Engineering innovative multi-layer films for laminated glass

© FraunhoferIMWS
Bonding of plastics to glass plates is also analyzed by means of falling ball tests.

Laminated glass, in which two glass plates are connected by means of plastic, provide unique opportunities for applications in the building trade and other industries. Use of traditional materials may be limited by demanding environmental conditions. Together with partners, the Fraunhofer Center for Silicon-Photovoltaics (CSP) intends to engineer innovative, reliable multi-layer films for applications like these. Intentional leveraging of optical, thermal, and mechanical properties makes them particularly lightweight and able to withstand extremely low temperatures.

The largest glass facades in the Apple headquarter in Cupertino/ California are 16 meters high. 174.000 m² of mirrored plate glass were consumed in Burj Khalifa in Dubai for the biggest building on earth. Spectacular architectural designs like these are made possible by advanced laminated glass. In this technology, two glass plates are bonded adhesively. Materials commonly used for these high-performance purposes are, for instance, films made of plastics, such as polyvinyl butyral (PVB).

In a joint project scheduled up to the end of 2021, the project partners – the film producer Folienwerk Wolfen GmbH, the Fraunhofer Center for Silicon-Photovoltaics (CSP), and the Institute for Mechanics of the Otto-von-Guericke-University (OvGU) Magdeburg – intend to develop a new material for applications like these. They rely on films based on silicones or ethylene vinyl acetate-co-polymers (EVA). Although these composite films have hitherto mostly been employed as encapsulation material for solar cell modules in photovoltaics, they also offer some advantages for applications in architecture and buildings/ construction compared to PVB, as in low-temperature applications, for example; they are also less sensitive to humidity.

Innovative encapsulation and plastic multi-layer films connecting glass plates using EVA are reliable and suitable for current trends in architecture that tend toward lighter and thinner glass plates. The materials must increasingly withstand very challenging environmental conditions, such as high UV radiation, extreme temperatures, frequent storms, heavy snow, special static requirements in regions with frequent earthquakes or full-on and spray contact with both fresh- and saltwater.

»To meet these requirements, we want to use and further adapt the advantages of the EVA materials to the special needs of building with glass. This demands comprehensive research and development of the optical, thermal and mechanical properties that we want to combine with the development of simulation tools to predict the material characteristics of laminated glass used under a wide range of loads and stresses«, says Ringo Köpge, project leader at the Fraunhofer CSP.

The project partners are concentrating on three cases of application. First, they focus on reducing weight, thereby relying on a fundamental benefit of using laminates: employing a suitable composite film, it is possible to significantly improve the properties of laminated glass in comparison with monolithic glass plates of the same thickness, without losing the safety and surface properties of the glass. The second application is aimed at finding and enhancing the resilience of the laminates to exposure to alternating temperatures, hail, dew, or ice. Anticipated measures to do this are, for instance, the suitable selection of film material and thickness, the use of additives, or the combination of EVA with other plastics.

The third range of application deals with disadvantages the safety glass currently made of laminated glass bonded by EVA layers may still have: it is not always completely transparent and may have a residual haze. The project intends to determine the reasons for this effect under various environmental conditions, using spectroscopic, thermal and materials science methods in the micro- and nanometer range. These findings will be used to define opportunities to influence and eliminate this phenomenon. The Fraunhofer CSP with its many years of experience in the fields of analytics and characterization of polymer materials, the design of technological processes to manufacture laminates, and the simulation of composite systems, is well prepared to find solutions to these problems. It is also possible to make use of findings obtained in a previous project, in which innovative encapsulation materials for use in extreme climate zones have already been researched.

First, the project partners will define a set of requirements to be fulfilled by innovative encapsulation and laminated glass bonded by polymer films and, based on this, will develop solutions on a laboratory scale, which, in turn, will be transferred to the pilot plant and industrial scale. Characterization, accelerated ageing tests, and the transfer to suitable simulation and predictive models will be performed for the material combinations used at the beginning of the development, the films, laminates and the demonstrators envisaged at the end. This strategy makes it possible to predict the long-tem characteristics and allows for technology transfer to other materials.

Various questions have to be answered: How do cracks occur? How does damage propagate? How do the EVA laminated glasses react to saltwater or temperatures equal to or less than -60 °C? »We will investigate the material’s performance – from deformation characteristics to strength evaluation – in detailed as we analyze the processing characteristics and the process parameters during production, such as viscosity and curing characteristics of films or the melting point pressure and the temperature profile of the melt. Only in this way can we succeed in engineering innovative laminated safety glass with intentional characteristics«, says Köpge in describing the span of the project’s focus.

The film producing company Folienwerk Wolfen has already used the EVA multi-layer film evguard® in manufacturing laminated glass that is used for collapse-proof or burglarproof laminated safety glass, façades, photovoltaic elements integrated into buildings, or as decoration elements or components for vehicle manufacturing. The Institute for Mechanics of the OvGU will carry out in-depth theoretical and numerical investigations taking into account the experimental data obtained in the project to be able to simulate even complex damaging processes, such as crack initiation, crack propagation, crack pattern, and de-lamination, and to develop a software tool for material characteristics’ prediction from these results.

Finally, the project aims to define suitable criteria for standardization and test instructions. The calculation models and standards currently available are frequently characterized by national requirements and are not able to completely address special climatic load cases. »Here we want to find approaches to solving problems and thus also simplify material development as a whole«, says Köpge.