Adhesion Properties of Paints on Applied Surfaces

In paint terminology, adhesion is the expression in force/area of the resistance that a cured paint film shows to mechanical removal from the solid surface to which it is applied. In multi-layer systems, the term inter-coat adhesion is expressed as the resistance of any paint film layer to separation from the layer beneath it. Good Adhesion (or inter-coat adhesion) is a requirement for the following coating performance criteria: • Elasticity, • Physical, chemical and external resistance, • Corrosion, moisture and water resistance are indispensable properties for the overall performance of the paint system.

A Paint's Good Adhesion Property Depends on the Following Parameters:

I. Surface Preparation: Prior to painting, depending on final quality expectations, it will be necessary to use one or a combination of the following surface preparation processes to increase adhesion, impart additional properties and improve performance characteristics. Grease, silicone or wax-based mold release agents, stains from hand contact, dust and dirt on the surface, and electric charges forming on the surface have very significant effects on paint performance and appearance, including adhesion.

Basic Surface Preparation Operations

Metals: Sandblasting, cleaning, degreasing, chemical treatments. Plastics: Degreasing, flame treatment, adhesion primer application. Wood: Cleaning, fumigation, sanding. Glass: Cleaning, degreasing, abrasion with cerium oxide.

II. Application and Curing Conditions:

Selection of the correct application method for each use (spraying, rolling, brushing, dipping, curtain coating, etc.) is important. Within the correct application method, viscosity, correct thinner and hardener types/mixing ratios, not exceeding the recommended pot life in 2K systems, and compliance with flash off and oven curing times and temperatures are very important not only for good adhesion but also for overall performance properties.

III. Selection of Appropriate Paint System:

Just as the materials to be painted have different physical and chemical properties, so do the end user's expectations for these materials differ. In particular, metal surfaces such as galvanized steel and aluminum, plastic surfaces such as polypropylene, polyethylene and PVC, and glass surfaces are critical examples for the selection of the correct paint systems from an adhesion perspective. Additionally, each product within the system must be compatible with one another. To better understand the subject, it will be helpful to recall some basic concepts in paint terminology:

1) Cohesion:

It is the force of attraction between molecules of a material. The cohesion of a paint film is the force of attraction that the molecules of a cured paint film exert on one another. Generally, the expectation is that the paint film does not break internally from mechanical effects nor does it separate from the surface to which it is applied. For this to happen, adhesion > cohesion, and as the difference between them increases without weakening cohesion, adhesion will be better.

2) Surface Tension:

For solids and liquids, the energy levels between molecules at interfaces are higher than the energy levels between molecules within the bulk. At liquid-gas interfaces, molecules are pushed toward the bulk; therefore, droplets assume a spherical shape which provides maximum volume with minimum surface area. This means that the molecules at the interface have higher potential energy. To increase the surface area of molecules within the bulk by moving to the interface, a force must be spent to overcome the attractive forces between molecules. The force required to increase unit area at a liquid-air interface is called surface tension and is directly proportional to surface energy. σ = Energy / area = force / length, dyn/cm or mN/m. High surface tension means the liquid has high cohesion.

3) Surface Wetting:

Regardless of which application method is used, good surface wetting by the paint is very necessary for good adhesion; however, it is an insufficient property on its own. The liquid paint must penetrate the indentations and roughness on the surface and wet as wide a surface area as possible, and mechanical anchoring of the cured film to the surface must be provided. Low-viscosity application will provide good penetration and, consequently, better adhesion. Another parameter for good surface wetting is that the surface tension of the surface to be painted must be higher than that of the paint. While direct measurement of surface tension of solid surfaces is not possible, it can be calculated indirectly by utilizing the contact angle created on the surface by a liquid droplet of known surface tension applied to the solid.

Figure 3. Contact angle and surface wettability on two surfaces—one without surface preparation (poorly wetted) and one with surface preparation (well wetted)

As the contact angle approaches zero, σL = σS and the wettability of the surface increases. This means that as the paint's surface tension decreases and/or the surface's surface tension increases, surface wettability increases. In multi-layer paint systems such as automotive coatings, the surface tension of each paint layer must be lower than the surface tension of the layer beneath it, and this surface tension relationship continues like stair steps up to the top layer.

Adhesion Tests

The degree of adhesion of paint films to a surface can be tested using two basic test methods: 1. Cross-Cut Test: BS/EN ISO 2409, ASTM D 3359 3302, 2. Pull-Off Test: EN1542/12004-2, ASTM D4541/D72342. TABLE 3: ADHESION PROPERTIES OF SOME ACRYLIC RESINS TO VARIOUS COMMON SURFACES:   Muhammet Demirkıran Sales Manager, Paints, Inks and Adhesives Group Kempro Kimyasal Mad. ve Dış. Tic. A.Ş.      

References 1." Basics of Coating Technology" by Dr.Artur Goldschmidt & Dr.Hans-Joachim Streitberger 2." Painting of Plastics" by Rose Ann Ryntz 3." Pigments, Paints,Polymer Coatings and printing inks" by R.A.Ryntz 4. "Organic Coatings: Science and Technology"; by Zeno W. Wicks,2007