KEY INSIGHTS

The article highlights odour elimination tactics

• Physical and chemical methods to remove residual monomers.
• Additives used to minimize odour during polymer processing.

BULLET POINTS

• Key reasons for the occurrence of odour in polymers
• Physical and chemical methods to eliminate these odours
• List of various commercial additives; manufacturer/supplier details and product, to eliminate the odour during polymer processing.

KEYWORDS

Odour control; Odour elimination; Physical methods; Chemical methods; Use of additives

DETAILED REPORT

The occurrence of odour is due to the following reasons: (i) the occurrence of residual monomer in the polymer during synthesis, and (ii) the generation of odour-causing molecules during processing. The details are as discussed below.

1. THE OCCURRENCE OF RESIDUAL MONOMER IN FINAL POLYMER DURING SYNTHESIS

Polymerization reactions often leave small amounts of unreacted monomers in the final polymer, and many of them are classified as VOCs. They may cause unpleasant odors, create environmental and safety concerns, and affect product quality.

Some monomers are toxic (e.g., vinyl chloride and acrylonitrile), produce strong and offensive odors (e.g., acrylates and methacrylates), and may pose a risk of explosion during storage or transportation.

These residual monomers are undesirable in the case of food packaging, biomedical materials, and interior paints, etc., where safety and low emissions are essential.

Beyond safety concerns, residual monomers can also negatively influence polymer performance by causing problems such as discoloration, reduced heat resistance, and dimensional instability. For these reasons, post-polymerization purification steps are necessary to minimize residual monomer content and ensure product safety and performance.

1.1 CAUSES OF RESIDUAL MONOMER ACCUMULATION:

The reasons are as follows:

• Cage effect: monomers get entrapped within growing chains
• Glass/ Vitrification effect: polymerization temperature is below the Tg of the polymer produced, and the entrapped monomer molecules can’t escape the cage. The rise in polymerization temperature could minimize the residual monomer but may also alter the properties of the polymer.
• Co-monomers presenting very different reactivity ratios
• De-propagation: Monomers also find their way into the final product due to depolymerization occurring at high temperature (ceiling point).
• Radical anchoring at the polymer particle surface (in emulsion polymerisation)

1.2 HOW TO REMOVE RESIDUAL MONOMER

There are several techniques for reducing or eliminating the residual monomer content, and each of them is discussed below. Nevertheless, choosing the best, or the most adequate, technique is not always an easy task, and one still observes a relative lack of scientific literature on this subject.

1. PHYSICAL METHODS –

DEVOLATILIZATION AND STRIPPING TECHNIQUES

Residual monomers are commonly removed from polymers using devolatilization and stripping processes, which rely on techniques such as elevated temperature, reduced pressure, solvents, or a stream of inert gas (e.g., nitrogen) or vapor to volatilize and remove unreacted monomers. Supercritical fluids – most commonly CO₂ is mostly used due to high diffusivity and solvency. Other light alkanes (butane, pentane) are also used.

This process can be carried out in equipment such as extruders, thin-film evaporators, falling-strand devolatilizers, or even in polymerization reactors. The efficiency of devolatilization generally increases with higher temperature and greater stripping gas flow rate.

Stripping is widely performed in batch or continuous tank strippers, vacuum strippers, cross-flow gas systems, or counter-current stripping columns. Process parameters such as steam temperature, feed temperature, pressure, and gas flow rate strongly influence stripping efficiency.

A patent discloses that in the removal of vinyl chloride monomer from PVC, maintaining a controlled temperature difference of 5–30 °C between steam and polymer slurry improves monomer removal while minimizing foam formation. In some processes, a post-polymerization redox initiation step (using peroxide–reducing agent systems) is first applied to convert remaining monomer, followed by vacuum or steam stripping at 50–80 °C, reducing residual monomer levels to very low concentrations without destabilizing the polymer system

Another patent uses a free radical generator in an amount and for a time sufficient to reduce the residual monomer content to 1500-6000 ppm, followed by vacuum/steam stripping to reduce the content to 5-500 ppm without destabilizing or degrading the emulsion. The free radical generators are the redox couples, such as peroxides like hydrogen peroxide, benzoyl peroxide, and the like, with reducing agents like sodium formaldehyde sulfoxylate, ascorbic acid, sodium metabisulfite, etc. The stripping is carried out at a temperature of 50-80 °C.

CATALYTIC OXIDATION

Residual monomers are removed by catalytic oxidation, where volatile organic compounds are oxidized in the presence of oxygen and catalysts such as Pt, Pd, or metal oxides (MnO₂, CuO, Co₃O₄) supported on alumina or silica. In practice, monomers are first released from the polymer by heating, vacuum, or devolatilization, and the resulting vapours are passed through a catalytic oxidizer (200–400°C) where they are converted to CO₂ and H₂O. Since direct oxidation within the polymer can cause degradation, this method is mainly used to treat off-gases from reactors, extruders, or stripping units and is often combined with devolatilization or stripping processes.

ADSORPTION

Adsorption using activated carbon, zeolites, and silica gel removes residual monomers by trapping volatile molecules on the high-surface-area porous structure of the adsorbent. Residual monomer migrates from the polymer matrix to the adsorbent due to a concentration gradient. The adsorbent containing the captured monomers is removed by filtration, sedimentation, or screening.

Adsorbent may be added during polymer processing in amounts that it won’t hamper overall polymer properties. In emulsion or solution polymerization, they are added to the latex and stirred to allow adsorption. The adsorbent is later removed by filtration or centrifugation. Adsorbents in the form of a fixed bed can also be used. Key factors affecting efficiency include temperature, surface area, and pore size of the adsorbent, contact time, and concentration of residual monomer. Examples of some other adsorbents are antimony pentoxide, zirconium hydroxide, and zirconium sulphate.

CONDENSATION

The condensation step removes residual monomers by cooling the vapours released from the polymer during stripping or devolatilization. The unreacted monomers evaporate from the polymer matrix, and the vapour stream is passed through a condenser, where cooling lowers the temperature below the monomer’s dew point. The vapours then condense into liquid form and are collected. This method is commonly used as a primary recovery step for monomers. Monomers like styrene and other acrylates can easily be condensed.

MEMBRANE SEPARATION

Membranes can be used to separate monomers from the polymer matrix. Pervaporation membranes (especially polydimethylsiloxane-based) are most commonly used. If monomers are removed in vapour phase streams, gas separation membranes are used. Inorganic membranes, such as zeolite or ceramic membranes is known for their high chemical and thermal stability for aggressive monomers. Nanofiltration membranes are used when the polymer is in solution form. These membranes allow small monomers and solvent to pass through while retaining the polymer chains.

ION EXCHANGE RESIN

Ion exchange is used for residual monomer removal when the monomer or impurity is ionic, which is common in aqueous polymer or latex systems. The polymer dispersion is passed through ion-exchange resins containing fixed charged groups that selectively capture oppositely charged monomers, initiator fragments, or ionic impurities. This method is effective for ionic species, but less suitable for neutral hydrophobic monomers such as styrene or many acrylates.

SPRAY DRYING

SPRAY-DRYING: polymer products are sprayed at a temperature above the boiling point of the monomer. As the surface area is increased significantly, the monomer is volatilized more easily.

CHEMICAL METHODS –

TEMPERATURE

The propagation constants and diffusion coefficients increase with temperature, thus favouring a faster conversion of the remaining monomer. However, maintaining the elevated temperature over a considerably long period of time leads to the possibility of product degradation or decomposition, as well as lowered production efficiency.

INITIATOR

The appropriate selection and addition policy of the initiator, or of the finishing catalyst, can reduce reaction time or further deplete residual monomer after reaching high monomer conversions. Redox initiators can also be used.

REACTIVE CO-MONOMER

Residual monomer can be reduced with the introduction, at the end of the reaction, of a more reactive co-monomer. For example, vinyl acetate is often used as a scavenger monomer in the polymerisation of acrylates. This is added with an aliquot of initiator at the advanced stage of polymerisation (conversion above 80%). Vinyl acetate and methyl methacrylate can also be added during the synthesis of ABS. While choosing the right candidate as a reactive co-monomer, the reactivity ratios should be considered.

CHEMICAL REMOVAL

Appropriate compounds may be used that react with the monomer’s double bond to produce derivatives that are easier to remove. Some of the examples of such compounds include ammonia, ammonium salt, alkylamine and/or one of their salts, hydroxylamine and/or their salts, hydrogen halide, acetoacetate, malonate, bromo-succinamide, pyridinium bromide, dioxane perbromide, permanganate, bichromate, chromate, selenium dioxide, alkali sulphite or ammonium sulphite, alkali or ammonium hydrogen sulphite or disulfite, or a thio compound and also ozone.

A patent describes a method for removing residual monomers from vinyl polymers prepared by suspension polymerization or is later suspended in an aqueous medium, followed by the addition of water-soluble inorganic salts such as sodium bisulfite, potassium bisulfite, or ammonium bisulfite (0.5–30 parts per 100 parts polymer) to solubilize the unreacted monomer. The process is carried out at temperatures from the polymer Tg up to about 150 °C, often with an oxidizing agent (e.g., organic peroxide) to enhance removal. The addition of dispersants such as polyvinyl alcohol or sodium polyacrylate is preferred.

USE OF LOW VOLATILE MONOMERS OR REACTIVE DILUENTS

The reactive diluent should be low in volatility, low activation energy, low in viscosity, and low in cost. Recent studies show that the use of 3,3,5-trimethylcyclohexyl salicylate methacrylate, benzyl salicylate methacrylate, as a replacement for monomers like isobornyl methacrylate and benzyl methacrylate.

Dimethyl itaconate, methacrylated lignin model compounds (LMCs, i.e., eugenol and guaiacol) monomers, Vanillin methacrylate, vanillyldimethacrylate, disosorbidedimethacrylate, cinnamyl methacrylate as a replacement of styrene reactive diluent

• GENERATION OF ODOUR-CAUSING MOLECULES DURING PROCESSING

During plastic processing, additives like plasticizers, flame retardants, etc., are used. These additives, and sometimes the polymers, exude unwanted odours in finished products, due to shearing coupled with high temperatures during processing. The by-products of polymer degradation, such as amines, phenols, mercaptans, aldehyde and ketones, are common contributors to unpleasant odours in plastics. The recycling process further intensifies these odours, causing the polymers to degrade further.

2.1 ODOUR REMOVING COMPOUNDS

• Oxidizing agents: Potassium permanganate neutralizes odor-causing compounds in processing environments. For aldehydes and other odorants, oxidizers such as chlorine dioxide, permanganates, peroxides, and borates/perborates may also be used.
• Encapsulation/ adsorption agents: Cyclodextrins form inclusion complexes with odor molecules; activated carbon (0.1–1%), molecular sieves (0.1–1%), zeolites, silicas, activated alumina, calcium carbonate, clay minerals, and related porous materials adsorb VOCs and malodors.
• Catalytic degradation: Metal oxides such as TiO₂ and ZnO catalyze the breakdown of VOCs and odorants.
• Stabilization strategies: Antioxidants (e.g., BHT), heat stabilizers, and radical stabilizers reduce odor formation by preventing polymer oxidation and degradation during processing.
• Acid–base neutralization: Acid neutralizers like sodium bicarbonate can be used.
• Fragrance masking: Essential oils may mask or neutralize unpleasant odors through their perfumery effect.
• Reactive odor scavengers: Polyethylene imines can remove aldehyde odors (e.g., from rancid fats in recycled plastics).
• Neutralizing agents: Compounds such as lauryl methacrylate (Metazene), biguanides, quaternary ammonium compounds, and esters of unsaturated monocarboxylic acids are used as odor-neutralizing additives.
• Zinc-based odor control: Zinc salts (especially zinc ricinoleate) are widely used to neutralize amine and sulfur-containing malodors. Other salts include zinc acetate, zinc lactate, zinc abietate, zinc borate, zinc caprylate, zinc chloride, zinc sulfate heptahydrate, zinc undecylenate, and zinc gluconate.
• Malodor counteractant (MOC) blend: A synergistic formulation containing 3-(3,3-dimethylcyclohexyl)-2-ethoxycarbonyl-butanoic acid, diphenyl oxide, butyl-4-tert-cyclohexanol, and 2-phenoxyethyl-2-methylpropanoate shows superior odor control; removing any component reduces effectiveness.
• Specific odor inhibitors: Norlimbanol (especially the isomer Timbrol) effectively suppresses trimethylamine odors.
• Additional odor-control agents: Metal ions or nanoparticles (Ag, Cu, Zn), enzymes, urease inhibitors, proteases, cationic surfactants (e.g., N-ethyl-N-soyamorpholinium ethosulfate), chelating agents such as EDTA, and combinations thereof may also be used for broad-spectrum malodor control.

2.2 ODOUR REACTING COMPOUNDS

These additives chemically react with odorants to reduce or eliminate odors. Key classes include aldehydes, formaldehyde-donor compounds, ketones, and oxidizing agents.

Aldehydes react with amines to give low-odor imines and with thiols to yield thioacetals. Examples include simple aromatic aldehydes like anisic aldehyde, o-allyl-vanillin, benzaldehyde, cuminic aldehyde, ethyl-aubepin, ethyl-vanillin, heliotropin, tolyl aldehyde, and vanillin. Or bulky substituted aldehydes like 3-(4-tert-butylphenyl)propanal, 2-methyl-3-(4-tert-butylphenyl)propanal, 2-methyl-3-(4-isopropylphenyl)propanal, 2,2-dimethyl-3-(4-ethylphenyl)propanal, cinnamic aldehyde, α-amyl-cinnamic aldehyde, α-hexyl-cinnamic aldehyde.

Formaldehyde donors react similarly with amines and thiols, producing larger, low-volatility molecules with little or no smell.

Flavonoids may also act as odor-reducing additives.

Terpenes/ essential oils (preferred additives): Include hydrocarbon terpenes, unsaturated terpenes, and oxygenated terpenes (terpenoids) such as alcohols, aldehydes, ketones, esters, acetates, ketals, and oxides; covering mono-, sesqui-, di-, tri-, and tetraterpenes.

Representative terpene derivatives:

• Alcohols: linalool, citronellol, terpineol (especially tertiary alcohols capable of forming odor-reactive carbocations)
• Oxides: 1,4-cineole, 1,8-cineole, linalool oxide
• Ketones: camphor

Terpenes for hydrocarbon odors: α-pinene, β-pinene, dipentene.

Terpenes for styrene odor: p-cymene and p-cymenene.

Common terpene odor-reducing additives include d-limonene, l-limonene, dl-limonene, pinene, carene, terpinolene, camphene, myrcene, and sabinene.

2.3 COMMERCIAL ADDITIVES FOR DEODORIZING MASTERBATCHES

Mechanism includes adsorption and absorption of odour-causing molecules, binding them to the surface. Others chemically react with odour-causing compounds to neutralize them. Certain products may simply mask by introducing a more pleasant fragrance. Odour-removing compounds should be chosen carefully to ensure compatibility with the base polymer and other additives, while also being effective at odour reduction.

Manufacturer/ supplier	Product	Comments
Raytop	RT-17, RT-567, RT-10126	It has a dual mechanism, working by adsorption and oxidation. Adsorbs VOCs generated during processing and oxidizes others to remove malodours.
BYK	BYK-MAX OR 4206	Granulated anti-odour additive for recycled PE. Contains odour-absorbing substances that neutralize H₂S, amines, and ammonia without affecting polymer properties
Evonik	TEGOSORB® A 30	It is a malodour absorber that works irreversibly with H2S, mercaptan, thioether, isovaleric acid, ammonia, and other common odour-causing chemistries. Non-toxic, biodegradable, and compatible with fragrances. It has no biocidal or fungicidal activity.
	TEGOSORB® CONC. 50	Fast and permanent malodour elimination for gel and liquid systems; soluble in surfactant systems and compatible with other ingredients
	TEGOSORB® PY 50 PE andTEGOSORB® PY 50 PP	Masterbatches based on polyethylene and polypropylene with zinc ricinoleate as odour absorber.
	TEGOSORB® B 80	Water-soluble odour absorber based on zinc ricinoleate with benzalkonium chloride.
	TEGOSORB® PY 88 TQ	Zinc ricinoleate based odour absorber with good thermal stability; suitable for polyolefins, rubbers and recycled polymers. It effectively control odours like hydrogen sulphide, mercaptan, thioethers, isovaleric acid, amines and ammonia.
ECOSORB	ECOSORB® 206	Ecosorb 206 reduces styrene odour by up to 98% within 20 minutes.
	ECOSORB® 303 A	Liquid additive for odour control in resins, polymers, oils and emuslions. It operates via emission suppression mechanisms for controlling odours (not a masking agent).
Ampacet corporation	Ampacet 101787	Porous adsorptive masterbatch for films (LDPE, EVA) used in packaging. Removes ammonium-based and polar odour molecules.
	OdorClearTM	Masterbatch range for post-consumer recycled plastics, trapping odours within the polymer matrix.
PROTECH MINERALS	Zeolite	Natural mineral with strong adsorption and ion-exchange capability for odour-causing molecules.
AVANZARE	avanNATUR ODORLESS	Additives for recycled thermoplastics; capture aromatic compounds, aldehydes, and carboxylic acids formed during peroxide processing. Suitable for EVA, EPDM, SBR, and NBR, as well as all polymers that have been treated with peroxides.
NICHEM	ODOGONE	It has a dual mechanism as it works by adsorption and oxidation. Adsorbs VOCs generated during processing and oxidizes others to remove malodours.
STRUKTOL	RP 17 and RP 53	It has a dual mechanism as it works by adsorption and oxidation. Adsorbs VOCs generated during processing and oxidizes others to remove malodours.

2.4 OTHER COMMERCIAL PRODUCTS

Some other additives that are mostly used in household products like fabric care, air fresheners, hand dishwashing, and surface care applications, etc. No literature is available on their use in polymer formulations, but they could be tried for out-of-the-box solutions.

Manufacturer/supplier	Product name
CRODA	FORESTALLTM
	ZINADORTM 35L
	ZINADORTM 22L
	NEUTRAFRESHTM
	HE100
OMI INDUSTRIES	ECOSORB® 206A
	ECOSORB® GEL
RUDOLF GROUP	RUCO®3811
AULICK CHEMICAL SOLUTIONS INC.	NITRA-NOX
	ROX-92
BURLINGTON CHEMICAL CO	BURCO® DEO-46G
MAXWELL ADDITIVES PVT. LTD.	OLKLIN-803
ALPHA AROMATICS	Metazene
	Meelium
	4-O-DORTM (N-soya-N-Ethyl Morpholinium Ethosulfate)
	DUO-2-0®

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