Low adhesion sealent 2

[daxmann]

We currently have an issue with filling the gap between our window and frames using low adhesion sealant PR-1428, Class A. The sealant is runny and hard to keep in place for forming. The assembly must be pressure tested so complete sealing is required. I'm thinking about using another low adhesion sealant MIL-S-8784 Class B. It looks to hold after forming much better but we have never used it before. Does anyone know about MIL-S-8784 Class B if it will hold pressure or if there is a better solution for our current sealing issue? Thank you for the help.

 

[WKTaylor]

daxmann...

You may want to consider an alternative [follow-me-through].

1. Prepare window frame for sealing, thus:
Clean-off old sealant residue.
Clean surface for new sealant.
Apply adhesion promoter to the window frame fay surfaces.
Apply a tough high adhesion pressure sealant, such as MIL-PRF-81733, AMS3265 [min 2-hr cure] etc to the window frame surfaces.

2. Prepare an old/unserviceable [but not substantially damaged] window pane for installation thus:
Clean-off old sealant residue.
Clean surface for new sealant.
Apply mold release agent [IE: wax, AMS3091, etc] to the entire window fay surfaces [and/or entire outer/fay surfaces] and window edges.

3. Bolt the old/prepared window pane in-place per window installation tech data, or...
Install all bolts/nuts finger-tight.
Torque-tighten in-stages, using a typical cross-pattern sequence, until each bolt is at final torque value.
WARNING. Accomplish final torque sequence, well-before sealant reaches the end of it's 'working life'.

4. Remove uncured sealant squeeze-out and undesirable 'butt-gap' sealant squeeze-out [suing plastic gap and surface scrape tools]. Allow the sealant to fully cure.

5. Remove the old window, revealing the formed-in-place sealant-gasket. Trim cured sealant IF/AS require to allow a new window installation [steps 6, 7 and 8].

6. Fasten a new window pane in-place WITHOUT sealant, but-otherwise, per tech data. NOTE. Final fastener torque may/may-not have-to-be reduced, slightly, for this step, since sealant squeeze-out will-not occur during reinstallation [for obvious reasons].

7. Lay a thread of high strength mono-filament fishing line or nylon string [~50-# test line or string] or 0.040 [min] CRES lock wire in the Butt-gap [between window and frame].
Ensure line/string/wire is wiped clean with dry cheese cloth before installation.
Ensure line/string/wire is long-enough to ensure the ends over-lap by at least 2-inches, at a specific location relative to the window pane... such as: lower-aft corner, upper-forward corner, lower-center edge, etc... every-window, same location, every-time [Your call].
Tack the line/string/wire in-place with window-compatible [adhesive] injection sealant.

8. Carefully inject-fill [above-flush] the butt-gap around window/frame with the same window compatible [adhesive] sealant used-to tack the line/string/wire in-place [step 7].
Ensure injected sealant is void free.
Wipe-off excess sealant with plastic wiper so butt-gap is filled/flush with window/skin.
Allow sealant to cure.

9. Window removal.
CAREFULLY gouge-out butt-gap sealant in area [corner, zone, etc], to expose at least one-end of the buried line/string/wire.
Pull-out the one-end of the buried line/string/wire and grab it with pliers.
With a steady/firm/smooth pull, remove the buried line/string/wire from the butt gap. This should effectively tear-out the entire butt-gap sealant around the window pane.
Unfasten the window pane and push it-out of the window frame.
Carefully scrape/clean-off the remaining butt-gap sealant still attached around the window frame periphery. The cured sealant which the window pane mounts against [steps 1--5] should be fresh/clean and only slightly impressed due-to the old window pane. DO NOT damage this 'form-in-place' sealant-gasket when cleaning-off the butt-gap sealant.

10. Install new window pane per steps 6, 7 and 8.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[daxmann]

Thank you wktaylor for spending so much time on your response. I should have explained that the window is aprox. 20" x 36" and in a door. It is never removed once installed so we only have one time to get it right. The gap between the window and its frames is causing us to find a better alternative for seal. Do you think MIL-PRF-81733, AMS3265 is a better choice for our needs than MIL-S-8784 Class B?

 

[WKTaylor]

daxmann...

Be extremely careful... The sealants I noted are not necessarily compatible with window materials [acrylics, polycarbonate, innerlayers, etc]. This is a selection that should be made with assistance of the window manufacturer and the sealant supplier.

When a sealant is fully cured [all solvents dissipated and chemical reactions completed] it essentially becomes inert rubber. The sealant I recommended for butt-gap seal around the window... but did not explicitly state... has to be compatible with the window materials when applied and curing.

This is still a problem with the Acft I'm supporting: even experienced field techs don't understand that sealants designated for window installation must be used since they are tested/certified to be fully compatible with the window materials... and are included not just to make 'logistics more complicated'. When the incompatible sealants are used, they often display immediate and/or long-term effects on the pane edges/innerlayers/etc by clouding/discoloring, crazing, or premature hardening of materials.

I will give an example from PPG website.

http://www.ppgaerospace.com/Products/Sealants.aspx

Note the wide variety of purposes that various sealants are formulated for. Suggest going into each of these categories and exploring the properties from the downloadable TDS. What is interesting is how sealant chemistry has changed over the years, due to a variety of factors including advances in formulation, EPA/ODC rules, industry demand for lower densities and enhanced resistance properties [heat-chemical-ozone/electrical resistance, etc.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[daxmann]

wktaylor, thank you for your concerns. I did some more research and found that sealants with chromate can cause crazing so we have decided to go with PR-1428, Class B. I want to thank you so much for all your help as it did point me in the right direction. Thanks again.

 

[tbuelna]

Interesting topic and excellent input from wktaylor. Based on the post above that the door window is 20" x 36", can we assume it is made from a plastic material rather than glass? I read some of the sealant specs noted, and some of them required using a surface primer treatment that would not be compatible with plastic materials. Mostly intended for non-corrosive sealing of metal components.