Adhesive bonding favorably relies on chemical bonding to hold substrates together. Every solid, liquid, and gas is made up of atoms that interact with each other through chemical bonding. In addition to determining the composition of the material, chemical bonding influences curing reactions, flexibility, polymer formation, thermal stability, hardness, moisture resistance, and substrate interactions. Understanding the primary types of chemical bonds provides a foundation for why some adhesives are flexible and others rigid, while some resist chemicals better than others.
Chemical bonding is possible through the activity of valence electrons, which are the electrons in the outermost shell of an atom. Atoms tend to gravitate toward stable, lower-energy electron arrangements, so when they interact with other atoms, they share, gain, lose, or redistribute electrons to form chemical bonds.
In this article, we explain the major types of chemical bonds, how they form, and why they are important to adhesive and sealant technology.
What is a Chemical Bond?
A chemical bond is the force that attracts and holds atoms together through the interaction of their valence electrons. In a sense, atoms are motivated to reach more stable and lower total energy states, so they combine with other atoms and form chemical bonds. This phenomenon is sufficient grounds for the invention of adhesives.
Attractive and repulsive electrostatic forces repeatedly act between electrons of nearby atoms but only form a stable bond when the competing forces reach an equilibrium that lowers the total energy of the combined structure.
This principle demonstrates why atoms naturally combine to form molecules, polymers, crystals, minerals, metals, and various other materials throughout engineering and nature.
Not all chemical bonds are created equal, some bonds share electrons while others transfer electrons. These differences create materials with remarkably different physical and chemical properties. While materials may appear completely different in their functionality, they attribute their properties to certain types of chemical bonds.
Why do Atoms Form Chemical Bonds?
Atoms form chemical bonds because bonded configurations are more energetically stable than individual atoms. Atoms all possess a certain amount of potential energy that is determined by their electron arrangement. Atoms may reduce this energy by interacting with nearby atoms through their valence electrons. Chemical bonds form when the attractive forces outweigh the repulsive forces required to produce a lower-energy arrangement. This tendency to gravitate to more energetically efficient states is referred to as the principle of minimum energy. Atoms do not deliberately try to complete their outer electron shells but respond to electrostatic forces acting upon them. When electron arrangements transition to a lower energy configuration, the bonded states become thermodynamically more stable than they were in their isolated atoms.
Every element has atoms with their own electron arrangement, and different elements have different bonding behaviors. Carbon favors covalent bonding that result in products such as polymers, plastics, and organic molecules. Sodium chlorine transfers electrons to produce ionic bonds while metals such as aluminum and copper contribute electrons to a shared structure, producing metallic bonding.
When scientists understand the different bonding behaviors of different elements, they can predict how materials will perform under mechanical stress, heat, electrical current, moisture, and chemical exposure.
Chemical bonding plays a role in virtually every stage of an adhesive’s life cycle. It determines polymerization during curing, cross-linking of networks, bond rigidity, and interactions between substrates.
Covalent Bonds
Covalent bonds are the primary type of chemical bonds in adhesive and polymer science. They are responsible for holding together the molecules that comprise most organic adhesives, plastics, sealants, coatings, elastomers, and composite materials. If it weren’t for covalent bonding, modern adhesive technology would not exist.
A covalent bond forms when two atoms share one or more pairs of valence electrons. They do not transfer electrons directly from one atom to another; instead, the shared electrons are attracted by the positively charged nuclei of both atoms. This sharing of electron density lowers the total energy of the system, producing a stable chemical bond.
Covalent bonds are mainly formed between non-metal elements such as carbon, oxygen, hydrogen, nitrogen, silicon, sulfur, phosphorus, and halogens. These elements make up most of the synthetic polymers and organic molecules used in adhesive technology.
One of the most important elements in covalent bonding is carbon. Carbon has four valence electrons, which enable it to form up to four stable covalent bonds with nearby atoms. This bonding versatility allows carbon to form long chains, branched structures, rings, and crosslinked three-dimensional networks. These molecular architectures form the structural spine of many adhesives, including epoxies, acrylics, polyurethanes, cyanoacrylates, silicones, and pressure-sensitive adhesives.
There are two types of covalent bonds, polar and non-polar. Polar covalent bonds form when the atoms differ in electronegativity, causing the shared electron density to shift toward the more electronegative atom. Polar groups can participate in hydrogen bonding, acid-base interactions, and dipole-dipole interactions which all promote adhesion between adhesives and substrates. Nonpolar covalent bonds form when the two atoms have similar electronegativities, which causes the atoms to share their electron density equally. Polar bonds are generally stronger than non-polar bonds.
During adhesive curing, covalent bonds contribute to cohesion because they help form polymer chains, which are large macromolecules. This process of linking three-dimensional networks continuously between individual molecules is referred to as crosslinking. Crosslinking is directly responsible for increasing strength, stiffness, chemical resistance, and heat resistance because it makes the polymer chains more resistant to deformation and damage.
Epoxy adhesives are an excellent example of crosslinking. Before epoxy cures, resin and curing agents are just small molecules. When they cure, reactive chemical groups form new covalent bonds that crosslink thousands of molecules together. The liquid mixture then gradually sets into a rigid polymer with versatile structural strength and durability.
Despite covalent bonds being the strongest chemical bonds, they do not always ensure good adhesion to every material. Other factors such as intermolecular forces and mechanical interlocking play a role in how the cured polymer interacts with the substrate.
Ionic Bonds
While covalent bonds form where atoms share electrons, ionic bonds form when atoms transfer electrons.
When atoms lose or gain valence electrons, they become electrically charged particles referred to as ions. This is largely where ionic bond gets its name from in the sense they are ions that bond together. A positively charged atom is a cation, and a negatively charged atom is an anion. When these atoms are oppositely charged, they attract one another through electrostatic forces, resulting in an ionic bond.
Ionic compounds do not form individual molecules but rather assemble into large repeating crystal lattices where alternating ions of opposite charges surround ions.
Common ionic compounds include sodium chloride, calcium carbonate, and magnesium oxide. Since the electrostatic attractions within these crystal lattices are strong, ionic compounds can withstand high temperatures due to their hard and durable nature. Although most structural adhesives are not ionic materials, ionic compounds are found in many substrates such as concrete, cement, ceramics, glass, minerals, brick, stone, and many fillers.
Products known as adhesion promoters are often useful because they interact strongly with oxide surfaces and ionic minerals while also supporting cohesion with the adhesive itself.
Understanding ionic bonding can tell us that ions are sensitive to water exposure, which often weakens their ionic interactions and makes them more conductive to electricity.
Metallic Bonds
Metallic bonds are the third main type of chemical bond, and they are responsible for the unique properties of metals including their strength, electrical conductivity, ductility, and thermal conductivity. They are important to study because metallic bonding matters when dealing with metal substrates such steel, copper, aluminum, and titanium.
Metallic bonds do not individually share or transfer specific electrons but work through a process of delocalization of valence electrons. The outermost (valence) electrons do not simply move between two atoms but are shared by the entire metal structure. This forms an electron sea or electron cloud surrounding the lattice of positively charged metal ions. A lattice is a three-dimensional arrangement of the ions in a crystalline solid.
The mobile cloud of electrons continuously attracts positively charged metal ions which hold the entire crystalline solid together. For instance, in the event a metal is bent, the atoms are able to move past each other without causing the entire structure to fracture. As a result, the electron cloud redistributes itself around the shifted ions allowing the bond to remain intact. This is why metals are not brittle but are malleable and ductile.
Although metallic bonds produce strong structural metals, they are not particularly easy to bond with using adhesives. Most metals will interact with oxygen in the environment and produce oxide layers such aluminum oxide, iron oxide, and titanium dioxide. These oxide layers are primarily responsible for interacting with adhesives and forming adhesive bonds.
Surface preparation is particularly important when working with metal oxides, through abrasion, solvent cleaning, anodizing, chemical etching, and plasma treatment. These help remove contaminants and produce a substrate surface that is ready for adhesion.
Coordinate Covalent Bonds
Most covalent bonds form when an atom contributes one electron to a shared pair, but not all covalent bonds form this way.
A coordinate covalent bond, also referred to as a dative covalent bond, forms when the same atom supplies both electrons in the shared pair. Once the bond is formed, it behaves like an ordinary covalent bond.
Coordinate covalent bonds occur commonly in chemistry, such as in metal complexes and catalysts. In adhesion science, coordinate covalent bonds are important at interfaces involving metal oxides because adhesion promoters contain elements such as oxygen, nitrogen, phosphorus, and sulfur, which have single pairs of electrons that interact with the metal surfaces and improve adhesion and corrosion resistance.
Coordinate covalent bonding is also found in certain adhesive curing systems. Certain curing agents contain atoms with a single pair of electrons which help form cured polymer networks. These networks transform liquid adhesive into a solid material with a solidified bond. Primary applications of coordinate covalent bonds are found in adhesion promoters, catalysts, primers, and curing systems.
Chemical Bonds Common in Adhesives and Sealants
One primary thing to understand is that chemical bonds create the adhesive itself, but it is the intermolecular forces that help the adhesive attach to the substrate. We cover intermolecular forces in a separate article.
Knowing this, we can conclude that every adhesive obtains its performance from the chemical bonds that hold its molecules together. The polymer structures that determine how an adhesive flows, wets, and cures almost all rely on covalent bonding to build themselves. How these covalent bonds arrange themselves determines the adhesive’s flexibility, durability, and chemical resistance.
Epoxy adhesives are a form of adhesive technology that is used in structural applications. They rely on covalent bonding through the crosslinking of molecules into rigid three-dimensional structures. This produces a bond that has excellent mechanical strength, stability, and heat/chemical resistance.
Similarly, polyurethane adhesives rely on covalent bonding where the polymer chains form through reactions between isocyanates and polyols. These crosslinking processes make polyurethanes suitable as flexible elastomers or structural adhesives.
Cyanoacrylate adhesives rely on covalent bonding through rapid polymerization, which also leads to a quick curing time.
Silicone adhesives consist of silicon-oxygen polymer structures which are exceptionally good at resisting UV radiation, moisture, weathering, and extreme temperatures.
We discuss adhesive types in a separate article, but the key takeaway is that covalent bonding is the primary type of chemical bond within adhesives themselves.
Key Takeaways and Conclusion
Outer electron shells do not always seek to complete themselves. Atoms rearrange in ways where the resulting bond is a lower total energy state. Likewise, bond strength varies depending on the elements involved; therefore, covalent bonds between different elements can have varying bond strengths. Bond strength is one aspect of what determines a successful bonding application because other factors such as surface wetting, curing conditions, substrate preparation, environmental exposure, and joint design also play a role.
From this article, we may infer that the forces that hold atoms together to create molecules, crystals, metals, polymers, and practically every material used in engineering are chemical bonds that form through distinct behaviors of valence electrons. Bonds form where the resulting redistribution produces a more stable, lower-energy arrangement.
The three primary types of chemical bonds are covalent, ionic, and metallic bonds. Covalent bonds form through the sharing of electron density, while ionic bonds form through the transfer of electrons through electrostatic attraction between oppositely charged ions. Metallic bonds form due to delocalized valence electrons, which create a common electron pool. Coordinate covalent bonds are a special type of covalent bond that forms when the same atom supplies both electrons in a shared pair.
For adhesive applications, chemical bonds determine how polymer structures form during curing, how crosslinked networks develop, and how materials acquire their mechanical properties. Other factors such as intermolecular attractions, mechanical interlocking, and diffusion are covered in separate articles.
