Acrylonitrile-butadiene copolymer (NBR) for conveyor belts, rollers & footwear is an abrasion-driven grade choice governed by rotating drum abrasion tests under ISO 4649:2022 and DIN 53516, followed by adjustment of bound acrylonitrile content and Mooney viscosity. NBR is produced by cold emulsion polymerization at 5–15 °C; bound acrylonitrile typically ranges from 18% to 50%, shifting the glass transition temperature from approximately -55 °C to -20 °C. For abrasion-driven applications, the specification sequence fixes volume loss on a rotating drum abrader, then selects acrylonitrile content for oil-swelling limits, and then sets Mooney viscosity to match specific mixing, calendering, or injection molding equipment.

Abrasion in NBR compounds proceeds by microcutting, surface fatigue crack growth, and oxidative degradation of the butadiene-rich phase. The DIN 53516 method reports volume loss in mm³ after a specimen is traversed 40 m against a rotating drum covered with 60-grit alumina paper; lower values indicate greater abrasion resistance. Representative NBR conveyor belt cover compounds containing 50 phr N330 carbon black, 5 phr zinc oxide, 1 phr stearic acid, and a semi-efficient sulfur cure produce DIN 53516 volume loss of 80–120 mm³, depending on acrylonitrile content and crosslink density. This is the central abrasion-driven grade choice: a compound for ore-handling belts may tolerate volume loss of 110–130 mm³ when oil resistance is moderate, while a roller cover in oily steel-mill service may require 90–110 mm³ combined with higher tear resistance.

Acrylonitrile content establishes the oil-abrasion trade-off

Bound acrylonitrile content is the first grade discriminator. At 18–24% acrylonitrile, NBR exhibits low-temperature flexibility with brittleness temperature near -45 °C to -55 °C by ASTM D2137, but immersion in ASTM D471 IRM 903 at 100 °C for 72 h produces volume swell of 35–55%. Abrasion resistance in this class is generally lower; DIN 53516 volume loss commonly exceeds 120 mm³ in a 60 Shore A compound, and the material is reserved for low-load applications where oil swell is not the controlling variable. At 28–34% acrylonitrile, oil swell drops to 15–30% and DIN 53516 volume loss stabilizes at 90–120 mm³ with N330; this is the standard window for conveyor belt cover rubbers and general-purpose industrial rollers. At 39–50% acrylonitrile, oil swell falls below 10%, but the glass transition rises to -20 °C to -30 °C, and low-temperature embrittlement becomes a risk below -10 °C. Compounds in this upper band can achieve DIN 53516 volume loss of 70–100 mm³ when reinforced with N220 at 55–60 phr, making them candidates for contact with cutting oils and fuel oils.

Conveyor belt cover compounds are mixed in an intermeshing internal mixer with chamber volumes from 1.5 L to 270 L, dumped to a two-roll mill, and sheeted for calendering at 70–90 °C. For a three-ply textile-reinforced belt carrying mineral slurries, the cover compound uses NBR with 28–33% acrylonitrile and Mooney viscosity ML 1+4 at 100 °C of 50–70. Filler loading of N330 or N550 at 45–55 phr is typical; N550 raises tensile elongation but increases DIN 53516 volume loss by 10–20% relative to N330. A sulfur cure with 1.5–2.0 phr sulfur, 1.0–1.5 phr N-cyclohexyl-2-benzothiazolesulfenamide, and 0.25–0.5 phr tetramethylthiuram disulfide balances scorch safety and crosslink density. Belt covers of this design typically show tensile strength of 15–20 MPa by ASTM D412, elongation at break of 400–550%, and DIN 53516 volume loss of 85–120 mm³. Calendering at 2–6 m/min with roll nip 0.5–1.5 mm embeds the fabric; Mooney viscosity below 50 may cause calender bagging, while values above 70 raise power draw and scorch risk at stock temperatures above 105 °C.

High-shear dispersion of N220 and N234 in high-acrylonitrile NBR determines the width of the acceptable abrasion window. In a 270-L intermeshing internal mixer, ram pressure of 0.5–0.6 MPa and rotor speed of 30–50 rpm are used for the first mixing stage; dump temperatures are held between 110 °C and 130 °C to incorporate carbon black without thermal degradation. Two-stage mixing is standard for roller cover compounds: the first stage drops at 120–130 °C, and the second stage adds curatives at 70–90 °C to avoid scorch. Moving die rheometer cure curves at 180 °C show tc90 of 2–4 min for conveyor belt cover compounds; press cure at 150 °C extends tc90 to 6–9 min. Filler dispersion quality, measured by optical microscopy or electrical conductivity, affects abrasion: undispersed agglomerates larger than 10 μm act as microcutting initiators and can increase DIN 53516 volume loss by 15–25%.

What controls roller cover fatigue resistance under sustained loads?

Roller cover compounds for steel-mill, paper, and textile nip rolls are selected for a combination of dynamic fatigue resistance, tear strength, and compression set. The preferred NBR grades contain 33–38% acrylonitrile and Mooney viscosity ML 1+4 at 100 °C of 70–100, allowing high filler loading while retaining enough green strength for spiral winding or compression molding. A production-scale roller cover formulation with N220 or N234 at 50–60 phr, zinc oxide 5 phr, stearic acid 1 phr, sulfur 1.5–2.0 phr, and sulfenamide accelerator 1.2–1.8 phr typically achieves ASTM D412 tensile strength of 18–22 MPa, ASTM D624 die C tear strength of 35–55 N/mm, and ASTM D395 compression set of 20–30% after 22 h at 100 °C. DIN 53516 volume loss for these covers falls between 90–110 mm³. The higher Mooney viscosity reduces cold flow during autoclave cure at 140–160 °C and maintains dimensional stability of the roller profile. However, factory trials have shown that Mooney viscosity above 100 increases mill time and creates localized hot spots in internal mixers with tangential rotors; dump temperatures above 120 °C can initiate premature crosslinking and cause surface roughness after grinding.

When footwear outsoles require DIN 53516 volume loss below 150 mm³

Footwear outsole compounds are formulated around the injection molding machine constraint as much as abrasion performance. NBR grades with 33–38% acrylonitrile and Mooney viscosity ML 1+4 at 100 °C of 40–60 are preferred because low viscosity permits mold filling at barrel temperatures of 70–90 °C and mold temperatures of 160–190 °C. A work-boot outsole compound containing N220 carbon black at 45–55 phr, silica at 10–15 phr, dioctyl phthalate or trioctyl trimellitate at 10–15 phr, and a sulfur/accelerator cure system can produce DIN 53516 volume loss of 100–150 mm³; compounds below 120 mm³ are commonly specified for oil-resistant safety footwear, while compounds below 100 mm³ require higher crosslink density and may sacrifice flex resistance. Mooney scorch at 120 °C is maintained above 10 min to avoid scorch in the runner manifold; mold release is controlled by a zinc stearate external lubricant. Published data for this specific configuration is limited, and laboratory abrasion tests on prototype compounds are therefore used to confirm DIN 53516 volume loss before mold release. Plasticizer migration kinetics must be considered for outsoles stored in contact with PVC shanks; aromatic plasticizers above 15 phr can migrate into the PVC within 30 days at 40 °C, softening the shank and reducing dimensional stability.

Compounding variables that shift DIN 53516 volume loss independently of polymer grade include filler surface area, crosslink density, and antioxidant loading. Increasing N220 from 40 phr to 55 phr can reduce volume loss by 15–25%, but decreases processing safety and tear resistance at low temperatures. Crosslink density, expressed as moving die rheometer maximum torque minus minimum torque, should be maintained within 3–6 dN·m for conveyor belt covers; values above this range increase abrasion resistance but reduce crack growth resistance. The antidegradant system for outdoor conveyor belts typically uses 6PPD at 1.5–2.0 phr and microcrystalline wax at 1.0–1.5 phr to control ozone attack under 50 pphm ozone exposure per ISO 1431-1. In black compounds, omission of these antidegradants results in visible ozone cracks within 72 h at 25% strain.

Batch-to-batch variation in NBR belt and roller compounds is controlled by Mooney viscosity specification of ±5 ML units and by monitoring moving die rheometer torque at tc90 against a qualified reference curve. Sulfur donor systems can show reversion at cure temperatures above 180 °C; storage in direct sunlight without 6PPD and microcrystalline wax leads to ozone cracking. NBR compounds are unsuitable for continuous exposure to ketones, chlorinated solvents, or strong oxidizing acids, and low-acrylonitrile grades swell above 60% in ASTM reference fuel C. Process release for production batches therefore requires DIN 53516 volume loss, Mooney viscosity, and ASTM D412 tensile properties to fall within approved lot ranges before calendering or injection molding begins.