Nitrile Butadiene Rubber (NBR) Thermogravimetric Analysis: Principles, Methods, and Applications

Thermogravimetric analysis (TGA) of nitrile butadiene rubber (NBR) is a core thermal testing technique used to evaluate decomposition behavior, thermal stability, and compositional makeup of NBR materials. By continuously monitoring sample mass as temperature rises under controlled atmospheres, TGA delivers quantitative data on volatile content, polymer fraction, carbon black loading, and inorganic ash. This article presents a comprehensive technical overview of NBR thermogravimetric analysis, including sample types, testing items, standardized methods, and practical industrial applications.

Technical Overview

Nitrile butadiene rubber (NBR) is a synthetic elastomer produced by the free-radical copolymerization of acrylonitrile (ACN) and butadiene. The acrylonitrile content, typically ranging from 18% to 50%, governs the oil resistance, low-temperature flexibility, and thermal behavior of the material. Because NBR is widely used in seals, O-rings, hoses, fuel system components, and industrial gloves, understanding its thermal decomposition profile is critical for formulation design, quality control, and service life assessment.

Thermogravimetric analysis measures the change in mass of a specimen as a function of temperature or time under a precisely controlled atmosphere and heating program. The resulting thermogravimetric (TG) curve, together with its first derivative (DTG), reveals decomposition stages, peak degradation temperatures, and residual fractions. For NBR, pyrolysis under nitrogen typically proceeds in multiple steps: an initial low-temperature loss of moisture and plasticizer or process oil below approximately 300°C, followed by the main degradation of the polymer backbone between roughly 380°C and 480°C, where butadiene-rich segments undergo cyclization and crosslinking while releasing butadiene, acrylonitrile, hydrogen cyanide, and light hydrocarbons. A subsequent high-temperature step between approximately 480°C and 550°C corresponds to further breakdown of cyclized and charred structures.

The acrylonitrile content has a measurable influence on the TGA profile. Grades with higher ACN content generally exhibit slightly higher onset decomposition temperatures and greater char residue tendency, reflecting the stabilizing effect of nitrile groups. When the atmosphere is switched from nitrogen to air or oxygen at the end of pyrolysis, carbon black and carbonaceous char oxidize, while inorganic fillers and ash remain as the final residue. This enables a complete compositional breakdown of NBR compounds in a single experiment.

Testing Samples

Raw NBR Gum and Latex

Unvulcanized NBR polymer, supplied as bale gum, crumb, powder, or latex, is tested to characterize the intrinsic thermal stability of the base elastomer, compare polymerization grades, or verify supplier specifications. Latex samples are typically pre-dried or film-cast before analysis to remove water in a controlled manner.

Cured NBR Compounds and Vulcanizates

Fully formulated and crosslinked NBR compounds containing carbon black, silica, plasticizers, antioxidants, and cure packages are analyzed to quantify filler loading, oil content, and polymer fraction. These samples represent the majority of industrial TGA submissions for quality control and reverse engineering purposes.

Finished NBR Products

Seals, O-rings, gaskets, diaphragms, fuel hoses, rolls, conveyor belts, and disposable gloves can be sectioned directly into small pieces for TGA. Testing finished articles is common for incoming inspection, batch conformity checks, and failure investigations of field-returned parts.

NBR Blends

NBR is frequently blended with PVC to improve ozone resistance and rigidity, or with other elastomers to balance properties. TGA, often combined with DTG peak deconvolution and complementary techniques such as DSC and FTIR, helps estimate blend ratios and assess phase-level degradation behavior.

Testing Items

Thermogravimetric analysis of NBR yields a broad set of quantitative parameters relevant to material performance and composition:

• Initial decomposition temperature and extrapolated onset temperature, indicating the practical upper service temperature limit.
• Maximum degradation rate temperature, determined from the DTG peak maximum of each decomposition step.
• Mass loss at specified temperatures or temperature intervals, used for batch-to-batch comparison.
• Volatile content, including moisture and process aids, lost below approximately 300°C.
• Plasticizer and extender oil content, typically observed between 200°C and 350°C.
• Polymer or elastomer fraction, corresponding to the main pyrolysis step under inert atmosphere.
• Carbon black and carbonaceous char content, measured by the mass loss after switching from nitrogen to air or oxygen.
• Inorganic ash and mineral filler residue remaining at the final test temperature.
• Residual char yield at 600°C under nitrogen, an indicator of charring tendency.
• Kinetic parameters such as apparent activation energy, estimated using Kissinger, Flynn–Wall–Ozawa, or Friedman methods from multi-heating-rate experiments.
• Oxidative induction temperature or time, evaluated under oxidative atmosphere to assess antioxidant effectiveness.

Testing Methods

Specimens are cut or punched into small pieces weighing approximately 5 to 20 mg to ensure uniform heat transfer, and placed in open platinum or alumina pans. The instrument is calibrated for temperature using standards such as indium, zinc, or the Curie point of nickel, and for mass using certified weights. Buoyancy correction is applied through a blank run when high accuracy is required.

The standard program for compositional analysis follows a two-atmosphere protocol:

• Equilibrate at ambient temperature or 30°C under nitrogen purge at 40 to 60 mL/min.
• Heat at a constant rate of 10°C/min to approximately 600°C and hold isothermally for several minutes to complete pyrolysis of the polymer fraction.
• Switch the purge gas to air or oxygen while holding at 600°C, or ramp to 800–900°C, to oxidize carbon black and char.
• Hold at the final temperature until a stable residue mass is achieved; the remaining mass represents inorganic ash and fillers.

Relevant standards include ASTM E1131 for compositional analysis by TGA, ASTM D6370 for rubber compositional analysis by thermogravimetry, ISO 11358 for thermogravimetry of polymers, and ISO 9924-1 for rubber and rubber products determination of composition by thermogravimetric analysis of vulcanizates and uncured compounds containing butadiene. When kinetics are required, a minimum of three heating rates, for example 5, 10, and 20°C/min, are run under identical atmospheric conditions. For pure thermal stability comparison, a single-run nitrogen-only program to 600°C with full DTG recording is sufficient. Data interpretation integrates the TG step heights, DTG peak positions, and final residue mass to produce a complete compositional and stability report.

Applications

NBR thermogravimetric analysis supports a wide range of industrial and research objectives:

• Quality control of incoming raw polymer and finished compounds against formulation specifications.
• Reverse engineering and competitive benchmarking by quantifying carbon black loading, oil level, and filler content.
• Failure analysis of degraded seals, hoses, and gaskets, where shifts in onset temperature and residue levels reveal thermal aging, oil extraction, or under-cure.
• Formulation development, comparing the effects of acrylonitrile content, filler types, plasticizers, and antioxidant systems on thermal endurance.
• Service life estimation by combining TGA kinetics with accelerated aging data and Arrhenius extrapolation.
• Verification of blend ratios in NBR/PVC and other multiphase systems.
• Support for regulatory documentation and material data sheets requiring quantified composition.

FAQ

What temperature does NBR start to decompose?

The main decomposition of NBR begins at approximately 380°C under nitrogen, with the DTG peak typically near 450°C. Volatiles such as moisture and process oil may be lost below 300°C, which does not represent polymer degradation. Maximum continuous service temperatures for NBR products are far lower, generally 100°C to 120°C depending on the formulation.

Can TGA determine the acrylonitrile content of NBR?

TGA alone does not directly quantify acrylonitrile content. However, the decomposition profile, char yield, and DTG peak shape correlate with ACN level, allowing comparative estimation against reference materials. Precise acrylonitrile determination is better performed by elemental nitrogen analysis, FTIR, or pyrolysis-GC/MS, with TGA serving as supporting evidence.

Why is the atmosphere switched from nitrogen to air during the test?

Under nitrogen, both carbon black filler and carbonaceous char from the polymer remain in the pan after pyrolysis. Switching to air or oxygen at high temperature combusts the carbonaceous material, leaving only inorganic ash. The mass loss during this oxidation step quantifies carbon black plus char, completing the compositional analysis.

How much sample is needed for NBR TGA testing?

A specimen of 5 to 20 mg is standard. Smaller samples improve resolution of overlapping decomposition steps, while slightly larger samples improve the signal for low-level residues. Homogeneity is important, so multi-point sampling is recommended for heterogeneous finished products.

What standards govern NBR thermogravimetric analysis?

The most commonly applied standards are ASTM E1131, ASTM D6370, ISO 11358, and ISO 9924-1. These standards define heating rates, purge gas conditions, atmosphere switching points, and calculation methods for volatiles, polymer, carbon black, and ash fractions.