Zero-span tensile strength tester measures the tensile strength of wet pulp.

This article introduces the method of measuring the tensile strength of wet pulp using a zero-span tensile strength tester. Traditional testing is mostly conducted after the pulp is dried, but wet-state testing can more directly reflect the bonding potential between fibers. This instrument accurately measures the intrinsic bonding strength of fibers by stretching wet pulp sheets under near-zero-span conditions, eliminating the influence of fiber slippage. The article details the testing principles, required equipment, specific operational steps, and key factors affecting the results, such as pulp dryness and fixture alignment. This method is primarily used to evaluate the fiber bonding capacity of different pulps, study the effects of processes and additives, and holds significant value for research, development, and production control in the papermaking industry.

Wet pulp tensile strength

In the pulp and paper industry, the tensile strength of wet pulp is one of the key indicators to evaluate its fiber bonding ability and the physical properties of the final paper. Traditional tensile strength testing is usually performed after drying, but the strength in the wet state is more directly reflective of the hydrogen bonding potential between fibers and the wet part process performance. The zero-distance tensile strength tester provides an accurate measurement method for this purpose, which effectively excludes the influence of fiber slip by stretching the wet slurry sheet under the clamping condition of near zero distance, so as to measure the bonding strength of the fiber itself.

Test Principle:

The core principle of zero-distance tensile strength testing is that the prepared wet slurry sample is clamped between two fixtures, and the initial clamping distance of the fixtures is very small (theoretically close to zero). During the test, the upper clamp is stretched upwards at a constant speed until the sample breaks. The instrument records the maximum force to be subjected to at the time of fracture. Due to the extremely short gripping distance, the elongation of the sample occurs mainly at the fibers themselves and their bonding points, rather than the rearrangement or slippage of the fibers. Therefore, the measured strength directly reflects the strength of the fiber-to-fiber bonding bond.

The zero-distance tensile strength (ZDT) of wet pulp is usually expressed as a force per unit width, and is calculated as follows:

ZDT = F_max / w

Among them, ZDT is the zero-distance tensile strength, which is measured in kilonewtons per meter (kN/m); F_maxis the maximum force value recorded in the test, in Newton (N); w is the width of the sample, in meters (m).

Instruments and materials

The main equipment required for testing is a zero-pitch tensile strength tester. The instrument typically includes a precision force sensor, a pair of grippers for precise alignment with minimal initial clearance, a constant speed tensile drive, and a data acquisition system. Standard fiber slippers, formers, and drying equipment are required for sample preparation (dry strength comparison is required). The test material is the wet pulp suspension to be tested.

Test steps

The testing process needs to be strictly standardized to ensure that the results are repeatable and comparable. The main steps are summarized as follows:

1. Sample preparation: The pulp suspension is loosened and dispersed using standard methods, then a quantitative uniform wet slurry sheet is formed on a standard former, and excess moisture is removed to the target dryness (typically around 20-30%) by pressing.

2. Sample Cutting: Cut the wet slurry sheet into strip samples with specified widths (such as 15mm) and length.

3. Instrument calibration: Before testing, the force sensor and displacement sensor of the instrument need to be calibrated.

4. Sample Loading and Testing: Carefully clamp the sample in the zero-pitch fixture to ensure that the sample is flat, vertical, and evenly forced. Start the test program and stretch it until it breaks.

5. Data Logging: Record the maximum force value (F) for each test_max) and fractures. Each slurry sample should be tested for at least 10 valid data, average.

6. Result Calculation: Calculate the zero-distance tensile strength according to the formula.

Influencing factors

The accuracy of the test results is affected by a number of factors that need to be controlled during operation:

The concentration of slurry is uniform	It affects the uniformity of the formation of wet slurry sheets, and it is necessary to standardize the loosening and forming process.
Wet slurry dryness (moisture content)	It is a critical parameter that affects the strength of the binding and needs to be precisely controlled and reported.
Fixture alignment with initial clearance	The fixture must be aligned and the initial clearance should meet the standard (typically < 0.5mm).
Stretch speed	A constant speed is set according to the selected standard (e.g. ISO or TAPPI method).
Test the temperature and humidity of the environment	May affect moisture evaporation, it is recommended to complete the test under standard temperature and humidity conditions or quickly.

In addition, wet slurry samples should be prepared and transferred to avoid accidental stretching or damage, and the gripping force of the fixture should be sufficient to prevent slippage but not crush the sample.

Application:

Zero-pitch tensile strength testing provides an important process control and R&D tool for the paper industry. By measuring the zero-distance tensile strength of wet pulp, the intrinsic fiber binding potential of different slurries (such as chemical pulp, mechanical pulp, waste pulp) can be evaluated; study the effects of pulping degree and chemical additives (such as wet strength agents and retention agents) on fiber bonding; Optimize the wet process parameters of papermaking and predict some physical strength of the finished paper. This method shifts the focus from fiber network structure to fiber-to-fiber bond quality, providing a unique perspective for understanding the formation mechanism of paper strength.

Epilogue

The tensile strength of wet pulp is determined by zero-distance tensile strength tester, which is a direct and effective analytical method. It compensates for some of the shortcomings of traditional dry state testing and can more sensitively reveal the binding properties between fibers. Strict adherence to standardized operating procedures and full consideration of influencing factors such as slurry condition and test conditions are the basis for obtaining reliable and comparable data. This technology has clear value for deepening the understanding of pulp properties and guiding paper production practices.

References

1. ISO 15754: Determination of zero-distance tensile strength of paper and cardboard.

2. TAPPI T231: Test method for the tensile strength of pulp at zero distance.

3. Papermaking analysis and testing (related technical book chapters).

4. A review of the research on the bonding mechanism of wet pulp fibers (related academic journal articles).