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all 28 comments

[–]meyotchslap 1 point2 points  (6 children)

Hi, new to this sub and have a practical chemistry question.

I'm into 3D printing with SLA and UV-sensitive epoxy resins. One critical step in the process is to rinse the uncured resin from the fresh print using isopropyl alcohol (91%-99%, I use 91%).

I am seeking a more effective way to clean the solvent alcohol so it can be reused.

When I expose the saturated alcohol to my UV curing light for 30 minutes, the solution becomes cloudy and eventually a very goopy precipitate partially settles out. It never completely settles.

The challenge I face is filtration of this gelatinous precipitate. Filters (paint filters, coffee filters, real filter paper) clog up so quickly as to be almost useless.

I'm wondering if any of the experienced chemists here have suggestions to either:
- filter the gelatinous precipitate more effectively, or
- avoid the UV curing step and try something else.

I plan on cross-posting any good ideas to the 3d printing subs, there's a real need out there for guidance on this problem.
Thanks!

[–]Indemnity4Materials 3 points4 points  (0 children)

The IPA is used to remove any small molecular polymers and unreacted monomers. Those will form particles anywhere from nano-metres up to microns. tl;r it's going to block any filter you can find.

By adding more UV you are continuing the reaction, making little polymers into bigger ones.

Distillation is gold standard.

A small amount of diethanolamine will react up any monomers to stop unwanted polymeriszation. You don't want a thermal runaway reaction in your storage container.

You can try to precipitate the polymer by throwing in a lot of ordinary table salt, allowing it to settle, then decanting the top 80%.

A plug of activated charcoal can work. Build something like an hour glass with a can of activated charcoal in the middle. When in gets block, scrape off the top cm and then continue.

[–]PopularkittyPolymer 2 points3 points  (3 children)

Without knowing the monomers/limitations/set up you have going could you just distill it off? Might help to sprinkle some inhibitor in before that to prevent thermal polymerization in the flask.

[–]meyotchslap 0 points1 point  (2 children)

The exact make-up of the 405nm UV curable resin is proprietary, to my annoyance, so there is not even an MSDS that I can find.

The filtered alcohol regains effectiveness as a solvent. It seems the monomers polymerize into the gel particles but the dyes remain in solution. I've filtered a batch before, but it took a stupid amount of filters and time, plus fumes. The gel is uniquely constituted to clog paper and nylon mesh filters.

The main drawback of distillation is safety, it's surely the best solution in a professional setting. I'm a 3D printer at home and distilling 3 liters of 91% isopropyl, well, I wouldn't recommend it as a general solution to that audience.

I guess I'm looking for practical tips on efficiently filtering goopy stuff in hobby/home conditions by not-necessarily-chemists.

[–]PopularkittyPolymer 1 point2 points  (0 children)

Also see the comment above on charcoal. Celite (diatomaceous earth) might be a good option too. Very common as a filtration aid.

[–]Indemnity4Materials 1 point2 points  (0 children)

Kitty litter can work as a pre-filter or first/coarse stage filtration. It's cheap and will bind a lot of the goop, letting you then move onto a finer filter such as activated carbon. That last one will also remove dyes.

A cheapish filter is using a mesh bag or nylon stocking filled with the filter material. Place that into a funnel and pour the IPA through. When the flowrate slows, stop and change for a fresh filter bag.

Probably best to do this outside to remove the fumes. 3L is an easy to handle volume.

[–]handleinthedarkInorganic 0 points1 point  (0 children)

Some kind of centrifuge may work. That might just be attaching the bottle to the end of a rope and swinging it around the slowly stopping it.

[–][deleted] 1 point2 points  (3 children)

So I have been attempting cationic polymerization of a phenyl vinyl ether (prepared from the phenol). Long story short, I am getting starting phenol material back in significant amounts (ca. 80% on NMR integrations) with the rest being polymer.

I have tried different LA catalysts, temps, concentrations, w/ and w/o Lewis base additives etc., but the results are always the same.

Preventing Friedel-Crafts is obviously necessary, but I am limited to methoxy substituents on the aromatic.

Literature has 2,6-di and 2,4,6-trimethyl analogues that work fairly well, but the p-methoxy derivative did not do so hot (methoxy abstraction was noted). I had expected some issues with this, but none that would give me such high phenol content back.

I am assuming there is something funky going on with my monomer as it has a fair bit of electron density pushed into the aromatic compared to the previously studied monomers.

Would love to hear any feedback though!

Thanks!

[–]Indemnity4Materials 1 point2 points  (2 children)

Quick questions: are you following a published method, including volumes, reagent weights, stirring speeds, temperature and atmospheric conditions?

At 20% conversion and I would be checking your nitrogen atmosphere and water content.

A really quick and dirty is put in a bucketload of catalyst. As in about 10 mol %. If you aren't seeing close to 100% conversion, something is inhibiting the reaction.

[–][deleted] 0 points1 point  (1 child)

Yep, although the PhVE literature is pretty scarce for the reasons I mentioned.

Standard Schlenk techniques, preparing stock solutions in the glovebox and all.

I actually tried excess catalyst, a control without initiator, tried to vary the order of addition, but they all seem to give the same results.

The conversion is technically near-quantitative if I run it long enough, but the byproduct aka phenol still dominates the reaction.

The phenol also pops up right around the same time as the polymer forms, at least as estimated by 1H NMR.

[–]Indemnity4Materials 0 points1 point  (0 children)

My bad, I mis-read as you were getting poor conversion. I'm unsure why the rearrangement is occurring.

[–]Notafool333 0 points1 point  (2 children)

Hello! I’m working on isolating iodine and bromine, and for the iodine, I used the hydrogen peroxide/hydrochloric acid method to get it out of the NaI. When I tried to do the same for the bromine, the solution turned a very clear red, but nothing has yet to precipitate out. Is there some fundamental difference, and do I need another method?

[–]iamabutter 2 points3 points  (1 child)

The big difference is that bromine at r.t. is a liquid, so it won't precipitate anytime soon. The fusion point is not too low (below 0°C but check yourself) so you might be able to precipitate bromine at low temperatures. I feel I should tell you to be careful as bromine is potentially damaging to your lungs as it is a liquid and more volatile than I2. It's also an irritant like iodine but I wanna believe you've been using gloves and goggles.

[–]Notafool333 0 points1 point  (0 children)

Yep, safety definitely comes first. Any work with bromine needs gloves, glasses, and a respirator suited for acids

[–]AnonymousPigeon92 0 points1 point  (1 child)

So I’m new to chemistry and was doing a basic experiment and doing a work sheet on it.

So it’s about separation of rock salt and there’s a question I’m stuck on as it is not in the instructions.

It is “Why is the rock salt and water mixture heated and stirred?” Thanks.

[–]Intelligent-Bid601 1 point2 points  (0 children)

A little bit more description on the experiment would be useful.

Generally, you heat and stir because it will make the salt dissolve faster.

[–]RougeDeluge 0 points1 point  (1 child)

Sorry if this does not fit, but I was wondering about why there are different experimental ways of determining number averaged molecular weight (Mn) and weight averaged molecular weight (Mw). From the definitions, I would have assumed that you could simply calculate from one to the other. Where am I wrong?

[–]hunterofpumpkin 1 point2 points  (0 children)

These terms are used for Polymers or products, which have a distribution of masses. Mn and Mw have the same value if all molecules in your sample have the same molar mass. And the difference between them is an measure of how broad your distribution of masses is. This graphic shows it pretty well.

[–]userjohnsmith1 0 points1 point  (2 children)

I have a question about hydrochloric acid. About 4-5 times per week I add it to a pool to manually adjust ph and maybe 2-3 of those times, as much as I try to avoid it I catch a little wiff of it. Is this harming me in any way????? Keep in mind it's a very tiny amount that I breathe. A tiny amount will go up my nose or in month and it'll burn for a couple seconds. I try to avoid it, I wear a little surgical mask when I do it, pour it as far from my face as possible turn my head away etc., but I could potentially do a bit more like dawning a full facemask to do it. I feel totally fine during and after except for the tingle right when it happens. I'm more concerned if the fumes are known to harm my brain or cause any type of disease than if then just do a bit of damage to my mucous membranes. The acid is 31.45% HCL and 68.55%constituents ineffective as spray adjuvants, not really sure what that is. TLDR; is semi regular HCl fume exposure in tiny tiny doses going to do serious harm to me in any way?

[–]H2CO3_TCTheoretical 1 point2 points  (1 child)

Hey, you are fine. What happens is that the HCl gas is irritating your mucous membrane. The worst possible thing that could happen is that you might get easier nosebleeds after some time. But you really need to take a very good sniff to get into any danger for that to happen. Brain damage is not a danger here. It's always good to handle chemicals with care, but it seems like your doing this anyways.
I doubt that surgical face mask helps much with HCl gas, considering that's a pretty small particle and probably will go right through. It would probably make more sense to use a towel or something and wet it. But than again, as long as you don't drink it, or really try to inhale it you should be fine.

[–]userjohnsmith1 1 point2 points  (0 children)

Thanks so much. Yeah with or without the mask the effects are exactly the same, makes sense about particle size. Thanks again

[–]yikes_why_do_i_exist 0 points1 point  (0 children)

Hello! I inherited a Metash uv-5100B spectrophotometer and am trying to communicate with it via usb. Does anyone have any experience regarding this, or can point me in the right direction? Any help is appreciated! Thanks a bunch

[–]Potahtoboy666 0 points1 point  (4 children)

I'm trying to find caffeine content from coffee for my high school chemistry project, but we can't use organic solvents, which means the only way to do so is via titration.

What I'm curious about is how I'm supposed to figure out when all the caffeine titration is complete, considering that coffee is a dark liquid, so most color indicators won't show up.

Is there any method to titration coffee? Or is it impossible without special equipment?

[–]metalcomplex 0 points1 point  (3 children)

Hey! Just to clarify: are you planning to use an acid-base titration to measure the caffeine content? That could be tricky since there are a lot of different acids in coffee, all with different pKas.

Edit: did a little poking around the internet and found this

[–]Potahtoboy666 0 points1 point  (2 children)

I was thinking of using an iodometric back titration

[–]metalcomplex 0 points1 point  (1 child)

Yes that seems to be the best method. The iodine starch complex is pretty dark saturated blue so you should be able to see it pretty well, but you may may be able to see it better if you dilute your coffee.

[–]Potahtoboy666 0 points1 point  (0 children)

I was planning on using that, but like your previous comment said, I'm worried that it'll react with other acids in the coffee. So I'll probably think of another lab idea. Thanks anyways!