Critical Cleaning and Sedimentary Rock

Cleaning processes tend to build up; one layer of cleaning is added to another. The overall cleaning process becomes something that resembles sedimentary rock (or, to the surface of the desk of the Rocket Scientist). Recently, a manufacturing engineer involved in optics fabrication, called to brainstorm and, frankly, to vent. (I am referring to the individual as he – the person could be she.) He saw nagging, intermittent issues with product yield, particularly after changing one cleaning agent. Read the story and my thoughts!

Extra Steps
During his review of the cleaning processes, the engineer who contacted us made very astute observations that may, eventually, lead to an effective process. Some cleaning steps made him ask: why is this step necessary? For example, he noticed two sequential process baths that contained two slightly different aqueous cleaning agents from two different companies. It was not clear why two different cleaning agents were selected, or even if the two cleaning agents are functionally distinct. He suspects that an extra cleaning step was added sometime in the undefined past. The additional cleaning step fixed a problem, so it was permanently added. Of course, the problem could have been transient or related to some other process control issue. While adding the extra cleaning step may not really have been necessary, the process “improvement” stuck. The cleaning process got longer, forming an additional layer of sedimentary rock.

A drop-in substitute (or maybe not)
Optics cleaning can be tricky because the glass or crystal substrates are typically held in place using what are referred to as blocking compounds. Blocking compounds can be polymeric material or a wax or even pitch (asphaltum). The challenge is to remove the blocking compound and polishing materials without leaving appreciable residue and without damaging the exquisitely-polished surface.

To remove the blocking material, the group had been using an aggressive halogenated solvent, one of the aggressive, effective solvents that are under current re-evaluation by the U.S. EPA in terms of risks of worker exposure (See our feature article in the June 2020 Clean Source). The optics group stopped using that solvent and moved to a blend primarily containing trans-1,2-dichloroethylene (trans-DCE). The blend was initially expected to be a drop-in substitute. Because trans-DCE is a low-flashpoint solvent, it is almost always sold blended with another inerting solvent that has weaker solvency for most industrial soils of interest. The inerting solvent suppresses flammability. I should make you aware that trans-DCE is also under regulatory review by the EPA, although that review is in a much earlier stage.

The bottom line was that the overall cleaning process was not as reliable as it had been. It turned out that the blend was almost – but not quite – a substitute. The solvency properties are slightly different. More significantly, the boiling point of trans-DCE blends are far lower than that of the solvent being replaced. Especially for optics, temperature is an important factor in successful soil removal. This is because removal of many of the blocking compounds depends on adequate heat, more than physical agitation. Physical forces like ultrasonic agitation or strong spray can damage the surface of the optics. According to Arrhenius’s Rule, as a rule of thumb, for every 10 Deg. C increase in the boiling point, the reaction rate and cleaning effectiveness doubles.

Examples of cleaning solvent boiling points

Solvent

Boiling point (C)

Perchloroethylene

121

Trichloroethylene

87

1-bromopropane (nPB, n-propylbromide)

71

Trans-DCE blend (Product A)

43

Trans-DCE blend (Product B)

48

d-limonene

178

The engineer Indicated that there have been a number of attempts to compensate for the performance of the blend. While many cleaning agents have been tested and cleaning steps have been added, achieving consistent, reliable coating remains a challenge.

For example, d-limonene, a terpene that has many interesting, favorable properties, is a promising candidate. It is a fairly aggressive solvent with a high boiling point. However, it can be difficult to remove from the substrate and thorough rinsing is imperative.

Simplifying a complex process. – somewhat
In the course of my career as the Cleaning Lady, I have observed that optics fabrication processes involve lots (and lots and lots) of cleaning steps. The cleaning process is complex and includes solvent and aqueous cleaning agents. It is not surprising to have multiple steps in optics fab. The glass or crystal has to be held in place and polished. There may be several polishing steps; and each step can introduce a different polishing compound that introduces residue. Hydrofluoric acid (HF) may be used to etch the optics. Alternatively, ammonium bifluoride which releases HF may be used for etching. HF poses immediate, acute worker exposure dangers in both the liquid and vapor forms. It has to be handled and removed carefully; hence, the need for additional cleaning steps.

Revamping this or any complex process involves a multi-pronged approach. Suggestions include asking legendary and iconic (i.e. older) engineers about the rationale for various steps. Even retired engineers are accessible via social networking. Learning more about the cleaning agents helps, including looking at the Technical Data Sheets (TDS) and Safety Data Sheets (SDS).

Is all the “good stuff” going away?
The optics engineer commented that they have taken away all the “good stuff.” Regulatory agencies have classically worked on a chemical by chemical basis. There is interest among some in the regulatory world in also looking at the cleaning process and the consequences of process change. Now could be an opportune time to speak up.

Sedimentary rock
If the cleaning process is working relatively well, why take the time to eliminate cleaning steps? There are many reasons to simplify the process. The more complex the cleaning process, the more difficult it is to control. Particularly if complex, often undisclosed formulations are involved, changes in cleaning agents can have a domino effect. The more cleaning steps, the greater the expense. True critical cleaning is value-added cleaning.

What about specs and customer requirements? If the cleaning process is unnecessarily complex and then a customer or an agency, like the military, restricts your freedom to change the process, you are faced with a process that is no longer sedimentary rock but more like metamorphic rock. The process, for better or worse, becomes set in stone!

Fortunately, in contrast to the perennial state of his desk, the Rocket Scientist is diligent about helping clients achieve cleaning processes that are lean and value added. Give us a call!

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