[TowerTalk] OT: Bonding to Teflon

[Larry McDavid]

The AG (see link below) primer for bonding Teflon TFE is interesting; 
this must be a relatively new material. All my experience bonding TFE 
has been with Tetra-Etch. The AG adhesive shown in the link seems to be 
a cyanoacrylate, judged from its rapid bond time. All my experience with 
cyanoacrylate adhesives for long-term use has has poor results; it is a 
useful fixturing adhesive but has not had long life, especially in 
high-humidity areas. The shelf life of cyanoacrylate adhesives is poor 
and you soon find the whole container has set up and is useless. There 
are some applications where cyanoacrylate adhesives work well--such as 
the rear view mirror mount on car windshields, but you can be sure those 
applications have excellent process control.

The AG link example tests bond strength in peel and shear but there is a 
lot of contact area in that joint; the strength in psi evaluated by that 
hanging masonry block is really not that high. And, there is no evidence 
the bond will survive for a long time if wet or in high humidity, 
conditions which are not a problem for urethane or epoxy adhesives. It 
is not clear if the AG primer will work with anything but the 
cyanoacrylate demonstrated.

But, to say that TFE cannot be bonded is just not true. MIL-Spec and 
NASA procedures use Tetra-Etch, which has metallic sodium in a 
hydrocarbon solvent; check the readily-available Tetra-Etch MSDS if you 
want to know more. But, including metallic sodium makes disposal difficult.

I've used a great deal of Tetra-Etch on structural TFE and on the white 
TFE jacket of MIL-Spec electronic cables. Applying and rinsing off 
Tetra-Etch does make the TFE bondable by epoxy and urethane adhesives, 
and probably by acrylic adhesives. Specs for using Tetra-Etch put a 
limit on the shelf life of an etched surface; I'd be uncertain about the 
bondability shelf life of pre-etched Teflon. My recollection from 
MIL-aero work is that the etched TFE must be bonded within two days at most.

Searching on-line, I see several common industrial sites that offer 
Tetra-etch for sale.

Teflon electronic "spaghetti" is not TFE, it is FEP, recognized by its 
translucent semi-transparency and bluish color tint. FEP can be etched 
by Tetra-Etch also.

Hydrofluoric (HF) acid does not attack TFE and it will not etch the TFE 
surface for bonding. Worse, HF is very dangerous to use, as even a tiny 
drop on your skin will eat through your flesh and there is not a whole 
lot you can do to prevent severe, long-lasting injury. Industrially, HF 
is used in many chemical reactions but under very careful control. You 
really, really don't want to fool around with HF! As someone here 
pointed out, it can be stored in Teflon, polyethylene and even lead 
metal, but not glass.

Larry

On 7/9/2020 8:39 PM, Gedas wrote:
> Did find this interesting. Need to find out if it is available in the US 
> (and if it is affordable). The chemical surface prep seems not to be as 
> scary as a lot of other techniques:
> 
> https://www.youtube.com/watch?v=Q6mU4excEE8
> 
> Gedas, W8BYA EN70JT...

-- 
Best wishes,

Larry McDavid W6FUB
Anaheim, California  (SE of Los Angeles, near Disneyland)