Determination of Acceleration Factor
Changes in the QS-value of the different sealants within set 2 with natural and artificial weathering carried out according to the conditions indicated in Table 7 are indicated in Table 8 (and shown in Fig. 9). Because exposure occurred only under dynamic conditions (with forced movement), the QS-value was determined at three points along the central axis of the test specimen, corresponding to movement amplitudes of 1.5%, 15%, and 25%. The MS-1 product showed adhesion failure at the 25 % extension/compression amplitude in the XWOM and SWOM exposures. The process of calculating the acceleration factor between accelerated weathering and natural outdoor weathering for each sealant and the specific artificial light source is indicated below using the MS-2 product as an example (see also Fig. 10).
• The QS-values of the specific product are converted to a threedimensional diagram as shown in Fig. 6.
• The time period derived for outdoor exposure (to reach a specific QS – value) was divided by the corresponding time periods for each light source as shown in Eq 3.
• The average acceleration factor for each artificial weathering condition was calculated by repeating the above-described process for the different extension/compression amplitudes and then averaging the results
K = t[outdoor]/t[light source] (3)
K = accelerated factor/ratio,
t[outdoor] = time period required in order to reach a specific QS-value in outdoor exposure, and
t[light source] = time period required in order to reach a specific QS-value in accelerated exposure.
The acceleration factors for the different light sources versus natural weathering are given in Table 9, and the irradiation time of each accelerated light source corresponding to one year of outdoor exposure is given in Table 10 as calculated according to Eq 4. As the silicone sealant product shows no degradation, no numerical acceleration factor can be determined. The other products show acceleration factors, approximately over 10 for XWOM and SWOM exposures and below 5 for UV exposure.
Irradiation time equivalent to one year of outdoor exposure
= 24 hours x 365 days/accelerated ratio (4)
TABLE 10—Irradiation time corresponding to one year.
Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by
The irradiation time in the various artificial weathering machines corresponding to one year of outdoor weathering (with the exception of the silicone sealant product that does not show any degradation in either artificial or natural weathering during the exposure period) are as follows: XWOM, 500 to 900 h; SWOM, 350 to 550 h; and UV, 1300 to 3250 h. No general acceleration factor for each artificial weathering method can be derived, and the evaluation of individual products is indispensable for the determination of the specific irradiation time.
(1) The functional equation of the QS-value that indicates the degree of surface degradation of sealants is obtained, and the equation provides a reasonable relationship between observed and calculated values.
(2) The acceleration factor for each artificial weathering light source against outdoor natural weathering is obtained by applying the QS – value fitting concept. By using this process, we are able to obtain an acceleration factor for each light source.
(3) Products of all sealant types tested show an acceleration factor between artificial weathering and natural weathering, with the exception of the silicone sealant, which did not show any surface degradation over the exposure period of this test program.