Table 1 shows a schematic of how various process variables affect adhesion. For a given tie-layer, increasing the contact time usually increases adhesion since there is a longer time for the chemical reaction and chain entanglement to occur. Contact time is affected by processing variables such as chill roll temperatures, line speed and die design.
The application process can also affect adhesion. High interfacial stress can lead to lower adhesion. Orientation during or after processing (e.g., thermoforming) increases interfacial stress and also reduces the number of reaction sites per unit area, usually leading to a decrease in adhesion. Properties of the tie-resin, such as shrinkage, physical properties and graft levels, are also important. If one layer in a structure has high shrinkage compared to the other layers, interfacial stress can reduce adhesion or the film may curl. Higher melt and die temperatures generally improve adhesion since the rate of reaction and diffusion (i.e., chain entanglement) is accelerated.
Figure 1 shows that increasing the heat-seal temperature increases the adhesion strength of modified EVA to nylon 6. While higher temperatures generally improve adhesion, it is important not to exceed the recommended processing temperatures due to potential processing problems, gel formation or even resin degradation. EVA resins are more sensitive to degradation compared to other polyolefins. Anhydride-modified polyolefins (AMP) tie-layers are typically processed at 400-450°F but non-EVA tie-layers can be processed as high as 620°F under controlled conditions. Thermal stability of the tie-layer will depend on both time and temperature in the extruder. The thickness of the tie-layer also affects adhesion. Increasing the thickness of the tie-layer, which increases the level of reactive sites and amount of material available to absorb energy, generally increases adhesion up to a plateau value. Figure 2 shows the beginning of the plateau region for a modified LLDPE tie-layer.
At a critical point, adhesion increases rapidly as a function of thickness as shown for a LLDPE tie-layer. Therefore, not using enough tie-layer could result in adhesion failures. It is critical to test products under the most extreme conditions including potential variations in processing temperature or tie-layer thickness.
The properties of the resin will also affect adhesion. Figure 3 shows the adhesion strength of various LLDPE tie-layer resins in a cast film structure of HDPE/tie/EVOH/tie/HDPE. This difference in adhesion performance between these LLDPE tie-layer resins can be attributed to the overall physical properties of the resin and the level of functionality. The physical properties of the resin can be enhanced with modifiers and the functionality can be adjusted for the application. Again, the application process can also affect adhesion.