Formulation development
Start from the part, not from the pellet

Set the target from the part

A recycled formulation has three inputs: variable recycled feedstock, additives that compensate, and the target set by the part. The formulation is where those three meet, and the output we need is a pellet specification that is stable enough and can be checked on every lot.

Before choosing any additive, agree the following with the customer:

• The part and its use: what loads it takes, indoor or outdoor, service temperature and expected life
• The process: injection, extrusion or blow moulding, which sets the MFI window the machine and mould can handle. See plastics processing
• The acceptance tests for the part: drop test, stiffness, dimensions, colour, odour. Use the part's criteria, not the values of virgin resin
• The target recycled share and how it is counted: some standards count PCR and PIR differently (see PIR vs PCR), and the customer may need specific documentation (see standards and certification)
• Regulatory limits: food-contact use of PCR PP and PE is very limited; check the rules of the target market before starting
• Volume and frequency: these set the lot size and the plan for blending feedstock lots

Characterise the feedstock

Recycled feedstock must be tested over several lots, not one, because what you are designing for is the range of the feed, not its average. If the feed is still dirty or heavily mixed, fix that first at sorting and preparation.

ACS does not have its own laboratory. We help define the test list and criteria; measurements are done at a partner or external lab, or at the customer's QC.

Formulation levers

No single additive fixes everything. Each lever addresses a specific problem and comes with a trade-off. Choose from the feedstock results and the part target.

1. Re-stabilisation

Phenolic primary antioxidants trap radicals over the service life, phosphite processing stabilisers protect during melting, and acid scavengers neutralise residual acids. Together they stop further degradation, but they do not reconnect chains that are already broken. The amount depends on the remaining OIT and the recycled share; reference ranges for an additive package are on the recycled PCR page.

2. Compatibilisers for PP/PE blends

PP and PE do not mix into one phase; they form two phases with weak adhesion. Compatibilisers, usually olefin copolymers or elastomers with affinity for both phases, strengthen the interface and reduce delamination and brittleness. Choose type and amount from the ratio measured by DSC.

3. Impact modifiers

Elastomers such as POE or EPDM restore toughness lost to degradation and mixed polymers. The trade-off is lower stiffness (modulus) and usually a lower MFI.

4. MFI adjustment

Blending a fixed share of virgin resin brings MFI onto target and keeps it stable, at the cost of a lower recycled share. For PP, controlled rheology (peroxide used to cut chains in a controlled way) raises MFI and narrows the molecular-weight distribution, which helps thin-wall injection moulding but reduces toughness. Lowering MFI is harder and is usually done by blending in lower-MFI pellets or flakes. Blending several lots before use always helps to reduce swings.

5. Odour control and degassing

Vacuum venting during melting pulls out volatiles, and odour absorbers bind part of what remains. Masking with fragrance does not remove the cause, and some customers do not accept it. Agree the odour criterion and how it is measured with the customer from the start.

6. Colour correction

Mixed-colour flakes tend to come out grey or brownish. Grey or black masterbatch hides colour differences between lots, and well-dispersed carbon black also protects against UV. But the achievable final colour depends on the starting flakes; light or bright colours need clear or natural flakes.

7. UV package for outdoor use

Recycled feedstock has already seen sun and heat and has little stabiliser left. Outdoor products such as pallets, crates and plant pots need fresh HALS and/or UVA; see plastic turning brittle in sunlight. Acidic contaminants reduce the effectiveness of some HALS types, so test the UV package in the full formulation, not on its own.

8. Fillers

Talc or CaCO₃ raises stiffness, reduces shrinkage and helps dimensional stability, at the cost of higher density, lower toughness and an ash value that must be stated in the spec. Count the filler already in the feed (from the ash result) before adding more.

Summary: lever, what it fixes, trade-off

Trial plan to a written spec

A recycled formulation is not finished when the lab results look good. It is finished when the part passes on the customer's machine and later lots keep passing in the same way.

1. Agree the target and criteria Part, process, acceptance tests and recycled share, written down in a short document before starting.
2. Characterise several feedstock lots Using the test list above, to know the range the formulation has to absorb.
3. Design starting formulations and run a pilot lot Usually 2–3 variants are compared, and the pellet properties are measured against the target.
4. Part trial on the customer's machine Same mould and settings as production. Compare three samples: the current material, the new formulation and straight virgin resin. Measure scrap, cycle and the part acceptance tests.
5. Adjust and trial again If it fails, work out whether the fix belongs in the feedstock, the formulation or the process settings.
6. Write the spec down Properties, accepted ranges, test methods (for example ISO 1133 for MFI), test frequency, retain samples, and the conditions under which a lot is rejected.
7. Repeat lots Watch the first lots closely, record results for every lot, keep retain samples and give advance notice when the feedstock source or formulation changes (change control). See from flakes to pellets and lot QC.

Cost per good part and the role of ACS

A well-formulated recycled grade can cost more per kilogram than raw recycled material, because it contains additives. What counts is the cost per good part, including scrap, cycle time and rejected deliveries. Put your plant's numbers into the Cost-in-Use Calculator and read the method in formulation economics.

ACS PCR / rPP compounds are sold under the Pure Earth brand. Circular grades are made in ACS's own plant (up to 3,000 t/year, being expanded) and are co-developed with customers following the steps on this page. We help define the test list and lot acceptance criteria; results are measured at a partner or external lab, or at the customer's QC.