In the 1960s, the automotive industry started installing windshields as a supporting construction part of the vehicle’s mechanics by utilizing adhesives. To conduct this procedure, special and highly viscous 1-C polyurethane adhesives were developed. As a result, they improved and accelerated the production process on the assembly line significantly and greatly contributed to automation.

Although the adhesive class of polyurethanes counts among the old adhesive solutions (after all, it dates back to Otto Bayer in 1937), it still required particular formulations and manufacturing to meet the high production line demands of automobile manufacturers to this day.

In the late 1960s, the development of a sealing system for insulating glass began, consisting of a primary sealing that was based on polyisobutylene, as well as polysulfide or silicone, which acted as a secondary sealing. Thanks to this 2-stage system technology, it was possible to design a fully automatic insulating glass production for the first time: From cutting the glass to the washing process, frame placement, gas filling, to connecting to insulating glass, with the so-called sealant plus secondary sealing depicting the final production step.

In the mid-80s, Dr. M.V-R – founder and CEO of IGK insulating glass adhesives – had the idea of applying the positive experiences with polyurethanes from the automotive industry to that of insulating glass. During both developments, the focus had always been on glass and the respective adhesion structure of the sealants used. Thus, he recognized the potential to start developing polyurethane sealants there. Back then, the insulating glass market accounted for about 90% of polysulfide, while silicone, hot-melt, and polymercaptan polymers were subdivided into the remaining market shares.

In 1988, he founded IGK-GmbH, which is currently located in Hasselroth in the greater Frankfurt area, convinced of the concept of using 2-Cpolyurethane sealants to develop better and cheaper compounds for insulating glass that were more efficient than its predecessors. Because the formulations and application technology of these industries differ significantly, it was crucial to conduct extensive development efforts, since not much from the adhesive technology of the automotive industry could be transferred.

Thus, the laboratory was and remains to be the core, even to this day. Here, numerous raw materials such as polymers and catalysts were tested with sustainability and resolve in order to find the famous “needle in a haystack”. Furthermore, the various side reactions in polyurethane chemistry needed to be controlled, while ensuring the desired reaction process, both in IGK’s own manufacturing as well as that of the customer. Additionally, the synergy of application-specific and final properties in the insulating glass industry represents a distinct challenge. It is required to “pack” the longest possible pot life into a formulation with quick curing. Those requirements are supposed to lead to good sealing properties concerning gas retention as well as moisture inlet and resistance to UV radiation.

That these high standards were met can be explained by the fact that IGK was enhanced, rebuilt, and automated several times. Due to these measures, IGK has established itself as the market leader for polyurethane sealants in the insulating glass industry while remaining to be active in many European countries. In the past two years, IGK internationalized sales further, which led to new branches in the USA and Russia.

In retrospect, it is no exaggeration when one says that the sealant market for insulating glass in Europe has changed by approximately 75% towards polyurethane – mostly due to the visionary idea of Dr. M. Vollrath-Rödiger!

Dr. R. Karrer 28.12.20

In the late 1960s/early 1970s, polysulfide sealants dominated the European industry from the get-go once the 2-stage sealing system for insulating glass was developed. The fundamental chemistry for this is over 180 years old by now, while the manufacturable elastomers are approximately half as old. Nowadays, 2-C polysulfide sealants are mostly utilized for insulating glass. However, due to their lack of chemical reactivity, they are also used as sealing masses in aircraft and automotive construction as well as for “water” proofing around gas stations to prevent petrol from seeping in.

Their immediate and early triumph in insulating glass has two reasons: On the one hand, the industry (and especially American raw material producers) was in search of new applications after this chemistry was replaced as missile fuel in the mid-1950s. And on the other hand, an application was found that offered the insulating glass manufacturer not only an easily applicable sealant but also provided a broad tolerance range for mixing the particular A and B components. In the late 1990s, polysulfide sealants dominated the insulating glass industry with a market share in Europe of more than 90%. From the 1980s to the 2000s, this market was sustained and controlled by 4 raw material manufacturers from the USA, Japan, Germany, and Russia. Today, only 2 are left and still deliver significant amounts to the insulating glass sealant industry.

That reflects on the development and use of the corresponding insulating glass sealants throughout recent years in Europe. As a result, the market share of polysulfide sealants has decreased noticeably in favor of polyurethane sealants, based on the sealings for windows, such as behind UV-protective frames, in Europe. The market share of silicones is primarily limited to the open and unprotected edge, which is mostly part of glass facade constructions. However, when gas-tightness plays a minor part, silicones are occasionally (or in certain countries) used as secondary sealants in IG units for windows.

In the USA, polysulfides – despite their prior widespread use – have been largely replaced by (reactive) hot-melts, whose operators appreciate their swift and easy use. First of all, decisive for the respective market penetration are the respective national standards and laws, along with offers of the machine and sealant manufacturers. Then, the price/performance ratio decides either for a sealant or against it. For many years, developers have been quite restricted since polysulfide polymers are expensive to manufacture in comparison and only have very few options in terms of variants for the insulating glass market. In contrast to polyurethane, polysulfide sealants have another massive disadvantage. This sealant’s UV/water resistance can only be achieved with immense efforts and high costs while the new standards increasingly require these properties.

In Russia, the replacement of polysulfide sealants has manifested as well – albeit through other very economical mercaptan groups (polymercaptan polymers or PUSH polymers). Here, a polyurethane backbone receives SH groups, which results in appropriate sealants. This chemistry is also relatively old, seeing as it was introduced in the USA during the mid-1960s. This product class is currently experiencing a comeback due to its low manufacturing costs. That is particularly true for the Russian market, which has its effect on the global market share of polysulfide sealants.

Simply put, in Europe the trend shifts from polysulfide to polyurethane, in the USA it shifts from polysulfide towards (reactive) hot-melt and polyurethane, and in Russia from polysulfide towards polymercaptan polymers.
In China – or Asia, in general –, it is quite difficult to conduct an accurate market analysis. A great number of local sealant suppliers are present there, while their products are mostly silicone-based, with some of them still being on the basis of solvents. European sealant qualities are rarely demanded and usually only when high-quality machines are in use. However, the Asian machine manufacturers have caught up with the competition in Europe, which shows in the current trend of buying both machines and sealants locally.

During the next few years, it remains to be seen which legal restrictions will be issued and if environmentally conscious measures and manufacturing are going to prevail.

IGK rises to these global trends with its product portfolio that has grown in recent years.

Dr. Randolf Karrer

For an industry newcomer, the European IG production looks quite simple and structured at first sight. At the beginning of the production, 6m times 3.21m large glass arrive via low loader. Afterward, a crane loads them onto the frames, and if necessary, they get transported to the glass cutting. The software takes over the glass management and assures a mostly automatic cutting process. For low-E glasses, it also provides automatic edge trimming, including garbage disposal of the excess glass. The IG production process continues with several stations, such as washing machine, inspection station, frame settling of the butylated spacers or TPS® order (if required), gas filling, and pressing in. These procedures look all quite simple, are mostly invisible, and are navigated nearly fully automatic. Afterward, the mostly finished insulating glass emerges once again and merely requires one more step – the edge sealing with a matching sealant. After a brief retention time in the warehouse, the finished product is usually ready to be sent to the window fitter or metalworker within 24 hours.

But none of this is as easy as it may seem at first sight. The customer order requires the accurate glass and a suitable coating in the correct position, if necessary. Furthermore, special glass types, such as ESG or VSG for soundproof or shatterproof glass, may be added. Due to the increased use of low-E coatings in the past years regarding the triple IG, mainly at positions 2 and 5 and/or sunscreen coating at position 1 (if necessary), this complexity already demonstrates the first major challenge. The matching soft coatings need to be removed quantitatively from each edge. As a result, the sealants that are going to be applied afterward will find a defined surface for an optimal adhesion structure.

The next step includes washing in the vertical washing machine, which removes dust and dirt. Usually, warm VE water is used during this process. The quality inspection of the washing water is almost always conducted by conductivity measurement, seeing as this usually does not pose an issue. However, the washing process changes the glass surface, both chemically and physically. Thus, the subsequent drying process is crucial. That is especially relevant for the following adhesion structure of the sealants. In the meantime, the widely spread inspection stations take over the automatized, visual control of the IG during the next step.

Up to here, almost only physical processes are involved, except for the base glass production by tin bath or atmosphere that occurred previously. These can also be controlled quite well by physical methods.
However, chemistry comes into play now: This happens while applying butyl to the spacer at the so-called “butyl station”, while the primary sealant is directly applied in the form of TPS® on glass, or also during the mechanical application of a prefabricated foam spacer at the lateral cut surface by means of acrylate adhesive.

All three types of applications contain physical processes as well as chemical raw materials. Their formulations or structures have a very detailed composition and are also subject to the respective local application conditions. Some factors, such as defective base materials, an inconsistent structure, or faulty manufacturing by the component manufacturer, can constitute a reason for complaint. An incorrect application temperature that is either too high or too low can, in turn, cause an uneven sealant application, a lack of adhesion, high rates of gas leakage, or so-called “butyl inlets” between the clearance in the insulating glass or window.

Furthermore, defects may appear at this cut surface, which can be caused by inadequate synergy of spacers and the application of butyl on the so-called “butyler”. This happens whenever a frequent change of the various “warm-edge”, and lately “multilayer” spacers, is taking place. Usually, these are curved and equipped with corner joints and should not have any oily fingerprint marks, which can lower the sealants’ adhesion.

Other unfavorable influences can be caused by uneven butyl application, irregular compression to the insulating glass, or higher/lower contact pressure in the gas filling press. Such causes negatively manifest themselves with too large thickness tolerances during the following quality control measurements – the so-called “package magnitude” around the IG unit.
One can also make enormous mistakes while using a secondary sealant. That begins with transport and storage conditions. A product that is either stored in direct sunlight, cold conditions, or outdoors can lead to misdirecting results. This applies analogously to the previously mentioned primary sealants that major users obtain from barrels. If stored outdoors at freezing temperatures, it requires up to 14 (!) days for them to “defrost” at the center of the barrel at room temperature.

Back to the secondary sealant though. No matter if polyurethane, polysulfide, or silicone is used as a 2-component system for the mechanical stabilization of the IG unit, it is crucial to set and control the correct mixing proportion at all times. The rinsing processes that take place before breaks and shift changes require surveillance and implementation according to the instructions of the sealant manufacturers. Special attention should be paid whenever the material needs to be changed. The reason for this is that a facade element with silicone sealant and corresponding UV resistance can be put under seal after an IG unit for a gas-filled window with polyurethane or polysulfide. If the same mixing units are utilized, then extensive rinsing processes are needed to avoid cases of incompatibility among the sealants.

Speaking of “incompatibility”: In the past 20 years, numerous complaints have occurred regarding reactions of incompatibility between sealants, not only among themselves but also regarding other parts of the window. Even though the industry has gained a lot of knowledge in the last 5-10 years, incompatible materials are still used together with edge joint sealants for weather sealings, for example, the so-called glued window, including burglary resistant glazing, or even for settling blocks (based on polystyrene). Due to the preparation of approval lists for several fields of application by the leading sealant manufacturers (including IGK), the complaints concerning cases of “incompatibility” regarding windows have reduced significantly. That is due to the fact that their laboratories apply the corresponding ift- and RAL-GMI-guidelines and lately also the EN-1279.1 (2018) standard.

As already mentioned, the correct application, as well as the complex chemical composition of the sealants, plays a crucial role in the production of a premium insulating glass. To ensure this standard in the future, IGK will continue to visit its raw material suppliers at the global production centers and validate them via audit, as it has done during the past years. On this basis, IGK will focus especially on chemical connections and manufacturing processes to offer its clients the best sealant quality possible.

Dr. Randolf Karrer

A few years after founding IGK in 1988, the term “warm-edge” was coined in the mid-1990s. It described the loss of heat within the insulating glass across the spacers with good heat conductivity (bad insulation). The term “K-value” which was used up until then, was replaced by the so-called “U-value”, describing in regard to insulating glass its heat transfer, including the respective window material. That resulted in a massive development boost concerning new spacers and led to the previously widely spread aluminum being increasingly replaced. As a consequence, new polymeric supporting materials, co-extrusion, and diverse stainless-steel qualities – also in the form of foiled products – appeared on the market. At the same time, TPS technology was introduced, and a few years later, so-called foam spacers, an American development, entered the European market.

During this time, the leading glass manufacturers developed better glass coatings, which allowed a significant reduction of the thermal transmittance values (U-values) for insulating glass below 0.7 W/m²K.
These developments concerning the insulating glass market can be interpreted as a sign of the so-called Energy Saving Ordinance, which was issued to serve as a monitoring tool of the German energy and climate protection policy in 2002. The various improvements and amendments from 2007, 2009, and 2013 (-2016) included different state promotional programs, not only for new buildings but also for the reconstruction of private and public buildings. In the ensuing years and until today, these provided the German construction growth a lasting peak.

The insulating glass industry – especially in Central Europe – has profited from these measures and developments as well, including a strong trend towards the triple insulating glass. Thus, this manufacturing technology, which was designed in Scandinavia for many years, experienced a new boost and was included in various calculations and presentations at international conferences, such as the Glass-Processing Days in Tampere, Finland (2011). It gave new developments in machines, sealants, and spacers room to enhance, which contained new challenges when combined by the insulating glass manufacturer.

In 2005, due to the newly developed adhesives and frame geometries – which led to the glued joints of the window frame with the insulating glass –, everyone was soon talking about the “adhesive window”. Diverse systems, such as rollover, rabbet base, or glass edge adherence, paved the way for new fields of application regarding silicone, polyurethane, and acrylate adhesives, as well as duct tapes. The main idea was the same with all adhesives; to transfer the rigidity of the glass to the frame. Furthermore, experts reasoned that the removal of the steel reinforcement in the case of some systems – especially regarding the window casement –, should also contribute to energy saving. Back then, such adhesion of glass and frame was heavily discussed. That applied particularly to the rabbet base adhesion, which broke with an old insulating glass principle of the ventilated rabbet base to avoid condensation in the window. Additionally, it posed a challenge regarding the synergy of the adhesive and its environment – especially when it came to the edge joint sealants. Nowadays, the adhesion of glass and frame is not discussed as frequently, although they are especially relevant for burglary resistant windows.
In the field of facades, the trend focused on larger windows, facilitating the architects’ aim for a frameless design and a free view at window displays of stores. Due to this demand, a 17m long insulating glass panel by Henze Glas was shown to the public at the glass fair in Düsseldorf in 2014. Such dimensions can only be produced safely and optically flawless via automatically appliable spacers such as IGK 611 (TPS®).
For 3 years, spacer suppliers for insulating glass have increased their focus on so-called multilayer spacers. Optimized for additional improved PSI values, they contribute to the further improvement of U-values and thermal insulation. The most skillful step of this manufacturing process is the application (adhering) of the several foil layers onto the respective supporting material. The insulating glass manufacturer has to deal with the challenge of achieving high speeds while bending the used spacers. Currently, a lot of work is put into achieving an ideal performance.

Regarding the edge joint materials, it is important to refer to the development and the increased usage of so-called “reactive 1-C sealants”. Those have formed new possibilities for edge joint sealing in the form of “reactive TPS®” as primary sealing or “reactive warm melt” as secondary sealing. In recent years, the EU Directives 848 (2012) and 852 (2017) represented major incisions, seeing as they completely prohibited the use of mercury as a catalyst in polyurethane adhesives and sealants.

Upon implementing the aforementioned First Energy Saving Ordinance in Germany in 2002, the EN-1279 insulating glass standard was also reissued, even though it took 16 years and numerous discussions in national and international committees until it resulted in the new regulation EN-1279 (2018). Although the goal was a consistent regulation concerning the quality assurance of insulating glass in Europe, this target was only achieved to a certain extent because of national lobbying and protection of jobs in the respective country.

Thus, the EN-1279 (2018) sets modern and further extended requirements for European insulating glass, but it is still partly surpassed in some aspects by various national standards. In recent years, the “surveillance” conducted by the international authorities has increased significantly at the component manufacturers, including the sealant suppliers, in the form of audits. Even though this increases costs, the insulating glass customer (from IGK) also benefits from it in the form of Cekal, ATG, or RAL-GMI certificates and adequate sealants (from IGK).

Dr. Randolf Karrer

In the mid-1980s, polysulfide was by far the best-selling insulating glass sealant worldwide. Each year, more than 90% of the sealant was applied as an outer joint sealing in the form of a fillet. For this purpose, the old elastomer chemistry, as well as 2-C sealing machines, were utilized. Back then, 4 raw material suppliers for polysulfide polymers dominated the industry while 3 leading European sealant producers put their material on the market.

During this period, manual sealing machines were increasingly migrated to semi-automatic machines. In this process, the glass was usually cut on separately constructed machines and manually driven to the washing machine in order to be combined with the spacer that was bent on the frame bender to form the insulating glass as a result. The manual sealers gathered several insulating glasses before subsequently sealing them with polysulfide in series. Here, polysulfide sealants had proven to work best. Due to their 2-component system, they could be applied “conveniently”. In other words, they had a long pot life that was achieved receptively by chemical deferrers. If desired, it was possible to opt for a quick hardening, requiring little to no chemical knowledge from the operator. Furthermore, cleaning the mixers was also an easy task, seeing as they did not get clogged excessively and had a long cycle time of up to 1 year.

Upon application of fully automatic sealings, which took place at the end of the 1980s/early 1990s, as well as adequate production of up to 800 IG units per shift, the mixing and dosing accuracy of the machine manufacturer was nowhere near where it is nowadays. Thus, a sealant was required that condoned mixing and dosing errors during automatic operation. That attribute applied to polysulfide as well, seeing as its so-called heterogeneous hardening mechanism permitted an overdosing or underdosing of 20% to 25%, give or take. Therefore, it was an excellent option for factories that had little chemical or mechanical expertise.

At the end of the 1980s, bold insulating glass manufacturers pioneered their first experiences with polyurethane as an alternative for polysulfide. As already mentioned, the lack of mixing and dosing accuracy of the sealing machines, the short pot life, as well as the quick contamination and wear of the mixing units, made it unavoidable to overthink the new sealant polyurethane as a joint compound material. However, many IG manufacturers were not ready yet, watching their colleagues instead as they had bad experiences. As a result, the market share of polysulfide in Europe reached its peak in the mid-1990s, accounting for about 93%. Silicone (mainly used for facades), hot melt, and some polyurethane were subdivided into the remaining contingent. There was no real chemical substitute for polysulfide within sight.

But why did a sudden transition from polysulfide to polyurethane occur at the turn of the millennium?
It all began with Morton, one of the leading polysulfide polymer manufacturers. According to their official statement, the company received such strict environmental regulations in the United States that they were forced to stop production. Overnight, their buyers did no longer receive enough material, leading to them being unable to produce the polysulfide sealant that was in such high demand.

At the same time, PU manufacturers, with IGK leading the way, had enhanced their products. However, those products were still far from the mercury-free PUs as we know them nowadays. Nonetheless, a supply gap had emerged that needed to be filled. During this time, the IGK technicians worked almost every weekend and updated the machines with great effort and costs. The entire mixers, valves, pumps, and supply pipes had to be changed to ensure that no chemical reactions could take place in the pipes that still contained polysulfide. The installation of the B-component had to be exchanged completely because the viscosities of the available PU-B (liquid) and PS-B (viscous) required other pumping and mixing units back then.

Due to the growing internationalization of the IG industry and stricter requirements in terms of standards, polyurethane displayed an advantage over polysulfide. French regulation encouraged the turnaround of PU significantly since its formulations passed the high standards of UV wet pollution in the climate test with flying colors. In contrast, polysulfide formulations were shown to absorb moisture while amplifying their size by up to 20%. As a result, the adhesion to the glass is insufficient, possibly leading to failed quality tests. That can especially be the case with low-cost polysulfide formulations.

Thus, the French market was the leading force in Europe at the beginning of the 2000s, holding a market share of > 90% per PU. Another crucial aspect was the fact that PU formulations were cheaper to produce than those using polysulfide. Simply put, manufacturers did not only save money but also achieved better quality. Despite the need to clean the mixers quite frequently, the high-speed 3-layered machines of the French IG manufacturers still displayed advantages in terms of PU.

Until 2006, a more or less balanced relationship between PU and PS as a secondary joint sealant formed in Europe. Due to the increased competitive pressure from Eastern Europe, including new investments in Poland, the production of insulating glass became cheaper. The open borders forced IG manufacturers to claim any cost savings. As already mentioned, PU is the clear winner in this situation, partly because it offers a better cost-benefit ratio.

After the world economic crisis from 2008 to 2010, IGK set new standards with its Hg-free polyurethane, which was introduced onto the market long before respective European regulations were implemented. The Hg-free version of the IGK 130 allowed for a smooth transition from PS to PU. After all, the no longer existing Hg, which acted as a catalyst, could not be contaminated by the rest of the polysulfide within the installation. Hence, the polysulfide inside the sealing machines could no longer prevent the hardening of PU. Now it was possible to cursorily clean the installments from the polysulfide and rinse them with the recently developed, pasty B-component of the IGK 130. That was followed up by modifying the correct mixing ratio before manufacturers could continue the production process without any further changes. This mechanism could often be carried out by the customers themselves, seeing as it was an affordable option that required no help from machine manufacturers. Technical support and guidance from IGK were available if required.

A further step towards the victory of polyurethane was the ongoing development of the IGK 130 formulation. Thanks to its long pot life, it assured a very long lifetime between the mixer’s cleaning processes and a very fast hardening. Because of these properties, the former PS technology was pushed aside for good.

Both technologies are still suitable for the production of insulating glass, according to the latest standards and requirements – however, polyurethane has the best cost-benefit ratio.

Dr. Randolf Karrer