Carbon residue determination quantifies the non-volatile char remaining after controlled pyrolysis of lignin-reinforced rubber compounds, providing a key indicator of thermal stability, flame retardancy, and bio-filler efficiency. This article outlines the applicable sample types, measurable parameters, standard test methods based on ISO, ASTM, and GB frameworks, and the industrial applications of carbon residue testing for lignin rubber materials. All testing described can be performed by accredited third-party laboratories using thermogravimetric analysis and gravimetric pyrolysis techniques.
Technical Overview
Lignin rubber refers to rubber composites in which lignin—a renewable, aromatic polyphenolic macromolecule derived from biomass—is incorporated as a reinforcing filler, crosslinking co-agent, or partial replacement for petroleum-based carbon black. During thermal degradation, the rubber matrix decomposes into volatile oils and gases, while the aromatic structure of lignin undergoes condensation, dehydrogenation, and partial graphitization, leaving a coherent carbonaceous char. The determination of carbon residue (also called char residue or pyrolysis residue) quantifies the mass percentage of this non-volatile residue after the material is heated to a specified end temperature under an inert atmosphere.
Carbon residue data are essential for evaluating the thermal stability and flame-retardant performance of lignin rubber, estimating the effective reinforcement contribution of lignin-derived char, and predicting the yield of solid carbon recovered from rubber pyrolysis processes. Because technical lignins typically leave 30–50% char at 600°C, whereas most hydrocarbon rubber chains volatilize almost completely, the measured residue directly reflects the lignin loading, dispersion quality, and char-forming ability of the compound. Reliable measurement requires strictly controlled atmosphere, heating rate, and final temperature, following internationally recognized standards such as ISO 11358, ASTM E1131, ISO 10370/ASTM D4530, and ISO 6964.
Testing Samples
Lignin-Filled Natural Rubber Compounds
Vulcanized or unvulcanized natural rubber (NR) compounds containing kraft lignin, organosolv lignin, or alkaline lignin at loadings typically ranging from 5 to 50 phr. Specimens are cut or ground into small pieces of 1–3 mm to ensure uniform heating and complete devolatilization during pyrolysis.
Lignin-Reinforced Synthetic Rubber Systems
Composites based on styrene-butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDM), and chloroprene rubber (CR), including hybrid filler systems such as lignin/silica and lignin/carbon black blends used in tire and industrial rubber goods.
Raw Lignin Fillers and Biochar Precursors
Unmodified and chemically modified lignin powders, including acetylated, phosphorylated, and silane-treated grades, tested as reference materials to establish the intrinsic char yield of the bio-filler and to benchmark compound formulation performance.
Finished Products and Pyrolysis Residues
Finished articles such as tire treads, seals, hoses, anti-vibration pads, and flame-retardant sheets, as well as solid residues recovered from laboratory or pilot-scale rubber pyrolysis reactors, evaluated for residual carbon content and potential reuse.
Testing Items
Depending on the selected method and engineering purpose, the following parameters can be determined and reported:
- Char residue (carbon residue) at specified end temperatures such as 500°C, 550°C, 600°C, or 800°C, expressed as a mass percentage of the original specimen
- Combustible carbonaceous residue, obtained by switching the atmosphere from nitrogen to air or oxygen after pyrolysis and recording the additional mass loss
- Ash content, representing the inorganic residue remaining after complete combustion of the char
- Volatile matter content, derived from the cumulative mass loss up to the defined pyrolysis temperature
- Thermogravimetric characteristic temperatures, including the 5% mass-loss onset temperature, the maximum decomposition rate temperature from the DTG curve, and full residual mass profiles
- Comparative char yield versus lignin loading, used to calculate the effective char-forming efficiency of the bio-filler
Testing Methods
Thermogravimetric Analysis (TGA) Under Nitrogen
Thermogravimetric analysis is the preferred technique for precise determination of pyrolysis residue. A specimen of approximately 5–10 mg is placed in an open platinum or alumina pan and heated from ambient temperature to 600°C, or up to 800–900°C when specified, at 10°C/min under a nitrogen purge of 40–60 mL/min, in accordance with ISO 11358 and ASTM E1131. The mass at the final temperature is recorded as the pyrolysis char residue. When the atmosphere is subsequently switched to air or oxygen, the char is combusted and the remaining mass is reported as ash; the difference between char and ash represents the combustible carbonaceous residue.
Micro Carbon Residue Method
For gram-scale specimens, the micro carbon residue method per ASTM D4530, ISO 10370, and GB/T 17144 is applied. A weighed sample is placed in a glass vial, heated to 500°C under a controlled nitrogen flow for a defined period, and then cooled in a desiccator. The carbonaceous residue is weighed, and the carbon residue value is calculated as the residue mass percentage, with correction applied for any diluent used. This method offers good repeatability and requires only small sample quantities.
Conradson Carbon Residue Method
The classical Conradson method (ASTM D189, ISO 6615, GB/T 268) applies destructive distillation with strong applied heat. The specimen in a porcelain crucible is heated by a gas burner or electric furnace following a prescribed heating sequence, and the carbonaceous mass remaining after the volatile matter has escaped is weighed. This method is mainly selected when correlation with legacy petroleum-type data or heavier feed materials is required.
Furnace Pyrolysis–Gravimetric Method for Vulcanizates
For cured lignin rubber compounds, a two-step gravimetric procedure based on ISO 6964, ASTM D297, and GB/T 3515 is used. The weighed specimen is pyrolyzed in a tube furnace at 550 ± 25°C under nitrogen, and the residue is weighed to obtain the pyrolysis carbon residue. The residue is then combusted in air at the same temperature, and the final inorganic residue is weighed as ash. The mass lost during combustion corresponds to the combustible char, allowing the lignin-derived carbon contribution to be separated from inorganic fillers and additives.
Applications
Carbon residue determination for lignin rubber supports material development and quality control across several industrial sectors:
- Green tire and rubber goods development: quantifying the reinforcing efficiency and char-forming contribution of lignin as a partial replacement for carbon black in tread and sidewall compounds
- Flame-retardant engineering: correlating char yield with limiting oxygen index, smoke suppression, and intumescent barrier performance in conveyor belts, cable sheathing, and sealing systems
- Pyrolysis and resource recovery: estimating the solid carbon yield of end-of-life rubber valorization processes and evaluating lignin-derived char as a recovered carbon black alternative
- Bio-filler quality control: verifying lot-to-lot consistency of technical lignins from kraft, organosolv, and biorefinery sources before compounding
- Research and development: establishing structure–property relationships between lignin modification, char morphology, and the thermal degradation kinetics of rubber composites
FAQ
What is the difference between carbon residue and ash?
Carbon residue is the carbonaceous char that remains after pyrolysis under an inert atmosphere and can still be burned if exposed to oxygen, whereas ash is the incombustible inorganic residue left after complete combustion of both the rubber and the char. Measuring both values distinguishes the combustible char from mineral fillers such as zinc oxide, silica, and calcium carbonate.
Why must pyrolysis be performed under nitrogen?
An inert nitrogen atmosphere prevents oxidative combustion. If oxygen were present, the carbonaceous char would ignite and burn along with the decomposing polymer, making it impossible to determine the true pyrolysis residue. Controlled switching from nitrogen to air in TGA further allows separate quantification of char and ash within a single experiment.
How does lignin content influence the measured char yield?
Because lignin possesses a highly aromatic, highly crosslinked structure, it leaves a substantially higher residue than hydrocarbon rubber chains. Increasing lignin loading generally raises the char residue in a near-linear manner, although dispersion quality, particle size, and chemical modification can cause deviations from ideal additivity.
What specimen size and temperature are typically used?
TGA measurements use approximately 5–10 mg of finely cut or ground material with a common end temperature of 600°C at a heating rate of 10°C/min. Furnace gravimetric methods use specimens of several hundred milligrams to a few grams pyrolyzed at 550 ± 25°C. Alternative end temperatures can be selected to match specific service conditions or research requirements.
Which standards govern this testing?
Commonly applied standards include ISO 11358 and ASTM E1131 for thermogravimetry, ASTM D4530, ISO 10370, and GB/T 17144 for micro carbon residue, ASTM D189, ISO 6615, and GB/T 268 for the Conradson method, and ISO 6964, ASTM D297, and GB/T 3515 for pyrolysis-based analysis of vulcanized rubber. Tests are conducted by accredited third-party laboratories under these recognized frameworks.
What factors can affect the accuracy of results?
Key influences include atmosphere purity and flow rate, heating rate, final temperature and hold time, specimen homogeneity and particle size, moisture and volatile content, and complete transfer of the residue during weighing. Strict adherence to standardized procedures, together with calibrated balances and thermocouples, ensures reproducible and traceable results.