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Bonding technologies: adhesive melt blowing.

A new method of bonding substrates has evolved in our industry. The process utilizes air to spin hot melt adhesive fibers and convey them onto a substrate for bonding - it is called adhesive melt blowing.

As early as 1980, attempts were made by companies familiar with the conventional Exxon Chemical melt blowing process to blow hot melt adhesive fibers onto nonwoven and disposable products to bond these substrates together. Most of these early studies were based on a combination of standard polymer dies and hot melt processing equipment. Problems soon arose due to die designs.

In the melt blowing of polymers, large amounts of air volume, velocity and heat are required to draw the viscous polymer into fine fibers. Typical sizes of these fibers are 1-10 micron. The early dies were designed to deliver such large air volumes with low pressure drops and had relatively large openings as air slots.

This configuration did not prove practical in the fiberization of hot melts. Due to the relatively low viscosity or high MFR of most commercial hot melts, air volumes of these magnitudes spin the hot melt material into fibers that are much too small to provide an effective bond or lamination. These fiber may be 0.3-2 micron. But these early tests revealed that there were also significant benefits to hot melt fibers and fiber structure for bonding substrates. A considerable development effort began by most of the hot melt equipment suppliers to create innovative methods for the fiberization of hot melts.

The process as we have come to know it today delivers relatively large hot melt filaments around 20-200 micron. These large filaments have proven to provide a more effective bond between substrates. Many industries have taken advantage of the new hot melt blowing technology. Systems are in use for a myriad of applications. A few include:

Baby diaper leg elastic

Diaper frontal tape

Diaper standing leg cuff

Diaper chassis construction

Diaper core stabilization

Diaper liquid transfer layer

Diaper outer cover lamination

Diaper elastic cuff lamination

Feminine napkin core stabilization

Feminine napkin adhesive strip

Industrial filtration bonding

Industrial filter material lamination

Filter mask lamination

Surgical gown lamination

Surgical drape lamination

Perishable products packaging

In most cases, determination of the proper lamination technique must be made on an application basis. However, it has been found through continuing application analysis that adhesive melt blowing is the most effective method of substrate bonding. These findings have been based on the following criteria: fiber size, fiber distribution, edge control, product aesthetics, minimal operator interface, air consumption, overall system maintenance, customer comments and satisfaction.

Fiber size. Fiber sizes can easily be controlled, from 20-200 micron, by changing the melt-to-air ratio. No stray fibers are generated due to the inherent stability of adhesive melt blown applicators.

Fiber distribution. If 100% coverage with consistent fiber size and distribution is key to a product's successful lamination, adhesive melt blowing is the clear method of choice. Product coverage can be maintained from 5-100%. This is a simple scientific function of draw down of the hot melt filament. Unlike swirl applicators, melt blowing dies are not susceptible to viscosity or temperature changes, since the filament is drawn in a straight line towards the substrate.

Edge control. Adhesive melt blown dies will provide [+/-.sup.1/16] inch typically.

Product aesthetics. Due to the nature of the melt blowing fiberization technique, visual properties of the laydown are excellent. Unlike swirl nozzles, 100% pattern coverage is achievable. In some aspects of the nonwovens industry, where the hot melt filaments provide a functioning product layer, only melt blowing can provide the correct filament structure. Light films and foils are not distorted by large swirls but bonded by millions of bond points.

Minimal operator interface. Reduction of operator involvement is also a prime concern. The next generation grid melt systems, utilizing touch screen graphics, network interfacing and modern accessibility have allowed the capability of remote system analysis, startup and troubleshooting. Reduced cavity die distribution channels, recessed air plates and hydraulic discharge features have reduced the necessity for cleanup to standard maintenance down days. Line speed tach following adhesive melt and air flow ramping functions have allowed the splice and run speed mode to be preset and the capability of constant add on weight regardless of line speed.

Air consumption. Melt blowing adhesive requires very low volumes of air flow and pressure. For example, J and M Laboratories has production ready dies requiring process air of 1 SCFM per inch of die at pressures of 10-25 PSI capable of producing fiber diameters in the range of 30-125 microns.

Maintenance requirements. In industry's early attempts at developing production capable adhesive melt blown lines, maintenance (die cleanliness, orifice clogging) proved to be a hindrance towards product acceptability. One approach to alleviating this problem has been to introduce filtration systems and release coatings placed strategically prior to both the transfer hose and die balancing chamber adhesive. Adhesive melt blown dies have relatively large orifices, typically 0.20-0.40 inches, compared to .010-.016 inch of swirl applicators. Environmental or external causes of orifice and air gap clogging due to adhesive runoff or fluff in the air have been virtually eliminated by both the concept of recessed air gaps and the hydraulic discharge feature.

About the author:

Martin Allen is vice president of J and M Laboratories, Dawsonville, GA, a manufacturer of adhesive melt blown application systems. Mr. Allen is one of the founders of the company, which has been involved in nonwovens since 1985. The company is in the process of constructing a new 125,000 sq. foot facility and 20 acre complex just north of Atlanta, GA. J and M has a worldwide sales and service network with offices in Europe, Japan, China and the U.S.
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Title Annotation:Nonwoven Bonding Technologies: There's More Than One way to Bond a Web
Author:Allen, Martin
Publication:Nonwovens Industry
Date:Oct 1, 1992
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