Autoclave

Summary

An autoclave is a machine used to carry out industrial and scientific processes requiring elevated temperature and pressure in relation to ambient pressure and/or temperature. Autoclaves are used before surgical procedures to perform sterilization and in the chemical industry to cure coatings and vulcanize rubber and for hydrothermal synthesis. Industrial autoclaves are used in industrial applications, especially in the manufacturing of composites. Many autoclaves are used to sterilize equipment and supplies by subjecting them to pressurized saturated steam at for around 30-60 minutes at a pressure of 15 psi above atmospheric pressure (205 kPa or 2.02 atm) depending on the size of the load and the contents. The autoclave was invented by Charles Chamberland in 1879, although a precursor known as the steam digester was created by Denis Papin in 1679. The name comes from Greek auto-, ultimately meaning self, and Latin clavis meaning key, thus a self-locking device. Uses St

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Stability and physical properties of alkylsilane and alkylphosphonate self-assembled monolayer (SAM) films on metal oxide substrates characterized at the micro- and nanoscale

Enamul Hoque

Self-assembled monolayer (SAM) films have attracted immense attention for both fundamental and applied research. A SAM is composed of a large number of molecules with a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film, and a spacer (backbone) chain group that connects the head and tail groups resulting in a coating. Interactions between spacer groups of different molecules, such as van der Waals forces and/or hydrogen bonding, hasten SAM film formation and contribute to its stability. In this dissertation, SAM and thin films have been formed onto copper and aluminum oxide surfaces by reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS), 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFTS), 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octylphosphonic acid (OP), decylphosphonic acid (DP), and octadecylphosphonic acid (ODP). The properties and stability of the films were investigated employing complementary surface analysis techniques: X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), friction force microscopy (FFM), a derivative of AFM, contact angle measurements (CAMs), and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS). The perfluoroalkylsilane SAM on Cu is found to be extremely hydrophobic typically having sessile drop static contact angles of more than 130° for pure water and a surface energy of 14 mJ/m2 (mN/m). FFM showed a significant reduction in the adhesive force and friction coefficient of PFMS modified Cu (PFMS/Cu) compared to unmodified Cu. Treatment by exposure to harsh conditions showed that PFMS/Cu SAM can withstand boiling nitric acid (pH=1.8), boiling water, and warm sodium hydroxide (pH=12, 60 °C) solutions for at least 30 minutes. Furthermore, no SAM degradation was observed when PFMS/Cu was exposed to warm nitric acid solution for up to 70 min at 60 °C or 50 min at 80 °C. XPS and FT-IRRAS data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (CuOH). For a PFTS modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water has been measured to be more than 125° and the surface energy to be typically less than 16 mJ/m2. Stability tests show that the PFTS/Cu film can survive in boiling pure water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 minutes, sodium hydroxide solution (pH 12, 70 °C) for 30 minutes, and autoclave conditions (steam at 134 °C and 3 atmospheres) for 15 minutes. The more commonly used self-assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. Extremely hydrophobic (low surface energy) and stable PFMS/Cu SAMs and PFTS/Cu films could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces or drop-wise condensation on heat exchange surfaces. XPS analysis confirmed the presence of perfluorinated and non-perfluorinated alkylphosphonate molecules on the PFDP, DP, and ODP SAMs deposited at the aluminum oxide coated silicon (Al/Si) surfaces. The sessile drop static contact angle of pure water on PFDP SAMs was typically more than 130° and on DP and ODP typically more than 125° indicating that the phosphonic acid SAMs reacted with Al samples were very hydrophobic. The surface roughness for PFDP/Al, DP/Al, ODP/Al, and bare Al was approximately 35 nm, as determined by AFM. The surface energy for PFDP/Al was determined to be approximately 11 mJ/m2 by the Zisman plot method compared to 21 mJ/m2 and 20 mJ/m2 for DP/Al and ODP/Al, respectively. PFDP/Al gave the lowest adhesion and friction force while bare Al gave the highest. The adhesion and friction forces for ODP/Al and DP/Al SAMs were in between. ODP, DP, and OP SAMs have been studied in detail on relatively flat aluminum oxide surfaces. The rms surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm, as determined by AFM. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115° and on OP/Al typically less than 105°. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 mJ/m2 and 22 mJ/m2, respectively, compared to 26 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by FFM to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable. The chemical stability of ODP/Al, PFDP/Al, DP/Al, OP/Al, and PFMS/Al SAMs has been inspected by exposure to warm nitric acid (pH 1.8, 30 min, 60-95 °C). The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid (PA) or silane molecule and the oxidized Al surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60-95 °C) show ODP/Al SAMs to be most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs. For PFTS/Al, stability tests demonstrate that modified aluminum is able to survive exposure to warm nitric acid (pH = 1.8, 60 °C, 30 min) indicating some degree of robustness. Hydrophobic, low adhesion, and robust aluminum surfaces have useful applications for micro/nano-electromechanical systems (MEMS/NEMS), such as digital micro-mirror devices (DMDs). These studies are expected to aid in the design and selection of proper lubricants for MEMS/NEMS.

EPFL2007

Vacuum-bag processing of honeycomb sandwich structures faces particular challenges compared to autoclave sandwich fabrication for aeronautics, a consequence of using, at the most, atmospheric pressure for their manufacturing. In particular, the adhesion of the skin to the honeycomb core is reduced and the skins tend to have higher values of porosity. On the other hand, autoclave processing is expensive, in particular for large structures, and a growing interest has been observed for vacuum-bag only processing of sandwich structures. During honeycomb sandwich cure, the skin represents a barrier to air flow between the honeycomb and the vacuum-bag so the air transport through the prepreg, adhesive and consumables or other additional materials may be a crucial factor to increase skin debulking, reduce void content and extract air from the honeycomb cells. The pressure inside the honeycomb is, therefore, a result of the through thickness permeability to air of the skins and vacuum pump work. The goal of this thesis is to develop methods for improving the low pressure processing of honeycomb sandwich structures by quantifying the through thickness air permeability of fibre reinforced composites during cure and analysing related processing issues and governing phenomena. An experimental method based on a falling pressure measurement was proposed to evaluate the through thickness air permeability of prepregs during cure, with two set-up variants. One concerns the measurement of the inherent air permeability and the other applies to processing conditions, implying the use of both consumables and vacuum-bag. The methods were successfully tested and fulfil the criterions used in soil science for the evaluation of the air permeability. Prepreg and adhesive permeabilities were determined separately and in skin combination. Prepreg permeability to air during cure varies between 10-19 m2 and 10-17 m2 and was found to depend on the evolution of the resin viscosity, which shifts from an initially immobile phase to a mobile one and then back to an immobile phase. The adhesive film was found to have very low initial permeability, at least two orders of magnitude lower than that of the prepreg. A range of initial skin through thickness air permeability could be achieved by perforating the prepreg plies, the adhesive layer, or a combination of both. An adhesive deposition method which results in the centre of honeycomb cells being free of adhesive was also tested. A corresponding range of achievable pressure was measured inside the honeycomb. The evolution of the through thickness permeability with the curing cycle was determined for each case. The adhesive layer was identified as the element that reduces the initial through thickness air permeability the most in skin manufacturing. This suggests that one of the ways of increasing the initial through thickness air permeability of the skin is to modify the permeability of the adhesive layer. The role of the resulting pressure inside the honeycomb on skin-core adhesion and skin quality was then evaluated. Skin permeability was found to control skin-core adhesion through the pressure drop in the honeycomb cells and potential outgassing of the adhesive layer. An optimal range of pressure inside the honeycomb was found to be between 40 kPa and 70 kPa. A process window is proposed for achieving the optimal pressure inside the honeycomb, as a function of the skin permeability and time of vacuum application. A semipreg alternating dry and resin impregnated areas along the fibre bed surface was also investigated as an alternative to prepreg in skin manufacturing. The semipreg through thickness air permeability before cure is approximately three orders of magnitude higher than that of a unidirectional prepreg impregnated with the same resin, thus allowing the reduction of air pressure in the honeycomb. A model was proposed for the air permeability change during cure, as dry areas get infiltrated. Due to resin pouring inside the honeycomb cells, this type of semipreg is viable as a skin only if combined with a material that has low permeability to resin, e.g., a prepreg. The resulting initial pressure in the honeycomb cells might then be tailored to specific needs. Finally, it was possible to produce sandwich samples combining a consolidation system, such as a prepreg, with an infiltration system, as wet lay-up or with a semipreg. The combination of a prepreg with a semipreg revealed to be a very practical and clean system, where it is possible to have the same resin in both materials. Moreover, it allows to tailor the initial honeycomb pressure as well as during cure, and to have the initial permeability assessed.

EPFL2009

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The use of carbon fibre reinforced plastics is steadily increasing in the aerospace industry as rising fuel costs and concerns over the environment push airframe manufacturers to improve aircraft efficiency. The high costs associated with manufacturing carbon fibre reinforced components in autoclaves have prompted interest in alternative out-of-autoclave processing methods. In this study a combined cost modelling and life-cycle assessment approach is applied to selected out-of-autoclave production scenarios. Out-of-autoclave specific "prepregs" and resin infused fabrics are cured in thermal and microwave ovens of comparable volume and assessed against a benchmark autoclave scenario. Results showed that materials, in particular carbon fibres, contributed most significantly to component cost and environmental impacts. Resin infusion processes were effective at reducing costs, as reinforcement fabrics and resin were less expensive. Due to the small contribution of energy to total cost, reductions in energy use did not lead to significant savings, although they did improve the environmental performance of the manufacturing process. Out-of-autoclave specific prepregs did not perform as well due to their higher costs, longer associated cycle times and the need for lengthy de-bulking operations. Microwave oven curing offered little in terms of cost reduction and environmental improvement as investment costs were comparable to those of an autoclave, and energy consumption was relatively high compared with traditional thermal oven use. Opportunities for improvement exist if investment costs can be reduced and additional work carried out to promote more efficient transfer of energy. Improvement of the carbon fibre production process would be the most effective approach for reducing impacts and costs from carbon fibre components. (C) 2012 Elsevier Ltd. All rights reserved.

Elsevier2012