Originally a post by sonoronos:
Casio G-Shock watches use polyurethane resin. Polyurethane resin is a cross-linked polymer, which depends on the reaction (crosslinking) of isocyanate groups to bond together polyols. The bulk properties of the material depends on the types of crosslinking as well as the type of polyols involved. Strength
The potential lifetime of polyurethane is indefinite, however, like any other crosslinked polymer, it has several common weaknesses. These weaknesses all revolve around depolymerization.
Ultraviolet degradation aka photodegradation:
UV radiation attacks polymer crosslinking through a variety of means, both photochemical and oxidative. Regardless of the methodology, UV destroys the chemical crosslinking that gives PUR its useful physical properties - causing both cosmetic damage (yellowing) and reduction in tensile strength (weakness) and elasticity (brittleness)
PUR is susceptible to hydrolysis, which is basically another attack on its crosslinked structure through the use of acids or bases in conjunction with water. The water will react with the organic structure of the polymer to combine water broken down into hydrogen or hydroxides with existing structures in the material causing those structures (bonds) to break - hence "hydrolysis".
Ozone is present everywhere but can be present in higher amounts in certain situations. This is another chemical attack where ozone reacts in a process called ozonolysis to break polymer bonds.
Ketones and other solvents will have little effect on crosslinked polymers, except for swelling.
Silicone oil and other things will not protect any of the major failure modes of PUR. What will protect PUR against degradation is to keep it out of sunlight and prevent exposure to acidic and basic solutions, especially at elevated temperatures. Also, storing in a vacuum helps.
You also cannot remove the yellow color cast from degraded clear polyurethane.
I'm sure organic chemists can fill in the details.