Off-grade polyethylene costs less because the producer does not guarantee it. The lot fell outside the melt index and density window of the grade it was meant to be, so it is sold without that grade’s name, without a warranty that it meets a specification, and without the compliance letters and listings issued for named grades. It costs the producer no less to make: it comes out of the same reactors, on the same feedstock, as prime polyethylene grades.
The names on an offer do not change that answer, because the producer’s own split has only two sides: inside the grade window or outside it. Off-spec, wide-spec, near-prime and transition resin are the trade’s labels for material on the outside, and we found no standard that fixes the meaning of any of them.
The discount is not a recycled-content discount either, which matters if you were hoping to claim one. Off-grade is virgin resin sold as a product, so in our reading it does not meet the US FTC Green Guides’ test for recycled content, which requires material recovered or diverted from the waste stream.
Where Off-Grade Polyethylene Comes From
Most off-grade polyethylene is made while a reactor changes from one grade to another.
The polymer made during each changeover fits neither the outgoing grade nor the incoming one, and it has to be sold at a lower price. Start-ups, process upsets and a missed additive feed put more material outside the window.
The producer decides which lots are prime. Each grade is defined by a target melt index and density, and the producer sets its own interval around each target: a lot inside every interval is released under the grade name, and anything outside is sold as off-grade.
What the Off-Grade Discount Pays For
The off-grade discount pays for three things the buyer gives up: properties held inside a known window, the producer’s warranty against that window, and the paperwork issued for the grade name.
Properties Outside the Grade Window
An off-grade lot is usually out on melt index or density, the two properties a polyethylene grade’s window is drawn on. It may also carry gels or a different additive package.
- Melt index above the window: the melt flows more freely. Tensile strength at yield, impact strength, stress-crack resistance and melt strength all fall, shrinkage drops and gloss rises. A melt index below the window moves each property the other way, with a more viscous melt.
- Density above the window: stiffness, tensile strength at yield and shrinkage rise. Impact strength, low-temperature toughness and stress-crack resistance fall. Density below the window gives a softer, tougher resin with lower yield strength.
- Gels: gels in polyethylene film spoil its appearance and act as stress concentrators where it fails.
- Additive package: a lot made while an additive feed was off may carry less antioxidant or a different one. The antioxidant decides how steady the melt flow stays through repeated extrusion passes, which matters when edge trim and regrind go back into the extruder.
No Grade Name, Warranty or Compliance Letters
Off-grade polyethylene loses every guarantee and document the producer issues against a grade name: the warranty that the lot meets its specification, the food-contact letter, pressure-pipe listings and drinking-water certification.
On a prime sale the producer warrants conformance to a specification and owes a remedy when the lot misses it. Off-grade is the material that already missed the specification, so there is nothing for such a warranty to be measured against.
A food-contact assurance letter names the resin by trade name. Formosa Plastics’ food-contact letter for its Formolene HDPE grades states that it does not apply to generic, non-branded, rebranded or wide-spec resin, whether sold by Formosa or by others.
That letter has legal value to a converter. Under section 303(c) of the US Federal Food, Drug and Cosmetic Act, a good-faith guarantee from the supplier protects the buyer from penalties if a food additive it received turns out to be adulterated or misbranded.
Pressure-pipe ratings are tied to a name in the same way. The Plastics Pipe Institute’s TR-4 listing records each rating under the producer’s company name and a named listing, and the rating holds only for that formulation. Off-grade sold without a grade name carries no listing into pressure pipe.
Drinking-water certification under NSF/ANSI/CAN 61 is equally narrow. It applies only to products made under controlled processes from authorized materials at the original production location.
The grade name also stood for more than the two numbers on a lot certificate. A transition lot whose certificate of analysis shows melt index and density inside the grade’s datasheet range is still not that grade, and it can run differently, because its copolymer composition shifts with how the producer ran the changeover.