As coating corporations work toward increasing margins, getting more out of the capital at the site, improving turns in inventory, and reducing environmental impact, they are running into the edge of material, expertise, and equipment capability. Hot melt adhesive coating is a great option for traditional solvent-based tape and label manufacturing, especially as the “green” label is providing a marketing edge in commerce.
But what are the limitations? How thin can an adhesive be stretched before the rheology of the polymer is detrimental to the final product? How do you best operate the equipment to maximize product yield? This presentation will exhibit the current direction hot melt adhesive manufacturing companies are headed and what the impact is for raw material suppliers and equipment manufacturers.
Worldwide growth in the use of hot melt adhesives has created a strong demand for process techniques and equipment systems that provide consistent, streak-free application of adhesives at high speed and with minimal environmental impact. Recent advances in technology and technique have added to the ability to coat hot melt adhesives economically, productively, and with improved product quality.
HOT MELT ADHESIVES
Hot melt adhesives are polymeric materials, such as ethylene-vinyl acetate (EVA), polyamides, and polyurethanes, that are applied at elevated temperatures and chemically bond to the substrate as they cool. They are used in a wide range of manufacturing, including flexible packaging, paperboard, textile, and other continuous web processes for which a solvent-free replacement can work. Continuous web coating is even more widespread in the tape, label, nonwoven, and other industries that employ pressure-sensitive adhesives (PSAs). Considered a subset of hot melts, PSAs are typically rubbers or acrylate copolymers that are tacky at room temperature and physically bond to the substrate as pressure is applied.
Why would a manufacturer want to utilize a hot melt adhesive over a solvent-based adhesive? With solvent-based adhesives there are inherent costs in the handling, solvent emissions, and solvent recovery. With hot melt adhesives, the costs are in the mastication and energy to heat the adhesive to processable temperatures. Comparatively, the solvent system is more expensive on a day-today operation and over the long term. The added benefit of not having to utilize solvents in the adhesive formulation takes out the EPA and health issues associated with solvent technology. Most importantly, the use of 100% solid adhesive eliminates the need for drying units, leading to increased production rates.
HOT MELT PROCESS
Depending on the sophistication of the chemical development group of a specific manufacturer, the adhesive may either be purchased or mixed on-site. If the adhesive is mixed on-site, the components (resin, tackifier, anti-oxidant, oils, etc.) will be mixed in a twin screw extruder and subsequently pumped to a melter that maintains a consistent temperature of the mixed adhesive prior to delivery to the coating head. If the adhesive is purchased, the twin screw extruder would be bypassed for direct melter delivery to the coating head.
After delivery of the adhesive to the substrate, different curing technologies may be required to lock-in the required properties of the adhesive. This may be as simple as a cooling drum or as complicated as an energy station (eBeam or UV) to cure the final product structure and morphology.
In addition to adhesive delivery and curing capability, standard web handling equipment and controls are required to maintain the substrate integrity, both entering the coating station and providing the highest quality product in roll form.
To compete with existing adhesive coated product, the manufacturer needs to be able to not only coat an adhesive without solvent, but provide improved performance and long term process cost reductions. This means faster and thinner wide adhesive coating production. What does faster mean? Current state of the art requires that the equipment and techniques can handle 3280 ft/min (1000 m/min). What does thin mean? Coatings that have equivalent tack and adhesion at 0.5 mil (0.0127 mm) are required. Both the speed and thickness of coating are needed to make an economical product, all while maintaining a streak-free coating appearance.
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