The primary function of an architectural sealant is to prevent the elements, wind, rain, heat, cold, etc…, from penetrating the skin of a building. The sealant must be capable of maintaining an impenetrable seal while at the same time allowing for thermally induced movement between dissimilar and similar substrates. The sealant must also be resistant to degradation caused by exposure to ultraviolet radiation, water, and heat. A secondary function would be to create an ascetically pleasing line at building substrate interfaces.

Technically speaking the term caulk is actually a verb that describes the process of sealing up a construction joint. Within the building trades the term is used loosely referring to almost any type of sealant material. The term is also used when referring to early “calks” such as oil based, latex, and butyl “calks”. The term sealant refers to an elastomeric material used to fill and seal an expansion joint. Sealant materials prevent the passage of water and allow for horizontal and lateral movement at the expansion joint.

The term elastomeric refers to the ability of a material to elongate when a stress is applied and to return to original shape when the stress is removed. An elastomeric material is commonly referred to as a material that has memory.

Whenever we in the sealant industry refer to adhesion the terms adhesive failure and cohesive failure will invariably come up. Cohesive failure refers to failure within the sealant itself, that is the sealant fails leaving behind sealant on the substrate. Adhesive failure refers to failure illustrated by the adhesive pulling away clean from the substrate In the architectural sealant industry a sealant is said to have acceptable adhesion if it exhibits cohesive failure when tested according to manufactures specifications. Cohesive failure is also required when conducting field adhesion tests.

Primers can be composed of any number of chemical compounds but in the end they can be thought of as a chemical bridge between an adhesive or sealant and the bonding substrate. This chemical bridge can be likened to a weld between two sections of metal. Most primers are specially designed molecules carried in a solvent vehicle. Once applied the solvent must evaporate and in some cases the active ingredients must undergo a chemical reaction. The time necessary for these processes to take place differs from one primer to another. It is important to consult the technical data sheet for proper drying times before applying sealant to primed surfaces to ensure a long lasting quality seal.

The ideal answer to this question is that primer should be used whenever cohesive failure is not achieved in the absence of primer. Of course this is assuming that sealant applicators conduct field adhesion testing. Or that all construction projects take advantage of adhesion testing services offered by most sealant manufactures. For specific primer recommendations, contact the Pecora Technical Services Group at 1-800-523-6688.

Generally speaking a sealant will increase in viscosity, that is, get thicker as the temperature decreases. When temperatures dip below 40°F the workability and application of a sealant can become difficult to impossible. Once the sealant is in the joint and temperatures are low the curing of the sealant can be substantially prolonged. When applying sealant at low temperatures, provisions should be made for the extra curing time necessary to form a fully cured elastomeric seal. At warmer temperatures, the opposite is true, that is, a sealant will drop in viscosity or get thinner at higher temperatures. Most sealants will also cure faster at higher temperatures. The optimum temperature range for sealant application is 50°F to 80°F.

The obvious difference is that a two-component sealant is composed of two parts, a base component and an activator component. The activator component is typically added to the base component and mixed for a prescribed period of time before it is ready for application. Two component sealants are typically packaged in buckets while a one-component sealant is typically packaged in a cartridge. Some of the not so obvious characteristics of one and two component sealants are listed below:

One Component	Two Component
No mixing required	Mixing required
No special equipment required	Bulk guns and mixing equipment required
Require exposure to atmospheric moisture to cure	Cure without exposure to atmosphere
Color added by manufacturer	Color added at job site
Limited shelf life	Extended shelf life
Cures from exposed surface down	Cures uniformly
Generally slower cure speed than two component	Generally faster cure speed than one component
Requires open cell backer rod	Can use open cell or closed cell backer rod