RSSV 4 2013.indd - Stazione Sperimentale del Vetro

4-2013
studies
studi
Rivista della Stazione Sperimentale del Vetro
In addition to working problems, rigid warm-edge
spacers present also some intrinsic differences compared
to aluminum spacers. Warm-edge spacers
have a different coefficient of linear thermal expansion
from aluminum. Changing temperatures can
result in a stronger stress on the butyl sealant, weakening
the most important defense of the glazing unit
from gas leakages and moisture penetration.
Table 1 here below shows the coefficients of linear
thermal expansion of warm-edge spacer components.
Only steel has a lower coefficient than aluminum.
Table 1. Coefficient of linear thermal expansion of
warm-edge spacer components
Spacers are made of composite materials. Their expansion
will therefore depend on the interaction of
those materials.
“Flexible” spacers:
- Homogeneity of application of butyl on foam back.
- Extrusion temperatures and homogeneity of shape
for the butyl types.
- Mechanical behavior during pressing.
- Joint closure.
- Material adhesion to the second sealant.
- High permeability of foams to the gas.
2. UNI 1279-2/3 tests on different compositions of
insulating glazing units
The Stazione Sperimentale del Vetro launched a
testing campaign to compare different materials -
aluminum spacers and “rigid” warm-edge spacers.
This campaign studied also the difference between
glazing units made only of float glass and glazing
units with low-emission glass which had undergone
edge-coating removal. The following tests on samples
manufactured in normal fabrication conditions
were performed:
- Dew point.
- Initial gas concentration.
- Final gas concentration.
- Evaluation of the penetration index (I %).
- Measure of gas leakage (Li %).
The test outcomes led to the following findings:
- Dew point: in some cases, condensation appears
on samples before ageing, whereas on the same
samples no condensation was reported after ageing.
According to one of the hypotheses formulated, the
desiccant agent for some types of spacers takes a
long time to absorb the moisture inside the glazing
unit, due to inappropriate holes “drilling” and subsequent
poor exchange of air in the cavity.
- Initial gas concentration: in general, the registered
values fell within the limits provided for in the standards
(+10%, - 5% with reference to the declared
concentration).
- Final gas concentration (after ageing): in some
cases a quite significant decrease (up to 10%) in gas
concentration was registered.
- Initial moisture level in desiccants: it sometimes
appeared high, proving that some types of spacers
absorb moisture during storage.
- Evaluation of the penetration index (I %) and Li
measure: the comparisons between the data collected
in ITT tests, those obtained repeating the tests
performed the year before and the tests carried out
with “rigid” warm-edge spacers of 45 manufacturers
are reported here below.
3. Penetration Index Values (I)
Samples with rigid warm-edge spacers and L.E.
have higher average I values and dispersion, which
are often over the limits permitted by the standards
(I ≤ 20), as shown in figure 1.
In addition to the working problems mentioned
above, one of the hypotheses is that this phenomenon
is due to different stresses on the permeability
profile by expansions of W.E. spacers subject
to temperature range imposed by ageing cycles
(+58°C; -18°C ). However, this hypothesis cannot
explain such a widespread and sometime serious
problem.
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