Acid Scavengers and Catalyst Neutralizers for Polymers: 4 Types, Mechanisms, Dosage and Selection

Acid scavengers are basic additives that neutralise the acidic residues left in a polymer by its catalyst, its chlorine content or its flame retardant; the 4 classes run from metal soaps such as calcium stearate, dosed at up to 1,000 ppm (0.1 wt%) in polyolefins, to magnesium-aluminium and calcium-aluminium layered double hydroxides and basic metal oxides. Acid that is not scavenged corrodes the extruder, shifts the colour and attacks the antioxidant package, so which class belongs in which polymer, and how much of it?

Acid scavengers are one of the 43 families of plastic additives and one of the 6 stabilizer families, next to antioxidants, PVC heat stabilizers, UV stabilizers, metal deactivators and hydrolysis stabilizers. An acid scavenger does not protect the polymer chain; it protects the additives that do.

This page defines an acid scavenger, traces the acid to its 3 sources, sets out the damage it causes, explains neutralization and interlayer anion exchange, describes the 4 types in order, sets calcium stearate against hydrotalcite and zinc oxide, gives the dosage in ppm and phr with the source type of every number, matches a scavenger to each polymer, covers the interactions with antioxidants and nucleating agents, names the test methods, sets out the EU and US food-contact positions, names the producers and closes with the complete list of 6 substance pages.

The 4 types of acid scavengers differ in how they bind the acid, what they leave behind and whether they move through the polymer.

Migration behaviour and comparative efficiency are supplier statements (Kisuma) unless marked otherwise. No published dosage range exists in our sources for types 2 to 4; see the dosage section.

What Is an Acid Scavenger in Polymers?#

An acid scavenger is a basic additive that neutralises the acidic residues in a plastic, above all the chloride-containing Ziegler-Natta catalyst residues in polyolefins and the hydrogen chloride released by PVC and halogenated flame retardants. If the acid is only a residue, why does it matter? Residual acid corrodes processing equipment, drives colour drift, deactivates phenolic antioxidants and hindered amine light stabilizers, and hydrolyses the phosphite in the stabilizer package, so a few hundred ppm of base protects a package that costs far more per kilogram than the base itself.

An acid scavenger is not an antioxidant, not a PVC heat stabilizer on its own, and not a metal deactivator. It does not intercept free radicals and it does not decompose hydroperoxides, so it does not stabilise the polymer against oxidation; it keeps the acid away from the substances that do that work. A PVC heat stabilizer binds hydrogen chloride as one of 3 jobs and also substitutes the labile chlorine atoms on the chain. A metal deactivator chelates the transition metal ion itself, rather than the acid the ion releases.

Acid scavenger, acid acceptor or catalyst neutralizer: are they the same?#

Yes: acid acceptor, acid neutralizer, antacid, HCl scavenger and catalyst neutralizer all name the same function, and the term chosen usually follows the polymer. Halogen scavenger is the sixth name in common use, and it appears wherever a halogenated flame retardant rather than a polymerisation catalyst is the acid source.

The vocabulary splits by polymer in a predictable way. In polyolefins the additive is a catalyst neutralizer or a catalyst deactivator, because the acid arrives with the catalyst. In PVC and in halogenated rubber compounds such as CR, CSM, FKM and NBR the same substance is an HCl scavenger or an acid acceptor, and Kisuma sells hydrotalcite into those roles as well as into flame-retardant compounds. In PLA and polyester systems the concept appears as an acid regulator, because there the polymer generates its own acid instead of inheriting it.

Where Does the Acid in a Polymer Come From?#

Acid reaches a plastic from 3 sources: chloride-containing catalyst residues left over from polymerisation, hydrogen chloride split off by PVC and halogenated flame retardants, and the carboxylic acid end groups formed when a polyester hydrolyses. The 3 sources differ in timing. Catalyst residue is present from the reactor onward, dehydrochlorination begins when the compound is heated, and hydrolytic acid builds through the service life of the part.

These acids are one branch of polymer degradation, alongside the thermal, oxidative and photo-oxidative routes, and they are the branch an acid scavenger addresses. The table below names the source, the acid released and the first thing that goes wrong in each case.

Ziegler-Natta catalyst residues in polyolefins#

Polypropylene and polyethylene carry titanium, aluminium and magnesium chloride residues from their Ziegler-Natta catalyst, and those chlorides release hydrogen chloride in the melt. High-activity catalysts leave little residue by mass, yet the residue is chloride, and chloride at the ppm level is enough to pit steel and to consume a stabilizer.

Timing decides how much damage the residue does. Espelage and colleagues (Polymers, 2025), working with polypropylene supplied by Borealis, found that more of the phosphite Irgafos 168 survived compounding when calcium stearate was present, and that the effect is particularly relevant immediately after polymerisation. The consequence for a resin producer is that the scavenger belongs in the first pelletising step rather than in the converter recipe, because by the time the resin reaches a converter the acid has already had its first opportunity.

Hydrogen chloride from PVC and halogenated flame retardants#

PVC starts to eliminate hydrogen chloride at 100-120 °C, and the released HCl catalyses further elimination, which is why every PVC compound carries an HCl-binding stabilizer system. The elimination runs along the chain and leaves conjugated polyene sequences that absorb visible light, so the colour change is the visible record of the acid.

In PVC the acid-binding job belongs to the stabilizer one-pack described on PVC heat stabilizers, where calcium soaps and hydrotalcite sit next to the zinc soap. Halogenated flame retardants create the same problem in polymers that contain no chlorine of their own, because a halogenated retardant works by releasing hydrogen halide into the flame and part of that halide is released early, in the extruder. Hydrotalcite is sold as a halogen scavenger for exactly those compounds.

Carboxylic acid end groups and hydrolysis in polyesters and PLA#

Polyesters and PLA generate their own acid: every hydrolysed ester bond leaves a carboxylic acid end group, and those end groups catalyse further hydrolysis. The process is autocatalytic, so a small rise in acid number accelerates the next step and the molecular weight falls with it.

Carbodiimides and the other hydrolysis stabilizers do the main work here, and a calcium hydrotalcite is added as the acid-regulating co-stabilizer. Hallstein, Metzsch-Zilligen and Pfaendner at Fraunhofer LBF in Darmstadt (Materials, 2024) combined calcium hydrotalcite with an aziridine inhibitor and improved both hydrolytic and thermal ageing stability in PLA.

What Damage Does Residual Acid Cause? Corrosion, Yellowing and Antioxidant Loss#

Residual acid causes 4 problems in a plastic: it corrodes processing equipment, shifts the colour, deactivates phenolic antioxidants and hindered amine light stabilizers, and hydrolyses the phosphite in the stabilizer package. The 4 problems are listed below in the order a compounder usually meets them.

• Equipment corrosion, on screws, dies and every metal surface the melt touches.
• Discoloration, as colour drift and yellowing during processing.
• Antioxidant and HALS antagonism, where the acid deactivates phenolic antioxidants and hindered amine light stabilizers.
• Phosphite hydrolysis, where the secondary antioxidant breaks down into acids, black specks and further corrosion.

The package under attack is described on antioxidants for plastics, where phenolic primaries and phosphite secondaries work as a pair. Losing either one shortens the service life of the part, which is why an acid scavenger is specified together with the antioxidant and not after it.

Corrosion is the damage with a number attached to it. Peter Greven reports that 500 ppm of calcium stearate prevented steel corrosion in a polypropylene test, a supplier test result rather than a published corrosion rate, and no corrosion-rate figures for unscavenged polyolefins are established in our sources.

Acid-driven colour drift belongs to the wider question of yellowing, pinking and gas fading in plastics, where the acid route is one of 3. Yellowing from residual catalyst acid, from phenolic antioxidant transformation products and from gas fading call for 3 different corrections, so the diagnosis comes before the formulation change.

How Does an Acid Scavenger Work?#

Acid scavengers work in 2 ways: a basic metal soap or oxide neutralises the acid and forms a metal salt, while a layered double hydroxide both neutralises and traps the chloride between its hydroxide layers. The difference is what happens to the chloride after the reaction, and that difference decides whether a soluble, hygroscopic salt stays in the compound or an insoluble mineral particle carries the chloride away from the melt.

Both routes need the base to reach the acid, which is a dispersion problem rather than a chemistry problem. A scavenger dosed at a few hundred ppm has to be distributed through the whole melt, so particle size, surface treatment and the point of addition change the result as much as the chemistry does.

Neutralization by metal soaps#

A metal soap neutralises acid by exchanging its carboxylate for the anion of the acid: calcium stearate binds hydrogen chloride and leaves calcium chloride and stearic acid behind. The same ligand exchange runs inside a PVC stabilizer system, where the calcium soap scavenges HCl from the zinc chloride formed by the zinc soap and so regenerates it: ZnCl2 + Ca(OOCR)2 gives Zn(OOCR)2 + CaCl2.

The by-product decides the side effects. Calcium chloride is hygroscopic, and Kisuma reports that the water it carries raises water carry-over in polypropylene raffia tape. The stearic acid released in the same reaction is a lubricant, which is one reason a metal soap changes processing behaviour as well as acid content, and migrating metal soaps can impair metallization and lamination in metallized BOPP, again on Kisuma's evidence.

Interlayer anion exchange in layered double hydroxides#

A layered double hydroxide binds acid twice: its hydroxide layers neutralise the proton, and its interlayer carbonate is exchanged for chloride, which stays locked between the layers. The captured chloride is therefore removed from the melt rather than dissolved in it, and Kisuma describes the capture as irreversible and the particle as non-migratory.

Jiang and colleagues (Materials, 2020) give the general layered-double-hydroxide formula as [M2+(1-x)M3+(x)(OH)2]x+(An-)x/n times mH2O, with an aluminium fraction x of 0.20 to 0.33, and note that the effect of hydrotalcite in PVC was first observed in the 1980s, as also reported by van der Ven and colleagues (Applied Clay Science, 2000). The idealised hydrotalcite composition is Mg6Al2(OH)16CO3 times 4H2O, and the exchangeable carbonate in that formula is what the chloride replaces.

4 Types of Acid Scavengers for Polymers#

The 4 types of acid scavengers for polymers are metal soaps, hydrotalcite and other magnesium-aluminium layered double hydroxides, hydrocalumite and calcium-aluminium layered double hydroxides, and basic metal oxides and hydroxides. Only the metal soaps migrate through the polymer; the 3 mineral classes stay where they are dispersed, which is why they are chosen for films that are printed, laminated or metallized (Kisuma). The 4 classes are described below in the order used throughout this page, which runs from the class the literature documents best, the metal soaps, to the class it documents least, the hydrocalumites, and is also roughly the order of use.

1. Metal soaps (metallic stearates)#

Metal soaps are metal salts of stearic acid, above all calcium stearate and zinc stearate, that neutralise acid by ion exchange and at the same time act as lubricants and release agents. Sodium stearate and calcium lactate belong to the same class and are used in the same role, and neither is documented in our source library, so this page names them as class members and adds nothing further about them.

The same substances are covered as processing additives on metal stearates, where their lubricant and release roles are set out. The dual function is the reason a metal soap is so common in polyolefins: one addition neutralises catalyst acid and improves the release of the melt from hot metal, and the formulator pays for one additive instead of two.

Calcium stearate#

Calcium stearate, calcium distearate (CAS 1592-23-0), is the acid scavenger the industry documents best, dosed at 0.05 to 0.20 % as an acid scavenger and at up to 1,000 ppm in general polyolefin practice. Its EC number is 216-472-8, its formula C36H70CaO4 and its molar mass 607.0 g/mol; it is a white powder that melts at 179 °C with a density of 1.12 g/cm3. The background text of patent EP2100916A1 describes metal stearates, and calcium stearate in particular, as the most commonly used scavengers in the plastics field, a patent statement rather than a measured market share.

The trade-offs follow from the by-product. Calcium chloride is hygroscopic, so Kisuma reports water carry-over in raffia tape, and migrating metal soaps can impair metallization and lamination in BOPP. Calcium stearate can also act as a pro-oxidant for polypropylene under natural exposure, according to a review in Materials in 2023. Incoming grades are specified on calcium content of 6.3 to 7.9 % (Peter Greven), ash of 9.2 to 12.0 % and free fatty acid of at most 3.0 % (Struktol), and the material is sold as CEASIT and LIGASTAR CA.

Zinc stearate#

Zinc stearate, zinc distearate (CAS 557-05-1), melts at 130 °C, far below calcium stearate, and is chosen where a low-melting metal soap disperses better or where a sodium-based nucleating agent rules calcium stearate out. Its EC number is 209-151-9, its formula C36H70O4Zn and its molar mass 632.3 g/mol, its density 1.095 g/cm3 at 25 °C, and Peter Greven grades carry 10.4 to 11.3 % zinc. The trade names are ZINCUM from Baerlocher and LIGASTAR ZN from Peter Greven.

No acid-scavenger dosage for zinc stearate is established in our sources; the 0.5 % quoted for most plastics in the Struktol technical data sheet is a general lubricant level, not a scavenger dose. The regulatory profile is where it differs from calcium stearate: the EU route is the same salt-of-stearic-acid entry, but zinc carries an Annex II metal limit of 5 mg/kg, as amended by Regulation (EU) 2020/1245.

2. Hydrotalcite and Mg-Al layered double hydroxides#

Hydrotalcite is a magnesium-aluminium layered double hydroxide (CAS 12304-65-3) that neutralises acid and then locks the chloride between its hydroxide layers instead of releasing a soluble salt. Most synthetic grades are registered under the second identity, CAS 11097-59-9 (EC 234-319-3), the mineral identity sits on the EC list as 602-916-1, and the idealised composition is Mg6Al2(OH)16CO3 times 4H2O.

Kisuma states that its DHT-4 grades are non-migratory, which gives better surface treatability, metallization and printing and lower haze in BOPP and BOPE film, and that the grade is 3 times more effective than traditional acid scavengers and present in more than 400,000 km2 of film a year. Those are supplier claims, not verified in our sources. The grade ladder runs from DHT-4A, the industry standard, through DHT-4V with an RSPO-certified vegetable surface treatment and DHT-4A-2 with better thermal stability, to the stearate-free DHT-4C, the most thermally stable and most potent grade in the supplier's description; ALCAMIZER is the PVC line. In a PVC formulation, Jiang and colleagues (Materials, 2020) combined 2.4 phr hydrotalcite with 0.3 phr zinc stearate and 0.3 phr zinc acetylacetonate and reached 190 min to complete blackening in static oven ageing at 180 °C, against a Congo red maximum of 46 min in the same study.

3. Hydrocalumite and calcium-aluminium layered double hydroxides#

Hydrocalumite and the other calcium-aluminium layered double hydroxides work by the same interlayer capture as hydrotalcite, with calcium in place of magnesium. The class appears in the family literature as a separate group, and the documented example in our sources are the calcium hydrotalcite Actilox CAH from Nabaltec.

Published data on this class is limited. Hallstein, Metzsch-Zilligen and Pfaendner at Fraunhofer LBF used Actilox CAH with an aziridine hydrolysis inhibitor in PLA and reported improved hydrolytic and thermal ageing stability (Materials, 2024). No CAS number, no grade ladder, no dosage range and no EU FCM entry for hydrocalumite is established in our sources, so this page names the class and its documented use and gives no numbers for it.

4. Basic metal oxides and hydroxides#

Basic metal oxides and hydroxides, zinc oxide, magnesium oxide, calcium oxide and calcium hydroxide, neutralise acid directly and leave a metal chloride and water behind. They serve as co-scavengers next to a soap or a layered double hydroxide, and as halogen scavengers in flame-retardant compounds, rather than as the sole scavenger of a polyolefin package.

Zinc oxide (CAS 1314-13-2, EC 215-222-5, 81.38 g/mol) melts at 1,974 °C with a density of 5.6 g/cm3 and carries the one harmonised hazard classification in this family, H400 and H410 under Index 030-013-00-7. Calcium oxide (CAS 1305-78-8, 56.08 g/mol) chemisorbs water irreversibly, CaO plus H2O giving Ca(OH)2, which makes the same particle a desiccant as well as a scavenger. All 4 are on the EU Union list in their own right: zinc oxide as FCM 402, magnesium oxide as FCM 397, calcium oxide as FCM 395 and calcium hydroxide as FCM 394.

Calcium Stearate vs Hydrotalcite vs Zinc Oxide: Which Acid Scavenger to Choose?#

Choose calcium stearate for general polyolefin compounding where its lubricating side effect is welcome, choose hydrotalcite when the film must be metallized, printed or laminated or when the calcium chloride by-product causes water carry-over, and use a basic oxide such as zinc oxide as a co-scavenger or halogen scavenger. The 8 criteria below separate the 3 candidates, and the decisive ones are the by-product, the migration behaviour and the metal limit in the target market.

The full head-to-head for polypropylene and polyethylene, including grade selection, is on acid scavengers for polyolefins. The choice is rarely exclusive in practice, because a stabilizer one-pack can carry a soap and a layered double hydroxide together, and the question a formulator actually answers is which of them carries the main load and which is present for a side function. Three of the 8 rows decide most cases. The by-product row rules calcium stearate out wherever water carry-over is a defect, in raffia tape above all. The migration row rules it out wherever the surface is printed, metallized or laminated. The metal-limit row decides which of the 3 survives a food-contact calculation, because zinc oxide and zinc stearate draw on the same 5 mg/kg zinc allowance while hydrotalcite draws on the 1 mg/kg aluminium allowance.

How Much Acid Scavenger Is Needed? Dosage in ppm and phr#

Acid scavengers are ppm-level additives: calcium stearate is dosed at 0.05 to 0.20 % (500 to 2,000 ppm) as an acid scavenger and at up to 1,000 ppm in general polyolefin practice, and 1,000 ppm equals 0.1 wt%. The first of those values is a Peter Greven supplier guideline and the second a secondary source, which is the honest description of the best-documented number in this family.

Polyolefin levels are given in ppm or wt%, PVC levels in PHR (parts per hundred resin), and the 2.4 phr hydrotalcite of the PVC study above follows that convention. The table gives every level our sources hold, with the kind of source attached to each row, because a regulatory ceiling, a supplier guideline and a research formulation are not the same thing.

A regulatory ceiling is not a recommended dosage. No published polyolefin dosage range exists in our sources for hydrotalcite, hydrocalumite or the basic oxides.

No published dosage range for hydrotalcite, hydrocalumite or the basic oxides in polyolefins was found in a primary source, so this page gives the regulatory ceilings and the grade-specific supplier values instead of a range.

The practical route around the gap is a trial series: set the calcium stearate level from the documented range, screen the mineral scavenger at the level its grade supplier recommends for that film, and confirm the result by colour, melt flow and corrosion checks rather than by arithmetic.

How is an acid scavenger dosed: powder, masterbatch or one-pack?#

Acid scavengers reach the polymer in 3 forms: as a powder metered at the resin plant or the compounder, as an additive masterbatch, or as part of a stabilizer one-pack. The choice follows the dosing accuracy available at the point of addition, not the chemistry of the scavenger.

At ppm level most converters use an additive masterbatch, because gravimetric accuracy below 0.1 wt% is hard to hold on the machine and a fine powder has to be dispersed rather than merely mixed. Powder dosing stays common upstream, at the resin plant, where the scavenger goes in at the pelletising step and the equipment is built for it.

Grade form matters at this step too. Hydrotalcite is sold as a plain powder and as surface-treated grades, the treatment chosen for dispersion and polymer compatibility, with DHT-4V carrying an RSPO-certified vegetable surface treatment and DHT-4C supplied stearate-free.

Ready-made blends such as SONGNOX 6280, a 93:7 mixture by weight of the phosphite SONGNOX 6260 with Mg/Al hydrotalcite, are one-pack additive systems in miniature. They remove one weighing step and they fix the ratio between the two components, which suits a converter who wants the buffered phosphite behaviour without formulating it.

Which Acid Scavenger Suits Each Polymer?#

The acid scavenger follows the acid source and the surface requirement: calcium stearate for general polypropylene and polyethylene compounding, hydrotalcite where the film is metallized or printed, a calcium hydrotalcite in PLA, and a calcium or zinc soap inside the PVC stabilizer one-pack. In polypropylene the scavenger is one member of a package that also carries a phenolic antioxidant with a phosphite, a thioester, HALS and a UV absorber outdoors, nucleating and clarifying agents, antistats, slip and antiblock additives and fillers.

Greenhouse and tunnel film is the application where the scavenger protects a different additive altogether: Kisuma describes an acid scavenger there as protection for the HALS against agrochemical attack, which makes the choice a light-stability decision rather than a corrosion decision.

How to select an acid scavenger in 6 steps#

Select an acid scavenger in 6 steps: identify the acid source, check the surface requirement, check the processing temperature, check the rest of the additive package, screen the metal limits, then set the level and confirm it by testing. Steps 1 and 2 rule out whole classes before any dosage is discussed.

1. Identify the acid source, whether catalyst residue, PVC, a halogenated flame retardant or ester hydrolysis, because the source sets the quantity and the timing of the acid.
2. Check whether the part is printed, metallized, laminated or coated, which rules migrating metal soaps out and points to a layered double hydroxide.
3. Check the processing temperature against the thermal stability of the grade, since the grade ladder exists largely for this reason.
4. Check the rest of the additive package for antagonism, above all sodium-based nucleating agents and phosphites.
5. Screen the food-contact position and the metal limits in each target market, including the Annex II limits of 1 mg/kg for aluminium and 5 mg/kg for zinc.
6. Set the level in ppm, dose it as a masterbatch or a one-pack, and confirm the result by colour, melt flow and corrosion checks.

The same 7-step logic for every additive family is worked through on how to select plastic additives. A formulator should run steps 1 to 3 before asking a supplier for a recommendation.

How Do Acid Scavengers Interact with Other Additives?#

Acid scavengers interact with 3 additive groups: they protect phenolic antioxidants, phosphites and HALS from acid, and they can help or hinder a nucleating agent depending on which metal the nucleator carries. Every one of those interactions is an exchange of a metal or a proton, which is why the identity of the metal decides the outcome. The 6 partner additives in the table below cover both directions: the 3 stabilizer classes the scavenger protects, and the 3 nucleation cases where the two additives compete for the same cation.

Acid scavengers, antioxidants and phosphite hydrolysis#

An acid scavenger protects the antioxidant package: acid deactivates phenolic antioxidants and hydrolyses phosphites, and calcium stearate left more Irgafos 168 intact after compounding in polypropylene. That finding comes from Espelage and colleagues (Polymers, 2025) on Borealis polypropylene, and it shows the scavenger paying for itself through the additive it saves.

Hydrolysis is the weak point of the secondary antioxidants covered on phosphite and phosphonite antioxidants, which is why SONGNOX 6280 blends a phosphite with hydrotalcite at 93:7 by weight. Phosphite hydrolysis produces acids, black specks, corrosion and feeding problems, and the 2 established fixes are a hydrolysis-resistant phosphite grade or a co-added hydrotalcite or amine that buffers the acid as it forms.

Recycled polyolefins bring both problems at once, because the acid is legacy acid and the antioxidant left in the material is already partly consumed. Songwon reports that XP 2121, a restabilization package containing an acid scavenger, tested at 0.15 to 0.4 wt% in recycled polypropylene, raised the days to embrittlement at 150 °C from about 25 for the unstabilised material to about 37 to 42 at 0.2 to 0.4 %, values read from a supplier chart and approximate for that reason.

Acid scavengers and nucleating agents#

The acid scavenger and the nucleating agent compete when both carry a metal: Simanke and colleagues (2016) found a negative interaction between calcium stearate and sodium benzoate at 1,000 to 2,200 ppm in polypropylene. The exchange runs between the calcium of the soap and the sodium of the nucleator, and the nucleating efficiency is what suffers.

Two substitutions solve it. Hydrotalcite is preferred where a phosphate-ester nucleating agent is present, because the mineral holds its chloride instead of trading a cation, and zinc stearate is the alternative metal soap where a sodium nucleator rules calcium stearate out.

Dual-function grades that nucleate and scavenge in one particle are covered on nucleating agents for polypropylene. HPN-20E is two thirds calcium cis-1,2-cyclohexanedicarboxylate and one third zinc stearate, the acid-scavenging component, in the composition reported by Sowiński and colleagues in 2015. Kisuma introduced Setogem RD in 2025 as a zinc-free dual-function nucleator and acid scavenger for polypropylene at 200 to 300 ppm, in place of talc or sodium benzoate dosed at several kilograms per tonne, and a quintinite treated with cis-1,2-cyclohexanedicarboxylic acid combines the same two functions under FDA notification FCN 2476.

How Is Acid Scavenging Performance Tested?#

Acid scavenging is judged indirectly by 4 measurements: melt flow drift, yellowness index, oxidative induction time and, in PVC, the Congo red thermal stability time. No direct acid-scavenging capacity standard is established in our sources, so every method below measures a consequence of unneutralised acid rather than the neutralisation itself.

Every method used on this site is indexed under testing plastic additives, with the current edition of each standard. Melt flow rate is run at 230 °C and 2.16 kg for polypropylene and at 190 °C and 2.16 kg for polyethylene, and the useful measurement for a scavenger is the drift across repeated extrusion passes rather than a single value, because acid damage accumulates. Two edition traps sit in this table. Colour is reported to ASTM E313-20, reapproved in 2025, and never to ASTM D1925, withdrawn in 1995 and still cited in old specifications. Oxidative induction time, run at 190 to 220 °C under ASTM D3895-19 or ISO 11357-6, measures the surviving antioxidant and is therefore read as a package result.

How Are Acid Scavengers Regulated for Food Contact?#

Acid scavengers are regulated in 3 layers: the EU Union list of Regulation (EU) No 10/2011 with its Annex II metal limits, the US 21 CFR food-additive and prior-sanction routes, and REACH registration and CLP classification. The overall migration limit of 10 mg/dm2 and the generic specific migration limit of 60 mg/kg apply to the finished material in the EU, whatever the additive.

Every instrument that touches a plastic additive is summarised in plastic additive regulations. For this family the EU layer is the one that needs the most care, because 3 of the 6 substances are authorised indirectly, as salts of an acid on the Union list, rather than under their own entries.

EU 10/2011: FCM numbers and metal limits#

In the EU the metal soaps are authorised indirectly, as salts of stearic acid (FCM No 106) under Article 6(3)(a) of Regulation (EU) No 10/2011, while hydrotalcite, zinc oxide and the other oxides carry their own FCM numbers. Article 6(3)(a) permits the salts of authorised acids, which is why no separate Union list entry for calcium stearate, zinc stearate or magnesium stearate is needed.

The migration-limit system behind these numbers is explained on EU 10/2011. The matrix below joins the EU route to the US one for the 6 substances of this page and the 3 further class members that carry an FCM number.

Union list values read from the EUR-Lex consolidated text of 16 March 2025; re-check against the current consolidation before use in a compliance file.

The limits on the metals are Annex II metal limits and not substance-specific migration limits, and the distinction changes how a compliance calculation is run. Hydrotalcite has no substance SML, and the Annex II aluminium limit of 1 mg/kg applies to the food or simulant as a whole, so every aluminium-bearing component counts against the same number.

Every additive limit in the Union list is tabulated on specific migration limits (SML) of plastic additives. Three class members named on this page, hydrocalumite, sodium stearate and calcium lactate, have no FCM entry established in our sources, and a compliance file needs that point checked at source rather than assumed.

FDA: GRAS listings and prior sanctions (21 CFR)#

In the United States the metal soaps and the basic oxides are cleared as generally recognised as safe substances rather than as polymer additives: calcium stearate under 21 CFR 184.1229, zinc stearate under 182.8994 and zinc oxide under 182.8991. Calcium oxide is listed under 184.1210 and magnesium stearate under 184.1440, which completes the GRAS set for this family.

A second route runs beside the GRAS listings. Calcium stearate and magnesium stearate are also prior-sanctioned under 21 CFR 181.29 as stabilizers, a category that predates the current food-additive system. One clerical detail is worth knowing: the CFR text at 184.1229 prints the CAS number 1529-23-0, a transposition of the correct 1592-23-0.

How the GRAS, prior-sanction, regulated-additive and FCN routes differ is mapped on FDA food contact rules. Hydrotalcite is the gap: it is not listed in 21 CFR 178.2010, its FCN status is unverified in our sources, and Kisuma's statement that DHT-4 grades hold global food-contact approvals is a supplier claim a buyer should see documented for the specific grade.

REACH and CLP status of acid scavengers#

No acid scavenger on this page is on the REACH Candidate List, and all of them except magnesium stearate carry active REACH registrations, from 10 dossiers for calcium stearate to 184 for zinc oxide. The 3 status points a regulatory file needs are set out below.

• Registration: 10 active dossiers for calcium stearate, 13 for zinc stearate plus 76 for fatty acids C16-18 zinc salts (EC 293-049-4), 10 for hydrotalcite, 184 for zinc oxide and 100 for calcium oxide, read from ECHA CHEM on 22 September 2026; no identifier for magnesium stearate could be located.
• Candidate List: none is a substance of very high concern, and hydrotalcite is also absent from the California Proposition 65 list of 31 July 2026.
• Classification: zinc oxide carries a harmonised CLP classification of H400 and H410; the stearates carry notifier aggregates only, calcium stearate not classified in 85.5 % of 3,808 notifications and zinc stearate not classified in 61.3 % of 2,108.

Registration duties, tonnage bands and the Candidate List process are explained in REACH and plastic additives.

Who Makes Acid Scavengers? Suppliers and Trade Names#

Acid scavengers come from 3 kinds of producer: metal-soap makers such as Baerlocher, Peter Greven and Struktol, layered-double-hydroxide specialists such as Kisuma Chemicals and Nabaltec, and stabilizer blenders such as Songwon that sell hydrotalcite already mixed with a phosphite. The 3 groups rarely overlap, because a metal soap plant and a layered double hydroxide plant run different chemistries on different feedstocks.

Grades, plant locations and certifications are compared on calcium and zinc stearate manufacturers and suppliers. Baerlocher is family-owned, has traded for more than 200 years and employs about 1,150 people; its headquarters city is not settled in our sources, so this page names the country only.

No market size, growth rate, price or producer ranking for acid scavengers or metal stearates is established in our sources, so this page names producers without ordering them. The full producer index for every family is the plastic additive manufacturers and suppliers directory.

Masterbatchers form the second supply layer, because a converter buying at ppm level buys a concentrate rather than a powder, and the concentrate maker chooses the grade.

Metal soaps are made from stearic acid, an oleochemical feedstock, which is why India is a centre of this family. Indian producers are listed with plastic additive manufacturers and suppliers in India.

Complete List of Acid Scavenger Substances (6 Pages)#

The 6 substances below carry an acid-scavenging function in plastics, and each one has its own page with identity, dosage and full regulatory status. Only zinc oxide is filed under this family in the site database; the other 5 are filed under lubricants, heat stabilizers or desiccants and appear here because of what they do, not because of where they are catalogued.

All 435 additives, including these 6, are searchable in the plastic additives database.

Are Acid Scavengers Safe, and How Do They Affect Migration and Recycling?#

The substances used as acid scavengers are among the least restricted plastic additives: none of them is on the REACH Candidate List, and calcium stearate, zinc stearate, zinc oxide and calcium oxide are all listed as generally recognised as safe for food use in the United States. The one hazard statement that belongs in this group is the harmonised aquatic classification of zinc oxide, H400 and H410, which does not extend to the stearates, whose classifications are notifier aggregates rather than harmonised entries.

Two questions remain open even with that regulatory position, and both are formulation questions rather than hazard questions: whether the scavenger leaves the plastic, and what it does to the material when the plastic is recycled.

Do acid scavengers migrate out of the plastic?#

The metal soaps migrate and the mineral scavengers do not: calcium stearate and zinc stearate move to the surface, which is what makes them lubricants as well as scavengers, while hydrotalcite stays where it is dispersed. Kisuma describes its layered double hydroxide particles as non-migratory, and the surface benefits the supplier claims for metallization and printing follow from that.

Diffusion models for every migrating additive are on additive migration in plastics, and the food-contact control here is the Annex II metal limit rather than a substance SML. A compliance calculation for zinc stearate therefore targets the 5 mg/kg zinc limit and one for hydrotalcite the 1 mg/kg aluminium limit, and both count every other source of that metal in the formulation.

Do acid scavengers affect plastic recycling?#

Acid scavengers help recycled polyolefins rather than hindering them: a restabilization package containing an acid scavenger raised the days to embrittlement of recycled polypropylene at 150 °C from about 25 to about 37 to 42 at 0.2 to 0.4 % (Songwon product data). Recycled polyolefins carry legacy acid from their first life and from any chlorinated contamination, and the antioxidant left in them is partly consumed, so the scavenger and the antioxidant are added together.

How each additive family affects sorting and reprocessing is set out under design for recycling. Calcium oxide has a second role in the same stream, as the active component of a desiccant masterbatch. No recyclability rating and no APR or RecyClass position for acid scavengers is established in our sources.

Is an acid scavenger the same as a PVC heat stabilizer or a metal deactivator?#

No: an acid scavenger only binds acid, while a PVC heat stabilizer also replaces labile chlorine atoms and a metal deactivator chelates the metal ion itself instead of the acid it releases. The 3 functions overlap often enough that the names are used loosely, and the distinction matters when a formulation is being corrected rather than described.

In a PVC one-pack the acid-binding component sits among the PVC co-stabilizers, next to the beta-diketones, polyols and epoxides, and hydrotalcite is a standard long-term component of calcium and zinc systems. The full system substitutes labile chlorine, binds the hydrogen chloride released and interrupts polyene growth, and acid binding is only the second of those 3 jobs.

The bridge runs through the same substances: calcium stearate and hydrotalcite are acid scavengers in a polyolefin and co-stabilizers in a PVC one-pack, and what changes is how much of the stabilisation work each carries.

Where copper is the problem rather than chloride, the answer is one of the metal deactivators such as a phenolic hydrazide. A metal deactivator chelates transition metal ions, above all copper, to stop them catalysing hydroperoxide decomposition, and wire and cable insulation on a copper conductor defines the family.

Is an HVAC acid scavenger the same thing?#

No: an HVAC acid scavenger is a filter-drier or additive that neutralises acid in a refrigeration circuit after a compressor burnout, while an acid scavenger for polymers is a solid additive compounded into the plastic itself. The two share a name and nothing else: one treats a refrigerant charge, the other is dispersed in a polymer melt at a few hundred ppm.