The adhesive strength is one of the important indexes to evaluate the adhesive quality
Guide: The adhesive strength is one of the important indicators to evaluate the adhesive quality. The adhesive strength is generally divided into shear strength, tensile strength, peel strength and impact strength according to the stress condition of adhesive in use.
0 1 Bonding strength Definition Bonding strength refers to the stress required to damage the interface between the adhesive and the object to be bonded or its adjacent parts under the action of external forces. Bonding strength is also called bonding strength. Bonding strength is the stress required for the destruction of the adhesive system. Its size depends not only on the bonding strength, mechanical properties of the adhesive, properties of the adhesive, and bonding process, but also on the joint type, stress conditions (type, size, direction, frequency), environmental factors (temperature, humidity, pressure, medium), test conditions, and experimental techniques. It can be seen that adhesive force is only one of the important factors that determine the adhesive strength, so adhesive strength and adhesive force are two concepts with completely different meanings and must not be confused.
The force acting on the adhesive layer of the adhesive joint under the external force can be summarized into four forms: shear, tension, uneven pulling and peeling. A. The external shear force is equal in magnitude and opposite in direction, basically parallel to the bonding surface, and evenly distributed on the entire bonding surface. B. Tension is also called uniform pulling, which is perpendicular to the bonding surface and evenly distributed on the whole bonding surface under the action of opposite tension. C. Uneven tearing is also called splitting. Although the direction of the external force is also perpendicular to the bonding surface, the distribution is uneven. D. The direction of the peeling external force is at an angle with the bonding surface. The above four forces are basically distributed on a straight line of the bonding surface. In the same adhesive system, there may be several forces at the same time, but the main problem is which one.
0 3 The classification of bonding strength depends on the stress of the bonding joint. The bonding strength can be divided into shear strength, tensile strength, non-uniform tear strength, peel strength, compressive strength, impact strength, bending strength, torsional strength, fatigue strength, creep resistance strength, etc.
A. Shear strength Shear strength refers to the shear force that can be borne by the unit bonding surface when the bonding part is damaged, and its unit is expressed in megapascals (MPa). Shear strength can be divided into tensile shear, compressive shear, torsional shear and flexural shear strength according to the stress mode during testing. The shear strength of adhesives with different properties is also different. In general, the shear strength of ductile adhesives is greater than that of flexible adhesives. A lot of tests show that the thinner the adhesive layer is, the higher the shear strength is. The environmental temperature and test speed have the greatest influence on the test conditions. With the increase of temperature, the shear strength decreases, and with the decrease of test speed, the shear strength decreases. This shows that the temperature and speed have an equivalent relationship, that is, increasing the test temperature is equivalent to reducing the loading speed.
B. Tensile strength Tensile strength, also known as uniform tensile strength and normal tensile strength, refers to the tensile force per unit area in megapascals (MPa) when the adhesive stress is broken. Because the tensile force is much more uniform than the shear force, the tensile strength of general adhesives is much higher than the shear strength. In the actual measurement, under the action of external force, the deformation of the adhesive is larger than that of the adherend, and in addition to the different axiality of the external force, the test piece is likely to have shear and also transverse compression, so simultaneous fracture may occur during the breaking. If the length of the specimen can be increased and the bonding area can be reduced, the effect of peeling at break can be reduced, and the stress distribution can be more uniform. The effects of elastic modulus, adhesive layer thickness, test temperature and loading speed on tensile strength are basically similar to shear strength.
C. Peel strength Peel strength refers to the maximum load that can be borne per unit width when the adhesive parts are separated under specified peeling conditions, and its unit is kN/m. There are various forms of peeling, generally including L-shaped peeling, U-shaped peeling, T-shaped peeling and curved surface peeling. With the change of stripping angle, the stripping form also changes. When the peel angle is less than or equal to 90 °, it is L-shaped peel; when it is greater than 90 ° or equal to 180 °, it is U-shaped peel. These two forms are suitable for the peeling of rigid materials and flexible materials. T-peel is used for peeling when two kinds of flexible materials are bonded. The peel strength is affected by the width and thickness of the test piece, the thickness of the adhesive layer, the peel strength, the peel angle and other factors.
D. Uneven pull off strength Uneven pull off strength indicates the maximum load that the adhesive joint can bear when subjected to uneven pull off force. Since the load is mostly concentrated on two edges or one edge of the adhesive layer, it is the force per unit length rather than unit area, and the unit is kN/m2.
E. Impact strength Impact strength refers to the maximum work consumed per unit bonding area when the bonding part is damaged by impact load, and the unit is kJ/m2. The impact strength is divided into bending impact, compression shear impact, tensile shear impact, torsional shear impact and T-peel impact according to different joint forms and stress modes. The impact strength is affected by the toughness of the adhesive, the thickness of the adhesive layer, the type of the adhesive, the size of the test piece, the impact angle, the environmental humidity, the test temperature, etc. The better the toughness of the adhesive, the higher the impact strength. When the modulus of adhesive is low, the impact strength increases with the increase of adhesive layer thickness.
F. Endurance strength Endurance strength refers to the maximum load that can be borne by the unit bonding area after the adhesive is subjected to static load for a long time, expressed in megapascals (MPa). The rupture strength is affected by loading stress and test temperature, and decreases with the increase of loading stress and temperature.
G. Fatigue strength Fatigue strength refers to the maximum stress that will not cause damage when a certain load is repeatedly applied to the adhesive joint for a specified number of times. Generally, the fatigue strength at 10 times is called the fatigue strength limit. Generally speaking, the adhesive with high shear strength has low peeling, bending and impact strength; The adhesive with high peel strength has high impact and bending strength. Different types of adhesives have different strength characteristics.
