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EVA films for photocells  #9(115), 2012

The new manufacturing technology of film materials of ethylene vinyl acetate (EVA), based on developed by the experts of KraussMaffei Berstorff GmbH concept of post-extrusion workshop, reduces prime cost and improves sustainability of production

Project manager of the Process engineering department for film and board extrusion,
Head of the trial center of Extrusion equipment department for foam, film and board extrusion,
Prof., Dr., Head of Extrusion equipment department for foam, film and board extrusion, KraussMaffei Berstorff GmbH,
Andrey VOLKOV,
Manager of Extrusion equipment department of the Moscow representative office of KraussMaffei Berstorff GmbH

Ethylene vinyl acetate is often used as a base polymer for production of technical films. EVA films have lower prime cost as compared to alternatives, showing at that in the most cases the required physical and mechanical properties. Therefore the films of ethylene vinyl acetate are widely applied in production of silicon solar cells as an encapsulant covering.

Until recently there existed restrictions in production of EVA films: maximum width - 1000 mm, shrinkage factor - up to 45 percent (whereas in extreme cases even up to 65 percent). Besides, there was a problem to return back to production process the trimmed edges of extruded film.

These problems were resolved by the specialists of KraussMaffei Berstorff GmbH while developing the new technology, claimed for a patent.

The new manufacturing technology of EVA film allows to produce films of up to 3200 mm width, with no limitations on trimmed edges’ processing (in-line-recycling) and without any loss in product quality. The standard capacity of extrusion lines by innovative technology is up to 1200 kg/hr.

Moreover, produced via new technology EVA films feature drastically low shrinkage – is has been reduced by up to 45 percent and is now lower than 5 percent. New films – even when the trimmed edge strips are fed directly back into production – show high transmission, being (along with shrinkage) the most important quality criteria for manufacturers of photovoltaic modules. Inline recycling of extruded film’s trimmings reduces both production cost and waste. The latter improves environmental aspects of such productions.

To demonstrate recyclability of innovative EVA films, extensive studies were conducted at the Fraunhofer Center for Silicon Photovoltaics (CSP) in Halle, dealing with development of photovoltaic technologies on the basis of silicon. These studies clearly showed that there was no loss of film quality, even upon repeated re-processing in extruder.

Such result originates mostly due to twin-screw extruder of the ZE UTX series, which homogenizes the raw material gently but thoroughly. The twin-screw extruder offers a further economic advantage by virtue of its ability to process individual components (say, in powdery state) instead of expensive masterbatches. This gives manufacturer more control over his choice of recipe and allows him to respond flexibly to different needs.

To obtain a homogeneous melt when production waste is used and also to prevent the peroxide cross-linking of extrudate, a special screw geometry was developed.

At present manufacturers of photovoltaic modules have began more actively use EVA films with double-sided embossing. The matter is that to prevent agglutination of extruded EVA film at a
winding stage, its layers are spaced with a paper tape. Double-sided embossing reduces the contact area and, as a consequence, the adhesive properties of the film which in turn allows winding without spacers.

Positioning of embossed film while assembling photovoltaic modules is carried out much faster than when using a smooth film. Besides, EVA film embossing simplifies deaeration of assembled photovoltaic module in a vacuum chamber.

The new technology of Krauss-Maffei Berstorff allows to fulfill, if necessary, double-sided embossing by specified template.

The important features of protective film in production of photovoltaic modules are the minimal shrinkage factor and high light transmission.

EVA films manufactured on the company’s extruder installation in Hanover in different production conditions, were subjected to relevant reflection and transmission test series in the above mentioned Frauenhofer CSP center.

The following samples of EVA films were tested:
 - film by formulation based on primary EVA - virgin EVA, silanes, peroxides and other additives (sample 0);
 - film by formulation as above, containing returned to production shredded trimmed edges in a weight fraction of 20 percent (sample A20);
 - film by formulation based on secondary EVA (sample RG3 – the material used as raw stock has undergone regranulation three times).

Examined samples of EVA films were laminated between two glass plates having thickness of 2.9 mm each. All tests on evaluation of light reflection/transmission factor were carried out in comparison with a reference specimen from glass of appropriate thickness.

The obtained results have proven the following:
 - all the three samples, prepared under very different recipes, have virtually identical profiles of light transmission; that’s a fact which proves that the EVA sheets are recyclable;
 - in the spectral range from 420 to 1100 nm the average light transmission coefficient is over 99 percent for all the three specimens; slightly attenuated transmission as compared with that of the pure glass, stems from reflection at the two glass/sheet interfaces.

Nowadays manufacturers of photovoltaic modules, facing the necessity to consider high shrinkage factor of conventional EVA sheets, are forced to seek for workaround solutions – like overdimensioning of encapsulant sheet and adjustment of laminating parameters in production process. Despite these efforts, high shrinkage can still give rise to quality problems - changes in molecular chain orientation in the sheet during shrinkage can lead to microcracks in the solar cells, and voids or defects in the sheet and thus to inferior quality, reduced efficiency and a shortened life span.

To determine the shrinkage factor of EVA films produced on the new extrusion line of KraussMaffei Berstorff, the two standard techniques were used. The results obtained show that the factor’s value in different temperature modes is only 2-5 percent (opposed to conventional values of up to 45 percent).

The presented analysis unambiguously shows economic and technological advantages of the new technology by KraussMaffei Berstorff, and lends itself admirably to production of EVA sheets – it concerns both the product of the new commercial formula and its recycling. KraussMaffei Berstorff also sees a potential market in laminated safety glass, which is commonly used in motor vehicles or building construction, where it could replace standard polyvinyl butyral (PVB) sheet – EVA films are more cost-effective and easier in production.

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