Applications of polyurethane adhesives

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The diversity of polyurethane adhesives provides solutions to many bonding challenges, and they are particularly suitable for applications where other types of adhesives cannot bond or struggle to bond. In addition, polyurethane adhesives also possess characteristics such as adjustable toughness, simple bonding process, excellent low-temperature resistance, and good stability. It is precisely due to the excellent bonding properties of polyurethane adhesives and their ability to bond with a variety of substrates that their application areas continue to expand, making them one of the fastest-growing adhesive types in recent years both domestically and internationally. 1. Polyurethane adhesives for automobiles: The introduction of large amounts of lightweight metals, composite materials, and plastics in new automobile designs has led to a continuous increase in the demand for adhesives and sealants used in vehicles. The polyurethane adhesives most widely used in automobiles include single-component, moisture-curing polyurethane sealants for assembling windshields; structural adhesives for bonding glass fiber reinforced plastics and sheet molding composites, two-component polyurethane adhesives for interior components, as well as water-based polyurethane adhesives. Automotive interior components are also a sector where the use of adhesives is increasing. For a considerable period of time in the past, China’s automobile manufacturing industry relied on importing polyurethane windshield adhesives from abroad; it was not until the \"Eighth Five-Year Plan\" period that the development of such adhesives was made a **key priority in scientific and technological research. Aqueous polyurethane adhesives, which are widely used in automobiles, refer to adhesives formed by dissolving or dispersing polyurethane in water. Most water-based polyurethanes are linear thermoplastic polyurethanes; due to the lack of cross-linking in their film layers and their low molecular weight, they exhibit poor properties such as water resistance, solvent resistance, and film strength. Therefore, it is necessary to modify them in order to improve their performance. Automobile repair varnishes prepared using hybrid dispersions of polyester and acrylic along with uretdiones and isocyanurates require no high-speed mixing equipment; they mix easily together and possess good adhesion properties. 2. Polyurethane adhesives for wood: With the rapid decline of global forest resources and the implementation of China’s natural forest protection programs, when small pieces of wood are combined to form larger boards, it is necessary that the adhesives possess bonding strength as well as durability and weather resistance that are superior to those of the wood itself. The amount of adhesive used has become an indicator of the technological advancement level of the wood industry. In the past, the wood adhesives used were mainly urea-formaldehyde resins based on formaldehyde, phenol-formaldehyde resins, and melamine-formaldehyde resins. However, due to the presence of free formaldehyde, these products would gradually release formaldehyde gas into the surrounding environment over time, causing environmental pollution. The wood processing industry has begun to turn its attention to the new, environmentally friendly adhesive polyurethane, in an effort to reduce environmental pollution. The one-component moisture-curing polyurethane adhesives used in the woodworking industry are liquid and are used at room temperature. It generally has high adhesion strength, good flexibility and water resistance, and can bond with many non-wood substrates (such as textile fibers, metals, plastics, rubber, etc.). The single-component polyurethane adhesives exhibited better dry and wet strengths in the tests compared to phenolic adhesives. Applying a hydroxymethylresorcinol (HMR) coupling agent to the surface of the substrate before bonding can improve the bonding strength; HMR can enhance the bonding strength of all thermosetting wood adhesives. When the wood surface is pre-coated with an HMR coupling agent, the strength and durability of one-component polyurethane adhesives can meet most stringent testing requirements. 3. Polyurethane adhesives for shoes: China is a major shoe-making country. After three generations of development in this field, rising global awareness of environmental protection and the intensification of the oil crisis have led to the emergence of the fourth generation of environmentally friendly solvent-free and water-based adhesives for shoes. The production process of water-based polyurethanes has become increasingly mature. For bonding some low-polarity shoe materials such as SBS, the peel strength of polyurethane adhesives does not meet the required standards. By modifying them with tackifying resins and other substances, polyurethane adhesives for shoes can be developed that exhibit high crystallinity, fast crystallization rates, high cohesion strength, and satisfactory peel strength. 4. Polyurethane adhesives for packaging – soft packaging, also known as soft cans – have surpassed hard packaging such as plastic bottles and glass jars due to their unique comprehensive advantages, including light weight and convenience, long shelf life, hygiene, ease of storage and transportation, simplicity in opening, reduced waste generation, and good shelf effect. Due to their excellent properties, polyurethane adhesives can bond film materials with different properties together to produce composite films for flexible packaging that possess various characteristics such as cold resistance, oil resistance, chemical resistance, transparency, and wear resistance. In both domestic and international markets, polyurethane adhesives have become the primary adhesives used in the processing of composite films for flexible packaging. In the domestic adhesive market, within the manufacturing industry of packaging composite films, polyurethane adhesives are used in the second largest volume, right after those used in the shoe-making industry. There are a wide variety of polyurethane adhesives used for packaging, such as water-based polyurethane adhesives, hot-melt polyurethane adhesives, solvent-based polyurethane adhesives, and solvent-free polyurethane adhesives. The commonly used polyurethane hot-melt adhesives can be further divided into 2 categories: 1) thermoplastic polyurethane elastomer hot-melt adhesives; 2) reactive hot-melt adhesives. The main drawback of thermoplastic hot melt adhesives is their high viscosity, which has a significant impact on the apparent quality of the coating. Reactive polyurethane hot-melt adhesives are a new type of adhesive developed on the basis of traditional hot-melts. They not only possess the advantages of traditional hot-melts, such as good initial adhesion and excellent post-curing properties, but also benefit from the versatility of polyurethane’s structural composition and its wide range of adjustable properties, enabling them to provide strong bonding strength to a variety of substrates. Furthermore, in terms of water-based polyurethane adhesives used for packaging, the use of emulsifiers or the introduction of hydrophilic ionic groups into the molecules reduces their water resistance; as a result, research on improving their water resistance has become a focus. At the same time, due to water’s high heat capacity, finding ways to increase its solid content in order to boost its drying speed is also one of the urgent problems that need to be addressed. 5. Polyurethane adhesives for construction: In addition to being non-toxic, pollution-free, and easy to use, polyurethane adhesives possess advantages that other adhesives cannot match, namely excellent resistance to low temperatures, solvents, aging, ozone, and bacteria. They play an important role in the application of construction flooring materials. It is widely used in elastic rubber mats, rigid rubber tiles, and sports fields with plastic running tracks. The new two-component polyurethane adhesive overcomes the conflict between shear strength and peel strength associated with traditional adhesives, enabling both properties to reach high levels simultaneously. It exhibits excellent performance in bonding steel plates used in construction, providing strong adhesion without significant deformation. Additionally, its curing speed can be adjusted at room temperature, making this polyurethane adhesive easy to use and suitable for a wide range of applications. Polyurethane adhesives are widely used in bonding PVC materials for construction, in the production of sandwich panels, and in building waterproof coatings. 6. Water-based inks using polyurethane adhesives are considered the preferred choice for environmentally friendly printing. Water-based polyurethane adhesives that can be cross-linked at room temperature can be used to produce single-component water-based polyurethane inks suitable for gravure printing. These inks are resistant to water in high-gloss areas, offer strong adhesion, allow for adjustable drying times, possess vivid colors and clear color gradients, are non-toxic and non-flammable, weather-resistant, and have a consistency that is easy to control. Compared with other water-based inks, they exhibit broad compatibility with various substrate materials. Polyurethane-polyacrylate, used as a binder in water-based inks, combines the properties of polyurethane adhesives and polyacrylates during the drying process of ink-coated films. The emulsion obtained through this modification also exhibits good adhesion to such films. The base films used in the color printing industry include polyester film, nylon film, polypropylene film, polyethylene film, and others. The key when choosing ink is that it must have good adhesion to all of these base films. A new type of polyurethane adhesive can be prepared by reacting a vinyl chloride-hydroxyethyl acrylate copolymer containing hydroxyl groups in its molecules with high-molecular-weight diols, low-molecular-weight chain extenders, and toluene diisocyanate; this adhesive can effectively improve the quality of color printing inks. 7. Polyurethane adhesives for book binding: Over the past 20 years, the use of hot-melt adhesives in book binding has seen rapid development. The main reason is the astonishing growth in the production of low-cost paperback books; simultaneously, a binding process without covers was developed. Due to the excellent adhesion and elasticity of EVA hot melt adhesive, it is used in the vast majority of book bindings in the domestic market. However, EVA hot melt adhesives are expensive, and they have drawbacks such as memory effect. Polyurethane hot-melt adhesives can overcome the aforementioned shortcomings. These adhesives have a simple production process and low costs; they possess good elasticity and toughness, as well as high bonding strength. They can be melted and reused repeatedly, resulting in high product utilization rates, and thus they offer broader application prospects. 8. Applications in railway construction: The installation of ballastless tracks (the development direction of high-speed rail engineering technology) will rely primarily on domestically produced polyurethane adhesives. For a single-track system, a protruding retaining block is installed every 5 meters; approximately 17.8 kg of polyurethane adhesive is required on each side of each such block. For the construction of one kilometer of double-track ballastless tracks, more than 7 tons of polyurethane adhesive are needed. In addition to railway construction, the production of high-speed trains has also **increased the demand for polyurethane adhesives. Polyurethane plays various essential roles in trains, such as bonding glass, securing floors, filling gaps, and providing waterproofing. In CRH3 high-speed trains, each car uses approximately 84.07 liters of polyurethane adhesive, which is equivalent to around 109.3 kg; this adhesive is mainly used for sealing window glass and for sealing certain filler areas. 9. Other polyurethane adhesives, thanks to their excellent bonding properties, are playing an increasingly important role in areas such as the bonding of aerospace components, the conservation and restoration of cultural artifacts, the military industry, stationery, and healthcare. Polyurethane adhesives are widely used in various fields due to their excellent properties, and are increasingly becoming an important synthetic material in human life. The global polyurethane adhesive industry is evolving to meet the requirements of environmental protection, safety and hygiene, as well as resource recycling. China’s polyurethane adhesive industry has also shown rapid growth, with significant progress being made in technology development. With the gradual expansion of polyurethane’s applications and the growing environmental awareness among humans. Polyurethane adhesives will play an increasingly important role.