Toilet paper smoothness testing is a critical quality control process that measures the surface evenness and texture of tissue products to ensure optimal softness, comfort, and printability. This technical article explains the principles behind smoothness measurement, the internationally recognized testing standards such as ISO 8791 and GB/T 456, and the practical methods used by manufacturers and accredited third-party laboratories to evaluate toilet paper surface quality.
Technical Overview
Smoothness is one of the most important surface properties of toilet paper, directly influencing consumer perception of softness, comfort during use, and overall product quality. Smoothness refers to the degree of surface evenness of the paper sheet, which is determined by factors such as fiber composition, pulp refining intensity, creping conditions, basis weight, calendering treatment, and the crepe structure characteristic of tissue manufacturing.
In toilet paper production, smoothness is typically evaluated by measuring the rate of air leakage between the paper surface and a flat reference surface under controlled pressure. Because toilet paper is a low-basis-weight, highly creped and porous material, smoothness testing must account for its compressibility and air permeability. For tissue products, surface smoothness is closely correlated with tactile softness, and manufacturers use smoothness data alongside softness panel testing to benchmark product grades.
Quantitatively, smoothness is expressed in units of time (seconds) in the Bekk method, or as air flow (mL/min) in the Bendtsen and Parker Print-Surf methods. Lower air flow values or longer leak times indicate smoother surfaces. These measurements enable objective comparison between production batches, raw material formulations, and competing products.
Testing Samples
Roll Toilet Paper (Bathroom Tissue)
Standard consumer roll toilet paper is the most common sample type. Specimens are taken from the outer, middle, and inner layers of the roll, avoiding the first few wraps which may be damaged or contaminated. Samples must be conditioned in a standard atmosphere of 23 ± 1 °C and 50 ± 2% relative humidity prior to testing, in accordance with ISO 187.
Ply-Separated and Multi-Ply Specimens
Because toilet paper is commonly manufactured in one, two, three, or four plies, testing can be performed on the complete multi-ply assembly as used by the consumer, or on individual separated plies to evaluate single-sheet surface quality. The test report must clearly state the ply configuration tested, as results differ significantly between configurations.
Jumbo Roll and Parent Tissue Reels
For in-process quality control, samples are taken from jumbo reels before converting. Testing parent reel samples allows paper mills to detect forming, pressing, Yankee coating, and creping irregularities early in the production chain, before value is added in the converting stage.
Core and Deckle Position Samples
Samples taken from different cross-machine positions (front edge, center, back edge) help identify cross-profile non-uniformities in smoothness caused by uneven drying, coating application, or crepe blade wear.
Testing Items
A comprehensive smoothness evaluation of toilet paper typically includes the following measured and derived parameters:
- Bekk smoothness value (seconds) — the time required for a specified volume of air to pass between the specimen surface and a ring-shaped glass anvil under standard pressure.
- Bendtsen roughness (mL/min) — the air flow rate escaping between the paper surface and a flat glass plate under a defined clamping pressure.
- Parker Print-Surf (PPS) roughness (µm) — air leakage measured across a land pattern simulating printing nips, suitable for embossed or printed tissue.
- Sheffield roughness (units) — an air-leak method applicable to coarse, absorbent, and creped grades.
- Smoothness two-sidedness — comparison of top (Yankee-side) and wire-side surfaces.
- Surface roughness profile (Ra, Rz) — optional stylus or optical profilometry data for research-level characterization.
- Related supporting tests — basis weight (ISO 12625-6), thickness (ISO 12625-3), tensile strength (ISO 12625-4), softness by panel evaluation (ISO 12625-18), and absorbency.
Testing Methods
Bekk Smoothness Method (ISO 8791-2 / GB/T 456)
The specimen is clamped against a precision-ground annular glass surface with a defined pressure. A vacuum is drawn, and the time required for 10 mL of air to leak between the paper and the glass land is measured electronically. The result is reported in seconds; a longer time indicates a smoother surface. Because toilet paper is highly permeable, testing is normally performed on multi-ply specimens or with backing films, and the test conditions must be documented. ISO 8791-2 specifies the applicability of the Bekk method to papers with a smoothness between approximately 15 s and 3000 s, and notes that the method may be adapted for absorbent grades with clearly stated modifications.
Bendtsen Method (ISO 8791-2, ISO 8791 Series Concept)
In the Bendtsen tester, the specimen is pressed against a flat glass plate by a weighted ring. Compressed air at a controlled pressure flows to the measuring head, and the air that escapes between the paper surface and the glass is measured by a float-type flowmeter in mL/min. Higher readings indicate rougher surfaces. The Bendtsen method is best suited to moderately rough papers and is widely used for mill-side process control of tissue base paper.
Parker Print-Surf Method (ISO 8791-4)
The PPS tester measures air leakage between the specimen and a structured measuring land while the paper is backed by an annular resilient backing ring that simulates printing press conditions. Clamping pressure and land width are selectable. The result, expressed in micrometers, represents mean roughness. PPS testing is particularly valuable for tissue products that receive printing, embossing, or lamination, because it evaluates the surface under nip-like compression comparable to converting operations.
Sheffield Method (ISO 8791-3)
The Sheffield tester uses a wide measurement range with air flowing between the specimen and a flat, lapped metal land. It is suited to coarse and absorbent papers, including creped hygiene grades, where Bekk or PPS measuring ranges may be exceeded. Results are reported in Sheffield units, with lower values indicating smoother surfaces.
Sample Preparation and Conditioning
At least ten valid measurements are taken on each side of the material, distributed across the specimen area and oriented consistently relative to the machine direction. Specimens must be free from wrinkles, embossing patterns, folds, and watermarks. Conditioning follows ISO 187 (23 °C, 50% RH), and results are reported as the arithmetic mean with standard deviation and the number of determinations.
Result Interpretation
Interpretation must consider that toilet paper smoothness is inherently lower than that of supercalendered fine papers due to the crepe structure and bulky, low-density formation. Manufacturers typically establish internal acceptance limits by grade: premium grades target higher smoothness (longer Bekk time or lower PPS/Bendtsen values), while economy grades allow higher roughness. Two-sidedness is assessed by comparing the Yankee-side and wire-side results, and cross-direction profiles are reviewed for uniformity.
Applications
- Product quality control — verifying that each production batch of toilet paper meets defined smoothness specifications before release.
- Raw material and furnish comparison — evaluating how different pulp types (virgin softwood, hardwood, recycled fiber, bamboo, straw pulp) and refining levels affect surface smoothness.
- Process optimization — correlating Yankee dryer temperature, crepe ratio, blade angle and wear, and coating chemistry with measured smoothness to tune manufacturing conditions.
- New product development — benchmarking prototype tissue structures against competitor or reference products during grade development.
- Compliance and certification support — providing objective test data for product standard conformity, such as GB/T 20810 for toilet paper, and for submissions to accredited third-party laboratories or regulatory bodies.
- Supplier and incoming material inspection — tissue converters testing parent reels from multiple paper mills to ensure consistent converting performance and end-product quality.
- Softness prediction — using smoothness in combination with thickness and stiffness data in softness models to reduce reliance on time-consuming sensory panel testing.
- Trade and export documentation — supplying internationally comparable test reports based on ISO methods for buyers and import inspectors.
FAQ
Why is smoothness important for toilet paper?
Smoothness is a major contributor to perceived softness and comfort. A smoother surface provides gentler skin contact, improves the uniformity of embossing and printing, and is often associated with a more premium product feel. Consistent smoothness also indicates a stable creping and forming process.
What is the difference between Bekk, Bendtsen, and PPS smoothness testing?
All three are air-leak principles, but they differ in measuring geometry and reporting units. Bekk measures the time for a fixed air volume to escape (seconds, higher is smoother). Bendtsen measures air flow rate (mL/min, lower is smoother) and suits rougher papers. PPS measures leakage across a land with resilient backing and reports micrometers, best reflecting printing nip conditions. The choice depends on the paper grade and the intended end use.
Can standard smoothness methods be applied directly to creped toilet paper?
Because toilet paper is creped, bulky, and air-permeable, the standard fine-paper procedures often require adaptation, such as testing multi-ply specimens, using impermeable backing films, or selecting the Sheffield/PPS range. Any modification must be clearly documented in the test report so results remain comparable and traceable.
How should samples be conditioned before testing?
Specimens must be conditioned in a standard atmosphere of 23 ± 1 °C and 50 ± 2% relative humidity until moisture equilibrium is reached, following ISO 187. Testing without proper conditioning can produce misleading results because humidity strongly affects tissue softness, compressibility, and air-leak behavior.
How many measurements are required for a reliable result?
A minimum of ten valid determinations per surface is customary, with more measurements recommended for highly creped or embossed products. The report should include the mean, standard deviation, number of tests, ply configuration, tested side, and the exact standard and modifications applied.
Which standards govern toilet paper smoothness testing?
The primary references are the ISO 8791 series (roughness/smoothness by air-leak methods), including ISO 8791-2 (Bendtsen), ISO 8791-3 (Sheffield), and ISO 8791-4 (Parker Print-Surf); GB/T 456 for Bekk smoothness; and the ISO 12625 tissue series for supporting properties. Product-level requirements are covered by standards such as GB/T 20810. Accredited third-party laboratories can perform these tests and issue internationally recognized reports.