Mixing in reactive extrusion of low‐density polyethylene melts: Linear vs. branched

Abstract

The melt flows of linear low‐density polyethylene (LLDPE) and branched low‐density polyethylene (LDPE) have been compared in a fully intermeshing co‐rotating twin‐screw extruder. The polyethylene melts were selected in order to investigate the effects of the melt rheology on the mixing. Their shear vicosity curves are quite similar, but the LDPE has a markedly higher apparent extensional viscosity over a wide range of stretch rates. The stagger of the paddles in the mixing zone of the extruder creates axial pressure‐driven axial flow can have significant extensional strain components. Residence time distributions obtained in the melt zones of the extruder with tracer dye reveal that the LDPE has a narrower residence time distribution than the LLDPE over a wide range of operating conditions. The axial dispersion for the LDPE is significantly lower than the axial dispersion for the LLDPE. This is attributed to the greater extensional viscosity of the LDPE. During the reactive extrusion process, solid maleic anhydride and polyethylene were added at the feed port but the peroxide provides better control of the crosslinking reaction. Residence time distributions measured for the chemically more reactive LLDPE melt indicate reduced levels of axial mixing with reaction. The reduction in mixing is due to a crosslinking reaction that occurs in parallel to the grafting reaction. This change in mixing is smaller than the difference in mixing between LDPE and LLDPE.

Flow behavior in a corotating twin‐screw extruder of pure polymers and blends: Characterization by fluorescence monitoring technique

“…Chen et al27 found that a polymer with lower viscosity (higher melt flow index) had longer t, but its axial mixing (dispersion) quality was not affected by viscosity. Polance and Jayaraman13 revealed that polymers with lower extensional viscosity had shorter t and better axial mixing quality. In the present study, we investigated t and Pe number (the latter being a measure of the axial mixing quality), given by eqs.…”

“…Generally, the RTD of pure polymers in TSE was widely investigated 4, 12–16. Its variation as a function of processing parameters, geometry of the equipment, and polymer properties (such as shear or extensional viscosity, elasticity, etc.)…”

“…The authors also showed that gear blocks performed better axial mixing than kneading blocks. Comparative measurements of RDT for low‐density polyethylene (LDPE) and linear low‐density polyethylene (LLDPE) revealed that LDPE had a narrower RTD and lower axial mixing than LLDPE because of its higher extensional viscosity 13. Elkouss et al14 studied the RTD for binary polymer blends with different and relatively similar viscosities.…”

Epoxy functionalized poly(lactide) reactive modifier for blown film applications

“…2,3 Chain extenders have been widely used to improve the rheological properties of polyesters by reacting with either the carboxylic acid and/or hydroxyl end groups. The blown film process requires polymer melts to exhibit a certain level of melt strength as well as nonlinear viscoelastic properties like strain hardening.…”

“…Long-chain branching or a small fraction of high-molecular-weight polymer can be introduced to neat PLA to improve these rheological properties. 2,3 Chain extenders have been widely used to improve the rheological properties of polyesters by reacting with either the carboxylic acid and/or hydroxyl end groups. For example, diisocyanates have proven to be an effective chain extender of PLA and improve its rheological properties.…”