An Overview to Adhesives Commonly Used in the Footwear Industry

In addition to the solvent-based adhesives introduced in the article A Complete Guide to Adhesives Commonly Used in the Footwear Industry, water-based adhesives and hot-melt adhesives covered in this paper are also applicable to the footwear manufacturing sector.

Water-Based Adhesives

Water-Based Neoprene Adhesives

Water-based neoprene adhesives, which contain no organic solvents, serve as an environmentally friendly alternative to solvent-based neoprene adhesives. However, the initial tack, freeze resistance, water resistance, and stability of current water-based neoprene adhesives are still far inferior to those of solvent-based neoprene adhesives, which limits their range of applications. Water-based neoprene adhesives for footwear are typically formulated by blending neoprene latex with water-emulsified tackifying resins.

Advantages of Water-Based Neoprene Adhesives:

Advantages of Water-Based Neoprene Adhesives:

1. Contains virtually no organic solvents, has minimal odor, poses no health risks to industrial workers, and is an environmentally friendly product.
2. Non-flammable, making it extremely safe during transportation, storage, and use; 
3. It offers a wide range of viscosity and concentration adjustments, providing excellent bonding performance;
4. Compared to solvent-based neoprene adhesives, it exhibits a tendency to form strings, making it suitable for various application methods such as roller coating, spray coating, and brush coating;
5. It is suitable for bonding materials susceptible to solvent corrosion, such as expanded polystyrene and polyurethane soft foam used in footwear.

Disadvantages of Water-Based Neoprene Adhesives

1. Because water evaporation absorbs a large amount of heat, water-based neoprene adhesives dry slowly; 
2. Their initial bond strength is lower than that of solvent-based neoprene adhesives; 
3. They have poor storage stability at low temperatures; 
4. The adhesive film has poor resistance to water, heat, and chemicals.

Water-Based Polyurethane Adhesives

Water-based polyurethane refers to a polyurethane solution formed by dissolving or dispersing polyurethane in water. Based on its appearance and particle size, it can be classified into aqueous polyurethane solutions, water-dispersed polyurethane, and water-emulsified polyurethane. Water-based polyurethane uses water as a dispersant and contains virtually no volatile organic compounds (VOCs). As a result, it offers advantages such as cost-effectiveness (low cost and resource conservation), safety (non-flammable and non-explosive), non-toxicity, environmental friendliness, minimal odor, and easy cleanup of residues after use, making it an environmentally friendly product.

Water-Based Natural Rubber Adhesives

Water-based natural rubber adhesives are formulated by blending natural rubber latex with additives such as tackifying resins. Natural rubber latex has a high solid content, excellent film-forming properties, and high self-adhesion, and its viscosity can be easily adjusted; as a result, it has been used as an adhesive for a long time. However, it has poor resistance to aging, oil, chemicals, and freezing.

Hot-Melt Adhesives

Hot-melt adhesives for footwear are primarily composed of thermoplastic resins, modifiers, and processing aids. Based on the chemical structure of the thermoplastic resins, hot-melt adhesives for footwear mainly include ethylene-vinyl acetate (EVA) hot-melt adhesives, amorphous α-olefin copolymer (APAO) hot-melt adhesives, copolyester (PES) hot-melt adhesives, polyamide (PA) hot-melt adhesives, and polyurethane (PU) hot-melt adhesives.

EVA Hot-Melt Adhesives

In ethylene-vinyl acetate (EVA) resins, the ratio of ethylene to vinyl acetate varies, resulting in significant differences in performance. The resins used in EVA hot-melt adhesives contain more than 28% vinyl acetate. In addition to EVA resin, EVA hot-melt adhesives for footwear also contain tackifying resins, viscosity and surface tension modifiers, antioxidants, and fillers. Tackifying resins primarily include terpenes, petroleum resins, rosin and modified rosin resins, and coumarone resins; viscosity and surface tension modifiers include paraffin, microcrystalline wax, and polyethylene wax; antioxidants are mainly hindered phenols, while fillers primarily consist of calcium carbonate, clay, and talc.

APAO Hot-Melt Adhesives

APAO is a general term for a series of amorphous poly-α-olefins with relatively low molecular weights, most of which are synthesized industrially using heterogeneous titanium-based catalysts. Depending on their intended use and polymerization process, the main product types include PP homopolymers, propylene-ethylene copolymers, propylene-1-butene copolymers, propylene-1-hexene copolymers, and ethylene-propylene-1-butene terpolymers.
The main components of APAO hot-melt adhesives for footwear include amorphous α-olefin copolymers, tackifying resins (petroleum resins, rosin resins, etc.), and fillers.

PES Hot-Melt Adhesives

Polyester (PES) is produced by the polycondensation of aromatic dicarboxylic acids, aliphatic dicarboxylic acids, and diols. Hot-melt adhesives made from PES for footwear applications primarily consist of PES resin and additives; the ratio of the polyester hot-melt adhesive matrix to other additives is generally not strictly defined. Commonly used diluents include styrene copolymer resin and petroleum resin; tackifiers include xylene resin and phenolic resin; and fillers include talc and calcium carbonate. Sometimes, to adapt to different working conditions, reinforcing agents such as glass fiber and carbon fiber, flame retardants such as brominated aromatic compounds and phosphorus compounds, and UV absorbers such as diphenyl ketone compounds may also be added.

PA Hot-Melt Adhesives

Polyamide hot-melt adhesives for footwear are low-molecular-weight polyamide hot-melt adhesives that offer excellent strength and toughness. These adhesives can be modified by adding epoxy resin and are typically manufactured into adhesive strips and wound onto rolls. They are suitable for applications such as bandages and leather edging.

PU Hot-Melt Adhesives

Polyurethane hot-melt adhesives are classified into two types: thermoplastic polyurethane (TPU) hot-melt adhesives and reactive moisture-curing polyurethane (PUR) hot-melt adhesives.
TPU hot melt adhesives can be produced in granular, powdered, or film form. During use, they are heated to a specific temperature to melt onto the substrate surface and then cure upon cooling. The curing process primarily involves physical cross-linking through hydrogen bonding within the composition, offering advantages such as high bonding strength, low-temperature resistance, and abrasion resistance.
PUR hot melt adhesives are synthesized primarily by reacting polyether or polyester polyols with diisocyanates to form an end-isocyanate-functional prepolymer. When the NCO content reaches a predetermined value, an appropriate amount of tackifying resins, fillers, and other additives are added. The cured adhesive layer exhibits high resistance to both high and low temperatures, excellent solvent resistance, and strong bonding strength, while also being solvent-free and non-toxic.