I am still trying to make a vacuum chamber for something that involves food .
Does anybody know if plexiglas (acrylic) releases any toxic fumes under a vacuum?? Please, I need to know. The last 2 schemes I had for sealing this
vessel didn't work out and I'm over budget not_important - 12-4-2007 at 19:27
The term "acrylic" is about as precise as "alcoholic beverage" Some acrylics are copolymers of an acrylic acid ester and styrene, there are a number
of acrylic acid derivatives used to make plastics. Plexiglas/Lucite is poly methyl methacrylate, but even that can still have additives for things
like UV stability.
So the answer is, it depends on the exact resin used and what additives there are. There are acrylic plastics approved for food applications, there
are other that are not. Some may outgas, others won't. Ask the manufacture of the plastic.
Unless you have pretty thick plastic it's not going to make it as a vacuum chamber of any size. Doing something that is strong enough to hold against
air pressure, is food safe, and easy to clean, is going to cost you if you're trying to do it in a single material in one piece. That's why I
suggested using a PE bucket inside a metal vac chamber, the bucket is food safe, and easy to remove and clean, cheap to replace. If you want it in one
piece, you will need rounded corners so it is easy to clean out, and that's going to be work or money.
[Edited on 13-4-2007 by not_important]tnhrbtnhb - 12-4-2007 at 21:51
Groan.... just ask the manufacturer... not like they're going to tell me or even know, though. They'd just say "oh no, it's not safe for food
contact", that's their default response. But I can't find any other options here..... It's not going to be in contact with the food,
it's like this,
I have a 3/4 inch plywood box with a 3/4 inch plexiglas top. it's 90 cm by 45 cm. internaly. I know it's kinda large. The top is so I can see
what's going on. I have the seal around the top taken care of using food grade silicone caulk.
Then that's my vacuum chamber and I can do whatever I want inside it, put cooling, heating elements, I've got a nice metal pot here to hold food etc.
But I can't get the plywood sealed with stuff I know won't release toxic fumes..... I've tried painting it with walnut oil, but it's not working out.
Linseed oil etc, they won't form an airproof covering.... polyurethane varnish is out 'cause there's all kinds of carcinogenic or worse stuff in that
and I can't for the life of me find out the quantities involved. I thought of soaking it in canola oil or something but it would go rancid
eventually. I can't get a hold of enough silicone caulk, and it's too delicate anyway. It doesn't have to be easy to clean.
I respect your PE bucket, but making a metal vac chamber is beyond what I can practically do, I think.
So I'm thinking that as a last resort, I could take some 1/8 inch plexiglas and glue that on the outside with caulk.
Maybe I'm asking the wrong question... anybody else have a better idea for sealing the plywood? I can get food grade silicone oil (LMW
polydimethylsiloxane since you guys are so picky), maybe I can just coat the plexiglas with silicone oil and it won't be able to outgas in any case?12AX7 - 12-4-2007 at 22:07
Liquids are still permeable to gas, and are prone to evaporating themselves (depending on just what kind of vacuum we're talking about here!).
How about...plastic contact cemented to the surface? I'm thinking HDPE, PP or another food-safe plastic. Problem is, PP and to a lesser extent HDPE
are almost impossible to bond...
Timtnhrbtnhb - 12-4-2007 at 22:34
I'm talking about 15 torr or something, not serious. Check out the "bonding to polyethylene" thread. The real problem with that is that a sheet this
size would be hard to get a hold of. Stuff like vapor barrier has small holes in it. But like pmma, PE by itself is not toxic but the end product
usually has additives in it.... Vapor barrier has flame retardants and probably some sort of fungicide, too. If only I could find out to what extent
these things outgas at least . I mean, it might not matter but I don't know.
Besides that none of the measurements deal with outgassing in a vacuum, only with food contact, which is another ball game.
I guess just being prudent might be the way to go, can't be paranoid all the time or I'll never get this done.... I've read the MSDS for plexiglas,
the only toxic component mentioned is acrylic acid, and that's not all that bad.... just as long as it's not neurotoxic or like bisphenol A (*glares
at tubes of epoxy*)or something. Who cares about carcinogens? That's wayyy long time from now..........
The trouble is I can't test the end product, and if I'm liable to be chronically exposed...not_important - 12-4-2007 at 23:00
Make a dummy top from more plywood. If you haven't managed to seal the seams, find a caulk that is OK. Caulk from the outside, briefly pull a mild
vacuum to pull the caulk in further. Let cure, check and recaulk as needed
Pump the box down partway - 1/2 atmosphere at the most. Oil the outside, let the pressure force the oil in. Drop the vacuum and let cure. Repeat.
Don't use linseed oil unless you are sure it doesn't have additives such as cobalt or manganese salts/soaps, I don't think you'll find any that use
lead anymore.
Alternatively, get polythene sheeting in a several mil thickness. Put the dummy lid on, start pumping the box down. Wrap it in the PE sheeting, then
use a heat gun to warm the sheeting allowing it to get sucked into pores in the wood. PE sheeting intended for temporary use - drop cloths and so on
- usually doesn't have much if any additives, they just add cost and do not add benefits for the application.
Hmmm .. heavy duty aluminium foil would work as well, but you'd still need an adhesive over the entire surface. You might be able to use the casein
white water based glues or the newer PVA water based glues; both generally have only mild warnings of the CYA sort; milk proteins aren't too toxic and
PVA iis fairly safe. Those glues often can be found as products intended for children to use.
[Edited on 13-4-2007 by not_important]12AX7 - 13-4-2007 at 00:08
You could also build it out of, say, HDPE cutting boards.
Probably way too flexible for a literal ton of force though (depending on just what the scale of this thing is).
Timindigofuzzy - 13-4-2007 at 00:46
Forgive me if this is a stupid idea, but why not just shrink-wrap the box? or use several layers of polyethylene plastic wrap (available at any
grocery store) and the heat gun method?not_important - 13-4-2007 at 01:47
You'd want to make sure the shrink wrap was not PVC based, as that typically has some volatiles. PE shrinkwrap would likely be OK.Ozone - 13-4-2007 at 19:04
Hello, glad to be back,
Acrylate monomers are notorious for very high conversion--this means no residual monomer in a properly made piece. Unfortunately, if you get it hot,
above its ceiling temperature, it will depolymerize and you will get some monomer. Ever heat PMMA, say by sanding it, and get the slight lacrymatory
essence of MMA?
Should be OK if you don't get it hot.
Sorry, I'm trying to type with a"long" cast on my arm .
Cheers,
O3BeanyBoy - 17-4-2007 at 03:52
Quote:
Originally posted by tnhrbtnhb
Maybe I'm asking the wrong question... anybody else have a better idea for sealing the plywood? I can get food grade silicone oil (LMW
polydimethylsiloxane since you guys are so picky), maybe I can just coat the plexiglas with silicone oil and it won't be able to outgas in any case?
How about covering it in paraffin? Dipping it seems out of the question, but pouring melted paraffin over it while you're pulling a vauum in it should
seal it up, I'd think... assuming you can rotate it w/vacuum applied, to cover each face....indigofuzzy - 20-4-2007 at 23:48
I'd wonder though, since paraffin is composed of long chain hydrocarbons, if any of these would become more volatile at reduced pressures.BeanyBoy - 26-4-2007 at 11:17
Quote:
Originally posted by indigofuzzy
I'd wonder though, since paraffin is composed of long chain hydrocarbons, if any of these would become more volatile at reduced pressures.
Paraffin will decompose by the actions of strong acids (see below)... but tnhrbtnhb said this was a food application; dunno offhand what strong acids
he muight need to use.
Fuming sulphuric acid converts the middle and higher members of the series into sulphonic acids and dissolves the lower members (R. A. Worstall, Amer.
Chem. Journ., 1898, 20, p. 664). Dilute nitric acid, when heated with the paraffins in a tube, converts them into secondary and tertiary
nitroderivatives (M. Konowalow, Ber., 1895, 28, p. 1852), whilst long boiling with strong nitric acid or nitro-sulphuric acid converts the middle and
higher members of the series partly into primary monoand di-nitro compounds and partly oxidizes them to carbonic, acetic, oxalic and succinic acids
(Worstall, ibid., 20, p. 202; 21, p. 211). Fuming nitric acid only reacts slowly with the normal paraffins at ordinary temperature, but with those
containing a tertiary carbon atom the reaction is very energetic, oxidation products (fatty acids and dibasic. acids) and a small quantity of
polynitro compounds are obtained (W. Markownikow, Centralblatt, 1899, r, p. 1064; Ber., 1899, 3 2, p. 144 1). Chlorine reacts with the paraffins,
readily substituting hydrogen. Isomeric hydrocarbons in this series first appear with butane, the number increasing rapidly as the complexity of the
molecule increases. For a means of determining the number of isomers see E. Cayley, Ber., 1875, 8, p.1056; F. Hermann, Ber., 1898, 31, p. 91.
OT, but this seems to mean my idea of using paraffin-soaked cork stoppers for making nitric acid won't work. Drat.
-BeanyBoyTwospoons - 26-4-2007 at 14:56
I'd be less worried about the plexiglass, and more worried about the glue used to bond the plywood! A lot of the glues used industially to bond ply
and particle board are known for releasing formaldehyde.
Why not just build the whole thing out of thick glass? Glass is pretty strong.
.
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