ISO 252 Conveyor belts Elements Adhesion Strength Peeling Test

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ISO 252 Conveyor Belts — Adhesion between Constitutive Elements — Test Methods

ISO 252 specifies two test methods (A and B) for measuring adhesion strength between the constituent layers of conveyor belts: between top/bottom covers and carcass, and between individual plies.

It applies to most textile‑reinforced conveyor belts, Not applicable for steel‑cord reinforcement belts, textile belts with full‑thickness tensile strength < 160 N/mm, light conveyor belts (ISO 21183‑1).

Test Principle

Using a constant‑rate traverse tensile testing machine, the test measures the force required to strip/peel covers from the carcass and separate adjacent plies. The force trace is recorded, and median peak force is used to calculate mean adhesion strength.

Mechanically: one end of the test strip is carefully separated by hand (cover from ply, or ply from ply) for a short distance to create two "legs," which are then clamped in the machine's grips. The machine pulls at a fixed crosshead speed, peeling/separating the interface while recording force continuously. The interface is unsupported during stripping (free peel, not backed by a rigid plate).

Because rubber-to-fabric and rubber-to-rubber adhesion produces a jagged, multi-peak trace (not a single clean break), the data is analysed statistically per ISO 6133 — the median peak force is taken as the mean adhesion force.

Two specific Test methods A & B:

Method A

(Ply-by-ply separation)

One interface at a time, working progressively through the belt cross-section.

On Test Piece 1 (longitudinal):

Hand-separate top cover ↔ first ply at one end → clamp both legs in the machine, Peel/strip, record force trace (≥ 8 peaks over ≥ 100 mm strip length),

Then hand-separate first ply ↔ second ply on the same piece, clamp, peel, record.

Continue ply-by-ply up to the middle of the test piece.

On Test Piece 2 (longitudinal):

Start from the bottom side — separate bottom cover ↔ adjacent ply, peel and record.

Work upward through the ply interfaces toward the middle.

ISO 252 Conveyor belts Elements Adhesion Strength Peeling Test

Method B

(Grouped / combined separation)

Peel two unseparated plies together as a group (or the cover + first ply as a group from the next ply), rather than isolating every single interface.

On Test Piece 1:

Separate top cover ↔ first ply → peel, record (same as Method A)

Then, on the remainder, strip two unseparated plies together from what's left (i.e., the cover+ply₁ combination or ply group peeled from the next ply down)

On Test Piece 2:

Start by separating the unseparated top cover + first ply from the second ply → peel, record

Continue similarly

Test Specimen Information

Shape & Dimensions	Width: (25 ± 0.5) mm — rectangular strip cut from the belt Length: ≥ 200 mm minimum, so that a length of ≥ 100 mm can be stripped Edges: Clean-cut (sawn or die-cut; edges square, not ragged)
Number of Test Pieces	Two test pieces in the longitudinal direction (always) Transverse test pieces (also two) may be conducted if agreed between manufacturer and purchaser.
Sample cutting	Taken ≥ 100 mm from the edges of the available belt sample From places as widely spaced as possible on the sample
Time Between Manufacture and Test	≥ 16 hours between completion of belt production and commencementof testing — and this 16 h period must include the conditioning time.

ISO 252 Conveyor belts Elements Adhesion Strength Peeling Test Equipment:

Item	Requirement
Tensile testing machine	Power-driven, constant-rate-of-traverse type, conforming to ISO 36 (which in turn references ISO 7500-1 for force verification). Must be capable of making an autographic (force–time or force–extension) record of the stripping force
Grips/Jaws	Sufficient to hold separated ends without slippage during peel
Rate of traverse	Driven jaw speed = (100 ± 10) mm/min
Support condition	Test piece shall be unsupported during the strip (free-hanging peel)
Sample cutting tools	Used to prepare the test sample according standard requirement.

Test Parameters & Stipulations

Parameter	Requirement
Crosshead / traverse speed	(100 ± 10) mm/min
Peel support	Unsupported (free peel)
Minimum peaks for analysis	Strip enough length to obtain ≥ 8 points of peak force on the autographic trace
Minimum strip length	≥ 100 mm of separated interface
Adhesion metric	Median peak force ÷ width → N/mm (ISO 6133 methodology)
Reporting rounding	Nearest 0.1 N/mm
Failure mode rule	If separation occurs inside a component (e.g., cover tears cohesively rather than peeling at the interface), it is classified as rupture of the component — not a valid interfacial adhesion value

Test procedure of ISO 252 Conveyor belts Elements Adhesion peeling test:

1, Condition specimens per ISO 18573.

2, Pre‑separate one end for gripping.

3, Mount in tensile machine; set speed to 100 ±10 mm/min.

4, Run strip test and record force trace.

5, Analyze peaks per ISO 6133.

6, Calculate mean and minimum adhesion (N/mm).

(Mean adhesion = median peak force (N) / specimen width (mm) → N/mm;

Minimum adhesion = lowest peak force (N) / specimen width (mm) → N/mm;

Report to nearest 0.1 N/mm.)

Industry Fields / Applications

Mining & bulk handling (coal, copper, iron ore, aggregates, ports)	Plies delaminating mid-operation = catastrophic spillage / downtime. Adhesion values are a specified acceptance requirement in most mining belt specs.
Belt manufacturing QC	Every multi-ply belt build requires adhesion checks: cover pull-off, ply-to-ply bond, especially after compound changes, calender adjustments, or skim-coat reformulation.
Procurement / third-party inspection (DNV, BV, independent labs)	ISO 252 is the contractually cited method for verifying that delivered belts meet minimum N/mm bond strength requirements.
R&D / compound development	Comparing rubber formulations (NR/SBR vs. FR compounds vs. heat/oil-resistant blends) — adhesion is thefirst indicator of whether the compound will survive flexing, impact, and moisture ingress
Forensics / failure investigation	When a belt delaminates, ISO 252 on retained material determines whether the bond was below spec (manufacturing) or destroyed by mechanical abuse / chemical attack / overheating.

Related Test Standard:

ISO 36	Rubber, vulcanized or thermoplastic — Determination of adhesion to textile fabrics— ISO 252's basic test conditions are in conformity with ISO 36
GB/T 6759	Conveyor belts - Adhesion between constitutive elements - Test methods
ISO 21183-1	Light conveyor belts— explicitly excluded from ISO 252 scope.
ISO 283	Textile conveyor belts — Full thickness tensile strength, elongation at break and elongation at the reference load — Test method

WDW Series Computer Control Electronic Universal Testing Machine made by UNITEDTEST range from 100N to 600KN load capacity with various models like single columns, table type, door frame type etc., is used to perform tension, compression, flexure/bending, shearing, peeling etc., test for metal and nonmetal specimens.

T-Peel Resistance Test Fixture for Adhesives is based on ASTM D1876 standard, used to measure the adhesion strength of adhesive maeterial. ASTM D1876 peel strength test method is commonly used to measure the strength of adhesive bonds between two sealed materials.

Webbing test fixture used for tensile testing of the webbing, belt test sample, determine the webbing, belt tensile strength, elongation etc., Webbing is a strong fabric woven as a flat strip or tube of varying width and fibres often used in place of rope.

Q1: What does this standard actually test?

A: ISO 252 measures the adhesion / bond strength inside a conveyor belt — specifically:

Cover ↕ Carcass (top cover to first ply, bottom cover to adjacent ply)

Ply ↕ Ply (between adjacent fabric plies in multi-ply belts)

It does this by peeling (stripping) those layers apart on a tensile machine at a controlled speed and expressing the result as N/mm of strip width (typically a 25 mm wide strip). The force trace is jagged with repeating peaks, so the result is analysed statistically per ISO 6133 — the median peak force becomes the official mean adhesion.

Note: This is not a tensile test. It's a peel / bond / delamination resistance test for the interfacesthat hold the belt structure together.

Q2: Why is adhesion testing important? What happens if bond strength is low? The belt has great fabric tensile strength — why do I also need ISO 252?

A: Because a conveyor belt is a laminate (covers + plies + skim rubber between plies). The tensile carcass can be excellent while the bond between layers is the weak link. Real-world failure isn't always tensile rupture — it's often delamination:

Carriage/idler impact → local ply separation

Flexing over pulleys thousands of times → fatigue opens weak bonds

Moisture / chemicals infiltrate at edges → bond degrades from the outside in

Poor skim-coat mixing, insufficient cure pressure, or entrained air → weak zones from new

When plies delaminate mid-operation: tracking fails, the belt telescopes, material spills, and you're down for days. ISO 252 is the early warning system that the internal glueis sound before the belt ever goes into service.

Q3: What types of belts does ISO 252 apply to — and NOT apply to?

A: Applies to: All conveyor belt constructions.

Not apply for 1), Steel cord belts (steel cords use different adhesion/pull-out methods)

2), Textile-reinforced belts with full-thickness tensile strength < 160 N/mm

3), Light conveyor belts (per ISO 21183-1).

Q4: How is the result calculated? What does "N/mm" mean here?

A: Same unit, completely different meaning:

ISO 283 (tensile) → N/mm = total rupture force ÷ belt width(carcass strength)

ISO 252 (adhesion) → N/mm = peel (stripping) force ÷ strip width(interface bond strength)

Mean adhesion (N/mm) = Median peak force (N) ÷ nominal width (25 mm)

Minimum adhesion (N/mm) = Lowest peak force (N) ÷ nominal width (25 mm).

Q5: Sometimes when I peel, the cover rips / tears throughthe rubber instead of separating cleanly at the bond line. How do I report that?

If the failure is at the interface (clean rubber-to-fabric or rubber-to-rubber bond line visible) → that's a valid adhesion value

If the cover or ply tears through itself (cohesive rupture in the rubber/compound) → that's not a valid bond measurement; it tells you the compoundis the limiting factor, not the bond.

Q6: When is a result not an adhesion failure?

A: If the sample breaks inside the cover or ply material (not at the bonding interface), it is material rupture, not adhesion failure.

Q7: What are the most common mistakes that give wrong/low adhesion?

#	Mistake	Effect
1	Strip not exactly 25±0.5 mm or edges ragged	Stress concentration → artificially low peel force
2	Conditioning skipped or wrong atmosphere	Rubber stiffness changes → N/mm drifts
3	Hand-start too violent — you've already damaged the bond before the machine records	Low first-peaks drag median down
4	Gripping too tight / too close to peel zone → crushing	Local damage mimics poor adhesion
5	Counting a cohesive cover tear as a valid adhesion value	You're measuring tear strength, not bond strength
6	Using < 8 peaks / insufficient strip length	ISO 6133 median becomes statistically unstable

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