1. Definitions
The 90-degree peel strength is a key mechanical index for evaluating the interfacial bonding properties of bonded joints, coatings or composite layers. It specifically refers to the average peeling force per unit width when the flexible object is peeled off from the rigid object at an angle of 90 degrees under the specified test conditions. Its mathematical expression is:
σ90 = F / b
Among them, σ90represents a 90-degree peel strength, usually measured in Newtons per millimeter or kiloNewtons per meter; F represents the average peel force measured during the peeling process; b represents the width of the specimen. Compared with the 180-degree peel test, the peel force action direction is perpendicular to the surface of the rigid adhesive during 90-degree peeling, which makes the stress state of the peeling area clearer, and the peeling force is mainly concentrated on the front end of peeling, so it can more clearly reflect the firmness of the interface bond.
2. Principle
The determination of the 90-degree peel strength is based on the basic principles of fracture mechanics and adhesion science. During the peeling process, the external force is transmitted to the bonding interface through the flexibility of the adhesive, so that the adhesion bond at the interface is gradually broken. From the energy point of view, the peeling process is the process of converting the work done by external forces into interfacial fracture energy, which is used to overcome the intermolecular forces, chemical bonding forces, and possible plastic deformation of materials near the interface.
When the flexible object is stretched in a 90-degree direction, the peeling front end creates a highly concentrated stress area. The stress state in this region can be simplified to the compound effect of interfacial normal tensile stress and tangential shear stress. As the peeling progresses, the stress concentration area moves forward, and the original bonding interface is gradually separated. The ideal 90-degree peel test requires no significant plastic elongation deformation of the flexible adhesive during the peel process to ensure that the measured force values truly reflect the bonding strength of the interface rather than the tensile behavior of the material itself.
According to the principle of energy balance, when the peeling reaches a steady state, the work done by the external force is equal to the fracture energy of the interface plus the energy consumed by the flexibility of the bending and deformation of the adhesive. Therefore, by measuring the average peel force during the steady-state peel phase and combining it with the specimen geometric parameters, the peel strength value that characterizes the interfacial bonding performance can be calculated.
3. Measurement method
Measurement of 90-degree peel strength follows strict standardized operating procedures to ensure accuracy, repeatability, and comparability of test results. Different industries have formulated corresponding test standard systems according to their own material characteristics and application needs, but the basic measurement process has common characteristics.
Sample preparation:Specimen preparation is the primary link that affects the measurement results. Flexible adhesive materials (e.g., films, sheets, fabrics) are usually bonded to adhesives (usually rigid plates, e.g., metal, glass, plastic plates) to form standard-sized specimens. During the bonding process, it is necessary to strictly control the uniformity of the thickness of the adhesive layer, the curing conditions and the bonding pressure to avoid defects such as bubbles and lack of glue. For finished materials, specimens of specified widths and lengths need to be cut from the products.
Test device:The 90-degree peel test is usually performed on a universal material testing machine equipped with a special fixture. The lower fixture of the testing machine is used to fix the rigid plate, and the upper fixture holds the free end of the flexible object. The key is that the upper clamp must be kept at a peeling angle of 90 degrees at all times by a specific mechanism (e.g. pulley system or movable beam). Modern testing machines are mostly equipped with a 90-degree peeling special test bench, which can move horizontally along the guide rail during the peeling process, so that the flexible adhesive is always perpendicular to the surface of the rigid plate and ensure the constant peeling angle.
Test process:The prepared specimen is installed on the testing machine, and the clamping position is adjusted so that the flexible part is perpendicular to the surface of the rigid plate. Set a constant peeling speed (usually 100 mm per minute or according to the standard) and start the testing machine for peeling. The testing machine automatically records the force-displacement curve throughout the peeling process. The peel force typically undergoes an initial peak, a stable peel phase, and an end phase. When calculating the peel strength, the force value data of the stable peel stage should be intercepted, and the average value should be obtained and divided by the width of the specimen.
Data logging:The measurement results should record the average, maximum, and minimum values of peel strength and the mode of peel failure. The failure modes can be divided into interface failure (adhesion failure), cohesive failure (adhesive layer itself failure) or substrate failure, and different failure modes correspond to different quality control judgment standards.
4. Influencing factors
The measurement results of 90-degree peel strength are intricately influenced by many factors, and understanding these factors is valuable for correctly interpreting test data and optimizing the bonding process.
Peel angle and rate:Precise control of the peel angle is crucial in the 90-degree peel test. A deviation of 90 degrees will lead to a change in the stress state, affecting the accuracy of the measurement results. The peeling rate also significantly affects the results: the response time of viscoelastic materials is insufficient during high-speed peeling, which may lead to an increase in the apparent peeling strength. Conversely, low-speed peeling may allow for more interfacial slack and may reduce peel strength. Therefore, the test must be carried out strictly at the rate specified by the standard.
Properties of flexible adhesive:The thickness, elastic modulus and yield behavior of the flexible adhesive have significant effects on the peel strength measurement. If the flexible material has obvious plastic elongation or necking during the peeling process, the measured force value includes the energy consumed by the material deformation, and cannot truly reflect the interfacial bonding strength. The ideal flexible adhesive should have a sufficiently high modulus and strength to maintain elastic deformation during the peeling process.
Adhesive Properties:The ontological mechanical properties, viscoelastic behavior and interface interaction with the adhesive jointly determine the peel strength. Adhesive layer thickness is an important parameter: there is usually an optimal thickness range, if it is too thin, the stress concentration is serious, and if it is too thick, it can lead to cohesive failure. The degree of curing of an adhesive directly affects its cohesive strength and interfacial bonding, and it is important to ensure that it is fully cured before testing.
Surface state of the adherent:The surface roughness, cleanliness and surface treatment of rigid adhesive directly affect the interfacial bonding strength between the adhesive and the adhesive. Surface contamination, oxide layers, or release agent residues can significantly reduce peel strength. Proper surface treatments, such as sanding, chemical etching, or undercoating, can enhance interfacial bonding and improve peel strength.
Environmental Conditions:The temperature and humidity of the test environment have a significant impact on the mechanical behavior of the bonding interface. The increase of temperature may lead to a decrease in the modulus of the adhesive, an increase in viscous flow, and a change in the peel failure mechanism. Increased humidity may cause interfacial hydrolysis or plasticization of the adhesive layer. Therefore, the 90-degree peel strength test must be performed under standard temperature and humidity conditions, or simulated as needed in a specific use environment.
5. Application
As a reliable indicator for evaluating the bonding properties of interfaces, the 90-degree peel strength plays a central role in product development, quality control and process optimization in many industrial fields.
Adhesives & Tapes Industry:This is the most widely used area of 90-degree peel strength. The adhesive performance evaluation of pressure-sensitive tapes, double-sided tapes, labels and protective films is based on the peel strength of 90 degrees or 180 degrees as the key quality index. By measuring peel strength, adhesive manufacturers can screen formulations, optimize processes, and ensure consistent adhesion performance across different materials being bonded.
Electronics and Optoelectronics:In the manufacturing process of smartphones, tablets, and wearable devices, adhesives are widely used to secure components in thin and lightweight designs. The interface and reliability of flexible circuit boards, optical transparent adhesives for touch screens, and fixing tapes for batteries are directly related to the service life and safety of products. The 90-degree peel test is widely used to evaluate the bond strength of these precision components due to its clear stress state.
Packaging & Printing Industry:The interlayer bonding strength of flexible packaging composite film and paper-plastic composite material is the basis for ensuring packaging integrity and protective performance. The 90-degree peel test can evaluate the fastness of composite between materials with different layers, which provides a basis for composite process optimization, material selection and quality control. The bond strength of the label to the substrate is also evaluated by the peel test.
Automotive & Aerospace:With the advancement of lightweight technology, glue bonding is increasingly widely used in body structures, interior parts fixing, and composite material connections. The 90-degree peel strength test is used to evaluate the adhesive properties of structural adhesives, screen surface treatment processes, and verify the durability and reliability of adhesive joints to ensure the safety of connections under complex working conditions.
Medical & Biomaterials:The adhesion performance of medical tapes, wound dressings, patches and other products to the skin is directly related to the comfort of use and the treatment effect. The 90-degree peel strength test is used to evaluate the adhesion of these products to the skin, optimizing the adhesive formulation and backing material to ensure sufficient adhesion strength while reducing pain and tissue damage during peeling.
Coatings & Coatings Industry:Adhesion between various functional, protective and decorative coatings and the substrate is key to determining the performance of the coating. The 90-degree peel test can be used to quantitatively evaluate the interface bond strength between the coating and the substrate, and provide a scientific basis for coating process optimization, coating formulation design and coating life prediction.
6. Summary
As the core mechanical parameter for characterizing the bonding properties of interfaces, the 90-degree peel strength occupies an important position in a wide range of fields from basic research to engineering applications with its clear stress state and good repeatability. Its physical nature is rooted in the science of fracture mechanics and adhesion, which translates abstract interface bonding forces into quantifiable engineering indicators by measuring the average peel force over a unit width. At the level of measurement methods, every link from sample preparation, test equipment to data processing needs to strictly follow standardized procedures to ensure the accuracy and comparability of results. Factors affecting 90-degree peel strength involve multiple dimensions such as material properties, geometric parameters, test conditions, and environmental factors, which require researchers and engineers to maintain a systematic mindset when designing tests and interpreting data. In the application field, the 90-degree peel strength has penetrated into all aspects of modern industries such as adhesive industry, electronic manufacturing, packaging and printing, automobile, aerospace and biomedical, and has become an indispensable technical means to evaluate product quality, optimize process design, and ensure safety in use. With the continuous emergence of new materials and processes, as well as the continuous progress of multi-scale characterization technology and theoretical simulation methods, the measurement principle and application scope of 90-degree peel strength will be further expanded and deepened, providing more solid support for interface science and engineering practice in various industries.