SAFECHEM’s global industry manager aviation, Richard Starkey says that when it comes to component cleaning, manufacturers need an unbiased mindset and a willingness to challenge preconceptions.
When it comes to aviation component cleaning and degreasing, the effectiveness of solvent cleaning is undisputed. Perchloroethylene and trichloroethylene have been long-established standards for decades. Modified alcohols, a newer alternative to chlorinated solvents, are also proven to deliver outstanding cleaning results. Nevertheless, despite the evident efficiency of solvent cleaning, I am seeing aviation manufacturers coming under significant pressure to move away from solvents to water-based cleaning – often in the name of achieving greater safety and sustainability.
Any efforts to bolster safety standard and environmental protection are to be advocated of course. What is troubling however is when companies make sweeping decisions based on a distorted view on how solvent and water cleaning work – with unintended costly consequences.
I used to consult a US manufacturer of precision motion control components and systems for military and commercial aircraft. The company had always achieved satisfying cleaning results through the use of both solvent and water cleaning systems in the operations. When it was acquired by a major corporate who had an internal policy not to use chlorinated solvents, it was forced to adopt water cleaning for all. Since there was no technical justification for the change, problems soon ensued. Not only has the company had to outsource cleaning work, it has to go as far as to altering its manufacturing to suit the water cleaning process. This has resulted in increased time and costs to get the job done, and the latest feedback I received was “the cleaning still isn’t consistent and working correctly”.
Another case in point is a UK-based manufacturer specialised in the full wet assembly of aerostructure components, including installation of captive nuts using riveting and bonding techniques, and heat and shrink installation of high-performance bushes and proof load testing. The company was pressured internally to switch from solvents to aqueous cleaning because of a corporate mandate to go ‘green’, plus an external perceived drive to adopt water-based cleaning solutions. A great challenge that they have been wrestling with since is the staining marks left on aluminium parts after cleaning – a common problem often seen with water cleaning that is not yet widely recognised.
The truth is there is no one universal solution that works for all. What’s more, every cleaning system has its own environmental impact that manifests in different ways. Whether it is solvent or water cleaning, both processes require prudence and diligence from users in ensuring worker safety, environmental protection and regulatory compliance. Making an informed decision means evaluating all the facts, data and key aspects objectively. And this must first start with overcoming preconceived notions.
The environmental impact
Many people tend to associate water with “being green”. However, water alone cannot clean off contaminations entirely. Most water-based cleaners contain surfactants, biocides, complexing agents, dyes, fragrances etc. Some of these compounds are not biologically degradable and, depending on their concentration, may be harmful to fauna and flora in surface waters.
Not to forget that water itself is a finite resource. Water-based cleaning also comes with significant energy demand. There is the requirement to operate high-pressure pumps, heat the cleaning water, dry the metal parts, as well as treat and purify used water for re-use or disposal. As a relatively slow-drying cleaner, water requires 10 times more of the latent heat of vaporisation (2259 J/g) than that of solvents (200-300 J/g).
Solvents in contrast have good drying behaviour and can penetrate easily into tight spaces. Of course, such properties can also bring inherent risks such as air emissions or ground penetration. The use of modern, closed machine technology, in combination with a closed loop solvent delivery system, is therefore key to enabling an emission-free and spill-free transport, storage and handling of solvents.
Solvent cleaning does not require heaters or blowers since parts readily come out dry from the cleaning machine due to the vacuum condition. While energy is required to keep machine operation under vacuum, this also lowers the boiling points of solvents in a vacuum, hence accelerating their evaporation and enabling quick drying of metal parts within a shorter cycle time.
Since solvents can be continuously recycled through built-in distillation in closed cleaning machines, there is less waste generated. The most modern solvent systems with up-to-date solvent management practices can remove 100kg of oil using less than 15kg of solvent . In comparison, to clean off 100kg of oil, approx. 100kg of water-based cleaner is required in a water-based system.
The cost of ownership
Many companies tend to focus on acquisition cost – but that is just one part of the equation. The true cost of ownership must take into account long-term operating costs such as chemicals, utility bills, floor space, waste disposal charges, to name a few.
Standard solvent machine technology and initial first fill chemicals often require a higher capital investment than water-based cleaning. But solvents can be continuously reused and recycled via the machine’s distillation unit. There are thus much lower solvent replacement volumes and fewer bath exchanges necessary. In aqueous cleaning, since dirt and soils are emulsified and rinsed off, aqueous baths that are not treated will have to be replaced frequently.
Another key cost factor is labour time for operation and process monitoring. Regardless of the cleaning method, process control and bath maintenance is crucial to ensuring process safety and optimal cleaning results. In solvent cleaning, only stabilisers are added (when necessary), so the chemistry in the vapor degreaser remains relatively consistent and requires little attention to ensure a constant cleaning performance. With water cleaning, since various chemicals such as builder and surfactants are added to make the cleaning of non-polar contaminations possible, their concentration requires consistent monitoring and adjustments.
The advantages of each
Both solvent and water cleaning are effective cleaning media, but the evaluation of their use must be tied to the specific context. Water-based cleaning can be an excellent choice for polar contaminations, like salt residues. The process can be more efficient when working wet in wet (e.g. water-based painting, galvanisation). Depending on the pH-values, it does not impact rubbers, plastics, or painted surfaces. Cleaning can also be combined with surface finishes such as phosphating, chromating or deposition of protective coatings. Because of the different formulations available, water cleaning can also be adapted to specific cleaning requirements.
Solvents on the other hand are first choice for cleaning off non-polar contaminants, like oils and greases. It also has universal compatibility with metals (please note that water-cleaning agents can be acidic, neutral or alkaline and are usually matched to specific metal types). Because of their good creeping capability, solvents are particularly suited to cleaning complex geometries or tiny parts. The continuous distillation and re-purification of solvents can significantly increase solvent lifespan, which means lower lifecycle costs (and a lower cost per-cleaned-part).
Eyes on the end prize
The aviation industry is known for its meticulousness, because every decision can have an impact on safety. Deciding on the right metal cleaning set-up is therefore no exception, because cleaning parts to the exacting standard is fundamental to them functioning properly. Considering what is at stake, aviation manufacturers must demonstrate an unbiased mindset and a willingness to challenge preconceptions. The end reward – a high performing cleaning process that delivers on cost, quality and sustainability – will no doubt be a prize worth striving for.