,2]. Such materials (antistatic, semi-conductive or conductive) are used in many practical applications as heating elements, temperature-dependent resistors and sensors, self-limiting electrical heaters and switching devices, antistatic materials for electromagnetic interference shielding of electronic devices, etc. [3–5]. High performance electrically conductive materials are formulated by mixing of different polymeric matrices with electrically conductive fillers. Carbon black [6], graphite [7], metals [8], metallized organic/inorganic fillers [9] and * Corresponding author. Tel./fax: +421 2 5477 2300. E-mail address: [email protected] (I. Krupa). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.053

carbon nanotubes [10] are the most frequently used electro-conductive fillers. Metal-coated inorganic fibers are often used as a substitute for metals [11–14]. The advantages are the easiness metallization of convenient substrates, high conductivity, low density and lower prices when compared to metals. Common application of such composites is EMI shielding materials. Among the most frequently metallized substrates are glass fibers, carbon fibers and mica [15]. Metals commonly employed for coatings are silver, copper and nickel. Silver is a very convenient material for metallization, due to its high electrical conductivity, and because its oxide is also conductive and therefore the exposure to the humidity does not significantly change its conductivity contrarily to alumina or copper [16]. Successful coatings of polymeric fillers can bring many advantages, including lower density of final filler, lower price, as well as variability in the shape of the filler [17,18]. An interesting alternative to the metallization of organic or inorganic fillers is

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their coating by conductive polymers. Recently, we reported an improvement of electrical conductivity of composites using cellulose fibers coated by polypyrrole [19]. In previous works, we investigated the complex behavior of high density polyethylene composites filled with silver-coated polyamide particles [18,20]. We have shown from electrical conductivity measurements that the percolation threshold of these composites was found close to 4 vol.% of filler. Composites filled with high filler content were highly electrically conductive and reached the value of 6.8102 S cm1 for 33 vol.% of filler what corresponds to the 12 vol.% of silver [18]. Similarly, Agoudjil et al. [15] reported a significant improvement of electrical conductivity of composites based on the EVA matrix when filled with silver-coated glass spheres. In this paper, we present the study of the electrical conductivity, mechanical properties as well adhesive properties of EVA copolymer filled with silver-coated wollastonite fibers. It is generally known that wollastonite itself is the industrially important mineral filler of the pyroxene mineral group. It is the one from only a few natural fillers which have fibrous or rod-like structure. Due to this morphology, wollastonite is frequently used in plastics applications because increases scratch resistance, thermal stability, welding strength, and decreases warpage and shrinkage [21]. It will be demonstrated that coating of wollastonite fibers by silver lead to the significant improvement of electrical conductivity of composites at low silver content.

2. Experimental 2.1. Sample preparation A commercial ethylene vinyl acetate copolymer (EVA), MiravithenÒ D 18,000 V (Leuna Polymer GmbH, Germany), containing 18 wt.% of vinyl acetate (VA) (a melt flow index 0.5 g/10 min, 190 °C/2.16 kg, melting point = 89 °C, the specific enthalpy of melting = 83 kJ/kg K) was chosen as the matrix, while silver-coated wollastonite was used as the filler. All the composites were prepared by melt mixing in a 30 ml mixing chamber of a Brabender Plasticorder PLE 331 (Germany) at 150 °C for 10 min at a mixing speed of 35 rpm.

2.2. Metallization of wollastonite fibers Metallization of wollastonite fibers was carried out using an electro-less metallization method [22]. Two solutions have been prepared, namely solution A consisting of silver nitride (30 g) dissolved in 1000 ml of distilled water and solution B obtained by dissolving sodium–potassium tartrate (50 g) in distilled water (900 ml). The bath for electro-less deposition of silver was prepared by mixing 100 ml of solution A and 20 ml of solution B at ambient temperature. Wollastonite fibers were coated with silver (reported in the text as: W-Ag) at ambient temperature

for 20 min, and then washed thoroughly using distilled water, filtered and dried at 60 °C in an oven for 12 h. 2.3. Electrical conductivity measurements Room temperature conductivity of composites was determined by a four-point method in a van der Pauw [23] arrangement using a Keithley 237 High-Voltage Source Measurement Unit and a Keithley 2010 Multimeter equipped with a 2000-SCAN 10 Channel Scanner Card. When the conductivity of the samples was below 104 S cm1, a two-point method using a Keithley 6517 electrometer was used. Before these measurements, circular gold electrodes were deposited on both sides of the pellets. DC electrical conductivity of metal-coated filler was measured on the compressed filler particles. The 1-mm thick discs with a diameter of 12 mm were prepared by compression of filler particles at room temperature under 200 MPa. A special six-point method (it simulates the exact fourpoint method in a van der Pauw [23] arrangement in the case that a sample is thin and has large cross-section area), with applied current of 40 mA was used to measure the DC electrical resistance of samples. The voltage was measured by the Solatron Shlumberger 7081 voltmeter. The averaging of measured data was done to suppress an electrical noise. 2.4. Mechanical tests One millimeter thick slabs were prepared by compression molding of the mixed composite using a laboratory press Fontijne SRA 100 (The Netherlands) at 170 °C for 3 min. Dog-bone specimens with a working area of 35  3.6  1 mm were cut from the slabs. The mechanical properties were measured at room temperature using an Instron 4301 (England) universal testing machine at a deformation rate of 10 mm min1 at room temperature. This machine was also used for measurement of the strength of adhesive joints, as discussed later. 2.5. Surface properties The surface properties of composites were evaluated by measurements of contact angles using re-distilled water drops placed on the surface of composites. The contact angles were measured using a Surface Energy Evaluation System (SEE) with CCD camera (Masaryk University, Czech Republic) and software for evaluation the results. The six drops of re-distilled water with a volume 3 ll were placed on the cleaned surface of EVA/W-Ag composite. The arithmetic mean of 6 measurements of contact angles for each kind of composite was used for determination of surface hydrophilicity of EVA/W-Ag composites. 2.6. Peel strength of adhesive joints The peel strength of adhesive joint of melted EVA/W-Ag composites to aluminum alloys was measured by peel test of the adhesive joint at a constant angle 90°. The composition of the aluminum alloy having composition AlMgSi 0.5

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was the following: 98.45-wt.% Al, 0.7-wt.% Mg, 0.5-wt.% Si, 0.2-wt.% Fe and 0.15-wt.% Mn. The dimensions of the aluminum slabs were 220  20 mm. The peel test was done in universal testing machine Instron 4301 (Instron, England) with an additive aluminum peeling wheel at a crosshead speed of 10 mm min1 and a length of the joint equal to 80 mm. The width and thickness of the EVA/Ag-coated wollastonite hot-melt adhesive layer were 20 mm and about 0.1 mm, respectively. The adhesive joints were prepared by fixing two aluminum alloys foils in the form of strips of 20  20  0.2 mm with melted EVA/Ag-coated wollastonite composite in hydraulic press Fontijne SR100 (Fontijne, Netherlands) at 120 °C for 1 min. The 90° peeling of the adhesive joints was performed along the length of adhesive joint. The peel strength of adhesive joint (N m1) was calculated according to equation:

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explained by the formation of a conductive path through the sample in such a way that the conductive particles, which create the path, are in contact at a filler concentration corresponding to the percolation threshold. It is seen from Fig. 2 that the percolation concentration is at around 8–10 vol.% of the filler. In order to be able to determine the percolation threshold more precisely, the percolation point has been arbitrarily identified as the inflexion point in an empirical fitting curve using Eq. (2) [22]:

log





rc ¼ Bð1  expða/f ÞÞn rm

ð2Þ

where P is the mean force of peeling (N) and b is the width of the adhesive joint (m).

where B, a, n are adjustable parameters, rc is the electrical conductivity of the composites, rm is the electrical conductivity of the polymeric matrix and /f is the filler volume fraction. Those parameters were found from the fitting of experimental data with Eq. (2) as follows: B = 17 ± 1, a = 29 ± 9 and n = 11 ± 9. The inflexion point (/i) is calculated according to Eq. (3). This point is identified as the percolation concentration (/c) [22]:

2.7. The specific density measurements

/i  /c ¼

Specific density of the silver-coated fibers was determined at room temperature using pycnometer and their value was found to be 5.7 g cm3, whereas the specific density of neat wollastonite is 2.90 g cm3. Specific densities of the composite samples were determined according to ASTM D153 using decane as the working substance and a Sartorius R160P balance.

Using the above mentioned parameters n and a, the percolation concentration /c = 8 vol.% of the filler, which corresponds to 3 vol.% of silver. The silver volume content in composites (/Agc) was calculated according to Eq. (4):

Am ¼

P b

ð1Þ

3. Results and discussion 3.1. Filler characterization The SEM micrograph of W-Ag is shown in Fig. 1a. It demonstrates fibrous morphology as it is typical for wollastonite. It is also seen that fibers are not continuously covered by compact silver layer, only the deposits of silver on the filler surface were observed. The electrical conductivity of this filler was found to be 4.8  105 S/m. The granulometric parameters of the filer are summarized in Table 1. The diameter of the neat wollastonite is 4 lm whereas the average length is 44 lm. The morphology of the composites was also investigated. In Fig. 1b, we present a SEM of composite filled with 20 wt.% of the filler which demonstrates good homogeneity of the sample. 3.2. Electrical conductivity The experimental values of the electrical conductivity obtained for all samples investigated. The measurements were performed at least two or three times for each sample. The dependence of the electrical conductivity of composites on the filler volume fraction is shown in Fig. 2. The percolation effect is observed in the dependence of conductivity versus filler content and manifests itself as a dramatic increase in conductivity by several orders of magnitude in a rather narrow concentration range of the filler around the percolation threshold [1,2]. This effect is

lnðnÞ a

/cAg ¼ /f /fAg

ð3Þ

ð4Þ /Agf

where /f and are the filler volume fraction and the volume fraction of silver deposited on the wollastonite fiber, respectively. /Agf can be calculated from the specific density of the filler (qf), and of well as its pure components, namely wollastonite (qw) and silver (qAg), according to Eq. (5):

/fAg ¼

qf  qw qAg  qw

ð5Þ

The specific density of silver, wollastonite and the filler are 10.5 g/cm3, 2.9 g/cm3 and 5.7 g/cm3, respectively. Thus, the volume portion of the silver in the filler was found to be 0.37. Obviously, the above mentioned Eqs. (2) and (3) are only empirical formulas. In reality, many different models of electrical conductivity of composites can be found in the literature, however, the most of them have a limited validity [1.2]. The most prominent one, which belongs to the groups of geometrical percolation models, is that of Kirkpatrick’s [28]. In this model, the correlation between the electrical conductivity of composites and the volume portion of the filler is given by Eq. (6):

rc ¼ rf ð/f  /c Þt

ð6Þ

where rc is electrical conductivity of the composites, rf is the conductivity of the filler, /f is the volume portion of the filler, /c is the percolation concentration, t is a parameter determining the power of the conductivity increase above /c (the so called critical exponent). Kirkpatrick

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Fig. 1. (a) SEMs of W-Ag; (b) secondary electrons SEMs of EVA/W-Ag composite (80/20 w/w).

Table 1 Granulometric parameters of particles of the filler D5 lm

D10 lm

D20 lm

D30 lm

D40 lm

D45 lm

D50 lm

D60 lm

D70 lm

D80 lm

D90 lm

2.5

5.0

12.0

18.9

23.6

25.9

28.2

32.7

37.6

43.5

52.5

Di – i% of volume of particles is smaller than dimension Di.

[24] gave the following values for the critical exponent t: t = 1.6 ± 0.1 (for the bond percolation model) t = 1.5 ± 0.1 (for the point percolation model). However, other critical exponents can also be found in the literature. According to Tchmutin et al. [25], the critical exponent for three-dimensional system is t = 1.6–1.9, while the percolation concentration is 0.17. We can estimate the value of percolation concentration /c, the value of critical exponent t and the value of electrical conductivity of the filler by fitting of experimental data with Eq. (6), where all parameters are adjustable. The following parameters were found: rf = (2.1 ± 0.7)  104 S cm1, t = 1.4 ± 0.5, /c = 0.1 v 0.03. R2 = 0.99202 (R is the coefficient of linear regression) It is seen that the prediction of electrical conductivity of the filler is one order lower than the

experimentally found value (4.8  105 S cm1). The estimation of parameters /c and t is in the line with what we would expect. 3.3. Mechanical properties The mechanical properties of EVA/W-Ag composites were characterized using standard measurement in the tensile mode. Young’s modulus, stress at break, yield stress as well as elongation at break were determined from the stress–strain curve. 3.3.1. Elongation at break Recently, we have paid attention to the investigation of the correlation between the dependence of elongation at

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According to an analysis of various sets of experimental data, it seems that there is a close relationship between the formation of an internal network of particles within a matrix, and a decrease in the elongation at break. In the case of conductive fillers, the increase of the network of particles can be characterized in terms of the percolation concentration. For a description of the dependence of elongation at the break versus filler content, recently we have suggested an equation (Eq. (7)) [22], which is able to predict the dependence of elongation at the break versus filler content on the base of knowledge of percolation concentration:



eb;c / ¼ exp  f eb;m /c Fig. 2. Dependence of the electrical conductivity (squares) upon filler volume fraction; straight line represents the fit according to Eq. (2).

break in the filler content and the electrical conductivity of those composites [18,22]. We have demonstrated a reasonable correlation between the increase in electrical conductivity and the decrease in the elongation at the break, as a consequence of the internal network of filler formation. The dependence of the elongation at break upon filler content in composites is shown in Fig. 3. The dependence clearly demonstrates a drop in drawability after addition of the filler. A decrease in elongation at break of polymers, filled with inorganic fillers, is obvious and always observed. A few models, which describe this behavior, were reported in the literature [26]. They assumed homogeneous distribution of the particles, and regular (usually spherical) shape and homogeneous drawing. The most well known model, describing the decrease in elongation at the break with an increase in filler volume fraction, is the Nielsen model [27]. This model is valid for composites filled with particles having a spherical shape and assuming perfect adhesion between the phases. Many types of fillers obviously do not fulfill these conditions.

Fig. 3. Dependence of elongation at break (squares) upon filler volume fraction; straight line represents the model defined by Eq. (7).

 ð7Þ

where eb;c and eb;m are the elongation at break of composites and the matrix, respectively. The exclusive parameter is the percolation concentration of the filler (/c). As seen in Fig. 3, the Eq. (7) excellently correlates with the experimental data. 3.3.2. The Young’s modulus The dependence of Young’s modulus on the filler content is shown in Fig. 4. It is clear from Fig. 4 that the stiffness of EVA/W-Ag composites, characterized by the Young’s modulus increases with an increase in the filler content in the whole concentration region. The maximum value of 171 MPa for the specimen filled with 29 vol.% of W-Ag is about five times higher than the one of the neat EVA. As it will be discussed later, the experimental data can be appropriately described by Nielsen model up to 20 vol.% of the filer. It is a common knowledge that the improvement in Young’s modulus is an expected consequence of the reinforcing effect of the inorganic filler. It has been established in the scientific literature that the improvement of tensile modulus depends not only on the Young’s modulus of components but also is influenced by the good dispersion of particles and good interfacial adhesion between filler and the matrix [27–29].

Fig. 4. Young’s modulus of composites (squares) and the models given by Eqs. (8 a), (8 b) and (8 c) vs. volume fraction of the filler.

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The need for a control and prediction of the filled polymers leads to a wide effort for a better estimation of their physical properties, knowing only the initial properties of their components, i.e. to determine their constitutive relations. A lot of models have appeared in scientific community [30,31] for the description of Young’s modulus of a material filled with various filler, but their validity has a limited application range. One of the most prominent models is Nielsen and Lewis model [27,32,33], which can be generally applied for different properties of composites such as Young’s modulus and also thermal and electrical conductivity of composites [34,35]. The model can be expressed by the set of Eqs. (8 a), (8 b) and (8 c):

Ec ¼ Em

B¼

1 þ AB/f 1  Bw/f

ð8aÞ

ðEf =Em Þ  1 ðEf =Em Þ þ A

w¼1þ

1  /m /2m

ð8bÞ ! /f

ð8cÞ

where Ec, Ef and Em are the Young’s modulus of the composite, filler and matrix, respectively, /f is the filler volume fraction, /m is the maximum packing fraction of the dispersed phase, A and B are constants, and w is the factor that enables the use of a reduced concentration scale to take into account the maximum packing fraction of the filler. The correct application of Nielsen model significantly depends on the careful selection of both parameters, A and /m. The values of these two parameters depend on the shape, geometry and orientation of the filler within the matrix [35]. In the respect to the present work, the values /m = 0.62 is reported for composites filled with randomly dispersed wollastonite fibers [35]. On the other hand, it must be recalled that this value is valid only for fully amorphous matrix. In the case of semicrystaline matrix, the crystalline part is not available for the distribution of the filler and therefore the above mentioned value must be recalculated considering only amorphous part of polymer. The degree of crystallinity of pure EVA was determined by DSC and has a value of 28 wt.% [36]. From the knowledge of the specific density of pure EVA (qEVA = 0.972 g/cm3 [36]) and from the density of crystalline phase of polyethylene’s component of EVA copolymer (qx = 0.997 g/cm3 [37]) we can estimate the volume portion of the crystalline phase of EVA (/x) from Eq. (9):

/x ¼

qEVA wx qx

polymeric continuous phase. The typical values of Young’s modulus for wollastonite vary from 303 to 530 GPa [21]. The Young’s modulus for EVA was determined to be about 33 MPa. We can see from Fig. 4 that the values of the Nielsen equation largely coincide with experimental data up to 20 vol.%. 3.3.3. The stress at break and yield stress The dependencies of the yield stress and the stress at break are shown in Fig. 5. It is seen that whereas the stress at break of composites significantly decreases with an increase in the filler content, the yield stress slightly increased (from 5.1 MPa for neat EVA to 7.4 MPa for composite filled with 60 wt.% (20 vol.%) of the filler. From stress–strain curves it is visible that the yielding occur up to 60 wt.% (20 vol.%) of the filler content. It indicates relatively week interfacial adhesion between polymer and filler. It is known that if polymer matrix-filler adhesion is poor, yielding is always observed, similarly as in the pure matrix [27]. In our case, the rupture before yielding was observed only for the highest filler content (70 wt.%). It is caused by steric hindrances of the filler which suppress the mobility of polymeric chains. However, an increase of the stress at break with an increase in filler content indicates the reinforcing effect of the fibers. On the other hand, the fibers should have at least the critical length, to be able to insure sharing of the force between fiber and matrix [26]. Aspect ratio has significant influence on stress at break. As reported in literature, the minimum aspect ratio should be at least 10 [26]; whereas average aspect ratio of wollastonite fibers used in this work is 11. As for the stress at break of composites, if interfacial interaction between polymer and filler is low, stress at break of composites should decrease where increasing the filler content. Nevertheless, if interfacial interaction between polymer and filler is sufficient (due to the use of coupling agents or due to an inherent activity of the filler), the stress at break dependence versus filler content has a increasing tendency or exhibits a minimum at a particular filler content. The increase in stress at break when filler

ð9Þ

Thus, the volume portion of crystalline phase in EVA equals 0.27. The amorphous volume portion is 0.73 and the maximum volume fraction of the filler (/ma) in EVA can be estimated as /ma = /x /m = 0.45. Concerning parameter A, according to the literature, we chose the value A = 4.93 which the best characterizes the fibers with aspect ratio equals to 11 [35]. The Eq. (8 b) indicates that B is close to 1 because the wollastonite is much more rigid material than the soft

Fig. 5. Stress at break (squares) and yield stress (circles) vs. filler volume fraction.

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content reaches some specific level is caused by a reinforcing effect of the active fillers. In our case, relatively short fibers which do not sufficiently interact with the matrix are not able to reinforce the composite. So, the main reason that the stress at break of composites decreases with the increase of filler contents is caused by a significant suppression of the orientation strengthening that is a common behavior of semi-crystalline polymers at high deformation. The filler presence influences negatively the orientation of the polymeric chains which induce the decrease of the deformation so that orientation of the matrix cannot occur [26]. Initially very ductile EVA co-polymer is becoming less and less deformable, loosing at first its ability of orientation hardening and finally, when the composite is filled with 70 wt.% of the filler, the material becomes brittle, without any yield point. Fig. 7. Dependence of the peel strength upon filler volume content.

3.4. Surface properties The dependence of contact angle of re-distilled water drops deposited on the surface of EVA/Ag-coated wollastonite vs. content of the filler is shown in Fig. 6. The presence of the Ag-coated wollastonite in EVA matrix leads to decrease of the contact angles due to growth of hydrophilicity on the surface of composites in the whole concentration range. 3.4.1. Strength of the adhesive joint The strength of the adhesive joints of EVA/Ag-coated wollastonite to aluminum was measured by 90° peel test. The results of peel test are illustrated in Fig. 7. The strength of the adhesive joint of EVA/Ag-coated wollastonite composites decreases when increasing filler content, i.e. from 319.3 N m1 for the pure EVA to 238.2 N m1 when filled with 70 wt.% of W-Ag. This fact is caused by a decrease of EVA content in the composites what results in the worst wettability of the tested aluminum foils. 4. Conclusions It was found that the composites based on the EVA copolymer filled with W-Ag became electrically conductive

above a filler volume fraction of 8%. It corresponds to the 3 vol.% of silver content. Composites filled with high filler content were highly electrically conductive and reached the value of 1.8  105 S m1 for 29 vol.% of fillers. The dependence of elongation at break on the volume filler fraction demonstrated a drop in drawability after addition of the filler. The experimental data are in excellent agreement with semi-empirical model define by a simple exponential decay given by Eq. (7). It was found that the stress at break of composites significantly decreases when the filler content increases, whereas the yield stress slightly increased (from 5.1 MPa for neat EVA to 7.4 MPa for the composite filled with 60 wt.% (20 vol.%) of fillers. From stress–strain curves it is visible that the yielding occurs up to 60 wt.% (20 vol.%) of fillers. This indicates relatively week interfacial adhesion between polymer and fillers. A decrease of contact angle of water on the EVA/W-Ag surface as a function of content was observed in all cases. A decrease in the strength of the adhesive joints to aluminum for increased in filler content was also found. Finally, a decrease of peel strength of adhesive joint of EVA/W-Ag composites to aluminum was observed due to reducing of wettability and adhesion of melted EVA-based composite after deposition on the aluminum surface. All the above mentioned results indicate that the EVA copolymer filled with W-Ag is a material with high electrical conductivity and appropriately good mechanical and adhesive properties. This behavior may enable to employ this kind of composites in various applications, for example as electrically conductive hot melt adhesives. Acknowledgements

Fig. 6. Dependence of the contact angle upon filler volume content.

The research was supported by the Scientific Grant Agency of the Ministry of Education of Slovak Republic and the Slovak Academy of Sciences (Project No. 2/6114/ 26). The Scientific and Technological Research Council of Turkey TUBITAK is acknowledged for the granting of V. Cecen postdoctoral study in the framework of TUBITAKBIDEB 2219-International Postdoctoral Research Scholarship Program. This work is also a part of the research plan

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