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How do UV light curable adhesives (such as Loctite's and Permabond's UV glues) work? What gives them the ability to join two non-porous surfaces such as glass-to-glass, glass-to-metal?

Also, do products marketed as 'liquid plastic' (such as Bondic and RapidFix UV) work/adhere the same way?

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UV-curables belong to a class of adhesives which "cure" or change from liquid to solid by a chemical reaction called cross-linking. In UV-curables, the reaction is photochemical: the liquid chemicals which form the adhesive receive the activation energy needed to trigger the crosslinking reaction from photons of light. By artful design of the chemistry, the photon energy needed to promote the reaction is moved into the UV range, where ordinary visible light will not activate the glue.

These glues generally contain a mix of liquid polymer resins which crosslink with each other when exposed to UV. They are usually blended with another component called a photochemical promoter; when struck by UV light, the promoter degrades into free radicals which then boost the reaction in the resins to speed up the cure and increase the strength of the cured glue.

Other chemistries are possible, and the photochemical component is often a cocktail of promoters and other ancillary chemicals which increase the sensitivity of the promoter to UV light or modify the reaction chemistry in specific ways to get the best cure.

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  • $\begingroup$ Thanks! How does the polymer adhere to glass? What is special about it that it works when adhering non-porous surfaces such as glass and metal together while other types of adhesives fail? With most glues, you need to scuff the surface for them to work. For Loctite and Permabond's UV glues, you do not need a rough surface for them to work. Why? $\endgroup$
    – Jet Blue
    Mar 30 '18 at 18:34
  • $\begingroup$ @jetblue, with the right composition of the glue resins, it is possible to make the glue so thoroughly "wet" the surfaces that it creates (somewhat weak) chemical bonds with them and adheres without roughening. Other glues rely on mechanical cohesion, where the roughened surfaces get interlocked with each other by the hardened resin that fills the spaces between them. In this case, the chemical reaction with the surface is not the major component of the glue line's strength. Still other glues (for plastics) contain softening agents that... $\endgroup$ Mar 30 '18 at 19:13
  • $\begingroup$ @jetblue, ...partially depolymerize the plastic surfaces. the glue then mixes with the softened plastic, and when it cures, the resulting joint is almost as strong as the plastic was before it broke. $\endgroup$ Mar 30 '18 at 19:14

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