Adhesives need to undergo some form of change after application to usefully bond to the substrate. A liquid adhesive may become a solid material, a soft adhesive may develop greater cohesive strength, or reactive ingredients may form a three-dimensional polymer network. The type of adhesive will determine what type of process the adhesive will undergo to form a bond.
Polymerization and curing are two important concepts for understanding the changes an adhesive goes through to form a bond. They are tightly related but do not refer to the same thing. Polymerization is a chemical process in which suitable molecular building blocks connect to form polymers. Curing is a general term that refers to the development of an adhesive towards its functional state and property.
For some adhesives, polymerization may be part of curing, while in others existing molecules or polymers become crosslinked. Some adhesives develop bond strength primarily through physical processes such as cooling or the loss of water. Knowing the distinctions between these processes will explain why adhesives have different application methods, working times, curing conditions, and final properties.
What is Polymerization?
Polymerization is a chemical process through which monomers or suitable molecular building blocks become chemically connected to form polymers. The resulting structure of polymers may take the form of long polymer chains, branched molecules, or interconnected polymer networks.
There are multiple types of polymerization which occur through distinct chemical mechanisms. Some polymers involve monomers containing carbon-carbon double bonds, while others involve reactions between other functional groups.
Polymerization is an important part of adhesive science because some adhesives contain reactive components that undergo polymerization upon adhesive application. As the reaction progresses, the molecular structure of the material changes. Small reactive molecules can form as part of large macromolecules, and the mechanical properties of adhesives can shift dramatically.
Polymerization itself does not determine if an adhesive will stick to a substrate. It mainly describes the process that occurs internally within the adhesive.
Polymerization vs. Adhesive Curing
Polymerization and curing are closely related, but the terms must not be used interchangeably.
Polymerization specially refers to the chemical process that results in the formation of polymers from suitable molecular building blocks such as monomers. Curing describes the broader development of the adhesive towards its functional state.
An acrylic adhesive, therefore, undergoes polymerization of reactive acrylic or methacrylic components during curing. Likewise, an epoxy adhesive can cure through reactions that create cross-linked networks. Moreover, a hot-melt adhesive may already contain polymers beforehand and may solidify upon cooling.
Every adhesive undergoes polymerization, and some adhesives may not undergo chemical curing, but every adhesive must undergo some change to reach its finished state.
Chain-Growth Polymerization
Chain-growth polymerization is a type of polymerization in which reactive centers cause monomer molecules to be added successively to growing polymer chains.
A simplified chain-growth process can be divided into three distinct stages: initiation, propagation, and termination. During the initiation phase, reactive species capable of starting polymer growth are produced. Once propagation starts, monomers repeatedly react at the growing chain end, making the polymer chain longer. Once the chain growth system has reached its growth potential, the termination phase stops further growth from occurring.
Acrylic and methacrylic adhesives generally use chain-growth polymerization because their monomers contain carbon-carbon double bonds that connect during polymerization.
Cyanoacrylate adhesives also use chain-growth polymerization, and small amounts of moisture can help initiate the process.
Step-Growth Polymerization
Step-growth polymerization happens when suitable functional groups of molecules react with one another, progressively creating larger molecular structures. Unlike chain growth polymerization, the growth does not depend on monomers being added to the chain end one at a time. Any molecule that is compatible may react with each other at any stage of the process.
Polyurethane chemistry is a common example of step growth polymerization in adhesives. Isocyanate groups react with hydroxyl functional groups to form urethane linkages. When molecules contain many reactive groups, these reactions will combine to form long chains, branching, and eventually network structures.
Step-growth polymerization illustrates that the type and number of reactive functional groups influence how molecules become connected during polymer formation and curing.
Crosslinking
Crosslinking is the formation of chemical connections between different molecules, polymer chains, or parts of a molecular network. When crosslinking occurs in greater quantities, three-dimensional networks may start to develop.
Epoxy is a common example of crosslinking in adhesives. Epoxy resins contain reactive epoxide groups, while curing agents contain the ability to react with these groups. As curing progresses, these groups react with each other to connect molecules in an increasingly interconnected network.
Crosslinking usually leads the adhesive to become stiffer and more rigid because molecular movement is restricted. This may be good because the bond will be more dimensionally stable, chemically resistant, and resistant to flow.
Too much crosslinking may have its downsides because it can cause some materials to become brittle and easy to break under sudden impact.
One-Part and Two-Part Reactive Adhesives
Reactive adhesives govern the curing process through either a one part or two-part system.
A one-part adhesive has all of the necessary formulation in a single container. The adhesive must remain stable but react when exposed to the appropriate curing condition. That condition may be atmospheric moisture, air, UV light, or heat.
A two-part adhesive keeps the important reactive components separate until application time. One component acts as a resin, and the other contains a hardener or curing agent. Once the two parts of the adhesive system are mixed, the curing process begins.
Two-Part Adhesive Curing
Two-part adhesives require two components to be mixed in correct proportions at the right time. Once these two components are mixed, they can begin to react and form the cured material.
The mixing ratio is important because it determines the relative quantities of reactive components. Incorrect mixing may interfere with stoichiometric relationships and leave an insufficient amount of one component compared to the other. This may affect the intended crosslinking networks and final properties of the assembly.
Thorough mixing is also important because even if the mixing ratio is correct, inadequate mixing may leave unequal concentrations of the components.
Once the components are mixed, there is a limited amount of time before the adhesive must be applied effectively. Too much elapsed time may cause the viscosity to increase and make the material unstable for normal application.
Moisture Curing
Some adhesives use environmental moisture as an aid to their curing process. These are known as moisture-curing adhesives.
Some one-part polyurethane adhesives contain isocyanate containing functional groups which participate in reactions with water to form chain extensions or network formations.
Some silicone sealants use atmospheric moisture by participating in hydrolysis, leading to the formation of siloxane linkages.
Cyanoacrylates are another moisture sensitive adhesive family where small amounts of moisture can initiate polymerization.
UV and Light Curing
Some adhesives cure when exposed to light of an appropriate wavelength. These formulations usually contain photoinitiators which absorb suitable light and produce reactive species that initiate polymerization or another curing reaction. The adhesive generally remains stable until it starts interacting with light.
Light curing can be advantageous because the adhesive can be positioned before exposure and cured when the components are properly aligned.
However, the light must be able to contact the adhesive. Configurations involving opaque substrates, thick sections, pigments, fillers, or complex joint geometries can reduce light penetration. Areas that do not receive sufficient light exposure may not cure properly.
Heat-Activated Curing
Heat can support the initiation or acceleration of curing for certain adhesives. Increasing temperature will generally increase the molecular motion and can accelerate chemical reactions. Some adhesive formulations generally stay stable at colder temperatures but cure more rapidly under warm temperatures.
Heat can also lower the viscosity of adhesives, influencing flow and wetting.
Endothermic refers to a process that absorbs heat from its surrounding environment. Heat-cured adhesives do not automatically undergo endothermic reactions. While heat may be required in the external environment, it does not indicate that the adhesive itself absorbs heat from its surroundings. Some curing reactions may be exothermic (heat-releasing) even though heat is necessary to initiate them.
Too much heat can also contribute to thermal stress or damage temperature-sensitive substrates, so temperature must be controlled.
Anaerobic Curing
Anaerobic adhesives are adhesives that remain stable in the presence of oxygen but cure when confined to a space where oxygen is not available.
They are commonly used in closely fitted metal assemblies such as threaded fasteners, retaining applications, and some flange-sealing applications.
Oxygen prevents the adhesive from undergoing polymerization, but when the adhesive is confined to an area with little to no oxygen, the curing process initiates.
Physical Setting vs. Chemical Curing
Hot-Melt Adhesives already contain polymeric materials. Heating softens or melts the adhesive so it can flow and wet the substrate. As the adhesive cools down, it solidifies and develops cohesive strength. Polymerization is not typically required for a standard thermoplastic hot melt to set.
Solvent-based adhesives can contain polymeric materials dissolved in organic solvents. After application, the solvent evaporates from the adhesive layer, increasing the concentration of the polymers and allowing the adhesive film to develop.
These adhesive processes are regarded as physical setting because the main strength-developing mechanism does not require polymerization through a chemical reaction.
The Stages of Adhesive Curing
The exact stages of curing differ by the type of adhesive, but a generic guideline can be developed to explain how an adhesive forms a bond.
First, the adhesive is applied to one or both surfaces. It must make sufficient contact with and wet the surface. The adhesive will then begin to set or react depending on its own specific mechanism. Reactive adhesives can polymerize or crosslink, while physically setting adhesives may lose solvent or begin cooling.
In reactive systems, the material will reach gelation, which results in an interconnected structure where flow slows down, and strength continues to develop.
Therefore, the process = Application > Wetting > Reaction or Setting > Gelation or Solidification > Strength Development > Final Properties.
Working Time, Gel Time, Fixture Time, and Full Cure
During adhesive curing, there are some key terms you must know and understand.
Working time is the useful period during which a mixed or activated adhesive can still be applied and manipulated as intended. After working time has elapsed, it is difficult to reposition the adhesive.
Gel time refers to the time required for a reactive material to reach its gelation point under specific conditions. At this stage, an interconnected structure has developed, and the material is no longer liquid.
Fixture time describes the period required for a bonded assembly to develop enough strength to remain positioned or be handled without the joint moving. Fixture strength does not represent the final strength.
Full cure describes the point at which the adhesive has reached the cure state. During this time, it will begin to develop its final properties.
How Temperature Affects Curing
Temperature can have a strong influence on adhesive curing.
For many chemically reactive adhesives, increasing the temperature increases the reaction rate. Warmer adhesives may cure faster than colder adhesives.
Temperature may also affect the viscosity, with higher temperatures contributing to lower viscosity and vice versa.
Temperatures outside the recommended range can prevent the adhesive from developing its intended properties.
How Moisture and Humidity Affect Curing
Moisture can either aid or interfere with curing depending on the adhesive.
For some adhesives, moisture is necessary to cure through reactions with water. Dry conditions for these adhesives will lead to slower or incomplete curing.
Some adhesives are negatively affected by excessive moisture. Water can interfere with surface preparation, react undesirably with certain ingredients, and produce unwanted by-products.
Humidity therefore is beneficial in certain contexts while harmful in others.
How Bond-Line Thickness Affects Curing
The thickness of the adhesive layer can impact curing in several ways.
In a moisture-curing adhesive, moisture may need to penetrate to reach reactive material from exposed surfaces or through permeable substrates. A thick bond line will have different permeability than a thin bond line.
Light curing adhesives rely on sufficient light penetration. A thicker bond line may receive less light exposure in deeper regions, especially if the formulation strongly scatters the curing wavelength.
Solvent-based and water-based adhesives rely on volatile material evaporating from the adhesive layer. Thick or poorly ventilated bond lines can make it difficult for volatile material to evaporate.
Reactive adhesives that release heat can cause thick sections or large masses to accumulate heat more rapidly than it can dissipate.
Therefore, thick adhesive layers are not automatically better than thinner layers; the effect will depend on the curing mechanism and formulation.
What Happens When an Adhesive Does Not Cure Properly
Improper curing can result from many reasons.
Two-part adhesives can be affected by improper curing because of incorrect mixing ratios and inadequate mixing. Light-curing adhesives may receive insufficient exposure. Moisture-curing adhesives may not receive sufficient water exposure. Low temperatures may slow down reactions.
Likewise, contamination may interfere with curing or bonding. Oils, release agents, cleaning residues, water, dust, or other substances can disrupt or prevent sufficient contact between the adhesive and substrate.
Old or improperly stored adhesives may cause reactive ingredients to degrade, separate, crystallize, or otherwise change.
Incomplete curing may result in soft, tacky, weak, or chemically unstable material. Even if the surface appears to have cured, deeper material may not have reached full strength capabilities.
Curing and Final Adhesive Properties
How curing initiates and progresses can impact the final properties of a reactive adhesive. As polymerization and crosslinking progress, cohesive strength can increase. Molecular mobility may decrease, and the material becomes more resistant to deformation. As the network develops, chemical resistance, heat resistance, stiffness, and dimensional stability can change.
However, it is not implied that curing is only a matter of stiffening the adhesive or crosslinking it as much as possible. Different applications require a different set of properties.
A rigid structural adhesive may require stiffness and dimensional stability. A sealant may require flexibility to accommodate movement. A pressure-sensitive adhesive requires viscoelastic behavior to maintain surface contact.
As a result, the purpose of an adhesive is to develop the molecular structure and properties to support the adhesive’s intended use.
Curing Does Not Automatically Mean Good Adhesion
Even if an adhesive successfully cures, it may still produce a weak bond.
Curing primarily concerns cohesive strength; adhesion depends on the interaction between the adhesive and substrate surface. Both cohesion and adhesion must be adequate to ensure success.
For example, an epoxy may cure well and form a crosslinked structure but still detach from a contaminated metal surface. The adhesive itself is cured, but the adhesive-substrate interface is weak. Inadequate wetting can create a similar problem where the uncured adhesive does not make sufficient contact with the substrate.
Weak boundary layers, surface contamination, poor preparation, improper surface chemistry, residual stresses, and unsuitable joint geometry can all contribute to failure despite the adhesive curing correctly.
Successful bonding is the result of cohesion and adhesion, meaning there must be sufficient strength within the adhesive and at the adhesive-substrate interface.
Conclusion
Polymerization and curing explain how adhesives transition from molecular building blocks into connected polymers capable of bonding substrates. Polymerization can occur through different mechanisms such as chain-growth and step-growth processes. Crosslinking can connect molecules or polymer chains into three-dimensional networks.
Adhesive curing is a broader topic than polymerization because an adhesive can cure through other types of chemical reactions too. The curing mechanism itself determines many characteristics of an adhesive, such as working time, environmental requirements, rate of strength development, and sensitivity to temperature, moisture, and light.
Understanding both polymerization and adhesive curing provides insights into how an adhesive goes from a chemical substance to a bonded assembly.
