The Basic Chemistry Of Laminating Adhesives
Laminating adhesives are supplied in a wide variety of chemistries and carriers. Often various chemistries can perform the same function in the end package and its up to the converter to choose the best system for their needs, and what may fit best with their application equipment. In addition, its important to understand the other components being used, IE films and inks and insure the adhesives perform with and are compatible with these. By having a basic understanding of these, will help the converter choose which chemistry offers the optimum benefit for their end use.
Let’s begin with the carrier or support vehicle for the functional resin/polymer. Adhesives may be known as water based, meaning water is the support vehicle for the resin system and the volatile component of the adhesive. Most often for water based the chemistry is acrylic, acrylic hybrid or polyurethane dispersions. Conversely solvent based adhesives use solvent as the carrier or support vehicle and is the volatile component of the adhesive. A wide range of solvents may be used but are most commonly ester or ketone in nature. Most often solvent based adhesives are polyether or polyester urethane and may be cure with either aliphatic or aromatic isocyanates. The last class of adhesives are known as solvent less. These systems do not use a volatile carrier but are 100% reactive. These are most commonly also polyether or polyester urethanes and may be cured with aromatic or aliphatic isocyanates (more on these differences in later paragraphs).
Next let’s compare and contrast the basic resin systems. The most widely used adhesives in the Industry are polyurethanes. They have a long track record of use, good overall adhesion properties and a wide performance profile. The basic building blocks of these are an isocyanate(either aromatic or aliphatic) and a polyol(either polyester or polyether). The reaction of these two create a urethane. Aromatic isocyanates, which are the most common, are unsaturated cyclic compounds. They offer fast reactivity, good heat and chemical resistance, and lower cost, but are prone to yellowing in UV light. In addition they are not typically used in packaging that require the highest FDA(21CFR 177.1390). Aliphatic isocyanates, which are less common, are higher in cost, have slower reactivity but have excellent UV resistance and can be used in higher FDA regulations such as the retort regulation mentioned above. The polyol component can vary in molecular weight and functionality but are of two basic types. Most common are the polyester polyols. A wide variety of these exist and they are formed by the reaction of an acid and glycol. They are typically straw colored and offer fast reactivity with very good adhesion, heat and chemical resistance. The less common polyether polyols are formed by a reaction of propylene or ethylene oxide and a glycol. They are water white in color and offer good low temperature properties, flexibility and hydrolysis resistance. Often a portion of the reaction is done to form the adhesive and is called a prepolymer. This is NCO terminated and is then cured with a polyol to form the cured adhesive. Conversely the adhesive component could be OH terminated and then cured with an isocyanate to form the cured adhesive.
The above adhesives can be supplied in solvent or be solvent free. Another option is that they are supplied in water and use many of the same building blocks. These adhesives are known in the industry as polyurethane dispersions(PUDS). However, many of the water based adhesives are known as acrylics or acrylic hybrids. These are emulsion polymers made by the free radical polymerization of acrylic monomers in water. Like the above-mentioned polyurethanes, a wide variety of acrylic monomers may be used. Once reacted they become a very high molecular weight polymer finely dispersed in water. They are milky white in appearance with low viscosity and high application solids. Due to their high molecular weight, they offer very high shear resistance and excellent weatherability. Recent advancements have pushed the performance of these adhesives well into the medium performance range with higher FDA compliance as well(21CFR 177.1395). One last chemistry to mention is also urethane based but the reaction/cure sites are different. 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 structures. In addition, side reactions with moisture or alcohol solvents in inks are eliminated. This chemistry is widely known but usually limited to more general performance type applications and only indirect food contact application (21CFR 175.105).
As one can see that there are many chemistry options available to choose from. From the carrier system to the types of backbones available. Having a very basic understanding of these will help guide the converter in the correct direction. Working closely with the adhesive manufacturer on these choices is essential. This will allow the converter to best optimize their adhesive choice for the range of applications and the equipment assets they have available.