Hotmelts with improved properties by integration of carbon nanotubes

Author’s Accepted Manuscript Hotmelts with improved properties by integration of carbon nanotubes Franziska Wehnert, Petra Pötschke, Irene Jansen

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S0143-7496(15)00093-7 http://dx.doi.org/10.1016/j.ijadhadh.2015.06.014 JAAD1675

To appear in: International Journal of Adhesion and Adhesives Received date: 22 May 2014 Accepted date: 23 June 2015 Cite this article as: Franziska Wehnert, Petra Pötschke and Irene Jansen, Hotmelts with improved properties by integration of carbon nanotubes, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2015.06.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Title:

Hotmelts with improved properties by integration of carbon nanotubes

Author/Membership:

Franziska Wehnert1, Petra Pötschke2, Irene Jansen3

1

Technische Universität Dresden, Institute of Manufacturing Technology

2

Leibniz-Institut für Polymerforschung Dresden e.V.

3

Fraunhofer-Institut für Werkstoff- und Strahltechnik (IWS) Dresden

Contact details:

Irene Jansen [email protected]. Phone: +49 351 83391 3017 Fax: +49 351 83391 3210

Fraunhofer IWS Dresden, Winterbergstraße 28, D-01277 Dresden, Germany

Keywords A: carbon nanotubes, hotmelt, B: composites, C: rheology, D: mechanical properties of adhesives

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Abstract With the aim of the development of conductive and mechanically improved adhesives, carbon nanotubes (CNTs) were dispersed by melt mixing into a non-reactive polyolefine based hotmelt adhesive. The composite materials, containing 0.5 to 5.0 wt% multi-walled CNTs (MWNTs), showed electrical percolation at about 0.75 wt%. Investigations of the mechanical properties using tensile tests resulted in a significant enhancement of Young’s modulus up to 372 % and nearly doubling of tensile strength at 5.0 wt%. Even if the hotmelt material is highly elastic compared to typical thermoplastic matrices, the melt mixing resulted in suitable CNT dispersion. The melt viscosity increased with CNT loading, however near the observed electrical percolation threshold the processability was not notably reduced. Most important, next to conductivity at low CNT loadings, also a significant enhancement in the shear strength of bonded joints of AlMg3 up to values of 250 % of the pure hotmelt could be obtained. The property profile can be tailored with CNT concentration, indicating the suitability of CNT addition into these hotmelt adhesives.

1

Introduction

Adhesive bonding as an energy efficient, effective and versatile joining technology offers new possibilities for the bonding of not easily weldable and traditionally non compatible items. It is one of the few solutions for bonding of hybrid materials. Furthermore there are many possible applications for adhesive bonding in highly advanced technology, e.g. lightweight construction. In this field the textile technology exhibits one of the very interesting sections where the integration of different functions within the textile is a challenging problem. These issues also prompt a need for further development of antistatic or conductive adhesives combined with mechanical stability as well as an improvement of adhesion. There are special adhesives designed for bonding of different substrates, e. g. epoxies, acrylates, polyurethanes

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and hotmelts. A distinction can be made between chemical and physical curing materials. Hotmelts are solvent-free one-component adhesives setting physically. This material class belongs to the group of thermoplastics, which consist predominantly of a linear chain like structure and exhibits amorphous or semicrystalline states. They may have different polymers or copolymers as a basis e. g. polyamides, saturated polyesters or ethylene-vinyl-acetatecopolymers. The polymer essentially determines the quality of the bond line regarding to adhesion, strength and temperature behaviour. The advantage of amorphous polyolefines as base materials for hotmelt adhesives derives from the saturated nature of these polymers exhibiting very good heat resistance of the melt during processing, low viscosities of the melt and advantageous adhesive strengths even when used to bond difficult combinable substrates. Because of these advantageous properties, amorphous polyolefines are used as the basis of pressure sensitive and contact adhesives.[1] To influence other important properties such as cohesive strength, viscosity and softening temperature further modifications to the adhesive are often necessary. [2]. Recent studies have considered the effects of inorganic fillers on the properties of adhesives. Commercial electrically conducting adhesives are often filled with silver particles. A combination of silver flakes and spherical silver particles have been shown to improve both thermal and the electrical conductivity, but result in a decrease in shear strength with increasing silver concentration.[3-5] The usage of carbon nanotubes (CNTs) as filler materials is founded in their characteristical properties, e.g. their highly advantageous electrical, mechanical and thermal properties [6-12]. The advantage of using CNTs as the reinforcing phase in nanocomposites arises from an improvement of composite properties at significantly lower filler levels than would be the case with conventional micron-sized filler materials. The addition of nanomaterials into polymers to produce composite materials significantly influences mechanical and barrier properties, flame retardancy and electrical conductivity. To achieve the reinforcing properties a preferably homogenous dispersion of the nanofiller within the -3-

polymer is required.[8,13-15] Different authors have reported on the behaviour of CNTs in polymeric matrices and about their tendency to remain in micrometer sized bundles and agglomerates.[11,16-18] Mechanical tests have shown that the existence of primary agglomerates of filler material negatively affects the mechanical properties of the composites since they act as imperfections. In the case of a homogenous dispersion an improvement of mechanical properties can be achieved [19]. At a particular concentration an increase in CNT dispersion can also improve electrical conductivity [15,20-26]. Recent studies have reported the effects of introducing CNTs into epoxy resins and other polymer types typically employed as the basis of adhesives such as polyurethanes and acrylates. Inserting carbon nanotubes into adhesives has thereby improved mechanical and electrical properties of the adhesives.[2,10,11,22,27,28] The aim of this study was the enhancement of the properties of a commercially used nonconductive hotmelt by incorporating multi-walled carbon nanotubes (MWNTs). Due to the addition of CNTs antistatic or conducting adhesives were expected which at the same time should exhibit improved tensile and shear strength properties. The incorporation was achieved by melt mixing in small scale and the processability of these modified hotmelts was studied using melt rheology. These composite materials have the potential for achieving better cohesion and adhesion towards textiles, and to obtain flexible and more stable bonded components. The possibility of integrating electrical properties into the adhesive formulations further establishes the development of new functional applications. These adhesives are for example suitable for electrostatic discharge of fixed carpets where the prevention of electrostatic charging of people is required.

2

Materials and methods

2.1 Materials

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Multi-walled carbon nanotubes (MWNTs, trade name: Baytubes® C 150P) with a purity of 95% (Bayer MaterialScience AG, Leverkusen) were employed. This kind of nanotubes is synthesised in a high-yield catalytic process based on chemical vapor deposition and has, according to the supplier’s data sheet, 3-15 walls, outer diameters in the range of 5-20 nm, inner diameters between 2 and 6 nm, and lengths in the range between 1 and > 10 µm. The MWNTs were incorporated into a granular hotmelt adhesive based on an amorphous poly-α-olefine featuring a high viscosity with the commercial description APAO 230.00 (Jowat AG, Detmold). The hotmelt used in this work consists of an amorphous poly-αolefine, synthesised via reactions with organic metal complexes. The basic structure is shown in Figure 1. The materials were predried at room temperature. H

R1

*

* H

R2

n

Figure 1. Chemical structure of the used hotmelt: poly-α-olefine, with R1=H and R2 as a linear or branched saturated aliphatic chain

2.2 Preparation of the MWNT/hotmelt composites Polymer nanocomposites containing 0, 0.5, 0.75, 1.0, 2.0, 3.0 and 5.0 wt% of MWNTs were prepared by melt mixing using a small-scale conical twin-screw microcompounder DSM Xplore 5 with 5 ml capacity at 130°C,400 rpm, for 10 min. For comparison, the unfilled hotmelt was treated under the same conditions. The temperature was selected to enable a relatively low viscosity for good wettability of the polymer on the nanotube surfaces but prevent thermal or oxidative degradation of the polymer. For electrical and mechanical testing the extruded strands were compression molded to plates of 60 mm diameter with 0.5 mm thickness using a Weber press at 130°C and a contact pressure of 50 kN maximum for 30 s. 2.3 Characterisation

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The electrical volume resistivity was measured at room temperature using thin sheets prepared by compression molding and cleaned with ethanol prior to testing. For samples with high resistivity (>107 Ohm⋅cm), the Resistivity Test Fixture 8009 (Keithley) was used in combination with the Keithley electrometer 6517A and pressed plates of 60 mm diameter were used. For higher conductive samples strips cut from the plates, with dimensions of 30x3 mm2, were investigated using a four point test fixture combined with a Keithley DMM 2000 electrometer. The measurement principles are based on the ASTM standards D 257 (insulating materials) and D4496 (moderately conductive materials). The measured resistivity values were converted into conductivity (reciprocal values). Tensile tests were carried out on small dogbone specimens at room temperature using UPM zwicki 2.5 (Zwick GmbH & Co KG) in accordance to the test standard DIN EN ISO 527-2/S2/20 with a testing of 20 mm/min and a testing speed of 1 mm/min for the determination of Young´s modulus. Samples were prepared by stamping five test pieces corresponding to the specimen 5B. One selected characteristic curve is shown in Fig. 3 for each composition, whereas Table 1 gives mean values and standard deviations. For investigation of the shear strength of the MWNT/hotmelt materials versus AlMg3 (pretreated by sand blasting, SACO Rocatector® Delta) the nanocomposite was pressed into onto plates of 300 µm thickness (press: Collin P300, at 130°C and 50 bar for 5 min), placed between the adherends, fixed and bonded at 150°C. Shear strength tests were performed using a Z 050 (Zwick GmbH & Co KG) in accordance with the test standard DIN EN ISO 1465. The quality of the MWNT dispersion was observed on surfaces of the pressed plates by SEM using the charge contrast mode (UltraPlus, Zeiss). Rheological measurements conducted on samples cut from pressed plates were performed in frequency sweeps in the linear-viscoelastic range according to the model of Maxwell using a CVOR150 (Malvern Instruments GmbH). Plate-plate geometry with a diameter of 25 mm was

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used with a melt film thickness adjusted to 1 mm. The measurements were performed under atmospheric conditions at 130°C.

3. Results and Discussion 3.1. Electrical behaviour of the modified adhesives The effect of MWNTs on the conductivity of the MWNT/hotmelt nanocomposites is presented in Figure 2. A change of the electrical properties of the hotmelt from nonconducting to conductive can be obtained by the addition of MWNTs, corresponding to the percolation theory. The nonconductive hotmelt exhibits increased conductivity values already at the lowest investigated CNT concentration of 0.5 wt%. Starting at 0.75 wt%, conductivity values within the antistatic discharge range above 10-10 Scm-1are reached and at 5.0 wt% the composite can be regarded as moderately electrically conductive (>10-4 S cm-1). The change of the volume resistivity is indicative of the vicinity of the particles to each other within the adhesive matrix that enables the system to improve its conductive properties. Reaching a specific critical concentration, these particles are close enough to each other to show connectivity and to form a network with increasing CNT content. The change from a nonconducting to a conducting system is constituted as described by the percolation threshold.[22,29] The results indicate that the nanotubes percolate electrically at around 0.75 wt%, as at this concentration the electrical conductivity is significantly enhanced.

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Figure 2. Dependence of electrical volume conductivity on the concentration of MWNT

3.2 Stress-strain behaviour of the modified adhesives The mechanical properties as obtained in stress-strain tests are summarised in Table 1. Figure 3 shows characteristic stress-strain curves of the pure hotmelt and the MWNT/hotmelt composites. The typical behaviour of the hotmelts is modified in the direction of higher mechanical stress values at given strain deformation, however the break occurs at lower strains with increasing CNT concentration. In addition, the Young´s modulus increased with CNT addition at 0.75 wt% to 278 % and at 5.0 wt% to 372 %. Also the tensile strength is increased with CNT addition to 159 % at 0.75 wt% and 186 % at 5.0 wt% loading, but elongation at the break is reduced significantly. This is the typical stress-strain behaviour of thermoplastics when adding CNTs, however due to the relatively low strength of the neat polyolefine the stress increase at 5.0 wt% loading is quite significant. Even if the values do not increase in a regular manner, in general the results show that the stiffness and strength of these adhesive materials are enhanced due to the addition of MWNTs. The effects, which are expected to be regularly increasing with MWNT content, are also influenced by the state of nanotube dispersion properties increasing as nanotubes are individualized from the as produced primary agglomerates. It would be expected, that the higher modulus and strength of -8-

the modified adhesives would also contribute to better mechanical properties of joints made with these modified materials.

Table 1. Mechanical properties of pure hotmelt and MWNT/hotmelt composites measured in tensile tests Filler content [wt%] 0 0.5 0.75 1.0 1.5 2.0 3.0 4.0 5.0

Young´s modulus [MPa] 7.4 ± 0.8 18.7 ± 0.6 20.6 ± 0.8 12.6 ± 0.2 12.5 ± 1.2 13.8 ± 1.6 25.5 ± 1.0 23.6 ± 1.1 27.6 ± 1.1

Tensile strength [MPa]

Elongation at break [%]

1.25 ± 0.03 1.68 ± 0.02 1.99 ± 0.03 1.57 ± 0.04 1.53 ± 0.03 1.69 ± 0.08 2.23 ± 0.03 2.19 ± 0.02 2.32 ± 0.05

181.8 ± 17.8 143.5 ± 5.2 129.8 ± 11.0 198.0 ± 28.9 107.0 ± 10.4 117.3 ± 5.8 140.7 ± 0.7 145.9 ± 20.5 114.1 ± 22.9

Figure 3. Selected representative stress-strain curves of the pure hotmelt and MWNT/hotmelt composites The effect of CNT addition on the strength of bonded single lap joints is shown in Figure 4. The shear strength increases with CNT content. At 3.0 wt% loading a maximum in shear strength of about 2.4 times of the value of the neat polyolefine is reached; above this content a decrease in shear strength is observed. At 0.75 wt% loading, representing the composites just -9-

above the percolation threshold, the shear strength increased to about 134 %. After conducting shear strength tests of the MWNT/hotmelt composites, the fracture surfaces were observed by optical microscopy (Figure 5). The fracture patterns exposed in all cases an adhesive failure between the adherents and the adhesive indicating that the improvement in shear strength originates from a modified interface between the nanocomposite and the AlMg3.

Figure 4. Dependence of the shear strength of joints between AlMg3 and MWNT/hotmelt with different amounts of MWNT

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Figure 5. Fracture pattern of joints bonded with MWNT modified hotmelt at different concentration

3.3 Morphology and structure of the modified adhesives SEM images taken in charge contrast mode (Figure 6) illustrate that the dispersion of the nanotubes within the hotmelt is relatively uniform. One has to consider that in this imaging mode only nanotubes contributing to the electrical network are visible and appear light. The micrographs show that electrical conductivity starts at 1.0 wt% (Figure 6(a)). Samples with lower CNT amounts did not show clear images. With increasing CNT content the network of the nanotubes contributing to electrical conduction becomes more homogenous. At 2.0 wt% more individualised CNTs can be observed in the vicinity of a few agglomerates (Figure 6(b)). At 3.0 wt% CNT concentration, better dispersed smaller agglomerates form the conducting pathways and only a few regions without any CNTs can be identified (Figure 6(c)). At 5.0 wt% CNT concentration the electrical conductive nanotube network occupies the whole observation area, indicating quite uniform dispersion and distribution within the polyolefine (Figure 6(d)). - 11 -

1.0 wt%

3.0 wt%

(a)

(c)

2.0 wt%

(b)

5.0 wt%

(d)

Figure 6. Characterisation of the MWNT dispersion within the polyolefine matrix at different loadings using SEM in charge contrast mode

3.4 Influence of the nanotubes on the rheological behaviour Rheological characterisation of the MWNT/hotmelt systems was carried out in order to elucidate the processability of the adhesive. The data obtained in frequency sweeps at low frequencies provides information about the nanotube network formation with increasing filler content. The rheological behaviour of the pure polyolefine and the MWNT/hotmelt composites with different MWNT amounts are shown in Figure 7. The graphs in Figure 7(a) demonstrate a decrease of the complex viscosity with increasing frequency and an increase of the viscosity dependent on the concentration of carbon nanotubes. Between 0 and 0.5 wt% MWNT content, the materials exhibit Newtonian behavior. However looking at the low frequency behavior, starting at 0.75 wt%, an increase in viscosity on lowering the frequency can be observed and the nanocomposites show a definitive shear thinning character. This effect is indicative of the formation of a filler network and can be regarded as rheological

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percolation, a transition where the nanotubes restrict the motion of the polymer matrix. At high frequencies, the viscosity increase with nanotube addition is only moderate so that the processing behaviour is not changed much. Further detailed rheological investigations of the loss and storage modulus facilitate a conclusion regarding the nanotube network formation, due to the sensitivity of the storage modulus of filled polymers adverse microstructural appearances.[30,31] It provides evidence of the percolation threshold.[32,33] The frequency dependence of the storage modulus G´ in log-log plots is shown in Figure 7(b) at a temperature of 130°C. MWNTs have a substantial effect on the flow behaviour of the adhesive, especially at low frequencies. The addition of MWNTs to the hotmelt leads to an increase in G´ up to 10,000 Pa at 5.0 wt%. At low nanotube concentrations (below 0.5 wt%) the viscoelastic properties of the dispersion are mainly determined by the polymer matrix. At higher concentrations the local mobility of individual polymer chains is affected by the nanotubes and at 1.5 wt% G´ shows a completely frequency independent plateau at low frequencies. This indicates a solid-like behaviour due to the interaction at the interface between CNTs and the polymer chains, accordant to the shear thinning behaviour shown at the η ∗ - plot Figure 7 [32,34]. By plotting the storage modulus as a function of the nanotube concentration at a fixed frequency, a rapid increase in elastic modulus of the composites compared to the pure hotmelt can be observed (Figure 8). This phenomenon can be related to a rheological percolation transition at which the nanotubes restrict the motion of the polymer matrix.

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(b)

(a)

Figure 7. Frequency dependence of the complex viscosity η ∗ (a), frequency dependence of the storage modulus G´ (b) at various MWNT concentration at 130°C. The results conducted from the rheological and the electrical investigations show that under the selected measurement conditions both percolation thresholds occur at very low loading (Figure 8). Whereas the electrical percolation threshold is strongly dependent on the pressing conditions, facilitating or hindering secondary agglomeration is shown to be favorable for network formation, with rheological characteristics strongly dependent on measurement temperature [31,35]. However, in many previous studies such comparisons have been made where, in the majority of cases, the electrical percolation threshold was determined at higher CNT concentrations than the rheological percolation threshold (measured at only one selected temperature).[7,33,36,37] In addition, electrical and rheological percolation thresholds are caused by different interactions within the composite. While rheological percolation originates from a combined nanotube and polymer network, electrical percolation is mainly affected by the carbon nanotube network.

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Figure 8. Electrical resistivity (squares) and storage modulus G´ (triangles) at a fixed measuring frequency of 0.1278 rad⋅s-1 in dependence on the nanotube loading

4

Conclusions

It has been demonstrated that integrating carbon nanotubes into hotmelts leads to good degrees of dispersion resulting in a significant increase of electrical conductivity at quite low loadings of about 1.0 wt%. Thus, the integration of carbon nanotubes significantly improves the electrical properties of adhesives which allow their use in connection with antistatic or conductive applications, e.g. in adhesive bonding of carpets or for containers where antistatic behaviour is necessary. In addition, an improvement of the mechanical properties of the composites has been shown. Investigations of shear strength between AlMg3 probes resulted in a considerable improvement of shear strength for the bonded joints, with a maximum value of 2.4 times the nonmodified hotmelt reached at the filler content of 3.0 wt%. However, as oscillatory melt rheological tests have indicated, the processing behaviour is limited by the increase of viscosity with rising CNT concentration. Starting at 3.0 wt% CNT loading, the composites feature increased melt viscosity and are consequently more difficult to apply. This effect diminishes at higher shear rates, as typical for material processing. In summary, composites filled with 1.0 wt% MWNT represent an excellent compromise, if antistatic hotmelt materials are required. - 15 -

Acknowledgements The authors want to thank Dr. Schneider and Mr. Scheibner for enabling tensile test measurements and Mrs. Boldt (all from the Leibniz Institute of Polymer Research Dresden) for SEM investigations with charge contrast imaging.

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