Help! What Sealant is Better than Dow 795 for polycarbonate??

[sy Dolce Vita]

We use a Tri-polymer called Geocil 2300 . Available at your roofing suppliers for under $5.00 a tube

[Dsanduril]

Originally Posted by Franziska

Sorry, but I do strongly disagree.

Never use silicone for this. Any trained boat builder will tell you this.
Silicone has only very very few uses on a boat.
...
Polyurethane or acrylic based sealant do not suffer from this.

I generally agree with limiting the use of silicone but both Sika 295 and Dow 795/995 are silicone based adhesives and these are the go-to for glazing and hatch lens installation even in the boating world. This may be in part because the polyurethanes and polysulfides are not fully compatible with acrylic and polycarbonate windows/lenses and acrylic sealants tend to shrink while curing, which does not help with the expansion/contraction of glazing materials.

[Daniel Grant]

GE-SSG2000Used on windows of skyscrapers. Replaced all my windows with Lexan and used GE-SSG 2000 and it has been at least 6 years with no leaks

[MJH]

Originally Posted by Chotu

Can anyone suggest a sealant better than Dow 795? I have large poly carbonate windows that move about an eighth of an inch each day due to heat. They have VHB tape holding them in place. That is working. But they also need to be sealed around the edges. And everything is cracking. Doubt 795 is failing. What is a more soft and supple type of sealant? One that will move with the window.

Sounds like a faulty design or misuse of poly carbonate for that large a window. I question the practicality of them offshore in a rough seas impact...I've seen some online photos of boats losing them...then again maybe you are not going offshore, you didn't share your plan.

My neighbor had a large fiberglass dodger built for his center cockpit Spencer 1300 (44') with the three piece windscreen made of tinted auto windshield glass; the dodger covered the entire cockpit. I always thought that was a great design as the windows never leaked, faded or cracked and cleaned up so much nicer than poly carbonate.

~ ~ _/) ~ ~ MJH

[Franziska]

Yes, but instead of standart silicone Sika does NOT create the issues I mentioned.
Typically people refer with silicone to the stuff they find in a DIY store. And that should really not be used onboard (with very very few exceptions).

By the way, in glazing it should be 295UV not 295.

Originally Posted by Dsanduril

I generally agree with limiting the use of silicone but both Sika 295 and Dow 795/995 are silicone based adhesives and these are the go-to for glazing and hatch lens installation even in the boating world. This may be in part because the polyurethanes and polysulfides are not fully compatible with acrylic and polycarbonate windows/lenses and acrylic sealants tend to shrink while curing, which does not help with the expansion/contraction of glazing materials.

[Phantastic]

Originally Posted by sy Dolce Vita

We use a Tri-polymer called Geocil 2300 . Available at your roofing suppliers for under $5.00 a tube

This is a solvent based formula- manufacture says do not use on polycarbinate or acrylic

[mdenize]

Originally Posted by Chotu

Was also considering epoxying a piece of u-channel along the top to simply direct water over the gap and onto the window where it runs off.

Any thoughts about that?

That's the sole purpose of the sealant here. Not to hold the window in place (the VHB tape that's unseen inside does that). Just need to redirect water into the face of the window skipping the crack where it meets the surface.

Maybe z channel?

Hey there Chotu, Well this works OK for houses, but do you use your boat as a boat? On my boats the water goes everywhere including up, no flashing system would work.

The sealant system I described works well 99% of the time, some of the windows are still sound after 20 Years. I do find it easier to remove the whole window than muck around if there is leak, To remove the glue layer I just use a very sharp craft knife and cut between the window and the aluminium but I do have more than 1/8 to play with. As others have said I think the lack of thickness is the problem with your sealant cracking and increasing this is the most likely way forward. Sadly any form of patching it up without removing the window is just delaying the inevitable

[Phantastic]

Glueline Thickness
Proper glueline thickness facilitates the installation of sealant and allows reduced stress on the structural
joint resulting from differential thermal movement. A minimum glueline of 1/4" (6.4 mm) is required,
but as the structural bite increases, the glueline should be increased to allow the sealant to be applied
easily and the panel to expand and contract when subjected to thermal movement. If structural bite
requirements exceed 3/4" (19 mm), the glueline thickness should be increased to a dimension greater
than 1/4" (6.4 mm). To facilitate filling of the structural joint, the bite-to-glueline thickness ratio should
be maintained at 3 to 1 or less.
13
All panels that are structurally glazed undergo repeated expansion and contraction due to variation in
temperature. Glueline thickness must be properly designed to accommodate these movements. The
thermal movement can be calculated for any panel or framing member if the length of the material,
material type (e.g., glass, aluminum) and coefficient of thermal expansion (CTE) is known.

Joint movement for a particular panel can be calculated as follows:
Movement (inches) = panel length (inches) x CTE (inches/inches/ºF) x temperature change (ºF)
For example, for a 4' by 8' high lite of glass fixed at the sill and a temperature change of 180ºF, glass
with a CTE of 5.1 x 10–6 will show movement of 0.088". Aluminum with a CTE of 13.2 x 10–6 will
move 0.228". Differential movement between the glass and aluminum will be 0.228" minus 0.088",
which is 0.14".
Movement (mm) = panel length (mm) x CTE (mm/mm/ºC) x temperature change (ºC)
For example, for a 1219 mm by 2438 mm high lite of glass fixed at the sill and a temperature change of
82ºC, glass with a CTE of 9.2 x 10-6 willshow movement of 1.84 mm. Aluminum with a CTE of 23.8 x
10-6 will move 4.76 mm. Differential movement between the glass and aluminum will be 4.76 mm
minus 1.84, which is 2.92 mm.
The dimension of glueline required (a) for the differential movement (b) can be calculated using the
Pythagorean theorem. Likewise, the allowable movement (b) for a particular glueline dimension (b) can
also be calculated. The new glueline thickness (c) is limited by the movement capability of the sealant
in shear in a structural joint configuration.


a2
+b2 = c2 where a = original glueline
b = joint movement
c c = new glueline after joint movement
a

b
For the example discussed above where differential movement of 0.14" (b) is expected, and the sealant
has an original glueline thickness of 0.25" (a), the sealant will elongate to a new glueline thickness of
0.287" (c). Extension of the sealant from 0.25" to 0.287" will be 14.8%.
For DOW CORNING 995 Silicone Structural Adhesive and DOW CORNING 795 Silicone Building
Sealant, maximum extension from thermal expansion is 15% in any structural silicone joint. For
DOW CORNING 983 Silicone Glazing and Curtainwall Adhesive/Sealant, maximum extension from
thermal expansion is 10% in any structural silicone joint. The lower allowable movement capability in
structural joints is due to the joint design, where the joint width, or glueline, is less than the joint depth
or structural bite. This is the reverse situation to weatherseals, which have higher movement capability.

[Phantastic]

Sealant Cure Requirements
In all silicone structural glazing applications, the silicone must be fully cured and adhered before the
adhesive is stressed. The exact time can be determined by fabricating several small samples that
replicate the joint design of the units. These test specimens should be cured along the curtainwall units.
These samples can be cut apart to determine the degree of cure as a function of time. These samples
would also be used to check the adhesion of the sealant to the substrates. Full frame deglazes are also
used to determine time to full cure, plus to test adhesion and joint fill (structural bite).

Construction Site (Field) Glazing
Temporary support of adjoining materials must be used during the cure of the structural silicone sealant.
This is to prevent any stress on the sealant prior to full development of sealant adhesion and strength.
DOW CORNING 995 Silicone Structural Sealant and DOW CORNING 795 Silicone Building Sealant
typically require 7-14 days but up to 28 days or more in cold, dry conditions, depending on joint size,
temperature, and relative humidity.

I think the above is your answer--You are stressing the joint with heat expansion prior to a full setup of the sealant. Do as was suggested- shade the area for 7 days to limit the expansion / contraction.

[Mike Banks]

Well, I only used heavy glass for mine--but that was held in place using a rubber strip, and there was a series of captive bolts welded to the inside of a cover gasket face that went through the cabin walls and a reinforcing plate, and the whole lot was bolted together sandwiching the glass as though it were in the jaws of a vice with rubber on each jaw. It never leaked.

Some trawler men I knew used polycarbonate and they used it glued into a metal frame--I think they used a product made by Sikoflex. It was really powerful stuff. One aapplies one layer to the frame, inserts ythe window material, puts another coat on the windoe w in its frame, adds the cover stripo, and the wholele is tightened down lightly and the job cleaned up. Later after the compound has set, the nuts are tightened again, but not overtightened.

Apparently the trick is to keep as much of the compound as possible both sides of the glass, yet have the glass evenly secured all around. The thickness of the rubbery set compound is what allows for small movements.

[Daniel Grant]

Bomar Uses GE-SSG2000 to seal the "glass" in their hatches

[Disailor]

I think a strip of teak surrounding & overlapping the poly would hide the joint & support the plastic thereby allowing a thick glazing compound. The screws securing the teak to the "wall" would be outside the perimeter of the poly.

[mainsil]

Look up this stuff. Works great for many boat jobs and it won't break the bank.

[Chotu]

Originally Posted by mdenize

Hey there Chotu, Well this works OK for houses, but do you use your boat as a boat? On my boats the water goes everywhere including up, no flashing system would work.

The sealant system I described works well 99% of the time, some of the windows are still sound after 20 Years. I do find it easier to remove the whole window than muck around if there is leak, To remove the glue layer I just use a very sharp craft knife and cut between the window and the aluminium but I do have more than 1/8 to play with. As others have said I think the lack of thickness is the problem with your sealant cracking and increasing this is the most likely way forward. Sadly any form of patching it up without removing the window is just delaying the inevitable

The windows are set using VHB tape pretty far back from the edges. They aren't going anywhere without a grinder.

[Chotu]

Originally Posted by Phantastic

Sealant Cure Requirements
In all silicone structural glazing applications, the silicone must be fully cured and adhered before the
adhesive is stressed. The exact time can be determined by fabricating several small samples that
replicate the joint design of the units. These test specimens should be cured along the curtainwall units.
These samples can be cut apart to determine the degree of cure as a function of time. These samples
would also be used to check the adhesion of the sealant to the substrates. Full frame deglazes are also
used to determine time to full cure, plus to test adhesion and joint fill (structural bite).

Construction Site (Field) Glazing
Temporary support of adjoining materials must be used during the cure of the structural silicone sealant.
This is to prevent any stress on the sealant prior to full development of sealant adhesion and strength.
DOW CORNING 995 Silicone Structural Sealant and DOW CORNING 795 Silicone Building Sealant
typically require 7-14 days but up to 28 days or more in cold, dry conditions, depending on joint size,
temperature, and relative humidity.

I think the above is your answer--You are stressing the joint with heat expansion prior to a full setup of the sealant. Do as was suggested- shade the area for 7 days to limit the expansion / contraction.

I'd agree. Plus the smaller than ideal bondline.