Experiment

It has been well noted that gradual changes in silicone structural sealant properties are affected by exposure to weather and environmental conditions as well as by the composition of sealant materials. Although silicone structural sealants are durable and not significantly affected by long term degradation under harsh conditions, under certain conditions gradual changes can be observed in some properties [7].

As mentioned previously, the principal driving force for the evaluation is to gain support for a Korean national guide for structural silicone sealant and structural glazing. So, evaluating actual performances of locally available struc­tural silicone sealant is critically important to reflect realistic values for the specification. The authors gathered various sealants from as many manufac­turers of structural silicone sealants used in the local Korean curtain wall mar­ket as possible.

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Sealant purpose

One-part

structural

Two-part

structural

Two-part IG secondary

One-part IG secondary

Company A

1

2

3

Not available

Company B

1

2

3

4

Company C

1-a

2-a

3-a

Not available

1-b

2-b

3-b

3-c

TABLE 1—Products evaluated (for nomenclature refer to text).

Table 1 illustrates the nomenclature for the products used in the evaluation. All products are silicones based on alkoxy cure technology using either one-part or two-part packaging. There are three major companies that supply structural silicone for curtain wall fabrication in Korea (labeled A to C).

In Table 1, “1" stands for one-part structural glazing sealant. “2" for two – part structural silicone sealant. “3" for two-part silicone insulating glass second­ary sealant, and “4" for one-part silicone insulating glass secondary sealant (see Fig. 1 for application details). The indices “a", “b," and “c" indicate different product names for the same application.

Our study made reference to the following industry standard for the prepa­ration and evaluation of structural silicone test specimens: ASTM C1135 [9] and ISO/FDIS 28278-1 [3]. Of particular note, the intended guide for a future Korea national standard, ISO/FDIS 28278-1 has “Annex B" which describes the requirements for use of structural glazing sealants or insulating glass sealants with exposed applications. Therefore, the testing specimens were prepared according to ASTM C1135 and subjected to weathering tests following the requirements in ISO/FDIS 28278-1. Because the pulling speed of tensile testing

Silicone

. Spacer Cutback;

IG Sealant Bite

Compatible Spacer

Weatherseal

Horizonta Mu lion

FIG. 1—Section detail of structural silicone glazing [8].

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for ISO/FDIS 28278-1 is 5 mm/min compared to 50 mm/min in ASTM C1135, initial properties in tensile testing were evaluated according to both ISO/FDIS 28278-1 and ASTM C1135 in order to detect any difference resulting from the pulling speed. Furthermore, ISO/FDIS 28278-1 suggests that ASTM C1135 can be used as an alternative test method for tensile testing.

Depending on the test method, the dimensions of the test specimen may vary, but the dimensions of the sealant geometry and bead size shall always be 12 mm x 12 mm x 50 mm for both ISO 28278 and ASTM C1135 standards. For this particular evaluation, test specimens were prepared according to ASTM C1135 using anodized aluminum as one support substrate for one side of the test specimen and float glass for the other support substrate (see Fig. 2). The purpose was that the adhesion performance could be evaluated by different weathering conditions simultaneously on two of the most common substrates used (aluminum and glass) along with the mechanical properties of structural silicone sealant.

Before commencing the weathering evaluation, the intrinsic properties of each silicone structural sealant were evaluated. Tensile properties were deter­mined along with the average values based on five test specimens for several temperature conditions (—20, 23, and 80°C) as well as tear strength. Test speci­mens were conditioned for 4 h at 80 and —20°C. Test specimens were then tested at the same temperature in a conditioned tensile testing machine. Additionally, cohesion/adhesion performances after exposure to artificial light through glass and to water, salt spray, and SO2 atmosphere were determined. For accelerated weathering, xenon-arc weathering was employed to simulate actinic radiation

FIG. 2—Tensile adhesion joint used in the evaluation.

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[10]. In future work, it is planned to evaluate and compare the effects of both xenon-arc weathering and fluorescent UV light weathering [11].