Abstract:Films made of metallocene catalyzed polyethylene (mPE), low-density polyethylene (LDPE), and their blend were prepared to investigate how LDPE influences the hot tack of film. Experimental results showed hot tack is independent of film thickness. The addition of 30 wt % of LDPE can increase the hot tack of mPE film. The thermograms of differential scanning calorimetry (DSC) suggest the respective partial melting and recrystallization of those smaller size crystals at the bond forming and joint fracture stages … Show more
“…The data about crystal size and melting can be used to investigate the sealing properties of the films obtained from these mPEs. One of the main applications of mPEs as mentioned previously is in multilayer flexible packaging where they can be used as sealant layer . Seal strength and seal initiation temperature are the most important criteria about a sealant film.…”
Section: Resultsmentioning
confidence: 99%
“…Shih et al studied sealability of mPE in blends with LLDPE. They stated that incorporation of mPE into LLDPE would reduce the seal initiation temperature and improves the hot tack strength.…”
Three single site catalysts metallocene polyethylenes (mPE) different in molecular structure were selected and films from them were produced using a semi‐industrial cast film extrusion line. Rheological and gel permeation chromatography tests were performed on the mPE resins to assess their molecular structure. Mechanical, physical and sealability properties of the films were evaluated as well as flexural cracking and particles encapsulation (caulkability) and the results are discussed in relation to the molecular structures of the resins. It was found that molecular weight and distribution of short chain branching (comonomer) on the backbone of the polyethylene chains are the main factors that control sealability, flexural cracking and mechanical properties. The placement of comomoner on medium length chains generated crystals with smaller size that show a lower melting peak. Sealing was controlled by crystal distribution, chain diffusion and entanglement formation at the interface. mPE with lower melting point and linear molecular structure showed greater hot tack and seal strength. Polydispersity along with molecular weight contributed to toughness and puncture resistance. Flexural cracking resistance was revealed to be related to crystallinity, tie chain density and more importantly to fraction of amorphous phase. The amorphous part could absorb flexural energy and hinders crack initiation and propagation. Seal through contamination (caulkability) was found to be related to flowability of the melt. The resin with stronger shear thinning in melt state and lower heat seal initiation temperature showed an improved caulkability.
“…The data about crystal size and melting can be used to investigate the sealing properties of the films obtained from these mPEs. One of the main applications of mPEs as mentioned previously is in multilayer flexible packaging where they can be used as sealant layer . Seal strength and seal initiation temperature are the most important criteria about a sealant film.…”
Section: Resultsmentioning
confidence: 99%
“…Shih et al studied sealability of mPE in blends with LLDPE. They stated that incorporation of mPE into LLDPE would reduce the seal initiation temperature and improves the hot tack strength.…”
Three single site catalysts metallocene polyethylenes (mPE) different in molecular structure were selected and films from them were produced using a semi‐industrial cast film extrusion line. Rheological and gel permeation chromatography tests were performed on the mPE resins to assess their molecular structure. Mechanical, physical and sealability properties of the films were evaluated as well as flexural cracking and particles encapsulation (caulkability) and the results are discussed in relation to the molecular structures of the resins. It was found that molecular weight and distribution of short chain branching (comonomer) on the backbone of the polyethylene chains are the main factors that control sealability, flexural cracking and mechanical properties. The placement of comomoner on medium length chains generated crystals with smaller size that show a lower melting peak. Sealing was controlled by crystal distribution, chain diffusion and entanglement formation at the interface. mPE with lower melting point and linear molecular structure showed greater hot tack and seal strength. Polydispersity along with molecular weight contributed to toughness and puncture resistance. Flexural cracking resistance was revealed to be related to crystallinity, tie chain density and more importantly to fraction of amorphous phase. The amorphous part could absorb flexural energy and hinders crack initiation and propagation. Seal through contamination (caulkability) was found to be related to flowability of the melt. The resin with stronger shear thinning in melt state and lower heat seal initiation temperature showed an improved caulkability.
“…Higher self-adhesion of metallocene polyolefin polymers compared to conventional PEs has already been reported. 46–49 Also, Figure 11 shows that all the polymers have the same elongational melt strength or strain hardening. Figure 11.Transient elongational viscosities at Hencky strain rate ɛ· = 1 s −1 , the dash-lines represent the linear behaviour where the transient elongational viscosity is equal to 3 times of the zero shear viscosity in shear flow (to facilitate the comparison between data, the curves have been shifted by a multiplication factor as indicated).…”
The influence of molecular architecture on interfacial self-adhesion above polyethylene film melt temperature was examined in this study. The investigated molecular structures include molecular weight (Mw), molecular weight distribution, long chain branch amount and distribution and short chain branch among and along polyethylene chains. The long chain branches concentration was quantified using gel permeation chromatography and short branches concentration using nuclear magnetic resonance techniques. The adhesion strength was measured immediately after melt bonding using a T-Peel test. The results showed that increasing Mw resulted in higher adhesion strength in linear metallocene ethylene α-olefins. Low long chain branch concentrations hinder reptation motion and diffusion, and result in lower adhesion strength. Low density polyethylene with highly branched chains yielded very low self-adhesion. A drastic difference in adhesion strength between metallocene and conventional linear low density polyethylene is attributed to homogeneity versus heterogeneity of composition distribution. The low interfacial self-adhesion in conventional polyethylene was concluded to be due to enrichment of highly branched low molecular weight chains at the film surface. These segregated chains at the interface diffuse before the high molecular weight chains located in the bulk.
“…The latter behavior is commonly explained in literature as the result of a reduced melt penetration depth derived from a lower flowability. 21 It can be hypothesized that in our systems the molecular interdiffusion of mLLDPE, having intrinsic narrow molecular weight distribution and Mw comparable to LLDPE, 10 which involve lower short-chain content in mLLDPE than in LLDPE, could be limited because long chains diffuse more slowly than short ones. In support of this, it is noticeable that the flow activation energy E a for mLLDPE is slightly higher than for LLDPE (see Table 1), as demonstrated elsewhere.…”
Multilayer films of a conventional linear low-density poly(ethylene/-octene) (LLDPE) and a metallocene linear low-density poly(ethylene/-hexene) (mLLDPE) copolymers were produced by cast co-extrusion process. Coextruded films were obtained by varying the position and the relative thicknesses of the two polymers. Mechanical (tensile and impact) tests, permeability, haze and hot tack measurements were carried out on the produced films in order to verify the effect of layer composition and position on the performances of the coextruded samples. Thermal and atomic force microscopy characterizations of the three-layer structures were also performed to correlate the resulting morphology with the film properties. The experiments demonstrated that, at fixed composition, the structures having the mLLDPE copolymer as external layers exhibit generally better mechanical performances and a widening in the sealability temperature window due to a lowering of 5 C in the seal-initiation temperature, with only a small increase (max 3.5%) in the film haze percentages. The oxygen permeability values do not seem to be significantly affected by the structure composition and layout.
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