Volume 40, Issue 3 March 2005
A Look at New Generations of Dual Seal IG Systems
by Gerhard Reichert
Thermoset sealants, such as polysulphide and silicone, have long been heralded for their high performance attributes in the insulating glass (IG) industry. IG spacers typically have incorporated a thermoplastic component, such as polyisobutylene or butyl, for dual-seal moisture vapor and gas barrier properties.
New generations of thermoset spacer technology offer a unique reversal of the common dual seal IG system, with thermoset technology as part of the spacer, and thermoplastic barrier sealants in the cavity behind. IG performance improvement—such as enhanced flexibility and durability, improved memory of the spacer/sealant system, overall window U-value performance, condensation resistance, argon or other inert gas retention, acoustic performance, automated manufacturing efficiencies and quality control benefits—are all possible.
Thomas D. Stetson, when inventing and patenting the concept of insulating glass in 1865, never intended on creating an insulating panel by using highly conductive spacer materials such as aluminum, steel or stainless steel. There was no hypocrisy in his design based on insulating wood or rope as the spacer material of choice.
Since the 1960s, conductive metal spacers have been used for IG in order to provide a reasonably durable packaging solution with an acceptable life expectancy. Only when using a metal spacer combined with a single-component, thermoplastic butyl primary extruded seal and a dual-component thermoset rubber sealant for anchoring the spacer and glass lites together, did the industry find it met the durability criteria. This system is often referred to as dual-seal metal spacer technology.
Metal Spacer Replacement
The problem remains that the spacer should be of an insulating nature to create insulating glass panels, as Stetson suggested more than 135 years ago. Past attempts to replace the metal spacer with insulating materials such as plastic or fiberglass profiles resulted in decreased durability due to either the moisture vapor transmittance rate (MVTR) permeability increase of the polymeric-based material, the increase in linear thermal coefficient of expansion or chemical fogging issues.
Novel approaches toward creating durable dual-seal IG systems by giving the spacer a thermoset component have opened the door to the combined advantage of optimized durability and energy savings. As a result of combining insulating thermoset technology with today’s low-E coatings and gas filling, along with the latest in IGU production process automation, an IG unit with optimum performance to cost ratio can be achieved.
Vertical insulating glass production lines have also been developed and adopted for manufacturing IG units using thermoset foam spacers.
One critical production advancement lies in the automated manufacture of high-performance argon gas filled units, where only one automatic gas fill press is required, filling one unit at a time, to produce more than 800 and up to 1200 IG units per eight-hour shift. In the past it has always been necessary to use tandem IG gas filling to achieve those kinds of numbers with traditional dual seal hollow spacer systems.
Another production control and quality control benefit is that the spacer is assembled to the glass in a just-in-time, lean manufacturing, pull-through method. There is no handling of spacer frames ahead of time, which can cause contamination or desiccant preloading.
Custom sizes of IG units can be produced in any order without slowing down and the spacer set- back tolerance to the edge of glass can be improved ten times from ± 2mm to ± 0.2 mm even on large units.
In the early 1980s a landmark 40-story office tower rose in the Pittsburgh skyline at One PPG Place. This John Burgee- and Philip Johnson-creation contains 19,750 pieces of glass in the tower alone. Nearly 1 million square feet of reflective silver Solarban 550 Twindow insulating glass units by PPG sheath the complex.
The PPG Twindow technology of these IG units utilizes some of the durability benefits associated with the thermoset concept. Traditional metal spacer was used with a unique twist.
A silicone sealant cleat system was gunned in behind the primary seal to anchor the metal spacer to the glass surfaces against possible spacer shifts or dimensional changes within the IG unit. This was then followed with the final sealant cavity filled-in with high-performance hot melt butyl. (Note: Unlike a butyl primary seal, a butyl secondary seal is gunned in hot at full temperature to both lites of glass and the spacer at one time.)
After 20 years of field service at One PPG Place there have been no reports of IG unit loss due to seal failure.
Such an example illustrates that by utilizing this reverse dual seal technology within the spacer front and high- performance hot-melt butyl as the secondary seal, IG unit durability can be achieved.
Gerhard Reichert is the vice president of business development for Edgetech I.G. Inc. based in Cambridge, Ohio.
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