This article will take an in-depth look at hot melt adhesives.
The article will bring more detail on topics such as:
This chapter delves into the details of hot melt adhesives, examining their production process and how they function.
Hot melt adhesives, commonly known as glue adhesives, are thermoplastic polymers that remain solid at room temperature but become liquid when heated beyond their softening threshold.
These adhesives have a relatively low softening temperature compared to other packaging materials, allowing them to be used across various industries. Turning into liquid upon heating, they become active and enable bonding between surfaces.
In solid form, hot melt adhesives come in shapes like beads, blocks, pellets, chips, pillows, or glue sticks. Depending on their application, they might be extruded through a nozzle, sprayed, or rolled onto surfaces.
The makeup of a hot melt adhesive affects its cost, performance, functionality, and availability. A typical hot melt comprises a polymer mixed with various additives, each serving different purposes.
Common additives include antioxidants, resins, plasticizers, and waxes, with other chemicals added to enhance specific properties of the adhesive.
Polymers are vital components of hot melts. Being long chains of specific molecules, polymers determine most physical traits of hot melt adhesives, such as heat resistance, flexibility, and strength.
The type of polymer, its molecular weight, and concentration influence properties like shear and impact resistance. Higher polymer content raises the viscosity, which refers to the thickness of the liquid, and enhances the adhesive's toughness and flexibility.
Resins crucially determine the tack, which refers to the stickiness and persistence of the adhesive after application. They also affect wetting properties, which control how the adhesive behaves in its liquid state on surfaces.
The choice of resin—based on factors like its softening point and compatibility with the primary polymer—affects the adhesive's performance. Resins such as rosin, its derivatives, and hydrogenated hydrocarbons are commonly used.
A higher resin content generally reduces tack and shortens drying time, while a lower resin content results in a more aggressive and durable hot melt.
Waxes primarily control the open time and set speed of hot melt adhesives. Open time refers to how long it takes to establish a bond, and set speed measures the quickness in achieving a sufficiently strong bond.
These parameters need to match production line requirements. Waxes also affect heat resistance and sub-ambient adhesion. Common waxes include synthetic, microcrystalline, and natural waxes, which vary by molecular weight, crystallinity, and melting point.
A lower wax content leads to higher viscosity, resulting in a more flexible and aggressive adhesive, while a reduced wax content lowers viscosity, decreases aggressive bonding, and accelerates setting.
Antioxidants prevent degradation in hot melts over time and include aromatic amines, phenols, BHT, phosphites, and phosphates. Mixed with stabilizers, these antioxidants do not change the adhesive's physical properties but protect it during its molten state and throughout compounding.
Plasticizers, used alongside resins and polymers, are crucial additives in hot melts. Acting as secondary base polymers, they boost toughness and flexibility. Typically, plasticizers are hydrocarbon oils that lower melt viscosity, enhancing wetting abilities with minimal aromatic content.
Beyond the main components, hot melts can incorporate various additives for specific applications, like biocides for bacterial inhibition, fillers to increase strength and decrease costs, or pigments, flame retardants, and even glitter for desired visual effects.
This chapter will cover the production process and characteristics of hot melt adhesives.
Hot melt adhesives are thermoplastic materials that become pliable or plastic when heated and solidify as they cool. They are free from any liquid components. The essential raw materials for producing hot melt adhesives include resins, polymers, plasticizers, waxes, and antioxidants. After these ingredients are combined, the adhesive solidifies into its final form, ready for use or distribution across various industries. Common types of hot melt adhesives include polyurethane, metallocene, EVA, and polyethylene.
Hot melt adhesives are heated until they reach a liquid state. This liquid is then applied to the target surface using methods such as spraying, rolling, or extrusion. Following application, the adhesive is allowed to briefly settle to achieve the ideal moisture level and temperature for effective bonding. The two surfaces are then joined, and in some cases, pressure is applied to ensure a robust and effective bond.
Key properties and attributes of hot melt adhesives include their molten appearance, failure temperature, softening point, viscosity, thermal stability, loop tack, adhesion specific to different substrates, and various mechanical characteristics.
Viscosity: Viscosity measures the thickness of a liquid and its resistance to flow. Liquids with high viscosity, such as oil, move slowly, whereas those with lower viscosity flow more easily. The viscosity of hot melts is influenced by the application temperature, typically between 250 and 350°F (121 to 176°C). Shear rate is a key factor in determining viscosity. Various instruments, such as capillary rheometers, Brookfield thermosel, and dynamic mechanical analyzers, are used to assess viscosity, which is measured in poise.
Molten Color: The color of hot melt adhesives is evaluated using numerical scales and both quantitative and subjective methods, including Hunter, Gardner, Saybolt, and the yellowness index.
Peel: Peel measures the force required to separate bonded surfaces. It is quantified in pounds per inch and can be tested at various angles and on different surfaces.
Failure Temperature: Failure temperature is the point at which a hot melt adhesive ceases to perform effectively. It is divided into peel adhesion failure temperature and shear adhesion failure temperature. Peel adhesion failure occurs when bonded surfaces can be easily separated at high temperatures, while shear adhesion failure involves the force applied as one surface slides over another. The strength of the adhesive is tested by mounting a specimen vertically, applying weight, and observing how long it takes for the surfaces to part.
Softening Point: The softening point is the temperature at which the hot melt begins to flow. It is influenced by the transition temperature of the base polymer and the type of wax used. Common methods to determine the softening point include the ring and ball test and the Mettler method.
Substrate-Specific Adhesion: This property varies depending on the material used. Bond strength characteristics are assessed using actual substrates and can be tested at ambient, elevated, and sub-ambient temperatures.
Thermal Stability: Thermal stability indicates how well a hot melt can endure various temperatures without degrading. This is assessed by heating the hot melt to its application temperature. A hot melt with high thermal stability remains stable at extreme temperatures, showing no significant decomposition. The test evaluates changes in viscosity, and the formation of charring, skin, edge rings, color shifts, and gel formation.
Cold Crack Formation: Cold crack formation tests determine at which low temperatures hot melts may develop cracks. Rubber-based hot melts typically show cracking in temperatures between 10 to 30°F (-12 to -1°C), while other types of hot melts can crack at temperatures ranging from 15 to 45°F (-9 to 7°C).
Loop Tack: Loop tack is used for evaluating pressure-sensitive hot melts that bond only when pressure is applied. This test measures the adhesive’s aggressiveness, with results expressed in ounces. A higher force needed to remove the hot melt indicates greater aggressiveness.
Mechanical Properties: Mechanical properties are crucial for evaluating a hot melt's performance. Key properties include the yield point, which is the stress level at which the hot melt begins to deform permanently, and tensile strength, which measures the force required to break a sample.
Hot melt adhesives are favored for their flexibility, ease of application, and lower melting points relative to other adhesive types. These properties make them highly suitable for a wide range of substrates and materials. Hot melt adhesives are particularly safe and effective for use on the following substrates:
Hot melt adhesives come in various types, including:
Ethylene vinyl acetate (EVA) serves as a polymer base commonly used in industrial hot melt adhesives for packaging. EVA is known for producing high-performance and competitive packaging adhesives. It is a popular choice due to its broad performance range, making it suitable for various applications. EVA can be used in both standard and low-temperature hot melt adhesives, offering flexibility across different conditions.
Polyethylene serves as a polymer base effective in tray sealing and carton applications. These adhesives are characterized by their low odor, light color, and easy release from metal surfaces, which facilitates cleanup. However, polyethylene is not well-suited for bonding substrates, making it more appropriate for traditional application temperatures.
Metallocene polymers have been around for decades, where they were integrated with other materials to enhance the stability of hot melt adhesives. These polymers offered excellent storage temperature stability in both hot and cold environments and quickly became a popular choice. Metallocenes minimized degradation due to heat exposure, reduced charring, and improved bonding performance across temperature extremes. While some current adhesives still use metallocene, there are now various formulations with differing stabilities, costs, and bonding characteristics compared to the original versions.
Amorphous poly alpha olefins (APAO) are primarily utilized in applications requiring extended working times or strong adhesion to plastics. These adhesives are known for their tacky, soft, and highly flexible properties, making them ideal for product assembly, including adding foam protection to corrugated packaging and bonding corrugated plastic containers.
Polyamide adhesives excel in applications requiring high-temperature resistance and quick assembly strength. They are particularly useful in scenarios demanding solvent and oil resistance. Ideal for wood, filter, and other high-performance applications, polyamides perform well in extreme temperatures, around 400 °F (204 °C). However, compared to other adhesive types, they may not exhibit the same level of stability under heat.
This chapter will explore the various applications and advantages of hot melt adhesives.
Hot melt adhesives find widespread use across various industries. Here are some key sectors that benefit significantly from these adhesives:
Spraying hot melt adhesives is an efficient and rapid method for application. This technique involves directing a continuous stream of adhesive to create specific patterns using compressed air. To maintain machine efficiency and avoid operational problems, it's important to oversee the application process carefully to prevent issues such as:
The process of cooling and setting hot melt adhesives starts with a spray nozzle designed for hot melts. This technique is especially effective for covering extensive surface areas.
Hot melt slot coating involves applying a layer of hot melt adhesive onto a substrate before it solidifies and cools. This method is typically used for adhesives that are pressure-sensitive, especially on thin substrates.
In the process of creating a hot melt bead or line, the nozzle dispenses a specific amount of adhesive before stopping momentarily and then restarting. This technique is mainly used when an adhesive needs to be applied precisely to a particular location and quickly compressed. Like with spray applications, the adhesive begins to cool immediately after being dispensed. Adhering to the recommended speed is essential to prevent any issues with bonding during this application.
While the benefits of hot melt adhesives can differ across various industries, there are several common advantages that are widely recognized. Some of these universal benefits are outlined below.
Hot melts are created from a variety of materials, leading to significant differences in cost, effectiveness, and the final characteristics of the adhesives. This diversity highlights the range of performance and functionality among different hot melt products.
Hot melts used for labels and tapes are typically pressure-sensitive. The labeling industry requires customizable products that offer a wide range of tack levels. A smaller tack range can be used for tapes, although they need a high level of shear resistance. Pressure-sensitive hot melts are now being used in place of solvent-based ones, mainly because they have a greater plasticizer and heat resistance.
Hot melts are commonly utilized in packaging, heat sealing, tray molding, and container labeling. Key attributes of these adhesives include extended pot life, adaptability in adhesion, minimal odor, lower application temperatures, and cost-efficiency. Specialized hot melts are employed for flexible packaging, such as small containers and lids.
In the graphic arts industry, hot melts are predominantly based on EVA and are typically applied at temperatures between 325 and 350 degrees Fahrenheit (163 to 176°C). These adhesives are known for their quick setting and reliable performance within a temperature range of 20 to 120 °F (-6.6 to 48.8 °C). They feature a unique property where they do not snap shut if released, akin to a mouse trap index. Additionally, graphic art hot melts resist ink solvents. Applying these adhesives requires precision, often using specialized tools, with the hot glue gun being the most frequently utilized.
Hot melt adhesive cleaners are very important to maintain and keep equipment running efficiently. There is a wide variety of hot melt cleaning systems and fluids from different manufacturers. Hot melt cleaners are offered for thermoplastic, polyurethane-based, polyamide-based, and polyolefin adhesives.
This adhesive exhibits excellent resistance to both extreme cold and heat. It offers robust bonding strength and superior vibration resistance, enhancing production efficiency and achieving peak strength more quickly than silicone alternatives.
Primarily used for laminating foam and mattresses, this adhesive cures rapidly, making it ideal for swift production. It features strong initial adhesion and good heat viscosity, along with excellent weather resistance.
Designed for bookbinding, this adhesive is suitable for binding reports, magazines, and hardcover books. It provides high bonding strength, weather resistance, and flexibility, ensuring the book remains flat.
This adhesive is highly effective for fabrics, offering strong adhesion, resistance to washing, and stability.
These adhesives are formulated to withstand high humidity and aging, making them suitable for various wood types and other materials.
Specifically intended for multi-layered materials, this adhesive provides strong, flexible bonding and long-lasting durability.
This adhesive is suitable and mainly used for laminating materials like aluminum boards, calcium silicate, bright PVC, etc.
This adhesive is ideal for bonding clear containers made of OPP and PVC. It features excellent adhesion, strong initial bonding, and impressive resistance to aging.
Electronic assembly adhesive is designed for use with various plastic materials in product assembly. It provides strong water resistance and ensures an airtight seal.
Hot melt adhesives find applications across diverse industries, including functional clothing, furniture laminating, and medical protection laminating adhesives.
Hot melt adhesives are also known as glue adhesives. They are thermoplastic polymer adhesives that, at room temperature, are in a solid state. Hot melt adhesives liquefy if they are heated at a temperature that is above their softening point. The use of hot melt adhesives has been of great impact on humanity. They are much safer and not hazardous for operators to use as compared to many other adhesives and thus make them a suitable and appropriate alternative.
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