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What is hot melt adhesive? Hot melt adhesive is the name of any polymer-based glue applied in a molten state. The application of adhesives is usually carried out through industrial glue guns, which work by triggering actions or finger pressure. Due to its versatility, hot melt adhesives are used in various environments, including packaging, binding, carton manufacturing, graphic arts, tapes and labels, product assembly, and the automotive industry.
There are many reasons why hot melt adhesive is attractive. On the one hand, it solidifies rapidly. In fact, the curing time can be adjusted according to the requirements of the work. Hot melt adhesive is safe and environmentally friendly to use. In addition to bonding surfaces, hot melt adhesive can also be used to fill gaps. It is also very convenient for transportation and storage.
Hot melt adhesive has become so popular that it has surpassed solvent-based adhesives. Solvent-based adhesives are mixtures of various components dissolved in solvents. When the solvent evaporates, the material will stick together. Polymer-based hot melt adhesives have solved many problems of solvent-based adhesives.
Compared with solvent-based adhesives, hot melt adhesives are faster, less costly, have stronger adhesion, and produce less volatile organic compounds.
Due to its chemical properties, hot melt adhesives can take various forms. This includes glue sticks, pellets, bags, cakes, drums, bricks, slats and pillows. Hot melt adhesives can also be applied in various ways, including through nozzles, extrusion, meltblown, spiral spraying, screen printing and slot die coating. Hot melt adhesive dispensing equipment can take the form of melt adhesive containers, vacuum conveyors, roller or bucket unloaders and pre-melt machines.
The raw materials used in hot melt adhesives can affect their functionality, cost, availability and performance. Generally, hot melt adhesives are composed of polymers (which can take various forms) and several additives. These additives include resins, waxes, antioxidants and plasticizers. Other chemicals can be added to endow the hot melt adhesive with more performance.
Polymer. The basic component of hot melt adhesive (and many types of glue) is polymer. These are long repetitive chains of certain molecules. Depending on the length of the chain and the type of molecule, they have different properties.
The main polymers used in hot melt adhesives are ethylene-vinyl acetate (EVA), polyolefins, polyamides and polyesters, styrene block copolymers, polyethylene and ethylene-methyl acrylate (EMA) or ethylene-n-butyl acrylate (EnBA).
Polymers endow hot melt adhesives with strength and flexibility, heat resistance, impact resistance and shear force. These characteristics mainly depend on the type, molecular weight and quantity of the polymer.
As the polymer content increases, you will achieve higher viscosity (a measure of liquid thickness), as well as greater flexibility and toughness. The lower the polymer content, the lower the viscosity.
Tackifying resin. The resin determines the viscosity of hot melt adhesive. Viscosity is a measure of the stickiness of an adhesive, essentially referring to the duration for which the adhesive remains adhered after application. Therefore, the resin controls the wettability of the adhesive (that is, the time it remains liquid when in contact with the substrate surface).
Resin also has an impact on the adhesion of hot melt adhesive. The selection of resin depends on its compatibility with the main polymer, softening point and specific adhesion. The main tackifying resins used in hot melt adhesives include rosin and hydrogenated rosin, C9, hydrogenated hydrocarbons, terpene phenolic resins, rosin esters and C5. Pure aromatic monomers are also used.
By using more tackifying resins, you can obtain hot melt adhesives with reduced viscosity and faster speed. By using a resin with less viscosity, you can obtain a tougher and more aggressive hot melt adhesive.
Wax. Hot melt wax mainly controls the solidification speed and open time. The opening hours refer to the time required to establish a bond. This can range from a few seconds to an endless duration (for some pressure-sensitive hot melt adhesives). Set the speed to measure the speed at which the hot melt adhesive forms a bond of a certain acceptable strength.
These two values must exactly match the parameters used in the production line. In addition to the solidification speed and open time, wax also affects the heat resistance and adhesion of hot melt adhesives in sub-environments (i.e., below the application temperature).
The main types of wax used in hot melt adhesives are natural wax, microcrystalline wax and synthetic wax. The properties of wax are determined by the percentage of crystallinity, melting point and molecular weight.
The lower the wax content, the higher the viscosity of the hot melt adhesive, the greater its flexibility and the stronger its adhesive force. Using less wax, hot melt adhesive will have a lower viscosity, a faster curing speed, and a smaller adhesive force.
Antioxidant. Antioxidants are used in hot melt adhesives mainly to protect the materials from degradation within their shelf life. Some commonly used antioxidants in hot melt adhesives include phenols, aromatic amines, phosphates, phosphites and BHT.
Adding a small amount of antioxidant together with the stabilizer will not affect the physical properties of the hot melt adhesive. They not only protect the hot melt adhesive during its shelf life, but also provide protection when it is in the molten state during application and mixing.
Plasticizer. In addition to base polymers and tackifying resins, plasticizers are common additives in hot melt adhesives. In fact, they are used as a second base polymer to provide greater flexibility and toughness for hot melt adhesives. Plasticizers are usually hydrocarbon oils with low aromatic hydrocarbon content and paraffin chemical properties.
Ideally, plasticizers should be low in volatility, transparent and odorless. By using plasticizers, hot melt adhesives can achieve lower melt viscosity and faster wetting.
In addition to these main components, hot melt adhesives also contain several other additives to endow them with certain desired properties. Fungicides can prevent bacterial growth, while fillers can increase volume and strength while reducing costs. Hot melt adhesives can also contain flame retardants, various pigments and even glitter.
The main defining characteristics of hot melt adhesives are their viscosity, melt color, failure temperature (for shear bonding and peel bonding), softening point, substrate specific adhesion force, thermal stability, cold crack formation, ring adhesion and various mechanical properties.
Viscosity:
. Viscosity is a measure of the thickness of a liquid or the magnitude of its resistance. High-viscosity liquids move very slowly (think of oil). Liquids with higher viscosity, such as water, flow more easily. The viscosity of hot melt adhesive is not a single value but depends on the application temperature (which can range from 250 to 350 degrees Fahrenheit).
It is also important to understand the shear rate (or the rate at which a layer of fluid changes speed as it moves over an adjacent surface) to calculate viscosity.
There are various devices available for testing the viscosity of hot melt adhesives. This includes Brookfield Thermosel (for low shear), dynamic mechanical analysis (for low and high shear), and capillary rheometers (for high and very high shear). The unit of viscosity is Poole .
The color of melting. Measure the color of hot melt adhesive within a numerical range using subjective and quantitative methods. This includes the Gardner, Hunter and Saybolt methods as well as the yellowness index.
Failure temperature. As the name suggests, the failure temperature is the temperature at which a hot melt adhesive stops working. There are two failure temperature measurement values used to characterize hot melt adhesives.
Peel bonding failure temperature. At higher temperatures, it is easier to peel off the two surfaces bonded with hot melt adhesive. PAFT measures the degree to which hot melt adhesives resist spalling at higher temperatures. In the test, the temperature started at 25 degrees Celsius and increased at a rate of 25 degrees per hour. For a sample of 1 inch x 1 inch x 0.01 inch, the weight used for inducing peeling is 100 grams.
Shear bonding failure temperature Shear force is a force that exists when one surface slides over another. In the shear test, the specimen is installed vertically and attached with a heavy object. How long does it take for the surfaces to separate from each other to tell you the strength of the hot melt adhesive. The measurement method of SAFT is the same as that of PAFT, but it uses shearing instead of peeling. The temperature is the same, but the weight is 500 grams.
Softening point. The softening point of hot melt adhesive (or any glue) is the temperature at which the glue begins to flow. The main determining factor for the softening point of hot melt adhesives is the melting point of the wax used or the transition temperature of the base polymer (in the case of hot melt adhesives based on styrene block copolymers). The universal method and the Mettler method are usually used to measure SP.
Substrate specific adhesion. This measure depends on which material you are trying to use the hot melt adhesive for. The bonding strength and other properties can be determined by using the actual substrate. You can also test the properties at different temperatures (ambient temperature, below ambient temperature and high temperature).
Thermal stability. The stability of hot melt adhesive under different temperature changes will tell you its overall durability. For thermal stability testing, you heat the hot melt to its pot temperature (the temperature at which it will boil if it is in liquid form). Hot melt adhesive with good tank stability will not caramelize or decompose at higher temperatures.
By using the thermal stability test, you can determine any changes in viscosity, color, carbonization, edge rings, skin formation and gel formation.
Low-temperature brittle fracture. At the other end of the temperature spectrum, some hot melt adhesives tend to crack. Tests are used to determine under what circumstances low-temperature cracking will occur. For EVa-based hot melt adhesives, use a temperature between 45 and 15 degrees Fahrenheit. For rubber-based hot melt adhesives, the temperature used is between 30 and 10 degrees.
The formation of cold cracks is of great significance to research, especially in the application of graphic art. This is because you can see the flexibility of the spine at a lower temperature.
Mechanical properties. There are various mechanical properties related to hot melting. These include tensile strength (how much force you need to apply to break the sample), yield point (how much stress can be applied before the hot melt adhesive deforms permanently), elongation at break (how long the sample will be stretched before breaking), and Young’s modulus (stress-strain ratio). Other attributes are combinations of these.
In some cases, cold adhesives are required. This is especially true when the substrate is highly sensitive to temperature. In these cases, it is recommended to use water-based adhesives.
The different materials used to manufacture hot melt adhesives and the resulting adhesive properties mean that there are significant differences in the functionality, performance and cost of hot melt adhesive products.
Tape and label hot melt adhesive. Hot melt adhesives used for tapes and labels are usually pressure-sensitive. For labels, you need products that can be customized, die-cut and offer a range of adhesive levels. For tape, you can use a smaller range of adhesiveness, but you need a high level of shear resistance.
Pressure-sensitive hot melt adhesives are beginning to replace solvent-based hot melt adhesives, partly because they have better heat resistance and resistance to plasticizers.
Packaging hot melt adhesive Hot melt adhesives are used in various forms of packaging, including box type, carton type and heat sealing. Pallet forming And container labels. The key performance feature of packaging hot melt adhesive is its versatility in bonding. Extend the pot life of (aging performance of hot melt adhesive); And their lower odor, cost and application temperature.
The packaging hot melt adhesive has the following characteristics: viscosity of 900 cps (at 350 degrees Fahrenheit), set speed of 1-2 seconds, open time of 3-4 seconds, PAFT at 135 degrees Fahrenheit and SAFT at 200 degrees Fahrenheit.
A special case of packaging hot melt adhesive – heat-sealing hot melt adhesive – is used in flexible packaging (such as LIDS and small containers), carpets and vehicles. During the heat sealing process, hot melt adhesive is applied to one surface, then this surface is bonded to another, and then heat and pressure are applied.
Graphic art hot melt adhesive. The hot melt adhesive used in the graphic is usually based on EVA and is applied between 325 and 350 degrees Fahrenheit. They can be set up quickly and perform well anywhere between 20 and 120 degrees Fahrenheit. Graphic hot melt adhesives also have the flat (or mousetrap index) property, which means they do not suddenly close when released. They are also resistant to ink solvents.
The application of hot melt adhesive is a delicate matter, and a series of tools can be used to achieve this goal. Spray guns are commonly used. Even when using the same spray gun, hot melt adhesive can be flexibly applied, for example, by choosing the nozzle.
Hot melt adhesive is everywhere.
Due to its unique properties and flexibility, hot melt adhesive is used in various environments. Packaging, graphics, automobiles, processing and many other industries have all been affected. As polymer technology becomes increasingly advanced, we can hope to see a wider range of applicability. More and more hot melt adhesives are being manufactured specifically for individual substrates.
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