Sciencemadness Discussion Board

Bonding PTFE

Swede - 30-12-2009 at 10:03

There are "kits" available from companies like U.S. Plastics that supposedly bond PTFE sheet and solid to other plastics, metal, etc. The problem is that they are expensive, around $75.

Googling the question brings up vague suggestions;
Quote:

Etch the PTFE with a sodium naphthalene dispersion


and the like. Or, they simply suggest alternative methods that don't actually involve a true bond. Has anyone had any success bonding PTFE using lab-made materials, or is the commercial kit the best answer?

Thanks!

watson.fawkes - 31-12-2009 at 07:32

Quote: Originally posted by Swede  
Or, they simply suggest alternative methods that don't actually involve a true bond. Has anyone had any success bonding PTFE using lab-made materials, or is the commercial kit the best answer?
What's the geometry of the application? Is there a reason that a mechanical arrangement won't work and a chemical one is necessary?

BASF - 31-12-2009 at 07:47

I have tried to weld ptfe foil using an impulse welder for plastic foils (~100 eur).
Results were not satisfactory because of the high temperature needed (~400 deg +) being beyond the abilities of these instruments (optimized for PE, PP foil).

Although not very common, i think that "welding" of ptfe can work, e.g. with a modified impuls welder which keeps the higher temp needed for a few minutes. (the resistance heating wire would have to be temperature-controlled by a PID-controller).

dann2 - 31-12-2009 at 14:21

Hello,
As far as I know you cannot get PTFE to melt as such, it starts to decompose before it melts. Are not PTFE items made from sintering PTFE powder?
If you were to obtain PTFE powder and put it between the parts to be bonded and heat to the necessary sintering temperature perhaps you will get bonding.
Not sure myself.

Dann2

densest - 31-12-2009 at 17:22

I've successfully closed PTFE tubing ends by heating until it becomes semitransparent then pressing the heated material together. There is a small window of temperature between softening to what looks like a jelly and decomposition. In the "jelly" state it can be molded and it adheres to itself. It would take some form of mold or tooling to make a precise shape.

The Na-naphthalene reagent chews off enough F to make some adhesives work. If you don't need pure PTFE it's more precise.

BTW, does anyone know how to make sodium naphthalide? I haven't found any references yet, and some naive experiments didn't work (Na metal and naphthalene in solution in hexanes or butyl diglyme).

not_important - 31-12-2009 at 18:12

You were close regarding sodium naphthalide, you need to use solvents such as THF or glyme/dimethoxyethane, groups larger than methyl reduce the effectiveness. Diglyme itself might work.

http://qchem.knu.ac.kr/enter/pdfs/jpc_109_20719_2005.pdf

and

United States Patent 3617218 , ignore the H2 gas addition as the purpose of the naphthalide is to combine with H2 to make NaH
Quote:

EXAMPLE 1

Naphthalene (1.66 g., 0.0130 m.) was dissolved in dry tetrahydrofuran (THF) to a concentration of 0.16 M. Sodium metal (1.50 g., 0.0652 m.) in pieces of about 0.2 g. in mass were added and the mixture was stirred vigorously at room temperature in an atmosphere of dry hydrogen at atmospheric pressure...

As the reaction neared completion the system changed from an initial dark green color to brown and finally to a red-brown to brown color. The reaction mixture was centrifuged and a gray substance contaminated with small quantities of a white solid was separated. This gray substance was shown by hydrolysis to be sodium hydride, yielding H 2 and NaOH in relative molar amounts of 0.98 and 0.95.

EXAMPLE 2

The materials and procedure were essentially the same as in example 1, the major difference being that a 1:1 molar ratio of sodium to naphthalene was used and these two were given time enough to react completely with formation of a solution of sodium naphthalide before exposure to hydrogen and TTIP. Thus, naphthalene (6.4 g., 0.050 m.) was dissolved in THF to a concentration of 0.63 M in an atmosphere of dry argon. Sodium metal (1.15 g., 0.0500 m.) was added and the mixture stirred at room temperature for five hours. The argon atmosphere was replaced by hydrogen. The vigorously stirred dark green solution of sodium naphthalide absorbed hydrogen very slowly....


densest - 31-12-2009 at 22:21

@not_important - thanks! Time to go to the welding shop for a tank of oxygen-free argon... There are tantalizing references in the patent literature to methods for electrolytic production of sodium metal from solutions using membrane-covered cathodes. The most coherent one I saw needed to initially complete the circuit by impregnating the cathode with sodium using the naphthalide. Since no commercial production of sodium using this method seems to exist, there are probably good reasons it doesn't work. Still, an aprotic solvent which dissolved sodium salts to a conductive ionic solution would be interesting. Molten NaNaphth and anhydrous sodium sulfate? That would be too easy... probably explosive. Any anion I can think of would react pretty violently. Oh well.

Eclectic - 1-1-2010 at 01:11

I've read of using ETFE (Tefzel) as hot melt adhesive for ptfe, but don't have actual references.

vulture - 1-1-2010 at 03:11

Welding will not work, as the PTFE will decompose, releasing extremely toxic gas.

Do realize that sodium naphtalide is probably pyrophoric, so you'll need inert conditions and dry solvents all the way.

Swede - 3-1-2010 at 08:16

It sounds like re-engineering, rather than bonding, is a better answer. I use PTFE mostly as long-lived fittings in electrochemical cells, and sometimes it would be handy to be able to bond them to another object as part of an overall fitting or construction.

dann2 - 15-1-2010 at 06:20


Hello,

Quote from
this thread.


Teflon Sintering


I use Teflon and other fluorpolymer compounds all the time and our company fabricates many systems requiring a Teflon (Dupont's trade name for PTFE), or PFA (properties similar to Teflon but easier to work with.

Teflon Flows and sinters well with ther right heat and pressure. This is frequently used to "weld" Teflon components together. Temperatures required for clean surfaces range from 400 to 600 F depending upon pressure applied. The chemical industry frequently makes Teflon envelope gaskets for industry by sintering 2 sheets of Teflon together with a stainless steel screen in between. This makes a better long term seal as the SS screen reduces the flow of the Teflon under pressure.

Just remember when sintering Teflon, the three variables are temperature, force, and time. More of one means that less of the other is required. Also, there must be absolutely NO oil or grease on the surfaces!

My favorite is PFA because it welds much easier. For PFA, you simply heat the two parts with a radiant heater until transparent and press them together. Teflon requires much more force and time.

When coating other materials with PTFE or other fluoropolymers, you either need a special primer and/or a very rough surface (both is better).

Dann2