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PSA: Balm separation repairs chemical warning!
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PostPosted: Tue Oct 03, 2023 6:53 pm    Post subject: Reply with quote
eggplant wrote:
There is probably an answer I'm missing by not reading the appropriate sources first, but are the anti-reflective coatings on the front and rear of a doublet effected by the process? Submerging in water and boiling etc.

I know the temperatures required to coat a lens are at least double of 200c, but still.


I will shy from heating doublet due to possible fracture and distortion. Especially with IR. First, glass will reflect most of IR, then absorption by glass is near zero. Assuming whatever get through is absorbed only by glue - this would create thin heated layer against colder glass. Not a good scenario. Stress may, if not crack then pit the glass. Unless thermal flux will even all thermal gradients, but then there is no point to try to heal thin layer only. If absolutely have to heat - I would rather go with slow oven. But then handling glass with cold tools may thermally stress it anyway. Preheating tools, working in the oven - too much complications for me.

I have never seeing coating on inner surfaces of doublets. There reflection reduction is mostly through index matching of optical glue (balm, epoxy, LOCA). Ideally index of glue should be somewhere between, but not precisely in center, of indexes of two pieces of doublet. But I do mostly old, >20 y.o. lenses. I guess theoretically, doublet/triplet inner coating may improve it further. But while index of external coating have to be between glass and air (wide range of selection), coating of inner mating surfaces of doublet need to be between indexes glass and glue, tiny range comparing to external coating. So I guess it is complication that nobody needs, performance improvement yield is too low.

Best of my knowledge, traditional coatings are various metal oxides. They are mostly passive, at room temperature. Never got any damaged by solvents, including Goof Off. Oxidation releases energy. To reverse - need to push energy back, to heat. I'm not sure if heating to 200C as suggested come close to start reversing coating formation process, Maybe some chemist can enlighten us?

Then, googling glass transition temperature: 140 °C to 370 °C. That is when it started to flow. At 200C it may start to flow, causing distortions. Even resting on support during heating may cause problems, I think. Good glass is polished to fraction of micron.

In my head I feel safer to not to heat the doublets. Just an opinion.


PostPosted: Wed Oct 04, 2023 10:02 pm    Post subject: Reply with quote
SalPro wrote:

In my head I feel safer to not to heat the doublets. Just an opinion.


I sure is safer ... but then I have lenses such as the said Sigma 3.5-4.5/50-200mm APO which

1) are absolutely un-useable because of the "blind" doublet and
2) aren't rare/expensive

So why not try? At least, after separating / cleaning / re-cementing, they are much much better than before.
Probably not "like new", but certainly much better than before!

S


PostPosted: Wed May 01, 2024 7:17 pm    Post subject: MC ban goes wider in US Reply with quote
April 30, 2024

https://www.epa.gov/newsreleases/biden-harris-administration-finalizes-ban-most-uses-methylene-chloride-protecting

https://www.yahoo.com/news/u-bans-most-uses-paint-185621034.html



I personally support this ban.
I still have old can of Goof Off.
What to use instead? I've tried many other solvents and found none other to separate doublets/triplets.


PostPosted: Thu May 02, 2024 9:11 am    Post subject: Re: MC ban goes wider in US Reply with quote
SalPro wrote:
April 30, 2024

https://www.epa.gov/newsreleases/biden-harris-administration-finalizes-ban-most-uses-methylene-chloride-protecting

https://www.yahoo.com/news/u-bans-most-uses-paint-185621034.html



I personally support this ban.
I still have old can of Goof Off.
What to use instead? I've tried many other solvents and found none other to separate doublets/triplets.


Looks like the US is following the European example; here methylene chloride has been banned for sale to the general public for several years already.

For separation of epoxy-cemented doublets I've tried every legal solvent I could easily obtain here; nothing worked. A prolonged 3 month bathing in acetone did seem to affect the epoxy a little bit, but still solidly stuck.

So I've got an epoxy-cemented doublet to separate and at this stage not much to lose; as a last resort I'll try Norland's recommended alternative method of heating the doublet in mineral oil for an hour at 400F; but it is risky and it may crack.


PostPosted: Thu May 02, 2024 3:40 pm    Post subject: Reply with quote
Yes, heating elements to separate them carries some risk of fracture, but I think it's overestimated by folks. The following is a very old heat-based method handed down to me that I have used many times without ever fracturing an element.

In my experience, this is relatively safe to use on flint/crown doublets up to 1.5"/40mm in diameter or so. After that, I'd be more concerned about the greater amount of flexional stresses on individual elements. I would also NOT use this on ED or other exotic glasses, as they are often much more brittle and inclined to fracture.

So here's the method:

1. Identify and remove the doublet that needs to be separated from its housing

2. remove any paint from ground glass edges with acetone wipe

3. scribe cheat marks on the ground glass edges to re-align the element later during recementing

4. Find a 65-70 watt incandescent bulb. These are a bit harder to find now, but certainly easier than a supply of methylene chloride. Put the bare bulb in a lamp where you have ready access to the sides and top.

5. Wipe any lingering oils of of the doublet and place it directly on top of the cold bulb. If the doublet has a concave side, it's a lot easier to balance it.

6. Carefully turn the bulb on. Keep it on for about 10 minutes. This will bring the doublet to a high temperature, but gently enough that it won't fracture.

7. After 10 minutes, carefully turn the bulb off so as not to knock over the doublet.

8. Allow it to cool untouched for a few minutes until it becomes cool enough to handle.

9. While it is handleable but still warm, and with clean hands or gloves, gently push the two sides of the doublet back and forth across each other. You'll see bubbles opening up in the cement where the surfaces are beginning to pull apart.

10. If the sides don't immediately separate, wipe the surfaces off and repeat steps 5-9. Usually it takes no more than 3 rounds of heating to get full separation.

11. Once the element is separated, you may have to use solvents (MEK/Acetone) or scrape remaining cement off the elements with a soft plastic tool. Urethane-based cements come off pretty easily with a fingernail. Epoxy-based ones can require some elbow grease.

12. Recement using machinist's blocks/prisms and the cheat marks made in step 3 to ensure alignment. I prefer Norland NOA61 to Canada Balsam, which takes weeks to months to dry, but may be preferable to first timers who don't feel confident in the cleanliness or alignment of their recements.

Anyway, as with any advice given on this board, YMMV, and I don't make any guarantees as to whether this will work in any particular applications. Any broken parts/mishaps/etc. are solely your own responsibility.


PostPosted: Thu May 02, 2024 4:22 pm    Post subject: Re: MC ban goes wider in US Reply with quote
As a paint stripper (by far its most common use), it's been banned in the U.S. for 5 years already. There was fairly well-publicized incident where a young man stripping paints from parts using it with inadequate ventilation died, which really drove the calls for the ban.

Incidentally, even the new restrictions do not ban its use in plastic welding. Micro-Mark Same Stuff, which is used to weld pieces of plastic models, is actually MCL-based and still widely available. Per unit volume, it's a LOT more expensive than the old MCL strippers used to be, but can handy in a pinch to remove bits of cement clinging to heat-separated elements. Simply brush it on with the built-in cap brushh, and the cement usually peels right off.

RokkorDoctor wrote:

Looks like the US is following the European example; here methylene chloride has been banned for sale to the general public for several years already.


PostPosted: Thu May 02, 2024 6:58 pm    Post subject: Re: MC ban goes wider in US Reply with quote
BrianSVP wrote:
As a paint stripper (by far its most common use), it's been banned in the U.S. for 5 years already. There was fairly well-publicized incident where a young man stripping paints from parts using it with inadequate ventilation died, which really drove the calls for the ban.

Incidentally, even the new restrictions do not ban its use in plastic welding. Micro-Mark Same Stuff, which is used to weld pieces of plastic models, is actually MCL-based and still widely available. Per unit volume, it's a LOT more expensive than the old MCL strippers used to be, but can handy in a pinch to remove bits of cement clinging to heat-separated elements. Simply brush it on with the built-in cap brushh, and the cement usually peels right off.

RokkorDoctor wrote:

Looks like the US is following the European example; here methylene chloride has been banned for sale to the general public for several years already.


Well, it wouldn't be the first time that chemicals used carefully and wisely by most ordinary people end up banned for sale to the general public because a few professionals were doing silly things with it. Professional builders refinishing bathtubs in a poorly ventilated bathrooms seems to have been the trigger in the UK... Rolling Eyes

Interesting that that particular plastic model cement in the US still has MCL in it; in Europe plastic model cement is all MEK-based, or a 50/50 mix of butyl acetate and acetone, as far as I know (there may be ones with other solvents also).


PostPosted: Fri May 03, 2024 3:38 pm    Post subject: Reply with quote
Consumer use in USA is banned for 5 years. Latest ban is for "most industrial applications".

Goof Off reformulate most of their products but most still just as dangerous as their original paint stripper. But their other products of little interest for lens application.

Curiously, in very pure form it is still available on US Amazon:

item B0CRPTP55W.

Solvent - Methylene Chloride 1 LTR (1)

4.0 out of 5 stars
50+ bought in past month
$42.00 ($1.24/Fl Oz)
FREE delivery May 9 - 13
Only 19 left in stock - order soon.

💀


PostPosted: Fri May 03, 2024 3:53 pm    Post subject: Reply with quote
https://en.wikipedia.org/wiki/Dichloromethane

Quote:
Regulation

In many countries, products containing DCM must carry labels warning of its health risks. Concerns about its health effects have led to a search for alternatives in many of its applications.[12][37]

In the European Union, the Scientific Committee on Occupational Exposure Limit Values (SCOEL) recommends an occupational exposure limit for DCM of 100 ppm (8-hour time-weighted average) and a short-term exposure limit of 200 ppm for a 15-minute period.[38] The European Parliament voted in 2009 to ban the use of DCM in paint-strippers for consumers and many professionals,[39] with the ban taking effect in December 2010.[40]

In February 2013, the US Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health warned that at least 14 bathtub refinishers have died since 2000 from DCM exposure. These workers had been working alone, in poorly ventilated bathrooms, with inadequate or no respiratory protection, and no training about the hazards of DCM.[9][30][41] OSHA has since then issued a DCM standard.[42]


On March 15, 2019, the US Environmental Protection Agency (EPA) issued a final rule to prohibit the manufacture (including importing and exporting), processing, and distribution of DCM in all paint removers for consumer use, effective in 180 days. However, it does not affect other products containing DCM, including many consumer products not intended for paint removal.

On April 20, 2023, the EPA proposed a widespread ban on the production of DCM with some exceptions for military and industrial uses.[43]


Quote:
Natural sources of dichloromethane include oceanic sources, macroalgae, wetlands, and volcanoes.[13] However, the majority of dichloromethane in the environment is the result of industrial emissions.[13]

Quote:
DCM's volatility and ability to dissolve a wide range of organic compounds makes it a useful solvent for many chemical processes.[12] In the food industry, it is used to decaffeinate coffee and tea as well as to prepare extracts of hops and other flavourings.[15][16] Its volatility has led to its use as an aerosol spray propellant and as a blowing agent for polyurethane foams.


Quote:
metabolized by the body to carbon monoxide potentially leading to carbon monoxide poisoning.[31] Acute exposure by inhalation has resulted in optic neuropathy[32] and hepatitis.[33] Prolonged skin contact can result in DCM dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns.[34]
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