PP and PE recycled pellets: what to check before buying — and when the next lot behaves differently

Short answer. You cannot judge a lot by the MFR and density from a data sheet alone. Before buying, agree what data you will get with every lot and ask for a sample from the lot you are buying. Afterwards, compare every delivery with the lot that worked: the lot documents (CoA), the MFR measured under the same conditions, labelled samples and the process records from both periods. Tests such as ash content, DSC or FTIR are not automatically required for every lot unless the agreed specification or inspection plan says so; otherwise order them when a specific symptom points to them. Acceptance limits are agreed with the supplier for your product and your process.

First, establish what exactly you are buying

The words in an offer describe different things and are not interchangeable:

• regrind — plastic ground mechanically, without remelting; the particle size depends on the mill screen;
• regranulate (recycled pellets) — plastic that has been remelted and pelletised again; whether and through which screen the melt was filtered is something to establish with the supplier;
• origin is separate information: post-industrial waste (PIR) or post-consumer material (PCR), the supplier’s own waste or material from a third party, and from which products.

Ask what the supplier calls a lot. ISO 15270 distinguishes a quantity with one unique designation from a quantity produced under conditions considered uniform; it also describes homogenisation, i.e. evening out the properties within the material. Ask plainly: is a lot one big bag, one day of production or a whole truckload? The answer decides what the lot certificate covers and whether two bags from one delivery should behave the same (glossary entry Lot / batch).

Before the first order, ask for a sample from the same lot you will buy. How much you need depends on what the trial is meant to check; a trial on an injection moulding machine is described in the Polish article Material trial on an injection moulding machine (in Polish).

Symptom, possible cause and what to compare

The table does not point to the cause. It helps you collect data from both deliveries before you change the next parameter or file a complaint. Process notes are assigned to the technology: injection moulding or extrusion, including blown film.

Material, process or measurement?

Injection moulding guides divide the causes differently. Autodesk Moldflow lists material causes of dimensional variation (including differences in properties and pigmentation), process causes and machine causes. The Mold-Masters hot-runner guide points mainly to machine and mould causes and, on the material side, to too much regrind. Neither says which cause applies to your line. So check in parallel:

• the material — the documents and results of both lots, the regrind share, storage;
• the process — the machine and the tool (mould or die), temperatures and, in injection moulding, also holding pressure, back pressure and cycle time;
• the measurement — the same method, conditions and instrument for both deliveries.

Where do differences in the material itself come from? A recyclate carries the variability of the stream it was made from. In a Danish study of household waste, the processability of PE and PP depended on the kinds of packaging in the stream. Then there is the processing history: in laboratory studies, successive processing cycles of PP shortened the chains and raised the MFR.

A study of 10 commercial PE and PP recyclates from five suppliers (Gall et al., 2021) found foreign components in all samples — from fillers to other polymers — in amounts that differed between suppliers. That study compared suppliers, not successive lots from one supplier, so it does not say how much deliveries from one source vary. That has to be checked on your own lots.

What a TDS says and what a lot document says

A technical data sheet (TDS) describes the grade, usually with typical values. Examples from public data sheets (not SEMANAT offers): the data sheet of the recyclate Purpolen PP Y55 (Borealis / mtm plastics, 2023) states that the values are typical and are not to be used for specifications, and both that sheet and the PP regrind data sheet of Attero (2021) note that suitability for the application is checked by the buyer. According to the Purpolen sheet the producer homogenised deliveries in truckloads and still reserved that colour may vary between lots.

A certificate of analysis (CoA) gives results for a specific lot. Ask which parameters are tested for every lot and which only periodically. The CoA of one lot does not confirm the parameters of the next, so it is worth collecting the certificates and comparing them lot by lot — that is also how the LyondellBasell polyolefin injection moulding guide uses them.

MFR: compare only comparable results

MFR is the mass of melt that flows through the die of a plastometer under defined conditions. The result depends on the temperature and the load, which are chosen according to the material standard. A number without conditions is not fit for comparison.

MFR describes flow at one point, at a shear rate much lower than in processing. Two lots with a similar MFR can therefore behave differently, and a change in MFR does not say where it came from. When comparing lots, check:

• the test conditions (temperature, load) and the standard;
• whether both results come from the same laboratory or from an agreed method;
• whether the difference is larger than the spread of the measurement itself.

More on the conditions and limits of MFR: MFR / MFI: what it means — and what it does not tell you.

How to label and store samples

With every delivery:

• take samples from several bags according to an agreed plan; for regrind, take them from different places in the bag;
• label every sample: the supplier’s lot number, delivery date, bag number, sampling point, sampling date, what it was used for;
• keep a reference sample from the lot that worked until the following deliveries are used up — in a sealed container, away from moisture, heat and light;
• link the sample to the process record of the day the material went onto the machine.

Sampling of plastics waste and recyclates for testing is described in CEN/TS 16010. For external tests, agree the sampling plan with the laboratory.

Three levels of data when buying and comparing deliveries

1. Basic data and documents with every delivery

• polymer and form (regranulate, regrind, flakes, agglomerate) and, for regranulate, whether the melt was filtered and through which screen;
• the origin of the stream and whether the source is constant;
• the lot number and what the supplier calls a lot;
• MFR with value, tolerance, temperature, load and standard;
• a CoA or lot declaration with the test methods;
• for regrind and flakes: particle size fraction, share of fines, bulk density with the method;
• moisture with the method, if it matters in your process — also for material stored in the cold, because moisture can condense on cold material;
• packaging and storage.

2. Tests chosen by symptom and risk

These tests are not automatically required for every lot unless the agreed specification or inspection plan says so. Otherwise you order them when a specific signal appears or the product demands it. Compare every result with the reference sample, measured by the same method.

A single test does not settle the cause of a problem on the machine.

3. Requirements agreed between supplier and buyer

The standards EN 15345:2025 (PP recyclates) and EN 15344:2025 (PE recyclates) name the main characteristics of recyclates and the methods for testing them. They serve as a common basis on which the parties agree a specification. This guide gives no acceptance limits — you set them for your product and your process. To agree:

• which parameters, with which tolerance, are tested for every lot and which periodically (inspection plan);
• test methods and conditions, including the laboratory that decides in case of a dispute;
• whether admixture of another polymer is acceptable and how it is determined;
• how the supplier informs you about a change of waste source or process;
• what happens with a lot outside the specification.

What to send the supplier

“The material is different” gives the supplier nothing to check. Prepare:

• the numbers of both lots — the one that worked and the problem lot — and their CoA;
• your own results with the method, conditions and laboratory;
• a description of the symptom: what, since when, on which machine, in which product;
• process records from both periods and what else changed (regrind share, drying, machine, mould or die);
• photos and the kept, labelled samples of both lots.

Such a set lets the supplier compare the lots on their side and lets you tell whether the material, the process or the measurement changed.

Questions we hear

Are a matching MFR and density enough to treat a lot as the same?
No. MFR is one measuring point of flow, and density depends on the filler and on admixture of a second polyolefin at the same time, so on its own it does not show what has changed. Compare with the reference lot and watch the behaviour in the process.

Does every lot need DSC or FTIR?
Not automatically. These tests are required if the agreed specification or inspection plan provides for them; otherwise match them to the symptom. With every delivery, compare the agreed basic data with the reference lot.

How much regrind can be added to virgin material?
The sources give no single value. LyondellBasell recommends controlling the regrind share carefully, and Autodesk Moldflow — limiting it to the level recommended by the material supplier. The acceptable share follows from the product requirements, the grade producer’s recommendations and a trial; what matters is that it stays constant.

How SEMANAT can help

We match PP and PE regranulate and regrind to the product and the process. The delivery documents you send us support material selection: they help establish what to ask about and which material may fit your specification. SEMANAT is not a laboratory — we do not carry out tests, issue expert opinions or settle complaints.

Selling regranulate or regrind? Describe the lot on the page for suppliers.

Limits of this guide

• The figures and examples come from public data sheets and publications (individual materials and studies), not from SEMANAT offers or tests. Do not transfer them directly to your material.
• The guide gives no acceptance limits, MFR tolerances, allowable moisture, regrind share or process parameters — they depend on the product, the process and the contract.
• Blown film has its own requirements for gels and filtration, which this guide does not cover. Regranulates of PA and other hygroscopic polymers need a different approach to moisture.
• Food contact, medical products and recycled-content declarations require documentation of origin and an assessment of legal requirements that this guide does not cover.