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Polyurea Adhesives and Sealants - All You Need to Know

Edward M. Petrie – Jul 31, 2020

TAGS:  Reactive Adhesives      Sealants    

Polyurea Adhesives and SealantsPolyurea adhesives and sealants offer fast-curing systems with interesting properties. Polyurea chemistry has been well developed for high-performance maintenance coatings. However, polyureas are not as well-known as adhesives or sealants except in applications that require exceptionally fast cure, such as high-volume assembly (e.g. RFID chips or fast-setting construction sealants).

Let’s review the chemistry of polyureas used in adhesive and selant formulations. This article assesses the advantages and disadvantages of these interesting polymers in current and future applications. To achieve adhesive and sealant products that have a fast cure with properties that are equivalent or superior to conventional polyurethane and epoxy systems, updated formulation principles will also be provided here.


Chemistry of Polyureas Versus Polyurethanes


Polyureas are similar to polyurethanes but they are based on reacting an isocyanate with a multifunctional amine rather than with a polyol (see figure below). The polyurea chemical reaction occurs quickly in a single step without added catalysts. This type of reaction offers advantages, such as:

  • It provides good bonding and physical properties within a few seconds to minutes at room temperature.
  • This unique “auto-catalytic” nature is one of the most notable characteristics of polyurea products.

Chemistry of Polyurea and Polyurethane
Chemical Reaction for a Polyurethane (Top) and Polyurea (Bottom)

With a conventional 2K polyurethane adhesive, the base chemicals consist of an isocyanate and a polyol with a mixing ratio of approximately 1:1. The 2K polyurea adhesives consist of isocyanate and an amine, also mixed at a ratio of approximately 1:1. The high reaction speed of a polyurea often requires dedicated mixing and dispensing technology.

Current developments in the industry have aimed at controlling (slowing) the gelation of polyurea without sacrificing performance properties.

The Polyurea Development Association (PDA) is the official trade association for the polyurea industry. Polyurea is defined by the PDA as a material derived from the reaction product of an isocyanate component and a resin component.

  • The isocyanate can be aromatic or aliphatic in nature. It can be a monomer, polymer, or any variant reaction of isocyanates, quasi-prepolymer, or a prepolymer.
  • The prepolymer or quasi-prepolymer can be made of an amine-terminated polymer resin or a hydroxy-terminated polymer resin.

The table below defines the differences between a polyurea, polyurethane, and polyurea/polyurethane hybrid.

Chemistry Characteristics
Polyurea
  • A polyurea provides a chemical backbone containing amine linkages.
  • Polyurea has been used as an industrial coating and sealant in severe environments with good chemical resistance.
  • For example, it is resistant to hydrocarbons and hydrogen sulfide gas and immersed sewage application.
Polyurethane
  • Usually these formulations have no amines in the polymer backbone.
  • All functionality is considered to be hydroxyl.
  • Polyurethanes show good longevity and are relatively inexpensive.
Polyurea/Polyurethane
  • This hybrid is the result of a chemical reaction between an isocyanate and a mixture of polyol and amine reactants.
  • Hybrid formation can display some of the negative problems associated with polyurethane chemistry (less resistance to immersion or extreme application temperature conditions).

Characteristics of Polyurea, Polyurethane, and Polyurea/Polyurethane Hybrids

The classic water/isocyanate reaction used with most moisture-cured polyurethane adhesives and sealants also produces urea groups at the end of the process. However, this reaction should not be considered a polyurea reaction since the mechanism is a two-step process. It is a much slower reaction and produces carbon dioxide as a byproduct.

The properties of polyurea, polyurethane, and polyurea/polyurethane products can be made comparable since there is a great degree of freedom in the formulation. The unique characteristics of polyurea systems are described below.



Properties of Polyurea Technology


The exceptional features of polyurea technology that can prove useful in adhesive and sealant formulations include:

  • Fast, consistent reactivity and cure
  • Moisture and temperature insensitivity during processing
  • Excellent physical properties/elastomeric qualities
  • Exceptionally low water absorption qualities
  • Hydrolytic stability
  • Thermal stability
  • Good UV and color stability (with aliphatic isocyanate types)
  • No solvents or VOCs required for thin coatings

The gel time of polyurea adhesive/sealant compositions is typically on the order of a few minutes, and it is consistent over a wide temperature range (-20°C to 60°C) since the polyurea polymer does not rely on a catalyst to complete the reaction. Furthermore, the cure characteristics are very uniform over a broad range of application humidity.

Unlike polyurethanes, polyureas are unaffected by ambient moisture during application and cure. This makes polyureas ideal for use in outdoor applications such as the building and construction sectors.

The table below provides for a performance comparison of common polymers.

Performance Polyurea Polyurethane Epoxy Thermoplastic Acrylic Polysulfide
Physical strength Low-High Low-Mid High Mid-High Low-Mid
Elongation High High Low Low-Mid Mid-High
Impact resistance High Mid-High Mid Mid-High Mid
Abrasion resistance High Mid-High Mid-High Mid-High Mid
Adhesion to concrete Low-High Low-Mid High Low-Mid Low-Mid
Cure shrinkage Low Low Low High Low
Permeability Low Mid-High Low Low-Mid Mid-High
UV resistance Mid-High Low-High Low High High
Creep Low High Low-Mid Low-Mid High
Temperature limit Low-High Mid Mid-High Mid Low-Mid
Chemical Resistance
Mineral acid Low-Mid Low-Mid Mid-High Mid Low-Mid
Organic acid Mid Low-Mid Mid Mid Low-Mid
Alkali Mid-High Low-Mid High High Low-Mid
Chlorinated solvent Low-Mid Low-Mid Low-High Low-Mid Low-Mid
Oxygenated solvent Low-Mid Low-Mid Low-High Low-Mid Low-Mid
Hydrocarbon solvent Low-Mid Low-Mid Mid-High Low-High Low-Mid
Salts High Mid-High High High Mid-High
Water High Low-High Mid-High Mid-High Low-High

Performance Comparison of Polyurea to Other Common Polymers2

Compared to polyurethanes, polyureas can have superior physical properties, such as:

  • High temperature performance
  • Adhesion
  • Low creep, and
  • Tear strength

Due to the remarkably high reactivity, polyureas are known to impart a non-sagging characteristic once it is applied even though it is mixed in a low viscosity liquid state. When compared to epoxies, polyurea systems can have higher elongation and better UV resistance depending on the formulation.


Disadvantages of Polyureas


The main disadvantages of polyurea in adhesive and sealant applications are:

  • They are extremely fast and usually require automated metering and mixing equipment.
  • Heat-sensitive substrates may also be damaged by the high exotherms generated with polyurea systems.
  • The polyether backbone structure of the polyurea molecules is also not suited for environments containing strong oxidizing chemicals. This is especially true for the aromatic-based polyurea products.
  • Polyureas tend to be more expensive than polyurethanes, but their fast cure rates are often worth the extra cost.


Formulation Refinements for Polyurea Systems


As with other adhesives and sealants, one should not consider the chemical class (e.g. “polyurea”) a specific material. Rather, it is a description of a technology that represents a variety of formulation possibilities to achieve the desired performance. This is done through the selection of various raw materials, much like polyurethane chemistry. The selection of the optimal raw materials can be a complex procedure, which is described elsewhere3.

Polyurea properties can range over a significant degree depending on the formulation employed. For example, typical polyurea sealant properties range in:

  • Hardness from 20-95 Shore A
  • Tensile strength from 180-1600 psi
  • Elongation from 150-1000%, and
  • Tear strength from 100-180 pli.

The table below shows the properties of two optimized polyurea formulations.

Components Formulation 1 Formulation 2
Part A Parts by weight
DESMODUR® E 210 100 100
Part B
Jeffamine® D-2000 Amine 41.28 38.00
Desmophen® NH 1420 47.67 51.75
Tinuvin® 292 0.24 0.25
Tinuvin® 1130 0.24 0.25
Tinuvin® 1135 0.49 0.50
Kronos TiO2 9.82 5.00
Properties (Mix Ratio: 1:1 by Volume)
Gel time, mins 6.1 9.5
Hardness, Shore A 91 91
Tensile strength, psi 1218 714
100% Modulus, psi 689 502
200% Modulus, psi 876 611
300% Modulus, psi 1091 720
Maximum elongation, % 352 450
Tear resistance, pli 341 288

Properties of Optimized Polyurea Formulations4

As indicated earlier, polyurea polymers are derived from the reaction product of an isocyanate component and a resin blend component. Like polyurethanes, the polyurea molecular backbone is composed of hard and soft segments.

  • The soft segment of the polyurea polymer is formed from multi-functional, high molecular weight amine-terminated polyether polyols.
  • The lower molecular weight aromatic diamine chain extenders are responsible for the hard segment.

The polyurea network consists of alternating flexible polyether blocks and short and stiff urea blocks. The urea segments associate to hard domains that are connected via hydrogen bonds. These domains act as physical crosslinks and provide a restorative force when the thermoplastic urea elastomer is stretched. However, urea resins can also be chemically crosslinked. For example, by reacting isocyanates with trifunctional amines, such as polyoxypropylene triamine.

The isocyanate component is often a soft block-prepolymer based on aromatic or aliphatic isocyanates, such as:


The isocyanates can be partially polymerized with higher molecular weight amines, such as amine-terminated ethylene or propylene oxide based polyethers (e.g. Jeffamine's® from Huntsman).

Isocyanate Grades

Common chain extenders are low molecular weight amines, such as:

  • Diethylenetriamine, triethylene tetraamine, or etheramines for aliphatic ureas.
  • Diethyl-toluene diamine, dimethylthio-toluene diamine, or N,N’-di(sec.butyl)- amino-biphenyl methane for aromatic ureas.

Polyureas made with aliphatic isocyanates are more UV-stable and are less susceptible to oxidation and degradation than polyureas made with aromatic isocyanates. However, they are more expensive and are only used when high UV stability is required, such as for exterior coatings. For most other applications, aromatic isocyanates are preferred.

Much of the formulation development for polyurea adhesives and sealants have focused on methods to moderate the fast cure. This allows greater freedom in application processes (e.g. longer working life and freedom from dedicated mixing/dispensing equipment) as well as better wetting and penetration into porous substrates.

Several processes have been developed to moderate cure and exotherm. However, even with slower setting formulations, polyurea adhesives and sealants generally require the use of two-component automatic metering and mixing equipment employing a static mixing tube with automatic flushing operation.

Formulation refinements to control the setting time of polyurea systems generally focus on the
effective concentration of the amino group in the reacting system.


Freedom to Specify Reactivity with the Fate of Amines


Aromatic amines, such as dimethylthio-toluene diamine or N, N’-di(sec.butyl)- amino-biphenyl methane slow down the reaction significantly, whereas most aliphatic amines react rapidly with isocyanates, often within a few seconds.

To slow down the reaction, aspartic ester amines are often used, which provide secondary amine functionalities that cure noticeably slower than most other aliphatic amines. The ester groups attached to the secondary amines greatly reduce the reactivity compared to typical secondary amines due to the steric hindrance of these groups.

  • The slower reaction kinetics of secondary, alkythiolated, and hindered primary aromatic diamine chain extenders, allow for a more controlled viscosity rise in the reaction mixture. This minimizes the initial viscosity of the reaction to permit more time for thorough mixing of the components before the mixture begins to gel. The slower reaction is not achieved by incorporation of hydroxyl-terminated resins, but through careful formulating of amine-terminated chemicals to achieve slower rates.

  • By modifying the traditional diamine structure and increasing molecular weight through formation of an oligomer, reactivity can be modified to the point where adhesive applications become feasible while still maintaining the well-known superior properties of the polyurea system. This can be accomplished by condensation of p-amino-benzoic acid with a difunctional diol and the formation of an amine functional telomer. The reactivity of the p-amino group with isocyanates is decreased by the carboxyl functionality5.

  • Bayer’s aspartic esters technology uses Desophen® NH1420 solvent-free amine functional resin in combination with DESMODUR® E 210 isocyanate to create a polyurea sealant that allows reactivity to be adjusted from approximately 2 to 30 minutes. The freedom to specify a fast or slow reactivity makes two-component polyurea a versatile solution for a wide spectrum of applications6.

  • Polyurea sealants have also been formulated with specially engineered MDI-based isocyanates, which contribute to extending pot life and helping create polyurea polymers to meet specific performance requirements7


Applications of Polyureas in Adhesives & Sealants


Two-component Polyurea Systems


Two-component polyurea systems can be formulated to set in as little as several seconds without the use of a catalyst. As a result, one of the first commercial applications for polyurea systems was in RIM applications in the automotive sector and fast-setting construction sealants.

Two-component polyurea elastomeric coatings have been available for some time for construction coatings.

Applications include:
  • Tank linings
  • Pipeline coatings
  • Flooring
  • Parking decks
  • Bridge coatings
  • Aquarium lining
  • Road striping
  • Oil platforms
  • Potable water storage tanks
  • Food processing facilities, and many others.

Adhesive and sealant applications are now just developing. Adhesion can be difficult, especially for the fastest-setting polyurea formulations. The fast set time does not allow sufficient wetting and penetration to occur to provide exceptionally high degrees of bond strength. However, set time can be managed with the proper formulation.

The high number of hydrogen atoms on the polyurea backbone (see figure above) provides a significant opportunity for hydrogen bonding to occur. Proper surface preparation and substrate pretreatment are always necessary.


Polyurea Sealants


Polyureas for Building Joints
Polyurea sealants have been claimed to provide moisture resistance equivalent to most polyurethane sealants with superior UV and color stability.

  • More importantly, they offer the option of low-temperature cures with a wide latitude of gel times.
  • They also offer low VOC and plasticizer-free formulations, making them an environmental friendly alternative to certain other types of sealants.

Polyurea is being successfully used as a multi-purpose joint fill, caulking, and sealant material. It can provide a flexible, formable, weather tight, and traffic resistant seal for all types of building joints. These include expansion joints and control joints in masonry floors, perimeter joints, panels and doors, water reservoirs, etc.

The polyurea sealant has excellent crack bridging properties with high elongation and tensile strength. The fast cure time and impermeability to moisture allows for a quicker installation with a wider application window.


Polyurea Adhesives


Polyurea adhesives are noted for high adhesive strength to wood and concrete substrates. They have been developed primarily for non-sagging products that can be conveniently manipulated in a low viscosity liquid form but then applied to vertical surfaces as a thick non-running material.

The auto industry provides a good example of the use of polyurea adhesives. During automotive manufacturing, structural adhesives can be subjected to temperatures of about 200°C. At these temperatures, many of the more common adhesives (e.g. polyurethanes) lose their integrity by foaming, cracking, and/or softening with a consequential loss in physical and adhesive properties. Polyureas can be mixed and applied via static mixers, cure quickly, and resist these adverse conditions.


Recent Developments Increase the Demand of Polyureas


Several new products have been developed in recent years that foretell increased demand for polyurea adhesives and sealants in the future.

  • Nolax (Switzerland) also found that fast-reacting polyurea adhesive bonds are very well to soft and hard woods. They provide:
    • High green strength in seconds
    • Full bond strength after 1-2 days of cure at room temperature.
    These adhesives are resistant to temperature of greater than 100°C and provide D4/C4 water resistance according to EN 204/2058.

  • Nolax has also developed a system consisting of a combination of polyurea and polyisocyanurate (PIR) reactions which leads to unique adhesives with the following properties:
    • Adhesion to a wide selection of substrates
    • High shear strength at elevated temperatures, and
    • High impact resistance depending on formulation9.

  • Several waterborne polyurethane urea dispersions have been prepared by mixing different amounts of two waterborne polyurethane urea dispersions made with polyester and polycarbonate diol. Their crystallinity, thermal, rheological, viscoelastic and adhesion properties depended on the segmented structure and degree of phase separation which were determined by the different content of the parent dispersions10.

    ETERNACOLL® polycarbonate diols (PCD) by UBE

  • Pressure sensitive urea-based adhesives are disclosed which are prepared by the polymerization of reactive oligomers. The reactive oligomers can be prepared from polyamines through chain extension reactions using diaryl carbonates followed by capping reactions. Adhesive articles, including optical adhesive articles may be prepared using the adhesives11.

This article was originally published on Feb 24, 2010 and updated on July 31, 2020.1



Formulating Advanced Reactive Hot Melt Adhesives with New Chemistries


Take the course by our Industry expert Edward M. Petrie where he will go beyond traditional PUR-based reactive hot melt adhesives to match today’s performance requirements (increased shelf life, pot life; difficult to bond substrates, a wider range of service temperatures, reduced free isocyanates (NCO)...).

Formulating Advanced Reactive Hot Melt Adhesives




References

  1. Petrie, E.M., “Polyurea Adhesives and Sealants”, SpecialChem, February 24, 2010.
  2. Adapted from “Polyurea Spray Coatings” by Dudley J. Primeaux II, Primeaux Associates, LLC, Elgin, Texas.
  3. Primeaux II, D.J., “Polyurea Elastomer Technology: History, Chemistry, and Basic Formulating Techniques”, Primeaux Associates, LLC, Elgin, Texas, also Primeaux II, D.J., “Two-Component Polyurea Coatings / Linings”, in Selecting Coatings for Industrial and Marine Structures, The Society for Protective Coatings, 2008.
  4. Bayer Material Science.
  5. Santosusso, T. M., “Room Temperature Curable Polyureas for Adhesive Applications”, Journal of Cellular Plastics, Vol. 27, No. 1, 1991, p. 48.
  6. Johnston, J., “Fast Curing Polyurea Sealants: Controllable Reactivity with Commercially Available Secondary Amines”, Thermoset Resin Formulators Assoc. Meeting, Savanah, GA, Sept. 10-11, 2007.
  7. Broekaert, M. (Huntsman Polyurethanes), “Modified MDI-Prepolymers Improve the Initial Physical Properties and Reduce the In-Service Time of Aromatic Polyurea Coatings”, Society of Manufacturing Engineers, 2000.
  8. Nolax, member of the Collanao Group, Sempach Station, Switzerland.
  9. PIR-a New Class of Structural Polyurea Based Adhesives, Nolax.com, October 2016.
  10. Fuensanta, M., and Jofre-Reche, J.A., “Structure and Adhesion Properties Before and after Hydrolytic Ageing of Polyurethane Urea Adhesives Made with Mixtures of Waterborne Polyurethane Dispersions”, International Journal of Adhesion and Adhesives, Vol. 85,. June 2018. WO 2009/085662.

3 Comments on "Polyurea Adhesives and Sealants - All You Need to Know"
Francesco V Jan 18, 2021
Very informative article, I'm intrigued by the polyaspartic opportunites. Thanks
Joseph S Aug 10, 2020
Excellent article
Sheila M Aug 6, 2020
Dear all, Can I use this same approach to overprint varnish on flexible food packaging? Thanks and regards, Sheila

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