Structural Sealants

Property

Terminology

The term “sealant” refers to wet seal in construction industry, includes weather sealant and load bearing/ structural sealant. This two terms if frequently used in façade construction.

Classification

1. Caulking: installed as filler between dissimilar materials in an interior controlled environment no subject to thermal or structural movement.
2. Sealant: exterior application, using high performance material (e.g. silicones), which are typically coloured.
3. Glazing material: used in construction of window panels or curtain walls, where higher strength is more important than movement capability.

• Water based sealant is those used in the interior application where not in contact with water, e.g. latex sealant.
• Solvent based/ oil based sealant is used on the external façade as it is able to resist weathering effect (rain and UV). Because of its load bearing capability, it is considered structural sealant; e.g. Butyl, Acrylic, Silicone, Polyurethane and Polysulphide.

Grade

According to ASTM C 920

• Grade P (pourable or selflevelling) sealant shall have flow characteristics such that when tested in accordance with test method C 639 it shall exhibit a smooth, level surface.
• Grade NS (non-sag) or gunnable sealant shall have flow characteristics such that when tested in accordance with test method C 639 it does not sag more than 4.8mm in vertical displacement. Also the sealant shall show no deformation in horizontal displacement.

Constituents

• Polymer
• Filler: part of sealant that give the ‘body’
• Cross linker: to link the polymer
• Adhesion promoter
• Plasticizer (solution / oil part of sealant)
• Pigment

Base Material

Acrylic

Factory mixed, one component materials polymerised from acrylic acid. These are not used on joints subject to high movement because of their relatively low movement capability. They commonly used in remedial applications with acrylic-based waterproof coatings and small movement joints such as windows and door perimeters.

Butyl

Produced by copolymerisation of isobutylene and isoprene rubbers. Butyl sealants are used in metal curtain wall construction because of their ability to function in very thin application. Although they have low-movement capability, they have excellent adhesion to most substrate and have excellent weathering resistance.

Latex

Latex sealants are typically acrylic emulsion or polyvinyl derivatives. It seldom used for exterior application since it is sensitive to water immersion and traffic. Typically used for interior purposes when a fast cured is desired for painting.

Polysulphide

Polysulphide materials are produced from synthetic polymers of polysulphides rubbers. It exceeds the movement capability of paints and therefore should not be painted. However, they are manufactured in both one and two component packaging in wide range of colour. It is considered as one of good performance sealants and thus suitable for wide range of applications, includes curtain wall joints, cast insitu and precast panels. Of all commercially available sealants, polysulphides are best suited for total-immersion joints, such as swimming pool, water and waste water treatment structures, fountains, and ponds.

Polysuphides should not be installed in joints that might have bituminous residue or contamination such as premold joint filler. It should also not be applied over oil-or solvent based joint sealants.

Polyurethane

Urethane sealants are polymers produced by chemical reactions formed by mixing di-isocynate with a hydroxyl. It ranges from one-component, self-levelling materials in a pourable grade for horizontal joints in plaza decks to two-component non-sagging materials used for vertical expansion joints. Urethane is not recommended for continual immersion application.

Two component urethanes are low-modulus sealants and have high joint movement capability averaging 25%. Polyurethane cannot be used in joints containing a polysulphide sealant or residue. Urethane sealant should also not be used in glazing applications of high performance glass, plastics, or acrylics. Joints contaminated with asphalts, tar or for release agents must be cleaned before sealing work.

Silicones

Silicones based sealants are derivatives of silicones polymers produced by combining silicone, oxygen, and organic materials. As it has high thermal stability, silicone is the most common sealant used in Singapore. Silicones are available in a wide range of compositions that are extremely effective in high-movement joints, including precast panels and expansion joints.

Silicone sealant cannot be used for below grade applications, horizontal applications subject to vehicular traffic, and water immersion joints. It is extremely important not to installed silicone materials that might bleed through a silicone. This includes oil, solvents, or plasticizer, which will cause staining and possible silicone failure.

Silicone materials exceed the movement capability of paints, and as most paints will not adhere to silicones, they should not be painted over. It has excellent adhesion to almost all building products including wood, ceramic, aluminium, and natural stones. It may be used in curtain wall joints (typically the high tensile-strength silicones with lower movement capability), precast concrete panel, expansion and control joints.

Precompressed foam sealant

Foam sealants are manufactured by impregnating open-cell polyurethane foam with chemical sealant containing neoprene rubbers. An adhesive is applied to one side and covered with a release paper. The foam is then compressed and supplied in rolls with various widths of up to 12 inches. During application, it swells and expands to fit tightly against both joint sides, allowing for any irregularities in joint width. Splices in material are prepared by overlapping or butting joint ends. This material eliminates the need for joint backing, primers, and tooling.

Foam sealant is compatible with other sealant materials and allows elastomeric sealant to be applied over the foam, providing good barrier in critical water proofing joints.

*The process of weathering is defined as the action of atmospheric elements in altering the colour, texture, composition or form of exposed objects, ultimately leading to disintegration or failure to perform a function. The well-known elements of weather are radiation, moisture, thermal conditions and gases.

Particularly for sealant material, the following weathering factors will affect the performance:

1. UV
2. Ozone + UV
3. Acid rain + Ozone + UV
4. Heat (especially for west and south orientation)
5. Building movement = thermal expansion & contraction + building settlement movement + building flexural movement + thermal shock movement (drop of temperature during a rain storm resulting in extreme short time movement in joint areas
6. Miscellaneous chemicals in the air

Adhesion Strength

BS 5889:1989:

The ability of sealant to bond to a particular substrate, including adhesion during substrate movement, since materials differ substantially in their adhesive strength to a particular substrate, manufacturer should be consulted for adhesion test samples on proposed substrate.

The “Adhesion in peel” test according to BS 5889:1989, the material shall not fail in adhesion over more than 25% of the test area. ASTM C 794 tests the adequacy of adhesion of sealant to substrates and other sealant types.

Cohesion Strength

BS 5889:1989:

The ability of a material’s molecular structure to stay together internally during movement. Cohesive strength has a direct bearing on elongation ability. Cohesive failure is when a sealant tears or splits apart due to excessive joint movement or improper installation.

According to BS 5889:1989, adhesion and cohesion shall meet:

• Initial test

• Cyclic adhesion test
• Adhesion and cohesion after exposure to UV radiation through glass

Elasticity

According to BS 5889:1989 the types of sealant according to the elastic recovery:

Shore Hardness

ASTM D 2240:

Shore hardness is resistance to impact by a durometer gauge. This property becomes important in selecting sealant subject to traffic or punctures, such as horizontal paver joints.

* Shore A Hardness

** Too rigid to be used in curtain wall or cladding system

According to ASTM C 920, the classification of hardness is as follows:

• Use T (traffic); shall have a hardness reading after being properly cured, of not less than 25 or more than 50 when tested in accordance with test method C 661.
• Use NT (non traffic); shall have a hardness reading, after being properly cured, of not less than 15 or more than 50 when tested in accordance with test method C 661.

Service Temperature Limit

According to ASTM C 920 on effect of heat aging; the sealant shall not lose more than 7% of its original weight or show any cracking or chalking when tested in accordance with test method C 1246.

Movement Capability

Sealant must have a cyclic movement capability of ±12% to ±25% through its useful service life. Refer to test methods specified in ASTM C719 or equivalent national standard.

Modulus

ASTM D412:

High, medium, and low modulus.

Modulus at 100% elongation: ± 0.4 MPa.

Tensile Strength

ASTM D412:

Tensile strength: 1.4-2 MPa

Substrate

Concrete and masonry

Newly placed concrete or masonry must be allowed to cure before applying sealant. If wet, it shall allow to dry at least 24 hours in good drying weather before application.

Stone

These surfaces generally provide good sealant adhesion. However, some stone should be primed before sealant application. If the surface area of stones appears to be flaking or dusty, it must be cleaned by either water blasting, sandblasting, or wire brushing before application.

Glass

These excellent surfaces for sealants once their surfaces are cleaned of contaminants and oils. Methyl ethyl ketone or alcohol is an ideal cleaner.

Painted and lacquered surfaces

Depending on where these surfaces are located and their exposure to weather, sealant should not be applied to flaking painted or lacquered surfaces. Sound painted should be first cleaned by wiping with a solvent to remove oil and dust.

Aluminium with a mill finish

A good degreasing solvent, such as trichloroethane or xylene, will clean these surfaces properly. A rub down with fine steel wool or fine emery cloth might permit better adhesion.

Aluminium with an anodised finish

Generally provides excellent surface for sealant application. However, it should be wiped down with methyl ethyl ketone or xylene to remove any surface contaminants.

Galvanized Steel

New galvanized surfaces present more difficult surfaces for adhesion than weathered galvanized surfaces. Consultation with sealant manufacturer is recommended.

Stainless Steel

This is difficult surface for adhesion purposes. Primers are often recommended along with solvent cleaning of the surface.

Staining

Possibility of sealant causing staining to certain substrates is tested in accordance with ASTM C1248 or D2203-01

Compatibility With Other Material (for façade)

Compatibility of sealant with other materials such as gasket, spacer, setting block, etc is tested in accordance with ASTM C1087. Adequacy of adhesion of sealant to substrates and other sealant types is tested in accordance with ASTM C794.