Plastic Compounding: The 5-Step Process, Equipment and How Additives Are Incorporated

Plastic compounding is the preparation of a plastic formulation by mixing polymers and additives in the molten state, and it runs in 5 steps: feeding, conveying and melting, mixing, strand cooling and pelletising. The output is a fully formulated compound in pellet form, so how does a compound differ from a masterbatch or a dry blend? A compound is fully formulated and ready to process without adding anything else, a masterbatch is a concentrate that carries 40 to 65 wt% additive in a carrier resin and is let down into natural polymer at 1 to 5 %, and a dry blend is unmelted PVC powder that has not yet passed through a melt. Every one of those forms has to survive the same thermal window: plastics melt and are processed between 150 and 320 °C, and every additive in the recipe has to be stable across that range.

This page works through the 5 steps in process order, the twin-screw extruders, co-kneaders and internal mixers that run them, the 4 routes by which an additive reaches the polymer, the additive families and dosage bands that belong at the compounding step, how the recipe changes from PVC to PP to PET, the 4 degradation modes the melt pass itself causes and how stabilizer packages answer them, how a finished compound is qualified against a specification, and what the process costs and who runs it. It sits under plastic formulation as the process-level page in that section, and it closes with what compounding means for a plastic's recyclability.

Steps 1, 2, 3 and 5 are the chain given in the Wikipedia entry Plastic compounding; devolatilisation, pressure build-up and melt filtration are separate unit operations on production compounding lines.

What Is Plastic Compounding?#

Plastic compounding is the preparation of plastic formulations by mixing or blending polymers and additives in a molten state, and the product is a pelletised compound that a converter can process without adding anything else. Polymer compounding names the same operation: both terms describe mixing a resin with its additive package under heat and shear until the melt is homogeneous, and both end in the same pelletised output. The process needs two distinct kinds of mixing, dispersive and distributive, plus heat management that keeps every ingredient inside its thermal window, and it runs on one of three equipment classes: co-kneaders, twin-screw extruders in co- and counter-rotating form, and internal mixers.

A compound is not the same thing as its two neighbouring forms. It is fully formulated, meaning every ingredient the converter needs is already in the pellet at its final concentration, where a masterbatch is instead a concentrate that gets diluted at the converter's machine and a dry blend is a powder mix that has never passed through a melt. Which polymer and additive combinations justify that extra melt pass depends on the family involved: high loadings, reinforcements and flame retardants are compounded because no other route can hold that much material accurately, and the 43 families of plastic additives that a compounder can dose into the melt each have their own reason for taking, or avoiding, that route.

Why are plastics compounded?#

Plastics are compounded for 5 reasons: to reach a property target, to cut material cost, to make the polymer processable, to set the appearance, and to meet a specification that names the compound rather than the resin.

• Property target, for example intumescent ammonium polyphosphate at 22 to 30 wt% of a polypropylene compound for a UL 94 V-0 fire rating, or aluminium trihydrate or magnesium dihydroxide at 160 to 185 phr (about 60 wt%) in a halogen-free flame-retardant cable compound.
• Material cost, where calcium carbonate at 20 to 40 % of a polypropylene compound replaces resin volume with a lower-cost mineral.
• Processability, where a rigid PVC pipe compound carries a lubricant package of paraffin wax, calcium stearate and polyethylene wax to control fusion during extrusion.
• Appearance, where a black polyethylene pressure pipe is compounded at 2.0 to 2.5 wt% carbon black for UV protection and colour consistency.
• Compliance, where a polyethylene pressure pipe compound must reach an oxidative induction time of at least 20 minutes at 210 °C under EN 12201-1 and ISO 4427-1, which itself requires a ready-made pre-compound rather than a dosed-at-the-converter recipe.

Each of these decisions is a plastic formulation problem before it is a machine problem.

What is the difference between compounding and extrusion?#

Compounding and extrusion use the same class of machine but have different outputs: a compounding line extrudes a melt in order to make pellets, while a converting line extrudes the same kind of melt in order to make a pipe, a film or a profile. Both processes push a molten polymer through a screw and a die, and both depend on the same barrel heating and screw-transport mechanics, but a compounding line ends at a pelletiser and a converting line ends at a forming tool. Twin-screw extruders are all but mandatory for compounding, because adequate mixing of the additive package into the resin is essential and a single-screw machine cannot deliver it reliably, while single-screw extruders remain common in converting, where the melt is already homogeneous and only needs to be shaped. In general plastics extrusion practice, a vented two-stage screw typically runs an L/D ratio of 36:1, a proportion chosen to give the melt enough residence length for both plasticating and devolatilising. Screw configuration, side feeding and barrel layout are covered in detail under twin-screw compounding.

Is compounding the same as making masterbatch?#

No: a masterbatch is a concentrate, not a finished material, and it carries 40 to 65 wt% additive in a carrier resin so that the converter can let it down into natural polymer at 1 to 5 %. Making masterbatch is itself a compounding operation, but the target composition is inverted: instead of a fully formulated pellet, the line produces a deliberately over-dosed pellet that only becomes usable once diluted. Masterbatch production commonly uses a split feed, with the carrier polymer fed through the main throat and pigments introduced by a twin-screw side feeder downstream of the melting zone, which gives gentler wetting and fewer agglomerates than feeding everything together. The alternative premix route blends every component into one dry mixture first and feeds the whole mix through a single volumetric feeder, which is simpler but gives less control over pigment dispersion. Types, carrier resins and let-down ratios are compared on masterbatch.

The 5 Steps of the Plastic Compounding Process#

The plastic compounding process runs in 5 steps: feeding the polymer and additives, conveying and melting them in the heated barrel zones, mixing the melt dispersively and distributively, cooling the extruded strands in a water bath or spray, and cutting them into pellets.

1. Feeding and dosing the polymer and the additives#

Feeding sets the recipe: every ingredient is dosed against the polymer, in phr where the polymer is always 100 parts, or in wt% and ppm of the finished compound. The polymer and the higher-dose ingredients typically enter through the main feed throat, while low-dose or heat-sensitive additives are introduced by a side feed further down the barrel, and the choice between premixing everything first and splitting the feed streams depends on how far apart the dosage levels sit. Converting between the two units is a fixed calculation: wt% of an ingredient equals its phr divided by the sum of all phr in the recipe, multiplied by 100.

The worked compound below is the range composition published by the Plastics Pipe Institute in TR-2 (2023) for a PVC pressure pipe, and it is the only fully sourced recipe in our source library, so no other compound formula on this page should be read as a universal PVC starting point.

Plastics Pipe Institute, TR-2 (2023), Appendix C. Total 108.03 parts; wt% = phr / 108.03 x 100.

Dosage sits in four bands across a compound, from ppm-level clarifiers and acid scavengers up through percent-level plasticizers and fillers, and feeding hardware has to hold each band to the accuracy the additive needs. The conversion in both directions is worked through on parts per hundred resin (phr).

2. Conveying and melting in the barrel zones#

The screws convey the feed forward while the barrel zones heat it, and plastics reach a melt state between 150 and 320 °C depending on the polymer. Solid pellets and powders are dragged forward by the screw flights, compacted, and progressively heated by a combination of barrel-wall conduction and shear from the screw itself, until the feed converts from discrete solid particles into a continuous melt somewhere along the barrel length.

The thermal window is narrow for some polymers and wide for others, and the recipe has to respect the tightest constraint in it. PVC loses hydrogen chloride slowly starting from 100 to 120 °C and degrades rapidly near 250 °C, which is why rigid PVC compounding stays at the bottom of the processing window and typically runs on counter-rotating machines that build the pressure needed at lower shear. The same sensitivity that limits the melt shows up in additive handling: phosphite antioxidants ship in aluminium-coated bags because they hydrolyse in ordinary moisture, and a stabilizer package that cannot survive the barrel's thermal history is the wrong package for that polymer. The mechanisms that start inside this window are set out on polymer degradation.

3. Dispersive and distributive mixing#

Mixing does two different jobs: dispersive mixing breaks pigment and filler agglomerates down to their primary particles, and distributive mixing spreads those particles evenly through the melt without making them smaller. Dispersive mixing needs high, localised shear stress to overcome the cohesive forces holding an agglomerate together, while distributive mixing relies on repeated splitting and recombination of the melt stream to carry particles into every part of the cross-section once they are already broken down.

• Dispersive mixing: breaks agglomerates of pigment, filler or additive particles down to their smallest units, typically in a high-shear zone such as a kneading block.
• Distributive mixing: spreads those already-broken particles uniformly through the melt without further reducing their size, typically through repeated flow splitting.

Wetting the pigment chemically is the job of dispersing agents for plastics and masterbatch, and mixing quality is judged downstream rather than in the barrel itself. Side-feeding pigments downstream of the melting zone, as Coperion describes for masterbatch production, gives gentler wetting and fewer agglomerates than feeding pigment through the main throat, though that specific advantage is documented for pigments and should not be assumed to extend to reinforcing fibres, whose feed position is set by fibre-length protection rather than wetting. Poor dispersion from insufficient energy during concentrate production is one of three named sources of gels in polyethylene film, and black masterbatch dispersion is reported per lot on a microdispersion scale from 1 to 5, with 1 the best rating, read on roughly 1.5 mil polyethylene film.

4. Devolatilisation, pressure build-up and melt filtration#

A production compounding line removes volatiles through a vent, builds melt pressure ahead of the die and screens the melt through a filter, and these three operations decide how clean the pellet is. Devolatilisation pulls off moisture, residual monomer and reaction by-products before they can turn into voids or surface defects in the finished pellet, and Coperion reports that side devolatilisation on a ZS-EG unit can raise line throughput by up to 30 %, since the vent no longer has to compete with the main melt channel for capacity.

The screen pack catches contamination and unmelted or degraded polymer fragments before they reach the die, and a fine pack is the first line of defence against gels. The documented remedies for gels include tighter screen packs, higher throughput, lower melt temperature, a modified screw design, smaller masterbatch pellets and a change of carrier resin, and identifying which of the three named gel sources is active has to come before any of those changes are made. Moisture is the specific concern for polyesters: excess moisture cleaves PET chains through hydrolysis, which is why polyester resins are dried or vented rather than compounded straight from the bag. Identifying the gel source before changing the screen pack is the subject of gels and fisheyes in plastic film.

5. Strand cooling and pelletising#

The melt leaves the die as strands, is quenched in a water bath or by spray, and is cut into pellets, which is the form in which every injection moulder and extruder expects to receive material. Cooling has to be fast enough to solidify the strand before cutting but controlled enough to avoid internal stress or a distorted cross-section, and the cut itself sets the pellet's length and, with it, its bulk flow behaviour in downstream hoppers and feeders.

Pellet geometry is not a cosmetic detail: smaller masterbatch pellets are one of the documented fixes for gels, because a smaller pellet disperses into the host resin faster and more completely during the converter's own melt pass. A compound leaves the pelletiser in the same physical form as the resin it replaces, ready to drop into equipment built around pellet handling. The same strand-and-cut line makes concentrates, as described on how masterbatch is made.

Which Machines Compound Plastics?#

Three machine classes compound plastics: twin-screw extruders in co-rotating and counter-rotating form, co-kneaders, and internal batch mixers, and the twin-screw extruder is all but mandatory for continuous compounding because adequate mixing is essential. Each class trades continuous throughput against shear intensity and batch flexibility differently, which is why a single plant commonly runs more than one class side by side rather than standardising on one machine for every recipe.

Machine classes from the Wikipedia entry Plastic compounding; the co-rotating and counter-rotating comparison from Plastics extrusion; throughput figures from the Coperion ZSK product page.

Co-rotating and counter-rotating twin-screw extruders#

Co-rotating and counter-rotating twin-screw extruders trade mixing against pressure: at the same cross-section and degree of intermeshing, the co-rotating machine reaches a higher axial velocity and a higher degree of mixing, while the counter-rotating machine builds more pressure. That trade-off is why co-rotating machines dominate polyolefin stabilisation, masterbatch and filled or reinforced compounds, where dispersion quality is the priority, while counter-rotating machines remain standard for rigid PVC, which arrives as a powder from PVC dry blending and needs the higher pressure build-up to convey and compress a dry powder feed rather than an already-molten resin.

Scale-up from laboratory trial to production line is predictable within a machine family because the geometry stays proportional. Coperion holds the diameter ratio between the outer and inner screw diameter, and the specific torque, constant across every size in its ZSK series, which is what lets a formulation trialled on a small line transfer to a production line without re-optimising the recipe. Coperion's own ZSK MEGAlab trial line runs from 200 gram batches up to 40 kg/h, the scale at which a new recipe is proven before it moves to production, and Coperion reports over 15,000 compounding systems installed worldwide and co-rotating twin-screw expertise dating back to the 1950s.

Co-kneaders and internal batch mixers#

Co-kneaders and internal batch mixers cover the work a continuous twin-screw line does badly: heat-sensitive or very highly filled compounds on the co-kneader, and small or frequently changed batches in the internal mixer. The co-kneader combines screw rotation with an axial oscillation that continuously renews the surface exposed to the barrel wall, which keeps peak shear lower for a given degree of mixing than a twin-screw extruder achieves, though our source library holds no throughput, shear or temperature figures for that machine class. The internal batch mixer, named after its inventor Fernley H. Banbury and dating to 1916, applies intensive batch shear between two counter-rotating rotors in an enclosed chamber, which suits short production runs and frequent recipe changes better than a continuous line that has to be purged between batches.

How feeder accuracy limits additive dosing#

Feeder accuracy sets the floor on how little of an additive a compounder can dose reliably, and a clarifier at 150 to 200 ppm or a slip agent at 500 to 1,200 ppm sits below what a volumetric feeder can hold to spec. Two feeder types cover that range between them.

• Gravimetric loss-in-weight feeders: weigh the feed continuously and adjust screw speed in real time, which is what ppm-level accuracy requires; a Coperion K-Tron SFT load cell can resolve weight changes at 8,000,000 to 1 in 20 milliseconds.
• Volumetric feeders: meter by screw revolution rather than weight, which is adequate for higher-dose ingredients but not for ppm-level additives.

Because volumetric feeders cannot hold a ppm-level set point, ppm-level additives are dosed as masterbatch or as a one-pack instead of neat, and every family's band is tabulated on additive dosage levels in plastics. Side feeders extend that accuracy to difficult bulk materials: a Coperion ZS-B MEGAfeed side feeder handles bulk densities down to 20 kg/m3, which is the range that low-density recyclate flake and light fillers fall into and that a main-throat feeder cannot dose consistently. Check a set point against the recipe with the PHR to weight percent calculator.

How Do Additives Get Into the Polymer? The 4 Incorporation Routes#

An additive reaches the polymer by 4 routes: melt compounding into a finished compound, masterbatch let down at the converter's machine, dry blending in powder form, and direct dosing of a neat powder or liquid.

1. Melt compound the full package when the loading is too high or the specification too tight for anything to be added later, since a finished compound is the only route with gravimetric loss-in-weight feeders built into the production line.
2. Let down a masterbatch at the converter's own machine, which gives more accurate dosing of expensive additives, cuts dust, keeps longer in storage than a solvent-based alternative, and lets several concentrates be combined at the press; a liquid masterbatch is dosed by peristaltic pump for accurate metering and fast colour changes.
3. Dry blend a PVC powder with its stabilizer, lubricant, filler and plasticizer package before the first heat history, since PVC's stabilizer has to be present before the resin ever sees a melt.
4. Dose a neat powder or liquid directly at the machine when the additive is cheap, high-dose and does not need the accuracy a masterbatch provides.

Powder, granule, pastille and liquid grades are compared on additive product forms, and pre-blended packages are one-pack additive systems. A compounder should choose the route by dose level, by the cost of the additive and by how often the colour changes, since those three factors, more than any single machine feature, decide whether melt compounding, masterbatch, dry blending or direct dosing is the right choice for a given recipe. Convert any let-down between per cent and ratio with the let-down ratio calculator.

Which Additives Are Added During Compounding, and at What Level?#

Additive levels in a compound span four orders of magnitude: clarifiers and acid scavengers sit at 150 to 1,000 ppm, stabilizers at 0.05 to 1 %, PVC stabilizers, lubricants and impact modifiers at 1 to 10 phr, and plasticizers, fillers and mineral flame retardants at 10 to 70 %. That span is why a single feeder cannot serve every ingredient in a recipe: the lowest band needs gravimetric accuracy and the highest band needs bulk-handling capacity, and the compounding step is where both extremes meet in one melt pass.

High-loading families push that upper band furthest. Aluminium trihydrate or magnesium dihydroxide reach 160 to 185 phr, about 60 wt%, in a halogen-free flame-retardant cable compound, intumescent ammonium polyphosphate sits at 22 to 30 wt% of polypropylene for a UL 94 V-0 rating, calcium carbonate runs 20 to 40 % of a filled polypropylene compound, and reinforcing fibres load at 15 to 30 wt%. At the low end, calcium stearate as an acid scavenger tops out around 1,000 ppm in polyolefins, and more Irgafos 168 antioxidant is retained after compounding when calcium stearate is present in the same recipe, a synergy documented by Espelage et al. in 2025. In polyamide compounding, a die-bearding and drool reducer such as Struktol TR 063A is dosed at 0.2 to 2.0 % to control resin buildup at the die face.

How Does Compounding Differ from Polymer to Polymer?#

The polymer decides three things about a compounding job: the unit the recipe is written in, the route the additives take, and the damage the melt pass does to the polymer itself. PVC formulations are written in phr because the resin arrives as a powder in variable-sized batches, while polyolefin and engineering-polymer formulations are usually written in wt% and ppm because the resin is already pelletised and stabilised before it reaches the compounder. Every polymer has its own guide under additives by polymer.

Two mechanisms explain most of the difference between rows. Polypropylene chain-scissions during compounding, so its melt flow rate rises with every extra melt pass, while polyethylene does the opposite and crosslinks, which shows up as gels rather than a flow-rate change. PET, PC, PA and PLA share a separate vulnerability: they hydrolyse, and excess moisture cleaves PET's chains specifically, which is why polyester compounding always starts with drying or in-line venting rather than direct feeding from the bag. Filled and reinforced compounds add a third complication regardless of the base polymer: talc, kaolin and silica adsorb antioxidants and light stabilizers on their surface acid sites, so a filled compound needs a higher stabilizer dose than the same polymer would need unfilled, and desiccant masterbatch, where used to dry a hygroscopic resin in line, is dosed at 1 % of masterbatch per 0.15 % of moisture to be removed.

How Does Compounding Degrade the Polymer, and How Is That Prevented?#

Compounding is itself a heat history, and 4 degradation modes run during a single melt pass: thermal, thermo-oxidative, thermo-mechanical and, for polyesters and polyamides, hydrolytic.

• Thermal degradation: side-group elimination and chain scission driven by heat alone, starting with HCl loss from PVC between 100 and 120 °C.
• Thermo-oxidative degradation: oxygen attacks the polymer chain during the melt pass, chain-scissioning polypropylene so its melt flow rate rises and crosslinking polyethylene into gels.
• Thermo-mechanical degradation: shear scission accumulates over repeated melt passes, and recycled polypropylene shows a rising melt flow rate pass by pass as a direct result.
• Hydrolytic degradation: moisture cleaves ester and amide bonds in PET, PC, PA and PLA during the melt pass if the resin was not adequately dried beforehand.

The additive answer to all four modes is a stabilizer package sized to the specific melt history the recipe will see, not a generic dose. Heat stabilizers and acid scavengers address the thermal mode, primary and secondary antioxidants address the thermo-oxidative mode, and drying combined with carbodiimides or chain extenders addresses the hydrolytic mode in polyesters and polyamides. Fillers complicate that sizing further, because talc, kaolin and silica adsorb antioxidants onto their acid sites and remove them from the melt before they can do their job, so a filled compound needs more stabilizer than an equivalent unfilled one at the same processing severity. The colour version of the same damage is on why plastics turn yellow or pink.

Recycled polyolefins expose the cumulative effect most clearly, because each additional melt pass adds another dose of thermo-mechanical and thermo-oxidative stress on top of whatever the polymer already carried, which can leave a compound over-stabilised rather than under-stabilised if a fixed top-up dose is repeated every cycle without adjustment. Symptom-by-symptom fixes are on troubleshooting additive-related defects.

How Is a Compound Qualified? Tests and Specifications#

A compound is released against a specification rather than a recipe, and five properties carry that specification: melt flow rate, oxidative induction time, ash content, dispersion and colour. A specification states an outcome, so two compounds with different recipes can both pass the same specification, and a compounder is free to change suppliers or loadings as long as the certificate of analysis still meets every listed property. Every method is indexed under testing plastic additives.

ASTM D3895 and ISO 11357-6 are not technically equivalent; always state which method a value comes from.

Five tests appear on a typical compound certificate.

1. Melt flow rate under ISO 1133-1 or ASTM D1238, run through a standard 8.000 mm by 2.095 mm die and reported in grams per 10 minutes, an empirical measure rather than a fundamental material property.
2. Oxidative induction time under ASTM D3895 or ISO 11357-6, which holds the compound at an elevated isothermal temperature under oxygen and times how long the stabilizer package delays the onset of oxidation.
3. Ash content under ASTM D5630 or ISO 3451, which burns off the polymer and weighs what remains to check the filler loading against the recipe.
4. Dispersion rating, read on a thin cast film and scored on a 1 to 5 microdispersion scale where 1 is the best result, backed by a filter pressure value where finer confirmation is needed.
5. Melt stability by torque rheometry, which tracks torque against time on a small heated mixing head to find the stable processing window and the point at which the compound starts to degrade.

What Does Compounding Cost, and Who Does It?#

Compounding is costed per function, not per kilogram of additive: the cost in use of a compound is the sum of each ingredient's weight fraction times its price, and converting that to a cost per litre by multiplying by density is what shows whether a filler actually saved money. A filler such as calcium carbonate lowers the cost per kilogram of a compound because it typically costs less than the resin it displaces, but it also raises the compound's density, so the cost saving per litre of moulded part is smaller than the cost saving per kilogram of pellet suggests. Price a formulation per kg and per litre with the additive dosage and cost-in-use calculator.

Compounding happens at two very different scales, and a formulation typically moves between them before it reaches full production. A trial line such as Coperion's ZSK MEGAlab runs from 200 gram batches up to 40 kg/h, the scale at which a new recipe is proven, while a production polyolefin line compounds at up to about 135 t/h for HDPE, a scale at which virgin resin commonly arrives already stabilised rather than compounded separately. Scale-up between the two is predictable because the diameter ratio and specific torque of a Coperion ZSK series stay constant across every size in the range, so a compounder should choose in-house compounding when the recipe is proprietary or changes often, and buy a ready-made compound when the loading is standard and volumes do not justify a dedicated line. The directory of plastic additive manufacturers and suppliers lists who makes each ingredient, and segment and regional data sit on plastic additives market.

Get quotes for the additives in your compound: one request, several verified suppliers. Send one request to several suppliers with the plastic additive supplier finder.

Does Compounding Make a Plastic Easier or Harder to Recycle?#

Compounding does both: it is the step that restabilizes a recyclate and makes it usable, and it is also the step that locks in the additives that make a plastic hard to sort or reprocess later. Restabilization was introduced as a formal practice by R. Pfaendner, first at Ciba and later at Fraunhofer LBF, who published a 30-year review of the field in 2022, and it exists because post-consumer and post-industrial polyolefins are generally not topped up with antioxidant by default, so a recyclate that skips restabilization enters its next melt pass with whatever stabilizer capacity survived the first life. Songwon reports that its SONGNOX PQ additive at 0.1 % can cut gel counts in recycled LLDPE from about 210 to about 150 per 1,525 cm2, a result read from the supplier's own chart. Only about 9 % of the plastic produced up to 2015 had been recycled at all, and about 1 % had been recycled more than once, and the Lancet Countdown's 2025 report still puts the global recycling figure below 10 %. The full list of additive choices that decide recyclability is on design for recycling.

How many times can a plastic be compounded?#

There is no fixed number of passes: each melt pass shortens chains by shear scission, and the limit is reached when the melt flow rate leaves the converter's window rather than after a set count. Shear scission accumulates cumulatively, so recycled polypropylene shows a rising melt flow rate pass by pass, and the practical ceiling on reprocessing is set by whichever property specification the compound has to meet. In a closed loop with 500 ppm of a primary antioxidant and 1,000 ppm of a secondary antioxidant added every cycle, Irgafos 168 builds from about 650 ppm after cycle 1 to over 1,200 ppm after cycle 5, according to Knoben et al. in a 2025 paper in Materials, while unstabilised open-loop regranulate in the same study still held over 150 ppm of intact antioxidant. Only about 1 % of the plastic produced up to 2015 had been recycled more than once. Topping the package back up is covered on restabilization of recycled plastics.

Which additives make a compound harder to recycle?#

Four additive choices make a compound harder to recycle: a degradable or pro-oxidant additive, a carbon-black colour that near-infrared sorters cannot see, a filler load that moves the compound out of its density band, and a stabilizer package too weak to survive a second melt pass.

• Degradable or pro-oxidant additives: oxo-degradable plastic products have been banned from the EU market since 3 July 2021 under Directive (EU) 2019/904 Article 5, and the Association of Plastic Recyclers treats any degradable additive as making polypropylene and polyethylene packaging non-recyclable.
• Carbon-black colour: near-infrared sorting struggles to identify black and strongly coloured plastics, and the Association of Plastic Recyclers sets its detectable-black threshold at an L value above 40 and a near-infrared reflectance above 10 %.
• Filler loads above the density band: high mineral loading moves a compound's density out of the sort window a recycler expects for that resin type.
• A stabilizer package too weak for a second pass: masterbatches above 40 % carbon black should be avoided, since over-concentration makes accurate let-down harder.

A detectable black is described on NIR-sortable black colorants.

Do compounders check recyclate for legacy additives?#

Yes: a recyclate stream carries additives that were legal when the article was made, so a compounder buying recovered rigid PVC works against a lead limit of 1.5 % that runs only until 28 May 2033, after which the general limit of 0.1 % applies. That schedule comes from REACH Annex XVII entry 63, introduced by Regulation (EU) 2023/923, which set the general lead-in-PVC limit below 0.1 % from 29 November 2024. A separate limit governs food-contact recyclate: EU food-contact plastics carry an overall migration limit of 10 mg/dm2 and a generic specific migration limit of 60 mg/kg under Regulation (EU) No 10/2011. Which substances arrive with the stream is set out on legacy additives in recycled plastic, and entry 63 and the 2033 derogation are explained on REACH Annex XVII restrictions.

A short history of compounding machines#

Compounding machinery has three dated milestones: Fernley H. Banbury's internal batch mixer of 1916, Paul Troester's thermoplastic extrusion in Hamburg in 1935, and the first twin-screw extruders built shortly afterwards by Roberto Colombo at LMP in Italy. Banbury's machine predates continuous compounding and remains the ancestor of today's internal batch mixers; it was originally built for rubber, which is also where the unit phr comes from. Troester's 1935 work in Hamburg showed that a screw could plasticate and convey a thermoplastic continuously rather than in a batch, opening the path to extrusion-based compounding, and Colombo's twin-screw design followed soon after, becoming the basis for the co-rotating and counter-rotating machines that dominate compounding today, a lineage Coperion continues, reporting co-rotating twin-screw expertise since the 1950s and over 15,000 compounding systems installed. The additive side of the same story is on history of plastic additives.

---