Ionomers, with their unique combination of high stiffness and excellent sealant properties, are found to provide greater versatility in downgauging than polyolefin sealants when trying to maintain or increase bending stiffness. As the industry drives towards downgauging and cost savings, the designer of multilayer composite structures will increasingly need to understand the engineering principles that influence stiffness and utilize materials with greater rigidity to achieve functionality without loss of stiffness. The stiffness of a material is characterized by its modulus, which in a tensile test is the initial slope of the stress vs. strain curve. The higher the modulus, the stiffer the material. The values vary over a range due to differences in material properties within a family of resins and how samples are prepared.
Moisture can affect the modulus of some materials, such as nylon.
This is because for a material to have good sealing properties, it must have a low melt point. The melt point of a polyolefin sealant is controlled by its crystallinity. This is illustrated for blown film in Figure 1 . The modulus of ionomers, however, are not controlled by crystallinity. Ionomers are acid copolymers that have been partially neutralized with sodium or zinc salts.