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Marie Curie Summer School EFFECT OF SILANE SURFACE TREATMENT Knowledge Based Materials ON DURABILITY OF ACRYLIC-ALUMINIUM BONDS Jesús R. Jimenez-Octavioa, M. Cano de Santayana-Ortegab and Juan C. del Real-Romerob a)Institute of Research in Technology; email@example.com b) Department of Mechanical Engineering “Multi-Phase Materials ” Pontificia Comillas University, Alberto Aguilera,23 28015 Madrid (Spain). 14-24 August 2006 Älvdalen, Sweden 1.- General Abstract 2.- Technical Abstract The use of adhesive bonding in the manufacture of load bearing Silane coupling agents are a environmental friendly alternative to other surface structures can provide numerous benefits in comparison to more traditional treatments which use toxic chemicals (hexavalent chromium compounds (VI), strong joining techniques such as mechanical fastening or welding. Thus, for acids, etc), such as CAE, CAA or PAA. Due to the toxic and carcinogenic nature of bonded aluminium structures, in order to promote long-term durability, hexavalent chromium [Cr(VI)] its use is limited or prohibited for environmental surface treatment of the alloy prior to bonding has generally been regarded regulations in several countries . Thus, the aim of this work was to investigate the as a vital component of the manufacturing process. effect of the organosilanes on the durability of aluminium joints bonded with toughened acrylic adhesives. R-Si(OR’)x Industrial Applications Automotive, aerospace, naval industries, etc. ⇓ Typically, a silane coupling R-Si(OR’)3 agent has a hydrolyzable alkoxy Materials processing Adhesive Surface Pretreatment 3H2O group such as methoxy (OCH3), Metals, polymers, ceramics, etc Epoxies, phenolics, urethanes and acrylics. Mechanical, chemical, electrochemical ethoxy (OC2H5) and acetoxy 1. HYDROLYSIS (OCOCH3) and an organofunctional 3.- Glossary 3R’OH group. The alkoxy groups, OR, hydrolyze in the aqueous Term Definition R-Si(OH)3 environment and form covalent Non-metallic substance capable of joining materials by surface bonding (adhesion), and the bond possessing adequate Adhesive internal strength (cohesion) 2Si(OH)x bonds with hydroxyl groups on the surface, as well as with other Adhesion The state in which the particles of the adhesive and the adherend are held together hydrolyzed silane molecules. The Cohesion The state in which the particles of the adhesive, or the adherend, are held together 2. CONDENSATION 2H2O organofunctional group, is An adhesive that is a solution of rubber-base polymers in methacrylate monomers. Acrylic adhesives bond quickly at expected to reacts with the organic Acrylic adhesive room temperature, and can adhere to oily surfaces, as well as to many types of materials, including copper, most plastics, glass, and wood. R R R polymer. | | | Epoxy adhesive Synthetic resin containing epoxide groups usually used as structural adhesives or encapsulants OH Si O Si O Si OH Silane coupling agents Durability The ability of an adhesive to maintain an adequate bond under the conditions of service that the joint has to withstand | | | therefore have the possibility of OH OH OH working as adhesion promoters Bondability Ability of a substrate to form a bond of specified properties with a specified adhesive under specified conditions. between the adhesive and the P2 etch Sulfo-ferric acid etch for surface treatment + metal surface. Typically, silane CAA Chromic acid anodising for surface treatment coupling agents with amine, epoxy, OH OH OH CAE Chromic acid etch for surface treatment (i.e. FPL: Forest Product Laboratory) | | | methacrylic, or vinyl SUBSTRATE organofunctional groups are used Mechanichal treatment in which a high-pressure airline projects grained grit onto the top surface of the metals producing Grit blasting a finish similar to cleft. for adhesion enhancement. Figure 1: Organosilane hydrolysis and condensation reaction mechanisms Table 1: Glossary 4.- Experimental Procedure 5.- Results and Discussion (a) (b) (c) Silane coupling agents used in this study were Several results are shown and briefly discussed supplied by Aldrich. Their chemical formulation and below, firstly single lap shear tests and secondly nomenclature are showed in Table 2. wedge tests. Failure Load (N) Name Abbrev. Formulation Figure 6: Images (SEM x 1000) of surfaces treated with: a) Grit blasting b) P2 etch c)Abrasion 7000 3-Glycidoxypropyltrimethoxysilane γ-GPS CH2CH(O)CH2OCH2CH2CH2Si(OCH3)3 6000 Over these lines, surface characterization images 5000 are shown (Figure 6). These pictures are taken by SEM 3-Aminopropyltrimethoxysilane H2NCH2CH2CH2Si(OCH5)3 As received (33% Rh-25ºC) γ-APS for different surface treatments. 4000 3-Methacryloxypropyltrimethoxysilane γ-MPS CH2=C–COOCH2CH2CH2Si(OCH3)3 3000 Finally, Figures 7 and 8 show fracture surfaces 2000 Table 2: Silane coupling agents used in this study and fracture energy evolution in wedge tests with 1000 different surface tretments. Crack Fracture 0 propagation after test The single-lap specimens were made according to Initial Crack Initial γ-GPS γ-APS γ-MPS P2 etch Initial crack the procedure described in UNE EN 1465 . The single- Surface treatment (a) crack propagation (b) crack (c) lap joint specimens, schematically shown in Figure 2, Control Humidity 95% were made from EN AW 6082-T6 aluminium alloy sheet. Figure 4: Failure load for lap shear specimens after 30 days of exposure at 95% HR and 50ºC Figures 4 and 5 show failure load data and 12,5 fracture surfaces in single-lap shear tests with different surface tretments. Figure 7: Fracture surfaces of test specimens after 30 days of exposure at 95% HR and 50ºC: Figure 2: Schematic of single-lap joint a) γ-APS b) γ-GPS c) γ-MPS (a) Wedge test specimens were prepared with some The surface treatmens γ-APS 6000 deviations to ASTM D3762 . Dimensions are showed and γ-GPS provide unsatisfactory 5000 in Figure 3. Adherend thickness was 8 mm to avoid results even with dry enviromental plastic deformation in specimens. The adhesive was conditions. Their behavior were much 4000 GIC (J/m ) 2 spread so as to leave at one end of the joint a length of worse than specimens without any 3000 19 mm bare and ready for the insertion of the wedge. treatment (as received). 8 mm 2000 Adhesive (b) (c) Cohesive (d) Cohesive failure failure 1000 0 0 10 20 30 40 50 60 70 ½ Time (hours ) 19 mm γ-APS γ-GPS γ-MPS P2 FPL GB Adhesive Adhesive failure failure 150 mm Figure 8: Fracture energy against exposure time for: γ-APS, γ-GPS, γ-MPS, P2 etch, FPL and Grit-blasting Figure 5: Fracture surfaces of test specimens after 30 days of Figure 3: Schematic drawing of Wedge test specimen cure at 95% HR and 50ºC: a) γ-APS b) γ-GPS c) γ-MPS d) P2 Results of this test are analogous to the Specimens were cured in a dessiccator at room previous one, showing deficient results for γ-APS and temperature for at least 7 days. When the joints were 6.- Conclusions γ-GPS. However, γ-MPS treatment present a fully cured wedges were driven fully into place by with satisfactory behaviour and, although FPL is slightly a tensile testing machine at a constant rate of 10 γ-APS and γ-GPS behavior with the acrylic better, γ-MPS avoids enviromental problems. mm/min. Measurement of initial crack length and crack adhesive used is deficient. These pretreatments The incompatibility of γ-APS and γ-GPS with the growth was carried out in both edges of each present unsatisfactory results even with dry acrylic adhesive seems to be the reason for this bad specimen. enviromental conditions. behavior. The fracture energy, GIc, was calculated from the mean Durability obtained with test specimens treated value of the total crack length in each case. with γ- Metacriloxipropiltrimetoxisilane are similar 8.- References to those obtained with P2 etch treatment. γ-MPS 1. Directive 200/53/EC of the European Parliament 2 3 3E ∆ h enhances adhesion between aluminium and acrylic and on the Council on end-of-life vehicles. Offical GIC = J. Eur. Commun. L170, 81-84 (2002). 16(a + 0, 6h) 4 adhesives; and less enviromental problems than Where: hexavalent chromium based compounds. 2. UNE EN 1465:1996 Standard. Determination of GIc is the Mode I fracture energy, tensile lap-shear strength of rigid-to-rigid bonded E is the Young's Modulus of the adherends (70.5 GPa for aluminium), 7.- Acknowledgements assemblies. (1996) 3. ASTM D 3762:98 Standard Test Method for ∆ is the displacement at the load point Marie Curie fellowship for financing my attendance to Adhesive-Bonded Surface Durability of Aluminum h is the adherend thickness the Knowledge Based Materials Summer School 2006. (Wedge Test).
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