The Mechanism of Adhesion Improvement of Elastomeric Silicone Sealants to Difficult-to-Bond Polymeric Substrates through Reactive or Interpenetrating Molecular Brushes

ABSTRACT: High-quality and durable adhesion of elastomeric adhesives to metallic, ceramic, and organic substrates is essential to a broad range of industries, e. g., building and construction, automotive, electronic, aerospace, biomedical, and others. The principles of engineering substrate surfaces through grafted connector molecules are discussed in this paper. In particu­lar, two important modes of interaction for surface-grafted “molecular brushes” are investigated and experimentally verified. It is demonstrated that the inclusion of silicone – and/or amine-terminated graft molecules, such as silanes or polyethyleneimines, at polymer interfaces, results in the formation of strong molecular bridges between a range of organic substrates and elas­tomeric sealants leading to significantly improved bonding. The technology has been successfully adopted by global automotive industry for improving adhesion of a variety of adhesives and coatings to polyolefinic substrates.

KEYWORDS: adhesion, surface engineering, durability, silicone adhesion, organic substrate, metallic substrate, molecular brushes, silicones, polyethyleneimines

Introduction

Elastomeric silicone sealants are an important class of structural adhesives commonly used by the building, automotive, aerospace, electronic, and other industries.

Specifically formulated silicone adhesives are presently the only materials that possess all the necessary properties for meeting the demanding needs of these industries: reliable adhesion to a variety of substrates, elastomeric proper­ties that allow accommodation of both thermal and structural movements of the bonded components, and, finally, adhesive and cohesive properties that are little affected by ultraviolet (UV) radiation and other environmental factors.

Whereas the outstanding bonding capacity of structural silicone adhesives to glass and metallic surfaces is well documented, the mechanism of adhesion to other types of substrates is not yet fully understood. For instance, various decorative or functional finishes, such as wet paints, powder coatings, or self­cleaning coatings applied to building facade components, exhibit a broad range of decorative and functional attributes but frequently may have surface proper­ties that adversely affect the adhesion of sealants.

One particular example of these are fluorinated powder coatings, which ex­hibit outstanding long-term durability guaranteed for up to 25-30 years and, yet, require careful assessment of adhesion performance and frequently need priming to ascertain long-term sealant adhesion and facade integrity.

It is well known throughout the industry that a great number of paint fin­ishes and metallic or polymeric substrates create various degrees of difficulty in attaining satisfactory long-term adhesion.

In many cases, the improvement of adhesion requires the use of a primer. It is applied to the substrate as a “tie layer" acting as a sealant/substrate compati – bilizer, which promotes improved adhesion. In some cases, however, this approach is unsuccessful, resulting in a need to change the substrate or the type of surface finish. Sometimes, despite good initial results, the adhesion problems become apparent during the service life of a structure.

This is caused by the lack of durable, chemical bonds between either the sil­icone adhesive and substrate, or across the interfaces between the primer and the substrate, or that between the primer and silicone adhesive.

In this paper, we demonstrate and examine an effective process that pro­vides the potential for improved adhesion of silicone adhesives to selected types of difficult-to-bond polymers, such as polypropylene (PP)-ethylene-vinyl acetate (EVA) blend, and polyacetal, through the use of surface-grafted, chemically reac­tive connector molecules based on organo-functional silanes. These results are then compared with those from other types of surface-grafted macromolecules.

The new surface-engineering process enables simple on-line surface engi­neering of a broad range of architectural substrates, e. g., paint finishes or powder coating, anodized aluminum, rigid plastics, polymeric films, etc. The chemical

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composition and the nano-scale topography of the substrate surface are con­trolled to optimize the durability of the adhesive/substrate interface.

This new process opens up new opportunities to the end-users of silicone – based and other generic types of adhesives. The following are regarded as the main potential advantages for any commercial application of this technology:

• Longer warranties for applications involving the adhesion of silicones to a wider range of substrates,

• Improved potential for exploiting problematic substrates, such as polyo­lefins, poly(vinylidene fluoride) (PVDF), siliconized coatings, organic – dyed anodized aluminum, stainless steel, etc.,

• Greatly reduced need for solvent-based primers,

• Simplification of the design techniques for structural silicone adhesives, as the sealant cohesive strength itself becomes the main design parame­ter because of the improved adhesion, and

• Simplification of the adhesive chemistry.

The technology discussed in this paper has been successfully adopted by the global automotive industry for improving adhesion of a variety of adhesives and coatings to polypropylene-based substrates (body-trim panels, instrument panel, and door-trim panels).