Polyurethane Resins (PU) for Adhesives & Sealants: A Comprehensive Guide

Strengths and Limitations of PU Adhesives

Polyurethane is a versatile polymeric material that can be tailored to meet the demands of a number of adhesive and sealant systems. They are used extensively and are well-known for their durability, tough bonds and high peel strength. PU resin adhesives also give the formulator the freedom to modify the modulus and elongation required by an adhesive to meet specific end use requirements/applications.Polyurethane adhesives' versatility has allowed them to grab a good market share in:

Footwear
Construction, and
General manufacturing industries

One of the largest applications of PU adhesives is in footwear where high bond strengths to PVC compositions are needed.

Strengths and Limitations of PU Adhesives

Benefits	Limitations
Extremely tough Varying cure time Display good flexibility at low temperatures (up to -157°C) Good resistance to solvents Good impact and abrasion resistance (Tensile shear: 2200 psi; T-peel: 80piw) Polyurethanes are excellent adhesives for a wide range of materials (most smooth, non-ferrous) Moderate cost	Sensitive to moisture both in cured and uncured state Application of a primer may be needed for adhesion to some substrates Poor elevated temperature resistance (max 79°C) May revert with heat and moisture Have a short pot life

For in-depth understanding of polyurethane adhesives, knowing different chemistries used to formulate polyurethane adhesives is the key. Let’s begin by exploring aliphatic isocyanates and their development which has extended the possibilities offered by polyurethane adhesives in terms of both performance and formulation flexibility.

Aliphatic Isocyanates – Functionality & Mechanism

Polyurethane adhesives based on aliphatic isocyanates exhibit:

Supreme mechanical properties
Good weathering resistance
Durability
Bonding to various substrates, and
They are user friendly as they don't release carcinogenic amines

However, traditional PU adhesives based on aromatic isocyanates, are not able to satisfy highly demanding specifications, such as extended durability or color stability in tough environment.

Some major aliphatic isocyanates comprise of:

Hexamethylene diisocyanate (HDI)
Dicyclohexylmethane 4,4'-diisocyanate (H12-MDI)

Aliphatic isocyanates are used to formulate high performance PU adhesives that demand high heat & humidity resistance, transparency and non-yellowing upon aging.

How to Use Aliphatic Isocyanates?

Polyurethane bonds are made from the reaction between an isocyanate (NCO) and chemicals containing active hydrogens (OH, COOH etc.). The choice of raw material is very large, enabling many combinations with a wide variety of properties.

Reaction to Form Polyurethane

By preparing customized prepolymers, the adhesive stiffness, elasticity and crosslinking properties can be tailored to suit specific needs.

Among properties that can be adjusted, viscosity is a key parameter to consider. Both solvent-based and waterborne ready-to-use type of products offer a wide range of viscosity to meet your application needs.

These prepolymers will be used as ready-to-use component to form the polyurethane chain with the whole range of properties needed in adhesive applications.

Properties Imparted by Aliphatic Isocyanates in PU Adhesives

Aliphatic isocyanates either used as building blocks or as crosslinkers to produce polyurethane adhesives, impact on the final performances of polyurethane adhesives. Thanks to their chemical nature, they bring the following benefits:

Durability and Color Stability

Polyurethane adhesives based on aliphatic isocyanates show exceptional durability and color stability thanks to:

The lack of phenyl group in the aliphatic isocyanates backbone. It makes polyurethane adhesives prepared from aliphatic isocyanates far less prone to oxidation than those based on aromatic isocyanates.
The superior resistance of the urethane bond when exposed to UV light.

The use of aliphatic isocyanates is particularly suitable when adhesives are visible from end-users. In outdoor applications, the material needs to keep the original color as long as possible in order to maintain aesthetics at its best level. In these typical applications, polyurethane adhesives made from aliphatic isocyanates are recommended for their non-yellowing property as for their exceptional physical resistance.

Aliphatic-based polyurethane adhesives will, as a result, keep their excellent mechanical properties and unchanged color and transparency during a longer time than when using aromatic isocyanates.

The following graph compares the color stability of PU adhesives based on aliphatic and aromatic isocyanates:

Impact of the Type of Isocyanates on Color Retention

Adhesion on a Wide Variety of Substrates

The challenge for adhesives is not only to adhere and show a good cohesion property, but to ensure a durable adhesion in severe conditions, such as humidity or heat. The urethane bond provides superior adhesion to a wide variety of substrates thanks to the following phenomena:

Hydrogen bonding with water and hydroxyl groups on the substrate surface
Physical interaction through van der Waals forces
Effective wetting of most substrates thanks to low surface energy

Polyurethane adhesives based on aliphatic isocyanates last over time even under harsh conditions thanks to the weathering resistance of urethane bonds.

Some applications where aliphatic isocyanates-based PU adhesives are widely used include wood and furniture, automotive/transportation, packaging, sealants, and others.

Heat & Humidity Resistance

In some applications such as kitchen furniture and sports shoes, adhesives must stick firmly, even in very hot and humid conditions.

Polyurethanes adhesives are the material of choice for such applications thanks to the heat and humidity resistance of the urethane bonding.

For example, when submitted to a EN 204 standard test, used to classify non-structural wood adhesives, many of the aliphatic isocyanates can lead to a D4 classification, which corresponds to adhesives submitted to extreme conditions.

D1	Adhesives have to withstand a dry resistance of 10 MPa minimum. This corresponds to dry conditions such as interior furniture or decoration.
D2	Adhesives should withstand a 3 hours immersion in water at 20°C, followed by drying and the test requires a minimum resistance of 8 MPa after drying.
D3	Adhesives must withstand several cycles including several 4-day immersion in water at 20°C and then drying, and the requirements are: initial resistance 10 MPa, wet resistance 2 MPa, and after drying 8 MPa. These requirements correspond to outside windows and doors.
D4	Adhesives should withstand several cycles including several 6-hour immersion in boiling water, and then drying. The minimum requirements are: initial resistance 10 MPa, wet 4 MPa and after drying 8 MPa. This is a very severe test which corresponds to extreme heat & humidity conditions: outside building parts, ship decks, kitchen and bathrooms furniture.

Transparency

Transparency is often required for adhesives used in applications, such as:

Hot melt
Transportation, and
Flexible packaging

Among the various adhesive solutions available on the market, aliphatic isocyanates provide high bonding performances combined with durable transparency.

Aliphatic isocyanates allow for the formulation of transparent adhesives and sealants. Thanks to exceptional durability and color stability, aliphatic-based polyurethane adhesives will stay transparent upon aging.

User-friendly

Today's trend in the adhesive and sealant industry, as in many other industries, is to formulate user-friendly products. In that extend, the use of aliphatic isocyanates will help formulators to meet these requirements by avoiding the release of hazardous amines but also by enabling easier formulation of waterborne adhesives.

No residual amine – Formulating polyurethane adhesives with aliphatic isocyanates will not release carcinogenic amines, such as MDA (methylene bisphenyl amine) or TDA (toluene diamine) as it happens with aromatic isocyanates.

Formulation of waterborne adhesives – Thanks to a unique and innovative technology, easy-to-use solutions are now available to switch to waterborne polyurethane systems. Exposure to aggressive solvents that enter in solvent-based polyurethane adhesive formulations can, as a result, be avoided. With the new generation of aliphatic isocyanate products, no more high-speed mixing equipment is required as these products are self-emulsifiable. And therefore, they are very easy to incorporate into waterborne coating formulations.

Reactivity Control

The use of aliphatic isocyanates allows a better reactivity control in comparison to the reactivity obtained with aromatic isocyanates. Further adjustments of the reactivity can be achieved with a proper selection of the aliphatic polyisocyanate functionality. A wide range of functionality is available in the market.

The reactivity control obtained when switching to an aliphatic polyisocyanate system will provide a longer open-time and bring more flexibility for applicators.

Applications of Aliphatic Isocyanates

Thanks to their unique properties, aliphatic isocyanates are very useful to formulate high performance adhesives used in industries, including wood and furniture, automotive/transportation, packaging, sealants and others.

Industry	Description
Wood	Aliphatic isocyanates are widely used to develop high performance adhesives for wood applications (wood-on-wood, wood-on-PVC or PVC laminated on wood), that require at least one of the following performances – durability, transparency and heat & humidity resistance.
Footwear	Footwear are often manufactured with wide range of materials and are submitted to numerous repetitive constraints under various climates during their service life. A suitable adhesive need thus to provide durable adhesion on many substrates to withstand these harsh conditions of service. Polyurethane adhesives based on aliphatic isocyanates are the products of first choice due to their excellent adhesion, exceptional durability and very good flexibility.
Transportation	Polyurethane adhesives based on aliphatic isocyanates are the solution of first choice to answer the highly demanding requirements of the transportation industry. They provide a very strong and very long-lasting adhesion on all kind of substrates, in harsh conditions (high humidity and temperature, presence of various detergents and chemicals).
Sealants	Sealants for glass production, bathroom or transportation applications need to withstand severe conditions and require excellent adhesion to various substrates, flexibility, chemical resistance and waterproof properties. The use of polyurethane sealants will help achieve these specifications.
Flexible Packaging	Polyurethane adhesives used in flexible packaging show a good adhesion on various types of materials (plastic films, aluminum, cardboard...) and their low viscosity makes them suitable for roller applications. When applications require heat & humidity resistance, transparency and non-yellowing upon ageing properties, aliphatic isocyanates will be of good help to formulate suitable adhesives. In addition, no hazardous amines are released when aliphatic isocyanates are used to formulate polyurethane adhesives.
Non-Woven	Thanks to their superior adhesion on a wide variety of varnishes and substrates, Polyurethane adhesives will help formulators developing high performance solutions for non-woven applications.

Overall, two options are available to formulate polyurethane adhesives based on aliphatic isocyanates, regarding the specificity of the application:

Monomers can be develop to build customized prepolymers
Aliphatic isocyanates can be used as ready-to-use adhesives

Ready-to-use Aliphatic Polyisocyanates – 1K and 2K Systems

Polyurethanes adhesives are made from macromolecules or polymers formed by the reaction between a polyisocyanate and another polymer, commonly known as a polyol - that contains OH groups.

When selecting an aliphatic polyisocyanate for your adhesive formulation, two main categories can be distinguished – two-component (or 2K) and heat-activated, one-component (or 1K).

1K Polyurethane Adhesives – One-component PUs are similar materials to the previous one, except that a blocked isocyanate is used to provide a storage stable one-pack formulations containing the polyol. After application of the adhesive on the substrate and during stoving at elevated temperature, the isocyanate is de-blocked and then it reacts with the polyol to form the PU network.

2K Polyurethane Adhesives – Two-component systems consist of a polyisocyanate and a polyol that are mixed just prior to application and cure at room temperature. Therefore, 2K polyurethane adhesives are used on:
- Substrates sensitive to heat (plastics, wood, cars....)
- Objects too big to be stoved (trains, airplanes, bridges...)

1K Systems	2K Systems
1k systems consist of isocyanate-containing prepolymers dissolved in a solvent carrier, and reaction with moisture occurs as the solvent evaporates.	2k systems consist of a polyol component (resin) and an isocyanate component (hardener) that must be mixed in a defined ratio prior to application.
Get easily cured by ambient moisture. Humidity conditions play a major role in the application of one component PU adhesives.	Cure faster and do not necessarily need moisture for curing.
Can be applied easily and require no mixing.	Do not cure completely if not mixed properly before application.
Used for the construction of sandwich elements of porous materials (wood, polystyrene foam, polyurethane foam and others) and laminated boards (plastics) or metals (aluminum) that are then used in partitioning walls and doors or side walls of caravans and trailers.	Used for large-surface adhesive bonds in vehicle superstructures (sandwich construction), facade elements, and ship building and container construction.
These adhesives are comparatively expensive.	2k systems are economical when compared to 1k PU adhesives.

Polyurethane Prepolymers – Mechanism & Applications

Polyurethane Prepolymers are formed by reacting the diisocyanate and polyol components which are common to all polyurethane adhesives. A pictorial representation is shown below.

The isocyanate used is most commonly either diphenylmethane diisocyanate (MDI) or toluene diisocyanate (TDI).
The polyol can be as simple as a glycol.

Production of Polyurethane Prepolymers

The composition of the reaction mixture is set such that an excess of isocyanate functionality will remain in the matrix that forms after all polyol alcohol groups have reacted. Often the excess of isocyanate to polyol is more than five-fold, with the isocyanate serving partly as a solvent to keep the polyol from gelling. Some consider that a prepolymer is only formed when the excess is less than two-fold, and when the excess is greater, they would call the resulting matrix a semi or quasi-prepolymer.

Once the prepolymer matrix has been fully formed non-reacted diisocyanate can be removed by vacuum distillation to eliminate any toxicity issues. As well as toxicity this impacts the prepolymers' viscosity, which depends on the chemical nature as well the amount of free isocyanate.

Lower molecular weight and solvent-like free isocyanates lower the prepolymer mixture's overall viscosity.

To ensure stability of the isocyanate and prevent further polymerization of the prepolymer, the reagents are added without heating under an inert atmosphere. The reaction mixture must be kept completely water-free at temperatures below 100°C to avoid urea formation and allophanate crosslinking.

To prevent this crosslinking when the prepolymer is being stored prior to use, stabilizers are added such as:

Benzoyl chloride
Acetyl chloride, or
p-toluene sulfonic acid

Properties of Polyurethane Prepolymers

Reactivity of the prepolymer depends almost exclusively on the nature of the isocyanate and those less sterically hindered, like MDI, are most reactive. TDI and isophorone diisocyanate are asymmetric molecules and the most reactive isocyanate group is used to form the prepolymer, giving the final matrix lower overall reactivity.

This minimizes the prepolymer reactivity with unreacted polyol and slows the initiation rate of further polymerization, leading to higher molecular weight macromolecules.
This also results in narrow molecular weight distribution and lower viscosity.

Check out all isocyanate groups used to produce PU-prepolymers here »

Viscosity is increased in polyurethane prepolymers generally by allophanate cross-linking, that can begin to occur at temperatures above 60°C.

Alkaline residues, which are sometimes present in polyether polyols for example, can catalyze this reaction and allow it to occur at these relatively low temperatures. The viscosities of tetramethylxylidene diisocyanate prepolymers are also very low due to steric hindrance at the isocyanate group's tertiary carbon that slows reaction initiation and hinders allophanate formation, even when processed at 125°C.

Polyurethane Polyols – Production & Features

Polyurethane polyols can be either polyether polyols or polyester polyols.

Polyether polyols are made by the reaction of epoxides with compounds having active hydrogen atom.
Polyester polyols are made by the polycondensation reaction of multifunctional carboxylic acids and polyhydroxyl compounds.

They can be further categorized according to their end use.

Polyether Polyols Production Process

Epoxides like ethylene oxide or propylene oxide react with the multifunctional initiator in the presence of a catalyst, which is either a strong base, such as potassium hydroxide or a double metal cyanide catalyst, such as zinc hexacyanocobaltate-t-butanol complex and yield polyether polyols.

Polyether Polyols Properties

Low temperature flexibility
Low viscosity
Polyethers assist to extend open time, reduce the viscosity and provide good low-temperature flexibility
Polyether polyols display improved hydrolytic stability and outstanding resistance to weak acids and bases
Polyether polyols show weak resistance to UV radiation and offer poor resistance to oils and fuels

Polyester Polyols Production Process

Polyester polyols are obtained by the condensation or step-growth polymerization of diols like diethylene glycol and dicarboxylic acids (or their derivatives) e.g. phthalic acid. The diols or triols, are first heated upto 60-90°C and then dicarboxylic acid is added and removal of the reaction water can be observed.

Polyester Polyols Properties

The properties of the polyester polyols largely depend the overall molecular weight of the polyester.

Good abrasion resistance
Offer good adhesion promotion
Crystalline polyester polyols impart quick-forming green-strength and rapid setting to the final adhesive while amorphous polyesters can increase open time
Polyesters polyols show excellent resistance to oil, solvents, grease and oxidation
Display good tear strength and dimensional stability
Polyester polyols are more expensive
Polyester polyols are highly viscous
Difficult to handle

Polyurethane Dispersions – Chemistry & Selection Tips

Chemically, Polyurethane Dispersions (PUD) are aqueous, anionic dispersions of high molecular weight aliphatic polyester polyurethanes, which are especially used to manufacture heat-activated adhesives. They are high performance adhesive raw materials comparable to polyurethane resins used in solvent-based adhesives.

The use of waterborne polyurethane dispersions has grown in recent years as they have replaced solvent borne polyurethanes in a number of application areas due to following benefits.

Fully reacted, linear polymers that are emulsified and dispersed in water
Have very low or no VOC (volatile organic content) emissions
Do not contain residual isocyanate
Non-flammable

Table below compares the properties of solvent based and waterborne polyurethane adhesives. One of the most important differences is the combination of high solids content and low viscosity in the aqueous dispersion.

Property	Solvent Based Polyurethane	Aqueous Polyurethane
Physical state	Solution	Particles dispersed in water
% Solids	15-18%	30-65%
Viscosity	50-1000	1000-5000
Polar monomeric groups	No	Yes
Toxicity, flammability, and explosion hazard	Yes	No
Coalescing solvent (cosolvent)	Unnecessary	Often necessary
Presence of foam	Sometimes	Yes; use of defoaming agents is usually necessary
Drying mechanism	Simple evaporation of solvent	Evaporation of water followed by interdiffusion of polymer particles
Drying time	Short (little energy required)	Long (more energy required)
Stability at low temperatures	Yes (freezing can be reversed)	No (Irreversible freezing)
Stability of the adhesive once applied (e.g., moisture and heat resistance)	High	Medium (unless crosslinked)
Adhesive properties	Excellent	Excellent

Property Comparison of Solvent-Based and Aqueous Polyurethane Adhesives

Chemistry of Polyurethane Dispersion

Raw materials for polyurethane dispersions are characteristic of polyurethane chemistry as shown in the figure below. Typically, polyester or polyether polyols are reacted with aliphatic or aromatic isocyanates. Acid or hydroxy functional reactive groups can be incorporated into the polymer backbone during the polymerization stage to provide enhanced performance. These reactive groups can also be crosslinked with dispersible isocyanates for use in two component adhesive systems.

General Reaction Mechanism of Polyurethane Chemistry

Emulsification and dispersion is accomplished by building hydrophilicity into the polymer backbone with either cationic or anionic groups or long hydrophilic polyol segments or, less frequently, through the use of external emulsifiers. There are several approaches to manufacturing polyurethane dispersions^1,2.

One-Component Systems

One-component systems are generally derived from fully reacted polyurethane pre-polymer dispersions or similar dispersions with blocked isocyanate end groups. A blocked isocyanate is an isocyanate which has been reacted with a material which will prevent its reaction at room temperature will permit that reaction to occur at higher temperatures.

The chemical blocking mechanism provides the isocyanate with protection from the water carriers and results in excellent shelf life. This type of adhesive is generally used for bonding non-porous substrates such as unlike metals and composites.

Two-Component Systems

Two-component systems also utilize fully reacted polyurethane pre-polymers along with a water dispersible isocyanate as a second component in the formulation. The isocyanate is added by the end-user and additional crosslinking is achieved after application, two component polyurethane dispersions are claimed to provide properties similar to solvent borne polyurethane adhesives.

Selecting Right Polyurethane Dispersion

The properties of a polyurethane dispersion are determined mainly by:

Choice of Raw Materials

The principal raw materials for polyurethane dispersions are:

Polyols (linear polyether, polyester, and other polyols)
Isocyanates and their derivatives which are generally classified as either aliphatic or aromatic

The primary selection choice of polyurethane dispersion will be based on the type of polyol and this is usually specified in the supplier’s PDS. Other properties of the dispersion are often not provided in the PDS. A wide range of linear polyether, polyester, and other polyols can be used.

Polyols are simply materials that contain two or more hydroxyl groups. Typically, low molecular weight polyols (mw <2000) provide the best adhesive properties. The most common polyols are polyether or polyester based polyols. The major differences between polyether and polyester based polyols are shown in Table below:

Property	Polyester	Polyether
Wear resistance	+	-
Load bearing, compression set, etc.	+	-
Low temperature flexibility	-	+
Hydrolytic stability	-	+
Heat aging	+	-
Swelling in oil, grease, solvents	+	-
O₂, O₃, and UV stability	+	-
Stability to radiation	+	-
Microbe and fungus resistance	-	+
Reactivity	+	-
Processing (low viscosity)	-	+
Low cost	-	+

Major Differences between Polyether and Polyester Based Polyols

Polyether Polyols

The polyether polyols are used in polyurethane adhesives because of performance and economics. Their Tg is about -60°C.

Therefore, the resulting adhesives have very good properties like:

Low temperature performance
Elongation
And impact properties

Polyether polyols are also more resistant to hydrolysis than the polyester based types.

Polyester Polyols

Polyester polyols are often used in formulating polyurethane dispersions and hot melt polyurethane adhesives for the shoe industry because of their high crystallinity resulting in high green (immediate) strength.

Polyester based polyols are available in many molecular structures ranging from linear to highly branched. With more branching, more hydroxyl functionality is available for crosslinking. Polyester polyols have higher tensile strength and greater heat resistance than polyether polyols, but they have poorer hydrolytic resistance, low temperature properties, and chemical resistance.

Isocyanate

The number of available isocyanates and their derivatives is very large, but are generally classified as either aliphatic or aromatic. Aliphatic isocyanates are preferred for polyurethane dispersions because of the lower reactivity of their isocyanate group with water. The most commonly used are 4,4'-Dicyclohexylmethane diisocyanate (H₁₂MDI) because of finer dispersion and better mechanical properties of the final product.

Aromatic isocyanates can also be used if a suitable preparation process is followed. These are sometimes used because of their reactivity and lower cost. However, the aromatic isocyanates are not as light stable as the aliphatics nor are they as resistant to oxidation.

Crystallinity

Polyurethane dispersions can be manufactured with crystalline as well as amorphous molecular backbones.

A crystalline (polyester) polyol is used and provides very open time when heat activated.
Amorphous and blends of crystalline and amorphous polyols are used to adjust performance properties.

Products also differ in heat resistance. The effects of amorphous versus crystalline content on heat activation temperatures, heat resistance, and other properties are shown in table below. It also indicates the suitability of each product for specific industry applications.

Property	Polyurethane Dispersion Product
Property	A	B	C	D	E	F
Heat activation	-	-	-	+	+	++
Crystallization	-	-	-	0	+	++
Initial peel strength	-	+	+	++	++	-
Tack at room temperature	+	+	-	-	-	-
Tack at 50°C	+	+	+	++	++	+
Hot tack life		₊	0	++	+	-
Heat resistance (1K system)	++	+	++	-	-	-
Applications
Furniture		+	++	++	++	+
Automobile				++	++	++
Shoe				++	++	++
Textile	++	++	+	+	+
Packaging	+	++
A: Clear, soft, amorphous polymer for wet bonding applications (e.g. textile) using at least one porous substrate B: Amorphous polymer for flexible bonding applications C: Fast crystallizing polymer optimized for higher application temperatures D: Higher solids level (50%) dispersion with a fast crystallization rate and a longer open time after heat activation than Product E E: Standard product with 40 % solids content with a fast crystallization rate used for a wide range of applications (furniture, automotive) F: Fast crystallizing product optimized for bonding operations with a low temperature requirement; crosslinker needed for heat resistance ++: Very high +: High 0: Medium -: Low --: Very low

Properties and Applications of Selected Polyurethane Dispersion with Properties Noted³

Ionic Content

Incorporating ionic groups into the polyurethane structure help in hydrophilic modification for dispersion in aqueous media. Therefore, polyurethane dispersions can also be classified by their ionic charge.

Ionic Charge	Stable pH Range	Characteristics
Anionic	> 7	Most common form of stabilization for commercial products. Dispersing agent is normally a bishydroxy carboxylic acid
Cationic	< 7	Normally based on alkylated or protonated tertiary amines
Non-ionic	Wide pH range	Have no polarity and are stable over a very wide pH range

Particle Size & Viscosity

The particle size of polyurethane dispersion can be varied from about 0.01 to 5 µm. Particle size also affects the appearance of the final film, from opaque (smaller particles) to milky white (larger particles).

Also, unlike solvent based systems, the viscosity of polyurethane dispersions is independent on molecular weight.

Dispersion Properties

The particle size of polyurethane dispersion can be varied from about 0.01 to 5 µm. It affects the properties of dispersions as:

Dispersions with relatively large particle size (>1 µm) are generally unstable with respect to sedimentation.
Dispersions with smaller average particle size are more useful as they are more storage stable and have a high surface energy.
Dispersions with smaller particle size have a stronger driving force for film formation.

Viscosity

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

The low viscosity and high solids content provides adhesive systems that can be easily sprayed or rolled onto a substrate and give formulators a high level of freedom in designing their formulation. The adhesive dispersion viscosity must:

Ensure good wetting
Should penetrate into the substrate, but not dive too deeply

Attention must also be paid to the relationship between viscosity and shear rate. For the formulator to adjust viscosity, thickeners are employed. Their effectiveness differs according to the type of thickener and the amount added.

Explore the importance and role of rheology modifiers along with their chemistries, selection process, formulation and testing guidelines in order to easily adjust the flow characteristics of your final adhesive formulations.

Rheology Modifiers Selection for Adhesives

Crosslinkable Systems: One-Component or Two-Component

Crosslinkable polyurethane dispersion adhesives can provide properties very similar to solvent borne polyurethane adhesives. They have very good moisture resistance and heat resistance compared to non-crosslinked dispersions.

Crosslinkable polyurethane dispersion adhesives can be formulated as either one-component (1K) systems or two-component (2K) systems where:

The one-component system is a polyurethane dispersion already containing a deactivated latent-reactive isocyanate.
The two-component system is made with a polyurethane dispersion which can be crosslinked with a second component, such as a water emulsifiable polyisocyanate.

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

2K PUD with Emulsifiable Polyisocyanate	1K PUD with Deactivated Isocyanate
Limited pot life once the polyisocyanate is emulsified Error in the mixing ratio is possible when the polyisocyanate is added Processing step must take place at the end-user	Stable if stored < 30°C Ready to use Long open time when applying the adhesive (if < 30°C) Parts to be bonded can be pre-coated and sold as such to the end-user. Bonding process is simplified as there is no adhesive handling at the end-user

Manufacturing Process Parameters with Crosslinkable Polyurethane Dispersions

Blending with Other Polymers

Polyurethane dispersions are generally more expensive than other waterborne systems. Therefore, the addition of less expensive resins will optimize the price/performance ratio of the final adhesive formulation. In some circumstances blending with other resins will also improve certain properties of the formulation for particular applications.

Acrylic latex resins are most commonly used to upgrade performance with respect to toughness, flexibility, abrasion resistance and film forming properties. Water dispersed rosin esters and modified terpene phenolics have also been used to improve creep resistance and tack. These tackifiers are especially useful in formulating polyurethane dispersion contact adhesives.

Thermoplastic Polyurethanes – Key Benefits & Types

Thermoplastic polyurethanes (TPU) are formed by linear polymeric chains comprising a small proportion of hard segments and soft segments. TPU offer benefits like:

Excellent initial and final bonding
Outstanding hot tack that can be tailored according to the required application, and much more.

TPU adhesives used in a wide range of applications. Industries select TPUs for their applications to benefit from their broad combination of properties, such as adhesion to difficult substrates, hot tack, formulation versatility, adjustable sealing temperature, etc.

Adhesion to Difficult Substrates

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

Leather, wood & metals
Rigid & plasticized PVC
Rubbers (TR, SBR, and CR)
Thermoplastic polyurethanes (TPUs)

They can also be used to bond some polyolefins (EVA, PE, PP) if these substrates previously receive an appropriate physical or chemical pre-treatment.

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

Download Case Study: Enhance the Adhesion of Polyurethanes with VPS 7161 – a Special Silane from Evonik

Hot Tack

In the automotive, furniture and footwear industries, the adhesives based on heat activated TPUs must have a high hot tack, in order to guarantee a perfect initial bonding. This property of TPUs is due to their highly crystalline structure.

Formulation Versatility

One of the main advantages when choosing TPUs is that TPUs can be tailor-made to meet the different specifications of diverse industrial applications. Their main physical properties can be adjusted over a wide range according to the application's demands.

The following TPU properties can be tailor-made:

Molecular weight
Crystallization rate
Thermoplasticity
Hot tack adjustment

This results in a wide range of possibilities when we select the product that will be the best choice, once the properties requested have been considered.

Adjustable Sealing Temperature

TPU adhesives are most frequently used in bonding operations where heat activation is part of the process. The optimum adhesive strength can be achieved at a given temperature of the adhesive film. Therefore, to reach such temperature is very important. These values change depending on the nature of the soft segment (polyol) that exists in the TPU.

Solvent-based TPU adhesives and adhesives based on water dispersible TPU pellets produce, after application and drying, non-blocking adhesive films, which can be made tacky by heating.

All of these properties fully comply with requirements of several application areas.

TPU-based Technologies for Most Demanding Applications

The four different TPU-based technologies for your most demanding applications.

TPU Hot Melt Adhesives

Thermoplastic polyurethane for hot melts are special lineal TPUs supplied in pellet form with high thermoplasticity and different crystallization rates. They can be applied by extrusion process or sintering. For many applications Thermoplastic Polyurethane for Hot Melts are favored for their versatile adhesive performance characteristics, improved economy and environmental friendliness.

Thermoplastic polyurethane for hot melts present the following advantages:

Better range of adhesion to different substrates (thanks to polar groups)
Wide crystallization range
Wide melting range (70°-140°C)
Provide excellent low temperature flex properties (low Tg)
High elasticity and softness

Water Dispersible TPU Pellets

The water dispersible pellets are thermoplastic polyurethane in pellet form, with ionomeric groups incorporated in. These TPUs can be firstly dissolved in acetone, and later by means of an emulsification process, it is possible to obtain an aqueous TPU dispersion. This is a Patented process.

Adhesive manufacturers can produce aqueous TPU dispersions themselves, with the following benefits:

Decrease shipment costs
Increase adhesive stability
Free choice of specific TPU dispersion properties to differentiate their adhesives

These water dispersible thermoplastic polyurethane dispersions are used to manufacture adhesives designed for markets such as the automotive, furniture and footwear industries.

TPU Adhesives Films

Based on thermoplastic polyurethane technology, a TPU family supplied in pellet form, has been created specifically to produce TPU adhesive films. These TPUs allow different bonding temperatures, and can be applied by either flat extrusion or blow extrusion process. The TPU adhesive film, produced with TPU, presents the following advantages:

High melting temperature not requested
High adhesion value
High green strength

Solvent-Based TPU Adhesives

These are formulated from linear crystalline TPU polymers in pellet form and are easily dissolved in a wide range of solvents. The selection of the solvents is often determined by the regulations of each country, and by the final physical characteristics of the desired adhesives.

A crosslinking agent (isocyanate) can be added to the original adhesive solution when the adhesive is applied by the end-user. Such systems can be used during a short pot-life of a few hours.

The following benefits must be presented by solvent based Thermoplastic Polyurethane Adhesives:

High green strength at low activation temperature
Easy heat activation
Good heat resistance
High final bonding strength

Polyurethane Grades for Adhesives

View a wide range of polyurethane polymers available today, analyze technical data of each product, get technical assistance or request samples.

References

Manock, H.L., “New Developments in Polyurethane and PU/Acrylic Dispersions”, Pigment & Resin Technology, Vol. 29, No. 3, 2000, pp. 143-151.
Kim, B.K., “Aqueous Polyurethane Dispersions”, Colloid Polymer Science, Vol. 274, 1996, pp. 599-611.
Dispercoll U Polyurethane Dispersions, Bayer Material Science, June 2005.
US Patent 5,834,554.
Manock, H.L., “New Developments in Polyurethane and PU/Acrylic Dispersions”, Pigment & Resin Technology, Vol. 29, No. 3, 2000, pp. 143-151.

Key Applications

Suppliers

Brands

Polyurethane Resins (PU) for Adhesives & Sealants: A Comprehensive Guide

Strengths and Limitations of PU Adhesives

Strengths and Limitations of PU Adhesives

Aliphatic Isocyanates – Functionality & Mechanism

How to Use Aliphatic Isocyanates?

Properties Imparted by Aliphatic Isocyanates in PU Adhesives

Durability and Color Stability

Adhesion on a Wide Variety of Substrates

Heat & Humidity Resistance

Transparency

User-friendly

Reactivity Control

Applications of Aliphatic Isocyanates

Ready-to-use Aliphatic Polyisocyanates – 1K and 2K Systems

Polyurethane Prepolymers – Mechanism & Applications

Properties of Polyurethane Prepolymers

Polyurethane Polyols – Production & Features

Polyether Polyols Production Process

Polyester Polyols Production Process

Polyurethane Dispersions – Chemistry & Selection Tips

Chemistry of Polyurethane Dispersion

One-Component Systems

Two-Component Systems

Selecting Right Polyurethane Dispersion

Choice of Raw Materials

Crystallinity

Ionic Content

Particle Size & Viscosity

Crosslinkable Systems: One-Component or Two-Component

Blending with Other Polymers

Thermoplastic Polyurethanes – Key Benefits & Types

Adhesion to Difficult Substrates

Hot Tack

Formulation Versatility

Adjustable Sealing Temperature

TPU-based Technologies for Most Demanding Applications

TPU Hot Melt Adhesives

Water Dispersible TPU Pellets

TPU Adhesives Films

Solvent-Based TPU Adhesives

Polyurethane Grades for Adhesives

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