Polyethylenes in blown films: Effect of molecular structure on sealability and crystallization kinetics

Abstract

Three types of polyethylene (PE), with different molecular structure, were selected and used in the production of films using a semi‐industrial blown film extrusion line. Carbon nuclear magnetic resonance and crystallization analysis fractionation tests were performed on the PE polymers to assess their molecular structure. Sealability properties of the films, as well as crystallization kinetics, were evaluated and the results are discussed according to the molecular structures of the polymers. It was found that distribution of short chain branching (average comonomer sequence nH or nO) on the backbone of the PE chains are the main factor controlling the beginning of sealability. The completely random placement of the comononer on the polymer backbone (monomer dispersity 100%) generated crystals with smaller size and a lower melting temperature. Sealing was controlled by crystal distribution, chain diffusion, and entanglement formation at the interface. PE with lower melting point and linear molecular structure showed greater hot tack strength. PE with long chain branching showed a delay on seal process leading to lower sealing speed. POLYM. ENG. SCI., 57:52–59, 2017. © 2016 Society of Plastics Engineers

Designing polymer crystallinity: An industrial perspective

“…In general, the actual distribution of short chain branches influences the beginning of sealability, or Sealing Initiation Temperature (SIT). A perfectly random inclusion of branches causes indeed the formation of smaller lamellae in blown films, as also suggested by lower melting temperature of the films in the first heating scan of DSC . This may seem to provide a target for the polymer design to obtain low SIT, however low melting temperature is undesirable as it makes processing more difficult.…”

Understanding the factors affecting the seal integrity in heat sealed flexible food packages: A review

“…However, when the polymer shows a slower crystallization behavior or when the applied seal temperatures are much higher than the crystallization temperature, the hot tack seal strength will be more related to the interdiffusion rate than the solidification. 53 Also according to the study of Simanke et al, between two metallocene linear LLDPE fractions, it can be observed that the sample having a higher crystallization temperature shows better hot tack performance than the other one. This feature can be attributed to the immediate recrystallization of the m-LLDPE molecules after the sealing process, because of its higher crystallization temperature.…”

Microstructure–sealing performance linkage of metallocene linear low density polyethylene and low density polyethylene blends

“…In the case of semicrystalline polymers like polyethylene, the sealing temperature must be near the melting temperature of the matrix polymer to allow sufficient mobility of polymer chains across the interface. The seal strength, failure mode, and appearance of these seals after cooling to room temperature are important parameters for end use applications 15–19 . It has been established that the crystallinity of polyolefins and ethylene copolymers affected the seal strength 16 .…”

“…It has been established that the crystallinity of polyolefins and ethylene copolymers affected the seal strength 16 . Furthermore, the heat seal strength depends on the extent of molecular penetration, that is, intermolecular diffusion, across the melt interface during the sealing process 15,19 . Thus, sealing behavior of the LLDPE/LDPE would be depending on the intermolecular diffusion of the LLDPE and LDPE polymer chains during sealing process.…”