Engineered rubber-to-metal devices enable key performance attributes for automotive, industrial and aerospace applications. These critical parts rely on a dependable bond being formed between the rubber and the substrate (typically metal) in order to be successfully used in service. Historically, several methods have been employed in an effort to achieve a dependable rubber-to-metal bond. Initial efforts utilized ebonite, brass plating, multiple tie coats, and polyisocyanates as a means for bonding; however, these methods each had their own shortcomings.
In 1956, a substantial advancement in rubber-to-metal bonding was made when LORD Corporation commercialized its Chemlok® 205/220 system. This primer/adhesive system allowed for bonding to a wide variety of available elastomers, while enabling substantial improvements in service performance. Since this time, numerous advancements have been made with rubber-to-metal adhesive systems to meet the changing requirements of the markets they serve.
Today, rubber-to-metal adhesives are commercially available in a wide variety of forms to meet the needs of the end user. Both one- and two-component systems can be used, and they are available under the following categories: solventbased, low-HAPs (Hazardous Air Pollutant), environmentally preferred, and aqueous (water-based). Materials Engineers must carefully consider the importance of selecting a proper adhesive system for their application as these bonds must be capable of withstanding potentially harsh exposure environments. This page will discuss the adhesion science involved in rubber-to-metal bonding as well as provide some general considerations for achieving a robust rubber-to-metal bond.
Historically, several bonding methods have been employed within the rubber industry in an effort to achieve a dependable and durable rubber-to-metal bond. Modern adhesive technologies utilize both singleand two-coat adhesive formulations, with the preference depending upon the service needs of the bonded assembly. For example, a two-coat system would be preferred for applications involving exposure to harsh environmental conditions, such as automotive applications. For the purposes of this paper, two-coat systems will be the focus.
Two-coat adhesive systems are comprised of a primer layer and an adhesive topcoat. The primer provides a means of obtaining robust adhesion to the metal substrate while providing the necessary reactivity with the adhesive topcoat. A primer also provides heat- and corrosion-resistant properties that are desirable for applications involving harsh environmental service conditions. A typical primer is comprised of film-forming polymers, crosslinkable resins, fillers, and a solvent or water-based carrier system.
An adhesive topcoat is applied to the dried primer. The topcoat provides a means of reacting with unsaturation in the elastomer backbone, typically during the molding process. The topcoat also must react with the primer layer such that adhesive and primer intercoat adhesion is achieved. A typical adhesive topcoat would be comprised of curatives, film-forming polymers, fillers, and a carrier system.
Within the bonded assembly, there are various interactions occurring at each of the interfaces such that overall adhesion is achieved. The primer-to-metal interface involves adsorption and/or chelation of the primer components at the surface of the metal. The primer-to adhesive interface obtains intercoat adhesion through the adsorption and/or diffusion of respective components within the layers. Crossbridging of reactive topcoat chemistries into the primer and the elastomer also occurs at each of these respective layers’ interfaces. Within the adhesive and primer layers, internal crosslinking of the polymeric systems also occur.
The layers of components within the bonded assembly are arranged in such a way that they are progressively decreasing in modulus from the primer to the elastomer. This can be seen in the bonded assembly illustrated in Figure 1. The reason for this arrangement is to provide a gradient in the stiffness change between the substrate and the elastomer, eliminating a sharp interface.