Footwear Upper-sole Bonding Strength Testing

Footwear upper-sole adhesion strength testing mainly conducts scientific testing and analysis on key performance indicators, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Core Testing Items

Peel strength test: This is the most direct quantitative indicator of the adhesion strength, calculated by measuring the force required to separate the upper from the sole at the bonding interface. It is usually expressed in "newtons per centimetre" (N/cm) or "kilogram-force per centimetre" (kgf/cm). This test aims to simulate the working conditions of toe curling during wearing or the upper and the sole edge being pulled by external forces, and is the core basis for judging the adhesion reliability.

Adhesion strength test after damp heat aging: Footwear is affected by temperature and humidity changes during wearing and storage. In this item, the specimen is placed in an environment of a specific temperature (such as 50 ℃±2 ℃) and humidity (such as 95%±5%) for a specified time (such as 48 hours), and after taking it out, its peel strength is tested within the specified time. This test is used to evaluate the durability and aging resistance of the adhesive and the bonding interface under damp heat conditions.

Cold resistance test: Evaluates the adhesion performance of footwear under low-temperature environments. The specimen is frozen in a low-temperature environment (such as -20 ℃±2 ℃) for a specified time, and after taking it out, the peel test is carried out immediately or within the specified time. Low temperature may cause the material to harden and become brittle, and the elasticity of the adhesive to decrease. This test can effectively reflect the service reliability of the product in cold climates.

Flexing resistance test: Simulates the working conditions of repeated bending of the forepart of the shoe during walking. The finished shoe or specimen is bent tens of thousands of times at a certain angle and frequency on a flexing testing machine, and then the bonding part is checked for opening, cracking, or the residual peel strength is directly tested. This item is especially important for sports shoes and casual shoes.

Water immersion test: The specimen is completely immersed in water at a specified temperature (such as immersion in room temperature water for 24 hours), and after taking it out and wiping it dry, its peel strength is tested. Water can penetrate and possibly damage the adhesion of some adhesives, or cause the shoe materials to absorb water and expand, generating internal stress. This item is used to evaluate the water resistance of the bonding structure.

Scope of Application

Footwear of various bonding processes: Suitable for leather shoes, sports shoes, casual shoes, safety shoes and work shoes produced by processes such as cold bonding, hot melt bonding and adhesive bonding. Whether it is a flat lamination or a complex three-dimensional side-wall bonding structure, its basic bonding quality needs to be evaluated through this test.

Different combinations of upper and sole materials: The test covers bonding combinations of various materials such as leather, synthetic leather, fabric, rubber, EVA (ethylene-vinyl acetate copolymer), PU (polyurethane), TPR (thermoplastic rubber) and PVC (polyvinyl chloride). Different materials have great differences in surface energy, polarity and porosity, and have different requirements for adhesives and pretreatment processes.

Finished shoes and prepared specimens: The test objects can be either complete finished shoes, from which standard test strips are cut, or bonding specimens specially prepared in the laboratory according to the actual production process, representing a specific material combination. The latter is often used for preliminary verification in process development and quality control.

Quality control and R&D evaluation: On the production line, regular sampling for upper-sole adhesion strength testing is a key link in quality control. In the product R&D stage, it is used to screen adhesives, optimize treatment agent formulations and determine the best curing process parameters (such as pressure, temperature and time).

Basis for conformity certification and inspection: This test is a mandatory or recommended requirement of footwear product standards in many countries or regions (such as GB/T, ISO, SATRA and EN), and is also one of the core test items for third-party testing and certification (such as protective footwear in CE certification) and for brand owners and buyers to carry out inspection.

Testing Methods

Peel test method (mainstream method): A peel testing machine is used. The upper and the sole are pre-peeled for a short section (about 25 mm), and respectively clamped in the upper and lower fixtures. The testing machine carries out 180° or 90° peeling at a constant speed (such as 100 mm/min±10 mm/min), continuously recording the force change during the peeling process until the bonding part is completely separated. The peel strength is obtained by calculating the average force value on the force-displacement curve.

Test specimen preparation: A specimen strip with a width of 10 mm or 20 mm and a length of not less than 100 mm is cut from the bonding part of the finished shoe. When cutting, it is necessary to ensure that the specimen contains the complete bonding interface and that the long side is parallel to the possible cracking direction. For parts where standard strips cannot be cut (such as the toe), simulated specimens can be prepared according to the relevant standards.

Environmental conditioning: Before the test, the specimen must be conditioned in a standard laboratory environment (such as a temperature of 23 ℃±2 ℃ and a relative humidity of 50%±5%) for at least 24 hours, to stabilize the material properties. This is an important premise for obtaining comparable data.

Execution of pretreatment procedures: According to the requirements of the test items, the conditioned specimen is subjected to corresponding pretreatments, such as damp heat aging, low-temperature freezing, water immersion or flexing tests. After the pretreatment, the peel test needs to be completed within the specified time window (such as within 2 minutes after freezing and within 1 hour after water immersion), to ensure that the results reflect the true state after the pretreatment.

Result calculation and recording: The initial peak and the force values at the end are removed from the peel curve, and the average value of the force in the middle stable peeling section is taken. The peel strength (average force value / specimen width) is calculated. At the same time, the peel failure type is recorded, such as material failure (tearing of the shoe material), cohesive failure of the adhesive layer (fracture of the adhesive layer) or interfacial adhesion failure (debonding), which is crucial for failure analysis.

Testing Standards

Domestic Standards:

1. GB/T 21396-2008, Footwear — Test method for the adhesion of the outsole to the upper — Peel resistance

2. GB/T 3903.3-2011, Footwear — General test methods — Determination of the strength of the bond between the upper and the sole

3. QB/T 2885, Footwear — Test methods for shoe components — Determination of the peel strength of the bond between the upper and the sole

4. GB 25036, Safety shoes

International Standards:

1. ISO 17708, Footwear — Test methods for whole shoe — Upper sole adhesion

2. SATRA TM411, Peel strength of shoe soles and unit soles (bonded construction)

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electronic peel testing machine: The core equipment, which needs to have sufficient range (usually 0-500 N) and accuracy (±1%). It is equipped with a programmable controller that can set a constant test speed and collect, display and output the force-displacement curve in real time. The fixtures should be designed to firmly clamp upper and sole materials of different thicknesses and hardnesses, preventing slipping during the test.

Environmental test chamber: Used for pretreatments such as damp heat aging and cold resistance. The damp heat aging chamber needs to be able to precisely control the temperature and humidity to the standard requirements (such as temperature ±1 ℃ and humidity ±3%). The high and low temperature test chamber needs to provide a stable low-temperature environment (such as a range of -40 ℃ to +150 ℃) and have rapid cooling capability.

Flexing testing machine: Used for flexing resistance testing. The machine can repeatedly bend the shoe or specimen at a set angle (usually 30° to 90°) and frequency (such as 100 times per minute). The counter records the number of bending times. After the test, the opening condition needs to be checked by visual inspection or with tools, or the specimen is moved to the peel testing machine to test the residual strength.

Specimen preparation tools: Including die cutters for precisely cutting the specimen, rotary cutting knives or sharp leather knives, and steel rulers. It is necessary to ensure that the cutting edge is neat and does not affect the bonding interface. In addition, auxiliary tools for marking and pre-peeling may be needed.

Data acquisition and analysis system: A computer software system supporting the peel testing machine. This system can not only control the operation of the machine, but also automatically analyze the peel curve, calculate the average force value, maximum and minimum values, and generate test reports containing curve diagrams and detailed data, improving the test efficiency and result accuracy.