Comparing Compression Density Measurement and Hydrostatic Weighing for Rubber Density Testing

Rubber density measurement is critical in ensuring the quality and consistency of rubber compounds, both uncured and vulcanized. Two prominent methods for determining rubber density are Compression Density Measurement and Hydrostatic Weighing. Each method offers distinct advantages tailored to specific testing environments and material states.

Compression Density Measurement

Measuring Principle and Procedure: Compression Density Measurement utilizes advanced instrumentation to compress a rubber sample, and through this process and calculations, achieves a density measurement.

The process begins by weighing the sample. Once weighed, the test specimen is transferred to the barrel where the instrument piston compresses air out of the sample. This attains the volume measurement due to the compression cylinder having a calibrated diameter and measurable displacement of the piston.

Applications and Advantages: This method is only suitable for uncured rubber compounds, raw polymers and elastomers, making it a preferred choice in laboratory or production settings. For major rubber mixing laboratories, the compression density method is typically the preferred choice. The primary benefits include:

• Masterbatch Density Calculation: Masterbatch rubber typically does not cure, therefore the uncured compression density method allows process engineers the ability to find deviations in uncured rubber.
• Speed and Automation: The process is quick and automated, minimizing manual intervention.
• High Reproducibility: Ensures consistent and precise measurements.
• Integrated Data Management: Options for built-in printers and barcode scanners enhance data handling and traceability.

Hydrostatic Weighing (ASTM D297)

Principle and Procedure: Hydrostatic Weighing, a time-tested method, measures the density of vulcanized rubber by leveraging the buoyant force exerted by a fluid. The procedure involves weighing the rubber sample in air and then submerging it in a liquid with known density (typically water) to measure its buoyant weight. Ethanal may be a choice for low density materials. The density is calculated based on the weight difference, which corresponds to the volume displaced by the sample in water.

Applications and Advantages: This method is widely recognized for its accuracy in measuring the density of solid, vulcanized rubber materials. Key advantages include:

• Established Standards: Defined by ASTM D297, ensuring reliable and consistent results.
• Versatility: Applicable to a variety of solid materials, including those that are insoluble or non-reactive in water.

Key Differences

1. Material State:
   • Compression Density Measurement: Only suited for uncured rubber compounds.
   • Hydrostatic Weighing: Only for vulcanized rubber. Typically
2. Measurement Environment:
   • Compression Density Measurement: Does not require a fluid medium; relies on mechanical compression.
   • Hydrostatic Weighing: Requires a fluid medium (water) and measures buoyant force.
3. Automation and Integration:
   • Compression Density Measurement: Highly automated with advanced data management features. Much faster testing process with no material scrap as rubber can be thrown back into the process.
   • Hydrostatic Weighing: Generally more manual, but automation is possible with the right equipment. Samples typically scrapped after testing.
4. Accuracy and Precision:
   • Both methods provide high accuracy and precision, with the choice dependent on material state and specific testing requirements.

The choice between Compression Density Measurement and Hydrostatic Weighing hinges on the material state and testing environment. For uncured rubber compounds, Compression Density Measurement offers automation and precision and be accomplished as an online production test – not subject to lab environment only.

For vulcanized rubber, Hydrostatic Weighing remains a robust and established method. This method may not be done as an online test, therefore, requires lab environment.

Understanding these methods allows for selecting the appropriate technique to ensure optimal rubber quality and consistency. Our goal at MonTech USA is to ensure the right instrument is assigned to the correct application.

To learn more about the RD 3000 Rubber Compression Density Tester, Click Here!

To learn more about the DM 3000 Hydrostatic Densimeter, Click Here!