The standard “MBN 10518 Test specifications for the adhesion of assembly bonds” developed by the Mercedes-Benz group is primarily based on the standard “ISO 9142 Adhesives - Guide for the selection of standard laboratory ageing conditions for testing adhesively bonded joints” developed by the International Organization for Standardization (ISO). This international standard covers laboratory ageing conditions under which adhesive bonds may be exposed to various environmental influences, be they climatic or chemical, and is used to evaluate the effects of these influences on specific properties.
Adhesive bonding is a process in which the joining of two or more parts is accomplished by solidifying or curing a non-metallic adhesive material placed between the mating surfaces of the parts. The surfaces being joined are not melted but may be heated.
Ageing conditions are applicable to bonded assemblies and can be used to establish a set of tests for the evaluation of an adhesive. The results obtained using the procedures described in this standard may not be valid for determining the service life of a bonded assembly because there is no direct relationship between the test results and the behavior of a bonded assembly under service conditions over a specified period of time. However, for certain specific applications, experience with the procedures may enable a correlation to be established. Ageing conditions should preferably be related to the specific application of the adhesive.
The MBN 10518 § 6.7 standard focuses on thermal aging with variation. Thermal aging is a long-term, irreversible process that affects the structure, composition, and morphology of various compounds at the high temperatures encountered in service. This phenomenon results from a combination of mechanical stress and chemical changes, with atmospheric oxygen-induced oxidation being a major contributing factor. High temperatures, particularly in plastic materials, cause long-chain chain scission, resulting in interactions between polymer chains and a subsequent reduction in chain length, which alters molecular weight and distribution. These changes adversely affect polymer properties, including reduced ductility, brittleness, chalking, discoloration, cracking, and changes in other desirable physical properties.
Adhesive forces are generally based on physical interactions, such as interactions between polar or polarizable groups, hydrogen bonds, or van der Waals forces. When bonding plastics, especially with solvent-based adhesives, diffusion processes can also play a role. In this case, the plastic on the substrate surface is dissolved by the solvent present in the adhesive. This leads to increased mobility of the polymer chains of the plastic, which in turn allows the adhesive chains to penetrate. As a result, additional interactions occur between the polymer chains of the adhesive and the substrate. A solid compound forms after the solvent evaporates. Chemical bonds are also important in certain adhesive/substrate combinations, for example, when bonding glass using silicone adhesives, wood using polyurethane adhesives, and aluminum using epoxy adhesives. Chemical bonding leads to significantly higher adhesion than physical bonding. In addition, the penetration of the liquid adhesive into the substrates can provide additional adhesion after curing.
Achieving adhesion between the adhesive and the substrate not only requires an adhesive with a suitable composition for the substrate, but also places high demands on the substrate surface. Due to the short range of the adhesive forces, the nature of the surface layer of the substrate is of great importance. It must be sufficiently firmly bonded to the body of the substrate. For example, many adhesives adhere well to a corroded steel surface. However, the corrosion layer (rust) is not firmly bonded to the substrate. Under load, failure can occur in the corroded material or between the rust layer and the uncorroded steel. The same applies to coated products. The adhesive must provide adhesion to the coating. The coating must also be sufficiently firmly bonded to the substrate.
The MBN 10518 standard provides a frequently applied test method for determining the adhesive strength of various assembly ties.
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