Adhesion is the ability of a material to form a joint between two substrates. It is one of the most crucial properties of coatings yet one of the most complex and challenging to understand. Without proper adhesion, all other coating characteristics deteriorate, and it cannot comply with its protective or decorative functions.

What are the Critical Factors that affect Adhesion?

Although the phenomenon of adhesion partly remains a puzzle until today, several parameters play a pivotal role:

1. Surface Roughness
   preparation of the surface such as roughening before applying the coating can either enhance or diminish adhesion. If the coating formulation has good substrate wetting and low viscosity upon application, it can effectively penetrate the surface irregularities and form ‘lock and key’ structures, thereby enhancing adhesion. On the other hand, if the coating does not wet the surface completely, voids with air or water droplets may be left, leading to a decrease in the interaction between the coating and substrate, compromising adhesion and corrosion performance.

2. Wetting and Surface Energy
   A coating formulation should be able to spread easily on the surface and form a homogeneous film upon solvent evaporation. A general rule is that good wetting is possible only if the liquid’s surface tension is lower than the substrate’s surface energy. For instance, the surface energy of metals is higher than that of plastics, making it easier for a coating formulation to wet a metal surface than a plastic one, given similar roughness. This is also why solvent-based formulations have better wettability than water-based ones (water surface tension is much higher than organic solvents). To enhance the wetting of water-based formulations, surface active agents are added to lower surface tension.

Surface energy and surface tension have the same units (N/m) and can be compared directly. The former is characteristic of solids, and the latter is specific to liquids.

3. Physical & Chemical Interactions

Strong interactions between the coating and surface are of utmost importance. The strongest adhesion can be achieved with the covalent (permanent) attachment of coating molecules to the substrate surface. In this case, the coating becomes permanently attached to the surface and can only be removed by thermal or oxidative degradation. The presence of functional groups that can create hydrogen bonding (carboxy, amine, hydroxy, urethane, amide, phosphate) can also significantly increase adhesion.

The high content of hydroxy groups formed during the cross-linking process and the presence of strong ether linkages are the reason for the excellent adhesion of epoxy resins.

How can Adhesion Properties of Epoxies be improved?

Now that we are familiar with the phenomenon of adhesion, we can consider specifics to improve the adhesion of epoxy resins:

• Lower molecular weight resins have higher molecular mobility than high molecular weight polymers. Hence, smaller molecules can better penetrate rough substrates, leading to better adhesion. Similarly, low-viscosity formulations allow to achieve better adhesion.
• High temperature-cured epoxies have higher adhesion than ambient temperature-cured epoxies. Viscosity decreases at increased temperature providing better wetting of substrate cavities and improving adhesion.
• Using a slow-evaporation solvent can ensure the time is given for polymer molecules to rearrange and form bonds with the surface. Therefore, the adhesion will be stronger than for the system with a fast-evaporating solvent.
• High rigidity of epoxy macromolecules permits only partial interaction of hydroxy groups with the substrate surface. Free hydroxy groups can interact with the rest of the coating, increasing cohesion. If all hydroxy groups of the resin interact with the substrate surface, poor interlayer adhesion within the coating will lead to delamination.

How do Epoxy Adhesives behave with Different Surfaces?

Epoxy resins have extensive applications in many industries, such as marine, automotive, aircraft, packaging, and architectural coatings and adhesives. Thus, they require good adhesion to substrates with a broad spectrum of properties—surface energy, roughness, and chemistry. Here, we consider each type of substrate, possible application challenges, and specifics for epoxy systems.

1. Epoxy Adhesion to Metals

Metal surfaces, such as steel and aluminum, are generally covered with metal oxides with high surface energy and polarity. The structure (phase) and thickness of metal oxide may be advantageous for adhesion or may even compromise it. Epoxy resins have different polar functional groups capable of forming hydrogen bonding (hydroxy, ether, and amine) with polar metal oxides. However, even dry metal surfaces contain tightly bound water layers, which must be removed before any coating application. Otherwise, adsorbed water layers may interfere with metal oxide/polymer interactions and reduce adhesion.

Epoxy primers are widely applied in the automotive, aerospace, and packaging industries, as well as architectural coatings. It is important to remember that proper metal surface pretreatment is crucial for good adhesion. As such, metals are often contaminated with oil after manufacturing (processing lubricants), which can be removed with solvent rinsing or high-pressure wash. Roughening using sandblasting or acid etching is also desired for architectural substrates to improve adhesion (removes weak oxide layers and roughens the surface), especially for stainless steel. Initially, when the formulation is liquid, epoxy and curing agents may penetrate substrate cavities and, upon curing, mechanically interlock with the substrate. However, sandblasting is unsuitable for automotive coatings due to high requirements for substrate smoothness.

2. Epoxy Adhesion to Wood

Clearcoats and glues are the most common applications of epoxy resins for wood substrates. The wood substrate can be coated with epoxy resin or soaked with resin, increasing its water and aging resistance and protecting it from microbial/fungi growth. Although wood is usually not difficult to bond, thanks to its high porosity and polarity of surface, there are several specifics to consider when applying epoxy resins on wood substrate. It is important to understand that substrate preparation will greatly affect the adhesion; for instance, too intense sanding can destroy the wood cells and cause weak interfacial adhesion (ultimately leading to delamination). Also, too extensive smoothing of a surface will not satisfy the requirements for surface roughness, leading to poor adhesion. Another important aspect is that wood is a so-called “living” substrate, constantly undergoing moisture absorption-desorption and changing substrate dimensions. Because polymer dimensions remain almost unchanged, this mismatch in polymer and wood dimensions may cause weak adhesion and delamination.

3. Epoxy Adhesion to Concrete

Epoxy resins are widely used as floor coatings for concrete, and many commercial products are available mostly as two-component systems (epoxy and cross-linking agent). Concrete is a highly porous material that can absorb a lot of moisture in humid conditions. To prevent that, epoxy resin is applied to a concrete foundation and can efficiently seal all the pores, increasing moisture resistance and preserving its mechanical properties. The disadvantage in this case is the poor UV-light resistance of epoxy resins, which limits their application to indoors only.

Nowadays, many systems have been developed to enhance mortar and concrete durability and aging resistance. For that purpose, epoxy resin can be used as a single binder or as a co-binder with cement. A formed composite is called polymer cement concrete (PCC) or polymer cement mortar (PCM). Such composites have enhanced mechanical properties, chemical resistance, and permeability compared to bare concrete and are extremely effective in repairing old concrete structures.

Excellent adhesion to metals, in combination with decreased water and Chloride ion permeability, provides increased corrosion resistance of rebar in polymer concrete composite.

4. Epoxy Adhesion to Plastics & Composites

Adhesion to plastic substrates is typically challenging due to their low surface energy and poor wetting. Most plastics have low surface energy and are difficult to wet. Additionally, molded thermoplastic parts are contaminated with mold release agents and require proper cleaning before a coating application. Flame and plasma surface treatments are effective methods to improve the wetting of plastics. This allows to oxidize the substrate surface and create polar groups (hydroxy, carboxy, ketone) to improve wetting and enhance interactions with the applied epoxy resin.

Another important example of adhesion to plastics is the adhesion to other coatings (intercoat adhesion). In this case, the low surface energy of the top coating, the presence of a solvent that can penetrate the primer coating, and high-temperature curing (above glass transition temperature) can improve adhesion. Similar principles apply to recoat adhesion – when the surface is coated multiple times with the same resin, which is particularly important for repairs.

A niche application for epoxy resins where adhesion is extremely important is the production of carbon and glass fiber-reinforced epoxy composites. These materials are excellent for applications where a high strength-to-weight ratio is required, such as aircraft industry and wind turbine blades. In the composite preparation, fiber-polymer (interfacial) adhesion is crucial to achieve high mechanical performance. To enhance interfacial adhesion, graphene oxide sheets are used. They possess polar functional groups (hydroxy, carboxy, carbonyl) that provide better interactions with epoxy resin than pristine carbon fibers. Similarly, glass fibers are treated with compounds such as aminosilanes to provide better dispersion in epoxy resin and, thus, adhesion.

Ultimately, increased compatibility of epoxy resin and fibers provides better stress dissipation and higher mechanical performance in composites.

5. Epoxy Adhesion to Glass

Glass is traditionally a challenging substrate for achieving good adhesion without coating delamination. Despite the high polarity of the glass surface, its excessive smoothness doesn’t satisfy the roughness requirement crucial for good adhesion. Special substrate treatment with reactive silanes can enhance adhesion. Silane molecules are deposited on the glass surface and are permanently attached to epoxy molecules during cross-linking, creating strong anchoring points that prevent coating detachment.

Conclusions

In conclusion, despite the complexity of the adhesion phenomenon, it is useful to familiarize oneself with the principles behind good adhesion – proper substrate pretreatment (decontamination and roughening), surface wetting (surface energy of epoxy resin must be lower than the surface energy of substrate), and compatibility between the epoxy resin and substrate (functional groups to provide good interactions between resin and substrate). We considered the application of epoxy resins on metal, wood, concrete, plastic, and glass substrates and discussed the specifics to enhance adhesion on each substrate. Understanding the substrate and resin characteristics provides valuable information on achieving good adhesion in specific applications.

References

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