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Market DriversMarket, consumer and regulatory factors are driving the demand for flexible packaging. New packaging trends in snack food, fresh-cut produce and baking-mix packaging; pouch replacements for rigid packaging; and the growth of medical and disposable single-dose pharmaceuticals all play a part. In addition, there is consumer demand for more convenient, space-saving packaging for home meals. The food industry also has stepped up its private-label marketing efforts, which require package differentiation, while major food companies are seeking lower-cost packaging alternatives. All of these trends feed into the growth of the flexible-packaging marketplace.
In addition, governmental regulatory concerns over the impact of solvated technologies on the environment have driven laminating-converter demands for waterborne or solvent-free alternatives in flexible packaging.
A Look At AdhesivesThe rigorous requirements of flexible-packaging laminations, including film-destruct bonds and excellent heat, water and UV resistance, have challenged providers of adhesives to create effective bonding solutions.
Flexible-packaging adhesives must provide excellent clarity and bond strength, and be resistant to heat, humidity and, in some cases, chemicals. In addition, they must resist tunneling (the localized separation or delamination of the two substrates being bonded). Tunneling often occurs when the two films differ in extensibility and stretch, or relax at different rates. These localized delaminations can have a detrimental effect on package appearance and a potentially damaging effect on the package contents; they remain a major concern for most converters.
Adhesive manufacturers have responded to the increasing demands of converters by developing numerous generations of flexible-packaging products, each offering improvements over their predecessors in terms of processing, performance and regulatory compliance.
Solventless Adhesive DevelopmentFirst-generation solventless laminating adhesives are one-part polyurethane moisture-curing products made from isocyanate prepolymers, which are high in viscosity, yielding good green bond strengths but requiring high application temperatures (typically 80ºC to 90ºC). In addition to their high application temperature, several drawbacks to these adhesives include the need for water-misting equipment to facilitate the adhesive reaction, poor appearance in the lamination (often caused by excessive carbon dioxide degassing from the isocyanate/moisture reaction) and inconsistent cure rates. These adhesives also contain high levels of monomer that, because of the elevated application temperature, can migrate through the sealant film and cure on the back side of the film. This cured adhesive layer can hinder the sealing properties, creating the undesirable phenomenon known as “anti-seal.”
Second-generation adhesives, which are two-part polyurethane adhesives, are composed of a polyurethane prepolymer and a polyol, both lower in viscosity than a first-generation system. These adhesives have the advantage of a consistent and rapid cure rate (24 to 48 hours, depending on film type and storage conditions). The limitations of these second-generation adhesives are low initial bond strengths and the presence of moderate to high residual isocyanate monomer. However, many of these second-generation products are used at lower application temperatures, reducing adhesive handling and processing issues. In addition, extraction/migration testing typically is performed on key laminates made with these types of adhesives to detect any possible migratory components of the adhesive. H.B. Fuller has developed sensitive analytical techniques for determining trace levels of any extractable components well into the parts-per-trillion range, and endorses only low or no migratory adhesives.
Third-generation laminating adhesives were developed to address the limitations of their predecessors by having a consistent cure rate, low residual monomer and increased initial bond strengths. Typical adhesives of this genre are based on moderate-viscosity polyurethane polymers that require 50ºC to 70ºC application temperatures. They feature higher-molecular-weight polymers that exhibit cure rates of approximately 48 hours (depending on film type and storage conditions) and low residual isocyanate monomer that virtually eliminates regulatory and safety issues associated with isocyanates. In addition, third-generation solventless adhesives do not cause anti-seal issues, as have some of their predecessors.
Fourth-generation solventless laminating adhesives offer continued improvement, further lowering the isocyanate-monomer content and viscosity without extending cure times. In addition, prototype two-part, solventless, fifth-generation, UV-curable products currently under development excel in radically reducing cure times while maintaining adhesion and resistance performance properties. The future holds promise for these new-generation adhesives.
Development Of Waterborne AdhesivesWaterborne polyurethane dispersions and acrylic emulsion-based laminating adhesives offer some advantages over their solventborne predecessors in that the monomeric portion of the polymer is completely reacted out prior to use in a converting operation. Typically, urethanes are reserved for higher-performing applications, whereas the acrylics tend to be used on less-stringent end-use structures. Both polyurethane and acrylic waterborne adhesives typically are crosslinked for applications where increased adhesion or resistance properties are required. Crosslinked waterborne polyurethane and acrylic adhesive systems often are used to replace solventborne adhesive systems.
Essential Non-Migratory AdhesivesBeyond offering top-notch performance, suppliers of laminating adhesives must exercise diligence in assuring the products are low to non-migratory in nature, since their end-use application cannot be controlled.
While adhesives actually represent only a small fraction of a typical food or medical package, they are subject to various important regulatory statutes that deal with this issue of migration. Packaging requirements are determined by the end-use conditions and are less stringent for room-temperature and below-room-temperature applications, such as dry good or refrigerated packaging, than they are for higher-temperature applications such as boil-in-bag and hot-fill condiment. Members of the flexible-packaging industry – end-users, converters and raw-material suppliers – continue to work together to ensure that regulatory and performance requirements are met. Heightened awareness of potential migratory components, ever-changing film developments and the move from solvent to non-solvent waterborne and solventless technologies have prompted increased testing for migratory components and a review of existing standards.
Potential extractable components from new adhesive systems include monomers, oligomers and any additives formulated into the adhesive. However, through the proper selection and use, monomers and proper control of polymerization factors (such as crosslink density and reactivity ratios), adhesives can be designed to be essentially non-migratory.
The inner-sealant film type as well as the adhesive composition play critical roles in controlling the relative amount of extractable components (if any) that could contaminate the package contents. In addition, potential extractable migrants are dependent on film and adhesive thickness, as well as package contents and end-use conditions. Dry goods would have little chance of absorbing migrants from the film or adhesive, whereas aqueous or fatty foods would have a greater tendency. The proper selection of films, printing inks and adhesives is critical for designing a reduced or non-migratory package for a desired packaging application.
ConclusionThe adhesive marketplace offers many opportunities for film-laminating converters to select products that not only meet end-use requirements, but improve package performance. Future formulations will continue to be developed to address new compliance measures and to meet the increasing demand for flexible packaging in place of rigid containers.
Additional information on H.B. Fuller® adhesives for flexible packaging is available from H.B. Fuller Company, Marketing Dept. 3530 Lexington Ave. N., St. Paul, MN 55126-8076; phone 888-HBFULLER; fax 651-236-3128; or visit the company’s Web site: www.hbfuller.com.