A Comprehensive Guide to Polyurethane (PU) Hot Melt Adhesives

What Are Polyurethane (PU) Hot Melt Adhesives?

Polyurethane hot melt adhesive, often called PUR, is a type of hot melt adhesive made from polyurethane resin or prepolymer as the main material, and various additives (such as thickeners, catalysts, antioxidants, and fillers) as ingredients. After processing methods such as mixing and injection molding, it can exist in the form of strips, particles, powders, films, etc. at room temperature.

Polyurethane hot melt adhesives possess excellent elasticity and high bonding strength, as well as resistance to solvents, high temperatures, aging, and abrasion. They are suitable for bonding a wide variety of materials and are particularly well-suited for applications in packaging, industrial assembly, construction, and automotive component bonding.

Two Classifications of Polyurethane (PU) Hot Melt Adhesives

Polyurethane hot melt adhesive can be divided into two categories: one is thermoplastic polyurethane elastomer (TPU) hot melt adhesive, also known as hot melt polyurethane hot melt adhesive; The other type is reactive polyurethane hot melt adhesive. According to different reaction mechanisms, reactive polyurethane hot melt adhesive can be divided into wet curing type and closed type.

TPU Hot Melt Adhesives

What Is TPU Hot Melt Adhesive?

TPU is a linear block copolymer synthesized through the polymerization of diols, diisocyanates, and chain extenders. By blending TPU with additives—such as colorants, plasticizers, and stabilizers—hot-melt adhesives are produced in various forms, including films, tapes, or powders. The synthesis methods employed include bulk polymerization and solution polymerization.

Two TPU Hot Melt Adhesive Formulations

Formulation 1: Poly(ethylene adipate) - 1,4-Butanediol - MDI Hot Melt Adhesive

1. Preparation of the adhesive film. Poly(ethylene adipate) (molecular weight ≈ 2000), 1,4-butanediol, and MDI are used, with the materials fed at a molar ratio of 1:2:3.

2. The Effects of Raw Material Ratio and Preheating Temperature on the Properties of Adhesive Films

a. Influence of Raw Material Ratio on Adhesive Film Performance

Ensuring the accuracy of material input during the reaction is of paramount importance; consequently, the molecular weight of the polyester polyol must be determined with precision. If the polyester is present in excess, the adhesive will exhibit a low softening point and form films with difficulty; conversely, if the MDI is in excess, the adhesive film will be prone to cross-linking.

b. Influence of Polyester Polyol Preheating Temperature on Reaction Performance

The preheating temperature of the raw materials must be appropriate. Since polyester polyol is used in a larger proportion than the other components, its preheating temperature exerts the most significant influence on the overall reaction, playing a decisive role. As indicated by the data in the table, as the preheating temperature of the polyester polyol increases from 100°C to 120°C, the maximum reaction temperature rises progressively from 158°C to 170°C; a temperature of 105°C is identified as the optimum.

③Application of Adhesive Film.The poly(ethylene adipate)-1,4-butanediol-MDI hot melt adhesive can bond a variety of materials, including both smooth and rough surfaces such as glass, fabrics, and metals. It is also applicable to the bonding of liquid crystal components in electronic instruments and seamless bonding of fabrics.

Formulation 2: Poly(ethylene adipate) - Neopentyl Glycol - HDI Hot Melt Adhesive

1,6-Hexanediol CAS 629-11-8

Polyester Polyol

①Preparation of adhesive pellets. This polyurethane hot melt adhesive is prepared by mixing 39.3 parts HDI, 13.2 parts 1,6-hexanediol, 7.8 parts neopentyl glycol, and 39.7 parts polyester polyol.

Reactive Polyurethane Hot Melt Adhesives

What Is Reactive Polyurethane Hot Melt Adhesive?

The primary component of reactive polyurethane hot-melt adhesives is an isocyanate-terminated polyurethane prepolymer synthesized from polyester or polyether polyols and polyisocyanates.

Upon hot melt application, the prepolymer undergoes rapid curing and exhibits a certain degree of initial bonding strength; subsequently, through the absorption of moisture, the terminal isocyanate groups react to form urea, biuret, and allophanate linkages, thereby establishing a cross-linked macromolecular network structure that demonstrates ideal heat and solvent resistance.

Reactive polyurethane hot-melt adhesives are based on isocyanate-terminated prepolymers, formulated with thermoplastic resins that do not react with isocyanates, tackifiers, antioxidants, catalysts, fillers, and other additives.

The dissociation temperature of the sealing agent in closed hot melt adhesive can reach over 100°C, which can cause bubbles to form in the adhesive layer. Heating the sealing agent before use increases the complexity and energy consumption of the process, and there may also be environmental pollution caused by the volatilization of the sealing agent, which limits its application.

Wet cured polyurethane hot melt adhesive is a single component, solvent-free adhesive that complies with environmental protection regulations, is easy to use, and has performance comparable to solvent reactive adhesives. Therefore, its development prospects are promising. A typical formula can refer to the following examples.

Two Reactive Polyurethane Hot Melt Adhesives Formulations

Formulation 1: Polyurethane Hot Melt Adhesive for Nylon Fabrics & Polyester Films

100 g of polytetrahydrofuran (molecular weight: 1000) and 45 g of MDI (methylene diphenyl diisocyanate) are added into a reactor. The mixture is heated to 80 °C under stirring and reacted for several hours to prepare a prepolymer, which has a viscosity of 5000 mPa·s at 25 °C. This prepolymer is blended with polystyrene oligomer to obtain the moisture-curable reactive polyurethane hot melt adhesive.

Formulation 2: Polyurethane Hot Melt Adhesive for Woodworking Applications

Polyoxypropylene polyol (molecular weight 400) reacts with MDI to prepare an isocyanate-terminated prepolymer which is liquid at room temperature. 850 g of this prepolymer is uniformly mixed with 94 g of another prepolymer synthesized from polyoxypropylene (molecular weight 4000) and MDI at approximately 120 °C to obtain a moisture-curable reactive polyurethane hot melt adhesive. This adhesive is suitable for wood bonding and features a viscosity of 1200 mPa·s at 130 °C.

Why Choose Kerton Chemicals as Your Supplier?

Specializing in industrial chemicals, advanced materials, cosmetic materials, and life science products, Kerton Chemicals delivers premium-quality products and professional, tailored chemical solutions for research, industrial, and life science needs, with in-depth technical insights into hot melt adhesives and other key chemical products. For seamless collaboration and customized chemical solutions, visit our official website at https://www.kerton-industry.com/ and connect with our team to meet your unique.