Tapes used in bonded glazed windows
- The market for bonded glazed windows is growing in Germany, Austria and Switzerland
- Static, dynamic and thermic loads must be considered when designing and producing windows
- There are a variety of technical reasons for the beneficial use of tape in making bonded glazed windows, including large size possibilities, necessary added stiffness, lack of need of adjustment, reliable service life, energy efficiency, noise reduction and resistance class (RC2)
- Economical reasons for using tapes in windows include no steel in the product, lower manufacturing costs, more automatisation, and the possibility of more ‘special products’
- As a certifying body, RAL-Gütegemeinschaft Kunststoff-Fensterprofilsysteme e.V. (GKFP) developed the standard RAL-GZ 716, part 2, which describes how bonded glazing must be designed and tested by the profile system manufacturer in order to be sold to the window manufacturer with a quality mark. The aim is to ensure the lasting fitness for use of the manufactured products
- This standard has been adopted in Germany, Austria and Switzerland
- GKFP-certified companies include systems manufacturers, i.e., manufacturers of plastic window profiles and systems, as well as manufacturers of seals, raw materials, film, bonding systems and machinery, and contract laminating companies.
6th article in Afera’s Sicily conference presentation series
In his presentation “The Outlook for the Building/Construction Industry in Conjunction with Adhesive Solutions: Tapes and Windows,” Bernard Elias, who is responsible for Quality Assurance at RAL-Gütegemeinschaft Kunststoff-Fensterprofilsysteme e.V. (GKFP), educated Afera’s audience on the uses, benefits and requirements of tapes in the design and construction of bonded glazed windows.
The bottom line: In the growing market of bonded glazed windows in Germany, Austria, Switzerland and beyond, engineers and manufacturers, as well as consumers, are taking notice of the many technical and economical reasons for using bonded glazing technology, in which tape can be used to fix the glass into the sash. As a certifying body, GKFP awards a quality mark to member companies which adhere to the quality and test requirements of RAL-GZ 716, part 2, the standard the association has developed.
The market for bonded glazing
In Europe and Russia, 120 million windows are installed each year in both new construction and renovations. This includes windows made of plastic (PVC), wood, aluminium and a combination of these three materials. In the DACH (Germany, Austria and Switzerland) market, 2.4 million windows were sold in 2011. In Austria, 70% of all windows had bonded glazing, while Switzerland had 40% and Germany, 6%. That means Austria had 1.5 million windows with bonded glazing, while Germany, which is much larger than Austria, only had 150,000. The number of instalments has picked up in Germany since 2011 however.
How to use tape in windows
Bonded glazing in a window consists of a plastic window in which the tape is used to fix the glass into the sash. The window has both outside and inside glazing on double- or triple-glazed glass.
Static load is becoming a bigger topic, because the trend is moving from double-glazed insulated glass to triple-glazed. Furthermore, architects and builders are enlarging windows to create more light, so the elements are growing heavier. These two factors contribute to greater static loads.
In Austria and Germany, you will find a lot of tilt and turn windows and few sliding doors and windows, which if opened and closed, carry greater dynamic loads. This must be taken into account when considering the design and instalment of larger windows.
How long will a plastic window last? Mr. Elias explained that for certification purposes the tested sample windows must withstand an open/close cycle of 10,000 repetitions. Following the open/close test, the window must be perfect for the approval test. Even this test probably does not cover the life cycle of a window, which should be several decades. All tests are geared towards permanent fitness for use, the basic idea of RAL quality assurance.
In terms of thermic load, outside temperature changes range from cold to warm and may differ greatly from inside temperatures. Thus there is a great thermic load on the complete structure, which is hard on the plastic window. This is where tapes can add benefit.
Why use tapes in windows?
There are a variety of technical reasons for the beneficial use of tape in making bonded glazed windows:
- Large sizes: Mr. Elias used the example of a maximum size window element of 2.3 metres high x 1.8 metres wide that was made and tested by Gelean, which uses tapes in their bonded glazing system STV. The element can take a great amount of weight on the sash.
- Needed stiffness: A tilt and turn window which hangs when opened should be avoided. Bonded glazing prevents this by creating a really stiff window when the glass is fixed inside of the sash.
- No need for adjustments: In the old-fashioned method of putting the glass into the sash with shims, the whole structure moves. The more often you open and close it, the more it hangs. Bonded glazing can prevent this.
- Reliable service life: You have a secure open/close during the entire service life.
- Energy efficiency: Plastic windows customarily have steel reinforcement, which is a good conductor of heat. This affects your insulating value negatively. If you omit steel in your window design and apply the necessary stiffness through the glass with bonded glazing, you will raise your energy efficiency.
- Noise reduction: This is achieved through the use of stiff glass.
- RC2: You can easily achieve resistance class 2, even on a PVC window, when you have bonded glazing, because you cannot crack the window due to its stiffness.
- No steel: If you take steel out of your window production, you can simplify your supply chain by cutting out one supplier.
- Cut manufacturing costs: Omitting steel from your production means that you can cut costs.
- More automatisation: Tapes and solvent glues can be applied by robots in window production. Gelean, the company mentioned above, applies their tape right after extrusion of the profile which is cut into 6-metre lengths. The window manufacturer wheels in the windows, takes off the protective film and fixes the glass.
- More ‘special products’: As mentioned above, architects like having large windows to create light in spaces. Designs such as dark frames are possible, because bonded glazing can prevent customary problems such as deformation due to heat build-up. All sorts of unusual designs are feasible.
Standards in the field
In 2006-2007, GKFP found bonded glazing on the market, but noticed that there wasn’t much standardisation. As a quality association, GKFP got its member companies which design plastic window systems or supply to them, to sit around the table and agree on a standard. RAL-GZ 716, part 2 was drafted. (Part 1 applies to conventional glazing.)
RAL-GZ 716, part 2 is a standard which describes how bonded glazing has to be designed and tested by the profile system manufacturer in order to be sold to the window manufacturer with a quality mark. The standard does not cover window production but all the steps leading up to it, such as design and design stress. The standard, which has been revised already, is acknowledged in the DACH region and has been adopted in Germany, Austria and Switzerland. Mr. Elias added that this was a good sign.
Some important elements of the standard:
- Clause 3: Fitness for purpose of the adhesive system: Those who drafted the standard drew ideas, such as the definition of test specimens, from European Technical Approvals Guideline (ETAG) 002, which is applicable for glass facades and silicon adhesives.
Clause 3 lists all possible bonding positions, including on the insulating glass edge and the glass itself. It also defines how to check if the adhesive on the tape is fit for purpose. A peel test is always involved. Depending on where you use your glue, however, you may have to gauge tensile strength, compression strength, and shear strength. All of these are included in a table which indicates the testing requirements to be fulfilled.
Regarding testing, samples are taken, and according to where they are used, i.e., the position at which the bonding takes place, they are aged. They are stored in the environment in which they would be used, exposed to such elements as sun, water and cleaning detergents. They are then checked for the strength of their bonding. Although no limits are delineated by the standard, designers receive all this information. They then must check that the properties that have been achieved meet the design stress figures required for their products.
- Clause 4: Compatibility of components: When using adhesives in a window, the interaction of various chemical elements must always be checked. This includes components that are in direct or indirect contact, such as tape; insulating glass, which has a primary and secondary seal; and gaskets which contain EPDM, thermoplastic elastomers or plasticised PVC. This clause delineates how, for example, you take your tape, pack your other component on the side, store it for 42 days at 70⁰, and then perform a peel test. It should be noted that in this area no legal values are required by the standard either, but the designers of the window systems must approve the results. The standard also requires that the manufacturers of the tapes and other elements verify that they are satisfied with the results of the testing performed on their products.
- Clause 5: The standard requires bonded glazed window elements to be built at maximum size according to the system description of the profile system designer. For example, a maximum size window must be constructed and tested for water and air tightness and wind pressure before the product type is certified.
Companies with a quality mark
Mr. Elias said that RAL-GZ 716, part 2 has been in place for about three years. GKFP has about 22 manufacturers which are certified for their bonded glazing systems, some of which incorporate tapes. These companies have all received a quality mark for plastic window profile systems and are listed on the association’s website.
Inspire the next generation with tapes
As many academic and vocational institutions do not teach the use of tapes in the building of windows or other engineered components, Mr. Elias suggested to those present to reach out to the systems in their countries by supplying them with information about working with tape and its capabilities, as well as samples. Students who are inspired by this material will take the knowledge and ideas with them into the workforce.
About Bernhard Elias
Bernard Elias is responsible for Quality Assurance at RAL-Gütegemeinschaft Kunststoff-Fensterprofilsysteme e.V. (GKFP), where he has coached several working groups which established the RAL-quality and testing requirements for plastic window profile systems. Mr. Elias formerly worked in a test and research institute for plastics engaged in the testing of pipes and piping systems. As a testing expert, he was delegated to national and international standardisation working groups. In Germany, he was appointed as auditor to the RAL quality association for plastic pipes. Mr. Elias obtained a degree in engineering for plastics technology in Vienna, Austria.
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