Volatile matter determination is a critical quality control test for NBR cork rubber, a composite gasket material combining nitrile butadiene rubber with cork granules. This article explains the gravimetric testing principles, applicable standards such as ISO 248 and ASTM D5668, sample preparation procedures, and how volatile content affects sealing performance, dimensional stability, and long-term reliability.
Technical Overview
NBR cork rubber is an engineered composite material consisting of cork granules bound within a nitrile butadiene rubber (NBR) matrix. The material combines the high compressibility, elastic recovery, and conformability of cork with the excellent oil resistance, fuel resistance, and mechanical strength of nitrile rubber. It is widely used for oil pan gaskets, valve cover seals, flange gaskets, and transformer sealing components. Volatile matter determination measures the percentage of mass lost when a conditioned specimen is heated under strictly controlled temperature and time conditions, quantifying moisture, residual processing aids, low-molecular-weight plasticizers, residual solvents, and other evaporable substances within the material.
Excessive volatile matter in NBR cork rubber signals potential manufacturing deficiencies such as insufficient curing, inadequate drying after production, or improper plasticizer loading. From a performance standpoint, high volatile content can cause gasket shrinkage after installation, loss of bolt torque retention, outgassing in sealed systems, reduced sealability over time, and dimensional instability. Reliable volatile matter data therefore supports incoming material inspection, batch release, process validation, and failure analysis.
Testing Samples
Sheet and Rolled Materials
Standard NBR cork rubber sheet stock in thicknesses typically ranging from 0.5 mm to 10 mm is the most common sample form. Specimens are cut from representative locations across the sheet to capture thickness direction and planar variability. Surface contamination, release agents, and protective films must be removed before specimen preparation.
Finished Gaskets and Stamped Parts
Complete gaskets, valve cover seals, oil pan gaskets, and other die-cut or molded components can be tested whole or as sectioned pieces. Testing finished parts verifies that post-processing steps such as cutting, anti-stick coating, and storage have not introduced additional volatile substances.
Compounded and Raw Stock
Uncured or partially cured compound, mixing mill sheet, and granular material may be tested to evaluate binder formulation consistency and drying effectiveness prior to vulcanization. For granular or shredded samples, uniform particle size improves drying reproducibility.
Specimen Preparation and Conditioning
Specimens are typically cut into small pieces of a few millimeters to increase the exposed surface area and accelerate the release of volatiles. A sample mass of approximately 2 g to 10 g is customary, depending on the test method. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity in accordance with ISO 23529 or ASTM D618 to ensure consistent initial moisture state.
Testing Items
A typical volatile matter determination program for NBR cork rubber includes the following measurable parameters:
- Total volatile matter content, expressed as mass loss percentage after heating
- Moisture content contribution, where separate low-temperature drying is performed
- Mass loss at specified temperatures, commonly 105 °C ± 2 °C, with optional elevated-temperature screening at 150 °C for thermal stability assessment
- Post-heating visual and dimensional examination for shrinkage, blistering, cracking, or delamination
- Repeat determinations for statistical evaluation of within-batch uniformity
Testing Methods
Hot Air Oven Gravimetric Method
The oven method is the most widely used approach for finished NBR cork rubber products. A weighed, conditioned specimen is placed in a forced-ventilation oven at 105 °C ± 2 °C for a defined period, commonly 1 hour to 3 hours depending on specimen thickness and specification requirements. The specimen is then cooled to ambient temperature in a desiccator and reweighed. The procedure references ISO 248-2 and ASTM D5668 as methodological bases, adapted to vulcanized composite products in line with principles found in ASTM D297 for rubber product analysis.
Hot Mill or Hot Press Method
For uncured compound and raw binder evaluation, the specimen may be passed through a heated mill or pressed between hot platens at approximately 100 °C to 105 °C for a short exposure, following the hot mill principle of ISO 248-1. The rapid surface renewal during milling accelerates volatile release and is useful for process control of mixing quality.
Thermogravimetric Analysis
Thermogravimetric analysis provides a high-resolution profile of mass loss as temperature ramps at a controlled rate, such as 10 °C per minute under nitrogen. It distinguishes moisture evolution below approximately 120 °C from oil and plasticizer loss at higher temperatures, making it valuable for failure investigation and formulation studies rather than routine quality release.
Calculation of Results
Volatile matter content is calculated from the mass difference before and after heating. The initial conditioned mass is denoted m1 and the final mass after heating and desiccator cooling is denoted m2. The volatile matter content, expressed as a percentage, equals (m1 minus m2) divided by m1, multiplied by 100. Duplicate determinations are standard, and the reported result is typically the mean value. Result precision depends on accurate balance resolution of 0.1 mg, stable oven temperature uniformity, and consistent cooling practice, since reabsorption of atmospheric moisture during weighing is a primary source of error.
Quality Acceptance Considerations
Acceptance limits for volatile matter depend on the product specification and end use. Sealing applications in enclosed systems or high-temperature service generally demand tighter control, frequently below 2% to 3% mass loss, while general industrial gasket grades may permit somewhat higher values. Consistent interlaboratory comparison requires agreement on heating temperature, duration, specimen geometry, and conditioning, since all of these factors influence the measured value.
Applications
Volatile matter determination supports quality assurance and engineering decisions across many NBR cork rubber applications:
- Automotive oil pan and valve cover gaskets, where shrinkage resistance and torque retention are essential
- Transformer and electrical enclosure gaskets, where outgassing must be minimized
- Fuel and oil system flange seals requiring long-term dimensional stability
- Incoming material inspection and supplier qualification for gasket converters
- Batch release testing during NBR cork rubber sheet manufacturing
- Failure analysis of leaking or degraded seals to identify excessive volatile loss or under-cure
- Research and development of cork-to-rubber ratios and plasticizer systems for improved compound stability
FAQ
What temperature and time are used for the standard oven test?
The most common condition is 105 °C ± 2 °C for 1 hour to 3 hours in a forced-air oven, followed by cooling in a desiccator before reweighing. The exact duration depends on specimen thickness and the governing specification.
Which standards govern this test?
ISO 248 (Parts 1, 2, and 3) and ASTM D5668 cover volatile matter determination in rubbers, while ASTM D297 provides chemical analysis guidance applicable to vulcanized products. Individual customer and industry specifications typically adapt these methods for NBR cork composite materials.
What does an abnormally high volatile matter result indicate?
Elevated results usually indicate retained moisture, excess plasticizer or processing oil, residual solvents, or insufficient vulcanization. Each cause carries different corrective actions, and follow-up testing such as thermogravimetric analysis helps separate moisture from oil-related losses.
How much sample is required?
A specimen mass of 2 g to 10 g is sufficient for routine testing. Finished gaskets can be sectioned into small pieces so that both faces and the core of the material are represented.
Why is cooling in a desiccator important?
A hot specimen rapidly reabsorbs atmospheric moisture, which biases the final mass upward and understates the true volatile content. Desiccator cooling to ambient temperature before weighing is therefore a mandatory step for accurate, repeatable results.
Can the same test evaluate long-term service stability?
Volatile matter determination is a short-term accelerated indicator rather than a full aging study. For service-life assessment, it is commonly combined with compression set testing, heat aging, and oil immersion testing to build a complete picture of material durability.