Previous in Forum: when to use argon gas or liquid nitrogen during metal processing?   Next in Forum: methane gas sensor using fiber bragg gratings
Close
Close
Close
15 comments
Power-User

Join Date: Feb 2010
Posts: 190
Good Answers: 2

Can I Seal Two Polymers With Different Coe's?

06/29/2010 7:48 AM

I have been trying to make a small solar collector using polymers ( acrylic and fiberglass or both pieces acrylic). The top of the collector will be acrylic and the bottom either fiberglass or acrylic. The problem I have is in trying to seal the two pieces. Since they have differing coefficients of expansion I have been told it will not compute. I am reaching for an air tight seal where I can evacuate the air and replace it with Argon. Or if the seal is good enough then I will evacuate the air as much as possible and leave it at that. Yes I know that acrylics are porous, but the loss is so small over time that I feel that it will either not matter, or it will outlive the life of the collector.

Does anyone have experience in successfully sealing two items with differing coe's?

__________________
The time has come the Walrus said to speak of many things, of shoes and ships and sealing wax, of cabbages and kings.
Register to Reply
Pathfinder Tags: fiberglass polymers sealants solar
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
2
Guru

Join Date: Oct 2008
Posts: 42308
Good Answers: 1666
#1

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 9:23 AM

Depends on the bond strength and the difference in CTE.

Obviously acrylic to acrylic is not a problem. Because of the glass reinforcement, "fiberglass" has a lower CTE. This bond may still be feasible, but mat require more surface area for an effective bond.

You will have to experiment, since there are too many variables to give meaningful, specific advice.

A GENERAL guide is that if your materials CTE's are within one order of magnitude of each other you probably can do it.

Register to Reply Good Answer (Score 2)
Power-User

Join Date: Feb 2010
Posts: 190
Good Answers: 2
#2
In reply to #1

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 10:18 AM

It's not hard to tell I am not a chemist. Thank you for being polite. The abbreviation is of course CTE for coefficient of thermal expansion and not coe (coefficient of expansion). Your answer is appropriate. I called Weld On and talked with them this morning and they suggested Weld On 16 which is for welding two acrylics. I should have called them in the first place. I was told it could not be done by a polymers 'expert' because of the difference in temperatures between the two surfaces. of course my first thought was to pose the question on CR4. My project is back on course, thank you.

__________________
The time has come the Walrus said to speak of many things, of shoes and ships and sealing wax, of cabbages and kings.
Register to Reply
Guru

Join Date: Oct 2008
Posts: 42308
Good Answers: 1666
#4
In reply to #2

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 11:09 AM

Thanks Doorman.

In years past I've bonded almost any plastic you can name to Aluminum, steel and many other thermally mismatched materials. And they survived rocket rides into space.

Here's something to keep in mind. You have a lower and upper service temp limit on your product. Do your bonding in the middle of the range. That way you negate some of the stress of expansion and contraction.

If you apply heat of, say 150°F to cure the adhesive, then when it cools to room temp it will be stressed.

Once again the more area in your bond line the stronger it will be.

I don't have an opinion about adhesive products, you'll have to trust someone to help you with that. Naturally, everyone you talk to will know the best material to use.

Pick one and follow the supplier's instructions.

Good luck.

Register to Reply
Guru
Hobbies - DIY Welding - New Member

Join Date: Feb 2007
Location: Next to the Prime Merridian (51°29'34.50"N 0°13'32.85"W)
Posts: 780
Good Answers: 1
#5
In reply to #4

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 1:00 PM

"I've bonded almost any plastic you can name to Aluminum, steel and many other thermally mismatched materials"

Could you give a general rule of thumb, if there's any, in bounding as I am a novice to this and to me both CTE & COE are something I never knew about, even they make sense. Are you saying that even different materials can be successfully bounded and be exposed to extreme temperature uses?

I have been thinking of building something of a kind myself in the near future and was thinking of placing the whole collector assembly into a box with vacuum in it. I am also puzzled if I could use aluminium for the bottom part and perspex to close it with or just use perspex through out.

__________________
Making mistake is part of learning.
Register to Reply
Guru

Join Date: Oct 2008
Posts: 42308
Good Answers: 1666
#6
In reply to #5

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 1:50 PM

Wiki glass transition temperature and CTE for your homework. Then go to Matweb or other materials site and look up the cte of your materials. You can calculate the expansion over your temp range and get some idea of the magnitude of the problem.

Once you exceed Tg of either material or adhesive the CTE grows rapidly and failure is almost certain.

As I said earlier, it's probably doable if there are the same number of zeros to the right of the decimal point for both materials. There are exceptions to this, but it's a start.

I'm at work now, so more will have to wait till later today.

Register to Reply
Guru
Hobbies - DIY Welding - Don't Know What Made The Old Title Attractive... Popular Science - Weaponology - New Member United States - US - Statue of Liberty - 60 Year Member

Join Date: Apr 2009
Location: Yellowstone Valley, in Big Sky Country
Posts: 7001
Good Answers: 284
#3
In reply to #1

Re: Can I Seal Two Polymers With Different Coe's?

06/29/2010 10:39 AM

Ah, lynlynch. As Jerry Reed says, "When you're hot, you're HOT"

I will vote you a 'Good Answer". Well and nicely done.

__________________
When you come to a fork in the road, take it. (Yogiism)
Register to Reply
Guru

Join Date: Dec 2005
Location: Etats Unis
Posts: 1871
Good Answers: 45
#7

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 1:22 AM

In the event you do not have an adequate surface area to sustain the stresses involved you might consider a resilient bonding compound or possibly a resilient inter-layer or gasket.

__________________
The hardest thing to overcome, is not knowing that you don't know.
Register to Reply
Guru
Hobbies - DIY Welding - New Member

Join Date: Feb 2007
Location: Next to the Prime Merridian (51°29'34.50"N 0°13'32.85"W)
Posts: 780
Good Answers: 1
#8
In reply to #7

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 8:04 AM

I guess you're rite about the gasket (or inter-layer?) could be the answer. I have a further question - what is the required vacuum to create a good thermal insulation and pressure would it equal to? I noticed the most ideal shape to resist external pressure is cylindrical but it is not always practical especially when the installation requires something different.

__________________
Making mistake is part of learning.
Register to Reply
Commentator

Join Date: May 2008
Location: St. Paul , MN
Posts: 79
Good Answers: 6
#9

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 8:35 AM

Although bond strength will be important you need to consider the amount of differential movement likely to be observed under the installed conditions. The sealant must then be chosen whereby it has enough elongation to handle the likely movement. Finally the width and thickness of the sealant bead should be designed such that the tensile properties of the sealant are not compromised. I would suspect a high quality silicone will do the trick provide the bead designed is large enough.

Good Luck.

Register to Reply
Power-User

Join Date: Mar 2008
Location: MA 01864, USA
Posts: 453
Good Answers: 7
#10

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 11:45 AM

You can do this if your are careful.

Acrylic to acrylic will not have any issue.

Acrylic to glass or glass fiber will be issue one way to do is

I am assuming you have glass fiber board and not the fiber or glass fiber shape.

Take a 400 grid sand paper and rough the surface

and then you will be able to make a good seal

For glass fiber to acrylic thinner and rougher glassfier surface will make good bonding of the two system.

__________________
Masyood
Register to Reply
Guru

Join Date: Oct 2008
Posts: 42308
Good Answers: 1666
#11
In reply to #10

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 12:54 PM

We haven't gotten to surface preparation yet. 180 grit or rougher is required to give the plastic enough "tooth" to adequately bond to the substrates.

Solvent clean etc. etc.

Register to Reply
Power-User
Popular Science - Biology - New Member

Join Date: Oct 2009
Location: Colorado - USA
Posts: 133
Good Answers: 15
#12

Re: Can I Seal Two Polymers With Different Coe's?

06/30/2010 7:26 PM

Michael,

I have successfully solved this kind sort of problem with a flexible sealant, but the one I used will degrade rapidly in sunlight (it was for epoxy circuit boards). You might consider a clear silicon caulk material. You'd need a precision drawdown bar to apply it, but it is flexible and can survive sunlight environments.

__________________
Life is not an illogicality, yet it is a trap for logicians.
Register to Reply
Anonymous Poster
#13
In reply to #12

Re: Can I Seal Two Polymers With Different Coe's?

07/01/2010 11:05 AM

Urethane for bonding autoglas in cars is the nicest bond, can be flexible and resistant to uv, cheap and safe to work with, I made a waterproof camera case once with polycarbonite using only this for adhesive, water cure is fast enough but not hard when done, MG retired peugeot mech,

Register to Reply Score 1 for Good Answer
Guru

Join Date: Oct 2008
Posts: 42308
Good Answers: 1666
#14
In reply to #13

Re: Can I Seal Two Polymers With Different Coe's?

07/01/2010 11:21 AM

Most excellent answer! With the CTE of steel much higher than glass, modern windshields seem to survive in any environment quite well.

Sign up, we need smart people to tip the scales back to the "normal" side. One of our more mischievous members has unleashed a "dumb" pandemic.

(If you hang around, this will ultimately make sense to you)

Register to Reply
Anonymous Poster
#15

Re: Can I Seal Two Polymers With Different Coe's?

07/02/2010 2:07 PM

My suggestion would be to use silicone based adhesive/sealant as they are know to survive environmental exposure in direct glazing applications on high rise buildings. Polyurethane in windsheild applications benefits from the "frit' (black coating all around the windsheild) being there, plus primers have to be used to bond the windshield to the car. Don't know if you have this flexibility, in any case you don't want to use primers on acrylics as they are solvent based and would swell the acrylic.

Register to Reply
Register to Reply 15 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (2); Daddio926 (1); Doorman (1); GKC (1); Isti80 (2); lyn (5); Masyood (1); Michael Rock (1); rcapper (1)

Previous in Forum: when to use argon gas or liquid nitrogen during metal processing?   Next in Forum: methane gas sensor using fiber bragg gratings

Advertisement