Detailed explanation of PET double-sided tape coating process

Adhesive strength refers to the stress required to cause damage to the interface or adjacent areas between the adhesive and the object in the bonded component under external forces, and is also known as adhesive strength. The adhesive strength is the stress required for the failure of the adhesive system, and its magnitude not only depends on the adhesive force, mechanical properties of the adhesive, properties of the adherend, bonding process, but also on the joint form, stress situation (type, size, direction, frequency), environmental factors (temperature, humidity, pressure, medium) and testing conditions, experimental techniques, etc. From this, it can be seen that adhesive force is only one of the important factors determining adhesive strength, so adhesive strength and adhesive force are two concepts with completely different meanings and cannot be confused. The force acting on the adhesive layer of a bonded joint under external forces can be summarized into four forms: shear, tension, uneven tearing, and peeling.

1. Cut. The external force is equal in magnitude and opposite in direction, basically parallel to the bonding surface, and evenly distributed throughout the entire bonding surface< Br/> 2. Stretch. Also known as uniform pulling, it is subjected to opposite pulling forces, perpendicular to the bonding surface, and evenly distributed throughout the entire bonding surface< Br/> 3. Uneven pulling apart. Also known as splitting, although the direction of external force is also perpendicular to the bonding surface, it is unevenly distributed< Br/> 4. Peel off. The direction of the external force is at a certain angle with the adhesive surface, and it is basically distributed on a straight line of the adhesive surface. The above four forces are likely to exist simultaneously in the same adhesive system, but it is only a matter of which one is the main one.

According to the different stress conditions of adhesive joints, adhesive strength can be specifically divided into shear strength, tensile strength, uneven tensile strength, peel strength, compressive strength, impact strength, bending strength, torsional strength, fatigue strength, creep resistance, etc.

Tensile strength, also known as uniform tensile strength or positive tensile strength, refers to the tensile force borne per unit area when the adhesive is subjected to force failure, expressed in megapascals (MPa). Because tensile stress is much more uniform than shear stress, the tensile strength of general adhesives is much higher than shear strength. In actual testing, under the action of external forces, the specimen may experience shear and lateral compression due to the larger deformation of the adhesive compared to the adherend, and the different axial properties of the external force. Therefore, simultaneous fracture may occur when pulled apart. If the length of the sample can be increased and the bonding area can be reduced, the impact of delamination during tearing can be reduced, making the stress distribution more uniform. The effects of elastic modulus, adhesive layer thickness, test temperature, and loading speed on tensile strength are basically similar to those on shear strength.

Peel strength is the maximum load that can be borne per unit width when the adhesive component separates under specified peeling conditions, expressed in kN/m. There are various forms of peeling, which can generally be divided into L-shaped peeling, U-shaped peeling, T-shaped peeling, and curved peeling, as shown in the following figure. As the peeling angle changes, the peeling form also changes. When the peeling angle is less than or equal to 90 °, it is an L-shaped peeling, and when it is greater than 90 ° or equal to 180 °, it is a U-shaped peeling. These two forms are suitable for delamination of rigid and flexible materials. T-shaped peeling is used for peeling when bonding two flexible materials. The peel strength is influenced by factors such as the width and thickness of the specimen, the thickness of the adhesive layer, the peel strength, and the peel angle.

Fatigue strength refers to the maximum stress at which a certain load is repeatedly applied to a bonded joint for a specified number of times without causing damage. The fatigue strength at 10 cycles is generally referred to as the fatigue strength limit. Generally speaking, adhesives with high shear strength always have lower strength in areas such as peeling, bending, and impact; Adhesives with high peel strength have higher impact and bending strength. Different types of adhesives also have significant differences in their strength characteristics.