Enhancement of Adhesion of Silicone Adhesives through Surface-Grafted Molecular Brushes

Untreated polymeric materials in the form of rigid plastic, flexible polymeric film or a decorative coating (e. g., wet paint or powder coating) are frequently not adequately receptive to reactive species available in elastomeric sealants, adhesives, or decorative coatings in terms of reactivity through hydrogen or covalent bonding because of a lack of reactive chemical functionalities at the substrate surface.

The above drawback can be partially overcome by surface activation of polymer surface by commodity “oxidative" processes, such as corona discharge

Cohesive

Fai ure zone

Adhesive

Failure zone

Surface graft density, cr (chains/nm2)

FIG. 6—Interfacial fracture energy versus surface graft density, a, of surface-grafted and chemically bonding (with adhesive) macromolecular connector chains [12].

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014 Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

or flame treatment, which create surface-functional sites, such as OH, C=O, and COOH groups, through which the polymer surface energy and, hence, its wett­ability by adhesives and coatings is improved.

The key drawbacks of such oxidative treatments are: (i) fast decay of surface effects owing to restructuring of the surface because of rotation of functional groups and, hence, short operating time for completing adhesive bonding, joint sealing, or coating, and (ii) a lack of control of substrate surface chemistry and, hence, its reactivity with specific sealants or adhesives.

In this work, we discuss our novel polymer surface-engineering process, which utilizes specific receptor sites created by corona discharge, flame, or oxy­gen plasma treatments, e. g., OH, C=O, and COOH groups, which are, in turn, capable of chemically reacting with designated functional groups available at the ends or branches of specific connector molecules.

The process [13-15] commonly known as SICOR (originally derived from the combination of silane and corona) for fabrication of polymer surfaces “tethered" by designated types and surface architecture of molecular brushes, comprises the following:

• surface oxidation, e. g., by flame, corona discharge, ozone, or UV treat­ment, which are the precursor activation processes for subsequent grafting of specific molecular chains, and

• application of silanes, organo-metallic [16-22], or other polyfunctional chemicals [16,17] containing atomic species or molecules capable of creating ionic or covalent bonds with the receptor groups on the oxi­dized polymer surface. These are applied to modify the properties of polymer surfaces such as surface chemistry and surface architecture in a desired manner.

As outlined in the section on “Surface-Grafted Connector Molecules" above, a certain surface density of macromolecular chains needs to be grafted onto the surface for maximizing the performance of the interphase. To facilitate this, the polymer surface first needs to be oxidized at a required energy input, E, deter­mined by the following expression

E = Ptn (8)

where P = power output (W) of the energy source, e. g., corona discharge or flame burner, t = time of exposure, per unit length, under the electrode or flame cone of width, d (mm) [t = d/V, where V = the treatment velocity of the substrate (mm/s)] and n = number of substrate passes through the energy source.

The energy output, Eu (mJ/mm2), per unit area of substrate is given by

where L is the length (mm) of the treating electrode, flame burner, or other energy source.

Figure 7 provides a schematic illustration of the SICOR process (silane – on-corona discharge treated polymer). In the first step of this process, surface hydroxyl or carboxyl groups are introduced to the substrate surface by

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014

Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.

oxidation. These subsequently provide attachment sites for organo-functional compounds such as silanes, titanates, and zirconates.

The functionality of these compounds, as illustrated in Fig. 7 [6], is chosen to provide a surface reactivity that is compatible with the adhesive, or any other material brought into contact with the surface-modified polymer.

The process allows for the continuous and inexpensive incorporation of a wide range of surface-functional groups onto the surface of a polymeric sub­strate. This provides the possibility of tailoring the surface chemistry of a poly­mer, without altering its bulk properties, to optimize the adhesion between the surface-engineered substrate and adhesive, or other materials.

Full details of theoretical and practical aspects of adhesion enhancement of a variety of adhesives to representative types of difficult-to-bond polymers such as polyolefines, e. g., low-density polyethylene (LDPE) or polypropylene, and to other engineering polymers through commodity surface-treatment processes such as corona discharge and flame treatment as well as our novel surface – tethering technology, the latter comprising surface activation by corona dis­charge or flame oxidation followed by surface grafting of an amino-functional silane or polyethylene imines as polymeric pre-cursors of molecular brushes, are presented in [16-21] and [32]. A rigorous discussion on surface chemistry of polyolefinic substrates subjected to these alternative types of treatment is also provided in Refs 19, 21, and 32, and hence, the authors of this paper refer the reader to these specific publications for details not covered in this paper.

Experimental