Quantification of Surface Degradation

The determination of the degradation by cracking and crazing was based on a 2 cm x 1 cm assessment area using the rating scheme provided in ISO 4628-4

[13] , with additional rating criteria introduced in order to determine the influ­ence of minute cracks on the surface of sealants (see Table 4 and Fig. 4). The degree of degradation (QS) was based on the product of the rating for the quan­tity (Q) and size (S) of cracks as provided in Eq 1

QSit) = Q(t)x S(t) (1)

TABLE 4—Crack rating index.

Rating

Quantity of Cracks (Q)

Rating

Size of Cracks (S)

0

None

0a

Not visible under x 100 magnification

0.5a

Trace

0.3a

Only visible under magnification up to x 50

1

Very few

0.5a

Only visible under magnification up to x 30

2

Few

1

Only visible under magnification up to x 10

3

Moderate

2

Just visible with normal corrected vision

4

Considerable

3

Clearly visible with normal corrected vision

5

Dense

4

Large cracks generally up to 1 mm wide

5

Very large cracks generally more than 1 mm wide

aNewly introduced ratings.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

where:

QS(t) = QS-value after t months,

Q(t) = rating Q after t months, and S(t) = rating of S after t months.

The approximate relationship between the QS-value and the surface condi­tion is indicated in Fig. 5. A QS-value above about 20 indicates considerable degradation by surface cracks, and a QS-value below 10 indicates few and smaller cracks. The relationship between the QS-value and the cracking portion analyzed by the binary imaging showed a reasonable correlation [6].

An outline of the modeling of the QS-value with weathering is indicated below [14]. For the modeling, the outdoor exposure test in Yamanashi Prefec­ture was carried out on sealant set 1 according to the conditions given in Table 5, and the QS-value for the different extension/compression amplitudes along the central axis of the test specimens was determined over time.

The change of the QS-value over time for the different movement exposures is given in Fig. 6, and the following conclusions can be drawn:

(1) For some sealants (MS-2 (SR), PU-1, and PU-2), the QS-value increases with the passage of time even when the test specimens are exposed to outdoor weathering in a static position (no forced movement). For

FIG. 5—Relationship of QS-value and surface conditions.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

TABLE 5—’Weather summary ofYamanashi Prefecture, set 1.

Temperature,°C

Max.

40.4

Min.

-7.8

Avg.

15.2

Accumulated total solar radiation, 0°

• MJ/m2

21 004

Accumulated precipitation, mm

4277

*Term: Jan. 16, 2003, through Jan. 16, 2007.

these sealants, forced movement (even at the small amplitude of ±1.5%) substantially accelerates the occurrence and worsens the degree of surface degradation.

(2) For all sealants (with the exception of SR-2), surface degradation occurs when the specimens are exposed to dynamic conditions (forced move­ment plus outdoor weathering). For some of these sealants, higher movement amplitudes worsen or accelerate the surface degradation.

з

d

HH

P-H

К v'(.om. rafea

FIG. 8—Relationship of QS-value between observation and calculation.

The change in the QS-value is then modeled as indicated in the steps below (see also Fig. 7).

• The QS-value is plotted for each material and exposure condition along the z-axis, with the progress of time being on the x-axis and the exten – sion/compression amplitude on the y-axis.

• The measured QS-values are fitted by a smooth line for every amplitude and exposure period.

• The curved surface then represents a three-dimensional model (response surface) of the QS-value as a function of the exposure time and amplitude of extension and compression exposure.

The QS-value change can be calculated according to Eq 2

QS(s • t) = (a • tb)x(1 + c • sd) (2)

where:

QS(e • t) = QS-value for an extension/compression ratio e (in %) after t months,

t = exposure time (in months),

e = extension/compression amplitude (as a percentage of joint with), and

a, b, c, and d = constants specific for each sealant.

In Eq 2, (a • tb) represents the QS-value under static conditions (without extension/compression movement) after t months, and (1 + c • ed) represents the acceleration effect caused by the joint movement.

The congruity between the calculated and observed QS-values is shown in Fig. 8, with the symbols ° and • indicating a gap between the two values and the c indicating identical values. The curve-fit achieved with Eq 2 for the QS-value of each product provides a reasonable relationship between observation and calculation over the exposure period of this test program. The equation derived for each sealant is given in Table 6. Because the silicone sealant SR-2 did not show any surface degradation, the constants a and c equal 0, whereas for all other products these constants are greater than 0.

TABLE 6—Equations for each sealant.

Sealant

Equation for QS(s ■ t)

SR-2

QS(s ■ t) = (0 x t10) x (1 + 0 x s10)

IB-2

QS(s ■ t) = (0.002 x t10) x (1 + 104 x s02)

MS-2(GP)

QS(s ■ t) = (0.005 x t10) x (1 + 40 x s02)

MS-2(SR)

QS(s ■ t) = (0.005 x t19) x (1 + 1.6 x s03)

MS-1

QS(s ■ t) = (0.02 x t10) x (1 + 4 x s14)

PS-2

QS(s ■ t) = (0.01 x t10) x (1 + 18 x s0 2)

PU-2

QS(s ■ t) = (3.8 x t06) x (1 + 0.4 x s0 4)

PU-1

QS(s ■ t) = (0.7 x t0 8) x (1 + 0.003 x s0 9)

Relationship between Outdoor and Accelerated Exposure

Sealants and Test Procedure

Accelerated artificial weathering exposure test and outdoor natural exposure test at Choshi-city were carried out on the sealant set 2.