Stretch film is a plastic LLDPE (linear low-density polyethylene) film with the capacity of straining and having elastic recovery. Stretch film is used as a wrapping foil for protecting materials within the agricultural or pallet industry. Stretch film needs to have characteristics like good tensile strength, strain, high retention forces, tack, adhesion, puncture or tear resistance and a high elongation at break. The stretch film is wrapped around the material in several layers until the stability of the pallet is adequate. The layers of stretch film will attach to each other due to the tacky characteristics of the film. This tacky surface can be created through the adding of polyisobutene (PIB) as a tack agent during the production process. Using the right PIB for this LLDPE foil could be challenging because of the thickness of the foil but even more important is the quality of the PIB used.
The two most used methods to produce LLDPE plastic films are bubble blown extrusion (see below) in a ring die and extrusion through a cast die. Blown films are offering a higher tear resistance and a higher load and stretch capacity. The cast films on the other hand will give a higher clarity. Between 1 and 4% polyisobutene (PIB) is co-extruded and as it is immiscible with polyethylene, it migrates in the roll to the surface within 48 hours. Auto-adhesion forces, also named cling forces, will increase within time, the migration of PIB within the foil is completed after a maximum of 2 weeks. The produced stretch film needs to be stored at a minimum temperature of 20’C within those two weeks to have all PIB migrated to the surface of the foil. Main factors that affect migration are the molecular weight, the grade of the polyisobutene (PIB) used, the PIB quality, the molecular structure of the LLDPE film and the density. Besides this the morphology and crystallinity of the LLDPE are both of an important influence on the adhesion. In general mono and co-extruded blown films do have the same qualities at the end, such as tensile strength and elongation.
PIB with a higher molecular weight (e.g. 4200 g/mol) diffuses much slower through LLDPE films in comparison with lower molecular weight PIB (e.g. 1300 g/mol). PIB migrates immediately and faster through the bulk of an LLDPE with a lower density (e.g. 0,900 g/cm3) in comparison with a LLDPE with a higher density (e.g. 0,930 g/cm3). This results in a higher cling film. PIB with a lower molecular weight (e.g. 1300 g/mol) will result in much higher final cling force compared to PIB with higher molecular weight (e.g. 4200 g/mol). Therefore, the PIB with a higher molecular weight is not suitable for stretch film.
Adhesion of the stretch film increases if stored at a higher temperature due to the fact PIB migration to the surface increases with a higher temperature. Because of this characteristic less PIB is used during the summer within the film stretch. The pressure within the roll will increase due to this migration. It is of high importance that wrapping the clingfilm is done tight enough.
Besides PIB creating tackiness, it can also be used as a gas barrier in LDPE wrap foil. As e.g. the clingfilm will keep food fresh and immune to oxidation. PIB is a non-staining, non-toxic product and certain producers of PIB are FDA approved.