Polyethylene back to polyethylene

A weighed sample of unfilled polyethylene is converted to isolated ethylene, that ethylene is polymerized back to polyethylene, and the new polymer matches the starting resin in a tensile test run the same day.

Polyethylene is the plastic made in the largest amount, and it has no ester or amide to unzip. Cracking it makes a mixture of olefins. Returning a mixture of ethylene and propylene to a cracker is not the same as handing back the monomer and making the same polymer again. The grade is the isolated ethylene, then a tensile bar of the polymer made from it.

Bi, Chen, Lin, Han, and co-workers report carbon yields of ethylene and propylene together of up to 79% from polyethylene, over two zeolite stages, the first at 260 °C and the second at 540 °C, with no added hydrogen and no added olefin. They state that conventional catalytic cracking keeps the combined ethylene and propylene yield below 25%, and that pulsed-heating routes have reached 36–45% combined. The catalyst remained active over five cycles in their test. The main text reports the sum. It does not give the ethylene share of that 79%, and it does not polymerize the ethylene back to polyethylene or report a tensile test of a remade resin.

A route that co-feeds ethylene to make propylene is a different transformation. This problem does not count propylene, and it does not allow an olefin co-feed.

The starting material is an unfilled high-density polyethylene, identified by grade. A portion is molded into ASTM D638 bars. At least five bars are tested. The median ultimate tensile strength and the median elongation at break are the reference.

A separate weighed charge of the same resin, at least 1.00 g, is converted in a reactor whose carrier gas is nitrogen or argon. No hydrogen and no olefin is co-fed. The hottest zone is recorded. Ethylene is isolated as a gas or a condensed liquid, not left as a peak in a chromatogram of a mixed stream. The isolated ethylene is at least 95 mol% pure by gas chromatography against a calibration mixture. Carbon in that isolated ethylene is the amount of isolated gas, from its mass or from its measured volume, pressure, and temperature, multiplied by the ethylene mole fraction from that chromatogram. That carbon is divided by the carbon charged in the polyethylene. The graded yield is this carbon yield, not the gross mass of an impure cut.

Identified products, including propylene, alkanes, and residue, are quantified. Together with the isolated ethylene they account for at least 90% of the carbon charged, or the run is invalid.

The isolated ethylene, and no other monomer, is polymerized by a written procedure to polyethylene. The product is molded into bars of the same geometry as the starting resin and tested on the same machine, the same day, at least five bars. The clock from charging the polyethylene to the end of that tensile test is at most 14 days.

Carbon in the isolated ethylene is at least 80% of the carbon charged. The remade polymer’s median ultimate tensile strength and median elongation at break are each at least 80% of the starting resin’s medians. Purity, carbon closure, and the 14-day clock hold.

Eighty percent carbon yield, 80% of each tensile median, 95 mol% purity, 90% carbon closure, 1.00 g, and 14 days are pass marks defined here. Eighty percent as ethylene alone is above the reported 79% combined ethylene-plus-propylene yield, so it is above either share of that sum. The tensile comparison is not a number that paper reports.

• A reactor with no polyethylene, taken through the same temperature program and the same carrier flow, produces ethylene amounting to less than 5% of the ethylene isolated from the real charge. If it does not, the ethylene did not come from the polymer.
• The starting resin and the remade polymer are molded to the same thickness and pulled at the same speed. ASTM D638 warns that preparation, speed, and environment change the comparison. Five specimens and the median, rather than the best bar, are the pass rule defined here.
• Gas chromatography of the isolated ethylene is calibrated the same day with a mixture of known ethylene content.

Any catalyst, including zeolites, at any loading. Any temperature the apparatus can hold, including a stage near the 540 °C cited above. A laboratory polymerization of the isolated ethylene, including a Ziegler–Natta or metallocene procedure in a rated pressure vessel. Nitrogen or argon as the carrier.

An olefin co-feed, a hydrogen co-feed, or any carbon-containing feed other than the polyethylene and the catalyst. Counting propylene, butenes, or a mixed C₂–C₃ stream as ethylene. A chromatogram yield of ethylene that is not isolated and then polymerized. Polymerizing purchased ethylene. Quoting a literature tensile strength for polyethylene instead of the side-by-side test. Fillers or fibers in the remade bars that the starting bars do not contain. A charge below 1.00 g.

Carbon in the isolated ethylene is at least 40% of the carbon charged, the isolated ethylene is at least 95 mol% pure, and the remade polymer’s median ultimate tensile strength is at least 80% of the starting median. Elongation need not meet the 80% comparison. Forty percent and the strength comparison are pass marks for this lesser bar. They are not a solution. The main text of the 79% paper does not report this remake. If the ethylene share of that 79% is later read off and is already above 40%, the yield half of this lesser bar is not beyond that paper, and the tensile half is the part that remains.

Combined ethylene and propylene yields up to 79%, the statement that conventional cracking is below 25% combined, the 36–45% pulsed-heating range, and the stage temperatures 260 °C and 540 °C: Bi, Chen, Lin, Han, and co-workers, Nature Chemical Engineering, 2025, doi:10.1038/s44286-025-00290-y (opens in a new tab). ASTM D638-22, astm.org/d0638-22.html (opens in a new tab), for the tensile method and the warning that comparative tests have to control preparation, speed, and environment. The 80% isolated-ethylene yield, the 40% partial yield, the tensile ratios, the 95 mol% purity, and the 14-day clock are pass marks defined here. Open point: the ethylene-versus-propylene split of the 79% is not stated in the main text opened for this draft, so this problem does not assign an ethylene yield to that paper.