Designing Soft Reactive Adhesives by Controlling Polymer Chemistry
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Designing Soft
Reactive Adhesives by Controlling Polymer Chemistry Agnès Aymonier and Eric Papon
Abstract Soft reactive adhesives (SRAs) are polymer-based materials (e.g., polyurethanes, polysiloxanes, polydienes) designed to be further vulcanized or slightly cross-linked through external activation (heat, moisture, oxygen, UV–visible irradiation, etc.), either at the time of their application or within a subsequent predefined period. They are used mainly as mastics, or sealing compounds, in a wide range of industrial and commercial fields such as construction, footwear, and the automotive industry. Generally deposited as thick films, SRAs behave as structural adhesives; their low elastic moduli accommodate large strains between the bonded parts without incurring permanent damage. Other outstanding attributes of SRAs are their resistance to solvents, their ability to withstand aggressive environments, and their ease of use. This article discusses examples of SRAs and, more specifically, shows how the cross-linking chemistry, mainly through step-growth polymerization, provides their primary advantages. Keywords: chain polymerization, polyurethanes (PURs), silicones, soft reactive adhesives (SRAs), step-growth polymerization.
Introduction Chemists working in the field of reactive adhesives, that is, adhesives that require a chemical reaction to cure, must provide technological solutions that fulfill two main conditions: The viscosity of the adhesive must be low enough to allow easy wetting of the adhesive on the substrates to be bonded and fast interfacial interactions; and The transformation of the uncured liquid adhesive into a solid bonded joint must be fast (by means of chain or step polymerization, initiated or activated under mild conditions). These principles are the key to finding the best compromise between the characteristics of a formulated polymer-based adhesive and the behavior of bonded joints. Reduced viscosity for better wetting may lead to a weaker bond, so a compromise between properties is often needed. Using the example of soft reactive adhesives (SRAs), we will show how polymer chemistry is able to provide adhesive manufac-
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turers with some innovative solutions. SRAs can be considered as structural adhesives; their low elastic moduli and high adhesion properties accommodate the relative motion of the substrates without damage. This behavior is useful in a wide range of applications for adhesives, mastics, and sealants in many industrial fields, for example, in the automotive, construction, and electronics industries.
Background Various reactive adhesive systems can be designed through good control of polymer chemistry. Two main polymerization mechanisms can be used: chain polymerization, based on the reactivity of monomers such as acrylates and cyanoacrylates; and step-growth polymerization, characterized by addition or condensation reactions, which are involved in the synthesis of polyurethanes or polyepoxides. In the case of chain polymerization processes, highly reactive adhesive
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