Hot melt applications continue to grow

Because our industry is dynamic and in a constant state of flux, how well and how quickly organizations adapt to change is more than a choice; it is a matter of survival. Manufacturers of hot melt adhesives and application equipment have quietly been entering into mutually beneficial partnerships with converters in an effort to get a competitive edge. A benefit of this often informal partnering trend is the accelerated pace of technological development in hot melts and the systems used to apply them.

Russ Anderson, director of sales and marketing for J and M Laboratories, Inc., Dawsonville, GA, and a veteran of 32 years in the hot melt business, reports that many recent developments have been customer-driven. The customer tells the adhesive or application system manufacturer what is needed, and the manufacturer in turn nudges its R&D departments in the proper direction.

"The customer has specific applications and needs, and the machinery and adhesive manufacturers work together to meet those needs," sums up Mark Zirkle, Nordson's marketing manager for graphic arts in the converting group. "Necessity, drives advances in hot melt technology."

Zirkle also said that as converters become more comfortable with the technology, they no longer look to the manufacturers for ideas but are beginning to provide much of the development impetus themselves. "Our customers are finding that if they marry a certain release liner to a specific imaging stock and then combine that with a special glue, it will give them a proprietary edge or a proprietary product to take to the marketplace."

Hot Melt Adhesives

"The equipment used to manufacture the hot melt adhesive itself has improved greatly," Anderson added. "The various components are blended in a vacuum and exposed to shorter heating cycles resulting in less stress on raw materials." He said that because of improved technology, chemists now can blend chemicals and resins that couldn't be used before. "Five years ago you could heat a hot melt for about 50 hours before it would begin to degrade. Today's hot melts are twice as thermally stable - pot life has been increased to 100 hours."

Zirkle said that five years ago converters couldn't use hot melts on clear film or applications that would be exposed to high ultraviolet levels because the hot melt tended to yellow or brown out. However, in the past few years new UV-resistant, rubber-based adhesives with the high tack of an acrylic have been developed to completely eliminate the yellowing problem.

There still are four basic formulations of hot melt - ethyl vinyl acetate, polyamid, polyester, and butyl - and they're available in bulk, granular, chips and stick form. Each has it own strengths, and all have been improved. "It's erroneous to think that one type of hot melt is always better than another," Jim Doherty, technical service manager for Bostik, said. "Certain applications call for a specific type of hot melt; one adhesive can't do everything."

The end user makes several choices with the application in mind, including open time, green strength or how long it takes for the adhesive to completely cure. "Hot melts are being used in applications that just a short time ago were only suitable for acrylics or cold glues," Zirkle said.

According to Doherty the biggest innovation in hot melt chemistry has been the development of polyurethane reactive hot melts, or PURs. PURs absorb ambient moisture from the atmosphere and crosslink into a super bond - they no longer need noxious isocynates to cure.

The typical hot melt is applied at 350 [degrees] or higher. PURs are applied at 250 [degrees], and there are formulations for room temperature application. Application temperature is important, especially in light of the industry-wide trend toward thinner, lighter substrates, because applying a 350 [degrees] hot melt adhesive onto a thin film will cause it to curl up and melt. The industry expects a 40% increase in use of PUs per year for the next five to six years.

Application Equipment

Application equipment has evolved to a much more user-friendly state. "If you looked at slot coating equipment three years ago, there were a lot of push/pull types of switches on everybody's equipment," Zirkle said. "The operator had to make a dozen different adjustments to create the proper coating profile." Zirkle adds that in the past two years these manual switches have disappeared, leaving a simple angle adjustment for the coating die or handwheels to move the coating head in and out against the substrate. Pattern coating of hot melt adhesives in the past two years has grown in terms of meeting the precise laydown of adhesives in register to a printers web, whether it be a form or label stock.

Major advances have been made in the precise placement of the adhesive. According to Zirkle, in the past the standard procedure was to flood-coat the full web and then die-cut. Screen transfer results in a big adhesives savings. "With pattern coating and screen coating you can apply adhesives precisely where you want them at speeds in the range of 350 to 400 fpm, which is what most flexo and letterpress printers can do," Zirkle said.

In recent years there has been a great deal of development in die tip and spraying patterns. Matt Pelham, marketing manager, for J & M Labs, Inc., offers hot melt applicators that provide high peel strength with the same strength and a softer hand. But the biggest advantage of the Dura Fiber Modulizer System to cost-conscious converters is the savings in adhesive. This new fiberization or melt blown method uses 35 percent less hot melt than a spiral die. "The melt blown adhesive hardly ever damages a substrate, because the air that fiberizes it also cools it down," said Pelham. "The fibers are a much smaller diameter than what's extruded from a typical swirl die."

Because polyurethanes begin their crosslinking sequence immediately upon exposure to air, enclosed delivery systems have been developed. Without them the adhesive would set up within the system and turn an expensive machine into a fused piece of junk. Different companies have different systems, but they're all enclosed systems in which the adhesive is heated in the tank.

Bob Degenkolb, hot melt technology specialist for Graco, believes the most innovative product introduced recently is the gridmelt system. These systems are fed by a heated drum that's turned upside-down. All the adhesive drips into a sump, and every drop is used. That's good for business, the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA).

Adhesive application equipment has been made more user-friendly in terms of maintenance as well. Zirkle explains that many components have been modularized. You now can remove the pump motor, die block, and even the entire glue containment pot as an individual component in seconds and take it to a workbench for servicing rather than asking an engineer to crawl under the machine and expose himself to a gooey adhesive heated to 400 [degrees].

In the past converters had a choice. They may have elected not to use hot melts because solvent-based adhesives cost less and the equipment needed to lay it clown was less expensive. The Clean Air Act has done away with much of this choice. The solvent-free, 100% solid and low-volatile-organic-compound qualities inherent in hot melts make them the viable alternative for many converters.


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