In hardening of adhesive, why do cracks form?

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In summary, the cracking and formation of bubbles in epoxy when exposed to UV light is caused by a combination of shrinkage due to thermal and cure processes. This can also be influenced by the presence of volatiles and UV cure agents. Acrylics, which have a high cure shrinkage, may fail in dental adhesives due to this factor.
  • #1
unscientific
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Specifically, shining UV-light onto epoxy hardens it, but sometimes cracks/holes/voids form, why? I want to know the physics behind this!
 
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  • #2
Probably mostly a matter of shrinkage.
 
  • #3
I don't know a lot about adhesives, but I'm wondering if evaporation might contribute since many of them contain volatiles.
 
  • #4
Yes - shrinkage due to evaporation.
 
  • #5
Sorry, Russ. I assumed (yeah, I know better than to do that) that you meant shrinkage due to chemical bonding reducing the overall volume.
 
  • #6
Shrinkage of epoxies during cure is almost always due to a combination of thermal and cure shrinkage. In light-cured systems, the resin can absorb a significant amount of thermal energy while it is liquid. The liquid resin will expand upon heating and begin to set in this thermally-expanded state. When the lamp is removed, the temperature returns to RT and the resin begins to shrink. If the polymerization is not building sufficient strength during this post initiation cure phase, cracks will result. If the polymer is of sufficent strength to withstand the thermal stresses, those stresses will be 'frozen' into the final fully cured product. Post cure cracking in this case will be a slow process but it can still occur.

The bubbles and other holidays in the final cured product can arise from a variety of causes. If the substrate is porous and contains volatiles, these can manifest during cure or post cure. Solvents from the uv cure package that one mixes into the resin can be a source of bubble-type holidays as well. If the UV cure agent is present in too high a concentration, it can shield underlying resin in thicker areas from the UV and those areas might be poorly cured or not cured at all just under the surface.

Cure shrinkage itself is due to the crosslinked polymer having a smaller molar volume than the uncured resin, as Danger has indicated. Acrylics tend to have the highest cure shrinkage of the polymers I am familiar with. I believe that acrylic acid itself has a cure shrinkage of approximately 20%!
 
  • #7
chemisttree said:
Cure shrinkage itself is due to the crosslinked polymer having a smaller molar volume than the uncured resin, as Danger has indicated.
That part is surprising to me - I didn't realize that that could be a significant factor in anything but gases.

Thanks for the info.
 
  • #8
Actually, I should have said that the molar volume of the repeating unit in the polymer is smaller than the molar volume of the monomer (or repeating unit) in the uncured resin. One of the reasons that acrylics fail as dental adhesives arises from their large cure shrinkage.
 

1. Why does hardening of adhesive result in the formation of cracks?

The hardening process of adhesives involves the evaporation of solvents or water, causing the adhesive to shrink. This shrinkage creates internal stresses within the adhesive, leading to the formation of cracks.

2. Can the use of certain adhesives prevent the formation of cracks during hardening?

Yes, certain adhesives that have a lower shrinkage rate or those that cure at a slower pace can reduce the likelihood of crack formation during hardening.

3. Are there any external factors that can contribute to the formation of cracks during hardening?

Yes, factors such as high temperature, humidity, or uneven application of the adhesive can increase the chances of cracks forming during hardening.

4. How do cracks in hardened adhesive affect the overall strength of the bond?

Cracks in hardened adhesive can significantly weaken the bond strength since they act as stress concentrators and reduce the contact area between the two surfaces being bonded.

5. Is there a way to repair or prevent cracks from forming in hardened adhesive?

To repair cracks, it is best to remove the hardened adhesive and reapply a fresh layer. To prevent cracks, one can use adhesives with lower shrinkage rates, control external factors, and ensure proper adhesive application techniques.

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