Adhesives and sealants
Adhesives are used in many different industries, such as in the automotive industry, the aerospace industry and even microelectronics, for joining or gluing metallic parts to other metallic substrates or plastics or ceramic components. In all of these applications, it is important that the strength of the surface-adhesive bond is maintained following the application of an adhesive to the substrate as any changes to this interface can potentially cause a failure of the bond often leading to issues with stability of the machine or vehicle to which it is attached.
One of the most common causes of failure to an oxide-adhesive bond is corrosion of the substrate to which it is attached. Exposure of the adhesive/substrate bond to water can cause a number of points of failure, including on a painted surface where blistering of the coated metal surface and filiform corrosion of the substrate occur due to water exposure. Blistering corrosion occurs when defects are present, either in the paint film, or due to a problem with the preparation of the substrate. Water can diffuse into the paint film through a rupture in the film, or due to the porosity of the paint film. When the water reaches the metal surface, corrosion will take place, weakening the bond between the paint and the substrate. This can then lead to further corrosion underneath the blistered area, causing the paint layer above to lift away from the surface, thereby causing a failure in the adhesive/substrate bond.
Filiform corrosion, or under film corrosion, is a type of corrosion which can occur under thin organic coatings, taking the form of thin, connected filaments. This type of corrosion occurs in relatively high humidity, and mainly in aluminium substrates. For filiform corrosion to take place, the coating has to be breached – exposing the surface to water and oxygen. This then allows for corrosion of the substrate to take place under the surface layer of the coating. As corrosion progresses, a “bubbling” of the coating occurs, lifting away the coating from the surface of the substrate, and leading to a detachment of the coating and adhesive from the metal, resulting in the failure of the bond. Whilst barrier corrosion methods can be used to prevent exposure of the surface to the environment, disruption to the paint surface will quickly lead to corrosion, and loss of adhesion. The best method for prevention of adhesive failure is to protect the surface of the substrate from corrosion, by using an active corrosion inhibitor, such as Inhibispheres®. Specially formulated through an emulsion polymerisation process, Inhibispheres® utilise organic or organometallic corrosion inhibitors which, when exposed to water, will be released gradually through the silica matrix, helping to heal the coating rupture and preventing further corrosion from occurring.
Inhibispheres® offer a green alternative to other more hazardous corrosion prevention methods, such as chromates and zinc dust, whilst still maintaining the corrosion protection needed. Utilising inhibitors which have been specially selected for multiple substrates and paint formulations, allowing for protection of steel and aluminium systems, including for solvent less coatings or high viscosity adhesives. The sustained release of inhibitor from the particles means that Inhibispheres® can actively prevent corrosion time and time again.
Sealants are polymeric materials made of silanes or siloxanes, vegetable oils and resins are utilized to prevent the penetration of moisture and corrosive materials through joints, bolts and surfaces. Generally, a sealant not only covers the surface but also fills the pores of the materials it coats, providing a barrier against corrosive environments. Sealants can be applied a wide range of surfaces such as bridges, driveways, parking ramps, garage floors and reinforced concrete. However, lab studies and practical field results, have shown that sealants without corrosion inhibitors only delay the onset of the corrosion process. For instance, commercial sealants do not protect the reinforced concrete effectively if the chloride concentration increases to more than 0.5% by the weight of the cement . It is therefore imperative to improve the sealant resistance to corrosion in different environments by incorporating some corrosion inhibitors in its formulation. Broadly, sealing treatments can be classed into two categories: organic and inorganic sealants. Organic resins with a high surface tension and viscosity are usually epoxy resin, silicone resins or phenol-based resins.
Accordingly the inclusion of Inhibispheres can help to mitigate corrosion. Their use allows formulators to include an environmentally benign and highly efficient inhibitor into a sealant. The organic and organometallic nature of the inhibitors used in Inhibispheres products have minimal effect in terms of their environmental impact and the controlled release aspect of the products allow for greater control of the release of the inhibitors into the environment. Due to the Inhibispheres products silica matrix, they can be incorporated into many different types of sealant chemistry without effecting the chemistry of the sealant.