The Importance of Reliable Adhesive Curing
Adhesive curing is the process through which an adhesive fully hardens and crystallises. It is through the curing process that adhesives form bonds with their substrates.
It’s vital that the curing process is handled with the correct care and expertise, in order to ensure a durable and strong chemical bond. If this step is neglected, the bonds between the adhesive and substrates may weaken quickly, ultimately leading to the failure of the bonded product.
Factors Influencing Adhesive Curing
Factors that affect adhesive curing include:
- Adhesive composition – adhesives with low water and high solid compositions will cure faster than adhesives with high water and low solid compositions.
- Substrate materials – closed-celled substrate materials tend to take longer to form bonds, as compared to open-celled substrates.
- Adhesive layer thickness – the greater thickness of the applied adhesive, the longer the time required for it to cure.
- Humidity – high humidity will slow down the evaporation rate of the water and liquid components within an adhesive, which can affect the cure time, depending on the adhesive.
- Temperature – higher temperatures will increase the evaporation rate of the water and liquid components within an adhesive, which can lead to faster cures, depending on the adhesive.
Different Curing Mechanisms
Different adhesives require different conditions in order to cure properly. Here we explore the main curing mechanisms:
Two-component adhesives typically cure at room-temperature conditions without the requirement for other factors. This is due to them containing a resin and a hardener, which mix together to give a solid chemical bond within minutes. The final strength of the bond, however, is generally reached after 24 to 72 hours.
It is important to maintain the specified ratio of the resin and hardener in order to obtain the optimal cure and physical characteristics of the adhesive.
Anaerobic curing adhesives cure under the absence of oxygen, by reacting with metal ions on the metal surface. The curing takes only a few minutes. The processing speed can be increased by additional light fixation or the use of activators or primers.
Anaerobic adhesives are thermosets. Therefore, the resulting bonds have high strength and high resistance to heat, oils, solvents and moisture. The bond can, however, be quite brittle and not suitable for flexible substrates.
They are often used for their sealing properties for mounting engines in the automotive industry, in addition to securing screws and bonding concentric parts in bearings and electric motors.
Heat curing adhesives require high temperatures for a specified period of time to achieve a full cure. Cured adhesives are hard and rigid but tough with excellent chemical and heat resistance. They can only be used to bond plastics that have softening points sufficiently above the cure temperature of the adhesive.
Heat curing of adhesives can be facilitated by air convection and tunnel ovens. The speed of some applications can be increased by using induction curing instead of conventional ovens. Heat cured adhesives are also typically cured between 60 to 200oC.
Structural adhesives and one-component epoxy adhesives are usually heat cured. Epoxy resin is used in many applications, including those in the automotive, aerospace, and metal fabrication sectors.
Moisture curing adhesives require interaction with humidity in order to cure into a solid bond. The speed of the curing process depends on the substrate geometry and the relative humidity in the environment but usually takes place within 72 hours.
Many silicone adhesives are typically moisture curing, as used in household applications and industrial bonding and sealing of glass and metal. They can cure using the humidity already present in the open air around them.
UV curing adhesives are typically extremely fast-curing, as they can be cured in seconds on exposure to light of the correct wavelength. This requires at least one of the substrates to be permeable to the wavelengths of light used to initiate the cure of the adhesive. With these adhesives, it’s important that the bond line is the correct distance from the lamp when curing, as the intensity of the radiation can affect the bond strength.
UV curing adhesives are typically used for bonding glass, transparent plastics and metal/plastic casings. Discover our extensive guide on UV-curing to learn more about the process, in addition to recommended adhesives.
Dual curing adhesives allow two curing mechanisms to be used simultaneously, enabling more effective and rapid curing. An example of this would be UV curing combined with heat or humidity curing. This method can be particularly useful in applications where one mechanism doesn’t cover the entire geometry of the bond surface, e.g. if UV light can’t reach the entire surface, leaving shadowed areas, which can then be cured using the existing humidity in the air. This can lead to a more sustainable and energy-efficient curing process.
Many two-component and anaerobic adhesives can typically be dual-cured to give a solid bond.
General Curing Tips
Here are some general good practices to keep in mind when curing adhesives:
- Ensure that the adhesive doesn’t flow out of the joint before it cures.
- Apply only the required amount of adhesive. An excessive amount will increase curing time, by increasing the bond thickness, therefore slowing down the process.
- The cure time of an adhesive starts when the adhesive is in the exact required conditions as specified by its datasheet; not immediately after it’s applied.
- If appropriate to the design and handling strength required, cyanoacrylate adhesives can be used as a way of fixturing the components/substrates in place, avoiding the use of jigs or clamps and potentially speeding up operations.
- Ensure that, when appropriate with the adhesive, substrate surfaces are properly prepared for bonding before an attempt at curing.
In addition to optimising the adhesive curing process, it’s essential that manufacturers optimise adhesive dispensing processes. This ensures maximum performance, efficiency and reliability of the adhesive; applicable to both manual and automated dispensing.