Introduction to Adhesive Viscosity and Strength

The adhesive strength is one of the critical indicators for evaluating the quality of adhesives. Adhesive strength is typically categorized based on the types of forces that adhesives experience during use, including shear strength, tensile strength, peel strength, and impact strength.

1. Definition of Adhesive Strength

Adhesive strength refers to the stress required to cause the adhesive in an adhered joint to fail or disrupt at the interface between the adhesive and the adherend or in its vicinity when subjected to an external force. Adhesive strength is also referred to as bonding strength.

Adhesive strength is the stress required for the adhesive system to fail when it is disrupted. Its magnitude depends not only on adhesion, the mechanical properties of the adhesive, the nature of the adherend, and the bonding process but also on joint design, the type, magnitude, direction, frequency of the applied forces, environmental factors (temperature, humidity, pressure, medium), test conditions, experimental techniques, and more. Hence, it is evident that adhesion is just one of the critical factors determining adhesive strength. Therefore, adhesive strength and adhesion are two entirely distinct concepts and should not be confused.

2. Types of Forces Acting on Bonded Joints

The forces exerted on bonded joints can be categorized into four forms: shear, tensile, uneven tearing, and peel.

(1) Shear: Equal and opposite forces act parallel to the bonded surface and are evenly distributed.

(2) Tensile: Also known as uniform tearing, it involves forces pulling in opposite directions, perpendicular to the bonded surface, and evenly distributed.

(3) Uneven Tearing: Also called cleavage, the forces act in the perpendicular direction to the bonded surface but are distributed unevenly.

(4) Peel: Forces are applied at an angle to the bonded surface and are primarily distributed along a straight line on the bonded surface.

In the same adhesive system, it is possible that several of these forces may coexist, and the dominant force depends on the specific application.

3. Classification of Adhesive Strength

Depending on the specific forces applied to the bonded joint, adhesive strength can be categorized into shear strength, tensile strength, uneven tearing strength, peel strength, compressive strength, impact strength, bending strength, torsional strength, fatigue strength, creep resistance, etc.
  (1) Shear Strength 

Shear strength refers to the force per unit area required to cause the adhesive in the bonded assembly to fail under shear stress conditions. It is typically measured in megapascals (MPa). Shear strength can be further categorized based on the mode of loading during testing, including tensile shear, compressive shear, torsional shear, and flexural shear strength. Different adhesive properties result in different shear strengths. Generally, tough adhesives have higher shear strength than flexible adhesives. Experiments have shown that thinner adhesive layers result in higher shear strength.

Factors like environmental temperature and test speed significantly affect shear strength. As temperature increases, shear strength decreases, and reducing the test speed leads to lower shear strength. This suggests an equivalent relationship between temperature and loading speed—raising the test temperature is akin to lowering the loading rate.

(2) Tensile Strength

Tensile strength, also known as uniform tearing or tensile shear strength, refers to the force per unit area that the bonded assembly can withstand before it undergoes tensile failure. It is typically measured in megapascals (MPa). Tensile strength is generally much higher than shear strength because tensile forces are distributed more uniformly. During actual testing, as a result of the greater deformation of the adhesive compared to the adherend and different axes of external forces, shear may occur along with lateral compression during the tear, potentially leading to simultaneous failure. Increasing the specimen's length and reducing the bonded area can help minimize the effect of peeling during tearing and lead to a more uniform stress distribution. The influences of elastic modulus, adhesive layer thickness, test temperature, and loading speed on tensile strength are similar to those on shear strength.

(3) Peel Strength 

Peel strength is the maximum load per unit width that an adhesive joint can withstand when separated under specified peel conditions. It is measured in kilonewtons per meter (kN/m). Peel failure can take various forms, such as L-type, U-type, T-type, and curved peel, depending on the angle of peeling. Changes in the peel angle can result in different peel modes. For instance, peel angles less than or equal to 90° result in L-type peeling, while angles greater than 90° or equal to 180° correspond to U-type peeling. These two forms are suitable for peeling rigid and flexible materials. T-type peeling is used when bonding two flexible materials. Peel strength is influenced by factors such as specimen width and thickness, adhesive layer thickness, peel angle, and peel strength.

(4) Uneven Tearing Strength

Uneven tearing strength represents the maximum load that a bonded joint can withstand when subjected to uneven tearing forces. Since the load is concentrated on the edges or one edge of the adhesive layer, it is measured in kilonewtons per square meter (kN/m2).

(5) Impact Strength

Impact strength refers to the maximum energy consumed per unit area when a bonded joint is subjected to impact loads before failure. It is measured in kilojoules per square meter (kJ/m2). Impact strength can be classified into various forms based on joint design and the mode of loading, including flexural impact, compressive shear impact, tensile shear impact, torsional shear impact, and T-peel impact strength. The magnitude of impact strength is influenced by factors such as adhesive toughness, adhesive layer thickness, adherend type, specimen dimensions, impact angle, environmental humidity, and test temperature. Greater adhesive toughness leads to higher impact strength. When the adhesive modulus is lower, increasing adhesive layer thickness improves impact strength.

(6) Creep Resistance

 Creep resistance refers to the maximum load per unit area that a bonded joint can withstand when subjected to long-term static loading. It is measured in megapascals (MPa). Creep resistance decreases with higher applied stress levels and elevated test temperatures.

(7) Fatigue Strength

 Fatigue strength represents the maximum stress that a bonded joint can endure when subjected to repeated loading for a specified number of cycles without failure. Typically, the fatigue limit is defined as the fatigue strength at 10 million cycles. Generally, adhesives with high shear strength have lower peel, bending, and impact strengths, while adhesives with high peel strength exhibit higher impact and bending strengths. Different types of adhesives exhibit varying strength characteristics.