Circular Economy
Common problems with recycled pellets, and how to manage them
Recycled pellets are not always worse than virgin, but they are always more variable. Almost every root cause goes back to an uneven feed. This page collects the common symptoms, their causes and the step that manages each one, from sorting through to the specification.
Marketing Department, Advanced Chem Solutions · 22 October 2026 · about 9 min read
Plastic waste is growing
According to the OECD Global Plastics Outlook: Policy Scenarios to 2060, global plastic waste was 353 million tonnes in 2019 and is projected to reach 1,014 million tonnes in 2060, almost triple, without new policies. Only 9 % was recycled in 2019, projected to rise to 17 % in 2060 (OECD press release, 3 June 2022).
In Thailand, according to the government's Roadmap on Plastic Waste Management 2018–2030 from the Pollution Control Department, the phase 2 action plan (2023–2027) targets 100 % recycling of target plastic products and no target plastic waste sent to landfill. These figures say that more recycled feedstock is coming. They do not say it will get better on its own.
Variability is the real problem
A virgin grade comes from one plant, one recipe, one set of conditions. Recycled pellets come from thousands of different parts, with different grades, colours, additives and service histories. Recycling is therefore the work of squeezing that spread step by step until the pellets fall inside a window the customer's machine can handle.
Problems, causes and fixes
The last column names the step that manages each problem: sorting · washing · degassing · filtration · formulation · specification. Most problems need several steps at once, and fixing at the source is always cheaper than fixing downstream.
| Problem | Root cause | How it is managed |
|---|---|---|
| Odour | Substances absorbed into the plastic during use (food, perfume, detergents), inks, adhesives and thermal degradation products | Sorting: keep strong-smelling sources out · Washing: hot wash with detergent · Degassing: vacuum vents, sometimes several · Formulation: odour absorbers · Specification: odour panel with an agreed method |
| Gels and black specks | Paper, aluminium, wood, rubber, higher-melting plastics, and polymer burnt or crosslinked in dead spots of the machine | Sorting and washing: remove contaminants before melting · Filtration: screen fineness matched to the end use, or a laser filter for dirty feed · Specification: specks per area on a test film or plaque |
| MFI drift lot to lot | Feed from different grades (high-MFI injection parts, low-MFI bottles and film) and chain scission from degradation | Sorting: separate by part type · Formulation: blend grades to adjust MFI, add stabiliser against degradation during processing · Silo homogenisation · Specification: MFI range with ISO 1133 conditions |
| Colour variation, grey tone | Mixed colours in the feed, printing inks and labels, yellowing from degradation | Sorting: colour sorting with optical sorters · Washing: label removal, ink reduction · Formulation: adjust with colour masterbatch or place the grade in dark-coloured parts · Specification: L*a*b* and ΔE against a reference lot |
| Brittleness, low impact | Chain scission from heat and UV in the first life, or PE mixed into PP, which is incompatible and phase-separates | Sorting: NIR to separate PE from PP · Formulation: stabiliser, compatibiliser or impact modifier · Specification: impact to ISO 179/180 plus DSC |
| Moisture, voids, splay | Washed flakes not dried enough, moisture-absorbing fillers, or moisture-sensitive polymers such as PET and PA in the mix | Washing and drying: dewater and dry fully · Degassing · Formulation: desiccant · Specification: moisture on every lot, moisture-proof packaging |
| PVC contamination | PVC bottles or labels in the feed. PVC decomposes at PP and PE processing temperatures, releasing acidic gas that causes black specks, yellowing and machine corrosion | Sorting: NIR and sink-float (PVC sinks) · Specification: check when the source changes · If it keeps appearing, fix the source, not the formulation |
| Metals, paper, sand | Caps, springs, paper labels, dirt picked up during collection | Sorting: magnets and metal detectors · Washing: sink-float removes heavy material · Filtration · Specification: ash to ISO 3451 |
| Feed that changes over time | Quantity and quality of feedstock change with the season, market prices and collection sources | Sorting: check incoming material before accepting it · Use several sources and hold feed stock · Formulation: adjust to that feed lot · Specification: tell the customer when the source changes |
| Price swings against virgin | Collection, sorting and washing costs do not fall with virgin prices; when virgin gets cheaper the gap narrows or reverses | Specification: specify only what the part really needs, do not over-specify · Work out cost per good part, including scrap, with the calculator |
| Regulatory and food-contact limits | Contaminants from the first use may migrate into food; assessed food-contact options for PCR PP and PE are very limited | Specification: use PCR in non-food parts, or ask for approval evidence for the process case by case; see Standards & certification |
| Overstated claims (greenwashing) | Counting in-process regrind as recycled, claiming shares that cannot be traced, or not disclosing mass balance | Specification: use the ISO 14021 definitions · keep lot records through the chain · word claims to match the evidence, and never cite a certificate you do not hold |
Part defects such as sink marks, warpage or flow lines may have nothing to do with the recycled pellets. Work through the causes step by step in the troubleshooting guide.
PE in PP: the invisible problem
PE and PP are both less dense than water, so a sink-float tank cannot separate them. The blend looks normal and its MFI may be on target, but PE and PP are incompatible. The material separates into two phases and the parts are more brittle than they should be, especially at low temperature.
- Detect: DSC shows separate melting peaks (PE at about 105–135 °C, PP homopolymer at about 160–165 °C)
- Prevent: NIR sorting before grinding, or sorting by part type at the source
- Mitigate: a compatibiliser or impact modifier in the formulation helps partly; confirm with impact testing
For the converter's view of recycled pellets that will not stay consistent, see Recycled PCR with inconsistent quality.
Why consistent quality makes recycling pay
Recycling only survives if someone buys the pellets again every month. A converter keeps using them only if they run without re-setting the machine for every lot, without extra scrap, and without complaints from their own customers.
- Lots are inconsistent the machine has to be re-set each time, scrap rises, stoppages increase
- Cost per good part rises the per-kilogram saving disappears into scrap
- The converter stops using it demand for recycled pellets falls
- Collection stops paying the waste goes back to landfill
The loop reverses when quality is consistent: converters use higher ratios, demand becomes predictable, and collection pays. Consistent quality is not a side issue; it is what makes the loop actually turn.
How to start without getting hurt
- Start with parts that tolerate variation: dark colours, non-glossy surfaces, no food contact
- Start at a low ratio and step up while recording scrap and mechanical properties
- Specify only the properties that matter for the part, as ranges, with test methods
- Trial at least two or three lots; never decide on a single lot
- Keep a sample of every lot you use, to compare when something goes wrong
Circular grades are made in ACS's own plant (up to 3,000 t/year, being expanded) and are co-developed with customers. There is no minimum production quantity, so trials can start small.
FAQ
Questions about recycled-pellet problems
Why can't PE and PP be separated by flotation?
Both PE and PP are less dense than water, so both float in a sink-float tank. The tank only separates sinking plastics such as PET and PVC from the polyolefins. Separating PE from PP needs near-infrared (NIR) sorting before grinding, or sorting by part type, and the result can be checked by DSC, which shows separate melting peaks for the two.
Can recycled pellets cost more than virgin?
Yes, at times. The cost of collecting, sorting, washing and pelletising does not fall when virgin prices fall, so when virgin gets cheaper the gap narrows or even reverses. Decide on cost per good part, including scrap and downtime, not on price per kilogram alone.
Parts became brittle after adding PCR. What should I check first?
Check three things first: DSC to see whether PE is mixed into the PP, OIT to see whether enough stabiliser is left, and MFI to see whether chains have been cut by degradation. Then choose the fix, for example better sorting, added stabiliser, a compatibiliser, or a lower PCR ratio.
How can I tell whether a recycled-content claim is overstated?
Ask which definition is used: under ISO 14021, regrind fed back into the same process is not recycled material. Ask for documents that trace from the lot number back to the material source, ask whether the share is physically segregated or mass balance, and if a certificate is cited, check its scope and expiry with the certification body.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
Having trouble with the recycled pellets you use? Send them to us
Send pellets or parts with the symptoms, the ratio you use and your current spec. We will help you tell whether the cause sits in the feed, the formulation or the process. Free of charge, no obligation to buy.
