Question 4 of 6: Polyethylene Grades — HDPE, LDPE, LLDPE

Question 4: Polyethylene Grades — HDPE, LDPE, LLDPE (20 marks: 5×4)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Morphology and its molecular origin. HDPE is essentially linear with very few short branches, so its chains pack into thick crystalline lamellae, giving high crystallinity (~70–90%) and high density (0.94–0.97 g/cm³). LDPE carries abundant long-chain branches (plus short branches), which are steric defects that disrupt crystalline registry, lowering crystallinity to ~40–55% and density to 0.91–0.94 g/cm³. LLDPE has an essentially linear backbone but a controlled population of short-chain branches from a comonomer, with no long-chain branching; crystallinity and density fall between HDPE and LDPE (0.915–0.925 g/cm³). The molecular origin is thus the type and amount of branching: none/short → dense crystalline packing; long-chain → the loosest packing.

(b) Polymerization mechanisms. HDPE is made by coordination (insertion) polymerization at low pressure over Ziegler–Natta, Phillips (supported chromium oxide) or metallocene catalysts, which propagate linear chains with tight stereo/regio control. LDPE is made by free-radical polymerization at high pressure (1000–3000 atm, 150–300 °C) with trace oxygen or peroxide initiators; intramolecular “backbiting” chain transfer produces the short branches and intermolecular transfer produces the long-chain branches. LLDPE is made by low-pressure coordination copolymerization of ethylene with an α-olefin comonomer (1-butene, 1-hexene, 1-octene) over Ziegler–Natta or metallocene catalysts, the comonomer content setting the short-branch density.

(c) Heterogeneous vs. homogeneous catalysts for HDPE. Heterogeneous Ziegler–Natta/Phillips catalysts present several kinds of active site with different reactivities, so they give a broad molecular-weight distribution (PDI often 4–30) and a broad, non-uniform comonomer/short-branch distribution across chains. Homogeneous single-site (metallocene) catalysts have one uniform active site, giving a narrow MWD (PDI ≈ 2), uniform comonomer incorporation, and better-controlled molecular architecture — hence more uniform, better-defined product properties (though broad-MWD resins can process more easily).

(d) Rising rate in early gas-phase Ziegler–Natta kinetics. As polymer forms on and within the porous catalyst particle, the growing polymer fragments the catalyst/support, continuously exposing fresh active sites that were initially buried and inaccessible to monomer. Site activation and the relief of monomer-diffusion limitations as the particle breaks open add to the effect, so the observed rate climbs before mass-transfer resistance and site deactivation eventually cause it to decay.

(e) Low per-pass conversion in LDPE reactors. Free-radical ethylene polymerization is strongly exothermic (~−3.3 to −3.6 MJ/kg), and at high pressure ethylene can undergo runaway decomposition to carbon, methane and hydrogen. Because heat-removal capacity is limited, conversion per pass must be kept low (~20% autoclave, ~40% tubular) to keep the temperature rise bounded, avoid decomposition/hot spots, and control molecular weight and branching (which are temperature sensitive). Unreacted ethylene is separated and recycled, so overall utilization is still high.