Sealant Durability/Stress-Strain Graphs

The curtain wall systems were tested within 3 months after their construction. It must also be shown that the sealant system chosen for this application would perform equally as well after many years of weathering. There is extensive pub­lished work already available on the durability of silicone sealants [12-14]. As part of this research effort, some of that work has been updated and expanded to further illustrate the durability of silicone sealants. In particular, tensile ad­hesion samples were tested before and after weathering as well as shear adhe­sion samples. Cyclic testing, using AC45 criterion [15], to 50psi (345 kPa) was also performed on sealants to show the effect of cyclical strain experienced dur­ing a seismic event.

The sealant that would generally be specified for curtain wall applications that are shop-fabricated is a high modulus, 25 % movement capability, two-part quick curing 100% silicone sealant. Additionally, a high modulus, 50% move­ment capability, single-component 100% silicone sealant was tested so that a single-component sealant would also have documented results for its suitability. Single-component sealants are generally easier to work with in the field and may be used for reglazing or stick built/field installed units.

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The tensile adhesion samples were constructed and tested to ASTM C1184 [16] criteria (Fig. 9). Per this criteria, the sample sealant dimension being tested is 2 in. (50 mm) long (left to right, Fig. 9) x 1/2 in. (12.5 mm) deep (front to back in Fig. 9) x 3/8 in. (9.5 mm) thick (top to bottom in Fig. 9). These are the same dimensions as for the shear sample, but the shear sample is pulled in the shear direction as shown in Fig. 10.

The movement rate for the pull testing was 0.5 in. (12.5 mm) per minute for both shear and adhesion samples in accordance with ASTM C1184 [16], Section 8.6. For the cyclical testing, tensile joints were prepared in the same configura­tion shown in Fig. 9. The graphs shown below in Figs. 11 and 12 summarize the tensile and shear testing results for the single-component and dual-component structural sealant, respectively. Based on the ultimate stress the sealants are ca­pable of withstanding and the cyclical testing discussed below, 50psi (345 kPa) is a reasonably acceptable stress level. Considering sealants are flexible materi­als, the behavior of the sealants from 0 to 50psi (0-345 kPa) is very repeatable and the sealants in this range are behaving elastically as the cyclical testing dis­cussed shows. The racking test results, discussed later, further validate the elas­tic behavior of the sealants in this stress range.

Seismic movement placed on the structural sealant bead attached to the inside of the glass lite primarily results in shear behavior. As documented by Zarghamee et al. [2], the shear modulus is approximately 1/4 that of tension. The ultimate strengths in either mode are very similar. For example, at 35 psi (241 kPa) in tension, the sealant strain is approximately 12% in tension and

FIG. 9—Typical tensile adhesion sample set up as installed on a tensile testing machine.

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FIG. 10—Typical shear adhesion sample set up as installed on a tensile testing

machine.

50 % in shear when examining Fig. 12 and the behavior of the dual-component sealant. At 35 psi (241 kPa) in tension for the single-component sealant (Fig. 11), the strain on the sealant is approximately 20% in tension and 70% in shear. This is actually favorable for seismic situations in that for a given strain or dis­placement during a seismic load, the associated stress on the sealant is lower in a shear mode than in a tensile mode. Therefore, using tensile data to design the sealant joint for seismic-induced stress is conservative when considering either a single – or dual-component sealant. Figure 12 also suggests that the behavior of the modulus varies with temperature in that the particular silicone tested at higher temperature shows higher modulus for the two-part sealant; such is not the case, however, for the one-part sealant as shown in Fig. 11. Although more softening of the modulus is generally expected with higher temperatures, the two-part sealant displays slightly different behavior but is within industry speci­fication for silicone used in SSG systems, and some new test results are avail­able in a recent report as well [17].

Per the testing method, the shear and tension results presented are based on pulling the sealant at a constant rate 0.5 in. (12.5 mm) per minute until

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— – * Tension at-29 °С

…………. Tension at 88 °С

——— Tension at 21 Day

RoomTemp

——— Tension after 5C0C

hrQUV

50 pSi ІІПе [M5 fcPat Shear at -29 °С

— * – Shear at 88 °С

— Shear at 21 Day RoomTemp

FIG. 12—Stress-strain curve for high modulus two-part sealant as tested in both shear and tension, at various environmental

conditions.

destruction. During a seismic event, the sealant will be stressed to a lower level than its ultimate strength, but more likely in a cyclic manner. Consequently, it was proposed to simulate cyclical testing on a tensile adhesion joint to deter­mine if there is any sealant softening after stressing the sealant repeatedly. Based on prior work [2], 50psi (345 kPa) was chosen as the cyclic stress level. Samples were prepared and pulled to 50psi (345 kPa), then allowed to relax, then pulled again to 50 psi (345 kPa) and allowed to relax for four cycles. On the fifth cycle, the samples were pulled to destruction. The strain rate for this test­ing was 1 in./min (25mm/min) [15]. This testing was a variation of a standard tensile adhesion test method (ASTM C1135 [18]) and was carried out as new research to look specifically at the effect of a sealant repeatedly reaching 50psi (345 kPa) in a short-term event. The ability of silicone sealants to withstand long-term cyclic movement due to thermal effects, from a durability standpoint, has already been addressed by an industry study in which sealants were cycled for 361500 times, estimating 50 to 100 years of durability depending on the envi­ronment the silicone would be exposed to [19]. The intent of the shorter term cyclic testing completed here was to assess any effect on ultimate sealant tensile strength when sealants are purposefully pulled to 50 psi (345 kPa) during a short-term event such as an earthquake, versus a long-term cyclic stress such as thermal movement over many years. It is understood that during a seismic event that the number of cycles (including low and high amplitudes) will likely exceed five and an intermediate level of cycles could be considered for future sealant testing.

This cyclic testing was performed in tension, as it has been shown in this research and prior industry studies [20] that tensile testing provides conserva­tive results relative to shear testing when evaluating design stresses for a seal­ant. It is understood that the results of cyclic testing in shear would provide valuable additional insight into sealant performance. As can be seen in Figs. 13 and 14, pulling either the 1 – or 2-component sealant to 50 psi (345 kPa) repeat­edly has very little effect on its ultimate strength. The cyclic testing was per­formed at room temperature, so the final result of the sample pulled to destruction can be compared to the room-temperature samples from the tensile adhesion testing referenced above. Five samples were cyclically pulled for each sealant, and the averages are presented in Table 1. One typical sample is depicted in the graphs for clarity.

The other important sealant properties to account for when considering seismic design are durability of the sealant and consistency of the sealant’s strength and modulus properties over time, which can be seen in the 5000-h QUV exposure (accelerated weathering testing under ultraviolet light frequen­cies and condensation) and extreme temperature exposure conditions (Tables 2 and 3). The weathering criteria included low-temperature exposure (-29°C), high-temperature exposure (88°C), and 5000 hr QUV exposure, cycling from 8hr at 60°C with ultraviolet (UV) exposure to 4 hr 50°C condensation. Modulus has been used here to indicate sealant stability across these conditions. The moduli reported have been calculated at 10 % sealant strain. Young’s modulus is calculated by taking the slope of a 0.2 % offset trend line through the sealant stress-strain curve, from 0 to 10% strain in this case. Although peak stress may

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FIG. 13—Stress-strain curve for cyclical testing of one-component medium modulus sealant to 50psi (345kPa) with final test to ultimate strength.

fluctuate somewhat, if the modulus is remaining relatively stable, this indicates sealant durability. Results are presented in Tables 2 and 3 for both the two – component and single-component silicone sealants, as tested in shear and tension.

FIG. 14—Stress-strain curve for cyclical testing of two-component high modulus seal­ant to 50psi (345 kPa) with final test to ultimate strength.

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TABLE 1—Sealant ultimate stress in tensile adhesion joint versus sealant ultimate stress after cyclical loading.

Ultimate Stress psi (kPa)

Ultimate Stress psi (kPa)

Sealant

of Single Pull Sample

of Cyclically Pulled Sample

Single-component silicone at room temperature

160 (1103)

185 (1275)

Two-component silicone at room temperature

166 (1145)

149(1027)

TABLE 2—Summary shear data: 21-day exposures).

room temperature (RT) cure (+ environmental

Sealant

Ultimate Stress psi (kPa)

Ultimate Strain (%)

Young’s Modulus psi (kPa)

Single-component silicone RT

133(917)

253

45 (310)

Single-component silicone RT + 1 hr 88°C + 3hr RT dwell

147(1013)

262

47 (324)

Single-component silicone RT + 1 hr -29° C

184(1268)

293

50 (345)

Two-component silicone RT

146(1006)

198

71 (490)

Two-component silicone RT + 1 hr 88°C + 3hr RT dwell

143 (986)

179

73 (503)

Two-component silicone RT + 1 hr -29°C

170(1172)

228

65 (448)

TABLE 3—Summary tensile adhesion data: (+ environmental exposures).

■ ASTM C-1135 data, 21-day RT cure

Ultimate

Young’s

Stress

Ultimate

Modulus

Sealant

psi (kPa)

Strain (%)

psi (kPa)

Single-component silicone RT

160(1103)

191

231 (1592)

Single-component silicone RT + 1 hr 88°C + 3hr RT dwell

160(1103)

171

236 (1627)

Single-component silicone RT + 1 hr -29°C

185 (1275)

214

230 (1586)

Single-component silicone RT + 5000 hr UV exposure

145 (1000)

171

200 (1379)

Two-component silicone RT

166(1144)

114

358 (2468)

Two-component silicone RT + 1 hr 88°C + 3hr RT dwell

175 (1206)

81

399 (2751)

Two-component silicone RT + 1 hr -29°C

151 (1041)

128

364 (2510)

Two-component silicone RT + 5000 hr UV exposure

177(1220)

179

401 (2765)

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It is common that sealants are assumed to stiffen at cold temperatures and soften at high temperatures, but in general silicones are still more stable across a variety of conditions than other sealant chemistries [13]. In particular, the dual-component sealant represented in Fig. 12 actually displays tensile behavior opposite from what may be expected by some stiffening within a certain range of (environmental) high-temperature exposure; but the modulus is not signifi­cantly affected (Table 3) and the ultimate strength across all conditions is still well above 50psi (345 kPa). It is a unique property of the sealant tested in this study but has posed no performance issue as the sealant has been used for structural glazing for many years in Europe and the United States. A recent test report [17] presents additional test data regarding this sealant behavior.

Additionally, sealant data is presented for 88°C exposure followed by a 3-hr room-temperature dwell time. As the focus of this paper is seismic behavior, the researchers found it very unlikely that a sealant in a construction project would come to an 88°C dwell time of 1 hr and simultaneously undergo a seismic event. However, the authors felt it was important to document any effects on the seal­ant if it had been exposed to a high-temperature event at some point prior to experiencing a separate seismic event. The important documented behavior is that the sealant can withstand high temperatures and return to its original performance.

The silicone sealants tested show modulus stability across both environ­mental conditions and cyclical testing. As further shown by this curtain wall rack testing, silicones provide the strength and flexibility to withstand signifi­cant seismic-induced movement. Furthermore, the weathering data presented here and in prior work substantiate that silicones provide the durability required for this stringent application.

Finally, the consistency of the results across all conditions, the high ulti­mate strength of the silicones, and the cyclic testing support the proposition made in 1996 [2] to employ a sealant design stress level of 50psi (345 kPa) for seismic design.