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What is hot melt adhesive? Hot melt adhesive is the name of any polymer based adhesive applied in a molten state. The application of adhesives is usually through industrial glue guns, which act by triggering actions or finger pressure. Due to its versatility, hot melt adhesive is used in various environments, including packaging, binding, carton manufacturing, graphic art, tape and labels, product assembly, and the automotive industry.
There are many reasons why hot melt adhesive is attractive. On the one hand, it is fast curing. In fact, the curing time can be adjusted according to the needs 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 a mixture of various components dissolved in a solvent. When the solvent evaporates, the material will stick. Polymer based hot melt adhesives have solved many problems with solvent based adhesives.
Compared with solvent based adhesives, hot melt adhesives have faster speed, lower cost, stronger viscosity, and produce fewer volatile organic compounds.
Due to its chemical properties, hot melt adhesive can come in various forms. This includes glue sticks, granules, bags, cakes, drums, bricks, Flat noodles and pillows. Hot melt adhesive can also be applied in various ways, including through nozzles, extrusion, melt spraying, spiral spraying, screen printing, and slot die coating. Hot melt adhesive dispensing equipment can be in the form of adhesive containers, vacuum conveyors, drum or bucket unloaders, and pre melting machines.
The raw materials used in hot melt adhesive can affect its functionality, cost, availability, and performance. Usually, hot melt adhesive is composed of polymers (which can come in various forms) and several additives. These additives include resins, waxes, antioxidants, and plasticizers. Other chemicals can be added to give hot melt adhesive more performance.
Polymer.The basic component of hot melt adhesive (and many types of glue) is polymer. These are long repeating chains of certain molecules, which have different properties depending on the length of the chain and the type of molecule.
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 strength. These characteristics mainly depend on the type, molecular weight, and quantity of the polymer.
As the polymer content increases, you will gain higher viscosity (a measure of liquid thickness), as well as greater flexibility and toughness. The lower the polymer content, the lower the viscosity.
Thickening resin.Resin determines the viscosity of hot melt adhesive. Stickiness is a measure of the stickiness of an adhesive, essentially the time it remains adhered after application. Therefore, resin controls the wettability of the adhesive (i.e., the time it remains liquid when in contact with the substrate surface).
Resin also affects the adhesion of hot melt adhesive. The choice of resin depends on its compatibility with the main polymer, softening point, and specific adhesion. The main thickening resins used in hot melt adhesives are 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 adhesive with reduced viscosity and faster speed. By using less viscous resin, you can obtain a tougher and more aggressive hot melt adhesive.
Wax.Hot melt wax mainly controls the solidification speed and opening time. The opening hours are the time required to establish bonds. 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 hot melt adhesive forms an acceptable strength bond.
These two values must match exactly with the parameters used in the production line. In addition to solidification speed and opening time, wax also affects the heat resistance and sub environmental (i.e. below application temperature) adhesion of hot melt adhesive.
The main types of wax used in hot melt adhesive are natural wax, microcrystalline wax, and synthetic wax. The characteristics of wax are determined by the crystallinity, melting point, and percentage of molecular weight.
The lower the wax content, the higher the viscosity, flexibility, and adhesion of the hot melt adhesive. Using less wax, hot melt adhesive will have lower viscosity, faster curing speed, and lower adhesion.
antioxidant.Antioxidants are used in hot melt adhesives mainly to protect the material from degradation during its shelf life. Some commonly used antioxidants in hot melt adhesives include phenols, aromatic amines, phosphates, hypophosphites, 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 in its molten state during application and mixing.
Plasticizer.Besides basic polymers and tackifiers, 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 content and paraffin chemical properties.
Ideally, plasticizers have low volatility, transparency, and no odor. By using plasticizers, hot melt adhesives can achieve lower melt viscosity and faster wetting.
In addition to these main components, hot melt adhesive also comes with several other additives to give it certain desired properties. Fungicides can prevent bacterial growth, while fillers can increase volume and strength while reducing costs. Hot melt adhesive can also contain flame retardants, various pigments, and even glitter.
The main defining characteristics of hot melt adhesive are its viscosity, melting color, failure temperature (used for shear bonding and peel bonding), softening point, substrate specific adhesion, thermal stability, cold crack formation, ring adhesion, and various mechanical properties.
viscosity。Viscosity is a measure of the thickness or resistance of a liquid. High viscosity liquids move very slowly (think of oil). Liquids with higher viscosity, such as water, are easier to flow. The viscosity of hot melt adhesive is not just a single value, but depends on the application temperature (which can range from 250 to 350 degrees Fahrenheit).
Understanding the shear rate (or the rate at which a layer of fluid changes velocity as it moves on adjacent surfaces) is also important for calculating viscosity.
There are various devices used to test the viscosity of hot melt adhesive. This includes Brookfield Thermosel (for low shear), dynamic mechanical analysis (for low and high shear), and capillary rheometer (for high and very high shear). The unit of viscosity ismoor。
Melting color。Measure the color of hot melt adhesive within a numerical range using subjective and quantitative methods. This includes 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 the hot melt adhesive stops working. There are two types of failure temperature measurements used to characterize hot melt adhesives.
Peel off adhesive failure temperature.At higher temperatures, it is easier topeel offTwo surfaces bonded with hot melt adhesive. PAFT measures the degree of anti peeling of hot melt adhesive at higher temperatures. In the test, the temperature starts at 25 degrees Celsius and increases at a rate of 25 degrees per hour. For a sample of 1 inch x 1 inch x 0.01 inch, the weight used to induce delamination is 100 grams.
Shear bonding failure temperature.Shear force is a force that exists when one surface slides over another surface. In the shear test, the specimen is vertically installed and a heavy object is attached. How long does it take for the surface 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 of the softening point of hot melt adhesive is the melting point of the wax used or the transition temperature of the base polymer (in the case of styrene block copolymer based hot melt adhesive). The global method and Mettler method are commonly used to measure SP.
Substrate specific adhesion.This measure depends on which material you are trying to use the hot melt adhesive for. The adhesive strength and other characteristics can be determined using actual substrates. You can also test the properties at different temperatures (ambient temperature, below ambient temperature, and high temperature).
Thermal stability.How stable is hot melt adhesive under different temperature changes, and 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 in liquid form). Hot melt adhesive with good can stability will not char or decompose at higher temperatures.
Using the thermal stability test, you can determine changes in viscosity, any changes in color, carbonization, rim rings, skin and gel formation.
low-temperature brittle fracture。At the other end of the temperature spectrum, some hot melt adhesives often crack. Testing is used to determine under what circumstances low-temperature cracking may occur. For EVA based hot melt adhesive, use a temperature between 45 and 15 degrees Fahrenheit. For rubber based hot melt adhesive, the temperature used is between 30 and 10 degrees.
The formation of cold cracks is important for research, especially in graphic art applications. This is because you can see the flexibility of the spine at lower temperatures.
Mechanical performance.There are various mechanical properties related to hot melt. 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 permanently deforms), elongation at break (how long the sample will stretch before fracture), and Young’s modulus (stress-strain ratio). The other attributes are a combination of these.
In some cases, cold adhesive is 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 adhesive 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 adhesive used for tape and labels is usually pressure sensitive. For labels, you need products that are customizable, die cut, and offer a range of adhesive levels. For tape, you can use a smaller adhesive range, but you need a high level of shear resistance.
Pressure sensitive hot melt adhesives are beginning to replace solvent based hot melt adhesives, partly due to their better heat resistance and plasticizer resistance.
Packaging hot melt adhesive。Hot melt adhesive is used for various forms of packaging, including box, carton, and heat sealing; Tray forming; And container labels. The key performance characteristic of packaging hot melt adhesive is its multifunctionality in bonding; extendApplicable period(Aging performance of hot melt adhesive); And their lower odor, cost, and application temperature.
Packaging hot melt adhesive has the following characteristics: 900 CPS viscosity (at 350 degrees Fahrenheit), 1-2 seconds set speed, 3-4 seconds open time, PAFT at 135 degrees Fahrenheit, and SAFT at 200 degrees Fahrenheit.
A special case of packaging hot melt adhesive – heat sealed hot melt adhesive – is used in flexible packaging (such as lids and small containers), carpets, and vehicles. During the heat sealing process, apply hot melt adhesive to one surface, then combine that surface with another, and apply heat and pressure.
Graphic art hot melt adhesive.The hot melt adhesive used in graphics is usually based on EVA and applied between 325 and 350 degrees Fahrenheit. They can be quickly set up and perform well anywhere between 20 and 120 degrees Fahrenheit. Graphic hot melt adhesive also has a flat (or mousetrap index) characteristic, which means they will not suddenly close when released. They are also resistant to ink solvents.
The application of hot melt adhesive is a delicate matter, and a range of tools can be used to achieve this goal, with spray guns being commonly used. Even with the same spray gun, hot melt adhesive can be flexibly applied, such as selecting a nozzle.
Due to its unique characteristics and flexibility, hot melt adhesive is used in various environments. Packaging, graphics, automotive, processing, and many other industries have 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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