The Ultimate Guide to Polyurethane Adhesives

Introduction to Polyurethane Adhesives

Polyurethane adhesives are a crucial component of the rapidly developing polyurethane resins, which possess excellent properties and are widely used in various applications, making them one of the key varieties of synthetic adhesives.

Polyurethane adhesives offer excellent shear strength and impact resistance, making them suitable for a wide range of structural bonding applications and exhibiting exceptional flexibility.

Polyurethane adhesive exhibits excellent rubber characteristics, allowing it to adapt to the different coefficients of thermal expansion of the substrate being bonded. It forms a soft-to-hard transition layer between the substrate, providing not only a strong adhesive force but also excellent cushioning and shock absorption functions. The low- and ultra-low-temperature performance of polyurethane adhesives exceeds that of all other types of adhesives.

A water-based polyurethane adhesive with a low VOC content, low or no environmental pollution, and other desirable characteristics is the key development direction for polyurethane adhesives.

Classification of Polyurethane Adhesives

• Classified according to the reaction composition, can be divided into polyisocyanate adhesives, polyurethane adhesives containing isocyanate groups, polyurethane adhesives containing hydroxyl groups, and polyurethane resin adhesives.
• According to the use and characteristics of the classification, the classification can be divided into general-purpose adhesives, adhesives for food packaging, adhesives for shoes, paper-plastic composite adhesives, construction adhesives, structural adhesives, ultra-low-temperature adhesives, foam-type adhesives, anaerobic adhesives, conductive adhesives, hot-melt adhesives, pressure-sensitive adhesives, closed-end adhesives, water-based adhesives, and sealing adhesives. The following are some of the most popular types of adhesives.

principle

No matter what type of polyurethane adhesive is used, the isocyanate groups in the system react with active hydrogen-containing substances, either within the system or outside, to form polyurethane groups or polyurethane, which significantly enhances the system’s strength and achieves the purpose of bonding.

Polyurethane Adhesives Product Characteristics

High performance waterborne polyurethane adhesive has the following characteristics.

1. Good water and media resistance.
2. High bonding strength and high initial adhesion.
3. Good storage stability.
4. Freezing and thawing resistance, as well as resistance to higher temperatures.
5. Fast drying speed and good film formation at low ambient temperature.
6. Good construction techniques.

To achieve the above points, in addition to synthesizing high-performance waterborne polyurethane, the choice of film-forming additives is also particularly important.

To achieve the above points, in addition to synthesizing high-performance waterborne polyurethane, the choice of film-forming additives is also particularly important.

The synthetic raw materials of polyurethane prepolymer are primarily oligomer polyols and diisocyanates. Oligomeric polyols are usually divided into two categories: polyether polyols and polyester polyols, and the prepolymer made from polyether polyols has good hydrolytic stability, good flexibility and elongation, and good low-temperature resistance, while the polyester polyol-type prepolymers have large cohesive force and high adhesive strength.

Polyurethane Adhesive Use

Scheno chemical polyurethane adhesive with high solid content, low viscosity, good process applicability, strong adhesive force, non-toxic and other advantages are applicable to aluminum foil (OPA), pre-treated polyethylene (HDPE, LDPE), polypropylene (OPP, CPP), polyester (PET), polyamide (PA, ONY) film, spraying the metal of the film, the cellophane composite between the two-component solvent-type Polyurethane adhesives.

Characteristics: This product offers good economy with low viscosity, excellent machinability and wettability, particularly a notable initial adhesion effect after compounding, good transparency and elasticity, as well as aging resistance and no odor.

Instructions for use: (weight) 5 parts of the main agent with 1 part of the curing agent: ethyl acetate 6-8-10 (according to the thickness of the coating film)

1. The solvent is suitable for ethyl acetate; the solvent content of water shall not exceed 300 ppm. This is not applicable to aromatic, toluene, or alcohol solvents.
2. dilution method: must be required by the solids content to the main agent to add solvent, stirring, and then add the curing agent to the diluted main agent,and stir thoroughly.
3. The applicable period: The applicable period of the formulated adhesive is determined by its solid content, storage temperature, and the presence of water in the diluent. Generally, formulated adhesives stored in closed containers can be used for about 1-2 days without a significant increase in viscosity. The use of adhesives or glue barrels in the old glue will have a negative impact on the quality of composite products.
4. Coating: It can be adapted to any dry laminating machine equipped with a light roller and anilox roller.
5. Coating amount: To accommodate the diverse uses and material composition requirements of composite products, the dry base coating amount can be selected between 2-6g/㎡. To carry out thermal processing or deep molding processing of the composite film, the amount of adhesive coating is larger, while for the opposite process, the amount of adhesive coating is smaller. The printed film coating amount is adjusted accordingly.
6. Working concentration: The working concentration is determined by the amount of glue coated on the flower roller. The higher the concentration, the better the economy; the. The general recommendation is to use a concentration of 25-40% as a choice between options.
7. Drying: To ensure the film is completely clean after the adhesive solvent has evaporated, it is necessary to pay attention to sufficient air volume, wind speed, and temperature. If the drying system is divided into three sections, the temperature gradient from the film inlet to the outlet is controlled at 50-60 ℃ to 70-80 ℃ or 60-80-90 ℃.
8. Bonding temperature: depends on the nature and quality of the composite substrate; using the highest possible temperature, generally controlled at 50-90 ℃. Films are susceptible to temperature at 50-60 ℃; other films can be used at 70-80 ℃.
9. Composite pressure: In the case of not damaging the film, the composite pressure should be increased as much as possible.
10. Aging: The main agent and curing agent in composite processing do not immediately exhibit high bond strength. The products need to be sent into the aging room for 24-48 hours, as the winter and summer operating environments differ. Please pay attention. Preliminary composite laminating film should be matured for at least one day as possible before rewinding or manufacturing three-layer laminating film.
11. Remaining adhesive liquid: it cannot be further used if it has been more than 8 hours old or if it exhibits a whitening, translucent, or thickening phenomenon.

Storage:
Store in unopened, well-packed containers in a sun-protected, dry location with a storage period of 8 months, and use within a short period after opening the package.

Film corona treatment requirements:
Polyethylene and polypropylene film must be corona treatment; polyethylene film surface tension should be as far as possible to reach 42 dynes and shall not be less than 40 dynes; polypropylene film surface tension should not be less than 39 dynes.

applications

The versatility of polyurethane adhesives provides a solution to many bonding challenges and is particularly suited to areas where other types of adhesives cannot bond or are difficult to bond.

In addition, polyurethane adhesives are characterized by adjustable toughness, easy bonding processes, excellent low-temperature resistance and excellent stability. It is due to the excellent adhesive properties of polyurethane adhesives and their adaptability to a variety of substrates, which has led to their field of application continuing to expand. In recent years, both domestically and abroad, polyurethane adhesives have become the fastest-growing adhesive.

1. Automotive polyurethane adhesives
   A new automotive structure is being introduced, which incorporates a large number of lightweight metals, composite materials, and plastics, resulting in the continued growth of automotive adhesives and sealants. The most widely used automotive polyurethane adhesives include assembled windshields with a one-component wet-curing polyurethane sealant, bonding of glass fiber-reinforced plastics and sheet molding composite materials, structural adhesives, two-component polyurethane adhesives, and water-based polyurethane adhesives for interior parts, among others. Automotive interior parts are also an area of growth in terms of adhesive usage. For a considerable period, China’s automobile manufacturing industry had been importing polyurethane windshield adhesives from abroad. However, during the “Eighth Five-Year Plan” period, the development of this adhesive was included in the national key scientific and technological research projects. Water-based polyurethane adhesive, widely used in automotive applications, refers to polyurethane dissolved in water or dispersed in water, resulting in the formation of a water-based adhesive. Most of the water-based polyurethane is linear thermoplastic polyurethane because its coating film is not crosslinked, resulting in a low molecular mass. Consequently, its performance in terms of water resistance, solvent resistance, film strength, and other properties remains poor, and it must be modified to improve its performance. The heterogeneous dispersion of polyester and acrylic acid cooperates with urea diketone and isocyanuric acid ester to prepare an automobile repair varnish that does not require high-speed mixing equipment and is easily mixed, exhibiting good adhesion properties.
2. Polyurethane adhesive for wood
   With the drastic reduction of worldwide forest resources and the implementation of China’s natural forest resources protection project, small wood spelling large boards require adhesive bonding strength, durability, and weathering properties that are better than those of the wood itself. The amount of adhesive has become a measure of the level of technological development of the wood industry. In the past, people used wood adhesive, mostly formaldehyde, as the main raw material for urea-formaldehyde resin, phenol-formaldehyde resin, and melamine formaldehyde resin. However, due to the presence of free formaldehyde, the use of these products during the period would gradually emit formaldehyde gas into the surrounding area, resulting in environmental pollution. The woodworking industry has begun to focus on the new environmentally friendly polyurethane adhesive in order to reduce environmental pollution. The one-component, moisture-curing polyurethane adhesives used in the woodworking industry are available in liquid form and can be applied at room temperature. They typically exhibit high bond strength, good flexibility, and water resistance and can bond with various non-wood substrates, such as textile fibers, metals, plastics, and rubber. The one-component polyurethane adhesive in the test showed that its dry and wet strength is better than that of the phenolic adhesive. Before bonding, the hydroxymethyl resorcinol (HMR) coupling agent can be coated on the surface of the bonding substrate to improve the bonding strength; HMR can strengthen 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 the most stringent test requirements.
3. Polyurethane Adhesives for Shoes
   China is a major shoe-making country, and the development of shoe adhesives has progressed through three generations. With the increasing global environmental consciousness and the intensification of the oil crisis, the emergence of the 4th generation of environmentally friendly, solvent-free, and water-based shoe adhesives has been prompted. The preparation process of water-based polyurethane has become increasingly mature. For the bonding of some low-polar shoe materials such as SBS, the peel strength of polyurethane adhesives cannot meet the requirements. Through the addition of tackifying resins and other modifications, polyurethane shoe adhesives with high crystallinity, rapid crystallization, and high cohesiveness and strength can be developed.
4. Packaging polyurethane adhesive
   Flexible packaging, also known as soft cans, offers several advantages over rigid packaging, including its lightweight and convenient design, long freshness, hygiene, ease of storage and transportation, easy opening, reduced waste, and a good shelf effect, among other unique features. As a result, it is now more popular than rigid packaging options such as plastic, glass bottles, and cans. Polyurethane adhesive, due to its excellent performance, can bond together the different properties of film materials to achieve cold, oil-resistant, drug-resistant, transparent, wear-resistant, and other flexible properties in composite films. In the domestic and international markets, polyurethane adhesives have become the main adhesive for flexible packaging with composite film processing. In the domestic adhesive market, packaging composite film manufacturing and polyurethane adhesive dosage are second only to the shoe industry, ranking second. Used in packaging polyurethane adhesive varieties, such as water-based polyurethane adhesive, hot melt polyurethane adhesive, solvent-based polyurethane adhesive, and solvent-free polyurethane adhesive. Among them, the commonly used polyurethane hot melt adhesives can be divided into 2 categories: (1) thermoplastic polyurethane elastomer hot melt adhesives and (2) reactive hot melt adhesives. The main disadvantage of thermoplastic hot melt adhesive is its high viscosity, which has a greater impact on the apparent quality of the coating. Reactive polyurethane hot melt adhesive is a new type of adhesive developed on the basis of traditional hot melt adhesive; it not only has the traditional hot melt adhesive initial viscosity and after-curing performance is excellent, but also has the composition of polyurethane structure of the advantages of the variable structure and the performance of the adjustable range of advantages of a wide range of substrates with excellent adhesive properties.
   Additionally, regarding water-based polyurethane adhesives for packaging, the use of emulsifiers or the introduction of hydrophilic ionic groups into the molecule reduces the adhesives’ water resistance. Consequently, research on improving water resistance has become a key focus. Meanwhile, due to the large heat capacity of water, increasing its solid content and thus improving its drying speed is also one of the urgent problems to be solved nowadays.
5. Construction polyurethane adhesive
   Polyurethane adhesive, in addition to being non-toxic, non-polluting, and easy to use, also offers other advantages that cannot be compared to its excellent low-temperature resistance, solvent resistance, aging resistance, ozone resistance, and bacterial resistance. These properties make it a valuable choice for building paving materials. It is widely used in elastic rubber floor mats, hard rubber floor tiles and laying plastic runway sports ground. The new two-component polyurethane adhesive breaks through the traditional adhesive shear strength and peel strength of the contradiction, which can make both at the same time achieve a higher use of strength in the construction of steel plate bonding embodies excellent performance, bonding firm and not easy to produce deformation, and room temperature adjustable curing speed, so that the polyurethane adhesive in the use of convenient and easy to use, widely used. Polyurethane adhesive is widely used in the construction of PVC materials, the production of sandwich panels, and the application of waterproof coatings.
6. Polyurethane adhesive for ink
   Water-based ink is listed as the first choice for environmentally friendly printing. Room temperature crosslinking waterborne polyurethane adhesive can be made suitable for gravure printing of one-component polyurethane water-based ink; the water-based ink has high light water resistance, strong adhesion, dry adjustable, bright colors, clear levels, non-toxic, non-combustible, maroon, viscous and easy to control, compared with other water-based inks, a wide range of substrate materials have a wide range of adaptability. Polyurethane-polyacrylate, as a water-based ink connecting material, combines the performance of polyurethane adhesive and polyacrylate in the ink film printing process. The emulsion obtained from film modification also exhibits good adhesion fastness.
   The color printing industry primarily uses polyester film, nylon film, polypropylene film, and polyethylene film as base films, among several other types. The key to choosing the ink is its response to these base films, ensuring good adhesion. The use of molecules containing hydroxyl vinyl chloride-hydroxyethyl acrylate copolymer and large molecule diol, as well as small molecule chain expander toluene diisocyanate, can result in the production of new polyurethane adhesives that effectively improve the quality of color printing ink.
7. Bookbinding with polyurethane adhesive
   Over the past 20 years, the application of hot melt adhesive in bookbinding has experienced rapid development. The reason is mainly low-priced paperback book production has grown phenomenally and, at the same time, produced a wireless binding process; due to the EVA hot melt adhesive has a very good adhesive and elasticity, the domestic market for the use of bookbinding hot melt adhesive, the vast majority of EVA-type hot melt adhesive. However, the price of EVA-type hot melt adhesive is expensive, and it also has memory and other shortcomings. Polyurethane hot melt adhesive can address the above defects. The adhesive production process is simple, with low cost, and the adhesive exhibits high elasticity, toughness, and bonding strength. Additionally, its adhesive can be repeatedly melted, resulting in a high product utilization rate and broader application prospects.
8. Railroad construction on the application of
   ballastless track laying (high-speed rail engineering technology development direction) with glue will be dominated by domestic polyurethane adhesive products, monorail embedded in every 5 meters of a convex baffle, each baffle on both sides of each side of the need to be infused with polyurethane adhesive about 17.8kg, each kilometer of double-track ballasted track construction needs polyurethane potting adhesive more than 7 tons. In addition to the laying of railroads, the production of high-speed trains for the use of polyurethane adhesive demand has also greatly increased; polyurethane in the vehicle assumed the role of glass bonding, floor bonding, embedded filler, sealing and waterproofing and another indispensable role in the vehicle, in accordance with the CRH3 as the basis of the train set, a single carriage with polyurethane adhesive of about 84.07L converted to about 109.3Kg, mainly applied to the vehicle It is mainly used for the adhesive sealing of window glass and the sealing of partially filled parts.
9. Others
   Polyurethane adhesives play an increasingly important role in the adhesion of aerospace equipment, the protection and restoration of cultural relics, the military industry, stationery supplies, and medical and healthcare altercations due to their excellent adhesive properties.Polyurethane adhesive, with its excellent performance, is widely used in various fields and is becoming an increasingly important synthetic material in human life. The global polyurethane adhesive industry is evolving in the direction of environmental protection, safety and health, and resource recycling, among other areas. China’s polyurethane adhesive industry has also demonstrated rapid growth momentum, accompanied by significant technological advancements. With the gradual expansion of polyurethane use and increasing awareness of environmental protection, Polyurethane adhesives will play an increasingly important role.