Volume 7 Issue 6 June 2006
When and Why Manufacturers Should Consider Hot-Melt Sealants
by Lori Enszer and Ken Rubis
Under constant pressure to improve manufacturing processes, door and window manufacturers face the challenge of selecting from an array of sealing technologies. In addition to traditional adhesive tapes and liquid sealants, manufacturers can now choose among a new generation of hot-melt sealants, which promise to improve performance and adhesion to substrates while reducing manufacturing time, material and labor costs.
Hot melts are fast-drying, nonvolatile sealants applied in a liquefied state to seal glass to its frame. While traditional wet sealants can take eight to 24 hours to set, hot melts typically have hold times between one and five minutes. This increases production speed capacity and decreases process inventory.
In window manufacturing, several characteristics and production factors will influence whether or not to use a hot melt, including sales volume and current manufacturing and automation process. Because the initial equipment investment can be significant manufacturers already using automated production have lower start-up costs than manufacturer who transition manual techniques to automated production and must invest in new machinery; manufacturers with already large volume sales achieve greater economies of scale than manufacturers with low sales volume when transitioning to hot-melt sealants.
Silicone vs. Organic
Most hot-melt sealants form strong bonds, set rapidly upon cooling and are relatively easy to handle, but their performance characteristics vary widely depending on their chemical structures. Hot-melt sealants fall into two primary groups: silicones and organics (butyls, acrylics and polyurethanes).
Three key requirements can help manufacturers determine which hot-melt material meets their needs most precisely: productivity requirements, adhesion speed and performance durability.
With a 500 to 800 windows-per-shift/line operation using one to two people, switching from tape glazing to automated glazing with a hot-melt assembly sealant yields a significant increase in productivity with no increase in labor. Silicone hot-melt sealants also offer manufacturers a range of advantages over acrylic and polyurethane hot-melt sealants:
• No need for priming or surface activation on most substrates including glass, PVC, wood and aluminum;
• Easy to process using automatic equipment; and
• Long pot life and long open time allow manufacturers to adjust glass as it is being set.
Organic hot melts offer a similar increase in productivity, but have a lower green strength and can be more difficult to clean up.
Silicone has low heat capacity, which allows silicone hot melt to cool very quickly, triggering a corresponding increase in viscosity (see graph).
Silicone hot melts that respond to a forced application to initiate a state change are called “reactive” silicones. Within 30 seconds after application the silicone cools and the viscosity rises to give “instant bonding.” This adhesive bond strength is from both the viscosity rise and the pressure sensitive adhesive (PSA) character of the material. The PSA is “built in” to the sealant to provide instant green strength, which allows windows to move from one processing step to the next without any hold time.
Once cured, silicone hot-melt sealants are much more flexible than organic sealants, thanks to their silicone-oxygen backbone. This flexibility results in lower stress on the sealant and on the bond line. Additionally, silicone sealants have lower glass transition temperatures than their organic counterparts. This allows the sealants to remain flexible well below 0°F, where many organic materials become brittle. Hot-melt silicones are compatible with most common construction materials and demonstrate excellent adhesion.
The inherent UV stability of silicone materials and their extreme weather resistance make them the material of choice for applications as increasingly recognized by building standards. Particularly in window bonding and back-bedding applications, silicones exhibit the following key performance advantages compared to alternative organic-based technologies:
• Unaffected by long term direct and indirect UV exposure;
• Chemical bond unaffected by typical high and low temperature conditions; and
• High movement capability minimizes the stress levels on the insulating glass unit.
Availability of newly developed, liquid-applied, fast-curing and high green strength hot- melt sealants present a unique opportunity for door and window manufacturers as well as construction assembly OEMs. By combining the merits of traditional wet silicone with the benefits of tape, hot melts enable users to face business challenges better by lowering the total cost of their product, particularly within the manufacturing process. Silicone hot melts also meet environmental low volatile organic compound (VOC) standards.
In a comparison between silicone and organic hot melts, organics have a lower initial cost, but silicones deliver longer performance against environmental factors such as UV radiation and temperature. They provide fast adhesion for increased productivity, making them an ideal choice for high-volume and long warranty window systems.
Lori Enszer and Ken Rubis serve as marketer and technical service engineer, respectively, for Dow Corning Corp.
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