In general, hot melts are applied by extrusion, lamination or spraying, and the high melt viscosity makes them ideal for porous and permeable substrates.
hot melt adhesivescan bond similar and different materials. They are applied to a starting material when they are in a liquid or molten state. After a short open time, the second material comes into contact.
At this point, the adhesives cool and set quickly, joining the two together. Hot melt is the name given to any polymer-based glue that is applied in the molten state. The application of the adhesive is usually done by an industrial glue gun, which works by the action of the trigger or the pressure of the finger. Hot melt is used in a variety of environments due to its versatility, including packaging, binding, carton manufacturing, graphic arts, tape and label, product assembly and in the automotive industry.
Hot melt adhesives are generally 100% solids formulations based on thermoplastic polymers. They are solid at room temperature and are activated by heating them above their softening point, where they are liquid and can therefore be processed. After application, they retain the ability to wet the substrate until they solidify. When they solidify, they return to a physical state that has structural integrity and can function as an adhesive.
The adhesive is applied by extrusion, lamination or spraying and bonding is carried out immediately after application or after reheating the solidified layer. The variety of polymers of this class is very wide and includes both natural and synthetic polymers. The high viscosity of the melt makes them particularly suitable for porous and permeable substrates that would otherwise be more difficult to bond with a solvent system. A feature of hot melts is that, when cooled, they quickly increase their internal strength, which allows quick assembly and subsequent processing.
Because they are based on thermoplastic polymers, hot melts can be repeatedly heated to melt and cooled to solidify. This property limits the temperature resistance of hot melt joints and they also have a tendency to deform when subjected to continuous stresses or high temperatures. On the plus side, these adhesives can be used to create bonded joints that are thermally removable and can also be reattached. Hot melt adhesives are heated to become liquid.
The liquid is applied to the initial surface by extruding, spraying or rolling the prescribed amount of application and then exposing it to a short “open time” to reach the optimum temperature and “humidity” to bond to the initial surface. After the prescribed opening time, the two surfaces come into contact with each other. At this point, the adhesive infiltrates the second surface and cools, returning to a stable solid state and bonding the two together. In some cases, compression may be necessary to promote optimal bonding.
Hot melt adhesives (HMA) are as numerous as they are versatile. HMAs are capable of bonding a variety of different substrates, including paper, rubbers, ceramics, metals, plastics, glass, wood and more. They are compatible with many structural polymers and have a high softening point along with a low melt viscosity. These include tensile strength (how much force does it need to apply to break a sample), yield point (how much stress can you apply to hot melt before it is permanently deformed), elongation at break (how long will a sample stretch before breaking), and Young's modulus (stress over ratio deformation).
Hot melts are used in various forms of packaging, including carton, carton and heat sealing; tray forming; and container labeling. Examples of hot melt adhesive compositions and their corresponding melting temperatures are shown in Table 12, 12. Most of the polyolefins used in hot melt adhesives today are synthesized by conventional Ziegler-Natta polymerization. In addition to these main ingredients, hot melt comes with several other additives to give it certain desired characteristics.
Unlike other adhesives, the shaping process is reversible and, at approximately 77°C, most hot melts begin to lose strength. They also provide the fastest method of bonding, and hot melt equipment occupies a much smaller factor footprint than other joining methods. Polyamide hot melt adhesives have lower melting points than polyamide (nylon) plastics, and this is achieved by using a mixture of monomers, which has the effect of reducing hydrogen bonds between N-H-OC chains and thus reducing the heat of fusión. Most commercially available hot melt adhesives require temperatures above 180°C to ensure complete melting of all components and achieve satisfactory application viscosity.
Must be liquid (molten) enough to wet substrates and hot enough to assemble before cooling. Today, hot melt adhesives are available in a variety of different types, allowing them to be used in a wide range of applications in various industries. Formulation options for hot melts are broad and dependent on the end use, e.g. styrene block polymers can be used for pressure sensitive, and polyolefins such as polyethylene, ethylene vinyl acetate and polypropylene can be used for coating and laminating as well as for end uses of filling and filling indicated above.
In addition, there are many forms of hot melt adhesives (pellets, blocks, ropes, sticks) and application patterns (dots, spiral spraying, random spraying, continuous lines, etc. This could provide a problem in that many textile substrates are heat sensitive, and conventional hot melt adhesives must be applied at high temperatures. . .