Acrylonitrile-butadiene copolymer (NBR) is a synthetic elastomer produced by emulsion copolymerization of acrylonitrile and 1,3-butadiene. The material is classified under ASTM D1418-21 as NBR and under ISO 1629:2013 within the R-family of diene rubbers. In glove manufacturing, NBR appears in two distinct physical forms: an aqueous NBR latex and a coagulated solid rubber. The choice between NBR latex and solid NBR sourcing determines the processing route, minimum film thickness, residual chemical profile, and applicable barrier-testing regime. Carboxylated NBR latex for dip-molding typically contains 28–30% bound acrylonitrile by mass, 43–45% total solids, and Brookfield viscosity at 25°C of 10–100 mPa·s measured according to ISO 1652:2020. Solid NBR is supplied as dried bales or crumbs with Mooney viscosity ML 1+4 at 100°C commonly in the range 20–90 units, determined per ISO 289-1:2015. The glass transition temperature of uncured NBR shifts with bound acrylonitrile content; grades with 20% ACN may show Tg near -40°C, while 40% ACN grades approach -20°C. That shift controls room-temperature stiffness and flexural cracking in finished gloves.

NBR latex remains dispersed in water from polymerization through compounding and dipping; it is therefore a direct film-forming feedstock. Solid NBR cannot be drawn into an unsupported film of 0.05–0.15 mm thickness by conventional two-roll milling, calendering, or extrusion without gauge variation and pinhole formation. This difference separates disposable NBR gloves, which are latex-dipped, from reusable NBR gloves, which are solid-rubber molded.

Why Does Carboxylated NBR Latex Dominate Thin-Film Glove Dipping?

Dip-molding of NBR gloves depends on the formation of a coherent wet gel at the former surface. The carboxylic acid comonomer is included at 3–6 wt% of the polymer feed to create carboxylated NBR latex. These carboxyl groups are neutralized by zinc oxide or calcium ions during compounding and gelling. Zinc oxide added at 1–3 phr contributes to ionic crosslink formation after water removal and during thermal cure. Without carboxylation, unsupported films from NBR latex exhibit lower green strength and poor tensile properties at the target thickness. In production-scale latex, the absence of natural coagulum and low gel content permit reproducible filterability through 20–50 µm mesh before the dipping tank. Latex storage is maintained at 5–35°C; freezing or shear-induced instability produces coagulum that clogs dipping filters.

The resulting gloves are specified under ASTM D6319-19, which requires a minimum tensile strength of 14 MPa and ultimate elongation of 500% both before and after accelerated aging. The accelerated aging condition in that specification uses 70 ± 2°C for 7 days. These values are attainable for latex films of 0.05–0.15 mm after curing at 110–130°C. The water-leak requirement of ASTM D5151-19 with an acceptance quality limit of 1.5 is met because latex particles coalesce into a continuous membrane free of the microvoids that characterize solvent-cast or re-emulsified solid rubber films.

In a continuous coagulant dipping line, ceramic or stainless-steel formers are heated to 50–70°C, submerged in an aqueous coagulant solution containing 10–30 wt% calcium nitrate, and then moved into the carboxylated NBR latex bath. Calcium nitrate destabilizes the latex and deposits a wet gel whose thickness is governed by latex total solids, dwell time, former temperature, and coagulant concentration. The wet gel is then leached in water tanks at 40–60°C to remove water-soluble residues. After drying and curing at 110–130°C for 10–20 min, the glove is stripped from the former and may undergo additional surface chlorination or polymer coating to reduce tack. Amine-based additives are generally avoided in carboxylated NBR latex compounding because their basic character can destabilize the anionic surfactant system and compete with zinc oxide for carboxylate sites. Published data for exact film thickness growth rates on production-scale machines are limited because former geometry, line speed, and latex rheology vary among installations.

Solid Rubber Sourcing Constraints for Wet-Film Formation

Solid NBR is produced by coagulation of NBR latex, followed by washing, drying, and baling. It is supplied as a dry elastomer with Mooney viscosity values that reflect higher molecular mass and chain entanglement. For thin unsupported films below 0.15 mm, solid NBR bales cannot be used directly. They must first be dissolved in ketone or aromatic solvents to create a rubber cement, or re-emulsified by high-shear mixing with surfactants and water. Solvent casting from NBR cement produces films with high residual solvent content and thickness variation; re-emulsified solid NBR has particle sizes above 1 µm, much larger than the original latex particles, and its films tend to contain microcracks when dried. Neither route is economically or technically equivalent to direct coagulant dipping of NBR latex on high-speed glove formers.

Where solid NBR does participate in glove manufacturing is in reusable hand protection, such as chemical-handling gloves, glove-box gauntlets, and heavy-duty sleeves. These articles are typically compression molded or injection molded at temperatures of 150–180°C and cured for 3–15 min, depending on wall thickness. Thickness is normally 0.4–3.0 mm, and the product is tested to chemical permeation and mechanical standards such as ISO 374-1:2016, EN 388:2016+A1:2018, and ISO 37:2017. Tensile strength for solid NBR compounds is commonly 10–20 MPa, with elongation at break of 300–600%, but these values depend on filler type, plasticizer level, and ACN content.

When Accelerated Aging and Tensile Set Dictate Formulation Choices

Compounding and curing decisions for NBR gloves are driven by the need to retain tensile properties after oxidative aging. ASTM D6319-19 requires that NBR examination gloves retain a minimum tensile strength of 14 MPa and elongation at break of 500% after accelerated aging at 70 ± 2°C for 7 days. Sulfur-cured carboxylated NBR latex systems using zinc dibutyldithiocarbamate or zinc mercaptobenzothiazole accelerators can achieve this, but residual thiuram and carbamate accelerators may pose Type IV sensitization risk. In contrast, sulfur-free zinc oxide crosslinking of carboxylated NBR latex reduces extractable accelerators but may require higher zinc oxide loadings, typically above 1.5 phr, to maintain tensile strength retention. The balance is evaluated by tensile modulus at 300% elongation and by elongation at break according to ISO 37:2017 or ASTM D412-16.

Formulation and curing of solid NBR reusable gloves follow a different optimization path. High ACN grades, often 33–45% bound acrylonitrile, are specified for resistance to aliphatic hydrocarbon oils and fuels, but they raise compound Tg and reduce low-temperature flexibility. The cured sheet is tested for volume swell after immersion in ASTM IRM 903 oil or Fuel C according to ASTM D471-16a. Higher ACN content increases tensile strength and permeation resistance but narrows the low-temperature service window. Published data for exact low-temperature brittleness rates across all commercial solid NBR grades are limited because plasticizer type and cure state shift the brittle point independently of ACN content.

Permeation Resistance, Leaching Residuals, and Type IV Allergy Limits

Chemical permeation through NBR gloves is assessed using ISO 374-1:2016 and EN 16523-1:2015, where breakthrough time is recorded at a permeation rate of 1.0 µg/cm²/min. NBR is generally more resistant to aliphatic hydrocarbons and oils than to polar solvents such as acetone or dimethylformamide; actual breakthrough time varies with ACN content, film thickness, and crosslink density. For disposable examination gloves, thickness is often at the lower end of the range, so permeation performance should not be inferred from solid NBR reusable gloves of greater thickness. Water-soluble extractables, including calcium nitrate, accelerators, and processing stabilizers, are reduced by leaching and chlorination. Residual accelerators are not fully specified by ASTM D6319-19, but skin sensitization risk is assessed following ISO 10993-10:2021 for medical devices. NBR contains no natural rubber latex proteins, which removes the Type I immediate allergy risk associated with latex gloves; Type IV delayed hypersensitivity to dithiocarbamate or thiuram accelerators remains an operational limitation.