Because water is a reactant, not merely a contaminant. The isocyanate component reacts with water in preference to much else, and that reaction produces carbon dioxide inside the curing mass.

The result is foaming: bubbles through the casting, a spongy texture, poor mechanical properties and lost insulation performance. Small amounts of moisture produce scattered voids rather than obvious foam, which is harder to spot and just as damaging.

Moisture arrives from humid air, damp components, hygroscopic fillers and even a wet mixing vessel. The defences are dry storage with containers sealed under nitrogen or kept tightly closed, dry parts, and degassing or vacuum mixing where the application justifies it.

Freedom from stress on the components. A cured elastomer deforms rather than transmitting load, so as temperatures change and materials expand at different rates, the resin absorbs the difference instead of pushing it into solder joints and fine wires.

It also damps vibration, which is why the chemistry is common in automotive, rail and mobile plant electronics where units live on something that shakes.

The corresponding limitation is mechanical support. A soft compound does not hold heavy components against shock loading the way a rigid one does, so assemblies with mass hanging on slender leads may still need mechanical fixing regardless of the potting.

By what the assembly needs to survive. Polyurethanes span an unusually wide range, from very soft gels that can be pushed aside with a finger, through rubbery elastomers, to grades approaching rigid plastic.

Soft gels suit delicate assemblies and anything that must remain accessible, since they protect against moisture and vibration while allowing a probe or a component to be worked free.

Mid-range elastomers suit most sealed units that see movement or thermal cycling. Firmer grades are chosen when handling robustness matters more than stress relief. Because hardness is quoted on both Shore A and Shore D scales depending on range, comparing figures means checking which scale is used.

Considerably better than most rigid alternatives, which is a large part of why it is specified for outdoor and transport applications. Formulations retain useful elasticity well below freezing, so an encapsulated unit that flexes on a warm day still flexes on a cold night.

A rigid encapsulant becomes stiffer and more brittle as it cools while continuing to contract, which is precisely when stress on components peaks.

Every polymer has a temperature at which it goes glassy, and grades differ widely in where that sits, so the figure to check is the low-temperature limit of the specific product rather than an assumption about the chemistry as a whole.

Hydrolysis, particularly the combination of heat and humidity together. Polyester-based polyurethanes are vulnerable to this: the polymer backbone is attacked by water over time at elevated temperature, and the material gradually softens, becomes sticky and loses strength.

Polyether-based grades resist hydrolysis far better and are the usual choice for warm, damp service.

Straightforward thermal limits apply as well, and continuous service ratings are generally lower than epoxy and much lower than silicone. Where an assembly runs hot continuously, this chemistry is often the wrong one regardless of how attractive its flexibility is at room temperature.

Often yes, and it is a genuine advantage over harder chemistries. Softer grades can be cut with a blade and peeled or dug back to expose a component, and because the material is elastic it tends to tear along a cut rather than shattering into the assembly.

Gels are the most forgiving, sometimes allowing a component to be lifted out and the material closed back over the repair.

Firmer grades are progressively harder work and adhere more strongly to the substrate. Success also depends on how the unit was designed: leaving access room around serviceable parts at the design stage is what makes later repair practical rather than theoretical.

Respiratory protection is the significant one, because the isocyanate component is a recognised respiratory sensitiser. Once someone is sensitised, subsequent exposure at very low levels can trigger a reaction, so the aim is to avoid exposure entirely rather than to keep it within a comfortable margin.

That means adequate extraction over mixing and dispensing, gloves and eye protection, and particular care with any process that produces aerosol or vapour, including spray dispensing and heated material.

Waste and part-used containers need controlled disposal, and spills are dealt with promptly since the material continues to react with atmospheric moisture wherever it lands.