Current Status and Development Trends of Polyurethane Adhesives in Major Application Fields

Through the analysis of polyurethane adhesive formulations, it can be seen that polyurethane adhesives contain urethane groups (-NHCOO-) or isocyanate groups (-NCO) in their molecular chains, which can be divided into two main categories: polyisocyanate and polyurethane. Regardless of the type, polyurethane adhesives bond by the reaction of isocyanate groups with substances containing active hydrogen within or outside the system, forming polyurethane or polyurea groups, thereby increasing the system’s strength and achieving bonding. Polyisocyanate molecules contain isocyanate (-NCO) and urethane (-NHCOO-) groups, making polyurethane adhesives highly reactive and polar, allowing them to bond with materials that contain active hydrogen, such as foams, plastics, wood, leather, fabrics, paper, ceramics (porous materials), as well as metals, glass, rubber, plastics, and other smooth materials, showing excellent chemical bonding properties.

Adhesives are formulated with a main adhesive, along with curing agents, plasticizers, fillers, solvents, preservatives, stabilizers, and coupling agents. Initially, adhesives were derived from natural materials such as starch, dextrin, bone glue, fish glue, etc., with water as the solvent, which were heated to prepare the adhesive. Due to their simple composition and poor applicability, they could not meet the diverse needs of various applications. With the advent of synthetic polymers, people began developing synthetic resin adhesives for various bonding purposes. In recent years, with rapid advancements in material development, a variety of more versatile adhesives have emerged, enriching the adhesive market.

1. Development Status

Polyurethane adhesives are a type of excellent adhesive with good flexibility, impact resistance, chemical resistance, wear resistance, and importantly, low-temperature resistance. By adjusting raw materials and formulations, various types of polyurethane adhesives suitable for bonding different materials and for different purposes can be designed. Polyurethane adhesives were first used in the military field in 1947, when Bayer successfully applied triphenylmethane triisocyanate for bonding metals and rubber, such as for tank tracks, laying the foundation for the polyurethane adhesive industry. Japan introduced German and American technology in 1954, began producing polyurethane materials in 1960, and successfully developed vinyl-based polyurethane waterborne adhesives in 1966, with industrial production commencing in 1981. Currently, Japan’s research and production of polyurethane adhesives is highly active, and it, along with the U.S. and Western Europe, is a major producer and exporter of polyurethane. After the 1980s, polyurethane adhesives developed rapidly, and the industry now boasts a wide range of types and applications.

China developed and produced triphenylmethane triisocyanate (Liekna glue) in 1956 and soon followed with toluene diisocyanate (TDI) and two-component solvent-based polyurethane adhesives, which remain some of the largest-volume products in the country today. Over the years, China has introduced many advanced production lines and products from abroad, with a large amount of imported polyurethane adhesives and supporting materials, accelerating domestic research and development. Especially after 1986, the polyurethane industry in China entered a period of rapid development. For a long time, polyurethane adhesives were very expensive, with prices 1–2 times higher than neoprene adhesives. In recent years, the price of polyurethane adhesives has continuously decreased, and now it is only about 20% higher than neoprene, which has created conditions for polyurethane adhesives to capture the neoprene market.

The production methods for polyurethane adhesives include solution-based and solid resin dissolution methods. Currently, there are over a hundred enterprises in mainland China producing shoe adhesives based on polyurethane, with more than twenty large-scale enterprises. Companies such as Nanguang and Bali have production capacities above 10,000 tons per year, mostly using imported resins from Spain, Germany, the U.S., and Taiwan to dissolve and produce polyurethane adhesives. Due to the increasing competition among production enterprises, some large adhesive manufacturers have started building solution-based production units for polyurethane adhesives, so the proportion of dissolved polyurethane adhesive production will decrease.

Currently, the most common polyurethane adhesives are solvent-based, which, like neoprene, pose toxicity and environmental pollution issues. In recent years, solvent-free polyurethane adhesives have emerged, using ketones, esters, and other mixed solvents. While these reduce toxicity, the problem has not been fundamentally solved.

Polyurethane adhesives can be classified according to their reaction components into polyisocyanate adhesives, polyurethane adhesives containing isocyanate groups, hydroxyl-functional polyurethane adhesives, and polyurethane resin adhesives. According to their applications and characteristics, they can be divided into general-purpose adhesives, food packaging adhesives, shoe adhesives, paper-plastic composite adhesives, construction adhesives, structural adhesives, ultra-low-temperature adhesives, foam adhesives, anaerobic adhesives, conductive adhesives, hot melt adhesives, pressure-sensitive adhesives, sealing adhesives, waterborne adhesives, and adhesive sealants.

2. Main Application Fields

2.1 Woodworking Industry

With the rapid depletion of global forest resources and China’s implementation of natural forest protection projects, the need for adhesives with better bonding strength and durability than the wood itself has increased, especially for splicing small timber into large panels. The quantity of adhesive used has become an indicator of the technological development level of the wood industry. Traditional wood adhesives were often urea-formaldehyde resins, phenolic resins, and melamine-formaldehyde resins, which, due to free formaldehyde emissions, caused environmental pollution. The woodworking industry has started shifting its focus to new, environmentally friendly adhesives—polyurethane adhesives—to reduce environmental impact. Single-component moisture-curing polyurethane adhesives, which are liquid at room temperature, exhibit high bonding strength, flexibility, and water resistance, and can bond various non-wood substrates (such as metals, plastics, rubbers, etc.). The strength of polyurethane adhesives in dry and wet conditions is superior to that of phenolic adhesives. Pre-coating the substrate with hydroxy-methylphenol (HMR) coupling agents can enhance bonding strength.

2.2 Footwear Industry

China is a major footwear manufacturing country, and after three generations of shoe adhesives, with increasing global environmental awareness and the escalation of the oil crisis, the fourth generation of solvent-free and water-based shoe adhesives has emerged. In recent years, the preparation process for water-based polyurethane has matured. For low-polarity materials like SBS, the peeling strength of polyurethane adhesives may not meet the requirements. Modifying the adhesive by adding tackifying resins has led to the development of high-crystallinity, fast-crystallizing, high cohesive strength, and better peel strength polyurethane adhesives for shoes.

2.3 Packaging Industry

Flexible packaging, due to its lightweight, long shelf life, hygiene, ease of storage and transportation, easy opening, low waste, and good shelf effect, has surpassed rigid packaging like plastic, glass bottles, and cans. Polyurethane adhesives, with their excellent properties, can bond different types of films to create flexible packaging materials with features like cold resistance, oil resistance, pharmaceutical resistance, transparency, and abrasion resistance. Polyurethane adhesives have become the primary adhesive for composite film processing in flexible packaging. Among the types used, water-based, hot-melt, solvent-based, and solvent-free polyurethane adhesives are common. The most widely used polyurethane hot-melt adhesives can be divided into two types: thermoplastic polyurethane hot-melt adhesives and reaction-type hot-melt adhesives. The main drawback of thermoplastic hot-melt adhesives is their high viscosity, which affects the coating quality. Reaction-type hot-melt adhesives developed from traditional hot-melt adhesives combine good initial tack and post-curing properties, with the flexibility and strength of polyurethane, offering superior bonding performance for multiple substrates.

2.4 Building and Flooring

Polyurethane adhesives, aside from being non-toxic and pollution-free, also have superior low-temperature resistance, solvent resistance, aging resistance, ozone resistance, and bacterial resistance, making them indispensable in the construction and flooring industry. They are widely used for elastic rubber mats, hard rubber floor tiles, and synthetic sports tracks. New two-component polyurethane adhesives have overcome the contradiction between shear strength and peel strength, achieving high levels of both in applications such as bonding steel plates for construction. These adhesives are stable, resistant to deformation, and cure at adjustable speeds at room temperature, making them highly convenient for use. Polyurethane adhesives are also applied to PVC material bonding, sandwich panel production, and waterproof coatings.

2.5 Other Applications

In the last 20 years, hot-melt adhesives have gained significant popularity in bookbinding, driven by the dramatic growth of low-cost paperback book production. Currently, EVA-based hot-melt adhesives dominate the bookbinding market, but they are costly and have issues like memory effects. Polyurethane hot-melt adhesives can overcome these disadvantages. They are cheaper, have good elasticity and toughness, high bonding strength, and can be remelted for reuse, offering significant potential in bookbinding.

Polyurethane adhesives, due to their excellent bonding characteristics, are increasingly important in aerospace materials, cultural heritage protection and restoration, military industries, stationery, medical and healthcare fields. They are becoming a critical synthetic material in daily life. Currently, the global polyurethane adhesive industry is shifting towards environmental protection, safety, health, and resource recycling. The polyurethane adhesive industry in China is also growing rapidly, with significant technological advancements.

3. Waterborne Polyurethane Green Adhesives and Their Development

Currently, the entire polyurethane adhesive industry is still predominantly solvent-based, but with the increasing awareness of safety and environmental protection, the research and development of waterborne polyurethane adhesives has accelerated.

3.1 Performance Characteristics

Waterborne polyurethane adhesives, while retaining the advantages of polyurethane, have the following unique features compared to solvent-based adhesives: they use water as the medium, which is harmless, non-flammable, odorless, and non-polluting, thus saving energy. These adhesives

Waterborne polyurethane adhesives have excellent water and medium resistance, high bonding strength, strong initial tack, good storage stability, resistance to freeze-thaw cycles, high-temperature resistance, fast drying speed, good film-forming ability at low temperatures, and good application processes.

3.2 Classification

Waterborne polyurethane adhesives can be classified in various ways:

1. By the Structure of the Polyurethane

According to the raw materials and structure, they can be divided into:Polyurethane emulsions, vinyl polyurethane emulsions, multi-isocyanate emulsions, and closed-type polyurethane emulsions.

According to the molecular structure, they can be divided into linear polyurethane emulsions and crosslinked polyurethane emulsions. Crosslinked types are further divided into internal and external crosslinking types.

2. By the Nature of Hydrophilic Groups

• Anionic: Most waterborne polyurethane adhesives are modified by adding carboxyl or sulfonate groups through chain extension.
• Cationic: These adhesives generally contain ammonium or sulfur ions in the main or side chains.
• Nonionic: These adhesives contain nonionic hydrophilic segments or hydrophilic groups in the molecules.

These adhesives contain both ionic and nonionic hydrophilic segments or groups.

3. By Preparation Method

Methods include self-emulsification, external emulsification, prepolymer method, acetone method, melt dispersion method, and ketone imine-methanone linking method.

4. By Usage Form

Single-component and two-component waterborne polyurethane adhesives.

3.3 Research on Performance Improvement

Compared to solvent-based adhesives, waterborne polyurethane adhesives have the advantages of being solvent-free, non-polluting, good film-forming ability, high bonding strength, and the ease of blending with other polymers, especially emulsion-based ones, making them easier to modify. Since the 1990s, they have gradually found applications in automotive interior bonding, kitchenware manufacturing, composite film production, footwear bonding, and garment processing. However, there are still several drawbacks that need to be addressed through various modifications to enhance their functionality.

Recent studies have focused on issues such as slow drying speed, low initial tack, poor wettability on non-polar substrates, poor water resistance, and inadequate heat resistance. Several improvement measures have been proposed, such as reducing the hydrophilic group content to improve water resistance while maintaining the stability of the emulsion. Research on improving waterborne polyurethane performance mainly focuses on modifications, including crosslinking, blending, copolymerization, and nanomaterial modifications.

3.3.1 Crosslinking Modification

Crosslinking is an effective way to improve the performance of waterborne polyurethane, enhancing the water resistance, heat resistance, and bonding strength of the adhesive film. Crosslinking can be either internal or external:

• Internal Crosslinking:This involves introducing crosslinking agents during synthesis, such as polyols with multiple functional groups, isocyanates, amines, or epoxy groups. However, internal crosslinking methods have the drawback of high viscosity in the prepolymer, making emulsification difficult.
• External Crosslinking:This involves adding crosslinking agents before use, followed by heating to induce chemical reactions and form a crosslinked adhesive film. Compared to internal crosslinking, the emulsions produced have better performance, but external crosslinking requires a two-component system, which is less convenient than single-component systems. Crosslinking agents such as formaldehyde, melamine-formaldehyde resin, epoxy compounds, and others can be used in external crosslinking, typically at high temperatures (120–180°C). Some crosslinking agents, such as N-vinylpyrrolidone and isocyanate, can react at room temperature, but certain agents like N-vinylpyrrolidone can be volatile and toxic, posing health risks.

3.3.2 Blending Modification

Blending modification utilizes the complementary effects of different resins to improve and enhance the performance of waterborne polyurethane adhesives. Waterborne polyurethane can be blended with other resins, such as epoxy resin emulsions, acrylic emulsions, chloroprene latex, polyvinyl acetate emulsions, urea-formaldehyde resins, etc., to produce new adhesive formulations with excellent overall properties. Research on blending polyurethane with epoxy resins, acrylic resins, and other materials has been particularly significant. Other modifications include the copolymerization of polyurethane with resins such as epoxy, organic silicon, fluorine-containing compounds, acrylic esters, carboxymethyl cellulose, polyvinyl alcohol, vinyl acetate, and styrene-butadiene rubber.

3.3.3 Nanomaterials

Nanomaterials possess unique properties such as surface effects, size effects, optical effects, and quantum size effects, which can impart new functionalities to materials. When particle size enters the nanoscale, the increased surface activity can enhance chemical and photocatalytic reactions, providing self-cleaning properties under UV light and oxygen. The functional groups on the surface of the nanomaterials can form secondary chemical bonds with the film-forming substance, increasing the rigidity and strength of the coating, improving scratch resistance. Modified high-surface-energy nanomaterials can simultaneously achieve hydrophobic and oleophobic properties.

Nanomaterial-modified waterborne polyurethane is mainly focused on the coatings sector, with relatively limited research in adhesives. However, with the increasing emphasis on safety, environmental protection, and green development, there is huge market potential for environmentally friendly waterborne polyurethane adhesives.

4. Development Trends

As an independent industry with a complete production chain, adhesives play an irreplaceable role in the packaging industry, driving technological progress and product development in the packaging and related sectors. The development and production of new adhesives with superior quality and performance have become a significant focus of research in the packaging industry, which has accelerated the growth of the adhesive industry.

Currently, solvent-based polyurethane adhesives dominate the market, particularly in food packaging composite films in China, where most adhesives used are solvent-based. This raises environmental pollution and safety concerns while consuming large amounts of energy. It is estimated that over 400,000 tons of organic solvents are emitted into the atmosphere annually in the food packaging industry. Organic solvents are flammable, volatile, and have strong odors, leading to air pollution and potential health risks. The solvents that cannot be fully eliminated remain in food packaging, potentially posing a greater risk to consumers than the food safety itself.

In recent years, demand for hot-melt adhesives in China has grown rapidly, particularly because they do not use organic solvents, have quick application times, and provide strong bonding, offering promising development prospects. In addition to traditional EVA hot-melt adhesives, the market should actively develop new types of polyester and polyamide-based hot-melt adhesives.

Internationally, solvent-free adhesives, such as epoxy adhesives, anaerobic adhesives, acrylate adhesives, and light-curing adhesives, are being increasingly used. Solvent-free polyurethane adhesives are applicable to various types of plastic composite packaging and provide excellent adhesion to aluminum foil, as well as good content resistance and high-temperature steam resistance, similar to ester-solvent polyurethane adhesives, with higher peel strength. Waterborne adhesives are favored due to their non-combustible, non-toxic, and non-polluting properties. However, they have some limitations, such as slow drying speed, poor water resistance, and high energy consumption. It will take at least 10 years for waterborne adhesives to completely replace ester-solvent polyurethane adhesives.

Therefore, in future production and research, efforts should be made to increase the solid content of waterborne adhesives, accelerate drying speed, and adopt crosslinking methods to improve their water resistance, expanding their applications and market share. With the growing environmental awareness and increasingly strict regulations, promoting the use of environmentally friendly adhesives is an inevitable trend. The development of environmentally friendly, energy-efficient adhesives with high technological content, high added value, and high performance will be a major focus in the coming years.