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D6125-97 – Standard Test Method Technical Guide
ASTM D6125-97 (Reapproved 2007) provides a comprehensive procedure for quantifying the bending resistance of paper and paperboard using a Gurley Type Tester. This method is critical for process control, converting operations, and end-use performance prediction. The instrument’s flexibility in specimen dimensions and applied force allows it to accommodate a wide range of material stiffnesses.
🛠️ Apparatus Specifications and Unit System
The Gurley Type Tester features a balanced pendulum pivoted at its center of gravity in jewel bearings. It includes multiple holes for adjusting the specimen bending length and a set of calibrated weights to vary the applied load. Force is reported in Gurley Units, which are numerically identical to the traditional milligrams of force (mgf). The standard provides an exact conversion to the International System of Units (SI) using the table below.
| 🟦 Gurley Units (mgf) | 📏 Force (mN) |
|---|---|
| 1 | 9.807 × 10⁻³ |
| 10 | 0.09807 |
| 100 | 0.9807 |
| 1000 | 9.807 |
🚨 User Responsibility: This standard does not purport to address all safety concerns. The user must establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
📐 Specimen Geometry and Directional Orientation
Strict adherence to Practices D585 (Sampling) and D685 (Conditioning) is required. The orientation of the test specimen relative to the clamp specifically defines the axis being measured. Bending resistance varies significantly between the machine direction (MD) and cross direction (CD) of the sheet.
| 🎯 Measured Axis | 📐 Clamp Orientation |
|---|---|
| Machine Direction (MD) Bending Resistance | MD of the paper perpendicular to the specimen clamp. |
| Cross Direction (CD) Bending Resistance | CD of the paper perpendicular to the specimen clamp. |
The user can vary the specimen length, width, and applied force within wide limits. The goal is always to achieve a valid scale deflection. The required deflection range is a critical parameter for data validity.
| 📏 Scale Deflection | ✅ Test Validity |
|---|---|
| Less than 1 | Invalid — Increase applied weight or specimen length. |
| Between 1 and 7 | Valid — Record the reading directly. |
| Greater than 7 | Invalid — Decrease applied weight or specimen length. |
⚙️ Method Limitations and Data Validity
This test method is specifically designed for materials that exhibit a crisp bending response. It is not recommended for soft, limp, or creped materials such as tissue or toweling. Furthermore, materials with a pronounced degree of curl will yield erroneous results. Carefully observe the lower limit of the method before testing lightweight grades.
⚡ Critical Lower Limit: Do not test products with a bending resistance below 1.39 Gurley Units or products that cannot achieve a deflection between 1 and 7 on the scale when using the lightest available pendulum weight.
💡 Terminology Note: Within this standard, the terms “bending resistance” and “stiffness” are used as synonyms. Gurley stiffness is the specific term for results obtained using this patented instrument, which distinguishes it from Taber stiffness or other related methods.
The method is supported by closely aligned TAPPI standards, including T 543 (Bending Resistance of Paper) and T 1200 (Interlaboratory Evaluation of Test Methods).
❓ Frequently Asked Questions
🔍 What is the minimum bending resistance required for this test?
Products with a bending resistance below 1.39 Gurley Units are explicitly excluded from this method. Any sample that cannot produce a pointer deflection between 1 and 7 using the lightest available weight is also considered outside the test method’s scope.
💡 How do I convert Gurley Units to standard force units?
The standard provides the exact conversion formula: Force (mN) = 9.807 × 10⁻³ × Gurley Units. This conversion is essential for reporting results in metric units or comparing data across different physical test instruments.
⚡ What distinguishes this method from Taber Stiffness?
The difference is instrument-specific. Gurley stiffness utilizes the W. & L. E. Gurley Co. patented pendulum tester, while Taber stiffness uses a distinct instrument patented by Taber Industries. Both measure bending resistance but under different loading geometries and defined units.
📌 Which conditioning standards are mandatory for this test method?
ASTM D685 (Practice for Conditioning Paper and Paper Products for Testing) is mandatory for sample environment control. Additionally, ASTM D585 (Practice for Sampling and Accepting a Single Lot) governs the sampling procedure required prior to testing.