"Window manufacturers also need to think about anisotropies"

05/31/2022 | Print view

Anisotropies are increasingly being criticised by planners and building owners, especially in commercial construction. - © LiteSentry — © LiteSentry

Anisotropies are increasingly being criticised by planners and building owners, especially in commercial construction.

The demands on facade glass in terms of safety and optical quality are increasing. Glass expert Hannes Spiß is a specialist for functional glass, especially for facade glass and large-format glass applications. In this interview, he explains why such glass is often made of safety glass, what this means in terms of optical qualities and how this also affects (upmarket) residential construction.

GW-News: Safety glass is increasingly being used in facades throughout Europe, why is that?

Hannes Spiß: The demand and application of safety glass has grown for several reasons. In addition, the normative requirements have also increased in recent years. Large-format glass panes in particular very often require the use of safety glass to reduce the potential risk of injury in the event of glass breakage. In the case of large panes, it may also be a static necessity to manufacture them from laminated safety glass with a higher rigidity. There is also a trend towards replacing thermally toughened glass (ESG/TVG) with laminated safety glass (LSG) made of float glass. This eliminates local distortions, so-called "roller waves", which can occur with tempered glass.

GW-News: What glass qualities are required for toughened glass?

Spiß: Expectations of optical, visual quality have risen sharply in recent years for all types of glass. The problem of "roller waves" has lessened recently due to improved furnace technology. However, what is becoming more and more the focus of building owners and contractors are anisotropies. These are patterns in thermally toughened glass that are particularly visible in polarised light, i.e. on clear days with a lot of sun. Here, too, it is possible today to manufacture ESG virtually anisotropy-free thanks to optimised production plants and processes.

GW-News: Do these requirements also apply to windows for residential buildings?

Spiß: Of course. Due to the increased quality awareness, these requirements are also increasingly finding their way into private residential construction. If large-format glass panes are required or installed there, they are often made of thermally toughened glass in order to meet the static requirements. However, this also increases the risk of the aforementioned optical impairments.

GW-News: When we talk about large-format glass, what other building elements are affected by anisotropies?

Spiß: In residential construction, for example, this concerns glass for large-format (living room) windows and especially also for sliding doors to balconies or terraces. These panes are often made of toughened safety glass and are larger and thicker in structure than standard window glass.

GW-News: So thicker glass is more likely to be affected?

Spiß: Exactly. Anisotropies and their intensity are directly related to the thickness of the glass. The thicker the ESG, the higher the risk of visible anisotropies. This in turn suggests that small-format windows with low glass thicknesses are less affected.

GW-News: How can such anisotropies be avoided?

Spiß: Anisotropies can be kept so low that they are not visually noticeable by optimising machine control and using modern production technology. Today there are measuring techniques and methods to determine anisotropies during production. These results can then be used to optimise the furnace control and improve the associated product quality.

The StrainScanner, which was jointly developed by arcon and ilis to detect anisotropies, makes it possible to detect them during production and readjust the ESG furnace if necessary.

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GW-News: What do the regulations say about anisotropies?

Spiß: The current standard situation says that anisotropies are production-related. However, this only partially corresponds to the current state of the art. Especially with regard to anisotropies, there are no European, normative sets of rules. However, there are currently several working groups with members from the glass industry as well as from science. The US has convened a normative working group and one can expect a draft standard by 2020. Regardless of this, there are already some tender documents from international architects and engineers that set higher requirements for anisotropy-free glass.

GW-News: What advice do you give to window and facade manufacturers in consultations with customers, what offers should they make?

Spiß: Anisotropies are often no longer tolerated and criticised by building owners, especially in commercial construction. The fabricators will have to face up to this. In addition, the demands on glass for facades and windows will continue to increase. I therefore recommend every window and facade manufacturer to deal intensively with such future topics, including anisotropies.

The interview was conducted by Matthias Rehberger