Improving melt strength of polypropylene by minimal branching and blending

Abstract

Branched polypropylenes (PPb) with markedly improved melt strength were produced without significantly affecting the processability of the original PP. A two‐step process of functionalization with MA and crosslinking with m‐XDA was used, both by batch mixing and by extrusion. Branching degrees of ~0.06 LCB/1000 monomer units or smaller were obtained. All PPbs display clear and significant strain hardening, being the PPb obtained by extrusion the one that shows the largest melt strength. This polymer has a zero‐shear‐rate viscosity slightly smaller than that of PP while its strain‐hardening index is about 10 times higher. Moreover, the nonlinear behavior of PP at elongation begins at a time similar to its terminal relaxation time or larger, while the ratio of these times reduces significantly with branching. PP/PPb blends were prepared to extend the range of obtainable melt strength in PP. They display rheological behavior between those of the mixed polymers with slight positive deviation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 137, 48845.

Non‐isothermal Crystallization Kinetics, Thermal, and Rheological Behavior of Linear/Branched Polypropylene Blends

“…This behavior can be attributed to the existence of complex and branched molecular structures and the influence of their long periods of relaxation. [ 30 ] Similar results were observed by Guapacha and collaborators, [ 31 ] who stated that the Newtonian region is outside the experimental frequency range used. At high frequencies, where the viscoelastic response is mainly due to the dynamics of short segments of the macromolecules, the complex viscosity of H301 and BPP converges to close values.…”

Coloration and branching of polypropylene by reactive processing

“…Melt strength is defined as the maximum force at which a molten thread can be drawn under standard conditions before it breaks 3 . This property becomes enhanced by the presence of long‐chain branches (LCB) in the molecular structure of an otherwise linear material 4,5 …”

“…Reactive melt modification of linear PP to introduce LCBs involves using polyfunctional substances that act as chain‐linking agents (CA) 8 . Additionally, this process frequently requires the presence of an initiator, such as organic peroxides 4,5,9–11 . When using peroxides, macroradicals are generated by the attack of decomposed peroxide to PP, which then may participate in combination reactions or react with CA generating long‐chain branched PP (PPb).…”

“…LCBs can also be generated using a previously functionalized PP, such as maleic anhydride grafted PP (PPg), by direct melt mixing with a CA 12,13 . Since molecular scission and recombination are two competing reactions in this process, type and concentration of both peroxide and CA are key variables for obtaining a PPb of appropriate molecular weight with no gel fraction 4,5,8–11,13 . Monomers with functional groups like epoxy, alcohol, amine, acrylate, triallyl phosphate, vinyl silane, etc., have been used as CA of PP 4,5,8,11–13 …”

“…Presently, we consider two organic dyes with capacity to act as CA, aiming to simultaneously improve both fade resistance and melt strength of a linear commercial PPg. The selected dyes, basic fuchsin and disperse blue, contain primary amines in their molecular structure, which should react with anhydride groups of PPg 5,8,13 . However, both dyes have melting point much higher than typical processing temperature of polyolefins.…”