Folding endurance measures the ability of toilet paper to withstand repeated folding under a defined tension before rupture, serving as a sensitive indicator of product durability, furnish quality, and production consistency. This article explains the test principle, applicable standards such as ISO 5626, TAPPI T 511, and GB/T 457, specimen preparation, instrument parameters, result expression, and typical applications, providing a practical reference for tissue manufacturers, converters, and accredited third-party laboratories.
Technical Overview
Folding endurance is defined as the number of double folds that a paper strip of specified width can withstand under a specified tensile load, while being folded back and forth through a defined angle, before it ruptures. For toilet paper, a lightweight, creped, and often multi-ply tissue product, folding endurance is a supplementary but highly informative physical property. Because fiber furnish, refining level, creping conditions, strength additives, and softening chemistry all influence the flexibility and internal bonding of tissue, folding endurance provides a fast and sensitive probe of both raw material quality and process stability.
Although folding endurance is generally not a mandatory item in toilet paper product standards such as GB/T 20810 or the ISO 12625 series, it is routinely evaluated during research and development, competitive benchmarking, incoming inspection, and failure analysis. Testing is performed using calibrated folding endurance testers of the MIT or Schopper type, either in manufacturers' quality control laboratories or by accredited third-party laboratories.
- Definition: the number of complete double folds sustained until breakage of the strip
- Common expression: arithmetic mean of double folds, or the logarithm (base 10) of the mean, reported separately for machine direction (MD) and cross direction (CD)
- Primary methods: ISO 5626, TAPPI T 511 (MIT instrument), TAPPI T 423 (Schopper instrument), and GB/T 457
- Supporting standards: ISO 186 / GB/T 450 for sampling and ISO 187 / GB/T 10739 for conditioning atmosphere
Testing Samples
Applicable samples cover the full tissue production and converting chain, from parent reels to finished consumer products. Representative sampling and careful specimen preparation are essential because creped tissue is easily damaged by handling, and fold counts are highly sensitive to surface defects.
Finished Toilet Paper Rolls
Consumer rolls, including single-ply and multi-ply products, embossed or plain, printed or unprinted. Specimens are cut from the usable width of the sheet, away from perforation lines, roll edges, splices, and heavily embossed zones unless the emboss pattern itself is the object of evaluation. Both machine direction and cross direction strips are prepared from each roll.
Jumbo Rolls and Base Tissue
Parent reels sampled before converting allow mills to monitor furnish consistency, refining, and creping stability. Sampling at several positions across the reel width helps detect cross-machine profile variation in basis weight, crepe structure, and moisture that would otherwise appear as scatter in fold counts.
Specimen Geometry and Quantity
Test strips are cut to a width of 15.0 mm with a tolerance of 0.1 mm and a length of at least 100 mm, cleanly and free of nicks, wrinkles, water marks, and visible crepe defects. In accordance with ISO 5626, at least 10 valid strips in each principal direction are tested per sample. Damaged strips are discarded and replaced, and the reasons for rejection are recorded.
Testing Items
The core deliverable of the determination is the folding endurance of the sample in each principal direction, supported by a statistical summary and, where required, a set of related physical properties that help interpret the fold data.
- Machine direction (MD) folding endurance, expressed in double folds
- Cross direction (CD) folding endurance, expressed in double folds
- Log10 of the mean double folds, as specified in ISO 5626
- Statistical summary: mean, range, standard deviation, and coefficient of variation
- Optional supporting tests: dry tensile strength, wet tensile strength, basis weight, thickness and ply bulk, absorbency, softness, and brightness
Testing Methods
Two instrument families are used worldwide for folding endurance determination: the MIT-type tester and the Schopper-type tester. Because the results produced by the two instruments are not interchangeable, the method and all test parameters must always be clearly stated in the report.
Sample Conditioning
Specimens are conditioned in the standard atmosphere specified in ISO 187 and GB/T 10739, namely 23 °C ± 1 °C and 50 % ± 2 % relative humidity, for at least 4 hours before testing. Thin tissue reaches equilibrium quickly, but multi-ply or laminated products may require longer. Folding endurance is strongly humidity dependent, with higher moisture generally increasing fold counts, so strict conditioning is essential. Gloves or tweezers should be used to prevent contamination of the strips with skin oils and moisture.
MIT Instrument Method
In the MIT method, described in TAPPI T 511 and referenced in GB/T 457, the strip is clamped vertically between the jaws of an oscillating folding head and a spring-loaded tension clamp. The folding head rotates through 135° on each side of the vertical position at approximately 175 double folds per minute, while a selectable tensile load between 4.9 N and 14.7 N is applied. For lightweight creped tissue, a reduced initial tension, commonly 4.9 N or another level agreed between the interested parties, is used so that the specimen does not fail almost immediately. The electronic counter stops automatically when the strip ruptures, and the number of double folds is recorded.
Schopper Instrument Method
In the Schopper method, specified in ISO 5626 and TAPPI T 423, the strip is folded backwards and forwards through 180° over a slotted folding blade at roughly 100 to 120 double folds per minute under a fixed spring tension of approximately 7.55 N. Since this fixed tension can exceed the strength of very light single-ply tissue, the method is more suitable for heavier multi-ply grades. If the Schopper method is applied to toilet paper, the resulting low fold counts must be interpreted with appropriate caution.
Calculation and Expression of Results
The mean number of double folds is calculated separately for the MD and CD populations. ISO 5626 additionally expresses folding endurance as the logarithm to base 10 of the mean double-fold count, because fold data follow an approximately log-normal distribution and can span several orders of magnitude. Standard deviation and coefficient of variation are reported to document scatter, which is characteristically high for this property. Results are only comparable when instrument type, tension, folding angle, direction, and conditioning history are identical.
Measurement Quality Assurance
Folding angle, applied tension, oscillation rate, and the electronic counter are verified regularly during calibration. Reference paper strips with known behavior are tested periodically to confirm instrument status. Operators must ensure that strips are clamped straight without pre-creasing, since misalignment and initial bends systematically reduce fold counts. All test parameters, specimen counts, and rejection criteria are documented in the final test report.
Applications
Folding endurance determination supports decision making across the entire tissue value chain, from fiber selection to finished product benchmarking. Its sensitivity to bonding and flexibility makes it particularly valuable as a process fingerprinting tool.
- Mill process monitoring: detecting changes in refining, crepe ratio, Yankee dryer coating, and drying conditions that affect sheet flexibility
- Furnish and additive optimization: evaluating softwood and hardwood fiber ratios, recycled fiber content, wet-strength resins, and debonding softeners that trade off strength against softness
- Converting and product development: quantifying the influence of embossing depth, ply bonding, and lamination on durability
- Incoming inspection and supplier qualification by converters and brand owners
- Competitive benchmarking and failure analysis, for example when sheets break excessively during converting or in use
FAQ
Why test folding endurance of a disposable product like toilet paper?
Because it is a rapid and sensitive composite indicator of furnish quality, internal bonding, and creping consistency. Small changes in fiber processing or chemical additives that are difficult to see in tensile data often appear clearly in fold counts, making the test valuable for process control even when the end product is disposable.
What is the difference between the MIT and Schopper methods?
The MIT tester folds a tensioned strip through 135° in each direction at approximately 175 double folds per minute with selectable tension, while the Schopper tester folds the strip through 180° over a blade at roughly 100 to 120 double folds per minute under a fixed spring tension of about 7.55 N. The two methods produce different numbers for the same paper and their results must never be compared directly.
Which tension should be selected for toilet paper?
The lowest calibrated setting that still gives clean clamping and a measurable result, commonly 4.9 N for lightweight single-ply tissue. Heavier multi-ply grades may be tested at 9.8 N. Whatever level is chosen, it must be held constant throughout a comparison series and stated in the report.
How many specimens are required?
ISO 5626 requires at least 10 valid strips in each principal direction per sample. For heterogeneous embossed products, additional strips improve confidence. Strips may only be rejected for identifiable physical defects, never simply because their counts are inconveniently low.
Why is the result sometimes reported as log10 of the double folds?
Fold counts are log-normally distributed and can vary over a wide range even for replicate specimens from the same sheet. The logarithmic expression stabilizes the variance and is the standardized reporting format in ISO 5626.
What values are typical for toilet paper?
Values are inherently low for creped tissue. Light single-ply toilet paper may sustain only a few double folds, while stronger multi-ply products can reach several tens of folds. Because absolute values depend strongly on the instrument, tension, and direction, comparisons are meaningful only between results obtained under identical, documented conditions.