Influence of different functionality degree grafting monomers on polypropylene melt radical branching reaction and their structure

Abstract

Long-chain-branched polypropylene (LCBPP) is one of polypropylenes (PPs) with high melt strength and good melt elasticity. Recently, due to its outstanding properties, LCBPP have been attracted increasingly attention in the field of development and characterization by the researchers all over the world. In this study, LCBPP was prepared by the melt radical branching reaction in a torque rheometer. The influences of various acrylate monomers with different functionality degrees on the structure and melt performance of PP products were investigated.The results indicated that grafting monomers with different functionality degrees made diverse influences on the branching density and branching chain length of branching PP products. With the increase of the functionality degree of grafting monomers, the branching level of PP products increased gradually and the "multiplicity" of branches became increasingly obvious. Besides, a higher reactivity of pentaerythritol triacrylate with hydroxyl than the similar molecular structured pentaerythritol tetraacrylate was confirmed. Furthermore, due to the high reactivity of dipentaerythritol penta(hexa)acrylate, branching and crosslinking reaction occurred simultaneously during the reaction process. As a result, the gel content increased and finally formed highly star branching structures with a shape of "dense and short." 1 However, the application of the ordinary commercial polypropylene in some processes, eg, thermoforming, extrusion foaming, and blow molding, is limited because of its linear structure and low melt strength (MS). Thus, lots of researches are focusing on improving its MS. 2-4Some researches proposed that a small quantity of long-chain branches (LCBs) in PP could reinforce the MS obviously, which is owing to the augment of entanglements among the macromolecule chains. [4][5][6] In addition, long-chain-branched polypropylene (LCBPP) exhibits obviously strain hardening behavior in the MS. 7,8 The LCB can be introduced via copolymerized reaction during the stage of propene polymerization or through the branching modification of linear commercial PP using a method of post-reactive process. Due to the inductive effect of methyl, the tertiary carbon atoms in PP own a great reactivity, which makes PP more vulnerable to be oxidized and thus then results in β-scission under the action of heat or high-energy rays. Therefore, the method of melt radical branching reaction, as a simple and effective way, is widely applied to prepare LCBPP. [5][6][7][8][9][10] Nonetheless, this reaction process must be controlled strictly because of the complexity of free radical reaction, which mainly included coupled reaction among PP chains and degradation caused by β-scission. 11Some grafting monomers are often added into the radical reaction system to stabilize the unstable tertiary carbon radicals. Those monomers are usually electron donors, such as styrene, acrylic amide, and acrylate. They can be combined with the PP tertiary carbon radicals and stabilize them via their conjugative ef...

Improving melt strength of polypropylene by minimal branching and blending

“…The number of branches that can be estimated from the intrinsic viscosity data of PPb5E using the model of Zimm and Stockmayer and considering g ε = [ η ] b /[ η ] l with ε = 0.75 is approximately 0.06 LCB/1000 monomer units. This low number of LCB shows that the aim has been accomplished, being the branching degree lower than the results usually reported in the literature for branched PPs obtained with standard branching processes that seek to improve the melt strength …”

“…Branching can be done directly during polymerization or by post‐reaction processing. The most frequently used post‐reaction mechanisms are irradiation and reactive processing . Particularly, reactive processing has aroused great interest because of its flexibility and relatively low cost as it can be implemented on an extruder, which is the most common equipment in industrial plastic processing.…”

“…These macroradicals are susceptible to β ‐scission and may react with functional monomers promoting the generation of branches and inhibiting the scission of the PP chains. The route frequently followed to obtain long‐chain branched PP (PPb) from PP involves one reactive step where a polyfunctional monomer is used together with a peroxide . In that sense, the most frequently used peroxides are dicumyl peroxide (DCP) and 2,5‐dimethyl‐2,5(tert‐butyl peroxy) hexane (DBPH), while the most chosen monomers are acrylates .…”

Influence of polypropylene topological structure evolution during melt branching reactive processing on its melt performances

“…It is obvious that there are 2 peaks appearing in the curve. The first peak is due to the melting of PP granules, and the second one is the reaction peak, which is absent in pristine PP, indicating that branching reaction took place in the mixing process.…”

“…Figure C shows the reaction mechanism possible . In fact, there exist some kinds of free radicals in the radical reaction system initiated by DBHP, and the unstable tertiary carbon radical can easily lead to β‐scission and degradation of PP chains.…”