Austrian researchers have analyzed the cracking susceptibility of polypropylene (PP) backsheets and concluded that they can potentially be used in solar module manufacturing, unlike the co-extruded polyamide backsheets that caused a lot of headaches for the solar industry in the past. In addition, PP backing sheets are claimed to have lower stiffness and greater flexibility than their laminated counterparts.
“For a few years, we have been testing polypropylene-based coextruded backing plates for its properties, performance and reliability,” says Gernot Oreski, lead author of the study. pv magazine. “The biggest risk for these new types of backing plates is the appearance of cracks after a few years of field use.”
In the article “Investigation of Fracture Susceptibility of Coextruded Polypropylene Backing Films for Solar Photovoltaic Modules”, published Solar energy materials and solar cellsThe scientists recalled that these backsheets have already been used in industry, leading to the so-called “polyamide backsheet disaster”, which they say refers to a series of faulty installations developed between 2009 and 2011.5 in several countries with this technology.
“I think it was a really innovative idea to use co-extruded backing sheets where each layer has different requirements and therefore different fillers and additives,” Oreski said. – However, the selection of materials and especially the qualification of the material was not done thoroughly. The test protocols commonly used at the time focused on exposure to single stresses, so the problematic long-term behavior of the polyamide was not revealed, so the cracking of the backing plates occurred after a few years.
“Meanwhile, new test protocols have been developed specifically for these types of backplane failures, and the solder coupon test is one of them,” he continued. “Unfortunately, this led to a reluctance in the PV industry to use polyamides, which is understandable, but also to the general use of co-extruded backing films.”
The researchers explained in the study that coextruded PP backsheets have lower stiffness and greater flexibility than laminated backsheets. Additionally, they noted that PP backsheets have low water vapor transmission rates comparable to their PET counterparts, but have high oxygen and acetic acid transmission rates.
Using a standard encapsulant, the researchers prepared test samples, which they called solder coupons, and subjected them to an accelerated aging test along with reference laminated backplanes.
They tested five different samples: a PP-based coextruded backing sheet; polyamide (PA) based co-extruded backing plate; laminated backing sheet based on polyethylene (PE), polyethylene terephthalate (PET) and polyvinylidene difluoride (PVDF); laminated backing sheet based on polyvinyl fluoride (PVF) and PET; and a laminated backsheet based on fluorinated coatings (fc), PET and PVDF.
The Austrian team then subjected the samples after lamination to five cycles of 500 hours of xenon and 100 thermal cycles in the temperature range of -40 C to 85 C. “Two samples from each coupon material combination were subjected to accelerated aging tests: In one sample, the back plate was directly irradiated, the front glass of the other sample was irradiated, it explained.
Testing showed that only the second and third listed backplate types showed serious cracking. In the other three types, on the other hand, there were no cracks or embrittlement of the outer layer.
“The results of this study confirm previously published results on the long-term stability of co-extruded PP-based backsheets, showing excellent stability against temperature, humidity and UV radiation,” the team said, referring to the remarkable group of the first type of backsheet. . “In general, co-extruded PP backsheets show great potential to replace standard PET-based backsheets in PV modules.”
The research team also includes researchers from the Austrian Institute of Chemistry and Technology, Silicon Austria Labs, Kyoto Solar and Borealis Polyolefine GmbH.
“Coextrusion allows the backplane to be tailored exactly to the requirements of a given situation, because the process itself is much more flexible than the lamination process,” Oreski said, pointing to the real possibilities coextruded backplanes can bring back to the solar industry. Approval of PP material.
