Proceedings of ISES Solar World Congress 2007: Solar Energy and Human Settlement
520
Therefore, the following single objectives were defined:
(1) Selection and basic characterization of potential
polymeric materials for solar absorber applications.
(2) Comprehensive characterization of selected materials
on specimen level as to their aging behavior under relevant
environmental conditions.
(3) Lab-scale and selective full-scale characterization of
long-term fracture behavior under environmental
conditions on component level.
(4) Interpretation of the aging behavior on base of polymer
physical relations and establishment of structure- property-
correlations.
While the single objective [1] was treated in a previous
paper [8], within this paper the focus is given to objective
[2] dealing with different test methods to characterize the
degradation behavior for selected polymeric materials.
2. BACKGROUND AND METHODOLOGY
For Nothern climates an all polymeric collector has been
developed consisting of a polycarbonate (PC) glazing and
an absorber made of PPE+PS. The University of Oslo (2)
(Norway) selected and developed the absorber material in
close collaboration with General Electric Plastics (GE)
(Bergen op Zoom, the Netherlands). This collector works
as a pressureless system with the drain-back principle,
reaching pressure levels of maximum 0.1 to 0.5 bar. As heat
carrier water is used and a lifetime of 20 years is intended.
Maximum temperatures of 80°C under operating conditions
and 140°C in air under stagnation have been recorded. For
our research these conditions have been considered and a
market study on thermoplastic materials was carried out. 12
different amorphous and semi-crystalline materials
including the currently used absorber material PPE+PS
were selected and investigated (objective (1)). For the
characterization, films with a thickness of 400 μm were
extruded according to the minimum wall thickness in the
Solarnor absorber and characterized as to the relevant
thermal, thermo-mechanical and mechanical properties.
After a basic characterization (8) 9 thermoplastics were
further investigated as to their ageing behavior on specimen
level (objective (2). In this paper the aging behavior of 3
polymers, including the PPE+PS type, polycarbonate (PC)
and polypropylene (PP) as the only semi-crystalline
polymer grade is described. Specimens were exposed to hot
water at 80 °C and to hot air at 140 °C for up to 16000 and
500 hours, respectively. To describe the aging behavior, the
reference and the aged materials were characterized by
using appropriate analytical (thermal analysis, size
exclusion chromatography and infrared (IR) spectroscopy)
and mechanical methods.
3. EXPERIMENTAL
3.1 Polymer Types
The following commercially available amorphous and semi-
crystalline polymers were chosen for the investigations:
Amorphous grades:
Polycarbonate (PC)
(trademark: Makrolon; manufacturer: Bayer, Leverkusen,
GER)
Poly (phenylene ether) / polystyrene (PPE+PS) blend
(trade-mark: Noryl; manufacturer: GE, Bergen op
Zoom, NL)
Semi-crystalline grade:
Polypropylene random copolymer (PP)
(trademark: Beta-PPR RA7050; manufacturer: Bo-
realis, Linz, A).
While PC from Bayer was supplied as films with a
thickness of about 500 μm, the other materials were cast to
films on a single screw extruder (Collin, Ebersberg,
Germany) with a chill roll unit. The thickness ranged from
420 to 480 μm. For tensile testing specimens according to
type S2 (DIN 53504 [9]) were cut with a puncher. The
overall length and the measuring length of the samples
were limited to 75 mm and 40 mm, respectively.
The film specimens were heat aged in hot air at 80°C for
125, 250, 375 and 500 hours and exposed to distilled water
at 80°C for 2000, 4000, 6000, 8000 and 16000 hours using
an air-circulating oven (Kendro, Vienna, A). While the
specimens exposed to hot air were placed on a metal grid,
the specimens exposed to hot water were stored in a water
filled glass jar covered by a screw top. The distilled water
was changed in 2000 hours intervals.
3.2 Differential Scanning Calorimetry (DSC)
Thermal analysis was carried out using a Mettler Toledo