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Chromic acid anodising (CAA), preceded by degreasing and acid etching is the European norm for obtaining optimal strength and durability when adhesively bonding aluminium alloys 1 .
Plasma polymers (PP) are being investigated as an alternative to chromate based processes because of the environmental compliance of the process and the potential superior properties offered by such materials.2,3 Furthermore, plasma preparation of the aluminium surface offers an integrated and controllable cleaning-deposition process which may represent an advantage over solvents, alkali cleaning and acid etching.4 Here, the use of carboxylic acid functionalised plasma polymer coatings is investigated,5,6 where the functionalities are intended to form chemical bonds with the substrate and the adhesive.7
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Figure 1: (a) Optical image of both sides of the non-aged joint failure
(b) aged joint showing visible metallic areas
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Materials and plasma pretreatments
- Substrate:
- AA6016 Aluminium
- Deposits:
- Plasma polymerised acrylic acid (ppAAc) at power 2 and 20W
Plasma polymerised acrylic acid-co-octadiene (ppAAc-co-Oct) at 10 and 40nm thickness at power 2W.
All plasma deposits employed were preceded by an Ar plasma etch at P = 100 W.
- Adhesive:
- Ciba Polymers XW1044-3, cured at 160°C for 60 min and then at 180°C for 30 min.
- SLS Testing:
- 3 repeats tested at 2 mm/min, 20°C at RH=50%.
- Ageing of joints:
- 1 week immersed in deionised water at 60°C.
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Plasma polymer durability performance compared with CAA pre-treatment.
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Figure 2: (above)
Failure stress of SLS joints before and after ageing in deionised water at 60°C for a week. The decrease in the initial bond strength is given in parentheses.
Samples: 2 is P=2W ppAAc, 20 is P=20W ppAAc and 2-co is P=2W ppAAc-co-Oct while CAA is chromic acid anodised
Figure 3: (right) -->
(a) Schematic illustrating failure of SLS joints on a macro- and
(b) nano-scale.
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Tensile testing of SLS joints
The failure stress of SLS joints, before and after ageing, is presented in Figure 2 for a range of plasma pretreatments and CAA.
Failure stress of ppAAc-co-Oct (10nm) = 21.9 ± 1.5 MPa, prior to ageing, which compared well with that of CAA, 22.5 ± 0.8 MPa.
Failure stress of a non-functional plasma polymer coating (plasma polymerised hexane (ppHex); [C]= 99 at% and [O]= 1 at%) is very low, 2.8 ± 0.5 MPa.
Non-aged joints
Failure occurred in the adhesive as shown schematically in Figure 3(a). Figure 1(a) shows an optical image of a typical non-aged joint failure.
Aged joints
Joint strength decreased after ageing: optical images Figure 1(b) showed presence of metallic regions indicating interfacial failure, possible loci of failure are shown schematically Figure 3(b)
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Figure 4: Transmission electron micrograph of an ultramicrotomed section from the metallic area of an aged joint; AA6016 pretreated with 40nm ppAAc 2W (including 1 minute Ar etch) and bonded with magnesium silicate filled epoxy-amine adhesive.
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Cross-sections from the fracture surface were examined using TEM. Figure 4 shows a typical structure of a metallic area of aged joint.
A distinct layer of PP between 20-40nm thick was observed bonded to the oxidised Al surface.
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Figure 5: Al 2p, Mg KLL and C 1s photoelectron images were acquired from a region of visible metallic and epoxy region. Elemental images were acquired at 160eV pass energy in 2 min (peak-background)
Figure 6: Chemical state images acquired at 286.5eV for the epoxy/ether component and at 289.1eV for the acid functionality. Images were acquired with 80eV pass energy in 5 min (peak-background)
Figure 7: shows C1s species of different concentrations consistent with the photoelectron images.
(move your mouse over the analysis areas to view spectra).
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XPS imaging of the fracture surface in both elemental and chemical states mode combined with small spot analysis revealed the lateral distribution of the species.
Non-aged
Particles of magnesium silicate in a matrix of amine-cured epoxy.
Aged
A representative image from the failure surface of an aged joint, including both a visually metallic area and adjacent resin, is presented in Figure 5.
Elemental imaging
The photoelectron images of Figure 5 show the presence of oxidised Al in the visually metallic region and a distribution of C in the adjacent epoxy region. Also visible in the XPS images but not apparent in optical microscopy are the magnesium silicate filler particles.
Chemical state imaging
The chemical state photoelectron images of Figure 6 taken in the same area were acquired at 289.1eV BE for the acid (C(=O)OX) functionality and at 286.5eV for the epoxy/ether (C-OX) functionality.
Small area analysis
Figure 7 shows C1s species of different concentrations consistent with the photoelectron images.
Summary of small spot analyses
Point 1 Metallic region.
Oxidised Al layer with an over layer of polymer. High resolution XPS reveals the C1s to be composed of several species shifted from the hydrocarbon component by 1.5, 3.1, 4.1 and 5.1eV associated with epoxy/ester/ether, carboxylate, acid/ester and carbonate functionalities respectively.
The metallic region therefore has a combination of contributions from PP and epoxy resin.
The presence of Al 2p signal indicates that PP film is either less than 10nm thick or inhomogeneous. TEM showed 20-40nm thick PP indicating PP is inhomogeneous on a lateral scale unresolvable by XPS.
Point 2 Particulate filler.
This area appears to have a significant overlayer of adhesive from the C1s peak with a slightly increased hydrocarbon contribution.
Point 3 Resin area.
The cohesive resin failure region is characterised by an epoxy/ether functionality at a 1.5 eV shift from the hydrocarbon peak
Epoxy side
Analysis of the epoxy side of the adhesive failure area, indicates a similar concentration of acid/ester functionalities and silicone as the complimentary surface illustrated in Figure 3. There is no contribution from the filler to the XPS analysis which is consistent with the presence of a particle-free region that has also been observed in electron micrographs of the intact interface region of the adhesive.
Variable nitrogen concentrations were observed in the metallic areas, in some cases up to 10 times greater than that generally found in the bulk adhesive. This suggests segregation of the amine hardener. Silicone was also identified at a low concentration. However, the data do not reveal whether segregation, or other factors are responsible for the interfacial failure.
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Promising durability and strength results were achieved from a range of carboxylic acid plasma polymers deposited on AA6016, one which compared favourably with the conventional CAA pretreatment process.
Comparison of the SLS strength of non-functional (ppHex) and functional plasma polymers (ppAAc and ppAAc-co-Oct) indicated that the carboxylic acid functionalities played an important role in forming a strong interfacial bond with the adhesive.
A combination of TEM and imaging XPS was required to determine that failure occurred at both the PP-aluminium oxide and PP-epoxy interfaces after ageing of epoxy bonded SLS joints.
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[1] G.W. Critchlow and D.M. Brewis, Int J.Adhesion and Adhesives, 16, 255 (1996)
[2] T.J. Lin, J.A. Antonelli, D.J. Yuang, H.K. Yasuda and F.T. Wang, Progress in Organic Coatings, 31, 351 (1997)
[3] R.H. Turner, I Segall, F.J. Boerio, G. D. Davis, J. Adhesion, 62, 1 (1997):
S Eufinger and W.J.Van Ooij and K.D Conners Surface and Interface Analysis, 24, 841 (1996)
[4] W. Petasch, B. Kegel, H. Schmid, K. Lendenmann and H. U. Keller, Surface and Coatings Technology, 97, 176 (1997).
[5] M. R. Alexander and T. M. Duc, J. Mat Chem., 8 (4), 937 (1998).
[6] M. R. Alexander and T. M. Duc, Polymer, 40 (20), 5479 (1999).
[7] M.R. Alexander, S. Payan and T.M. Duc, Surface and Interface Analysis, 26,. 961 (1998).
This poster was presented at ASA-6
"Loughborough Adhesion" Conference, Loughborough University, UK, 18th - 20th April 2000
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