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 explore what epoxy adhesives are, how they are manufactured, and how they function.
Epoxy: Epoxy glue is a thermosetting adhesive composed of a resin or epoxy polymer and a hardener. It is used to bond a variety of surfaces together, creating a strong, permanent, and robust bond capable of withstanding extreme stress and weather conditions.
Adhesive: Refers to a substance used for sticking objects or surfaces together.
Epoxy adhesives are among the most widely used industrial adhesives and are renowned for their versatility as structural adhesives. The strength of the cured product, combined with their remarkable ability to bond to a wide variety of materials, makes epoxy adhesives highly popular. Epoxy resin glue solutions are also easy to customize to meet the specific property requirements of any project.
Epoxy adhesives are composed of various types of epoxy resin, which determine the adhesive's fundamental properties. For applications requiring high temperature resistance, heat-resistant epoxy resin is the ideal choice, while flexible epoxy resin is best suited for situations where movement is possible.
When evaluating the efficacy of an epoxy adhesive, it's helpful to consider the general composition of its compounds. Epoxies are produced through the polymerization of two initial components: the resin and the hardener. Epoxy adhesives consist primarily of epoxy resin and a curing agent. Additional components such as filler, toughener, plasticizer, and other additives including 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 | Curability |
Modifying | Filler | Property Modification |
Modifying | Toughener | Toughening |
Modifying | Plasticizer | Flexibility |
Additive | Coupling Agent | Adhesion |
Additive | Colorant | Color |
Table 1: Epoxy Adhesive Compositions
Epoxy resins are primarily produced by reacting active hydrogen from phenols, alcohols, amines, and acids with epichlorohydrin (ECH) under carefully controlled conditions. Epoxy resin can also be created by oxidizing an olefin with peroxide, similar to the process used to produce cycloaliphatic epoxy resins.
Bisphenol A diglycidyl ether, also known as bisphenol A type epoxy resin, was the first commercially available epoxy resin and remains the most widely used today. This type of epoxy resin is expected to account for approximately 75% of the epoxy resin used in industry by volume.
Bisphenol A diglycidyl ether, the most common epoxy resin used in epoxy adhesives, features a chemical structure with several functional groups that contribute to its important properties.
Curing begins when the resin is mixed with a specific catalyst. This process, an exothermic reaction, occurs as molecular chains react at chemically active sites. The covalent bonds formed between the epoxide groups of the resin and the amine groups of the hardener or catalyst enable the polymer to cross-link, which determines the epoxy's stiffness and strength. By controlling curing conditions such as temperature and selecting appropriate resin and hardener chemicals, it is possible to adjust mechanical strength, as well as thermal, electrical, and chemical resistance. This versatility allows epoxy adhesives to be used in a wide range of applications and operating conditions.
Epoxy adhesives adhere to a wide range of materials, with their properties determined by the system's chemistry and the type of cross-linking employed. Key performance characteristics include exceptional chemical and heat resistance, excellent adhesion and water resistance, and satisfactory mechanical and electrical insulating properties.
Epoxy adhesives, widely used as structural glues, are commonly available in 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 strong adhesion to metals, high strength, and resistance to environmental and harsh chemical conditions. In practice, these adhesives are often used as alternatives to welding and rivets.
One-component systems are pre-catalyzed and require only mild heat to cure, which enhances efficiency and minimizes the risk of errors due to air entrapment. These systems generally have a shorter cure time compared to two-component systems. In contrast, two-component epoxy systems require catalyzation at ambient temperatures, which can be accelerated by heat. The application of heat stimulates further cross-linking, leading to improved properties.
Two-component solutions can bond most surfaces effectively. They are highly stable due to their ability to withstand a constant weight or force over extended periods and their excellent resistance to physical and chemical effects.
They are incredibly versatile and can be used for bonding, sealing, coating, and encapsulation across various industries, including electronics, medical devices, and aerospace, among others. Special formulations offer additional features such as flame retardancy, cryogenic serviceability, fast curing, and high-temperature resistance.
Some considerations when choosing epoxy adhesives include:
For optimal adhesion, an adhesive must thoroughly "wet out" the surface to be bonded. When an adhesive "wets out," it flows over and covers the surface, increasing the contact area and enhancing the attractive forces between the adhesive and the bonding surface.
The following are key design guidelines to consider when creating an adhesive joint:
Bond strength can be significantly affected by common conditions such as temperature, moisture, chemical fluids, and environmental weathering. Therefore, candidate adhesives should be tested under simulated service conditions where potential degrading factors are present.
When analyzing adhesives, it is crucial to assess not only their performance but also their production requirements. Consider the application method for the material: will it be applied as a one-component or two-component system?
Early consideration of dispensing, mixing, and application methods can save significant time and money in the long run and help prevent production delays.
Ultimately, evaluating adhesives involves selecting one (or a few) options and testing them in the actual application before making a final decision. While established tests can compare relative qualities, there is no substitute for personally sampling and testing the products yourself.
To achieve optimal adhesive performance for a given application, first determine the appropriate cure schedule. Consider the following major guidelines:
Epoxies are often supplied in two parts, which are weighed and mixed according to the required quantity at the time of use. This approach allows users to extend the adhesive's shelf life and minimize waste.
Many adhesives are now available in dual-barrel cartridges with static mixing tips, which dispense the two components pre-mixed in the correct ratio providing improved convenience. This allows users to apply a homogeneous, consistently blended adhesive. Additionally, epoxies are sometimes offered in a frozen, premixed form that must be thawed before use. This method of delivery is especially common in the electronics industry.
Chemical and flame resistance are important considerations when selecting adhesives. One of the key characteristics of epoxies is their resilience to a variety of substances. They can withstand exposure to harsh chemicals and conditions, including acids, bases, organic solvents, fuels, fluids, and both fresh and saltwater.
Some epoxies are specifically manufactured to meet flame-resistance specifications, such as those required by safety certification standards like Underwriters Laboratories and others. Specialty epoxies are also designed to extend the lifespan of devices in harsh environments.
Epoxies are commonly used in electronic and electromechanical assemblies. They can bond materials while maintaining electrical or thermal conductivity between them, making them suitable for these applications.
Epoxies, for example, can bond heat sinks to electronic components that generate heat during operation. A thermally conductive epoxy enhances heat dissipation by allowing more effective thermal transfer to the heat sink. Additionally, electrically conductive epoxies can be used to join components that require an electrical connection for signal transmission or static electricity discharge.
These bonding properties enable design and process engineers to create more reliable and high-performing devices.
This chapter will explore the different classes and types of epoxy adhesives.
Epoxy adhesives are available in both one-component and two-component systems. Both types form exceptionally strong bonds but differ in their curing processes, final properties, and applications. Generally, two-component epoxy resin adhesives are more durable than single-component systems.
Before application, the two components of two-component (2K) epoxy resin adhesives must be mixed together. This reaction causes the epoxy adhesive to cure. Two-component epoxy resin adhesives offer greater adaptability and are used across various market segments. However, they can be more challenging to handle since the resin and catalyst need to be mixed. The working time of these adhesives can range from a few minutes to several hours, depending on the specific composition of the industrial epoxy adhesive.
Two-component (2K) epoxy resin adhesives cure at room temperature, but the curing process can be accelerated by applying heat or external energy sources like UV radiation, depending on the formulation. Epoxy adhesives achieve their strongest bonds when the curing period is extended.
Two-component (2K) epoxy glue solutions can adhere to nearly any surface, including wood, metals, plastics, ceramics, and various 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°F).
In contrast to two-component (2K) epoxy glue, one-component epoxy resin adhesives are straightforward to use as they do not require mixing. They typically have a paste-like consistency, making them suitable for trowel application or bead extrusion. These adhesives cure at elevated temperatures, generally between 120°C and 175°C, depending on the formulation.
Single-component heat-cure adhesive systems efficiently fill and seal gaps, especially between metal surfaces. These adhesives are also available as heat-curing film adhesives, which are ideal for bonding or laminating large areas.
These films are utilized in assembly compounds within the aerospace sector. One-component epoxy resin adhesives are also commonly employed as construction adhesives, including for tile installation.
The various types of epoxy adhesives include:
DGEBA epoxy, or diglycidyl ether of bisphenol A (BPA), is the earliest and most significant epoxy resin used in adhesive formulations, largely due to its low raw material cost. Additionally, this resin is compatible with a wide range of catalysts, enabling the creation of a diverse range of properties.
DGEBA epoxy-based industrial adhesives can cure at ambient temperature or with the addition of heat. DGEBA epoxy resin is available in various forms, including low molecular weight liquids, high molecular weight semi-solids and solids, and brominated resins. The brominated resins are specifically used in applications that require ignition resistance, such as circuit boards and other flame-retardant applications.
Epoxies are naturally hydrophobic and cannot dissolve in water. However, water dispersibility can be achieved through chemical modification or emulsification. Emulsification, which is commonly used for resins in aqueous epoxy adhesives, involves using a surfactant to separate the water from the resin. The mechanical and chemical stability of the adhesive depends on the surfactant used and the manufacturing parameters employed.
Two types of epoxy acrylate resins used in adhesives are vinyl ester and a specific resin designed for radiation curing. Although they are epoxies, these resins behave more like polyester resins. They are easy to manufacture, cure quickly at room temperature, and can be cured with peroxides.
Epoxy acrylate resins have lower viscosity, increased flexibility, and better wetting properties compared to traditional DGEBA epoxies. However, they tend to shrink more during curing. These resins can also be cured using external radiation sources, such as ultraviolet (UV) light or electron beams. Generally, epoxy acrylate resins exhibit low viscosity and vapor pressure in adhesive applications.
Epoxy glues and adhesives are not typically known for their flexibility. However, long-chain aliphatic epoxy resins can enhance the flexibility of epoxy adhesives. While these flexible resins improve flexibility, they also reduce the adhesive's hardness. Consequently, flexible epoxy resins are usually blended with other epoxies to create a tougher and stronger adhesive that still maintains flexibility. By incorporating 10-30% aliphatic epoxy resin, the desired level of flexibility can be achieved without significantly compromising other properties. Flexible epoxy resin adhesives are effective for applications such as laminating safety glass, absorbing vibration and sound, and encapsulating electrical and other delicate components that undergo temperature cycling.
Epoxy novolac adhesives are renowned for their superior chemical and thermal resistance compared to other epoxies. They also offer better adhesion to surfaces than BPA epoxies. To fully develop these qualities, epoxy novolac adhesives must be cured at high temperatures. When cured at room temperature, they achieve properties similar to those of DGEBA epoxy adhesives. The thermal stability of the cured bond is influenced by the length of the curing cycle.
Epoxy novolac resins are typically difficult to process due to their high viscosity. Recently, low viscosity alternatives have been developed to ease manufacturing, although these products have a lower epoxy resin content. Despite this, epoxy novolac is almost exclusively used as a two-component (2K) epoxy glue.
Epoxy adhesives designed for room temperature curing are typically provided in a two-component package, with the epoxy resin in one part and the curing agent in the other. When these components are mixed, the epoxy resin rapidly reacts with the curing agent at room temperature to form a strong, cross-linked thermoset structure that adheres tightly to substrates. The pot life and curing time can be adjusted by using different types of curing chemicals.
Mercaptan compounds are commonly used as curing agents for fast room temperature curable epoxy adhesives due to their rapid reaction with epoxy resin, especially in the presence of basic chemicals such as tertiary amines or imidazoles as accelerators. The epoxy resin reacts with the mercaptan group through a poly-addition reaction mechanism. Fixture time at room temperature can range from as short as 15 minutes to as long as 30 minutes, with complete curing taking 24 hours. Due to their limited working life, which can be as short as 5 to 10 minutes, precautions must be taken during application.
In epoxy resin technology, aliphatic polyamines are the most commonly used curing agents. Several modified polyamine-type curing agents have been commercialized to improve curability, handling, and other physical properties. These agents react with an epoxide in a polyaddition mechanism, involving the active hydrogen of primary and secondary amines. By selecting the appropriate curing agent, fixture time and work life can be adjusted to suit specific needs.
UV-curable epoxy adhesives cure rapidly and have been effectively used in various electronics assembly and general bonding applications. These include image sensor module assembly, display panel and module assembly, and other scenarios where high adhesion performance and fast production speeds are crucial. Recently, a range of UV-cationic epoxy adhesives and UV-acrylate hybrid thermal cure epoxy adhesives have been introduced to further enhance performance and versatility.
When compared to typical UV acrylate adhesives, UV cure epoxy adhesives have no oxygen inhibition issue, reduced curing shrinkage, and improved adherence.
UV-cationic epoxy adhesives primarily consist of epoxy resin and a cationic photo-initiator. Cycloaliphatic epoxy resins are often used in these adhesives due to their faster cationic polymerization rate compared to traditional bisphenol A diglycidyl ether type resins. In UV epoxy adhesives, cationic photo-initiators absorb UV light to generate strong acids. These acids react with the epoxy resin to produce cationic species, which then induce the homopolymerization of the epoxy resin.
UV-cationic epoxy adhesives exhibit reduced cure shrinkage compared to traditional acrylate-based UV adhesives due to their epoxy structure. Additionally, they do not experience surface cure issues related to oxygen inhibition, which affects free radical polymerization, because they cure via cationic polymerization.
UV-cationic epoxy adhesives are not suitable for bonding basic substrates that interfere with cationic polymerization. Additionally, these adhesives may require more time to cure. In practice, a post-thermal cure is often employed after UV radiation to ensure complete curing and achieve satisfactory adhesion performance.
Acrylate compositions are among the most commonly used for UV-curable adhesives. The key components of acrylate-based UV cure adhesives are acrylate monomers, acrylate oligomers, and radical photo-initiators. These adhesives cure almost instantaneously, within seconds, under UV light. However, acrylate-based UV cure adhesives have some drawbacks, including surface cure issues, shadow cure problems, excessive cure shrinkage, and limited humidity reliability.
UV and thermal cure hybrid epoxy adhesives have been developed and commercialized for over two decades by combining UV acrylate and thermal cure epoxy compositions. These hybrid adhesives typically include acrylic monomers, epoxy resins, photo-initiators, and epoxy curing agents. By integrating both UV acrylate and thermal cure epoxy components, these adhesives leverage the benefits of both types. Compared to standard UV acrylate adhesives, the inclusion of epoxy components can significantly enhance adhesion reliability.
Production efficiency can be significantly enhanced with UV and thermal cure hybrid epoxy adhesives by reducing fixture time to just seconds with UV radiation, compared to the extended curing times required for thermal cure epoxy adhesives. Additionally, the lower concentration of free radical curable acrylate compositions in these hybrids can mitigate some of the issues associated with traditional UV acrylate adhesives, such as surface cure problems, shadow cure issues, and excessive cure shrinkage.
In some cases, a thermal initiator, such as peroxide, is included in the hybrid adhesive formulation to ensure the curing of acrylate compositions that have not been exposed to UV light or are in shadowed areas where UV light cannot penetrate.
This chapter will explore the various applications and advantages of epoxy adhesives.
The applications of epoxy adhesives include:
The benefits of epoxy adhesives include:
The drawbacks of epoxy adhesives include:
Epoxy adhesives offer excellent adherence to a wide range of surfaces and are among the most commonly used structural adhesives. They can be cured at room temperature, elevated temperatures, or via UV light radiation, depending on the type of curing agent used. Both one-component and two-component epoxy adhesives are available and have been widely utilized in various industrial applications for bonding metals, concrete, glass, ceramics, plastics, wood, and other materials.
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