This article will take an in-depth look at epoxy adhesives.
The article will bring more detail on topics such as:
This chapter provides an overview of epoxy adhesives, detailing their creation process and operational mechanisms.
Epoxy: Epoxy glue is a thermosetting adhesive composed of an epoxy polymer resin and a hardener. It facilitates the adhesion or joining of diverse surfaces, creating a durable, lasting bond capable of withstanding extreme stresses and environmental conditions.
Adhesive: Pertains to the process of sticking to surfaces or objects.
Epoxy adhesives are the dominant choice in the realm of industrial adhesives, and they are known for being among the most versatile structural adhesives available. Their popularity is largely due to the strength of the cured product and their remarkable ability to bond with numerous materials. Additionally, these epoxy resin glue formulations are easily tailored to meet specific project requirements.
Various types of epoxy resin are used in formulating epoxy adhesives, which define the glue's core properties. For instance, a heat-resistant epoxy resin is optimal where temperature resilience is needed, whereas a flexible epoxy resin is preferable if movement comes into play.
To assess the effectiveness of an epoxy adhesive, examining the basic makeup of its compounds is insightful. Epoxies are generated through the polymerization of a blend of resin and hardener. The primary components of epoxy adhesives are epoxy resin and a curing agent, though additional ingredients such as fillers, tougheners, plasticizers, and other additives like silane coupling agents, defoamers, and colorants can be incorporated as needed.
| Constituent | Ingredient | Main Role |
|---|---|---|
| Primary | Epoxy resin, reactive diluent | Adhesive base |
| Primary | Curing agent or catalyst, accelerator | Curing enhancement |
| Modifying | Filler | Altering properties |
| Modifying | Toughener | Durability enhancement |
| Modifying | Plasticizer | Increase flexibility |
| Additive | Coupling Agent | Optimizes adhesion |
| Additive | Colorant | Adds color |
Table 1: Epoxy Adhesive Components
The primary production of epoxy resins involves reacting active hydrogens from phenols, alcohols, amines, and acids with epichlorohydrin (ECH) under tightly controlled conditions. An alternative method involves using peroxide to oxidize an olefin, producing cycloaliphatic epoxy resins similarly.
Bisphenol A diglycidyl ether, often termed bisphenol A type epoxy resin, was the pioneer commercially produced epoxy resin and remains the most prevalent, making up approximately 75% of industrial epoxy resin use by volume.
This type of epoxy resin, commonly used in epoxy adhesives, owes its utility to its chemical structure composed of numerous functional groups and crucial properties.
Curing starts when the resin mixes with a designated catalyst, triggering an exothermic process as molecular chains engage at chemically active sites. The resulting covalent bonds between the epoxy resin's epoxy groups and the hardener's amine groups allow the polymer to cross-link, impacting the epoxy's rigidity and strength. By adjusting curing conditions like temperature and choosing specific resins and hardeners, it is possible to customize mechanical strength and resistance traits to meet diverse applications and operational contexts.
Epoxy adhesives adhere to many material types, and their characteristics are influenced by the system's chemistry and the specific cross-linking employed. These adhesives excel in chemical and heat resistance, adhesion, water resistance, and offer suitable mechanical and electrical insulation properties.
As the most common structural adhesives, epoxy types are available in one-component or two-component systems. One-component epoxies generally cure at temperatures between 250 and 300°F, resulting in products with substantial strength, excellent metal bonding, and strong environmental and chemical resistance, making them a preferred alternative to welding and riveting.
Pre-catalyzed one-component systems cure with mild heat, boosting efficiency and minimizing errors caused by air entrapment. The cure time for these systems is shorter compared to two-component systems. For two-component epoxy systems, catalyzation can occur at ambient temperatures, with heat further enhancing cross-linking and improving properties.
Offering versatility, two-component solutions can bond most surfaces and stand out for enduring constant force or weight over prolonged periods, resisting physical and chemical influences, and thus showcasing remarkable stability.
Highly adaptable, these epoxies serve various applications such as bonding, sealing, coating, and encapsulation across industries like electronics, medical devices, and aerospace. Special formulations offer properties like flame retardancy, cryogenic serviceability, rapid curing, and high-temperature resistance.
Some crucial factors to keep in mind when selecting epoxy adhesives are:
For optimal adhesion, an adhesive should adequately 'wet out' the surface, allowing it to spread and cover the surface area effectively, enhancing the contact and bonding forces between the adhesive and the substrate.
Key design elements to consider for an adhesive joint include:
Bond strength is influenced by conditions like temperature, moisture, chemicals, and weather exposure. It's advisable to test potential adhesives under conditions mimicking the actual service environment when degrading factors are likely.
Assessing adhesives involves not only performance but also considerations for their application. What application method will be employed? Is it a single or dual-component system?
Early planning of dispensing, mixing, and applying methods can lead to considerable time and cost savings, preventing potential production delays.
Ultimately, evaluating adhesives necessitates selecting and testing several candidates in the application, comparing properties through standardized tests. Sampling and personal trials are critical for informed decision-making.
Determining an appropriate cure schedule is vital for achieving optimal adhesive performance. Considerations include:
Epoxies are often paired and mixed as needed to extend their shelf life and minimize waste. Newer dual-barrel cartridges with static mixers allow dispensing premixed components in correct ratios for consistent application quality. In electronics, pre-mixed, frozen epoxies are popular, needing thawing right before use.
Resistance to chemicals and flames is critical in some applications. A primary feature of epoxies is their resilience to a broad array of substances like acids, bases, solvents, fuels, fluids, and both salt and freshwater, suitable for harsh chemical exposure and environments.
Certain epoxies also meet flammability standards for applications adhering to rigorous specifications (e.g., Underwriters Laboratories, an international safety certification organization). Specialized epoxies can enhance device longevity in challenging settings.
Epoxies often connect electronic and electromechanical assemblies to maintain electrical or thermal conductivity. These adhesives can attach heat sinks to components producing thermal energy during operation, ensuring better heat distribution for effectiveness.
Electrically conductive epoxies establish connections requiring electric conductivity for signal transfer or static discharge, easing the creation of reliable, high-performance devices for design and process engineers.
This chapter will discuss the classes and types of epoxy adhesives.
Epoxy adhesives come in one and two component glue systems, both of which generate exceptionally strong connections but differ in curing process, ultimate qualities, and applications.
In general, two-component epoxy resin adhesives are more durable than single-component systems.
Prior to application, the two components of the 2K epoxy resin adhesives must be mixed together. The components react with one other, causing the epoxy adhesive to cure. The adaptability of two-component epoxy resin adhesives is greater, as they are used in all market segments. They're a little more difficult to deal with because the two components, resin and catalyst, must be mixed together, and the worklife varies from a few minutes to several hours depending on the industrial epoxy adhesive composition.
These 2K epoxy resin adhesives cure at room temperature, but depending on the formulation, the process can be accelerated by applying heat or an external source of energy such as UV radiation. Epoxy adhesives cure to the strongest bonds when the curing period is the longest.
2K epoxy glue solutions have the advantage of adhering to nearly any surface, including wood, metals, plastics, ceramics, and many types of rubber. They are also resistant to physical and chemical impacts, and can withstand temperatures ranging from 95 to 200°C (200 to 390℉).
In contrast to 2K epoxy glue, one component epoxy resin adhesives are simple to use because they do not require mixing. They usually have a paste-like consistency, making them ideal for trowel application or bead extrusion. These adhesives cure in elevated temperatures that are between 120°C and 175°C depending on the formulation.
The single component heat cure adhesive system efficiently fills and seals gaps, particularly between metal surfaces. One part epoxy adhesives are also found as heat curing film adhesives. The film adhesives are the best utilized where the area is large for bonding or laminating purposes.
These films are used in assembly compounds in the aerospace sector. One-part epoxy resin adhesives are also commonly used as construction epoxy adhesives, which can be used for installing tiles.
The various types of epoxy adhesives include:
DGEBA epoxy, or diglycidyl ether of bisphenol A (BPA), is the earliest epoxy resin and by far the most important in epoxy adhesive formulations, not least because of its low raw material cost. Furthermore, this resin is compatible with a wide range of catalysts, allowing for the creation of a diverse spectrum of properties.
DGEBA epoxy-based industrial epoxy adhesive solutions cure at ambient temperature or with the addition of heat. DGEBA epoxy resin is available in a variety of forms, including low molecular weight liquids, high molecular weight semi-solids and solids, and brominated resins. The latter are generally employed in applications that necessitate ignition resistance. As a result, brominated resins are commonly used in circuit boards and other applications that require flame retardance.
Epoxies are hydrophobic by nature and hence cannot dissolve in water. Regardless, water dispersibility can be imparted to epoxies via chemical modification or emulsification. The latter is generally employed for resins in aqueous epoxy adhesives, and it is accomplished by using a surfactant to separate the water from the resin. The mechanical and chemical stability of the adhesive is determined by the surfactant used and the manufacturing parameters used.
Two types of epoxy acrylate resins are utilized to make adhesives: vinyl ester and a specific resin for radiation curing. Despite being epoxies, these resins behave more like polyester resins. They're simple to make, cure quickly at room temperature, and can be cured with peroxides.
These epoxy resins have a lower viscosity, more flexibility, and better wetting properties in adhesives than traditional DEGBA epoxies. They do shrink more than any other epoxy adhesive when they cure. Radiation from an external source of energy, such as ultraviolet (UV) or electron beam, can also cure epoxy acrylate resins (EB). The viscosity and vapor pressure of epoxy resin adhesives are typically low.
Epoxy glues and adhesives aren't recognized for being particularly flexible. Long-chain aliphatic epoxy resins, on the other hand, allow epoxy adhesives to be made more flexible. Nonetheless, the use of this flexible epoxy resin reduces the glue's hardness. As a result, flexible epoxy resins are typically blended as modifiers with other epoxies to create a tougher, stronger, but still flexible solution. When 10-30% of the resin is aliphatic epoxy, the requisite level of flexibility can be attained without significantly affecting other qualities. These are the most effective flexible epoxy adhesive methods available. Flexible epoxy resin adhesives are used for laminating safety glass, absorbing vibration and sound, and encapsulating electrical components and other delicate components that require temperature cycling, among other applications.
Epoxy novolac adhesives are well-known for being the most chemically and thermally resistant epoxies. They also adhere to surfaces better than BPA epoxies. The epoxy novolac must be cured at a high temperature in order to fully acquire these qualities. The qualities that can be reached when cured at room temperature are similar to those of DGEBA epoxy adhesives. The length of the curing cycle influences the thermal stability of the cured bond.
Due to their high viscosity, epoxy novolac resins are generally difficult to manufacture. To make processing easier, low viscosity alternatives have lately been created. These products, on the other hand, have a lower epoxy resin content. In any event, epoxy novolac is virtually entirely made up of 2K epoxy glue.
Epoxy adhesives for room temperature cure are typically manufactured and supplied in a two-component package, with the epoxy resin component parked in one resin part and the curing agent parked in the other hardener part. When these two elements are mixed together, epoxy resin reacts fast with the curing agent at room temperature to form a cross-linked strong thermoset structure that can tightly attach adhesion substrates. Pot life and cure time can be customized by using several types of curing chemicals.
Because its reaction with epoxy resin is very fast in the presence of modest amounts of basic chemicals such as tertiary amine or imidazole as an accelerator, mercaptan compounds are commonly used as curing agents for fast room temperature curable epoxy adhesives. Through a poly-addition reaction mechanism, epoxy resin reacts equivalently with the mercaptan group. At room temperature, the fixture time can be as short as 30 minutes or as long as 15 minutes. It will take 24 hours for the remedy to complete. Precautions must be taken due to its limited working life of 10 or perhaps 5 minutes.
In epoxy resin technology, aliphatic polyamines are the most often utilized curing agents. Curing agent providers have commercialized a number of modified polyamine type curing agents with adjustments to curability, handling, or other physical features for ease of use. The active hydrogen of primary and secondary amines combines with an epoxide in a polyaddition mechanism in the same way. Fixture time and work life can be changed using a suitable curing agent in combination.
UV-curable epoxy adhesives cure quickly and have been successfully used in a variety of new electronics assembly and general bonding applications, such as image sensor module assembly, display panel and module assembly, and other applications where high adhesion performance and fast production speed are required. In recent years, a variety of UV cationic epoxy adhesives and UV acrylate hybrid thermal cure epoxy adhesives have been introduced.
When compared to typical UV acrylate adhesives, UV cure epoxy adhesives have no oxygen inhibition issue, reduced curing shrinkage, and improved adherence.
Epoxy resin and cationic photo-initiator are the main components of UV cationic epoxy adhesives. Because cycloaliphatic type epoxy resins have a faster cationic polymerization rate than conventional bisphenol A diglycidyl ether type epoxy resins, they are commonly used in UV cationic epoxy adhesives. Cationic photo-initiators in UV epoxy adhesives absorb UV light to produce strong acid, which reacts with epoxy to produce cationic, which can induce epoxy resin homopolymerization.
UV cationic epoxy adhesives feature reduced cure shrinkage than traditional acrylate-based UV adhesives due to the epoxy structure and no surface cure issue due to oxygen inhibition of free radical polymerization because they cure via cationic polymerization.
UV cationic epoxy adhesives, on the other hand, are not appropriate for bonding basic substrates that end cationic polymerization. UV cationic epoxy adhesives will require more time to cure. In practice, after UV radiation, a post thermal cure of UV cationic epoxy adhesives is usually utilized to ensure full cure and satisfactory adhesion performance.
Acrylate compositions are the most often used UV cure adhesives. The essential components of acrylate-based UV cure adhesives are acrylate monomer, acrylate oligomer, and radical photo-initiator. UV cure adhesives based on acrylates cure instantly, in seconds. Surface cure issues, shadow cure issues, excessive cure shrinkage, and poor humidity reliability are all drawbacks of UV cure acrylate adhesives.
UV and thermal cure hybrid epoxy adhesives have been produced and commercialized for over two decades by combining UV acrylate and thermal cure epoxy compositions. UV and thermal cure hybrid adhesives are typically made up of acrylic monomer, epoxy resin, photo-initiator, and epoxy curing agent. UV hybrid epoxy adhesives combine the benefits of both the UV acrylate percentage and the thermal cure epoxy part in one product. When compared to standard UV acrylate adhesives, the introduction of epoxy composition could significantly improve adhesion dependability.
Meanwhile, production efficiency might be greatly improved by reducing fixture time by UV radiation to seconds, as opposed to the dozens of minutes required for thermal cure epoxy adhesives. Because of the decreased concentration of free radical curable acrylate compositions, the surface cure issue, shadow cure issue, and cure shrinkage problem of acrylate UV adhesives could be improved to some extent.
In some circumstances, a thermal initiator such as peroxide is used in the hybrid adhesive formulation to ensure the curing of acrylate compositions that have not been exposed to UV light or that are in shadow areas where UV light cannot reach.
This chapter will discuss the applications and benefits of epoxy adhesives.
The applications of epoxy adhesives include:
The benefits of epoxy adhesives include:
The drawbacks of epoxy adhesives include:
Epoxy adhesives provide excellent adherence to a wide range of surfaces and are the most commonly used structural adhesives. Epoxy adhesives can be cured at room temperature, at increased temperatures, or via UV light radiation, depending on the type of curing agent used. Numerous epoxy adhesives, either one-component or two-component, have been marketed and widely utilized in various industrial production and applications for bonding metals, concrete, glass, ceramics, concrete, many plastics, wood, and other materials.
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