Unveiling the Wonders of Polyurethane Adhesives and Sealants

In the realm of modern materials, polyurethane (PU) stands out as a true marvel, especially when it comes to adhesives and sealants. With its remarkable properties and diverse applications, PU has become a cornerstone in various industries. In this blog post, we will delve deep into the world of polyurethane adhesives and sealants, exploring their characteristics, advantages, and different types.

What Makes Polyurethane Special?

Polyurethane is a versatile polymer material renowned for its durability, strong bonding capabilities, and high peel strength. These qualities make it an ideal choice for formulating adhesives and sealants that can withstand a wide range of conditions, from extreme temperatures to exposure to chemicals, water, and humidity.

One of the key factors that contribute to the popularity of polyurethane adhesives is their excellent adhesion to a broad spectrum of substrates. Whether it’s metal, plastic, wood, or concrete, polyurethane can form a robust bond, making it suitable for numerous applications. In the footwear industry, for example, polyurethane adhesives are highly valued for their ability to provide strong adhesion to PVC formulations.

The Advantages of Polyurethane Adhesives and Sealants

1. Flexibility and Elasticity: Many polyurethane formulations offer outstanding flexibility and elasticity, allowing them to adapt to movement and vibration without cracking or losing adhesion. This property is particularly crucial in construction and automotive applications, where materials are often subjected to dynamic forces.
2. Toughness and Abrasion Resistance: Polyurethanes are known for their toughness and resistance to abrasion, impact, and wear. They can withstand the rigors of transportation, machinery, and outdoor environments, making them a reliable choice for demanding applications.
3. Chemical Resistance: Polyurethane adhesives and sealants exhibit excellent resistance to a variety of chemicals, solvents, oils, and other harsh environments. This makes them suitable for industrial and chemical processing applications, where exposure to corrosive substances is common.
4. Temperature Resistance: The performance and properties of polyurethanes can be maintained over a wide temperature range, from extremely low temperatures to high heat. This makes them suitable for applications in cold storage facilities, high-temperature industrial processes, and everything in between.
5. UV and Weather Resistance: Many polyurethane systems are designed to provide long-lasting performance in outdoor applications. They are resistant to UV radiation and weathering, ensuring that they maintain their integrity and functionality over time.
6. Diverse Curing Options: Polyurethanes can be formulated as either one-component (moisture-curing) or two-component (reactive) systems. This allows for different curing mechanisms and application methods, providing flexibility to meet the specific requirements of each project.
7. Customizable Performance: By adjusting the raw materials used in the formulation, it is possible to customize the properties of polyurethane adhesives and sealants to meet the specific needs of different applications. This includes properties such as viscosity, adhesion strength, and curing time.
8. Environmental Options: Some polyurethane systems offer lower environmental impact, with reduced volatile organic compound (VOC) emissions and compliance with stricter regulations. This makes them a more sustainable choice for environmentally conscious applications.
9. Cost-Effectiveness: Despite their high performance, polyurethane adhesives and sealants often provide a good balance between performance and cost. They can offer long-term durability and reliability, making them a cost-effective solution compared to other adhesive technologies.

The Building Blocks of Polyurethane

At the heart of polyurethane chemistry are two key components: isocyanates and polyols. Isocyanates are compounds containing the -NCO group, which can be either aliphatic or aromatic. Polyols, on the other hand, are compounds containing multiple -OH groups and can be classified as polyether-based or polyester-based.

The choice of isocyanates and polyols plays a crucial role in determining the final properties of the polyurethane resin. Aliphatic isocyanates, for example, offer superior durability and color stability compared to aromatic isocyanates, making them a preferred choice for applications where long-term exposure to sunlight and weathering is expected.

Polyether polyols are known for their good low-temperature flexibility, low viscosity, and improved hydrolysis stability. They are often used in applications where these properties are essential, such as in the production of flexible foams and adhesives for cold environments.

Polyester polyols, on the other hand, exhibit excellent abrasion resistance, adhesion promotion, and resistance to oils, solvents, and oxidation. They are commonly used in applications where high mechanical strength and durability are required, such as in the production of coatings, sealants, and adhesives for industrial applications.

Different Types of Polyurethane Adhesives and Sealants

1. Polyurethane Prepolymers: Polyurethane prepolymers are formed by reacting diisocyanates and polyols, with the isocyanate functional groups remaining in excess. The properties of the prepolymer, such as its reactivity and viscosity, can be adjusted by controlling the ratio of isocyanates to polyols and the type of isocyanates used.
2. One-Component Polyurethane Adhesives: One-component (1K) polyurethane adhesives use blocked isocyanates to provide a storage-stable, single-pack formulation. When applied to a substrate and heated, the isocyanates are解封, allowing them to react with the polyols and form a polyurethane network. These adhesives are commonly used for bonding porous materials and in applications where heat curing is possible.
3. Two-Component Polyurethane Adhesives: Two-component (2K) polyurethane adhesives consist of a polyisocyanate and a polyol, which are mixed together before application. The two components react with each other at room temperature to form a polyurethane bond. These adhesives are suitable for bonding heat-sensitive substrates and large objects that cannot be heated for curing.
4. Polyurethane Dispersions (PUDs): Polyurethane dispersions are high-molecular-weight aliphatic polyester polyurethanes dispersed in water. They are used to produce heat-activated adhesives and offer several advantages over solvent-based polyurethanes, including low VOC emissions, non-flammability, and good adhesion to a variety of substrates.
5. Thermoplastic Polyurethane (TPU) Adhesives: Thermoplastic polyurethane adhesives are made from linear polymer chains containing hard and soft segments. They offer excellent initial and final bond strength, high thermal tack, and the ability to be customized to meet the specific requirements of different applications. TPU adhesives are commonly used in industries such as automotive, furniture, and footwear.

Choosing the Right Polyurethane Adhesive or Sealant

When selecting a polyurethane adhesive or sealant for a specific application, several factors need to be considered, including:

1. Substrate Compatibility: The adhesive or sealant should have good adhesion to the substrate material being bonded.
2. Application Requirements: Consider the specific requirements of the application, such as the type of bonding (structural or non-structural), the environmental conditions, and the desired performance characteristics.
3. Curing Conditions: Determine whether a one-component or two-component system is suitable, based on the available curing methods and the time constraints of the project.
4. Environmental Factors: Consider the environmental impact of the adhesive or sealant, including VOC emissions and compliance with relevant regulations.
5. Cost: Evaluate the cost of the adhesive or sealant in relation to its performance and durability, to ensure that it provides a cost-effective solution for the application.

Polyurethane (abbreviated as PU) is widely used in formulating adhesives and sealants. Adhesives based on polyurethane resin exhibit durability, strong adhesion, and high peel strength. They also have good strength at low and high temperatures, and are resistant to chemicals, water, and humidity.

When choosing the right polyurethane resin, you should consider the following factors:

• Selection of raw materials (e.g., polyols, isocyanates, prepolymers…)
• Application Requirements (e.g., adhesion, viscosity, durability…)
• Substrate Compatibility (e.g., wood, metal, plastic…)
• Curing conditions (e.g., one-component, two-component)
• Environmental factors (e.g., solvent-based vs. water-based)

I. What is polyurethane?

Polyurethane is a versatile polymer material and is selected for adhesives and sealants due to its durability, strong bonding force, and high peel strength. Polyurethane resin gives you the freedom to modify the modulus and elongation to meet the needs of specific applications.

Polyurethane adhesives have a significant market share in the footwear, construction, and general manufacturing industries. One of the largest applications of polyurethane adhesives is in the footwear industry, where high bond strength to PVC formulations is required.

II. Why should polyurethane be chosen?

The following are some of the key reasons why polyurethane is used in these applications:

• Excellent Adhesion: Exhibits outstanding adhesion to a wide range of substrates. These substrates include metals, plastics, wood, concrete, and various other materials. This versatility makes them suitable for a variety of applications.
• Flexibility and Elasticity: Many polyurethane formulations offer excellent flexibility and elasticity. This enables them to adapt to movement and vibration without cracking or losing adhesion. This is particularly important in construction and automotive applications.
• **Toughness and Abrasion Resistance**: Polyurethanes are renowned for their toughness and resistance to abrasion, impact, and wear. They are suitable for demanding applications in transportation, machinery, and outdoor environments.
• Chemical Resistance: Provide resistance to various chemicals, solvents, oils, and other harsh environments. This makes them suitable for industrial and chemical processing applications.
• Temperature Resistance: The properties and performance of polyurethanes can be maintained over a wide temperature range. This can range from low-temperature to high-temperature environments.
• UV and Weather Resistance: Many polyurethane systems ensure long-lasting performance in outdoor applications.
• Diverse Curing Options: Polyurethanes can be formulated as one-component (moisture-curing) or two-component (reactive) systems. This allows for different curing mechanisms and application methods to meet various requirements.
• Customizable Performance: By adjusting the raw materials, you can customize the final product. The types of basic formulations may include isocyanates, polyols, and other additives.
• Environmental Options: Some polyurethane systems offer lower environmental impact. They can also comply with more stringent regulations.
• Cost – effectiveness: Polyurethane adhesives and sealants typically offer a good balance between performance and cost. This makes them more economically viable for various applications compared to other technologies.

III. Advantages and Limitations of Polyurethane

Understanding the advantages and disadvantages of polyurethane resin is crucial for making the right material selection. Choosing the appropriate polyurethane raw materials can optimize the formulation to enhance the properties required to address any weaknesses.

The following are some of the advantages and disadvantages of polyurethane (PU) resin that you should know:

Understanding the above characteristics helps ensure the stability of product quality and performance. At the same time, it can also meet regulatory requirements while maintaining cost – effectiveness.

IV. Basic Constituent Units of Polyurethane

Polyurethane resin is formed by the reaction of isocyanates and polyols, creating urethane bonds.

Isocyanates are compounds containing the -NCO group. They can be aliphatic or aromatic. Polyols are compounds containing multiple -OH groups. They can be polyether-based or polyester-based.

The choice of isocyanates and polyol compounds determines the final properties of polyurethane resins. Therefore, a deep understanding of their chemical properties is crucial. Let’s start with the aliphatic structure.

1. Aliphatic vs Aromatic Isocyanates – What’s the Difference?

Aliphatic isocyanates are compounds with a linear or cycloaliphatic structure. They are used as building blocks or crosslinking agents. Aliphatic isocyanates are used as building blocks or crosslinking agents to influence the final properties of polyurethane adhesives.

Aromatic isocyanates contain one or more benzene rings in their molecular structure. Traditional polyurethane adhesives based on aromatic isocyanates cannot meet the requirements for long-term durability or color stability in harsh environments.

Some properties that make aliphatic isocyanates superior to aromatic isocyanates include:

• Durability and color stability

Polyurethane adhesives based on aliphatic isocyanates exhibit excellent durability and color stability, which is attributed to:

– The aliphatic isocyanate backbone lacks phenyl groups. This makes polyurethane adhesives prepared from aliphatic isocyanates less prone to oxidation compared to adhesives based on aromatic isocyanates.
- The urethane bond exhibits excellent resistance when exposed to ultraviolet light.

In adhesive applications visible to the end user, aliphatic isocyanates are particularly suitable. In outdoor applications, the material needs to maintain its original color for as long as possible to maintain optimal aesthetics. In these typical applications, polyurethane adhesives made from aliphatic isocyanates are recommended because of their non-yellowing properties and excellent physical resistance.

Therefore, aliphatic-based polyurethane adhesives will maintain their excellent mechanical properties as well as unchanged color and transparency for a longer time than those using aromatic isocyanates.

The following figure compares the color stability of polyurethane adhesives based on aliphatic and aromatic isocyanates:

• Adhesion on various substrates

The challenges faced by adhesives are not only to adhere and exhibit good cohesive properties, but also to ensure long-lasting adhesion under harsh conditions such as humidity or high temperature. Urethane bonds have excellent adhesion to a wide variety of substrates due to the following properties:

• Form hydrogen bonds with water and hydroxyl groups on the surface of the substrate
• Physically interact through van der Waals forces
• Due to its low surface energy, it can effectively wet most substrates.

Polyurethane adhesives based on aliphatic isocyanates can be durable even under harsh conditions, thanks to the weather resistance of the urethane bond. Aliphatic isocyanate – based polyurethane adhesives are widely used in the fields of wood and furniture, automotive/transportation, packaging, sealants, etc.

• Heat resistance and moisture resistance

In some applications such as kitchen furniture and sports shoes, the adhesive must adhere firmly even under very hot and humid conditions.

Due to the heat resistance and moisture resistance of the carbamate bond, polyurethane adhesives become the preferred material for such applications.

For example, when conducting the EN 204 standard test, which is used to classify non-structural wood adhesives, many aliphatic isocyanates can achieve the D4 classification, which corresponds to adhesives used under extreme conditions.

• D1: The adhesive must withstand a dry tensile strength of at least 10 MPa. This corresponds to dry environments, such as indoor furniture or decoration.
• D2: The adhesive shall be able to withstand immersion in water at 20°C for 3 hours, followed by drying. After drying, the test requires a minimum tensile strength of 8 MPa.
• D3: The adhesive must withstand multiple cycles, including soaking in water at 20°C for 4 days several times, followed by drying. The requirements are: an initial tensile strength of 10 MPa, a wet tensile strength of 2 MPa, and 8 MPa after drying. These requirements correspond to outdoor doors and windows.
• D4: The adhesive shall be able to withstand multiple cycles, including being immersed in boiling water for 6 hours several times, and then dried. The minimum requirements are: an initial tensile strength of 10 MPa, 4 MPa when wet, and 8 MPa after drying. This is a very strict test, corresponding to extreme high-temperature and high-humidity environments: outdoor building components, decks, kitchen and bathroom furniture.
• Transparency

In some applications, such as:

• Hot melt adhesive
• Transportation
• Flexible packaging

Generally, adhesives are required to have transparency. Among various adhesive solutions on the market, aliphatic isocyanates offer a combination of high bonding performance and long-lasting transparency.

Aliphatic isocyanates can be used to formulate transparent adhesives and sealants. Due to their excellent durability and color stability, aliphatic – based polyurethane adhesives remain transparent after aging.

• User-friendliness

Today, in the adhesives and sealants industry, as in many other industries, the trend is to formulate user-friendly products. In this regard, the use of aliphatic isocyanates will help formulators meet these requirements by avoiding the release of harmful amines and facilitating the formulation of water-based adhesives.

• No residual amines: Formulating polyurethane adhesives with aliphatic isocyanates does not release carcinogenic amines such as MDA (methylenedianiline) or TDA (toluenediamine), which is the case with aromatic isocyanates.
• Preparation of Water-based Adhesives: Thanks to unique and innovative technologies, it is now easy to switch to a water-based polyurethane system. As a result, exposure to harmful solvents used in solvent-based polyurethane adhesive formulations can be avoided. With the new generation of aliphatic isocyanate products, high-speed mixing equipment is no longer required as these products can self-emulsify. Therefore, they can be very easily incorporated into water-based coating formulations.
• Reactive control

Compared with the reactivity obtained using aromatic isocyanates, better control of reactivity can be achieved using aliphatic isocyanates. The reactivity can be further adjusted by appropriately selecting the functionality of the aliphatic polyisocyanates. There are various functionalities available on the market.

The reactivity control obtained when switching from an aromatic isocyanate system to an aliphatic isocyanate system will provide a longer open time and bring more flexibility to the constructors.

Overall, depending on the specificity of the application, there are two options for formulating aliphatic isocyanate-based polyurethane adhesives:

• Monomers can be developed to construct customized prepolymers.
• Aliphatic isocyanates can be used as ready-to-use adhesives.

2、Polyols

Polyurethane polyols are further classified into polyethers, polyesters, caprolactones, polybutadienes, and polyacrylates. They can be selected according to the intended end – use.

• Polyether Polyols: Production and Characteristics

Polyether polyols are made by reacting epoxides with compounds containing active hydrogen atoms. Epoxides such as ethylene oxide or propylene oxide react with polyfunctional initiators in the presence of a catalyst, which can be a strong base such as potassium hydroxide or a double metal cyanide catalyst such as zinc hexacyanocobaltate tert-butyl alcohol complex, to produce polyether polyols.

The main characteristics of polyether polyols include:

• Good low – temperature flexibility
• Low viscosity
• Prolong the open time
• Improve hydrolysis stability
• Excellent resistance to weak acids and weak bases
• Weak resistance to ultraviolet radiation, oil and fuel
• Polyester Polyols: Production and Properties

Polyester polyols are obtained through the condensation or step – growth polymerization reaction of diols (such as ethylene glycol) and dicarboxylic acids (or their derivatives) such as phthalic acid. First, heat the diol or triol to 60 – 90°C, and then add the dicarboxylic acid. The removal of reaction water can be observed.

The main properties of polyester polyols depend to a large extent on the overall molecular weight of the polyester.

• Good abrasion resistance
• Good adhesion promotion
• Excellent resistance to oil, solvents, greases and oxidation
• Good tear strength and dimensional stability
• The price is relatively high
• High viscosity
• Difficult to handle

Crystalline polyester polyols provide the final adhesive with rapidly formed initial strength and fast curing, while amorphous polyesters can increase the open time.

• Caprolactone Polyurethane

Caprolactone polyurethane incorporates polycaprolactone (PCL) as a key component in its structure. Polycaprolactone is a biodegradable and biocompatible polyester. PCL is renowned for its excellent flexibility, low melting point, and good thermal stability.

They are synthesized through the ring – opening polymerization of ε – caprolactone, which is a cyclic ester monomer. In the synthesis of caprolactone polyurethane:

• Polycaprolactone polyols are used as soft segment components,
• Isocyanates (such as MDI or TDI) are used as hard segment components.

These components together produce unique properties derived from the polycaprolactone segment.

• Polybutadiene polyurethane

Polybutadiene polyurethane uses polybutadiene as one of the main components during the synthesis process. Polybutadiene is a synthetic rubber polymer produced by polymerizing 1,3-butadiene monomers.

In the production of polybutadiene polyurethane, polybutadiene – based polyols are used as the soft – segment components. These polyols are combined with isocyanates (such as MDI or TDI) as the hard – segment components.

The key properties and advantages of polybutadiene polyurethane include:

• Toughness and impact resistance
• Wear resistance
• Fuel and solvent resistance
• Low – temperature performance
• Shock absorption performance

Polybutadiene polyurethanes may have limitations in some applications. This is due to their poor hydrolytic stability and limited compatibility with certain additives or fillers. Therefore, compatibility tests are required when incorporating these polyurethanes into specific applications.

V. Polyurethane Prepolymer

Polyurethane prepolymers are formed by reacting the diisocyanate and polyol components common to all polyurethane adhesives. The following is a schematic diagram.

The isocyanates used are usually diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI). The polyols can be as simple as diols.

1. Composition of Polyurethane Prepolymer

The composition of the reaction mixture is set such that in the matrix formed after the reaction of all the polyol alcohol groups, the isocyanate functional groups will remain in excess. Typically, the excess ratio of isocyanate to polyol is more than five times, and the isocyanate portion acts as a solvent to prevent the gelation of the polyol. Some people believe that a prepolymer is formed only when the excess is less than two times, and when the excess is greater, they call the formed matrix a semi – prepolymer or quasi – prepolymer.

Once the prepolymer matrix is fully formed, unreacted diisocyanate can be removed by vacuum distillation to eliminate any toxicity issues. Besides toxicity, this also affects the viscosity of the prepolymer, which depends on the chemical properties and the amount of free isocyanate.

Low molecular weight and solvent-like free isocyanates reduce the overall viscosity of the prepolymer mixture.

To ensure the stability of isocyanates and prevent further polymerization of the prepolymer, the reagent is added under an inert atmosphere without heating. The reaction mixture must be completely anhydrous at temperatures below 100°C to avoid the formation of urea and allophanate cross-linking.

To prevent this crosslinking during the storage of the prepolymer prior to use, stabilizers are added, such as:

• Benzoyl chloride
• acetyl chloride, or
• p-Toluenesulfonic acid

2. Properties of Polyurethane Prepolymers

The reactivity of the prepolymer almost entirely depends on the nature of the isocyanate. Isocyanates with less steric hindrance, such as MDI, have the highest reactivity. TDI and isophorone diisocyanate are asymmetric molecules. Using the most reactive isocyanate groups to form the prepolymer reduces the overall reactivity of the final matrix.

This minimizes the reactivity of the prepolymer with unreacted polyols, slows down the initiation rate of further polymerization, resulting in macromolecules with higher molecular weights. This also leads to a narrower molecular weight distribution and lower viscosity.

In polyurethane prepolymers, the viscosity is usually increased through cross – linking with isophenyl cyanate anhydride, which can start to occur at temperatures above 60°C.

For example, the basic residues sometimes present in polyether polyols can catalyze this reaction and enable it to occur at these relatively low temperatures. Due to the steric hindrance at the tertiary carbon of the isocyanate group, which slows down the reaction initiation and hinders the formation of isophenyl cyanate anhydride, the viscosity of the tetramethylxylene diisocyanate prepolymer is very low even when processed at 125°C.

By preparing customized prepolymers, the stiffness, elasticity, and cross – linking properties of the adhesive can be adjusted to meet specific requirements. Among the properties that can be adjusted, viscosity is a key parameter to be considered. Both solvent – based and water – based ready – to – use products offer a range of viscosities to meet your application needs.

These prepolymers will serve as ready-to-use components to form polyurethane chains, meeting all the required properties for adhesive applications.

VI. Polyurethanes Based on Curing Mechanisms

When choosing a polyurethane resin based on the curing mechanism, two main categories can be distinguished – two-component (or 2K) and thermally activated one-component (or 1K).

1. One-component polyurethane

1K polyurethane adhesives are similar to aliphatic isocyanates. The difference is that blocked isocyanates are used to provide a storage – stable one – pack formulation containing polyols. When the adhesive is applied to the substrate and during the high – temperature baking process, the isocyanates are deblocked and then react with the polyols to form a polyurethane network.

Sandwich elements for porous materials (wood, polystyrene foam, polyurethane foam, etc.) and structures of laminates (plastic) or metals (aluminum), which are subsequently used in partition walls and doors or the side walls of RVs and trailers.

2. Two-component polyurethane

2K polyurethane adhesives are composed of polyisocyanates and polyols. They are mixed before application and cured at room temperature. Therefore, 2K polyurethane adhesives are used for:

• Thermally sensitive substrates (plastics, wood, automobiles, etc.)
• Large objects that cannot be baked (trains, airplanes, bridges, etc.)

Used for large-area bonding in vehicle superstructures (sandwich structures), facade elements, and the manufacture of ships and containers.

Key differences between 1K and 2K polyurethanes

VII. Finding the Direction with Polyurethane Dispersions

From a chemical perspective, Polyurethane Dispersions (PUDs) are anionic dispersions of high molecular weight aliphatic polyester polyurethanes in water, specifically used for the manufacture of heat-activated adhesives. They are high-performance adhesive raw materials that can be compared to the polyurethane resins used in solvent-based adhesives.

In recent years, the usage of waterborne polyurethane dispersions has increased as they have replaced solvent-based polyurethanes in many application areas for the following reasons:

• Fully reacted linear polymers, emulsified and dispersed in water
• Almost no or no volatile organic compound (VOC) emissions
• Free of residual isocyanates
• Non-flammable

The following table compares the properties of solvent – based and water – based polyurethane adhesives. One of the most important differences is the combination of high solid content and low viscosity in water – based dispersions.

1. Raw materials used in the manufacture of PUDs

The raw materials of polyurethane dispersions possess the characteristics of polyurethane chemistry, as shown in the figure below. Typically, polyester or polyether polyols react with aliphatic or aromatic isocyanates. During the polymerization stage, reactive groups with acid or hydroxyl functionality can be introduced into the polymer backbone to provide enhanced properties. These reactive groups can also be crosslinked with dispersible isocyanates for two-component adhesive systems.

2. Emulsification and Dispersion of Polyurethane Dispersions

The emulsification and dispersion of polyurethane dispersions are achieved by introducing hydrophilic groups (cationic or anionic groups) or long hydrophilic polyol segments into the polymer backbone, or less commonly, by using external emulsifiers. There are several methods for manufacturing polyurethane dispersions.

3. Application of Polyether Polyols in Polyurethane Dispersions

Polyether polyols are used in polyurethane adhesives due to their performance and cost – effectiveness. Their glass transition temperature (Tg) is approximately – 60°C. The resulting adhesives have the following excellent properties:

• Low-temperature performance
• Extensibility
• Impact resistance

In addition, polyether polyols are more resistant to hydrolysis than polyester-based polyols.

4. Application of Polyester Polyols in Polyurethane Dispersions

Polyester polyols are commonly used to formulate polyurethane dispersions and hot – melt polyurethane adhesives in the footwear industry because they have high crystallinity, which results in high initial (instantaneous) strength.

Polyester – based polyols have a variety of molecular structures, ranging from linear to highly branched. The higher the degree of branching, the more hydroxyl functional groups are available for cross – linking. Compared with polyether polyols, polyester polyols have higher tensile strength and better heat resistance, but they have poorer hydrolysis stability, low – temperature performance, and chemical stability.

When choosing a polyurethane dispersion, the main basis is the type of polyol, which is usually specified in the supplier’s Product Data Sheet (PDS). The properties of other dispersions are generally not provided in the PDS.

Polyols are materials containing two or more hydroxyl groups. Generally, polyols with low molecular weight (molecular weight less than 2000) provide the best adhesion performance. The most common polyols are polyether – based or polyester – based. The main differences between polyether – based and polyester – based polyols are shown in the following table:

5、Isocyanate

There is a wide variety of selectable isocyanates and their derivatives, which are generally classified into two major categories: aliphatic and aromatic. Due to the relatively low reactivity of the isocyanate groups in aliphatic isocyanates with water, they are the preferred choice for polyurethane dispersions. Among them, 4,4′-dicyclohexylmethane diisocyanate (H12MDI) is the most commonly used because it can make the dispersion finer and the final product has better mechanical properties.

If a suitable preparation process is adopted, aromatic isocyanates can also be used. They are sometimes selected because of their high reactivity and low cost. However, compared with aliphatic isocyanates, aromatic isocyanates have poor light stability and weak antioxidant ability.

6. Consideration factors for selecting suitable polyurethane dispersions

The performance of polyurethane dispersions is mainly determined by the following factors:

• Crystallinity

Polyurethane dispersions can be manufactured using either crystalline or amorphous molecular backbones.

• The use of crystalline (polyester) polyols can provide a longer open time when thermally activated.
• Use amorphous polyols as well as mixtures of crystalline and amorphous polyols to adjust the performance characteristics.

Products also vary in terms of heat resistance. The following table shows the influence of the amorphous and crystalline contents on the thermal activation temperature, heat resistance, and other properties, and indicates the specific industry applications for which each product is suitable.

7、Ionic content

Introducing ionic groups into the polyurethane structure helps with hydrophilic modification for dispersion in aqueous media. Therefore, polyurethane dispersions can also be classified according to their ionic charges.

8、 Particle size

The particle size of the polyurethane dispersion can vary from approximately 0.01 to 5 microns. The particle size also affects the appearance of the final film, ranging from opaque (smaller particles) to milky white (larger particles). In addition, unlike solvent – based systems, the viscosity of the polyurethane dispersion is independent of the molecular weight.

9、 Characteristics of the dispersion

The particle size of the polyurethane dispersion can vary from approximately 0.01 to 5 microns. Its impact on the properties of the dispersion is as follows:

• Dispersions with relatively large particle sizes (> 1 micron) are usually unstable in terms of sedimentation.
• Dispersions with a relatively small average particle size are more useful because they are more resistant to storage and have a higher surface energy.
• Dispersions with smaller particle sizes have a stronger film – forming driving force.

10、Viscosity

Unlike solvent – based systems, the viscosity of polyurethane dispersions is independent of molecular weight. Water – based polyurethane dispersions have a relatively low viscosity (50 – 1000 centipoise) at room temperature, with a typical solid content of 30 – 50%.

Low viscosity and high solid content provide the convenience for the adhesive system to be easily sprayed or rolled onto the substrate, and offer a high degree of freedom to formulators when designing formulations. The viscosity of the adhesive dispersion must:

• Ensure good wettability
• It can penetrate into the substrate, but not too deeply.

It is also necessary to pay attention to the relationship between viscosity and shear rate. To adjust the viscosity, formulators will use thickeners. Their effects vary depending on the type of thickener and the amount added.

11. Blending with Other Polymers

Polyurethane dispersions are usually more expensive than other water-based systems. Therefore, adding cheaper resins will optimize the price/performance ratio of the final adhesive formulation. In some cases, mixing with other resins will also improve certain properties of the formulation to meet the needs of specific applications.

• Acrylic emulsion resins are most commonly used to improve toughness and flexibility, abrasion resistance, and film – forming properties.
• Water-dispersible rosin esters and modified terpene phenolic resins are also used to improve creep resistance and tackiness. These tackifiers are particularly suitable for formulating polyurethane dispersion contact adhesives.

12. One-component PUD system

One – component systems usually originate from fully – reacted polyurethane prepolymer dispersions or similar dispersions with blocked isocyanate end – groups. Blocked isocyanates are isocyanates that react with a material which prevents their reaction at room temperature but allows the reaction to occur at higher temperatures. One – component systems are polyurethane dispersions that already contain inactivated potentially reactive isocyanates.

This chemical blocking mechanism provides protection for the isocyanate against the aqueous carrier, thus achieving an excellent shelf life. This type of adhesive is commonly used to bond non-porous substrates, such as different metals and composite materials.

13. Two-component PUD system

Two – component systems also utilize fully – reacted polyurethane prepolymers, as well as water – emulsifiable polyisocyanates as the second component in the formulation. The isocyanate is added by the end – user, and additional cross – linking is achieved after application. Two – component polyurethane dispersions claim to provide similar properties to solvent – based polyurethane adhesives.

Both systems can provide the same level of heat resistance, good initial and final bond strength, and long-term durability. The choice between the two systems will be mainly determined by the manufacturing process parameters used to assemble the parts to be bonded, as shown in the following table:

VIII. Starting Formulation of Waterborne PUD

The following table shows the starting formulation of a waterborne polyurethane for heat-sealing adhesives. By adding isocyanate, it can be formulated into a thermoplastic or thermosetting system.

Heat the mixture of polyol and isocyanate to 80°C and maintain for 2.5 hours. When the isocyanate content reaches approximately 2.6%, disperse the NCO – terminated sulfonated polyurethane prepolymer in 1138 grams of deionized water. The water temperature before dispersion is 40°C. The pH value of the resulting dispersion is 6.5.

The following table shows the requirements of common substrates and applications for polyurethane dispersion adhesives. The application will determine the following properties of the adhesive film.

IX. Innovative Thermoplastic Polyurethane (TPU) Technology for Adhesive Solutions

Thermoplastic polyurethane (TPU) consists of linear polymer chains containing a small amount of hard segments and soft segments. TPU has the following advantages:

• Excellent initial and final bonding performance
• Excellent hot tack, which can be customized according to the required application, etc.

1. Key properties of thermoplastic polyurethane (TPU)

Thermoplastic polyurethane (TPU) adhesives are widely used in various fields. Industries choose TPU for their applications to take advantage of its wide range of performance combinations, such as adhesion to difficult – to – bond substrates, hot tack, formulation diversity, adjustable sealing temperature, etc.

• Adhesion to difficult – to – bond substrates

TPU adhesives provide excellent initial and final bonding performance and have outstanding adhesion to many substrates, such as:

• Leather, wood and metal
• Rigid and plasticized PVC
• Rubber (natural rubber, styrene-butadiene rubber and neoprene)
• Thermoplastic Polyurethane (TPU)

If these substrates are properly pre-treated physically or chemically in advance, TPU adhesives can also be used to bond certain polyolefins (such as EVA, PE, PP).

TPU adhesives have been particularly successful in bonding leather, wood, metal, rubber, etc.

• Thermal tackiness

In the automotive, furniture, and footwear industries, adhesives based on heat-activated TPU must have high hot tack to ensure a perfect initial bond. This property of TPU is attributed to its highly crystalline structure.

• Formulation diversity

One of the main advantages of choosing TPU is that TPU can be customized according to the different specifications of various industrial applications. Their main physical properties can be adjusted within a wide range according to application requirements.

The following characteristics of TPU can be customized:

• Molecular weight
• crystallization rate
• Thermoplasticity
• Thermal Viscosity Adjustment

This gives us a wide range of possibilities when choosing the most suitable product, once the required characteristics are taken into account.

• Adjustable sealing temperature

TPU adhesives are most commonly used in bonding operations where thermal activation is part of the process. Optimal bond strength can be achieved at a given temperature of the adhesive film. Therefore, it is very important to reach such a temperature. These values vary depending on the nature of the soft segments (polyols) present in the TPU.

After application and drying, solvent – based TPU adhesives and adhesives based on water – dispersed TPU particles produce non – sticky adhesive films, which can be made sticky by heating.

All of these characteristics fully meet the requirements of multiple application fields.

2. TPU Technology for the Most Challenging Applications

• TPU hot melt adhesive

The thermoplastic polyurethane for hot melting is a special linear TPU, supplied in granular form, with high thermoplasticity and different crystallization rates. They can be applied through extrusion processes or sintering. For many applications, thermoplastic polyurethane for hot melting is favored due to its versatile adhesion performance characteristics, improved economic efficiency, and environmental friendliness.

Thermoplastic polyurethanes for hot melt have the following advantages:

• It has a wider range of adhesion to different substrates (thanks to polar groups)
• Wide crystallization range
• Wide melting range (70° – 140°C)
• Provide excellent low – temperature flexibility (low Tg)
• High elasticity and softness
• Water-dispersed TPU particles

Water-dispersed particles are thermoplastic polyurethanes in the form of particles containing ionic groups. These TPUs can be first dissolved in acetone, and then through an emulsification process, an aqueous TPU dispersion can be obtained. This is a patented process.

Adhesive manufacturers can produce waterborne TPU dispersions on their own, which has the following benefits:

• Reduce transportation costs
• Improve the stability of adhesives
• Freely choose specific TPU dispersion characteristics to distinguish their adhesives

These water-dispersible thermoplastic polyurethane dispersions are used to produce adhesives for the automotive, furniture, footwear and other industry markets.

• TPU Adhesive Film

Based on thermoplastic polyurethane technology, a family of TPUs supplied in granular form has been created, specifically for the production of TPU adhesive films. These TPUs allow for different bonding temperatures and can be applied via flat extrusion or blow molding processes. TPU adhesive films produced using TPU offer the following advantages:

• No high melting temperature is required
• High adhesion value
• High initial strength
• Solvent-based TPU adhesive

These adhesives are formulated based on linear crystalline TPU polymers in granular form and can be easily dissolved in a wide range of solvents. The choice of solvent is usually determined by the regulations of various countries and the final physical properties of the required adhesive.

When the end user applies the adhesive, a crosslinking agent (isocyanate) can be added to the original adhesive solution. This system can be used within a short pot life of a few hours.

Solvent – based thermoplastic polyurethane adhesives must have the following advantages:

• Have high initial strength at low activation temperature
• Easy to thermally activate
• Good heat resistance
• High final bonding strength

In conclusion, polyurethane adhesives and sealants offer a wide range of benefits and applications, making them an essential component in many industries. By understanding their properties, advantages, and different types, you can make an informed decision when choosing the right polyurethane adhesive or sealant for your specific needs. Whether you’re working on a construction project, manufacturing products, or developing new materials, polyurethane adhesives and sealants are sure to play a crucial role in achieving your goals.