are glass and polyester resin incompatible? 3

[niksmith]

I am presently sticking glass( of 4 mm thickness ) using polyester resin onto fibre glass of around 2mm thickness . However the problem I have is that many of the pieces seem to break after some days .
My feeling is that there is an incompatibility in the coeffecient of expansions of the two materials namely glass and resin leading to this cracking during shipment by air .
I would like to know if someone has had a similar experience and the possible solution for this problem .

Thanks in advance

 

[patprimmer]

There is a significant difference in the co-efficients of expansion , with the resin being considerably higher.

I am sure you can find each by a google search.

I am surprised that the stress build up is enough to break the glass rather than break the bond, and or stretch the polyester.

Some shrinkage of the polyester will occur on curing, creating some preload, which will be increased stress at lower temperatures.

Regards

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[CoryPad]

Glass-reinforced polyester is a very common material. This combination is what is referred to generically as "fiberglass". The trick to compatibility is the use of a silane coupling agent on the fibers. One end of the silane adheres well to the glass fiber, the other end promotes adhesion to the polyester.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[niksmith]

Thank you Patprimmer ,actually I was thinking that the expansion is the only reason for the cracks which I observed .The curing shrinkage is a new information though .

Actually Cory I am doing two things here -firstly I am creating the fibre glass using the resin as you described at the same time I am laying it in conatct with a glass sheet of thickness of 4mm .So the polyester resin which is reinforced with the fibre is also simultaneoulsy adhering to the glass sheet . The silane can promote the adhesion with the glass but can it avoid the stress development between the glass and the resin ?
Is there some resin which would be more compatible . Epoxy resin has a much higher expansion too .But can it be amde compatible in some way .

Regards,

 

[patprimmer]

Mineral fillers will inhibit the shrinkage and co-efficient of expansion to some extent. The glass fibre also reduces shrinkage and co-efficient of expansion, but more effectively in the direction of lay of the fibre. Although the glass fibre reduces the shrinkage and co-efficient of expansion of the polyester, it is not so effective so as to avoid the difference in these properties vs glass plate, but it does reduce the difference.

Some mineral filler might reduce the difference to an extent that avoids breakage.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[rnd2]

Epoxy generally does not shrink in cure anywhere near the amount polyesters do. Some vinyl esters shrink as much as 10% by volume unless heavily filled as patprimmer suggests.
If you are stuck using polyesters, here is something you may consider. Add 10~15% by weight one part uncured polyurethane elastomer such as Sikaflex or equivalent, to the polyester resin and mix for a few minutes until it is completely dissolved. The result is a more flexible material than regular polyester, and should be more forgiving against the glass.

 

[Demon3]

First let me say that the answers you received already are really excellent and probably deserving of some shiny stars. Even so I would like to add some comments.

Pat is right that fillers generally have thermal expansion coefficients about 1/10 that of polymers. That means adding the glass fiber helps in that regard, bringing the CTE of the filled resin closer to that of the glass sheet. As Pat said, adding a mineral filler (an isotropic one like CaCO3) would help reduce the CTE further.

The second factor is the shrinkage on cure. A friend of mine did a study on how that affects bonding (J. Lange, KTH Stockholm). Anyway, what he showed is that the curing during shrinkage builds a stress at the coating / substrate interface and that tries to delaminate the joint. At the start of cure the shrinkage is there but the resin is liquid so there is no force built up. As cure proceeds the total force at the interface builds. Think of it as a spring, the force is the shrinkage x the modulus of the resin at that point in the cure. Here the glass fibers hurt you a lot because they increase the modulus of the material by 100% or even more. They only do that in the direction they are aligned too so it's important to align them evenly in all directions. Or can you remove the glass fibres? What do they give to the final product? Consider mica instead.

Regarding silanes they can help adhesion a lot, especially if water gets in to the bond area. The idea of adding something soft was also a nice one (e.g. PU) but if you can afford to lower the modulus by adding a soft component then I would suggest it's cheaper and better to just leave out or reduce the glass fiber amount.

Hope that helps.

 

[niksmith]

Thank you PatPrimmer,rnd2 and Demon3 for those very useful replies .I'd happily give stars for those .
But please also let me know what would be the approximate percentage of filler to be added to the polyester resin in order to have good adhesion properties and also in order to be able to lower the shrinkage during the cure .I think I will try the suggestions regarding the alignment of the fibres and about the use of the PU elastomer and see the changes in the final product .

 

[rnd2]

There really is no fixed rule because fillers vary so much in shape and density. To really pack a resin with fillers requires a combination of diffent particle sizes, much like road base. One single paticle size is inefficent. Think of a container filled with marbles. There is still plenty of space between the marbles for smaller particles like sand to enter. In turn, clay can fill the space between the sand.

 

[Demon3]

Hello,

Thanks for the stars. You asked about adding filler to decrease shrinkage and that may work but as I mentioned it's a balance because the glass fiber as well as reducing shrinkage, will actually hurt adhesion because it increases the modulus and therefore increases the force at the interface that tries to delaminate the bond.

Can you specify what overall properties you want? Does the polymer layer have to be transparent / translucent? What do you want from the polymer layer mechanically? Can you accept another material instead of polyester?

As a first guess I would suggest adding the soft component as you planned and perhaps replacing some percentage of the glass fiber with glass beads. The glass beads help reduce shrinake but without increasing the modulus nearly as much. (Potters is one supplier of glass beads)

 

[Pud]

Hi,

Can you not just stick it on with silicon rubber adhesive?

This will give a flexible joint...

Rgds

Harry

 

[niksmith]

Thanks for the input Demon3 and Pud . The layer was initially meant to act as a support since we were handling big sheets which were quite fragile ( 2-3mm thickness and 1m x 0.5m dimensions ) . This helped us during the transporation ( less breakages ) and also in mounting these pieces on cement .
The colour and appearance of the coating is not important in any way .I am ready to try out other material other than polyester but feel that this is the cheaper option compared to epoxy resin .
Demon3 can you please explain about the increase in the modulus in simpler terms ,I fail to follow you on that matter .

 

[Demon3]

Hello,

Sure, I can explain. There is a force at the interface of the resin and the glass. The force is caused by the shrinkage of the resin as it cures. The resin contracts and the glass doesn't and that builds a force that is trying to rip the resin off the glass. Can you picture that? The magnitude of that force depends on the amount the resin shrank by and the modulus of the resin. Imagine a spring, the force in a spring is the modulus (stiffness) of the spring multiplied by how much I compress the spring by. The interface is the same. That means there are three answers to your delamination problem.

1. Make the interfacial bonding so good that the delamination force I described is not enough to rip the resin off the glass (e.g. certain silanes could help)

2. Make the resin shrink less (e.g. fillers or choose a resin that does not shrink on cure like some cyclic monomers that ring open on cure)

3. Make the resin softer (e.g. add less filler, add a soft component or change from polyester to a soft resin like silicone)

 

[niksmith]

Demon 3 ,thank you very much for the explanation .The concept you were conveying is absolutely clear .I am trying out the various suggestions one after the other and will let you know my observations as the results come . You also mention regarding 'cyclic monomers that ring open on cure ' .Does that mean that they do not develop any stress ? Are these cyclic monomers easily available in the market ? Are they more expensive compared to polyester or epoxy resins ? Can you suggest one cyclic monomer which I could try out .

 

[Demon3]

Hello,

Glad to have helped! It's actually even slightly more complicated that what I explained because the actual force at the interface is an integral. It's the modulus at a given time multiplied by the shrinkage, then as cure proceeds the modulus goes up and the shrinkage does too so you have to add that new modulus x shrinkage to the previous one and so on until cure is complete.

Regarding cyclic monomers they are available but not that common as far as I know. I have seen an aliphatic cylclic epoxy (yes I know epoxy groups themselves are cyclic before someone points it out). The cylcic monomers tend to expand on cure whereas most monomers give shrinkage. If you blend them correctly you can get a material that gives neither shrinkage nor expansion. I know some work has been done to make such materials for dental fillings.

I am pretty sure that using a softer adhesive for example will be a better solution for you than exotic adhesives with cyclic monomers.

Good luck on your quest and tell us how you do.

 

[rnd2]

After a bit more time to think about what is going on here is another option.
Regular glass sheet is a rigid material and very prone to cracking if overflexed.
For reasons unknown to us you need one side of the glass untouched but it is OK to clad the other side with just about anything cost effective that will help prevent the glass from over flexing.
The current method is an insitu lay up of a 2mm thick polyester fiberglass laminate to the glass sheet. However, due to high resin shrinkage, the bond creates bending stress causing the glass sheet to bend slightly towards the fiberglass. You can easily verify this. This pre-loading between the laminate of glass and fiberglass is causing an unacceptable number of breakages during subsequent handling.
To overcome the problem, a simple, effective solution would be to pre-cast or even buy in flat sheets of 2mm thick fibreglass and simply bond them directly to the glass sheet.

 

[niksmith]

Hi to all of you ,
I posted two other messaged today and thought of updating you on the progress obtained as as result of the suggestions I got on this thread . By adding filler in the resin,by placing the fibres bidirectionally and by leaving some gaps in between the fibre groups I was able to solve the above said problem of the big sheet of glass bending and cracking.
A big thanks to all of you for all the suggestions you offered .