ISO 1408 Rubber Carbon Black Content Test by Pyrolytic Degradation Methods

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ISO 1408: Rubber -- Determination of Carbon Black Content -- Pyrolytic and Chemical Degradation Methods

ISO 1408 specifies three methods for determining the carbon black content of rubber compounds: a pyrolytic method (A) and two chemical degradation methods (B and C). It provides precise procedures for quantifying carbon black, a critical reinforcing filler in rubber formulations.

ISO 1408 Rubber Carbon Black Content Test by Pyrolytic Degradation Methods

Test methods and principles:

All three methods are based on the gravimetric method, calculating the carbon black content by measuring the mass difference before and after a specific treatment.

l Remove organic matrix: Either by pyrolysis (Method A) or chemical oxidation (Methods B and C).

l Isolate residue: Consists of carbon black + acid-insoluble mineral fillers.

l Quantify carbon black: Burn off carbon black in air/oxygen at 850°C; mass loss = carbon black content.

1, Principle of Method A: Heat the extracted rubber sample in a nitrogen flow at 850°C to pyrolyze it, and weigh the mass of the non-volatile residue (including carbon black and ash). Then, burn off the carbon black in air or oxygen at the same temperature and weigh again. The difference between the two weights is the mass of carbon black.

2, Principle of Method B: Oxidize and dissolve the organic components and acid-soluble inorganic substances in the rubber using nitric acid. Filter and wash the residue (carbon black and acid-insoluble mineral fillers), first dry it to a constant weight at 850°C in nitrogen, and then burn off the carbon black in air at the same temperature. The difference between the two weights is the mass of carbon black.

3, Principle of Method C: Swell the sample with hot dichlorobenzene and then oxidize the organic matter with tert-butyl hydroperoxide. Subsequent steps (filtration, washing, drying in nitrogen, burning in air) are similar to Method B, and the carbon black content is calculated based on the weight difference.

Calculation Formula (all methods):

Carbon Black Content (%)=(m1−m2)/m0×100

m0 = mass of extracted, dried test piece (g); m1 = mass of residue after drying at 850°C in nitrogen (g); m2 = mass of residue after burning off carbon black in air (g)

Method	Type	Primary Application	Key Characteristics
Method A	Pyrolytic	Preferred for most rubbers (polyisoprene, polybutadiene, SBR, butyl rubber, EPDM, etc.)	Least hazardous, avoids chemical degradation artifacts
Method B	Chemical (Nitric acid oxidation)	Samples not amenable to Method A; unsaturated rubbers (except isobutylene-isoprene copolymers)	Effective for samples with interfering substances
Method C	Chemical (tert-Butyl hydroperoxide oxidation)	Isobutylene-isoprene copolymers and EPDM when A and B fail	More hazardous, requires special safety precautions

Test Equipment for ISO 1408:

Methods A Carbon content tester	Tube Furnace System: Includes quartz or ceramic combustion tubes, a horizontal tube furnace (maintaining 850°C ± 25°C), an air intake system (providing nitrogen, oxygen/air), and a steam absorption device. Combustion Boat: Made of silicon, used to hold samples for pyrolysis and combustion. Typical tube furnace assembly
Methods B/C	Gooch crucible: A crucible with a bottom filter layer made of inert, heat-resistant filtering material, used for filtering and holding residues. Tube furnace system: Used for high-temperature heating of the Gooch crucible under a controlled atmosphere (nitrogen/air). Apparatus specific to Method C: Flat-bottom flasks with ground glass joints, an air condenser, and a water condenser, used for reflux oxidation reactions. Extraction apparatus: A Soxhlet extractor or similar equipment used for sample pretreatment. Fume hood: All operations involving solvents, acids, and high temperatures must be carried out in a fume hood, which is an important safety device.
Analytical balance	precision: ±0.1 mg
Muffle furnace	850°C ±25°C, thermostatically controlled.
Heating Oven	100°C ±3°C, for drying extracted samples

Test Application and limitations:

ISO 1408 applies to:

Raw rubber compounds: Quality control during production

Vulcanized rubber products: Performance verification and failure analysis

Recycled rubber: Carbon black content assessment for material reuse

Specialty rubbers: Silicone rubbers, fluorosilicone rubbers, chlorosulfonated polyethylenes (<30% chlorine)

Limitations:

Method A cannot be used for chloroprene rubber or NBR with >30% acrylonitrile content

Method B unsuitable for isobutylene-isoprene copolymers

Method C relatively hazardous, used only when A and B fail.

General Test Procedures:

Method A (Pyrolytic)

1) Weigh the dried extracted sample (approximately 0.1-0.5 g).

2) Place the sample into a combustion boat and put it in the low-temperature region of a combustion tube with nitrogen gas flowing.

3) After purging the system with nitrogen, slowly push the combustion boat into the 850°C heating zone for pyrolysis.

4) After pyrolysis is complete, cool under nitrogen and weigh (mass m1).

5) Heat in air or oxygen (or in a muffle furnace) at 850°C to burn off the carbon black.

6) After cooling, weigh again (mass m2).

7) Calculate the carbon black percentage by the formula (m1 - m2) / m0 × 100.

Method B (Chemical)

1) Weigh the dried sample for extraction (about 0.3-0.5 g).

2) Oxidize and decompose by heating with nitric acid at room temperature or in a water bath.

3) Hot filter the reaction solution into a pre-weighed Gooch crucible, leaving the residue in the beaker.

4) Wash the residue in the beaker multiple times with hot nitric acid, acetone, acetone-chloroform mixture, chloroform, sodium hydroxide solution, hydrochloric acid solution, etc., and filter to ensure quantitative transfer.

5) Dry the Gooch crucible with the residue under a nitrogen atmosphere at 850°C to constant weight, cool, and weigh (m1).

6) Switch to air or oxygen and heat at 850°C until the carbon black is completely oxidized and disappears, then cool and weigh (m2).

7) Calculate the carbon black content using the same formula.

Method C (Chemical)

1) Weigh the sample (about 0.3–0.5 g) and place it in a flask containing dichlorobenzene for reflux swelling.

2) Add tert-butyl hydroperoxide solution and reflux to oxidize.

3) Add toluene (or acetone) to dilute, and let it stand to allow insoluble substances to settle.

4) Filter into a Gooch crucible, and wash sequentially with toluene, acetone, nitric acid, hydrochloric acid, etc.

5) The subsequent drying, combustion, weighing, and calculation steps are the same as in Method B.

Related Standards with ISO 1408:

ASTM D1619	Standard Test Methods for Carbon Black—Sulfur Content
GB/T 3515	Rubber -- Determination of carbon black content -- Pyrolytic method (Chinese standard, modified from ISO 1408:1995)
JIS K6227	Rubber -- Determination of carbon black content -- Pyrolytic and chemical degradation methods.
NBN T 31-021	Rubber - Determination of carbon black content-Pyrolytic and Chemical degradation methods

Carbon black content tester is used to measure the carbon content in PE, PP, PB etc. , sample under nitrogen condition, through high temperature pyrolysis, then measure the weight to get the carbon level.

Ash Content Test Furnace (Muffle Furnace) analysis and determination and general small steel quenching, annealing, tempering and other heat treatment heating, high temperature furnace can also be used for metal, ceramic sintering, dissolution, analysis and other high temperature heating.

Q1: What is the core purpose of ISO 1408?

A: ISO 1408 specifies standardized test methods to accurately measure the carbon black content in raw and vulcanized rubber compounds. It provides three reliable procedures (pyrolysis and chemical oxidation) to quantify carbon black, a key reinforcing filler in rubber materials.

Q2: Why is the ISO 1408 test so important for rubber materials?

A: Carbon black directly dominates rubber performance:

It reinforces rubber, improving tensile strength, wear resistance, and durability.

It affects processing viscosity, curing behavior, and final product stability.

Accurate content testing ensures batch consistency, quality control, and compliance with material specifications.

It supports formula optimization, cost control, and failure analysis for rubber products.

Q3: How many test methods are included in ISO 1408, and what are they?

A: Three methods are provided:

Method A (Pyrolytic): Thermal decomposition in nitrogen, the most widely used and safest method.

Method B (Chemical): Oxidation with nitric acid, for rubbers not suitable for Method A.

Method C (Chemical): Oxidation with tert-butyl hydroperoxide, for special rubbers like butyl rubber.

Q4: Which test method should I select for my rubber sample?

A: Choose Method A for most general rubbers (SBR, NR, BR, EPDM, etc.).

Choose Method B for unsaturated rubbers excluding isobutylene-isoprene copolymers.

Choose Method C only for butyl rubber and EPDM when Methods A and B are invalid.

Q5: What are the most critical pieces of test equipment?

A: Key equipment includes an analytical balance, tube/muffle furnace (850 °C), extraction apparatus, desiccator, and method-specific tools like combustion boats, filtration crucibles, or reflux condensers.

Q6: What are common causes of inaccurate test results?

A: Incomplete extraction of soluble components.

Improper furnace temperature or gas flow (Method A).

Incomplete oxidation of the rubber matrix.

Residual moisture in samples or residues.

Loss of residue during filtration or transfer.

Q7: The standard describes three methods (A, B, C). Which one should I choose for my sample?

A: The choice is strictly defined by the polymer type and compound ingredients, as outlined in the document's Scope:

Method A (Pyrolytic) is the preferred and primary method. Use it for common polymers like Natural Rubber (NR), Styrene-Butadiene Rubber (SBR), Butyl Rubber (IIR), EPDM, etc., unless the compound contains materials like lead/cobalt salts, bitumen, or phenolic resins, which can form a carbonaceous char during pyrolysis and falsely inflate the result.

Method B (Nitric Acid Oxidation) is for samples not suitable for Method A, typically for unsaturated rubbers. It cannot be used for Isobutylene-Isoprene (Butyl) rubbers.

Method C (tert-Butyl Hydroperoxide Oxidation) is considered more hazardous and is a last resort. It is specifically intended for Isobutylene-Isoprene copolymers (IIR) and Ethylene-Propylene copolymers (EPM) when Methods A and B fail.

Q8: My result seems lower than expected. What could cause an underestimation of carbon black?

A: The document itself provides a key reason in the notes:

Volatiles in the Carbon Black: The carbon black, as purchased, may contain matter volatile at 850°C. This volatile component is lost during the pyrolysis/drying step in nitrogen (when obtaining mass m1), leading to a final result that is low by that amount. A correction can be applied if the carbon black type and origin are known.

Other potential reasons (based on testing knowledge):

Incomplete Oxidation in Method B/C: If the organic polymer is not fully destroyed by the acid/peroxide, some unoxidized polymer may remain with the residue, causing an overestimation, not underestimation. The document warns about this for Method B.

Loss of Carbon Black During Filtration: Fine carbon black particles may pass through the filter in Methods B and C if the Gooch crucible preparation is faulty.

Over-oxidation in Method B: Prolonged or overly vigorous heating with nitric acid can partially oxidize the carbon black itself to CO2, causing loss. The standard specifically warns about this for fine particle-size carbons.

ISO 1408 Rubber Carbon content tester Operation Video:

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