The following content provides insights into the different chemistry options for Bostik's Laminating Adhesives and the factors a converter should consider when deciding which adhesive to use. It is also important to fully understand the other components used to create a package, such as films and inks, and ensure the adhesives perform well and are compatible with these components. Understanding these fundamentals can help converters choose which laminating adhesive is optimal for their end use.
Carrier Selection
Consideration begins with the carrier or support vehicle for the functional resin or polymer in the laminating adhesive. Some adhesives may be water-based, meaning water is both the support vehicle for the resin system and the volatile component of the adhesive. For water-based laminating adhesives, the chemistry is often acrylic, acrylic hybrid, or polyurethane dispersion.
Conversely, solvent-based laminating adhesives use a solvent as the carrier or support vehicle and is the volatile component of the adhesive. A wide range of solvents may be used, but most are ester or ketone in nature. Solvent-based adhesives are often polyether or polyester urethane and may be cured with either aliphatic or aromatic isocyanates.
The last class of laminating adhesives is known as solventless. These adhesive systems do not use a volatile carrier and are 100% reactive. Like solvent-based adhesives, they are most commonly polyether or polyester urethanes and may be cured with aromatic or aliphatic isocyanates.
Polurethane Laminating Adhesives
Polyurethanes are the most widely used laminating adhesive resins in the industry. They have a long track record of use, good overall adhesion properties, and a wide performance profile. The basic building blocks of polyurethanes are isocyanates and polyols. The reaction of these two creates urethane.
Aromatic isocyanates, the standard building blocks for laminating adhesive urethanes, are unsaturated cyclic compounds. They offer fast reactivity, good temperature stability, chemical resistance to laminates, and cost less. However, they are prone to yellowing in UV light and are not typically used in packaging that requires the highest FDA compliance. Aliphatic isocyanates, which are less common, are more expensive and have slower reactivity but excellent UV resistance. They can be used in applications with higher FDA requirements, such as hot fill and retort packaging.
The polyol component can vary in molecular weight and functionality but are of two basic types. Polyester polyols are the most common chemicals, available in a wide variety and formed by an acid and glycol reaction. Polyester polyols are typically straw-colored and offer fast reactivity with very good adhesion along with excellent heat and chemical resistance for laminates. Polyether polyols are less common, formed by a reaction of propylene or ethylene oxide and glycol. They are water-white and offer good low-temperature properties, flexibility, and hydrolysis resistance to laminates.
Often, a portion of the reaction between the isocyanate and polyol is done to form the adhesive; this is called a prepolymer. This NCO-terminated prepolymer is then cured with a polyol to form the cured adhesive. Alternatively, the adhesive component may be OH terminated and subsequently cured with an isocyanate to form the cured polyurethane adhesive.
These polyurethane-based adhesives can be supplied in a solvent or be solvent free. They may also be supplied in water and use many of the same building blocks, where they are known as polyurethane dispersions (PUDs). Another chemistry to note is also urethane based, but the reaction sites differ from what has been described. Instead of the NCO and OH reaction sites, this chemistry has amine and epoxy sites. This type of reaction eliminates the potential for outgassing in barrier-to-barrier laminate structures. Also, side reactions with moisture or alcohol solvents in inks are eliminated. This chemistry is widely known but usually limited to more general-performance applications that are indirect food contacting and are not exposed to high heat or chemicals.
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