The principle of hot melt adhesive

Hot melt adhesive is also known as glue adhesive. They are thermoplastic polymer adhesives that are solid at room temperature. If heated to a temperature above its softening point, the hot melt adhesive will liquefy.

Compared with various packaging components, their softening temperatures are relatively low. The liquefaction of hot melt adhesives allows them to be used and applied in many fields. When in liquid form, hot metal adhesives are activated and allow for surface bonding. Hot melt adhesives exist in various forms in their solid state. They can be granular, block, sheet, film-like, pillow-like or stick-like. Depending on the application, it can be extruded, sprayed or rolled onto the surface from the nozzle.

Raw materials for hot melt adhesives

The raw materials in hot melt adhesives can affect their cost, performance, functionality and availability. Generally, hot melt adhesives are composed of polymers and various forms of additives.

Some of these additives are antioxidants, resins, plasticizers and waxes. Other chemicals can be added to endow the hot melt adhesive with more performance.

Polymer in hot melt adhesive

Polymers are regarded as the basic components of hot melt adhesives. All types of glue are polymers. These are repetitions and long chains of specific molecules with different properties. Polymers are responsible for most of the physical properties in hot melt adhesives.

Polymers endow hot melt adhesives with heat resistance, flexibility, strength, as well as shear and impact resistance. The above-mentioned characteristics largely depend on the type, molecular weight and quantity of the polymer. A high content of polymers in the hot melt means high viscosity. Viscosity is defined as a measure of the thickness of a liquid. Higher polymer content also means greater toughness and flexibility.

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Resin is responsible for the adhesion of hot melt adhesive. Viscosity is defined as a measure of the adhesive’s stickiness, usually the period during which the adhesive remains intact after use. In addition, the resin controls the wetting of the adhesive. This means that the resin controls the time for which the hot melt adhesive remains liquid when in contact with the surface substrate.

Resin has a significant impact on the properties of hot melt. The selection of resin depends on various factors, such as its softening point, compatibility with major polymers, and specific adhesive force. Most of the resin types used in hot melt adhesives are rosin and hydrogenated rosin, rosin esters, hydrogenated hydrocarbons, terpene phenolic, C5 and C9.

Some melts also use aromatic monomers. Adding more resin will produce a hot melt with the least viscosity and a faster drying speed. On the contrary, less resin means a more aggressive and tougher hot melt adhesive.

Wax in hot melt adhesive

The open time and solidification speed in hot melt adhesives are mainly controlled by wax. The open time is defined as the time required to establish a bond. When dealing with some pressure-sensitive hot melt adhesives, the time range is from a few seconds to a long-lasting duration. The purpose of setting the speed is to measure the speed at which the hot melt adhesive forms a bond of acceptable strength.

The obtained values must be exactly the same as the parameters used in the production line. Wax also affects the heat resistance of hot melt adhesives and their adhesion at temperatures below ambient temperature. The commonly used wax types in hot melt adhesives include synthetic wax, microcrystalline wax and natural wax. The quantities or percentages of molecular weight, crystallinity and melting point determine the properties of wax.

Hot melt adhesive will exhibit greater viscosity while having a lower wax content. As a result, it becomes more flexible and highly aggressive when bonding. When the wax content is low, hot melt adhesive will exhibit less erosiveness, lower viscosity and solidify more quickly during bonding.

Antioxidant in Hot melt adhesive

The main use of antioxidants in hot melt adhesives is to prevent the material from degrading over time. Generally, the antioxidants used in hot melt adhesives are aromatic amines, phenols, BHT, phosphites and phosphates. Antioxidants are added together with stabilizers in very small amounts. They do not affect the physical properties of the hot melt adhesive. Antioxidants help protect hot melt adhesives in their molten state during application and when mixing hot melt adhesives.

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Apart from resins and polymers, plasticizers are one of the most commonly used additives in hot melt adhesives. They are used as the second base polymer to provide greater toughness and flexibility for hot melt adhesives. In most cases, they are hydrocarbon oils with a very low content of aromatic hydrocarbons, and their chemical properties are the same as those of paraffin. Plasticizers are renowned for their low volatility. They have no odor. Plasticizers can reduce melt viscosity and accelerate wetting speed. In addition to the main components, many other additives are also used in the formation of hot melt adhesives to develop specific desired properties. For instance, fungicides help prevent the growth of bacteria; The filler increases strength and volume while reducing cost. Finally, hot melt adhesives can be used in combination with various pigments, flame retardants and even glitters.