Sam Rostami graduated from Imperial College, London. He has over 20 years experience in chemistry and physical chemistry of polymers, polymer surfaces, interfaces and adhesion. He has worked in ICI Advanced Materials and Acrylics R&D Departments on all aspects of polymers, blends and composites, reaching the position of Business Research Associate in ICI. He has a wealth of experience in fundamental research, new product development and characterisation of multi-component materials, sufaces and interfaces. He is co-editor and author of a book on 'Multi-component Polymeric Systems', author of five chapters in technical books, over fifty published technical papers and eight patents. He joined Advanced Materials and Processes Department of TWI in 2001 as a Principal Project Leader in polymers and adhesives. He has since worked on numerous industrial projects including surface modification of metals for metal-polymer and metal-adhesive bonding.
Ewen Kellar graduated from Edinburgh University in Chemistry and followed this with an MSc and PhD in Physical Chemistry at the University of East Anglia. He has worked as a postdoctoral assistant at Univeristy College of Swansea and in the Materials Department of Queen Mary and Westfield College, London. He joined TWI in 1996 in the Advanced Materials and Processes Department as a Senior Oroject Leader in Adhesives Technology. Since then he has worked on the analysis, characterisation and testing of engineering polymers, adhesives, surface pre-treatment of metals and composite materials.
It has been found that a popular aerospace aluminium alloy, with a new method of surface preparation involving chemical bonding, has produced adhesive durability results comparable with those made using the existing, and soon to be restricted, industry standard.
The aerospace industry has successfully used adhesive bonding of aluminium alloys for many years. As Sam Rostami and Ewen Kellar explain the key issues of high strength and durability have been addressed through the implementation of highly controlled, rigorous surface pre-treatment prior to the bonding process.
The chromic/sulphuric acid etch followed by an anodisation process to produce a controlled oxide layer is a favoured and common surface pre-treatment that is used in industry. Chromic/sulphuric acid etching involves the use of hexavalent chromium, Cr 6+ , compounds. However, Cr 6+ compounds present in the chromic/sulphuric acid etching solution are hazardous, cancer-causing substances. Recent European Union Directives will soon restrict the use of such acid etching in the metals pre-treatment industry. In general, these directives will hold manufacturers responsible for the hazardous materials that are produced and used. Alternative methods to the chromic/sulphuric acid surface pre-treatment will be needed in the near future.
Several new processes are being investigated as possible alternatives to those described. In this report, a new approach to surface pre-treatment for aluminium alloys has been investigated. The process is based on in-situ creation of aluminium hydroxide (pseudo-boehmite) and the subsequent use of its hydroxyl groups in a condensation reaction with epoxy organosilanes, see Fig.1. Although the organosilane has been used in the past for surface pre-treatment of metals, the creation of this hydroxide layer is the key distinguishing factor in this investigation. After treatment, epoxy functional groups at the aluminium surface become available for incorporation in organic reactions such as adhesive bonding, corrosion protection and/or decorative coatings. The effects of the new surface pre-treatment on the durability performance of bonded joints are evaluated.
The formation of covalent bonds (-Si-O-substrate) between the organosilane and aluminium has been established elsewhere using time of flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS).
In similar work to that herein, Rider and Arnott recently reported the durability performance of adhesively-bonded aluminium alloys (2000, 5000 and 7000 series) that were surface pre-treated using dry grit blast cleaning followed by immersion in boiling water and organosilane functionalisation.
In this report we demonstrate a new method of hot wet grit blast cleaning, that has never been used before, as a faster alternative to cold grit blast cleaning and boiling water treatment.
Experimental
Aluminium 2024 T-3, gamma-glycidoxypropyltrimethoxysilane (GPS - Silquest 187) and FM ® 73 epoxy adhesive film supplied by Cytec Engineered Materials and XD4600 epoxy adhesives supplied by Dow Automotive were used.
Surface Preparations
Prior to any other surface pre-treatment, all alloy specimens were degreased by immersion in a standard degreasing solution (10 vol% of sodium hydroxide in de-ionised water) at 55±1°C for 10 minutes. They were washed with de-ionised water for 15 minutes and dried in an oven at 50±1°C for 15 minutes. Chromic/sulphuric acid etching with and without an anodisation process was used as the control for the organosilane functionalised aluminium.
Further surface pre-treatments were adopted from one, or a combination of, the following processes.
- Dry grit blasting (GB)
- The plates were sprayed for 40 seconds with 50µm aluminium oxide particles dispersed in air/nitrogen at four bar pressure. - Chromic acid etching (CAE)
- The etching treatment was carried out in chromic/sulphuric acid solution for 30 minutes at 65±1°C, followed by washing with de-ionised water for 15 minutes and drying in an air oven at 65±1°C for 10 minutes. - Anodising (ANOD)
- The plates were electrolytically treated in 100g/l of phosphoric acid solution at 10 volts for 30 minutes at 65±1°C. They were then washed in de-ionised water for 15 minutes and dried in an air oven at 65±1°C for 10 minutes.
Surface functionalisation
The following steps are used in organosilane functionalisation of aluminium surfaces:
- Boiling water treatment (WB).
- The plates were immersed in boiling de-ionised water for 10 minutes and then allowed to cool to ambient temperature.
- Freshly made organosilane solutions (1wt% in de-ionised water, adjusted to pH5 using acetic acid) were applied onto the aluminium plates for 10 minutes, within 10 to 30 minutes of its preparation. The organosilane coated plates were then dried in an air oven at 93±1°C for 60 minutes.
Durability Test
A standard wedge rupture test (ASTM: D37628) was used to assess the durability performance of the specimens that had been surface pre-treated. After placing the wedge, the specimens were left to stabilise for one hour, before the first measurements of crack growth were taken. They were then transferred into a humidity chamber at 50±1°C/98%RH. They were removed from the chamber, allowed to cool down to ambient temperature and stabilise for one hour. The crack was located and measured using a magnifying lens. The specimens were returned to the chamber and the experiment was continued for up to 800 hours with repeat crack length measurements at regular intervals.
Release energy calculation
Exposure of the adhesively bonded aluminium to hot and humid conditions accelerates interfacial dissociation depending on the surface pre-treatment used. The loss of adhesion strength may be measured via the length of the crack growth as a function of the exposure time. Furthermore to accelerate failure in the wedge rupture test the joint is initially stressed. The length of crack growth is normalised against material properties and the geometry used to calculate the release energy, G 1 , from the following equation.
G 1 = 1000 (Ed 2 h 3 /16)[3(a + 0.6h) 2 + h 2 ]/[(a + 0.6h) 3 + a h 2 ] 2
In this equation, G 1 is in (kJ/m 2 ), d is the joint thickness (mm), E and h are the aluminium elastic modulus (GPa) and thickness (mm) and a is the measured crack length (mm). The adhesives thickness is t = d-2h.
The above equation assumes that the wedge test specimens can be described as simple cantilever beams. Furthermore, no distinction has been made between adhesive or cohesive mechanisms of failure.
Results
Energy release as a function of ageing time for aluminium 2024 T-3 that had been surface treated by seven different methods is shown in Fig.2. The chromic/sulphuric acid etched and anodised specimens perform best in this case, whereas the chromic/sulphuric acid etched only or dry grit blast cleaned show the poorest durability performance. The dry grit blast cleaned plus organosilane functionalised specimens and also dry grit blast cleaned plus boiling water treated plus functionalised organosilane samples performed well in this test. Surprisingly, the dry grit blast cleaned and organosilane functionalised specimens, without the water boiling, performed well in this test. The absorption of atmospheric moisture on the freshly prepared surface of aluminium 2024 T-3, however, may be adequate to catalyse the condensation reaction between organosilane and aluminium. Furthermore, initial trials using hot grit blasting is shown to speed up the pre-treatment process over the original cold grit blasting and water boiling.
Conclusions
Based on the experimental results shown in this report one can conclude that:
Aluminium 2024 T-3 alloy functionalised with organosilane, GPS, after dry grit blast cleaning and boiling water treatment has provided bonded joints with durability only slightly lower than made by the industry standard chromic/sulphuric acid etching and anodisation processes.
