Developing a Bond Between Adhesives and Overlays Proper surface preparation and adhesive use will prevent delamination. By Larry Sloan and George Carter
The Adhesive and Sealant Council estimates the total U.S. market size in 2002 for lamination adhesives used for decorative overlays (excluding saturated papers and edgebanding) at 54 million dry pounds. This figure is predicted to grow 2.5 percent annually over the next five years. The array of adhesive types is shown in the following table. These figures exclude wood veneer and assembly adhesives, as well as all packaging film-to-film adhesives and similar type adhesives.Overlay Shipment Figures Excluded from the data table are saturated papers that are used for thermofused melamine panels. These overlays do not require an adhesive since the resin compound saturated into the paper acts as the bonding agent by cross-linking with the wood fibers on the surface of the substrate.
The right amount of the best adhesive is dependent upon several factors. These include: type and overall condition of laminating equipment (which may include nip roll distance from the adhesive spreader, as well as operating temperature and pressure); whether the adhesive is applied to a web overlay, board core, or both; how the adhesive is applied (i.e., combined wet, semi-wet, or thermal); weight of the adhesive coating; spread roller type; the production line speeds (fpm); drying conditions (type, temperature, and time); ancillary in-line equipment (such as sanders, cutters, and trimmers); and end-use specifications and standards (i.e., how will the end product be used and what environmental conditions will the product be exposed to).Most adhesive suppliers offer key strengths in specific market niches. Many also conduct testing to ensure the proper adhesive is selected. Larger adhesive companies often have very sophisticated testing labs that can test for industry wide accepted standards. Examples are humidity cycling tests, 180 F heat aging tests, and HPVA type 2 water resistance tests. If industry standards are not well defined, adhesive companies can compare testing results to historical data. By relying on the experience of the adhesive supplier, a company can gain a good perspective on an adhesive’s expected performance. Price is also a consideration. The end-user may need to figure in the price of storage and application equipment to come up with the total cost of using a particular adhesive. Don’t forget to include labor and utilities as well. A typical PURHM (polyurethane hotmelt) adhesive can be seven times more expensive per pound than a PVA (polyvinyl acetate), but may save time and money in the long run.
In-plant environmental conditions can have an effect on the lamination process. For example, what type of equipment is being used? What are operating temperatures and airflow requirements for proper bonding to occur? Consider the ventilation and dust control situation — excessive particulates in the air can adversely affect lamination, and ultimately lead to delamination.Environmental factors also dictate adhesive selection. Depending upon state and local regulations, adhesives containing high levels of volatile organic compounds may be banned. Regional safety codes may also dictate how adhesives are handled and disposed of, thus eliminating certain technologies. Regulations limiting the use of products containing potentially hazardous VOCs may necessitate the use of other products that may seem less ideal. As an example, solvent-based contact cements have been under attack for many years. Fortunately, however, many alternative adhesive formulations have been developed which oftentimes perform better than the incumbents. The adhesive industry is constantly researching ways to “build a better product” that will stay ahead of the environmental curve without compromising performance. Other ConsiderationsIn addition to environmental considerations, in-use and in-transit conditions also should be considered. Low temperatures can create challenges for certain types of adhesives, such as water-based products. Other products, including those that are polyurethane-based, are more impervious to damage during shipment. In general, freezing should be avoided, as it may affect ultimate performance when the adhesive thaws out. Always check with your supplier before using any products that may have frozen during shipment. Wide temperature swings inside a production plant should also be considered. For example, typical PVA glues will have a minimum use temperature. If ambient temperatures fall below this point, the adhesive may not perform as well and result in a weaker bond joint. Another example is PURHM formulations. Initial green strength and total process time can be affected if temperatures in the plant fluctuate too much. Of course, storage and feed equipment must be operated according to manufacturers’ guidelines. Tank/tote storage conditions, pumping rates and pressures and application monitoring all need to be considered.
There are several new trends in adhesives that are positively impacting the laminating market. According to Chicago Glue Machine, “foaming water-based glues may reduce adhesive usage by 30 to 40 percent, increase operating line speeds, and ‘sit up’ better on the board, thus resulting in a superior laminated product.” These technologies are considered “cost effective for high-production environments with lower coating weights, improved mileage, lower energy costs, and reduced reject rates leading to higher productivity,” the company says.According to Franklin International, fast-setting, water-based adhesives can be processed more quickly through heated roll press systems. The equipment is reasonably priced and process speeds are greatly improved. In addition, there is a new class of hot press PVA adhesives that permits bonds to be formed, even if the glue is dry before the high pressure laminate is laid up. According to Franklin International, this helps compensate for line stoppages without wasted material. It also permits companies to continue to glue up panels even if the hot press is a bottleneck. Reactive hotmelt and polyurethane technologies also speed up processing, and are appropriate when gluing difficult or non-porous substrates. Other technologies already on the market include: high solids, web coat, pre-applied and heat- and pressure-activated products. Larry Sloan is the director of marketing and member services for The Adhesive and Sealant Council. For information, contact the ASC at (301) 986-9700 or visit www.ascouncil.org. George Carter is the executive director of the Laminating Materials Assn. Contact the LMA at (201) 664-2700. |
Developing a Bond Between Adhesives and Overlays
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