Packaging films or any plastic products that are thin and are required to travel on a machine's metal surface at a fast speed, require slip agents. Slip agents are the special additives that improve machinability of such thin plastic products. Polyolefins are the largest candidate for packaging films that are typically thin and are used on packaging machines, making them the major user of slip agents.
Polymer nature and morphology, as well as crystallinity are the most important variables. Slip behaviour of plastic product depends upon the type of polymer. Slip molecules diffuse mainly through amorphous regions of a polyolefin, so migration is slower in more crystalline resins like HDPE and PP than in LDPE or LLDPE. LDPE being more amorphous compared to LLDPE and HDPE require larger addition of slip agent in order to achieve the same performance. Highly amorphous and tacky polymers, such as lower density mLLDPE grades (<0.92 g/cc) require high loadings of slip. Polar polymers like EVA or EMA can interact with the functional groups of a slip and retard its migration. Slip agents essentially perform only on the surface of a polymer and are therefore required to migrate from the matrix onto the surface in order to be effective. Migration of slips through polyolefins is slower in a wound roll than an unwound one. The greater the winding tension, the slower the diffusion rates.
Typically amides of different molecular weights are used as slip agents. Oleamide was the first slip agent and is still used in LDPE and to some extent in LDPE. However, being more volatile compared to erucamide, it is slowly losing its share to erucamide. However, in order to balance the migration, a combination of fast migratory and slower migratory amides such as oleamide and erucamide are often recommended and used. Erucamide being more thermally stable can perform at somewhat higher temperatures compared to oleamide, and being increasingly preferred.
Other slips based on secondary amides have almost twice the molecular weight of primary amides and thus migrate more slowly in polyolefins. Traditional primary and secondary amides are offered commercially in LDPE, LLDPE and mLLDPE carriers.
The recent developments of non-migratory slip additives, however, are threatening amides to give away its stronghold in this market. Newer non-migratory slip agents essentially have very large molecules. Typically, new slip masterbatch grades may be comprised of blends of primary slips or primary and secondary amide slips, as well as a variety of carrier resins such as PP, EMA and EVA.
The slip drawn away from the PE film surface can cause the COF to increase to 1.0 or more, which makes film movement more difficult through vertical form-fill-seal, printing, and other converting equipment. The loss of COF is more pronounced in laminated films that sit for a time before being converted or printed.
A recently introduced type of slip can avert this problem by remaining in the PE layer. 30 day tests found that some slip agent holds COF steady before and after lamination. This slip was compared with a conventional erucamide and a non-migratory slip in films with a three-layer coextruded structure commonly found in flexible food packaging—mLLDPE seal layer plus two octene-LLDPE layers. The films were laminated to a PET film with a solvent-based polyurethane adhesive. COF in the samples containing the new slip settled in at 0.2 before and after lamination. COF in the films with the standard erucamide jumped more than five-fold to between 0.68 and 0.85 after lamination, while COF in films with the non-migratory slip rose modestly from about 0.35 to about 0.5.
Experience indicates that the new slip formulation should perform similarly in other film structures, such as coextruded LLDPE films with an EVA sealant layer. Non-migratory slips typically yield COFs from 0.20 to 0.40, depending on use level. Besides the COF remains fairly consistent on a prolonged storage.
Unlike migratory slips, these types are only needed in the outer layers of multilayer films. That makes non-migratory slips more cost-effective in multilayer films, even though higher levels (1-2%) are required in those outer layers. Non-migratory slips reduce the need to overdose the amount of slip to create a reservoir in the film to allow sufficient migration to the surface. Also, the consistent COF with non-migratory slips eliminates one process variable, which helps reduce rejects during converting.
Non-migratory slips are stable at high temperature and may not adversely affect the heat sealing of a PE film if done at the appropriate temperature. They can also be used in cast films and withstand the elevated temperatures of hot filling, shrink tunnels, and other operations. Films made with non-migratory slips have good seal strength.
Non-migratory slip masterbatches are available with 10% and 20% slip for LLDPE blown films, LDPE cast films and extrusion coatings, and PP cast films. These slips can transfer across film surfaces on tightly wound rolls and they reduce film clarity more than primary or secondary amides.