The world population will rapidly grow from 7 to 9 billion by 2050 and this will parallel a surging annual plastics consumption from today's 350 million tons to well beyond 1 billion tons. The switch from a linear economy with its throwaway culture to a circular economy with efficient reuse of waste plastics is therefore mandatory. Hydrocarbon polymers, accounting for more than half the world's plastics production, enable closed‐loop recycling and effective product‐stewardship systems. High‐molar‐mass hydrocarbons serve as highly versatile, cost‐, resource‐, eco‐ and energy‐efficient, durable lightweight materials produced by solvent‐free, environmentally benign catalytic olefin polymerization. Nanophase separation and alignment of unentangled hydrocarbon polymers afford 100% recyclable self‐reinforcing all‐hydrocarbon composites without requiring the addition of either alien fibers or hazardous nanoparticles. Recycling of durable hydrocarbons is far superior to biodegradation. The facile thermal degradation enables liquefaction and quantitative recovery of low molar mass hydrocarbon oil and gas. Teamed up with biomass‐to‐liquid and carbon dioxide‐to‐fuel conversions, powered by renewable energy, waste hydrocarbons serve as renewable hydrocarbon feedstocks for the synthesis of high molar mass hydrocarbon materials. Herein, an overview is given on how innovations in catalyst and process technology enable tailoring of advanced recyclable hydrocarbon materials meeting the needs of sustainable development and a circular economy.
Tailored polyethylene reactor blend additives (RB) with ultrabroad bimodal molar mass distributions comprise nanophase-separated ultrahigh molar mass polyethylene (UHMWPE) uniformly dispersed in polyethylene wax. During injection molding of high-density polyethylene (HDPE) together with variable amounts of the nanophase-separated HDPE wax/ UHMWPE (70/30) additive (RB30) flow-induced oriented crystallization affords shish-kebab fiber-like UHMWPE nanostructures accounting for efficient HDPE self-reinforcement. RB additives are readily prepared by ethylene polymerization on silica-supported two-site chromium catalysts which simultaneously produce HDPE wax together with disentangled nanoplateletlike UHMWPE. The presence of HDPE wax is essential for lowering melt viscosity at high UHMWPE content. Since HDPE wax crystallizes onto extended-chain UHMWPE shish to form kebab structures, high HDPE wax content is tolerated without encountering emission problems and impairing mechanical properties as observed in the absence of UHMWPE. This in situ reinforcement substantially improves HDPE toughness/stiffness/strength balance as reflected by simultaneously increased Young's modulus (+365%), tensile strength (+392%), and impact resistance (+197%). The performance of self-reinforced polyethylene (PE-SRC) is far superior to that of melt-blended UHMWPE/HDPE and the majority of PE nanocomposites. Neither hazardous UHMWPE nanoparticles nor alien inorganic nanofillers are required.
Tailoring trimodal
polyethylene (PE) molar mass distributions by
means of ethylene polymerization on three-site catalysts, supported
on functionalized graphene (FG), enables nanophase separation during
polymerization and melt processing, paralleled by PE self-reinforcement.
Typically, FG/MAO-supported three-site catalysts combine bis(iminopyridyl)chromium
trichloride (CrBIP), producing PE wax having high crystallization
rate, and quinolylcyclopentadienylchromium dichloride
(CrQCp), forming in situ ultrahigh molecular weight PE (UHMWPE) nanostructures,
with bis(iminopyridyl)iron dichloride (FeBIP) or bis(tert-butyl cyclopentadienyl)zirconium (ZrCp), respectively,
producing HDPE with variable intermediate molar mass. During injection
molding, the formation of shish-kebab fiber-like extended-chain UHMWPE
structures, as verified by SEM, AFM, and DSC, account for effective
self-reinforcement. Only in the presence of high UHMWPE content, PE
wax, usually an unwanted byproduct in HDPE synthesis, functions as
a built-in processing aid and enables the incorporation of much higher
UHMWPE contents (30 wt %) than previously thought to be tolerable
in injection molding. Whereas the incorporation of UHMWPE/PE wax blends
improves stiffness and strength, the simultaneous FG dispersion accounts
for substantially higher impact strength.
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