Types of Hot-Melt Adhesives

Hot-melt adhesives (HMA) are a type of adhesive that is solid at room temperature but melts into a viscous fluid when heated. After being applied to and wetting the substrates to be bonded, the bond is formed through compression and cooling. They are made by melting and mixing thermoplastic materials with small amounts of tackifiers, plasticizers, fillers, and antioxidants. Available in powder, granule, stick, and block forms, they feature rapid curing, are non-polluting, and pose no environmental hazards; for these reasons, hot melt adhesives are often referred to as “green adhesives".

Classification of Hot-Melt Adhesives

Classification by Form

Based on the requirements of hot-melt adhesive applicators, the common forms of hot-melt adhesives can be categorised into blocks, granules, sticks, powders, strands, nets, and films (sheets). Among these, block and granular hot-melt adhesives have the highest demand; block hot-melt adhesives are primarily available in cubes, cylinders, and rectangles; and granular hot-melt adhesives are primarily diamond-shaped, prismatic, or spherical—with spherical being the most common. They come in a variety of colours and can be classified by appearance as transparent, translucent, or opaque. To use them, they must be melted and then applied by scraping or spraying, and a hot-melt tank is required; hot-melt adhesive sticks are applied using a hot-melt glue gun. Common specifications include a diameter of 11 mm and a length of approximately 150–300 mm. They are mostly yellow or transparent and are primarily used for manual applications, such as bonding metals, electronic products, plastic components, and toys; powdered hot melt adhesives typically have a particle size of 80–250 μm and are primarily white in colour. They are applied using heat-pressing methods for bonding materials and are used in textiles, automotive interiors, and powder coatings; mesh hot-melt adhesives, also known as double-sided adhesive liners, are primarily used for bonding lightweight garments and can also be used in hat and shoe manufacturing, electronic components, and automotive manufacturing; Film (sheet) hot-melt adhesives typically have a thickness of 0.35–0.76 mm and are used for bonding glass, solar cells, automotive glass, and aircraft panels.

Classification by Base Resin

Hot-melt adhesives can be classified based on their base resin into ethylene-vinyl acetate copolymer (EVA) hot-melt adhesives, styrene block copolymer (SBC) hot-melt adhesives, polyester (PES) hot-melt adhesives, polyamide (PA) hot-melt adhesives, polyurethane (PU) hot-melt adhesives, and polyolefin (PO) hot-melt adhesives, among others.

EVA

Ethylene-vinyl acetate (EVA) hot-melt adhesives are formulated from EVA copolymers, tackifying resins, waxes, plasticisers, antioxidants, fillers, and other components. With a wide range of applications and high consumption, they are one of the most important types of hot-melt adhesives. EVA hot-melt adhesives are widely used in bookbinding, food packaging, bonding plastics, restoring ceramics, mounting, and shaping garments and footwear, as well as in the automotive and building materials industries. Graft copolymerisation of EVA with maleic anhydride significantly improves the bond strength when bonding polar materials.

EVA resin is a thermoplastic resin with limited resistance to high and low temperatures and a low softening point. Furthermore, most tackifiers are low-molecular-weight polymers that merely serve to reduce melt viscosity and improve wetting ability and bonding performance; they are not inherently heat-resistant, which contributes to the drawback of EVA hot-melt adhesives being susceptible to both high and low temperatures. At the same time, EVA hot-melt adhesives also suffer from low bonding performance, lack of resistance to fatty oils, a tendency to form a skin, and are not well-suited for large-area coating and bonding applications.

Polyester (PES) Hot-Melt Adhesives

PES for hot-melt adhesives refers to a copolyester obtained through the polycondensation reaction of two or more dicarboxylic acids and diols. With the continuous development of the global economy, polyester hot-melt adhesives can be broadly categorised according to market demands into environmentally friendly, reaction-crosslinked, high-temperature-resistant, and multifunctional types. PES hot-melt adhesives are the most widely used type in the footwear industry. They offer advantages such as high bonding strength, fast curing speed, and excellent toughness and can be used in virtually all bonding processes during shoe manufacturing. PES hot-melt adhesives are also frequently used to bond labels to clothing, shoes, and hats. By copolymerising polyester with polycarboxylic acids and primary diamines, a block copolymer known as polyesteramide hot-melt adhesive can be produced. Since its macromolecular chains contain both ester and amide bonds, it possesses the common characteristics of both polyester and polyamide hot-melt adhesives. It is widely used for bonding fabrics and is resistant to both dry cleaning and wet washing. In addition, polyester hot-melt adhesives offer advantages such as good thermal stability, excellent cold and heat resistance, good insulating properties, chemical resistance, and environmental friendliness. They are widely used in industries such as packaging, bookbinding, plastics, ceramics, automotive, and building materials.

PA

PA used in hot-melt adhesives is a linear thermoplastic polymer containing repeating —CONH— amide functional groups in its main chain. It is typically produced by the polymerisation of polyene aliphatic compounds (such as ethylenediamine and diethylenetriamine). In practical applications, most PA hot-melt adhesives utilize copolyamide resins to meet various application requirements. Copolymerisation disrupts the regularity of the molecular chains and partially breaks hydrogen bonds, thereby reducing crystallinity and lowering the melting point. By varying the molar ratios, PA hot-melt adhesives with high (180–190°C), medium (140–150°C), and low (105–110°C) global softening points can be produced. Polyamide hot-melt adhesives exhibit excellent heat resistance, oil resistance, cold resistance, electrical properties, and resistance to chemicals and media. They are odourless, leave no colour stains, and cure rapidly. Polyamides have good affinity with a variety of metallic and non-metallic materials, offer excellent adhesion, and are highly compatible with other resins; therefore, they can be blended with other resins to improve their performance.

PU

Hot-melt polyurethane adhesives are prepared using linear polymers with urethane as the repeating structural unit as the base resin. They feature excellent strength and elasticity, high bond strength, and resistance to abrasion and solvents, making them suitable for bonding a wide variety of materials. They hold significant potential for growth in the bookbinding (seamless binding) and footwear industries. PU hot-melt adhesives are generally classified into two major categories: thermoplastic elastomeric and moisture-curing reactive types. Thermoplastic elastomeric PU hot-melt adhesives are typically composed primarily of hydroxyl-terminated polyurethane elastomers, while moisture-curing PU hot-melt adhesives consist mainly of isocyanate-terminated polyurethane polymers. Moisture-curing PU hot-melt adhesives offer advantages such as fast curing, solvent-free formulation, low energy consumption, pollution-free processing, and ease of application. However, the presence of active free —NCO groups in their composition results in poor storage stability. Additionally, CO₂ is generated during the curing process, and the resulting bubbles can adversely affect the adhesive’s performance. PU hot-melt adhesives feature a tough adhesive film, good impact resistance, high peel strength, good resistance to extremely low temperatures, and excellent oil resistance and abrasion resistance. Their disadvantages include a high number of bubbles in the finished product, complex operation, and the need for vacuum casting.

PO

This category includes polyethylene (PE), polypropylene (PP), polyolefins (POE), and other polyolefins. PE is a material with low surface energy and, consequently, low bonding strength; however, it is inexpensive and widely available, so it is still used in some industries. The quality of random PP-based hot-melt adhesives is often compromised by the presence of impurities in the polymer. Furthermore, these adhesives cure slowly and have poor heat resistance; consequently, they are rarely used on their own today. Polyolefin-based hot-melt adhesives are primarily composed of olefin copolymers or modified polyolefins.