Acrylonitrile-butadiene copolymer (NBR, CAS 9003-18-3) is an emulsion-polymerized elastomer whose bound acrylonitrile content is controlled between approximately 18% and 50% by monomer feed ratio. The remaining repeat units derive from 1,3-butadiene, producing a random copolymer with pendant nitrile groups and backbone unsaturation. Production trains typically consist of a continuous stirred-tank polymerization battery, shortstop and monomer recovery stages, coagulation, washing, and dewatering; a vacuum steam stripper reduces residual acrylonitrile in the latex. Grade selection is governed by ISO 4658:2020 and ASTM D3187-06, which define raw-polymer evaluation and test formulations. In compounded goods, NBR is specified for aliphatic oil resistance, low gas permeability, and a service temperature window that narrows as bound acrylonitrile increases.

Raw-polymer testing under ISO 4658:2020 uses a two-roll mill mixing cycle with cooling water at 30 °C and a carbon black masterbatch according to the standard recipe. Mooney viscosity ML 1+4 at 100 °C is determined by ISO 289-1:2015; commercial NBR grades commonly range from 30 MU to 90 MU. High-ACN grades generate greater shear heating during mixing because pendant nitrile groups increase internal friction and polarity. Without cooling-water jackets on the inlet roll and batch cooling after dump from an internal mixer, mill-surface temperatures can exceed 120 °C, approaching the scorch regime for sulfur-accelerated compounds. NBR compound design frequently uses sulfur at 1.2–1.8 phr with sulfenamide accelerators at 1.0–1.5 phr, or dicumyl peroxide at 2.5–4.0 phr where lower compression set is required. Peroxide-cured NBR compounds must avoid amine-based antioxidants that preferentially consume free radicals; acidic fillers such as unneutralized silica can retard cure.

Is NBR a Polymer of Low Concern? REACH Registration Exemptions Explained

REACH does not define a “polymer of low concern” as a registration category. Article 2(9) of Regulation (EC) No 1907/2006 exempts polymers as substances from Title II registration and Title VI evaluation. Consequently, an NBR polymer cannot be registered as low concern under REACH; it is either a polymer within the meaning of Article 3(5) and therefore outside direct registration, or its monomers are subject to separate obligations. That position is frequently misunderstood because the term “polymer of low concern” originates in OECD and non-EU new-chemical notification schemes, not in EU REACH. Under Article 3(5), NBR meets the polymer definition because it contains a distribution of molecules with at least three monomer units covalently bound to another monomer unit or other reactant, and the molecules of identical molecular weight do not constitute more than 50% w/w of the substance.

For an importer of NBR, the regulatory file therefore contains no REACH registration number for the polymer itself. The obligation shifts to the monomers acrylonitrile and 1,3-butadiene. Article 6(3) requires a polymer manufacturer or importer to submit a registration for the monomer substances or other substances from which the polymer is derived, if those substances have not already been registered by an actor up the supply chain. That obligation applies only when the polymer consists of at least 2% w/w of the monomer substance in the form of monomeric units and chemically bound substances, and the total quantity of that monomer substance makes up at least 1 tonne per year. Both thresholds are exceeded in most commercial NBR trade because bound acrylonitrile is almost always above 18% w/w and bound butadiene above 50% w/w.

The Article 6(3) calculation is based on the mass of monomeric units in the polymer, not on residual free monomer. A low-ACN NBR grade containing 20% bound acrylonitrile would only trigger acrylonitrile registration if the annual import volume retains at least 1 tonne of bound acrylonitrile, equivalent to 5 tonnes of that NBR grade. For a medium-ACN grade at 33% bound acrylonitrile, the polymer volume threshold falls to approximately 3 tonnes per year. The same calculation applies to butadiene, although most NBR grades contain more butadiene than acrylonitrile. Registration coverage is supply-chain specific. If both monomers are already registered by an upstream EU manufacturer or importer, the downstream polymer importer may rely on that registration and is positioned as a downstream user, provided its uses are within the exposure scenario or are addressed by a downstream user chemical safety report.

When NBR Contains Acrylonitrile and Butadiene, Article 6(3) Triggers Monomer Obligations

Because acrylonitrile and 1,3-butadiene are hazardous monomers, their registration dossiers include exposure scenarios that may restrict certain downstream activities. Acrylonitrile carries a harmonised CLP classification including Carc. 1B H350, Flam. Liq. 2 H225, Acute Tox. 3 H301/H311/H331, Skin Irrit. 2 H315, Eye Irrit. 2 H319, Skin Sens. 1 H317 and STOT SE 3 H335. 1,3-Butadiene carries Flam. Gas 1 H220, Press. Gas, Carc. 1A H350 and Muta. 1B H340. These classifications do not automatically attach to the NBR polymer, but free monomer above generic concentration limits may change classification and SDS disclosure. An EU importer should verify that the supplier’s monomer registration covers the imported NBR grade, the annual tonnage, and the downstream uses in compounding, calendering, extrusion, injection molding, or latex dipping.

On an industrial vacuum steam stripper operating at latex temperature 65–75 °C and vacuum below 20 kPa, residual acrylonitrile can be reduced to below 10 mg/kg in stripped NBR. If stripping temperature is lowered or steam-to-latex ratio falls below 0.1 kg/kg, residual values may exceed specification. Residual monomer is determined by headspace gas chromatography; production-scale release decisions commonly require certificates for residual acrylonitrile and 1,3-butadiene for each batch. Non-stripped high-temperature grades or latex supplied for dipped goods may contain higher free monomer and should be evaluated separately. In processing, NBR should be pre-dried at 80 °C for 2–4 h when ambient relative humidity exceeds 60% to avoid porosity in extrudates and molded parts.

Downstream Safety Data Sheet and Candidate List Interactions

Where residual acrylonitrile or 1,3-butadiene remains above 0.1% w/w in a preparation, CLP classification thresholds may require hazard communication and may affect SDS Section 3.2. In practice, stripped NBR resin is below 0.1% w/w; however, NBR compounds containing free acrylonitrile as an impurity from non-stripped latex or as a residual from co-agents should be evaluated. Candidate List obligations under REACH Article 33 apply to articles containing an SVHC above 0.1% w/w. Bound monomer units are not substances; they do not themselves make an article an SVHC-containing article. Residual free monomer is the relevant analytical target. An article containing NBR may still have SVHC communication duties if other additives, pigments, plasticizers, or residual monomers exceed the Article 33 threshold.

NBR is not exempt from REACH restriction obligations. Extender oils used in NBR compounds must be selected as low-PAH types where REACH Annex XVII entry 50 applies, with benzo[a]pyrene below 1 mg/kg and the sum of the eight listed PAHs below 10 mg/kg. The polymer exemption under Article 2(9) does not extend to restricted additives or to monomers that are intentionally released during processing. Compounders and converters must therefore maintain separate compliance evidence for polymer exemption, monomer registration coverage, CLP classification, and restricted substances in the finished compound.

OECD Polymer of Low Concern Does Not Align With the REACH Registration Boundary

The OECD polymer of low concern assessment uses molecular-weight distribution, oligomer content, water absorption, and reactive functional-group criteria. NBR can have number-average molecular weight above 100,000 g/mol and low water absorption, but the low-concern designation does not determine EU registration. An NBR grade that meets OECD low-concern criteria is still a polymer under REACH and is exempt from direct registration; conversely, an NBR grade that fails OECD low-concern assessment because of a reactive additive package is also exempt from direct polymer registration under REACH. The operational burden under EU law shifts to monomer registration, hazard communication, restriction compliance, and, where relevant, authorisation duties for SVHC-bearing residual substances.

For EU importers, the practical regulatory file for NBR includes bound monomer mass balance, residual monomer data, CLP-relevant impurity profile, and confirmation of upstream monomer registration. Carboxylated NBR grades containing carboxylic acid comonomers above 2% w/w require the same Article 6(3) analysis for the acid monomer if the 1 tonne per year threshold is met. In all cases, the absence of a REACH registration number for the NBR polymer is not a data gap; it is the expected outcome of the Article 2(9) polymer exemption. The larger compliance risk is usually unregistered monomer coverage or an SDS that does not reflect residual free monomer in the supplied grade.