In coatings, construction, and other adhesive applications, adhesion and bond strength are two critical performance indicators. Although both involve the bonding effect between materials, they differ in terms of definition, test method, scope of application, and influencing factors. This article will elaborate on the difference between adhesion and bond strength in light of relevant standards and technical backgrounds to help better understand these two concepts.
1. Definition and test of adhesion
Adhesion, as defined by GB/T 5206-2015 / ISO 4618: 2014, refers to the adhesion phenomenon that occurs through the action of molecular forces at the interface between a solid surface and another material. In simple terms, adhesion refers to the adhesion between the coating or adhesive and the substrate, which determines whether the coating can adhere stably to the surface of the substrate.
Pull-off adhesion is a commonly used test method to measure adhesion. This method tests the force required for the failure of the coating or the adhesion between the coating and the substrate by applying a vertical tensile force at a defined rate. The test results are usually measured in MPa and reflect the degree of adhesion of the coating and the interlayer adhesion between the coatings. This method can be used not only to test the adhesion of the coating, but also to comprehensively evaluate the overall adhesion of the coating.
Relevant standards:
GB/T 5210-2006 / IDT, ISO 4624: 2004
ASTM D4541-2022 Test Method for Determining the Tear-Off Strength of Coated Layers by Portable Adhesion Tester
ASTM D7234-2021 Test Method for Pull-up Adhesion Strength of Concrete Coatings Using a Portable Pull-Off Adhesion Tester
Pull-up adhesion testing is commonly performed on hard substrates such as metals and plastics, and is suitable for thinner coatings, and is mainly performed in a laboratory environment.
2. Definition and test of bond strength
Bonding strength refers to the maximum tensile force that the bonded material can withstand, usually expressed as the tensile force per unit area. It is an important indicator to measure the performance of adhesive materials in the bonding process. The strength of the bond depends on a number of factors, including the nature of the adhesive, the amount of coating, the type of substrate, and the application environment.
Unlike adhesion, the test of bond strength is often used for materials such as thicker coatings or putty. The test method focuses on measuring the tensile strength of the binder layer, especially in the fields of architectural coatings, fire retardant coatings and floor materials. Common bond strength test methods include tensile bond strength test, which is often used to evaluate the adhesion performance of basic materials such as concrete and masonry.
Relevant standards:
JC/T 2079-2011
JG/T 24-2018
GB/T 9779-2015
GB/T 16777-2008
Bond strength usually does not need to describe the failure state, and the test results directly reflect the tensile strength of the material, which is widely used in the construction industry.
3. The main difference between adhesion and bond strength
Test Object:
Adhesion is mainly used for thinner coatings, commonly used in surface treatments such as industrial paints, focusing on the adhesion effect of the coating to the substrate (such as metal, plastic, etc.).
The adhesive strength is mostly used for thicker coatings, putty and floor materials, etc., and the test objects are usually architectural coatings, floor coatings, fire retardant coatings, etc., mainly to investigate the adhesion performance of the coating in a large area.
Substrate Type:
Adhesion is usually tested on hard materials such as metals and plastics.
Bond strength is mostly used on rougher substrates such as concrete, mortar, masonry, etc., and in some cases, especially on mortar substrates, the test results may be distorted due to the strength limitations of the substrate itself.
Destruction Mode:
The adhesion results not only reflect the tensile value of the coating, but also describe the mode of coating failure, such as the failure of substrate cohesion, the failure of adhesion between coatings, etc. By observing the failure state, the adhesion of the coating and the matching properties of the coating can be further analyzed.
Bond strength test results typically report only the maximum tensile force value, focusing on the strength of the material itself rather than the failure mode.
Influencing factors:
Adhesion is greatly affected by factors such as coating thickness, surface roughness, and adhesive wetting ability. In the case of thin coatings, the surface treatment and uniformity of the coating are particularly critical.
Adhesion StrengthIn addition to the adhesive's compatibility with the substrate, the surface treatment and the roughness of the substrate can also significantly affect the bond strength. Thicker coatings can have defects such as bubbles or uneven application, resulting in a loss of strength.
Adhesion and bond strength, two important measures of a material's adhesion performance, have many similarities, but they differ in what they are tested, how they are used, and where they are used. Adhesion is mainly used in the field of industrial paints and coatings, focusing on testing the adhesion of thin coatings, while bond strength is more used to evaluate the performance of architectural coatings and other thicker coatings. Each of the two plays an important role in practical applications, and understanding their differences can help to select the appropriate test methods and evaluation criteria for different fields and environments.
Further reading:
Coating adhesion is a key indicator to evaluate the adhesion ability of the coating to the surface or coating of the coated object, and good adhesion is essential for the protective effect of the coating. Coating adhesion is affected by a number of factors, including the adhesion of the coating to the substrate, the quality of the application, and the type of substrate. Adhesion is usually classified as chemical, mechanical, and polar.
Common coating adhesion testing methods include:
Circular Drawing Method – Drawing circles on the coating at a uniform speed using a stylus to assess adhesion by observing the degree of damage to the coating. This method is suitable for testing thin coatings and can reduce operating errors using electrically driven instruments.
Scratching method – by cutting a lattice on the coating, the degree of damage to the coating is checked. This method evaluates the adhesion of the coating in different directions, commonly found on thickly coated materials.
Adhesive tape method – Adhesion is assessed by attaching and tearing off the tape and observing the peeling off of the coating. This method is simple and is commonly used for rapid testing in the field.
Pull off method – the scraper is applied to the coating and the load is gradually increased until the coating comes off. This method allows for more accurate quantification of coating adhesion and is suitable for the evaluation of thicker coatings.
