Sciencemadness Discussion Board

Making a fritted glass gas diffusion tube from ground glass

RogueRose - 17-2-2019 at 21:41

I need a number of gas diffusion tubes to make a series of absorption vessels and/or scrubbers and the diffusion tubes are crazy expensive so I thought I would give a try at making the bubbles.

I have lots of broken boro glassware i've kept and have smashed a bunch up (after cleaning very well) and I'm making "frit" (ground glass) and am going to separate it by size with wire mesh. The frit can be made by heating the glass to a temp right around the melting point (slightly below I believe) and allowing to cool.

I've made a mold for the bubbler part, it's an old brass cartridge rifle round with the neck and base cut off, so it is just a thin tube, the diameter I want the bubbler to be (~10mm - I'm also making a 6-7mm mold). I have steel plugs (sliced off a bolt, fits perfectly inside the mold) that will be used to hold the glass inside the mold. I plan to drill a hole in the center of one plug (maybe 1/8" diameter) and place a metal rod in the hole - this will be used to create an air channel inside the ground glass (it doesn't run the entire length of the mold, only about 2/3 - 3/4 of the way).

So I will place the plug w/ 1/8" rod inside the brass mold, then fill with the sifted glass. Then place plug on other end to seal the mold (IDK if this is necessary yet). I have two pieces of aluminum 2" x 3.5" x 1.25", each with a ~10mmm hole drilled in it to receive the plug/mold assembly. It will sit in this Al holder while it is heated. IDK if I need to place the block on the top of the mold/plug or not, it will help compress the glass but that may not be necessary.

I plan to heat the brass and plugs with a propane torch and or an acetylene torch (both at once maybe??) and see how things turn out. My biggest concern is how to determine how long I need to heat the mold. the brass is very thin and it hold up extremely well to prolonged heat of the acetylene torch (I've tested it a few times) and I suspect it should transfer heat well. This is the part where I need any advice from anyone who has worked with glass. I've done a little work, joining rods/tube, bending, etc, but never worked with ground glass (especially in a mold).

Any suggestions on methodology, design or the process are welcome. If this goes well and the process is fairly easy, I'll offer these to members if they want some of them, I'm working on making a setup with 24/40 joint as well as using normal media bottles with a GL45 cap among others.

Ubya - 17-2-2019 at 23:28

the softening point for borosilicate glass is 820°C, brass melts at 900, and aluminium at 600, be careful with temperature control. a kiln or furnace should be better to control the temperature versus a direct propane or acetylene flame

RogueRose - 18-2-2019 at 00:23

Quote: Originally posted by Ubya  
the softening point for borosilicate glass is 820°C, brass melts at 900, and aluminium at 600, be careful with temperature control. a kiln or furnace should be better to control the temperature versus a direct propane or acetylene flame


Thanks for the heads up. I considered using steel tubing and using an induction coil to heat the tubing but I guess a kiln might give better temp control. That's a good suggestion.

I'm not sure where I found the temp for melting borosilicate but to make a frit I read that you don't get it to melting point but below the temp. IIRC, the temp was 1250-1300F for boro to get to the point where it starts to get tacky/sticky. I don't think that is with a flux or anything either and there is also some fluxes that can be used that help make the bond (at lower temps) and it can later be dissolved away.

If the brass doesn't work, I'll move to copper tubing I guess, IDK if steel tubing would be better, I guess I could probably find SS tubing that would work as well.

I think the glass needs to get to the "glass transition temperature" (Tg) which is well below melting, where bonds in the glass start to re-form and become somewhat fluid. At this point, if pressure is applied then the pieces of glass that are in contact with each other will form bonds on the molecular level (like a crystal growing). At least, that is what I took from the page/video I watched some time ago. This page explains the transition point pretty well.
https://pslc.ws/macrog/tg.htm

This page describes how glass can deform at the transition point, below the MP, when stress is applied
Quote:

4.4.4 Measurement of Tg
Glass transition temperatures can be measured by many techniques. Not all methods will yield the same value because this transition is rate dependent. Polymer segments will respond to an applied stress by flowing past each other if the sample is deformed slowly enough to allow such movements to take place at the experimental temperature. Such deformation will not be recovered when the stress is released if the experiment has been performed above Tg. If the rate at which the specimen is deformed in a particular experiment is too fast to allow the macromolecular segments to respond by flowing, the polymer will be observed to be glassy. It will either break before the test is completed or recover its original dimensions when the stress is removed. In either event, the experimental temperature will have been indicated to be below Tg. As a consequence, observed glass transition temperatures vary directly with the rates of the experiments in which they are measured.


1.2.1 Glass Transition Temperature, Tg
The glass transition temperature, often called Tg, is an important property when considering polymers for a particular end-use. Glass transition temperature is the temperature, below which the physical properties of plastics change to those of a glassy or crystalline state. Above Tg they behave like rubbery materials. Below the Tg a plastic’s molecules have relatively little mobility. Tg is usually applicable to wholly or partially amorphous plastics. A plastic’s properties can be dramatically different above and below its Tg. The value of the glass transition temperature depends on the strain rate and cooling or heating rate, so there cannot be an exact value for Tg

https://www.sciencedirect.com/topics/chemistry/glass-transit...

[Edited on 2-18-2019 by RogueRose]

[Edited on 2-18-2019 by RogueRose]

[Edited on 2-18-2019 by RogueRose]

Chemetix - 18-2-2019 at 15:03

Going glass to metal seal for this application is a complex way of going about the task. For a start the metal has to have a very similar co-efficient of thermal expansion and even then the small differences between the glass and metal mean you will need some engineering of the seal geometry and precise technique to get this right.

Alternatively, grind and sieve some borosilicate glass and put some of the desired size in a test tube (borosilicate also). Heat using a propane air torch until there are no loose particles of glass. Slowly withdraw from the flame to allow even cooling and then, when cool, grind the end of the test tube off and leave a disc of frit exposed. You might have to have a few goes to get the technique right.

RogueRose - 18-2-2019 at 20:17

Quote: Originally posted by Chemetix  
Going glass to metal seal for this application is a complex way of going about the task. For a start the metal has to have a very similar co-efficient of thermal expansion and even then the small differences between the glass and metal mean you will need some engineering of the seal geometry and precise technique to get this right.

Alternatively, grind and sieve some borosilicate glass and put some of the desired size in a test tube (borosilicate also). Heat using a propane air torch until there are no loose particles of glass. Slowly withdraw from the flame to allow even cooling and then, when cool, grind the end of the test tube off and leave a disc of frit exposed. You might have to have a few goes to get the technique right.


I'm sorry I didn't explain what I was planning on doing once the fritted end was made. Once it cools inside the mold, I'll push it out and then put it in another tube to hold it while the end with the air passage is heated to the point it can be joined with a glass tube of similar diameter. This will be done the same way joining 2 glass rods or tubes is done. I've done this a number of times and it is fairly easy to do.

So the end product will be a 12" glass tub with a 1.5" fritted bubbler on the end so total about 13.5"

Chemetix - 18-2-2019 at 22:59

"Once it cools inside the mold, I'll push it out and then put it in another tube to hold it while the end with the air passage is heated to the point it can be joined with a glass tube of similar diameter."

That makes little more sense, but graphite is probably going to be more glass friendly than brass or metals for a mold.