Novel methods, incorporating pre- and post-anodising steps, for the replacement of the Bengough–Stuart chromic acid anodising process in structural bonding applications

The present investigation focuses on novel anodising processes which are being developed for the replacement of the hexavalent chromium containing 40/50V Bengough-Stuart process, with particular emphasis on their resultant performance in structurally bonded systems used in the demanding and harsh environments encountered on operational aircraft. An electrolytic phosphoric acid based deoxidiser (EPAD)has been studied in combination with a standard sulphuric acid anodise. It has been shown that the EPAD provides an open porous structure in order to enhance adhesion to the modified sulphuric acid anodised (SAA) surface. Additionally, a post anodising (PAD) treatment has been used to aid structural adhesion in combination with the SAA processes. 2 As a control, the standard 40/50V Bengough-Stuart chromic acid anodising (CAA) has been used as a baseline performance indicator in adhesion tests. Single lap shear (SLS) and modified Boeing wedge tests were used to determine adhesion performance. SLS tests were used to determine initial, dry joint strengths whilst wedge test joints immersed in deionised water for up to 100 hours gave a measure of joint durability. Overall, excellent initial joint strengths and durability have been found with both EPAD plus SAA and PAD plus SAA processes suggesting that these environmentally benign treatments may be used as possible drop-in replacements for the currently used hexavalent chromium process. Electron microscopy has been used to investigate the topographical changes introduced to the surface by the various surface pretreatments under investigation to provide an explanation for the observed adhesion test results

Optimisation of boric sulphuric acid anodising based processes for metal-to-metal adhesive bonding

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State of the art hexavalent chromium free surface pretreatments for aluminium alloys

At the present time, the phosphoric acid anodising (PAA) process [1] is widely used in America for the pretreatment of aluminium alloys used in adhesively bonded structures. With the replacement of the Forest Products Laboratory (FPL) etch in the anodising line with a hexavalent chromium free alternative [2], this has solved many of the environmental problems associated with the ever increasing regulations enforced by both national and local authorities. However, this route has never been favoured in the European aircraft industry due to the superior bond durability offered by chromic acid anodising (CAA) relative to PAA in corrosive environments. Furthermore, CAA has been shown to have twice the anodising throwing power and generally higher peel strengths to that of PAA [3]

Friendlier surface treatments - for metals

With only a few exceptions, some degree of surface treatment is applied to all metal surfaces prior to adhesive bonding. The surface treatment applied will depend upon the requirements of the bond and service conditions that it will see and will generally be chosen on a “fit-for-purpose” basis. The minimum preparation which is usually carried out might include a simple degrease to remove processing aids, such as oils and waxes, and contaminants. However, it is recognised that the current state-of-the-art processes for structural or semi-structural metal bonding are highly complex, multi-stage treatments including conversion coating and anodic oxidation. Alternatives to the commonly-used degreasing processes are sought for many reasons, for example: established processes may not be adequate for difficult-to-remove materials; the processes may use VOCs; they can be carcinogenic or ozone depleting. Regarding the higher treatments, the anodising processes, in particular, are difficult, time consuming and costly to carry out. There are also legislative drivers which make utilisation of the more complex processes, especially those which utilise hexavalent chromium, highly undesirable. Other factors such as energy and chemical disposal costs also deserve consideration when considering the need for environmental or operator “friendly” processes. This paper will consider a number of alternative friendly surface treatments which might be considered as drop-in replacements for the current industrial standards. The friendlier surface treatments include two simple cleaning methods, namely: seaweed-based cleaners and CO2 laser ablation. In addition, to cover the spectrum of processes, two novel anodising methods will also be discussed. These are based upon electrolytic phosphoric acid deoxidising plus sulphuric acid anodising (EPAD+SAA) and alternating currentdirect current (ACDC) anodising

The potential of kaolin as a reinforcing filler for rubber composites with new sulfur cure systems

The effect of a large amount of kaolin (China clay) on the viscosity, cure, hardness, Young’s modulus, tensile strength, elongation at break, stored energy density at break, tear energy and compression set resistance of some sulfur-cured natural rubber, polybutadiene rubber and ethylene-propylene-diene rubber composites was investigated. The kaolin surface had been pre-treated with 3-mercaptopropyltrimethoxysilane to improve its dispersion in the rubbers. For natural rubber, the hardness and Young’s modulus improved, tensile strength and tear energy were unchanged and the remaining properties deteriorated when kaolin was added. The viscosity increased and the scorch and optimum cure times decreased whilst the cure rate rose with kaolin. For polybutadiene rubber and ethylene-propylene-diene rubber, with the exception of the compression set resistance, all the properties including the viscosity gained from the kaolin. The kaolin was found to be extending or non-reinforcing filler for natural rubber, and highly reinforcing for polybutadiene rubber and EPDM. In addition, the scorch and optimum cure times and cure rate of polybutadiene rubber benefitted, whereas with the exception of the scorch time, the optimum cure time and cure rate of ethylene-propylenediene rubber were adversely affected by kaolin

Environmentally-friendly surface treatments

The presentation will focus upon both existing and novel developmental processes for the replacement of the hexavalent chromium containing 40/50V Bengough-Stuart anodise (CAA), with particular emphasis on their resultant performance in structurally bonded systems. Two systems of particular interest are based upon a phosphoric acid based electrolytic deoxidiser (EPAD) studied in combination with a standard sulphuric acid anodise and an alternating current-direct current phosphoric-sulphuric (ACDCPSAA) anodise in a benign electrolyte. It has been shown that the EPAD provides an open porous structure in order to enhance adhesion to the modified sulphuric acid anodised (SAA) surface. Additionally, a post anodising (PAD) treatment has been used to further aid structural adhesion in combination with the aforementioned processes. As a control, the standard 40/50V Bengough-Stuart chromic acid anodising (CAA) has been used as a baseline performance indicator in adhesion tests. Adhesion levels have been established using single lap shear and modified wedge test configurations. Overall, excellent initial joint strengths and durability have been found with both EPAD and ACDCPSAA, suggesting that these environmentally benign treatments may be used as possible drop-in replacements for the currently used CAA process

A Simple and Sequential Strategy for the Introduction of Complexity and Hierarchy in Hydrogen-Bonded Organic Framework (HOF) Crystals for Environmental Applications

Hydrogen-bonded organic frameworks (HOFs) are a new class of crystalline porous organic molecular materials (POMMs) with great potential for a diverse range of applications. HOFs face common challenges to POMMs, and in general to purely organic crystals, that is, the difficulty of integrating complexity in crystals. Herein, we propose a simple and sequential strategy for the formation of HOFs with hierarchical superstructures. The strategy is based on controlling the assembly conditions, avoiding the use of any surface functionalization or template, which allows to obtain hierarchical crystalline porous superstructures in an easy manner. As proof of concept, we obtained the first example of core–shell (HOF-on-HOF) crystals and HOFs with hierarchical superstructures having superhydrophobicity and trapping abilities for the capture of persistent water contaminants such as oils and microplastics. We expect that this strategy could serve as inspiration for the construction of more intricate multiscale structures that could greatly expand the library of HOF materials

Strategies for the replacement of chromic acid anodising for the structural bonding of aluminium alloys

The 40/50V Bengough-Stuart chromic acid anodise process is widely used in demanding applications as a prebond treatment. This process has a number of disadvantages and its replacement is the subject of much interest in the aerospace, automotive and defence sectors, amongst others. This paper details a number of modifications to the standard boric-sulphuric acid anodising (BSAA) process specifically to achieve satisfactory structural bond performance. These included: variations in the deoxidiser and anodising parameters, and; the use of a post anodising dip. It has been demonstrated in these studies that there are three possible methods of providing excellent durability using a variation of the standard BSAA process: the use of an electrolytic phosphoric acid deoxidiser (EPAD); a high temperature anodise at 35°C, and; the use of a post anodise phosphoric acid dip (PAD)

The effect of pore size and porosity on mechanical properties and biological response of porous titanium scaffolds

The effect of pore size and porosity on elastic modulus, strength, cell attachment and cell proliferation was studied for Ti porous scaffolds manufactured via powder metallurgy and sintering. Porous scaffolds were prepared in two ranges of porosities so that their mechanical properties could mimic those of cortical and trabecular bone respectively. Space-holder engineered pore size distributions were carefully determined to study the impact that small changes in pore size may have on mechanical and biological behaviour. The Young’s moduli and compressive strengths were correlated with the relative porosity. Linear, power and exponential regressions were studied to confirm the predictability in the characterisation of the manufactured scaffolds and therefore establish them as a design tool for customisation of devices to suit patients’ needs. The correlations were stronger for the linear and the power law regressions and poor for the exponential regressions. The optimal pore microarchitecture (i.e. pore size and porosity) for scaffolds to be used in bone grafting for cortical bone was set to <212μm with volumetric porosity values of 27-37%, and for trabecular tissues to 300-500μm with volumetric porosity values of 54-58%. The pore size range 212-300μm with volumetric porosity values of 38-56% was reported as the least favourable to cell proliferation in the longitudinal study of 12 days of incubation

Hierarchical Assembly of a Micro- and Macroporous Hydrogen-Bonded Organic Framework with Tailored Single-Crystal Size

Porous organic molecular materials represent an emergent field of research in Chemistry and Materials Science due to their unique combination of properties. To enhance their performance and expand the number of applications, the incorporation of hierarchical porosity is required, as exclusive microporosity entails several limitations. However, the integration of macropores in porous organic molecular materials is still an outstanding challenge. Herein, we report the first example of a hydrogen-bonded organic framework (MM-TPY) with hierarchical skeletal morphology, containing stable micro- and macroporosity. The crystal size, from micro to centimetre scale, can be controlled in a single step without using additives or templates. The mechanism of assembly during the crystal formation is compatible with a skeletal crystal growth. As proof of concept, we employed the hierarchical porosity as a platform for the dual, sequential and selective co-recognition of molecular species and microparticles.J.F.-S. thanks “Ramón y Cajal“ program (RYC2019-02794-I), MINECO (Spain) (Projects PID2019−104778GB−I00 and Excellence Unit “Maria de Maeztu” CEX2019−000919−M) and Generalitat Valenciana (SEJI/2020/034). E.V.R.F. thanks Ministerio de Ciencia e innovación (PID2020-116998RB-I00) and Ministerio de Educación y Formación Profesional (PRX21/00407)