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ISO 27588:2012 – Very Low Rubber Hardness (VLRH) Testing
Rubber — Determination of dead-load hardness using the VLRH scale
1. The Very Low Rubber Hardness (VLRH) Scale
ISO 27588:2012 specifies a dead-load method for determining the hardness of very soft vulcanized or thermoplastic rubbers using the Very Low Rubber Hardness (VLRH) scale. This standard is essential for characterizing ultra-soft elastomeric materials where traditional durometer methods (Shore A, IRHD) lack sufficient resolution and accuracy at the low end of the hardness range.
The VLRH scale provides a linear relationship between indentation depth and hardness value, unlike the non-linear scales used in traditional methods. This linearity simplifies data interpretation and enables more precise differentiation among very soft rubber compounds. The standard includes a normative annex (Annex A) detailing the mathematical relationship between indentation depth and VLRH values, along with the derivation of the scale.
The VLRH method is particularly valuable for hardness values below 30 IRHD or 30 Shore A, where traditional methods lose discrimination. For extremely soft materials such as gel-filled pads, soft seals, and vibration dampening components, VLRH provides the needed measurement resolution.
| Hardness Scale | Range | Load (N) | Indentor Geometry | Best For |
|---|---|---|---|---|
| VLRH | 0 – 100 | 0.30 + 5.40 | Sphere R 2.5 mm | Ultra-soft (below 30 IRHD) |
| IRHD (Micro) | 0 – 100 | 0.30 + 5.40 | Sphere R 0.395 mm | Thin specimens |
| IRHD (Normal) | 0 – 100 | 0.30 + 5.40 | Sphere R 2.5 mm | Standard rubbers |
| Shore A | 0 – 100 | Spring-based | Frustum 35 deg | General purpose |
2. Apparatus, Calibration, and Test Procedure
The standard specifies the dead-load hardness tester apparatus, consisting of a indentor with a 2.5 mm radius spherical tip, a test foot, and a mechanism for applying specified contact and total loads. The contact load is 0.30 N and the total load is 5.70 N (0.30 N contact + 5.40 N additional), giving a total applied force of 5.70 N. The indentation depth is measured after 30 seconds of total load application.
A critical addition in this second edition (replacing ISO 27588:2008) is the calibration schedule detailed in Annex B. This annex specifies the calibration procedures, frequency, and acceptance criteria for the apparatus, ensuring measurement traceability and interlaboratory reproducibility. Calibration involves verifying the applied forces, indentation depth measurement system, and the geometry of the indentor.
Calibration is essential for obtaining reliable VLRH measurements. The standard requires that the hardness tester be calibrated at least annually, with more frequent verification recommended for high-usage instruments. Special attention must be paid to the spherical indentor tip, which can wear or become damaged with regular use.
3. Engineering Applications and Material Design Insights
The VLRH scale is particularly valuable in industries where ultra-soft elastomers are critical to product performance. Applications include soft-touch grips, ergonomic interfaces, vibration isolation mounts, medical device components, soft seals and gaskets, and shock-absorbing padding. The ability to precisely characterize materials in the very low hardness range enables engineers to optimize material formulations for specific performance requirements.
The time-dependent nature of rubber hardness measurements is addressed by the standard’s requirement for a 30-second dwell time before reading. This accounts for the viscoelastic creep behavior inherent in rubber materials. For materials with significant creep, multiple readings at different locations on the same specimen can provide insight into material consistency and the effects of processing conditions.
The linear VLRH scale enables direct comparison between materials and facilitates the development of specification limits for quality control. Unlike the compressed low-end scale of Shore A, where a difference of 1 unit can represent a significant change in material properties, VLRH provides uniform resolution across the entire measurement range.
VLRH measurements are highly sensitive to specimen thickness and surface condition. The standard requires a minimum specimen thickness of 6 mm. Thinner specimens may produce erroneously high hardness readings due to the “bottoming out” effect, where the indentor senses the rigid support surface beneath the specimen.
4. Frequently Asked Questions
Q: How does VLRH differ from traditional Shore 00 hardness?
A> Shore 00 uses a different indentor geometry and spring-based loading, while VLRH uses a dead-load (weight-based) system with a spherical indentor. VLRH provides better reproducibility and a more linear scale, making it superior for very soft materials.
A> Shore 00 uses a different indentor geometry and spring-based loading, while VLRH uses a dead-load (weight-based) system with a spherical indentor. VLRH provides better reproducibility and a more linear scale, making it superior for very soft materials.
Q: What is the minimum specimen thickness required for VLRH testing?
A: The standard requires a minimum thickness of 6 mm. For thinner specimens, multiple layers can be stacked, provided they are in good contact and the stacking does not introduce air gaps.
A: The standard requires a minimum thickness of 6 mm. For thinner specimens, multiple layers can be stacked, provided they are in good contact and the stacking does not introduce air gaps.
Q: Can VLRH values be converted to IRHD or Shore A values?
A: While approximate correlation charts exist, direct conversion is not recommended due to the different measurement principles and scale definitions. For critical applications, always measure using the appropriate scale directly.
A: While approximate correlation charts exist, direct conversion is not recommended due to the different measurement principles and scale definitions. For critical applications, always measure using the appropriate scale directly.
Q: Why was the calibration annex added in the 2012 edition?
A: The calibration schedule (Annex B) was added based on industry experience showing that uncalibrated instruments produce significant measurement variability. The annex provides a standardized framework for ensuring traceable, reproducible measurements across different laboratories and instruments.
A: The calibration schedule (Annex B) was added based on industry experience showing that uncalibrated instruments produce significant measurement variability. The annex provides a standardized framework for ensuring traceable, reproducible measurements across different laboratories and instruments.
The Very Low Rubber Hardness (VLRH) method specified in ISO 27588 addresses a critical gap in elastomer testing technology. Traditional hardness measurement methods such as Shore A and IRHD lose resolution and accuracy at the low end of the hardness scale, making them unsuitable for characterizing ultra-soft elastomeric materials. The VLRH method provides a linear measurement scale throughout the entire 0 to 100 range, enabling precise differentiation among materials that would appear nearly identical when measured by conventional methods.
The dead-load principle used in the VLRH method provides several advantages over spring-based methods. Dead-load instruments apply a constant force that is not affected by the indentation depth or the material stiffness, eliminating a significant source of measurement variability. The total applied force of 5.70 N (0.30 N contact force plus 5.40 N additional force) provides adequate indentation even for very soft materials while maintaining acceptable sensitivity for harder compounds within the VLRH range.
The spherical indentor geometry with a 2.5 mm radius provides optimal contact conditions for soft elastomers. The spherical shape ensures consistent contact geometry regardless of indentation depth and avoids the stress concentration issues associated with sharp indentors. The relationship between indentation depth and VLRH value is defined by a linear equation, which simplifies calibration and data interpretation compared to the logarithmic or empirical scales used in other hardness methods.
Specimen preparation and conditioning significantly affect VLRH measurements. The standard requires a minimum specimen thickness of 6 mm to prevent the indentor from sensing the rigid support surface beneath the specimen. Specimens must be conditioned at standard laboratory temperature (23 +/- 2 degrees C) for at least 24 hours before testing. The time interval between vulcanization and testing must also be controlled, typically 16 to 72 hours, to allow for post-cure stabilization of the rubber compound.
The calibration schedule specified in Annex B is essential for maintaining measurement accuracy. Daily verification using reference blocks provides immediate feedback on instrument condition, while monthly calibration of force and displacement systems identifies any long-term drift. Annual comprehensive calibration by an accredited laboratory ensures traceability to national standards and provides confidence in the accuracy of measurement results.
