Hot Melt Adhesives
Hot melt adhesives are also known as glue adhesives. They are thermoplastic polymer adhesives that are solid at room temperature. Hot melt adhesives liquefy if heated to a temperature above their softening point...
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This article will take an in-depth look at epoxy adhesives.
The article will bring more detail on topics such as:
This chapter will discuss what epoxy adhesives are, their manufacturing, and how they function.
Epoxy: Epoxy glue is a thermosetting adhesive made up of a resin or epoxy polymer and a hardener that is used to adhere or join a range of surfaces together with a strong, permanent, and robust bond that can tolerate extreme stress and weather conditions.
Adhesive: refers to sticking, be it on a surface or object.
Epoxy adhesives are the most widely used industrial adhesives, as well as the most adaptable structural adhesives. The tenacity of the cured product, as well as their incredible ability to stick to a wide range of materials, contribute to the popularity of epoxy adhesive. Epoxy resin glue solutions are very simple to customize to satisfy the specific property requirements of each project.
Epoxy adhesives are made with several epoxy adhesive resin types, which define the glue's fundamental features. When high temperature resistance is necessary, a heat resistant epoxy resin is the ideal choice, whereas a flexible epoxy resin is the best choice when movement is possible.
When evaluating the efficacy of an epoxy adhesive, it's helpful to look at the general composition of the compounds that make it up. Polymerization of a mixture of two initial components, the resin and the hardener, produces epoxies. Epoxy adhesives consist primarily of epoxy resin and a curing agent. Filler, toughener, plasticizer, and additional additives including silane coupling agent, deformer, and colorant, among others, can be added as needed.
|Primary||Epoxy resin, reactive diluent||Adhesive base|
|Primary||Curing agent or catalyst, accelerator||Curability|
Table 1: Epoxy Adhesive Compositions
Epoxy resins are primarily made by reacting active hydrogen from phenols, alcohols, amines, and acids with epichlorohydrin, commonly abbreviated as ECH, under carefully regulated conditions. Epoxy resin can also be made by oxidizing an olefin with peroxide, similar to how cycloaliphatic epoxy resins are made.
Bisphenol A diglycidyl ether, sometimes known as bisphenol A type epoxy resin, was the first commercially available epoxy resin and is still the most extensively used today. This form of epoxy resin is expected to account for roughly 75% of epoxy resin used in industry on a volume basis.
Bisphenol A diglycidyl ether, the most common epoxy resin used in epoxy adhesives, has a chemical structure and important properties of numerous functional groups.
Curing begins when the resin is combined with a specific catalyst. Curing is the exothermic reaction that occurs when molecular chains react at chemically active locations. The covalent connections that form between the epoxide groups of the resin and the amine groups of the hardener or catalyst as a result of this combination allow the polymer to cross-link, dictating the stiffness and strength of the epoxy. The ability to change mechanical strength qualities and thermal, electrical, and chemical resistance by monitoring curing conditions by temperature and choice of resin and hardener chemicals is possible. As a result, epoxy adhesives meet a wide range of applications and operating circumstances.
Epoxy adhesives stick to a wide range of materials, and their qualities are determined by the system's chemistry and the type of cross-linking available. Exceptional chemical and heat resistance, good adhesion and water resistance, and satisfactory mechanical and electrical insulating qualities are just a few of the most significant performance requirements.
Epoxy adhesives, the most generally used structural type glue, are commonly available as one-component or two-component systems. One-component epoxy adhesives are typically cured at temperatures between 250 and 300°F, resulting in a product with strength, strong metal adhesion, and environmental and harsh chemical resistance. In reality, this product is frequently used instead of welding and rivets.
One-component systems are pre-catalyzed and only require mild heat to cure, increasing efficiency and reducing the risk of mistake caused by air entrapment. The cure time for one component system is less than that of two-component systems. Catalyzation occurs at ambient temperatures and can be increased by heat in two-component epoxy systems. Heat stimulates further cross-linking, which results in better properties.
Most surfaces can be bonded with two-component solutions. Because of their capacity to endure a constant weight or force for an extended period of time, as well as their resistance to physical and chemical effects, they are highly stable. Two-component epoxies are highly stable due to their great capacity to endure a steady weight or strain over an extended period of time, as well as their excellent resistance to physical and chemical effects.
They're incredibly adaptable, and can be used for bonding, sealing, coating, and encapsulation in a variety of industries, including electronics, medical devices, and aerospace, to name a few. Flame retardant, cryogenically serviceable, fast curing, high temperature resistant, and more are all features of special formulations.
Some of the considerations when choosing epoxy adhesives include:
An adhesive must thoroughly "wet out" the surface to be bonded for optimal adhesion. When an adhesive "wets out," it flows and covers a surface in order to increase the contact area and attractive forces between the adhesive and the bonding surface.
The following are the primary design guidelines to keep in mind when creating an adhesive joint:
Many common conditions, such as temperature, moisture, chemical fluids, and outside weathering, have an impact on bond strength. Candidate adhesives should be tested under simulated service conditions in situations where probable degrading factors are present.
When analyzing adhesives, it's crucial to ensure that they operate not just in terms of performance but also in terms of production. What method will be used to apply the material? Is it a one or two-component system?
Early consideration of dispensing, mixing, and applying concepts can save a lot of time and money in the long run, as well as prevent production delays.
In the end, evaluating adhesives boils down to picking one (or a few) and putting them to the test in the application before making a decision. There are established tests for comparing relative qualities. As previously stated, there is no replacement for personally sampling and testing products.
In order to get the best adhesive performance for a given application, first determine the right cure schedule. The following are the major guidelines to consider:
Epoxies are often delivered in two parts, which are weighed and combined for the quantity required at the time of usage. This means that a user can extend the adhesive's shelf life and avoid wasting it.
Many drugs are now available in dual-barrel cartridges with static mixing tips that dispense the two components — which are premixed in the right mix ratio — for improved convenience. This allows the user to apply a homogeneous, repeatable blended adhesive. Epoxies are sometimes offered in a frozen, premixed form that must be thawed immediately before use. In the electronics business, this mode of delivery is highly prevalent.
Chemical and flame resistance are both important considerations. Epoxies' resilience to a variety of substances is one of their most essential characteristics. Many of these compounds, including acids, bases, organic solvents, fuels, fluids, and fresh and saltwater, are now appropriate for exposure to harsh chemicals and conditions.
Some epoxies are also manufactured to meet flame-resistance specifications, such as those for applications that must adhere to strict guidelines (such as for the Underwriters Laboratories, a global safety certification and others). Specialty epoxies also extend the life of devices in hostile settings.
Electronic and electromechanical assemblies typically use adhesives. Epoxies can be used to bond materials that need to retain electrical or thermal conductivity between them in these applications.
Epoxies, for example, can bond heat sinks to electronic assembly parts that create heat during operation. To promote heat dissipation, a thermally conductive epoxy allows for more effective thermal transfer to the heat sink. Electrically conductive epoxies can be used to join components that require an electrical connection for the transmission of electrical signals or the discharge of static electricity.
Design and process engineers may now construct more dependable and high-performing devices thanks to these bonding properties.
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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