Accelerated Weathering

To assess their usefulness in predicting real world weathering results, the seal­ants exposed in the El Paso warehouse exposure were also subjected to several accelerated weathering tests. Results are presented in Tables 2-4 and in Figs. 8-11.

Weathering of Plaques in Xenon Arc and Fluorescent UV Weathering Devices – The results of the accelerated weathering of sealant plaques exposed in the xenon arc and fluorescent UV weathering devices are detailed in Table 2 and in Figs. 8 and 9. In the fluorescent UV, the polyurethane sealant plaque starts to craze after approximately 500 h and exhibits severe crazing after 2200 h (Fig. 8). These crazes extend as far as 1 mm into the bulk of the specimen. The

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FIG. 7—Acrylic sealant in a horizontal parapet joint showing slight surface wrinkling or crazing.

acrylic sealant plaque, in contrast, exhibits no visible flaws after an extended exposure period of 6400 h in the fluorescent UV (Fig. 8). The polyurethane sealant in the xenon arc device looks similar to that in the fluorescent UV, exhibiting significant crazing after 2200 h. The acrylic sealant plaque exhibits no visible flaws in the xenon arc device after a comparable exposure period. Although there was no attempt to quantify crazing as a function of exposure time, the accelerated weathering results for both the acrylic and polyurethane sealants are consistent with the appearances of these two sealants after exterior exposure in El Paso. The early crazing of the polyurethane sealant plaque after accelerated weathering is similar to the crazing seen on the vertical joints of the polyurethane sealant after 3 years in El Paso. The deep crazing of the polyure­thane sealant plaque after longer periods of accelerated weathering is consis­tent with the appearance of the polyurethane sealant in the parapet joints after 3 years in El Paso.

TABLE 2—Accelerated weathering results.

Sealant

Weathering Apparatus

Hours

Visual Appearance

Polyurethane

Fluorescent UV

2200

Severe crazing

Acrylic

Fluorescent UV

2200

No flaws

Acrylic

Fluorescent UV

6400

No flaws

Polyurethane

Xenon arc

2200

Severe crazing

Acrylic

Xenon arc

2200

No flaws

Sealant

Weathering Apparatus

Cycles

Hours

Failure3

Visual Appearance

Polyurethane

Fluorescent UV

5

3916

0.35 in.2 (226 mm2) C at interface

Fine surface crazing

Acrylic

Fluorescent UV

5

3916

None

No flaws

Polyurethane

Xenon arc

5

3571

1.25 in.2 (806 mm2) C at interface

Severe crazing; up to 3 mm deep

Acrylic

Xenon arc

5

3571

None

Slight crazing

aFailure is reported as the total area of cohesive plus adhesive failure over three specimens. C = cohesive failure.

 

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TABLE 4—Appearance of sealants after 1 year of exterior exposure in static joints.

Sealant

Coating

Dirt Pickupa

Crazinga

Chalkinga

Polyurethane

No

80

70

30

Yes

67

NA

NA

Acrylic

No

63

100

85

Yes

83

NA

NA

aQualitative scale where 100 represents no change from the initial appearance.

The polyurethane sealant softens during accelerated weathering in both the fluorescent UV and the xenon arc. This phenomenon was not quantified due to the fact that the sealant plaques used for weathering are too thin for reliable hardness measurements. However, the softening is pronounced, and the weath­ered polyurethane sealant after 2200 h feels gummy and significantly less tough than when it was originally exposed. This observation is consistent with the softening of the polyurethane sealant in the parapet joints in El Paso. The hardness of the acrylic sealant appears to change little during accelerated weathering.

In addition to general weathering, sealant color change was also evaluated as a function of exposure time in the fluorescent UV (Fig. 9). During the first 1500 h of exposure the polyurethane sealant yellows and darkens, as measured by changes in L* and b*. After 1500 h this trend is reversed as the sealant starts to chalk and, as a consequence, lighten. The color change of the polyurethane sealant during accelerated weathering was not noted in the El Paso warehouse exposure presumably because it was obscured by early dirt pickup. However,

image281

FIG. 8—The acrylic sealant after 6400 h in the fluorescent UV apparatus (left) and the polyurethane sealant after 2200 h (right). Plaque width is 1.5 in. (38 mm).

image282

FIG. 9—Change in sealant color during accelerated weathering in the fluorescent UV apparatus.

 

image283

FIG. 10—Photograph of the acrylic sealant {left) and the polyurethane sealant {right) after 3571 h of weathering in the xenon arc device and five cycles of ±25 % cyclic movement. Sealant joint dimensions are 0.5 X 0.5 X 2 in.3 (12 X 12 X 51 mm3), and the sealants are slightly flexed to show crazing and loss of adhesion.

 

image284

FIG. 11—Polyurethane sealant {top) and acrylic sealant (bottom) in painted wood channels. The sealant on the left hand side of each channel is unpainted. The sealant on the right hand side of each channel is coated with an EWC. The channel width is 3/4 in. {19 mm).

the chalking of the polyurethane sealant seen during accelerated weathering is consistent with the chalking observed in the field. The color of the acrylic seal­ant remains essentially unchanged after exposure in the fluorescent UV, and chalking after 2200 h is insignificant. This lack of chalking is likewise consis­tent with observations of the acrylic sealant after 3 years of exterior exposure in El Paso.

ASTM C1519-04 Durability—The results of ASTM C1519-04 durability test­ing are reported in Table 3 and in Fig. 10. After five cycles and 3916 h of weathering in the fluorescent UV, the ASTM C1519 polyurethane sealant H block specimens exhibit fine surface crazing and have a total of 0.35 in.2 (226 mm2) cohesive failure at the interface with the aluminum substrate. Under the same conditions, the acrylic sealant specimens exhibit no surface degradation and no adhesive or cohesive failure. After five cycles and 3517 h in the xenon arc apparatus, the ASTM C1519 polyurethane sealant H block speci­mens are severely crazed, with the crazes extending up to 3 mm into the bulk of the specimens. The specimens also have a total of 1.25 in.2 (806 mm2) cohe­sive failure at the substrate interface (Fig. 10). The acrylic sealant specimens exhibit slight surface crazing after xenon arc weathering but show no signs of adhesive or cohesive failure (Fig. 10).

The ASTM C1519 polyurethane sealant H block specimens weathered in the fluorescent UV (Table 3) show substantially less surface degradation after 3916 h of exposure than the plaques of polyurethane sealant weathered for 2200 h in the fluorescent UV (Table 2). The faster surface degradation of the polyurethane sealant plaques may be due to the thinner cross section of the plaques and to the greater impact of heat and moisture on bulk sealant prop­erties. Or it may be that the heat capacity of the H block specimens is higher than that of the plaques, resulting in less condensation during fluorescent UV exposure and less overall exposure to surface wetting. The greater degradation in the thin cross sections is consistent with the installer’s observations that some polyurethane sealants in the field tend to “burn-though” or degrade when applied in thin cross sections or over backer rod that has been installed at an insufficient depth.

The ASTM C1519 polyurethane sealant H block specimens weathered in the xenon arc apparatus (Table 3) generally look similar to the plaques of poly­urethane sealant weathered in the same device (Table 2). Both develop a dense network of cracks. However crack depths vary from roughly 1 mm in the ex­posed plaques to as much as the 3 mm in the ASTM C1519 H block specimens. The greater crack depths in the ASTM C1519 H block specimens are likely due to the longer exposure time (3571 h vs 2200 h) and to the repeated cycles of ±25 % joint movement that are part of the ASTM C1519 durability test.

The degradation of the ASTM C1519 polyurethane sealant H block speci­mens is significantly greater after weathering in the xenon arc device than it is after weathering in the fluorescent UV (Table 3). This may be due to the broader spectrum of irradiance of the xenon arc light source or to the fact that speci­mens in the xenon arc see prolonged exposure to significantly higher tempera­tures than they do in the fluorescent UV. A comparison of accelerated to exte­rior weathering of the polyurethane sealant indicates that testing according to ASTM C1519 in the xenon arc apparatus (Fig. 10) generally predicts the worst of the UV degradation seen in the El Paso exposure (Fig. 6).

The ASTM C1519 acrylic sealant H block specimens weathered in the fluo­rescent UV look similar after 3916 h of exposure to the plaque of acrylic sealant weathered for 6400 h in the fluorescent UV. Neither exhibits any signs of sur­face degradation and the accelerated thin film degradation of the polyurethane sealant does not happen to the acrylic sealant. The lack of degradation after both accelerated fluorescent UV exposures is consistent with the lack of degra­dation of the vertical acrylic sealant joints after 3 years of exterior exposure in El Paso.

The ASTM C1519 acrylic sealant H block specimens weathered in the xenon arc apparatus exhibit a slight amount of surface crazing, while the plaque of acrylic sealant weathered in the same device has none. This is likely due to the longer exposure of the ASTM C1519 specimens (3571 h vs 2200 h) and to the repeated cycles of ± 25 % joint movement. The fact that the ASTM C1519 acrylic sealant H block specimens craze slightly after xenon arc weath­ering but not after similar times of fluorescent UV weathering may be due to the full daylight spectrum of the xenon arc lamp or to a greater exposure to elevated temperatures. Testing according to ASTM C1519 in the xenon arc ap­paratus (Fig. 10) predicts the slightly greater crazing of the acrylic sealant in the horizontal parapet joints in El Paso (Fig. 7).

The ASTM C1519 polyurethane sealant H block specimens soften signifi­cantly after weathering in the xenon arc apparatus but do not appear to soften after weathering in the fluorescent UV. The softening after xenon arc exposure is consistent with that seen after thin plaque weathering and after exposure in the parapet joints in the El Paso warehouse. The fact that noticeable softening of the polyurethane sealant only occurs on specimens which exhibit substantial surface crazing suggests that there is a connection between these two phenom­ena.

Exterior Exposure in Static Joints—The results of the Spring House Farm exposure in static joints are summarized in Table 4 and in Fig. 11. The un­coated polyurethane sealant picks up relatively little dirt but crazes and discol­ors upon exposure. This discoloration is consistent with that seen in the fluo­rescent UV exposure—initial yellowing followed by whitening due to chalking. The uncoated acrylic sealant shows no signs of degradation but picks up more dirt than the polyurethane sealant. These dirt pickup observations differ from the El Paso data, which indicate that the two sealants are very similar in ap­pearance after 1, 2, and 3 years of exterior exposure. The reduced dirt pickup of the polyurethane sealant in the static joint relative to the exterior joints in El Paso may be due to the fact that the sealant exposed in the static joint was cured for several weeks prior to exposure, thus eliminating the early high tack, high dirt pickup phase of the polyurethane cure. The increased dirt pickup of the acrylic sealant in the static joint relative to the exterior joints in El Paso may be due to the different climate of the southeast PA exposure or to the fact that the static joints were exposed horizontally, which allows for less run-off and self-cleaning than do the vertical tilt-up joints in the El Paso exposure.

The EWC appears to be compatible with both the polyurethane and the acrylic sealants, with no apparent debonding at the coating/sealant interface. The EWC coated polyurethane sealant exhibits substantial dirt pickup. This, presumably, is due to plasticizer migration from the polyurethane sealant into the EWC. The EWC coated acrylic sealant exhibits very little dirt pickup due to the lack of plasticizer in the acrylic formulation.