Traditional insulating glass edge seals
Traditionally constructed IGUs consist of two or more glass panes and one or more rectangular hollow spacer tubes bent to fit the final insulating glass unit, then filled with desiccant and applied with poly-isobutylene sealant (also named primary sealant, PIB) on the sides of the spacer bar. This is then applied to the glass by positioning one sheet of glass on an IG assembly line before being conveyed to the platen press where it is then coupled with the other glass sheet. The glass cavity is ...
Traditional insulating glass edge seals
Traditionally constructed IGUs consist of two or more glass panes and one or more rectangular hollow spacer tubes bent to fit the final insulating glass unit, then filled with desiccant and applied with poly-isobutylene sealant (also named primary sealant, PIB) on the sides of the spacer bar. This is then applied to the glass by positioning one sheet of glass on an IG assembly line before being conveyed to the platen press where it is then coupled with the other glass sheet. The glass cavity is usually filled with argon gas in the platen press to enhance thermal performance. After gas filling and pressing, the insulating glass unit is then sealed with a typically elastic, chemically curing sealant such as polyurethane, polysulfide or silicone.
The trouble with triples
Despite the rise in popularity of triple glazing, the traditional IGU production line is not really set up to manufacture triple IGUs effectively. One of the most obvious hurdles is the addition of process. There is 50 per cent more glass to toughen and cut, and if you are using a box spacer there will be twice as much of that to cut, PIB and add desiccant to. All of these additional processes slow down production and, as a result, cost more money.
The German Federal Association for Architectural Glazing gives as a guideline a tolerance of the following: “The permissible deviation of the spacer(s) in relation to the parallel straight glass edge or to other spacers (e.g. in three-layer insulating glass) is 4 millimetres up to an edge length of 2.5 metres. For longer edge lengths the permissible deviation is 6 millimetres.”
Rigid spacer bars only adhere to the sealant once it is cured, so the PIB acts as the mounting help for the spacer during IGU construction. The PIB is thus effectively the only ‘structural’ bonding element until the sealant has cured which can result in slippage or sheer movement of the centre pane during production. The quality and durability of an insulating glass unit with a rigid spacer bar is mainly dependent on the amount and evenness of the PIB applied as the primary seal onto the sides of the spacer bar. There is also a potential issue of PIB distortion if it comes into contact with operatives’ fingers during assembly, or when it can be squeezed out by the barometric and thermal movement of IGU and can bleed into the sightline during its service life.
Insulating glass edge seals with flexible spacer systems
A typical IGU manufactured using flexible rubber spacer systems is different from an IGU manufactured with rigid spacers. It is not only the production process that is different but also the edge seal bond line construction. The bond line adhesion to the glass in edge seals with rubber foam spacers typically have a pressure sensitive adhesive (PSA) towards the cavity of the insulating glass. Encapsulated in a pocket in the foam spacer, the PIB, which just serves as the moisture and gas barrier, is trapped between the PSA towards the inside of the insulating glass and the sealant towards the outside of the unit. As a result, the PIB can become as hot as the unit may become due to sun radiation, but can go nowhere because it is essentially trapped. As the spacer system is a product for architectural application, it has to be made from silicone rubber to make it UV-stable. The PSA level, the PIB and the outer seal give the edge seal system a three-level adhesive bond line, which, as well as being flexible, give the unit structural strength right from the start of construction. The cured silicone sealant then gives the unit excellent service life expectation.
The robotic application of the rubber foam spacer is also the key to very high precision of spacer application, especially obvious when triple IGs are built and the two spacers lie parallel to each other. The same precision in parallelity is difficult to achieve with bent rigid box spacers. This bond line construction and the manufacturing process also enable the glass to behave differently in the barometric, temperature and seasonal pressure changes. Because the spacer is practically elastic, with the elastic polymer secondary seal to the outside, the complete edge seal of the IG becomes elastic, although spacer bonding is ensured by the PSA. This makes any pressure changes to the IGU easier to bear due to lower stress, as the spacer does not act like a pivot in the same way as rigid spacers do.
Thermoplastic spacer systems
Much of what I am saying here about flexible rubber spacers also applies to flexible, thermoplastic spacers. However, there are some significant differences.
The bond line construction for the edgeseal with thermoplastic spacers is just two levels – the thermoplastic spacer and the outer seal. In many cases, there is no barrier layer between the spacer and the sealant, therefore, when using this system, you need to make sure that there is sufficient desiccant capacity to dry out the interpane cavity and to keep it dry. The outer seal must also be compatible to the spacer mastic and must seal the insulating glass flawlessly. The thermoplastic nature of the spacer also has some effects on temperature stability. Whereas cured rubber spacers remain in the same structural condition over a wide temperature bandwidth, thermoplastic products tend to become rigid in low temperatures and softer in high temperatures. Moreover, thermoplastics do not have the same memory effect as rubber products. This means they may deform after compression, whereas rubber products always tend to go back into their old shape.
Flexible rubber foam spacers
Processing flat pane glass to insulating glass
The production of insulating glass with a flexible rubber spacer system is streamlined compared to rigid box spacers. The extra tasks of preparing the spacer profile in a frame, filling it with desiccant and applying PIB are eradicated. The only additional step with flexible rubber foam profiles is the application of the PIB, which is performed simultaneously with the positioning of the spacer to the glass. In this way there is no possibility to jeopardize, deform or otherwise damage the PIB beads. The additional benefits are higher efficiencies compared with rigid bar manufacturer as less manpower requirements through state of the art automation.
Processing bent or curved glass to insulating glass
The flexible, but strongly bonding nature of rubber foam spacers and the pre-applied PIB bead as primary seal, enable rubber foam spacers to be exceedingly suitable for architecturally bent and curved glass applications. There are special rubber foam spacer products available that have an extended PSA and butyl bond line. This is to accommodate for the bending differences in parallelism of the glass panes, the higher stiffness and the heavier weight of the glasses.
Practically every shape that is possible to be bent in a certain parallelism in glass can be assembled using flexible rubber spacers, which also offer great aesthetics and durability.
Climatic loads and coupling effect, especially for curved insulating glass
Climatic loads in flat plane glass have been investigated in the past and software is now available to simulate these loads in FEM under different surrounding parameters.
With insulating glass constructed with bent or curved glass, these loads have a different effect to the insulating glass, as the stiffness of the glass is changed by the radius of the bending. Furthermore, it becomes even more complicated if the glass is not just bent cylindrically but also in the third dimension (spherical). As typical FEM software is not able to relate to these differences, it has to be proven in the individual cases that the edge seal is suitable (thick enough) for the structural and climatic loads. Current tests looking into how the stiffness of the edge seal changes the stress on it indicate that a flexible edge seal may bring benefits over rigid spacer edge seals.
The application of thermally bent glass in architectural applications has also been described in a German Flat Glass Association guideline, and contains regulatory and legal boundaries, building physics, tolerances, dimensioning of bent glass, and glazing. The aim is to start to evaluate calculation bases so as to create a means to calculate IGU construction for bent insulating glass that has common acceptance throughout the industry.