Volatile matter determination is an essential quality control test for nitrile butadiene rubber (NBR), quantifying the moisture, residual monomers, and other volatile components that directly influence processing behavior and finished product quality. Accurate volatile content analysis, performed according to international standards such as ISO 248-1 and ASTM D5668, helps manufacturers prevent porosity defects, ensure precise compounding, and verify supplier conformity. This article presents a complete technical guide to NBR volatile matter testing, covering testing principles, sample preparation, measurement procedures, calculation methods, and practical applications.
Technical Overview
Nitrile butadiene rubber (NBR) is a synthetic elastomer copolymerized from acrylonitrile and butadiene, widely valued for its excellent resistance to oils, fuels, and greases. During cold emulsion polymerization, coagulation, washing, and drying, small quantities of water, unreacted monomers, emulsifier residues, and low-molecular-weight organic substances remain trapped in the rubber matrix. These substances are collectively defined as volatile matter, and their content is typically expressed as a mass percentage lost under specified heating conditions.
Excessive volatile matter in NBR creates serious manufacturing problems, including porosity and blistering in vulcanized articles, weighing errors during compounding, poor dimensional stability of extruded profiles, and condensation inside packaging during storage. For this reason, commercial NBR grades are commonly specified with a volatile matter limit of 0.5 percent or less, although certain powder and crumb grades may permit up to 1.0 percent depending on the application.
Volatile matter determination for NBR is governed by recognized international and national standards, including ISO 248-1 (hot-mill and oven methods), ASTM D5668, and GB/T 24131. These standards define sample preparation, heating temperature and duration, weighing procedures, and result calculation to ensure reproducible interlaboratory agreement.
Testing Samples
Testing samples must be representative of the entire lot. Sampling is generally performed in accordance with ISO 1795 or the contractual agreement between supplier and purchaser. The principal sample types submitted for volatile matter determination include the following.
Raw NBR Bale Rubber
Bale-form NBR is the most common sample type. A representative portion is cut from one or more bales, then homogenized by passing through a clean laboratory mill at room temperature to ensure uniform distribution of volatiles. The homogenized sheet is cut into small pieces of approximately 2 mm or less before testing.
Powdered and Crumb NBR
Powder and crumb grades, frequently produced by spray drying or coagulation processes, tend to carry higher and less uniform volatile content. Multiple grab samples are taken from different locations, blended thoroughly, and reduced by coning and quartering to obtain a test portion of 2 to 10 grams.
NBR Compounds and Blends
Filled compounds, NBR/PVC blends, and masterbatches can also be tested, but results must be interpreted with caution because plasticizers, processing oils, and accelerators may partially volatilize under test conditions. The oven method at 105 degrees Celsius is preferred for these materials to minimize thermal loss of compounding ingredients.
Testing Items
The volatile matter determination service covers the following measurable parameters:
- Total volatile matter content, expressed as mass percentage of the original sample
- Moisture contribution within the total volatile fraction
- Constant-mass verification through repeated drying cycles
- Conformity assessment against supplier specifications or purchase contracts
- Comparative analysis between incoming lots for trend monitoring
Testing Methods
Two standard procedures are accepted for NBR volatile matter determination: the hot air oven method and the hot mill method. Both rely on gravimetric measurement of mass loss and require an analytical balance with a resolution of 0.1 mg.
Hot Air Oven Method
The oven method is the most widely used and applicable to all NBR forms. The procedure is as follows:
- Prepare the sample by milling and cutting to pieces of 2 mm or smaller to expose maximum surface area
- Weigh a clean, dry weighing bottle or dish, then add 2 to 10 grams of sample and record the combined mass
- Place the open container in a forced-ventilation oven maintained at 105 plus or minus 2 degrees Celsius for 1 hour
- Transfer the covered container to a desiccator and cool for 30 minutes, then reweigh
- Repeat drying cycles of 30 minutes until the mass change between consecutive weighings is 0.5 mg or less, indicating constant mass
Hot Mill Method
The hot mill method is faster and well suited to production quality control. The sample is passed repeatedly through a laboratory mill whose roll surface temperature is controlled at 100 plus or minus 5 degrees Celsius, maintaining a rolling bank to ensure uniform heating. The sheet is weighed after each sequence of passes, and milling continues until two consecutive masses differ by 0.1 gram or less. Care must be taken to avoid over-milling, because prolonged heat exposure can oxidize or crosslink NBR and distort the result.
Calculation and Reporting
Volatile matter content is calculated using the following formula, where m0 is the initial sample mass and m1 is the final mass after drying to constant weight:
Volatile matter content (%) = (m0 − m1) / m0 × 100
Reports should include the method used, drying temperature and duration, number of drying cycles, individual weighing results, calculated percentage, and the applicable standard. Typical repeatability within a single laboratory is 0.05 to 0.10 percentage points when temperature uniformity, desiccator efficiency, and weighing speed are properly controlled.
Applications
Volatile matter determination for NBR serves critical functions across the rubber industry:
- Incoming raw material inspection at seal, O-ring, gasket, hose, and fuel-line manufacturing plants
- Release testing by NBR producers to certify lot conformity before shipment
- Troubleshooting of porosity, blistering, and bubble defects in molded and extruded goods
- Verification of drying efficiency in polymerization and finishing operations
- Research and formulation work involving moisture-sensitive curing systems
- Trade compliance, supplier qualification, and long-term storage stability monitoring
FAQ
Q: What is the typical acceptance limit for volatile matter in NBR? A: Most commercial bale grades are specified at 0.5 percent maximum, while powder and crumb grades commonly allow up to 1.0 percent. The contractual specification always takes precedence.
Q: Which method is more accurate, the oven method or the hot mill method? A: The oven method generally provides better precision and interlaboratory reproducibility, while the hot mill method is faster and preferred for rapid in-process checks.
Q: Why must the sample be cooled in a desiccator before weighing? A: A hot sample absorbs atmospheric moisture during weighing, which artificially lowers the measured mass loss. Desiccator cooling ensures the sample reaches ambient temperature in a dry environment.
Q: Why should drying temperatures above 105 degrees Celsius be avoided? A: Excessive temperature can initiate oxidation or thermal crosslinking in NBR, which changes the polymer mass and produces inaccurate, non-reproducible results.
Q: Is volatile matter the same as ash or extractable content? A: No. Volatile matter measures mass lost by heating, ash measures inorganic residue after combustion, and extractables measure soluble substances removed by solvents. Each requires a separate dedicated test.
