Discussion

According to our findings, in the accelerated weathering test, cohesion/adhesion properties after exposure to artificial light through glass and to water there is no

TABLE 4—Salt spray evaluation results.

Max. point stress, MPa

Young’s modulus, MPa

Break point elongation, mm

Elongation,

%

Failure

type

A-1

1.096

0.89

30.8

256.8

Cohesive

A-2

0.879

2.361

18.2

151.4

Cohesive

A-3

0.828

1.794

24.4

203.2

Cohesive

B-4

0.486

2.514

2

16.8

Adhesive

B-1

0.724

1.527

14.3

119.3

Cohesive

B-2

0.839

1.998

10.2

85.1

Cohesive

B-3

0.263

2.268

0.7

5.8

Adhesive

C-3-a

1.131

2.255

13.6

113.3

Cohesive

C-1-a

0.526

1.039

16.3

135.8

Cohesive

C-3-b

0.817

2.637

9.4

78.2

Cohesive

C-3-c

1.074

1.499

31.1

258.9

Cohesive

C-2-a

0.948

1.669

23.8

198.3

Cohesive

C-2-b

0.969

1.707

27.7

230.6

Cohesive

C-1-b

1.106

1.371

17.6

147

Cohesive

consistent pattern for the changes in tensile strength as a function of duration of exposure. For example, for some sealants noticeable changes occur already after 1000 to 3400 h of UV exposure, while other show little change over the whole 5000 h period. Furthermore, some sealants showed an increase in tensile strength after exposure, while others had almost no change with exposure.

When one-part structural silicone sealants are compared to each other, the sealant B-4 shows the largest tensile strength change after 1000 h of weathering. In the case of C-1-a, it shows consistently lower tensile strength from initial to 5000 h of weathering. Although C-1-a and C-1-b had different tensile strength values for every measurement point, they had a similar trend for the changes over the total weathering period.

As expected, the effects of combined weathering (water and xenon-arc expo­sure) are more detrimental to the mechanical properties of the sealant material and its adhesion to the glass surface. From the current evaluation, it was noted that high temperature could be a critical weathering conditions important for the assessment of the sealant, and B-4, a one-part structural sealant, had simi­larly low tensile strength after 80°C exposure and xenon-arc weathering. Test specimens were conditioned for 4 h at 80°C and measured for tensile strength in the same conditioned testing machine. It implies that high temperature expo­sure for structural glazing sealant can have significant and similar impact on sealant durability as artificial weathering does. However, it is not clearly under­stood how high temperature exposure could give such material behaviors for structural silicone sealant. More verification of sealant behaviors at different elevated temperature conditions and water immersion could be the focus of the next study.

Conclusions

Structural glazing was introduced to Korea three decades ago, but there is no industry standard available for structural silicone sealant and the structural glazing method. Therefore the establishment of national guideline for silicone structural sealant based on ISO/FDIS 28278-1 and 2 was suggested.

Industry specialists working on local construction projects do not have any durability information for structural silicone sealants used in Korea. Therefore various structural silicone sealants from three major suppliers were gathered and evaluated in accordance to the requirements suggested by international industry standards. Three out of 14 evaluated products could not pass the mini­mum requirements suggested by ISO/FDIS 28278-1. The data generated will provide a good reference for judging the actual performances of structural sili­cone sealants used in Korea. Also the evaluation results invoked a urgency for the implementation of a national guideline as soon as possible because of sev­eral super-tall buildings are being planned based on the structural glazing method.

Although some products routinely pass the relevant national standard (i. e., ASTM C1184) as this is commonly used in the US and many Asian countries, they do have some difficulty in meeting the adhesion and durability require­ments of ISO/FDIS 28278-1. These shortcomings have been noted by the current

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evaluation and the products need to be improved for meeting this international guideline and soon to be adapted Korea national standards.

One key finding from the evaluation is that different pulling speed with ten­sile adhesion joint according to ISO/FDIS 28278-1 and ASTM C1135 did not give significant affect tensile values.

Some products had a significant difference between the values at high tem­perature (80°C) and low temperature (—20°C). Also, high temperature exposure by itself appears to be enough to have negative impact on the durability of some structural silicone sealants. Similar results for these products could be seen af­ter xenon-arc/water spray weathering. It implies that artificial weathering and high temperature exposure are important tactics for the durability of structural silicone sealants.

A structural silicone sealant is only a small portion of all materials used at the job site when compared to other building materials, thus its durability can be easily overlooked by industry specialists. However, if it fails to meet the dura­bility expectations, there might be a human made disaster such as falling of glasses. Therefore, the industry specialist should be aware of these concerns because the structural glazing system will be exposed to harsh conditions dur­ing the life cycle of the building.