5.2.2 Factors for Bond line and Assembly Strength

Calculating the bond line or assembly strength is a complicated yet essential step in the validation of an adhesive for a specified application. Documents such as TDS, white papers and adhesive/chemistry specifications will contain adhesive properties, benefits and limitations, specific substrate bond strengths, end user-specific requirements and real application testing data. The starting point for any assembly strength calculation or assessment should be the relevant technical literature provided. Technical data sheets (TDS) and Test Data

A product-specific TDS is a technical document containing the product description and some basic facts about the product, uncured and cured characteristics of the adhesive, general information on surface preparation and recommended application methods. The TDS will also provide product-specific technical information, test data regarding the performance of cured adhesives on common substrates and environmental resistance data. The American Society for Testing and Materials (ASTM) standards or International Organization for Standardization (ISO) test methods associated with the test data are commonly provided. The data published on the TDS is gathered using production batches to ensure it is as applicable as possible to real-world applications.

When trying to determine suitable adhesives and sealants, a product TDS is a great resource to gain an understanding of the product’s benefits and limitations; typically, these will be listed as hardness, shear strength, elongation at break, chemical resistance and temperature resistance. Data relating to adhesive performance when cured has been measured under specified conditions (such as temperature and humidity), which are typically listed on the TDS.

Bond strength data are typically listed in [N/mm2] and [psi] and in the ‘Typical Performance of Cured Material’ section. Physical properties of the cured adhesive under specified conditions, independent of substrate, are also listed; these include shore hardness, elongation, tensile strength and other physical test data where applicable. The specific data listed on a TDS will vary by adhesive type and chemistry; however, the most important characteristics of a specified adhesive or chemistry will always be included. This section also contains cured bond strength data as it relates to various common or representative substrates; again, these data are measured under standardised curing and testing conditions which are typically listed on the TDS.

Bond strength data are gathered based on standardised test methods such as ASTM, ISO and DIN depending on the type of stress the bond is exposed to. The specifications of test procedures, such as repetitions, temperature and substrate type, are typically specified in the standard test method documentation. The majority of the reliable adhesive joint strength tests fall into four categories: tensile, shear, and peel and compression.15

An analysis of the TDS of the adhesive will provide a starting point for conducting preliminary trials under production conditions. While laboratory testing may help validate adhesives in a specific application, these production trials are an essential step before a final decision can be reached.16

When relying on TDS information, it is important to remember that many of the characteristics listed on the data sheet may be affected by multiple factors such as cure time, substrate type, surface preparation methods, bond line gap, bond area, application temperature and relative humidity (see Section 4). It is absolutely critical to understand that real-world production or maintenance conditions may not always match the controlled conditions under which the adhesive has been cured and tested.

15) A. Pizzi, and K. L. Mittal. Handbook of Adhesive Technology, M. Dekker, 2003.

16) Loctite North America. Worldwide Design Handbook. Rocky Hill: Loctite European Group and Loctite North America, 1998.

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