Sealants

I was told to always say sealant, not caulk.  Here is a little research about what products are currently available.  Caulk originated in boat making; Sealant was first used in building construction.  Both products serve the same purpose: to keep water and air from getting within a joint.  In construction, some manufacturers today use caulk as an all-purpose term and sealant to refer to high-performance products.  In commercial construction, it's always sealant.  Here is an attempt to identify the current options for application of architectural sealants.

Sealant Failure - To better understand the selection process for sealants, first understand the common reasons for sealant failure that need to be prevented.  The following failures usually happen for one of two reasons; either the substrate was not properly prepared or the wrong product was selected.

• Adhesive: bond between caulk and substrate fails
• Cohesive: the caulk itself tears
• Substrate: substrate breaks

Joint geometry -  Higher performance sealants require a backer rod to create a width to depth ratio of 2 to 1.

Factors for sealant selection:

• What material is the sealant adhering to?
• How much it is likely to move?
• Will mold and mildew be present?
• Will it need to resist weathering?
• Will temperature and humidity affect the installation?
• Should it match a specific color or texture?
• Will it need to resist abrasion?

Types

• Water Based
  • Latex products including acrylic and vinyl
  • Apply and let cure in warm (40°F or more) and dry conditions
  • Good choice for most interior applications
  • Easy workability: smooth, thin consistency
  • Paintable
  • Adhere to most building materials
  • Silicones and Polyurethanes have superior performance, but some newer water based products provided added performance.
  • Spray foam - water based sealants are also available.  Not for moisture resistance.

• Polyurethanes
  • Difficult to apply in cold weather
  • Stringy
  • Toxic
  • The only sealant that can stand up to traffic and abrasion.
  • Expanding polyurethane foams are also available.

• Silicones
  • Best choice for metal, glass, tile and cold weather
  • Adheres to non-porous substrate best
  • [typically] cannot be painted
  • It tears.  If sealant if damaged, cannot repair the existing installation.
  • Adheres poorly to wood
  • Handles extreme weather well.
  • Remains flexible for joint movement
  • Stinks, but not toxic
  • Unaffected by mold and mildew
  • Resists UV degradation

• Synthetic Rubber
  • Most flexibe and clearest curing
  • Ideal for exterior joints likely to expand and contract
  • Solvent based can be applied in wet and cold weather
  • Resist mold and mildew
  • (Low modulus) Stretch and recover easily without breaking.  Since they stretch easily, there is less likelihood that the bond with substrate will break.
  • Best product for weathering exterior wood
  • Paintable with water based paint
  • Flammable until cured
  • Do not meet VOC (volatile organic compound) regulations and should not be used indoors

• Butyls
  • Very messy
  • Stretch but do not recover
  • Most water resistant product available
  • Unattractive tar-like appearance
  • Used in gutters, roof flashing & anywhere needed below grade
  • Never completely hardens. can withstand movement of a shear joint

• Unclassified Hybrids
  • Modified-silicone polymers
  • Combine chemistry and benefits of water-base, silicone, and polyurethane
  • Expensive, but high-performance
  • Like latex and acrylic, they are easy to apply
  • Like polyurethane, incredibly durable and adhere to most any substrate
  • Like silicone, can be applied in extreme temperatures and withstand water immediately
  • Flexible and paintable

References:
Fine Homebuilding
Exterior Building Maintenance of New York City School Buildings
Construction sealants and adhesives on Google Books