Both natural and synthetic adhesives are used in packaging. The various types of adhesive that are commonly used in packaging and their share of the packaging market are shown in Table 3.1.
Table 3.1: Packaging Adhesives - Type and Market Share
Adhesives rarely contain a single component, and most are heavily formulated materials with a number of ingredients. These ingredients modify the properties of the adhesive so that a wide range of properties may come from a single family of adhesives. The modifiers are generally used to:
modify viscosity for coating,
control open time,
stabilize the bond against environmental factors such as moisture, UV, or heat.
Typically a packaging adhesive formulation may contain two or more of the following components:
Binder - the base polymer
Carrier - water or organic solvent
Plasticizers - to flexibilize the adhesive and improve low temperature properties
Tackifiers - to improve the pressure sensitive nature of the adhesive and give a correct balance between adhesive and cohesive strength
Fillers - to reduce cost and to inhibit excessive penetration into porous substrates
Thickeners - to adjust viscosity for easier application
Surfactants - to improve wetting or to provide stability to emulsions
Biocides and fungicides - to protect the adhesive against mold and other living organisms
Waxes - to adjust melt viscosity in hot melt formulations
Stabilizers - to protect the adhesive against moisture, UV, or heat from the storage or service environments.
Selection of a particular adhesive system depends on a number of factors, all of which can lead to problems unless they are properly understood or controlled. Common chemical, physical, and performance factors are shown in Table 3.2.
Table 3.2: Factors Affecting the Selection of Flexible Packaging Adhesives
It should be noted that the adhesive is often chosen for more than only its bonding ability. In addition to holding the substrates together, the adhesive might have to perform certain other functions that are necessary to the success of the final product. These additional functions could include: