As the name suggests, laminated glass is composed of multiple layers of glass with an interlayer in between to hold the panes together. With a vast variety of different interlayers available on the market today, how can you make the right choice?
We get often asked what the difference between laminating PVB and EVA foils is – and it’s an excellent question because deciding between PVB and EVA can also significantly affect the type of production equipment that you need.
Before we compare the main characteristics of the foils, it’s crucial to understand how adhesion level between the foil and glass affects safety performance of the laminated glass.
At first it might sound that it’s best to have as strong bond between the glass and interlayer as possible, as that will cause the laminate to stick together very strongly. While it’s true that high adhesion level helps to reduce the risk of delamination over time, for example in open edge applications where water is present, the tradeoff is worse impact resistance which means worse safety performance.
Lower adhesion will allow the laminate to absorb more energy while still staying intact, so adhesion level is directly related to safety of the glass. The effect of adhesion level to impact resistance and resistance to delamination is illustrated in the chart below. Low adhesion level results in good impact resistance performance while high adhesion results in good open edge performance. In practice, most often the most important thing is to find correct balance between impact resistance and adhesion level for each application.
Note that the comparison here is related to foils and their behavior: safety of the final laminate is also related to the sandwich composition (glass thicknesses, whether glasses are tempered or not, foil thickness used, amount of foils used, etc.). Being able to evaluate what kind of characteristics you need for different applications will help you reach optimal performance with the final product.
Different types of foils have different kinds of properties when it comes to adhesion level achieved during processing. Standards define the level of safety performance required for each use case from the final laminate. Consequently, standards do not normally specify which kind of foil you actually need to use as long as the end product fulfills safety requirements set for the application. In the end it’s up to the designers and glass processors to agree what kind of product should be used.
Invented over 90 years ago, PVB (polyvinyl butyral) has been the main laminating glass material and it is still by far the most used foil in the market. During the years there has been a lot of development done on PVB foils, making PVB a valid choice for almost any kind of application.
PVB characteristics include:
The wide variety of different kinds of PVB makes it a valid choice for most applications, while being easy to produce and handle. While normal PVB is a good interlayer to use in most applications, it’s good to acknowledge that it’s possible to further improve performance in different applications by selecting the correct PVB type to be used.
The main downside of using normal PVB is that it has poor water resistance. It delaminates easily if exposed to humid environments for a long time. This means that normal PVB is not the best choice for open edge applications. For open edge applications make sure to use either stiff or high adhesion PVB that has better water resistance or other interlayer types.
The storage and processing conditions for PVB interlayers are somewhat demanding – however storage is easier with interleaved PVB that is the most used foil type in the architectural market nowadays. When PVB is being stored, make sure that the storage area is temperature and humidity controlled. With interleaved PVB the temperature should be kept around 18–22 °C, and humidity maintained at around 25% (check optimal storage conditions from your supplier).
Proper storage of PVB is extremely important to guarantee that correct adhesion level between the glass and foil is achieved. There are also different types of PVB designed to develop different levels of adhesion to provide optimal performance in different use cases. Low adhesion level can help in achieving better impact resistance while high adhesion can help with open edge applications.
EVA (ethylene-vinyl acetate) is a rather new interlayer type compared to PVB. Nowadays EVA’s market share in the architectural market is around 5%. The chemical composition of EVA allows the internal molecules to generate very strong bonds, ensuring better performance in special conditions, such as high temperature or humidity.
EVA characteristics include:
EVA is typically used in interior glass and decorative applications and it can also be used in some open edge applications. High adhesion level of this interlayer type increases end-product resistance to delamination and also its durability. Due to high adhesion level in low temperatures, EVA is often used in applications where materials (such as pictures, leaves, etc.) are embedded in the laminate for decorative purposes.
The downside of high adhesiveness of EVA is that it leads to poor impact resistance performance, thus normal EVA does not often fulfill the requirements of laminated safety glass. Consequently, EVA interlayers might not be the best option if they need to support fractured glass for a long time. Therefore, it is not the most suitable choice for applications with strict safety requirements and structural applications. If EVA is used in applications where there are strict safety standards in place, make sure to verify the performance of the selected sandwich composition.
Note, too, that EVA interlayers are typically used with vacuum bagging equipment. The upside is that a vacuum bagging system represents a smaller initial investment. As a downside, it provides very limited capacity compared to nip-roll lines. EVA can also be tricky to process, as it tends to flow out of the laminate during heating. Before you choose, it is important to weigh all the pros and cons in your particular case.
It is also important to remember that not all EVA or PVB foils are the same. Various products with slightly different properties might be listed under these acronyms. So, be sure to look at the manufacturers’ datasheets before selecting the right one for you. Also remember that in the end actual performance of the end product is what matters – so whichever foil type you use, make sure to test that local safety requirements are met.
The main characteristics and differences between PVB and EVA interlayers are shown in the table below.
For the complete table and a detailed description of the structural interlayers, such as SentryGlas®, please check out the Laminating Line Buyer’s Guide.
Keep in mind that laminated glass is a complex product. Its mechanical properties may change considerably depending on the type of interlayer applied.
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