Rubber Volatile Matter Testing: Standards, Methods, and Applications

Rubber volatile matter testing determines the percentage of volatile substances—primarily moisture, residual monomers, solvents, and low-molecular-weight components—released when rubber is heated under controlled conditions. Accurate volatile matter data is essential for raw rubber specification compliance, quality control, and fair trade settlement across the rubber and tire industries. This article explains the testing principle, applicable standards such as ISO 248 and ASTM D5668, sample types, test methods, and typical applications of rubber volatile matter determination.

Technical Overview

Volatile matter in rubber refers to the mass fraction of substances that evaporate when a rubber sample is heated under standardized conditions. In raw natural and synthetic rubbers, these volatiles typically consist of absorbed moisture, residual monomers and oligomers, emulsifier residues, coagulation acids, processing oils, and other low-molecular-weight components. In compounded rubber and rubber chemicals, volatile matter may also include moisture picked up during storage and handling as well as thermally unstable additives.

Controlling volatile matter content is critical for downstream processing and final product quality. Excessive volatiles can cause porosity, blistering, and sponge-like defects in vulcanized articles, degrade dimensional stability during extrusion or molding, reduce bond strength in rubber-to-metal and rubber-to-fabric assemblies, and interfere with vulcanization kinetics. For these reasons, virtually all raw rubber specifications—including technically specified natural rubber (TSR) grades and bale-form synthetic rubbers—place explicit limits on volatile matter, commonly in the range of 0.5% to 1.0% by mass.

Several international and national standards govern the determination, including ISO 248-1 and ISO 248-2 for raw rubber, ASTM D5668 for rubber compounding materials, ASTM D1278 for rubber from natural sources, GB/T 3517 for rubber in the Chinese national system, and ASTM D1509 or ISO 1126 for loss on heating of carbon black. Accredited third-party laboratories routinely perform these tests for certification, trade settlement, and quality assurance purposes.

Testing Samples

Natural Rubber

Natural rubber products are among the most frequently tested materials for volatile matter. Typical sample types include ribbed smoked sheet (RSS) grades, technically specified natural rubber such as SMR, STR, and SVR grades, crepe rubber, skim rubber, cup lump, and natural rubber latex solids. Because moisture content varies considerably across grades and storage conditions, homogenization before testing is particularly important for these materials.

Synthetic Rubber

Synthetic elastomers tested include styrene-butadiene rubber (SBR, including oil-extended grades), polybutadiene rubber (BR), nitrile butadiene rubber (NBR), ethylene-propylene rubber (EPDM and EPM), isoprene rubber (IR), chloroprene rubber (CR), butyl and halobutyl rubbers (IIR, BIIR, CIIR), acrylic rubber (ACM), fluoroelastomers (FKM), and silicone rubber (VMQ). Emulsion-polymerized rubbers generally carry higher volatile levels due to residual water and coagulation chemicals than solution-polymerized types.

Rubber Compounds, Masterbatch, and Reclaimed Rubber

Mixed compounds, masterbatch materials, and reclaimed or devulcanized rubber are tested to verify drying efficiency and to monitor moisture pickup during storage. For these materials, volatile matter data supports mixing process control and helps prevent porosity and delamination in finished goods.

Rubber Chemicals and Compounding Ingredients

Fillers, reinforcing agents, and rubber chemicals—including carbon black, precipitated silica, clays, zinc oxide, accelerators, and antioxidants—are tested for loss on heating or moisture content, since excessive volatiles in these ingredients directly transfer into the compound and can cause dispersion problems and cure deviations.

Testing Items

The core deliverable of rubber volatile matter testing is the volatile matter content, expressed as the mass percentage lost by the sample after heating under the specified conditions. Depending on the material and the client's requirements, typical test items include:

• Volatile matter content of raw natural and synthetic rubber
• Volatile matter content of oil-extended rubber
• Loss on heating (heating loss) of rubber compounds and masterbatch
• Volatile matter or heating loss of carbon black and mineral fillers
• Moisture content of rubber chemicals and additives
• Volatile matter of reclaimed rubber and rubber powder

The result is calculated from the mass of the sample before and after heating: Volatile matter content (%) = (m0 − m1) / m0 × 100, where m0 is the initial mass and m1 is the mass after heating and cooling in a desiccator. Results are reported to two decimal places together with the test method, temperature, and heating time used.

Testing Methods

Oven Method

The oven method described in ISO 248-1, ASTM D5668, ASTM D1278, and GB/T 3517 is the most widely used procedure. A homogenized test portion, typically around 10 g cut into small pieces, is weighed into a suitable container, heated in a ventilated oven at 105 °C ± 5 °C for the period specified in the standard (commonly one to two hours depending on the rubber type), cooled to ambient temperature in a desiccator, and reweighed. The drying, cooling, and weighing cycle is repeated until a stable mass is achieved, and the volatile matter content is calculated from the mass loss.

Hot-Mill Method

In the hot-mill method of ISO 248-1, the homogenized sample is passed repeatedly through a laboratory open mill with heated rolls, typically at approximately 100 °C to 105 °C. The mechanical working combined with heat drives off moisture and other volatiles, and the sample is weighed after successive passes until the mass stabilizes. This method simultaneously homogenizes the sample, which makes it well suited to natural rubber and other materials requiring mastication before analysis.

Thermogravimetric and Rapid Moisture Analyzer Methods

ISO 248-2 defines thermogravimetric procedures in which a small sample is heated while its mass is monitored continuously on a balance, either in a dedicated thermogravimetric analyzer or in an infrared or halogen rapid moisture analyzer. The test ends automatically when mass loss reaches a defined stability criterion. These methods require only a few grams of sample and typically deliver results within 10 to 30 minutes, making them attractive for routine in-process quality control, although results should be correlated with the classical oven method for specification purposes.

Complementary Techniques

Where differentiation between moisture and other volatiles is required, Karl Fischer titration can quantify water specifically. Full thermogravimetric analysis under controlled atmospheres can additionally separate oil, polymer, carbon black, and ash fractions, providing a complete compositional profile of compounded rubber. Key factors affecting accuracy in all methods include sample homogenization, particle size and thickness, precision of temperature and time control, weighing accuracy at ambient humidity, and consistent cooling in a desiccator before reweighing.

Applications

Rubber volatile matter testing supports quality decisions at nearly every stage of the rubber value chain. Principal applications include:

• Certification of raw natural and synthetic rubber against grade specifications before shipment and acceptance
• Incoming material inspection at tire, hose, belt, seal, and general rubber goods manufacturing plants
• Trade settlement and contract disputes where rubber is bought and sold subject to volatile matter limits
• Process control of drying, mixing, and storage operations to prevent moisture-related defects
• Investigation of product failures such as porosity, blistering, delamination, and poor adhesion
• Verification of rubber chemicals and fillers against supplier specifications
• Research and development programs for compound formulation and material substitution

Manufacturers, traders, and end users frequently engage accredited third-party laboratories to obtain independent, standards-based volatile matter reports for lot acceptance, regulatory submissions, and long-term supplier qualification programs.

FAQ

What is volatile matter in rubber?

Volatile matter is the fraction of a rubber sample that evaporates under standardized heating, consisting mainly of moisture along with residual monomers, solvents, emulsifier residues, and low-molecular-weight processing components. It is expressed as the percentage of mass lost during the test.

Which standards are used for rubber volatile matter testing?

The principal standards are ISO 248-1 (hot-mill and oven methods), ISO 248-2 (thermogravimetric methods), ASTM D5668 (rubber compounding materials), ASTM D1278 (natural rubber), and GB/T 3517. For carbon black and fillers, ASTM D1509 and ISO 1126 cover loss on heating.

What temperature and heating time are used in the oven method?

The oven method is normally performed at 105 °C ± 5 °C. Heating time is specified by the applicable standard and typically ranges from one to two hours, with repeat cycles until the sample reaches constant mass.

Why is high volatile matter content a problem?

Excessive volatiles can cause porosity, blisters, and sink marks in vulcanized products, reduce tensile and adhesion properties, disturb cure behavior, and shorten shelf life. It also reduces the effective polymer content paid for in commercial transactions.

How should samples be prepared and submitted?

Samples should be representative of the lot, taken from the interior of bales away from surface contamination, sealed in moisture-tight packaging, and submitted with details of the rubber type and grade. Accredited third-party laboratories homogenize and test the material according to the requested standard.

How long does a typical test take?

Classical oven-based determinations, including homogenization, drying cycles, and conditioning, are usually completed within one working day. Rapid thermogravimetric or moisture analyzer methods can provide indicative results in less than one hour, although specification testing is generally confirmed by the reference oven method.