Polyurethane (PU) adhesives refer to adhesives that contain urethane groups (-NHCOO-) or isocyanate groups (-NCO) in their molecular chains. Polyurethane adhesives are divided into two major categories: polyisocyanates and polyurethanes.

The polyisocyanate molecular chain contains isocyanate groups (-NCO) and carbamate groups (-NH-COO-), hence polyurethane adhesives exhibit high reactivity and polarity.

It has excellent chemical adhesion to substrates containing active hydrogen, such as foams, plastics, wood, leather, fabrics, paper, ceramics, and other porous materials, as well as to smooth-surfaced materials like metals, glass, rubber, and plastics.

PU adhesive bonding principle

Its core reaction mechanism is the chemical reaction between isocyanate groups and substances containing active hydrogen, producing polyurethane groups. This reaction gives polyurethane adhesives high strength and flexibility.

PU adhesive characteristics

1. Advantages of PU adhesive

1) The isocyanate group (-NCO) and urethane group (-NHCOO) in PU molecules, which have strong polarity and chemical activity, exhibit excellent chemical adhesion with materials containing active hydrogen, such as foamed plastics, wood, leather, fabrics, paper, ceramics, and other porous materials, as well as smooth-surfaced materials like metals, glass, rubber, and plastics.

2) By adjusting the formulation of polyurethane resin, the ratio and structure of soft segments to hard segments in the molecular chain can be controlled, producing adhesives with different hardness and elongation. The bonding layer can be adjusted from flexible to rigid, thus meeting the bonding requirements of different materials.

3) Can be cured by heating or at room temperature. The bonding process is simple, with good operability.

4) Curing belongs to addition polymerization, with no side reactions, thus it is not easy to cause defects in the bonding layer.

5) Polyisocyanate adhesives can dissolve in almost all organic materials, and due to the small molecular volume and easy diffusion of isocyanates, polyisocyanate adhesives can penetrate into the bonded materials, thereby enhancing adhesion.

6) When polyisocyanate adhesives are used to bond rubber and metal, not only is the bond strong, but it also forms a soft-hard transition layer between the rubber and metal, thus resulting in lower internal stress in the bond and producing superior fatigue resistance.

7) Polyurethane adhesives have good wear resistance, water resistance, oil resistance, solvent resistance, chemical resistance, ozone resistance, and bacterial resistance.

8) Its low-temperature and ultra-low-temperature performance exceeds that of all other types of adhesives. The bonding layer can be used at -196°C (liquid nitrogen temperature), and even at -253°C (liquid helium temperature).

2. Disadvantages of PU adhesive

Poor heat resistance, easily hydrolyzed under high temperature and high humidity, reducing bonding strength.

2) -NHCOO- is toxic, and precautions should be taken when using it.

classification of PU adhesive

polyisocyanate adhesive

Polyisocyanate adhesives are adhesives composed of polyisocyanate monomers or their low molecular weight derivatives, and they are early products in polyurethane adhesives.

Polyisocyanate adhesives belong to the category of reactive adhesives, with high bonding strength, especially suitable for bonding metal with rubber, fibers, etc. This type of adhesive has the following main characteristics:

It has high reactivity and can form covalent bonds with many adhered materials that have active hydrogen atoms on their surfaces, such as metals, rubber, fibers, wood, leather, plastics, etc. After curing, it contains urethane, urea bonds, and highly polar bonds and groups, which easily form secondary bonds with the substrate. The combined effect of these chemical and physical adhesive forces results in high bonding strength between the adhered substrates.

2) It can cure at room temperature or by heating, easily forming a cross-linked structure with good heat and solvent resistance.

3) Contains a large number of free isocyanate groups, is sensitive to moisture, is toxic, usually contains organic solvents, and should be stored with attention to waterproofing and moisture protection. Proper ventilation is required during operation.

4) Due to the small relative molecular mass of polyisocyanate compounds and the high mass fraction of -NCO groups, the cured adhesive layer has high hardness and brittleness. Therefore, rubber solutions, polyethers, polyesters, and other oligomers are commonly used for modification or as cross-linking curing agents for various adhesives.

Diisocyanates produced industrially, such as MDI, TDI, XDI, dimethoxydiphenyl diisocyanate (DADI), methyl hexanoate-2,6-diisocyanate (LDI), etc., can all be used directly as adhesives for bonding metal and rubber. Currently, the most widely used polyisocyanate adhesives are tris(1-isocyanato-1-methylethyl)benzene, tris(4-isocyanatophenyl) thiophosphate, and the adduct of trimethylolpropane-TDI.

2. Two-component polyurethane adhesive

Two-component polyurethane adhesives are the most important category in polyurethane adhesives, with wide applications and large usage. They usually consist of two components, Component A (main agent) being the hydroxyl component, and Component B (curing agent) being the component containing free isocyanate groups. The two components are packaged separately and can be mixed in a certain ratio before use. In some cases, the main agent is a polyurethane prepolymer with terminal -NCO, and the curing agent is a low molecular weight polyol or polyamine. Components A and B are mixed in a certain ratio to form polyurethane resin.

Two-component polyurethane adhesive has the following characteristics:

1) The adhesive, after the two components are mixed, undergoes a cross-linking reaction to produce a cured product.

2) During preparation, the raw material composition and relative molecular mass of the two components can be adjusted to achieve a suitable viscosity at room temperature, allowing for the production of high solid content or solvent-free two-component adhesives.

3) Usually curable at room temperature, the curing speed can be adjusted by selecting the raw materials for preparing the adhesive or adding a catalyst. Generally, two-component polyurethane adhesives have a high initial bonding strength, which can meet the requirements of structural adhesives.

4) The amount of the two components can be adjusted within a certain range. The NCO/OH molar ratio of the two components is generally greater than or equal to 1. During curing, part of the -NCO groups participate in the curing reaction of the adhesive, generating chemical adhesion; excess -NCO groups, when heated and cured, can also produce urethane, biuret, etc., increasing the crosslink density, thereby enhancing the cohesive strength and heat resistance of the adhesive layer.

For solvent-free two-component polyurethane adhesives, since the initial relative molecular mass of each component is not large, generally, an NCO/OH molar ratio equal to or slightly greater than 1 is favorable for complete curing. Especially when bonding seals, it should be noted that the -NCO component should not be in excess.

For solvent-based two-component adhesives, the main agent has a relatively large molecular weight, and the initial adhesion performance is better. The usage of the two components can be adjusted over a wide range, and the NCO/OH molar ratio can be less than 1 or several times greater than 1. In cases where the -NCO component (curing agent) is in excess, polyisocyanates self-polymerize to form a tough bonding layer, suitable for bonding hard materials; when the -NCO component is used in smaller amounts, the adhesive layer is soft and can be used for bonding soft materials such as leather and fabrics.

3. one-component polyurethane adhesive

One-component polyurethane adhesive is a solvent-free reactive adhesive, whose main components include isocyanate, polyether or polyester, etc. This type of adhesive can react with water in the air within an appropriate temperature range to form a high-strength solid sealant, a process generally known as moisture curing. The adhesive has the following types:

terminal isocyanate-based polyurethane prepolymer, divided into moisture-curing type and solvent-free type.

2) Thermoplastic polyurethane, divided into solvent-based, water-based, hot-melt, and reactive hot-melt types.

3) Acrylate-polyurethane, divided into UV, electron beam, light or radiation curing types and anaerobic type.

4) Closed-type polyurethane, which is heat-cured.

Among them, the moisture-curing type is the mainstream.

terminal isocyanate polyurethane prepolymer

Terminal isocyanate-based polyurethane prepolymer system, utilizing the characteristic that the isocyanate groups at the ends of the polymer can react with moisture in the air or on the adherend, causing the polyurethane adhesive layer to crosslink and cure, hence it is named moisture-cured polyurethane adhesive.

Moisture-curing adhesives often have bubbles in the adhesive layer due to the release of carbon dioxide, leading to defects. Carbon dioxide absorbers or adsorbents are often added to the adhesive to overcome these drawbacks.

Moisture-cured polyurethane adhesives are often made by reacting polyether polyols with toluene diisocyanate or diphenylmethane diisocyanate, etc., and are good adhesives for wood, civil engineering, and structural applications; they are also commonly used as sealants and play an important role in the automotive, construction, and machinery industries.

2) thermoplastic polyurethane

Thermoplastic polyurethane, also known as isocyanate-modified polyurethane, is a linear carbamate polymer with terminal hydroxyl groups. This type of polymer, due to its inherent adhesion properties and strength, can be formulated alone for use as a thermoplastic resin adhesive.

solvent-based

Polyester-based thermoplastic polyurethane solution adhesives have good performance. The high polarity of the ester group gives the adhesive bonding properties to a variety of materials (especially plastics), and increasing the glass transition temperature of the polyester can improve the heat resistance of the adhesive.

(2) water-based type

The greatest advantage of water-based polyurethane adhesives is that they are not prone to poisoning or catching fire, and are suitable for substrates that are easily eroded by organic solvents.

Water-based polyurethane adhesives can be used as wood adhesives, capable of bonding wood at room temperature or higher temperatures, with advantages such as high initial tack, excellent bonding strength, and usability on relatively moist wood; the adhesive layer has good heat resistance, hot water resistance, and aging resistance, no free formaldehyde release, does not pollute the environment, and does not contaminate or corrode wood, all of which are beyond the reach of traditional wood adhesives.

When used as a fabric treatment agent and adhesive, the fabric treated with water-based polyurethane containing ionic groups has breathability; after cross-linking, it also has waterproof function.

(3) thermoplastic type

Polyurethane hot melt adhesives are mostly made from polyester polyols, MDI, and the chain extender 1,4-butanediol, and can also be made from polyether and TDI or MDI. Their main application is in the form of powder for the production of fabric linings. They have good dry and wet wash resistance.

(4) reactive hot melt adhesive

Polyurethane reactive hot melt adhesives contain reactive functional groups and possess the properties of both reactive and hot melt adhesives. Mainly moisture-cured, they are solvent-free and in a liquid form; the melting temperature is lower than that of general hot melt adhesives (170~200°C), allowing for low-temperature application (<120°C) with good operability; they have a long open time and practical life, making them easy to use. They exhibit excellent resistance to heat, cold, water vapor, chemicals, and solvents, maintaining the inherent characteristics of polyurethane adhesives; however, they are more sensitive to moisture, requiring strict handling during production, storage, transportation, and use, which somewhat limits their application.

The main components of polyurethane reactive hot melt adhesive are isocyanate-terminated prepolymers, combined with thermoplastic resins that do not react with isocyanates, tackifying resins, and fillers, plasticizers, antioxidants, UV absorbers, flame retardants, mildew inhibitors, coupling agents, and catalysts. Sagging resistance agents can be added if necessary.

acrylate-polyurethane

React (meth)acrylic hydroxyethyl ester or (methyl) acrylic hydroxypropyl ester, etc., with isocyanate-terminated prepolymer to produce polyurethane containing acrylic double bonds. This can be used as a base material to formulate anaerobic or light-curing adhesives. Generally, such adhesives are classified as acrylic adhesives.