Case 3

Good Practices

Joint

Design [7-8]

Proper joint design and sealant selection are the fundamentals for good sealant installation. In order to determine a suitable sealant for the joint, the following information is required:

a. Joint width and movement

To find out what the sealant’s plus-minus movement ability as determined by ASTM C719, the following equation can be used:

Joint size = 100 x Expected joint movement + tolerance (ASTM)

Joint width = ( total expected movement x 100) / Movement accommodation factor (BS)

Since there is an inversely proportional relationship between the joint size and the sealant movement ability, the smaller the joint size, the higher the movement ability of the sealant and vice versa.

b. Joint depth

For most cases where sealants are in butt movement joints, 5mm is the most practical minimum sealant depth as given by the British Standards, however the most ideal sealant depth would be half of the total joint width. (Figure 1).

For cases when the joint depth exceeds the ideal dimension, things like backer-rod can be used to provide a backing for the sealant to rest on.

However the use of a backer-rod can modify the sealant profile from a rectangle or square to a circular one.

For such cases, the sealant depth should be considered to be at the center of the curved face where the sealant depth is closest to the backer-rod.

Material Selection

Each sealant type depending on the formulation has its own relative characteristics and merits or limitations.
Below provides information on the various types of sealants applicable to curtain walling system [9].

Movement capability

Sealant must have a cyclic movement capability of ± 12% to ±25% through its useful service life (Refer ASTM C719 or equivalent).

Weather resistance

Sealants should not exceed a Shore A hardness of 45 after heat ageing for 6 weeks at 70°C. (Refer ASTM C719 or equivalent).

Sealant compatibility [9]

The following test should be carried out in advance of installation to evaluate:

• adequacy of adhesion of sealant to substrates and other sealant types (ASTM C794)
• possibility of sealant causing staining to certain substrates (ASTM C1248)
• compatibility of sealant with gaskets, spacers setting blocks etc. (ASTM C1087)

Applications

Sealant Application

Other than the sealant, there are several factors and auxiliary materials to note during sealant application to ensure proper installation.

a. Cleaners 

• Cleaning of joint surface before priming and sealant installation is important to the success of sealants.
• Cleaners are solvents that enable surface contamination to be removed from non-porous surfaces. Due to the chemical nature of the cleaners, it is reactive to plastics and therefore care should be taken during the cleaning of plastics substrate.

b. Primers

• Primers are reactive chemicals that are dispersed in solvent.
• Applied to substrate to ensure good adhesion between the sealant and the surface by changing the nature of the area of contact.
• Not always recommended but for cases when they are, it is essential that they be used.
• Proper storage of primers is important to ensure that the primers are in good condition.
• Due to the reactive nature of primers, handling and safety procedures are important and should always be observed.

c. Bond breakers

• Bond breakers are used to prevent the sealant from adhering to the back of the joint cavity and to ensure that the sealants in the movement joints are free to stretch and relax.
• They come in either the form of a tape or a rod. Tape is used when the depth is too shallow to accommodate a rod whereas the rod is used for deeper joints where the depth of sealant requires controlling. They are normally made out of polyethylene, which is will not reactive with sealants. The bond breaker rods can also act as back-up materials.
• The difference between bond breakers and back-up materials is that the primary function of back-up materials is only to limit the cavity depth and to ensure that the correct width: depth ratio is maintained.