Coated or laminated paper processes typically require high temperature curing which creates moisture imbalances between the paper substrate and the coating or laminated layer(s)

To remain competitive in today’s marketplace, paper based label manufacturers must be able to produce a stable, flat product. This requires controlling the moisture content in all layers of the final product to equilibrium. Coated or laminated paper processes typically require high temperature curing which creates moisture imbalances between the paper substrate and the coating or laminated layer(s). These imbalances, if significant, cause static electricity buildup, web brittleness and curl. All of these conditions can negatively affect product quality and production throughput. In the current economy, label producers need to coat efficiently, effectively, and with improved economies for margins and the marketplace. One often overlooked, yet important piece of equipment in this equation is the remoisturizer. Identifying what conditions are critical in this unit operation for maintaining process stability is essential. Conditions to consider include: the proper amount of moisture for the web; machine speed and protection of the coated surface from contamination. Most label manufacturers employ some means of remoisturizing the web after curing to bring the moisture in the substrate, the coating, and the laminated layers into equilibrium. Existing solutions include exposing the coated and cured web to steam showers, water sprays or reverse roll coaters to impart the required amount of moisture. In this presentation, we will be comparing the various remoisturization techniques and providing data to assess these technologies. LOW MOISTURE AND CURL A reduction in the moisture content of coated paper which occurs during the curing process can cause downstream production problems such as web brittleness, static electricity buildup and curl. Although brittleness and static electricity can cause immediate problems with web breaks and promoting fires or explosions, curl is not as easy to detect and it can significantly impact the efficiency of downstream processes. Therefore, we will focus this paper on how to address this problem. Curl occurs when layers of fiber within a sheet of paper or layers of laminated paper expand or contract unevenly. Uneven shrinkage or expansion causes a bending moment because of the differences in shear forces. There are a number of variables in coating or laminating processes that can cause curl. For example: differences in tension between two laminated layers can cause curl. Relative humidity differences in operating environments can also cause curl. Or, differences in the % moisture between each layer of laminated paper can cause curl. (This is particularly evident in pressure sensitive label manufacturing since high cure temperatures are often used for silicone coated release liners and solvent based adhesive coated face stock.) THE PROBLEMS THEY CAUSE In today’s competitive marketplace customers want label stock they purchase to “lay flat”. This is not just an aesthetic issue but more importantly it is so that customers can efficiently and cost effectively process label stock into a finished product. For example: pressure sensitive label stock will undergo printing before it is eventually attached to its intended surface. Curled stock can jam equipment, slow printing and cause other inefficiencies in downstream processing. When faced with choices of which supplier to purchase from, customers will choose manufacturers who can produce stable products to “lay flat” specifications.