Processing Magazine

Tips for selecting hot melt sealant technologies

November 13, 2006

Why Consider a Hot Melt?
Under constant pressure to improve manufacturing processes, 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 manufacturing, several characteristics and production factors will influence the decision to use a hot melt or not, 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; manufacturers with larger volume sales achieve greater economies of scale 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, or criteria, can help manufacturers determine which hot melt material meets their needs most precisely: productivity requirements, adhesion speed, and performance durability.

Productivity
With a line operation using one to two people, switching from tape glazing to automated glazing with a hot melt assembly sealant yield 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.

Adhesion
Silicone has low heat capacity, which allows silicone hot melt to cool very quickly, triggering a corresponding increase in viscosity. 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 products to move from one processing step to the next with no 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 to PVC, aluminum, painted and treated wood, performance paints and many glass substrates. Silicone hot melt sealants are applied without priming.

Durability
Silicone sealants have long been the “gold standard” in glazing performance. They are the sealants of choice in high-end operations because they deliver the best durability, flexibility and longevity. 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. 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 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 better face business challenges 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 a variety of applications.

Ross Noel is a senior applications engineer responsible for identifying sealant technologies to solve customer needs in the automotive and industrial assembly and maintenance markets. During his 25 years with Dow Corning, Mr. Noel has worked on a variety of projects from facilities engineering to application engineering. He has held leadership roles in both medical and electrical products at Dow Corning and holds six patents in pressure sensitive adhesion technology. Mr. Noel holds a degree in Electrical Engineering from Michigan State University.