Skip to main content

Historic Preservation - Technical Procedures

Spectitle:

Sealants: Characteristics, Uses And Problems

Procedure code:

0790002S

Source:

20Th Century Building Materials (Ed. Tom Jester, Nps)

Division:

Thermal And Moisture Protection

Section:

Joint Sealers

Last Modified:

02/24/2012

Details:

Sealants: Characteristics, Uses And Problems



SEALANTS: CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of sealants and identifies typical problems
associated with this material along with common causes of its
deterioration.  For guidance on replacing deteriorated sealant, see
07900-03-R.


INTRODUCTION

Characteristics of sealants:

-    Typically made of synthetic elastomeric materials.

-    May be single-component (no mixing required), or
    multicomponent (mixing required).

-    Good adhesion

-    Good cohesion

-    Good elasticity

-    Good weathering characteristics

-    Common types of sealants include silicone, polyurethane,
    polysulfide, acrylic, latex and butyl-based.


TYPICAL USES

Typical historical and current uses for sealants include:

-    Prior to 1950, oil- and resin-based caulks were the most
    common building joint sealants; typically used in bearing
    masonry construction; these were not considered elastomeric
    sealants due to their limited movability.

-    More elastic sealants were used after 1950 due to the
    popularity of curtain wall construction, which tends to move
    more than masonry construction.

-    Polysulfide sealants: First widely used elastomeric sealant.

-    Silicone sealants: First developed in the 1950s as two-
    component products; first silicone building sealant (one
    component) was developed by Dow Corning about 1960; typically
    used for nonporous surfaces with a high factor of movement;
    common in metal and glass cladding systems.

-    Silicone sealants were also used for structural joints in
    storefronts in place of mullions; this was common in the
    1960s.

-    Butyl sealants: available in construction in the mid-1950s.

-    Acrylic sealants: Available in the 1920s; first acrylic
    sealant for buildings developed by Tremco Manufacturing
    Company in 1958; typically used in small-scale construction
    under conditions requiring limited movement.

-    Urethane sealants: Typically used in joints requiring abrasion
    resistance; first ones were multicomponent types; typically
    used for porous surfaces with a high factor of movement such
    as cladding joints.

-    Latex sealants: Available in the 1960s; typically used in
    light building construction and residential construction under
    conditions requiring limited movement.

-    Isobutyl-based sealants: Typically used for glazing joints.

-    Sealants are commonly used in joints between individual stone
    or metal panels, between stone panels and flashing, at
    expansion and coping joints in masonry, around window and door
    openings, and in joints at horizontal surfaces.

-    Butt-joints are the most commonly used, but other types
    include fillet joints, lap joints, glazing beads, and glazing
    heel beads.


NATURAL OR INHERENT PROBLEMS

-    Staining: Common with silicone sealants; visible as dirt on
    the sealant, or staining on the adjacent masonry where the
    sealant's plasticizer has migrated into the substrate.


VANDALISM OR HUMAN-INDUCED PROBLEMS

-    Weathering: Deterioration of sealants can be caused by
    prolonged exposure to water, ultraviolet light, and freeze-
    thaw cycles; evidence of weathering appears in the form of
    chalking, discoloration, cracking, or softening.

-    Loss of Adhesion: When the sealant separates from the
    substrate; this may be caused by the presence of coatings or
    contaminants that prevent proper adhesion; adhesion is also a
    problem - especially with silicone sealants - if exposed to
    prolonged periods of wetting; poor adhesion may also result
    from poor surface preparation, incompatibility of substrate
    with selected sealant, or incompatibility of old sealant with
    new sealant - UNDERSTANDING OF THE SUBSTRATE
   AND SEALANT PROPERTIES IS ESSENTIAL.

-    cohesive failure: this means deterioration of the internal
    integrity of the sealant; cracking parallel to the interface
    of the joint is an indication of this type of failure

-    Inappropriate Choice of Sealant and Improper Joint Design:
    Correct installation of the bond-breaker tape or compressible
    foam backer rod is important in preventing the sealant from
    adhering to the sides of the joint; temperature is also
    important when installing sealant - it shouldn't be too hot or
    too cool, otherwise the width of the joint will not permit the
    installed amount of sealant to accommodate expansion and
    contraction.

-    Uncured Sealant: Most common in multicomponent urethane
    sealants; sealant that is uncured is often due to incomplete
    or improper mixing of the sealant components, or from using
    materials that have outlived their shelf life.

-    Bubbling and Blistering of the Surface: Common in single-
    component and multi-component urethane sealants; in single-
    component types, this can caused by curing at high
    temperatures or high humidity; in multi-component types, this
    can be caused by curing at high temperatures, which affects
    both the cure and durability of the sealant.

                         END OF SECTION