This article will take an in-depth look at hot melt adhesives.
The article will bring more detail on topics such as:
This chapter will discuss hot melt adhesives, their manufacturing process, and how they function.
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.
Their softening temperature is relatively low compared to various packaging components. The liquefaction of hot melt adhesives allows for their use and application in many sectors. When in the liquid state, hot metal adhesives activate and allow the binding of the surface.
Hot melt adhesives are present in many forms in their solid state. They can be beads, blocks, pellets, chips, pillows, or glue sticks. Depending on the application, the holt melt can be extruded from a nozzle, sprayed, or rolled onto surfaces.
The raw materials in a hot melt affect its cost, performance, function, and availability. Generally, the hot melt is made up of a polymer and many additives that can come in various forms.
Some of these additives are antioxidants, resins, plasticizers, and waxes. Other chemicals can be added to give the hot melt more properties.
Polymers are considered to be the basic constituents of the hot melt. Various types of glue are polymers. These are repetitive and long chains of specific molecules which have different properties. Polymers are responsible for most of the physical properties found in hot melts.
Polymers give the hot melt its heat resistance, flexibility, strength, and shear and impact resistance. The above characteristics depend greatly on the polymer type, molecular weight, and amount. A high content of polymers in hot melt means high viscosity. Viscosity is defined as the measure of liquid thickness. A greater content of polymer also means greater toughness and flexibility.
Resins are responsible for the tack of the hot melt. Tack is defined as the measure of adhesive stickiness, which is generally the period the adhesive stays intact after its application. In addition, resins are in control of the wetting of an adhesive. This means resins control the period it remains a liquid while the hot melt is in contact with the surface substrate.
Resins have a great effect on the nature of a hot melt. The selection of a resin depends on various factors such as its softening point, its compatibility with the main polymer, and the specific adhesion. Most of the resin types used in a hot melt are rosin and hydrogenated rosin, rosin ester, hydrogenated hydrocarbon, terpene phenolic, C5, and C9.
Some melts also use aromatic monomers. Adding more resin creates a hot melt with minimized tack and a much faster drying speed. Conversely, less resin implies a hot melt that is more aggressive and tougher.
The open time and set speed in a hot melt are primarily controlled by wax. Open time is defined as the amount of time it takes for a bond to be made. The time ranges from a few seconds to a long-lasting duration when dealing with some hot melts that are pressure sensitive. The set speed's purpose is to measure how quickly the hot melt can form a bond of acceptable strength.
The values obtained have to be in perfect harmony with the parameters used in the production lines. Wax also influences the heat resistance and sub-ambient adhesion of the hot melt. The wax types usually used in hot melts are synthetic waxes, microcrystalline waxes, and natural waxes. The amount or percentage of the molecular weight, crystallinity, and melting point determine the characteristics of the wax.
The hot melt will show a greater viscosity with lower content of wax. As a result, it will be more flexible and very aggressive when bonding. When there is less wax content, the hot melt will show less aggressiveness when bonding, lower viscosity, and set much faster.
The main use of antioxidants in hot melts is to protect the material from degrading over time. Usually, the antioxidants used in hot melts are aromatic amines, phenols, BHT, phosphites, and phosphates. Antioxidants are added together with stabilizers in very small quantities. They do not affect the physical properties of the hot melt. The antioxidants help protect the hot melt during its molten state when its application occurs and when the hot melt is being compounded.
Plasticizers are one of the most used additives in hot melts besides resins and polymers. They are used as a second base polymer to provide greater toughness and flexibility to the hot melt. Most often, they are hydrocarbon oils with little aromatic content, and their chemical characteristics are the same as those of paraffin. Plasticizers are well known for their low volatility. They have no odor, and plasticizers allow for lower melt viscosity and faster wetting. In addition to the main ingredients, many other additives are used in hot melt adhesive formation to develop specific desired characteristics. For example, biocides help prevent bacteria growth; fillers add strength and bulk and, at the same time, reduce cost; and finally, hot melts can be combined with various pigments, flame retardants, and even glitter.
This chapter will discuss the manufacture and properties of hot melt adhesives.
Hot melt adhesives are thermoplastic plastics; they become malleable or plastic upon heating and harden upon cooling. Hot melt adhesives do not contain any form of liquid. The raw materials required for their formation are resins, polymers, plasticizers, waxes, and antioxidants. Once these are in place, the hot melt adhesive hardens itself into the end product, ready for delivery or use in many industries. Polyurethane, metallocene, EVA hot melt, and polyethylene are common hot melt adhesives.
The defining properties and characteristics of hot melt are its molten color, failure temperature, softening point, viscosity, thermal stability, loop track, substrate-specific adhesion, and other mechanical properties.
Viscosity: Viscosity is the measure of the liquid thickness and how much resistance it offers to flow. Liquids that have a high viscosity move very slowly, like oil. Higher viscosity liquids have the advantage of flowing easily. The hot melt viscosity depends greatly on the application temperature, ranging from 250 to 350 °F (121 to 176 °C). The shear rate is an important factor to consider to figure out viscosity. Many devices are used to test for the viscosity of a hot melt. These devices include capillary rheometers, the Brookfield thermosel, and dynamic mechanical analysis. The unit used to measure viscosity is called poise.
Molten Color: A numerical scale is used to measure the hot melt's color using quantitative and subjective procedures. These include Hunter, Gardner, Saybolt, and the yellowness index methods.
Peel: Peel is defined as the measure of the force required to break the bond between two surfaces that are bonded together. The units for measuring peel are pounds per inch. Peel can be measured at different angles as well as for different surfaces.
Failure Temperature Definition: The failure temperature is the temperature at which the hot melt stops working. Failure temperature is categorized into two types: peel adhesion failure temperature and shear adhesion failure temperature. Under peel adhesion failure temperature, it becomes very easy to peel off the surfaces bonded with the hot melt at great temperatures. Under shear adhesion failure temperature, shear is considered to be a force found on one surface as it slides over the other. A specimen is mounted vertically during a shear test, and a weight is attached. The strength of the hot melt is affected by the period it takes for the surfaces to separate from each other.
Softening Point: The softening point of any glue or hot melt is considered the temperature where the hot melt starts to flow. The main determinants of the softening point of hot melt are the base polymer's transition temperature and the wax used. The methods that are used greatly are the ring and ball and the Mettler methods.
Substrate-Specific Adhesion: This depends greatly on the kind of material that is being used. Actual substrates can be used to determine characteristics like bond strength. The property can be tested at different temperatures that are at ambient, elevated, and sub-ambient levels.
Thermal Stability: The stability of hot melt under different temperatures shows how durable it will be overall. The thermal stability test is performed by heating the hot melt to its pot temperature. A hot melt that has good stability does not decompose at extreme temperatures. The thermal stability test helps to assess the change in viscosity and the formation of charring, skin, edge rings, changes in color, and gel.
Cold Crack Formation: Some hot melts tend to crack at the end of the temperature spectrum. Tests are carried out to determine the temperatures at which the cracking occurs. Rubber-based hot melts are associated with temperatures in the range of 10 to 30 °F (-12 to -1 °C), while other hot melts are used in temperature ranges of about 15 to 45 °F (-9 to 7 °C).
Loop Tack: Loop tack is mainly used for hot melts that are pressure sensitive. These are hot melts that only bond when there is an application of pressure. The loop track measures the aggressiveness of the hot melt. The units of measurement are ounces. If there is a need for a higher force for a hot melt to be pulled, then it is considered to be more aggressive.
Mechanical Properties: Mechanical properties are very significant to a hot melt. These mechanical properties are the yield point, the amount of stress that can be applied to the hot melt before it permanently deforms, and the tensile strength, the amount of force required to break a sample.
Due to their flexibility, ease of application, and low melting points as compared to other adhesive types, hot melt adhesives are appropriate and suitable for many different substrates or materials. Hot melt adhesives are much safer to use on the following substrates:
The types of hot melt adhesives include:
Ethylene vinyl acetate (EVA) is a polymer base that is used in the standard packaging of industrial hot melt adhesives. EVA produces high-performing and very competitive packaging hot melt additives. Ethylene vinyl acetate is considered the main option for many applications because it offers one of the greatest performance ranges across all current applications available. Ethylene vinyl acetate can work in both traditional application temperature and low application temperature hot melt adhesives.
Polyethylene is a polymer base that works well for tray sealing and carton applications. These adhesives have a very low odor and are light-colored and easily released from metal surfaces for easy cleanup. Polyethylene is not ideal for bonding substrates and is therefore suitable to be used in traditional application temperatures.
Metallocene polymers existed decades ago. They were combined with other components to provide great stability to hot melt products. They gave storage temperature strength under both hot and cold conditions. They became the fastest-growing polymer base. The metallocene minimized the breakdown from heat exposure and reduced char while expanding the cold and heat bond performance. Some of the offerings available today have metallocene in the adhesives; however, many variants with different stabilities, price points, and bond performance are not the same as in the original products.
Amorphous poly alpha olefins (APAO) are used mostly where there is a long working time or where there is a need for adhesion to plastics. These types of adhesives offer a tacky, soft, and very flexible texture. This makes them a very good choice for assembling products, including adding foam protection to corrugated packaging and bonding corrugated plastic containers.
Polyamides adhesives are excellent where there is a need for high-temperature resistance. They are also useful in scenarios that need quick assembly strength and solvent and oil resistance. Polyamides are considered an excellent choice for wood, filter, and other applications requiring high performance. Polyamides require extreme temperature applications of about 400 °F (204 °C). However, they do not show much stability when brought to heat, as do other adhesive types.
This chapter will discuss the applications and benefits of hot melt adhesives.
Hot melt adhesives have numerous applications in a diversity of industries. Below are some of the major industries that benefit from hot melt additives.
Hot melt adhesive spray application is very effective, fast, and efficient. They are dispensed by the manipulation of a continuous bead of adhesive to form a pattern with air that is compressed. In order to maximize the uptime of the machine, applications must be monitored to avoid problems and challenges such as:
The cooling and setting of the adhesives begin with the use of a hot melt spray nozzle. The method is most appropriate for very large surface areas.
Hot melt slot coating involves the application of a layer of hot melt adhesive to the substrate. This is done before they are solidified and cooled. Hot melt slot coating is usually used when working with adhesives which are pressure sensitive on very thin substrates.
When used in the creation of a hot melt bead or line, the nozzle will eject a certain amount of adhesive and cut itself off before it fires again. This method applies mostly when an adhesive is required in a certain spot and can be brought under compression quickly. As soon as the hot adhesive dispenses, as in spray applications, it starts to cool. When performing this application, working within the set speed is crucial to avoid any bonding challenges.
The advantages and benefits of hot melt adhesives vary greatly with different industries; however, some universal benefits are mentioned below.
Many different materials are used to produce hot melts. The resulting properties of the hot melt adhesives show that there is much diversity in the cost, performance, and function of the end products of hot melts.
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.
There is a well-recognized use of the various forms of hot melts in the packaging, heat sealing, tray forming, and container labeling. The main performance characteristics of hot melt packaging are their increased pot life (defined as the aging performance of the hot melt adhesives), their versatility in adhesion properties, their low odor, low application temperature, and cost. For flexible packages and vehicles, a special type of packaging hot melts are used, taking, for example, small containers and lids.
The hot melts used in graphics are usually EVA based. They are applied at temperatures with a range of 325 to 350 degrees Fahrenheit (163 to 176 °C). Graphic arts hot melts are fast and perform very well in the temperature ranges of 20 to 120 °F (-6.6 to 48.8 °C). They also consist of mouse trap index characteristics, meaning they do not snap shut in the event they are released. Graphic art hot melts are resistant to ink solvents. The application of the hot melt matter is very delicate, and a range of tools are present to accomplish this end. The most commonly used is the gun.
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 is very resistant to both cold and hot temperatures. The bonding strength is strong, and the vibration resistance is high. It improves production efficiency and reaches its main strength much faster than silicone.
Mattress assembly adhesive is mostly applicable for the lamination of foam material and mattresses. It is a fast curing type that is suitable for fast production. The initial adhesion is high, and the heating viscosity is good as well. The product offers great weather resistance.
The bookbinding adhesive is applicable for bookbinding uses. It is usually used for printing reports, magazines, and hardcover books. The bonding strength and the weather resistance are high. It is very flexible and can maintain the flatness of the book.
This adhesive product consists of strong adhesion properties to fabrics. It has washing resistance and is stable.
Woodworking wrapping adhesives consist of high humidity and aging resistance properties. They are capable of covering various types of wood and other material.
The product is designed specifically for materials or structures that are multi-layered. They have a high and flexible bonding force and durability.
This adhesive is suitable and mainly used for laminating materials like aluminum boards, calcium silicate, bright PVC, etc.
This type of adhesive is used for bonding transparent boxes like OPP and PVC. It has good adhesion properties, high aging resistance, and high initial strength.
Electronic assembly adhesive is used in the assembly of products that are suitable for many plastic materials. It has good water resistance and is airtight.
Other products of hot melt adhesives are being used in various industrial sectors. Some of these are functional clothing adhesives, furnishings laminating adhesives, 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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