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Fabricating structural adhesive bonds with high electrical conductivity
International Journal of Adhesion & Adhesives 74 (2017) 70–76
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Fabricating structural adhesive bonds with high electrical conductivity Zhongjie ZHAO a b
a,b
b,⁎
, Xiaosu YI , Guijun XIAN
MARK
a
School of Civil Engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin 150090, PR China AVIC Composites Co., Ltd, 66 Shuanghe Road, Shunyi District, Beijing 101300, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Structural adhesive B. Electrical conductivity C. Nylon textile D. Tetrapod-like zinc oxide E. Two-step-functionalization
In this paper we present a new approach, called Two-Step-Functionalization approach to integrate electric conductivity function into structural adhesive bonds. A nylon textile is used for supporting polymer adhesive film and it is especially surface-coated with silver (Ag) to produce an electrically conductive adhesive film. This is the first-step-functionalization and it occurs at micro-scale. The functionalized film is then surface-loaded on the both sides with three-dimensional tetrapod-like zinc oxide (TZnOs) particles coated again with Ag to form a sandwich structure, where the both thin surface layers are TZnO-rich. The sandwich film is then positioned between two aluminum adherends and bonded at the curing condition. In the context, this is the second-stepfunctionalization, and this occurs at nano-scale. Obviously, both the functionalization provides not only significant mechanical reinforcing efficiency, but also electrical connection between the bonded joints. Eventual electrical isolation effect is avoided which often occurs if the interface between the adhesive film and metal adherend is wetted by the adhesive in a low viscosity window during the curing process. As demonstrated, the adhesive bonded joints using a very limited amount of Ag as coating material shows a unique high electrical conductivity and better mechanical properties as compared to baseline counterparts.
1. Introduction Electrically conductive adhesives are being used for interconnecting and mounting electronic components on circuit carriers for several decades. A series of investigations and inventions related to various mixtures of conductive and non-conductive substances are made already in the first half of the last century [1,2]. More recently in 2000, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded with Nobel Prize for their discovery of electrically-conducting polymers initially with silvery shining films of polyacetylene [3]. The polymers may present a new material option for electrically conductive adhesives. In general, these adhesives are considered as functional polymers rather than structural materials. On the other hand, structural adhesive bonding has stood the test of time and is now firmly established as indispensable and viable manufacturing technologies for aircraft and automotive industries. Bonded joints are of great importance due to the advantages in terms of their light weight corresponding to the mechanical fastening and efficiency of joining, good damping and fatigue characteristics [4–8]. A typical structural requirement for the applications is high shear strength, modulus and toughness of the adhesive bonds. In structural applications, adhesive bonds are generally electrically isolated due to the nature that most structural adhesives are thermosetting polymers
⁎
Corresponding author. E-mail address: [email protected] (X. YI).
http://dx.doi.org/10.1016/j.ijadhadh.2017.01.002 Received 29 February 2016; Accepted 26 November 2016 Available online 04 January 2017 0143-7496/ © 2017 Elsevier Ltd. All rights reserved.
unfilled with any electrically conductive component. Currently, there is an increasing need in developing electrically conductive adhesives in order to, for example, preventing lightning strike damages for aircraft [9]. Reportedly, aircraft would inevitably confront the direct impacts of the lightning as often as once every 3000 flight hours, which might be induced by the high potential of the nimbus clouds [10,11]. To prevent the lightning strike damages, metallic wires and silver pastes are used to electrically connect the adhesive bonds in order to make aircraft structures electrically conductive. However, new design requirements for modern aircraft have triggered a search for bonding technologies that remain not only high mechanical performance, but are also additionally electrically conductive, and fully compatible with state-of-the-art aircraft manufacturing system [12–14]. Therefore, inherently conductive and high mechanical performance adhesive technologies are in urgent demand. Electrical conductivity of filled polymers is based on the percolation mechanism [15–19]. As reported by Do et al., electrical conductivity of carbon nanotubes filled polymer can be greatly enhanced by “soldering” the lapping regions together to form an electrical network [20]. Wu et al. dispersed silver nanowires (AgNWs) to an epoxy paste, and hence developed a high strength conductive epoxy adhesive. The large aspect ratio and the excellent conductivity of AgNWs enabled them to achieve an electrical resistivity of 1.2×10−4 Ω·cm, a lap shear strength
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of 18 MPa, and a filler volume fraction of about 54% [21]. Note that, typical filler volume fraction of traditional isotropic conductive adhesive is approximately 70%. Unfortunately, the performance figures of the filled polymers reported are not found to be transferable to the polymer structural adhesives [12–14]. Recently, a so-called the Functional Interleaf Technology (FIT) [22] is developed by depositing AgNWs on surface of a thermoplastic textile to fabricate a conductive reinforcing preform. The preform is then placed between carbon fiber layers to form a laminated system. As reported, with a low AgNW percolation threshold of about 0.3 g/m2, the through-thickness conductivity of the cured laminate is increased from 0.12 S/cm to 1.39 S/cm, the mode I and mode II interlaminar fracture toughness are improved from 306 J/m2 to 667 J/m2, and from 718 J/m2 to 2345 J/m2, respectively [23,24]. Inspired by the FIT technology, in this paper, we present a Two-Step-Functionalization method to develop high-performance adhesive technology with simultaneously high electrical conductivity and mechanical properties.
were used as reducing agents. A representative sample is shown in Fig. 1(a). Before silvering, the nylon textiles were activated by tin dichloride and palladium chloride. Similar procedure was also applied to coat the TZnO particles, Fig. 1(c). 2.3. Assembling the sandwich adhesive structure The nylon textile Ag-coated was then impregnated with an epoxy adhesive by hot pressing at 90 °C, under 3–5 MPa to produce an electrically conductive adhesive film. The adhesive used had the same chemical formulation as the product of SY-14. Afterwards, AgNWs and TZnOs Ag-coated were scattered on the both sides of the conductive adhesive film and slightly compressed to produce a sandwich adhesive film having enhanced surface electrical conductivity. A representative sample is found in Fig. 1(b). 2.4. Bonding
2. Materials and experimental methods The sandwich adhesive film was placed between two aluminum plates sand-blasted. Compared to the commonly used aluminum adherend anodized, the sand-blasted one was more electrically conductive but less chemically affinitive to the adhesive. The thickness of the bond line was controlled by the adhesive films having the textile support. The bonded joints were then cured in an oven. The curing condition was 180 °C/2.5hr, under a curing pressure of 0.3 MPa, according to the specification of the epoxy adhesive. Representative samples are shown in Fig. 1(d) and (e), respectively.
2.1. Material preparation The nylon textiles 200# was purchased from Beijing PFM Screen Trading Co., Ltd; the Tetrapod like zinc oxides AT-01 from Chengdu Crystrealm Co., Ltd; the silver nanowires CST-NW-S70 from Suzhou Cold Stones Technology Co., Ltd; the aeronautical grade structural epoxy adhesive film SY-14 from Beijing Institute of Aeronautical Materials; and the state-of-the-art conductive reference Loctite 3800 was purchased from Henkel AG & Co. KGaA, respectively. The adherend used in the study was 2024 aluminum alloy plate, which was purchased from Beijing Institute of Aeronautical Materials.
2.5. Property characterization Mechanical testing was carried out on an Instron 5948 Microtester. The microstructure was observed via a FEI Quanta 600 Scanning Electron Microscope. The elemental analysis and EDS mapping were performed using an Oxford IE350 Energy Dispersive Spectrometer. The viscosity was measured using an Anton Paar Physica MCR 101
2.2. Surface coating of nylon textiles and TZnO particles with silver The nylon textiles were chemically coated using silver nitrate and aqueous ammonia to prepare a salt solution; glucose and tartaric acid
(b)
(a)
Electrically conductive sandwich adhesive film
Nylon textilecoated with Ag and impregnated
(d)
Lap Shear (Thick Adherends)
(e)
(c) Coating of TZnOs with Ag
(f)
Lap Shear
Floating Roller Peel
Fig. 1. Representative photographs of the samples as-produced and studied. (a) Nylon-textile coated with Ag is impregnated with SY-14 structural adhesive. (b) The film is then surfaceloaded with TZnOs Ag-coated to form a sandwich adhesive film. (c) Slurry-coating of TZnOs with Ag. (d) Lap-shear bonds with thick adherends. (e) A lap-shear bond. (f) A sample after floating roller peel test.
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two aluminum joints. Each of the two aluminum adherends was then electrically connected to the Multimeter with 4 wires. Simultaneous measurement reveals that the pre-assembled bond showed an initially electrical resistance of about 1 Ω, Fig. 3(a). However, as the cure process advances, the resistance observed suddenly increases after the temperature increases to a level higher than 170 °C, where the viscosity of the adhesive reaches its minimum. Consequently, the initial electrically conductive bond becomes finally electrically isolated after the curing. The switch-like transition phenomenon is expected to be caused by wetting of the epoxy adhesive at a low viscosity level of about 300 Pa s, leading to an electrical isolation of all electrically conductive contacts in the interfaces between the conductive adhesive film and the aluminum adherend. The shear stress-shear strain curve of the bond using the nylon textile supported adhesive film indicates a higher stiffness, Fig. 3(b). The stress turns then to a plateau which is about 10% lower than that of the baseline bond. Note that the stress-strain curve is bondline specific using thick and stiff adherends (ASTM D5656 [27]), rather than that using thin-wall lap sample (ASTM D3165 [25]). It is known that the mechanical data cannot be directly compared because of the multi-axel conditions, if the thin-wall lap bond used. It is also well known that the thickness of the bondline is a sensitive factor affecting the bonding performance [33–35]. In Fig. 3(b), the control sample has a bondline thickness of about 165 μm whereas the electrically conductive bondline using the conductive film nylon textile supported shows a thickness of about 220 μm. To our comparative study, the difference was, however, considered and normalized by the Eq. 2 for the shear strain.
Rheometry. The square resistance and bulk electrical conductivity were tested by an Agilent 34410A Digital Multimeter. All mechanical tests of the bonded joints were carried out in accordance with the industrial standards and test specifications. The lap shear strength of the samples (Fig. 1(e)) was tested according to ASTM D3165 [25], and the floating-roller-peel-strength (Fig. 1(f)) according to ASTM D6862 [26], with a peeling angle of 90° and a peeling speed of 10 cm/min. The shear stress-strain plot of bonded samples with thick and stiff adherends (Fig. 1(d)) was acquired according to ASTM D5656 [27], with a bonding area of 10mm×5mm. To study the electrical conductivity, the square resistance of the conductive adhesive film prepared was acquired from the resistance reading of the Multimeter. While testing, two copper blocks were pressed onto the film in parallel. The contacting area was 1 cm×2 cm, and the parallel distance of the copper blocks were 2 cm. The square resistance tests were performed by four-point probe method. The aluminum bonds prepared had an aerial size of 2.5 cm×1.2 cm. The two adherends were electrically connected to the Multimeter via 4 wires. The bulk conductivity of the bonds was calculated according to Eq. 1.
σ = L /(R0 × A)… … … … … … … … … … … … … … … … … …
(1)
Where:σ=bulk conductivity, S/cmL=thickness of the bondline, cmR0=observed resistance of the test sample (tested by four point probe method), ΩA =bonding area of the test sample, cm2 3. Results and discussions
γUL = (dUL − d m )/ t … … … … … … … … … … … … … … … … … …
3.1. Preparation of electrically conductive adhesive as first-stepfunctionalization
(2)
Where:=shear strain at ultimate length (UL), dimensionless=displacement of the test sample at UL, cm=displacement of the metal adherend at UL, cm. In the study, it was neglected.t=thickness of the bondline, cm The benefit of using the nylon textile is obvious, even if it affects adversely the mechanical strength in an acceptable range. If coated with Ag, the textile provides as template of an ideal network to ensure the electrical conductivity other than the statistic percolation mechanism, the amount of Ag and the conductivity can also be precisely and optimally designed and controlled.
In commercial structural adhesive technologies, polymer nonwoven (veil) is commonly used to retain the adhesives from its flowing away, to accelerate wetting of adherends, and to control the bondline thickness [28–32]. In the present study, we used polyamide (Nylon) textile material instead of a nonwoven polymer. The unique feature of the textile used in this study is, chemically coat it with silver (Ag) to produce an electrically conductive and good mechanical support for adhesive film. As measured, the areal density of the nylon textile was increased from 18 g/m2 to 24 g/m2 before and after the Ag-coating, respectively. As can be seen in Figs. 1(a) and 2, the nylon textile was fully coated with Ag. The coating thickness of Ag on each fiber was less than 15 μm. The apparent square resistance of the textile Ag-coated was about 0.06Ω/square. The textile remained tough and flexible after the silvering. The original thickness of the nylon fiber was about 35– 40 μm; we call this step of processes First-Step-Functionalization at micro-scale. The textile was then hot-pressed into an 180 °C curing epoxy adhesive film and the film was subsequently used to adhesively bond
3.2. Preparation of electrically conductive interface layer as SecondStep-Functionalization There were two options to make the interface between conductive adhesive and metal adherend electrically conductive while maintaining the mechanical properties of the bond is less affected. The first one was to deposit silver nanowires (AgNWs) of higher volume fraction on the adhesive film surfaces to especially add more electrical contacts, and
Fig. 2. (a) SEM images of the nylon textile chemically coated with Ag, and (b), a zoom-in SEM image of (a).Overall, the textile is well-coated with Ag.
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Fig. 3. (a) Resistance-viscosity-temperature plot of the textile supported adhesive bond during a curing process, showing a switch-like transition of the resistance in dependence of the cure-temperature, and (b), shear stress – strain curves of two bonds in comparison between a baseline (control) and an electrically conductive bond with the same epoxy adhesive reinforced by the nylon-textile.
single whisker in the structure is about 10-50 μm. During the curing and bonding process, both the AgNWs and TZnOs are densely compressed in the 15 μm thin boundary layers between the adherend and adhesive film, and both of them are fully wetted by the adhesive, as shown in the figures. However, the one-dimensional AgNWs might be easily oriented parallel to the flat surface of the adherend and easily isolated by the polymer wetting, whilst the TZnOs may mechanically and electrically bridge the aluminum adherend and the conductive adhesive film crossing the boundary layer due to their structural size and rigidity. The assumption is partially verified by the observation of the representative cross-section SEM images of the joints with the AgNW- and TZnO-adhesive, respectively. For the AgNW-adhesive bond, Figs. 4(b) and (c), polymer-rich zones become visible along the interface between the adherend and adhesive, leading to an electrical isolation. They are marked by red arrows. A separation is also observed between the nylon fiber and adhesive, caused by chemical etching for sample preparation prior to the SEM analysis. For the TZnO-adhesive bond, polymer-rich zones are also visible in the fiber/adhesive interface. Fig. 4(e) and (f) shows the cross-section of the joint with TZnO-adhesive. Original and complete TZnO structure is rarely recognized. They may be mechanically fragmented by the compression during the bonding process. Consequently, a TZnO-rich zone is developed in the interface. It is believed that there are numerous mechanical and electrical contacts in the interface zone, helping to strengthen mechanically the interface and establish the electrical paths. This was found to be true, as obviously shown in EDS mapping image in Fig. 5(a). Overall, the Ag elements can be found in the adhesive bondline. They are particularly concentrated around the nylon fibers and in the TZnO-rich interface between the aluminum adherend and adhesive film, suggesting a complete electrical network established across the matrix. Obviously, the Ag observed comes from the coatings on the nylon textile and on the TZnOs, respectively. Fig. 5(b) shows a localized EDS mapping on the fractured surface of the electrically conductive bond. At a first look, it is a cohesive failure observed in the “weak layer [38]” of the adhesive close to the nylon fiber. In this region, numerous well-distributed Ag elements are also found and they are particularly marked in an artificial rectangular frame. The result implies a good bonding strength between the adhesive and the fiber, and the bonding apparently is mechanically less affected by incorporating Ag into the interface. The bond strength and electrical conductivity are thus improved significantly and simultaneously, see Table 1. Table 1 report also data of a bonded baseline sample, SY-14 as control, with the same epoxy adhesive reinforced by nylon textile, and a
another one was to use rigid three-dimensional conductive component, for example Ag-coated tetrapod-like ZnO particles (Fig. 1(c)) instead of using AgNWs. Through this modification, the adhesive film surfaceloaded on the both sides showed an initial thickness of about 400 μm; consisting of two AgNW- or TZnO-rich surfaces, each of them was about 15 μm in thickness. In this paper, the first one is simply denoted as AgNW-adhesive and the last one as TZnO-adhesive, respectively. Both of them are obviously a sandwich system (Fig. 1(b)). Test results on adhesively bonded aluminum joints using the sandwich bondline show that both functional additives are effective to improve the bond lap strength. As shown in Table 1, using the AgNW-adhesive with a local mass fraction of about 25% in the two thin boundary layers, the average lap shear strength of the thin-wall lap bonds reaches a level of 27.1 MPa, and a floating-roller-peel-strength of 4.5 N/mm. However, the final bonds cured become electrically isolated. In contrast to them, the bonds with the TZnO-adhesive demonstrate improvement in both the bond strength and electrical conductivity in the same time, depending on the local mass fraction of TZnOs. Generally, the lap shear strength decreases with the TZnOs, whereas the electrical conductivity increases. The conductivity difference observed can be ascribed to the structure difference between the one-dimensional AgNWs and threedimensional TZnOs, if we compare them in Fig. 4(a) and (d). The AgNWs used in the study show an average diameter of about 70 nm and lengths in a range of 35-45 μm, Fig. 4(a), whereas the TZnOs used can be considered as a four single-whisker structure grown from one juncture in a tetrapod-like manner, Fig. 4(d) [36]. The length range of a
Table 1 Comparison of typical mechanical and electrical properties of adhesively bonded aluminum joints studied. Samples
Lap shear strength [MPa]
Floating roller peel strength [N/mm]
Electrical conductivity [S/ cm]
AgNW-adhesive, Surfacial mass fraction of 25%
27.1
4.5
∼0
12.5% TZnO-adhesive, Surfacial mass 25.0% 37.5% fraction of SY-14adhesive with nylon textile State-of-the-art commercial conductive reference
30.8 26.8 18.9 29.4
– 4.2 – 5.9
< 0.1 1700 2100 ∼0
8.5
∼0
2000
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Fig. 4. (a) SEM image of AgNWs used; (b) cross section of an Al-bond with AgNW-adhesive, and (c), a zoom-in image of (b); (d) SEM image of TZnO particles used; (e) cross section of an Al-bond with TZnO-adhesive, and (f), a zoom-in image of (e).(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
bondline is highly reinforced and electrically networked, resulting in high bond strengths and electrical conductivity. In this approach, Ag-coating is especially effective and materialsaving. It can provide efficient electrical conductivity with only limited quantities, for example, with an overall weight of 13.8% of Ag plus ZnO, the adhesive bond achieves a conductivity level of 1700 S/cm, and shear strength of 26.8 MPa, where surfacial TZnOs mass fraction is about 25%, Table 1. In the same time, the good mechanical strengths shown in Table 1 are expected to be promising for structural adhesive applications [37]. It is furthermore envisaged that the adhesive technology developed is not only applicable for metal bonding, but also for honeycomb structure and bonding of dissimilar materials. Technologically, the bondline structure responsible for high mechanical properties and high electrical conductivity is established in two steps, we call hence this approach Two-Step-Functionalization. The process using the micro-scale textile and nano-scale TZnO particles as templates is obviously beneficial because no ball milling
state-of-the-art commercial conductive adhesive product with a similar chemistry and technical profile. It can easily figure out by comparison that the TZnO-adhesive bonded samples provide balanced high performance in mechanical as well as electrical properties.
3.3. Two-step-functionalization Upon the results presented and discussed above, Fig. 6 illustrates schematically the sandwich bondline structure with a central adhesive film nylon-textile supported, together with SEM microphotographs of the TZnO-rich surface and a representative cross-section of the bond. On the TZnO-rich surfaces-produced, numerous reinforcing TZnOs particles including complete TZnOs are observable. It is obvious that the particles play a bi-functional role as mechanical reinforcement and as electrical conductors ascribed to their structural sizes, rigidity and concentration. As a result of the synergic effect of the nylon-textile in the center and the TZnO-rich boundary layers in the interfaces, the
Fig. 5. (a) EDS mapping image of a TZnO-adhesive bonded Al-joint showing Ag elements distributed in the bondline, with a contrasted insert image; and (b), fractured surface of the bond showing a localized EDS mapping.
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Boundary layer (interface), 7.5% thick, TZnO-rich;TZnOs are Ag-coated for electrical conductivity. Second-StepFunctionalization
Adhesive film, 85% thick, Nylon-textilesupported, the fibers are Ag-coated for electrical conductivity. First-Step-Function
Boundary layer (interface), 7.5% thick, TZnO-rich;TZnOs are Ag-coated for electrical conductivity. Second-StepFunctionalization Fig. 6. Schematic illustration of the sandwich bondline structure consisting of an adhesive film nylon-textile supported in the center and two TZnO-rich boundary layers as interfaces ordered on the both surfaces, with respective text explanation and SEM microphotographs. Both the textile and TZnOs are coated by Ag for electrical conductivity. [10] Kasemir HW, Kasemir Heinz-Wolfram. Mazur V, Ruhnke LH, editors. His Collected Works. USA: American Geophysical Union; 2012. p. 418–28. [11] Willett JC, Park G, Krider EP, Peng GS, Simmons FS Law GW. Triggered lightning risk assessment for reusable launch vehicles at the southwest regional and Oklahoma spaceports.In: Proceedings of the 86th AMS Annual Meeting, Atlanta, Georgia; 19-30; 2006. [12] Ghosh K, Maiti SN. Mechanical properties of silver-powder-filled polypropylene composites. J Appl Polym Sci 1996;60(3):323–31. [13] Novák I, Krupa I, Chodák I. Electroconductive adhesives based on epoxy and polyurethane resins filled with silver-coated inorganic fillers. Synth Met 2004;144(1):13–9. [14] Rao Y, Lu DQ, Wong CP. A study of impact performance of conductive adhesives. Int J AdhesAdhes 2004;24(5):449–53. [15] Mclachlan DS. Equation for the conductivity of binary mixtures with anisotropic grain structures. J Phys C-Solid State Phys 1987;20:856–77. [16] Mclachlan DS. Measurement and analysis of a model dual-conductivity medium using a generalized effective medium theory. J Phys C-Solid State Phys 1988;21:1521–32. [17] Horyak GL, Patrissi CJ, Martin CR. Fabrication, characterization, and optical properties of golden nanoparticle/porous alumina composites: the non-scattering maxwell-garnett limit. J Phys Chem B 1997;101(9):1548–88. [18] Zhang C, Yi XS, Yui H, Asai S, Sumita M. Selective location and double percolation of short carbon fiber filled polymer blends: high-density polyethylene/isotactic polypropylene. Mater Lett 1998;36(1-4):186–90. [19] Shi FH, Li J, Zhang BY, QiuX S, Gu LY. Preparation and Electrical Properties of Graphene/PEK-C Films. Adv Mater Res 2015;1102:107–12. [20] Estrada D Do JW, Chang Xie X, Mallek N.J N, Rogers Girolami GS, Pop E JA, Lyding JW. Nanosoldering carbon nanotube junctions by local chemical vapor deposition for improved device performance. Nano Lett 2013;13:5844–50. [21] Wu HP, Wu XJ, Liu JF, Zhang GQ, Wang YW, Zeng YW, Jing JZ. Development of a novel isotropic conductive adhesive filled with silver nanowires. J Compos Mater 2006;40(21):1961–9. [22] Lin Y. Functionalized interleaf technology in carbon-fibre-reinforced composites for aircraft applications. Natl Sci Rev 2014;1(1):7–8. [23] GuoM C, Yi XS. The production of tough, electrically conductive carbon fibercomposite laminates for use in airframes. Carbon 2013;58:238–51. [24] Guo MC, Yi XS, Liu G, Liu LP. Simultaneously increasing the electrical conductivity and fracture toughness of carbon-fiber composites by using silver nanowires-loaded interleaves. Compos Sci Technol 2014;97:27–33. [25] ASTM D3165-07. Standard Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies; 2014. [26] ASTM D6862-11. Standard Test Method for 90 Degree Peel Resistance of Adhesives; 2016. [27] ASTM D5656-10, Standard Test Method for Thick-Adherend Metal Lap-Shear Joints for Determination of the Stress-Strain Behavior of Adhesives in Shear by Tension Loading. [28] Brewis DM, Comyn J, Cope BC, Moloney AC. Further studies on the effect of carriers on the performance of aluminum alloy joints bonded with an epoxide polyamide adhesive. Polymer 1980;21(12):1477–9. [29] Gouri C, Ramaswamy R, Ninan KN. Studies on the adhesive properties of solid elastomer-modified novolac epoxy resin. Int J Adhes Adhes 2000;20(4):305–14. [30] Gouri C. Elastomer modification of epoxy based film adhesives: adhesive and mechanical properties. J Adhes Sci Techol 2002;16(12):1569–83. [31] Schroeder JA, Ahmed T, Chaudhry B, Shepard S. Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography. Compos Part A-Appl S 2002;33(11):1511–7. [32] Tahania M, Yousefsani SA. On thermomechanical stress analysis of adhesively
and ultrasonic dispersion are needed as the traditional process does. The process control becomes easy and a better bonding quality could be easily achieved. 4. Conclusions In this paper, a new adhesive technology, called Two-StepFunctionalization adhesive technology, has been presented to develop high-strength structural bonding with high electrical conductivity. A nylon textile is first used for supporting and reinforcing adhesive film. It provides in the same time a template for surface-coating with silver to produce an electrically conductive adhesive film. The electrical function is thus primarily integrated into the structural supporting textile. This is the First-Step-Functionalization at micro-scale. The conductive adhesive film nylon-textile-supported is then surfaceloaded with three-dimensional tetrapod-like zinc oxide particles (TZnOs) coated also with Ag to form a sandwich adhesive film having an enhanced surface electrical conductivity. The surface structure is particularly effective to prevent adhesive wetting, which often occurs during the curing if the viscosity of the adhesive falls into a critical low level with the increasing temperature. This is the Second-StepFunctionalization at nano-scale. As demonstrated, the aluminum adhesive joints using the Two-Step-Functionalized approach shows balanced high electrical conductivity and mechanical properties by using only limited quantity of Ag, compared to a state-of-the-art product. References [1] Norman HRH. Conductive Rubbers and Plastics: Their Production, Application and Test Methods. New York: Elsevier Pub. Co.; 1970. p. 1–22. [2] Bulgin D. Electrically conductive rubber. Rubber ChemTechnol 1946;19(3):667–95. [3] Chiang CK, Fincher C, Park YW, Heeger AJ, Shirakawa H, Louis EJ, Gau SC, MacDiarmid AG. Electrical conductivity in doped polyacetylene. Phys Rev Lett 1977;39(17):1098–101. [4] Lee LH. Adhesive Bonding.. New York: Plenum Press; 1991. p. 239–84. [5] He XC. A review of finite element analysis of adhesively bonded joints. Int J Adhes Adhes 2011;31(4):248–64. [6] Schindel DW. Air-coupled ultrasonic measurements of adhesively bonded multilayer structures. Ultrasonics 1999;37(3):185–200. [7] Bai JM, Sun CT. The Effect of viscoelastic adhesive layers on structural damping of sandwich beams. Mech Struct Mach 1995;23(6):1–16. [8] Hansen SW, Spies RD. Structural damping in laminated beams due to interfacial slip. J Sound Vib 1997;204(2):183–202. [9] Feraboli P, Miller M. Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike. Compos Part A-Appl S 2009;40(67):954–67.
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mechanical properties of the adhesives. J Adhes Sci Technol 2014;28(11):1055–71. [36] Dai Y, Zhang Y, Lia QK, Nan CW. Synthesis and optical properties of tetrapod-like zinc oxide nanorods. Chem Phys Lett 2002;358(1-2):83–6. [37] Sargent JP. Adherend suface morphology and its influence on the peel strength of adhesive joints onded with modified phenolic and epoxy structural adhesives. Int J Adhes Adhes 1994;14(1):21–30. [38] Mahoney CL. Surface preparation for adhesive bonding. In: Skeist Irving, editor. Handbook of Adhesives. New York: Springer; 1990. p. 74–93.
bonded composite joints in presence of an interfacial void. Compos Struc 2015;130:116–23. [33] Hahn O, Schlimmer M, Yi XS. Mechanisches Verhalten der Klebschicht einschnittig ueberlappter Metallklebverbindungen im Zugversuch. Kunststoffe. 1986;76:9, [804/808]. [34] Sun YT, Pugno N, Gong BM. A simplified hardening cohesive zone model for bondline thickness dependence on adhesive joints. Int J Fract 2015;194(1):37–44. [35] Yang S, Xua W, Liang LH, Wang TC, Wei YG. An experimental study on the dependence of the strength of adhesively bonded joints with thickness and
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Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Fabricating structural adhesive bonds with high electrical conductivity Zhongjie ZHAO a b
a,b
b,⁎
, Xiaosu YI , Guijun XIAN
MARK
a
School of Civil Engineering, Harbin Institute of Technology, 73 Huanghe Road, Harbin 150090, PR China AVIC Composites Co., Ltd, 66 Shuanghe Road, Shunyi District, Beijing 101300, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: A. Structural adhesive B. Electrical conductivity C. Nylon textile D. Tetrapod-like zinc oxide E. Two-step-functionalization
In this paper we present a new approach, called Two-Step-Functionalization approach to integrate electric conductivity function into structural adhesive bonds. A nylon textile is used for supporting polymer adhesive film and it is especially surface-coated with silver (Ag) to produce an electrically conductive adhesive film. This is the first-step-functionalization and it occurs at micro-scale. The functionalized film is then surface-loaded on the both sides with three-dimensional tetrapod-like zinc oxide (TZnOs) particles coated again with Ag to form a sandwich structure, where the both thin surface layers are TZnO-rich. The sandwich film is then positioned between two aluminum adherends and bonded at the curing condition. In the context, this is the second-stepfunctionalization, and this occurs at nano-scale. Obviously, both the functionalization provides not only significant mechanical reinforcing efficiency, but also electrical connection between the bonded joints. Eventual electrical isolation effect is avoided which often occurs if the interface between the adhesive film and metal adherend is wetted by the adhesive in a low viscosity window during the curing process. As demonstrated, the adhesive bonded joints using a very limited amount of Ag as coating material shows a unique high electrical conductivity and better mechanical properties as compared to baseline counterparts.
1. Introduction Electrically conductive adhesives are being used for interconnecting and mounting electronic components on circuit carriers for several decades. A series of investigations and inventions related to various mixtures of conductive and non-conductive substances are made already in the first half of the last century [1,2]. More recently in 2000, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa were awarded with Nobel Prize for their discovery of electrically-conducting polymers initially with silvery shining films of polyacetylene [3]. The polymers may present a new material option for electrically conductive adhesives. In general, these adhesives are considered as functional polymers rather than structural materials. On the other hand, structural adhesive bonding has stood the test of time and is now firmly established as indispensable and viable manufacturing technologies for aircraft and automotive industries. Bonded joints are of great importance due to the advantages in terms of their light weight corresponding to the mechanical fastening and efficiency of joining, good damping and fatigue characteristics [4–8]. A typical structural requirement for the applications is high shear strength, modulus and toughness of the adhesive bonds. In structural applications, adhesive bonds are generally electrically isolated due to the nature that most structural adhesives are thermosetting polymers
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Corresponding author. E-mail address: [email protected] (X. YI).
http://dx.doi.org/10.1016/j.ijadhadh.2017.01.002 Received 29 February 2016; Accepted 26 November 2016 Available online 04 January 2017 0143-7496/ © 2017 Elsevier Ltd. All rights reserved.
unfilled with any electrically conductive component. Currently, there is an increasing need in developing electrically conductive adhesives in order to, for example, preventing lightning strike damages for aircraft [9]. Reportedly, aircraft would inevitably confront the direct impacts of the lightning as often as once every 3000 flight hours, which might be induced by the high potential of the nimbus clouds [10,11]. To prevent the lightning strike damages, metallic wires and silver pastes are used to electrically connect the adhesive bonds in order to make aircraft structures electrically conductive. However, new design requirements for modern aircraft have triggered a search for bonding technologies that remain not only high mechanical performance, but are also additionally electrically conductive, and fully compatible with state-of-the-art aircraft manufacturing system [12–14]. Therefore, inherently conductive and high mechanical performance adhesive technologies are in urgent demand. Electrical conductivity of filled polymers is based on the percolation mechanism [15–19]. As reported by Do et al., electrical conductivity of carbon nanotubes filled polymer can be greatly enhanced by “soldering” the lapping regions together to form an electrical network [20]. Wu et al. dispersed silver nanowires (AgNWs) to an epoxy paste, and hence developed a high strength conductive epoxy adhesive. The large aspect ratio and the excellent conductivity of AgNWs enabled them to achieve an electrical resistivity of 1.2×10−4 Ω·cm, a lap shear strength
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of 18 MPa, and a filler volume fraction of about 54% [21]. Note that, typical filler volume fraction of traditional isotropic conductive adhesive is approximately 70%. Unfortunately, the performance figures of the filled polymers reported are not found to be transferable to the polymer structural adhesives [12–14]. Recently, a so-called the Functional Interleaf Technology (FIT) [22] is developed by depositing AgNWs on surface of a thermoplastic textile to fabricate a conductive reinforcing preform. The preform is then placed between carbon fiber layers to form a laminated system. As reported, with a low AgNW percolation threshold of about 0.3 g/m2, the through-thickness conductivity of the cured laminate is increased from 0.12 S/cm to 1.39 S/cm, the mode I and mode II interlaminar fracture toughness are improved from 306 J/m2 to 667 J/m2, and from 718 J/m2 to 2345 J/m2, respectively [23,24]. Inspired by the FIT technology, in this paper, we present a Two-Step-Functionalization method to develop high-performance adhesive technology with simultaneously high electrical conductivity and mechanical properties.
were used as reducing agents. A representative sample is shown in Fig. 1(a). Before silvering, the nylon textiles were activated by tin dichloride and palladium chloride. Similar procedure was also applied to coat the TZnO particles, Fig. 1(c). 2.3. Assembling the sandwich adhesive structure The nylon textile Ag-coated was then impregnated with an epoxy adhesive by hot pressing at 90 °C, under 3–5 MPa to produce an electrically conductive adhesive film. The adhesive used had the same chemical formulation as the product of SY-14. Afterwards, AgNWs and TZnOs Ag-coated were scattered on the both sides of the conductive adhesive film and slightly compressed to produce a sandwich adhesive film having enhanced surface electrical conductivity. A representative sample is found in Fig. 1(b). 2.4. Bonding
2. Materials and experimental methods The sandwich adhesive film was placed between two aluminum plates sand-blasted. Compared to the commonly used aluminum adherend anodized, the sand-blasted one was more electrically conductive but less chemically affinitive to the adhesive. The thickness of the bond line was controlled by the adhesive films having the textile support. The bonded joints were then cured in an oven. The curing condition was 180 °C/2.5hr, under a curing pressure of 0.3 MPa, according to the specification of the epoxy adhesive. Representative samples are shown in Fig. 1(d) and (e), respectively.
2.1. Material preparation The nylon textiles 200# was purchased from Beijing PFM Screen Trading Co., Ltd; the Tetrapod like zinc oxides AT-01 from Chengdu Crystrealm Co., Ltd; the silver nanowires CST-NW-S70 from Suzhou Cold Stones Technology Co., Ltd; the aeronautical grade structural epoxy adhesive film SY-14 from Beijing Institute of Aeronautical Materials; and the state-of-the-art conductive reference Loctite 3800 was purchased from Henkel AG & Co. KGaA, respectively. The adherend used in the study was 2024 aluminum alloy plate, which was purchased from Beijing Institute of Aeronautical Materials.
2.5. Property characterization Mechanical testing was carried out on an Instron 5948 Microtester. The microstructure was observed via a FEI Quanta 600 Scanning Electron Microscope. The elemental analysis and EDS mapping were performed using an Oxford IE350 Energy Dispersive Spectrometer. The viscosity was measured using an Anton Paar Physica MCR 101
2.2. Surface coating of nylon textiles and TZnO particles with silver The nylon textiles were chemically coated using silver nitrate and aqueous ammonia to prepare a salt solution; glucose and tartaric acid
(b)
(a)
Electrically conductive sandwich adhesive film
Nylon textilecoated with Ag and impregnated
(d)
Lap Shear (Thick Adherends)
(e)
(c) Coating of TZnOs with Ag
(f)
Lap Shear
Floating Roller Peel
Fig. 1. Representative photographs of the samples as-produced and studied. (a) Nylon-textile coated with Ag is impregnated with SY-14 structural adhesive. (b) The film is then surfaceloaded with TZnOs Ag-coated to form a sandwich adhesive film. (c) Slurry-coating of TZnOs with Ag. (d) Lap-shear bonds with thick adherends. (e) A lap-shear bond. (f) A sample after floating roller peel test.
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two aluminum joints. Each of the two aluminum adherends was then electrically connected to the Multimeter with 4 wires. Simultaneous measurement reveals that the pre-assembled bond showed an initially electrical resistance of about 1 Ω, Fig. 3(a). However, as the cure process advances, the resistance observed suddenly increases after the temperature increases to a level higher than 170 °C, where the viscosity of the adhesive reaches its minimum. Consequently, the initial electrically conductive bond becomes finally electrically isolated after the curing. The switch-like transition phenomenon is expected to be caused by wetting of the epoxy adhesive at a low viscosity level of about 300 Pa s, leading to an electrical isolation of all electrically conductive contacts in the interfaces between the conductive adhesive film and the aluminum adherend. The shear stress-shear strain curve of the bond using the nylon textile supported adhesive film indicates a higher stiffness, Fig. 3(b). The stress turns then to a plateau which is about 10% lower than that of the baseline bond. Note that the stress-strain curve is bondline specific using thick and stiff adherends (ASTM D5656 [27]), rather than that using thin-wall lap sample (ASTM D3165 [25]). It is known that the mechanical data cannot be directly compared because of the multi-axel conditions, if the thin-wall lap bond used. It is also well known that the thickness of the bondline is a sensitive factor affecting the bonding performance [33–35]. In Fig. 3(b), the control sample has a bondline thickness of about 165 μm whereas the electrically conductive bondline using the conductive film nylon textile supported shows a thickness of about 220 μm. To our comparative study, the difference was, however, considered and normalized by the Eq. 2 for the shear strain.
Rheometry. The square resistance and bulk electrical conductivity were tested by an Agilent 34410A Digital Multimeter. All mechanical tests of the bonded joints were carried out in accordance with the industrial standards and test specifications. The lap shear strength of the samples (Fig. 1(e)) was tested according to ASTM D3165 [25], and the floating-roller-peel-strength (Fig. 1(f)) according to ASTM D6862 [26], with a peeling angle of 90° and a peeling speed of 10 cm/min. The shear stress-strain plot of bonded samples with thick and stiff adherends (Fig. 1(d)) was acquired according to ASTM D5656 [27], with a bonding area of 10mm×5mm. To study the electrical conductivity, the square resistance of the conductive adhesive film prepared was acquired from the resistance reading of the Multimeter. While testing, two copper blocks were pressed onto the film in parallel. The contacting area was 1 cm×2 cm, and the parallel distance of the copper blocks were 2 cm. The square resistance tests were performed by four-point probe method. The aluminum bonds prepared had an aerial size of 2.5 cm×1.2 cm. The two adherends were electrically connected to the Multimeter via 4 wires. The bulk conductivity of the bonds was calculated according to Eq. 1.
σ = L /(R0 × A)… … … … … … … … … … … … … … … … … …
(1)
Where:σ=bulk conductivity, S/cmL=thickness of the bondline, cmR0=observed resistance of the test sample (tested by four point probe method), ΩA =bonding area of the test sample, cm2 3. Results and discussions
γUL = (dUL − d m )/ t … … … … … … … … … … … … … … … … … …
3.1. Preparation of electrically conductive adhesive as first-stepfunctionalization
(2)
Where:=shear strain at ultimate length (UL), dimensionless=displacement of the test sample at UL, cm=displacement of the metal adherend at UL, cm. In the study, it was neglected.t=thickness of the bondline, cm The benefit of using the nylon textile is obvious, even if it affects adversely the mechanical strength in an acceptable range. If coated with Ag, the textile provides as template of an ideal network to ensure the electrical conductivity other than the statistic percolation mechanism, the amount of Ag and the conductivity can also be precisely and optimally designed and controlled.
In commercial structural adhesive technologies, polymer nonwoven (veil) is commonly used to retain the adhesives from its flowing away, to accelerate wetting of adherends, and to control the bondline thickness [28–32]. In the present study, we used polyamide (Nylon) textile material instead of a nonwoven polymer. The unique feature of the textile used in this study is, chemically coat it with silver (Ag) to produce an electrically conductive and good mechanical support for adhesive film. As measured, the areal density of the nylon textile was increased from 18 g/m2 to 24 g/m2 before and after the Ag-coating, respectively. As can be seen in Figs. 1(a) and 2, the nylon textile was fully coated with Ag. The coating thickness of Ag on each fiber was less than 15 μm. The apparent square resistance of the textile Ag-coated was about 0.06Ω/square. The textile remained tough and flexible after the silvering. The original thickness of the nylon fiber was about 35– 40 μm; we call this step of processes First-Step-Functionalization at micro-scale. The textile was then hot-pressed into an 180 °C curing epoxy adhesive film and the film was subsequently used to adhesively bond
3.2. Preparation of electrically conductive interface layer as SecondStep-Functionalization There were two options to make the interface between conductive adhesive and metal adherend electrically conductive while maintaining the mechanical properties of the bond is less affected. The first one was to deposit silver nanowires (AgNWs) of higher volume fraction on the adhesive film surfaces to especially add more electrical contacts, and
Fig. 2. (a) SEM images of the nylon textile chemically coated with Ag, and (b), a zoom-in SEM image of (a).Overall, the textile is well-coated with Ag.
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Fig. 3. (a) Resistance-viscosity-temperature plot of the textile supported adhesive bond during a curing process, showing a switch-like transition of the resistance in dependence of the cure-temperature, and (b), shear stress – strain curves of two bonds in comparison between a baseline (control) and an electrically conductive bond with the same epoxy adhesive reinforced by the nylon-textile.
single whisker in the structure is about 10-50 μm. During the curing and bonding process, both the AgNWs and TZnOs are densely compressed in the 15 μm thin boundary layers between the adherend and adhesive film, and both of them are fully wetted by the adhesive, as shown in the figures. However, the one-dimensional AgNWs might be easily oriented parallel to the flat surface of the adherend and easily isolated by the polymer wetting, whilst the TZnOs may mechanically and electrically bridge the aluminum adherend and the conductive adhesive film crossing the boundary layer due to their structural size and rigidity. The assumption is partially verified by the observation of the representative cross-section SEM images of the joints with the AgNW- and TZnO-adhesive, respectively. For the AgNW-adhesive bond, Figs. 4(b) and (c), polymer-rich zones become visible along the interface between the adherend and adhesive, leading to an electrical isolation. They are marked by red arrows. A separation is also observed between the nylon fiber and adhesive, caused by chemical etching for sample preparation prior to the SEM analysis. For the TZnO-adhesive bond, polymer-rich zones are also visible in the fiber/adhesive interface. Fig. 4(e) and (f) shows the cross-section of the joint with TZnO-adhesive. Original and complete TZnO structure is rarely recognized. They may be mechanically fragmented by the compression during the bonding process. Consequently, a TZnO-rich zone is developed in the interface. It is believed that there are numerous mechanical and electrical contacts in the interface zone, helping to strengthen mechanically the interface and establish the electrical paths. This was found to be true, as obviously shown in EDS mapping image in Fig. 5(a). Overall, the Ag elements can be found in the adhesive bondline. They are particularly concentrated around the nylon fibers and in the TZnO-rich interface between the aluminum adherend and adhesive film, suggesting a complete electrical network established across the matrix. Obviously, the Ag observed comes from the coatings on the nylon textile and on the TZnOs, respectively. Fig. 5(b) shows a localized EDS mapping on the fractured surface of the electrically conductive bond. At a first look, it is a cohesive failure observed in the “weak layer [38]” of the adhesive close to the nylon fiber. In this region, numerous well-distributed Ag elements are also found and they are particularly marked in an artificial rectangular frame. The result implies a good bonding strength between the adhesive and the fiber, and the bonding apparently is mechanically less affected by incorporating Ag into the interface. The bond strength and electrical conductivity are thus improved significantly and simultaneously, see Table 1. Table 1 report also data of a bonded baseline sample, SY-14 as control, with the same epoxy adhesive reinforced by nylon textile, and a
another one was to use rigid three-dimensional conductive component, for example Ag-coated tetrapod-like ZnO particles (Fig. 1(c)) instead of using AgNWs. Through this modification, the adhesive film surfaceloaded on the both sides showed an initial thickness of about 400 μm; consisting of two AgNW- or TZnO-rich surfaces, each of them was about 15 μm in thickness. In this paper, the first one is simply denoted as AgNW-adhesive and the last one as TZnO-adhesive, respectively. Both of them are obviously a sandwich system (Fig. 1(b)). Test results on adhesively bonded aluminum joints using the sandwich bondline show that both functional additives are effective to improve the bond lap strength. As shown in Table 1, using the AgNW-adhesive with a local mass fraction of about 25% in the two thin boundary layers, the average lap shear strength of the thin-wall lap bonds reaches a level of 27.1 MPa, and a floating-roller-peel-strength of 4.5 N/mm. However, the final bonds cured become electrically isolated. In contrast to them, the bonds with the TZnO-adhesive demonstrate improvement in both the bond strength and electrical conductivity in the same time, depending on the local mass fraction of TZnOs. Generally, the lap shear strength decreases with the TZnOs, whereas the electrical conductivity increases. The conductivity difference observed can be ascribed to the structure difference between the one-dimensional AgNWs and threedimensional TZnOs, if we compare them in Fig. 4(a) and (d). The AgNWs used in the study show an average diameter of about 70 nm and lengths in a range of 35-45 μm, Fig. 4(a), whereas the TZnOs used can be considered as a four single-whisker structure grown from one juncture in a tetrapod-like manner, Fig. 4(d) [36]. The length range of a
Table 1 Comparison of typical mechanical and electrical properties of adhesively bonded aluminum joints studied. Samples
Lap shear strength [MPa]
Floating roller peel strength [N/mm]
Electrical conductivity [S/ cm]
AgNW-adhesive, Surfacial mass fraction of 25%
27.1
4.5
∼0
12.5% TZnO-adhesive, Surfacial mass 25.0% 37.5% fraction of SY-14adhesive with nylon textile State-of-the-art commercial conductive reference
30.8 26.8 18.9 29.4
– 4.2 – 5.9
< 0.1 1700 2100 ∼0
8.5
∼0
2000
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Fig. 4. (a) SEM image of AgNWs used; (b) cross section of an Al-bond with AgNW-adhesive, and (c), a zoom-in image of (b); (d) SEM image of TZnO particles used; (e) cross section of an Al-bond with TZnO-adhesive, and (f), a zoom-in image of (e).(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
bondline is highly reinforced and electrically networked, resulting in high bond strengths and electrical conductivity. In this approach, Ag-coating is especially effective and materialsaving. It can provide efficient electrical conductivity with only limited quantities, for example, with an overall weight of 13.8% of Ag plus ZnO, the adhesive bond achieves a conductivity level of 1700 S/cm, and shear strength of 26.8 MPa, where surfacial TZnOs mass fraction is about 25%, Table 1. In the same time, the good mechanical strengths shown in Table 1 are expected to be promising for structural adhesive applications [37]. It is furthermore envisaged that the adhesive technology developed is not only applicable for metal bonding, but also for honeycomb structure and bonding of dissimilar materials. Technologically, the bondline structure responsible for high mechanical properties and high electrical conductivity is established in two steps, we call hence this approach Two-Step-Functionalization. The process using the micro-scale textile and nano-scale TZnO particles as templates is obviously beneficial because no ball milling
state-of-the-art commercial conductive adhesive product with a similar chemistry and technical profile. It can easily figure out by comparison that the TZnO-adhesive bonded samples provide balanced high performance in mechanical as well as electrical properties.
3.3. Two-step-functionalization Upon the results presented and discussed above, Fig. 6 illustrates schematically the sandwich bondline structure with a central adhesive film nylon-textile supported, together with SEM microphotographs of the TZnO-rich surface and a representative cross-section of the bond. On the TZnO-rich surfaces-produced, numerous reinforcing TZnOs particles including complete TZnOs are observable. It is obvious that the particles play a bi-functional role as mechanical reinforcement and as electrical conductors ascribed to their structural sizes, rigidity and concentration. As a result of the synergic effect of the nylon-textile in the center and the TZnO-rich boundary layers in the interfaces, the
Fig. 5. (a) EDS mapping image of a TZnO-adhesive bonded Al-joint showing Ag elements distributed in the bondline, with a contrasted insert image; and (b), fractured surface of the bond showing a localized EDS mapping.
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Boundary layer (interface), 7.5% thick, TZnO-rich;TZnOs are Ag-coated for electrical conductivity. Second-StepFunctionalization
Adhesive film, 85% thick, Nylon-textilesupported, the fibers are Ag-coated for electrical conductivity. First-Step-Function
Boundary layer (interface), 7.5% thick, TZnO-rich;TZnOs are Ag-coated for electrical conductivity. Second-StepFunctionalization Fig. 6. Schematic illustration of the sandwich bondline structure consisting of an adhesive film nylon-textile supported in the center and two TZnO-rich boundary layers as interfaces ordered on the both surfaces, with respective text explanation and SEM microphotographs. Both the textile and TZnOs are coated by Ag for electrical conductivity. [10] Kasemir HW, Kasemir Heinz-Wolfram. Mazur V, Ruhnke LH, editors. His Collected Works. USA: American Geophysical Union; 2012. p. 418–28. [11] Willett JC, Park G, Krider EP, Peng GS, Simmons FS Law GW. Triggered lightning risk assessment for reusable launch vehicles at the southwest regional and Oklahoma spaceports.In: Proceedings of the 86th AMS Annual Meeting, Atlanta, Georgia; 19-30; 2006. [12] Ghosh K, Maiti SN. Mechanical properties of silver-powder-filled polypropylene composites. J Appl Polym Sci 1996;60(3):323–31. [13] Novák I, Krupa I, Chodák I. Electroconductive adhesives based on epoxy and polyurethane resins filled with silver-coated inorganic fillers. Synth Met 2004;144(1):13–9. [14] Rao Y, Lu DQ, Wong CP. A study of impact performance of conductive adhesives. Int J AdhesAdhes 2004;24(5):449–53. [15] Mclachlan DS. Equation for the conductivity of binary mixtures with anisotropic grain structures. J Phys C-Solid State Phys 1987;20:856–77. [16] Mclachlan DS. Measurement and analysis of a model dual-conductivity medium using a generalized effective medium theory. J Phys C-Solid State Phys 1988;21:1521–32. [17] Horyak GL, Patrissi CJ, Martin CR. Fabrication, characterization, and optical properties of golden nanoparticle/porous alumina composites: the non-scattering maxwell-garnett limit. J Phys Chem B 1997;101(9):1548–88. [18] Zhang C, Yi XS, Yui H, Asai S, Sumita M. Selective location and double percolation of short carbon fiber filled polymer blends: high-density polyethylene/isotactic polypropylene. Mater Lett 1998;36(1-4):186–90. [19] Shi FH, Li J, Zhang BY, QiuX S, Gu LY. Preparation and Electrical Properties of Graphene/PEK-C Films. Adv Mater Res 2015;1102:107–12. [20] Estrada D Do JW, Chang Xie X, Mallek N.J N, Rogers Girolami GS, Pop E JA, Lyding JW. Nanosoldering carbon nanotube junctions by local chemical vapor deposition for improved device performance. Nano Lett 2013;13:5844–50. [21] Wu HP, Wu XJ, Liu JF, Zhang GQ, Wang YW, Zeng YW, Jing JZ. Development of a novel isotropic conductive adhesive filled with silver nanowires. J Compos Mater 2006;40(21):1961–9. [22] Lin Y. Functionalized interleaf technology in carbon-fibre-reinforced composites for aircraft applications. Natl Sci Rev 2014;1(1):7–8. [23] GuoM C, Yi XS. The production of tough, electrically conductive carbon fibercomposite laminates for use in airframes. Carbon 2013;58:238–51. [24] Guo MC, Yi XS, Liu G, Liu LP. Simultaneously increasing the electrical conductivity and fracture toughness of carbon-fiber composites by using silver nanowires-loaded interleaves. Compos Sci Technol 2014;97:27–33. [25] ASTM D3165-07. Standard Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies; 2014. [26] ASTM D6862-11. Standard Test Method for 90 Degree Peel Resistance of Adhesives; 2016. [27] ASTM D5656-10, Standard Test Method for Thick-Adherend Metal Lap-Shear Joints for Determination of the Stress-Strain Behavior of Adhesives in Shear by Tension Loading. [28] Brewis DM, Comyn J, Cope BC, Moloney AC. Further studies on the effect of carriers on the performance of aluminum alloy joints bonded with an epoxide polyamide adhesive. Polymer 1980;21(12):1477–9. [29] Gouri C, Ramaswamy R, Ninan KN. Studies on the adhesive properties of solid elastomer-modified novolac epoxy resin. Int J Adhes Adhes 2000;20(4):305–14. [30] Gouri C. Elastomer modification of epoxy based film adhesives: adhesive and mechanical properties. J Adhes Sci Techol 2002;16(12):1569–83. [31] Schroeder JA, Ahmed T, Chaudhry B, Shepard S. Non-destructive testing of structural composites and adhesively bonded composite joints: pulsed thermography. Compos Part A-Appl S 2002;33(11):1511–7. [32] Tahania M, Yousefsani SA. On thermomechanical stress analysis of adhesively
and ultrasonic dispersion are needed as the traditional process does. The process control becomes easy and a better bonding quality could be easily achieved. 4. Conclusions In this paper, a new adhesive technology, called Two-StepFunctionalization adhesive technology, has been presented to develop high-strength structural bonding with high electrical conductivity. A nylon textile is first used for supporting and reinforcing adhesive film. It provides in the same time a template for surface-coating with silver to produce an electrically conductive adhesive film. The electrical function is thus primarily integrated into the structural supporting textile. This is the First-Step-Functionalization at micro-scale. The conductive adhesive film nylon-textile-supported is then surfaceloaded with three-dimensional tetrapod-like zinc oxide particles (TZnOs) coated also with Ag to form a sandwich adhesive film having an enhanced surface electrical conductivity. The surface structure is particularly effective to prevent adhesive wetting, which often occurs during the curing if the viscosity of the adhesive falls into a critical low level with the increasing temperature. This is the Second-StepFunctionalization at nano-scale. As demonstrated, the aluminum adhesive joints using the Two-Step-Functionalized approach shows balanced high electrical conductivity and mechanical properties by using only limited quantity of Ag, compared to a state-of-the-art product. References [1] Norman HRH. Conductive Rubbers and Plastics: Their Production, Application and Test Methods. New York: Elsevier Pub. Co.; 1970. p. 1–22. [2] Bulgin D. Electrically conductive rubber. Rubber ChemTechnol 1946;19(3):667–95. [3] Chiang CK, Fincher C, Park YW, Heeger AJ, Shirakawa H, Louis EJ, Gau SC, MacDiarmid AG. Electrical conductivity in doped polyacetylene. Phys Rev Lett 1977;39(17):1098–101. [4] Lee LH. Adhesive Bonding.. New York: Plenum Press; 1991. p. 239–84. [5] He XC. A review of finite element analysis of adhesively bonded joints. Int J Adhes Adhes 2011;31(4):248–64. [6] Schindel DW. Air-coupled ultrasonic measurements of adhesively bonded multilayer structures. Ultrasonics 1999;37(3):185–200. [7] Bai JM, Sun CT. The Effect of viscoelastic adhesive layers on structural damping of sandwich beams. Mech Struct Mach 1995;23(6):1–16. [8] Hansen SW, Spies RD. Structural damping in laminated beams due to interfacial slip. J Sound Vib 1997;204(2):183–202. [9] Feraboli P, Miller M. Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike. Compos Part A-Appl S 2009;40(67):954–67.
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mechanical properties of the adhesives. J Adhes Sci Technol 2014;28(11):1055–71. [36] Dai Y, Zhang Y, Lia QK, Nan CW. Synthesis and optical properties of tetrapod-like zinc oxide nanorods. Chem Phys Lett 2002;358(1-2):83–6. [37] Sargent JP. Adherend suface morphology and its influence on the peel strength of adhesive joints onded with modified phenolic and epoxy structural adhesives. Int J Adhes Adhes 1994;14(1):21–30. [38] Mahoney CL. Surface preparation for adhesive bonding. In: Skeist Irving, editor. Handbook of Adhesives. New York: Springer; 1990. p. 74–93.
bonded composite joints in presence of an interfacial void. Compos Struc 2015;130:116–23. [33] Hahn O, Schlimmer M, Yi XS. Mechanisches Verhalten der Klebschicht einschnittig ueberlappter Metallklebverbindungen im Zugversuch. Kunststoffe. 1986;76:9, [804/808]. [34] Sun YT, Pugno N, Gong BM. A simplified hardening cohesive zone model for bondline thickness dependence on adhesive joints. Int J Fract 2015;194(1):37–44. [35] Yang S, Xua W, Liang LH, Wang TC, Wei YG. An experimental study on the dependence of the strength of adhesively bonded joints with thickness and
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