Adhesion Promoters

Adhesion promoters are additives that increase the mechanical separation resistance of paint or coating from any surface. In essence, they increase the adhesive strength of paint or coating. Most often, these compounds contain two different functional groups. The first interacts with the surface while the second interacts with the binder of the paint or coating. Examples of different coupling agents include trihydrolyzable silanes; mono-, di- and tetrahydrolyzable titanates; and chromium complexes found in nature. Adhesion Promoters. These additives are particularly important for metal surfaces to be painted or coated, especially in terms of their instability and lack of stability. Pure metal can always oxidize to form metal oxide on its surface. This oxidation process accelerates in environments exposed to moisture, oxygen and salts. Nearly all paints and coatings contain micro-voids into which small molecules such as oxygen and water, as well as ionic substances, can diffuse. If the paint or coating remains bonded to the metal, then the damage caused by these diffusing substances will not occur. In other words, corrosion will be prevented. For this reason, it is very important to maximize adhesion to the substrate. In some materials, the surface can be mechanically roughened to increase the surface area for physical absorption. Because of their tight bonding to phosphated surfaces, zinc/iron phosphates and various other materials used in chemical pre-treatment will delay metal access and thus prevent corrosion. Organofunctional silanes are typically used as adhesion promoters in paints or coatings because they provide a polar functional group that contributes to bonding to a mineral surface. They can also be hydrolyzed and provide surface activity along with wetting ability. Silanes are moisture-sensitive and hydrolyze to silanol over time. While this is not a problem in solvent-based paint and coating systems, it can cause problems in water-based systems. Silanes react with the surface in a way that creates covalent bonds both with the polymer and along the surface. Silane adhesion promoters are generally used in urethane, epoxy, acrylic and latex systems. A range of silane adhesion promoters are used in the market. Their reactivity characteristics differ depending on the resin or adhesive. Silanes can be produced with amine, epoxy, mercaptan and other materials, generally in more than one functional group. Some examples are given in Table 1. Silane adhesion promoters increase initial adhesive strength and simultaneously stabilize the surface to improve the durability and permanence of bonded fasteners in moist aging environments.

Table 2 shows lap shear values demonstrating the improvement in bond strength when silane coupling additives are added, particularly to nitrile phenolic type adhesive formulations. Silane-based coupling agents also have the ability to increase environmental resistance of aluminum, titanium and stainless steel joints.

Table 3 lists some general organosilanes. These commercially available silanes represent only a small portion of organosilanes. The structural formula of glycidoxypropyltrimethoxysilane (GPTMS), the most frequently used organosilan adhesion promoter for bonding to metal surfaces, is shown below.

Organosilanes form strongly adsorbed films on inorganic surfaces. The durability of these films depends largely on the chemical and physical character of the surface. Organosilanes are quite effective at bonding to active metals containing aluminum, steel, cadmium, copper, nickel and zinc. This also applies to aluminum oxide and treated polymer surfaces where plasma or treatment processes provide hydroxyl functionality. Figure 1 shows the relative effect of surface type on the effectiveness of organosilan coupling agents in improving adhesion to inorganic surfaces. It should not be forgotten that organosilanes bind excellently to smooth, high-energy surfaces and this is much less the case for rough, irregular surfaces. Organosilan adhesion promoters are generally not suitable for bonding polymers to surfaces lacking active hydroxyl functionality. In addition, no application is made for the bonding of graphite or precious metals such as gold, silver and platinum to polymers. These surfaces have no hydroxy "arms". However, organosilanes can bind on polymeric surfaces in the following cases: 1. When there are inorganic fillers or reinforcement buffers on the surface (usually through processing or wear), 2. When the plastic material itself provides hydroxyl functionality along the polymer chain, 3. When the plastic material is subjected to surface treatment to create hydroxyl functionality (for example; corona and plasma treatment). Organosilan adhesion promoters can be applied directly to the surface in a manner similar to primers or can be applied by mixing with the adhesive itself. When applied directly to the surface, the film thickness is very thin as a single coat. When mixed with the adhesive, the coupling agent has the ability to penetrate the surface and can react with the surface as the adhesive cures. Silanes form strongly adsorbed polysiloxane films on ceramic and metal surfaces. The chemical and mechanical durability of these films depends largely on application parameters such as solution concentration, solution pH, drying time and temperature. The quality of the applied surface can also affect polysiloxane film structure. Silanes are applied as a primer coat by brush, dipping, wiping or spraying to the layer surface. Film thickness is less than 8 microns. The solvent in the silane system is removed by drying at 50–60 °C for 10 minutes. The advantage of the primer application method is effective use of silane material and minimal stability problems. Disadvantages are that it is a two-step process and the transparency of the silane coating is difficult to see except for pigmented ones. Therefore, doubt arises as to whether the entire surface is coated. Recipient inorganic surfaces consisting of elements such as Si, Al, Ti and Fe are surfaces that can bond to hydroxyl groups. In contrast, surfaces such as boron and alkaline earth metal oxides, which are non-accepting, do not form stable covalent bonds with silanols. A range of different commercial silane coupling agents are used in paints and coatings. They are generally effective at usage rates of 0.05 to 1.0%. Methacrylic phosphate monomers are used that enhance adhesion to metal, concrete, glass and other inorganic surfaces and can be used in both water and solvent-based formulations. Some methacrylic phosphate monomers not only enhance metal adhesion but also significantly increase corrosion resistance. Additionally, there are acrylic phosphate functional monomers that enhance adhesion to different metal surfaces. Acrylic reactive groups provide higher reaction rates in UV and EB cure applications. Other adhesion promoters found in the market can be listed as follows: • Titanates (for example, isopropyl tris-[N-ethyl aminoethylamino] titanate), • Zircoaluminates, • Zirconates, • Aryl/alkyl phosphate esters, • Special metal organic compounds. Care must be taken when using titanates and zirconates in water-based systems due to their excessive moisture sensitivity. It is claimed that such a problem does not exist in neo-alkoxy products. Alkyl/aryl phosphate esters, zirconaluminates and metal organic adhesion promoters are stable in water-based paint systems. They are chemically quite different in structure and therefore must be evaluated separately in formulations. Epoxy/methoxy functional additives are effective in promoting adhesion of various paint and coating systems to glass, aluminum and steel surfaces. Methacrylate/methoxy functional additives provide adhesion of free-radical cured resins such as polyacrylates to inorganic surfaces. Epoxy functional silanes increase the adhesion and water resistance of a range of paint and coating systems to inorganic surfaces. When adhesion to glass or metal surfaces is required, amine/methoxy functional additives increase the adhesion and water resistance of the paint and adhesive. Titanates, zirconates and aluminates are used as coupling agents. Titanates serve as an organometallic chemical bridge between two different phases. For example, they can be used as an adhesion promoter for paint on a metal surface or between an inorganic pigment and a polymer binder. These additives serve as an alternative to silane coupling agents for silane-non-reactive surfaces such as CaCO₃, carbon black and phthalo blue. All adhesion promoters have different properties. They are two-functional surface-active agents. They are generally used at the lowest possible concentrations based on solid resin. Adhesion promoters generally consist of molecules carrying short organic chains with different chemical composition at both ends of the chain. One end is an organofunctional group specifically compatible with the adhesive in question. The other end of the chain is an inorganic functional group specifically compatible with the surface in question. The best results can be obtained by using adhesion promoter as a surface primer. However, some effects can be added to the adhesive. Titanate type adhesion promoters have six different functions: 1. To provide a hydrophobic and oleophilic bonding to create an atomic monolayer on inorganic surfaces, thereby ensuring complete distribution and dispersion of pigments and fillers with the lowest shear and working energy. It improves adhesion, provides noticeably lower system viscosity and changes critical pigment volume concentration (CPVC), 2. It provides lower baking times and lower baking temperatures. It increases mandrel flexibility and reverse impact resistance, 3. It forms heteroatom phosphate, prevents corrosion and has flame-retarding effect, 4. It provides alkyl/aryl functionality by creating polarity for adhesion and compatibility, 5. It provides thermoset functionality that intensifies cross-linking and hardness degree, 6. It creates molecular structure for a stable organometallic bond. The six functions of titanate (or zirconates) compared to silanes are given below. This comparison may help explain performance differences. Function (1) creates surface reaction mechanisms with filler/fiber layer. Functions (2)–(6) involve polymer/curing reactivity. In the simplest terms, the function (1) mechanism can be called proton (H⁺) through reactive solvolysis (mono alkyl) or coordinative (neo alkoxy) without requiring condensation water. The silane function (1) mechanism can be called hydroxyl (OH) reactive through a silanol-siloxane mechanism requiring condensation water. Silane's silanol-siloxane, water condensation mechanism is limited at temperatures below approximately 100 °C, thus eliminating the possibility of in situ reaction of thermoplastic or elastomer above 100 °C. Solvents that provide homogeneous mixing of two immiscible liquids are also called coupling agents. M. Namık Kayaalp Chemical Engineer Ecelak Boya Kimya San. Tic. Ltd. Şti.

References:

1. SpecialChem | Edward M. Petrie - Mar 21, 2011.

2. Cohen, L.B., "Adhesion Promoters", Industrial Finishing, July 1993.

3. Plueddeman, E.P., Organosilane Coupling Agents, Plenum Press, New York, 1982.

4. Parker, A. A., "A Technical Review of Organosilanes and Adhesion", Polymer Synergies LLC, Mullica Hill, NJ, 2001.

5. MacMillan, J.H., "Using Organosilanes as Adhesion Promoters", presentation of United Chemical Technologies, Inc. 2004.

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