Sciencemadness Discussion Board

How to glue polyethylene together

tnhrbtnhb - 17-2-2007 at 21:31

How can I glue or epoxy a copper tube in a block of polyethylene? I have been to the hardware store, no epoxies they had (a good selection) or glue will work on polyethylene.

This is for attaching copper tubing to a polyethylene vacuum vessel....

[Edited on 18-2-2007 by tnhrbtnhb]

Ozone - 17-2-2007 at 21:43

Welding plastic usually involves solvation. Solvation is not someting PE likes to do. You could drill, tap and screw in a retaining bracket, though.

It would be helpful, however, from the outside-helpers' point of view, if you were more detailed with your description of the experiment.

Cheers,

O3

GhostofUnintentionalChaos - 17-2-2007 at 21:49

I've seen normal superglue (ethyl cyanoacrylate) form a powerful bond between plastics and metal, but it would be extremely important that you are not trying to fill a gap and that the surfaces be smooth. To make sure it is sealed, you could put a partial vacuum on the finished, dried apparatus and liberally apply more glue to the joint. The reduced pressure should help capillary more glue into any holes that were not filled the first time.

Edit: What O3 said about the PE might be relevant here however. Assuming this does work, a thick ring of silicone caulk would probably also help (assuming it won't be handling high temps). Since it is flexible, it would have a tendency to be forced into any gaps by the atmospheric pressure, blocking off leaks, should they occur.

[Edited on 18-2-2007 by GhostofUnintentionalChaos]

Ozone - 17-2-2007 at 21:54

Please note: "partial vacuum"; water vapor from the air is the polymerization initiator with cyanoacrylates. The bond might be tight if you are pulling on it, but will snap off with the least application of oblique force. Again, is this OK, or is your intended use requiring stability under torque?

Cheers,

O3

not_important - 17-2-2007 at 22:43

Polyethylene is just about pure hydrocarbon, like paraffin wax not a lot sticks well to it. It also flexs a bit, which means that ridgid glues often do not work well as they break away when the PE is flexed; most cyanoacrylates fall in that category.

To glue two pieces of PE together you use a hot glue gun, which melts sticks of ployethylene. This also can be used to glue other stuff to PE, for metal it may be helpful to warm the metal before gluing.

There is a type of polyethylene glue that is solvent based. The 'Goop' brand is one, if you squirt some out onto a surface and then leave it along for a week, you'll have a blob of PE left behind. The solvents will soften a PE base that is being glued, but not to the extent that most solvent-plastic glues do to plastics such as PVC or polystyrene. The joins will not be really strong.


You can also used mechanical methods, washers and nuts on threaded rods or tubes. If you do this then a glue can be used just to seal the connection, silicones or polyurethane glues should work OK, orthe 'Goop' types; considering roughing the polyethylene where the glue will be applied. A soft washer under a rigid washer might do the job, depends on how hard a vacuum you are pulling.

You also might consider using plastic fittings rather than directly connecting copper pipe. If this is for the connection to the pump, and you are not going below 10 mmHG, go with plastic fittings and vacuum tubing/hose.

Ozone - 18-2-2007 at 09:10

Nice elegant solution!

Since, erm, I've never thought about the composition of hot melt glue stick I thought I'd post it here:

Wiki:

A common material for the glue sticks (eg. the light amber colored Thermogrip GS51, GS52, and GS53) is ethylene-vinyl acetate (EVA) copolymer.[1] The vinyl acetate monomer content is about 18-29 weight % of the polymer. Various additives are usually present, eg. a tackifying resin and wax. Other base materials may be based on polyethylene, polypropylene, polyamide, or polyester, or various copolymers.[2]

This might work quite well for LDPE, but I worry somewhat about the bond strength with (no doubt LDPE sticks) and HDPE (which is only slightly branched and quite crystalline). Perhaps if the glue was kept hot and intimate with the substrate for a fairly long time (to give the chains enough time to equilibrate, which is a slow process) would yield a better bond.

Cheers,

O3

tnhrbtnhb - 18-2-2007 at 13:59

Quote:
Originally posted by Ozone
Welding plastic usually involves solvation. Solvation is not someting PE likes to do. You could drill, tap and screw in a retaining bracket, though.

It would be helpful, however, from the outside-helpers' point of view, if you were more detailed with your description of the experiment.

Cheers,

O3

Allright. What I'm doing is this:
The point is to boil water out of a dough or batter to make no (well, not very much) -heat bread. So, it should rise like a popover - the water vapor is the leavener, or if all else fails I can use baking powder. I have already dealt with other issues, like flour not absorbing water at low temps.

to do this, I:
-Take a vessel(A)(which can hold about 25 litres or more) that has a large opening at the top, this holds the dough or batter.

-Take another vessel, B (I am using a pressure cooker, so no strength problems there) with a volume of about 5 litres.

-Seal both vessels, then connect them with a pipe.

-Connect a vacuum pump to one vessel.

-Have a method of heating vessel A and cooling vessel B. All the energy required to boil the water gets dissipated in vessels B, so it's quite a bit, but I figure I can cool it with tap water.

- Evacuate the vessels down to the vapor pressure of water at 10deg C or so. It doesn't have to be that low, but that's my goal. I am using an air conditioner compressor as a vacuum pump. The ingredients and vessels are both about 10 degrees at the start.

-close a valve to the vacuum pump to prevent air from entering that way.

- heat vessel A, water boils, enters vessel B and condenses.


Right now, the only suitable vessel A I can find is a PE winemaking pail. The way I have done connections with small pipes to vessels in the past is usually to take a block of stuff (in the it was usually wood, but....) drill a hole in it, epoxy the tube in the hole with some sticking out the other side.
Then drill a hole in the vessel, put the tube through that, and just glue the block to the vessel's outer wall. Seems to be the quickest and sturdiest, if a rather obvious, way.
The only blocks of totally nonporous stuff I can find are PE, as well, so.... I need to be able to glue to them somehow.

So, what I’m gathering from your posts, what might work is :
Some kind of retaiinng bracket (?), tapping, drilling holes…. The wall of vessel A is pretty thin, I don’t have tapping&die stuff. :(
Glue gun aka hot melt glue.J
Superglue. Says on the side that it won’t bond plastic, though. Also, quite brittle and no good at filling spacesL….. Maybe back it up with silicone caulk (or sealant?) :\
“goop”- not too strong
Mechanical methods (maybe take some hollow threaded rod, run the tubing through, epoxy it, then bolt the thing through a hole to the vessel and seal with silicone sealant or something)

I think Hot melt glue sounds best, of these. Some won’t do for the connection from A to B, which is a large tube (about 4 cm inner diameter) to allow water vapor to flow ; the pressure difference is only a few tens of torr.

Ozone - 18-2-2007 at 14:10

Ah. How about heating your tube with a torch (propane or MAPP) and pushing it through the PE? This should seat and seal fairly well. An then, maybe, go the extra mile with the silicone or hot glue (LDPE type)? Alternatively, perhaps drilling a hole just a tiny bit smaller than your tube and hammering the tube in to create a "compression" seal?

Hmm,

O3

BCPneumatics - 11-3-2007 at 07:48

What is the thickest part of the pail that you can attach the tube through? Tapping a hole and using an NPT to Flare or Compression adapter might be the answer.
A 1/4" NPT tap is around $6- not much more than most of the epoxies out there. The brass fitting shouldn't cost more than two dollars. (all USD)

Sauron - 11-3-2007 at 11:08

What will be service temperature of the copper tube?

Cyanocrylates won't hold up to heat. If this is for RT then they can't be beat, the bond does not rely on solvation nor porosity so binding nonporous and disimilar materials like Cu and PE works. But if this Cu pipe is your fractionating column and will operate at 80-100 C then the superglue will let go.

Also acetone dissolved the stuff if you ever have to get if off (of your parts, or your skin). Don't forget eye protection when working with it.

jimmyboy - 11-3-2007 at 13:04

From what i understand you can just use a soldering iron to melt polyethylene together -- kind of weld it.. in a sense.. trace with the iron tip and make any designs you wish.. but in your case -- connecting poly and copper together - i am guessing that both the tubes are roughly the same size - get a small tube that will fit inside both and is nice and tight - and use some pull-ties or a worm clamp to secure them - either that or a copper valve with a plastic fitting already on it..

FrankRizzo - 11-3-2007 at 14:50

PE plastics can be cemented together using epoxies, but the surface of the plastic must be treated (etched) to form a porous oxizided layer first.

Etching soln:
Sulfuric acid (conc.) 89
Sodium Dichromate 4
Water 7

Keep the plastic surface in contact with the etching solution for 45min.-1hr depending on the depth of etch desired.

Magpie - 11-3-2007 at 14:52

Standard industrial practice would be to weld a half coupling to the outside of the vessel, then drill through the vessel wall using the half coupling as guide.

A half coupling is just a coupling cut in half. It could be made of copper with threaded ends. Or be copper with a female end for soldering ("sweating") to plain end copper pipe.

I don't know the material of vessel "A". For welding you might have to braze, or even go to silver solder. You could probably hire this welding done for not too much $.

[Edited on by Magpie]

Mr. Wizard - 11-3-2007 at 16:22

I recall seeing a thread about welding polyethylene plastics using spin welding or heating the ends of a tube with a metal plate and then firmly pushing the heated ends together with a tool or jig, until they cooled. Another tool available is a hot air welding gun, with a variety of plastic sticks to fill the gap. The instructions maintain it is VERY important to use exactly the same composition in the weld stick as the base material. This means you can't use Polypropylene on Polyethylene, even though it may look like it works, it will break when cool. All that said, I have never done it.

bio2 - 11-3-2007 at 16:23

.....PE plastics can be cemented together using epoxies, but the surface of the plastic must be treated (etched) to form a porous oxizided layer first.........

The oxidized layer can also be formed with judiscious application of a flame, Scorching but not burning or melting.

Then roughen with sandpaper, wipe with acetone or alcohol, and apply a medium setting time (15-20 minutes)flexible 2 part epoxy. Clamping as needed.

There are special epoxies and especially for PE and PP that have been developed in recent years made for boat decks and such. .Even these only work if surface preparation is done properly.

ciscosdad - 20-3-2007 at 14:28

Vacuum problems:


What provision have you made to support the 25l container against the vacuum you intend to apply?
Even the partial collapse of the sides will put a lot of strain on any glued joints you have, unless you do it all through the lid and support the lid with a disk of plywood (or whatever).
If you take that approach, you could sandwich the lid between 2 disks of plywood (silicone sealed), held together with screws. Then the problem simplifies to attaching your vacuum fittings to the plywood.
Then you only have the problem of supportng the remainder of the container. 25l fermenters will collapse under vacuum.

Have you any idea what this no heat bread will taste like? Or is that beside the point?

YT2095 - 21-3-2007 at 00:33

do you HAVE TO glue directly to the PE?
I`m thinking about using another material to adhere to instead, if you can imagine a nut and bolt, you drill through the PE put a rubber washer around the hole and put the bolt through, then place another washer over the bolt and the screw the nut on tightly, the difference here is that the Bolt is Hollow :)

probably not the best explaination but I`ve seen it done, and these fittings are fairly easy to get hold off.

tnhrbtnhb - 24-3-2007 at 10:01

Quote:
What provision have you made to support the 25l container against the vacuum you intend to apply?
Even the partial collapse of the sides will put a lot of strain on any glued joints you have, unless you do it all through the lid and support the lid with a disk of plywood (or whatever).
If you take that approach, you could sandwich the lid between 2 disks of plywood (silicone sealed), held together with screws. Then the problem simplifies to attaching your vacuum fittings to the plywood.
Then you only have the problem of supportng the remainder of the container. 25l fermenters will collapse under vacuum.

Have you any idea what this no heat bread will taste like? Or is that beside the point?


Thanks, I wish I knew this before I bought the pail :|, I thought it might not. The pail actually collapses by buckling at the side first, so reinforcing the top and bottom doesn't look like it would have helped much anyway.

As for what no heat bread tastes like, of course it depends on your recipe :). I've made some normal bread in the kitchen and sometimes the dough tastes better than the result, so I reckon it should be okay.

In case you're wondering, I've decided to use an airproof plywood box to just encase everything instead of having the exterior of the cooking vessel at atmospheric. I.e. the heater, vessel A and B both go entirely in the plywood box. I'm still wondering if I can use silica gel to absorb the water vapor instead of condensing it....


anyway, I guess I ought to post the info I've collected about bonding to PE

Apparently hot melt glue / “glue gun glue” works fairly well. It’s not so much a matter of the adhesive involved as the temperature of it.
Apparently you can also use some sort of flame-treatment to make the
surface of polyethylene easier to bond to, directions at
http://www.marinetex.com/PRODUCT%20PAGE_files/All%20PolyDura...
20Info/Poly%20Dura%20Directions.htm , they describe it for use with
their product, but apparently it works for use with other adhesives as
well (I don’t know if it works for all or what, though.)
I also copy it to below

Directions for standard use: (Specific polyethylene bonding directions below.)
1. Note caution information on dispenser.
2. Clean and roughen all bonding surfaces with MEK.
3. Remove cap by twisting counterclockwise 1/4 turn, then pull out.
4. Push Static Mixer (nozzle) on to the cartridge, twist 1/4 turn clockwise to lock.
5. Insert epoxy cartridge in Dispensing Gun or Caulking Gun Adaptor.
6. Discard the first 3" of material ejected from the epoxy cartridge. You will notice that this material is not consistent in color and should not be used for bonding.
7. Apply epoxy to bonding surfaces, press together leaving epoxy between joints.
8. Remove Static Mixer and replace cap on nozzle.
9. Clean un-cured epoxy with rubbing alcohol.
10. Working time is 15-30 minutes, material will set in 1-2 hours, full cure in 24 hours at 72°F.
11. Longer cure times at lower temperatures.

Directions for bonding polyethylene using flame treating:

1. Fit a propane torch with a flame spreader.

2. Following the operating cautions of the propane torch, ignite the flame.

3. Observe the flame in a darkened room, noting the primary (bright blue) and secondary (faint yellow) portions of the flame (see drawing.)

4. Adjust the flame so that the primary flame is contained within the spreader, and the secondary flame is 1-1/2" beyond the spreader (see drawing.)

5. Treat the polyethylene to be bonded with the tip of the secondary flame by passing it over the polyethylene in 5 gentle strokes. Total exposure to the flame should be 2-3 seconds (.5 second per stroke.) This light exposure should not deform or melt the polyethylene in any way.

6. Test the polyethylene for bond readiness by wetting it with water. If the water runs off immediately, the treatment was not effective. If the water sheets on the surface, the surface is ready for bonding. If unsure, compare the water's action on the treated area with the untreated area.

7. Bond joints per the above directions within 1 hour after flame treating. Always prepare test bonds to be certain that flame treating is effective with your material.


Loctite sells 2 products that specifically say they can bond to polyethylene. There is the ”all plastics” superglue product that comes with an activator, and the “all plastics” epoxy product that needs no surface preparation. According to loctite, if you apply the activator that comes with the superglue you should then be able to use a wide range of adhesives from there on, instead of the superglue. They don’t give any guarantees, though, and recommend that you test out any combinations first.

Whoever said welding works is also right, though I don’t know exactly what technique would work best, HDPE melts at something like 80 Deg C, so heat to somewhere above there and hold the pieces together while ensuring the plastic sticks, I suppose.







YT: I think that is basically what magpie is suggesting.


[Edited on 24-3-2007 by tnhrbtnhb]