Lignin rubber is an increasingly important class of sustainable elastomer composites in which lignin partially replaces carbon black or silica, and its thermal decomposition can release corrosive gases and condensates that attack metals, electronics, and process equipment. A systematic corrosivity assessment of lignin rubber pyrolysis products, based on recognized methods such as IEC 60754, ISO 11358, ASTM D130, and ISO 11845, quantifies the corrosive potential of evolved gases, pyrolysis oils, and chars. This article outlines the sample types, testing items, analytical methods, acceptance criteria, and industrial applications of such evaluations, providing a practical reference for material developers, pyrolysis plant operators, and automotive and electronics reliability engineers.
Technical Overview
Lignin rubber refers to elastomer compounds, typically based on natural rubber (NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), ethylene-propylene-diene rubber (EPDM), or nitrile rubber (NBR), in which lignin derived from pulp, paper, and biorefinery streams is incorporated as a reinforcing bio-filler, co-crosslinker, or partial replacement for carbon black and silica. These compounds are used in tires, seals, gaskets, anti-vibration mounts, footwear, and hose components. During high-temperature processing, service overheating, fire exposure, or end-of-life pyrolysis for material and energy recovery, lignin rubber undergoes thermal decomposition that generates a complex mixture of gases, condensable liquids, and solid char.
The pyrolysis atmosphere of lignin rubber typically contains methoxyphenols, cresols, and catechols released from the lignin macromolecule; isoprene, limonene, and light aromatics from the rubber backbone; hydrogen sulfide (H2S), carbonyl sulfide (COS), sulfur dioxide (SO2), and mercaptans from sulfur-based vulcanization systems; organic acids such as formic and acetic acid; amine species from accelerators and antioxidants; and hydrogen chloride or hydrogen bromide where halogenated additives are present. These species attack copper, silver, brass, steel, zinc, and aluminum through acid dissolution, sulfide film formation, pitting, and tarnishing. Condensed pyrolysis oils with a high total acid number can corrode reactor walls, condensers, and piping, while sulfur and chlorine residues can degrade the quality of recovered carbon black.
A professional corrosivity assessment integrates thermal decomposition profiling, standardized evolved-gas corrosivity testing, chemical speciation, copper strip testing, and metal coupon exposure to generate a temperature-dependent corrosivity classification. The evaluation supports material selection, protective equipment design, electronics reliability engineering, and compliance demonstration for lignin-based rubber materials destined for demanding thermal environments.
Testing Samples
Representative sampling is essential because corrosivity depends strongly on lignin type, filler loading, cure system, and additive package. Typical programs require approximately 50 to 200 grams of material per test condition, with specimens conditioned and documented according to formulation details such as lignin loading in phr and crosslinking chemistry.
Raw and Compounded Materials
Untreated or modified lignin fillers, including kraft lignin, organosolv lignin, lignosulfonates, and enzymatic hydrolysis residues, together with lignin biochar, masterbatches, and uncured compound sheets, are tested to establish the corrosive contribution of each ingredient before vulcanization.
Vulcanized Lignin Rubber Compounds
Cured plaques, tiles, and finished components such as tire treads, sidewalls, seals, gaskets, anti-vibration elements, and hose liners are evaluated. Comparative formulations covering conventional sulfur, semi-efficient and efficient vulcanization systems, and peroxide or sulfur-free cure systems allow the corrosive influence of the crosslinking chemistry to be isolated.
Pyrolysis Product Streams
For end-of-life and recycling scenarios, samples of light and heavy pyrolysis oil fractions, the aqueous phase, tar deposits, recovered carbon black or char, and non-condensable gases collected in inert gas bags or cylinders are assessed directly for acidity, sulfur and halogen content, and metal attack behavior.
Metal Coupon Targets
Standardized coupons of copper, silver, brass, mild steel, galvanized steel, and aluminum in defined surface finishes and accurately pre-weighed condition serve as the corroding targets for outgassing and condensate exposure tests.
Testing Items
The assessment portfolio typically covers the following measurable parameters:
- Thermogravimetric decomposition profile and identification of pyrolysis stage temperatures
- pH value of the aqueous absorption solution from evolved pyrolysis and combustion gases
- Conductivity of the aqueous absorption solution, normalized per method requirements
- Halogen acid evolution content, expressed as hydrochloric acid equivalent
- Concentrations of sulfur species, including H2S, SO2, COS, and mercaptans
- Concentrations of organic acids, phenolic compounds, ammonia, and amine species
- Total acid number (TAN) and sulfur and chlorine content of pyrolysis oils and condensates
- Copper strip corrosion rating on a 1a to 4c scale for pyrolysis oils and fractions
- Coupon mass loss per unit area, calculated corrosion rate, and tarnish or pitting classification
- Surface film composition on exposed metals and, where relevant, contact resistance drift of connectors
Testing Methods
Thermal Decomposition Profiling
Thermogravimetric analysis is performed according to ISO 11358 or ASTM E1131 under nitrogen from ambient temperature to 600 to 800 degrees Celsius at a controlled heating rate. The resulting mass-loss curve identifies the principal decomposition stages, including the broad lignin degradation interval and the rubber backbone devolatilization window, and defines the isothermal hold temperatures, for example 300, 450, and 600 degrees Celsius, used in subsequent furnace testing.
Evolved-Gas Corrosivity by pH and Conductivity
The core corrosivity measurement follows the principle of IEC 60754-2, EN 50267-2-2, and GB/T 17650.2. A weighed specimen is pyrolyzed and combusted in a calibrated tube furnace, typically operated between 800 and 950 degrees Celsius depending on the method edition, and the evolved gases are swept into a defined volume of deionized water. The pH and conductivity of the absorption solution are then measured. Widely applied acceptance thresholds for low-corrosivity materials are a pH of 4.3 or higher and a conductivity of 10 microsiemens per millimeter or lower. Staged testing at lower furnace temperatures simulates processing and condensate formation conditions relevant to real equipment.
Halogen Acid Content
Where halogenated rubber systems or additives are present, halogen acid evolution is determined according to IEC 60754-1 and GB/T 17650.1, with a common low-halogen criterion of 0.5 percent or less expressed as hydrochloric acid equivalent.
Gas and Condensate Speciation
TGA-FTIR provides real-time evolution profiles for H2S, SO2, phenolic species, and carboxylic acids, while pyrolysis-gas chromatography-mass spectrometry quantifies individual compounds such as guaiacol-type methoxyphenols and cresols. Ion chromatography in line with ISO 10304-1 determines chloride, bromide, sulfate, sulfite, formate, and acetate in absorption solutions, and gas detector tubes or gas chromatography with appropriate detectors quantify H2S and ammonia in non-condensable gas streams.
Copper Strip and Coupon Exposure Tests
Pyrolysis oils and condensate fractions are evaluated with the copper strip test adapted from ASTM D130 and ISO 2160, in which a polished strip is exposed at approximately 100 degrees Celsius for three hours and rated from 1a, slight tarnish, to 4c, severe corrosion. Static sealed-vessel outgassing tests expose pre-weighed metal coupons to the material at temperatures from 85 to 200 degrees Celsius. After exposure, coupons are cleaned according to ISO 8407 and evaluated for mass loss and corrosion rate according to ISO 11845, with tarnish and pitting graded against established scales. Electronics-relevant validation may add the flowing mixed gas corrosion test of IEC 60068-2-60, also referenced as GB/T 2423.51, and the silver tarnish evaluation of ISO 4538, while finished assemblies with rubber parts can be assessed under ISO 9227 or GB/T 10125 salt spray exposure.
Condensate, Char, and Surface Characterization
The total acid number of pyrolysis oils is measured by potentiometric titration per ASTM D664, sulfur content by ASTM D4294, and leachable chloride and sulfate in chars by aqueous extraction followed by ion chromatography. Corrosion films on exposed coupons are identified by scanning electron microscopy with energy-dispersive spectroscopy or X-ray photoelectron spectroscopy, and contact resistance of connectors can be measured according to IEC 60512-2-1 where electrical reliability is in scope.
Corrosivity Classification and Reporting
Results are consolidated into a temperature-dependent corrosivity class, for example low, moderate, or high, based on combined pH, conductivity, halogen acid, copper strip, and coupon corrosion rate criteria. Reports include decomposition profiles, species-specific emissions data, coupon morphology, benchmarking against reference carbon black or silica compounds, and practical mitigation recommendations such as cure system selection or condenser metallurgy upgrades.
Applications
Corrosivity assessment of lignin rubber pyrolysis products serves a wide range of engineering and commercial purposes:
- Design and maintenance of end-of-life tire and rubber pyrolysis plants, including reactor, condenser, and piping alloy selection and inspection interval planning
- Research and development of lignin-reinforced green tires and industrial rubber goods, with benchmarking against carbon black and silica reference compounds
- Optimization of vulcanization systems, including efficient and peroxide cures, to minimize sulfur-containing off-gassing
- Automotive interior and underhood reliability engineering for rubber components installed near electronic control units, connectors, and sensors
- Qualification of low-smoke zero-halogen and specialty cable compounds using the IEC 60754 test suite
- Supplier qualification, material datasheets, and documentation supporting sustainability certifications for bio-based elastomer products
- Failure analysis and forensic investigation of corrosion incidents traced to thermal degradation of rubber parts
FAQ
Q: Why are the pyrolysis products of lignin rubber considered corrosive? A: Thermal decomposition releases acidic and sulfur-containing species, including organic acids from lignin and H2S, SO2, and mercaptans from sulfur cure systems, which form sulfide films on copper and silver, lower the pH of condensed phases, and accelerate general and pitting corrosion of engineering metals.
Q: At what temperatures is corrosivity evaluated? A: A staged approach is used: approximately 150 to 200 degrees Celsius for processing simulation, 350 to 500 degrees Celsius for main devolatilization, up to 600 degrees Celsius for char-forming stages, and 800 to 950 degrees Celsius for standardized furnace corrosivity tests in line with IEC 60754-type methods.
Q: What are the typical acceptance criteria? A: Commonly applied thresholds include a pH of 4.3 or higher, conductivity of 10 microsiemens per millimeter or lower, halogen acid content of 0.5 percent or less, and a copper strip rating of 1b or better, with coupon corrosion rate limits defined case by case according to the application specification.
Q: How much sample is required and what is the typical turnaround time? A: Approximately 50 to 200 grams per test condition is sufficient for most programs, and standard scopes are typically completed within five to ten working days, while extended coupon and mixed flowing gas campaigns may require several weeks.
Q: Can lignin rubber be directly compared with conventional compounds? A: Yes, side-by-side testing of lignin-filled and reference carbon black or silica compounds under identical thermal profiles allows the incremental corrosive contribution of lignin and its interaction with the cure system to be quantified.
Q: Are pyrolysis chars and recovered carbon black also evaluated? A: Yes, chars are assessed for leachable chloride and sulfate, residual sulfur, and acidity, because these residues influence the quality of recovered carbon black and the corrosivity of downstream storage and handling equipment.